Release of immunoreactive [Met5]enkephalin and cholecystokinin from the spinal cord by beta-endorphin administered intraventricularly in the pentobarbital-anesthetized rat.

Beta-endorphin (4 micrograms) injected i.v.t. induced the release of immunoreactive [Met5]enkephalin and sulfated cholecystokinin octapeptide (CCK-8s) from the spinal cord in the pentobarbital-anesthetized rat. The inhibition of the tail-flick response induced by beta-endorphin given i.v.t. was antagonized by i.t. injection of CCK-8s and potentiated by i.t. administration of the antibody to CCK-8s. The findings give support to the hypothesis that endogenous CCK-8s in the spinal cord may play a negative feedback role in modulating beta-endorphin-induced antinociception.