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 ol ivary nucleus (SON) receives inputs from nucleus angularis (NA) and nucleus laminaris (NL) and projects back to NA, NL, and nucleus magnocellularis (N M). The reciprocal connections between the SON and NL are of particular int erest because they constitute a feedback circuit for coincidence detection. In the present study, the chick SON was investigated. In vivo tracing stud ies show that the SON projects predominantly to the ipsilateral NM, NL, and NA. In vitro whole-cell recording reveals single-cell morphology, firing p roperties, and postsynaptic responses. SON neurons are morphologically and physiologically suited for temporal integration; their firing patterns do n ot reflect the temporal structure of their excitatory inputs. Of most inter est, direct stimulation of the SON evokes long-lasting inhibition in NL neu rons. The inhibition blocks both intrinsic spike generation and orthodromic ally evoked activity in NL neurons and can be eliminated by bicuculline met hiodide, a potent antagonist for GABA, receptor-mediated neurotransmission. These results strongly suggest that the SON provides GABAergic inhibitory feedback to laminaris neurons. We discuss a mechanism whereby SON-evoked GA BAergic inhibition can influence the coding of interaural time differences for sound localization in the avian auditory brainstem.