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

Disruption of Physiological Rhythms Persist Following Cessation of Cigarette Smoke Exposure in Mice

Background

Physiological rhythms in mammals are essential for maintaining health, whereas disruptions may cause or exacerbate disease pathogenesis. As such, our objective was to characterize how cigarette smoke exposure affects physiological rhythms of otherwise healthy mice using telemetry and cosinor analysis.

Methods

Female BALB/c mice were implanted with telemetry devices to measure body temperature, heart rate, systolic blood pressure (SBP), and activity. Following baseline measurements, mice were exposed to cigarette smoke for approximately 50 min twice daily during weekdays over 24 weeks. Physiological parameters were recorded after 1, 4, 8, and 24 weeks of exposure or after 4 weeks cessation following 4 weeks of cigarette smoke exposure.

Results

Acute cigarette smoke exposure resulted in anapyrexia, and bradycardia, with divergent effects on SBP. Long term, cigarette smoke exposure disrupted physiological rhythms after just 1 week, which persisted across 24 weeks of exposure (as shown by mixed effects on mesor, amplitude, acrophase, and goodness-of-fit using cosinor analysis). Four weeks of cessation was insufficient to allow full recovery of rhythms.

Conclusion

Our characterization of the pathophysiology of cigarette smoke exposure on physiological rhythms of mice suggests that rhythm disruption may precede and contribute to disease pathogenesis. These findings provide a clear rationale and guide for the future use of chronotherapeutics.

Moderate and severe hypoxia elicit divergent effects on cardiovascular function and physiological rhythms

KEY POINTS

In the present study, we provide evidence for divergent physiological responses to moderate compared to severe hypoxia, addressing an important knowledge gap related to severity, duration and after-effects of hypoxia encountered in cardiopulmonary situations. The physiological responses to moderate and severe hypoxia were not proportional, linear or concurrent with the time-of-day. Hypoxia elicited severity-dependent physiological responses that either persisted or fluctuated throughout normoxic recovery. The physiological basis for these distinct cardiovascular responses implicates a shift in the sympathovagal set point and probably not molecular changes at the artery resulting from hypoxic stress.

ABSTRACT

Hypoxia is both a consequence and cause of many acute and chronic diseases. Severe hypoxia causes hypertension with cardiovascular sequelae; however, the rare studies using moderate severities of hypoxia indicate that it can be beneficial, suggesting that hypoxia may not always be detrimental. Comparisons between studies are difficult because of the varied classifications of hypoxic severities, methods of delivery and use of anaesthetics. Thus, to investigate the long-term effects of moderate hypoxia on cardiovascular health, radiotelemetry was used to obtain in vivo physiological measurements in unanaesthetized mice during 24 h of either moderate (FIO2=0.15) or severe (FIO2=0.09) hypoxia, followed by 72 h of normoxic recovery. Systolic blood pressure was decreased during recovery following moderate hypoxia but increased following severe hypoxia. Moderate and severe hypoxia increased haeme oxygenase-1 expression during recovery, suggesting parity in hypoxic stress at the level of the artery. Severe but not moderate hypoxia increased the low/high frequency ratio of heart rate variability 72 h post-hypoxia, indicating a shift in sympathovagal balance. Moderate hypoxia dampened the amplitude of circadian rhythm, whereas severe disrupted rhythm during the entire insult, with perturbations persisting throughout normoxic recovery. Thus, hypoxic severity differentially regulates circadian blood pressure.

Involvement of endogenous hydrogen sulfide (H2S) in the rostral ventrolateral medulla (RVLM) in hypoxia-induced hypothermia

Hypoxia evokes a regulated decrease in deep body temperature (Tb). Hydrogen sulfide (H2S), a signaling molecule that belongs to the gasotransmitter family, has been demonstrated to participate in several brain-mediated responses. Rostral ventrolateral medulla (RVLM) is a brainstem region involved in thermoregulation. Recently, it has been shown that exogenous H2S modulates RVLM activity. In the present study, we investigated whether endogenously produced H2S in the RVLM plays a role in the control of hypoxia-induced hypothermia. Tb was measured before and after bilateral microinjection of aminooxyacetate (AOA, 0.2, 1 and 2 pmol/100 nl, a cystathionine β-synthase, CBS, inhibitor) or vehicle into the RVLM followed by a 60-min normoxia (21% inspired O2) or hypoxia (7% inspired O2) exposure. Microinjection of AOA or vehicle did not change Tb during normoxia. Exposure to hypoxia evoked a typical decrease in Tb. Microinjection of AOA (2 pmol) into the RVLM followed by hypoxia significantly attenuated the decrease in Tb. Thus, endogenous H2S in the RVLM seems to play no role in the maintenance of basal Tb, whereas during hypoxia this gas plays a cryogenic role. Moreover, RVLM homogenates of rats exposed to hypoxia exhibited a decreased rate of H2S production. Our data are consistent with the notion that during hypoxia H2S synthesis is diminished in the RVLM facilitating hypothermia.

