Library

Notes: 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.

Notes: The growth hormone (GH) axis is sensitive to alteration in body weight and there is evidence that central noradrenergic systems regulate neurones that produce growth hormone-releasing hormone (GHRH) and somatostatin (SRIF). This study reports semiquantitative estimates of the noradrenergic input to neuroendocrine GHRH and SRIF neurones in the sheep of different body weights. We also studied the effects of altered body weight on expression of dopamine β-hydroxylase (DBH), the enzyme that produces noradrenalin from dopamine. Ovariectomised ewes were made Lean (39.6 ± 2.6 kg; Mean ± SEM) by dietary restriction, whereas Normally Fed animals (61.2 ± 0.8 kg) were maintained on a regular diet. Brains were perfused for immunohistochemistry and in situ hybridisation. The Mean ± SEM number of GHRH-immunoreactive (-IR) cells was lower in Normally Fed (65 ± 7) than in Lean (115 ± 14) animals, whereas the number of SRIF-IR cells was similar in the two groups (Normally Fed, 196 ± 17; Lean 230 ± 21). Confocal microscopic analysis revealed that the percentage of GHRH-IR cells (Normally Fed 36 ± 1.5% versus Lean 32 ± 4.6%) and percentage of SRIF-IR cells (Normally Fed 30 ± 40.4% versus Lean 32 ± 2.3%) contacted by noradrenergic fibres did not change with body weight. FluoroGold retrograde tracer injections confirmed that noradrenergic projections to the arcuate nucleus are from ventrolateral medulla and noradrenergic projections to periventricular nucleus arise from the ventrolateral medulla, nucleus of solitary tract, locus coeruleus (LC) and the parabrachial nucleus (PBN). DBH expressing cells were identified using immunohistochemistry and in situ hybridisation and the level of expression (silver grains/cell) quantified by image analysis. The number of DBH cells was similar in Normally Fed and Lean animals, but the level of expression/cell was lower (P 〈 0.02) in the PBN and LC of Lean animals. These results provide an anatomical basis for the noradrenergic regulation of GHRH and SRIF cells and GH secretion. Altered activity or noradrenergic neurones in the PBN and LC that occur with reduced body weight may be relevant to the control of GH axis.