Excess recovery heat production by isolated muscles from mice overexpressing uncoupling protein-3

Body temperature regulation and drugs of abuse

Phenethylamine-induced hyperthermia can occur following exposure to several different types of illicit stimulants, such as amphetamine, methamphetamine, 3,4-methylenedioxymethamphetamine ("Molly"), synthetic cathinones ("bath salts"), and N-methoxybenyl ("NBOMe"), to name a few. Peripheral norepinephrine release mediated by these sympathomimetic agents induces a double-edged sword of heat accumulation through β-adrenoreceptor-dependent activation of uncoupling protein (UCP1 and 3)-regulated thermogenesis and loss of heat dissipation through α₁-adrenoreceptor-mediated vasoconstriction. Additionally, thyroid hormones are important determinants of the capacity of thermogenesis induced by phenethylamines through the regulation of free fatty acid release and the transcriptional activation of a host of metabolic genes, including adrenergic receptors and mitochondrial uncoupling proteins. Here, we review the central and peripheral mechanistic "triggers" of phenethylamine-induced hyperthermia and outline potential pharmacologic interventions for managing phenethylamine-induced hyperthermia based on these recently discovered hyperthermia mediators.

3,4-Methylenedioxymethamphetamine induces a hyperthermic and hypermetabolic crisis in pigs with and without a genetic disposition for malignant hyperthermia

BACKGROUND

Clinical symptoms of acute 3,4-methylenedioxymethamphetamine (MDMA) intoxication and malignant hyperthermia have many similarities. At present, however, there is contradictory evidence concerning the malignant hyperthermia trigger potency of MDMA.

OBJECTIVE

This study was designed to investigate whether MDMA has malignant hyperthermia trigger potential and leads to malignant hyperthermia in pigs with or without a genetic predisposition to the condition. In addition, the therapeutic effectiveness of a new dantrolene sodium suspension was examined.

DESIGN

Experimental study, using an animal model of Piétrain pigs.

SETTINGS

Institute for Research in Operative Medicine, University of Witten/Herdecke, Hospital Cologne Merheim, Cologne, Germany, October 2006 to February 2007. Trigger-free anaesthesia was performed on seven malignant hyperthermia-susceptible and six malignant hyperthermia-normal Piétrain pigs, and cumulative doses of MDMA were administered to each animal.

INTERVENTIONS

After achieving predefined malignant hyperthermia criteria, standardised therapy was initiated; dantrolene sodium suspension (5 mg kg(-1)) was administered and the injection was repeated after 24 min.

MAIN OUTCOME MEASURES

The malignant hyperthermia trigger potency of MDMA was analysed by monitoring pH, PaCO2 and temperature. In addition, concentrations of thyroid hormone, mitochondrial uncoupling protein 3, noradrenaline and free fatty acids during administration of MDMA and dantrolene sodium suspension were analysed.

RESULTS

MDMA administration led to fulminant hypermetabolic and hyperthermic responses in malignant hyperthermia-susceptible and malignant hyperthermia-normal pigs, with significant decreases in pH (susceptible: pH 7.21 ± 0.11, normal: pH 7.21 ± 0.07), severe hypercapnia (susceptible: paCO2 10.3 ± 3.5 kPa, normal: paCO2 9.8 ± 1.7 kPa), and hyperthermia (susceptible: 40.6 ± 2.0°C, normal: 40.1 ± 0.4°C). There were no significant differences in changes in clinical and laboratory variables between groups. The dantrolene therapy regimen was effective in treating the MDMA-induced metabolic crises.

CONCLUSION

MDMA is not a classic trigger for the development of malignant hyperthermia reactions in pigs. MDMA intoxication leads to severe, long-lasting hyperthermia and hypermetabolism in both malignant hyperthermia-susceptible and hyperthermia-normal pigs, with life-threatening malignant hyperthermia-like symptoms which are responsive to supportive treatment and dantrolene sodium suspension.

