Fall In Body Temperature Research Articles

Effect of Blood Transfusion in Combination with Dextran-40 and Hypertonic Saline Solution on Cardiopulmonary Haemodynamics of Endotoxin (Lipopolysaccharide) Shock in Buffalo Calves

The intravenous (i.v.) infusion of lipopolysaccharide (LPS) of E. coli endotoxin in buffalo calves (n = 15) at 5 microg/kg bw per h for 3 h caused a significant (p<0.05) fall in plasma volume, blood volume, haematocrit haemoglobin, and systolic, diastolic and pulse pressure, mean arterial pressure and central venous pressure (CVP), with a marked rise in respiration. Treatment with a combination of i.v. infusion of 7.2% hypertonic saline solution, Plasmex-D-40 (Dextran-40) and blood successfully alleviated hypovolaemia, and raised systolic, diastolic and pulse pressure, mean arterial pressure and central venous pressure. The whole blood was collected from apparently healthy male buffalo calves 24 h prior to infusion and was transfused without cross-matching. No significant fall in haemoglobin, haematocrit and body temperature was observed after transfusion. All these values tended to remain near normal levels. However, this combination of treatment had no effect on high respiratory rate. A one-time blood transfusion did not evoke any cross-reaction and was helpful in raising haematocrit and haemoglobin close to pre-infusion values. The general symptoms of restlessness, respiratory distress, profuse salivation, violent movement of the ears, snoring, intermittent struggle, etc. were markedly reduced. All the treated animals became quiet and lay with eyes open and survived the 7 h of observation.

Unmasking of α 1 adrenoceptor induced hypnogenic response from medial preoptic area

Earlier studies had indicated the possibility of the involvement of the α 1 adrenergic receptors on the medial preoptic (mPOA) neurons in hypnogenesis. Microinjection of α 1 agonist methoxamine (0.05 μg) into the mPOA of the rats, maintained at an ambient temperature ( T amb) of 24 °C, did not produce any significant change in sleep–wakefulness ( S– W), except for an arousal of short duration, which coincided with the steep fall in body temperature ( T b). It was probably not possible to elicit sleep, because of hypothermia simultaneously produced by this drug. To test this hypothesis, experiments were conducted in an environment where the effects of hypothermia could be partially compensated for. When the rats were allowed to select their T amb, they moved over to a compartment having a higher T amb of 30 °C, on administration of methoxamine into the mPOA. Simultaneously, there was induction of sleep, which lasted for 60 min. Thus, it was possible to demonstrate the hypnogenic action of methoxamine, which was masked by hypothermic action of the drug. The findings indicate that the α 1 adrenergic receptors in the mPOA are involved in the induction of sleep.

Daily torpor in a pregnant dunnart ( Sminthopsis macroura Dasyuridae: Marsupialia)

Mammalian reproduction is an energetically expensive process. In addition to normal energetic costs for maintenance, locomotion and thermoregulation, reproduction requires energy expenditure to acquire and process nutrients, produce milk, and transferring nutrients to growing offspring with the consequent increase in metabolic rate (MR) (Farmer 2003). Torpor in heterothermic mammals, on the other hand, results in an overall reduction of energy expenditure and is characterised by a pronounced fall in body temperature and MR (Geiser and Ruf 1995). For most mammals, these different energetic, and also hormonal, demands appear to require a temporal sequence of reproduction and torpor within the yearly schedule and thus, torpor usually occurs during the nonreproductive season (Goldman et al. 1986; Barnes 1996; Stamper et al. 1998; Mzilikazi and Lovegrove 2002). Not surprisingly then, there is a widely held view that torpor and reproduction in mammals are mutually exclusive processes. Nevertheless, members of a few mammalian taxa, including echidnas (Monotremata), some small dasyurids and feathertail gliders (Marsupialia), and some small insectivores and bats (Placentalia), are known to enter torpor while pregnant and/or lactating (Racey 1973; Morton 1978; Geiser 1996;

Thermoregulation during endotoxemia in mice lacking inducible nitric oxide synthase: Involvment of the arginine-vasopressin

