Epileptogenesis-induced changes of hippocampal-piriform connectivity.

OBJECTIVE: Tissue remodeling has been described in brain circuits that are involved in the generation and propagation of epileptic seizures. Human and animal studies suggest that the anterior piriform cortex (aPC) is crucial for seizure expression in focal epilepsies. Here, we investigate the effect of kainic-acid (KA)-induced seizures on the effective connectivity of the aPC with bilateral hippocampal CA3 regions using cerebro-cerebral evoked potentials (CCEPs). METHODS: Adult male Sprague-Dawley rats were implanted with a tripolar electrode in the left aPC for stimulation and recording, and with unipolar recording electrodes in bilateral CA3 regions. Single pulse stimulations were given to the aPC and CCEPs were averaged before KA injections and after the emergence of spontaneous recurrent seizures (SRS). Similar recordings at equivalent time intervals were obtained from animals that received saline injections instead of KA (controls). RESULTS: In the experimental group, the percentage change of increased amplitude of the contralateral (but not ipsilateral) CA3 CCEPs between pre-KA injection and after the emergence of SRS was significantly greater than in controls. No significant single-pulse-induced spectral change responses were observed in either epileptic or control rats when comparing pre- and post-stimulus time intervals. Also, we found no correlation between seizure frequency and the extent of amplitude changes in the CCEPs. CONCLUSIONS: In the KA model, epileptogenesis results in plastic changes that manifest as an amplification of evoked potential amplitudes recorded in the contralateral hippocampus in response to single-pulse stimulation of the aPC. These results suggest epileptogenesis-induced facilitation of interhemispheric connectivity between the aPC and the hippocampus. Since the amplitude increase of the contralateral CCEP is a possible in vivo biomarker of epilepsy, any intervention (e.g. neuromodulatory) that can reverse this phenomenon may hold a potential antiepileptic efficacy.

The effects of ammonia stimulation on kainate-induced status epilepticus and anterior piriform cortex electrophysiology.

OBJECTIVE: Strong olfactory stimulation (OS) with such substances as toluene or ammonia has been reported to suppress seizures. We aimed to investigate the role of ammonia stimulation on acute kainic acid (KA)-induced seizures. We also investigated any possible effects of ammonia stimulation on the electrophysiology of the anterior piriform cortex (APC). METHODS: Adult male Sprague-Dawley rats were implanted with bilateral hippocampal electrodes and an electrode in the left APC. Animals were exposed to either distilled water (control) or ammonia stimulation for 20 s every 5 min during KA induction of status epilepticus (SE). The electroencephalogram (EEG) was analyzed for seizure frequency, duration, severity, and total KA doses given prior to reaching SE. Seizure-free EEG epochs that coincided with OS were chosen and analyzed via wavelet analysis for any spectral changes. RESULTS: We found no significant differences in seizure frequency, duration, severity, or administered KA doses before SE between the groups. In the experimental group, a wavelet analysis of variance (WANOVA) revealed a significant stimulation-induced increase of power in the delta and alpha bands prior to the first KA injection and higher power in the delta and theta bands after KA injection. CONCLUSIONS: Whereas the spectral analysis of the APC revealed specific OS-induced changes, our findings suggest that OS with ammonia does not result in altering the threshold of attaining KA-induced SE. This does not rule out a potential role for OS in reducing recurrent seizures in the KA or other epilepsy models.

Antiepileptic effects of electrical stimulation of the piriform cortex.

Deep brain stimulation (DBS) may help control seizures in individuals with medically intractable epilepsy who are not candidates for resective surgery. The current review focuses on some preclinical studies of DBS of the piriform cortex (PC), an area involved in the generation and maintenance of seizures, as a potential therapeutic option for refractory epilepsy. We also present findings suggesting the safety of low frequency stimulation (LFS) of the PC on memory. A variety of LFS parameters have been suggested as an effective treatment strategy for refractory epilepsy. In generalized epilepsy, however, recent studies suggest that LFS may exacerbate seizures and high frequency stimulation (HFS) might be an alternative. Hence, further studies are required to explore the potential therapeutic targets and proper stimulation parameters for the successful translation of DBS of the PC to the clinic.

