The Nigrotegmental Circuit in an Scn1a+/- Mouse Model of Dravet Syndrome - Project Summary/ Abstract Although most epilepsy research focuses on neocortical and limbic structures – i.e., typical seizure onset zones – subcortical regions that significantly regulate the onset and spread of seizures may also be effective therapeutic targets in epilepsy. The nigrotegmental pathway, consisting of the substantia nigra pars reticulata (SNr) and its connections to the cholinergic brainstem pedunculopontine nucleus (PPN), plays a crucial role in the regulation of movement and arousal, and may also represent a promising target for seizure modulation. The SNr is predominantly composed of GABAergic neurons and exerts powerful control over both forebrain and midbrain networks through its inhibitory output. This effectively positions the SNr as a central “choke point” within the basal ganglia and many lines of evidence demonstrate that SNr inhibition produces robust seizure suppression across multiple epilepsy models. Although activating cholinergic PPN neurons promotes cortical desynchronization and Rapid Eye Movement (REM) sleep – both of which are considered seizure-protective – pharmacological manipulation of the PPN during seizures has yielded mixed results, with both pro- and anti- convulsant effects observed. This highlights the need for further research into the mechanisms responsible for SNr-mediated seizure suppression and the role of its downstream cellular targets in the PPN during seizures. The goal of this proposal is to establish the role of the nigrotegmental circuit in seizure initiation and severity using a well-characterized Scn1a+/- mouse model of Dravet Syndrome, a severe developmental and epileptic encephalopathy distinguished by pharmaco-resistant and temperature-sensitive seizures. First, we will activate the SNr→PPN circuit both in slice and in vivo to confirm that SNr activation leads to inhibition of the PPN. Next, we will record from each region while evoking seizures (with hyperthermia) to determine the endogenous activity of the SNr and PPN, in addition to the effect of SNr activation on PPN activity during the peri-ictal period. Finally, we will determine the impact of SNr inhibition and PPN activation on seizure initiation and severity in Scn1a+/- mice. We will achieve these aims using a combination of techniques – in vivo fiber photometry, slice and in vivo electrophysiology, circuit-specific opto- and chemogenetics, and seizure-triggered closed-loop optogenetics. This approach will allow us to gain cell-type specific knowledge of SNr projections to the PPN and to manipulate these regions with superior spatial and temporal resolution. The proposed research will meaningfully advance our understanding of nigrotegmental circuitry and its role in modulating seizures in Dravet Syndrome. The findings gleaned from this proposal will collectively form the basis for novel, targeted therapies for the treatment of seizures in Dravet Syndrome and other forms of epilepsy.
