Descending inhibitory influences from periaqueductal gray, nucleus raphe magnus, and adjacent reticular formation. I. Effects on lumbar spinal cord nociceptive and nonnociceptive neurons

1. This study examined the inhibitory effects of conditioning stimuli delivered to the periaqueductal gray (PAG), nucleus cuneiformis (CU), nucleus raphe magnus (NRM), nucleus reticularis gigantocellularis (NGC), and nucleus reticularis magnocellularis (NMC) on functionally identified neurons of the lumbar spinal cord dorsal horn in chloralose-anesthetized or decerebrate cats. 2. Neurons were classified according to their responses to a variety of cutaneous stimuli as low-threshold mechanoreceptive (LTM), wide dynamic range (WDR), or nociceptive specific (NS). The major aim of this study was to determine whether there was a difference in the effectiveness of the brain stem stimulation-produced inhibition of nociceptive (noci) neurons (consisting of both WDR and NS neurons) and the LTM non-nociceptive (nonnoci) neurons. There were no statistical differences in the susceptibility of WDR and NS neurons to brain stem-induced inhibition. 3. Most neurons tested could be inhibited by stimulation of any of the brain stem regions tested. In all cases the percentage of noci neurons inhibited from a given region was higher than the percentage of nonnoci neurons; however, this difference was only statistically significant in the case of NMC stimulation. 4. Threshold current intensities necessary to produce inhibition were determined for each neuron from each stimulation site. Although there was a trend for noci neurons to require slightly lower current intensities, there was in fact no statistically significant difference in the inhibitory thresholds between noci and nonnoci neurons for any of the regions tested. 5. A comparison of the mean threshold currents for the five regions studied revealed that the lowest stimulation currents were obtained in NMC with NRM, CU, NGC, and PAG, each requiring progressively higher current intensities in order to produce inhibition. 6. These results indicate that stimulation in PAG and NRM not only inhibits the responses of noci neurons but also those of nonnoci neurons. Moreover, stimulation in reticular regions adjacent to these two regions is effective in inhibiting the responses of both noci and nonnoci neurons.