Derived from the same prohormone, obestatin has been reported to exert effects on food intake that oppose those of ghrelin. The obestatin receptor GPR39 is present in brain and pituitary gland. Since the gene encoding those two peptides is expressed also in those tissues, we examined further the possible actions of obestatin in vivo and in vitro. Intracerebroventricular administration of obestatin inhibited water drinking in ad libitum-fed and -watered rats, and in food-and water-deprived animals. The effects on water drinking preceded and were more pronounced than any effect on food intake, and did not appear to be the result of altered locomotor/behavioral activity. In addition, obestatin inhibited ANG II-induced water drinking in animals provided free access to water and food. Current-clamp recordings from cultured, subfornical organ neurons revealed significant effects of the peptide on membrane potential, suggesting this as a potential site of action. In pituitary cell cultures, log molar concentrations of obestatin ranging from 1.0 pM to 100 nM failed to alter basal growth hormone (GH) secretion. In addition, 100 nM obestatin failed to interfere with the stimulation of GH secretion by GH-releasing hormone or ghrelin and did not alter the inhibition by somatostatin in vitro. We conclude that obestatin does not act in pituitary gland to regulate GH secretion but may act in brain to alter thirst mechanisms. Importantly, in rats the effects of obestatin on food intake may be secondary to an action of the peptide to inhibit water drinking.

Samson WK, Bagley SL, Ferguson AV, White MM. Hypocretin/ orexin type 1 receptor in brain: role in cardiovascular control and the neuroendocrine response to immobilization stress. Am J Physiol Regul Integr Comp Physiol 292: R382-R387, 2007. First published August 10, 2006; doi:10.1152/ajpregu.00496.2006.-Hypocretin/orexin acts pharmacologically in the hypothalamus to stimulate stress hormone secretion at least in part by an action in the hypothalamic paraventricular nucleus, where the peptide's receptors have been localized. In addition, orexin acts in the brain to increase sympathetic tone and, therefore, mean arterial pressure and heart rate. We provide evidence for the role of endogenously produced hypocretin/orexin in the physiological response to immobilization stress and identify the receptor subtype responsible for this action of the peptide. Antagonism of the orexin type 1 receptor (OX 1R) in the brain prevented the ACTHstimulating effect of centrally administered hypocretin/orexin. Furthermore, pretreatment of animals with the OX 1R antagonist blocked the ACTH response to immobilization/restraint stress. The OX 1R antagonist did not, however, block the pharmacological or physiological release of prolactin in these two models. Antagonism of the OX 1R also blocked the central action of orexin to elevate mean arterial pressures and heart rates in conscious rats. These data suggest receptor subtype-selective responses to hypocretin/orexin and provide further evidence for the importance of endogenously produced peptide in the physiological control of stress hormone secretion. autonomic function; hypothalamus; adrenocorticotropin; prolactin IN ADDITION TO THE WELL-CHRONICLED pharmacological actions of the hypocretins/orexins on arousal state (6,23,29,36), the peptides (referred to here simply as orexins) exert a variety of behavioral (24), autonomic (7,15,34), and endocrine (1, 5, 7, 13, 22, 26 -28, 31, 32) actions mediated by the two characterized binding proteins, the orexin type 1 (OX 1 R) and the orexin type 2 (OX 2 R) receptors (29). Although orexin A and orexin B have many common pharmacological effects, some actions unique to one and not the other have been reported (7,36). Since orexin A binds with higher affinity to the OX 1 R than orexin B and both isoforms bind with apparently equal affinity to the OX 2 R, many investigators have narrowed their attention to the pharmacological effects of orexin A. Our original studies focused on endocrine, cardiovascular, and behavioral actions of orexin A in the hypothalamus and brain stem (7,32,37,38). We demonstrated direct neuronal actions of orexin in the hypothalamic paraventricular nucleus (PVN) and established that the action of exogenous orexin to stimulate the hypothalamic-pituitary-adrenal (HPA) axis was mediated at least in part by the release of corticotrophin-releasing hormone (CRH) (32). We have attempted to identify the receptor subtype responsible for the cardiovascular and neuroendocrine actions of these peptides and provide evidence for the i...

