Beta-endorphin tolerance is inhibited by oxytocin.

The repeated administration of beta-endorphin to mice resulted in the development of tolerance to the analgesic effect of the opioid peptide. While SC injections of oxytocin failed to modify the magnitude or the duration of the analgesic effect of a single intracerebroventricular (ICV) beta-endorphin injection, the development of tolerance to beta-endorphin was antagonized by SC and ICV oxytocin treatment. Since both oxytocin and beta-endorphin are endogenous peptides in the brain, the data raise the possibility that endogenous oxytocin might be involved in the sensitivity of the central nervous system towards repeated or prolonged opioid stimulation.