Encoding of noxious stimulus intensity by putative pain modulating neuronsin the rostral ventromedial medulla and by simultaneously recorded nociceptive neurons in the spinal dorsal horn of rats

Neurons in the nucleus raphe magnus and adjacent structures of the rostral ventromedial medulla (RVM) are involved in the control of nociceptive trans mission. In the RVM the so-called on-cells are excited, and the so-called o ff-cells are inhibited, by noxious stimuli applied almost anywhere on the b ody surface, thus showing that they receive information from spinal and tri geminal nociceptive neurons. In deeply anesthetized rats, recordings were m ade from RVM neurons that resembled on- and off-cells (herein called putati ve on- and off-cells) in order to investigate (1) how they encode the inten sity of thermal noxious stimuli (46-56 degreesC) applied to a hindpaw, and (2) how their encoding properties relate to those of simultaneously recorde d spinal neurons. In 39 of 98 cases, a graded increase in the stimulus temp erature caused a monotonic decrease in the response latency of putative on- cells, putative off-cells and spinal neurons, while the response discharge rate monotonically increased for putative on-cells and spinal neurons and d ecreased for putative off-cells. In the majority of simultaneous recordings of RVM and spinal neurons, the latency and discharge rate of the putative on- or off-cell were highly correlated with the latency and discharge rate of the spinal neuron, and the stimulus/response slopes were similar. These results show that putative on- and off-cells can encode the stimulus intens ity in terms of response latency and discharge rate, and suggest that such encoding closely reflects spinal neuronal encoding. This may be relevant fo r the transmission and modulation of pain information by RVM neurons. (C) 2 001 International Association for the Study of Pain. Published by Elsevier Science B.V. All rights reserved.