Neonatal capsaicin treatment induces invasion of the substantia gelatinosa by the terminal arborizations of hair follicle afferents in the rat dorsal horn.

Capsaicin, administered on the day of birth, was found to alter laminar distribution, but not the receptive field properties or the morphology of the collateral arborizations of hair follicle afferents (HFAs) intra-axonally injected with horseradish peroxidase (HRP). Of the 65 HFA terminal arbors in capsaicin treated rats, 46 (71%) were found to enter the substantia gelatinosa (in control rats, 44/165, 27%). All of the collaterals projected to somatotopically normal areas of cord. Dorsal horn shrinkage (21%), as estimated by planimetric measurements of Nissl and acetylcholinesterase-stained material, was only a partial explanation of this result. This idea was supported by the statistically significant increase (27%, P less than 0.05) in the absolute dorsoventral length of collaterals. The results show that the destruction of unmyelinated fibres during the early postnatal period by capsaicin induces HFA invasion into the area that C fibres normally occupy. This invasion suggests that the laminar termination sites for different primary afferent fibres are not altogether specified and that intact neonatal primary afferents have the capacity to sprout into denervated regions of spinal cord.