Proposed mechanisms of neuroprotection for nicotine in Parkinson's disease

Abstract

Parkinson's disease (PD) is a neurological disorder that is characterized by the death of dopaminergic neurons in the substantia nigra. Despite extensive research, the exact cause of PD is unknown, and current treatment options are centered on symptom management rather than disease modification. Most, though not all, epidemiologic studies have demonstrated reduced risk of development of PD among smokers, generating interest in nicotine, a key component of tobacco. Many preclinical investigations have investigated nicotine's neuroprotective properties, especially through its interaction with nicotinic acetylcholine receptors in the central nervous system. Nicotine has been linked to a variety of cellular activities, including neurotransmitter release, neuronal survival, and anti-inflammatory responses. Animal studies in PD models have indicated that nicotine administration can attenuate the degeneration of dopaminergic neurons and ameliorate behavioral abnormalities. Clinical investigations evaluating nicotine as a treatment for PD have yielded mixed results in terms of efficacy. Thus, central questions remain about the effects of nicotine in patients with established PD, and neither nicotine nor smoking are recommended for treatment or prevention of PD. Further research on the multiple proposed mechanisms of nicotine is required, with particular emphasis on elucidating symptomatic versus neuroprotective effects. The aim of this scoping review is to provide a comprehensive discussion of the proposed mechanisms of neuroprotection for nicotine in Parkinson's disease.

Plain language summary

The aim of this review is to summarize the current scientific evidence on nicotine in PD. We describe the large body of scientific articles that address the ways in which nicotine may affect Parkinson's disease (PD). Epidemiologic studies have shown that people who smoke have a lower risk of developing PD, and there are multiple potential explanations for why this may be the case. Because nicotine is one active ingredient in tobacco, many studies have used cellular or animal models of PD to try to determine how nicotine may reduce risk of PD or may be of benefit in established PD. Many of these studies suggest that nicotine can prevent or reduce injury to dopaminergic neurons after exposure to toxic chemicals. However, many questions remain, and clinical trials in which patients with PD are treated with nicotine have largely not shown benefit to patients. Thus, neither smoking nor nicotine treatment are recommended for either prevention or treatment of PD at this time.

Introduction

Parkinson's disease (PD) is a neurodegenerative disorder that is characterized by aggregation of α-synuclein and loss of nigrostriatal dopaminergic neurons. ¹ There are multiple known genetic and environmental risk factors for PD, ¹ but the ways in which these risk factors ultimately cause disease pathogenesis are poorly understood. Many epidemiologic studies have demonstrated an inverse association between smoking and risk of PD (Table 1), which has begged the question of whether nicotine, the presumed active ingredient in tobacco, is neuroprotective in PD. In this narrative review, we will focus primarily on the many proposed mechanisms of action by which nicotine may exert neuroprotective effects in animal models of PD, with briefer discussions of the epidemiologic literature and clinical trials. We conclude by highlighting several outstanding questions that require further investigation.

Table 1.

Meta-analyses and systematic reviews on the epidemiology of smoking and PD.

Meta-analyses and systematic reviews addressing smoking and risk of PD
Citation	Year	Study design	Outcomes: relative risk (RR), pooled risk estimate, hazard ratio (HR) or odds ratio (OR), with (95% confidence interval)
2	2022	Summarized 29 published MR studies and 123 de novo MR analyses of 57 distinct primary outcomes	OR of PD 0.83 (0.72–0.95) for genetic liability to smoking initiation
3	2021	Meta-analysis of 14 articles with 697,185 total patients	RR of PD 0.83 (0.72–0.95) for secondhand smoking exposure
4	2021	Overview of 46 systematic reviews	RR of PD 0.64 (0.6–0.69) for ever smokers
5	2021	Systematic review of 53 studies	HR of PD 0.41 (0.28–0.61) in never-smoking smokeless tobacco users, with lower risk for moderate-heavy smokeless tobacco users and long-term duration
6	2021	Systematic analysis of 14 umbrella reviews (203 unique meta-analyses)	RR of PD 0.64 (0.60–0.69) in smokers
7	2020	Systematic review and meta-analysis of 21 studies	RR of PD 0.73 (0.56–0.95) for exposure to second-hand smoking
8	2016	Meta-analysis using 893 articles	RR of PD 0.46 (0.42–0.51) or 0.41 (0.34–0.48), depending on model used, in current smokers
9	2015	61 case-control and 8 cohort studies	RR of PD 0.59 (0.56–0.62) for ever smokers RR of PD 0.66 (0.49–0.88) for those smoking more than 30 pack-years RR of PD 0.39 (0.29–0.53) for those smoking less than 30 pack-years
10	2012	Systematic review of 202 articles and meta-analysis of 173 studies	RR of PD 0.44 (0.39–50) for current smokers
11	2011	Meta-analysis of 54 epidemiological studies	Risk estimate for PD 0.31 (0.25–0.38) for current smokers Risk estimate for PD 0.72 (0.63–0.83) for former smokers Risk estimate for PD 0.55 (0.51–0.59) for ever smokers
12	2007	Pooled analysis of PD using 8 US case-control and 3 cohort studies	OR of PD 0.48 (0.36–0.65) for current smokers in case-control studies OR of PD 0.67 (0.55–0.82) for former smokers in case-control studies OR of PD 0.61 (0.51–0.72) for ever smokers in case-control studies OR of PD 0.28 (0.14–0.56) for current smokers in cohort studies OR of PD 0.71 (0.50–1.00) for former smokers in cohort studies OR of PD 0.59 (0.43–0.83) for ever smokers in cohort studies
13	2007	Meta-analysis of 16 prospective studies	RR of PD 0.51 (0.43–0.61) for smoking
14	2004	Pooled results of 7 prospective studies	RR of PD 0.51 (0.43–0.61) in ever smokers RR of PD 0.35 (0.26–0.47) in current smokers RR of PD 0.66 (0.49–0.88) in former smokers
15	2004	Review of 26 studies (21 case-control, 4 cohort, and 1 cross-sectional)	Risk estimate of PD 0.37 (0.33–0.41) for current smokers Risk estimate of PD 0.84 (0.76–0.92) for former smokers
16	2003	Pooled three case-control studies	Risk estimate of PD 0.82 (0.44–1.53) in ever smokers with positive family history of PD Risk estimate of PD 0.77 (0.59–1.010 in ever smokers with negative PD family history
17	2002	Four case-control studies in Chinese patients were pooled	OR of PD 0.77 (0.60–0.97) for smoking
18	2002	Systematic review including 44 case-control and 4 cohort studies	RR of PD 0.59 (0.54–0.63) for ever smokers RR of PD 0.80 (0.69–0.93) for past smokers RR of PD 0.39 (0.32–0.47) for current smokers
19	2001	Summarized risk estimate using 46 studies	Risk estimate for PD 0.54 (0.49–0.60)
Meta-analyses and systematic reviews addressing the effect of smoking on cognition among patients with PD
20	2019	Meta-analysis of 32 articles and 18 factors	RR of cognitive impairment in patients with PD 1.31 (1.14–1.5) for smoking
21	2016	Meta-analysis of 25 studies	RR of dementia in patients with PD 1.93 (1.15–3.26) in patients in ever versus never smokers

MR: Mendelian randomization.

Methods

This scoping review is intended to summarize the large body of scientific literature on smoking, nicotine, and PD. PubMed was searched for the terms “smoking and Parkinson's disease” and “nicotine and Parkinson's disease.” Results from each search were exported to a Microsoft Excel spreadsheet using the “Send to collections” function in PubMed. Results from each search term were merged, and duplicate results were excluded, leaving 1481 studies for potential inclusion (Figure 1). Of those, we excluded manuscripts that were deemed not on topic, were not peer-reviewed, were not available in English, or were not available to us in full text. Of the remaining 743 studies, we prioritized focusing on original research, rather than other narrative reviews or commentaries. The 507 original research studies fell into three broad categories: 1) epidemiologic studies examining a potential inverse relationship between smoking and PD, 2) studies involving cellular and animal models of PD in which nicotine's potential benefit to dopaminergic neurons was assessed, and 3) clinical trials in which patients with PD were given nicotine in an attempt to alleviate symptoms. Each of these topics is included in this review. For the epidemiologic studies, we have included meta-analyses and systematic reviews of the so that readers can critically appraise the strength of this body of literature. We have also included Mendelian randomization studies, as they address the critical issue of whether the inverse relationship between smoking and Parkinson's disease is correlative or causative. We largely did not include individual epidemiologic studies due to the size of this body of literature and because the meta-analyses themselves capture the individual studies. For studies on cellular and animal models of PD, we prioritized high quality mechanistic research, while also trying to convey the full scope of models used, from cell culture to rodents. Finally, for the clinical trials, we prioritized inclusion of trials that enrolled the largest number of patients. The intent of the review was to be reasonably, but not absolutely, comprehensive, prioritizing what were, in our opinion, the highest quality and most relevant studies for inclusion.

Figure 1.

