Altered gene expressions involved in energy expenditure in 5-HT(2C) receptor mutant mice

Interactions between Lateral Hypothalamic Orexin and Dorsal Raphe Circuitry in Energy Balance

Orexin/hypocretin terminals innervate the dorsal raphe nucleus (DRN), which projects to motor control areas important for spontaneous physical activity (SPA) and energy expenditure (EE). Orexin receptors are expressed in the DRN, and obesity-resistant (OR) rats show higher expression of these receptors in the DRN and elevated SPA/EE. We hypothesized that orexin-A in the DRN enhances SPA/EE and that DRN-GABA modulates the effect of orexin-A on SPA/EE. We manipulated orexin tone in the DRN either through direct injection of orexin-A or through the chemogenetic activation of lateral-hypothalamic (LH) orexin neurons. In the orexin neuron activation experiment, fifteen minutes prior to the chemogenetic activation of orexin neurons, the mice received either the GABA-agonist muscimol or antagonist bicuculline injected into the DRN, and SPA/EE was monitored for 24 h. In a separate experiment, orexin-A was injected into the DRN to study the direct effect of DRN orexin on SPA/EE. We found that the activation of orexin neurons elevates SPA/EE, and manipulation of GABA in the DRN does not alter the SPA response to orexin neuron activation. Similarly, intra-DRN orexin-A enhanced SPA and EE in the mice. These results suggest that orexin-A in the DRN facilitates negative energy balance by increasing physical activity-induced EE, and that modulation of DRN orexin-A is a potential strategy to promote SPA and EE.

The Regulatory Role of the Central and Peripheral Serotonin Network on Feeding Signals in Metabolic Diseases

Central and peripheral serotonin (5-hydroxytryptamine, 5-HT) regulate feeding signals for energy metabolism. Disruption of central 5-HT signaling via 5-HT2C receptors (5-HT2CRs) induces leptin-independent hyperphagia in mice, leading to late-onset obesity, insulin resistance, and impaired glucose tolerance. 5-HT2CR mutant mice are more responsive than wild-type mice to a high-fat diet, exhibiting earlier-onset obesity and type 2 diabetes. High-fat and high-carbohydrate diets increase plasma 5-HT and fibroblast growth factor-21 (FGF21) levels. Plasma 5-HT and FGF21 levels are increased in rodents and humans with obesity, type 2 diabetes, and non-alcohol fatty liver diseases (NAFLD). The increases in plasma FGF21 and hepatic FGF21 expression precede hyperinsulinemia, insulin resistance, hyperglycemia, and weight gain in mice fed a high-fat diet. Nutritional, pharmacologic, or genetic inhibition of peripheral 5-HT synthesis via tryptophan hydroxylase 1 (Tph1) decreases hepatic FGF21 expression and plasma FGF21 levels in mice. Thus, perturbing central 5-HT signaling via 5-HT2CRs alters feeding behavior. Increased energy intake via a high-fat diet and/or high-carbohydrate diet can upregulate gut-derived 5-HT synthesis via Tph1. Peripheral 5-HT upregulates hepatic FGF21 expression and plasma FGF21 levels, leading to metabolic diseases such as obesity, insulin resistance, type 2 diabetes, and NAFLD. The 5-HT network in the brain–gut–liver axis regulates feeding signals and may be involved in the development and/or prevention of metabolic diseases.

Orexin, serotonin, and energy balance

The lateral hypothalamus is critical for the control of ingestive behavior and spontaneous physical activity (SPA), as lesion or stimulation of this region alters these behaviors. Evidence points to lateral hypothalamic orexin neurons as modulators of feeding and SPA. These neurons affect a broad range of systems, and project to multiple brain regions such as the dorsal raphe nucleus, which contains serotoninergic neurons (DRN) important to energy homeostasis. Physical activity is comprised of intentional exercise and SPA. These are opposite ends of a continuum of physical activity intensity and structure. Non‐goal‐oriented behaviors, such as fidgeting, standing, and ambulating, constitute SPA in humans, and reflect a propensity for activity separate from intentional activity, such as high‐intensity voluntary exercise. In animals, SPA is activity not influenced by rewards such as food or a running wheel. Spontaneous physical activity in humans and animals burns calories and could theoretically be manipulated pharmacologically to expend calories and protect against obesity. The DRN neurons receive orexin inputs, and project heavily onto cortical and subcortical areas involved in movement, feeding and energy expenditure (EE). This review discusses the function of hypothalamic orexin in energy‐homeostasis, the interaction with DRN serotonin neurons, and the role of this orexin‐serotonin axis in regulating food intake, SPA, and EE. In addition, we discuss possible brain areas involved in orexin–serotonin cross‐talk; the role of serotonin receptors, transporters and uptake‐inhibitors in the pathogenesis and treatment of obesity; animal models of obesity with impaired serotonin‐function; single‐nucleotide polymorphisms in the serotonin system and obesity; and future directions in the orexin–serotonin field.

