Effects of nitric oxide synthase inhibitors on the febrile response to muramyl dipeptide and lipopolysaccharide in rats

The effect of systemic iNOS inhibition during human endotoxemia on the development of tolerance to different TLR-stimuli

The phenomenon of repeated exposure to endotoxin resulting in diminished release of pro-inflammatory cytokines is called endotoxin tolerance, in which there is a putative role for nitric oxide (NO). We investigated the effect of selective inducible NO-synthase (iNOS) inhibition during experimental human endotoxemia on the development of tolerance to various Toll-like receptor (TLR) agonists ex vivo. Volunteers received 2 ng/kg Escherichia coli endotoxin in the absence ( n = 7) or presence ( n = 7) of the selective iNOS inhibitor aminoguanidine (bolus 5 mM followed by a continuous infusion of 1.5 mmol/h). At 0, 2 and 4 h, blood samples were drawn for ex vivo stimulation with different TLR agonists. Experimental endotoxemia did not induce tolerance to TLR-2 and TLR-7 stimulation. In TLR-3, TLR-4 and TLR-5 stimulated whole blood, pro- and anti-inflammatory cytokine release was attenuated at 4 h, indicating that endotoxin-induced tolerance is not confined to subsequent TLR-4 stimulation alone. Aminoguanidine-treated subjects also developed tolerance to TLR-4 stimulation. In contrast, tolerance to TLR-3 stimulation did not occur for IL-10, and tolerance in TLR-5 stimulated blood did not develop for both pro- and anti-inflammatory cytokines. The role of NO in the development of tolerance is different for the various TLRs stimulated and pro- and anti-inflammatory cytokines measured.

Stress- and lipopolysaccharide-induced c-fos expression and nNOS in hypothalamic neurons projecting to medullary raphe in rats: a triple immunofluorescent labeling study

Neurons in the rostral raphe pallidus (rRP) have been proposed to mediate experimental stress-induced tachycardia and fever in rats, and projections from the dorsomedial hypothalamus (DMH) may signal their activation in these settings. Thus, we examined c-fos expression evoked by air jet/restraint stress and restraint stress or by systemic administration of lipopolysaccharide (10 microg/kg and 100 microg/kg) as well as the distribution of the neuronal nitric oxide synthase (nNOS) in neurons retrogradely labeled from the raphe with cholera toxin B in key hypothalamic regions. Many neurons in the medial preoptic area and the dorsal area of the DMH were retrogradely labeled, and approximately half of those in the medial preoptic area and moderate numbers in the dorsal DMH were also positive for nNOS. Either stress paradigm or dose of lipopolysaccharide increased the number of c-fos-positive neurons and nNOS/c-fos double-labeled neurons in all regions examined. However, retrogradely labeled neurons positive for c-fos were increased only in the dorsal DMH and adjoining region in both stressed and lipopolysaccharide-treated groups, and triple-labeled neurons were found only in this area in rats subjected to either stress paradigm. Thus, hypothalamic neurons that project to the rRP and express c-fos in response to either experimental stress or systemic inflammation are found only in the dorsal DMH, and many of those activated by stress contain nNOS, suggesting that nitric oxide may play a role in signaling in this pathway.

Capsaicin and nitric oxide synthase inhibitor interact to evoke a hypothermic synergy

The present study investigated the effect of a drug combination of capsaicin and L-NAME on hypothermia in rats. Capsaicin administration (0.1, 0.25, 0.5, 1 and 2mg/kg, i.m.) caused a significant hypothermia. L-NAME (50mg/kg, i.p.), a nonspecific nitric oxide synthase (NOS) inhibitor, was ineffective. For combined administration, progressively increasing doses of capsaicin (0.1, 0.25, 0.5, 1 and 2mg/kg, i.p.) were given with a non-hypothermic dose of L-NAME (50mg/kg, i.p.). Experiments revealed that L-NAME (50mg/kg, i.p.) enhanced the hypothermic response to capsaicin (0.25, 0.5, 1, and 2mg/kg, i.m.). Comparison of the graded dose-effect curves for capsaicin alone and capsaicin plus L-NAME revealed a significant difference (P<0.05), thus indicating synergy for the drug interaction. To determine if L-NAME acted centrally, a fixed dose of L-NAME (1mg/rat, i.c.v.) was given with graded doses of capsaicin (0.25, 0.5, 1, and 2mg/kg, i.m.). L-NAME (1mg/rat, i.c.v.) only enhanced the hypothermia at a single dose of capsaicin (0.5mg/kg, i.m.). The super-additive hypothermia produced by the concurrent administration of capsaicin and L-NAME (50mg/kg, i.p.) is the first evidence of synergy for a drug combination of capsaicin and a NOS inhibitor. The synergy is apparent only when L-NAME is given systemically, thus indicating that the inhibition of peripheral NO production enhances the hypothermic response to capsaicin.

