iBAT Temperature Research Articles

Conjugated linoleic acid (CLA) reduces HFD-induced obesity by enhancing BAT thermogenesis and iWAT browning via the CD36-AMPK pathway.

The antiobesity effect of conjugated linoleic acid (CLA) has been reported. However, the underlying mechanisms have not been fully clarified. Thus, this study aimed to investigate the effects of CLA on thermogenesis of interscapular brown adipose tissue (iBAT) and browning of inguinal subcutaneous white adipose tissue (iWAT) and explore the possible signaling pathway. The in vivo results showed that CLA enhanced the O2 consumption and heat production in HFD (high-fat diet)-fed female mice by roughly 38%. Meanwhile, CLA increased the average iBAT temperature by 2°C at the room temperature and cold exposure, respectively. Correspondingly, CLA caused 1.6-and 2.4-fold increases in the expression of UCP1 (uncoupling protein 1) of BAT and iWAT, respectively, suggesting the activated iBAT thermogenesis and iWAT browning in HFD-fed female mice. Meanwhile, CLA could promote the formation of brown and beige adipocytes in differentiated stromal vascular cells (SVCs) isolated from iBAT and iWAT (the expressions of UCP1 were promoted by about twofold changes). In possible mechanisms, CLA stimulated the expression of CD36 and the activation of theAMPK pathway in mice iBAT and iWAT as well as the differentiated SVCs. However, inhibition of CD36 and AMPK (adenosine 5'-monophosphate-activated protein kinase) abolished the promotive effects of CLA on brown and beige adipocytes formation. Hence, we showed that CLA reduced HFD-induced obesity through enhancing iBAT thermogenesis and iWAT browning via theCD36-AMPK pathway.

Constitutive Serotonin Tone as a Modulator of Brown Adipose Tissue Thermogenesis: A Rat Study.

Brown adipose tissue (BAT), an important regulator of thermogenic and metabolic processes, is considered a promising target to combat metabolic disorders. The neurotransmitter and hormone serotonin (5HT) is a major modulator of energy homeostasis, with its central and peripheral pools acting in opposing ways. To better understand how individual variations in 5HT homeostasis influence the thermogenic functionality of BAT, we used a rat model consisting of two sublines with constitutively increased (high-5HT) or decreased (low-5HT) whole-body 5HT tone, developed by selective breeding for platelet 5HT parameters. We have shown that animals with constitutively low 5HT activity maintained at a standard housing temperature (22 °C) have greater interscapular BAT (iBAT) mass and higher iBAT metabolic activity (as evidenced by measurements of iBAT temperature and glucose uptake), accompanied by increased iBAT mRNA expression of key thermogenic genes, compared to animals with high 5HT tone. In response to further thermogenic challenges-intermittent cold exposure or treatment with a β3-adrenergic agonist-5HT sublines show several functional and molecular differences linking constitutively low endogenous 5HT tone to higher BAT activity/capacity. Overall, the results support a role of 5-HT in the control of BAT thermogenesis They also suggest that individuals with lower 5HT activity may be more sensitive to β3-adrenergic drugs.

1525-P: Ventromedial Hypothalamic Nucleus Subset Stimulates Tissue Thermogenesis via Preoptic Outputs

