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Abstract

Introduction: Prostaglandins (PGs) are involved in nociception and mast cell degranulation. Prostaglandin D₂ (PGD₂) is a vasodilatator released during mast cell degranulation. The headache-eliciting effect of PGD₂ has not been studied in man.

Subjects and methods: Twelve healthy volunteers were randomly allocated to receive intravenous infusion of 384 ng/kg/min PGD₂ over 25 min in a placebo-controlled, double-blind cross-over study. We recorded headache intensity and associated symptoms, velocity in the middle cerebral artery (V_MCA) and diameter of the superficial temporal artery (STA) and radial artery (RA) using ultrasonography.

Results: In the period 0–14 h, 11 subjects reported headache on PGD₂ compared to one subject on placebo (P = 0.002). During the in-hospital phase (0–120 min), the area under the headache curve was larger on PGD₂ compared to placebo (P < 0.05). Median peak headache, 1 (0–1), occurred 10 min after start of PGD₂ infusion. There was no difference in incidence of headache in the post-hospital phase between PGD₂ (n = 3) and placebo (n = 1). There was a decrease in V_MCA (P < 0.001), increase in STA (P < 0.001) and RA (P < 0.006) diameter during PGD₂ infusion compared to placebo. Peak decrease in V_MCA was 28.3% after 10 min and peak increase in STA was 55.7% after 20 min on the PGD₂ day. Conclusions: The present study shows that PGD₂ is a very strong vasodilator of MCA, STA and RA, but causes only mild headache.

Keywords

Prostaglandin headache arterial dilatation cerebral infusion

Introduction

In the last 10 years, nociception around cephalic blood vessels and mast cell degranulation have become ‘hot’ topics in migraine pathophysiology (1–4). In particular, dura mater and meningeal nociceptors are believed to play a pivotal role in the generation of head pain (5,6). Dural mast cells are densely lined close to meningeal perivascular afferents and animal models of inflammatory pain have shown that prostanoids are involved in nociception and mast cell degranulation (7). We have previously studied the ability of prostaglandin I₂ (PGI₂) and prostaglandin E₂ (PGE₂) to cause head pain in normal human volunteers, demonstrating that PGI₂ (8) and PGE₂ (9) elicit headache and cephalic vasodilatation. Prostaglandin D₂ (PGD₂) is a vasodilator released by mast cell degranulation (10–12); however, in contrast to PGI₂ and PGE₂, it is not known to activate or sensitize dural sensory afferents (13). PGD₂ synthesis is initiated upon arachidonic acid release from the plasma membrane by the action of phospholipase A₂. Arachidonic acid is then converted to prostaglandin G₂ and prostaglandin H₂ by cyclooxygenases and the final isomerisation step is executed by the specific prostaglandin D synthase to form the active PGD₂ (14). PGD₂ has a short half-life in plasma of (≤2 min) (15,16) and intracellular responses are mediated via G-protein-coupled receptors named DP-receptors (17).

The headache-inducing effect of PGD₂ has not previously been explored in humans, but headache has been reported as an adverse event during PGD₂ infusion (18,19). We aimed to study the headache-eliciting effect of PGD₂ and to correlate any headache to the effect of PGD₂ on cerebral and extracerebral arteries in healthy subjects in a placebo-controlled, double-blind, cross-over study. We hypothezised that PGD₂ would strongly dilate cranial arteries, and that this would be accompanied by a pronounced headache.

