Selected contribution: ambient temperature for experiments in rats: a new method for determining the zone of thermal neutrality

Brain hyperthermia induced by MDMA (ecstasy): modulation by environmental conditions

Drugs of abuse, such as 3,4-methylenedioxymethamphetamine (MDMA), often have more powerful effects during states of increased activation and under specific environmental conditions. Because hyperthermia is a major complication of MDMA use and a factor potentiating neurotoxicity, we examined the effects of this drug (9 mg/kg, sc; approximately one-fifth of the known LD(50) in rats) on brain [nucleus accumbens (Nacc) and hippocampus (Hippo)] and muscle (musculus temporalis) temperatures in male rats under conditions that either model human drug use (social interaction with female, warm temperature) or restrict heat dissipation from the brain (chronic occlusion of jugular veins). Under quiet resting conditions at 23 degrees C, MDMA induced a moderate but prolonged hyperthermia. Both NAcc and Hippo showed more rapid and stronger temperature increases than muscle, suggesting metabolic neural activation as a primary cause of brain hyperthermia. During social interaction with a female, brain hyperthermia induced by MDMA was significantly potentiated (+89%). Brain hyperthermia induced by MDMA was also strongly potentiated (+188%) in animals with chronically occluded jugular veins, suggesting impaired cerebral outflow enhances intrabrain heat accumulation. At 29 degrees C, MDMA pushed temperatures in the brain to its biological limits (>41 degrees C; +268%), resulting in fatalities in most (83%) tested animals. Therefore, by inducing metabolic brain activation and restricting heat dissipation, MDMA use under 'party' conditions may be much more dangerous than under standard laboratory conditions.

Acetaminophen-induced hypothermia in mice is mediated by a prostaglandin endoperoxide synthase 1 gene-derived protein

Acetaminophen is a widely used antipyretic analgesic, reducing fever caused by bacterial and viral infections and by clinical trauma such as cancer or stroke. In rare cases in humans, e.g., in febrile children or HIV or stroke patients, acetaminophen causes hypothermia while therapeutic blood levels of the drug are maintained. In C57/BL6 mice, acetaminophen caused hypothermia that was dose related and maximum (>2 degrees C below normal) with a dose of 300 mg/kg. The reduction and recovery of body temperature was paralleled by a fall of >90% and a subsequent rise of prostaglandin (PG)E(2) concentrations in the brain. In cyclooxygenase (COX)-2(-/-) mice, acetaminophen (300 mg/kg) produced hypothermia accompanied by a reduction in brain PGE(2) levels, whereas in COX-1(-/-) mice, the hypothermia to this dose of acetaminophen was attenuated. The brains of COX-1(-/-) mice had approximately 70% lower levels of PGE(2) than those of WT animals, and these levels were not reduced further by acetaminophen. The putative selective COX-3 inhibitors antipyrine and aminopyrine also reduced basal body temperature and brain PGE(2) levels in normal mice. We propose that acetaminophen is a selective inhibitor of a COX-1 variant and this enzyme is involved in the continual synthesis of PGE(2) that maintains a normal body temperature. Thus, acetaminophen reduces basal body temperature below normal in mice most likely by inhibiting COX-3.

Thermoeffector neuronal pathways in fever: a study in rats showing a new role of the locus coeruleus

It is known that brain noradrenaline (norepinephrine) mediates fever, but the neuronal group involved is unknown. We studied the role of the major noradrenergic nucleus, the locus coeruleus (LC), in lipopolysaccharide (LPS)-induced fever. Male Wistar rats had their LC completely ablated electrolytically or their catecholaminergic LC neurones selectively lesioned by microinjection of 6-hydroxydopamine; the controls were sham-operated. Both lesions resulted in a marked attenuation of LPS (1 or 10 microg kg(-1), i.v.) fever at a subneutral (23 degrees C) ambient temperature (Ta). Because electrolytic and chemical lesions produced similar effects, the role of the LC in fever was further investigated using electrolytic lesions only. The levels of prostaglandin (PG) E2, the terminal mediator of fever, were equally raised in the anteroventral third ventricular region of LC-lesioned and sham-operated rats during the course of LPS fever, indicating that LC neurones are not involved in febrigenic signalling to the brain. To investigate the potential involvement of the LC in an efferent thermoregulatory neuronal pathway, the thermoregulatory response to PGE(2) (25 ng, i.c.v.) was studied at a subneutral (23 degrees C, when fever is brought about by thermogenesis) or neutral (28 degrees C, when fever is brought about by tail skin vasoconstriction) Ta. The PGE2-induced increases in metabolic rate (an index of thermogenesis) and fever were attenuated in LC-lesioned rats at 23 degrees C, whereas PGE2-induced skin vasoconstriction and fever normally developed in LC-lesioned rats at 28 degrees C. The LC-lesioned rats had attenuated PGE2 thermogenesis despite the fact that they were fully capable of activating thermogenesis in response to noradrenaline and cold exposure. It is concluded that LC neurones are part of a neuronal network that is specifically activated by PGE2 to increase thermogenesis and produce fever.

