Naloxone blocks opioid peptide release in N. accumbens and amygdala elicited by morphine injected into periaqueductal gray.

It has been proposed that a serial, unidirectional circuit from the PAG to the N. accumbens and the amygdala are involved in antinociception and that enkephalins (ENK) and beta-endorphin (beta-EP) act as neurotransmitters in this circuitry. In the present study, we measured the release of ENK and beta-EP by simultaneous push-pull perfusion of the N. accumbens and amygdala after microinjection of morphine into the PAG. Morphine administration elicited an increase in immunoreactive ENK and beta-EP in both the N. accumbens and the amygdala, which was antagonized in each nucleus by perfusion with naloxone. These data suggest that the three nuclei were not serially connected and that they may take part in one and the same antinociceptive system with an "all or none" character.