Design and methods

Pilot experiment

Before the main experiment, we conducted an open pilot study to find the optimal dose of PGD₂ that would cause a mild-to-moderate headache on a verbal rating scale (VRS) from 0 to 10 (0, no headache and 10, worst imaginable headache), without intolerable adverse effects and with detectable changes in mean blood flow velocity of the middle cerebral artery (V_MCA) by transcranial Doppler (TCD). Three subjects (1 male and 2 females) received intravenous PGD₂ in step-wise increasing doses of 128, 192, 256 and 384 ng/kg/min. Each infusion lasted 25 min followed by a 30-min wash-out. The pilot experiment showed a prolonged dilatation of the STA and MCA over time after infusion. To avoid a possible carry-over effect in MCA and STA recordings, we decided to include another two subjects (1 male and 1 female) with 256 and 384 ng/kg/min with infusion time 25 min followed by a 90-min wash-out. The results of the pilot study showed that 384 ng/kg/min of PGD₂ induced a mild-to-moderate headache in five subjects (VRS 1; range 1–3), was well tolerated and led to a 38.3% decrease in V_MCA.

Main experiment

We recruited 12 healthy subjects (6 males and 6 females), mean age 24 years (range, 18–33 years). Exclusion criteria were: a history of migraine or any other type of headache (except episodic tension type headache less than once a month); any daily medication apart from oral contraceptives; and serious somatic or psychiatric diseases. Two subjects had a family history of migraine. The study was approved by the Ethics Committee of the County of Copenhagen (KA-20060044), Danish Medicines Agency, and the Danish Data Protection Agency and was undertaken in accordance with the Helsinki Declaration of 1964, as revised in Edinburgh in 2000. The study was registered on <www.clinicaltrials.gov> and monitored by the Good Clinical Practice (GCP) unit at Copenhagen University Hospital. All subjects gave informed consent to participate in the study.

Experimental design

In a double-blind, placebo-controlled, cross-over design, the subjects were randomly allocated to receive PGD₂ 384 ng/kg/min (Sigma) or placebo (isotonic saline) over 25 min on 2 days, separated by at least a week. The central pharmacy performed the randomisation and prepared the study drug. The randomisation code remained in the hospital during the study and was not available to the investigators until the study was complete.

All subjects reported to the laboratory at 08:30 h headache free. Coffee, tea, cocoa or other methylxanthine-containing foods or beverages were not allowed for at least 8 h before start of the study. All procedures were performed in a quiet room and room-temperature was between 22.7–26.3°C. The subjects were placed in the supine position and a venous catheter (Venflon®) was inserted into the right antecubital vein for infusion. The subjects then rested for 30 min before baseline recordings. After baseline measurements, infusion started using a time and volume controlled infusion pump (Braun Perfusor, Melsungen, Germany). Headache intensity, accompanying symptoms, V_MCA, diameter of the superficial temporal artery and radial artery, end-tidal partial pressure of pCO₂ (P_etCO₂), adverse events and vital signs were recorded at baseline and then every 10 min until 120 min after start of infusion.

The subjects were carefully instructed to complete a headache diary with accompanying symptoms according to the International Headache Society (20), including questions concerning pre-monitory symptoms (tired, yawning, stiff neck, blurred vision, thirst, intolerant/irritable, emotional, difficulty with concentration, any other symptoms) (21) and allodynia (combing, shaving, shower, cold, ear-rings, eyeglasses, heat, contact lenses, tight clothes, pillow, ponytail, necklace) (22) and any rescue medication every hour until 12 h after discharge from the hospital. Subjects were allowed to take rescue medication of their own choice at any time.

Headache intensity

Headache intensity was recorded on a verbal rating scale (VRS) from 0–10 (0, no headache; 1, a very mild headache [including a feeling of pressing or throbbing – pre-pain]; 5, moderate headache; 10, worst imaginable headache) (23).

Middle cerebral artery blood flow velocity

V_MCA was recorded bilaterally by transcranial Doppler (TCD) with hand-held 2-MHz probes (Multidop X; DWL, Sipplingen, Germany), as previously described (24,25). All recordings were done by the same skilled examiner (TW). P_etCO₂ (end-tidal CO₂) was recorded simultaneously to the TCD measurements using an open mask, that caused no respiratory resistance (ProPac Encore® Welch Allyn Protocol, Beaverton, USA).

