Increases In Brown Adipose Tissue Sympathetic Nerve Activity Research Articles

Mediobasal hypothalamic neurons contribute to the control of brown adipose tissue sympathetic nerve activity and cutaneous vasoconstriction

The mediobasal hypothalamus (MBH) contains heterogeneous neuronal populations that regulate food intake and energy expenditure. However, the role of MBH neurons in the neural control of thermoeffector activity for thermoregulation is not known. This study sought to determine the effects of modulating the activity of MBH neurons on the sympathetic outflow to brown adipose tissue (BAT), BAT thermogenesis, and cutaneous vasomotion. Pharmacological inhibition of MBH neurons by local administration of muscimol, a GABAA receptor agonist, reduced skin cooling-evoked BAT thermogenesis, expired CO2, body temperature, heart rate, and mean arterial pressure, while blockade of GABAA receptors by nanoinjection of bicuculline in the MBH induced large increases in BAT sympathetic nerve activity (SNA), BAT temperature, body temperature, expired CO2, heart rate, and cutaneous vasoconstriction. Neurons in the MBH send projections to neurons in the dorsal hypothalamic area and dorsomedial hypothalamus (DMH), which excite sympathetic premotor neurons in the rostral raphe pallidus area (rRPa) that control sympathetic outflow to BAT. The increases in BAT SNA, BAT temperature, and expired CO2 elicited by blockade of GABAA receptors in the MBH were reversed by blocking excitatory amino acid receptors in the DMH or in the rRPa. Together, our data show that MBH neurons provide a modest contribution to BAT thermogenesis for cold defense, while GABAergic disinhibition of these neurons produces large increases in the sympathetic outflow to BAT, and cutaneous vasoconstriction. Activation of glutamate receptors on BAT thermogenesis-promoting neurons of the DMH and rRPa is necessary for the increased sympathetic outflow to BAT evoked by disinhibition of MBH neurons. These data demonstrate neural mechanisms that contribute to the control of thermoeffector activity, and may have important implications for regulating body temperature and energy expenditure.

Systemic Interleukin‐1β Elicits Cyclooxygenase‐Independent Fever

Interleukin‐1β (IL‐1β) induces neuroimmune responses that help to fight infections. IL‐1β‐induced fever is not abolished by cyclooxygenase (COX) inhibitors used to treat inflammation and the neural pathways responsible for COX‐independent fever are still unknown. Considering that neurons that reside in the rostral raphe pallidus area (rRPa) and in the dorsomedial hypothalamus (DMH) play a key role in the control of thermoeffectors, we hypothesized that neurons in these areas contribute to COX‐independent fever elicited by intravenous (iv) IL‐1β. Female and male Sprague Dawley rats were anesthetized with urethane and α‐chloralose (ur/ch) or isoflurane (iso) and treated with saline or indomethacin, prior to receiving iv IL‐1β. Systemic IL‐1β increased brown adipose tissue (BAT) sympathetic nerve activity (SNA) (+2852 ± 2020% baseline (BL), p = 0.003), BAT temperature (TBAT; +2.6 ± 1.3°C, p < 0.001), expired CO2 (+1.0 ± 0.4%, p < 0.001), core body temperature (TCORE; +0.6 ± 0.2°C, p < 0.001), heart rate (HR; +102 ± 42 bpm, p < 0.001), mean arterial pressure (MAP; +11 ± 7 mmHg, p = 0.002), and paw temperature (TPAW; ‐6.9 ± 1.2°C, p < 0.001), which is an index of cutaneous vasoconstriction (CVC). Indomethacin did not prevent IL‐1β‐evoked increases in BAT SNA (+2545 ± 756% BL, p < 0.001), TBAT (+2.9 ± 1.3°C, p < 0.001), expired CO2 (+1.2 ± 0.6%, p = 0.003), TCORE (+0.9 ± 0.5°C, p = 0.005), HR (+117 ± 47 bpm, p < 0.001), MAP (+27 ± 20 mmHg, p = 0.01), or TPAW (‐5.9 ± 3.4°C, p = 0.02). Nanoinjection of muscimol in the rRPa prevented IL‐1β‐evoked increases in BAT SNA (p < 0.01), TBAT (p < 0.01), expired CO2 (p < 0.01) and TCORE (p < 0.01), but not in HR (p = 0.08), in comparison to the group that received IL‐1β only. Nanoinjection of muscimol in the rRPa in indomethacin‐pretreated rats prevented IL‐1β‐evoked increases in BAT SNA (p < 0.01), TBAT (p < 0.01), expired CO2 (p < 0.01), TCORE (p < 0.01), and TPAW (p = 0.03), but not in HR (p = 0.12), in comparison to an indomethacin‐pretreated group that received IL‐1β. Nanoinjections of glutamate receptor antagonists in the DMH markedly reduced IL‐1β‐evoked increases in BAT SNA (‐838 ± 542% BL, p = 0.03), TBAT (‐1.1 ± 0.4°C, p = 0.003), expired CO2 (‐0.4 ± 0.1%, p = 0.009) and HR (‐38 ± 20 bpm, p = 0.01), and reduced MAP (‐4 ± ‐2 mmHg, p = 0.02). Plasma PGE2 levels following iv IL‐1β were significantly higher (by ~3 fold) in rats that were not anesthetized when receiving IL‐1b (263.1 ± 136.4 pg/ml) compared to all other groups of rats that were maintained under anesthesia for 2 h (ur/ch anesthetized, saline pretreated: 96.7 ± 30.8 pg/ml, p = 0.007, or indomethacin pretreated: 119.7 ± 51.9 pg/ml, p = 0.02; iso anesthetized, saline pretreated: 82.5 ± 21.2 pg/ml, p = 0.003, or indomethacin‐pretreated: 108.4 ± 39.1 pg/ml, p = 0.01). Therefore, we suggest that the anesthetics used inhibited the systemic production of PGE2. Together, our data suggest that a glutamatergic input to the DMH is necessary for providing the excitatory drive for increasing BAT thermogenesis and tachycardia in response to circulating IL‐1β, likely via rRPa neurons. Additionally, IL‐1β‐evoked CVC requires activation of neurons in the rRPa. These data define critical components of the neural circuit for systemic IL‐1β‐induced fever and provide a foundation for elucidating additional brain mechanisms responsible for COX‐independent fever.