Background We have tested the hypothesis that nitric oxide (NO) arising from inducible nitric oxide synthase (iNOS) plays a role in hypothermia during endotoxemic shock, by regulating vasopressin (AVP) release. Methods and results 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 hours after LPS administration. One hour after LPS administration we observed a significant drop in body temperature (hypothermic response) in WT, which remained until the eighth hour after LPS injection, returning the close to the basal level. In iNOS KO mice, we found a significant fall in body temperature after the first hour of LPS administration, however the hypothermic response persisted until the end of the twenty-four hour of experiment. The pre-treatment with (b-mercapto-b,b-cyclopentamethylenepropionyl1, O-Et-Tyr2, Val4, Arg8-Vasopressin), an AVP V1 receptor blocker (10 μg/kg) administered i.p., abolished the persistent hypothermia induced by LPS in iNOS KO mice, suggesting the regulation of iNOS under the vasopressin release in this patophysiological model. Conclusion In conclusion, our data indicate that the inducible NOS isoform plays an inhibitory role in AVP release during endotoxemia, which seems to be critical for the thermoregulation during this pathophysiological state. Clinical Pharmacology & Therapeutics (2005) 77, P9–P9; doi: 10.1016/j.clpt.2004.11.037

Respiratory herpes simplex virus type 1 infection/colonisation in the critically ill: marker or mediator?

The clinical significance and pulmonary pathogenicity of herpes simplex virus type 1 (HSV-1) in mechanically ventilated, critically ill patients are unclear. To determine the clinical features and course of respiratory HSV-1 infections/colonisations in the critically ill, in order to evaluate the contribution to outcome. A retrospective cohort study in the intensive care unit of an university hospital, involving 22 patients with a HSV-1 isolated from bronchoalveolar lavage (BAL) fluid, divided into survivors (n = 13) and non-survivors (n = 9). All patients except for one survivor had been intubated and were mechanically ventilated. Non-survivors had acquired HSV-1 sooner on mechanical ventilation than survivors. Prior chronic heart disease was more prevalent in non-survivors than in survivors and, at the time of HSV-1 isolation, the mean creatinine level was higher (P < 0.05) in the former. Survivors had a somewhat greater fall in body temperature after a 10-day course of antiviral therapy than non-survivors, but the lung radiographic abnormalities prior to and after the course did not differ. There were no major differences in cardiorespiratory variables between outcome groups and causes of death and were judged not to relate, in general, to HSV-1. Critically ill patients in whom HSV-1 from BAL is isolated, have about 40% chance of dying, mainly because of severe underlying disease and comorbidity, which may predispose to endogenous reactivation of the virus. There is no clinical evidence for direct cardiorespiratory pathogenicity and beneficial effects of antiviral therapy. HSV-1 isolated from lung secretions may thus be a marker rather than a mediator of severe illness.

Metabolic rate and body temperature reduction during hibernation and daily torpor.

Although it is well established that during periods of torpor heterothermic mammals and birds can reduce metabolic rates (MR) substantially, the mechanisms causing the reduction of MR remain a controversial subject. The comparative analysis provided here suggests that MR reduction depends on patterns of torpor used, the state of torpor, and body mass. Daily heterotherms, which are species that enter daily torpor exclusively, appear to rely mostly on the fall of body temperature (Tb) for MR reduction, perhaps with the exception of very small species and at high torpor Tb, where some metabolic inhibition may be used. In contrast, hibernators (species capable of prolonged torpor bouts) rely extensively on metabolic inhibition, in addition to Tb effects, to reduce MR to a fraction of that observed in daily heterotherms. In small hibernators, metabolic inhibition and the large fall of Tb are employed to maximize energy conservation, whereas in large hibernators, metabolic inhibition appears to be employed to facilitate MR and Tb reduction at torpor onset. Over the ambient temperature (Ta) range where torpid heterotherms are thermo-conforming, the Tb-Ta differential is more or less constant despite a decline of MR with Ta; however, in thermo-regulating torpid individuals, the Tb-Ta differential is maintained by a proportional increase of MR as during normothermia, albeit at a lower Tb. Thermal conductance in most torpid thermo-regulating individuals is similar to that in normothermic individuals despite the substantially lower MR in the former. However, conductance is low when deeply torpid animals are thermo-conforming probably because of peripheral vasoconstriction.