Diameter of the superficial temporal artery and radial artery

Diameter of the frontal branch of the left superficial temporal artery (STA) and the left radial artery was measured by a high-resolution ultrasonography unit (20 MHz, bandwidth 15 MHz; Dermascan C; Cortex Technology, Hadsund, Denmark) as previously described (25,26).

Vital signs

Heart rate (HR) and blood pressure were measured every 10 min by an auto-inflatable cuff (ProPac Encore®; Welch Allyn Protocol). ECG (Cardiofax V; Nihon-Cohden, Japan) was monitored on an LCD screen and recorded on paper every 10 min.

Data analysis and statistics

Headache score are presented as median and quartiles. Vascular baseline variables are presented as mean ± SD and vascular peak responses as mean and 95% CI, unless otherwise stated. Baseline was beforehand defined as an average of T-10 and T0 before the start of infusion.

We defined an in-hospital phase, consisting of an infusion phase as a period from 0 to 30 min (0–30 min) because of a short half-life (≤2 min) of PGD₂ in plasma (15,16), and a post-infusion phase as a period from 30 to 120 min (30–120 min). Furthermore, a post-hospital phase was defined as the period of 2–14 h after start of the infusion.

Primary end-points were incidence of any headache on each experimental day, and differences in response calculated as area under the curve (AUC) according to the trapezium rule (27) for headache score (AUC_headache), VMCA (AUC_VMCA), diameter of STA (AUC_STA) between PGD₂ and placebo. Secondary end-points were differences in response in P_etCO₂ (AUC_{P_et} _CO₂), diameter of RA (AUC_RA), mean arterial blood pressure (MAP; AUC_MAP), systolic blood pressure (SBP; AUC_SBP), diastolic blood pressure (DBP; AUC_DBP), and heart rate (HR; AUC_HR) between PGD₂ and placebo. Baseline was subtracted before calculating the AUC to reduce variation between sessions within subject.

P_etCO₂ measured during TCD acquisition were also analysed for changes over time for each dose separately with univariate analysis of variance (ANOVA) with the fixed factors subject and time. To reduce mass significance, the following time points were selected for analysis (T0, T20 and T60). If overall differences were found, Dunnett’s test was applied to characterise which time points were different from baseline. V_MCA was corrected for significant changes in P_etCO₂ according to Markwalder et al (28).

To test the differences between incidence of headache and AEs, we used McNemar’s test. To test differences between variables we used the Wilcoxon signed rank test for headache score and a paired, two-way t-test for vascular data. We tested for period and carry-over effects for all baseline variables with Mann–Whitney test (headache score) and independent t-test. All analyses were performed with SPSS for Mac v11.0 (Chicago, IL, USA). Five percent (P < 0.05) was accepted as the level of significance.

Results

All subjects completed the study. At times 40 min and 50 min, subject 3 reported a severe lower abdominal pain that mimicked menstrual pain and it was impossible to score headache (including associated symptoms), TCD, and diameter of STA and RA. Only BP and heart rate were measured at time 40 min. Subject 8 did not return the headache diary on the placebo day despite encouragement. This subject denied headache or other symptoms in the post-hospital phase, but we decided to interpret this as missing data. Otherwise, data were complete. There was no carry-over or period effect for baseline values of headache, V_MCA, STA, RA, MAP, SBP, DBP or HR (P > 0.05). We found no difference in baseline V_MCA recordings between the left and right side on either active or placebo days (P > 0.05) and, therefore, used the average of both sides in the statistical calculations (Table 1).

Table 1.