Variation in the uncoupling protein 2 and 3 genes and human performance

Uncoupling proteins 2 and 3 (UCP2 and UCP3) may negatively regulate mitochondrial ATP synthesis and, through this, influence human physical performance. However, human data relating to both these issues remain sparse. Examining the association of common variants in the UCP3/2 locus with performance phenotypes offers one means of investigation. The efficiency of skeletal muscle contraction, delta efficiency (DE), was assessed by cycle ergometry in 85 young, healthy, sedentary adults both before and after a period of endurance training. Of these, 58 were successfully genotyped for the UCP3-55C>T (rs1800849) and 61 for the UCP2-866G>A (rs659366) variant. At baseline, UCP genotype was unrelated to any physical characteristic, including DE. However, the UCP2-866G>A variant was independently and strongly associated with the DE response to physical training, with UCP2-866A allele carriers exhibiting a greater increase in DE with training (absolute change in DE of -0.2 ± 3.6% vs. 1.7 ± 2.8% vs. 2.3 ± 3.7% for GG vs. GA vs. AA, respectively; P = 0.02 for A allele carriers vs. GG homozygotes). In multivariate analysis, there was a significant interaction between UCP2-866G>A and UCP3-55C>T genotypes in determining changes in DE (adjusted R(2) = 0.137; P value for interaction = 0.003), which was independent of the effect of either single polymorphism or baseline characteristics. In conclusion, common genetic variation at the UCP3/2 gene locus is associated with training-related improvements in DE, an index of skeletal muscle performance. Such effects may be mediated through differences in the coupling of mitochondrial energy transduction in human skeletal muscle, but further mechanistic studies are required to delineate this potential role.

Cell based in vitro and ex vivo models in metabolic disease drug discovery: nice to have or critical path?

BACKGROUND

The use of cellular models as tools in drug discovery is almost universal. However, in disease areas such as metabolic diseases, are they relevant to the process and do they add value?

OBJECTIVE

In this article, we explore the variety of cellular models now used in drug discovery in metabolic diseases as revealed by publication. We have tried to make some connections between drug phenotypes in these models with clinical parallels. We also ask the question as to whether such models add value in the drug discovery process. This overview is not about recombinant cell systems used in target-based screening; rather, we focus on in vitro, including ex vivo, models as physiological systems in drug discovery in obesity and diabetes.

CONCLUSION

In terms of building target confidence, in vitro models are often the only mechanistic link to human systems early in a projects life. Many of the current targets in metabolic diseases in the early discovery phase are not yet clinically supported, let alone validated. In this respect, therefore, in vitro models warrant a place in the critical path in early discovery. In terms of any predictive role for decision-making today, this is much more difficult and is more likely pushed to a supporting role as part of a wider package. However, there is a rapid rate of advancement in this field and future developments hold much promise.

Influence of dietary fats on Ecstasy-induced hyperthermia

BACKGROUND AND PURPOSE

Studies were designed to examine the effects of dietary fats on metabolic effects of 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy). These effects included hyperthermia, expression of uncoupling protein (UCP1 and 3) in brown adipose tissue or skeletal muscle and plasma free fatty acid (FFA) levels.

EXPERIMENTAL APPROACH

Male Sprague-Dawley rats were fed either a high-fat diet (HFD, 60% kcal) or a lower fat isocaloric controlled diet (LFD, 10% kcal) for 28 days before MDMA challenge.

KEY RESULTS

No significant differences were observed between LFD and HFD groups in terms of body weight, plasma thyroxine (T4) levels and expression of brown fat UCP1 or skeletal muscle UCP3 protein. HFD significantly raised levels of circulating FFA and potentiated the thermogenesis induced by MDMA (10 mg kg(-1), s.c.), compared to the effects of the LFD. Moreover, 30 and 60 min after MDMA administration, plasma FFA levels decreased in HFD animals, but were markedly elevated in the LFD group.

CONCLUSIONS AND IMPLICATIONS

These results indicate that high-fat feeding regulates MDMA-induced thermogenesis by augmenting the activation of UCP rather than its expression.