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.

Platelet-activating factor: a previously unrecognized mediator of fever

Lipopolysaccharide (LPS)-induced systemic inflammation is accompanied by either hypothermia (prevails when the ambient temperature (Ta) is subneutral) or fever (prevails when Ta is neutral or higher). Because platelet-activating factor (PAF) is a proximal mediator of LPS inflammation, it should mediate both thermoregulatory responses to LPS. That PAF possesses hypothermic activity and mediates LPS-induced hypothermia is known. We asked whether PAF possesses pyrogenic activity (Expt 1) and mediates LPS fever (Expt 2). The study was conducted in Long-Evans rats implanted with jugular catheters. A complex with bovine serum albumin (BSA) was infused as a physiologically relevant form of PAF; free (aggregated) PAF was used as a control. In Expt 1, either form of PAF caused hypothermia when infused (83 pmol kg-1 min-1, 60 min, i.v.) at a subneutral Ta of 20 degrees C, but the response to the PAF-BSA complex (-4.5 +/- 0.5 degrees C, nadir) was ~4 times larger than that to free PAF. At a neutral Ta of 30 degrees C, both forms caused fever preceded by tail skin vasoconstriction, but the febrile response to PAF-BSA (1.0 +/- 0.1 degrees C, peak) was > 2 times higher than that to free PAF. Both the hypothermic (at 20 degrees C) and febrile (at 30 degrees C) responses to PAF-BSA started when the total amount of PAF infused was extremely small, < 830 pmol kg-1. In Expt 2 (conducted at 30 degrees C), the PAF receptor antagonist BN 52021 (29 micromol kg-1, i.v.) had no thermal effect of itself. However, it strongly (~2 times) attenuated the febrile response to PAF (5 nmol kg-1, i.v.), implying that this response involves the PAF receptor and is not due to a detergent-like effect of PAF on cell membranes. BN 52021 (but not its vehicle) was similarly effective in attenuating LPS (10 microg kg-1, i.v.) fever. It is concluded that PAF is a highly potent endogenous pyrogenic substance and a mediator of LPS fever.

Warm temperatures activate TRPV4 in mouse 308 keratinocytes

Mammalian survival requires constant monitoring of environmental and body temperature. Recently, several members of the transient receptor potential vanilloid (TRPV) subfamily of ion channels have been identified that can be gated by increases in temperature into the warm (TRPV3 and TRPV4) or painfully hot (TRPV1 and TRPV2) range. In rodents, TRPV3 and TRPV4 proteins have not been detected in sensory neurons but are highly expressed in skin epidermal keratinocytes. Here, we show that in response to warm temperatures (>32 degrees C), the mouse 308 keratinocyte cell line exhibits nonselective transmembrane cationic currents and Ca2+ influx. Both TRPV3 and TRPV4 are expressed in 308 cells. However, the warmth-evoked responses we observe most closely resemble those mediated by recombinant TRPV4 on the basis of their electrophysiological properties and sensitivity to osmolarity and the phorbol ester, 4alpha-phorbol-12,13-didecanoate. Together, these data support the notion that keratinocytes are capable of detecting modest temperature elevations, strongly suggest that TRPV4 participates in these responses, and define a system for the cell biological analysis of warmth transduction.

The organum vasculosum laminae terminalis in immune-to-brain febrigenic signaling: a reappraisal of lesion experiments

