Activity of nociceptive modulatory neurons in the rostral ventromedial medulla associated with volume expansion-induced antinociception.

Abstract

A wide range of environmental stimuli have been shown to induce antinociception. Investigation of the brain regions contributing to environmentally induced antinociception (EIA) has focused primarily on the effect of disrupting neural transmission at various CNS sites. In contrast, the present study analyzed changes in the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM) following induction of antinociception by a physiological stimulus, volume expansion. Previous research indicates that 2 classes of RVM neuron, the on- and off-cells, facilitate and inhibit nociception, respectively. In the present study, volume expansion induced by intravenous administration of Ficoll inhibited the tail-flick reflex in 17 of 21 rats and simultaneously increased the firing rate of off-cells and decreased that of on-cells. Changes in cell activity occurred only in those rats in which antinociception was produced. Lidocaine inactivation of the RVM did not block antinociception produced by Ficoll infusion. These findings demonstrate that, although antinociception produced by physiological activation of vagal afferents is associated with an increase in off-cell and a decrease in on-cell activity, these changes are not required for volume expansion-induced antinociception.