Systemic serotonin inhibits brown adipose tissue sympathetic nerve activity via a GABA input to the dorsomedial hypothalamus, not via 5HT1A receptor activation in raphe pallidus.

Serotonin (5-hydroxytryptamine, 5-HT), an important neurotransmitter and hormone, modulates many physiological functions including body temperature. We investigated neural mechanisms involved in the inhibition of brown adipose tissue (BAT) sympathetic nerve activity (SNA) and BAT thermogenesis evoked by 5-HT. Electrophysiological recordings, intravenous (iv) injections and nanoinjections in the brains of anaesthetized rats. Cooling-evoked increases in BAT SNA were inhibited by the intra-rostral raphé pallidus (rRPa) and the iv administration of the 5-HT1A receptor agonist, 8-OH-DPAT or 5-HT. The intra-rRPa 5-HT, the intra-rRPa and the iv 8-OH-DPAT, but not the iv 5-HT-induced inhibition of BAT SNA were prevented by nanoinjection of a 5-HT1A receptor antagonist in the rRPa. The increase in BAT SNA evoked by nanoinjection of NMDA in the rRPa was not inhibited by iv 5-HT, indicating that iv 5-HT does not inhibit BAT SNA by acting in the rRPa or in the sympathetic pathway distal to the rRPa. In contrast, under a warm condition, blockade of 5HT1A receptors in the rRPa increased BAT SNA and BAT thermogenesis, suggesting that endogenous 5-HT in the rRPa contributes to the suppression of BAT SNA and BAT thermogenesis. The increases in BAT SNA and BAT thermogenesis evoked by nanoinjection of NMDA in the dorsomedial hypothalamus (DMH) were inhibited by iv 5-HT, but those following bicuculline nanoinjection in the DMH were not inhibited. The systemic 5-HT-induced inhibition of BAT SNA requires a GABAergic inhibition of BAT sympathoexcitatory neurones in the DMH. In addition, during warming, 5-HT released endogenously in rRPa inhibits BAT SNA.

Abstract 083: Divergent Metabolic and Cardiovascular Effects of FGF21

Fibroblast growth factor 21 (FGF21) is a pleiotropic hormone that regulates energy homeostasis. We previously demonstrated that FGF21 act in the brain to increase thermogenic sympathetic nerve activity (SNA), and that this effect required for the weight-reducing action of this hormone. However, the role of FGF21 in cardiovascular sympathetic control has not been investigated. To address this, we examined the effect of FGF21 on renal SNA recorded simultaneously with thermogenic brown adipose tissue (BAT) SNA in mice. In line with our previous findings, intravenous infusion of FGF21 (1mg/kg for 30 min) led to a significant increase in BAT SNA (182±33%, p<0.05), but not renal SNA (-35±16%). Intracerebroventricular administration of FGF21 (1 μg) that increased BAT SNA also failed to increase renal SNA (-14±28%) in mice. Next, we tested the effect of FGF21 on telemetry blood pressure and heart rate in mice. Chronic treatment with FGF21 (1mg/kg BID, IP) for 5 days led to a significant decrease in body weight (p=0.01), but caused no significant change in blood pressure (MAP: 3±3 mmHg vs 4±4 mmHg in vehicle control group, p=0.43) and heart rate (16±24 bpm vs 14±24 bpm in controls, p=0.5). Similarly, chronic treatment with the potent long-acting rhFGF21 analog (10mg/kg, SC) decreased body weight (p=0.002), but failed to alter blood pressure (p=0.14) and heart rate (p=0.25). Finally, we studied transgenic mice that overexpress FGF21 leading to ~5-10 fold higher circulating FGF21 than in the control mice. Interestingly, transgenic FGF21 mice displayed lower blood pressure (MAP: 104±1 vs 117±1 mmHg in controls, p<0.001) without change in heart rate (p=0.09). Consistent with the lower body weight (17.7± 0.8g vs 34.4± 1.3g in wild type controls), transgenic FGF21 mice displayed a 125% greater (p<0.001) baseline BAT SNA relative to controls whereas renal SNA was only 16% higher (p=0.02) in the transgenic mice. Together, our findings show contrasting effects of FGF21 on SNA controlling metabolism relative to blood pressure. The selective action of FGF21 may contribute to the differential sympathetic regulation of metabolism vs cardiovascular function.

