Local targets of T-stellate cells in the ventral cochlear nucleus.

T-stellate cells in the ventral cochlear nucleus (VCN) are known to have local axon collaterals that terminate in the vicinity of their dendrites and cell bodies within the same isofrequency lamina in parallel with the auditory nerve fibers that innervate them. Excitatory synaptic connections between stellate cells within an isofrequency lamina are hypothesized to be involved in the nitric oxide-mediated upregulation of T-stellate responses to their auditory input. This could serve as a mechanism of variable gain control in the enhancement of responses to vowel spectral peaks. Previous studies have provided indirect evidence for these possible synaptic interconnections between T-stellate cells, but unequivocal identification has yet to be established. Here, we used retrograde neuronal tracing with adeno-associated viral vector or biotinylated dextran amine injected into the inferior colliculus (IC) to detect the postsynaptic target of T-stellate cells within the VCN. We show that backfilled T-stellate cell axons make monosynapatic connections on the labeled cell bodies and dendrites of other labeled T-stellate cells within an isofrequency lamina. Electron microscopy revealed that T-stellate terminals can also make synapses on structures not retrogradely labeled from the IC. Glycine antibodies combined with the viral labeling indicated that these nonbackfilled structures that the labeled T-stellate terminals were synapsing on are most likely the cell bodies and dendrites of two size categories of glycinergic VCN cells, whose sizes and relative numbers indicated they are the D- and L-stellate cells. These cells are known to provide inhibitory inputs back onto T-stellate cells. Our data indicate that, in addition to their auditory nerve input, T-stellate cells provide a second modulatable excitatory input to both inhibitory and excitatory cells in a VCN isofrequency lamina and may play a significant role in acoustic information processing.