Endocrine

, Volume 14, Issue 1, pp 21–27

Interactions of growth hormone secretagogues and growth hormone-releasing hormone/somatostatin

Article

DOI: 10.1385/ENDO:14:1:021

Cite this article as:
Tannenbaum, G.S. & Bowers, C.Y. Endocr (2001) 14: 21. doi:10.1385/ENDO:14:1:021

Abstract

The class of novel synthetic compounds termed growth hormone secretagogues (GHSs) act in the hypothalamus through, as yet, unknown pathways. We performed physiologic and histochemical studies to further understand how the GHS system interacts with the well-established somatostatin (SRIF)/growth hormone-releasing hormone (GHRH) neuroendocrine system for regulating pulsatile GH secretion. Comparison of the GH-releasing activities of the hexapeptide growth hormone-releasing peptide-6 (GHRP-6) and GHRH administered intravenously to conscious adult male rats showed that the pattern of GH responsiveness to GHRP-6 was markedly time-dependent, similar to that observed with GHRH. Immunoneutralization of endogenous SRIF reversed the blunted GH response to GHRP-6 at trough times, suggesting that GHRP-6 neither disrupts nor inhibits the cyclical release of endogenous hypothalamic SRIF. By striking contrast, passive immunization with anti-GHRH serum virtually obliterated the GH responses to GHRP-6, irrespective of the time of administration. These findings suggest that the GHSs do not act by altering SRIF release but, rather, stimulate GH release via GHRH-dependent pathways. Our dual chromogenic and autoradiographic in situ hybridization experiments revealed that a subpopulation of GHRH mRNA-containing neurons in the arcuate (Arc) nucleus and ventromedial nucleus (VMN) of the hypothalamus expressed the GHS receptor (GHS-R) gene. These results provide strong anatomic evidence that GHSs may directly stimulate GHRH release into hypophyseal portal blood, and thereby influence GH secretion, through interaction with the GHS-R on GHRH-containing neurons. Altogether, these findings support the notion that an additional neuroendocrine pathway may exist to regulate pulsatile GH secretion, possibly through the influence of the newly discovered GHS natural peptide, ghrelin.

Key Words

Pulsatile growth hormonesomatostatingrowth hormone-releasing hormonegrowth hormone secretagogue receptorcolocalization

Copyright information

© Humana Press Inc. 2001

Authors and Affiliations

  1. 1.Departments of Pediatrics and of Neurology and NeurosurgeryMcGill UniversityMontrealCanada
  2. 2.Department of Medicine, Division of EndocrinologyTulane University Medical CenterNew Orleans
  3. 3.Neuropeptide Physiology LaboratoryMcGill University-Montreal Children's Hospital Research InstitutetMontrealCanada