Neocortical inhibitory interneuron subtypes display distinct responses to synchrony and rate of inputs

Populations of neurons in the neocortex can carry information with both the synchrony and the rate of their spikes. However, it is unknown whether distinct subtypes of neurons in the cortical microcircuit are more sensitive to information carried by synchrony versus rate. Here, we address this question using patterned optical stimulation in slices of barrel cortex from transgenic mouse lines labelling distinct interneuron populations: fast-spiking parvalbumin-positive (PV+) and somatostatin-positive (SST+) interneurons. We use optical stimulation of channelrhodopsin-2 (ChR2) expressing excitatory neurons in layer 2/3 in order to encode a random 1-bit signal in either the synchrony or rate of activity in presynaptic cells. We then examine the mutual information between this 1-bit signal and the voltage and spiking responses in PV+ and SST+ interneurons. Generally, we find that both interneuron types carry more information than GFP negative (GFP-) control cells. More specifically, we find that for a synchrony encoding, PV+ interneurons carry more information in the first 5 milliseconds, while both interneuron subtypes carry more information than negative controls in their later response. We also find that for a rate encoding, SST+ interneurons carry more information than either PV+ or negative controls after several milliseconds. These data demonstrate that inhibitory interneuron subtypes in the neocortex have distinct responses to information carried by synchrony versus rates of spiking.