Peter Rowell - US grants

Studies have shown that the terminals of specialized neurons of the brain contain presynaptic nicotinic receptors which, when stimulated, cause the release of neurotransmitters. The nicotine-induced release of dopamine from terminals of the mesolimbic and nigrostriatal systems is thought to produce the positively reinforcing properties of nicotine responsible for cigarette smoking. Although considerable research has been conducted on the molecular structure, localization and ligand binding properties of central nicotinic receptors; their functional properties have not been well characterized. Our recent studies have shown that prior nicotine treatment results in a desensitization of nicotine-stimulated dopamine release from striatal synaptosomes. Using the technique of in vitro superfusion, we plan to extend our studies to investigate the effects of intermittent schedules of treatment, temperature dependence and regional differences in nicotine-inducted desensitization of neurotransmitter release. In addition, the effect of the active nicotine metabolite, nornicotine, will be investigated, as will the apparent ability of low concentrations of nicotine to enhance basal neurotransmitter release with prolonged exposure. The ability of nicotine to desensitize receptor in vivo has been suggested as the mechanism by which chronic administration of the drug results in the development of nicotine tolerance and increased nicotinic receptor density. In particular, the concentration of nicotine, brain area affected, and time-course of treatment appear to be important determinants of the drug's effect. In this project we plan to investigate the ability of various nicotine administration protocol to upregulate nicotinic receptors in rat striatum and cerebral cortex. This will be coupled with measurements of plasma and brain levels of nicotine and nornicotine during administration. The overall objective of these studies is to characterize the effects of prior nicotine exposure with respect to desensitization and long-term regulation of central nicotinic receptors. This characterization will increase our understanding of how nicotine influences central neurotransmission and how the responses which it produces are affected by the time course, schedule and duration of its delivery. The processes of receptor desensitization and upregulation could have important implications on cigarette smoking where nicotine is taken at intermittent schedules for long periods of time. A better understanding of the dynamics of nicotine's effects on neurotransmitter systems in the brain is crucial in our attempts to develop successful smoking cessation programs as well as our ability to understand the central reinforcing pathways upon which nicotinic drugs exert their effects.

Cigarette smoking is a major public health problem in the United States, and nicotine is the principal pharmacological component of tobacco which reinforces smoking behavior. There is compelling evidence that nicotine produces its reinforcing effects by acting on nicotinic cholinergic receptors located on mesolimbic neurons in the brain which release dopamine from terminals of the nucleus accumbens. Studies of nicotinic receptors in other brain areas or in peripheral nerves have shown that, while brief exposure to nicotine produces receptor activation, prolonged exposure to even very low concentrations of nicotine (below the threshold for stimulation) results in receptor desensitization, whereupon subsequent exposure to nicotine does not produce the characteristic functional response. When one considers the blood and brain concentration of nicotine which are present in cigarette smokers throughout the day, it would seem that their receptors should be in a continual state of desensitization. How, then, could nicotine produce any significant reinforcing activity if the receptors upon which it acts have been rendered non-functional? There are a number of possible explanations which will be investigated in the present project. (1) It is possible that the receptors on the mesolimbic neurons do not display the desensitization characteristics determined in previous studies in other tissues and preparations. (2) It is also likely that, even during prolonged desensitization, the nicotine receptors on mesolimbic neurons exhibit sufficient basal or persistent activity to remain functionally responsive to nicotine. (3) There is evidence that glutamate neurons at the somatodendrites or the terminals of mesolimbic neurons can contribute to and perhaps enhance nicotine-stimulated dopamine release. (4) It is possible that the increased number of nicotinic receptors which result from chronic drug administration produces an increased response or that these receptors display different desensitization characteristics. These possibilities will be investigated by measuring the functional activity of mesolimbic receptors in intact slices of the ventral tegmental area and nucleus accumbens from nicotine-treated and nontreated rats. The nicotine-stimulated efflux of rubidium following various nicotine treatment schedules, coupled with assays of nicotine brain levels and receptors, will provide valuable new information about how mesolimbic neurons respond to nicotine. These studies are important as we attempt to understand the mechanisms responsible for cigarette smoking and nicotine dependence, as new nicotine replacement therapies become available for smoking cessation, and as nicotine and nicotine analogs find a place in therapeutics for such diseases as Alzheimer's, Parkinson's, ulcerative cholitis, schizophrenia and others.