Central administration of obestatin fails to show inhibitory effects on food and water intake in mice

Abstract

Obestatin is a ghrelin-associated peptide hormone with presumed anorexigenic and inhibitory effect on gastric propulsive motility activity. Recent literature, however, discloses much contestation over satiety and gastrointestinal motility-related functionalities of obestatin. In addition, antidipsinogenic effects in rodents by obestatin were recently reported. The present study was set up to bring more clarity into the contested effects of obestatin on food and water intake. Additionally, the stability of obestatin in brain tissue homogenate was investigated. The in vitro incubation of obestatin in brain homogenates revealed disappearance half-life times of 19 min for crude brain homogenate to 27 min for brain membrane homogenate. For the behavioural studies, male C57Bl/6 mice were intracerebroventricularly treated with 0.2 nmol murine amidated obestatin or vehicle at the age of 3 months. An additional group of mice was treated with 0.3 nmol of corticotropin releasing factor (CRF) as a positive control of suppression of food intake. Food and water intake were studied over a period of 5 h in metabolic cages. Under our experimental conditions, no suppressive effects of obestatin on food or water intake were observed, whereas CRF evoked a significant suppression of food intake, which proves the internal validity of the study design.

Introduction

A complex network of peptide hormones secreted from various tissues and organs like gut, brain, adipose tissue, pancreas and thyroid gland regulates food intake and energy metabolism [1]. Obestatin is a recently identified 23-amino acid gut-derived peptide encoded by the preproghrelin gene. Although originating from the same precursor as the widely studied appetite-stimulating peptide ghrelin, obestatin was reported to bring on antithetical physiological effects with regards to food intake and body weight regulation. The initial publication by Zhang et al. [2] reported that human obestatin suppressed food intake in fasted mice and reduced spontaneous body weight gain in lean mice after intraperitoneal (i.p.) or intracerebroventricular (i.c.v.) administration. Furthermore, peripheral administration of obestatin was shown to suppress gastric emptying. In vitro, obestatin suppressed jejunal activity and counteracted the stimulatory effect of ghrelin. Obestatin was hypothesized to exert its physiological effects through activation of the orphan G-protein coupled receptor GPR39.

Several subsequent studies, with few exceptions [3], [4], [5], [6], failed to corroborate the initially reported anorexigenic properties of obestatin [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]; for a summary; see Table 1]. In addition, the putative effect of exogenous obestatin on gastrointestinal motility has been questioned [7], [13], [21], [22], [23], [24], [25], as well as the presumed interaction between ghrelin and obestatin in the regulation of food intake and energy metabolism [8], [10]. Furthermore, the interrelationship between obestatin, obesity and diabetes has been reviewed [26].

The GPR39 receptor may not be deorphanised yet based on negative studies employing GPR39 transfected cell lines [27], [28], GPR39 knockout mice [11], and lack of GPR39 receptor expression in the murine hypothalamus [29].

In addition, other physiological actions of obestatin were reported. Samson et al. [20] reported that i.c.v. administration of obestatin inhibits water consumption related to food intake or stimulated by the dipsogenic peptide hormone angiotensin II. The same study failed to show effects of obestatin administration on food intake, locomotor activity and stereotypic behaviours. Other studies showed that obestatin regulated sleep [30], affected cell proliferation [31], [32], [33], increased the secretion of pancreatic juice enzymes [34], promoted survival of pancreatic β-cells [35], and inhibited glucose-induced insulin secretion [36].

As stated by Gourcerol et al. [37], above-mentioned observations on food and water intake await further confirmation. Therefore, the aim of the present study was to analyse the effect of i.c.v. administration of murine amidated obestatin in C57BL/6 mice on feeding and drinking behaviour during 5 h, employing metabolic cages. Corticotropin releasing factor (CRF) was used as a positive control for suppression of food intake [38], [39], in order to verify the internal validity of our study design. In addition, the in vitro brain metabolic stability of obestatin was determined.