Pronounced changes in the activity of nociceptive modulatory neurons in the rostral ventromedial medulla in response to prolonged thermal noxious stimuli.

1. Brain regions that inhibit nociception can be activated by various environmental stimuli, including prolonged noxious stimuli. The present study tested the effect of such a prolonged noxious stimulus on the activity of nociceptive modulatory neurons in the rostral ventromedial medulla (RVM). These neurons, called ON- and OFF-cells because of their respective burst and pause in activity associated with nocifensor reflexes, have been shown to facilitate and inhibit nociception, respectively. 2. Single-unit activity of ON- and OFF-cells was assessed in lightly halothane- or barbiturate-anesthetized rats exposed to prolonged noxious heat (50 degrees C water). This prolonged noxious stimulus caused an increase in ON-cell and a decrease in OFF-cell activity regardless of anesthetic (halothane or barbiturate) or stimulus location (hindpaw or tail). 3. Surprisingly, and despite the consistent changes in RVM cell activity, the prolonged noxious stimulus caused different effects depending on the reflex used to assess nociception. The hindpaw withdrawal reflex was facilitated when the tail was immersed in hot water, whereas the tail flick reflex was inhibited when the hindpaw was immersed in hot water (see preceding manuscript). Lidocaine inactivation of the RVM shortened the latency for both reflexes but had no effect on tail flick inhibition produced by the noxious conditioning stimulus. In contrast, lidocaine inactivation of the RVM completely reversed the hindpaw reflex facilitation produced by tail heat, indicating the involvement of RVM ON-cells in facilitation of this reflex. 4. These data demonstrate that RVM neurons respond in a consistent manner to noxious stimuli whether applied for a brief or prolonged time: ON-cell activity increases and OFF-cell activity decreases. Moreover, the activation of RVM ON-cells produced by a noxious stimulus is sufficient to enhance some nocifensor reflexes, whereas neural structures other than the RVM appear to mediate the antinociceptive effects produced by a prolonged noxious stimulus.