CHARACTERIZATION OF MUSCARINIC BINDING-SITES IN THE ADULT AND DEVELOPING RAT COCHLEA

The maturation of the cholinergic innervation of the rat cochlea is as sociated with a transient increase in the muscarinic-receptor activate d inositol phosphate synthesis. In order to investigate the mechanisms involved in this transient enhancement of the inositol phosphate resp onse, the binding properties of the cochlear muscarinic receptors were studied during rat cochlear development. Incubating the membranes fro m 4-day-old, 12-day-old and adult cochleas with [H-3]quinuclidinyl ben zylate indicates that their respective, mean concentrations of cholino ceptors are 454 +/- 51 (+/- SEM), 39 +/- 2 and 42 +/- 3 fmol/mg of pro tein. The dissociation constants at equilibrium are 207 + 80, 42 + 7 a nd 28 +/- 3 pM for the binding sites of the 4-day-old, 12-day-old and adult cochleas, respectively. Pharmacological characterization of the binding, using selective antagonists, shows that M3 cholinoceptors are expressed in developing and adult cochleas. The data demonstrate that changes in muscarinic receptor affinity and number do not correlate w ith the previously observed peak of the inositol phosphate metabolism. The transient enhanced inositol phosphate response is therefore not d ue to changes in cholinoceptors, but probably due to alterations invol ving the intrinsic activity of the phospholipase C and/or the efficacy of coupling of the transduction system.