Thermoregulatory role of inducible nitric oxide synthase in lipopolysaccharide-induced hypothermia

We have tested the hypothesis that nitric oxide (NO) arising from inducible nitric oxide synthase (iNOS) plays a role in hypothermia during endotoxemia by regulating vasopressin (AVP) release. Wild-type (WT) and iNOS knockout mice (KO) were intraperitoneally injected with either saline or Escherichia coli lipopolysaccharide (LPS) 10.0 mg/kg in a final volume of 0.02 mL. Body temperature was measured continuously by biotelemetry during 24 h after injection. Three hours after LPS administration, we observed a significant drop in body temperature (hypothermic response) in WT mice, which remained until the seventh hour, returning then close to the basal level. In iNOS KO mice, we found a significant fall in body temperature after the fourth hour of LPS administration; however, the hypothermic response persisted until the end of the 24 h of the experiment. The pre-treatment with beta-mercapto-beta,beta-cyclopentamethylenepropionyl(1), O-Et-Tyr2, Val4, Arg8-Vasopressin, an AVP V1 receptor antagonist (10 microg/kg) administered intraperitoneally, abolished the persistent hypothermia induced by LPS in iNOS KO mice, suggesting the regulation of iNOS under the vasopressin release in this experimental model. In conclusion, our data suggest that the iNOS isoform plays a role in LPS-induced hypothermia, apparently through the regulation of AVP release.

Nitric oxide synthase mediates delta opioid receptor-induced hypothermia in rats

The role of nitric oxide (NO) production in delta opioid receptor-induced hypothermia has not been reported. The present study investigated the effect of nitric oxide synthase (NOS) inhibitors on the hypothermic effect of (+)-4-[(aR)-a-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-80), a nonpeptide delta opioid agonist. SNC-80 (35 mg/kg, i.p.) administered to rats caused a significant hypothermia. N-nitro-L-arginine methyl ester (L-NAME) (10, 25 and 50 mg/kg, i.p.), a NOS inhibitor, and 7-nitroindazole (7-NI) (5 and 10 mg/kg, i.p.), a neuronal NOS inhibitor, were ineffective. For combined administration, L-NAME (50 mg/kg, i.p.) or 7-NI (10 mg/kg, i.p.) attenuated SNC-80-evoked hypothermia. To determine the involvement of central NOS, L-NAME (0.25, 0.5 and 1 mg/rat) was administered i.c.v. 30 min prior to SNC-80 (35 mg/kg, i.p.). Experiments revealed that L-NAME (1 mg/rat, i.c.v.) attenuated SNC-80-induced hypothermia. The present data demonstrate that central NO production is necessary for delta opioid receptor-induced hypothermia.

Lipopolysaccharide-induced fever in Pekin ducks is mediated by prostaglandins and nitric oxide and modulated by adrenocortical hormones

Information on avian fever is limited, and, in particular, very little is known about the mediators and modulators of the febrile response in birds. Therefore, in this study, the possible mediatory roles of nitric oxide (NO) and prostaglandins (PGs), together with a potential modulatory role for adrenocortical hormones in the generation of fever was investigated in conscious Pekin ducks. Their body temperatures were continuously measured by abdominally implanted temperature-sensitive data loggers. The febrile response induced by intramuscular injection of LPS at a dose of 100 μg/kg was compared with and without inhibition of NO production by N-nitro-l-arginine methyl ester (l-NAME), inhibition of PG synthesis (by diclofenac), and elevation of circulating concentrations of dexamethasone and corticosterone (by exogenous administration). LPS administration induced a marked, monophasic fever with a rise in temperature of more than 1°C after 3–4 h. In the presence of l-NAME, diclofenac, and adrenocorticoids at doses that had no effect upon normal body temperature in afebrile ducks, there was a significant inhibition of the LPS-induced fever. In addition, during the febrile response, the blood concentration of corticosterone was significantly elevated (from a basal level of 73.6 ± 9.8 ng/ml to a peak level of 132.6 ± 16.5 ng/ml). The results strongly suggest that the synthesis of both NO and PGs is a vital step in the generation of fever in birds and that the magnitude of the response is subject to modulation by adrenocorticoids.