Mean baseline values (± SD) of blood flow velocity in the middle cerebral artery (V_MCA), P_etCO₂, diameter of superficial temporal artery (STA) and radial artery (RA), mean arterial blood pressure (MAP), systolic blood pressure (SBP), diastolic blood pressure (DBP), heart rate (HR), in 12 healthy subjects on PGD₂ and placebo days

Variables	PGD₂	Placebo	P-value
V_MCA (cm/s)	72.3 ± 13.0	70.9 ± 12.2	0.342
P_etCO₂ (mmHg)	34.8 ± 2.7	35.2 ± 2.7	0.389
STA (mm)	1.04 ± 0.18	1.12 ± 0.20	0.019
RA (mm)	2.25 ± 0.26	2.37 ± 0.25	0.007
MAP (mmHg)	69.0 ± 4.9	71.4 ± 8.1	0.089
SBP (mmHg)	101.5 ± 6.6	103.6 ± 8.2	0.270
DBP (mmHg)	52.7 ± 6.0	55.2 ± 9.0	0.056
HR (beats/min)	59.6 ± 7.6	58.8 ± 5.5	0.718

P-value: paired t-test.

Headache

On the PGD₂ day (0–14 h), 11 subjects (92%) reported headache compared to one subject (8%) on placebo (P = 0.002, McNemar test).

During the in hospital phase (0–120 min), 10 subjects reported headache on the PGD₂ day and no subject reported headache on the placebo day (P = 0.002, McNemar test; Table 2). During the infusion phase, the AUC_{headache 0–30 min} on the PGD₂ day, 25 (20–30), was larger than on the placebo day, 0 (0–0; P = 0.005; Fig. 1). During the post-infusion phase, the AUC_{headache 30–120 min} on the PGD₂ day, 45 (15–60), was larger than on the placebo day, 0 (0–0; P = 0.008; Fig. 1). The median peak headache, 1 (0–1), occurred 10 min after start of PGD₂ infusion.

Figure 1.

Median (filled squares) and individual headache scores using a verbal rating scale (VRS) on PGD₂ (open squares) and placebo (crosses) days. There was a higher headache response in the infusion (AUC_{headache 0–30 min}; P = 0.005) and post-infusion (AUC_{headache 30–120 min}; P = 0.008) phases on PGD₂ day compared to placebo day.

Table 2.

Clinical characteristics of PGD₂-induced headache in 12 healthy subjects

Subject	Time (h)	Headache localisation/ intensity/quality/aggravation	Nausea/vomit/ photo/phonophobia	Gender	Age (years)
1				M	21
a		Bilat/1/throb+pres/Yes	Yes/–/–/–
b		–/–/–/–	–/–/–/–
2				M	29
a		Bilat/1/const+pres/–	Yes/–/–/–
b		–/–/–/–	–/–/–/–
3				F	24
a		Bilat/2/const+pres/Yes	Yes/–/–/–
b		–/–/–/–	–/–/–/–
4				M	23
a		Bilat/1/throb+pres/Yes	Yes/–/–/–
b		–/–/–/–	–/–/–/–
5				F	33
a		Bilat/4/throb+pres/–	Yes/–/–/–
b		–/–/–/–	–/–/–/–
6				M	25
a		Bilat/3/const+pres/–	Yes/–/–/–
b	3	Bilat/1/throb/Yes	Yes/–/–/–
7				F	29
a		Bilat/1/throb/Yes	Yes/–/–/–
b		–/–/–/–	–/–/–/–
8				M	22
a		Bilat/2/throb/Yes	Yes/–/Yes/–
b		Headache diary not returned
9				F	18
a		–/–/–/–	–/–/–/–
b	3	NS/1/NS/No	–/–/–/–
10				F	24
a		–/–/–/Yes	–/–/–/–
b		–/–/–/–	–/–/–/–
11				F	26
a		Bilat/2/pres/–	Yes/–/–/–
b		–/–/–/–	–/–/–/–
12				M	18
a		Bilat/2/pres/Yes	Yes/–/–/–
b	3	Bilat/3/throb/Yes	–/–/–/–

a, Peak headache on PGD₂ day, in-hospital phase (0–120 min).

b, Peak headache on PGD₂ day, post-hospital phase (2–14 h).

Throb, throbbing; pres, pressing; bilat, bilateral; F, female; M, male; NS, not stated.

Subject 10 had a bilateral headache score of 2 immediately after coughing with no associated symptoms.