Systemic Administration of Serotonin Reduces the Excitability of the Raphé Pallidus‐Brown Adipose Tissue Sympathetic Nerve Pathway

Activation of 5‐HT1A receptors in the raphé pallidus (RPa) inhibits the skin cooling‐evoked increases in brown adipose tissue (BAT) sympathetic nerve activity (SNA). Our aim was to determine whether intravenous (iv) 5‐HT would affect BAT SNA and if 5‐HT1A receptors in the RPa play a role in mediating any of the effects of iv 5‐HT. In urethane/α‐chloralose anesthetized, neuromuscularly blocked, ventilated rats, electrical stimulation (100–150 μA, single 1 ms pulses) of the RPa evoked compound potentials in BAT SNA. Iv infusion of 5‐HT (500 μg/h) reduced the amplitude of the RPa‐evoked compound potentials in BAT SNA. Skin cooling‐evoked increases in BAT SNA were also inhibited by the iv infusion of 5‐HT, and by an iv bolus of the 5‐HT1A/7 receptor agonist, 8‐OHDPAT (100 μg in 50 μl). Nanoinjection of the 5‐HT1A receptor antagonist, WAY100635 (10 mM, 100 nl), in the RPa completely prevented the iv 8‐OHDPAT‐induced inhibition of BAT SNA, but not that following iv 5‐HT. In conclusion, iv 5‐HT reduces the excitability of the sympathetic pathway from the RPa to BAT, but this effect is not mediated by activation of 5‐HT1A receptors in the RPa. In contrast, activation of 5‐HT1A receptors in the RPa plays a key role in the inhibition of cold‐evoked BAT SNA by iv administration of 8‐OHDPAT.Support or Funding InformationSupported by São Paulo Research Foundation (FAPESP) grant #2017/24232‐9 (C.M.D.M.) and by NIH R01‐NS091066 (S.F.M.).This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Tonic inhibition of brown adipose tissue sympathetic nerve activity via muscarinic acetylcholine receptors in the rostral raphe pallidus.

A tonically active, muscarinic cholinergic inhibition of rostral raphe pallidus (rRPa) neurons influences thermogenesis of brown adipose tissue (BAT) independent of ambient temperature conditions. The tonically active cholinergic input to rRPa originates caudal to the hypothalamus. Muscarinic acetylcholine receptor (mAChR) activation in rRPa contributes to the inhibition of BAT sympathetic nerve activity (SNA) evoked by activation of neurons in the rostral ventrolateral medulla (RVLM). The RVLM is not the sole source of the muscarinic cholinergic input to rRPa. Activation of GABA receptors in rRPa does not mediate the cholinergic inhibition of BAT SNA. We sought to determine if body temperature and energy expenditure are influenced by a cholinergic input to neurons in the rostral raphe pallidus (rRPa), the site of sympathetic premotor neurons controlling thermogenesis of brown adipose tissue (BAT). Nanoinjections of the muscarinic acetylcholine receptor (mAChR) agonist, oxotremorine, or the cholinesterase inhibitor, neostigmine (NEOS), in the rRPa of anaesthetized rats decreased cold-evoked BAT sympathetic nerve activity (SNA, nadirs: -72 and -95%), BAT temperature (Tbat, -0.5 and -0.6°C), expired CO2 (Exp. CO2 , -0.3 and -0.5%) and heart rate (HR, -22 and -41bpm). NEOS into rRPa reversed the increase in BAT SNA evoked by blockade of GABA receptors in rRPa. Nanoinjections of the mAChR antagonist, scopolamine (SCOP), in the rRPa of warm rats increased BAT SNA (peak: +1087%), Tbat (+1.8°C), Exp. CO2 (+0.7%), core temperature (Tcore, +0.5°C) and HR (+54bpm). SCOP nanoinjections in rRPa produced similar activations of BAT during cold exposure, following a brain transection caudal to the hypothalamus, and during the blockade of glutamate receptors in rRPa. We conclude that a tonically active cholinergic input to the rRPa inhibits BAT SNA via activation of local mAChR. The inhibition of BAT SNA mediated by mAChR in rRPa does not depend on activation of GABA receptors in rRPa. The increase in BAT SNA following mAChR blockade in rRPa does not depend on the activity of neurons in the hypothalamus or on glutamate receptor activation in rRPa.

