Are 'neutral cells' in the rostral ventro-medial medulla subtypes of on- and off-cells?

The classification of cells in the rostra ventromedial medulla (RVM) is bas ed on the response pattern to noxious tail heat: on-cell activity increased , off-cell activity decreased, and activity of neutral cells is unaffected by noxious heat tail stimulation. It is generally assumed that on-, off- an d neutral cells respond equally to noxious stimulation applied anywhere on the body surface, but so far this assumption has not been systematically ex amined. In the present study the effects of thermal and mechanical stimuli applied to the tail, the extremities and the orofacial region on the extrac ellularly recorded activity of 14 neutral cells were investigated in lightl y anesthetized rats. Although the neutral cells did not respond to noxious tail heat, all of them responded to most of the other stimuli in an on- or off-manner. Especially cell responses to pinch stimuli applied to the skin of the ear, the forehead and the nose differed from the neutral behavior. T he fact that the neutral cells in the present study responded in an off- or on-manner by applying noxious stimuli different from noxious tail heat sug gests that these cells are possibly subtypes of on- and off-cells in the RV M. (C) 2000 Elsevier Science Ireland Ltd and the Japan Neuroscience Society . All rights reserved.