In spite of much evidence for the existence of nicotinic cholinergic transmission in the central nervous system, and in spite of the availability of detailed molecular information on nicotinic acetylcholine receptors from peripheral target organs (electric tissue and muscle), no unambiguous identification of the neuronal nicotinic acetylcholine receptor has been accomplished. Brain tissue contains binding sites for -bungarotoxin, a potent blocker of nicotinic receptors in neuroeffector junctions, and it has been proposed that this 'toxin receptor' represents the neuronal nicotinic receptor; however, the failure of curarimimetic neurotoxins to inhibit synaptic transmission between neurons has cast doubt on this assumption. To settle this question we have attempted, without success, to impart acetylcholine sensitivity to synthetic membranes by incorporation of the -bungarotoxin binding macromolecule purified form chick brain, under conditions compatible with reconstitution of the peripheral acetylcholine receptor. Recently binding sites for acetylcholine have been described in rat brain, which have a nicotinic pharmacology and differ from toxin binding sites. We have found such sites in the avian brain, especially the optic lobe; we have solubilized them with full retention of binding activity; and we have obtained preliminary evidence that they reside on a macromolecule different from the toxin receptor. We plan to purify this receptor by means of conventional and biospecific (affinity) procedures. Important objectives of our research will be to obtain primary structure information, and to test the proposition that this binding protein functions like a nicotinic receptor in the neuromuscular junction, by analyzing its transport properties, using a reconstitution protocol, and by examining its architecture. This research will contribute to a characterization of the cholinergic system in the CNS and thereby, it is hoped, to a better understanding of various pathological processes such as Alzheimer's disease and nicotine addiction.
