Time-related facilitation and suppression of drinking by muscarinic anticholinergic drugs in water non-deprived rats.

Effects of tertiary anticholinergic drugs, atropine (1.3, 2.5, 5 and 10 mg/kg, s.c.) and scopolamine (0.13, 0.25, 0.5 and 1 mg/kg, s.c.), and a quaternary anticholinergic drug, methylatropine (1.3, 2.5, 5 and 10 mg/kg, s.c.), on the drinking behavior were investigated in water non-deprived rats that were housed in a 12-hr light-dark situation (light period: 6:00-18:00) with a free access to food. Atropine, 5 mg/kg, and scopolamine, 0.13 and 0.25 mg/kg, administered at 12:00, significantly increased the drinking during the 12:00-18:00 period. Furthermore, lower to medium doses of atropine increased the drinking during the 18:00-6:00 period. In contrast, the drinking did not change during the 12:00-18:00 period, but decreased during the 18:00-6:00 period in a dose-dependent manner after administration of methylatropine at 12:00, whereas the drinking during the 18:00-24:00 period decreased in a dose-dependent manner when both the tertiary and quaternary drugs were administered at 18:00. However, the drinking during the 24:00-6:00 period increased in the rats that were administered atropine at the dose of 5 or 10 mg/kg at 18:00, while the drinking still decreased after methylatropine at the same time. The present results suggest that in water non-deprived rats, central muscarinic cholinergic blockade is effective for both increasing and decreasing drinking behavior, depending on the doses, when the drug is administered, and time span between the drug administration and the behavior observation. It is also suggested that peripheral cholinergic blockade monotonously suppresses the drinking behavior.