Decreased brown adipose tissue thermogenic activity following a reduction in brain serotonin by intraventricular p-chlorophenylalanine

Low Protein Diets and Energy Balance: Mechanisms of Action on Energy Intake and Expenditure

Low protein diets are associated with increased lifespan and improved cardiometabolic health primarily in rodents, and likely improve human health. There is strong evidence that moderate to severe reduction in dietary protein content markedly influences caloric intake and energy expenditure, which is often followed by a decrease in body weight and adiposity in animal models. While the neuroendocrine signals that trigger hyperphagic responses to protein restriction are better understood, there is accumulating evidence that increased sympathetic flux to brown adipose tissue, fibroblast growth factor-21 and serotonergic signaling are important for the thermogenic effects of low protein diets. This mini-review specifically focuses on the effect of low protein diets with variable carbohydrate and lipid content on energy intake and expenditure, and the underlying mechanisms of actions by these diets. Understanding the mechanisms by which protein restriction influences energy balance may unveil novel approaches for treating metabolic disorders in humans and improve production efficiency in domestic animals.

TPH2 in the Dorsal Raphe Nuclei Regulates Energy Balance in a Sex-Dependent Manner

AbstractCentral 5-hydroxytryptamine (5-HT), which is primarily synthesized by tryptophan hydroxylase 2 (TPH2) in the dorsal Raphe nuclei (DRN), plays a pivotal role in the regulation of food intake and body weight. However, the physiological functions of TPH2 on energy balance have not been consistently demonstrated. Here we systematically investigated the effects of TPH2 on energy homeostasis in adult male and female mice. We found that the DRN harbors a similar amount of TPH2+ cells in control male and female mice. Adult-onset TPH2 deletion in the DRN promotes hyperphagia and body weight gain only in male mice, but not in female mice. Ablation of TPH2 reduces hypothalamic pro-opiomelanocortin (POMC) neuronal activity robustly in males, but only to a modest degree in females. Deprivation of estrogen by ovariectomy (OVX) causes comparable food intake and weight gain in female control and DRN-specific TPH2 knockout mice. Nevertheless, disruption of TPH2 blunts the anorexigenic effects of exogenous estradiol (E2) and abolishes E2-induced activation of POMC neurons in OVX female mice, indicating that TPH2 is indispensable for E2 to activate POMC neurons and to suppress appetite. Together, our study revealed that TPH2 in the DRN contributes to energy balance regulation in a sexually dimorphic manner.

Serotonergic Control of Metabolic Homeostasis

New treatments are urgently needed to address the current epidemic of obesity and diabetes. Recent studies have highlighted multiple pathways whereby serotonin (5-HT) modulates energy homeostasis, leading to a renewed interest in the identification of 5-HT-based therapies for metabolic disease. This review aims to synthesize pharmacological and genetic studies that have found diverse functions of both central and peripheral 5-HT in the control of food intake, thermogenesis, and glucose and lipid metabolism. We also discuss the potential benefits of targeting the 5-HT system to combat metabolic disease.

Traveling from the hypothalamus to the adipose tissue: The thermogenic pathway

Brown adipose tissue (BAT) is a specialized tissue critical for non-shivering thermogenesis producing heat through mitochondrial uncoupling; whereas white adipose tissue (WAT) is responsible of energy storage in the form of triglycerides. Another type of fat has been described, the beige adipose tissue; this tissue emerges in existing WAT depots but with thermogenic ability, a phenomenon known as browning. Several peripheral signals relaying information about energy status act in the brain, particularly the hypothalamus, to regulate thermogenesis in BAT and browning of WAT. Different hypothalamic areas have the capacity to regulate the thermogenic process in brown and beige adipocytes through the sympathetic nervous system (SNS). This review discusses important concepts and discoveries about the central control of thermogenesis as a trip that starts in the hypothalamus, and taking the sympathetic roads to reach brown and beige fat to modulate thermogenic functions.

