Localization of nicotinic cholinergic receptors in rat brain: autoradiographic studies with [3H]cytisine.
ABSTRACT There is a great deal of interest in the role of nicotinic acetylcholine receptors in the central nervous system, although their function is not well understood at present. Currently, central nicotinic receptors can be classified broadly as either alpha-bungarotoxin binding sites with low affinity for acetylcholine agonists, or as high-affinity agonist binding sites with low affinity for alpha-bungarotoxin. Neuronal nicotinic receptors with a high affinity for agonists are distributed widely in the central nervous system. Evidence from molecular biology and electrophysiology suggests that multiple nicotinic receptor types exist in the brain. In this study we have used the agonist [3H]cytisine as a ligand for autoradiography to generate a detailed quantitative map of the high-affinity agonist binding nicotinic receptor in the rat brain. Optimized binding conditions, characterization of the kinetic and equilibrium binding properties, and demonstration of the nicotinic pharmacology of this binding site in tissue sections confirm the usefulness of [3H]Cytisine as a ligand for nicotinic receptor autoradiography. [3H]Cytisine autoradiography provides excellent anatomic resolution with very low non-specific binding. This property has allowed us to describe variations in receptor density within subnuclei and gradients of receptor density in larger brain regions. Data from several studies suggest that the predominant high-affinity agonist binding nicotine receptor in the central nervous system is composed of the alpha 4 and beta 2 subunits. The data in the current study are consistent with the suggestion that [3H]cytisine labels only the alpha 4 beta 2 nicotinic receptor with high affinity, offering the possibility of localizing a specific nicotinic receptor subtype in the central nervous system. In summary, we characterize the optimum experimental conditions for the use of [3H]cytisine in tissue section autoradiography. [3H]Cytisine proves to be an excellent marker for nicotinic cholinergic receptors with a very high affinity and very low background. We provide a detailed quantitative characterization of nicotinic receptor density in the rat central nervous system and we find there are significant variations and gradients in receptor density within specific brain regions, including subregions previously thought to be homogeneous.
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ABSTRACT: Nicotinic acetylcholine receptors (nAChRs) mediate a variety of brain functions. Findings from postmortem studies and clinical investigations have implicated them in the pathophysiology and treatment of Alzheimer's and Parkinson's diseases and other CNS disorders (e.g. Tourette's syndrome, epilepsy, nicotine dependence). Therefore, it ultimately might be useful to image nAChRs noninvasively for diagnosis, for studies on how changes in nAChRs might contribute to cerebral disorders, for development of therapies targeted at nAChRs, and to monitor the effects of such treatments. To date, only (S)-(-)-nicotine, radiolabeled with 11C, has been used for external imaging of nAChRs in human subjects. Since this radiotracer presents drawbacks, new ligands, with more favorable properties, have been synthesized and tested. Three general classes of compounds, namely, nicotine and its analogs, epibatidine and related compounds, and 3-pyridyl ether compounds, including A-85380, have been evaluated. Analogs of A-85380 appear to be the most promising candidates because of their low toxicity and high selectivity for the alpha4beta2 subtype of nAChRs.Behavioural Brain Research 09/2000; 113(1-2):143-57. · 3.42 Impact Factor
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ABSTRACT: Tobacco use in adolescence represents one of the major challenges to the future of public health. Whereas numerous studies have explored the consequences of fetal or adult nicotine effects, little or no basic research has been conducted for nicotine during adolescence, the stage at which regular cigarette use typically begins. This review describes the recent development of a model of adolescent nicotine administration in rats that recapitulates the plasma levels of nicotine found in smokers. Adolescent nicotine evoked CNS nicotinic receptor up-regulation with a distinctly different regional pattern from that seen in the adult; increased receptor expression in male rats persisted for more than a month after discontinuing nicotine administration. We also identified evidence of cell damage and changes in cell size in the female hippocampus. These changes were accompanied by alterations in synaptic activity of cholinergic, noradrenergic, dopaminergic and serotonergic systems during nicotine administration and persisting for extended periods after the termination of exposure; behavioral alterations were commensurate with the neurochemical changes. In each case, the effects of adolescent nicotine differed not only from the adult, but also from the those seen after fetal exposure, indicating that adolescence represents a unique period of vulnerability for nicotine-induced misprogramming of brain cell development and synaptic function. Effects of nicotine on critical components of reward pathways and circuits involved in learning, memory and mood are likely to contribute to increased addictive properties and long-term behavioral problems seen in adolescent smokers.Neurotoxicology and Teratology 24(3):369-84. · 2.98 Impact Factor