Is the response pattern of on- and off-cells in the rostral ventromedial medulla to noxious stimulation independent of stimulation site?

The classification of on- and off-cells in the rostral ventromedial medulla is based on the response pattern to noxious tail heat. It is generally assumed that on- and off-cells respond equally to noxious stimulation anywhere on the body surface, but so far this assumption has not been systematically examined. In the present study the effects of noxious thermal and mechanical stimuli applied to the tail, the extremities and the craniofacial region on the extracellularly recorded activity of 13 on- and 23 off-cells were investigated in lightly anesthetized rats. In 3 out of 13 on-cells and 11 out of 23 off-cells the response pattern evoked by noxious stimulation of the extremities or the craniofacial region differed from the response pattern elicited by noxious tail heat. In comparison with the response pattern to noxious tail heat, stimulation of the extremities or the craniofacial region reproducibly evoked opposite reactions in 2 on- and 9 off-cells and did not change neuronal activity in one on- and 2 off-cells. The results of the present study raise the question of whether the response pattern of on- and off-cells in the rostral ventromedial medulla can be sufficiently predicted by a classification that is exclusively based on the cellular behavior to noxious heat stimulation of the tail.