Antiepileptic effects of electrical stimulation of the piriform cortex

Emerging approaches in neurostimulation for epilepsy

PURPOSE OF REVIEW

Neurostimulation is a quickly growing treatment approach for epilepsy patients. We summarize recent approaches to provide a perspective on the future of neurostimulation.

RECENT FINDINGS

Invasive stimulation for treatment of focal epilepsy includes vagus nerve stimulation, responsive neurostimulation of the cortex and deep brain stimulation of the anterior nucleus of the thalamus. A wide range of other targets have been considered, including centromedian, central lateral and pulvinar thalamic nuclei; medial septum, nucleus accumbens, subthalamic nucleus, cerebellum, fornicodorsocommissure and piriform cortex. Stimulation for generalized onset seizures and mixed epilepsies as well as increased efforts focusing on paediatric populations have emerged. Hardware with more permanently implanted lead options and sensing capabilities is emerging. A wider variety of programming approaches than typically used may improve patient outcomes. Finally, noninvasive brain stimulation with its favourable risk profile offers the potential to treat increasingly diverse epilepsy patients.

SUMMARY

Neurostimulation for the treatment of epilepsy is surprisingly varied. Flexibility and reversibility of neurostimulation allows for rapid innovation. There remains a continued need for excitability biomarkers to guide treatment and innovation. Neurostimulation, a part of bioelectronic medicine, offers distinctive benefits as well as unique challenges.

Intracranial investigation of piriform cortex epilepsy during odor presentation

The piriform cortex (PC) is part of the olfactory system, principally receiving input from the lateral olfactory tract and projecting to downstream components of the olfactory network, including the amygdala. Based on preclinical studies, PC is vulnerable to injury and can be easily kindled as an onset site for seizures. While the role of PC in human epilepsy has been studied indirectly and the subject of speculation, cases of demonstrated PC seizure onset from direct intracranial recording are rare. We present a pediatric patient with drug-resistant focal reflex epilepsy and right mesial temporal sclerosis with habitual seizures triggered by coconut aroma. The patient underwent stereoelectroencephalography with implantation of olfactory cortices including PC, through which we identified PC seizure onset, mapped high-frequency activity associated with presentation of olfactory stimuli and performance on cognitive tasks, and reproduced habitual seizures via cortical stimulation of PC. Coconut odor did not trigger seizures in our work with the patient. Surgical workup resulted in resection of the patient's right amygdala, PC, and mesial temporal pole, following which she has been seizure free for 20 months without functional decline in cognition or smell. Histological findings from resected tissue showed astrogliosis and subpial gliosis.

The adjustment mechanism of the spike and wave discharges in thalamic neurons: a simulation analysis

Different from many previous theoretical studies, this paper explores the regulatory mechanism of the spike and wave discharges (SWDs) in the reticular thalamic nucleus (TRN) by a dynamic computational model. We observe that the SWDs appears in the TRN by changing the coupling weights and delays in the thalamocortical circuit. The abundant poly-spikes wave discharges is also induced when the delay increases to large enough. These discharges can be inhibited by tuning the inhibitory output from the basal ganglia to the thalamus. The mechanisms of these waves can be explained in this model together with simulation results, which are different from the mechanisms in the cortex. The TRN is an important target in treating epilepsy, and the results may be a theoretical evidence for experimental study in the future.

Social defeat drives hyperexcitation of the piriform cortex to induce learning and memory impairment but not mood-related disorders in mice

