Electrophysiological evidence for an excitatory projection from ventromedial forebrain structures on to raphe- and reticulo-spinal neurones in the rat.

Spinally projecting neurones in nucleus raphe magnus (NRM) and the adjacent reticular formation of the medulla were identified by their antidromic responses to electrical stimulation in the lumbosacral spinal cord. Identified raphe-spinal and reticulo-spinal neurones were then tested for the effects of electrical stimulation at sites in the ventromedial forebrain, including the anterior hypothalamus and preoptic area (AH/POA). The results of these experiments have demonstrated that a considerable proportion of raphe- and reticulo-spinal neurones receive an excitatory input from the AH/POA. It is suggested that activity in this descending pathway might contribute to the inhibitory effects of AH/POA stimulation on the nociceptor-evoked activities of spinal dorsal horn neurones.