Cholinergic mechanisms in the cat vestibular system.

-A neuropharmacological stud) WBS undertaken in the cat to obtain further evidence of the role ofacetylcholine in vestibular function. It was shown that the vestibular nerve contains about half or less the enzymatic activity of choline acetyltransferase. acetylcholinesterase. cholinesterase and total cholinesterase than nucleus vestlbularls later&s (NVL). Field potentials as well as discharge of single neurones of nucleus vestibularis lateralis (NVL) were studied in locally anaesthetized. decamethonium immobilized ventilated cats before, during and after vestibular nerve and reticular formation stimulation. Various cholinergic agonists and antagonists and the adrcnergic agonists L-DOPA and (+)-amphetamine were then given intravenously to determine their effects. Three ma_jor evoked potentials to vestibular nerve stimulation were recorded in NVL. These potentials were labelled lV1, Nz and IV, on the basis of polarity and latency. Although the N, and lV3 waves were not much affected. the N, wave was dramatically enhanced by physostigmine and reduced by scopolamine. About half of NVL neurones excited by vestibular nerve stimulation responded to muscarinic cholinergic drugs. Nucleus wstibularis lateralis responses to reticular formation (RF) stimulation were primarily nicotinic and were blocked by the nicotinic antagonist. mecam!lamine. but not trimethadinium. Some neurones excited by RF stimulation were enhanced by L-DOPA or (+ )amphetamine. The data indicate that cholinergic mechanisms are strongly involved in vestibular function. Adrenergic mechanisms are also involved. but to a much lesser extent.