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Metabolic Diseases > Molecular and Cellular Physiology

5-HT Receptors and Temperature Homeostasis

The present review summarizes the data concerning the influence of serotonin (5-HT) receptors on body temperature in warm-blooded animals and on processes associated with its maintenance. This review includes the most important part of investigations from the first studies to the latest ones. The established results on the pharmacological activation of 5-HT1A, 5-HT3, 5-HT7 and 5-HT2 receptor types are discussed. Such activation of the first 3 type of receptors causes a decrease in body temperature, whereas the 5-HT2 activation causes its increase. Physiological mechanisms leading to changes in body temperature as a result of 5-HT receptors' activation are discussed. In case of 5-HT1A receptor, they include an inhibition of shivering and non-shivering thermogenesis, as well simultaneous increase of peripheral blood flow, i.e., the processes of heat production and heat loss. The physiological processes mediated by 5-HT2 receptor are opposite to those of the 5-HT1A receptor. Mechanisms of 5-HT3 and 5-HT7 receptor participation in these processes are yet to be studied in more detail. Some facts indicating that in natural conditions, without pharmacological impact, these 5-HT receptors are important links in the system of temperature homeostasis, are also discussed.

Role of the Oxytocin Receptor Expressed in the Rostral Medullary Raphe in Thermoregulation During Cold Conditions

Recent papers have reported that oxytocin (Oxt) and the oxytocin receptor (Oxtr) may be involved in the regulation of food intake in mammals. We therefore suspected the Oxt/Oxtr system to be involved in energy homeostasis. In previous studies, we found a tendency toward obesity in Oxtr-deficient (Oxtr (-/-)) mice, as well as impaired thermoregulation when these mice were exposed to cold conditions. In the present study, we observed the expression of Oxtr in the rostral medullary raphe (RMR), the brain region known to control thermogenesis in brown adipose tissue (BAT). Through immunohistochemistry, we detected neurons expressing Oxtr and c-Fos in the RMR of mice exposed to cold conditions. Up to 40% of Oxtr-positive neurons in RMR were classified as glutamatergic neurons, as shown by immunostaining using anti-VGLUT3 antibody. In addition, mice with exclusive expression of Oxtr in the RMR were generated by injecting an AAV-Oxtr vector into the RMR region of Oxtr (-/-) mice. We confirmed the recovery of thermoregulatory ability in the manipulated mice during exposure to cold conditions. Moreover, mice with RMR-specific expression of Oxtr lost the typical morphological change in BAT observed in Oxtr (-/-) mice. Additionally, increased expression of the β3-adrenergic receptor gene, Adrb3, was observed in BAT. These results are the first to show the critical role of RMR Oxtr expression in thermoregulation during cold conditions.

Social isolation affects the development of obesity and type 2 diabetes in mice

Social isolation is associated with increased risks of mortality and morbidity. In this study, we show that chronic individual housing accelerated body weight gain and adiposity in KK mice but not C57BL6J mice, and fully developed diabetes in KKA(y) mice. Individually housed KK and KKA(y) mice increased body weight gain over the initial 2 wk without increased daily average food consumption compared with group-housed animals. The individually housed KK and KKA(y) mice then gradually increased food consumption for the next 1 wk. The chronic social isolation-induced obesity (SIO) was associated with hyperleptinemia and lower plasma corticosterone and active ghrelin levels but not hyperinsulinemia. Elevated plasma leptin in the SIO suppressed expression of 5-HT2C receptor in white adipose tissue. The SIO was also associated with decreased expression of beta3-adrenergic receptors in white adipose tissue and hypothalamic leptin receptor, which might be secondary to the enhanced adiposity. Interestingly, social isolation acutely reduced food consumption and body weight gain compared with group-housed obese db/db mice with leptin receptor deficiency. Social isolation-induced hyperglycemia in KKA(y) mice was associated with increased expression of hepatic gluconeogenetic genes independent of insulin. These findings suggest that social isolation promotes obesity due to primary decreased energy expenditure and secondary increased food consumption, which are independent of the disturbed leptin signaling, in KK mice, and develops into insulin-independent diabetes associated with increased expression of hepatic gluconeogenetic genes in KKA(y) mice. Thus, social isolation can be included in the environmental factors that contribute to the development of obesity and type 2 diabetes.