Glycinergic inhibition of BAT sympathetic premotor neurons in rostral raphe pallidus.

The rostral raphe pallidus (rRPa) contains sympathetic premotor neurons controlling thermogenesis in brown adipose tissue (BAT). We sought to determine whether a tonic activation of glycineA receptors (GlyAR) in the rRPa contributes to the inhibitory regulation of BAT sympathetic nerve activity (SNA) and of cardiovascular parameters in anesthetized rats. Nanoinjection of the GlyAR antagonist, strychnine (STR), into the rRPa of intact rats increased BAT SNA (peak: +495%), BAT temperature (TBAT, +1.1°C), expired CO2, (+0.4%), core body temperature (TCORE, +0.2°C), mean arterial pressure (MAP, +4 mmHg), and heart rate (HR, +57 beats/min). STR into rRPa in rats with a postdorsomedial hypothalamus transection produced similar increases in BAT thermogenic and cardiovascular parameters. Glycine nanoinjection into the rRPa evoked a potent inhibition of the cooling-evoked increases in BAT SNA (nadir: -74%), TBAT (-0.2°C), TCORE (-0.2°C), expired CO2 (-0.2%), MAP (-8 mmHg), and HR (-22 beats/min) but had no effect on the increases in these variables evoked by STR nanoinjection into rRPa. Nanoinjection of GABA into the rRPa inhibited the STR-evoked BAT SNA (nadir: -86%) and reduced the expired CO2 (-0.4%). Blockade of glutamate receptors in rRPa reduced the STR-evoked increases in BAT SNA (nadir: -61%), TBAT (-0.5°C), expired CO2 (-0.3%), MAP (-9 mmHg), and HR (-33 beats/min). We conclude that a tonically active glycinergic input to the rRPa contributes to the inhibitory regulation of the discharge of BAT sympathetic premotor neurons and of BAT thermogenesis and energy expenditure.

Abstract 041: mTORC1 is Required for Leptin-induced Sympathetic Activation to the Kidney but Not Brown Adipose Tissue

The adipocyte-derived hormone leptin plays a critical role in the regulation of energy homeostasis through its action in the brain to decrease food intake and promote energy expenditure by increasing sympathetic nerve activity (SNA) to the thermogenic brown adipose tissue (BAT). Leptin also increases SNA to cardiovascular organs including the kidney and raises arterial pressure. However, it is unclear whether leptin controls regional SNA via conserved or distinct molecular mechanisms. Multiple intracellular pathways have been associated with leptin signaling including the mechanistic target of rapamycin complex 1 (mTORC1), which has been proposed as a critical determinant of leptin action. Here, we assessed the contribution of mTORC1 signaling to leptin-evoked regional sympathetic activation. Simultaneous multifiber recording of renal and BAT SNA in anesthetized C57BL/6J mice showed that intracerebroventricular (ICV) administration of leptin (2μg, n=5) increased both renal (170±34%) and BAT (208±37%) SNA. Interestingly, ICV pre-treatment with the mTORC1 inhibitor (rapamycin, 5ng, n=6) abolished the leptin-induced increase in renal (10±6%, P<0.05 vs controls) but not BAT (226±31%) SNA. Next, we used conditional knockout mice that lack the critical mTORC1 subunit, Raptor, specifically in leptin receptor (LRb)-expressing cells (LRb Cre /Raptor fl/fl ) to determine the long-term effects of disrupting mTORC1 signaling on leptin-evoked increase in regional SNA. We confirmed the inability of leptin to activate mTORC1 signaling in LRb-expressing cells of LRb Cre /Raptor fl/fl mice relative to controls using immunohistochemical staining of phosphorylated ribosomal S6, a downstream target of mTORC1. We observed a significant increase in renal SNA in response to ICV leptin in control mice (127±16%, n=9), but not in LRb Cre /Raptor fl/fl mice (-4±15%, n=9, P<0.05 vs controls). Conversely, ICV leptin-induced increase in BAT SNA was not different in LRb Cre /Raptor fl/fl mice (109±27%, n=5) vs. littermate controls (173±52%, n=4). Our data suggest a critical role for mTORC1 signaling in selectively mediating the cardiovascular sympathetic but not the thermogenic actions of leptin, with important implications for obesity-associated hypertension.

Respiratory modulation of brown adipose tissue sympathetic nerve activity during activation of dorsomedial hypothalamic neurons

Activation of dorsomedial hypothalamic (DMH) neurons contributes to the increases in brown adipose tissue (BAT) sympathetic nerve activity (SNA), BAT thermogenesis and heart rate (HR) induced by skin cooling and during the febrile response mimicked by injection of PGE2 into the medial preoptic area. Neurons in the same DMH region mediate increases in HR, and potentially other autonomic variables, during stress responses. In this study, the relationship between BAT SNA and phrenic nerve activity (PHR) was characterized during activation of localized populations of DMH neurons with nanoinjections of NMDA in urethane/chloralose-anesthetized, vagotomized, artificially-ventilated rats. NMDA in DMH usually elicited increases both in BAT SNA and in PHR amplitude and rate, although the optimal sites for evoking maximal increases in BAT SNA and in PHR were rarely the same. The bursts in evoked BAT SNA displayed a strong respiratory modulation with a phase relationship to the PHR which was not different from that observed during skin cooling-evoked activation of BAT SNA. These data are consistent with the availability of a descending drive to BAT and to respiration from likely separate populations of DMH neurons and with an influence of central respiratory networks on BAT SNA that is mediated at a site caudal to the DMH. Supported by NIH grant NS40987.

