The overall goals of this proposal are to determine what response transformations occur between lower nuclei and the inferior colliculus (IC), what impact each lower nucleus has in causing those transformations and what functional significance might accrue to those transformations. This will be accomplished by evaluating the processing of species-specific signals in the brainstem auditory system of Mexican free-tailed bats. These are highly social mammals that employ a rich repertoire of communication calls for a wide variety of social interactions, including mother-infant interactions, courtship, agonistic encounters, and territoriality. Studies conducted during the previous grant period have shown that responses of IC neurons to social communication signals undergo substantial transformations, and those transformations are strongly shaped by inhibition. The proposed studies will investigate the specific inhibitory influences that lower nuclei have on the IC. Particular attention will be given to three main sources of inhibitory innervation; 1) the ipsilateral DNLL; 2) the columnar division of the ventral nucleus of the lateral lemniscus (VNLLc); and 3) its multipolar division (VNLLm). The DNLL provides GABAergic inhibition to the IC while the VNI.I. divisions provide either glycinergic (VNLLc) or a mixed glycineric and GABAergic innervation (VNLLm) to the IC. Together, the projections from ipsilateral DNLL and the two divisions of the VNLL provide more than half of the total inhibitory innervation of the IC. Following from the above, experiments are proposed that will answer 2 main questions. First, how do neurons in the two regions of the VNLL respond to species-specific calls and how are their response features, evoked by both tones and complex signals, shaped by the excitatory and inhibitory innervation they receive? These questions will be answered by recording from neurons in each VNLL region before and then while inhibition is blocked by iontophoretic application of bicuculline or strychnine. Second, what influences do the GABAergic inputs from the ipsilateral DNLL have on IC responses evoked by both tones and complex signals, and how do those influences compare to the influences of glycinergic and/or GABAergic inhibition fi'om the VNLL subdivisions? These questions will be answered by recording from IC neurons before, during and after reversible inactivation of each of the lower nuclei. Taken together these data will provide a more complete picture of the inhibitory discharge trains that act on the IC in response to simple and complex signals. They will also reveal what impacts the inhibition from each nucleus have in shaping IC responses to simple and complex signals, including communication signals that convey unique categories of information. The elucidation of the processing of communication calls may provide insights into the processing of speech. The roles of inhibition in this processing may be especially relevant for the decline of speech perception in the elderly, which is correlated with age related loss of GABAergic neurons in the auditory system.