Uncoupling protein 3 and physical activity: the role of uncoupling protein 3 in energy metabolism revisited

Physical activity influences energy metabolism in human subjects by increasing activity-induced energy expenditure and resting metabolic rate for several hours after exercise. On the other hand, physical activity increases mechanical energy efficiency, suggesting that trained subjects would need less energy for daily activities. The underlying mechanism by which physical activity influences energy metabolism is largely unknown. The skeletal muscle-specific homologue of uncoupling protein (UCP) 1, UCP3, could possibly play a major role in energy expenditure. UCP3 is, like UCP1, able to uncouple respiration from ATP production. A strong link or association between theUCP3gene and energy metabolism was found. Furthermore, UCP3 mRNA expression is related to sleeping metabolic rate, and thyroid hormone, a powerful stimulator of energy expenditure, up regulates UCP3. Finally, mice overexpressing UCP3 are hyperphagic but lean. These findings indicated that UCP3 is related to energy metabolism and that UCP3 could have a role in the effect of physical activity on energy expenditure. Thus, acute exercise up regulates UCP3, whereas endurance training results in the down-regulation of UCP3 protein content. Only a minimal amount of physical activity is needed for down-regulation of UCP3. Moreover, there is very strong evidence that UCP3 is negatively related to mechanical energy efficiency, suggesting that the down-regulation of UCP3 with training increases mechanical energy efficiency. Taken together, although the exact function of UCP3 is still unknown, exercise and training studies clearly show that under certain circumstances UCP3 is strongly related to human energy metabolism, possibly as a secondary effect of its (yet) unknown primary function.

The Mu class glutathione transferase is abundant in striated muscle and is an isoform-specific regulator of ryanodine receptor calcium channels

Members of the glutathione transferase (GST) structural family are novel regulators of cardiac ryanodine receptor (RyR) calcium channels. We present the first detailed report of the effect of endogenous muscle GST on skeletal and cardiac RyRs. An Mu class glutathione transferase is specifically expressed in human muscle. An hGSTM2-2-like protein was isolated from rabbit skeletal muscle and sheep heart, at concentrations of approximately 17-93 microM. When added to the cytoplasmic side of RyRs, hGSTM2-2 and GST isolated from skeletal or cardiac muscle, modified channel activity in an RyR isoform-specific manner. High activity skeletal RyR1 channels were inactivated at positive potentials or activated at negative potentials by hGSTM2-2 (8-30 microM). Inactivation became faster as the positive voltage was increased. Channels recovered from inactivation when the voltage was reversed, but recovery times were significantly slowed in the presence of hGSTM2-2 and muscle GSTs. Low activity RyR1 channels were activated at both potentials. In contrast, hGSTM2-2 and GSTs isolated from muscle (1-30 microM) in the cytoplasmic solution, caused a voltage-independent inhibition of cardiac RyR2 channels. The results suggest that the major GST isoform expressed in muscle regulates Ca2+ signalling in skeletal and cardiac muscle and conserves Ca2+ stores in the sarcoplasmic reticulum.

Roles of norepinephrine, free Fatty acids, thyroid status, and skeletal muscle uncoupling protein 3 expression in sympathomimetic-induced thermogenesis