Mechanisms linking depression co-morbid with obesity: An approach for serotonergic type 3 receptor antagonist as novel therapeutic intervention

Despite of the enormous research, therapeutic treatment for depression has always been a serious issue. Even though depression and obesity are individual abnormal health conditions, each act as a triggering factor for the other. Obese individuals are twice prone to develop depression than that of non-obese persons. The exact mechanism how obesity increases the risk for depression still remains an area of interest for research in neuropsychopharmacology. Depression and obesity share some common pathological pathways such as hyperactivity of hypothalamic-pituitary-adrenal (HPA) axis, dysregulation of oxidant/antioxidant system balance, higher level of inflammatory cytokines, leptin resistance, altered plasma glucose, insulin resistance, reduced neuronal brain derived neurotrophic factor (BDNF) and decreased serotonergic neurotransmission in various regions of brain. The antidepressant-like effect of 5-HT3 receptor antagonists through allosteric modulation of serotonergic pathways is well evident from several research investigations belonging to our and some in other laboratories. Furthermore, serotonin regulates diet intake, leptin, corticosterone, inflammatory mechanisms, altered plasma glucose, insulin resistance and BDNF concentration in brain. The present review deals with various biological mechanisms involved in depression co-morbid with obesity and 5-HT3 receptor antagonists by modulation of serotonergic system as a therapeutic target for such co-morbid disorder.

Central serotonergic neurons activate and recruit thermogenic brown and beige fat and regulate glucose and lipid homeostasis

Thermogenic brown and beige adipocytes convert chemical energy to heat by metabolizing glucose and lipids. Serotonin (5-HT) neurons in the CNS are essential for thermoregulation and accordingly may control metabolic activity of thermogenic fat. To test this, we generated mice in which the human diphtheria toxin receptor (DTR) was selectively expressed in central 5-HT neurons. Treatment with diphtheria toxin (DT) eliminated 5-HT neurons and caused loss of thermoregulation, brown adipose tissue (BAT) steatosis, and a >50% decrease in uncoupling protein 1 (Ucp1) expression in BAT and inguinal white adipose tissue (WAT). In parallel, blood glucose increased 3.5-fold, free fatty acids 13.4-fold, and triglycerides 6.5-fold. Similar BAT and beige fat defects occurred in Lmx1b(f/f)ePet1(Cre) mice in which 5-HT neurons fail to develop in utero. We conclude 5-HT neurons play a major role in regulating glucose and lipid homeostasis, in part through recruitment and metabolic activation of brown and beige adipocytes.

Serotonin potentiates sympathetic responses evoked by spinal NMDA

In urethane-chloralose anaesthetized, neuromuscularly blocked, ventilated rats, we examined the effects on sympathetic outflow to brown adipose tissue (BAT) of separate and simultaneous spinal microinjections of NMDA and serotonin. Microinjection of NMDA (12 pmol) into the right T4 spinal intermediolateral nucleus (IML) immediately increased ipsilateral brown adipose tissue (BAT) sympathetic nerve activity (SNA; peak: +546% of control), BAT thermogenesis (+0.8 degrees C) and heart rate (+53 beats min-1), whereas microinjection of a lower dose of NMDA (1.2 pmol) did not change any of the recorded variables. Microinjection of 5-hydroxytryptamine (5-HT, 2 nmol) into the T4 IML increased BAT SNA (peak: +342% of control) at a long latency (mean onset: 23 min). The long latency 5-HT-evoked increase in BAT SNA was prevented by microinjection of methysergide (600 pmol) into the T4 IML. The increases in BAT SNA evoked by T4 IML microinjections of NMDA (12 pmol) were significantly potentiated (two to three times larger than the response to NMDA alone) following T4 IML microinjections of 5-HT (100 pmol to 2 nmol, but not 20 pmol). Also, microinjection of 5-HT (200 pmol) converted the subthreshold dose of NMDA (1.2 pmol) into an effective dose for increasing BAT SNA and heart rate. The 5-HT-mediated potentiation of the increase in BAT SNA evoked by microinjection of NMDA into the T4 IML was reversed by microinjection of methysergide (600 pmol) into the T4 IML. These results demonstrate that BAT SNA and thermogenesis can be driven by activation of spinal excitatory amino acid or 5-HT receptors and that concomitant activation of spinal NMDA and 5-HT receptors can act synergistically to markedly increase BAT SNA and thermogenesis.

