Cerebellar unit activity during generalized penicillin epilepsy in the awake cat

The cerebellum's understated role and influences in the epilepsies

Approximately 1 in 26 people will develop epilepsy in their lifetime, but current treatment options leave as many as half of all epilepsy patients with uncontrolled seizures. In addition to the burden of the seizures themselves, chronic epilepsy can be associated with cognitive deficits, structural changes, and devastating negative outcomes such as sudden unexpected death in epilepsy (SUDEP). Thus, major challenges in epilepsy research surround the need to both develop new therapeutic targets for intervention as well as shed light on the mechanisms by which chronic epilepsy can lead to comorbidities and negative outcomes. Despite not being traditionally associated with epilepsy or seizures, the cerebellum has emerged as not only a brain region that can serve as an important target for seizure control, but one that may also be profoundly impacted by chronic epilepsy. Here, we discuss targeting the cerebellum for potential therapeutic intervention and discuss pathway insights gained from recent optogenetic studies. We then review observations of cerebellar alterations during seizures and in chronic epilepsy, as well as the potential for the cerebellum to be a seizure focus. Cerebellar alterations in epilepsy may be critical to patient outcomes, highlighting the need for a more comprehensive understanding and appreciation of the cerebellum in the epilepsies.

The cerebellum and epilepsy

Epilepsy is the fourth most common neurological disorder, but current treatment options provide limited efficacy and carry the potential for problematic adverse effects. There is an immense need to develop new therapeutic interventions in epilepsy, and targeting areas outside the seizure focus for neuromodulation has shown therapeutic value. While not traditionally associated with epilepsy, anatomical, clinical, and electrophysiological studies suggest the cerebellum can play a role in seizure networks, and importantly, may be a potential therapeutic target for seizure control. However, previous interventions targeting the cerebellum in both preclinical and clinical studies have produced mixed effects on seizures. These inconsistent results may be due in part to the lack of specificity inherent with open-loop electrical stimulation interventions. More recent studies, using more targeted closed-loop optogenetic approaches, suggest the possibility of robust seizure inhibition via cerebellar modulation for a range of seizure types. Therefore, while the mechanisms of cerebellar inhibition of seizures have yet to be fully elucidated, the cerebellum should be thoroughly revisited as a potential target for therapeutic intervention in epilepsy. This article is part of the Special Issue "NEWroscience 2018.

Zones in the cerebellar cortex. Their organization and potential relevance to cerebellar stimulation

The organization of zones in the cerebellar cortex, as reflected by the arrangement of cortical efferent and afferent fibers, was reviewed with special emphasis on the anterior lobe. There is conclusive evidence for the existence of at least six, and possibly as many as nine, ipsilateral rostrocaudally oriented cortical zones, each having specific afferent and efferent connections. The topography afferent and efferent fibers of the cerebellar cortex and/or the spatial arrangement of cortical zones have not been given appropriate attention in both experimental and clinical studies dealing with focal electrical stimulation of the cerebellum. It is suggested that inattention to the relationships between electrode placement and zones in the cerebellar cortex may be partially responsible for the sometimes capricious results during and/or subsequent cerebellar cortical stimulation.

The activity of cerebellar neurones during epileptiform activity induced by penicillin in the cerebral cortex of the rat

In urethane anaesthetized rats an epileptiform focus was made in one cerebral cortex with penicillin (25 U). The relationship between epileptiform spikes in the cerebral cortex and the activity of cerebellar cortical and nuclear neurones have been investigated by means of extracellular recording techniques and computation of peri-event histograms and 'CUSUMs' (cumulative sums of differences from the mean). In the cerebellar cortex two distinct patterns of evoked activity were recorded: (a) In units showing little or no spontaneous activity, a brief evoked discharge followed the epileptiform spike. (b) In units showing steady spontaneous activity, a period of evoked inhibition followed the epileptiform spike. These included Purkinje cells and probably inhibitory interneurones. In the intracerebellar nuclei, cells with marked spontaneous activity showed a prolonged period of increased discharge following the epileptiform spike. It is concluded that the output of the intracerebellar nuclei is increased by epileptiform activity in the cerebral cortex. The significance of these results is discussed.

Selective amplitude histograms: a statistical approach to EEG-single unit relationships in generalized epilepsy

Statistical analysis techniques that permit detection and quantification of EEG-single unit correlations were employed in a study of generalized penicillin epilepsy in the cat. Single unit activity was recorded in cerebral and cerebellar cortices and compared to the locally recorded EEG. It was found that during generalized paroxysmal activity the background EEG was significantly related to the time of occurrence of unit discharge. The degree of synchrony of various units differed but tended to increase as the duration and extent of paroxysmal activity increased. These relationships were usually not evident upon visual inspection of the original unprocessed data but were consistently detected by the analysis techniques described here. The results demonstrate the usefulness of a statistical approach to the analysis of single unit data and suggest that, not only is there a significant relationship between paroxysmal events and neuronal activity in generalized penicillin epilepsy, but also an overall trend to closer synchronization of EEG and single unit discharges during nonparoxysmal periods.

Neuronal firing patterns during generalized penicillin epilepsy in the awake cat

Single unit activity was recorded in the neocortex of awake cats during spontaneous spike wave discharges. Ongoing activity was interrupted during paroxysmal events and was replaced by (1) bursts of action potentials coincident with the EEG spike and cessation of firing during the wave, or (2) cessation of firing for the duration of the paroxysm. Other units which displayed little or no background activity were recruited to fire during the EEG spike. Surface cerebellar stimulation at high frequencies led to decreased neuronal activity while single pulse shocks resulted in short latency activation. These results were compared to studies of spike wave activity elicited by electrical stimulation.