Changes in energy status ignite neural circuits that maintain body weight by engaging peripheral mechanisms that control energy expenditure; obesity results for when these responses are no longer sufficient. The key neural circuits that stimulate energy expenditure have not yet been established, but previous research has identified several critical sites (e.g. preoptic area (POA) and ventromedial hypothalamus (VMN)) that could be part of an interconnected system that controls tissue thermogenesis, which is essential for body weight control. Activating SF1-cre VMN cells in male diet-induced obese mice reduces body fat by 25% in 72 hours. This function is likely mediated at least in part by release of the peptide pituitary adenylate cyclase-activating polypeptide (PACAP), since knockout of PACAP in the VMN induces obesity at thermoneutrality (30C) and suppresses temperature above both the brown and beige adipose tissue in response to feeding. Because some VMN cells densely project to the POA, we hypothesized that VMN PACAP cells activate tissue thermogenesis via preoptic outputs. We placed temperature-sensing microchips above a traditional brown (interscapular brown adipose tissue (iBAT) and beige (inguinal white adipose tissue (iWAT) adipose tissue in Adcyap1(gene for PACAP)cre mice with DIO-ChR2 in the VMN and fiber optic implants in two major VMN outputs: the POA and the bed nucleus of the stria terminalis (BST). Activating VMN fibers in the POA that contain PACAP in both male and female mice, but not the BST, increased iBAT and iWAT temperature, and the effect was most robust when the mice were fasted. In summary, these data indicate that VMN PACAP connections with the POA is important for the induction of tissue thermogenesis, particularly when feeding conditions change. VMN PACAP, as well as associated signaling targets, are critical components to energy balance by activating adipose tissue mechanisms that stimulate energy expenditure. Disclosure A.J.Elmendorf: Other Relationship; Eli Lilly and Company. C.A.Mahler: Stock/Shareholder; AbbVie Inc., Pfizer Inc., AstraZeneca. B.Lorentz: Research Support; Eli Lilly and Company. J.Flak: Research Support; Eli Lilly and Company. Funding American Diabetes Association (Pathway 17-INI-15 to J.F.); Ralph W. and Grace M. Showalter Research Trust; Indiana University; Indiana Biosciences Research Institute

Direct detection of brown adipose tissue thermogenesis in UCP1\u2212/\u2212 mice by hyperpolarized 129Xe MR thermometry

Brown adipose tissue (BAT) is a type of fat specialized in non-shivering thermogenesis. While non-shivering thermogenesis is mediated primarily by uncoupling protein 1 (UCP1), the development of the UCP1 knockout mouse has enabled the study of possible UCP1-independent non-shivering thermogenic mechanisms, whose existence has been shown so far only indirectly in white adipose tissue and still continues to be a matter of debate in BAT. In this study, by using magnetic resonance thermometry with hyperpolarized xenon, we produce the first direct evidence of UCP1-independent BAT thermogenesis in knockout mice. We found that, following adrenergic stimulation, the BAT temperature of knockout mice increases more and faster than rectal temperature. While with this study we cannot exclude or separate the physiological effect of norepinephrine on core body temperature, the fast increase of iBAT temperature seems to suggest the existence of a possible UCP1-independent thermogenic mechanism responsible for this temperature increase.

The medial preoptic nucleus as a site of the thermogenic and metabolic actions of melanotan II in male rats.

The present study was designed to investigate the role of the medial preoptic nucleus (MPO) as a site of the thermogenic and metabolic effects of the α-melanocyte-stimulating hormone analog melanotan II (MTII). We also assessed the involvement of the dorsomedial hypothalamic nucleus (DMH) by investigating the effects of the MPO infusion of MTII in rats with DMH lesions produced by kainic acid. Infusion of MTII in the MPO led to increases in interscapular brown adipose tissue (iBAT) temperature and iBAT uptake of 14C-bromopalmitate. Both increases were blocked by DMH lesions. iBAT temperature increase (area under curve) and 14C-bromopalmitate uptake emerged as two correlated variables (r = 0.63, P < 0.001). DMH lesions also blocked MTII-induced expression of mRNAs coding for proteins involved in 1) thermogenesis [type II iodothyronine deiodinase (Dio2) and peroxisome proliferator-activated receptor gamma coactivator 1-α (Pgc1α)], 2) lipolysis [hormone-sensitive lipase (Hsl)], and 3) lipogenesis [diacylglycerol-O-acyltransferase 2 (Dgat2), fatty acid synthase (Fas)], in iBAT of rats killed 1 h after MPO infusion of MTII. MTII also stimulated expression of genes in iWAT but only in rats with DMH lesions. These genes included glucose transporter member 4 (Glut4), glycerol-3-phosphate acyltransferase 3 (Gpat3), Dgat1, Dgat2, triglyceride lipase (Atgl), Hsl, and carnitine palmitoyltransferase 1β (Cpt1β). Altogether, the present results reveal the MPO as a site of the thermogenic and metabolic actions of MTII. They also contribute to establish the MPO-DMH duet as a significant target for melanocortins to modulate energy homeostasis.

