Physiological survey of medullary raphe and magnocellular reticular neurons in the anesthetized rat

The present study was designed to provide a detailed and quantitative description of the physiological characteristics of neurons in the medullary raphe magnus (RM) and adjacent nucleus reticularis magnocellularis (NRMC) under anesthetized conditions. The background discharge and noxious stimulus-evoked responses of RM and NRMC neurons were recorded in rats lightly anesthetized with isoflurane. All cells that were isolated successfully were studied. After recording background discharge, the neuronal response to repeated noxious thermal and noxious mechanical stimulation of the tail was recorded. Most cells were identified as nonserotonergic by their irregular or rapid background discharge pattern. Because the spontaneous discharge of most RM nonserotonergic cells contained pauses and bursts, a comparison between the change in rate evoked by tail heat and the range of rate changes that occur spontaneously was used to classify cells. The mean responses of ON and OFF cells were more than four times the standard deviation of the changes in rate observed spontaneously, ON cells were excited in 86% of the tail heat trials tested. Similarly, OFF cells were inhibited in 97% of the noxious tail heat trials tested. The heat-evoked changes in ON and OFF cell discharge varied over more than two orders of magnitude and were greater in cells with greater rates of background discharge. The heat-evoked responses of ON and OFF cells had durations of tens of seconds to minutes and were always sustained beyond the visible motor response. Most ON and OFF cells responded to noxious tail clamp in a manner that was similar to their response to noxious heat. More than half of the NEUTRAL cells that were unresponsive to noxious heat were responsive to noxious tail clamp. A minority of ON, OFF, and NEUTRAL cells responded to innocuous brush stimulation with weak, transient responses. Although many cells discharged too infrequently to be classified, units with physiological properties that were different from those described above were rare. In conclusion, most RM and NRMC cells belong to three nonserotonergic physiological cell classes that can be distinguished from each other by the consistency, not the magnitude, of their responses to repeated noxious thermal stimulation. Because most of the heat-evoked change in ON and OFF cell discharge occurs after the conclusion of the initial motor withdrawal, ON and OFF cells are likely to principally modulate the response to subsequent noxious insults.