Dihydrocapsaicin-induced hypothermia and substance P depletion

Administration of dihydrocapsaicin to rats resulted in a dose-dependent (0.5-10 mg/kg s.c.) hypothermia. Dihydrocapsaicin was approximately 65% more effective in producing hypothermia than capsaicin. Desensitization and cross-tolerance occurred to the hypothermic effects of both capsaicin and dihydrocapsaicin. Repeated administration of either dihydrocapsaicin or capsaicin resulted in chemogenic antinociception but not marked thermal antinociception. In addition, repeated administration of dihydrocapsaicin, like capsaicin, resulted in depletion of substance P from dorsal root ganglia and dorsal spinal cord but not from the hypothalamus, corpus striatum or ventral spinal cord. These data indicate that dihydrocapsaicin, or radiolabelled dihydrocapsaicin, may be a useful tool for investigating the mechanisms by which capsaicin alters thermoregulation and primary afferent neuron function.