CHOLINERGIC AND PURINERGIC NEUROHUMORAL SIGNALING IN THE INNER-EAR - A MOLECULAR PHYSIOLOGICAL ANALYSIS

The ability to identify the expression of the protein subunits which a ssemble to form ionotropic receptors for acetylcholine and extracellul ar adenosine 5'-triphosphate (ATP) in individual cells of the inner ea r provides examples of the high resolution and exquisite sensitivity w hich molecular biology brings to the study of hearing and balance. The data from these studies provide both fine detail with respect to the classification of the elements involved and an overview of the sites o f potential interaction of both extracellular and intracellular signal ling pathways. The high sensitivity necessitates a molecular physiolog ical approach when using these techniques so that these data on the si te and extent of expression can be balanced against functional signifi cance. With the demonstration of expression of the alpha 9 subunit of the nicotinic acetylcholine receptor in cochlear outer hair cells, mol ecular biology has provided an explanation for the unusual cholinergic receptor pharmacology of the olivocochlear efferent innervation which has confounded investigators for decades. In addition, a role for ext racellular ATP as a signalling molecule regulating electrochemical gra dients and neurotransmission within the inner ear is supported by the extent of P2 receptor expression in this tissue, data which beg for in tense functional study.