The Claustrum in Relation to Seizures and Electrical Stimulation.

The neural mechanisms of altered consciousness that accompanies most epileptic seizures are not known. We have reported alteration of consciousness resulting from electrical stimulation of the claustrum via a depth electrode in a woman with refractory focal epilepsy. Additionally, there are reports that suggest possible claustral involvement in focal epilepsy, including MRI findings of bilaterally increased T2 signal intensity in patients with status epilepticus (SE). Although its cytoarchitecture and connectivity have been studied extensively, the precise role of the claustrum in consciousness processing, and, thus, its contribution to the semiology of dyscognitive seizures are still elusive. To investigate the role of the claustrum in rats, we studied the effect of high-frequency stimulation (HFS) of the claustrum on performance in the operant chamber. We also studied the inter-claustral and the claustro-hippocampal connectivity through cerebro-cerebral evoked potentials (CCEPs), and investigated the involvement of the claustrum in kainate (KA)-induced seizures. We found that HFS of the claustrum decreased the performance in the operant task in a manner that was proportional to the current intensity used. In this article, we present previously unpublished data about the effect of stimulating extra-claustral regions in the operant chamber task as a control experiment. In these animals, stimulation of the corpus callosum, the largest interhemispheric commissure, as well as the orbitofrontal cortex in the vicinity of the claustrum did not produce that same effect as with claustral stimulation. Additionally, CCEPs established the presence of effective connectivity between both claustra, as well as between the claustrum and bilateral hippocampi indicating that these connections may be part of the circuitry involved in alteration of consciousness in limbic seizures. Lastly, some seizures induced by KA injections showed an early involvement of the claustrum with later propagation to the hippocampi. Further work is needed to clarify the exact role of the claustrum in mediating alteration of consciousness during epileptic seizures.

A pilot study of the role of the claustrum in attention and seizures in rats.

OBJECTIVE: The claustrum has been implicated in consciousness, and MRIs of patients with status epilepticus have shown increased claustral signal intensity. In an attempt to investigate the role of claustrum in cognition and seizures, we (1) assessed the effect of high-frequency stimulation (HFS) of the claustrum on performance in the operant chamber; (2) studied interclaustral and claustrohippocampal connectivity through cerebro-cerebral evoked potentials (CCEPs); and (3) investigated the role of claustrum in kainate-induced (KA) seizures. METHODS: Adult male Sprague-Dawley rats were trained in operant conditioning and implanted with electrodes in bilateral claustra and hippocampi. Claustrum HFS (50 Hz) was delivered bilaterally and unilaterally with increasing intensities from 50 to 1000 µA, and performance scores were assessed. CCEPs were studied by averaging the responses to bipolar stimulations, 1-ms wide pulses at 0.1 Hz to the claustrum. KA seizures were analyzed on video-EEG recordings. RESULTS: Generalized Estimating Equations analysis revealed that claustral stimulation reduced task performance scores relative to rest sessions (bilateral: -15.8 percentage points, p < 0.0001; unilateral: -15.2, p < 0.0001). With some stimulations, the rats showed a stimulus-locked decrease in attentiveness and, occasionally, an inability to complete the operant task. CCEPs demonstrated interclaustral and claustrohippocampal connectivity. Some KA seizures appeared to originate from the claustrum. CONCLUSIONS: Findings from the operant conditioning task suggest stimulation of the claustrum can alter attention or awareness. CCEPs demonstrated connectivity between the two claustra and between the claustrum and the hippocampi. Such connectivity may be part of the circuitry that underlies the alteration of awareness in limbic seizures. Lastly, KA seizures showed early involvement of the claustrum, a finding that also supports a possible role of the claustrum in the alteration of consciousness that accompanies dyscognitive seizures.

Effects of low-frequency electrical stimulation of the anterior piriform cortex on kainate-induced seizures in rats.