Intragastric administration of capsiate, a transient receptor potential channel agonist, triggers thermogenic sympathetic responses

The sympathetic thermoregulatory system controls the magnitude of adaptive thermogenesis in correspondence with the environmental temperature or the state of energy intake and plays a key role in determining the resultant energy storage. However, the nature of the trigger initiating this reflex arc remains to be determined. Here, using capsiate, a digestion-vulnerable capsaicin analog, we examined the involvement of specific activation of transient receptor potential (TRP) channels within the gastrointestinal tract in the thermogenic sympathetic system by measuring the efferent activity of the postganglionic sympathetic nerve innervating brown adipose tissue (BAT) in anesthetized rats. Intragastric administration of capsiate resulted in a time- and dose-dependent increase in integrated BAT sympathetic nerve activity (SNA) over 180 min, which was characterized by an emergence of sporadic high-activity phases composed of low-frequency bursts. This increase in BAT SNA was abolished by blockade of TRP channels as well as of sympathetic ganglionic transmission and was inhibited by ablation of the gastrointestinal vagus nerve. The activation of SNA was delimited to BAT and did not occur in the heart or pancreas. These results point to a neural pathway enabling the selective activation of the central network regulating the BAT SNA in response to a specific stimulation of gastrointestinal TRP channels and offer important implications for understanding the dietary-dependent regulation of energy metabolism and control of obesity.

Inhibition of brown adipose tissue thermogenesis by neurons in the ventrolateral medulla and in the nucleus tractus solitarius

Neurons in the ventrolateral medulla (VLM) and in the nucleus tractus solitarius (NTS) play important roles in the regulation of cardiovascular and other autonomic functions. In the present study, we demonstrate an inhibition of brown adipose tissue (BAT) thermogenesis evoked by activation of neurons in the VLM, as well as by neurons in the intermediate NTS, of chloralose/urethane-anesthetized, artificially ventilated rats. Activation of neurons in either rostral VLM or caudal VLM with N-methyl-d-aspartate (12 nmol) reversed the cold-evoked increase in BAT sympathetic nerve activity (SNA), BAT temperature, and end-expired CO(2). Disinhibition of neurons in either VLM or NTS with the GABA(A) receptor antagonist, bicuculline (30 pmol), reversed the increases in BAT SNA, BAT temperature, and end-expired CO(2) that were elicited 1) by cold defense; 2) during the febrile model of nanoinjection of prostaglandin E(2) into the medial preoptic area; 3) by activation of neurons in the dorsomedial hypothalamus or in the rostral raphe pallidus (rRPa); or 4) by the micro-opioid receptor agonist fentanyl. Combined, but not separate, inhibitions of neurons in the VLM and in the NTS, with the GABA(A) receptor agonist, muscimol (120 pmol/site), produced increases in BAT SNA, BAT temperature, and expired CO(2), which were reversed by nanoinjection of glycine (30 nmol) into the rRPa. These findings suggest that VLM and NTS contain neurons whose activation inhibits BAT thermogenesis, that these neurons receive GABAergic inputs that are active under these experimental conditions, and that neurons in both sites contribute to the tonic inhibition of sympathetic premotor neuronal activity in the rRPa that maintains a low level of BAT thermogenesis in normothermic conditions.

Neural circuits underlying the inhibition of sympathetically‐mediated thermogenesis in brown adipose tissue (BAT) induced by atypical antipsychotic drugs

Administration of atypical antipsychotic drugs suppresses sympathetic activation of metabolism in BAT, which may contribute to the weight gain observed with maintenance on these medications. This study investigated the neural circuitry responsible for the antipsychotic drug‐evoked inhibition of the sympathetic outflow to BAT. In urethane/chloralose anesthetized rats, the increase in BAT sympathetic nerve activity (SNA) evoked by cooling the core temperature to ~35.5°C was completely inhibited by the atypical antipsychotic drug, Clozapine (Cloz; 3mg/kg, iv). Bilateral microinjection of muscimol (2mM, 120nl/side) into the ventrolateral medulla (VLM) reversed or prevented the Cloz‐evoked inhibition of BAT SNA. In contrast to the effect of Cloz on the cooling response, the increase in BAT SNA evoked by blockade of GABAA receptors within the rostral raphe pallidus (RPa) was not inhibited by Cloz (3–6mg/kg, iv). These results suggest that neurons within the VLM play a role in the inhibition of BAT SNA evoked by atypical antipsychotic drugs, possibly by activating a GABAergic input to the RPa. Supported by NIH grant DK082558 and an American Heart Association SDG.