Thyroid hormone (TH) plays a fundamental role in thermoregulation, yet the molecular mediators of its effects are not fully defined. Recently, skeletal muscle (SKM) uncoupling protein (UCP) 3 was shown to be an important mediator of the thermogenic effects of the widely abused sympathomimetic agents 3,4-methylenedioxymethamphetamine (MDMA; Ecstasy) and methamphetamine. Expression of UCP3 is regulated by TH. Activation of UCP3 is indirectly regulated by norepinephrine (NE) and is dependent upon the availability of free fatty acids (FFAs). We hypothesized that UCP3 may be a molecular link between TH and hyperthermia, requiring increased levels of both NE and FFAs to accomplish the thermogenic effect. Here, we demonstrate that MDMA (40 mg/kg s.c.) significantly increases plasma FFA levels 30 min after treatment. Pharmacologically increasing NE levels through the inhibition of phenylethanolamine N-methyltransferase with +/-2,3-dichloro-alpha-methylbenzylamine potentiated the hyperthermic effects of a 20 mg/kg dose of MDMA. Using Western blots and regression analysis, we further illustrated that chronic hyperthyroidism in rats potentiates the hyperthermic effects of MDMA and increases levels of SKM UCP3 protein in a linear fashion according to levels of circulating plasma TH. Conversely, chronic hypothyroidism results in a hypothermic response to MDMA that is directly proportionate to decreased UCP3 expression. Acute TH supplementation did not change the skeletal muscle UCP3 expression levels or temperature responses to MDMA. These findings suggest that, although MDMA-induced hyperthermia appears to result from increased NE and FFA levels, susceptibility is ultimately determined by TH regulation of UCP3-dependent thermogenesis.

Regulation of mitochondrial uncoupling proteins in mouse inner ear ganglion cells in response to systemic kanamycin challenge

Mitochondrial uncoupling proteins are a proton transporter family involved in regulation mitochondrial superoxide and ATP production. Uncoupling proteins are expressed by rat spiral ganglion and vestibular ganglion cells [Hear Res 196 (2004) 39]. This study tests the hypothesis that uncoupling protein expression is up-regulated in response to the reactive oxygen species challenge imposed by kanamycin and antioxidant (2,3-dihydroxybenzoate) treatment in mice. In control C57BL/6, CBA/J and BALB/c mice, mRNA for uncoupling protein 1, uncoupling protein 2, uncoupling protein 3, Slc25a27 (uncoupling protein 4) and Slc25a14 (uncoupling protein 5/BMCP1) was expressed in the spiral and vestibular ganglia. After kanamycin-treatment (700 mg/kg twice daily for 14 days s.c.), uncoupling protein 2 and uncoupling protein 3 mRNA expression increased significantly in spiral and vestibular ganglia and kidney, but was unaffected in cerebral cortex. Significant Slc25a27 (uncoupling protein 4) mRNA up-regulation was also observed in spiral and vestibular ganglia, but not in kidney or cerebral cortex. These effects were blocked by simultaneous administration of kanamycin and 2,3-dihydroxybenzoate (300 mg/kg twice daily for 14 days s.c.). Western immunoblotting and immunohistochemistry confirmed the uncoupling protein 2 and uncoupling protein 3 up-regulation in inner ear. Finally, 2,3-dihydroxybenzoate treatment alone produced an upregulation of uncoupling protein 1 mRNA in the spiral ganglion, vestibular ganglion and cerebral cortex, but not the kidney. Uncoupling protein 2 and uncoupling protein 3 upregulation in the kidney and inner ear ganglia likely reflects their general role as a feedback pathway to reduce mitochondrial superoxide generation. Slc25a27 (uncoupling protein 4) upregulation in the inner ear ganglia, by contrast, is likely to be a secondary response to kanamycin-induced hair cell death. We propose that increased uncoupling protein 2, uncoupling protein 3 and Slc25a27 expression has several neuroprotective effects via reduction in mitochondrial superoxide generation and local thermogenesis, including: (1) reducing mean ROS load to prevent apoptosis, (2) increasing signal-to-noise characteristics of intracellular ROS signaling pathways (e.g. lipoxygenases, growth factor and transcription factors), (3) heat-related alteration of enzyme kinetics and (4) promotion of cell depolarization (activation of heat-gated ion channels).