Thermoregulatory responses of rats to conventional preparations of lipopolysaccharide are caused by lipopolysaccharide per se— not by lipoprotein contaminants

LPS preparations cause a variety of body temperature (Tb) responses: monophasic fever, different phases of polyphasic fever, and hypothermia. Conventional (c) LPS preparations contain highly active lipoprotein contaminants (endotoxin proteins). Whereas LPS signals predominantly via the Toll-like receptor (TLR) 4, endotoxin proteins signal via TLR2. Several TLR2-dependent responses of immunocytes to cLPS in vitro are triggered by endotoxin proteins and not by LPS itself. We tested whether any Tb response to cLPS from Escherichia coli 055:B5 is triggered by non-TLR4-signaling contaminants. A decontaminated (d) LPS preparation (free of endotoxin proteins) was produced by subjecting cLPS to phenol-water reextraction. The presence of non-TLR4-signaling contaminants in cLPS (and their absence in dLPS) was confirmed by showing that cLPS (but not dLPS) induced IL-1β expression in the spleen and increased serum levels of TNF-α and IL-1β of C3H/HeJ mice; these mice bear a nonfunctional TLR4. Yet, both cLPS and dLPS caused cytokine responses in C3H/HeOuJ mice; these mice bear a fully functional TLR4. We then studied the Tb responses to cLPS and dLPS in Wistar rats preimplanted with jugular catheters. At a neutral ambient temperature (30°C), a low (0.1 μg/kg iv) dose of cLPS caused a monophasic fever, whereas a moderate (10 μg/kg iv) dose produced a polyphasic fever. In the cold (20°C), a high (500 μg/kg iv) dose of cLPS caused hypothermia. All Tb responses to dLPS were identical to those of cLPS. We conclude that all known Tb responses to LPS preparations are triggered by LPS per se and not by non-TLR4-signaling contaminants of such preparations.

Febrigenic signaling to the brain does not involve nitric oxide

1. The involvement of peripheral nitric oxide (NO) in febrigenic signaling to the brain has been proposed because peripherally administered NO synthase (NOS) inhibitors attenuate lipopolysaccharide (LPS)-induced fever in rodents. However, how the unstable molecule of NO can reach the brain to trigger fever is unclear. It is also unclear whether NOS inhibitors attenuate fever by blocking febrigenic signaling or, alternatively, by suppressing thermogenesis in brown fat. 2. Male Wistar rats were chronically implanted with jugular catheters; their colonic and tail skin temperatures (T(c) and T(sk)) were monitored. 3. Study 1 was designed to determine whether the relatively stable, physiologically relevant forms of NO, that is, S-nitrosoalbumin (SNA) and S-nitrosoglutathione (SNG), are pyrogenic and whether they enhance LPS fever. At a neutral ambient temperature (T(a)) of 31 degrees C, afebrile or LPS (1 microg kg(-1), i.v.)-treated rats were infused i.v. with SNA (0.34 or 4.1 micromol kg(-1); the controls received NaNO(2) and albumin) or SNG (10 or 60 micromol kg(-1); the controls received glutathione). T(c) of SNA- or SNG-treated rats never exceeded that of the controls. 4. In Study 2, we tested whether the known fever-attenuating effect of the NOS inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME) at a subneutral T(a) (when fever is brought about by thermogenesis) also occurs at a neutral T(a) (when fever is brought about by skin vasoconstriction). At a subneutral T(a) of 24 degrees C, L-NAME (2.5 mg kg(-1), i.v.) attenuated LPS (10 microg kg(-1), i.v.) fever, presumably by inhibiting thermogenesis. At 31 degrees C, L-NAME enhanced LPS fever by augmenting skin vasoconstriction (T(sk) fall). 5. In summary, both SNA and SNG had no pyrogenic effect of their own and failed to enhance LPS fever; peripheral L-NAME attenuated only fever brought about by increased thermogenesis. It is concluded that NO is uninvolved in febrigenic signaling to the brain.