The organum vasculosum laminae terminalis (OVLT) has been proposed to serve as the interface for blood-to-brain febrigenic signaling, because ablation of this structure affects the febrile response. However, lesioning the OVLT causes many "side effects" not fully accounted for in the fever literature. By placing OVLT-lesioned rats on intensive rehydration therapy, we attempted to prevent these side effects and to evaluate the febrile response in their absence. After the OVLT of Sprague-Dawley rats was lesioned electrolytically, the rats were given access to 5% sucrose for 1 wk to stimulate drinking. Sucrose consumption and body mass were monitored. The animals were examined twice a day for signs of dehydration and treated with isotonic saline (50 ml/kg sc) when indicated. This protocol eliminated mortality but not several acute and chronic side effects stemming from the lesion. The acute effects included adipsia and gross (14% of body weight) emaciation; chronic effects included hypernatremia, hyperosmolality, a suppressed drinking response to hypertonic saline, and previously unrecognized marked (by approximately 2 degrees C) and long-lasting (>3 wk) hyperthermia. Because the hyperthermia was not accompanied by tail skin vasoconstriction, it likely reflected increased thermogenesis. After the rats recovered from the acute (but not chronic) side effects, their febrile response to IL-1beta (500 ng/kg iv) was tested. The sham-operated rats developed typical monophasic fevers ( approximately 0.5 degrees C), the lesioned rats did not. However, the absence of the febrile response in the OVLT-lesioned rats likely resulted from the untreatable side effects. For example, hyperthermia at the time of pyrogen injection was high enough (39-40 degrees C) to solely prevent fever from developing. Hence, the changed febrile responsiveness of OVLT-lesioned animals is given an alternative interpretation, unrelated to febrigenic signaling to the brain.

Role for the cholecystokinin-A receptor in fever: a study of a mutant rat strain and a pharmacological analysis

The involvement of the cholecystokinin (CCK)-A receptor in fever was studied. The polyphasic febrile responses to lipopolysaccharide (LPS; 10 microg kg-1, I.V.) were compared between wild-type Long-Evans (LE) rats and the CCK-A-receptor-deficient Otsuka LE Tokushima Fatty (OLETF) rats. The response of the wild-type rats was biphasic, which is typical for LE rats. Phases 1 and 2 of the response of the OLETF rats were similar to those of the LE rats, but the OLETF rats also developed a robust phase 3. This late enhancement of the febrile response could reflect either the absence of the A receptor per se or a secondary trait of the mutant strain. To distinguish between these possibilities, we conducted a pharmacological analysis. We studied whether the normally low phase 3 of LE rats can be enhanced by a CCK-A-receptor antagonist, sodium lorglumide (4.3 microg kg-1 min-1, 120 min, I.V.), and whether the normally high phase 3 of Wistar rats can be attenuated by a CCK-A receptor agonist, sulphated CCK-8 (up to 0.17 microg kg-1 min-1, 120 min, I.V.). The dose of sodium lorglumide used was sufficient to increase food intake (to block satiety), but it did not affect the fever response. In both febrile and afebrile rats, CCK-8 induced dose-dependent skin vasodilatation and decreased body temperature, but it failed to produce any effects specific for phase 3. We conclude that the exaggeration of phase 3 in OLETF rats reflects a secondary trait of this strain and not the lack of the CCK-A receptor per se. None of the three known phases of the febrile response of rats to LPS requires the CCK-A receptor.

Expression of genes controlling transport and catabolism of prostaglandin E2 in lipopolysaccharide fever

Prostaglandin (PG) E(2) is a principal downstream mediator of fever and other symptoms of systemic inflammation. Its inactivation occurs in peripheral tissues, primarily the lungs and liver, via carrier-mediated cellular uptake and enzymatic oxidation. We hypothesized that inactivation of PGE(2) is suppressed during LPS fever and that transcriptional downregulation of PGE(2) carriers and catabolizing enzymes contributes to this suppression. Fever was induced in inbred Wistar-Kyoto rats by intravenous LPS (50 microg/kg); the controls received saline. Samples of the liver, lungs, and hypothalamus were harvested 0, 0.5, 1.5, and 5 h postinjection. The expression of the two principal transmembrane PGE(2) carriers (PG transporter and multispecific organic anion transporter) and the two key PGE(2)-inactivating enzymes [15-hydroxy-PG dehydrogenase (15-PGDH) and carbonyl reductase] was quantified by RT-PCR. All four genes of interest were downregulated in peripheral tissues (but not the brain) during fever. Most remarkably, the expression of hepatic 15-PGDH was decreased 26-fold 5 h post-LPS, whereas expression of pulmonary 15-PGDH was downregulated (as much as 18-fold) throughout the entire febrile course. The transcriptional downregulation of several proteins involved in PGE(2) inactivation, first reported here, is an unrecognized mechanism of systemic inflammation. By increasing the blood-brain gradient of PGE(2), this mechanism likely facilitates penetration of PGE(2) into the brain and prevents its elimination from the brain.