The PVH as a Site of CB1-Mediated Stimulation of Thermogenesis by MC4R Agonism in Male Rats

The present study was designed to investigate the involvement of the cannabinoid receptor 1 (CB1) in the stimulating effects of the melanocortin-4 receptor (MC4R) agonism on whole-body and brown adipose tissue (BAT) thermogenesis. In a first series of experiments, whole-body and BAT thermogenesis were investigated in rats infused in the third ventricle of the brain with the MC4R agonist melanotan II (MTII) and the CB1 agonist δ9-tetrahydrocannabinol (δ(9)-THC) or the CB1 antagonist AM251. Whole-body thermogenesis was measured by indirect calorimetry and BAT thermogenesis assessed from interscapular BAT (iBAT) temperature. δ(9)-THC blunted the effects of MTII on energy expenditure and iBAT temperature, whereas AM251 tended to potentiate the MTII effects. δ(9)-THC also blocked the stimulating effect of MTII on (14)C-bromopalmitate and (3)H-deoxyglucose uptakes in iBAT. Additionally, δ(9)-THC attenuated the stimulating effect of MTII on the expression of peroxisome proliferator-activated receptor-γ coactivator 1-α (Pgc1α), type II iodothyronine deiodinase (Dio2), carnitine palmitoyltransferase 1B (Cpt1b), and uncoupling protein 1 (Ucp1). In a second series of experiments, we addressed the involvement of the paraventricular hypothalamic nucleus (PVH) in the CB1-mediated effects of MTII on iBAT thermogenesis, which were assessed following the infusion of MTII in the PVH and δ(9)-THC or AM251 in the fourth ventricle of the brain. We demonstrated the ability of δ(9)-THC to blunt MTII-induced iBAT temperature elevation. δ(9)-THC also blocked the PVH effect of MTII on (14)C-bromopalmitate uptake as well as on Pgc1α and Dio2 expression in iBAT. Altogether the results of this study demonstrate the involvement of the PVH in the CB1-mediated stimulating effects of the MC4R agonist MTII on whole-body and BAT thermogenesis.

Inhibition of the dorsomedial hypothalamus, but not the medullary raphe pallidus, decreases hyperthermia and mortality from MDMA given in a warm environment.

The central mechanisms through which 3,4-methylenedioxymethamphetamine (MDMA) mediates life-threatening hyperthermia when taken in a warm environment are not well described. It is assumed that MDMA alters normal thermoregulatory circuits resulting in increased heat production through interscapular brown adipose tissue (iBAT) and decreased heat dissipation through cutaneous vasoconstriction. We studied the role of the dorsomedial hypothalamus (DMH) and medullary raphe pallidus (mRPa) in mediating iBAT, tail blood flow, and locomotor effects produced by MDMA. Rats were instrumented with guide cannulas targeting either the DMH or the mRPa brain regions involved in regulating iBAT and cutaneous vascular beds. In all animals, core temperature and locomotion were recorded with surgically implanted telemetric transmitters; additionally, either iBAT temperature (via telemetric transmitter) or tail artery blood flow (via tail artery Doppler cuff) were also recorded. Animals were placed in an environmental chamber at 32°C and microinjected with either control or the gamma-aminobutyric acid (GABA) agonist muscimol (80 pmol) followed by an intravenous injection of saline or MDMA (7.5 mg kg−1). To prevent undue suffering, a core temperature of 41°C was chosen as the surrogate marker of mortality. Inhibition of the DMH, but not the mRPa, prevented mortality and attenuated hyperthermia and locomotion. Inhibition of either the DMH or the mRPa did not affect iBAT temperature increases or tail blood flow decreases. While MDMA increases iBAT thermogenesis and decreases heat dissipation through cutaneous vasoconstriction, thermoregulatory brain regions known to mediate these effects are not involved. Rather, the finding that inhibiting the DMH decreases both locomotion and body temperature suggests that locomotion may be a key central contributor to MDMA-evoked hyperthermia.

