Pharmacological evidence that nitric oxide can act as an endogenous antipyretic factor in endotoxin-induced fever in rabbits

The nitric oxide pathway is an important modulator of stress-induced fever in rats

Psychological stress evokes a number of physiological responses, including a rise in body temperature (T(b)), which has been suggested to be the result of an elevation in the thermoregulatory set point, i.e., a fever. This response seems to share similar mechanisms with infectious fever. A growing number of studies have provided evidence that nitric oxide (NO) has a modulatory role in infectious fever, but no report exists about the participation of NO in stress fever. Thus, the present study aimed to verify the hypothesis that NO modulates stress fever by using restraint stress as a model. To this end, we tested the effects of the non-specific NO synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) or its inactive enantiomer N(G)-nitro-D-arginine methyl ester (D-NAME) on colonic T(b) of restrained or unrestrained rats. A rapid increase in T(b) was observed when animals were submitted to restraint. Intravenous (i.v.) injection of L-NAME at a dose (10 mg/kg) that caused no change in T(b) when administered alone significantly attenuated the elevation in T(b) elicited by stress, indicating that the NO pathway may mediate stress fever. Moreover, intracerebroventricular (i.c.v.) L-NAME (250 microg/microl) caused a rise in T(b) of euthermic animals and enhanced stress fever, supporting that NO in the central nervous system (CNS) leads to a reduction in T(b) and, therefore, this is unlikely to be the site where NO may mediate stress fever. Taken together, these data indicate that the NO pathway plays an important role in modulating restraint stress-induced fever in rats.

Role of neuronal nitric oxide synthase in hypoxia-induced anapyrexia in rats

Anapyrexia (a regulated decrease in body temperature) is a response to hypoxia that occurs in organisms ranging from protozoans to mammals, but very little is known about the mechanisms involved. Recently, it has been shown that the NO pathway plays a major role in hypoxia-induced anapyrexia. However, very little is known about which of the three different nitric oxide synthase isoforms (neuronal, endothelial, or inducible) is involved. The present study was designed to test the hypothesis that neuronal nitric oxide synthase (nNOS) plays a role in hypoxia-induced anapyrexia. Body core temperature (T(c)) of awake, unrestrained rats was measured continuously using biotelemetry. Rats were submitted to hypoxia, 7-nitroindazole (7-NI; a selective nNOS inhibitor) injection, or both treatments together. Control animals received vehicle injections of the same volume. We observed a significant (P < 0.05) reduction in T(c) of approximately 2.8 degrees C after hypoxia (7% inspired O(2)), whereas intraperitoneal injection of 7-NI at 25 mg/kg caused no significant change in T(c). 7-NI at 30 mg/kg elicited a reduction in T(c) and was abandoned in further experiments. When the two treatments were combined (25 mg/kg of 7-NI and 7% inspired O(2)), we observed a significant attenuation of hypoxia-induced anapyrexia. The data indicate that nNOS plays a role in hypoxia-induced anapyrexia.

Role of nitric oxide in hypoxia inhibition of fever

Hypoxia causes a regulated decrease in body temperature (T(b)), and nitric oxide (NO) is now known to participate in hypoxia-induced hypothermia. Hypoxia also inhibits lipopolysaccharide (LPS)-induced fever. We tested the hypothesis that NO may participate in the hypoxia inhibition of fever. The rectal temperature of awake, unrestrained rats was measured before and after injection of LPS, with or without concomitant exposure to hypoxia, in an experimental group treated with N(omega)-nitro-L-arginine (L-NNA) for 4 consecutive days before the experiment and in a saline-treated group (control). L-NNA is a nonspecific NO synthase inhibitor that blocks NO production. LPS caused a dose-dependent typical biphasic rise in T(b) that was completely prevented by hypoxia (7% inspired oxygen). L-NNA caused a significant drop in T(b) during days 2-4 of treatment. When LPS was injected into L-NNA-treated rats, inhibition of fever was observed. Moreover, the effect of hypoxia during fever was significantly reduced. The data indicate that the NO pathway plays a role in hypoxia inhibition of fever.

Dose-dependent attenuation of lipopolysaccharide-fever by inhibitors of inducible nitric oxide-synthase in guinea pigs

Different doses of aminoguanidine or S-methylisothiourea, both predominantly inhibitors of the inducible form of nitric oxide (NO)-synthase, were injected into the arterial circulation of guinea pigs alone or along with 10 microg/kg bacterial lipopolysaccharide. Doses of 10 mg/kg, 50 mg/kg or 250 mg/kg aminoguanidine per se had no influence on abdominal temperature of guinea pigs. Only the highest dose of aminoguanidine (250 mg/kg) completely suppressed the first phase of the biphasic febrile response to lipopolysaccharide-injections. Lipopolysaccharide-fever was not modulated by administration of 10 mg/kg or 50 mg/kg aminoguanidine, when compared to fever in response to injections of lipopolysaccharide along with solvent. Doses of 10 mg/kg or 50 mg/kg S-methylisothiourea did not alter abdominal temperature while a dose of 250 mg/kg S-methylisothiourea had a lethal effect in guinea pigs. The febrile response to lipopolysaccharide was unimpaired by administration of 10 mg/kg S-methylisothiourea, while a dose of 50 mg/kg again attenuated fever predominantly by a suppression of the first fever phase. None of the applied doses of aminoguanidine or S-methylisothiourea resulted in a significant attenuation of the lipopolysaccharide-induced circulating cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-6. The drugs themselves, without lipopolysaccharide-injections, did not enhance or reduce circulating levels of the investigated cytokines. The results indicate that endogenous NO may participate in the induction of lipopolysaccharide-fever and that fever suppression by systemic administration of NO-synthase inhibitors occurs independently from the lipopolysaccharide-induced circulating cytokines.

