Our goals are to identify neurotransmitters in the inner ear, to establish the mechanisms by which they act, and to characterize their roles in the physiology of hair-cell organs. In order to identify the hair-cell transmitter and other neuroactive substances in the inner ear, we have looked in extracts of fish inner ears for substances that can excite afferent fibers innervating hair cells. During the previous project period we purified a low-molecular-weight substance that is a candidate to be the hair-cell transmitter. It is concentrated in hair-cell tissue and excites afferent nerve fibers. During the next project period, we intend to identify the chemical structure of this substance and to determine its cellular mechanisms of action. During the previous project period we also found in inner-ear extracts a peptide, tentatively identified as calcitonin gene-related peptide (CGRP), that also excites afferent neurons. CGRP has been found in efferent fibers in the inner ears of several species. CGRP is the only peptide thus far that both has been found in efferent nerve fibers and has been demonstrated to affect the discharge rate of afferent nerve fibers. CGRP is thus the most promising candidate for studies of peptidergic efferent neurotransmitters. In this project period, we will examine the cellular mechanisms of action and functional roles of CGRP as well those of GABA, another putative efferent neurotransmitter. Understanding the functional roles of neurotransmitters and their cellular mechanisms of action will lead to increased understanding about both normal function and pathological states of the ear. This understanding will also help provide a basis for pharmacologic intervention in diseases of the ear. Drugs that affect the synthesis, storage, release, postsynaptic actions, uptake, or degradation of neurotransmitters have proven to be some of the most useful tools both for increasing our understanding of physiological processes and for intervening therapeutically in diseases of the nervous system. The knowledge developed in this project will be crucial for development of drugs that affect these neurotransmitter-related processes in the ear.
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