Hydrogen sulfide as a cryogenic mediator of hypoxia-induced anapyrexia

Hypoxia causes a regulated decrease in body temperature (Tb), a response that has been aptly called anapyrexia, but the mechanisms involved are not completely understood. The roles played by nitric oxide (NO) and other neurotransmitters have been documented during hypoxia-induced anapyrexia, but no information exists with respect to hydrogen sulfide (H(2)S), a gaseous molecule endogenously produced by cystathionine β-synthase (CBS). We tested the hypothesis that H(2)S production is enhanced during hypoxia and that the gas acts in the anteroventral preoptic region (AVPO; the most important thermosensitive and thermointegrative region of the CNS) modulating hypoxia-induced anapyrexia. Thus, we assessed CBS and nitric oxide synthase (NOS) activities [by means of H(2)S and nitrite/nitrate (NO(x)) production, respectively] as well as cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) levels in the anteroventral third ventricle region (AV3V; where the AVPO is located) during normoxia and hypoxia. Furthermore, we evaluated the effects of pharmacological modifiers of the H(2)S pathway given i.c.v. or intra-AVPO. I.c.v. or intra-AVPO microinjection of CBS inhibitor caused no change in Tb under normoxia but significantly attenuated hypoxia-induced anapyrexia. During hypoxia there were concurrent increases in H(2)S production, which could be prevented by CBS inhibitor, indicating the endogenous source of the gas. cAMP concentration, but not cGMP and NO(x), correlated with CBS activity. CBS inhibition increased NOS activity, whereas H(2)S donor decreased NO(x) production. In conclusion, hypoxia activates H(2)S endogenous production through the CBS-H(2)S pathway in the AVPO, having a cryogenic effect. Moreover, the present data are consistent with the notion that the two gaseous molecules, H(2)S and NO, play a key role in mediating the drop in Tb caused by hypoxia and that a fine-balanced interplay between NOS-NO and CBS-H(2)S pathways takes place in the AVPO of rats exposed to hypoxia.

nNOS is involved in behavioral thermoregulation of newborn rats during hypoxia

The present study was undertaken to investigate the role of nitric oxide (NO) pathway in the behavioral thermoregulation of newborn rats in cold and hypoxia. We predicted that injection of L-NAME (non-selective NO synthase (NOS) inhibitor) and SMTC (neuronal NOS (nNOS) inhibitor) would restore the huddling behavior and eliminate the reduction of Tb caused by hypoxia. Experiments were performed on Wistar rat pups of 7-9 days old. We measured Tb and analyzed the huddling behavior by means of the calculation of the total surface area occupied by 5 pups and the number of single pups grouped in the center of a chamber at 20 degrees C, before and after L-NAME, SMTC or their respective vehicles (D-NAME and saline) s.c. injections. Subsequently, the pups were exposed to hypoxia (10% O(2)) during 30 min, whereas control animals were kept under normoxia. The experiments were monitored by a digital camera. All animals were hypothermic when exposed to 20 degrees C. There was no significant difference in Tb, total area and number of single pups in normoxia after treatments. During hypoxia, the drop in Tb was higher in control groups, and this effect was attenuated by L-NAME and SMTC injections. Hypoxia increased the area occupied by the pups in saline, D-NAME and L-NAME groups, while SMTC attenuated this response. The data indicate that NO pathway is involved in the inhibition of huddling behavior and in the reduction of Tb caused by hypoxia, but plays no role during normoxia. Furthermore, NO seems to arise from the nNOS isoform.

Hypoxia induces a functionally significant and translationally efficient neuronal NO synthase mRNA variant

We tested the hypothesis that induction of neuronal NO synthase (nNOS) impairs vascular smooth muscle contractility after hypoxia. nNOS protein was increased in aorta, mesenteric arterioles, pulmonary arteries, brain, and diaphragm from rats exposed to 8% O2 for 48 hours and in human aortic SMCs after hypoxic incubation (1% O2). Ca-dependent NO synthase activity was increased in endothelium-denuded aortic segments from hypoxia-exposed rats. N-nitro-L-arginine methyl ester enhanced the contractile responses of endothelium-denuded aortic rings and mesenteric arterioles from hypoxia-exposed but not normoxic rats (P < 0.05). The hypoxia-inducible mRNA transcript expressed by human cells was found to contain a novel 5'-untranslated region, consistent with activation of transcription in the genomic region contiguous with exon 2. Translational efficiency of this transcript is markedly increased compared with previously described human nNOS mRNAs. Transgenic mice possessing a lacZ reporter construct under control of these genomic sequences demonstrated expression of the construct after exposure to hypoxia (8% O2, 48 hours) in the aorta, mesenteric arterioles, renal papilla, and brain. These results reveal a novel human nNOS promoter that confers the ability to rapidly upregulate nNOS expression in response to hypoxia with a functionally significant effect on vascular smooth muscle contraction.

