Simultaneous multi- and single-unit recordings in the rostral ventromedial medulla of ketamine-anaesthetized rats, and the cross-correlogram analysis of their interactions.

In anaesthetized rats, neurons located in the rostral ventromedial medulla (RVM) are classified by their differential responses to noxious stimulation. "On" cells are excited by noxious stimulation, "off" cells are inhibited and "neutral" cells are unaffected. Past research has primarily investigated the activity of these cells in lightly barbiturate anaesthetized animals while inducing a tail flick reflex. We recorded neuronal activity in the RVM of deeply ketamine-xylazine anaesthetized rats using a novel method of recording single- and multiunit activity simultaneously with one electrode. Under this anaesthetic paradigm we were able to record RVM cells which responded to noxious tail heating in the same manner as on and off cells without inducing a tail flick. A cross-correlogram analysis was done between the two levels of activity. It demonstrated that the RVM is a highly coordinated system in both the spontaneous period and the noxious evoked activity period. Through this technique it was also shown that both on and off cells fire simultaneously in the ketamine-xylazine paradigm.