Complementary roles for parvalbumin and somatostatin interneurons in the generation of hippocampal gamma oscillations

Gamma-frequency oscillations (30-120 Hz) can be separated into fast (>60 Hz) and slow oscillations, with different roles in neuronal encoding and information transfer. While synaptic inhibition is important for synchronization across the gamma-frequency range, the role of distinct interneuronal subtypes in fast and slow gamma states remains unclear. Here, we used optogenetics to examine the involvement of parvalbumin (PV+) and somatostatin (SST+) expressing interneurons in gamma oscillations in the mouse hippocampal CA3 ex vivo. Disrupting either PV+ or STT+ interneuron activity, via either photo-inhibition or photo-excitation, led to a decrease in the power of cholinergically-induced slow gamma oscillations. Furthermore, photo-excitation of SST+ interneurons induced fast gamma oscillations, which depended on both synaptic excitation and inhibition. Our findings support a critical role for both PV+ and SST+ interneurons in slow hippocampal gamma oscillations, and further suggest that STT+ interneurons are capable of switching the network between slow and fast gamma states.