The Nucleus of the Brachium of the Inferior Colliculus Contains Thalamic-Like and Midbrain-Like Subregions.

The ventral division of the medial geniculate body (MGV) is the core auditory thalamic nucleus and is bordered dorsally, medially, and ventrally by shell nuclei; a caudal shell counterpart has not been identified. The nucleus of the brachium of the inferior colliculus (nBIC), long viewed as a rostral extension of the midbrain IC, contains neurons that project to auditory and adjacent cortex, blurring the thalamus-midbrain boundary. We hypothesized that the nBIC contains both thalamic-like and midbrain-like neurons. We combined cytoarchitectural, neuroanatomical, and molecular approaches to test the hypothesis: Viral tracing to differentially tag thalamocortical and tectothalamic populations, retrograde cortical labeling, in situ hybridization for VGLUT2 and VGAT, and immunohistochemistry for GAD67, calretinin, and parvalbumin. The nBIC segregated into two zones: a rostrolateral region virtually devoid of GABAergic cells and enriched for glutamatergic neurons, and a caudomedial region containing intermingled GABAergic and glutamatergic neurons. Thalamocortical nBIC neurons were confined to the rostrolateral zone, whereas tectothalamic neurons were concentrated caudomedially; spatial analyses confirmed segregation. Cortically projecting nBIC neurons expressed VGLUT2 mRNA and lacked collaterals in the medial geniculate or superior colliculus, instead arborizing in the striatum and auditory cortex. Calcium-binding markers further differentiated the zones, with calretinin enriched rostrally and parvalbumin increasing caudally. These data indicate that, although indistinct by Nissl cytoarchitecture, the nBIC is a chimeric structure composed of a thalamus-like, thalamocortical rostrolateral zone and a midbrain-like caudomedial zone, suggesting a caudal shell-type complement to the MGV.