Interferon-alpha (IFN-alpha), a cytokine acting as an endogenous pyrog
en and a putative activator of the opioid system, binds to opiate rece
ptors in vitro. The mu opioid receptor antagonist, naloxone hydrochlor
ide (NLX), attenuates IFN-alpha-induced increases in the firing rate o
f cold-sensitive neurons within thermosensitive areas of the brain. Th
e influence of NLX on fevers induced by central endogenous pyrogens wa
s investigated in rats. Subcutaneous (SQ) injection of NLX (1 mg/kg) w
as made 30 min prior to intracerebroventricular (ICV) injection of IFN
-alpha 2(b) (7900 IU). Alternatively, NLX (10 or 80 mu g) was microinj
ected ICV 30 min prior to administration of IFN-alpha 2(b). Administer
ed SQ, NLX attenuated the febrile response to IFN-alpha 2(b). In contr
ast, central (ICV) NLX did not attenuate fevers induced by IFN-alpha 2
(b). Animals previously exposed to both IFN-alpha 2(b) and NLX (SQ or
ICV) subsequently lost their sensitivity to this cytokine, and also sh
owed diminished reactivity to human recombinant interleukin-1 beta (hr
IL-1 beta; 10 ng) and prostaglandin E(2) (PGE(2); 250 ng). These resul
ts suggest that systemic and central elements of the opioid system may
play differential roles in temperature regulation. Previous administr
ation of NLX and IFN-alpha 2(b) may alter the sensitivity of the CNS t
o subsequent injections of different pyrogens. (C) 1997 Elsevier Scien
ce Ltd.