Approach to inclusion of manuscripts. PubMed was searched on October 1, 2024 for the terms “Smoking and Parkinson's disease” and “Nicotine and Parkinson's disease,” which yielded 483 and 1177 publications, respectively. The results were merged and duplicates deleted, leaving 1483 publications. Of these 738 were omitted, leaving 743. Of these, we prioritized original research. 507 original research manuscripts were categorized by topic and evaluated for inclusion in the review. These manuscripts fall into the 3 broad categories of epidemiologic studies, mechanistic studies involving cellular and animal models of PD, and clinical trials. The highest quality manuscripts were selected for inclusion. This figure was created with BioRender Olsen, A. (2025) https://BioRender.com/eyvunlm.

Epidemiology of smoking and Parkinson's disease

Hundreds of epidemiological studies dating back to the 1960s have analyzed the relationship between cigarette smoking and the development of PD. These studies comprise various methods, including case-control, cohort, and twin studies. Most have demonstrated that risk of development of PD is inversely correlated with smoking. Multiple studies support the idea that the inverse relationship between smoking and PD might be the strongest environmental association with PD, among many that have been examined.^8,22 Summarizing this substantial literature consisting of hundreds of epidemiologic studies is beyond the scope of this review. Instead, we provide readers with a table summarizing 20 meta-analyses and systematic reviews (Table 1).

Although most studies associate smoking behavior with decreased risk of developing PD, not all studies confirm these findings, and there is a risk that publication bias has influenced the literature. Noyce et al. 2019 examined 12 exposures and risk of PD, and found an inverse association between smoking and PD, but only of nominal significance. ²³ Further, Xue et al. found that the number of cigarettes smoked was positively associated with PD, and cessation of smoking was inversely associated with PD. ²⁴ With these exceptions, the relationship between smoking and PD risk is robust across many epidemiologic studies. There are issues that complicate the interpretation of the relationship, however. First, there is the question of whether the inverse relationship between smoking and PD is causal or merely correlative. This has been addressed in part by Mendelian randomization studies and studies that examine a dose-response effect between the amount or duration of smoking and PD. Second, there is the issue of other factors that can modify the association between smoking and PD, which include both environmental and genetic factors. We briefly discuss these issues in the sub-sections below.

Mendelian randomization studies

One potential hypothesis underlying the inverse relationship between smoking and PD is that patients who are predisposed to develop PD tend to avoid risky or habit-forming behaviors like smoking. To attempt to determine a causal, rather than correlative, inverse relationship between smoking and PD, several Mendelian randomization studies have been conducted that support a causal inverse association between smoking and PD.

Noyce et al. developed a PD mendelian randomization research portal that investigated a large number of exposures, finding a nominally significant inverse association between smoking and PD ²³ Domenighetti et al. identified an inverse association between smoking initiation and PD that was not explained by reverse causation, confounding, or survival or incidence-prevalence biases. ²⁵ Further supporting the inverse relationship between smoking and PD, another study used a split-sample MR to determine that ever-smoking reduced the risk of PD with an odds ratio of 0.955. ²⁶ Dominguez-Baleon et al. found a causal inverse association between current (but not former) smoking and PD and postulated that this might indicate a stronger predisposition to habit-forming behaviors in those who are unable to quit. ²⁷ However, this result could also be explained by a dose-response to smoking. Further arguing against risk aversion as the underlying explanation, Grover et al. found a causal inverse association between ever smoking and PD but found that other risky behaviors beyond smoking were positively associated with PD. ²⁸ Sieruin et al. examined personality directly, finding that neuroticism was not directly associated with PD but was associated with smoking initiation, which in turn was inversely associated with PD. ²⁹ Finally, a meta-analysis of 29 Mendelian randomization studies and de novo Mendelian randomization of 57 primary outcomes identified a causative association with genetic liability to smoking and PD. ² In contrast to the above studies, Xue et al. found that genetically predicted smoking cessation was associated with a decreased risk of PD (OR 0.41). ²⁴

Collectively, the majority of the above Mendelian randomization studies support a causal inverse relationship between smoking and risk of PD. Still, caveats remain about the method of Mendelian randomization, including selection and survival biases, pleiotropy, and the possibility that single nucleotide polymorphisms (SNPs) influence the level of environmental exposure in the first place. ³⁰

Evidence for a dose-response relationship

Further lending credibility to a causal inverse relationship between smoking and PD is evidence for a dose-response relationship between the amount of smoking and risk of PD as well as between the duration of smoking and risk of PD. Mappin-Kasirer et al. found an inverse dose-response relationship between the daily amount of tobacco smoked and the risk of PD in the British Doctors cohort study. ³¹ Compared to never-smokers, people who had quit smoking 10 or more years ago had a 15% lower risk of PD. ³¹ Those who quit 0 to 9 years ago had a 29% lower risk than never-smokers. Therefore, the protective effect of smoking on the risk of PD was attenuated with increasing duration of time since quitting smoking. ³¹

Similarly, another group reported a 58% lower risk of developing PD in current smokers compared to never-smokers, and a 41% lower risk in ever-smokers compared to never-smokers. ⁹ Finally, van der Mark et al. found that there was an inverse association between total pack-years of smoking, longer smoking duration, and shorter time since cessation and PD risk. ³² The studies described above only included genetically unrelated patients, leaving questions as to whether these differences were solely due to nicotine intake or if genetic factors could be involved. A twin study attempted to understand this relationship by comparing PD patients with twins to unrelated controls as well as to their other twin. The authors identified a protective effect of smoking on PD when comparing PD patients to unrelated controls (OR 0.69), which was somewhat attenuated when comparing PD patients to twin controls (OR 0.81), demonstrating some degree of genetic confounding. ³³ Overall, these studies demonstrate a strong inverse correlation between smoking intensity and duration and PD risk but do not rule out genetic confounders.

Environmental modifiers of the relationship between smoking and PD

Environmental factors may influence the association between PD risk and smoking. Kim and colleagues found that the risk of PD was lower in those who quit smoking and had a body mass index (BMI) increase or BMI maintenance, whereas people with a BMI decrease still had a lower incidence of PD development, but this was not as robust as the other groups. ³⁴ When assessing weight change alone, BMI increase but not BMI decrease significantly increased the PD risk compared to BMI maintenance. ³⁴ Post-cessation weight gain did not offset the protective impact of smoking on PD development, but overall weight gain predicted an increased risk of PD. ³⁴

Some studies have shown the potential for alcohol to contribute to a decreased risk of developing PD, but the results are mixed. A meta-analysis by Zhang et al. suggested that alcohol intake might be inversely associated with PD risk. ³⁵ In this study, beer appeared to have a stronger effect than liquor, but these results only applied to men, as almost no difference was observed in women from any type of alcohol. ³⁵ The authors also pointed out that alcohol intake is often accompanied by smoking and caffeine intake which are also thought to reduce the risk of PD development. ³⁵ Therefore, it is unknown if these findings result from alcohol alone or its association with smoking and caffeine. ³⁵ Similarly, Palacios et al. ³² found a correlation between high caffeine intake, smoking and alcohol, but determined that total alcohol intake was not associated with the risk of developing PD. ³⁶ The authors suggest that these data demonstrate an aversion to addicting substances in people predisposed to develop PD later in life. ³⁶ These findings were supported once again by another team who found no association between baseline or lifetime alcohol consumption and PD risk; however, this study did not specifically account for differences in the amount of alcohol consumed. ³⁷ Although most studies did not specify the amount of alcohol consumed by patients, Jung et al. did, finding that light to moderate alcohol consumption resulted in a decreased risk of PD, while heavy drinking did not, demonstrating a U-shaped association based on the amount of alcohol consumed. ³⁸ Based on these studies it is unclear if alcohol and caffeine interact with smoking to influence risk of PD.

Potential gene interactions affecting the relationship between smoking and PD

Several investigations have examined the influence of genetic modifiers of the relationship between smoking and PD. The most common reported gene-environment interactions include nicotinic acetylcholinergic receptors,^39–42 monoamine oxidase B (MAO-B),^43,44 the HLA-DRB1 major histocompatibility complex,^45,46 haptoglobin,^47–49 glutathione S-transferase (GST) family members,^50–52 nitric oxide synthetase family members,^53–55 and other neurodegenerative disease proteins (Figure 2).^56–62 These putative interactions require confirmation in additional studies and diverse populations.

Figure 2.

