Parvalbumin and GABA Microcircuits in the Mouse Superior Colliculus.

The mammalian superior colliculus (SC) is a sensorimotor midbrain structure responsible for orienting behaviors. Although many SC features are known, details of its intrinsic microcircuits are lacking. We used transgenic mice expressing reporter genes in parvalbumin-positive (PV) and gamma aminobutyric acid-positive (GABA) neurons to test the hypothesis that PV neurons co-localize GABA and form inhibitory circuits within the SC. We found more PV neurons in the superficial compared to the intermediate SC, although a larger percentage of PV neurons co-expressed GABA in the latter. Unlike PV neurons, PV/GABA neurons showed predominantly rapidly inactivating spiking patterns. Optogenetic activation of PV neurons revealed direct and feedforward GABAergic inhibitory synaptic responses, as well as excitatory glutamatergic synapses. We propose that PV neurons in the SC may be specialized for a variety of circuit functions within the SC rather than forming a homogeneous, GABAergic neuronal subtype as they appear to in other regions of the brain.