EXTENT OF NOCICEPTIVE DERMATOMES IN ADULT RATS IS NOT PRIMARILY MAINTAINED BY AXONAL COMPETITION

Nociceptive innervation territories of individual peripheral and spina l nerves in the skin of the rat hind paw were investigated. In additio n, the hypothesis that competitive interactions among the axons from a djacent dorsal root ganglia (DRG) play an important role in maintenanc e of dermatomal extent in adult animals was tested. The area of innerv ation territories of individual spinal and peripheral nerves was deter mined by nociceptive pinch test of the skin after extirpation of adjac ent DRGs or transection of adjacent peripheral nerves, respectively. P ositions of nociceptive dermatomes and innervation territories of peri pheral nerves were similar to the territories innervated by the C-fibe rs described earlier by dye extravasation technique. In contrast, our results convincingly demonstrated substantial overlap of nociceptive ( probably A delta) fibers from adjacent dermatomes in which the autonom ous innervation areas were only about one-half of the maximal areas. N ociceptive territories of peripheral nerves overlapped, too. According ly, we could find no autonomous innervation area of the sural nerve. T wo weeks after extirpation of adjacent DRGs, the area of each of the i solated dermatomes L3, L4, and L5 increased only by about 10%, and it did not change detectably during the next 6 months. The results of our study (a) support the view that innervation fields supplied by the no ciceptive (probably A delta) fibers are greater and display more overl ap than those supplied by the C-fibers of the same nerve and (b) sugge st that axonal competition for innervation territory is not decisive f or maintenance of dermatomal borders in the adult rat. (C) 1998 Academ ic Press.