Potential genetic modifiers of smoking and PD. Many genes have been purported to have a relationship with both smoking and risk of PD. Confirmatory studies are required to validate these interactions. Abbreviations and references: CHRNA3 = cholinergic receptor alpha 3 subunit, ⁴⁰ CHRNA4 = cholinergic receptor alpha 4 subunit, ⁴² CHRNA5 = cholinergic receptor alpha 5 subunit, ⁴¹ CHRNB3 = cholinergic receptor beta 3 subunit, ³⁹ GSTM1 = glutathione S-transferase mu 1, ⁵⁰ NAT2 = N-acetyltransferase 2, ⁵⁰ GSTP1 = glutathione S-transferase pi 1,^50,51 GSTO1 = glutathione S-transferase omega 1, ⁵² GSTO2 = glutathione S-transferase omega 2, ⁵² NOS1 = nitric oxide synthase 1, ⁵⁵ NOS2A = nitric oxide synthase 2,^53,54 haptoglobin,^47–49 NDUFV2 = NADH:ubiquinone oxidoreductase core subunit V2, ⁶³ APP = amyloid precursor protein, ⁵⁶ SNCA = alpha-synuclein, ⁶¹ LRRK2 = leucine rich repeat kinase 2,^58,62 PINK1 = PTEN-induced kinase 1, ⁵⁷ PARK16 = Parkinson disease 16, ⁶⁰ MAO-B = monoamine oxidase B,^43,44 COMT = catechol-O-methyltransferase, ¹¹ MTHFR = methylenetetrahydrofolate reductase, ⁶⁴ SV2C = synaptic vesicle glycoprotein 2C, ⁶⁵ SLC17A6 = solute carrier family 17 member 6, ⁶⁶ RXRA = retinoid X receptor alpha, ⁶⁶ LINE-1 methylation = long-interspersed nuclear element 1, ⁶⁷ HLA-DRB1 = major histocompatibility complex, class II, DR beta.^45,46 This figure was created in Biorender. Olsen, A. (2025) https://BioRender.com/xoy8ym4.

In summary, the epidemiologic literature provides robust support for an inverse association between smoking and PD, with Mendelian randomization and dose-response studies further strengthening the evidence for causality rather than mere correlation. Questions remain regarding the degree to which genetic and environmental factors confound this interaction. Further, the underlying mechanism by which smoking might decrease the risk of PD is unclear. In the following section, we discuss the evidence that nicotine is responsible for this effect.

Is nicotine the active agent in tobacco?

There are many chemical components in tobacco smoke that could be responsible for the inverse association between PD and smoking, including carbon monoxide, ⁶⁸ inhibitors of monoamine oxidase B (MAO-B) and catechol O-methyltransferase (COMT), as well as activators of the Nrf2 pathway. ⁶⁹ However, most research has focused on nicotine, for several reasons. First, studies have demonstrated that the protective effect of tobacco extends to smokeless tobacco, suggesting that the protective effect is in the tobacco product itself and dose not require the formation or inhalation of smoke. ⁵ Second, a small body of research suggests that the protective effect extends to foods that contain nicotine. ⁷⁰ Third, there is considerable overlap between the distribution of the nicotinic acetylcholinergic system and the dopaminergic system in the striatum. ⁷¹ While dopaminergic dysfunction is the primary deficit in PD, there is evidence of dysfunction in other neurotransmitter systems, including the cholinergic system.^72,73 This impairment plays a role in the cognitive and motor symptoms of PD. The anatomical proximity of the acetylcholinergic system and the dopaminergic system may have functional relevance, because studies have shown that nicotine enhances striatal dopamine release in vitro and in vivo. ⁷⁴ Finally, nicotine protects against the degenerative effects of toxic insults in different experimental systems, as discussed in detail below.^75,76

Nicotine's potential neuroprotective properties have been investigated in numerous studies. Animal studies have revealed that nicotine may protect dopaminergic neurons from damage (Tables 2 and 3). Further, nicotine's ability to stimulate dopamine release suggests the potential for symptomatic benefit, although this has not been fully realized in clinical trials (Table 4). The addictive nature of nicotine raises concerns about its usage as a therapeutic agent, however, and, researchers are investigating alternate therapeutic approaches, such as use of nicotinic agonists and analogs.^77,78 Varenicline, a nicotinic acetylcholinergic receptor partial agonist, is the most effective treatment for tobacco use disorder^78,79 and may act by stimulating dopaminergic transmission. ⁸⁰ In a positron emission tomography (PET) imaging study, dopamine was increased after treatment with varenicline in smokers who were currently abstinent, and the increase in dopamine was correlated with decreases in ratings of the positive effects of smoking, although the correlation was just shy of statistical significance (p = 0.057). ⁸⁰

Table 2.

Evidence suggests that nicotine protects against nigrostriatal damage in toxic rodent animal models of PD.

Ref.	Species/model	Nicotine treatment	Positive outcomes	Negative outcomes
81	Male Sprague–Dawley rats 6-OHDA	1 mg/kg nicotine, subcutaneous administered 4 h before and 20, 44 and 68 h after the 6-μg 6-OHDA injection	Prevented the striatal dopamine loss	Ineffective in severe lesions (10-μg 6-OHDA injection) No effect on increased dopamine turnover
82	1.Male Sprague-Dawley rats 6-OHDA 2. α4 nAChR subunit knockout mice (sex not mentioned) Methamphetamine	1. A range of doses of chronic nicotine (0.75, 1.5, 3.0 and 30.0 mg/kg/day, s.c., 14 days) following 6-OHDA (10 μg intrastriatal unilaterally) in rats. 2. Acute nicotine (1 mg/kg, s.c.) pretreatment before methamphetamine (5 mg/kg, i.p., three doses at 2 h intervals) in wild-type and α4 nAChR subunit knockout mice.	1. Chronic low dose nicotine (0.75 and 1.5 mg/kg/day) protected against 6-OHDA-induced nigrostriatal degeneration. 2. Acute nicotine treatment protected against methamphetamine-induced loss of striatal dopaminergic nerve terminals in wild-type mice	High doses of nicotine were ineffective (3.0 and 30.0 mg/kg/day)
83	Male Sprague Dawley rats 6-OHDA	1 mg/kg nicotine subcutaneously, 4 h before and 20, 44 and 68 h after 6-OHDA administration.	Prevented the 6-OHDA effects on striatal DA tissue levels	Didn’t show any effect on SNc
84	Female Sprague Dawley rats 6-OHDA	1 mg/kg nicotine injected intraperitoneally 48, 24 and 4 h before and 4, 24,48 and 72 h after 6-OHDA injection	Protective effect on lipid peroxidation and hydroxyl radical production. Increased TH in the striatum.	No significant effect on the TH-negative area
85	Male C57Bl/6 mice MPTP	0.2 or 2 mg/kg nicotine injected i.p., during a 4-week period, five times a day at 2-h intervals	Reduced loss of TH-positive cell bodies in SNc	No preservation of TH immunostaining of nerve terminals in the striatum
86	Male Sprague–Dawley rats 6-OHDA	0.6 and 0.8 mg kg s.c. nicotine at 24 h and 30 min before lesioning with 6 μg 6-OHDA, then continued daily for 12 days post lesion	Reduced the number of amphetamine-induced rotations, reduced the loss of striatal TH immunoreactivity	Didn’t significantly protect TH-positive nigral cells in 6-OHDA lesioned animal
87	Male C57Bl/6 mice Paraquat	Ascending dose of 25 μg/ml to 400 μg/ml nicotine in drinking water	Partially protected against paraquat induced dopaminergic nigral cell loss and decrease in the striatal dopamine transporter	N/A
88	Sprague-Dawley rats 6-OHDA	2 mg/kg/d nicotine after 4 weeks of 6-OHDA lesion	Decreased dopamine release in both intact and lesioned striatum	N/A
89	Brain-specific SIRT6 transgenic mice MPTP	200 μg/ml nicotine, orally for 3 weeks, starting two weeks prior to MPTP injections	N/A	Nicotine did not rescue MPTP-induced DA neuron death in SIRT6 brain-specific knockout mice
90	Male C57Bl/6Rj DATiCRE and wild-type mice	10 mg/kg/day nicotine for 3 weeks	Increased the tonic activity of VTA DA neurons	N/A
91	Male C57BL6 mice MPTP	2.5 mg/kg/d nicotine for 5 d, i.p., 30 min before MPTP injection)	Improved MPTP-induced motor impairment, apoptosis, and TH activity. Reduced the accumulation of phosphorylated α-synuclein in the substantia nigra and striatal regions	N/A
92	Male C57Bl/6J mice 6-OHDA	0.4 mg/kg and 0.8 mg/kg nicotine, subcutaneous	TH expression level was increased in striatum.	N/A
93	Male, Sprague–Dawley rats 6-OHDA	0.4 mg/kg/day nicotine for 7 or 14 days	Decreased the apomorphine-induced contralateral rotations, altered striatal glutamate function	N/A
94	1. Male Sprague-Dawley rats 6-OHDA	1. Doses of 12.5, 25, 50 and 100 μg/ml nicotine (rat) in drinking water.	1. Nicotine pre-treatment decreased amphetamine-induced rotational behaviours in unilateral 6-OHDA-lesioned rats. 2. Nicotine pre-treatment improved the striatal dopamine transporter and nAChRs in 6-OHDA-lesioned rats	Nicotine treatment was found to prevent ongoing dopaminergic damage but failed to restore neurons or function after the damage was complete. Nicotine post-treatment did not improve rotational behaviors or striatal dopamine transporter.
95	C57BL/6J male and female mice 6-OHDA	300 μg/ml nicotine in drinking water for 7 to 8 weeks	Inhibited the development of LID restored [3H] dopamine release in the lesioned ventral striatum	Basal [3H] dopamine release from dorsal striatum samples was slightly but not statistically significantly increased
96	Male wistar rats 6-OHDA	0.3 mg/kg nicotine per day for 5 consecutive days i.p.	Improved short-term memory, increased conditioned avoidance responses, decreased number of escape failures in the shuttle-box task, increased the specific activity of glutathione peroxidase, decreased lipid peroxidation	No significant differences in SOD-specific activity
97	Albino male Wistar rats Rotenone	1.5 mg/kg nicotine daily for 11 days	Upregulated the expression of TH, downregulated the expression of α-synuclein and GFAP, increased striatal DA levels, increased MDA and TAC levels	N/A
98	Male Sprague-Dawley rats Rotenone	1 mg/kg nicotine, s.c. daily injection, starting 5 days before rotenone and continuing for 30 days	Attenuated dopaminergic cell loss in the SNc, partially prevented the reduction of dopamine levels in the striatum, improved the motor deficits	N/A
99	Male Sprague-Dawley rats 6-OHDA	Nicotine infusion (0.125 mg/kg per h) for one week	N/A	Did not significantly alter bFGF mRNA levels. 6-OHDA-induced depletion of dopamine in the neostriatum was not affected by nicotine treatment
100	Male C57BL/6 black mice, MPTP	Nicotine (0.25 mg/kg), twice a day at 2 h intervals for 7 days	Restored the TH expression in SNc Suppressed the ER stress induced by MPTP	N/A

