Anatomical and neurochemical profiles of somatostatin-positive neurons in the mouse inferior colliculus

The inferior colliculus (IC) integrates auditory information through a complex interplay of excitatory and inhibitory neurons, and provides both excitatory and inhibitory inputs to the medial geniculate body (MGB). Although IC projection neurons are predominantly glutamatergic, accumulating evidence suggests that they comprised heterogeneous subpopulations with distinct morphological and functional properties. Somatostatin (SST)-expressing neurons represent one such glutamatergic subpopulation; however, their characteristics and circuit organization remain poorly understood. In this study, we examined the projection domains within the MGB and the local circuits of SST-expressing neurons in the IC. SST-expressing neurons in the central nucleus of the IC (CNIC) project mainly to the ventral division of the MGB (MGv) and posterior limiting nucleus (POL), whereas those in the external (ECIC) and dorsal (DCIC) nuclei primarily target the POL. The SST axon terminals in the MGv were large, formed clusters, and positive for cholecystokinin (CCK), whereas those projecting to the POL were small, non-clustered, and CCK-negative. The local axons of SST-positive neurons in the IC formed vesicular glutamate transporter 2-positive basket-like terminals around large γ-amino butyric acid (GABA)-ergic neurons, a hallmark morphological profile of GABAergic projection neurons to the MGB. This finding indicates that SST-expressing neurons provide strong excitatory input to these inhibitory projection neurons. Collectively, our results show that IC SST-expressing neurons shape thalamic processing not only through direct excitatory inputs to the MGv and POL, but also indirectly via strong excitation of local GABAergic projection neurons, thereby positioning SST neurons as key regulators of IC-MGB communication.