ACTIVATION BY CUTANEOUS OR VISCERAL NOXIOUS-STIMULATION OF SPINAL NEURONS PROJECTING TO THE MEDULLARY DORSAL RETICULAR NUCLEUS IN THE RAT -A C-FOS STUDY

The involvement of spinal neurons in the transmission of cutaneous and visceral nociceptive input to the medullary dorsal reticular nucleus was studied. Rats were injected with cholera toxin subunit B in the le ft dorsal reticular nucleus and subjected 4 days later to noxious mech anical, thermal or chemical stimulation of the proximal internal aspec t of the left thigh, or to chemical stimulation of the urinary bladder , Sections of spinal segments T-13-L-3 were processed immunocytochemic ally for cholera toxin subunit B and Fos protein. The percentage of do uble-labelled cells in the population of Fos-positive cells was higher in lamina I (1-4%) than in deeper laminae (0-0.7%) following all stim uli. The percentage of double-labelled cells in the population of retr ogradely labelled cells was 30-53% in lamina I and 0-5% in laminae III -X. Visceral stimulation activated more retrogradely labelled lamina I cells than any kind of cutaneous stimulation. Pyramidal cells were ac tivated in higher numbers than multipolar and flattened cells after th ermal cutaneous or visceral stimulation, and in lower numbers than mul tipolar cells after mechanical stimulation. These results suggest that , in the experimental conditions used, spinal cord cells conveying nox ious input to the dorsal reticular nucleus are concentrated in lamina I. They further indicate that the spinal-dorsal reticular nucleus path way plays a major role in the transmission of nociceptive visceral inp ut, and point to the preferential involvement of pyramidal cells in cu taneous thermal and visceral processing.