6-OHDA: 6-hydoxydopamine; s.c.: subcutaneous; i.p.: intraperitoneal; nAChR: nicotinic acetylcholinergic receptor; TH: tyrosine hydroxylase; SNc: substantia nigra pars compacta; MPTP: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; SIRT6: Sirtuin 6; DA: dopamine; LID: levodopa-induced dyskinesia; GFAP: glial fibrillary acidic protein; MDA: malondialdehyde; TAC: total antioxidant capacity; bFGF: Basic fibroblast growth factor; ER: Endoplasmic reticulum.

Table 3.

Evidence suggests that nicotine protects against nigrostriatal damage in toxic non-human primate animal models of PD.

Ref.	Species/model	Nicotine treatment	Positive outcomes	Negative outcomes
101	Female squirrel monkeys MPTP	Ascending doses of nicotine in drinking water, beginning with 25 μg/ml for 1 week, 50 μg/ml for 1 week, and increasing by 50 μg increments/week over ∼3–4 months up to a final concentration of 650 μg/ml	Striatal dopamine level and TH expression level increased.	No effect on MPTP-induced cell loss in the substantia nigra. Nicotine did not alter monoamine oxidase activity.
102	Female squirrel monkeys MPTP	Ascending doses of nicotine in the drinking water, beginning with 25 μg/ml for 1 week, 50 μg/ml for 1 week, and increasing by 50 μg increments/week over ∼3–4 months up to a final concentration of 650 μg/ml	Normalized MPTP-induced increases in dopamine turnover in subregions of striatum, increases in fractional K + -evoked striatal dopamine release, and increases in fractional nAChR-evoked dopamine release in striatum. Reduced the threshold for corticostriatal long term depression induction.	No significant effect on MPTP-induced dopamine turnover in dorsal putamen.
103	Male and female squirrel monkeys MPTP	Nicotine was started at a dose of 50 μg/ml for 1 wk, increased to 150 μg/ml for 1 wk and then to a final concentration of 300 μg/ml via drinking water	Improved L-dopa-induced dyskinesias	The reduction in LIDs observed in mild-moderately parkinsonian monkeys was lost when nicotine was removed, showing the effect is reversible.
94	Female squirrel monkeys MPTP	10 μg/ml nicotine gradually increased to a final concentration of 650 μg/ml over 3 weeks	Nicotine post-treatment did not improve striatal dopamine levels and nAChRs in MPTP-lesioned monkeys	Nicotine treatment was found to prevent ongoing dopaminergic damage but failed to restore neurons or function after the damage was complete.
104	Female squirrel monkeys MPTP	Nicotine was started at 25 μg/ml and gradually increased to 650 μg/ml over a 3-to-4-month period in drinking water.	Lead to a greater increase in α3/α6β2* and α4β2* nAChRs in lesioned monkeys compared to controls. Reduced the number of extracellular neuromelanin deposits in the substantia nigra of MPTP-lesioned monkey	No effect on the dopamine transporter or the number of TH-positive cells in the SN. Significantly increased resting and activated microglia, with a somewhat greater increase with nigrostriatal damage.
105	Adult male Maccaca mulatta monkey, MPTP	Nicotine ditartrate (0.5 mg/kg) and the selective nAChR agonists (SIB- SIB-1553A (0.5 mg/kg) administered intramuscularly	Improved cognitive performance. Nicotine increased DA reuptake in the medial prefrontal cortex (mPFC) and striatum	Nicotine did not affect levodopa's improvement of motor symptoms.
106	Adult male Macaca mulatta monkeys, MPTP	Three doses of selective nAChR agonist SIB-1553A were used (0.05, 0.1 and 0.5 mg ⁄ kg for a minimum of 4 days. Nicotine ditartrate was used at three dose of 0.25, 0.5 and 1 mg ⁄ kg.	Both nicotine and SIB-1553A caused a dose-dependent enhancement of performance on the focused attention (cued reaction time) task	Nicotine failed to improve performance on the attention set-shifting task, which assesses cognitive flexibility.

6-OHDA: 6-hydoxydopamine; s.c.: subcutaneous; i.p.: intraperitoneal; nAChR: nicotinic acetylcholinergic receptor; TH: tyrosine hydroxylase; SN: substantia nigra; MPTP: 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; SIRT6: Sirtuin 6; DA: dopamine; LID: levodopa-induced dyskinesia; GFAP: glial fibrillary acidic protein; MDA: malondialdehyde; TAC: total antioxidant capacity; bFGF: Basic fibroblast growth factor; ER: Endoplasmic reticulum.

Table 4.

Results of nicotine in clinical trials.

Ref.	Study design	Intervention	Outcomes	Number of patients
107	Short-term double-blind, randomized controlled trial	Nicotine polacrilex resin gum (2 mg). Three pieces of gum administered at 2-h intervals over 4 ½ hours. Each piece was chewed for 30 min	No statistically significant differences between groups in an overall motor score or for subscores of tremor, rigidity, and bradykinesia.	48 PD patients
108	Double-blind placebo-controlled, cross-over trail	Nicotine gum (4 mg) administered for 30 min on two separate days	Systolic blood pressure increased within 10 min of administration and remained elevated for 90 min. Note: motor symptoms were not examined	10 PD patients with low blood pressure
109	Double-blind cross-over design	12-h exposure to nicotine patch (35 mg).	Nicotine exposure resulted in a worsening of OFF UPDRS III compared to placebo	16 PD patients
110	Double-blind, placebo-controlled, crossover design	Transdermal nicotine patched (7 mg) were administered for 24 h.	Nicotine normalized reaction-time differences for expected versus unexpected words in PD patients. Note: motor symptoms were not examined	10 non-smokers with PD, 16 non-smoking matched controls
111	Double-blind, placebo-controlled, crossover design	Transdermal nicotine patches (7 mg) for 24 h.	Nicotine improved compromised semantic processing in PD. Note: motor symptoms were not examined	12 non-smokers with PD, 17 non-smoker controls
112	Observational study	Nicotine was administered transdermally with increasing daily doses over 3 months up to 105 mg/day, and then decreased progressively	OFF and ON UPDRS III score and levodopa-equivalent doses improved at 3 months. At the one-year mark, the motor improvement was less pronounced.	6 PD patients
113	Open-label	Transdermal nicotine patch over 25 days in increasing titrated doses	Nicotine was tolerated poorly. Most of the patients withdrew from the study. In those remaining, nicotine had no positive or negative effect on motor or cognitive functioning.	22 non-smoking PD patients
114	Open-label	Patients received transdermal nicotine patches for 25 days (7 mg for first 11 days, then 14 mg for 11 days, and 21 mg for the last 3 days.	Nicotine was poorly tolerated. Many patients discontinued participation due to side effects or perceived worsening of symptoms.	22 non-smoking PD patients
115	Double-blind, placebo-controlled multicenter trial	Patients received one-year transdermal nicotine patches	Nicotine did not slow progression in early PD based on total UPDRS.	163 PD patients
116	Randomized, double-blind, placebo-controlled trial	Patients received transdermal nicotine patches (17.5 mg during week 1 and 35 mg during weeks 2 and 3), or placebo patches. Then patients went three weeks without any patch.	No significant effects in the Columbia University Rating Scale, the Webster scale, the Schwab-England scale, a timed walking test, an instrumental test for fine motor skills and hand tremor, and with the Hamilton Depression Scale.	32 non-smoking PD patients
117	Open-label study	The treated group received increasing doses of nicotine to reach 90 mg/day by 11 weeks. Dosage was then maintained for 28 weeks and then reduced over 6 weeks. Final evaluation was preformed 6 weeks following a washout period.	No significant difference in OFF UPDRS III between the nicotine-treated and non-treated groups.	40 PD patients
118	Pilot open-label trial	Daily doses of transdermal nicotine up to 105 mg/day over 17 weeks	ON-UPDRS total and subscores were significantly decreased. OFF-UPDRS III was not significantly different.	6 PD patients

