There is considerable evidence that the periaqueductal grey and the dorsal raphe contribute to an endogenous analgesia system and to the regulation of a wide variety of other responses, many of which involve spinal sites of action. To map the areas of the periaqueductal grey and dorsal raphe which contain neurons that project to the spinal cord, wheat germ agglutinin conjugated to horseradish peroxidase was injected into hemisected spinal cords in rat, cat, and monkey. After cervical or lumbar injections labelled neurons were found in the periaqueductal grey and dorsal raphe in all species examined. In the rat, labelling of the dorsal raphe is sparse but numerous labelled neurons are present in the mid and rostral periaqueductal grey. In the cat, the number of retrogradely-labelled neurons in both the dorsal raphe and the periaqueductal grey are considerable. In the monkey, like the rat, the labelling in the dorsal raphe was light but numerous labelled neurons were present in the periaqueductal grey and the adjacent nucleus cuneiformis. Injections into the lumbar spinal cord produced the same pattern of labelling as seen after cervical level injections with approximately 40% fewer labelled cells in all areas. Thus, while each species had a similar pattern of spinal projections from the periaqueductal grey and dorsal raphe, quantitative differences were evident among the species examined. These results suggest that the number of periaqueductal grey and dorsal raphe neurons projecting to the spinal cord in the rat, cat and monkey are considerably more numerous than previously reported and that the effects described during the stimulation of these regions could be, at least partly, due to the involvement of these direct pathways.
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