Respiratory, metabolic and cardiac functions are altered by disinhibition of subregions of the medial prefrontal cortex

The prefrontal cortex (PFC) is referred to as the visceral motor cortex; however, little is known about whether this region influences respiratory or metabolic outflows. The aim of this study was to describe simultaneous changes in respiratory, metabolic and cardiovascular functions evoked by disinhibition of the medial PFC (mPFC) and adjacent lateral septal nucleus (LSN). In urethane-anaesthetized rats, bicuculline methiodide was microinjected (2 mm; GABA-A receptor antagonist) into 90 sites in the mPFC at 0.72-4.00 mm from bregma. Phrenic nerve amplitude and frequency, arterial pressure, heart rate, splanchnic and lumbar sympathetic nerve activities (SNA), expired CO2, and core and brown adipose tissue temperatures were measured. Novel findings included disturbances to respiratory rhythm evoked from all subregions of the mPFC. Injections into the cingulate cortex evoked reductions in central respiratory function exclusively, whereas in ventral sites, particularly the infralimbic region, increases in respiratory drive and frequency, and metabolic and cardiac outflows were evoked. Disinhibition of sites in surrounding regions revealed that the LSN could evoke cardiovascular changes accompanied by distinct oscillations in SNA, as well as increases in respiratory amplitude. We show that activation of neurons within the mPFC and LSN influence respiratory, metabolic and cardiac outflows in a site-dependent manner. This study has implications with respect to the altered PFC neuronal activity seen in stress-related and mental health disorders, and suggests how basic physiological systems may be affected.

The renin–angiotensin system (RAS) in adaptive thermogenesis

It has been reported that the inhibition of angiotensin converter enzyme (ACE) by captopril reduces body weight gain in rats. Recent studies have demonstrated that interscapular brown adipose tissue (iBAT) produces angiotensin II (ANG II), which facilitates sympathetic neurotransmission (SN). In our study we evaluated the role of iBAT in mediate the effect of captopril on body weight. For that we treated rats with 2mg/day of captopril (CAP) in the drinking water for 4 weeks. Captopril treated rats gained less weight when compared to control (105±14 vs. 82±7 CAP; p&lt;0.01). When exposed to cold (4°C), controls and captopril treated rats kept their body temperature within normal values (35.1±1 vs. 36.3±1°C CAP). Food intake was similar for both groups, suggesting an increase in metabolic efficiency in rats treated with Captopril. Interestingly, increases in iBAT temperature after 20′ of Norepinefrine i.v. infusion was significantly higher in Captopril treated rat when compared to controls (1.54±0.1 vs. 2.14±0.2°C CAP, p&gt;0.05). In conclusion, our data suggest that Captopril reduces body weigh by activating facultative thermogenesis through brown adipose tissue.

Leptin Action in the Dorsomedial Hypothalamus Increases Sympathetic Tone to Brown Adipose Tissue in Spite of Systemic Leptin Resistance

Leptin regulates body weight in mice by decreasing appetite and increasing sympathetic nerve activity (SNA), which increases energy expenditure in interscapular brown adipose tissue (iBAT). Diet-induced obese mice (DIO) are resistant to the anorectic actions of leptin. We evaluated whether leptin still stimulated sympathetic outflow in DIO mice. We measured iBAT temperature as a marker of SNA. We found that obese hyperleptinemic mice have higher iBAT temperature than mice on regular diet. Conversely, obese leptin-deficient ob/ob mice have lower iBAT temperature. Additionally, leptin increased SNA in obese (DIO and ob/ob) and control mice, despite DIO mice being resistant to anorectic action of leptin. We demonstrated that neurons in the dorsomedial hypothalamus (DMH) of DIO mice mediate the thermogenic responses to hyperleptinemia in obese mammals because blockade of leptin receptors in the DMH prevented the thermogenic effects of leptin. Peripheral Melotan II (MTII) injection increased iBAT temperature, but it was blunted by blockade of DMH melanocortin receptors (MC4Rs) by injecting agouti-related peptide (AgRP) directly into the DMH, suggesting a physiological role of the DMH on temperature regulation in animals with normal body weight. Nevertheless, obese mice without a functional melanocortin system (MC4R KO mice) have an increased sympathetic outflow to iBAT compared with their littermates, suggesting that higher leptin levels drive sympathoexcitation to iBAT by a melanocortin-independent pathway. Because the sympathetic nervous system contributes in regulating blood pressure, heart rate, and hepatic glucose production, selective leptin resistance may be a crucial mechanism linking adiposity and metabolic syndrome.