Aim Orexin producing neurons, located primarily in the perifornical region of the lateral hypothalamus, project to a wide spectrum of brain sites where they influence numerous behaviors as well as modulating the neuroendocrine and autonomic responses to stress. While some of the actions of orexin appear to be mediated via the type 1 receptor, some are not, including its action on the release of one stress hormone, prolactin. We describe here the ability of orexin to increase locomotor behaviors and identify the importance of both receptor subtypes in these actions. Methods Rats were tested for their behavioral responses to the central activation of both the type 1 (OX1R) and type 2 (OX2R) receptor (ICV orexin A), as compared to OX2R activation using a relatively selective OX2R agonist in the absence or presence of an orexin receptor antagonist that possesses highest affinity for the OX1R. Results Increases in locomotor activity were observed, effects which were expressed by not only orexin A, which binds to both the OX1R and the OX2R receptors, but also by the relatively selective OX2R agonist, [(Ala11, Leu15)-orexin B]. Furthermore the OX1R selective antagonist only partially blocked the action of orexin A on most locomotor behaviors and did not block the actions of [(Ala11, Leu15)-orexin B]. Conclusion We conclude that orexin A exerts its effects on locomotor behavior via both the OX1R and OX2R and that agonism or antagonism of only one of these receptors for therapeutic purposes (i.e. sleep disorders) would not provide selectivity in terms of associated behavioral side effects.

Obestatin, a product of post-translational processing of the ghrelin prohormone, has been reported to act in the brain to inhibit thirst. We extended our initial studies on water drinking by examining the effects of obestatin on hypovolemia-induced water and saline drinking and vasopressin release in male rats. Intracerebroventricular administration of obestatin significantly inhibited water, but not saline (0 . 3 M NaCl) drinking in response to a hypovolemic challenge. Obestatin also inhibited, in a dose-related fashion, dehydration-induced vasopressin secretion without affecting plasma oxytocin levels. Vasopressin release induced by central angiotensin II administration was attenuated significantly by prior administration of obestatin. Finally, central administration of an antiserum specific to obestatin resulted in an exaggerated basal vasopressin release and an increased vasopressin response to overnight water deprivation. Antiserum treatment also resulted in significantly increased ad libitum water drinking and drinking in response to dehydration. We conclude that this product of post-translational processing of the ghrelin prohormone may be an important contributor to the physiologic regulation of fluid and electrolyte homeostasis.

Exaggerated thirst and salt appetite occurs when endogenous, brain-derived adrenomedullin (AM) production is compromised. In addition, the arginine vasopressin (AVP) response to hypovolemia is compromised. We hypothesized that AM acts in the hypothalamus to control oxytocin (OT) release and that the inhibitory action of AM on salt appetite is mediated via its effects on OT release in the rat. When plasma tonicity was elevated with sodium, ribozymeinduced compromise of central AM production significantly blunted the release of OT into plasma. OT responses to elevation of plasma osmolality without concomitant change in plasma sodium levels were not altered by compromise of AM production. Thus, brain-derived AM controls OT release in response to altered plasma sodium levels.Furthermore, central AM-induced inhibition of NaCl intake can be reversed by pretreatment with an OT antagonist, and the increase in NaCl appetite seen following ribozyme compromise of central AM can be attenuated with central OT administration. These data support the hypothesis that endogenous, brain-derived AM is an essential participant in the hypothalamic response to hypernatremia via its actions on OT-expressing neurons. Together with our previous reports of the effects of AM on AVP secretion and ingestive behaviors, our results suggest that endogenous AM is a physiologically relevant regulator of the endocrine and behavioral mechanisms that maintain fluid and electrolyte homeostasis.