Responses of neurons in the ventromedial midbrain to noxious mechanical stimuli.

Neurons of the ventromedial midbrain in pentobarbital-anesthetized rats were examined by extracellular recording for responses to mechanical stimulation of the skin. Responses were absent from neurons clearly located in the interpeduncular nucleus (IPN) (n = 20), and from 92% of linear raphe (LR) neurons (n = 26). However, 37% of neurons in the ventral tegmental area of Tsai (VTA) (n = 38) and 63% of neurons in the small interfascicular nucleus (IF) (n = 9) were inhibited, often recovering with a delay of 1-2 min. A few cells (n = 4) were weakly excited in these 4 nuclei; none responded to innocuous mechanical stimulation of the skin. It is concluded that noxious cutaneous stimuli will not modify (by feedback) any influence of the IPN on pain perception, but could dampen behavior-reinforcing effects of the VTA and IF.