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

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The working hypothesis that the periaqueductal gray (PAG), N. accumbens and amygdala were connected serially in a unidirectional loop for antinociception, in which Met-enkephalin and β-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 β-endorphin in the PAG and N. accumbens. When the perfusion fluid contained 3 μM of naloxone, the release of enkephalins and β-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.