Studies that were described as clinical trials but included 2 or fewer patients were excluded from review. One study was excluded from review because we were unable to obtain full text access. ¹¹⁹

Other studies have indicated that using nicotine analogs, which are less harmful than nicotine, can provide some of the same advantages as nicotine. ⁷⁷ In an in vitro PD model, nicotine analogs such as 3-(pyridin-3-yl)-3a,4,5,6,7,7a-hexahidrobenzo[d]isoxazole and (E)-nicotinaldehyde O-cinnamyloxime were studied. Their interactions with α7 nicotinic acetylcholine receptors were predicted by computational analysis. Pre-treatment with nicotine or (E)-nicotinaldehyde O-cinnamyloxime (10 μM) improved cell viability and reduced the production of reactive oxygen species (ROS) in SH-SY5Y cells exposed to rotenone. These results suggest potential neuroprotective effects of nicotine analogs against oxidative damage in brain pathologies. ⁷⁷

The complicated interplay between nicotine and PD requires further investigation to understand the underlying mechanisms and to better develop effective and well-tolerated treatment strategies. In the sections below, we discuss the current state of knowledge on nicotine's role in PD, covering both its potential benefits and the difficulties involved with its usage in clinical settings. We discuss promising pre-clinical research that suggests nicotine and/or nicotinic acetylcholine receptor (nAChR) agonists may be effective for reducing levodopa-induced dyskinesias in PD and protecting against long-term nigrostriatal damage.

The neuroprotective role of nicotine in PD models

Cellular models of PD

Nicotine demonstrates neuroprotective effects in PD by increasing neuronal growth and protecting against toxicants such as 1-methyl-4-phenylpyridinium (MPP 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 6-hydroxydopamine (6-OHDA).^92,120 It enhances cell viability and inhibits neurotoxicity in cellular models of PD. Nicotine induces the expression of thioredoxin (Trx-1), which inhibits oxidative stress and apoptosis, and also suppresses endoplasmic reticulum (ER) stress through the α7nAChR and phosphoinositide 3-kinase (PI3K) pathway in an in vitro PD model. ¹²⁰

In cultured mouse ventral midbrain neurons, nicotine supresses both the tunicamycin-induced endoplasmic reticulum (ER) stress and unfolded protein response (UPR). These effects are mediated by higher expression of nAChR and pharmacological chaperoning in the endoplasmic reticulum (ER). ¹²¹

Furthermore, nicotine reduces MPP + -induced neurotoxicity in SH-SY5Y cells by regulating oxidative stress and mitochondrial apoptosis, potentially through nAChR activation and monoamine oxidase B (MAO-B) inhibition. ¹²² In SH-SY5Y cells, nicotine significantly reduces cytochrome C release and mitochondrial swelling induced by a mixture of neurotoxins, including MPP + and 6-OHDA. This receptor-independent neuroprotective mechanism was achieved by inhibiting the production of ROS and mitochondrial complex I electron efflux. ¹²³ Nicotine decreases nicotinamide adenine dinucleotide (NADH) consumption and prevents electron transport from NADH to complex I by binding to complex I and competing with NADH. ¹²⁴ Nicotine may target not only complex I but also complex II, III, or IV. ¹²⁵

Additionally, nicotine can inhibit salsolinol-induced toxicity in dopaminergic neuroblastoma cells. ¹²⁶ Salsolinol induces apoptosis, and inhibition by nicotine may be primarily attributed to its antiapoptotic properties, which are mediated by targeting α7 nAChR. Further, nicotine reduces the level of apoptosis-related markers like cleaved caspase 3 and cleaved Poly (ADP-ribose) polymerase-1 (PARP-1).^92,126 Nicotine has also been investigated using primary dopaminergic cell cultures and N18TG2 neuroblastoma cells treated with rotenone. Pretreatment with nicotine improves the survival of tyrosine hydroxylase-positive neurons, increases neurite length and count, and modulates glutathione levels. Interestingly, this neuroprotective effect is not dependent on nAChR activation.^98,127

Overall, findings in cell culture support a protective effect of nicotine on dopaminergic neurons, other neurons, and neuron-like cells. Further studies are warranted to elucidate the effects of nicotine on other cell types. Furthermore, although α-synuclein aggregation is a hallmark of idiopathic PD, relatively few investigations have explored the potential therapeutic effect of nicotine on α-synuclein toxicity. Nicotine has been shown to reduce the α-synuclein aggregation kinetics in both yeast models and cell-free systems.^128–130 According to structural studies, nicotine and α-synuclein form a transitory complex that modifies α-synuclein's natural structure and prevents the development of oligomers.^35,128,129 Chronic nicotine therapy decreased α-synuclein aggregation, neuroinflammation, and neurodegeneration in human iPSC-derived neuronal models and transgenic mice expressing α-synuclein. These neuroprotective effects were mediated by activation of α4β2 nicotinic receptors (nAChRs), since α4 agonists and α4-specific antagonists both lowered nicotine-induced calcium response. ¹³¹

Drosophila models of PD

Drosophila melanogaster shares many genetic homologs with vertebrates and has proved to be a valuable tool for the study of human diseases. ¹³² Several Drosophila models of PD have been created. ¹³³ For example, mutation of the Drosophila ortholog of the PRKN gene, parkin (park), results in selective loss of dopaminergic neurons, decreased motor function, loss of olfaction, reduced lifespan, and mitochondrial dysfunction.^134–136 Such Drosophila PD models have been used to identify potential therapeutics for PD, including nicotine.^137–139 Nicotine has shown beneficial effects for park25-expressing flies and other models of PD. In park25 mutants, nicotine therapy improves motor behavior. On the other hand, nicotine negatively impacts control flies, making them more likely to drop and decreasing their tendency to climb downward. ¹⁴⁰

Synphilin-1 (Sph-1) is a component of Lewy bodies and has been implicated in the progression of PD. ¹⁴¹ Expression of Sph-1 in Drosophila neurons induces motor impairment, olfactory loss, neuronal death, and dysregulation of the dopaminergic neural system. ¹³⁸ Nicotine exhibits protective effects in flies expressing Sph-1, reducing motor parkinsonian symptoms and slowing dopaminergic neuron loss. This effect is mediated by nicotine's stimulation of dopamine synthesis and release, reducing oxidative stress and toxic dopamine degradation products. However, nicotine does not benefit control flies and may even shorten their lifespan and induce motor impairment. The differential effect may be due to nicotine's ability to enhance cell metabolism in metabolically challenged dopaminergic neurons, while acting as an excitotoxic insult in control flies. ¹³⁸

We examined nicotine in flies that express the human α-synuclein gene under a pan-neuronal driver, finding that nicotine rescued dopaminergic neuron loss, motor impairment, α-synuclein aggregation, and deficits in the autophagy system. ¹⁴² This improvement was dependent on intact expression of Drosophila orthologs of the SV2C gene, which had previously been suggested to interact with nicotine in a toxic model of PD in which flies are treated with paraquat. ⁶⁵ Thus, nicotine improves neurodegeneration in both genetic and toxic Drosophila models of PD.

Toxin-induced mammalian models of PD

In animal studies, nicotine ameliorates motor, cognitive, and memory deficits and reduces nigrostriatal damage in rodent and monkey models of PD.^85,143 Results of many animal studies using toxic models of PD and focusing on motor symptoms and striatonigral degeneration are summarized in Tables 2 and 3 so that readers can compare different treatment paradigms and model systems.

In addition to improving striatonigral degeneration, nicotine may have benefits for motor complications (dyskinesias) associated with PD as well as for non-motor symptoms.^{88,91,103,144,145} Nicotine pretreatment has been shown to improve anxiety-like behavior and dyskinesia produced by MPTP in PD mice. The mechanism of action involves reducing the accumulation of pathological phosphorylated-α-synuclein, improving nigrostriatal dopaminergic damage, alleviating apoptosis, and inhibiting pro-inflammatory factor expression and microglial activation in the substantia nigra and striatal regions of mouse brains under MPTP stimulation. ⁹¹ Moreover, nicotine administration has been found to alleviate deficits in odor discrimination and odor detection, thus improving olfactory impairment caused by MPTP treatment. ¹⁴⁶ Finally, nicotine administration reduces levodopa-induced dyskinesias (LIDs) in nonhuman primates, whether administered before they develop or after they are established. A maximal reduction (60–70% decrease) in LIDs is observed as early as 5 weeks into treatment. ¹⁰³

Studies of nicotine in humans

While it is inherently difficult to prove a neuroprotective role for nicotine in living humans, imaging and post-mortem studies can shed some light on potential mechanisms of action for nicotine.

