Capsaicin treatment in adult Wistar-Kyoto and spontaneously hypertensive rats: neurochemical effects in the spinal cord

Abstract

The effects of s.c. capsaicin treatment on the thoracic spinal cord contents of immunoreactive substance P (ISP), norepinephrine (NE) and serotonin (5-HT) were investigated in adult male Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Capsaicin administered s.c. significantly reduced the concentration of ISP in the thoracic spinal cords from rats of both the WKY and SHR strains. The s.c. administration of capsaicin significantly increased the NE contents of the thoracic spinal cords in rats of both the WKY and SHR strains, and both vehicle- and capsaicin-treated SHR animals exhibited significantly greater thoracic spinal cord NE concentrations than did rats from the corresponding WKY treatment groups. In contrast, s.c. capsaicin treatment significantly increased the concentration of 5-HT only in the thoracic spinal cords of SHR subjects. These results strongly suggest that the neurochemical effects of s.c. capsaicin administration on the spinal cord were not specific to ISP-containing neurons since significant changes in the concentrations of NE and 5-HT were also produced by capsaicin treatment. These capsaicin induced changes in the concentrations of putative neurotransmitter substances in the spinal cord are related to previously reported alterations in nociception and cardiovascular regulation produced by s.c. capsaicin administration. Physiological and pharmacological mechanisms possibly relating the neurochemical changes produced by capsaicin to the behavioral and cardiovascular effects of the drug previously reported are advanced and discussed.