The Central Nervous System Sites Mediating the Orexigenic Actions of Ghrelin

Abstract

The peptide hormone ghrelin is important for both homeostatic and hedonic eating behaviors, and its orexigenic actions occur mainly via binding to the only known ghrelin receptor, the growth hormone secretagogue receptor (GHSR). GHSRs are located in several distinct regions of the central nervous system. This review discusses those central nervous system sites that have been found to play critical roles in the orexigenic actions of ghrelin, including hypothalamic nuclei, the hippocampus, the amygdala, the caudal brain stem, and midbrain dopaminergic neurons. Hopefully, this review can be used as a stepping stone for the reader wanting to gain a clearer understanding of the central nervous system sites of direct ghrelin action on feeding behavior, and as inspiration for future studies to provide an even-more-detailed map of the neurocircuitry controlling eating and body weight.

A Significant Role of the Truncated Ghrelin Receptor GHS-R1b in Ghrelin-induced Signaling in Neurons

“…Human cDNAs for GHS-R1a, GHSR1b, cannabinoid CB 1 receptor (CB1R), corticotropin-releasing factor CRF 1 receptor (CRF1R), or adenosine A 1 receptor (A1R), cloned into pcDNA3.1, were amplified without their stop codons using sense and antisense primers harboring EcoRI and KpnI sites to clone GHS-R1a, GHS-R1b and CRF1R in the pRLuc-N1 vector (pRLuc-N1, PerkinElmer Life Sciences) or in the pEYFP-N1 vector (enhanced yellow variant of GFP, Clontech); HindIII and BamHI sites to clone A1R in the pcDNA3.1cRluc8-vector; BamHI and EcoRI sites to clone CB1R in the pcDNA3.1RLuc vector; or EcoRI and KpnI sites to clone GHS-R1a receptors in a GFP 2 -containing vector (p-GFP 2 , Packard BioScience, Meridien, CT). Amplified fragments were subcloned to be in-frame with restriction sites of pRLuc-N1, pEYFP-N1, or p-GFP 2 vectors to provide plasmids that express proteins fused to RLuc, YFP, or GFP 2 on the C-terminal end (GHS-R1a-Rluc, GHS-R1b-Rluc, CB1R-Rluc, CRF1R-Rluc, GHS-R1a-YFP, GHS-R1b-YFP, or GHS-R1a-GFP 2 ).…”

“…Resonance Energy Transfer-based Assays-For BRET assays, HEK-293T cells were transiently co-transfected with a constant cDNA encoding for receptor-Rluc and with increasing amounts of cDNA corresponding to receptor-YFP or receptor-GFP 2 . To control the cell number, the sample protein concentration was determined using a Bradford assay kit (Bio-Rad) using bovine serum albumin dilutions as standards.…”

“…The complemented YFP Venus or Rluc8 expression and BRET were quantified as described above. For sequence resonance energy transfer (SRET) assays (14), HEK-293T cells were transiently co-transfected with constant amounts of cDNAs encoding for both the receptor fused to RLuc or GFP 2 and with increasingly amounts of cDNA corresponding to the receptor fused to YFP. Using aliquots of transfected cells (20 g of protein), different determinations were performed in parallel: quantification of protein-YFP expression and quantification of protein-Rluc expression as described above.…”

“…Ghrelin is an orexigenic hormone, an internal signal for the animal to engage in food-directed behavior (1,2). It is produced by stomach oxyntic cells, which provide plasma levels that fluctuate diurnally with a peak in the day and trough at night.…”

“…Notably, oxyntic cells qualify as food-entrained oscillators, and ghrelin plasma levels increase during anticipated mealtimes and decrease after meals (1). These and other less well characterized central neuronal functions of ghrelin depend on its ability to cross the blood-brain barrier by still unclear mechanisms and reaching ghrelin receptors localized in specific brain areas, such as the hypothalamus, hippocampus, amygdala, mesencephalic dopaminergic regions, and striatum (2)(3)(4).…”