Naloxone blocks the release of opioid peptides in periaqueductal gray and N. accumbens induced by intra-amygdaloid injection of morphine.

The working hypothesis that the periaqueductal gray (PAG), N. accumbens and amygdala were connected serially in a unidirectional loop for antinociception, in which Metenkephalin and beta-endorphin were considered to be two important analgesic neurotransmitters, was examined by simultaneously perfusing the PAG and N. accumbens after microinjection of morphine into the amygdala. Intra-amygdaloid injection of morphine increased the release of enkephalins and beta-endorphin in the PAG and N. accumbens. When the perfusion fluid contained 3 microM of naloxone, the release of enkephalins and beta-endorphin was reduced in both the PAG and the N. accumbens. These results do not support the hypothesis of a unidirectional loop and its putative sequence.