Neurons in the paraventricular nucleus of the hypothalamus inhibit sympathetic outflow to brown adipose tissue

The paraventricular nucleus of the hypothalamus (PVH) plays an important role in energy homeostasis, regulating neuroendocrine, behavioral, and autonomic functions. However, the role of the PVH in regulating thermogenesis and energy expenditure in brown adipose tissue (BAT) is unclear. The present study investigated the effect of activating neurons within the PVH on BAT thermogenesis. In urethane- and chloralose-anesthetized, artificially ventilated rats maintained at a core body temperature of 37.0-38.0 degrees C, microinjection of N-methyl-d-aspartate (NMDA, 12 pmol in 60 nl) in the PVH did not increase BAT sympathetic nerve activity (SNA) or BAT thermogenesis. In contrast, the increase in BAT SNA evoked by body cooling was completely reversed by microinjection of NMDA in the PVH. Additionally, the increases in BAT SNA evoked by body cooling, by microinjection of prostaglandin E(2) (170 pmol in 60 nl) in the medial preoptic area or by microinjection of bicuculline (30 pmol in 60 nl) in the dorsomedial hypothalamus were completely reversed by microinjection of bicuculline (30 pmol in 60 nl) in the PVH. Although the increases in BAT SNA and thermogenesis evoked by microinjection of NMDA (12 pmol in 60 nl) in the raphe pallidus (RPa) was markedly attenuated following microinjection of bicuculline (30 pmol) in the PVH, the increases in BAT SNA and thermogenesis evoked by microinjection of bicuculline (30 pmol in 60 nl) in the RPa were unaffected by microinjection of bicuculline in the PVH. These results demonstrate that disinhibition of neurons in the PVH inhibits BAT SNA likely via activation of a GABAergic input to BAT sympathetic premotor neurons in the RPa.

Central efferent pathways mediating skin cooling-evoked sympathetic thermogenesis in brown adipose tissue

Control of thermoregulatory effectors by the autonomic nervous system is a critical component of rapid cold-defense responses, which are triggered by thermal information from the skin. However, the central autonomic mechanism driving thermoregulatory effector responses to skin thermal signals remains to be determined. Here, we examined the involvement of several autonomic brain regions in sympathetic thermogenic responses in brown adipose tissue (BAT) to skin cooling in urethane-chloralose-anesthetized rats by monitoring thermogenic [BAT sympathetic nerve activity (SNA) and BAT temperature], metabolic (expired CO(2)), and cardiovascular (arterial pressure and heart rate) parameters. Acute skin cooling, which did not reduce either rectal (core) or brain temperature, evoked increases in BAT SNA, BAT temperature, expired CO(2), and heart rate. Skin cooling-evoked thermogenic, metabolic, and heart rate responses were inhibited by bilateral microinjections of bicuculline (GABA(A) receptor antagonist) into the preoptic area (POA), by bilateral microinjections of muscimol (GABA(A) receptor agonist) into the dorsomedial hypothalamic nucleus (DMH), or by microinjection of muscimol, glycine, 8-OH-DPAT (5-HT(1A) receptor agonist), or kynurenate (nonselective antagonist for ionotropic excitatory amino acid receptors) into the rostral raphe pallidus nucleus (rRPa) but not by bilateral muscimol injections into the lateral/dorsolateral part or ventrolateral part of the caudal periaqueductal gray. These results implicate the POA, DMH, and rRPa in the central efferent pathways for thermogenic, metabolic, and cardiac responses to skin cooling, and suggest that these pathways can be modulated by serotonergic inputs to the medullary raphe.

Neurons in the dorsomedial hypothalamus (DMH) are required for leptin‐induced activation of brown adipose tissue (BAT) thermogenesis

The increased sympathetic nerve activity (SNA) to BAT stimulated by intravenous (iv) administration of leptin was inhibited by microinjection of the 5-HT1A receptor agonist, 8-OH-DPAT, into the raphe pallidus (RPa), indicating that neurons in RPa, presumably BAT sympathetic premotor neurons, are required for leptin-induced activation of BAT thermogenesis. In present study, we tested the hypothesis that DMH neurons comprised a requisite synaptic link in the central neural pathway for leptin-induced BAT thermogenesis. In six anesthetized rats maintained at a core temperature of 35.5°C, iv administration of leptin (1.0 mg/kg) increased BAT SNA by +4635% of control, BAT temperature by +3.0°C as well as RSNA by +164% of control. Subsequent inhibition of DMH neurons by microinjection of glycine (0.5 M/60 nl) reversed the leptin-induced BAT thermogenesis, but had no effect on the leptin-induced increase in RSNA. Similarly, intracerebroventricular (icv) administration of leptin (10μg/10μl) increased BAT SNA by 3516% of control, BAT temperature by +2.3°C and RSNA by +154% of control at a core temperature of 36.1°C. Icv leptin-induced increases in BAT SNA and BAT thermogenesis, but not those in RSNA, were reversed by subsequent bilaterally microinjection of muscimol (2 mM/60 nl) into the DMH. These results indicate that the increased energy expenditure in BAT by stimulation of central leptin receptors requires activation of neurons in the DMH and that the leptin-evoked stimulation of renovascular function uses a sympathoexcitatory pathway that does not include DMH neurons. Supported by NIH DK57838 and DK20378.