Tolerance to lipopolysaccharide is related to the nitric oxide pathway

Repeated administration of lipopolysaccharide (LPS) induces a refractory state to its usual pyrogenic effects which is called endotoxin tolerance. We tested the hypothesis that nitric oxide (NO) participates in the endotoxin tolerance. Single injection of LPS resulted in an elevation in body temperature (Tb), whereas a significant reduction of the thermoregulatory response to LPS was observed to repeated administration of LPS (administered at 48 h intervals). Intracerebroventricular (i.c.v.) injection of L-NAME (a non-selective NO inhibitor of nitric oxide synthesis) markedly enhanced the febrile response to LPS in tolerant rats. The data suggest that NO pathway in the central nervous system plays a role in endotoxin tolerance.

Nitric Oxide and Body Temperature Control

Pharmacological studies of thermoregulatory effector and neuronal responses indicate that nitric oxide (NO) may have differential roles in the control of body temperature and during fever. Histochemical analysis of site-specific changes in NO synthase activity in defined states of thermal stimulation appears a promising approach to unravel the underlying hypothalamic neuronal cytoarchitecture.

Nitric oxide as a peripheral and central mediator in temperature regulation

In animals including humans nitric oxide (NO) serves as a biological messenger both peripherally at neuroeffector junctions and in the central nervous system where it modulates neuronal activity. Evidence for the involvement of NO in homeostatic control is accumulating also for temperature regulation in homeotherms. In the periphery an auxiliary role in the vasomotor control of convective heat transfer to heat dissipating surfaces and modulation of thermoregulatory heat generation, especially in brown adipose tissue as the site of nonshivering thermogenesis, are discussed as NO actions. At the central level a thermolytic role of NO in thermoregulation as well as in fever is assumed, however, experimental data opposing this view suggest that topical specificity may be important. At the level of single neurons, the observed interrelationships between thermosensitivity and responsiveness to NO are still not consistent enough to reconcile these data with the effects of NO-donors and inhibitors of NO-synthase on temperature regulation.

Effect of nitric oxide synthase inhibition on hypercapnia-induced hypothermia and hyperventilation

Hypercapnia elicits hypothermia in a number of vertebrates, but the mechanisms involved are not well understood. In the present study, we assessed the participation of the nitric oxide (NO) pathway in hypercapnia-induced hypothermia and hyperventilation by means of NO synthase inhibition by using Nomega-nitro-L-arginine (L-NNA). Measurements of ventilation, body temperature, and oxygen consumption were performed in awake unrestrained rats before and after L-NNA injection (intraperitoneally) and L-NNA injection followed by hypercapnia (5% CO2). Control animals received saline injections. L-NNA altered the breathing pattern during the control situation but not during hypercapnia. A significant (P < 0.05) drop in body temperature was measured after both L-NNA (40 mg/kg) and 5% inspired CO2, with a drop in oxygen consumption in the first situation but not in the second. Hypercapnia had no effect on L-NNA-induced hypothermia. The ventilatory response to hypercapnia was not changed by L-NNA, even though L-NNA caused a drop in body temperature. The present data indicate that the two responses elicited by hypercapnia, i.e., hyperventilation and hypothermia, do not share NO as a common mediator. However, the L-arginine-NO pathway participates, although in an unrelated way, in respiratory function and thermoregulation.

Central application of a nitric oxide donor activates heat defense in the rabbit

In chronically instrumented, conscious rabbits at moderately warm ambient thermal conditions, infusion of the NO-donor SIN-1 into the anterio-ventral 3rd cerebral ventricle (1-2 microg/min per kg BW, 2-4 microl/min, 30 min) initiated a co-ordinated activation of autonomic heat loss mechanisms, as indicated by the rise in ear skin temperature and by increases in panting frequency and respiratory evaporative water loss, while oxygen consumption decreased slightly. The heat loss responses were similar to those attributed to NO in studies employing systemic application of NO-donors. Different from NO acting peripherally, which causes arterial hypotension and tachycardia, centrally acting NO induced arterial hypertension and bradycardia. The observation of the same heat loss responses despite opposing circulatory actions suggests that NO is specifically involved in thermoregulation as a central activator of heat defense mechanisms.

Physiological and pathophysiological roles of nitric oxide in the central nervous system

Nitric oxide (NO) is produced by three distinct isoforms of nitric oxide synthases in the central nervous system. Here, the roles of nitric oxide in the central nervous system are reviewed under physiological and pathophysiological conditions. Under physiological conditions, NO plays a role in the regulation of cerebral blood flow and autoregulation, blood flow-metabolism coupling, neurotransmission, memory formation, modulation of neuroendocrine functions, and behavioral activity. Impairment of the NO-mediated cerebrovascular vasodilatation occurs during ischemia-reperfusion, diabetes, hypertension, subararchnoid hemorrhage, and various forms of shock. Enhancement of NO production in the brain occurs during stoke, seizures, and acute and chronic inflammatory and neurodegenerative disorders. The alterations of the expression of the various isoforms of nitric oxide synthases under the above conditions are discussed. Moreover, the molecular mechanisms of NO and peroxynitrite induced cellular injury are delineated. Finally, the current strategies available for selective pharmacological manipulation of individual nitric oxide synthase isoforms are discussed.