Laminar segregation of excitatory and inhibitory projections from the inferior colliculus to the superior colliculus.

The superior colliculus (SC) integrates multimodal spatial information to guide orientation behaviors. Visual inputs to superficial layers drive these behaviors through circuits in deeper layers, which also receive input from other sensory modalities. The inferior colliculus (IC), a central midbrain hub of auditory system, provides both VGluT2-expressing excitatory and VGAT/GAD67-expressing inhibitory projections to the SC, but the organization of these projections remains incompletely understood. In this study, we systematically mapped excitatory and inhibitory IC projections to the SC. Cre-dependent viral tracing in VGluT2-Cre and VGAT-Cre mice was used to label excitatory and inhibitory IC axons, respectively. In addition, retrograde tracing combined with fluorescent in situ hybridization for VGluT2 and GAD67 mRNAs was used to identify IC neurons projecting to the superficial SC layers, deeper SC layers, or medial geniculate body (MGB). Both excitatory and inhibitory IC projections innervated superficial and deep SC layers; however, inhibitory projections showed a modest bias toward superficial layers, whereas excitatory projections preferentially targeted deeper layers. Retrograde tracing further revealed that SC-projecting inhibitory neurons were located more frequently in rostral shell subdivisions. The proportion of inhibitory projection neurons was higher for superficial SC than for deep SC and MGB. These findings reveal a structural organization of IC projections to the SC that differs from the classical IC-MGB auditory pathway and suggest a circuit mechanism through which auditory signals may modulate visual processing in the SC.