During the post-hospital phase (2–14 h), three subjects reported headache on the PGD₂ day and one subject reported headache on the placebo day (P = 0.625, McNemar test; Table 2). There was no difference in AUC_{headache 2–14 h} between the PGD₂ day, 0 (0–0.38), and the placebo day, 0 (0–0; P = 0.715). None of the subjects reporting headache in the post-hospital phase had a family history of migraine.

Table 3.

Timing of peak median headache and peak vascular responses on PGD₂ and corresponding values on placebo day in healthy subjects

	PGD₂ day healthy subjects		Placebo day healthy subjects
Maximal responses	Peak time (min)	Peak response compared to baseline	Peak response compared to baseline
Headache (VRS)	10	1	0
V_MCA (cm/s)	10	–28.3% (–33.6 to –22.9)	3.5% (0.7 to 6.3)
STA (mm)	20	55.7% (46.7 to 64.6)	4.5% (–0.2 to 9.3)
RA (mm)	20	16.3% (6.9 to 25.7)	–0.3% (–4.2 to 3.6)
HR (beats/min)	10	45.4% (34.1 to 56.7)	–1.3% (–5.0 to 2.4)
MAP (mmHg)	90	–3.2% (–6.8 to 0.5)	2.4% (–1.3 to 6.1)

Median headache is given on a Verbal Rating Scale (VRS). Maximal mean percentage changes from baseline (95% CI) in vascular variables. The middle cerebral artery blood flow velocity (V_MCA) is corrected for changes in P_etCO₂ (28).

Middle cerebral artery velocity

The AUC_VMCA was smaller on PGD₂ than on placebo during both the infusion (P < 0.001) and post-infusion (P = 0.018) phases (Fig. 2). There was a difference between the AUC_PetCO2 recordings during TCD scans between PGD₂ and placebo days during the infusion phase (–9.2 mmHg; –11.8 to –6.5, 95% CI, P < 0.001) and no difference in the post-infusion phase (–0.9 mmHg; –9.2 to –7.5, 95%CI, P = 0.824). ANOVA showed a change over time in P_etCO₂ on PGD₂ day (P < 0.001) and the placebo day (P < 0.001). Post-hoc analysis revealed a change in P_etCO₂ from baseline at the time 10, 20, 30 and 40 min on PGD₂ (P < 0.05) and at the time 110 min on placebo (P < 0.05).

Figure 2.

Individual and mean flow velocities (cm/s) in the middle cerebral arteries (V_MCA) corrected for changes in P_etCO₂ (28) on PGD₂ compared to placebo. There was a decrease in AUC_{VMCA 0–30 min} (P < 0.001) on PGD₂ indicating dilatation of MCA (49).

Figure 3.

Individual and mean diameter (mm) of the superficial temporal artery (STA) on PGD₂ compared to placebo. There was an increase in AUC_{STA 0–30 min} (P < 0.001) on PGD₂.

Correction for P_etCO₂ changes did not change the results of AUC_VMCA during the infusion (P < 0.001) and post-infusion (P = 0.005) phases. Peak response corrected for changes in P_etCO₂ is shown in Table 3.

Superficial temporal artery and radial artery

The AUC_{STA 0–30 min} during the infusion phase (P < 0.001) and the AUC_{STA 30–120 min} during the post-infusion phase (P = 0.001) were larger on PGD₂ day compared to placebo day. The AUC_{RA 0–30 min} during the infusion phase was larger on PGD₂ day compared to placebo (P < 0.006) and there was no difference in AUC_{RA 30–120 min} during the post-infusion phase (P = 0.435). Peak responses are shown in Table 3.

Allodynia and premonitory symptoms

On the PGD₂ day the following premonitory symptoms were reported: tiredness (91%), yawning (73%), blurred vision (9%), thirst (9%), difficulty with concentration (9%), and chest pain (9%). On the placebo day, premonitory symptoms were reported as tiredness (50%), yawning (42%), and difficulty with concentration (8%).

