Somatostatin Inhibits Alpha-2-Adrenergic-Induced Secretion of Growth Hormone-Releasing Hormone

Abstract

The purpose of this experiment was to determine the role of growth hormone-releasing hormone (GHRH) and somatostatin (SRIH) neurons in mediating α₂-adrenergic receptor-induced stimulation of growth hormone (GH) secretion in cattle. Our first objective was to determine if stimulation of α₂-adrenergic receptors increases activity of GHRH neurons in the arcuate nucleus (ARC) and/or decreases activity of SRIH neurons in periventricular (PeVN) and ARC nuclei. Clonidine (an α₂-adrenergic agonist) or vehicle (saline) were injected i.v. into steers and dual-label immunohistochemistry was performed to quantify the number of GHRH and SRIH neurons expressing Fos and Fos-related antigens (Fos/FRA) as markers of neuronal activity. Clonidine increased concentrations of GH in serum and decreased activity of SRIH neurons in the PeVN, but not in the ARC. Clonidine did not alter activity of GHRH neurons in the ARC. Our second objective was to determine if clonidine decreases secretion of SRIH from perifused slices of hypothalami, which contain perikarya and terminals of GHRH and SRIH neurons, and from explants of hypophysial stalk alone, which contain only terminals of GHRH and SRIH neurons. Clonidine failed to alter release of GHRH or SRIH from hypothalamic slices, but stimulated release of GHRH from explants of hypophysial stalk. Blockade of SRIH receptors enabled clonidine to stimulate release of GHRH from slices of hypothalami, but also stimulated release of SRIH. These results suggest that α₂-adrenergic-induced secretion of GH occurs via a dual mechanism involving inhibition of SRIH neurons in the PeVN and direct stimulation of GHRH release from axon terminals in the median eminence.

Noradrenergic Control of Arginine Vasopressin Release from the Ewe Hypothalamus In Vitro: Sensitivity to Oestradiol

“…Previous, in vitro perifusion studies using bovine hypothalamic slices have observed similar effects of α 2 ‐adrenoreceptor or dopamine receptor compounds on growth hormone release for 60–80 min after the end of exposure (West et al. 1997; McMahon et al. 2001).…”

A novel stereotaxic approach to the hypothalamus for the use of push-pull perfusion cannula in Holstein calves

“…The release of GHRH and SS from the hypothalamus in vivo has been demonstrated using two different techniques in rats (hypophyseal portal blood collection) (Plotsky and Vale, 1985;Watanobe and Habu, 2002), sheep (hypophyseal portal blood collection) (Frohman et al, 1990;Magnan et al, 1993), pigs (hypophyseal portal blood collection) (Drisko et al, 1998) and most recently in monkeys (push-pull perfusion of the stalk-median eminence) (Nakamura et al, 2003). In cattle, GHRH and/or SS secretion has been determined by in vitro experiments, such as perifusion of bovine hypothalamic slices (McMahon et al, 2001a;West et al, 1997) or tissue blocks (Ogwuegbu et al, 1991), or immunocytochemistry (McMahon et al, 2001a. Besides those approaches, indirect in vivo approaches like immunoneutralization of GHRH or SS have been reported (Trout and Schanbacher, 1990).…”

Regulation of Growth Hormone Secretion in Nursing Ewes: An Involvement of μ‐Receptor Subtype

“…Bero and Kuhn (1986) postulated that GH response to opioids is mediated by α‐2‐adrenergic receptor. Secretion of GH induced by α‐2‐adrenergic receptor might occur via a dual mechanism involving either inhibition of SRIF neurons in the periventricular nucleus or direct stimulation of GHRH release from axon terminals in the median eminence (McMahon et al. 2001a,b).…”