A quantitative study of the effects of neonatal capsaicin treatment and of subsequent peripheral nerve transection in the adult rat.

The effects of unilateral intercostal nerve transection have been studied in adult rats neonatally treated with capsaicin. Two rats were treated with capsaicin-free vehicle and served as controls. It is shown that capsaicin treatment leads to a dorsal root ganglion cell loss of about 43%, and about a 72% loss of unmyelinated dorsal root fibers. Myelinated dorsal root fibers remain unaffected. The cell size spectrum shows a 27% increase in median cell area in the capsaicin-treated rats, indicating a loss of preferentially smaller cells. A subsequent peripheral nerve transection resulted in a further cell loss of about 30% with no major change in cell size spectrum. The peripheral nerve transection also somewhat further reduces both the number of unmyelinated and myelinated dorsal root axons. The data indicate that neurons which die after a peripheral nerve transection belong to both the groups of cells, which are capsaicin-sensitive and capsaicin-resistant.