Chapter 8: Sympathetic Neuropharmacology and Adrenergic Agonists

Sympathetic Neuropharmacology and Adrenergic Agonists further elucidates the mechanisms of neurotransmitter termination, primarily through neuronal reuptake via the catecholamine transporter and metabolic degradation by enzymes such as monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT). A significant portion of the chapter is dedicated to the classification and function of adrenoceptors, which are G-protein coupled receptors divided into alpha and beta subtypes. The text explains how signal transduction varies by subtype, with alpha-1 receptors utilizing the phospholipase C and IP3 pathway to mediate smooth muscle contraction, while alpha-2 receptors generally inhibit adenylyl cyclase to reduce neurotransmitter release. Conversely, beta receptors activate adenylyl cyclase to increase cyclic AMP, leading to cardiac stimulation (beta-1), smooth muscle relaxation (beta-2), and lipolysis or bladder relaxation (beta-3). The chapter also covers dopamine receptors (D1 and D2) and imidazoline receptors. Pharmacological agents are categorized based on their mechanism of action: direct-acting agonists like the catecholamines (epinephrine, norepinephrine, isoproterenol, dobutamine) and noncatecholamines (phenylephrine, clonidine, albuterol); indirect-acting agonists such as cocaine and amphetamine which enhance synaptic neurotransmitter levels by blocking reuptake or promoting release; and mixed-acting agonists like ephedrine. The clinical applications of these drugs are thoroughly examined, highlighting the use of epinephrine in anaphylaxis and cardiac arrest, norepinephrine and dopamine in various forms of shock (septic, cardiogenic, hypovolemic), and selective beta-2 agonists for asthma and COPD management. Additionally, the text explores the use of alpha agonists for nasal decongestion and hypotension, and central alpha-2 agonists for hypertension, sedation, and opioid withdrawal. The chapter concludes by explaining the baroreceptor reflex, describing how drug-induced changes in blood pressure trigger compensatory changes in heart rate, such as reflex bradycardia or tachycardia.