A spinomedullary projection terminating in the dorsal reticular nucleus of the rat

Spinal afferents to the medullary dorsal reticular nucleus were studied using the following retrograde tracers: horseradish peroxidase (diluted in dimethylsulfoxide), wheat germ agglutinin conjugated with horseradish peroxidase, and cholera toxin subunit B. Spinal cord cells projecting to that medullary region were located predominantly in medial lamina I and lamina X. Cell labelling was moderate in the medial part of laminae II-IV and sparse throughout laminae V-VII. Labelling was predominantly ipsilateral in the dorsal horn and bilateral in laminae VII and X. After mechanical lesions of the dorsal white matter which severed most of the ipsilateral cuneate fasciculus, the numbers of superficial dorsal horn cells that were labelled from the dorsal reticular nucleus were considerably decreased caudal to the lesion, which suggests that their axons utilize mostly the cuneate fasciculus. Since the medullary dorsal reticular nucleus of the rat has a predominant population of nociceptive specific neurons, it is suggested that this spino-dorsomedullary reticular pathway is involved in pain processing.