There was no difference in reported allodynic symptoms between the two experimental days (P > 0.05, McNemar test). On the PGD₂ day, one subject found exposure to heat unpleasant.

Arterial blood pressure and heart rate

There was no difference in MAP during either the infusion (AUC_{MAP 0–30 min}, P = 0.400) or post infusion phase (AUC_{MAP 30–120 min}, P = 0.196). There was an increase in SBP during the infusion phase (AUC_{SBP 0–30 min}, P = 0.027) and no difference in the post-infusion phase (AUC_{SBP 30–120 min}, P = 0.213). There was a drop in DBP during the infusion phase (AUC_{DBP 0–30 min}, P = 0.015) and no difference in the post-infusion period (AUC_{DBP 30–120 min}, P = 0.077). There was an increase in heart rate in the infusion phase (AUC_{HR 0–30 min}, P < 0.001) and no difference in the post-infusion phase (AUC_{HR 30–120 min}, P = 0.162) – see Figure 4 and peak responses in Table 3.

Figure 4.

Mean systolic blood pressure (SBP; filled triangles), diastolic blood pressure (DBP; open circles), mean arterial blood pressure (MAP; open squares) and heart rate (HR; filled squares) on PGD₂ compared to placebo. There was an increase in SBP (AUC_{SBP 0–30 min}; P = 0.027), a decrease in DBP (AUC_{DBP 0–30 min}; P = 0.015), and an increase in heart rate (AUC_{HR 0–30 min}; P < 0.001).

Figure 5.

Median headache (filled squares) and mean percentage changes from baseline for the middle cerebral artery blood flow velocity (V_MCA) (open squares) corrected for changes in P_etCO₂, (28) diameter of the superficial temporal artery (STA) (filled triangles) and diameter of the radial artery (open circles) on PGD₂ day. Peak decrease in V_MCA was 28.3% at 10 min compared to baseline indicating dilatation of MCA (49). Peak increase in STA diameter was 55.7% at 20 min compared to baseline. Peak increase in RA was 16.3% at 20 min compared to baseline.

Adverse events

All subjects reported adverse events and data are presented in Table 4. Headache, nausea, flushing, palpitations, heat sensation and nasal congestion were very common.

Table 4.

Adverse events were recorded and reported during in-hospital (0–120 min) period on PGD₂ and placebo days (n = 12)

Symptoms	PGD₂	P-value
Headache	10	0.002
Nausea	10	0.002
Vomit	1	1.0
Photophobia	1	1.0
Flushing	12	<0.001
Palpitation	11	0.001
Heat sensation	12	<0.001
Abdominal pain	1	1.0
Feeling cold	3	0.250
Nasal congestion	10	0.002
Dizziness	1	1.0
Light shortness of breath	3	0.250
Tired	1	1.0
Painful i.v. access	1	1.0
Saliva feels more viscous than normal	1	1.0
Light pressure on collum	1	1.0
Feeling hungry	1	1.0
Low abdominal pain which mimics menstruation	2	0.5

P-value, McNemar test.

Discussion

The major finding in the present study is that PGD₂ induced a marked dilatation of intracranial (MCA), extracranial (STA) and peripheral (RA) arteries, but only a mild headache.

Human models of headache or migraine

We have validated and extensively used a human provocation model in normal volunteers (23,25) and in migraine sufferers (29–31) for more than 25 years and have learnt that normal volunteers should be used in the first place (25,32–34). Only substances that produce substantial headache in healthy volunteers have the potential to induce migraine in migraine sufferers and only such substances should, therefore, be tested in migraine sufferers. We have used this model to validate nitric oxide synthase (NOS) inhibition (35) and calcitonin gene related peptide (CGRP) receptor antagonism (36) as anti-migraine targets. Furthermore, we have suggested from human experimentation that cyclic adenosine monophosphate (cAMP) (37) and cyclic guanosine monophosphate (cGMP) (38) signalling pathways may play a crucial role in generation of head pain and that inhibition of these pathways may be effective in the treatment of migraine pain.

