The superior olivary nucleus and its influence on nucleus laminaris: a source of inhibitory feedback for coincidence detection in the avian auditory brainstem.

Located in the ventrolateral region of the avian brainstem, the superior olivary nucleus (SON) receives inputs from nucleus angularis (NA) and nucleus laminaris (NL) and projects back to NA, NL, and nucleus magnocellularis (NM). The reciprocal connections between the SON and NL are of particular interest because they constitute a feedback circuit for coincidence detection. In the present study, the chick SON was investigated. In vivo tracing studies show that the SON projects predominantly to the ipsilateral NM, NL, and NA. In vitro whole-cell recording reveals single-cell morphology, firing properties, and postsynaptic responses. SON neurons are morphologically and physiologically suited for temporal integration; their firing patterns do not reflect the temporal structure of their excitatory inputs. Of most interest, direct stimulation of the SON evokes long-lasting inhibition in NL neurons. The inhibition blocks both intrinsic spike generation and orthodromically evoked activity in NL neurons and can be eliminated by bicuculline methiodide, a potent antagonist for GABAA receptor-mediated neurotransmission. These results strongly suggest that the SON provides GABAergic inhibitory feedback to laminaris neurons. We discuss a mechanism whereby SON-evoked GABAergic inhibition can influence the coding of interaural time differences for sound localization in the avian auditory brainstem.