Research reportIncreases in melanin-concentrating hormone and MCH receptor levels in the hypothalamus of dietary-obese rats
Introduction
Melanin-concentrating hormone (MCH) is a 19-amino acid cyclic peptide originally isolated from teleost fish, in which it regulates skin pigmentation [36]. MCH is also found in the mammalian brain where it is synthesized predominantly in neurons located in the zona incerta and lateral hypothalamic area (LHA) [11], [64]. MCH-expressing neurons have extensive fields of projection, with axons terminating throughout the central nervous system [11]. The mammalian MCH gene encodes several other putative neuropeptides including neuropeptide EI (NEI), neuropeptide GE (NGE) [45], and an alternatively spliced product termed MCH-gene-overprinted-polypeptide (MGOP) [70]. The functions of these peptides are yet to be elucidated, but it has been suggested they may regulate the translation of MCH mRNA [70] or antagonize the action of MCH [12], [58].
In mammals, MCH appears to be involved in a variety of higher functions including sensory processing [43], stress responses [35], [49], and learning [42]. Recent evidence also points to a role in energy homeostasis. The LHA has long been regarded as a ‘hunger center’, as lesions here can produce temporary hypophagia and a decrease in body weight [4]. Acute injection of MCH, either intracerebroventricularly (icv) [51], [53] or directly into the paraventricular nucleus (PVN), arcuate nucleus (ARC), or dorsomedial nucleus (DMH) [1], [54], produces a transient, dose-dependent increase in food intake. Chronic central infusion of MCH increases food intake and body weight and stimulates adiposity [19], [34]. Furthermore, mice carrying a targeted deletion of the MCH gene are hypophagic, with increased metabolic activity and weight loss [61]; conversely, transgenic mice that overexpress MCH are obese and insulin-resistant [41]. The expression of MCH is regulated by fasting and by plasma leptin concentrations. MCH mRNA is overexpressed in the hypothalamus of leptin-deficient ob/ob mice which are hyperphagic and obese [51]. Fasting further increases MCH mRNA levels in this model and also in normal rodents [29], [51], [71]. Leptin replacement in ob/ob mice and fasted rats restores MCH mRNA levels to normal. Additionally, the hyperphagia elicited by the central administration of MCH is abolished by prior injection of leptin [55]. Leptin could directly target MCH neurons in the LHA or via indirect projections from areas such as the ARC [20], [21].
The first MCH receptor (MCHR1) was identified as the orphan G protein-coupled receptor SLC-1 [5], [17], [39], [40], [57], [62], which is not activated by any other known peptide [17]. MCHR1 is widely expressed in the rat brain, with high levels in the hypothalamus, thalamus, olfactory cortex, amygdala, and hippocampus [30]. The activation of MCHR1 generates diverse intracellular signaling responses via multiple G proteins [5], [17], [28], [39], [40], [57], [62]. Selective MCHR1 antagonists suppress the food intake induced by icv administration of MCH [13], [68], as well as the consumption of palatable food [13]. The expression of MCHR1 is increased in both fasted and ob/ob mice but is unchanged in MCH knockout mice [38]. It has therefore been proposed that MCHR1 is regulated by leptin and not by MCH, unlike other G protein-coupled receptors which are inversely regulated by the availability of ligands [63]. A second receptor (MCHR2) has been identified in humans, dogs, ferrets, and monkeys. It has 32% homology with MCHR1 and is predominantly expressed in the brain [3], [31], [44], [52], [56], [73].
The aim of this study was to examine further how MCH and MCHR1 expression in the rat hypothalamus is related to energy balance and to the adiposity signals, leptin and insulin. We chose two contrasting models of altered nutritional status, namely, dietary obesity and food restriction. Both states induce marked and diametrically opposite changes in leptin and insulin and also affect various hypothalamic peptides including neuropeptide Y (NPY) [14], [77], agouti-related peptide (AGRP) [27], and pro-opiomelanocortin (POMC) [14], [37]. As a positive control, we measured POMC mRNA levels in the dietary-obese and food-restricted brain samples. In a second experiment, we measured MCH peptide levels in discrete hypothalamic areas of dietary-obese rats. This is an important model because it reflects the most common form of human obesity more than the genetically obese rodents such as the ob/ob mouse or fa/fa Zucker rat. However, in situ hybridisation studies have reported that MCH mRNA levels are not significantly different in dietary-obese rats when compared to control rats, as well as rats resistant to hypercaloric and high fat diets [22], [69]. Previous studies have examined the effect of starvation on MCH and MCHR1 mRNA levels [10], [29], [50], but the effects of food restriction have not been reported.
Section snippets
Animals
Twelve-week-old male Wistar rats (Charles River, Kent, UK) were housed in groups of two or three in a room maintained on a 12-h light–dark cycle (lights on at 0700 h) at 19–22 °C. All animals had free access to water. At the end of each study, rats were sacrificed during the early light phase (1000 h) by CO2 inhalation, blood was collected, centrifuged, and the plasma stored for subsequent assay of glucose (glucose oxidase method, Boehringer Mannheim UK, Lewes, Sussex, UK), insulin
Dietary obesity
Rats presented with the highly palatable diet responded with variable hyperphagia and ultimately developed obesity ranging in intensity from moderate to severe. To examine the role of MCH in determining this altered susceptibility to the development of dietary induced obesity, the diet-fed rats were divided into a ‘high fat gain’ group (HG), which represented the top 50% of the animals, and a ‘low fat gain’ group (LG), comprising the rest (Table 2). Total fat-pad mass was significantly
MCH and MCHR1 mRNA levels in dietary obesity
In Experiment 1, we examined the effects of chronic dietary obesity and food restriction on the expression of MCH and its receptor, MCHR1, in the whole rat hypothalamus. As MCH expression is limited to the LHA, subdividing the hypothalamus into anatomical regions would not have provided any new information. Previous studies have found that MCH expression is increased in the ob/ob mouse [51] and fa/fa rat [66] and that the peptide could contribute to sustained hyperphagia in these models. Here,
Acknowledgements
We are grateful to AstraZeneca for supporting this research, and we would especially like to thank Drs. Thomas Berglindh and Gunnar Skogman for their valuable contributions.
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2015, NeuropeptidesCitation Excerpt :On the other hand, LH consists of two interesting and unique types of neurons: cells synthesizing orexins and cells producing a melanin-concentrating hormone (MCH). MCH is a hormone with very strong anorexigenic activity, which is often considered to play a leading role in obesity pathogenesis (Elliott et al., 2004; Ito et al., 2003; Kowalski et al., 2006). It was observed that prolonged MCH injection into rodent CSF causes obesity (Gomori et al., 2003).