Hypothalamic paraventricular injections of ghrelin: effect on feeding and c-Fos immunoreactivity
Introduction
Ghrelin, a novel 28-amino acid peptide, is synthesized in the periphery (stomach) [7], [12] and in the brain [12], [15], and is capable of crossing the blood brain barrier [3]. It acts as an endogenous ligand of the growth hormone secretagogue (GHS) receptor [9]. Aside from the initially studied function of this peptide as a GHS [1], [6], [9], recent studies have consistently shown that ghrelin is a potent inducer of food consumption [5], [14], [18], [28], [29]. It has been found that circulating levels of ghrelin increase following a 48-h fast, and infusion of glucose into the stomach decreases plasma-ghrelin concentration [28]. Also, ghrelin is released shortly before a meal in schedule-fed animals [24].
The organization of central circuitry appears to play an important role in integrating orexigenic effects of ghrelin. Perikarya synthesizing this peptide are localized in the hypothalamic arcuate nucleus (ARC) [12], [15] and project to brain sites involved in feeding regulation that contain the GHS receptor [8], [9], [17]. The few studies published thus far, have indicated that the paraventricular nucleus of the hypothalamus (PVN), as part of a larger feeding-related neural network, appears to mediate orexigenic properties of ghrelin. The PVN, which receives ARC-derived innervation [10], contains the GHS receptor [17], [30]. Direct injection of ghrelin into this site generates a robust feeding response [16], [29]. In addition, intracerebroventricularly (i.c.v.) administered ghrelin has been reported to increase c-Fos immunoreactivity (IR), a marker of neuronal activation, in the PVN and other brain regions such as the ARC, dorsomedial hypothalamic nucleus (DMH), lateral hypothalamus (LH), nucleus of the solitary tract (NTS) and area postrema (AP) [14], [18].
Based on the above, we hypothesized that direct injection of ghrelin into the PVN would not only robustly stimulate feeding, but would also “activate” the PVN, as well as other brain regions that are part of the feeding-related central circuitry. Thus, we infused ghrelin into the PVN and measured food intake and also assessed c-Fos IR levels in a variety of brain sites involved in the regulation of consummatory behavior.
Section snippets
Animals
Adult male Sprague–Dawley rats (Charles Rivers Laboratories, Wilmington, MA) weighing approximately 300 g at the beginning of the experiment were used in the studies. Animals were housed individually in wire-mesh cages with a 12:12 LD schedule (lights on at 07:00) in a temperature (21 °C) and humidity-controlled facility. Food (Rodent Chow, Teklad, Indianapolis, IN) and tap water were available ad libitum except when stated otherwise.
Surgeries
Rats were equipped with an indwelling stainless steel cannula
Results
Administration of 0.03 and 0.3 nmol ghrelin into the PVN resulted in an increase in feeding by approximately 1.25 g (P=0.0079) and 1.75 g (P=0.0004), respectively, during the first hour post-injection. During the second hour, only animals treated with the highest dose of the peptide consumed significantly more chow than controls (P=0.0267). No dose of the peptide affected 2–4 h consumption. In addition, cumulative 0–4 h food intake was significantly higher in rats treated with 0.3 nmol ghrelin
Discussion
Recently published data have implicated central nervous system circuitry in the mediation of ghrelin’s orexigenic effect. The few extant injection studies demonstrate the role of the hypothalamus in ghrelin-induced hyperphagia [14], [16], [18], [26], [29]. Within this central structure, the PVN is an area of crucial importance in energy metabolism (for review, see Ref. [11]). Our current data demonstrate that direct injection of ghrelin into the PVN (0.03 and 0.3 nmol) results in a short-lived,
Acknowledgements
This project was supported by NIH, National Research Service Award T32DA07097 from the National Institute of Drug Abuse, by the Department of Veterans Affairs, by the National Institute of Drug abuse Grant DA-03999, and by the National Institutes of Diabetes and Digestive and Kidney Disease Grant DK-42698 and P30 DK-50456.
References (30)
- et al.
Ghrelin is an appetite-stimulatory signal from stomach with structural resemblance to motilin
Gastroenterology
(2001) - et al.
Central effects of a novel acylated peptide, ghrelin, on growth hormone release in rats
Biochem. Biophys. Res. Commun.
(2000) - et al.
A-like cells in the rat stomach contain ghrelin and do not operate under gastrin control
Regul. Pept.
(2001) - et al.
Distribution of mRNA encoding the growth hormone secretagogue receptor in brain and peripheral tissues
Brain Res. Mol. Brain. Res.
(1997) - et al.
Neuropeptide Y projection from arcuate nucleus to parvocellular division of paraventricular nucleus: specific relation to the ingestion of carbohydrate
Brain Res.
(1993) - et al.
Immunocytochemical observation of ghrelin-containing neurons in the rat arcuate nucleus
Neurosci. Lett.
(2002) - et al.
Ghrelin injected into the paraventricular nucleus of the hypothalamus of male rats induces feeding but not penile erection
Neurosci. Lett.
(2002) - et al.
Connections of the hypothalamic paraventricular nucleus with the neurohypophysis, median eminence, amygdala, lateral septum and midbrain periaqueductal gray: an electrophysiological study in the rat
Brain Res.
(1981) - et al.
A transient ghrelin surge occurs just before feeding in a scheduled meal-fed sheep
Biochem. Biophys. Res. Commun.
(2002) - et al.
Molecular cloning and gene expression of growth hormone-releasing peptide receptor in rat tissues
Peptides
(1998)
Endocrine activities of ghrelin, a natural growth hormone secretagogue (GHS), in humans: comparison and interactions with hexarelin, a nonnatural peptidyl GHS, and GH-releasing hormone
J. Clin. Endocrinol. Metab.
Extent and direction of ghrelin transport across the blood-brain barrier is determined by its unique primary structure
J. Pharmacol. Exp. Ther.
Neuropeptide Y in hypothalamic paraventricular nucleus: a center coordinating energy metabolism
Am. J. Physiol.
A preprandial rise in plasma ghrelin levels suggests a role in meal initiation in humans
Diabetes
A receptor in pituitary and hypothalamus that functions in growth hormone release
Science
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