Morphine, pentazocine and naloxone effects on responding under a multiple schedule of reinforcement in rhesus monkeys and pigeons.

The effects of morphine, pentazocine, naloxone and combinations of these drugs on schedule-controlled behavior were examined in rhesus monkeys and pigeons. The order of potency in decreasing response rates under a multiple 5-minute fixed-interval, 30-response fixed-ratio schedule in both monkeys and pigeons was morphine greater than pentazocine greater than naloxone. Compared to monkeys, pigeons were less sensitive to morphine and pentazocine and slightly more sensitive to naloxone. In monkeys and pigeons, as the morphine dose increased, higher naloxone doses were required to restore responding to or near control levels. In pigeons, however, the response rate decreases caused by naloxone prevented the complete antagonism of the highest morphine dose and limited the range of naloxone doses over which complete antagonism of morphine occurred. Antagonism of pentazocine by naloxone generally was greatest at the lowest pentazocine dose tested in both monkeys and pigeons. At higher pentazocine doses, if any antagonism was obtained, it was slight, and the amount of antagonism typically did not increase as the naloxone dose increased. Pentazocine generally failed to antagonize morphine in monkeys and pigeons.