SR59230A, a beta-3 adrenoceptor antagonist, inhibits ultradian brown adipose tissue thermogenesis and interrupts associated episodic brain and body heating

Brown adipose tissue (BAT) thermogenesis occurs episodically in an ultradian manner approximately every 80-100 min during the waking phase of the circadian cycle, together with highly correlated increases in brain and body temperatures, suggesting that BAT thermogenesis contributes to brain and body temperature increases. We investigated this in conscious Sprague-Dawley rats by determining whether inhibition of BAT thermogenesis via blockade of beta-3 adrenoceptors with SR59230A interrupts ultradian episodic increases in brain and body temperatures and whether SR59230A acts on BAT itself or via sympathetic neural control of BAT. Interscapular BAT (iBAT), brain, and body temperatures, tail artery blood flow, and heart rate were measured in unrestrained rats. SR59230A (1, 5, or 10 mg/kg ip), but not vehicle, decreased iBAT, body, and brain temperatures in a dose-dependent fashion (log-linear regression P < 0.01, R(2) = 0.3, 0.4, and 0.4, respectively, n = 10). Ultradian increases in BAT, brain, and body temperature were interrupted by administration of SR59230A (10 mg/kg ip) compared with vehicle, resuming after 162 ± 24 min (means ± SE, n = 10). SR59230A (10 mg/kg ip) caused a transient bradycardia without any increase in tail artery blood flow. In anesthetized rats, SR59230A reduced cooling-induced increases in iBAT temperature without affecting cooling-induced increases in iBAT sympathetic nerve discharge. Inhibition of BAT thermogenesis by SR59230A, thus, reflects direct blockade of beta-3 adrenoceptors in BAT. Interruption of episodic ultradian increases in body and brain temperature by SR59230A suggests that BAT thermogenesis makes a substantial contribution to these increases.

When administered to rats in a cold environment, 3,4-methylenedioxymethamphetamine reduces brown adipose tissue thermogenesis and increases tail blood flow: Effects of pretreatment with 5-HT1A and dopamine D2 antagonists

When given in a warm environment MDMA (3,4-methylenedioxymethamphetamine, ecstasy) causes hyperthermia by increasing interscapular brown adipose tissue (iBAT) heat production and decreasing heat loss via cutaneous vasoconstriction. When given in a cold environment, however, MDMA causes hypothermia by an unknown mechanism. This paper addresses these mechanisms and in addition examines whether antagonists at 5-HT1A and D2 receptors reduce the hypothermic action of MDMA. Male Sprague–Dawley rats instrumented with a Doppler probe for measuring tail blood flow, and probes for measuring core and iBAT temperatures, were placed in a temperature-controlled chamber. The chamber temperature was reduced to 10 °C and vehicle (0.5 ml Ringer), the 5-HT1A antagonist WAY 100635 (0.5 mg/kg), the D2 antagonist spiperone (20 μg/kg), or the combination of Way 100635 and spiperone were injected s.c. Thirty minutes later the antagonists were injected again along with MDMA (10 mg/kg) or vehicle. MDMA reduced core body temperature by preventing cold-elicited iBAT thermogenesis and by transiently reversing cold-elicited cutaneous vasoconstriction. Pretreatment with WAY 100635 prevented MDMA induced increases in tail blood flow, and briefly attenuated MDMA's effects on iBAT and core temperature. While spiperone alone failed to affect any of the parameters, the combination of spiperone and WAY 100635 decreased MDMA-mediated hypothermia by attenuating both the effects on tail blood flow and iBAT thermogenesis. MDMA's prevention of cold-induced iBAT thermogenesis appears to have a central origin as it rapidly reverses cold-induced increases in iBAT sympathetic nerve discharge in anesthetized rats. Our results demonstrate that MDMA in a cold environment reduces core body temperature by inhibiting iBAT thermogenesis and tail artery vasoconstriction and suggest that mechanisms by which this occurs include the activation of 5-HT1A and dopamine D2 receptors.

