Capsaicin-induced attenuation of sensory nerve function in mice

Administration of multiple subcutaneous injections of capsaicin to anaesthetised mice is an effective means of impairing sensory nerve function [27–30]. On day one, mice were anaesthetized (130 mg/kg ketamine and 13 mg/kg xylazine, i.p.) and administered the bronchodilators terbutaline (0.1 mg/kg, i.p.) and theophylline (10 mg/kg, s.c.). Twenty mins later, mice were injected subcutaneously with capsaicin (25 mg/kg, s.c.) in the neck region between the shoulder blades. On days two and three, the procedure was repeated with an increased dose of capsaicin (50 mg/kg, s.c) (capsaicin-pretreated). Mice were left for 10 days before being intranasally inoculated with IAV ± SLIGRL-amide, as described above. Control mice received injections of vehicle (1:1:8 vol/vol/vol of ethanol/Tween 80/saline) (sham-pretreated). The protocol is shown in Fig. 3a. The effectiveness of the capsaicin desensitization protocol was determined in isometric tension recording studies by examining the capsaicin-induced relaxation responses in mouse isolated tracheal smooth muscle preparations.

Effect of sensory nerve dysfunction caused by in vivo capsaicin treatment on the anti-IAV activity of SLIGRL-amide in mice. a Protocol for capsaicin-induced dysfunction of sensory nerves. On 3 consecutive days, groups of mice were anaesthetised and injected subcutaneously with capsaicin or vehicle (Sham). Ten days later, mice were anaesthetised and intranasally inoculated with IAV in the presence or absence of SLIGRL-amide. A further four days later, BAL was performed on each euthanased mouse and the total numbers of leukocytes recovered was determined. b & c Isometric tension recordings performed on day 13 show that capsaicin pretreatment (using the protocol in a) abolishes the characteristic relaxant response to capsaicin (10 μM), indicative of sensory nerve dysfunction. d & e Total BAL leukocyte numbers recovered from IAV-inoculated mice that had been sham-pretreated (d) or pretreated with capsaicin to induce sensory nerve dysfunction. Shown are mean ± s.e.mean of 3–7 mice per group. *, indicates P <0.05, compared to respective IAV + saline control (one-way ANOVA)