L-NAME (N omega-nitro-L-arginine methyl ester), a nitric-oxide synthase inhibitor, and WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenyl-carbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one], a cannabinoid agonist, interact to evoke synergistic hypothermia

Cannabinoids evoke profound hypothermia in rats by activating central CB(1) receptors. Nitric oxide (NO), a prominent second messenger in central and peripheral neurons, also plays a crucial role in thermoregulation, with previous studies suggesting pyretic and antipyretic functions. Dense nitric-oxide synthase (NOS) staining and CB(1) receptor immunoreactivity have been detected in regions of the hypothalamus that regulate body temperature, suggesting that intimate NO-cannabinoid associations may exist in the central nervous system. The present study investigated the effect of N(omega)-nitro-L-arginine methyl ester (L-NAME), a NO synthase inhibitor, on the hypothermic response to WIN 55212-2 [4,5-dihydro-2-methyl-4(4-morpholinylmethyl)-1-(1-naphthalenylcarbonyl)-6H-pyrrolo[3,2,1ij]quinolin-6-one], a selective cannabinoid agonist, in rats. WIN 55212-2 (1-5 mg/kg, i.m.) produced dose-dependent hypothermia that peaked 45 to 90 min post-injection. L-NAME (10-100 mg/kg, i.m.) by itself did not significantly alter body temperature. However, a nonhypothermic dose of L-NAME (50 mg/kg) potentiated the hypothermia caused by WIN 55212-2 (0.5-5 mg/kg). The augmentation was strongly synergistic, indicated by a 2.5-fold increase in the relative potency of WIN 55212-2. The inactive enantiomer of WIN 55212-2, WIN 55212-3 [S-(-)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazinyl]-(1-napthanlenyl) methanone mesylate] (5 mg/kg, i.m.), did not produce hypothermia in the absence or presence of L-NAME (50 mg/kg), confirming that cannabinoid receptors mediated the synergy. The present data are the first evidence that drug combinations of NOS blockers and cannabinoid agonists produce synergistic hypothermia. Thus, NO and cannabinoid systems may interact to induce superadditive hypothermia.

Genetic Models in Applied Physiology. Differential role of nitric oxide synthase isoforms in fever of different etiologies: studies using Nos gene-deficient mice

Male C57BL/6J mice deficient in nitric oxide synthase (NOS) genes (knockout) and control (wild-type) mice were implanted intra-abdominally with battery-operated miniature biotelemeters (model VMFH MiniMitter, Sunriver, OR) to monitor changes in body temperature. Intravenous injection of lipopolysaccharide (LPS; 50 microg/kg) was used to trigger fever in response to systemic inflammation in mice. To induce a febrile response to localized inflammation, the mice were injected subcutaneously with pure turpentine oil (30 microl/animal) into the left hindlimb. Oral administration (gavage) of N(G)-monomethyl-l-arginine (l-NMMA) for 3 days (80 mg. kg(-1). day(-1) in corn oil) before injection of pyrogens was used to inhibit all three NOSs (N(G)-monomethyl-d-arginine acetate salt and corn oil were used as control). In normal male C57BL/6J mice, l-NMMA inhibited the LPS-induced fever by approximately 60%, whereas it augmented fever by approximately 65% in mice injected with turpentine. Challenging the respective NOS knockout mice with LPS and with l-NMMA revealed that inducible NOS and neuronal NOS isoforms are responsible for the induction of fever to LPS, whereas endothelial NOS (eNOS) is not involved. In contrast, none of the NOS isoforms appeared to trigger fever to turpentine. Inhibition of eNOS, however, exacerbates fever in mice treated with l-NMMA and turpentine, indicating that eNOS participates in the antipyretic mechanism. These data support the hypothesis that nitric oxide is a regulator of fever. Its action differs, however, depending on the pyrogen used and the NOS isoform.