Rostral ventromedial periaqueductal gray: A source of inhibition of the sympathetic outflow to brown adipose tissue

Central inhibitory pathways play a significant role in determining the level of sympathetic outflow to the cold defense efferents in mammals. We tested the hypothesis that neurons in the rostral ventromedial periaqueductal gray (rvmPAG) are a source of inhibitory regulation of the sympathetic nerve activity (SNA) to brown adipose tissue (BAT). In urethane/chloralose-anesthetized, paralyzed, artificially ventilated rats, microinjection of PGE 2 (200 pmol in 70 nl) into the medial preoptic area (POA) or microinjection of the GABA A antagonists, bicuculline or SR95531 (60 pmol in 60 nl), into the dorsomedial hypothalamic area (DMH) increased BAT SNA by +853 ± 176 and +898 ± 249% of control, respectively. These evoked increases in BAT SNA were reversed by microinjection of bicuculline (60 pmol in 60 nl) into the rvmPAG at the level of the posterior commissure. Microinjection of muscimol (160 pmol in 80 nl) into the rvmPAG increased BAT SNA by an amount (+191 ± 92% of control) that was significantly ( P < 0.05) smaller than the peak increase observed after bicuculline microinjection into the rostral raphe pallidus (+1340 ± 547% of control), but not different from that observed after transaction of the midbrain posterior to the rvmPAG (+423 ± 123% of control). We conclude that the rvmPAG contains neurons that exert an inhibitory influence on the sympathetic outflow to BAT. These BAT sympathoinhibitory neurons are, themselves, under a tonic GABAergic inhibition. Blockade of this tonic inhibition reveals an inhibitory influence of rvmPAG neurons that is capable of reversing BAT SNA activations from POA or from DMH. Augmenting the tonic inhibition of rvmPAG neurons elicits a modest increase in BAT SNA. Neurons in rvmPAG provide some, but not all, of the tonic inhibition regulating the discharge of BAT sympathetic premotor neurons in RPa and ultimately the level of thermogenesis in BAT.

Brown adipose tissue thermogenesis contributes to fentanyl-evoked hyperthermia

Mu-opioid receptor activation increases body temperature and affects cardiovascular function. In the present study, fentanyl was administered intravenously [100 mug/kg (300 nmol/kg) iv] and intracerebroventricularly [3.4 mug (10 nmol) in 10 microl icv] in urethane-chloralose-anesthetized, artificially ventilated rats. Increases in brown adipose tissue (BAT) sympathetic nerve activity (SNA) (peak, +326% of control), BAT temperature (peak, +0.8 degrees C), renal SNA (peak, +146% of control), and heart rate (HR; peak, +32 beats/min) produced by intravenous fentanyl were abolished by premamillary transection of the neuraxis but were mimicked by intracerebroventricular administration of fentanyl, which also increased arterial pressure (AP; peak, +12 mmHg). Pretreatment with the opioid antagonist naloxone (100 nmol in 10 microl icv) eliminated the intracerebroventricular fentanyl-evoked responses. Microinjection of glycine (0.5 M, 60 nl) to inhibit local neurons in the rostral raphe pallidus (RPa) selectively reversed the intracerebroventricular fentanyl-evoked increases in BAT SNA and HR, while the fentanyl-evoked excitation in RSNA, the pressor responses, and the tachycardic responses were reversed by inhibition of neurons in the rostral ventrolateral medulla (RVLM). Prior inhibition of neurons in the dorsomedial hypothalamus eliminated the intracerebroventricular fentanyl-evoked increases in BAT SNA, BAT temperature, and HR, but not those in RSNA or AP. These results indicate that activation of central mu-opioid receptors with fentanyl can elicit BAT thermogenesis and cardiovascular stimulation through excitation of the sympathetic outflows to BAT, kidney, and heart. Activation of neurons in the rostral RPa and RVLM are essential for the increases in BAT thermogenesis and renal sympathoexcitation, respectively, induced by activation of central mu-opioid receptors. BAT thermogenesis could contribute to fentanyl-evoked hyperthermia, particularly in infants where BAT plays a significant role in thermoregulation.

