ROLE OF THE NUCLEUS RAPHE OBSCURUS IN THE INHIBITION OF ROSTRAL VENTROLATERAL MEDULLARY NEURONS INDUCED BY STIMULATION IN THE VENTROLATERALPERIAQUEDUCTAL GREY-MATTER OF THE RABBIT

Experiments have been carried out to investigate the pathways which me diate the inhibitory influence of the ventrolateral periaqueductal gre y matter (vlatPAG) on neurones in the rostral ventrolateral medulla (R VLM). In anaesthetized rabbits, 20-ms trains of electrical stimulation in the vlatPAG inhibited ongoing activity of neurones in the RVLM to <30% of the control level. The inhibition was blocked after microinjec tions of 500-1000 nl local anaesthetic (4% lignocaine, n = 9) or 0.2 M glycine (n = 4) into nucleus raphe obscurus (NRO). We suggest that th e inhibitory influence of the vlatPAG on neurones in the RVLM is media ted by a relay in NRO.