Encoding of noxious stimulus intensity by putative pain modulating neurons in 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 transmission. In the RVM the so-called on-cells are excited, and the so-called off-cells are inhibited, by noxious stimuli applied almost anywhere on the body surface, thus showing that they receive information from spinal and trigeminal nociceptive neurons. In deeply anesthetized rats, recordings were made from RVM neurons that resembled on- and off-cells (herein called putative on- and off-cells) in order to investigate (1) how they encode the intensity of thermal noxious stimuli (46--56 degrees C) applied to a hindpaw, and (2) how their encoding properties relate to those of simultaneously recorded spinal neurons. In 49 of 98 cases, a graded increase in the stimulus temperature 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 decreased 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 intensity in terms of response latency and discharge rate, and suggest that such encoding closely reflects spinal neuronal encoding. This may be relevant for the transmission and modulation of pain information by RVM neurons.