Imaging studies in humans

A study employing dopamine transporter (DAT) positron emission tomography in 282 male patients with de novo PD revealed that current smokers exhibited elevated mean DAT activity in the posterior and ventral putamen relative to ex-smokers and never-smokers. This implies that active smoking might safeguard against dopamine neuronal degeneration in the striatum. ¹⁴⁷ Similarly, a small study assessing dopamine transporter imaging in PD patients receiving transdermal nicotine therapy revealed a decelerated reduction in striatal dopamine transporter density and notable motor enhancements following three months of high-dose nicotine treatment. Despite a partial reduction in motor benefits after one year, the gradual decline in binding potentials was associated with enhanced motor scores, indicating nicotine's possible influence on striatal dopamine. ¹¹² In contrast, Gigante et al. found a decreased [123]FP-CIT uptake in the putamen in current smokers with PD, despite them also have lower UPDRS-III scores. ¹⁴⁸ PET imaging studies evaluating nAChRs in PD patients demonstrated a notable reduction in receptor density in the striatum and substantia nigra relative to healthy controls. No relationship was observed between receptor density and the severity of PD, however. ¹⁴⁹ Further, a study that used quantitative susceptibility mapping (QSM) found that smoking correlated with reduced brain iron levels in PD patients. ¹⁵⁰ These imaging studies, while preliminary, suggest that patients with PD may have reduced nAChR expression and that smoking or nicotine may affect the dopaminergic system and brain iron levels in PD patients.

Post-mortem studies

Post-mortem studies have identified reductions in nicotinic receptors that occur with age as well as in specific neurodegenerative diseases. ¹⁵¹ One such study investigated nicotinic receptor density in PD and Alzheimer's disease (AD) brain tissue, demonstrating a decrease in nAChRs relative to controls. The decline was particularly significant in brain regions linked to cognitive and motor activities, including the hippocampus and cortex, with the most considerable loss observed in the striatum. ¹⁵² Another post-mortem study examined high-affinity nicotine binding by autoradiography in brain regions impacted by AD, Lewy body dementia (LBD), and PD, finding substantial reductions in the substantia nigra and dorsolateral tegmentum in both LBD and PD. In the hippocampus, receptor binding was reduced in specific hippocampal layers in AD and LBD, with decreased binding observed in cortical layers in AD but not in LBD. These data indicate a strong correlation between the loss of nicotinic receptors and the principal clinical characteristics of these neurodegenerative illnesses. ¹⁵³ Furthermore, Lange et al. observed diminished choline acetyltransferase activity in the frontal cortex and hippocampus of PD patients, signifying cholinergic system dysfunction. ¹⁵⁴

Nicotine in clinical trials

Based on the epidemiologic and preclinical evidence presented above, clinical trials have begun assessing the use of nicotine to treat the motor symptoms of PD (Table 4). Most clinical trials have focused on nicotine as a symptomatic therapy for motor symptoms, rather than a disease-modifying therapy, though in the largest nicotine trial to date, Oertel and colleagues assigned patients to transdermal nicotine patches or a placebo for one year and did not observe any slowed progression in early PD patients. ¹¹⁵ In Villafane et al., patients received increasing daily oral doses of nicotine in the form of NC001 (nicotine bitartrate dihydrate) over 14 weeks, followed by a maintenance period of 4 weeks, and then received a gradual decrease in doses). ¹¹⁸ All the patients in this study improved their motor scores and most required fewer dopaminergic drugs following their nicotine treatment. ¹¹⁸ These researchers later performed a phase 2 trial where high doses of transdermal nicotine were administered to PD patients. ¹¹⁷ Overall, patients tolerated the high doses of nicotine and saw some benefits. ¹¹⁷ Although there was no improvement in OFF Unified Parkinson Disease Rating Scale (OFF-UPDRS) scores after 6 months of treatment, there were some benefits such as reduced OFF-UPDRS-II scores and improved UPDRS-IV scores. ¹¹⁷ Overall, these patients also demonstrated a decrease in their dose of L-DOPA following treatment. ¹¹⁷ Itti et al. focused on the performance of dopamine transporter imaging in patients following nicotine therapy in patients with high motor impairments. ¹¹² These patients received increasing transdermal doses of nicotine for three months and were imaged before treatment, again at three months on nicotine, and then again at one year following treatment. ¹¹² These patients showed improved motor scores at both three months and one year and received lower doses of dopaminergic drugs. ¹¹² Kyaw et al. also showed improved symptoms in PD patients following treatment using a nicotine patch or gum. ¹⁵⁵

Oral nicotine intake has also been tested on its ability to reduce falls and freezing gait in PD patients. ¹⁵⁶ When NC001, nicotine bitartrate, was administered to 30 PD patients, they exhibited a significant reduction in falls and freeing of gait compared to placebo patients when assessed under the UPDRS Part II. ¹⁵⁶ There was also a nonsignificant improvement in dyskinesia when patients were treated with nicotine compared to the placebo. ¹⁵⁶ Multiple animal studies replicated these results as their goal was to reduce L-DOPA-induced dyskinesias (LIDs) using nicotine as a treatment. ¹⁵⁷ In eleven studies combining 181 animals, there was an overall reduction in LIDs in the nicotine-treated group compared to the controls. ¹⁵⁷ This reduction of dyskinesia may be produced by interactions at the β2* nicotinic receptor. ¹⁴⁵ This evidence is supported by a study where β2^−/− mice did not demonstrate the same dyskinesia recovery when administered nicotine compared to wild type mice. ¹⁴⁵ This indicates that β2* nAChRs might play an important role in this treatment. ¹⁴⁵

Although multiple studies have supported the use of transdermal nicotine patches for the treatment of PD motor symptoms, there are also studies rejecting the efficacy of this treatment. When patients were given NicoDerm patches for 25 days, followed by a two-week washout period there was no change in motor or cognitive performance when comparing nicotine-treated patients to those who received the placebo. ¹¹³ The authors recognized their differing results from those of other studies and attributed them to potential methodological factors. ¹¹³ Transdermal nicotine patches were also tested as an add-on treatment to L-DOPA in a 12-week randomized, double-blind study, which demonstrated no significant difference in fine motor skills, a timed walking task, or hand tremor in the PD patients given the nicotine patches. ¹¹⁶ In this study, patients had to be considered non-smokers for two years preceding the study, whereas this exclusion criteria was not present in other studies. ¹¹⁶ These results were supported by Clemens et al., where patients in a clinical trial were given either nicotine gum or a placebo to chew three times at 2-h intervals. ¹⁰⁷ When symptoms were evaluated before and after each trial there appeared to be no change in the overall motor scores of patients in either group. ¹⁰⁷ Another study evaluated 16 patients following 12 h of exposure to transdermal nicotine patches (35 mg of nicotine) or a placebo. ¹⁰⁹ Each patient received both treatments and most experienced a worsening of motor symptoms according to the UPDRS III sum scores following treatment with nicotine. ¹⁰⁹ The authors acknowledged the differing results from other studies and suggested further studies to establish a dose-response relationship to better understand the efficacy of nicotine treatments. ¹⁰⁹ Therefore, determining which of these studies utilized the best methodology is the challenge when attempting to understand the efficacy of nicotine treatments.

Although most studies have focused on the use of nicotine to treat the motor symptoms of PD, a couple have examined cognitive or autonomic symptoms. In Holmes et al., non-smoking controls and PD patients were administered nicotine via a transdermal patch or a placebo and tested for semantic processing ability via a test where they would expect target words from specified semantic categories based on primes, along with unexpected targets. ¹¹⁰ Overall, an expectancy effect was not observed in PD patients administered the placebo, but it was present and comparable to controls for those given nicotine. ¹¹⁰ This indicates that nicotine may improve semantic processing in PD patients. ¹¹⁰ Another study assessed the use of nicotine for treating lowered blood pressure associated with PD. ¹⁰⁸ PD can cause low blood pressure due to the denervation of the sympathetic nervous system and reduced levels of norepinephrine. ¹⁵⁸ Following the administration of nicotine gum, systolic BP increased within 10 min of administration and remained elevated for about 90 min, ¹⁰⁸ suggesting nicotine as a possible treatment for orthostatic hypotension. These results will need to be confirmed with additional trials.

Collectively, these studies provide some support for the potential use of nicotine, either orally or transdermally, as a symptomatic therapy for improving motor abilities and decrease the need for dopaminergic drugs. However, larger trials are needed, and there is no evidence to date that nicotine acts a disease-modifying therapy in PD patients. Further, side effects from nicotine limited its tolerability in several trials.

