Atropine as an inhibitor of voluntary ethanol intake in male rats.
A single ethanol injection (2.0 g/kg, i.p.) administered to male rats will reduce voluntary ethanol intake (water vs. ethanol 10%) if tested for 24 hr, 6 days after the injection. Maximal inhibition, when compared to saline-injected controls (S), is seen after ethanol injections (E) once a week for 5-6 weeks. At this maximal inhibition atropine (A) or saline (S) was injected i.p. prior to the voluntary 24 hr intake of ethanol. In experiment 1, the doses 1 and 2 mg/kg were used, and in experiment 2, 4 and 8 mg/kg. Each experiment consisted of groups AE, SE, AS and SS, according to the above descriptions. In group AS, atropine reduced 24 hr voluntary ethanol intake with a maximal effect following the 4 mg/kg dose. In group AE, atropine did not increase the inhibitory effect induced by the ethanol injections. Total fluid intake was not influenced. After the treatment period a continuous choice (water vs. 10% ethanol) was offered for 8 (exp. 1) or 11 (exp. 2) weeks. During these evaluation periods the ethanol intake in group SE was always significantly smaller than the corresponding intake in group SS. In the atropine-injected groups (AE and AS) a similar significant difference in voluntary ethanol intake was observed at the beginning but not at the end of the evaluation periods. At the end of exp. 2 a significantly larger number of muscarinic binding sites was obtained in the cortex but not in the striatum of group SE when compared with groups AS and SS. In group AS a negative correlation was found in the striatum between the ethanol intake recorded after the atropine injection in week 5 and 6 and muscarinic binding sites. Atropine, probably by acting on muscarinic binding sites, seemed to have a specific inhibitory effect on voluntary 24 hr ethanol intake, which was similar to the effect induced by the ethanol injections.