Intranasal delivery of a ghrelin mimetic engages the brain ghrelin signalling system in mice.

Abstract

Ghrelin, the endogenous ligand of the growth hormone secretagogue receptor (GHSR), promotes food intake, other feeding behaviours and stimulates growth hormone (GH) release from the pituitary. Growth hormone secretagogues (GHS), such as GHRP-6 and MK-0677, are synthetic GHSR ligands that activate orexigenic Neuropeptide Y neurons that co-express Agouti-Related Peptide (AgRP) in the arcuate nucleus of the hypothalamus when administered systemically. Systemic GHRP-6 also stimulates GH release in humans and rats. Thus, GHS and ghrelin have therapeutic relevance in patients who could benefit from its orexigenic and/or GH-releasing effects. This study examined whether intranasal delivery of ghrelin, GHRP-6, or MK-0677 engages the brain ghrelin signalling system. Effective compounds and doses were selected based on increased food intake after intranasal application in mice. Only GHRP-6 (5 mg/kg) increased food intake without adverse effects, prompting detailed analysis of meal patterns, neuronal activation in the arcuate nucleus (via Fos mapping) and neurochemical identification of c-fos mRNA-expressing neurons using RNAscope. We also assessed the impact of intranasal GHRP-6 on serum GH levels. Intranasal GHRP-6 increased food intake by increasing meal frequency and size. Fos expression in the arcuate nucleus was higher in GHRP-6-treated mice than saline controls. When examining the neurochemical identity of c-fos-mRNA-expressing neurons, we found co-expression with 63.5±1.9% Ghsr-mRNA, 79±6.8% Agrp-mRNA and 11.4±2.5% Ghrh-mRNA, demonstrating GHRP-6's ability to engage arcuate nucleus neurons involved in food intake and GH release. Additionally, intranasal GHRP-6 elevated GH serum levels. These findings suggest that intranasal GHRP-6, but not ghrelin or MK-0677, can engage the brain ghrelin signalling system.