Comparison of PGI₂ and PGE₂

Over the past 5 years, we have extensively studied headache eliciting properties of prostaglandins (PGs). Given that PGs have been examined in different individuals, comparisons between studies are limited. However, all studies used similar design including recording of headache and vascular (TCD and Dermascan) responses and we found that the headache eliciting effect of PGD₂ in terms of incidence and median intensity is similar to other PGs (Table 5) (8,9). In contrast, arterial responses such as decrease in MCA velocity and increase of STA diameter are more pronounced both in peak response and duration after PGD₂ than after infusion of PGE₂ and PGI₂. This discrepancy is particularly interesting because mechanical dilatation of cerebral arteries triggers head pain (5,39), possibly via activation of stretch-sensitive perivascular nociceptors (40). The throbbing nature of migraine, dilation of STA (41) and MCA (42), combined with the fact that external compression of arteries (43,44) and vasoconstrictors (45) alleviate head pain and migraine, support the view that a large and prolonged arterial dilatation such as observed in the present study might be responsible for the reported headache. However, a severe headache would have been expected compared to other PGs if dilatation of major cephalic arteries per se were the major contributor to head pain. This suggests a minor role of vasodilatation in the generation of head pain. A possible difference between PGs in migraine induction should be examined in future studies in migraine sufferers.

Table 5.

Timing of headache and vascular responses on prostaglandin E₂ (PGE₂) and prostaglandin I₂ (PGI₂, prostacyclin) in studies with identical design are given for comparison (8,9)

	PGE₂ Healthy subjects		PGI₂ Healthy subjects
Maximal responses	Peak time (min)	Peak response compared to baseline	Peak time (min)	Peak response compared to baseline
Headache (VRS)	40	2	20	1
V_MCA (cm/s)	20	–13.9% (–20.6 to –7.3)	10	–4.6% (–7.6 to –1.6)
STA (mm)	10	23.5% (14.0 to 37.8)	20	38.8% (26.5 to 51.1)
RA (mm)	10	4.4% (–0.2 to 9.1)	20	6.9% (3.1 to 10.6)
HR (beats/min)	20	63.3% (46.3 to 80.4)	20	44.4% (32.5 to 56.3)
MAP (mmHg)	10	–4.5% (–7.6 to –1.3)	10	–3.7% (–8.4 to –0.8)

Peak median headache values are given on a Verbal Rating Scale (VRS). Maximal mean percentage changes from baseline (95% CI) in vascular variables. The middle cerebral artery blood flow velocity (V_MCA) is corrected for changes in P_etCO₂ (28).

Possible mechanisms of PGD₂-induced vasodilatation and headache

Experimental studies in humans in vivo and in vitro have shown that PGD₂ acts as a vasodilator via DP-receptors (46–48). In agreement with this, we observed a marked dilatation of STA and a marked decrease in V_MCA. If regional blood flow in the MCA area is constant, a decrease in V_MCA is caused by dilatation of MCA (49). For technical reasons, we did not have the possibility to record regional or global CBF and can, therefore, not completely exclude that velocity in MCA was decreased due to a decrease in CBF. Activation of the DP-receptor by PGD₂ increases intracellular cAMP (14) which induces vasodilatation via opening of Ca²⁺-dependent K⁺ channels in rats and rabbits (50,51). PGI₂ and PGE₂ mediate dilatation via IP, EP2 and EP4 receptors also coupled to cAMP formation and opening of K_ATP and BK_Ca channels (52–55). Interestingly, PGD₂, PGI₂ and PGE₂ induce a Ca²⁺-dependent CGRP release in rat trigeminal neurons via DP, IP, EP2 and EP4 receptors (56). CGRP might, therefore, contribute to headache in the present study, but CGRP release cannot explain the discrepancy between mild headache intensity and marked vasodilatation. Therefore, only studies using CGRP antagonists will reveal the true role of CGRP in PGD₂-induced headache.

