Involvement of Interleukin-1β, Nerve Growth Factor, and Prostaglandin-E2in the Hyperalgesia Induced by Intraplantar Injections of Low Doses of Thymulin

Abstract

The effect of various doses of intraplantar thymulin injection, on nociceptive thresholds, in the hind paw of rats was assessed using different pain tests. As little as 0.5 ng thymulin resulted in localized mechanical hyperalgesia as assessed by the paw pressure test and thermal hyperalgesia as assessed by the paw immersion, hot plate, and tail flick tests. The highest dose of thymulin (10 ng) reduced both paw pressure and paw immersion latencies in the noninjected paw also. Thymulin (5 ng) also resulted in significant elevation in the levels of interleukin-1β (IL-1β) and nerve growth factor (NGF) levels in the injected paw. Both dexamethasone and indomethacin reversed thymulin-induced hyperalgesia. Also interleukin-1 receptor antagonist (IL-1ra) and a polyclonal anti-NGF antiserum significantly reduced thymulin-induced hyperalgesia. On the other hand, the tripeptide lys-D-pro-val (known to antagonize IL-1β and PGE2induced hyperalgesia) reversed the hyperalgesia due to thymulin. In conclusion, thymulin induces localized hyperalgesia which is mediated by PGE2-dependent mechanisms and this pathway could be either partially dependent on or totally independent of IL-1β mechanisms.