Caffeine reduces hypnotic effects of alcohol through adenosine A 2A receptor blockade

The role of the adenosine A 2A receptor in the hypnotic effects of ethanol was assessed in mice. The duration of the loss of righting reflex following acute ethanol administration was shorter for A 2A receptor-deficient mice (A 2AR KO) than for wild-type mice (A 2AR WT), whereas the fall in body temperature was not different between the two phenotypes. In contrast, the duration of the loss of righting reflex was increased in A 2AR KO mice versus controls after administration of pentobarbital. Dipyridamole, an inhibitor of adenosine uptake, increased the sleep time observed following administration of ethanol in CD1 mice and in A 2AR WT but not in A 2AR KO mice. SCH 58261, a selective A 2A receptor antagonist, unlike DPCPX, a selective A 1 receptor antagonist, shortened the duration of the loss of righting reflex induced by ethanol, thus mimicking the lack of receptor in deficient mice. Finally, the non-selective adenosine receptor antagonist caffeine (25 mg/kg) reduced ethanol-induced hypnotic effects. These results indicate that the activation of A 2A receptors that follows an increase in extracellular adenosine levels caused by the administration of high doses of ethanol plays a role in its hypnotic effects. Thus, A 2A receptor antagonists may be useful therapeutic agents for alleviating ethylic coma.

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.

Transient peripheral warming accompanies the hypoxic metabolic response in the golden-mantled ground squirrel

The hypoxic metabolic response of mammals involves a reversible metabolic suppression, possibly brought about by a reduction in the body temperature set-point. In the present study we tested the hypothesis that this is accompanied by a transient increase in heat loss that facilitates the decline in body temperature and metabolic rate. Peripheral heat distribution was assessed using infrared thermography to measure the surface temperatures of the golden-mantled ground squirrel at three different ambient temperatures (10, 22 and 30 degrees C). During early hypoxic exposure, surface temperatures increased dramatically in the feet, ears and nose, and this increase was more dramatic and prolonged at 22 degrees C than at the other two temperatures. These increases were associated with a fall in metabolic rate. Following this initial increase, surface temperatures decreased back to control values, and at 10 degrees C, the surface temperatures of the eyes and body decreased below normoxic levels. Subsequent normoxic recovery was not accompanied by transient changes in surface temperatures, despite large increases in metabolic rate associated with post-hypoxic shivering and thermogenesis. The temporal changes in surface temperature suggest that peripheral blood flow is initially increased during hypoxia, shifting heat away from the core to the periphery and thus facilitating cooling. These results are consistent with the hypothesis that hypoxia leads to a regulated fall in body temperature.

Changes in Body Temperature and Sleep‐Wakefulness AfterIntrapreoptic Injection of Methoxamine in Rats

Several pieces of evidence suggest that the noradrenergic afferents in the medial preoptic area produce sleep and hypothermia by acting on α1 adrenergic receptors. On the other hand, in a few studies monitoring body temperature with a rectal probe, preoptic injection of the α1 adrenergic agonist methoxamine produced contradictory changes in body temperature and sleep-wakefulness. Such contradictions call for the re-examination of methoxamine induced body temperature changes using a better technique like telemetric recording. In the present study, we monitored body temperature and sleep-wakefulness simultaneously after the micro-injection of 0.5, 1, and 2 μmol methoxamine, into the medial preoptic area of adult male Wistar rats. Methoxamine injection produced hypothermia but no major change in sleep-wakefulness during the 3 hours after drug injection, except for a short period (15 min) of sleep after 120 min of injection. A short period of wakefulness, coinciding with the maximum fall in body temperature (30 min after injection) occurred when methoxamine was administered at higher doses. The results of this study indicate that 1 adrenergic receptors participate in preoptically mediated thermoregulatory measures that reduce body temperature. Hypothermia induced by methoxamine might have masked the hypnogenic action of this drug.

Mu-1 opioid receptor stimulation decreases body temperature in conscious, unrestrained neonatal rats.

