UCSF

Fast spiking interneurons (FSIs) are thought to play a critical role in the normal function of the striatum, which is the input nucleus to the basal ganglia. In vitro studies have determined that FSIs are a major source of inhibition onto striatal principal cells, or medium spiny neurons (MSNs). However, in vivo studies have produced conflicting results regarding how FSI activity modulates MSN activity in the awake animal. Here, we use optogenetic techniques and acute head-fixed extracellular recordings to resolve existing controversy in our understanding of FSI-MSN interactions. We find that the net effect of FSI activity is to suppress MSNs, and in the process, discover an opsin-independent effect of light on physiology and behavior. We find strong evidence that this effect is mediated by temperature, and determine parameters for the safe use of light in neuroscience experiments. We then investigate the specific manner in which FSIs influence MSN activity by recording the sensory responses of striatal units in the auditory striatum. Here, we find that MSNs are more frequency selective for pure tone stimuli than neighboring FSIs, and exhibit differences in their responses to continuous stimuli when compared with FSIs. Optogenetic inhibition of FSIs results in a disinhibition of MSN responses to pure tones, increasing the magnitude of their responses and reducing their frequency selectivity. These results suggest that FSI activity in the auditory striatum serves to shape the activity of surrounding MSNs and increase their frequency selectivity.