Distinct Forebrain and Caudal Brainstem Contributions to the Neuropeptide Y Mediation of Ghrelin Hyperphagia

Abstract

G hrelin, the endogenous ligand for the growth hormone secretagogue (GHS) receptor (1), induces a hyperphagic response when delivered centrally (2-6). GHS receptors are located on neuropeptide Y (NPY)/agouti-related protein neurons in the arcuate nucleus of the hypothalamus (ARC) (7), and it is generally held that projections from these neurons to those in the paraventricular nucleus (PVN) and perhaps in other hypothalamic regions are an essential link in the downstream mediation of the orexigenic actions of ghrelin (8,9). Consistent with this view is evidence showing that ghrelin induces c-fos expression (5,10 -12) and NPY mRNA expression in ARC (10,13,14) and stimulates electrophysiological activity in NPY/agouti-related protein neurons (8,15). Central administration of either ghrelin or NPY also activates c-fos expression in the PVN (16 -18), an area with strong expression of NPY-Y1 and Y5 receptors (19). Functional support for the mediation of ghrelin hyperphagia by NPY systems comes from pharmacological studies in which NPY antibodies (14) or receptor antagonists (3,5,20,21) were shown to abolish ghrelin-induced feeding. The results of these studies, in which ligands were delivered to forebrain ventricles, are consistent with the view that the relevant GHS and NPY receptors are contained within the hypothalamus.The anatomical extent of the substrates supporting the ghrelin-NPY interaction relevant to feeding control, however, remains open to question. Attribution of a hypothalamic site of action for the pharmacological studies is tenuous, given that the caudal flow of cerebrospinal fluid (CSF) through the ventricular system carries ligands to behaviorally potent receptor populations in the caudal brainstem (CBS). Thus, with ghrelin and the NPY receptor antagonist both delivered to the third or lateral ventricles (20,21), it is possible that CBS receptors and circuits contribute to either or both the primary ghrelin effect and its reversal. GHS receptors are clearly represented in the CBS (22,23), as are the NPY receptor subtypes most commonly implicated in the control of ingestive behavior (24 -27). Targeted stimulation of brainstem GHS receptors (6) or NPY receptors (28,29), moreover, yields robust hyperphagic responses at low doses. Furthermore, as shown below, delivery of NPY antagonists to the fourth (hindbrain) ventricle completely reverses the hyperphagic response observed when ghrelin is administered through the same cannula. The parity of the results derived from

Caudal brainstem delivery of ghrelin induces fos expression in the nucleus of the solitary tract, but not in the arcuate or paraventricular nuclei of the hypothalamus

“…In support of the former case, recruitment of forebrain structures by hindbrain injections has been demonstrated for other feeding-relevant peptides (e.g., urocortin, Daniels et al, 2004) and, with specific respect to ghrelin, fourth-ventricular injections of ghrelin resulted in increased NPY mRNA in the arcuate nucleus (Kinzig et al, 2006). In support of the latter case, we have shown previously that hyperphagic responses to hindbrain injections of ghrelin require NPY Y1 and Y5 receptor activation in the hindbrain, but do not require activation of these receptors in the forebrain (Faulconbridge et al, 2005). As such, the hyperphagic response to fourth-i.c.v.…”

“…The caudal brainstem contributes to the hyperphagic response to both central (Faulconbridge et al, 2005) and peripheral (Date et al, 2006) ghrelin delivery. Forebrain ventricular ghrelin injections increase neural activity in the hypothalamus (e.g., Arc and PVN) as well as the hindbrain (e.g., NTS) (Lawrence et al, 2002;Nakazato et al, 2001;Olszewski et al, 2003a;Olszewski et al, 2003b), but the anatomical distribution of Fos expression resulting from hindbrain injection of ghrelin had not been examined.…”

“…Ritter et al (2001), for example, showed that activation of this pathway is necessary for the hyperphagic response to glucoregulatory challenges. The co-expression of NPY in 81% of the relevant TH neurons in the CBS (Stornetta et al, 1999) makes this system a likely target of ghrelin action, given the dependence of ghrelin hyperphagia on NPY recruitment (Faulconbridge et al, 2005). Several studies directly implicate catecholaminergic pathways arising from the NTS in the hyperphagic response to peripheral or forebrain administration of ghrelin (Abizaid et al, 2006;Brunetti et al, 2002;Date et al, 2006;Jerlhag et al, 2006).…”

“…In spite of the focus on forebrain areas, there is ample evidence for multiple sites of ghrelin action, including portions of the caudal brainstem (CBS) (Date et al, 2006;Faulconbridge et al, 2003;Faulconbridge et al, 2005;Lin et al, 2004). Several CBS nuclei express GHS-R, including the area postrema, the nucleus of the solitary tract (NTS) and the dorsal motor nucleus of the vagus nerve, which together compose the dorsal vagal complex (DVC) (Guan et al, 1997;Zigman et al, 2006).…”

“…In the present study, we asked if hindbrain ghrelin administration activates neurons within the hypothalamus and/or the caudal brainstem, using a paradigm that we had applied previously to evaluate ingestive responses. Specifically, Fos-immunoreactivity was measured in the NTS, the Arc, and the PVN after fourth-i.c.v injections of ghrelin at a dose (150pmol) shown to reliably induce a hyperphagic response (Faulconbridge et al, 2003;Faulconbridge et al, 2005).…”