Effects of ethylketazocine and morphine on schedule-controlled behavior in pigeons and squirrel monkeys.

The behavioral effects of morphine and ethylketazocine were compared in squirrel monkeys and pigeons responding under fixed-interval schedules of food presentation, and in monkeys responding under fixed-interval schedules of electric-shock presentation. Both morphine and ethylketazocine produced dose-related decreases in rates of responding maintained by food presentation in either species. However, intermediate doses of both drugs increased rates of responding maintained by shock presentation in monkeys; at higher doses both drugs decreased rates of shock-maintained responding. In monkeys and pigeons, the dose-effect curves for both ethylketazocine and morphine were shifted to a comparable degree of naloxone. Maximal effects of morphine were observed 16 to 24 min after injection and typically lasted the entire 40-min observation period. Effects of ethylketazocine had a faster onset (maximal effects were observed by 8 to 16 min after injection), and shorter duration (effects diminished by 24 to 40 min after injection). Ethylketazocine was 30 to 100 times more potent than morphine under either the schedule of food or shock presentation in squirrel monkeys, whereas the two drugs had similar potencies in pigeons. Thus, the effects of at least one of the drugs may be mediated differently in one of the two species.