mu opioid receptors in the ventrolateral periaqueductal gray mediate stress-induced analgesia but not immobility in rat pups

Rat pups become immobile and analgesic when exposed to an adult male rat. T he aim of this study was to determine whether these reactions are under the control of endogenous opioids and to determine the role of the midbrain pe riaqueductal gray (PAG), which mediates stress-induced immobility and analg esia in adult animals. In Experiment 1, 14-day-old rats were injected syste mically with the general opioid receptor antagonist naltrexone (1 mg/kg), w hich blocked male-induced analgesia to thermal stimulation but did not affe ct immobility. In Experiment 2, the selective mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP; 50 or 100 ng/200 nl) was mi croinjected into the ventrolateral and lateral PAG. CTOP suppressed male-in duced analgesia when injected into the ventrolateral PAG. Male-induced immo bility was not affected by CTOP. Male proximity therefore seems to induce a nalgesia in rat pups by releasing endogenous opioids that bind to mu opioid receptors in the ventrolateral PAG.