The action of cholinomimetic substances on impulse conduction in the habenulointerpeduncular pathway of the rat in vitro.

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Abstract
The effects of some cholinomimetic substances and their antagonists on the peak height of compound action potentials recorded from the terminal region of the habenulointerpeduncular pathway were studied using a rat brain slice preparation. Carbachol and acetylcholine (ACh) depressed the peak height of the compound action potential and increased the latency to peak. The nicotinic agonists nicotine and dimethylphenylpiperazinium depressed the peak height of the compound action potential while muscarinic and glutamate had no effect. The depressant effect of carbachol was blocked by the nicotinic antagonists hexamethonium, mecamylamine and d-tubocurarine but not by atropine. Physostigmine enhanced the effects of ACh and, to a lesser extent, carbachol. In the presence of physostigmine, carbachol or ACh initiated a spontaneous oscillation of the amplitude of the compound action potential which was Ca2+ dependent and was blocked by mecamylamine. Depression of the amplitude of the compound action potential is due to activation of presynaptic nicotinic receptors. Possible cholinergic mechanisms in the habenulointerpeduncular pathway are discussed.