Comparison of the neurochemistry of the endogenous opioid systems in two brainstem pain-processing centers

SCH-32615 is a new enkephalinase inhibitor whose analgesic effects were examined following its stereotactic microinjection into the periaqueductal gray (PAG) and the ventromedial medulla (VMM) regions of the brainstem of the rat. SCH-32615 produced a strong, dose-dependent, naloxone-reversible analgesia to thermal noxious stimuli as measured by the hot plate test (HP; supraspinal analgesia) and the tail flick test (TF; spinal analgesia). The peak analgesic effect was seen within 10 min and remained for 45-60 min. ED50 were for PAG, HP = 10.7 micrograms and TF = 17.3 micrograms, and for VMM, HP = 5.7 micrograms and TF = 7.2 micrograms. Using the irreversible mu receptor antagonist, beta-funaltrexamine, it was found that the endogenous enkephalins in the PAG produce their analgesic effects by acting at only one receptor subtype (the mu receptor) while in the VMM both mu and delta opioid receptors are involved (not through the delta alone as previously believed).