Anabaseine Is a Potent Agonist on Muscle and Neuronal Alpha-Bungarotoxin-Sensitive Nicotinic Receptors 1 WILLIAM R. KEM, VLADIMIR M. MAHNIR, ROGER L. PAPKE and CHRISTOPHER J. LINGLE Department of Pharmacology and Therapeutics (W.R.K., V.M.M., R.L.P.), College of Medicine, University of Florida, Gainesville, Florida and Anesthesia Research Unit (C.J.L.), Department of Anesthesiology, Washington University School of Medicine, St. Louis, Missouri Accepted for publication June 19, 1997 ABSTRACT We assessed the pharmacological activity of anabaseine, a toxin found in certain animal venoms, relative to nicotine and anabasine on a variety of vertebrate nicotinic receptors, using cultured cells, the Xenopus oocyte expression system, contractility assays with skeletal and smooth muscle strips containing nicotinic receptors and in vivo rat prostration assay involving direct injection into the lateral ventricle of the brain. Anabaseine stimulated every subtype of nicotinic receptor that was tested. It was the most potent frog skeletal muscle nicotinic receptor agonist. At higher concentra- tions it also blocked the BC 3 H1 (adult mouse) muscle type recep- tor ion channel. The affinities of the three nicotinoid compounds for rat brain membrane alpha-bungarotoxin binding sites and their potencies for stimulating Xenopus oocyte homomeric alpha7 re- ceptors, expressed in terms of their active monocation concen- trations, displayed the same rank order, anabaseine.anabasine. nicotine. Although the maximum currents generated by anabaseine and anabasine at alpha7 receptors were equivalent to that of acetylcholine, the maximum response to nicotine was only about 65% of the acetylcholine response. At alpha4-beta2 recep- tors the affinities and apparent efficacies of anabaseine and anabasine were much less than that of nicotine. Anabaseine, nicotine and anabasine were nearly equipotent on sympathetic (PC12) receptors, although parasympathetic (myenteric plexus) receptors were much more sensitive to anabaseine and nicotine but less sensitive to anabasine. These differences suggest that there may be different subunit combinations in these two auto- nomic nicotinic receptors. The preferential interactions of anabaseine, anabasine and nicotine with different receptor sub- types provides molecular clues that should be helpful in the design of selective nicotinic agonists. Neuronal nicotinic receptors have attracted much interest during the past few years, largely due to the discovery that the Alzheimer’s brain loses many of its nicotinic receptors by the time of death, whereas muscarinic receptors are much less affected (Kellar et al., 1987; Araujo et al., 1988). So far, therapeutic approaches directed toward cholinergic systems in the brain have focused on stimulation of postsyaptic mus- carinic cholinergic receptors, either directly with muscarinic agonists or indirectly by cholinesterase inhibition. Unfortu- nately these two strategies have thus far yielded only modest improvements in the cognitive functions of Alzheimer’s pa- tients. Stimulation of brain nicotinic receptors has been shown to enhance cognitive function in lower mammals (Woodruff-Pak et al., 1994; Arendash et al., 1995a, b; Decker et al., 1995; Bjugstad et al., in press), consistent with the idea that these nicotinic receptors may be potential targets treat- ment of Alzheimer’s and other dementias (Newhouse et al., 1993). Molecular biological studies have revealed a plethora of nicotinic receptor subunits in the vertebrate brain (Papke, 1993; McGehee and Role, 1995; Lindstrom, 1996; Albuquer- que et al., 1997). Although there is still little understanding of the functional consequences of this receptor multiplicity, several labs are investigating the pharmacological properties of the predominant nicotinic receptor subtypes occurring in the nervous system to provide a rational basis for the design of compounds selective for particular nicotinic receptor sub- types that influence particular mental or motor functions (Decker et al., 1995; de Fiebre et al., 1995). Flores et al. (1992) have shown that the major receptor subtype displaying high nicotine, cytisine and methylcarbamyl-choline affinity in the rat brain is the alpha4-beta2 combination. A major receptor subtype showing low affinity for nicotine but high affinity for BTX contains alpha7 subunits (Wonnacott, 1986; Luetje et al., 1990). Alpha7-containing receptors have been implicated in cognitive processes affected by hippocampal function, in- cluding sensory gating and spatial memory (Luntz-Leybman et al., 1992; Bjugstad et al., in press). Pharmacological investigations of nicotinic receptors have been facilitated by the availability of many potent natural Received for publication January 7, 1997. 1 This work was supported by Taiho Pharmaceutical Company, Ltd., To- kushima, Japan. ABBREVIATIONS: ACh, acetylcholine; BTX, alpha-bungarotoxin; DHBE, dihydro-B-erythroidine; i.c.v., intracerebroventricular; 3 H-MCC; 3 H- methylcarbamylcholine; TC, d-tubocurarine; TXX, tetrodotoxin. 0022-3565/97/2833-0979$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 283, No. 3 Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 283:979 –992, 1997 979