Effects of Narcotic Analgesics on Water and Food Intake in Normal Rats

S.c. morphine (0.5 to 8.0 mg/kg) in normal satiated rats induces water and food intake (1,3) beginning 1–2 hours and terminated 6 hours after injection, when given during the day (light) but not during the night (dark) period (4,5). Water and food ingestion is depressed between 6 and 24 hours after injection (3). The drinking and eating responses appear to be independent of each other. A second morphine injection 6 hours after the first induces drinking, but no eating. Bilateral nephrectomy suppresses the drinking, but not the eating response (4). With daily repetition of the injections, no tolerance develops within 17 days for the drinking and eating responses to 0.5 to 16 mg/kg of morphine (2,3).