DESCENDING PROJECTIONS FROM THE CAUDAL MEDULLA-OBLONGATA TO THE SUPERFICIAL OR DEEP DORSAL HORN OF THE RAT SPINAL-CORD

The location of neurons in the caudal medulla oblongata that project t o the superficial or deep dorsal horn was studied in the rat, by means of retrograde labelling from confined spinal injection sites. The tra cer cholera toxin subunit B was injected into laminae I-III (fuve rats ) or I-V (three rats) at C-4-7 spinal segments. Neurons projecting to the superficial dorsal horn were located in the dorsomedial part of th e dorsal reticular nucleus ipsilaterally, the subnucleus commissuralis of the nucleus tractus solitarius bilaterally, and a region occupying the lateralmost part of the ventrolateral reticular formation between the lateral reticular nucleus and the caudal pole of the spinal trige minal nucleus, pars caudalis, bilaterally. Neurons projecting to the d eep dorsal horn, which were only labelled when laminae I-V were filled by the tracer, occurred in the dorsomedial and ventrolateral parts of the dorsal reticular nucleus and in the ventral reticular nucleus bil aterally. A few cells were located in the above described lateralmost portion of the ventrolateral reticular formation bilaterally and in th e ventral portion of the ipsilateral cuneate nucleus. In the light of previous data demonstrating that dorsal horn neurons project to the do rsal reticular nucleus, the ventrolateral reticular formation, and the nucleus tractus solitarius, and that neurons in these three medullary regions are involved in pain inhibition at the spinal level, the desc ending projections demonstrated here suggest the occurrence of spine - medullary-spinal loops mediating the analgesic actions elicited in eac h nucleus upon the arrival of nociceptive input from the dorsal horn.