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Submitted on May 23, 2005
Accepted on July 26, 2005
Prince Henry's Institute of Medical Research, P.O. Box 5152, Clayton, VIC 3168, Australia; AgResearch Invermay, Private Bag 5003, Mosgiel, New Zealand; Institut National de la Sante et de la Recherche Medicale U.378, Institut Francois Magendie, 1 rue Camille Saint-Saens, F-33077 Cedex, Bordeaux, France
* To whom correspondence should be addressed. E-mail: iain.clarke{at}med.monash.edu.au.
Various neuropeptides and neurotransmitters affect growth hormone (GH) secretion by acting on growth hormone releasing hormone (GHRH) and somatostatin (SRIF) cells. GH secretion is also affected by alteration in adiposity, which could be via modulation of GHRH and SRIF cells. We quantified co-localization of neuropeptides in GHRH and SRIF cells and afferent projections to these cells in Lean (food restricted) and Normally Fed sheep (n = 4/group). The number of GHRH-immunoreactive (IR) cells in the arcuate nucleus was higher in Lean animals, but the number of SRIF-IR cells in periventricular nucleus was similar in the two groups. A subpopulation of GHRH-IR cells co-localized neuropeptide Y in Lean animals, but this was not seen in Normally Fed animals. GHRH/galanin co-localization was higher in Lean animals with no difference in numbers of GHRH/tyrosine hydroxylase (TH) or GHRH/galanin-like peptide cells. SRIF/enkephalin co-localization was lower in Lean animals. The % of GHRH neurons receiving SRIF input was similar in Lean and Normally Fed animals, but more GHRH cells received input from enkephalin afferents in Normally Fed animals. The % of SRIF cells receiving GHRH, neuropeptide Y, galanin and orexin afferents was higher in Lean animals. These findings provide an anatomical evidence of central mechanism/s by which appetite regulating peptides and dopamine could regulate GH secretion. Increased input to SRIF cells in Lean animals may be inhibitory and permissive of increased GH. The appearance of NPY in GHRH cells of Lean animals may be a mechanism for regulation of increasing GH secretion with reduced adiposity.
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