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Analgesia induced by morphine administered to the nucleus accumbens was blocked by naloxone of [Met~(5)] enkephalin antiserum injected into the periaqueductal grey of the rabbit

Yu Longchuan, Shi Yushun, Han Jisheng

Department of Physiology, Beijing Medical University. Beijing, China

Abstract

We have shown in rabbits an ascending neuronal pathway of analgesia from
periaqueductal grey (PAG) to the nucleus accumbens, using 5-hydroxytryptamine (5-HT) and [Met~(5)] enkephalin (ME) as neurotransmitters.The present study was performed to explore the possible existence of a descending analgesic
pathway from nucleus accumbens to PAG.Pain threshold was measured by the latency of the escape response (ERL) to radiant heat applied on the skin of the snout. Rabbits equipped with chronic intracranial cannulae were given morphine into nucleus accumbens to induce analgesia, which was followed microinjection of naloxone or antiserum against enkephalin into PAG 20 min later. It was found that injection of morphine,20#mu#g/1#mu#l, into nucleus accunbens produced an increase in ERL over 80%, lasting for a period of more than 50 min. This effect of morphine was partially blocked by naloxone administered to PAG in a dose-ependent manner (0.5,1.0,2.0#mu#g/#mu#l/site). A statistically significant blockade of the morphine analgesia was also achieved by intra-PAG injection of antiserum against ME (1#mu#l/site), but not by the same amount of normal rabbit serum or antiserum against [Leu~(5)] enkephalin. These results suggest the existence of a descending pathway of analgesia from nucleus accumbens to PAG, using ME as neurotransmitter in the PAG area. This descending pathway of analgesia, in connection with the ascending pathway from PAG to nucleus accumbens, may form a meso-limbic loop of analgesia. The implication of the possible existence of the neuronal loop in acupuncture analgesia and morphine analgesia is discussed.

Key words: Nucleus accumbens;Periaqueductal grey (PAG);Morphine;Naloxone;[Met_(5)] enkephalin antiserum;Meso-limbic loop of analgesia

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Citing This Article：

Yu Longchuan, Shi Yushun, Han Jisheng. Analgesia induced by morphine administered to the nucleus accumbens was blocked by naloxone of [Met~(5)] enkephalin antiserum injected into the periaqueductal grey of the rabbit. Acta Physiol Sin 1987; 39 (4): (in Chinese with English abstract).