Constant infusions of morphine and intakes of sweetened ethanol solution among rats.

Previous studies have indicated that injections of small doses of morphine increase rats' intake of solutions containing ethanol when rats have a choice of either water or a solution containing ethanol. In this experiment, rats which were implanted with osmotic pumps that delivered constant infusions of morphine (0.6 mg/kg/hr across 24 days) had elevated daily intakes of ethanol, as compared to controls, from the second day of opportunity to take the alcoholic beverage until the pumps were removed. In addition, half of the rats with pumps infusing morphine also received injections of morphine (1.0 mg/kg) just before the 1.5-hr opportunity to take alcoholic beverage or water every day for 8 days. Across this 8-day period, these rats took a mean of 5.18 g of pure ethanol/kg of body weight (g/kg) during the 1.5-hr opportunity to take the alcoholic beverage. This was reliably more than the mean of 4.02 g/kg that their counterparts (having morphine pumps and receiving injections of saline) took across the same period. These data support the hypothesis that a surfeit of opioidergic ligand may potentiate drinking of alcoholic beverages.

A small dose of morphine increases intake of and preference for isotonic saline among rats.

Water-deprived rats were given hourly opportunities to ingest physiological saline and water for a number of days until they were taking substantial amounts of both solutions. Prior to some opportunities to ingest, they were injected with either morphine (2.0 mg/kg) or a placebo. Across a variety of procedures, morphine increased intake of and, in 1-hr tests, increased preference for 0.9% NaCl. Intake of 1.5% NaCl also increased after administration of morphine. These data suggest that endogenous opioids are involved in sodium intake. These data also provide further support for the idea that one or more of the endogenous opioid systems are involved in the regulation of ingestion.