Regulation of Neuronal Nicotinic Receptor Expression by Novel Ligands and Mechanisms
Hussmann, Gregory Patrick
Chronic nicotine administration increases the density of α4β2 neuronal nicotinic acetylcholine receptors (nAChRs) in brain. Nicotine up-regulates these receptors in numerous research models, including cultured cells, rodent brains, and post-mortem human brains from smokers. The mechanism(s) of nAChR up-regulation involve ligand-binding to the agonist recognition site; thus, nAChR ligands other than nicotine also up-regulate receptors. Neuronal nAChR up-regulation has implications that range from nicotine dependence to therapeutics for neurodegenerative disease. Therefore, research that defines novel mechanisms and/or drugs that influence nAChR regulation is greatly desired.In this dissertation, we report two separate discoveries in the field of neuronal nAChR regulation. The first discovery is that muscarinic receptor activity augments ligand-induced up-regulation of α4β2 nAChR binding densities and subunit protein in transfected HEK293 cells. Muscarinic receptors augment nAChR up-regulation by increasing both α4 and β2 subunit mRNA levels. Furthermore, the increase in nAChR subunit mRNA appears to occur through post-transcriptional mechanisms. This research implies the possibility of modulating nAChR expression in brain through mechanisms that complement ligand-binding.Our second discovery is that sazetidine-A, a high-affinity ligand at β2-containing nAChRs, deceases nicotine self-administration without up-regulating nAChRs in rodent brains. In contrast, chronic nicotine and varenicline administration do up-regulate receptors. To understand why sazetidine-A does not up-regulate nAChRs in brain, we developed a new radioligand binding assay that measures the concentration of the drug in rodent brain ex vivo. In addition, we adapted another radioligand binding assay to measure the receptor occupancy of sazetidine-A in rodent brain. Based on these assays, sazetidine-A reaches brain concentrations that are known to activate and desensitize α4β2 nAChRs. In addition, sazetidine-A occupies the majority of nAChR binding sites in brain. Thus, sazetidine-A administration does not up-regulate nAChRs despite reaching brain levels that occupy, activate, and desensitize these receptors. This study implies that, unlike varenicline, sazetidine-A may promote smoking cessation in humans while it also allows nAChRs to down-regulate back to basal levels. Allowing receptor down-regulation during smoking cessation therapy may be a more effective strategy for preventing relapse.
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