Dopamine D2 receptor stimulation inhibits cold-initiated thermogenesis in brown adipose tissue in conscious rats

Dopamine D2-like receptor agonists cause hypothermia. We investigated whether inhibiting heat production by interscapular brown adipose tissue (iBAT), a major thermogenic organ in rats, contributes to hypothermia caused by dopamine D2-like receptor agonists. Temperature of iBAT and tail artery blood flow were measured in conscious rats. Activity in postganglionic sympathetic nerves supplying iBAT was assessed in anesthetized rats. Conscious rats were housed in a warm cage maintained at 26–28 °C and then transferred to a cold cage at 5–10 °C to induce iBAT thermogenesis. Cold exposure increased iBAT temperature (+0.7±0.1 °C, 30 min after transferring to the cold cage, P<0.01, n=54). The mixed dopamine D2/D3 receptor agonist, 7-hydroxy-2-(di-n-propylamino)tetralin (7-OH-DPAT, 0.5 mg/kg s.c.) reversed the cold-induced increase in iBAT temperature (−2.8±0.2 °C at 30 min after 7-OH-DPAT treatment during cold exposure vs. +0.3±0.1 °C at 30 min after vehicle treatment during cold exposure, n=8). These temperature changes were blocked by pre-treatment with the D2 receptor antagonists spiperone (20 μg/kg i.p.) and L-741,626 (2.5 mg/kg i.p.), but not by the selective D3 receptor antagonist SB-277011A (10 mg/kg i.p.). Another mixed dopamine D2/D3 receptor agonist, quinpirole (0.5 mg/kg s.c.) also reversed cold-induced iBAT thermogenesis, and this effect was also prevented by pre-treatment with spiperone, but not with a peripherally acting dopamine receptor antagonist, domperidone (2 mg/kg s.c.). Neither 7-OH-DPAT nor quinpirole reversed cutaneous vasoconstriction elicited by cold exposure. In anesthetized rats, quinpirole (0.5 mg/kg i.v.) abolished iBAT sympathetic nerve discharge elicited by cooling the trunk, and this change was reversed by spiperone (20 μg/kg i.v.). These results demonstrate that activation of CNS dopamine D2 receptors inhibits sympathetically-mediated iBAT thermogenesis in response to cold exposure. Furthermore, they suggest that in rats hypothermia induced by dopamine D2 receptor agonists in cold environments is mainly due to decreased heat production rather than to increased heat loss.

Clozapine reverses increased brown adipose tissue thermogenesis induced by 3,4-methylenedioxymethamphetamine and by cold exposure in conscious rats

Clozapine, an atypical antipsychotic agent important for the treatment of schizophrenia, has marked inhibitory effects on sympathetic outflow to the thermoregulatory cutaneous circulation. In rabbits clozapine reverses ear pinna vasoconstriction induced either by administration of MDMA (3,4-methylenedioxymethamphetamine, ecstasy) or by exposing the animal to a cold environment. In rats, both these procedures are known to increase sympathetic activation of interscapular brown adipose tissue (iBAT) thermogenesis, important for heat production in the rat. In the present study in conscious rats we determined whether clozapine reduces iBAT thermogenesis induced by MDMA and by exposure to cold. We designed our study so that we could also determine effects of clozapine on the acute (stress-induced) increases in iBAT thermogenesis initiated by the process of s.c. injection. MDMA increased iBAT temperature (+1.7±0.2 °C after 90 min, P<0.01, n=14 measurements from seven rats each studied on two occasions). Clozapine acutely reversed the MDMA-elicited increase in iBAT temperature (−1.3±0.2 °C 60 min after clozapine treatment following MDMA versus +0.3±0.2 °C for 60 min after vehicle treatment following MDMA, P<0.01, n=7). Clozapine also reduced stress-induced increases in iBAT temperature, as well as increases elicited by exposing rats to a cold (5 °C) environment. Results, taken together with our previous findings, suggest that MDMA activates the sympathetic thermoregulatory outputs (including the output to iBAT) that defend body temperature against cold exposure and that increase body temperature in response to environmental stress. Clozapine’s marked inhibition of iBAT thermogenesis may provide a clue to its marked tendency to cause obesity when used to treat humans with mental disorders including schizophrenia. Our demonstration in rats that clozapine decreases sympathetically-mediated increases in iBAT temperature elicited by MDMA adds to the likelihood that clozapine and clozapine-like agents might be therapeutically effective in life threatening hyperthermia induced by MDMA in humans.

