Thermoregulatory response to hypoxia after inhibition of the central heme oxygenase–carbon monoxide pathway

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.

Age specific effect of MK-801 on hypoxic body temperature regulation in rats

Hypoxic exposure produces a consistent decrease of rectal temperature (Tb), which is recognized as a potent protective response. While some of the neural mechanisms underlying this response have recently been described, it remains poorly known how these mechanisms evolve during post-natal development. We recently reported that in rat pups NMDA glutamate receptor limits Tb drop upon hypoxic exposure, an effect that has not been reported by others in adult rats. Accordingly, we tested the hypothesis that the implication of NMDA receptors on temperature control during hypoxic exposure evolves during development. To this aim, we evaluated the hypoxic (30 min - 12% O(2)) responses of Tb, metabolic rate, and ventilation in rats after injection of vehicle, or the NMDA receptor antagonist MK-801, at different ages (post-natal days 4, 10, 20 and 2-3 month-old - P4, P10, P20 and P60). MK-801 amplified the magnitude of the hypoxic-induced Tb drop in P4, P10 and P20 rats, but this effect was not apparent in adults. In P20 rats MK-801 tripled the hypoxic induced Tb drop, which was 0.5 degrees C in control and 1.4 degrees C in treated rats (p<0.0001). This effect was specific to temperature regulation, and was not accompanied by similar changes of other recorded parameters. MK-801 induced a significant decrease of the hypoxic ventilatory response in adults only. We conclude that NMDA glutamate receptor acts as a counter-regulatory factor that limits the hypoxic-induced drop of rectal temperature during post-natal development in rats.

Heme oxygenase-carbon monoxide pathway is involved in regulation of respiration in medullary slice of neonatal rats

Carbon monoxide (CO) is a novel biological messenger molecule. It is well known that CO can be synthesized in mammalian cells. In addition, CO is also demonstrated to participate in many physiological processes, such as vasomotion, thermoregulation and respiratory regulation. The purpose of our present study was to investigate the role of heme oxygenase-carbon monoxide (HO-CO) pathway in central regulation of respiration. The experiments were carried out on the medullary slices of neonatal Sprague-Dawley rats. The discharge activity of the hypoglossal rootlets was recorded to indicate the central rhythmic respiratory activity and its duration (DD), interval (DI), frequency (DF) and integrated amplitude (IA) were analyzed. The slices were perfused with ZnPP-9 (a potent inhibitor of heme oxygenase), CO and hemin (substrate of heme oxygenase), respectively, to observe their effects on respiratory activity. The results obtained were as follows: ZnPP-9 could decrease DD, DI and IA, and increase DF (P<0.05); exogenous CO caused a decrease in DD and DF, and an increase in DI and IA (P<0.05); in response to hemin, DI and IA decreased, DF increased (P<0.05), and DD did not change significantly (P>0.05); administration of both ZnPP-9 and hemin could decrease DI, and increase DF (P<0.05), but did not affect DD and IA significantly (P>0.05). It can be concluded from the results above that the HO-CO pathway may be involved in the regulation of rhythmic respiration at the level of medulla oblongata.

Physiology of temperature regulation: comparative aspects

Few environmental factors have a larger influence on animal energetics than temperature, a fact that makes thermoregulation a very important process for survival. In general, endothermic species, i.e., mammals and birds, maintain a constant body temperature (Tb) in fluctuating environmental temperatures using autonomic and behavioural mechanisms. Most of the knowledge on thermoregulatory physiology has emerged from studies using mammalian species, particularly rats. However, studies with all vertebrate groups are essential for a more complete understanding of the mechanisms involved in the regulation of Tb. Ectothermic vertebrates-fish, amphibians and reptiles-thermoregulate essentially by behavioural mechanisms. With few exceptions, both endotherms and ectotherms develop fever (a regulated increase in Tb) in response to exogenous pyrogens, and regulated hypothermia (anapyrexia) in response to hypoxia. This review focuses on the mechanisms, particularly neuromediators and regions in the central nervous system, involved in thermoregulation in vertebrates, in conditions of euthermia, fever and anapyrexia.

Activation of NMDA receptors prevents excessive metabolic decrease in hypoxic rat pups

We tested the hypothesis that glutamate NMDA receptors may help maintain metabolic rate and body temperature during acute or chronic hypoxic exposure in newborn rats. We recorded ventilation, metabolism ((.)V(O(2)) -- ((.)V(CO(2)) and rectal temperature, under normoxia, acute hypoxia (30 min -- 12% O(2)), or following 10 days of chronic hypoxia, in 10 days old male and female rats, receiving saline i.p. injection or the NMDA receptor antagonist MK-801. Acute hypoxia decreased rectal temperature and metabolism, and increased ventilation, and (.)V(E)/((.)V(O(2) and (.)V(E)/((.)V(CO(2) to the same extent in males and females. MK-801 injection amplified the metabolic decrease under acute (in males and females) and chronic (in males) hypoxia, prevented the increase of minute ventilation, while (.)V(E)/((.)V(O(2) or (.)V(E)/((.)V(CO(2)remained constant. Hence, NMDA glutamate receptors help to maintain metabolic rate, minute ventilation and body temperature at a determined level in acute (males and females) and chronic hypoxia (males only).

Role of the brain heme oxygenase-carbon monoxide pathway in stress fever in rats

This study was aimed at testing the hypothesis that the brain heme oxygenase (HO)-carbon monoxide (CO) pathway plays a role in stress fever. To this end, the effect of the HO inhibitor, zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG), on restraint-induced fever was tested. Intracerebroventricular ZnDPBG (200 nmol) did not affect the body core temperature of unrestrained rats, but markedly attenuated restraint-induced fever. However, at the same dose, intraperitoneal ZnDPBG did not affect the febrile response to restraint. Taken together, these results indicate that the brain HO-CO pathway plays a major role in the genesis of stress fever in rats.

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.

Central heme oxygenase-carbon monoxide pathway in the control of breathing under normoxia and hypoxia

Endogenously carbon monoxide (CO) arises from the catabolism of heme to biliverdin, free iron and CO, a process catalyzed by the enzyme heme oxygenase (HO). In the present study, we tested the hypothesis that the central HO-CO pathway plays a role in hypoxia-induced hyperventilation. To this end, we used intracerebroventricular (i.c.v.) injections of the HO inhibitor zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG; 200 nmol) and of heme-lysinate (152 nmol), which is known to induce the HO pathway, and measured respiratory frequency (f), tidal volume (VT) and pulmonary ventilation (VE) by body plethysmograph in conscious rats. Hypoxia (7% inspired oxygen) evoked a typical increase in VE by either raising f and VT, ZnDPBG or its vehicle caused no change in basal VE and did not affect the increase in VE elicited by hypoxia. Conformably, i.c.v. heme-lysinate did not affect VE as well. These results do not support the hypothesis that the HO-CO pathway in the central nervous system is involved in hypoxia-induced hyperventilation.