A possible cholinergic link in olivo-cochlear inhibition

Electrical stimulation of the crossed olivo-cochlear bundle (OCB) of the anesthetized cat produces an inhibition of sound-evoked VIII nerve action potentials. The OCB has been shown to contain acctylcholinesterase (Churchillet al., 1956) and synaptic vesicles (Engstroem, 1958). It was therefore suggested that the OCB exerted its inhibitory effect by neurohumoral means and that the neurohumor might be acetylcholine (ACh). Hemicholinium number 3 (HC-3) was employed in the present study as a means for testing the hypothesis that ACh is involved in OCB-induced inhibition.

Cats anesthetized with pentobarbital and with left middle ear muscle tendons severed were employed. Sound-evoked potentials were recorded from the left cochlear round window. The crossed OCB was stimulated electrically. All experimental drugs were injected via a cannula in the left axillary artery. Atropine (0.5–5 mg/kg), dihydro-β-erythroidine (2–10 mg/kg) and mecamylamine (2–5 mg/kg) infections did not antagonize OCB-induced inhibition. HC-3 (5 mg/kg) in the majority of cats reduced the OCB-induced inhibition. Choline injections and resting the preparation momentarily reversed the effect of HC-3. Under similar conditions, d-tubocurarine (2 mg/kg) produced no change in OCB-induccd inhibition.