LOCALIZATION OF NICOTINIC CHOLINERGIC RECEPTORS IN RAT-BRAIN - AUTORADIOGRAPHIC STUDIES WITH [H-3] CYTISINE

There is a great deal of interest in the role of nicotinic acetylcholi ne receptors in the central nervous system, although their function is not well understood at present. Currently, central nicotinic receptor s can be classified broadly as either alpha-bungarotoxin binding sites with low affinity for acetylcholine agonists, or as high-affinity ago nist 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 generate a brain. In this study we have used the agonist [H-3]cytisine as a ligand for autoradiography to derailed quantitative map of the high-affinity agonist binding nicotinic receptor in the ra t brain. Optimized binding conditions, characterization of the kinetic and equilibrium binding properties, and demonstration of the nicotini c pharmacology of this binding site in tissue sections confirm the use fulness of [3H]cytisine as a ligand for nicotinic receptor autoradiogr aphy. [3H]Cytisine autoradiography provides excellent anatomic resolut ion with very low non-specific binding. This property has allowed us t o describe variations in receptor density within subnuclei and gradien ts of receptor density in larger brain regions. Data from several stud ies suggest that the predominant high-affinity agonist binding nicotin ic receptor in the central nervous system is composed of the alpha 4 a nd beta 2 subunits. The data in the current study are consistent with the suggestion that [H-3]cytisine labels only the alpha 4 beta 2 nicot inic receptor with high affinity, offering the possibility of localizi ng a specific nicotinic receptor subtype in the central nervous system . In summary, we characterize the optimum experimental conditions for the use of [H-3]cytisine in tissue section autoradiography. [3H]Cytisi ne proves to be an excellent marker for nicotinic cholinergic receptor s with a very high affinity and very low background. We provide a deta iled quantitative characterization of nicotinic receptor density in th e rat central nervous system and we find there are significant variati ons and gradients in receptor density within specific brain regions, i ncluding subregions previously thought to be homogeneous.