Characterization of a putative nicotinic acetylcholine receptor in mammalian brain

The preponderance of evidence suggests that neurons respond electrophysio-logically to ACh and ACh antagonists in a manner consistent with the existence of AChRs, and at least some of these AChRs are nicotinic in nature. The binding of neurotoxins to brain tissue generally occurs in areas where other cholinergic markers exist. The biochemical properties of binding sites are remarkably similar to muscle tissues in size, the presence of a sulf hydryl group that alters ligand affinities, pharmacology, and glycoprotein nature. The dissimilarities, however, are also unmistakable: the association and dissociation rates of toxin binding are different, the inability of Butx to block electrophysiological activity in some tissues, the ratio of toxin sites blocked by MBTA and the apparent lack of antigenic similarity between peripheral nAChRs and brain. In at least one area the relationship to other cholinergic markers has been questioned. It is not likely that brain possesses non-functional sites with such similarity to nAChRs, but it is possible that some sites are non-synaptic and possibly non-specific (see ref. 74 for a discussion of some of these topics). We believe it is possible, however, that the dissimilarities between toxin binding sites in brain and muscle tissue reflect a structural difference between nAChRs. It is likely that toxins bind to nAChRs but do not prevent ACh binding to all sites. Since ACh has apparently not undergone evolutionary change, it is likely that evolutionary changes in cholinergic functionring are reflected in receptor specialization ³⁶. The neuromuscular synapse evolved for a highly specialized function requiring fast, excitatory post-synaptic depolarization. The muscle nAChR probably evolved from a primitive, possibly less specific, form of a molecule sensitive to acetylcholine. Although little is known regarding phylogenetic changes in receptor sites, evidence has recently been presented that the four Torpedo subunits are not immunologically distinct and may have evolved from a common precursor¹¹⁶. While some CNS neurons may have evolved with similar needs, other CNS neuronal groups might necessitate both the possibility of excitation and inhibition. Other synapses may have retained simpler evolutionary traits of mixed nicotinic-muscarinic responsiveness. There is, also, evidence for the interaction of ACh with neuromodulators and other putative neurotransmitters. It is presently not clear whether the characteristics of a receptor are similar when it is pre-synaptic, non-synaptic or inhibitory, in contrast to the peripheral excitatory post-synaptic nAChR. Future studies must determine if currently used neurotoxins bind to a functional CNS nAChR site in any of these cases. We must also find a ligand that specifically binds to and blocks nAChRs that are not affected by Butx. Only then will we be able to address the central questions. Specifically, do Butx and similar neurotoxins bind to an nAChR?, and are there nAChRs which are not bound by currently known neurotoxins?