Our knowledge of cochlear efferent function is derived almost exclusively from studies of the medial olivocochlear neurons to the outer hair cells in the organ of Corti. There is a second, less well-understood efferent pathway which arises from lateral olivocochlear neurons which are the source of the efferent synapses which typically contact each spiral ganglion cell dendrite close to its synaptic junction with an inner hair cell. Lateral olivocochlear neurons have proven difficult to access with current physiological and neurochemical techniques. Pilot data in this laboratory indicate that, despite the fact that both lateral and medial efferent terminals contain cholinergic enzymes, only efferent fibers belonging to the lateral efferent system degenerate following intracochlear infusion of 1 microM of the cholinergic neurotoxin, ethylcholine aziridinium ion (AF64A). The first set of experiments proposed in this application are directed at extending this observation. We will investigate the ultrastructural changes in the organ of Corti in chinchillas following the intracochlear infusion of AF64A at several survival times. Sound-evoked cochlear potentials will be used to monitor the functional integrity of the cochlea before, during and after micro- perfusion with the AF64A solution. As a test of the specificity of AF64A on the lateral afferent system, we will monitor the integrity of the medial efferent system by measuring the effect of its activity on cochlear potentials. The observation that medial efferent terminals appear to survive AF64A treatment could reflect a low rate of uptake of the toxin resulting from the low discharge rates found in these neurons. In order to test this hypothesis, we will infuse 1 microM AF64A while simultaneously stimulating the ears with a broadband acoustic signal. We hypothesize that under these conditions of stimulation, the medial efferents will be activated and may accumulate the toxin and degenerate. The cholinergic receptor(s) in the medial and the lateral efferent systems have not been characterized. Increased sensitivity to AF64A suggests that there may be differences in the two systems that are reflected in the neurotransmitter receptor type. In this application we propose to investigate the kind of cholinergic receptor on outer hair cells (OHCs). We will test the hypothesis that the cholinergic receptor on OHCs is a nicotinic cholinergic receptor with similarity to the receptor at skeletal muscle. We will test this hypothesis by studying the cross-reactivity of antibodies specific to nicotinic receptors with OHCs and by investigating sodium flux in OHCs following exposure to cholinergic agonists and antagonists.
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