Inhibitors of nitric oxide synthesis block cold-induced thermogenesis in rats

N-nitro-arginine methyl ester (L-NAME), an unspecific nitric oxide synthase inhibitor, was administered to individually caged Sprague-Dawley rats exposed to cold (18 degrees C) and thermoneutral (30 degrees C) environmental temperatures during the active phase of the animals' circadian cycle. Unrestrained rats were administered intraperitoneal injections of 100 mg x kg-1 L-NAME or 1 mL x kg-1 saline. Telemetry was used to measure abdominal temperature. On a separate occasion, metabolic rate and evaporative water loss were measured using indirect calorimetery, before and after the injection of 100 mg x kg-1 L-NAME, in rats exposed to the two environments. Injection of L-NAME had no significant effect on body temperature, metabolic rate, or evaporative water loss in rats exposed to the 30 degrees C environment. In the 18 degrees C environment, L-NAME injection caused a prolonged fall in body temperature ( F(1,12) = 17.43, P = 0.001) and a significant decrease in metabolic rate (Student's t test, P = 0.001) and evaporative water loss (one-sample t test, P = 0.04). Therefore, the effects that systemic injection of L-NAME has on body temperature are dependent on environmental temperature, with nitric oxide synthase inhibition seemingly preventing the metabolic component of cold defence.

Nitric oxide in the rostral ventrolateral medulla modulates hyperpnea but not anapyrexia induced by hypoxia

Hypoxia causes hyperpnea and anapyrexia (a regulated decrease in body temperature, T(b)) but the mechanisms involved are not well understood. The nitric oxide (NO) pathway is involved in hypoxia-induced anapyrexia and hyperpnea, but the site(s) of action is not known. Nitric oxide synthase is present in the rostral ventrolateral medulla (RVLM), which is a nucleus in the medulla oblongata involved in control of breathing, and RVLM neurons have been suggested to have intrinsic hypoxic chemosensitivity. Therefore, we examined the effects of inhibition of the NO pathway in the RVLM on hypoxic hyperpnea and anapyrexia. Ventilation (VE) and body temperature (T(b)) were measured before and after bilateral microinjection of N-monomethyl-L-arginine (L-NMMA, 12.5 microg/0.1 microl, a nonselective nitric oxide synthase inhibitor) into the RVLM, followed by a 120-min period of hypoxic exposure. Control rats received microinjection of saline (vehicle). Under normoxia, L-NMMA treatment did not affect VE or T(b). Typical hypoxia-induced hyperpnea and anapyrexia were observed after saline treatment. L-NMMA treatment reduced the ventilatory response to hypoxia but did not affect hypoxia-induced anapyrexia. These data suggest that nitric oxide in the RVLM is involved in the ventilatory response to hypoxia, exercising an excitatory modulation of the RVLM neurons, but plays no role in hypoxia-induced anapyrexia.

The effect of oxygen and adenosine on lizard thermoregulation

A regulated decrease in internal body temperature (Tb) appears to play a protective role against metabolic disruptions such as exposure to ambient hypoxia. This study examined the possibility that Tb depression is initiated when low internal oxygen levels trigger the release of adenosine, a neural modulator known to influence thermoregulation. We measured selected Tb of Anolis sagrei in a thermal gradient under varied ambient oxygen conditions and following the administration of the adenosine receptor antagonist 8-cyclopentyltheophylline (CPT). The average decrease in Tb observed following exposure to hypoxia (<10% O2) and following exhaustive exercise were 5 degrees and 3 degrees C, respectively, suggesting a role of oxygen availability on initiation of regulated hypothermia. When A. sagrei were run to exhaustion and recovered in hyperoxic (>95% O2) conditions, exercise-induced Tb depression was abolished. Administration of CPT similarly abolished decreased Tb due to both exercise and hypoxia. Trials using Dipsosaurus dorsalis indicate that elevated ambient oxygen during exercise does not influence blood pH or lactate accumulation, suggesting that these factors do not initiate changes in thermoregulatory setpoint following exhaustive exercise. We suggest that when oxygen is limiting, a decrease in arterial oxygen may trigger the release of adenosine, thereby altering the thermoregulatory setpoint.