Brown adipose tissue: function and physiological significance

The function of brown adipose tissue is to transfer energy from food into heat; physiologically, both the heat produced and the resulting decrease in metabolic efficiency can be of significance. Both the acute activity of the tissue, i.e., the heat production, and the recruitment process in the tissue (that results in a higher thermogenic capacity) are under the control of norepinephrine released from sympathetic nerves. In thermoregulatory thermogenesis, brown adipose tissue is essential for classical nonshivering thermogenesis (this phenomenon does not exist in the absence of functional brown adipose tissue), as well as for the cold acclimation-recruited norepinephrine-induced thermogenesis. Heat production from brown adipose tissue is activated whenever the organism is in need of extra heat, e.g., postnatally, during entry into a febrile state, and during arousal from hibernation, and the rate of thermogenesis is centrally controlled via a pathway initiated in the hypothalamus. Feeding as such also results in activation of brown adipose tissue; a series of diets, apparently all characterized by being low in protein, result in a leptin-dependent recruitment of the tissue; this metaboloregulatory thermogenesis is also under hypothalamic control. When the tissue is active, high amounts of lipids and glucose are combusted in the tissue. The development of brown adipose tissue with its characteristic protein, uncoupling protein-1 (UCP1), was probably determinative for the evolutionary success of mammals, as its thermogenesis enhances neonatal survival and allows for active life even in cold surroundings.

Behavioral and physiological responses to stress are affected by high-fat feeding in male rats

Interactions between monoaminergic neurochemistry and macronutrient intake have been frequently shown. Because monoaminergic systems in the brain are also closely involved in behavioral and physiological stress responses it can be hypothesized that differences in the macronutrient composition of diets are reflected in these responses. The present studies, therefore, were designed to assess the consequences of a change in dietary macronutrient composition on a variety of physiological and behavioral responses (both acute and long-term) to a number of stressors. The effect of chronic high-fat (HF; 61% kcal from fat) feeding on the stress responses was compared with controls receiving regular high-carbohydrate (HC; 63% kcal from carbohydrates) laboratory chow. Rats were kept on this diet for at least 2 months before they were exposed to either psychological (social defeat) or physiological (lipopolysaccharide, LPS, administration) stress. At baseline, chronic HF feeding caused a slight, but significantly reduction in body temperature relative to that observed in HC-fed rats. Following social defeat or LPS injection, HF feeding caused a faster recovery of the body temperature increase relative to animals on the HC diet. Stress-induced suppression of home cage locomotor activity and body weight gain were also reduced by HF feeding. The serotonergic 5-HT(1a) receptor hyposensitivity that was observed in HC-fed rats 2 weeks after stress was absent in the HF regimen. Although the present results cannot be readily interpreted as showing purely beneficial effects of high-fat diets on stress responsivity, the findings in the present study do encourage further investigation of possible ameliorating effects of high-fat diets on aspects of the behavioral and physiological response stress.

Leptin increases serotonin turnover by inhibition of brain nitric oxide synthesis

Leptin administration inhibits diencephalic nitric oxide synthase (NOS) activity and increases brain serotonin (5-HT) metabolism in mice. We evaluated food intake, body-weight gain, diencephalic NOS activity, and diencephalic content of tryptophan (TRP), 5-HT, hydroxyindoleacetic acid (5-HIAA), and 5-HIAA/5-HT ratio after intracerebroventricular (ICV) or intraperitoneal (IP) leptin injection in mice. Five consecutive days of ICV or IP leptin injections induced a significant reduction in neuronal NOS (nNOS) activity, and caused a dose-dependent increase of 5-HT, 5-HIAA, and the 5-HIAA/5-HT ratio. Diencephalic 5-HT metabolism showed a significant increase in 5-HT, 5-HIAA, and the 5-HIAA/5-HT ratio 3 hours after a single leptin injection. This effect was maintained for 3 hours and had disappeared by 12 hours after injection. After a single IP leptin injection, the peak for 5-HT, 5-HIAA, and the 5-HIAA/5-HT ratio was achieved at 6 hours. Single injections of ICV or IP leptin significantly increased diencephalic 5-HT content. Leptin-induced 5-HT increase was antagonized by the coadministration of L-arginine only when the latter was ICV injected, whereas D-arginine did not influence leptin effects on brain 5-HT content. Finally, in nNOS-knockout mice, the appetite-suppressant activity of leptin was strongly reduced, and the leptin-induced increase in brain 5-HT metabolism was completely abolished. Our results indicate that the L-arginine/NO pathway is involved in mediating leptin effects on feeding behavior, and demonstrate that nNOS activity is required for the effects of leptin on brain 5-HT turnover.

