Abstracts

Rationale: Generalized absence epilepsy is the most common type of epilepsy during childhood. The striatum is the main basal ganglia input structure and, although it is not directly involved with the genesis of absence seizures, it has important connections with cortical and subcortical brain sites, including a direct and an indirect projection to substantia nigra pars reticulata (SNr). Inhibition of the SNpr suppresses absence seizures, leading to the hypothesis that modulation of specific striatal pathways may be a viable approach to control of generalized absence seizures. Here, we tested if optogenetic activation of specific striatal projections can attenuate generalized absence seizures induced by gamma-butyrolactone (GBL) in male and female rats.

Methods: 40 Males and female Sprague-Dawley rats were submitted to stereotaxic surgery for bilateral cortical electrodes implantation over the frontal, parietal, and cerebellum (reference and ground). Fiber optics were bilaterally implanted over the dorsal striatum. To optogenetically activate specific neuronal populations from striatal direct or indirect pathways, a retrograde virus (AAVrg-hSyn-Chronos-GFP) was bilaterally injected into the SNr or into the globus pallidus (GP), respectively. Opsins were allowed to express for 3 weeks. For absence seizures, rats received intraperitoneal injection of GBL (100 mg/kg) and were tested under 5, 20, and 100 Hz (blue light 450nm). We also performed additional test sessions to compare open loop (continuous) and closed loop (on-demand) light delivery.


Results: In males and females, optogenetic activation of direct pathway (open loop, 5, 20, and 100 Hz) reduced the number of spike-and-wave discharges (SWD), and the total time seized during session (p< 0.5), while activation of indirect pathway (open loop 5 and 20 Hz) increased number, mean duration, and total time seized (p< 0.05). In the closed loop protocol, activation of striatal direct pathway (5 and 100 Hz) attenuated absence epilepsy manifestation in both sexes, reducing SWD mean duration (p< 0.05), but activation of the indirect pathway increased SWDs duration (p< 0.05). Animals were submitted to control test session with no light delivery and no alteration was found in seizure expression (p >0.05). In separate sessions we monitored electrographic activity changes over the cortex before, during, and after light delivery without evoking seizures; we found no alteration in the peak normalized power frequency, suggesting that optogenetic activation of these pathways cannot disrupt cortical activity in the absence of seizures.


Conclusions: Therefore, our results strongly suggest that striatal direct and indirect pathways might have opposite roles on absence epilepsy manifestation. In fact, activation of striatal direct pathway suppresses SWDs, while activation of striatal indirect pathway facilitates absence seizures. Additionally, our closed loop data suggest that, under specific testing conditions, on-demand activation of specific striatal pathways is sufficient to increase or decrease seizure duration in the GBL model of absence epilepsy.

Funding: R01 NS 097762