Activation of 5-HT1A receptors in raphe pallidus inhibits leptin-evoked increases in brown adipose tissue thermogenesis

To elucidate the central neural pathways contributing to the thermogenic component of the autonomic response to intravenous administration of leptin, experiments were conducted in urethane-chloralose-anesthetized, ventilated rats to address 1) the role of neurons in the rostral ventromedial medulla, including raphe pallidus (RPa), in the leptin-evoked stimulation of brown adipose tissue (BAT) sympathetic nerve activity (SNA); and 2) the potential thermolytic effect of 5-hydroxytryptamine(1A) (5-HT(1A)) receptors on RPa neurons that influence BAT thermogenesis. Leptin (1 mg/kg) administration increased BAT SNA by 1,219% of control, BAT temperature by 2.8 degrees C, expired CO(2) by 1.8%, heart rate by 90 beats/min, and mean arterial pressure by 12 mmHg. Microinjection of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) into RPa resulted in a prompt and sustained reversal of the leptin-evoked stimulation of BAT SNA, BAT thermogenesis, and heart rate, with these variables returning to their pre-leptin control levels. Subsequent microinjection of the selective 5-HT(1A) receptor antagonist WAY-100635 into RPa reversed the BAT thermolytic effects of 8-OH-DPAT, returning BAT SNA and BAT temperature to the elevated levels after leptin. In conclusion, activation of neurons in RPa, possibly BAT sympathetic premotor neurons, is essential for the increases in BAT SNA, BAT thermogenesis, and heart rate stimulated by intravenous administration of leptin. Neurons in RPa express 5-HT(1A) receptors whose activation leads to reversal of the BAT thermogenic and the cardiovascular responses to intravenous leptin, possibly through hyperpolarization of local sympathetic premotor neurons. These results contribute to our understanding of central neural substrates for the augmented energy expenditure stimulated by leptin.

Differential regulation of sympathetic outflows to vasoconstrictor and thermoregulatory effectors.

The medullary premotor neurons determining the sympathetic outflow regulating cardiac function and vasoconstriction are located in the rostral ventrolateral medulla (RVLM). The present study sought evidence for differential characteristics and baroreceptor reflex sensitivities between the sympathetic nerve activity (SNA) controlling brown adipose tissue (BAT) metabolism and thermogenesis and cardiovascular SNA such as that controlling mesenteric vasoconstriction via the splanchnic (SPL) nerve. The tonic discharge of sympathetic nerves is determined by the inputs to functionally specific sympathetic preganglionic neurons from supraspinal populations of premotor neurons. Under normothermic conditions, BAT SNA was nearly silent, while SPL SNA exhibited sustained, large-amplitude bursts. Disinhibition of neurons in the rostral raphe pallidus (RPa), a potential site of sympathetic premotor neurons controlling BAT SNA, or icv injection of prostaglandin E2, a pyrogenic stimulus, elicited a dramatic increase in BAT SNA. SPL SNA was strongly influenced by the baroreceptor reflex as indicated by a high coherence to the arterial pressure, while activated BAT SNA exhibited no correlation with the arterial pressure. Since these characteristics and reflex responses in sympathetic outflow have been shown to arise from the ongoing or altered discharge of sympathetic premotor neurons, the marked differences between SPL SNA and BAT SNA provide strong evidence supporting the hypothesis that vasoconstriction and thermogenesis (metabolism) are controlled by distinct populations of sympathetic premotor neurons, the former in the RVLM and strongly baroreceptor-modulated and the latter potentially in the RPa exhibiting little influence of baroreceptor reflex activation.

Differential regulation of brown adipose and splanchnic sympathetic outflows in rat: roles of raphe and rostral ventrolateral medulla neurons.

1. The medullary premotor neurons determining the sympathetic outflow regulating cardiac function and vasoconstriction are located in the rostral ventrolateral medulla (RVLM). The present study sought evidence for an alternative location for the sympathetic premotor neurons determining the sympathetic nerve activity (SNA) controlling brown adipose tissue (BAT) metabolism and thermogenesis. 2. The tonic discharge on sympathetic nerves is determined by the inputs to functionally specific sympathetic preganglionic neurons from supraspinal populations of premotor neurons. Under normothermic conditions, BAT SNA was nearly silent, while splanchnic (SPL) SNA, controlling mesenteric vasoconstriction, exhibited sustained large-amplitude bursts. 3. The rostral raphe pallidus (RPa) contains potential sympathetic premotor neurons that project to the region of sympathetic preganglionic neurons in the thoracic spinal cord. Disinhibition of neurons in RPa elicited a dramatic increase in BAT SNA, with only a small rise in SPL SNA. 4. Splanchnic SNA was strongly influenced by the baroreceptor reflex, as indicated by a high coherence with the arterial pressure wave, a significant amplitude modulation over the time-course of the cardiac cycle and a marked inhibition of SPL SNA during a sustained increase in arterial pressure. When activated, the bursts in BAT SNA exhibited no correlation with arterial pressure and were not affected by increases in arterial pressure. 5. Because these characteristics and reflex responses in sympathetic outflow have been shown to arise from the on-going or altered discharge of sympathetic premotor neurons, the marked differences between SPL and BAT SNA provide strong evidence supporting the hypothesis that vasoconstriction and thermogenesis (metabolism) are controlled by distinct populations of sympathetic premotor neurons, the former in the RVLM and the latter, potentially, in the RPa.