FUNCTIONAL ORGANIZATION OF THE INFERIOR COLLICULUS

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DESCRIPTION (Investigator's Abstract): The chief goal of these studies is to evaluate how inputs that originate from multiple lower auditory nuclei shape the response properties of collicular neurons in the mammalian auditory system. Specifically the investigators will: 1) Determine which of the cells that project to the E1 region of the colliculus, and to the 60 kHz regions of the DNLL and INLL, are GABAergic and which are glycinergic. The DNLL and INLL are the two nuclei of the lateral lemniscus that project to the 60 kHz region of the colliculus. This will be accomplished by first, in an individual bat, identifying the appropriate region in the nucleus of interest with electrophysiological recordings, and then making a focal injection of HRP. Subsequent processing will co-localize the retrogradely transported HRP with antibodies directed against glycine, GABA and glutamate decarboxylase (GAD), the synthetic enzyme for GABA. 2) Determine the monaural and binaural response properties of INLL and DNLL neurons by recording single unit activity while presenting 60 kHz tone bursts with speakers inserted into the ears. 3) Determine the functional consequences of the inhibitory inputs to the colliculus by constructing the spatial receptive fields of 60 kHz E1 neurons from their monaural and binaural response properties, which will then be compared to the changes in receptive field properties induced by iontophoretic application of bicuculling and/or strychnine. Thus, they will assess the differential roles of GABAergic and glycinergic innervation in shaping the response properties and spatial receptive fields of collicular E1 neurons, and identify the source or sources, of the GABAergic (or glycinergic) influence. Finally, by evaluating the response properties of 60 kHz neurons in DNLL and INLL and reconstructing their receptive fields, they will obtain further insights into the transformations that occur along the neuraxis, and how lower nuclei may be exerting their inhibitory influence on their targets in the 60 kHz contour of the inferior colliculus.