COMPARISON OF OPIATE-PEPTIDE-INDUCED AND OPIOID-PEPTIDE-INDUCED IMMOBILITY

Intraventricular administration of the endogenous opioid peptide β-endorphin produces a profound state of immobilization in rats characterized by the absence of spontaneous movement, loss of the righting response and extreme generalized muscular rigidity. The immobility syndrome induced by the opioid peptides β-endorphin and D-Met2-Pro5-enkephalinamide was compared with the behavioral profile prodced by subcutaneous and intraventricular administration of the opiates, morphine, methadone and etonitazene. The results indicate a close similarity between the pattern of effects caused by the opiates and opioid peptides. The immobility syndrome could also be produced by injection of β-endorphin into the ventromedial periaqueductal gray, but not into the caudate, globus pallidus, amygdala or dorsolateral periaqueductal gray. The resemblance between the opiate- and β-endorphin-induced profiles suggests that their effects are mediated through common mechanisms. © 1979.