Adiposity and serum leptin increase in fatty (fa/fa) BNZ neonates without decreased VMH serotonergic activity

Decreased ventromedial hypothalamic (VMH) serotonergic activity occurs in genetic and diet-induced animal models of obesity. We previously found that this activity was lower in adult and in 12-day-old Zucker fa/fa vs. Fa/Fa pups, the fa/fa animals being identified by their greater adiposity. In the present study, we evaluated fa/fa rats (Brown Norway-Zucker hybrids) at ages 2, 4, 7, and 12 days to test the hypothesis that lower VMH serotonergic activity occurs before increased adiposity and/or attenuated energy expenditure. Our results negate this hypothesis. VMH serotonergic activity showed no consistent genotype differences even at 12 days of age. In contrast, by day 7, fa/fa vs. Fa/Fa pups had higher serum leptin concentrations, greater percent body fat, lower resting and cold-induced energy expenditure, and lower activity of brown fat thyroxine 5'-deiodinase, an enzyme that converts thyroxine to triiodothyronine. We conclude that the onset of increased adiposity induced by the fa gene does not require decreased VMH serotonergic activity and that the lower serotonergic activity seen in older fa/fa pups may be secondary to metabolic consequences of the disruption of the leptin regulatory pathway.

Adrenalectomy increases serotonin turnover in brains of obese Zucker rats

Because adrenalectomy tends to normalize many metabolic abnormalities of obese Zucker rats, we hypothesized that it would also normalize the depressed serotonergic turnover in their ventromedial nucleus (VMN). Lean (Fa/Fa) and obese (fa/fa) male Zucker rats were adrenalectomized or sham operated when 5 wks old and sacrificed at 11 wks. Their brains were frozen, and 13 areas were dissected for HPLC-EC analysis of monoamines and metabolites. Consistent with previous studies, VMN serotonin turnover (indexed by 5-HIAA/5-HT) was lower in obese than lean sham-operated rats. Monoamine and metabolite concentrations were altered in several other brain areas as well. Adrenalectomy reduced percent body fat and elevated VMN serotonergic turnover more in obese than in lean rats. It also stimulated serotonergic turnover in almost every brain area examined. We conclude that in obese Zucker rats: monoaminergic activity is altered in several brain areas involved in regulating energy balance; adrenalectomy normalizes the reduced VMN serotonergic turnover seen in the obese rats; and adrenalectomy results in a generalized increase in central serotonergic turnover. These data are consistent with serotonin's role in inhibiting food intake and enhancing sympathetic stimulation of energy metabolism.

Medial basal hypothalamic monoamine activity associated with intracerebroventricular p-chlorophenylalanine-induced hyperphagia

There is evidence for reciprocal interactions between the brain monoamine neurotransmitters serotonin and noradrenaline which may play a critical role in homeostasis. The aim of the present study was to establish the effect of drug-induced damage to the serotoninergic system on noradrenergic activity in the hypothalamus. Bilateral intracerebroventricular injections of p-chlorophenylalanine (PCPA; 3 mg/kg in 2 x 6 microliters) were made to induce destruction in the serotoninergic system. Relative to saline-injected controls, PCPA-injected rats began overeating by 3 days postinjection. On day 10, when the experimental rats were consuming approximately 120% that of controls, animals were 4-h food deprived, sacrificed and the medial basal hypothalamus was removed for later analysis (by gas chromatography/mass spectrometry) of noradrenaline (NA), serotonin (5-HT) and dopamine (DA) and their principal metabolites dihydroxyphenylethyleneglycol (DHPG), 5-hydroxyindoleacetic acid (5-HIAA) and 3,4-dihydroxyphenylacetic acid (DOPAC), respectively. The ratio of metabolite to monoamine provided an index of functional activity. Trunk blood was collected for analysis of serum insulin and glucose. PCPA-injected animals had higher levels of DHPG (P less than 0.05), an increase in the DHPG/NA ratio (P less than 0.02), lower serum insulin (P less than 0.05) and increased serum glucose (P less than 0.05). There were significant correlations between noradrenergic activity (DHPG/NA ratio) and: (1) food intake (day 9 and 10 average; r = 0.62, P less than 0.05); and (2) serum glucose (r = 0.59, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)