Demonstrating morphine's potentiating effects on sucrose-intake.

Mildly deprived rats (18-hr of deprivation of water) were given the opportunity to take a solution of sucrose daily for periods of either 10, 18, 31, 56, or 100 min. After daily intakes stabilized and prior to a session, rats were given an injection of either morphine sulfate, 1.0 mg/kg, or naloxone hydrochloride, 2.5 mg/kg, an agonist and an antagonist, respectively, at the opioid receptors. Naloxone, as expected, decreased intakes regardless of the test-session's length. Morphine decreased intakes of the shorter sessions, but increased intake of the longest session. Subsequently, injections of morphine were given 56 min into a 100-min session. These injections also increased intake. Morphine's effects in potentiating intake seem to have special relevance with respect to the continuance of ingestion. Variations across experiments, in duration of test-sessions, could account for the variations in conclusions drawn about whether or not morphine and other agonists potentiate intake of ingesta.