Distribution of primary afferent fibers was studied in intact and neonatally capsaicin treated rats by the application of horseradish peroxidase to the central branch of the transected lumbar dorsal roots. Coarse primary afferent fibers entered the spinal cord through the larger medial portion of the rootlet and arborized in the deeper part of the dorsal horn (laminae III and IV). Fine fibers reached the spinal cord through the smaller lateral portion of the rootlet and arborized in the superficial portion of the dorsal horn (lamina I and outer portion of lamina II). The technique used was inadequate to stain fine, unmyelinated primary afferent fibers terminating in the larger inner portion of lamina II. After neonatal capsaicin treatment (50 mg/kg) the flame-shaped arborizations of thick primary afferent fibers terminating in intact rat in laminae III and IV spread dorsally and occupied the inner portion of lamina II in the larger lateral sector of the dorsal horn. Medially the dense arborization of a different type of thick primary afferent fibers sprouted up to the white-gray border. The border between the lateral and medial sector was sharp and only slightly varied in localization from experiment to experiment. The sprouting fibers established complicated synaptic contacts with dendrites and axon terminals. The rearrangement of primary afferent fibers after neonatal capsaicin treatment confirmed earlier results and revealed a mediolateral difference in the fiber organization of the dorsal horn indicating differences in the projection from hairy vs non-hairy skin areas.
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