Clinical studies have shown that social defeat is an important cause of mood-related disorders, accompanied by learning and memory impairment in humans. The mechanism of mood-related disorders has been widely studied. However, the specific neural network involved in learning and memory impairment caused by social defeat remains unclear. In this study, behavioral test results showed that the mice induced both learning and memory impairments and mood-related disorders after exposure to chronic social defeat stress (CSDS). c-Fos immunofluorescence and fiber photometry recording confirmed that CaMKIIα expressing neurons of the piriform cortex (PC) were selectively activated by exposure to CSDS. Next, chemogenetics and optogenetics were performed to activate PC CaMKIIα expressing neurons, which showed learning and memory impairment but not mood-related disorders. Furthermore, chemogenetic inhibition of PC CaMKIIα expressing neurons significantly alleviated learning and memory impairment induced by exposure to CSDS but did not relieve mood-related disorders. Therefore, our data suggest that the overactivation of PC CaMKIIα expressing neurons mediates CSDS-induced learning and memory impairment, but not mood-related disorders, and provides a potential therapeutic target for learning and memory impairment induced by social defeat.

Long-term seizure outcome during continuous bipolar hippocampal deep brain stimulation in patients with temporal lobe epilepsy with or without mesial temporal sclerosis: An observational, open-label study

OBJECTIVE

We present the findings related to seizure outcome during hippocampal deep brain stimulation (Hip-DBS) in patients with refractory temporal lobe epilepsy.

METHODS

Twenty-five patients submitted to Hip-DBS were studied. All patients were evaluated with interictal and ictal electroencephalography (EEG) and high-resolution 1.5 T magnetic resonance imaging (MRI). The hippocampus was targeted directly on MRI using a posterior occipital burr hole approach. Bipolar continuous stimulation was ramped up until 3.0 V (300 µs, 130 Hz). Patients were considered responders if at least 50% seizure frequency reduction was obtained.

RESULTS

Median age was 39 years; median follow-up time was 57 months (16 women). All patients had focal with impaired awareness seizure (FIAS) and 23 patients had focal aware seizure (FAS). Baseline median FAS and FIAS frequency was 8. Ictal EEG showed unilateral (n = 10) or bilateral (n = 15) seizure onset. MRI showed unilateral (n = 11) or bilateral (n = 8) mesial temporal sclerosis (MTS) and was normal in six6 patients. Fifteen patients were submitted to bilateral and 10 patients to unilateral Hip-DBS. Median reduction in FAS frequency was 66%. Eighteen patients with FAS were considered responders and five (21%) were free of FAS. Median FIAS frequency (n = 25) reduction was 91%. Twenty-two patients were considered responders and eight (32%) were free of FIAS. FIAS were significantly more reduced then FAS (P = .017). There was no relation between any contact's position within the hippocampus and outcome for either FAS (P = .727) or FIAS (P = .410). There was no difference in outcome in patients submitted to either unilateral or bilateral Hip-DBS regarding FAS (P = .978) or FIAS (P = .693).

SIGNIFICANCE

Hip-DBS significantly reduced the frequency of both FAS and FIAS in this cohort of patients with refractory temporal lobe epilepsy. Hip-DBS might represent a good therapeutic option in such patients not amenable to resective surgery.

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.

Deep Brain Stimulation Improves Motor Function in Rats with Spinal Cord Injury by Increasing Synaptic Plasticity

OBJECTIVE

To investigate the effect of deep brain stimulation (DBS) on rats with spinal cord injury (SCI) and its possible molecular mechanism.

METHODS

A rat SCI model was prepared using a modified Allen method. The animals were randomly divided into 3 groups (n = 12 per group): the sham group, the SCI group, and the SCI + DBS group. Then, DBS was applied to the rats in the SCI + DBS group for half an hour per day for 4 weeks. Basso, Beattie, and Bresnahan scores were used to assess spinal function.

RESULTS

DBS significantly improved hindlimb motor function in SCI rats, and the protein expression levels of brain-derived neurotrophic factor, the mammalian target of rapamycin, tropomyosin-related kinase B, protein kinase B, p70 ribosomal S6 protein kinase, postsynaptic density protein 95, and synaptophysin increased correspondingly.

CONCLUSIONS

DBS improves motor function in rats with SCI by increasing synaptic plasticity via tropomyosin-related kinase B-protein kinase B-mammalian target of rapamycin pathway.