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Submitted on December 20, 2005
Accepted on May 11, 2006
Department of Physiology, Department of Medicine, Nippon Medical School 1-1-5 Sendagi, Bunkyo-ku, Tokyo, 113-8602 Japan
* To whom correspondence should be addressed. E-mail: asuka{at}nms.ac.jp.
Growth hormone (GH) secretagogue (GHS)/ghrelin stimulates GH secretion by binding mainly to its receptor (GHS-R) on GH-releasing hormone (GHRH) neurons in the arcuate nucleus (Arc) of the hypothalamus. GHRH, somatostatin and neuropeptide Y (NPY) in the hypothalamus are involved in the regulatory mechanism of GH secretion. We previously created transgenic (Tg) rats whose GHS-R expression is reduced in the Arc, showing lower body weight and shorter nose-tail length. GH secretion is decreased in female Tg rats. To clarify how GHS-R affects GHRH expression in the Arc, we compared the numbers of GHS-R-positive, GHRH and NPY neurons between Tg and wild-type (WT) rats. Immunohistochemical analysis showed that the numbers of GHS-R-positive neurons, GHRH neurons and GHS-R-positive GHRH neurons were reduced in Tg rats, while the numbers of NPY neurons and GHS-R-positive NPY neurons did not differ between the two groups. The numbers of Fos-positive and Fos-positive GHRH neurons in response to KP-102 were decreased in Tg rats. Competitive RT-PCR analysis of GHRH mRNA expression in the cultured hypothalamic neurons showed that KP-102 increased NPY mRNA expression level, and that NPY decreased GHRH mRNA expression level. KP-102 increased GHRH mRNA expression level in the presence of anti-NPY IgG. GH increased somatostatin mRNA expression. Further, GH and somatostatin decreased GHRH mRNA expression while KP-102 showed no significant effect on somatostatin mRNA expression. These results suggest that GHS-R is involved in the up-regulation of GHRH and NPY expression, and that NPY, somatostatin, and GH suppress GHRH expression. It is also suggested that the reduction of GHRH neurons of Tg rats is induced by a decrease in GHS-R expression.
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