The influence of mu-selective opioid agonists on neonatal thermoregulatory mechanisms has received little attention. Opioid treatment in adult subjects can cause either hyper- or hypothermia, depending on the experimental conditions, the strain of rat used, and the dose and route of administration of the drug. The present study assessed the effect of two mu opioid agonists on body temperature in neonatal Wistar rats aged 2 to 13 days. Rat pups were administered either saline or one of the two mu-selective opioid agonists, dermorphin (0.4 mg/kg) or fentanyl (0.06 mg/kg), by subcutaneous injection. Continuous rectal temperatures were measured both prior to and following drug or saline injection in freely moving, conscious animals. Ambient temperature in a plethysmograph chamber was maintained within or close to the thermoneutral zone for pups (32 degrees C). To distinguish between mu-1 and mu-2 effects, all animals received either saline or 10 mg/kg of the irreversible mu-1 antagonist naloxonazine (NALZ) 1 day prior to agonist administration. NALZ on its own had no effect on body temperature. Dermorphin and fentanyl both caused a fall in body temperature in pups of all age groups. The temperature decreases ranged from 0.8 degrees -2.2 degrees C. These opioid-induced changes were inhibited by NALZ pretreatment. Although there was no evidence for endogenous mu-1 opioid activity, this study indicated that stimulation of mu-1 opioid receptors causes a decrease in body temperature in conscious, unrestrained neonatal rats under or close to thermoneutral conditions.

Diurnal fluctuations of heart rate, body temperature and locomotor activity in the house musk shrew (Suncus murinus).

Diurnal fluctuations of heart rate (HR), body temperature (BT) and locomotor activity (LA) in the unanaesthetized and unrestrained house musk shrew (Suncus murinus) were studied using a telemetry system. Six adult male shrews (Jic:SUN) weighing 60-70 g were used in the present study. They were housed under conditions of 24 C and a 12/12-hr light-dark cycle. HR, BT and LA were recorded over 10 days, following the post-implantation period (10 days or more) of the telemetric transmitter. A clear nocturnal rhythm of LA was shown, while intermittent and short-term LA were shown during the light period. The mean HR was 323.5 +/- 8.8 bpm in the light period and 354.3 +/- 5.2 bpm in the dark period, and the fluctuation of HR showed a nocturnal pattern. A nocturnal pattern was also observed in BT fluctuation, and all animals lowered their body temperature from 35-37 C to approximately 30 C or below, mostly during the light period. The fall of body temperature progressed over 2-3 hr, and then rose to the baseline temperature rapidly within approximately 30 min. While the body temperature fell, HR markedly decreased to approximately 100 bpm. These results suggest that the shrew has unique physiological properties in maintaining metabolic balance which are anticipated to be caused by the dramatic alteration of the autonomic nervous function.

Prooxidant–antioxidant balance in rats under hypothermia combined with modified hemoglobin–oxygen affinity

Deep hypothermia (fall of body temperature by more than 10°C) is associated with the serious perturbation of oxygen delivery and body prooxidant–antioxidant balance. The blood oxygen-binding properties determine the conditions of oxygen diffusion to tissues and the values of tissue PO 2 and have a specific role in the complex system of antioxidant defense. Our aim was to evaluate the prooxidant–antioxidant balance under the conditions of modified hemoglobin–oxygen affinity (HOA) in a rat experimental model of hypothermia (HOA was increased by sodium cyanate (NaOCN) and decreased by sodium o-iodobenzoate (OISB)). The preliminary shift of oxyhemoglobin dissociation curve (ODC) leftwards during the hypothermia was accompanied by the shift of prooxidant–antioxidant balance towards activation of the lipid peroxidation (LPO), and the ODC shift rightwards was accompanied by the LPO lowering. HOA decrease is favorable during the deep hypothermia due to the optimized oxygen flux to tissues and its lower fraction spent for the free radical reactions that is associated with the less marked distresses of prooxidant–antioxidant balance.

Time memory for heat exposure limited to a fixed daily time in heat-acclimated rats and humans

After daily heat exposure for 4–5 h limited to a fixed time for more than 5 consecutive days, body core temperature of rats and humans drops before and during the period they were previously exposed to heat. The fall in body temperature persists for a few days after terminating the timed daily heat exposure. Additionally, various thermoregulatory changes to resist heat are brought about especially during the specific period. It is, therefore, hypothesized that the thermoregulatory system can memorize the time for heat exposure and according to the memory, thermoregulatory responses for neutralizing heat stress are induced around the period corresponding to that of the previous heat exposure time even without actual heat exposure.

