Quantitative electron-microscopic analyses of pulpal nerve fibres in the mouse lower incisor after neonatal capsaicin treatment.

A single dose of capsaicin (50 mg/kg) was injected subcutaneously into four mice on day 2 of life; four untreated mice were used as controls. Six months later, a drop of 30 microM capsaicin was instilled on to the cornea of all the mice and the number of times the eyes were wiped was counted to assess the effect of capsaicin on trigeminal sensory neurones. Ultrathin cross-sections were made of the apical pulp of the incisors on both sides of control (n = 8) and capsaicin-treated animals (n = 8). Electron micrographs of pulp nerves were taken and enlarged to a final magnification of x34,000. The numbers of unmyelinated axons in the pulps of all 16 incisors and of unmyelinated axons per Schwann cell in the pulps of four incisors each from the control and capsaicin-treated groups were counted. The short diameters of unmyelinated axons were measured with a computer-operated image analyser. The number of eye wipings was eight-fold less in the capsaicin-treated than in the normal group. This finding clearly indicated that capsaicin irreversibly affected the chemogenic nociceptive trigeminal neurones. The mean number of unmyelinated axons was 345 in controls and 217 (37.1% reduction) in capsaicin-treated animals. The number of unmyelinated axons of less than 0.6 microns dia was 41.5% less in capsaicin-treated mice than in controls. Thus, fine unmyelinated axons in the mouse incisor pulp are capsaicin sensitive, and they are assumed to be nociceptive fibres conveying pain stimuli from the tooth. Capsaicin affected Schwann cells, even those with few unmyelinated axons.(ABSTRACT TRUNCATED AT 250 WORDS)