OBJECTIVE: Recent evidence in animals and humans suggests that low-frequency stimulation (LFS) has significant antiepileptic properties. The anterior piriform cortex (APC) is a highly susceptible seizure-trigger zone and may be critical for the initiation and propagation of seizures originating from cortical and limbic foci. We used the kainic acid (KA) seizure model in rats to assess the therapeutic effect of LFS of the APC on seizures. METHODS: Adult male Sprague-Dawley rats were implanted with electrodes in the left APC and recording electrodes bilaterally in the hippocampal CA3 regions. Rats were monitored continuously with video-EEG after the emergence of spontaneous recurrent seizures that followed induction of status epilepticus by intraperitoneal KA. After two weeks of baseline recordings to determine seizure frequency, LFS of the APC was applied 60-min On 15-min Off, for two weeks with 1Hz biphasic square waves, 0.2ms pulse width, at 200µA. Another 2-week period of video-EEG monitoring was done after the cessation of LFS to study the carry-over effect. Changes in seizure frequency, severity, and duration between baseline, during LFS, and post-LFS were analyzed using the Poisson regression model. RESULTS: Overall seizure frequency decreased during the post-LFS period to 5% of that at baseline (p=0.003). Severe seizures (stages 4 and 5 on the Racine scale) decreased to 0% of the baseline during the post-LFS period. CONCLUSIONS: Two weeks of LFS of the APC reduced spontaneous seizure frequency and severity in the KA model with the effect outlasting the stimulation. Our findings suggest that the APC can be an important therapeutic target for stimulation in epilepsy.

Acute and spontaneous seizure onset zones in the intraperitoneal kainic acid model.

OBJECTIVE: Hippocampal monitoring is often used in the intraperitoneal kainic acid (KA) seizure model for detection and quantification of early ictal activity. Here, we investigated extra-hippocampal seizure onset zones (SOZs) in this model. METHODS: Eight male Sprague Dawley rats implanted with depth electrodes were continuously recorded during intraperitoneal KA injections until status epilepticus (SE) was induced. Another group of four rats was monitored chronically up to two weeks after emergence of spontaneous recurrent seizures. All rats had hippocampal electrodes. Other sampled brain regions included, among others, the claustrum, piriform cortex, and orbital cortex. Seizures recorded with video-EEG were visually analyzed. RESULTS: In the 58 seizures recorded during KA injections, the SOZ was extrahippocampal in 7 (12%), diffuse in 29 (50%), and hippocampal in 22 (38%). Of the 14 spontaneous seizures recorded, none were solely extrahippocampal, 10 (71%) were diffuse, and 4 (29%) were of hippocampal onset. All extra-hippocampal seizures propagated to the hippocampus within 4 to 50s (mean=14, n=7). No distinctive semiological manifestations correlated with the SOZs. SIGNIFICANCE: We conclude that seizures can have multifocal SOZs in the KA model. This finding is important to consider when using this model, among other purposes, to screen for new therapies, study pharmacoresistance, or investigate comorbidities of epilepsy.

Yawning induced by focal electrical stimulation in the human brain.

The primary function of yawning is not fully understood. We report a case in which electrical stimulation of the putamen in the human brain consistently elicited yawning. A 46-year-old woman with intractable epilepsy had invasive depth electrode monitoring and cortical stimulation mapping as part of her presurgical epilepsy evaluation. The first two contacts of a depth electrode that was intended to sample the left insula were in contact with the putamen. Stimulation of these contacts at 6mA and 8mA consistently elicited yawning on two separate days. Engagement in arithmetic and motor tasks during stimulation did not result in yawning. When considering the role of the putamen in motor control and its extensive connectivity to cortical and brainstem regions, our findings suggest that it plays a key role in the execution of motor movements necessitated by yawning. Furthermore, given the role of the anterior insula in attention and focused tasks, activation of this area while engaged in arithmetic and motor tasks could inhibit the putaminal processing necessary for yawning. Many have hypothesized the function of yawning; however, it remains debatable whether yawning serves a primarily physiological or communicative function or perhaps both.