Abstracts

Rationale: Inhibition of the substantia nigra pars reticulata (SNpr) is potently anticonvulsant in a wide array of animal models. Similar effects have been reported by pharmacological activation of the striatum (caudate and putamen [CPu]). The striatum is composed of two populations of GABAergic projection neurons: (1) direct pathway neurons, which directly inhibit the SNpr, and (2) indirect pathway neurons, which result in activation of SNpr. Thus, the anticonvulsant effect achieved by nonspecific pharmacological activation of the striatum is somewhat surprising. To directly probe this phenomenon, selective targeting of direct and indirect pathway neurons is required, an advance made possible through optogenetic approaches and the recent availability of cre-driver rats enabling selective targeting of these populations. As a first step toward dissociating the direct-indirect pathway contribution, we sought to replicate the anticonvulsant effects reported with pharmacological microinjection in the striatum using an optogenetic approach. To extend these findings, we examined the anticonvulsant profile in two models of epilepsy: 1) amygdala kindling and 2) IP γ-butyrolactone (GBL) (100 mg/kg).   Methods: Adult male Sprague-Dawley rats were injected bilaterally with AAV5-hSyn-hChR2(H134R)-mCherry into the anterior dorsal striatum and fiber optic implants were placed at the same coordinates. In one group of rats (kindling), a bipolar stimulating/recording electrode was placed in the basolateral amygdala. In the second group (GBL) epidural screw electrodes were placed over the cortex. After a period of 3 weeks to allow for opsin expression, we evoked seizures in the respective models to determine the baseline pattern of electrographic and behavioral seizure activity; for the amygdala kindling group, all animals were stimulated until they reached five Racine Stage 5 seizures. On a subsequent day, rats were tested with the optogenetic intervention (100 Hz light [473 nm, 11 mW]). At the conclusion of the study, viral expression (through mCherry fluorescence) and fiber optic placement were validated histologically. Results: Kindling: Under baseline conditions, all animals displayed Racine Stage 5 seizures. Following optogenetic activation of the striatum, this was significantly decreased (P < 0.05, Wilcoxon test). Most animals displayed no behavioral seizure activity when tested with optogenetic stimulation. Afterdischarge duration was likewise decreased during trials with optogenetic stimulation as compared to baseline trials (P < 0.05). GBL: Animals that received GBL injection (n=8) displayed electrographic (spike-and-wave) discharges for a mean of 25% of the test session. Optogenetic stimulation significantly decreased the duration of spike-and-wave seizure activity in these animals (P < 0.05, paired t-test). Conclusions: These results demonstrate that optogenetic activation of CPu exerts seizure suppressive effects in multiple rat models of epilepsy. These data are consistent with prior reports using pharmacological approaches. These data provide further rationale for a detailed assessment of direct- and indirect pathway contribution to seizure control. Funding: R01NS097762 to PAF