The activity of neurons in the rostral medulla of the rat during withdrawal from noxious heat

Abstract

Neurons of the rostral ventromedial medulla (RVM) have been implicated in the modulation of nociceptive transmission. In order to further analyze their role in pain behavior, we studied their activity while eliciting the tail flick reflex with noxious heat. Recording sites were regions in the RVM from which microstimulation (less than or equal to 10 microA, 400 mu sec, 50 Hz continuous pulse trains) inhibited the tail flick reflex. Extracellular unit activity and tail temperature were recorded, stored, and plotted with reference to either the time of tail flick or the time when the stimulating temperature reached 45 degrees C. Neuronal discharges were found to be either increased (on-cells), decreased (off-cells), or unchanged around the time of the tail flick. The decreases in discharge were more closely correlated with the tail flick behavior than with the temperature of the stimulus. These off-cells were located at sites of lowest threshold for tail flick inhibition and tended to be ventral to on-cells. We propose that off-cells must pause if the tail flick is to occur, and that this pausing allows the transmission of nociceptive input through spinal reflex loops.

Evidence for two classes of nociceptive modulating neurons in the periaqueductal gray

“…The presence of numerous PAG neurons that do not show TF-related activity distinguishes this region from the RVM, where the majority of cells studied exhibited TF-related changes in activity (Fields et al, 1983a;Barbaro et al, 1986). Although a role for these neurons in pain modulation should not be ruled out, it is also possible that they are involved in one or more of the many functions that have been attributed to the PAG.…”

“…In the RVM, 2 classes of cells have been identified that show changes in activity at the time of the tail-flick response (TF) elicited by noxious heat (Fields et al, 1983a). We now report that neurons in the PAG region also show changes in activity related to TF.…”

“…The majority of cells were located by observation of spontaneous activity; however, noxious pinch of a paw delivered approximately once in each 100 pm advance of the microelectrode was used as a search stimulus. Each cell's responses to noxious pinch and to non-noxious cutaneous stimuli (brisk brush, tap) were determined, and it was then characterized according to the classification system applied to cells of the RVM by Fields et al (1983a). Cell activity, time of TF occurrence, and tail temperature were stored (Data General Nova 2) during 5 repetitions of the heat stimulus.…”

“…Like the PAG, this area is associated with modulation of nociception. Most neurons in RVM belong to 1 of 2 classes of cells whose members show abrupt changes in firing just prior to the occurrence of the tail-flick reflex (TF) evoked by noxious heat: Cells of 1 class (Off-cells) pause, and those of the other class (On-cells) accelerate, immediately before the TF (Fields et al, 1983a). For several reasons, it is likely that cells of both classes are involved in modulation of dorsal horn nociceptive processing and in opiate analgesia.…”

“…For several reasons, it is likely that cells of both classes are involved in modulation of dorsal horn nociceptive processing and in opiate analgesia. Both Off-and On-cells are found in regions of the medulla having the lowest threshold for inhibition of the TF by electrical stimulation (Fields et al, 1983a), and a significant proportion of both cell classes projects to the spinal cord (Vanegas et al, 1984a). Furthermore, the activity of both On-and Off-cells is altered by systemically administered morphine given in doses sufficient to inhibit the TF: Off-cell activity is increased (Fields et al, 1983b), while that of On-cells is decreased (Barbaro et al, 1986).…”