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1 The effect of acute and chronic (16 days) administration of nicotine on cortical acetylcholine (ACh) release, gross behaviour and brain nicotinic binding sites was investigated in rats and guinea-pigs.2 The drug, injected either subcutaneously (0.45-0.90mgkg-1) or intracerebroventricularly (1, 3 and 5 ig) increased the cortical ACh release, in a dose-dependent manner, through mecamylaminesensitive receptors for 1-2 h in both species. 3 Chronic treatment significantly increased basal ACh release in the rat and slightly lowered it in the guinea-pig, but the response to a challenging dose of nicotine was proportionally maintained in both species. 4 The number of nicotinic receptors was four times higher in the rat than in the guinea-pig and was not dependent on the radioligand used ([3H]-nicotine or [3H]-ACh, in the presence of atropine) to determine this. The nicotinic binding sites showed an apparent increase in chronically treated rats but no change in guinea-pigs. 5 Tolerance to the inhibitory effect of the drug, assessed with the T maze test, was found in the rat. No apparent change in gross behaviour was detected in the guinea-pig. 6 It is concluded that chronic nicotine treatment causes evident tolerance to its inhibitory effect on behaviour in the rat, but no adaptation to its excitatory properties on the cholinergic brain structures in rats and guinea-pigs.

Abstract— —A method to measure the rate of acetylcholine turnover in mouse brain in vivo has been developed. It is based on the formation of labelled acetylcholine from intravenously injected labelled choline. The isotopic dilution of choline in the brain has been measured by assaying endogenous choline in the brain by an enzymatic method using tritium‐labelled acetyl‐CoA and purified choline acetyltransferase. The rate of acetylcholine turnover in the brain could be calculated at 50 n‐moles acetylcholine/g/min in conscious mice. In anaesthetized mice and in mice treated with oxotremorine, a decrease of acetylcholine turnover to about 10 n‐moles/g/min was found.

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