Anti-inflammatory effect of the ghrelin agonist growth hormone-releasing peptide-2 (GHRP-2) in arthritic rats

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Abstract
Chronic arthritis induces hypermetabolism and cachexia. Ghrelin is a gastrointestinal hormone that has been proposed as a treatment to prevent cachexia. The aim of this work was to examine the effect of administration of the ghrelin agonist growth hormone-releasing peptide-2 (GHRP-2) to arthritic rats. Male Wistar rats were injected with Freund’s adjuvant, and 15 days later arthritic and control rats were daily injected with GHRP-2 (100 μg/kg) or with saline for 8 days. Arthritis induced an increase in serum ghrelin ( P < 0.01) and a decrease in serum concentrations of leptin ( P < 0.01), whereas GHRP-2 administration increased serum concentrations of leptin. GHRP-2 increased food intake in control rats but not in arthritic rats. However, in arthritic rats GHRP-2 administration ameliorated the external symptoms of arthritis, as it decreased the arthritis score (10.4 ± 0.8 vs. 13.42 ± 0.47, P < 0.01) and the paw volume. In addition, circulating IL-6 and nitrites/nitrates were increased by arthritis, and GHRP-2 treatment decreased the serum IL-6 levels ( P < 0.01). To elucidate whether GHRP-2 is able to modulate IL-6 release directly on immune cells, peritoneal macrophage cultures were incubated with GHRP-2 or ghrelin, the endogenous ligand of the growth hormone (GH) secretagogue receptor. Both GHRP-2 (10−7 M) and ghrelin (10−7 M) prevented endotoxin-induced IL-6 and decreased nitrite/nitrate release from peritoneal macrophages in vitro. These data suggest that GHRP-2 administration has an anti-inflammatory effect in arthritic rats that seems to be mediated by ghrelin receptors directly on immune cells.