The role of the sympathetic nervous system and uncoupling proteins in the thermogenesis induced by 3,4-methylenedioxymethamphetamine

Body temperature regulation involves a homeostatic balance between heat production and dissipation. Sympathetic agents such as 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) can disrupt this balance and as a result produce an often life-threatening hyperthermia. The hyperthermia induced by MDMA appears to result from the activation of the sympathetic nervous system (SNS) and the hypothalamic-pituitary-thyroid/adrenal axis. Norepinephrine release mediated by MDMA creates a double-edged sword of heat generation through activation of uncoupling protein (UCP3) along with alpha1- and beta3-adrenoreceptors and loss of heat dissipation through SNS-mediated vasoconstriction. This review examines cellular mechanisms involved in MDMA-induced thermogenesis from UCP activation to vasoconstriction and how these mechanisms are related to other thermogenic conditions and potential treatment modalities.

Localization of the mitochondrial uncoupling protein family in the rat inner ear

Uncoupling proteins (UCPs) are a proton transporter family located in the mitochondrial inner membrane. The molecular expression and activity of UCPs in brown adipose tissue and skeletal muscle are regulated by factors as diverse as chronic overeating and cold exposure, suggesting roles in energy expenditure and heat production. Although UCP2, UCP4 and brain mitochondrial carrier protein-1 (BMCP-1, i.e. UCP5) mRNAs are expressed in the central nervous system, their central function is unknown. This study presents the first evidence on localization and quantitative expression of UCPs in the rat inner ear by real-time PCR and immunohistochemistry. Real-time PCR studies revealed that UCP2 mRNA was expressed in the vestibular and spiral ganglia more abundantly than any other UCP. Neocortex, by contrast, contained UCP2 and UCP4 equally. Notably, UCP3 and UCP4 mRNAs were expressed in inner ear ganglia, but brain UCP3 mRNA expression level was undetectable by simple PCR. Immunohistochemical studies confirmed that both UCP2- and UCP3-like immunoreactivities were detected in vestibular and spiral ganglion cells and co-localized with a mitochondrial marker, MitoFluorGreen. According to previous reports, UCP2 and UCP3 are thermogenic in yeast and brain UCP2 has been suggested to modulate pre- and post-synaptic events by axonal thermogenesis. It has also been reported recently that UCP2 and UCP3 responses to superoxide application may be an antioxidant protective mechanism. Therefore, it is suggested that mitochondrial UCPs (UCP2, UCP3, UCP4) may play both a protective role against oxidative damage and a thermal signaling role in the eighth nerve.

Slow skeletal muscles of the mouse have greater initial efficiency than fast muscles but the same net efficiency

The aim of this study was to determine whether the net efficiency of mammalian muscles depends on muscle fibre type. Experiments were performed in vitro (35 degrees C) using bundles of muscle fibres from the slow-twitch soleus and fast-twitch extensor digitorum longus (EDL) muscles of the mouse. The contraction protocol consisted of 10 brief contractions, with a cyclic length change in each contraction cycle. Work output and heat production were measured and enthalpy output (work + heat) was used as the index of energy expenditure. Initial efficiency was defined as the ratio of work output to enthalpy output during the first 1 s of activity. Net efficiency was defined as the ratio of the total work produced in all the contractions to the total, suprabasal enthalpy produced in response to the contraction series, i.e. net efficiency incorporates both initial and recovery metabolism. Initial efficiency was greater in soleus (30 +/- 1%; n=6) than EDL (23 +/- 1%; n=6) but there was no difference in net efficiency between the two muscles (12.6 +/- 0.7% for soleus and 11.7 +/- 0.5% for EDL). Therefore, more recovery heat was produced per unit of initial energy expenditure in soleus than EDL. The calculated efficiency of oxidative phosphorylation was lower in soleus than EDL. The difference in recovery metabolism between soleus and EDL is unlikely to be due to effects of changes in intracellular pH on the enthalpy change associated with PCr hydrolysis. It is suggested that the functionally important specialization of slow-twitch muscle is its low rate of energy use rather than high efficiency.