Beyond PD: Nicotine in other α-synucleinopathies

α-synucleinopathies are a class of neurodegenerative diseases that are defined by pathologic aggregated α-synuclein protein, including not only PD but also dementia with Lewy bodies (DLB) and multiple system atrophy (MSA).^159,160 Although the inverse association between smoking and PD is well-established, far less is known about smoking and DLB or MSA. Smoking may protect against development of Lewy pathology, ⁹⁹ and Goodheart et al. found that smoking was associated with lower odds of diagnosis of DLB. ¹⁰⁰ Vanacore et al. found that MSA patients smoke less than population controls and did not see similar effects in patients with progressive supranuclear palsy (PSP). ¹⁰⁵ However, Jackson et al. found that smoking was associated with earlier age of onset in MSA, ¹⁰⁶ and, in a meta-analysis of smoking and MSA, smoking was associated with decreased prevalence of MSA, but the findings were not statistically significant. ¹⁶¹ Thus, many more studies are required to define the relationship between smoking and α-synucleinopathies beyond PD.

Mechanistic research employing animal and cellular models may provide an additional link between smoking and α-synucleinopathies, as some studies suggest that nicotine acts on α-synuclein. Nicotine has demonstrated the ability to diminish α-synuclein aggregation and its related toxicity in multiple models of α-synucleinopathy.^128,162,163 In α-synuclein-expressing Drosophila, nicotine enhanced locomotor activity, maintained dopaminergic neuron populations, and reduced α-synuclein aggregation. The protective effects diminished upon the knockdown of certain synaptic vesicle glycoproteins (SV2L1 and SV2L2), indicating that nicotine's neuroprotective mechanisms may be associated with synaptic vesicle function. ¹⁴²

A notable study has shown that nicotine diminishes α-synuclein aggregation and its related disease in transgenic mice that overexpress α-synuclein. In these animals, which express α-synuclein via the PDGFα promoter, nicotine administration improved neurodegenerative markers and motor performance. The study also showed, for the first time, that nicotine protects human induced pluripotent stem cell (iPSC)-derived dopaminergic neurons from α-synuclein-preformed fibril (PFF)-induced toxicity. ¹³¹

Moreover, chronic nicotine administration significantly increased the number of Nurr1 + cells in the substantia nigra pars reticulata (SNr) and enhanced mobility in transgenic mice overexpressing human α-synuclein (hα-syn+). Subsequent analysis showed that PD-related motor deficits were lessened in hα-syn + mice with increased Nurr1 overexpression in the substantia nigra. This study offers further evidence that PD-related locomotor impairment is prevented by overexpressing Nurr1, which does so by decreasing the number of TH + (tyrosine hydroxylase-positive) neurons that express hα-syn. ¹⁶⁴

Thus, while few studies address the relationship between smoking and α-synucleinopathies beyond PD, nicotine's effects on α-synuclein aggregation, inflammation, and the survival of dopaminergic neurons make it a promising candidate for future investigation as a potential therapeutic approach for α-synucleinopathies.

Potential mechanisms of neuroprotection

The exact cause of PD is unknown,^165–167 but it is believed to result from a complex interaction between genetic and environmental factors, which leads to various neurodegenerative mechanisms, including oxidative stress, mitochondrial deficits, inflammation, glutamate excitotoxicity, neurotrophic factor deprivation, and dysfunction of the ubiquitin–proteasome and lysosomal-autophagy systems. Efforts are ongoing to discover drugs that can slow the progression of PD by targeting these underlying neurodegenerative mechanisms. These include agents aimed at enhancing mitochondrial function, promoting trophic activity, modulating inflammatory processes, preventing protein aggregation, and inhibiting apoptosis. In this section, we further explore potential mechanisms of neuroprotection by nicotine (Figure 3).

Figure 3.

Potential neuroprotective effects of nicotine. Most studies report neuroprotective effects involving nAChR-dependent mechanisms, though receptor independent mechanisms have also been suggested. The different purported mechanisms may be occurring in parallel in dopaminergic neurons or may involve different cell types. Proposed neuroprotective effects include: (1) increased intracellular calcium and dopamine release,^168–177 (2) reduced neuroinflammation,^178,179 (3) activation of the cAMP response element-binding (CREB) pathway and increased brain-derived neurotrophic factor (BDNF) expression,^180–183 (4) reduction of monoamine oxidase B (MAOB) levels, ¹⁸⁴ (5) activation of the mitogen-activated protein kinase (MAPK)/p53 pathway, increased expression of phosphorylated c-Jun N-terminal kinase (p-JNK) relative to JNK, and decreased ratio of phosphorylated extracellular signal-regulated kinase (p-ERK) to ERK. This figure was created with BioRender. Olsen, A. (2025) https://BioRender.com/0obisw9.

Relationship between cholinergic and dopaminergic systems

Several investigations have demonstrated a significant overlap between the acetylcholine interneurons in the striatum and the terminals of the ascending nigrostriatal dopaminergic projections from the substantia nigra. ⁷¹ Only a small number of neurons in the striatum are cholinergic neurons, but they are physically large and notable for their dendritic arborization, which creates a dense network in the striatum with a distribution similar to dopaminergic terminals. Furthermore, cholinergic markers (acetylcholine, choline acetyltransferase, and acetylcholinesterase) and dopaminergic markers (dopamine, tyrosine hydroxylase, and dopamine transporter) have a similar distribution. ⁷⁵ Acetylcholine, which is tonically secreted by cholinergic interneurons, controls the release of neurotransmitters from striatal dopaminergic neurons and, to a lesser extent, from cortical glutamatergic terminals. ¹⁸⁵

Nicotinic receptors that control the action of striatal dopamine are made up of different combinations of α and β subunits, requiring 4 subunits to function. Whereas the β subunits regulate acetylcholine binding, the α subunits express cholinergic recognition sites. α2 through α7 and β2 through β4 are among the subunits that enable the expression of numerous receptor subtypes.^74,186 Major receptor subtypes in the striatum of rodents and nonhuman primates are heteromeric α4β2* and α6β2* receptors, and homomeric α7 receptors; asterisks denote possible extra subunits. Only the striatum and a few other brain areas contain α6β2* receptors. Research indicates that α6β2* receptors are mainly made up of α4β2 subunits, while α4β2* receptors are predominantly made up of α6α4β2β3 and α6β2β3 subunits.^187–189 On dopaminergic terminals, there are receptors for both α6β2* and α4β2*, which control the release of dopamine in the striatum. Approximately 75% of nicotinic receptor-evoked dopamine release in nonhuman primates is attributed to α6β2* receptors, with the remaining 25% coming from α4β2* receptors; in rodents, this distribution is the opposite.^185,190,191 Furthermore, dopamine release is indirectly mediated by a smaller population of α7 receptors on striatal glutamatergic afferents from the brain.^192,193 Drug development aimed at restoring striatal dopamine function requires a thorough understanding of the distribution and functionality of these receptor subtypes.

The mechanism(s) whereby nicotinic receptor stimulation results in neuroprotection might involve an initial rise in intracellular calcium concentration that subsequently affects many downstream pathways and processes, such as caspases, kinases, cAMP response-element-binding protein, apoptotic signaling, immune modulation and the nitric-oxide-cGMP pathway.^{178,194–196} These in turn may modify neurons excitability and/or activate trophic factors to improve nigrostriatal integrity. ¹⁸⁰ Additionally, nicotine may provide protection by inducing the production of dopamine, which attenuates the neurotoxic action of MPP + (the active metabolite of MPTP) by competing with it for uptake into striatal dopamine-containing terminals.^101,102 Nicotine may also provide protection through non-receptor-related mechanisms, such as antioxidant properties ¹⁹⁷ and inducing brain cytochrome P50 (CYP) enzymes. ¹⁹⁸

Nicotine enhances dopamine release

Dopamine release and neurotransmission mediated by nicotine are triggered by a series of molecular and cellular events. When nicotine binds nAChRs, the receptor undergoes a change in conformation that opens the ion channel associated with the receptor. As a result, there is an influx of positively charged ions such as calcium and sodium into the neuron. ¹⁶⁸ The influx of calcium into the presynaptic neuron is a key step in the mechanism. Calcium acts as a second messenger, playing a crucial role in triggering the release of neurotransmitters. ¹⁶⁹ The elevated intracellular calcium levels lead to the fusion of presynaptic membrane with vesicles containing neurotransmitters, such as acetylcholine. Acetylcholine is then released into the synaptic cleft, which requires the SNAP receptor (SNARE) protein system.^170–172

Dopaminergic neurons in the substantia nigra and ventral tegmental area (VTA) also express nAChRs.^173,174 When acetylcholine is released from presynaptic terminals and binds to nAChRs on dopaminergic neurons, it triggers the membrane of the neuron to depolarize. If the depolarization reaches a particular threshold, it triggers the initiation of an action potential. The action potential passes through the axon of the dopaminergic neuron and reaches the nerve terminals, also known as axon terminals, it triggers the dopamine release into the synaptic cleft. Dopamine binds to dopamine receptors on postsynaptic neurons when it is released into the synaptic cleft.^175,176

Nicotine reduces inflammation

PD patients’ substantia nigra contains reactive astrocytes, a marker of neuroinflammation.^199,200 In mouse primary cultured astrocytes, pretreatment with nicotine suppressed MPP ⁺ - or lipopolysaccharide (LPS)-induced production of TNF-α and astrocyte activation. ¹⁷⁹ Similarly, in rats, a chronic nicotine pretreatment demonstrated a significant reduction of dopaminergic neuronal loss in the substantia nigra after LPS. This reduction was accompanied by a decrease in tumour necrosis factor α (TNF-α) production. This study clearly showed that nicotine protects dopaminergic neurons from damage via modulating microglial activation, which is an anti-inflammatory mechanism. ¹⁷⁸

