Complex and spatially segregated auditory inputs of the mouse superior colliculus

Although the visual circuits in the superior colliculus (SC) have been thoroughly examined, the auditory circuits lack equivalent scrutiny. SC neurons receiving auditory inputs in mice were characterized and three distinguishable types of neurons were found. The auditory pathways from external nuclei of the inferior colliculus (IC) were characterized, and a novel direct inhibitory connection and an excitation that drives feed-forward inhibitory circuits within the SC were found. The direct excitatory and inhibitory inputs exhibited distinct arbourization patterns in the SC. These findings suggest functional differences between excitatory and inhibitory sensory information that targets the auditory SC.

The superior colliculus (SC) is a midbrain structure that integrates auditory, somatosensory and visual inputs to drive orientation movements. While much is known about how visual information is processed in the superficial layers of the SC, little is known about the SC circuits in the deep layers that process auditory inputs. We therefore characterized intrinsic neuronal properties in the auditory-recipient layer of the SC (stratum griseum profundum; SGP) and confirmed three electrophysiologically defined clusters of neurons, consistent with literature from other SC layers. To determine the types of inputs to the SGP, we expressed Channelrhodopsin-2 in the nucleus of the brachium of the inferior colliculus (nBIC) and external cortex of the inferior colliculus (ECIC) and optically stimulated these pathways while recording from SGP neurons. Probing the connections in this manner, we described a monosynaptic excitation that additionally drives feed-forward inhibition via circuits intrinsic to the SC. Moreover, we found a profound long-range monosynaptic inhibition in 100% of recorded SGP neurons, a surprising finding considering that only about 15% of SGP-projecting neurons in the nBIC/ECIC are inhibitory. Furthermore, we found spatial differences in the cell body locations as well as axon trajectories between the monosynaptic excitatory and inhibitory inputs, suggesting that these inputs may be functionally distinct. Taking this together with recent anatomical evidence suggesting an auditory excitation from the nBIC and a GABAergic multimodal inhibition from the ECIC, we propose that sensory integration in the SGP is more multifaceted than previously thought.