Thermogenic effect of YM348, a novel 5-HT2C-receptor agonist, in rats

We have investigated the effect of S-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine (YM348), a novel 5-HT(2C)-receptor agonist, on body temperature and energy expenditure in Wistar rats. m-Chlorophenylpiperazine (mCPP) and S-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (RO 60-0175) were used as reference 5-HT(2C)-receptor agonists. Administration of YM348, mCPP and RO 60-0175 dose-dependently and significantly increased body temperature in rats. YM348- or RO 60-0175-induced hyperthermia was significantly attenuated by the non-selective 5-HT(2)-receptor antagonist methysergide and the selective 5-HT(2C)-receptor antagonist SB242084, but not by the selective 5-HT(2A)-receptor antagonist MDL100907. mCPP-induced hyperthermia was significantly attenuated by methysergide, SB242084 and MDL100907. In addition to the increase in body temperature, YM348, mCPP and RO 60-0175 produced dose-related and significant increases in energy expenditure. YM348-, mCPP- and RO 60-0175-induced increases in energy expenditure were significantly attenuated by methysergide and SB242084 but not by MDL100907. These results suggested that 5-HT(2C)-receptor stimulation increased body temperature and energy expenditure and that the 5-HT(2C)-receptor was the target receptor in the thermogenic effect of YM348 in Wistar rats.

Agonist diversity in 5-HT(2C) receptor-mediated weight control in rats

RATIONALE

Food intake and energy expenditure are the two main determinants of body weight. Given that 5-HT(2C) receptor agonists are reported to have effects on both energy expenditure and food intake, this strongly suggests that 5-HT(2C) receptor agonists have excellent potential for development as antiobesitiy drugs. One important issue in antiobesity drug development is whether the effects of the compound are maintained during chronic drug treatment.

OBJECTIVES

The purpose of the present study was to investigate the effect of repeated oral administration of three 5-HT(2C) receptor agonists, m-chlorophenylpiperazine (mCPP), d(S)-2-(6-chloro-5-fluoroindol-1-yl)-1-methylethylamine (RO60-0175) and (S)-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine (YM348), on food intake and energy expenditure in rats.

RESULTS

In the food intake study, mCPP, RO60-0175 and YM348 decreased food intake in a dose-dependent manner on day 1 of administration. On day 14 of repeated administration, the hypophagic effect of YM348 was lost and that of mCPP was reduced. In contrast, the hypophagic effect of RO60-0175 was maintained even after repeated administration. The hypophagic effects of all agonists were significantly inhibited by a 5-HT(2C) receptor antagonist, SB242084. In contrast to the hypophagic effects, no drug tolerance developed with respect to the hyperthermic effects of mCPP, RO60-0175, and YM348. The hyperthermic effects of these drugs were also inhibited by SB242084.

CONCLUSIONS

Together, the difference between compounds in their hypophagic effects and the similarity in their hyperthermic effects suggest a diversity in agonists in 5-HT(2C) receptor-mediated weight control in rats.

Antiobesity effect of YM348, a novel 5-HT2C receptor agonist, in Zucker rats

The purpose of the present study was to investigate the potency of (S)-2-(7-ethyl-1H-furo[2,3-g]indazol-1-yl)-1-methylethylamine (YM348), a 5-HT2C receptor agonist, as an antiobesity agent in Zucker rats. Single oral administration of YM348 at 0.1, 0.3, 1 and 3 mg/kg significantly reduced food intake in a dose-dependent manner. This effect of YM348 on food intake was inhibited by SB242084, a selective 5-HT2C receptor antagonist. In addition, single administration of YM348 significantly increased body temperature and calorie expenditure at doses of 0.3, 1 and 3 mg/kg, and 1 and 3 mg/kg p.o., respectively. The increasing effect of YM348 on body temperature and calorie expenditure was inhibited by SB242084. Chronic subcutaneous infusion of YM348 (3 and 30 mg/kg/day) for 2 weeks also decreased food intake. However, this hypophagic effect of YM348 was marked during the initial week of infusion but only minor in the second. In contrast, no diminution of effect on body temperature and calorie expenditure was seen on repeated administration of YM348 (1 mg/kg p.o.). Two weeks' subcutaneous infusion of YM348 (3 and 30 mg/kg/day) resulted in a significant decrease in body weight gain throughout the experiment. These results suggest that the maintenance of thermogenesis contributed to the reduced body weight by YM348. The ability of YM348 to decrease body weight in Zucker rats suggests its strong potential for development as an antiobesity agent in humans.