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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 left dorsal reticular nucleus and subjected 4 days later to noxious mechanical, thermal or chemical stimulation of the proximal internal aspect of the left thigh, or to chemical stimulation of the urinary bladder. Sections of spinal segments T13‐L3 were processed immunocytochemically for cholera toxin subunit B and Fos protein. The percentage of double‐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 stimuli. The percentage of double‐labelled cells in the population of retrogradely 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 activated in higher numbers than multipolar and flattened cells after thermal cutaneous or visceral stimulation, and in lower numbers than multipolar cells after mechanical stimulation. These results suggest that, in the experimental conditions used, spinal cord cells conveying noxious input to the dorsal reticular nucleus are concentrated in lamina I. They further indicate that the spinaldorsal reticular nucleus pathway plays a major role in the transmission of nociceptive visceral input, and point to the preferential involvement of pyramidal cells in cutaneous thermal and visceral processing.