Thermogenesis in brown adipose tissue: Increase by 5-HT2A receptor activation and decrease by 5-HT1A receptor activation in conscious rats

Body temperature is decreased by 5-hydroxytryptamine 1A (5-HT1A) agonists and increased by 5-HT2A agonists. The present study determined whether changes in interscapular brown adipose tissue (iBAT) thermogenesis contribute to these effects in conscious unrestrained animals. Male Sprague-Dawley rats were pre-instrumented for measurement of iBAT and core temperature and tail artery blood flow one week before experiments. In the first series of experiments, rats were transferred from warm (25–28 °C) to cold (5–10 °C) environments. This increased iBAT temperature (+1.3 ± 0.2 °C, P < 0.01, n = 7) and reduced tail artery flow. Injection of the 5-HT1A agonist, 8-OH-DPAT (8-hydroxy-2-(di- n-propylamino)tetralin, 0.5 mg/kg, s.c.) reversed the increase in iBAT thermogenesis (−1.5 ± 0.4 °C, P < 0.01, n = 6), and decreased core temperature (−1.5 ± 0.4 °C, P < 0.01, n = 6). Pre-treatment with WAY-100635 ( N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)- N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride), a 5-HT1A antagonist, prevented effects of 8-OH-DPAT. In the second series of experiments, injection of a 5-HT2A agonist, DOI ( R(−)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride, 0.1 mg/kg, s.c.) increased both iBAT (+1.9 ± 0.1 °C, P < 0.01, n = 7) and core temperatures (+1.4 ± 0.2 °C, P < 0.01, n = 7), and decreased tail artery blood flow. Subsequent injection of SR 46349B ( trans-4-((3 Z)3-[(2-dimethylaminoethyl)oxyimino]-3-(2-fluorophenyl) propen-1-yl)-phenol, hemifumarate, 0.5 mg/kg, s.c.), a 5-HT2A antagonist, reduced all these changes. Results indicate that activation of 5-HT1A receptors reduces sympathetic outflow to BAT and that activation of 5-HT2A receptors increases this outflow. Changes in core temperature mediated by brain/spinal pathways regulated by 5-HT1A and 5-HT2A receptors reflect coordinated changes in BAT-mediated heat production as well as changes in heat dissipation via the thermoregulatory cutaneous vascular beds.

Suppression of PGE(2) fever at near term: reduced thermogenesis but not enhanced vasopressin antipyresis.

Fevers are known to be suppressed near term in the mother, but the mechanism responsible for this phenomenon is not understood. We tested the hypothesis that the suppression of fever at term is a result of enhanced vasopressin-induced antipyresis. Effects of intracerebroventricular prostaglandin E(2) (PGE(2)) were examined in rats at gestational days 16-17 and 19-20 (near term) and days 1-2 postpartum. PGE(2) (50 ng) elevated body and interscapular brown adipose tissue (iBAT) temperatures and increased sympathetic nerve activity to the iBAT. PGE(2)-induced changes in iBAT temperature and nerve activity, as well as in rectal temperature, were reduced or eliminated near term, and responses were recovered in the postpartum period. Blood pressure and heart rate changes induced by central PGE(2) were also decreased at near term. Coinfusion of Manning compound, a V(1) vasopressin receptor antagonist, with PGE(2) throughout the peripartum period did not reverse the suppressed iBAT temperature and nerve activity or body temperature responses to PGE(2). Microdialysis experiments revealed unchanged terminal release of vasopressin in the ventral septal area after PGE(2) infusion in either pregnant or parturient rats. These results suggest that fever reduction at near term is not associated with enhanced vasopressin antipyresis, but may be a result of reduced sympathetic tone and in particular a reduced sympathetic drive to the iBAT. This finding may reflect a generalized reduction in autonomic output around the time of parturition.

