Abstracts

Rationale: Epilepsy is the second most prevalent neurological disorder, affecting approximately 2 million people in the United States. One form of epilepsy, absence epilepsy, is characterized by typical absence seizures (ASs). ASs are brief (3-30 second) nonconvulsive epileptic events that consist of sudden impairment of consciousness accompanied by a generalized synchronous, bilateral, 2.5-4 Hz spike and slow-wave dischargers (SWDs) in the electroencephalogram (EEG). Various subcortical structures play a critical role in modulating seizures, including substantia nigra pars reticulata (SNpr) and superior colliculus (SC). These structures display broad-spectrum anti-seizure effects, however, the mechanism by which they produce changes in brain-wide excitability are poorly understood. Both structures provide input to pedunculopontine nucleus (PPN), a brainstem nucleus that is a critical part of the ascending reticular activating system. Activation of the PPN can trigger potent desynchronization of the cerebral cortex. Thus, the PPN is both anatomically and functionally positioned to mediate the anticonvulsant effects of SNpr and SC. Methods: The PPN contains a variety of cell types, including cholinergic projection neurons, GABA neurons and glutamatergic neurons. Here, we evaluated the anti-absence seizure effects of optogenetic activation in different neuron populations of PPN. We used ChAT::Cre and GAD::Cre Long-Evans rats transfected with AAV-hSyn-FLEX-Chronos-GFP  or AAV-hSyn-FLEX-ArchT-GFP to activate or inhibit cholinergic and GABAergic neurons of PPN. To study the role of glutamatergic neurons, we injected AAV-CamKII-Chr2-mCheery or AAV-CamKII-ArchT-mCherry in wild-type Long Evans rats to activate or inhibit glutamatergic cells. Absence seizures were evoked by systemic administration of γ-butyrolactone - GBL (SWD-thalamocortical/absence seizures model) and electrographic seizures were recorded on a within-subject basis (i.e., with and without optogenetic activation/silencing). Results: Optogenetic activation (5 Hz) of ChAT+ neurons suppressed cortical SWD absence seizures. However, inhibition of cholinergic neurons had no effect on number of absence seizures. Meanwhile, optogenetic activation of GABAergic neurons of PPN increased absence seizures, whereas optogenetic silencing of GABAergic neurons was without effect on absence seizures. Finally, activation or inhibition of glutamatergic neurons from the PPN have no effect of the number of absence seizures. Parallel studies are underway using DREADD-mediated modulation of PPN neuronal populations and examining the effect of these populations on other seizure types. Conclusions: These data indicate that different populations of neurons in the PPN exert diverse effects in the modulation of seizures. Using optogenetic neuronal activation in cholinergic neurons from PPN is a promising target for seizures control of epilepsy. Funding: NIH