Periaqueductal grey lesions reduce morphine analgesia in the rat

Many recent experiments have implicated the periaqueductal grey matter (PAG) as a major site of action of morphine in producing analgesia. This hypothesis was tested by investigating the effect of PAG lesions on morphine analgesia. Histologically verified electrolytic lesions were made in the rat mesencephalon which destroyed a large part of the PAG. Rats were subsequently tested for morphine analgesia using the tail flick method. Analgesin produced by morphine administration was markedly reduced in the lesioned rats. Pain thresholds were not altered by the lesions.