Activity of neurons in the rostral medulla of the halothane-anesthetized rat during withdrawal from noxious heat.

The physiological and pharmacological properties of two classes of putative nociceptive modulatory neurons have been extensively characterized in the rostral ventromedial medulla (RVM) of the barbiturate-anesthetized rat. 'On-cells' show a burst of activity, and 'off-cells' a sudden pause immediately preceding the occurrence of nocifensor reflexes. In the present study, we have characterized the reflex-related activity of RVM neurons in halothane-anesthetized rats to determine whether the properties of these neurons are dependent on barbiturate anesthesia. Both on- and off-cells were identified in this preparation. Repeated noxious stimulation was associated with a high level of ongoing activity in on-cells, and a low level in off-cells. These data thus demonstrate that the previously described reflex-related changes in RVM neuron activity are not specific to barbiturate-anesthetized preparations, and that a failure to demonstrate off-cells in some studies may result from these neurons being inactive following repeated testing with noxious stimuli.