[The prolonged analgesic effect of an antiserum to beta-endorphin in rats]

Intravenous injection of beta-endorphin antiserum lowered the threshold of the tail-shock reaction to nociceptive electrocutaneous stimulation in rats in the first 1-2 days and raised it in the following 2-3 months. Naloxone injection reversed the analgesic effect of antiserum to beta-endorphin. Administration of normal serum did not change the threshold of the nociceptive reaction and naloxone failed to alter its effect. It is suggested that the secondary protracted effect of antiserum to beta-endorphin is a result of activation of the endogenous opioid system as a rebound-effect to diminished liberation of beta-endorphin in the first phase of the action of its antiserum.