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.