The long-term objective of this proposal is to determine the cellular mechanisms that underlie normal and aberrant development of synaptic transmission in the central auditory system. The research plan is divided into three areas: (1) Dysfunction of glycinergic and glutamatergic synapses: whole-cell patch clamp analysis. The effects of sensorineural hearing loss on the development of central auditory function will be examined in 2 animal models: bilateral cochlear damage in the gerbil, and a genetically deaf mouse, jerker. Using a brain slic preparation, the strength of glycinergic synapses from the medial nucleu of the trapezoid body (MNTB) and glutamatergic synapses from the anteroventral cochlear nucleus (AVCN) will be evaluated within the lateral superior olive (LSO). Whole-cell voltage-clamp recordings will be obtained from LSO neurons while monitoring the MNTB- or AVCN-evoked response. The amplitude and duration of synaptic currents will serve as a direct measure of synaptic strength. Since Ca++ influx serves as an important signal during neuronal development, the presence of voltage- gated Ca++ currents will also be examined under voltage-clamp conditions A second set of current-clamp recordings will be performed to determine the global response to afferent stimulation. Each recorded neuron will be labelled with biocytin to determine whether dendritic atrophy correlates with decreased synaptic strength. (2) Dysfunction of gamma- aminobutyric acid-releasing synapses: whole-cell patch clamp analysis. The strength of GABAergic synapses from the dorsal nucleus of the latera lemniscus (DNLL) to the inferior colliculus (IC) will be evaluated following bilateral cochlear damage in the gerbil. The amplitude and duration of synaptic currents will, again, be obtained with whole-cell voltage clamp recordings in a brain slice preparation. (3) The trophic influence of inhibitory synapses: A tissue culture assay system. In order to extend our previous in vivo observations, the maturation of LSO dendritic arbors will be examined using an organotypic culture of the ventral auditory brainstem containing MNTB afferents. Following pharmacological manipulation of either the glycinergic synapses, the glutamatergic synapses, or the external and internal Ca++ concentrations the morphology of LSO dendrites will be evaluated with a computer- assisted morphometry system. The proposed experiments will demonstrate how central auditory synaptic transmission is affected by hearing loss, and suggest the cellular mechanisms by which synaptic transmission regulates neuronal maturation.