Role of L-glutamate in systemic AVP-induced hypothermia

It has been reported that systemic injection of arginine vasopressin (AVP) induces a drop in body core temperature (T(c)), but little is known about the mechanisms involved. Because glutamate is an important excitatory neurotransmitter involved in a number of thermoregulatory actions, in the present study, we tested the hypothesis that glutamate plays a role in systemic AVP-induced hypothermia. Wistar rats were pretreated intracerebroventricularly (icv) with kynurenic acid, an antagonist of l-glutamate ionotropic receptors, alpha-methyl-(4-carboxyphenyl)glycine (MCPG), an antagonist of l-glutamate metabotropic receptors, or saline 15 min before intravenous injection of AVP (2 microg/kg) or saline. T(c), brown adipose tissue (BAT) temperature, blood pressure, heart rate, and tail skin temperature were measured continuously. Administration of saline icv followed by intravenous AVP caused a significant drop in T(c) brought about by a reduction in BAT thermogenesis and an increase in heat loss through the tail. MCPG treatment (icv) did not affect the fall in T(c) induced by AVP. Treatment with kynurenic acid (icv) abolished AVP-induced hypothermia but did not affect the AVP-evoked rise in blood pressure or drop in heart rate and BAT temperature. Heat loss through the tail was significantly reduced in animals injected with AVP and pretrated with kynurenic acid. These data indicate that ionotropic receptors of l-glutamate in the central nervous system participate in peripheral AVP-induced hypothermia by affecting heat loss through the tail.

Prostaglandin E(2)-synthesizing enzymes in fever: differential transcriptional regulation

The febrile response to lipopolysaccharide (LPS) consists of three phases (phases I-III), all requiring de novo synthesis of prostaglandin (PG) E(2). The major mechanism for activation of PGE(2)-synthesizing enzymes is transcriptional upregulation. The triphasic febrile response of Wistar-Kyoto rats to intravenous LPS (50 microg/kg) was studied. Using real-time RT-PCR, the expression of seven PGE(2)-synthesizing enzymes in the LPS-processing organs (liver and lungs) and the brain "febrigenic center" (hypothalamus) was quantified. Phase I involved transcriptional upregulation of the functionally coupled cyclooxygenase (COX)-2 and microsomal (m) PGE synthase (PGES) in the liver and lungs. Phase II entailed robust upregulation of all enzymes of the major inflammatory pathway, i.e., secretory (s) phospholipase (PL) A(2)-IIA --> COX-2 --> mPGES, in both the periphery and brain. Phase III was accompanied by the induction of cytosolic (c) PLA(2)-alpha in the hypothalamus, further upregulation of sPLA(2)-IIA and mPGES in the hypothalamus and liver, and a decrease in the expression of COX-1 and COX-2 in all tissues studied. Neither sPLA(2)-V nor cPGES was induced by LPS. The high magnitude of upregulation of mPGES and sPLA(2)-IIA (1,257-fold and 133-fold, respectively) makes these enzymes attractive targets for anti-inflammatory therapy.

Rat tail skin temperature monitored noninvasively by radiotelemetry: characterization by examination of vasomotor responses to thermomodulatory agents

INTRODUCTION

Measurement of tail skin temperature (T(sk)) of the rat can provide important information on mechanisms pertaining to physiology, pharmacology, and toxicology. T(sk) is largely under control of peripheral vascular tone, which is also sensitive to most experimental manipulations such as handling and restraint. Hence, it is extremely difficult to acquire long-term measurements of T(sk) that are free of artifacts from experimental manipulation. The purpose of this study is to demonstrate the utility of a radiotelemetric probe to provide continuous, long-term measurements of T(sk) in undisturbed rats.

METHODS

A telemetry probe is placed on the base of the tail and secured with a protective guard to prevent the rat from chewing on the probe. T(sk) is continuously monitored with standard radiotelemetric software and computer technology. Core temperature (T(c)) is monitored in duplicate sets of rats at the same time but the current system does not allow for simultaneous measurement of T(sk) and T(c) from the same animal. Rats were subjected to a variety of experimental manipulations to demonstrate the utility of the probe.

RESULTS/DISCUSSION

A marked increase in T(sk) was seen during the transition from light to dark phase, reflecting an increase in heat loss to lower T(c); a decrease in T(sk) during the development of endotoxin-induced fever, reflecting a reduction in heat loss to facilitate an elevation in T(c); an increase in T(sk) following exposure to the organophosphate insecticide chlorpyrifos, reflecting an increase in heat loss to facilitate a hypothermia response; and a direct effect of increasing ambient temperature on T(sk). The probe is relatively inexpensive and is used with no surgery and provides long-term measurement (e.g., >24 h) of T(sk) in unrestrained rats.