Role of capsaicin-sensitive afferents in fever and cytokine responses during systemic and local inflammation in rats.

OBJECTIVE: Peripheral afferents play an important role in fever. In the present study, we investigated the role of capsaicin-sensitive afferents in fever and cytokine responses during systemic (induced by intraperitoneal lipopolysaccharide, LPS) and local (induced by injection of Freund's incomplete adjuvant, FIA, into the paw) inflammation. METHODS: Fevers in rats (8-10 weeks of age) whose capsaicin-sensitive afferents were depleted by neonatal capsaicin (50 mg/kg) treatment were compared to those of rats treated as neonates with vehicle. To investigate a possible involvement of cytokines, plasma levels of interleukin-6 (IL-6) and tumor necrosis factor (TNF) were measured during LPS- and FIA-induced fever in rats after capsaicin-induced desensitization. Body temperature was measured by biotelemetry. IL-6 and TNF bioactivities in plasma were determined using bioassays. RESULTS: The initial but not the late phase of LPS (50 microg/kg)-induced fever was markedly higher (approximately 1.0 degree C) in rats whose capsaicin-sensitive neurons were destroyed by neonatal capsaicin treatment. Capsaicin-induced desensitization also resulted in significantly higher plasma levels of IL-6 and TNF 1 but not 4 h after LPS challenge. In contrast, the day after injection with FIA (0.1 ml), rats treated with capsaicin had significantly lower body temperatures compared with vehicle-treated animals. No differences were found in plasma levels of IL-6 and TNF between capsaicin- and vehicle-treated animals in response to FIA. CONCLUSIONS: These data indicate that the role of capsaicin-sensitive afferents in fever depends on the type of inflammatory response. During systemic inflammation, capsaicin-sensitive afferents may be involved in modulating fever by regulating the levels of pyrogenic cytokines. During local inflammation, the late phase of fever is partially mediated via capsaicin-sensitive afferents.