The capsaicin test in mice for evaluating tachykinin antagonists in the spinal cord

A capsaicin test involving peripheral nociception, which produces behaviour similar to that elicited by formalin, is described in mice. Capsaicin was injected subcutaneously (s.c.) into the dorsal surface of a hindpaw and the time the animals spent licking the paw was recorded. Doses of capsaicin of 6.25–1600 ng induced nociception, during a period of 5 min, starting immediately after injection and disappearing completely at 10 min. Intrathecally (i.t.) administered [ d-Arg 1, d-Trp 7,9, Leu 11]substance P (spantide), a tachykinin antagonist and [ d-Phe 7, d-His 9]substance P (6–11), a selective antagonist of substance P (SP), inhibited the capsaicin-induced behaviour, in a dose-dependent manner. This licking behaviour was also inhibited by intrathecal administration of SP antiserum but not by somatostatin (SOM) antiserum. Intrathecal pretreatment with capsaicin resulted in a marked reduction of the licking response, following subcutaneous injection of capsaicin into the paw. Capsaicin-induced licking was not affected by intrathecal administration of cyclo[7-aminoheptanoyl-Phe- d-Trp-Lys-(OBz)-Thr], a SOM antagonist and by intrathecal pretreatment with cysteamine, a SOM depletor. This nociceptive test may allow discrimination between SP- and SOM-mediated responses in the spinal cord of the mouse.