Evidence that α 2 -Adrenergic Pathways Play a Major Role in Growth Hormone (GH) Neuroregulation: α 2 -Adrenergic Agonism Counteracts the Inhibitory Effect of Muscarinic Cholinergic Receptor Blockade on the GH Response to GH-Releasing Hormone, while α 2 -Adrenergic Blockade Diminishes the Potentiating Effect of Increased Cholinergic Tone on such Stimulation in Normal Men*