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.