Somatostatin-Expressing Interneurons Co-Release GABA and Glutamate onto Different Postsynaptic Targets in the Striatum.

The functional contribution of somatostatin-expressing interneurons (SST-INs) to the synaptic organization of the striatum is poorly understood. Using electrophysiological recordings, optogenetic stimulation, and single-cell PCR analysis, we investigated functional patterns of synaptic connectivity in striatal SST-INs expressing channelrhodopsin-2. Photostimulation of these cells induced both glutamatergic excitatory postsynaptic currents (EPSCs) and GABAergic inhibitory postsynaptic currents (IPSCs) in striatal spiny projection neurons (SPNs) and fast-spiking interneurons (FSIs). The two synaptic components showed equally fast onset latencies, suggesting a mechanism of co-transmission. Accordingly, single-cell PCR analysis revealed that individual striatal SST-INs expressed mRNAs for both glutamate and GABA vesicular transporters (VGLUT1 and VGAT, respectively). During relatively prolonged optical stimuli (0.5-1s), IPSC arrays consistently outlasted EPSCs. As a result, photostimulation of SST-INs caused a transient burst of action potentials followed by a prolonged inhibition in postsynaptic cells. These data suggest that striatal SST-INs are specialized to locally project synapses exerting a composite excitatory and inhibitory effect through GABA/glutamate co-transmission onto different postsynaptic targets.