Specific thermal responsiveness of ventromedial hypothalamic neurons to localized scrotal heating and cooling in rats.

1. The specificity of thermoresponsive ventromedial hypothalamic (VMH) neurons to localized, incremental scrotal thermal cooling and heating of urethane-anaesthetized male Sprague-Dawley rats (maintained at 37 degrees C colonically) was investigated. 2. Ventromedial hypothalamic extracellular neuronal activity and surface (scalp) electroencephalogram (EEG) activity from the parietal region were recorded. Intrascapular brown adipose tissue (TIBAT), surface tail (Tt) and scrotal (Tsc) temperatures, where thermal stimulation was evoked, were also monitored. 3. One hundred and twenty-five VMH neurons were recorded, with forty (32%) VMH neurons classified as warm-responsive neurons (WRNs), twenty-three (18%) as coldresponsive neurons (CRNs) and sixty-two (50%) as thermal non-responsive neurons (TNRNs) based on their thermal coefficients. Of VMH WRNs, 60% (i.e. 24) were classified as having biphasic neuronal activity responses, as were 60% (i.e. 14 of 23) of the CRNs. Forty per cent of WRNs and CRNs were classified as having monophasic changes in neuronal activity. 4. Scrotal heating or cooling from 5 to 40 degrees C resulted in specific firing rate changes of VMH WRNs and CRNs without any associated change in EEG activity (i.e. no significant change in EEG frequency or amplitude from initial baseline EEG activity when Tsc was 20 degrees C). EEG desynchronization (increased EEG frequency, decreased amplitude) was only observed when scrotal temperatures were at 45 degrees C or after each tail pinch (noxious stimulation) but not with scrotal brushing (mechanical stimulation). 5. With core temperature maintained at 37 degrees C, localized, scrotal heating and cooling of rats did not induce IBAT temperature changes indicative of brown adipose tissue activation, but delayed changes in tail temperature, indicative of vasoactive effector responses, did occur.

Brown adipose tissue thermogenesis is activated by electrical and chemical ( l-glutamate) stimulation of the ventromedial hypothalamic nucleus in cold-acclimated rats

Experiments were conducted to determine if both electrical and chemical stimulation of the ventromedial hypathalamic nucleus (VMH) could activate brown adipose tissue (BAT) thermogenesis. Age-matched, room-acclimated (21 °C) and cold-acclimated (4 °C for 3 weeks prior to testing) male Sprague-Dawley rats were given unilateral electrical or chemical stimulation to the VMH by way of a ‘chemotrode apparatus’. The devised ‘chemotrode’ allowed both electrical stimulation (insulated piano wire stimulating electrode) and chemical stimulation (23 gauge stainless steel intracranial cannula of equal length) to be performed at the same VMH site using a common 19 gauge stainless steel outer guide tube. The first unilateral VMH electrical stimulation (0.5 ms pulse, 50 Hz and 120 μA for 30 s) caused no significant rise in interscapular brown adipose tissue temperature (T bat ) colonic (T c) or tail surface temperatures (T t), compared to respective prestimulation control values in rats acclimated to 21 °C. In the 4 °C-acclimated group the first VMH electrical stimulation caused a significant rise in IBAT temperature. l-Glutamate administration to the same VMH site (60 nmol in 600 nl volume) also caused a significant increase in IBAT temperature in the 4 °C but not the 21 °C-acclimated rats. The rise in IBAT temperature following the l-glutamate injection to the 4 °C-acclimated group was similar to that found following the first electrical stimulation to this group. Interestingly, a second unilateral electrical stimulation of the VMH to 4 °C-acclimated rats could not evoke a similar increase in IBAT temperature suggesting that overall l-glutamate was acting in vivo as an excitotoxin. The results indicate that both unilateral chemical and electrical stimulation of the VMH of 4 °C but not 21 °C acclimated, anesthetized male Sprague-Dawley rats can evoke significant rises in IBAT temperature, indicative of activation of BAT thermogenesis by this CNS site. These results infer that cold acclimation causes facilitation of BAT thermogenesis peripherally and/or centrally such that subsequent electrical or chemical stimulation of the VMH can then evoke significant rises in IBAT temperature in those animals.