A neurochemical mechanism for hypoxia-induced anapyrexia

Hypoxia evokes a regulated decrease in body temperature, a response that has been termed anapyrexia, but the mechanisms involved are poorly understood. Therefore, the present study was undertaken to test the hypothesis that hypoxia-induced anapyrexia results from the activation of cAMP- and cGMP-dependent pathways in the preoptic region (PO). Adult male Wistar rats weighing 230-260 g were used. Body temperature was monitored by biotelemetry, and the levels of cAMP and cGMP were determined in the anteroventral third ventricular region (AV3V), where the PO is located. Using immunohistochemistry, we observed that the PO contains a high density of cAMP- and cGMP-containing cells. Interestingly, hypoxia exposure raised the levels of cAMP and cGMP in the AV3V. Intra-PO microinjection of Rp-cAMPS, an inhibitor of cAMP-dependent protein kinase, attenuated hypoxia-induced anapyrexia. Similarly, intra-PO microinjection of the mixed beta-adrenoceptor/serotonin (5-HT(1A)) receptor antagonist propranolol also impaired the drop in body temperature in response to hypoxia. The reduction in body temperature evoked by intra-PO serotonin, but not epinephrine, was blocked by Rp-cAMPS, indicating the involvement of a preoptic serotonin-cAMP pathway in the development of anapyrexia. Moreover, microinjection of N(G)-monomethyl-l-arginine, an inhibitor of nitric oxide (NO) synthesis, or Rp-cGMPS, an inhibitor of cGMP-dependent protein kinase, into the PO also attenuated hypoxia-induced anapyrexia. In conclusion, the present study supports that hypoxia-induced anapyrexia results from the activation of the serotonin-cAMP and NO-cGMP pathways in the PO.

Hypoxia-induced anapyrexia: implications and putative mediators

Hypoxia elicits an array of compensatory responses in animals ranging from protozoa to mammals. Central among these responses is anapyrexia, the regulated decrease of body temperature. The importance of anapyrexia lies in the fact that it reduces oxygen consumption, increases the affinity of hemoglobin for oxygen, and blunts the energetically costly responses to hypoxia. The mechanisms of anapyrexia are of intense interest to physiologists. Several substances, among them lactate, adenosine, opioids, and nitric oxide, have been suggested as putative mediators of anapyrexia, and most appear to act in the central nervous system. Moreover, there is evidence that the drop in body temperature in response to hypoxia, unlike the ventilatory response to hypoxia, does not depend on the activation of peripheral chemoreceptors. The current knowledge of the mechanisms of hypoxia-induced anapyrexia are reviewed.

Involvement of neuronal nitric oxide synthase in restraint stress-induced fever in rats

Nitric oxide (NO) has been shown to be an important modulator of the febrile response to pyrogens and to psychological stress. In the present study, we aimed to identify the nitric oxide synthase (NOS) isoform (neuronal or inducible, nNOS and iNOS, respectively) involved in restraint stress fever. Colonic temperature (Tc) was measured in unanesthetized rats before and after treatment with the more selective nNOS inhibitor 7-nitroindazole or with the selective iNOS inhibitor aminoguanidine (AG) under unrestrained or restrained conditions. Intraperitoneal injection of AG (25 or 50 mg/kg) did not affect restraint fever, indicating that iNOS is unlikely to be involved in restraint fever. On the other hand, intraperitoneal injection of 7-nitroindazole (25 mg/kg) significantly attenuated the rise in the Tc caused by restraint stress, whereas it caused no change in Tc of euthermic animals. These data show that NO produced by nNOS plays an important role in the genesis of restraint stress-induced fever.

Effects of intracerebroventricular administration of 7-nitroindazole on tolerance to ethanol

Previous studies have shown that nitric oxide (NO) is involved in the development of rapid tolerance to the motor incoordination produced by ethanol. In order to further investigate this involvement, three experiments were undertaken using the tilt-plane and the hypothermia tests. The first demonstrated that 7-nitroindazole (7-NI), a preferential neuronal NO synthase (nNOS) inhibitor, injected by intracerebroventricular (i.c.v.) route, blocked the development of rapid tolerance to ethanol-induced motor incoordination. This effect was prevented by i.c.v. injection of L-arginine. The second experiment showed that D-arginine did not influence the blockade of tolerance produced by 7-NI. The third experiment revealed that i.c.v. injection of 7-NI also blocked the development of tolerance to the hypothermic effect of ethanol. These results support the hypothesis that nNOS-derived NO participates in the development of rapid tolerance to ethanol.