Thermoregulation of transgenic growth hormone mice

Transgenic growth hormone (TG) mice (Mus musculus L., 1758) obtain enhanced growth via compensatory feeding at intermediate sizes and via higher growth efficiency. The latter involves diverting resources from other functions such as locomotion and wakefulness. Thermogenesis is a major expense for small mammals, so we explored whether TG mice express a trade-off between growth and thermoregulation. TG mice are hypothermic and cannot maintain their body temperature under cold stress. TG mice showed initial enlargement of brown adipose tissue and subsequent age-related decreases not seen in controls. Some TG mice became torpid after fasting durations not known to affect other mice. On a high-calorie diet, TG mice had higher body temperatures even though controls did not. Our background strain developed obesity on a high-protein and high-fat diet, and on a diet supplemented with carbohydrates, whereas TG mice never developed obesity. White adipose tissue deposits of TG females were relatively larger, but those of TG males were relatively smaller, than those of controls fed standard food. We also found significant effects of the three experimental diets, as well as gender, age, body mass, ambient temperature, and behavioural activity, on rectal temperatures of TG mice and controls in a large breeding colony. Thermogenesis of TG mice fed standard food appears energetically constrained, likely contributing to enhanced growth efficiency.

Attenuation of 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy)-induced rhabdomyolysis with alpha1- plus beta3-adrenoreceptor antagonists

1. Studies were designed to examine the effects of alpha(1) (alpha(1)AR)- plus beta(3)-adrenoreceptor (beta(3)AR) antagonists on 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy)-induced hyperthermia and measures of rhabdomyolysis (creatine kinase (CK)) and renal function (blood urea nitrogen (BUN) and serum creatinine (sCr)) in male Sprague-Dawley rats. 2. MDMA (40 mg x kg(-1), s.c.) induced a rapid and robust increase in rectal temperature, which was significantly attenuated by pretreatment with the alpha(1)AR antagonist prazosin (100 microg x kg(-1), i.p.) plus the beta(3)AR antagonist SR59230A (5 mg x kg(-1), i.p.). 3. CK levels significantly increased (peaking at 4 h) after MDMA treatment and were blocked by the combination of prazosin plus SR59230A. 4. At 4 h after MDMA treatment, BUN and sCr levels were also significantly increased and could be prevented by this combination of alpha(1)AR- plus beta(3)AR-antagonists. 5. The results from this study suggest that alpha(1)AR and beta(3)AR play a critical role in the etiology of MDMA-mediated hyperthermia and subsequent rhabdomyolysis.

Cardiac energetics: sense and nonsense

1. The background to current ideas in cardiac energetics is outlined and, in the genomic era, the need is stressed for detailed knowledge of mouse heart mechanics and energetics. 2. The mouse heart is clearly different to the rat in terms of its excitation-contraction (EC) coupling and the common assumption that heart rate difference between mice and humans will account for the eightfold difference in myocardial oxygen consumption is wrong, because the energy per beat of the mouse heart is approximately one-third that of the human heart. 3. In vivo evidence suggests that there may well be an eightfold species difference in the non-beating metabolism of mice and human hearts. It is speculated that the magnitude of basal metabolism in the heart is regulatable and that, in the absence of perfusion, it falls to approximately one-quarter of its in vivo rate and that in clinical conditions, such as hibernation, it probably decreases; its magnitude may be controlled by the endothelium. 4. The active energy balance sheet is briefly discussed and it is suggested that the activation heat accounts for 20-25% of the active energy per beat and cross-bridge turnover accounts for the balance. It is argued that force, not shortening, is the major determinant of cardiac energy usage. 5. The outcome of recent cardiac modelling with variants of the Huxley and Hill/Eisenberg models is described. It has been necessary to invoke 'loose coupling' to replicate the low cardiac energy flux measured at low afterloads (medium to high velocities of shortening). 6. Lastly, some of the unexplained or 'nonsense' energetic data are outlined and eight unsolved problems in cardiac energetics are discussed.

