Capsaicin in adult frogs: effects on nociceptive responses to cutaneous stimuli and on nervous tissue concentrations of immunoreactive substance P, somatostatin and cholecystokinin.

Adult frogs (Rana esculenta) were given subcutaneous injections of 10, 20, 30, 50 and 100 mg/kg capsaicin in sequential order over 5 days, or the vehicle only. The nociceptive thresholds to electrical, thermal and chemical stimuli were measured before, and 1, 5 and 24 h after each injection. Capsaicin was followed by a dose-related reduction of nociceptive responses to all stimuli, but these effects lasted for only 1-5 h after the given injection. Water/acetic extracts of undivided brains and spinal cords were prepared at the corresponding time periods for the radioimmunoassay of peptides. Spinal cord concentrations of immunoreactive substance P were essentially unaffected by capsaicin, while those of immunoreactive somatostatin were significantly increased after the second for fourth injections (20, 30 and 50 mg/kg) of capsaicin. Brain extracts showed an increase of somatostatin and substance P concentrations after the dose of 50 mg/kg. In an additional experiment, immunoreactive substance P, somatostatin and cholecystokinin were measured in tissue samples taken at 2 and 10 min, and 1, 5 and 24 h after a single dose of either 50 mg/kg capsaicin or the vehicle. The only significant effect of capsaicin was an increase of immunoreactive somatostatin concentration in brain homogenates at 5 h, while the vehicle in itself elicited major variations of all three peptides in spinal cord and/or brain. These results indicate that capsaicin reduces the nociceptive responses to cutaneous stimuli in adult frogs.(ABSTRACT TRUNCATED AT 250 WORDS)