Testosterone mediates sex difference in hypothermia and cholinesterase inhibition by rivastigmine

The fall in body temperature and inhibition of hypothalamic cholinesterase induced by rivastigmine (a pseudo-reversible carbamate inhibitor) were compared in male and female rats. In males, 1.5 mg/kg lowered body temperature by 1 °C and in females by 3.2 °C ( P<0.001) and inhibited cholinesterase by 65% and 74%, respectively ( P<0.05). Pilocarpine (20 mg/kg) decreased body temperature by 1.1 °C in males and 1.9 °C in females ( P<0.05). Orchidectomy, but not ovariectomy, abolished the sex difference in the hypothermic effect of pilocarpine and the enzyme inhibition induced by rivastigmine, but not in its effect on body temperature. Testosterone (10 mg/rat) decreased the cholinesterase inhibition and the temperature reduction induced by rivastigmine in gonadectomised males and females, but that induced by pilocarpine in males only. In conclusion, rivastigmine causes less inhibition of cholinesterase because testosterone may interfere with its entry into the brain. Testosterone may further decrease the temperature-lowering effect of rivastigmine and acetylcholine receptor agonists in males by an action at a receptor level.

Motion sickness potentiates core cooling during immersion in humans.

1. The present study tested the hypothesis that motion sickness affects thermoregulatory responses to cooling in humans. 2. Ten healthy male volunteers underwent three separate head-out immersions in 28 degrees C water after different preparatory procedures. In the 'control' procedure immersion was preceded by a rest period. In the 'motion sickness' procedure immersion was preceded by provocation of motion sickness in a human centrifuge. This comprised rapid and repeated alterations of the gravitational (G-) stress in the head-to-foot direction, plus a standardized regimen of head movements at increased G-stress. In the 'G-control' procedure, the subjects were exposed to similar G-stress, but without the motion sickness provocation. 3. During immersion mean skin temperature, rectal temperature, the difference in temperature between the forearm and 3rd digit of the right hand (DeltaT(forearm-fingertip)), oxygen uptake and heart rate were recorded. Subjects provided ratings of temperature perception, thermal comfort and level of motion sickness discomfort at regular intervals. 4. No differences were observed in any of the variables between control and G-control procedures. In the motion sickness procedure, the DeltaT(forearm-fingertip) response was significantly attenuated, indicating a blunted vasoconstrictor response, and rectal temperature decreased at a faster rate. No other differences were observed. 5. Motion sickness attenuates the vasoconstrictor response to skin and core cooling, thereby enhancing heat loss and the magnitude of the fall in deep body temperature. Motion sickness may predispose individuals to hypothermia, and have significant implications for survival time in maritime accidents.

Hypoxic metabolic response of the golden-mantled ground squirrel.

We examined the magnitude of the hypoxic metabolic response in golden-mantled ground squirrels to determine whether the shift in thermoregulatory set point (T(set)) and subsequent fall in body temperature (T(b)) and metabolic rate observed in small mammals were greater in a species that routinely experiences hypoxic burrows and hibernates. We measured the effects of changing ambient temperature (T(a); 6--29 degrees C) on metabolism (O(2) consumption and CO(2) production), T(b), ventilation, and heart rate in normoxia and hypoxia (7% O(2)). The magnitude of the hypoxia-induced falls in T(b) and metabolism of the squirrels was larger than that of other rodents. Metabolic rate was not simply suppressed but was regulated to assist the initial fall in T(b) and then acted to slow this fall and stabilize T(b) at a new, lower level. When T(a) was reduced during 7% O(2), animals were able to maintain or elevate their metabolic rates, suggesting that O(2) was not limiting. The slope of the relationship between temperature-corrected O(2) consumption and T(a) extrapolated to a T(set) in hypoxia equals the actual T(b). The data suggest that T(set) was proportionately related to T(a) in hypoxia and that there was a shift from increasing ventilation to increasing O(2) extraction as the primary strategy employed to meet increasing metabolic demands under hypoxia. The animals were neither hypothermic nor hypometabolic, as T(b) and metabolic rate appeared to be tightly regulated at new but lower levels as a result of a coordinated hypoxic metabolic response.

