Changes in expression of the genes for the leptin receptor and the growth hormone-releasing peptide/ghrelin receptor in the hypothalamic arcuate nucleus with long-term manipulation of adiposity by dietary means.

Changes in leptin and ghrelin levels occur with alterations in adiposity, but signalling may be affected by levels of the relevant receptors. We measured expression of the leptin receptor (Ob-Rb) and the ghrelin/growth hormone releasing peptide receptor (GHS-R) in the arcuate nucleus of sheep held at either high or low levels of adiposity. Plasma growth hormone (GH) levels were lower in Fat animals and higher in Lean animals. Plasma insulin and leptin levels were higher in Fat animals and lower in Lean animals. Frozen hypothalamic sections of arcuate nucleus were extracted and mRNA levels measured for mRNA for Ob-Rb and GHS-R. Gene expression for both Ob-Rb and GHS-R was higher in Lean animals than in Fat animals, with no difference in expression between Fat and Normal animals. A second group of animals (n = 4 per group) was used for double-labelling immunohistochemistry to determine whether the increase in Ob-Rb gene expression was translated into Ob-Rb protein and to ascertain whether this effect is localised to the cells of the arcuate nucleus that produce either neuropeptide Y (NPY) and/or pro-opiomelanocortin-derived peptides. Lean animals displayed a 255% increase in immunoreactive NPY cells (P < 0.005), a 167% increase in cells with Ob-Rb (P < 0.037) protein and a 344% increase in cells that were staining for both NPY and Ob-Rb (P < 0.02). There was no difference between the Normal and Lean animals in the number of cells that were detected with an adrenocorticotrophic hormone (ACTH) antibody or the number of ACTH-immunoreactive cells that also stained for Ob-Rb. Finally, we measured plasma ghrelin levels in Normal, Fat and Lean ewes (n = 4/group); levels were higher (P < 0.05) in Fat animals than in Lean animals. We conclude that lowering body weight leads to increased expression of Ob-Rb, ghrelin/GHS-R expression and proportion of NPY cells that express Ob-Rb in the arcuate nucleus. This may be an adaptive mechanism to increase responsivity to both leptin and ghrelin.

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