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Abstract
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 µg/kg)-induced
fever was markedly higher (∼1.0°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.