Pharmacology Of A Novel Class Of Allosteric Modulators For The Alpha4 Beta2 Sub-Type Of Neuronal Nicotinic Acetylcholine Receptors

Abstract

Neuronal nicotinic acetylcholine receptors (nAChRs) are members of a large family of ligand gated ion channels that mediate inhibitory and excitatory neural transmission in the central nervous system (CNS). The nicotinic subfamily has been implicated in a range of neurological disorders including autism, Alzheimer's disease and nicotine addiction; diseases that are currently both challenging and costly to treat. Despite the apparent importance of nAChRs in these disorders, only a limited number of drugs are currently available for altering nicotinic signalling in the CNS. No drug therapies are currently available that specifically target autism and only a limited number of drugs are available for the treatment of Alzheimer's disease. This thesis presents a novel class of nAChR ligands based on the natural product desformylflustrabromine (dFBr). Desformylflustrabromine (dFBr), a metabolite of the marine bryozoan Flustra Foliacea, was previously identified as an allosteric modulator of the alpha4beta2 subtype of nAChRs. In collaboration with Dr. Richard Glennon at the Virginia Commonwealth University, College of Pharmacy, we developed a synthetic dFBr and evaluated its interaction with two of the most common subtypes of nAChRS, alpha7 and alpha4beta2 (Chapter 2). We confirmed that dFBr is the active component of Flustra Foliacea and identified an additional inhibitory action that becomes evident as dFBr concentrations are increased beyond 10muM. This inhibition was not previously reported. Synthetic dFBr appears significantly more potent at potentiation of alpha4beta2 receptors then reported for the natural extract and shows only inhibitory action on alpha7 receptors. Multiple analogues of dFBr were designed and synthesized to determine the structure activity relation (SAR) for dFBr's action on alpha4beta2 receptors (Chapter 3). We identified three analogues capable of potentiating responses of acetylcholine. The majority of compounds inhibited responses on both alpha4beta2 and alpha7 receptors. The data presented here provide important information for determining a preliminary pharmacophore for dFBr and provide direction for the design of additional analogues on the path to development of more potent and potentially therapeutically useful analogues. To better understand the relationship of dFBr to other nAChR modulators, we also compared the action of dFBr to that of physostigmine, zinc and 17 beta-estradiol (Chapter 4). These compounds are thought to act at three different binding sites on nAChRs. All three compounds increase responses of alpha4beta2 receptors to acetylcholine. Our data show that dFBr is distinct from the clinically used modulator physostigmine but suggests similarities in mechanism with zinc and 17-beta-estradiol. These data provide important information regarding the mechanism of dFBr modulation and provide direction for future site directed mutagenesis studies that will identify the dFBr binding site. Identification of the binding site is critical for the development of receptor models that will facilitate computer assisted drug design.