Abstract
Buckmaster PS, Reyes B, Kahn T, Wyeth M. J Neurosci. 2022;42(39):7482-7495. doi:10.1523/JNEUROSCI.0429-22.2022. PMID: 35995562, PMCID: PMC9525166. Temporal lobe epilepsy is common, but mechanisms of seizure initiation are unclear. We evaluated seizure initiation in female and male rats that had been systemically treated with pilocarpine, a widely used model of temporal lobe epilepsy. Local field potential (LFP) recordings from many brain regions revealed variable sites of earliest recorded seizure activity, but mostly the ventral hippocampal formation. To test whether inactivation of the ventral hippocampal formation would reduce seizures, mini-osmotic pumps were used to continually and focally deliver TTX. High doses of TTX infused unilaterally into the ventral hippocampal formation blocked seizures reversibly but also reduced LFP amplitudes in remote brain regions, indicating distant effects. A lower dose did not reduce LFP amplitudes in remote brain regions but did not reduce seizures when infused unilaterally. Instead, seizures tended to initiate in the contralateral ventral hippocampal formation. Bilateral infusion of the lower dose into the ventral hippocampal formation reduced seizure frequency 85%. Similar bilateral treatment in the amygdala was not effective. Bilateral infusion of the dorsal hippocampus reduced seizure frequency, but only 17%. Together, these findings reveal that the ventral hippocampal formation is a primary bilaterally independent epileptogenic zone, and the dorsal hippocampus is a secondary epileptogenic zone in pilocarpine-treated rats. This is consistent with many human patients, and the results further validate the LFP method for identifying seizure onset zones. Finally, the findings are more consistent with a focal mechanism of ictogenesis rather than one involving a network of interdependent nodes.
Commentary
The electrical activity that pulses through the brain during a seizure propagates at breathtaking speed. During large-scale seizures observed in multiple brain structures (i.e., generalized), the brain often appears to instantaneously and synchronously explode with electrical activity. But does it really? Clearly, some discrete neuronal population—even if numerically small—must initiate a seizure, right? Although the hunt for seizure-initiating sites began in earnest nearly a hundred years ago, with the application of then-novel EEG recording techniques to epilepsy research, we still struggle to resolve such sites or to determine if such sites even exist.
Temporal lobe epilepsy (TLE) is the most common form of adult epilepsy. Temporal lobe epilepsy is characterized by spontaneously recurring seizures, cognitive deficits, and structural remodeling of the limbic system. Despite significant efforts, we continue to have an incomplete understanding of the pathophysiology of TLE, few effective therapeutics, and a high incidence of seizures that remain drug-resistant. Resolving the brain structures—if any—that reliably initiate seizures remains an essential goal of TLE research, and one that if achieved can likely lead to more effective surgical and pharmacological treatments.
Herein, we highlight the most recent attempt to resolve seizure initiating sites in TLE. 1 The authors build on their previous study (Toyoda et al), 2 wherein local field potentials (LFPs) were recorded from 9 brain regions in pilocarpine-treated rats. Local field potentials survey electric potentials from populations of neurons near the electrode and provide insights into the origins of a seizure. Toyoda et al 2 concluded that the ventral hippocampal formation (VHF) commonly produces the earliest seizure activity, a conclusion consistent with prior animal studies. 3 However, prior studies have also reported that the olfactory cortex, amygdala, 3 midline dorsal thalamus, 4 and specific parts of the hippocampal formation 5 can also generate the first bouts of activity during TLE seizures. Now, by selectively abolishing activity in putative seizure-initiating sites with the sodium channel blocker tetrodotoxin (TTX), the latest study provides compelling evidence that TLE seizures predominantly initiate in the VHF. However, some quite surprising findings were also made along the way.
Buckmaster et al systematically evaluated the effects of uni- and bilateral, TTX-mediated inactivation of the VHF, amygdala, and dorsal hippocampus on the number and duration of spontaneous seizures recorded in pilocarpine-treated rats. Seizures were measured before, during, and after TTX was delivered for multiple days via cannulae implanted into the aforementioned structures. Altogether, over 18 000 seizures in 30 animals were recorded. After a dose–response experiment, 0.1 μM TTX was selected as an effective and selective dose, but only when delivered bilaterally; unilateral delivery of 0.1 μM TTX did not abolish seizures. Unilateral delivery required h igh doses of TTX (1, 5, 10 μM) to decrease seizure frequency but was proven to have off-site effects and therefore affect structures beyond the target. The authors found that bilateral infusion of 0.1 μM TTX into the VHF decreased the number of seizures by 85%. Infusion of the same dose of TTX to the Dorsal Hippocampus (DH) decreased the seizure frequency by 17% and the amygdala by 20%. Even when 0.1 μM TTX was delivered unilaterally into the hemisphere containing the VHF with earliest activity, the activity in the contralateral VHF was likely sufficient to initiate the seizure. In short, the study appears to support the hypothesis that seizure abatement is effective only with bilateral VHF inhibition.
Despite providing an effective approach to silence neurons, targeted TTX delivery has its caveats. Indeed, the ability to restrict the diffusion of TTX to just the targeted region is likely impossible, particularly because the cell density of the amygdala and DH is different than in VHF. Therefore, future studies should consider the use of optogenetic and/or chemogenetic approaches to more selectively modulate the activity of neuronal populations. Spatial considerations also apply to recording techniques. Since LFPs ignore physical boundaries, uncertainty remains about the exact source of the recorded signal and only general conclusions regarding the recording locations can likely be made. Local field potential reflect the activities of inputs to the recorded area, not the spike data which represents the output. This input is mostly composed of sustained currents in the tissue: synaptic and somato-dendritic currents. Therefore early seizure activity measured by LFP in the VHF could reflect the activation of the structure by another source. Finally, although the pilocarpine model captures many features of human TLE, the question of how well the model really recapitulates human TLE will likely always remain. In TLE patients, the coactivation of multiple structures may be required for successful seizure initiation (González Otárula et al, e.g., mesial, lateral, mesial-lateral, and temporal-perisylvian structures). 6 Thus, pilocarpine-treated rats may only represent one form of TLE. On this note, however, the lack of better animal models for TLE continue to make the pilocarpine model relevant.
In summary, this study suggests that the VHF appears to serve as a primary node for seizure initiation in pilocarpine-treated rats. It remains possible that the ventral hippocampus plays a greater role in ictogenesis than the dorsal hippocampus due to its higher cortical connectivity. Nevertheless, this study motivates further investigation into the specific cell types and microcircuits that endow the VHF with the capacity to reliably initiate seizures. Such mechanisms can then be leveraged to develop better therapeutics to treat seizures. Moreover, the successful identification of a seizure-initiating site provides the hope that analogous sites exist for other forms of epilepsy. The hunt continues.