In spontaneous migraine attacks, an unknown direct or indirect mechanism may cause release of inflammatory mediators from meningeal mast cells and subsequent activation and sensitization of trigeminal afferents (1,7). Degranulation of mast cells causes release of different substances and PGD₂ is the major product (10–12). PGs alone do not trigger mast cell degranulation (57). Therefore, if mast cell degranulation is important to pain in neurovascular headaches, one would expect a more severe headache after PGD₂ than after other PGs. This was not the case suggesting that PGD₂ release from mast cells may not be a key factor in migraine pain. PGD₂ only weakly induces sensitisation of dural and peripheral sensory afferents after topical application compared to PGI₂ and PGE₂ (13,58–61). This weak pro-nociceptive effect of PGD₂ is, therefore, a likely explanation of the discrepancy between vasodilatation and pain in the present study. A central effect of PGD₂ is possible because PGD₂ may modulate the permeability of the blood–brain barrier, but only 0.1% of prostanoids are detectable in the brain (15,62) after intravenous infusion. Intrathecal PGD₂ has both pronociceptive and antinociceptive effects (63–66) and interpretation is, therefore, difficult. PGD₂ illustrates the complexity of prostanoid signalling because DP1 receptor agonists induce a weak sensitisation, but they decrease sensitisation induced by PGE₂ (67). Furthermore, DP1 antagonism increases sensitisation caused by PGE₂ (67). The cAMP–PKA pathway may be involved in the pathogenesis of neurovascular headaches (37,68) and the present results indicate differences in intracellular coupling to the cAMP–PKA pathway and regulation of DP1, IP, EP2 and EP4 receptors. Given the number of PGs and PG receptors, a mechanism of internal control between PGs probably exists.

Most of our subjects reported nausea (83%) and nasal congestion (83%; Table 4). The DP1 receptor is involved in the emesis reflex (69,70). PGD₂ causes nasal vasodilatation via DP receptors and previous literature considers PGD₂ to be one of the major mediators contributing to nasal congestion (47,48). Thus, PGD₂ may contribute to nausea and nasal congestion in primary headaches.

Adverse events of PGD₂ might compromise blindness of the present study (Table 4), but adverse events were caused by the physiological response to PGD₂ and could not have been avoided. The present double-blind approach was the best possible way, although not ideal, of coping with methodological error.

Conclusions

PGD₂ is the strongest dilator of cerebral and extracerebral arteries studied so far in human models of headache. Nevertheless, it did not cause more headache than other prostanoids. We suggest that the weak pronociceptive effect of PGD₂ is a likely explanation of the discrepancy between vasodilatation and head pain in the present study. PGD₂ causes considerable amounts of nausea and nasal congestion and may be the cause of these symptoms in spontaneous migraine attacks.

Footnotes

Acknowledgements

The authors thank laboratory technicians Lene Elkjær and Winnie Grønning for their dedicated and excellent assistance. The study was supported by grants from Mauritzen La Fountaine Fond, the Danish Headache Society and the Lundbeck Centre of Neurovascular Signalling (LUCENS).

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Discrepancy between strong cephalic arterial dilatation and mild headache caused by prostaglandin D 2 (PGD 2 )

Abstract

Keywords

Introduction

Design and methods

Pilot experiment

Main experiment

Experimental design

Headache intensity

Middle cerebral artery blood flow velocity

Diameter of the superficial temporal artery and radial artery

Vital signs

Data analysis and statistics

Results

Headache

Middle cerebral artery velocity

Superficial temporal artery and radial artery

Allodynia and premonitory symptoms

Arterial blood pressure and heart rate

Adverse events

Discussion

Human models of headache or migraine

Comparison of PGI2 and PGE2

Possible mechanisms of PGD2-induced vasodilatation and headache

Conclusions

Footnotes

Acknowledgements

References

Comparison of PGI₂ and PGE₂

Possible mechanisms of PGD₂-induced vasodilatation and headache