SOME BEHAVIORAL-EFFECTS OF MORPHINE, NALOXONE AND NALORPHINE IN SQUIRREL-MONKEY AND PIGEON

Abstract

Morphine, naloxone and nalorphine were studied for their effects on the performance of squirrel monkeys and pigeons responding under multiple fixed-interval (FI), fixed-ratio (FR) schedules of food presentation. Morphine generally produced only dose-related decreases in responding in both monkeys and pigeons; monkeys were 10 times more sensitive to morphine than pigeons. The only effect of lower doses of naloxone (0.01-1 mg/kg, monkeys; 1-10 mg/kg, pigeons) was to increase FI responding in some pigeons. Higher doses of naloxone (10-56 mg/kg), produced gross disturbances such as tremors and vomiting and decreased FI and FR responding of both monkeys and pigeons. Nalorphine had strikingly different effects on the behavior of the 2 spp. In the pigeons, nalorphine consistently increased both FI and FR response rates at doses from 0.3 to 10 mg/kg and decreased responding only at doses of 30-100 mg/kg. Nalorphine did not increase responding at any dose in the monkeys, and decreased responding at doses as low as 0.1 mg/kg. In both the monkeys and the pigeons, nalorphine was only 1/10 as potent as naloxone in antagonizing the effects of morphine on FI and FR responding. Decreasing response rates caused by nalorphine appeared to limit further its usefulness as a morphine antagonist. Antagonism of the rate-decreasing effects of morphine on FI and FR responding occurred over a narrower range of doses with nalorphine than with naloxone, especially in monkeys.