Administration of a single dose of capsaicin (50 mg/kg; sc) to two-day-old rats resulted in a loss of between 85 to 95% of the small diameter unmyelinated primary afferent fibres from lumbar dorsal roots. The numbers of myelinated fibres in the dorsal roots were not significantly affected. The substance P and somatostatin contents of the dorsal roots were decreased by 85 to 95%. The extent of depletion of these peptides from the dorsal horn of the spinal cord was somewhat less; substance P was decreased by 55% and somatostatin by 19%. Histochemical studies showed that fluoride-resistant acid phosphatase was virtually absent from the dorsal horn of capsaicin-treated animals. There was also a conspicuous absence of substance P-positive fibres from layers I and II of the dorsal horn and of fibres traversing these layers to deeper regions.

Colchicine pretreatment revealed the presence of substance P-containing perikarya within layers III–IV. The plexus of substance P-positive elements which normally surrounded these neurons was absent after capsaicin treatment, suggesting that they receive a substance P-containing primary afferent innervation.

The neurotoxicity of capsaicin after a single dose of 50 mg/kg in neonatal animals appears to be restricted largely to unmyelinated primary afferent fibres. The present results suggest that both substance P and somatostatin are contained in primary afferent sensory neurons which give rise to unmyelinated fibres. It can be further concluded that about 50% of substance P and only about 20% of the somatostatin in the dorsal horn appears to be of primary afferent origin.