Anti-inflammatory pathway activation is one possible explanation for the lower prevalence of PD among tobacco users. ⁹ Nicotine may protect substantia nigra dopaminergic neurons by activating α7 nAChRs expressed by microglia or astrocytes, which reduces neuroinflammation. ⁷⁷ Activation of α7 nAChR improves both the degeneration of dopaminergic neurons and associated neuroinflammation. For example, in mice, nicotine reduced the movement impairments induced by MPTP, and these effects were linked to a decrease in the degeneration of dopaminergic neurons. In addition, nicotine decreased astrocyte and microglia activation in the substantia nigra. ¹⁷⁹ Further, an α7 nAChR antagonist lessened the effects of nicotine on inflammatory markers and dopaminergic neurons degeneration. ¹⁷⁹ Additionally, α7 nAChR selective agonists lowered neuroinflammation and dopaminergic cell death in the striatum of rats given 6-OHDA ²⁰¹ and in the substantia nigra of mice given MPTP. ²⁰² Finally, co-cultures of mesencephalic neurons and microglia that had received a nicotine pretreatment showed a marked reduction in the loss of dopaminergic neurons, whereas α-bungarotoxin, an α7 nAChR selective blocker, significantly inhibited the effects of nicotine on dopaminergic neuronal loss and microglial activation. ¹⁷⁸

Nicotine stimulates neurotrophic factors

Studies indicate that nicotine affects important neurotrophic factors that are essential for neuronal survival, growth, and function. These factors include glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and fibroblast growth factor-2 (FGF-2). ¹⁸¹ Nicotine stimulates the expression of BDNF and FGF-2 in the rat striatum, which may be one mechanism responsible for nicotine's neuroprotective benefits. ¹⁸⁰

Nicotine affects BDNF expression both directly and indirectly . Nicotine stimulates the cAMP response element-binding protein (CREB) pathway by activating nAChRs, which increases the expression of BDNF. ¹⁸² Moreover, nicotine increases glutamatergic transmission, which may aid in the induction of BDNF. ¹⁸³ Additionally, it regulates dopamine release, which has an indirect impact on BDNF levels. Finally, by lowering neuroinflammation, nicotine's anti-inflammatory qualities may indirectly encourage the development of BDNF.

Nicotine may reduce oxidative stress

Nicotine has been reported to have both pro- and anti-oxidant effects in the peripheral and central nervous system. ²⁰³ The antioxidant properties of nicotine may involve nicotinic acetylcholinergic receptors, or nicotine may act extracellularly as a free radical scavenger. ¹⁹⁷ Moreover, it has been stated that low concentrations of nicotine may act as an antioxidant, while a high dose of nicotine may induce neurotoxicity and stimulate oxidative stress. ²⁰⁴ In a study of human volunteers, PET of the brain showed a 40% decrease in the level of MAO-B in smokers relative to non-smokers or past smokers, suggesting that smoking may reduce free radical formation by reduction of MAO-B levels. ¹⁸⁴

Nicotine's effects on hydroxyl radical levels have been investigated in both the MPTP mouse model and in a cell-free in vitro Fenton system. ²⁰⁵ In the in vitro experiment, nicotine and α-phenyl-N-tert-butyl nitrone (PBN) dose dependently (1.0–5 mM) reduced hydroxyl radicals, suggesting that nicotine can act as a free radical scavenger. However, in vivo a low dose of 0.1 mg/kg nicotine failed to rescue MPTP-induced dopamine depletion, and a higher dose of 0.4 mg/kg enhanced dopamine depletion. However, ROS and lipid peroxidation in the brains of the MPTP model mice and those treated with nicotine were not measured in this study, so whether nicotine affected oxidative stress in this model is unknown.

Nicotine modulates apoptosis

Apoptosis is one mechanism of cell death of dopaminergic neurons in PD patients.^206,207 Nicotine can activate pro-survival pathways, including the mitogen-activated protein kinase (MAPK) pathway,^208–210 the Wnt/beta-catenin pathway, ²¹¹ and the protein kinase C (PKC) pathway. ²¹² Of these pathways, the main signaling mechanism responsible for nicotine induced cell proliferation is the extracellular signal related (ERK)/MAPK pathway. ²¹³ Nicotine triggers ERK/MAPK and CREB signaling in neurons, including hippocampal neurons, ²¹⁴ as well as in cell lines like SH-SY5Y cells. ²¹⁵ In vitro studies have shown that nicotine inhibits the mitochondrial apoptotic pathway through the activation of α7-nAChRs, thereby preventing H₂O₂-induced apoptosis. ²¹⁶ In SH-SY5Y cells, the activation of the α7-nAChRs/MAPK/p53 axis by pretreatment with nicotine or the α7-nAChR agonist PNU-282987 demonstrated neuroprotective antiapoptotic effects against MPP + treatment. ²¹⁷ In this study, while PNU-282987 and nicotine both reduced the Bax/Bcl-2 ratio and prevented MPP + -induced apoptosis, PNU-282987 also prevented MPP + -induced phosphorylation of p65 and p53, whereas nicotine only prevented phosphorylation of p53 and not p65. Both the p53 and p65 transcription factors play a key role in the induction of apoptosis. ²¹⁷ A recent study also found that apoptosis was significantly increased in MPTP-induced mice, and nicotine pretreatment significantly ameliorated these changes in the substantia nigra. ⁹¹ MPTP-treated mice demonstrated alterations in c-Jun N-Terminal Kinase (JNK) and phosphorylated JNK as well as ERK and phosphorylated ERK ratios in the substantia nigra, and nicotine pretreatment prohibited these changes, suggesting that nicotine might produce neuroprotective effects in MPTP-induced PD animals by modifying the JNK and ERK signalling pathways in the substantia nigra.

Conclusions and areas for future investigation

As discussed in this review, a large body of epidemiologic evidence supports an inverse association between smoking and the risk of PD. This has led to great interest in nicotine as a possible neuroprotective agent, and many animal studies have offered potential mechanisms by which nicotine may rescue or prevent damage to dopaminergic neurons. However, the results of nicotine in clinical trials for PD patients are mixed, and neither smoking nor nicotine are recommended for the treatment or prevention of PD. The negative effects of smoking on human health are numerous and indisputable (Figure 4). Further, nicotine is highly addictive. Nevertheless, we believe there to be value in ongoing investigation of potential neuroprotective mechanisms of nicotine, with the hope being that a better understanding of mechanism could lead to development of safer therapies. There are many outstanding questions regarding nicotine, and we conclude this review by highlighting a few areas in which additional research is needed, in our opinion:

Beyond nicotine, do other ingredients in tobacco influence the inverse association between smoking and PD? How might these other ingredients attenuate or potentiate the effects of nicotine?

To what extent do gene-environment interactions or other poly-exposures determine an individual's response to nicotine/smoking? Understanding these interactions may be of great importance for a future of precision medicine, in which the people most likely to benefit from nicotine can be enrolled in clinical trials.

Most animal studies of nicotine have relied on toxic models (rotenone, 6-OHDA, MPTP). These studies should be complemented by studies in genetic models of PD, which may have greater construct validity.

Most studies report nAChR dependent mechanisms involving the α7 nAChR, but some have identified roles for the α4β2* and α6β2* receptors, and still others have identified receptor-independent mechanisms. ¹⁶³ The degree to which different nAChR receptors contribute to neuroprotective effects is an area of ongoing investigation.

Most studies of nicotine in PD have focused on its effects on dopaminergic neurons. Studies on other neurons and glial cells are of interest in dissecting any non-cell-autonomous mechanisms of neuroprotection.

Most studies in both animal models and patients with PD have focused on motor symptoms, though there is reason to believe that nicotine could be of benefit for cognitive and autonomic symptoms. Additional studies exploring nicotine in non-motor symptoms are needed.

The most promising clinical trial data to date suggests that nicotine may improve motor symptoms in PD, but the results are mixed, perhaps in part due to different drug regimens. Standardization of methodology should be employed in future trials, and attempts to dissect out symptomatic versus disease-modifying effects will be critical.

Figure 4.

Negative effects of smoking on human health. Smoking is the leading cause of preventable deaths worldwide. ²¹⁸ It causes lung damage, ²¹⁹ cardiovascular damage, ²²⁰ cancer,^221–224 obstetric issues,^225–227 immune dysfunction, ²²⁸ and accelerated aging.^229–231 This figure was adapted from Silva, J. (2025) Risk Factors For Cancer - Smoking. https://app.biorender.com/biorender-templates/details/t-652df36132b3f9fbc7a69372-risk-factors-for-cancer-smoking. The citation for the new figure is Olsen, A. (2025) https://BioRender.com/n9rj6wp.