Intrinsic and synaptic properties of the dorsomedial nucleus of the intercollicular complex, an area known to be involved in distance call production in Bengalese finches.
The dorsomedial nucleus of the intercollicular complex (DM) of the midbrain in the Bengalese finch is essential for the vocal production of distance calls that have sexually-dimorphic acoustic structures in the adult. Anatomical tracing of the vocal control system shows that DM neurons of adult males receive axonal inputs from the robust nucleus of the archistriatum (RA), and the inputs are considered to be crucial for the male-typical features of distance calls. In order to investigate the neural mechanisms underlying distance call patterns of male finches in DM, we characterized neurons in DM and examined their synaptic responses to RA inputs in brain slice preparations. By using whole-cell recording techniques, we could classify at least three types of neurons based on electrophysiological and morphological characteristics. Type I neurons exhibited regular and high-frequency trains of action potentials in response to depolarizing current pulses. Type II neurons had large somata and action potential trains accommodating during depolarization. Type III neurons were characterized by a few spikes followed by a slow depolarization during current injection. Their somata were markedly small and their axons often projected toward the contralateral DM or the thalamic nucleus uvaeformis (Uva). In all these cell types, electrical stimulation of an area including DM-projecting RA axons often elicited both excitatory postsynaptic currents (EPSCs) mediated mainly by non-NMDA glutamate receptors and inhibitory postsynaptic currents (IPSCs) mediated by GABA(A) receptors. These intrinsic properties of DM neurons and their excitatory and inhibitory synaptic inputs may play important roles in generating the acoustic patterns of distance calls in male finches.