Intravenous beta-endorphin: behavioral and physiological effects in conscious monkeys.

Beta-endorphin (0.7 and 2.8 mg/kg) and morphine (0.15 and 0.60 mg/kg) were administered intravenously to rhesus monkeys responding on an operant schedule. Beta-endorphin injections resulted in dose-dependent effects which included marked, but relatively brief disruptions in behavioral responding, decreases in systolic blood pressure, and more protracted increases in heart rate. Morphine injections were followed by much longer duration decreases in response rates and systolic blood pressure, and an irregular but largely deceleratory heart rate response. On a molar basis, beta-endorphin was approximately twice as potent as morphine. It was concluded that intravenously administered beta-endorphin exerts behavioral and physiological effects in the unanesthetized primate.