13 June 2001
The purpose of this experiment was to determine the role of growth hormone-releasing hormone (GHRH) and somatostatin (SRIH) neurons in mediating α<sub>2</sub>-adrenergic receptor-induced stimulation of growth hormone (GH) secretion in cattle. Our first objective was to determine if stimulation of α<sub>2</sub>-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 α<sub>2</sub>-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 α<sub>2</sub>-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.