Effects of adenosine administration on spike-wave discharge frequency in genetically epileptic rats.

1. In the present study, the effects of the administration of adenosine on absence seizures were investigated in the Wistar Albino Glaxo/Rijswijk (WAG/Rij) strain of rats, which are an adequate model for human absence epilepsy. 2. After baseline electroencephalogram (EEG) recording, adenosine was injected intraperitoneally (i.p.) at doses of 6, 30 and 120 mg/kg and EEG recordings were continued for 1 h postinjection. In addition, to enable evaluation of the contribution of peripheral factors to the occurrence of spike-wave discharges (SWD) after adenosine injection, arterial blood pressure and rectal temperature were also recorded. 3. Injection of adenosine induced a dose-dependent increase in the appearance of SWD. The number and total duration of SWD were found to be significantly increased after 30 and 120 mg/kg adenosine (P < 0.05). Adenosine also induced a fall in both blood pressure and body temperature in all experimental groups. 4. These results show that peripheral administration of adenosine promotes absence seizures. It is likely that the lowering of blood pressure and body temperature and the activation of sensory afferents after adenosine injection may contribute to the facilitation of SWD observed in the present study.

Opposite regulation of body temperature by cholinergic input to the paraventricular nucleus and supraoptic nucleus in rats

Hypothalamic cholinergic system plays an important role in the regulation of body temperature and fluid balance. We have previously shown that cholinergic stimulation of the anterior hypothalamus and preoptic area was accompanied by a fall in body temperature, increased water intake, and increased Fos protein in the paraventricular nucleus (PVN) and supraoptic nucleus (SON). In the present study, to estimate the role played by cholinergic input to the PVN and SON in thermoregulation and water intake, we used microdialysis for cholinergic stimulation with neostigmine and analysis of the nucleus, and also investigated immunoreactivity for c-Fos protein in the brain. This stimulation increased extracellular concentration of acetylcholine in these nuclei. Stimulation of the PVN decreased body temperature and increased water intake. On the other hand, stimulation of the SON increased body temperature. Both in PVN-stimulated and SON-stimulated rats, c-Fos-like immunoreactivity (Fos-IR) was evident in the PVN, SON and certain regions including locus coeruleus (LC), area postrema and nucleus of the solitary tract (NTS). Addition of atropine to the dialysis medium attenuated the increase of Fos-IR and suppressed the cholinergic stimulation-induced responses in body temperature and water intake. These results suggest that cholinergic muscarinic mechanisms in PVN and SON play an opposite function in the regulation of body temperature. The same neuronal pathway including LC and NTS may participate in an advance both in hypothermia and in hyperthermia.

Effect of Ambient Temperature on the Body Temperature Rhythm of Male Gray Mouse Lemurs (Microcebus murinus)

In order to cope with the seasonal variations in ambient temperature and food availability in the natural habitat, gray mouse lemurs (Microcebus murinus) exhibit adaptive energy-saving mechanisms similar to those in hibernating species with seasonal and daily heterothermia. To determine thermoregulatory responses, via telemetry we recorded body temperature and locomotor activity variations during the breeding season in three captive male mouse lemurs kept at ambient temperatures (Ta) ranging from 18° to 34°C. Rhythms in body temperature and locomotor activity were clearly exhibited regardless of ambient temperature. As a increased, mean body temperature increased from 36.5 ± 0.1°C to 37.6 ± 0.3°C, with significant change in the amplitude of the body temperature rhythm when a rose above 28°C. Effects of a were mostly due to changes in the fall in body temperature occurring daily at the beginning of the light phase when the subjects entered diurnal sleep. The daily decrease in body temperature was not modified by exposure to ambient temperatures from 18°C to 28°C whereas it disappeared under warmer condition. Changes in locomotor activity levels only delayed the occurrence of thermoregulatory modulation. These results strongly suggest that, during the breeding season, the thermoneutral zone of mouse lemurs is close to 28°C and that the diurnal fall in body temperature could be considered as an important adaptive energy-saving mechanism adjusted to ecological constraints.