The effects of beta-(Tyr9)melanotropin-(9-18) and H-Phe-Ile-Tyr-His-Ser-Tyr-Lys-OH on the analgesic action of morphine.

beta-(Tyr9)melanotropin-(9-18), which itself has no analgesic action, as measured by the tail-flick and hot-plate methods, decreased morphine-induced analgesia following intracerebroventricular injection. H-Phe-Ile-Tyr-His-Ser-Tyr-Lys-OH heptapeptide, which has weak CRF-like activity, had no action on analgesia and was not able to modify morphine-induced analgesia. Compared with ACTH1-24, which in a subcutaneous dose of 100 micrograms/rat decreased morphine-induced analgesia, the same dose of H-Phe-Ile-Tyr-His-Ser-Tyr-Lys-OH was ineffective.