Muscle type difference in the regulation of UCP3 under cold conditions

We found opposite regulation of uncoupling protein 3 (UCP3) in slow-twitch soleus and fast-twitch gastrocnemius muscles of rats during cold exposure. Namely, the UCP3 mRNA level was downregulated in the soleus muscles, but upregulated in the gastrocnemius muscles after a 24-h cold exposure. In the analysis of UCP3 protein, we first succeeded in detecting UCP3 short-form as well as the long-form in vivo, which levels were decreased markedly in the cold-exposed soleus muscles. However, the levels of UCP3 and cytochrome oxidase subunit IV were well maintained in the cold-exposed gastrocnemius muscles with a rise in the total mitochondrial protein level, suggesting an increase of total oxidative ability. The fast-twitch muscle rather than the slow-twitch one may play an important role in adaptive responses, including thermogenesis under acute cold exposure.

Hypothalamic-pituitary-thyroid axis and sympathetic nervous system involvement in hyperthermia induced by 3,4-methylenedioxymethamphetamine (Ecstasy)

An acute and potentially life-threatening complication associated with the recreational use of the 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) is hyperthermia. In the present study, Sprague-Dawley rats treated with MDMA (40 mg/kg s.c.) responded with a significant increase (maximal at 1 h) in rectal and skeletal muscle temperatures that lasted for at least 3 h post-treatment. Hypophysectomized (HYPO) and thyroparathyroidectomized (TX) animals treated with MDMA (40 mg/kg s.c.) did not become hyperthermic and in fact displayed a significant hypothermia. The HYPO and TX animals were also resistant to the serotonergic neurotoxic effects of MDMA assessed by serotonin measurements 4 to 7 days later in the striatum and hippocampus. MDMA (40 mg/kg s.c.) induced a significant increase in thyroxine levels 1 h post-treatment. Thyroid hormone replacement in TX animals returned the hyperthermic response seen after MDMA. Prazosin, an alpha(1)-antagonist (0.2 mg/kg i.p.), administered 30 min before MDMA significantly attenuated the MDMA-induced increase in rectal temperature, but had no effect on skeletal muscle temperature. Cyanopindolol, a beta(3)-antagonist (4 mg/kg s.c.), administered 30 min before MDMA (40 mg/kg s.c.) significantly attenuated the increase in skeletal muscle temperature, but had no effect on the rise in rectal temperature. The combination of prazosin and cyanopindolol resulted in an abolishment of MDMA-induced hyperthermia. The mechanisms of thermogenesis induced by MDMA seem to result from an interaction between the hypothalamic-pituitary-thyroid axis and the sympathetic nervous system, wherein mechanisms leading to core and skeletal muscle hyperthermia after MDMA exposure seem to be differentially regulated by alpha(1)- and beta(3)-adrenergic receptors.

The mitochondrial uncoupling proteins

The uncoupling proteins (UCPs) are transporters, present in the mitochondrial inner membrane, that mediate a regulated discharge of the proton gradient that is generated by the respiratory chain. This energy-dissipatory mechanism can serve functions such as thermogenesis, maintenance of the redox balance, or reduction in the production of reactive oxygen species. Some UCP homologs may not act as true uncouplers, however, and their activity has yet to be defined. The UCPs are integral membrane proteins, each with a molecular mass of 31-34 kDa and a tripartite structure in which a region of around 100 residues is repeated three times; each repeat codes for two transmembrane segments and a long hydrophilic loop. The functional carrier unit is a homodimer. So far, 45 genes encoding members of the UCP family have been described, and they can be grouped into six families. Most of the described genes are from mammals, but UCP genes have also been found in fish, birds and plants, and there is also functional evidence to suggest their presence in fungi and protozoa. UCPs are encoded in their mature form by nuclear genes and, unlike many nuclear-encoded mitochondrial proteins, they lack a cleavable mitochondrial import signal. The information for mitochondrial targeting resides in the first loop that protrudes into the mitochondrial matrix; the second matrix loop is essential for insertion of the protein into the inner mitochondrial membrane. UCPs are regulated at both the transcriptional level and by activation and inhibition in the mitochondrion.