RESPONSES OF NEURONS IN THE MEDULLARY RAPHE NUCLEI TO INPUTS FROM VISCERAL NOCICEPTORS AND THE VENTROLATERAL PERIAQUEDUCTAL GREY IN THE RAT

The ventrolateral periaqueductal grey matter (PAG) is believed to have a role in mediating cardiovascular responses to noxious visceral stim uli. The present study was carried out as a first stage in establishin g whether the ventrolateral PAG may exert these influences after a rel ay in the caudal medullary raphe nuclei (nucleus raphe obscurus and nu cleus raphe pallidus). Single unit extracellular recordings were made from neurones in the caudal raphe nuclei and, for comparison, in the m ore rostral nucleus raphe magnus in Saffan-anaesthetized and paralysed rats. Neurones in the mid-line medulla were tested for their response s to electrical stimulation at chemically identified depressor sites i n the ventrolateral PAG and to noxious visceral stimuli (distensions o f the urinary bladder and electrical stimulation of the greater splanc hnic nerve). Fifty-two per cent of caudal and 74% of rostral mid-line neurones gave short latency excitatory responses to stimulation of dep ressor sites in the ventrolateral PAG. Of the neurones that were also tested with noxious visceral stimuli, 5% of the caudal and 47% of the rostral neurones responded to bladder distension, while 33 and 35%, re spectively, of caudal and rostral neurones responded to splanchnic ner ve stimulation. These results indicate that many mid-line medullary ne urones receive inputs from both the ventrolateral PAG and visceral noc iceptors and may, therefore, be part of the output pathway by which th e ventrolateral PAG produces integrated physiological responses to nox ious visceral stimuli.