Dorsal raphe stimulation modulates nociceptive responses in thalamic parafascicular neurons via an ascending pathway: further studies on ascending pain modulation pathways.

A study on the nociceptive responses of single cells within the nucleus parafascicularis (PF) thalami of the rat was undertaken to clarify the reported observations of a pain suppression pathway to this nucleus from the dorsal raphe (DR) nucleus. Two types of nociceptive neuron were identified in the PF which were classified as 'nociceptive-on' and 'nociceptive-off' neurons, respectively. DR stimulation exhibits a simple monophasic 'dose-dependent' relationship between the degree of the inhibition elicited and the stimulation intensity used on the 'nociceptive-off' cells. In contrast, biphasic effects following DR stimulation on the 'nociceptive-on' cells was obtained, with low intensities eliciting suppression while high intensities excited the cells. These effects of low intensity DR stimulation upon the responses of the 'nociceptive-on' cells were diminished but not prevented by transection of the well-known bulbospinal inhibitory fibers descending in the dorsal half of the spinal cord, while the effects of DR stimulation upon the 'nociceptive-off' cells remain unchanged following spinal transection. Thus, our results show that DR stimulation modulates the nociceptive responsiveness of the PF by way of supraspinal pathways in addition to the previously described descending paths.