The goal of this project is to determine the molecular mechanism by which two abused drugs, nicotine and cocaine, interact with neuronal nicotinic acetylcholine receptors (nAChRs) in the peripheral and central nervous systems. Cocaine exerts its euphoric, addictive nature through the dopamine transport system, but some of its toxic effects may occur by way of some subtypes of neuronal nAChRs. nAChRs regulate signal transmission between many central nervous system neurons and at the neuromuscular junction, and both nicotine and cocaine affect their function. Reports in the literature have suggested that cocaine and other local anesthetics, when administered intraventricularly, are antagonistic to the behavioral and pharmacological effects of nicotine.
The aim of this proposal is to determine the chemical mechanism of by which nicotine activates and cocaine inhibits neuronal nAChRs expressed in Xenopus oocytes. Preliminary results have demonstrated that different neuronal nicotinic receptor subtypes are differentially affected by cocaine. The affinity of cocaine for some neuronal subtypes is well within the range of concentrations of cocaine expected to be present during a cocaine overdose and, possibly, during the recreational use of cocaine. Thus specific nicotinic receptor subtypes may be expected to play a role in the toxicity of cocaine. The site to which cocaine binds on wild type and mutated nAChRs will be characterized functionally and chemically. The approaches adopted include (I) construction of receptors with varying subunit composition, chimeras, and subunits with peptide regions altered by site-directed mutations, and (ii) chemical kinetic measurements using a new technique developed by our laboratories, with a 50 to 100 micro-second time resolution that allows one to determine the effect of the drugs on the elementary steps of the receptor mediated reactions. The molecular mechanism, including knowledge of the concentraton at which nicotine and cocaine affect specific nAChRs, will provide a basis for mechanism-based design of therapeutic agents that will alleviate some effects of the drugs without themselves having deleterious effects. The knowledge gained is expected to contribute to the rational treatment of the millions of individuals adversely affected by cocaine and nicotine.
