CHOLINE-ACETYLTRANSFERASE, GLUTAMATE DECARBOXYLASE AND TYROSINE-HYDROXYLASE IN COCHLEA AND COCHLEAR NUCLEUS OF GUINEA-PIG

Activities of choline acetyltransferase (ChAc), glutamate decarboxylase (GAD) and tyrosine hydroxylase (TH), enzymes catalyzing the synthesis of acetylcholine (ACh), GABA and catecholamines, respectively, were measured in the cochlea and cochlear nucleus of the guinea pig. ChAc activity in the organ of Corti, 3rd turn, was 1270 pmol ACh formed/min per mg protein (ChAc, 1270) and was higher than in turn 4 (ChAc, 543). ChAc activity was higher when the preparation included the inner hair cell region than when not. GAD activity in samples of turn 3 and 4 combined was low, 0.17 nmol GABA formed/min per mg protein (GAD, 0.17). All 3 enzymes were low in the auditory nerve: ChAc, 1.7, GAD, 0.10 and TH, 1.0 pmol dopa formed/min mg protein. In the cochlear nucleus, the values were: ChAc, 129; GAD, 1.70; and TH, 2.7. The distribution of ChAc activity in the organ of Corti suggested that th olivocohlear nerve fibers may be cholinergic. Because of low GAD in the cochlea, GABA is unlikely to be a transmitter in the organ of Corti. It is unlikely that ACh, GABA or a catecholamine is a transmitter between the auditory nerve and the cochlear nucleus.