Muscarinic synapses in the hypothalamus

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Abstract
It has been reported that drinking may be selectively induced by direct application of cholinergic crystalline substances (carbachol, acetylcholine capped with physostigmine) into the hypothalamus. We confirmed and extended this finding by demonstrating that this central cholinergic effect was based on a muscarinic, and not a nicotinic, action. Minute quantities (10–20 µg) of crystalline substances were introduced into the brains of 28 water-satiated rats through permanently indwelling cannulas. Muscarine had an effect equal to carbachol; strong drinking was elicited within 5–10 min and persisted with variable intensity for almost an hour. Nicotine had the same small, apparently nonspecific effect as sodium chloride—about twice as much drinking as when under no drug, but only 20% of the effect of carbachol or muscarine. Serotonin, potassium chloride, and sucrose had no measurable effects. The effects of carbachol and muscarine were largely blocked by prior application of atropine into the brain. It was concluded that chemical stimulation of muscarinic receptors in the hypothalamus can elicit drinking.

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