THE INHIBITORY EFFECT OF THE VENTROLATERAL PERIQUEDUCTAL GRAY-MATTER ON NEURONS IN THE ROSTRAL VENTROLATERAL MEDULLA INVOLVES A RELAY IN THE MEDULLARY RAPHE NUCLEI

Experiments were carried out in urethane-anaesthetised rats to determi ne whether the inhibition of neurones in the rostral ventrolateral med ulla (RVLM) induced by stimulation in the ventrolateral periaqueductal grey matter (PAG), is mediated via a relay in the medullary raphe nuc lei. Electrical stimulation in the ventrolateral part of the PAG (20-m s trains, 7 pulses, 5-100 muA) inhibited ongoing activity of neurones in the RVLM for periods of 10-120 ms (mean 43.6 ms). The duration of t he inhibition was reduced by 51.1% after microinjection of GABA (40-16 0 nmol in volumes of 200-400 nl, 9/12 sites), but not 165 mM NaCl (8/8 sites) in nucleus raphe magnus (NRM) and the rostral half of nucleus raphe obscurus (NRO). In a further series of experiments, activation o f neuronal perikarya at 17 sites in NRM or NRO by microinjection of d, l-homocysteic acid (5-40 nmol in volumes of 50-400 nl) inhibited ongoi ng activity of 9 out of 14 neurones in the RVLM, the other 5 being exc ited. We suggest that the inhibitory effect on neurones in the RVLM, w hich can be evoked by stimulation in the ventrolateral PAG, is mediate d indirectly by activation of an inhibitory projection to the RVLM fro m NRM and the rostral half of NRO.