Investigation of the possible pharmacologically active forms of the nicotinic acetylcholine receptor agonist anabaseine

Authors: ANDRUD, KRISTIN - University Of Florida; XING, HONG - University Of Florida; BJARNE, GABRIELSEN - University Of Florida; BLOOM, LINDA - University Of Florida; MAHNIR, VLADMIR - University Of Florida; Lee, Stephen; Green, Benedict - Ben; LINDSTROM, JON - University Of Pennsylvania; KEM, WILLIAM - University Of Florida

Submitted to: Marine Drugs
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/24/2019
Publication Date: 10/29/2019
Citation: Andrud, K., Xing, H., Bjarne, G., Bloom, L., Mahnir, V., Lee, S.T., Green, B.T., Lindstrom, J., Kem, W. 2019. Investigation of the possible pharmacologically active forms of the nicotinic acetylcholine receptor agonist anabaseine. Marine Drugs. 17:614. https://doi.org/10.3390/md17110614.
DOI: https://doi.org/10.3390/md17110614

Interpretive Summary: Anabaseine, an alkaloid with chemical and pharmacological properties similar to nicotine is a potent agonist at a variety of nicotinic acetylcholine receptors, particularly the vertebrate skeletal muscle and a7 neuronal nAChRs that display high affinities for the snake toxin a-bungarotoxin. Anabaseine is also a weak partial agonist at a4ß2 nAChRs that modulate cognitive and addiction neuronal circuits in the brain. However, since two monocationic forms of anabaseine coexist at equal concentrations regardless of pH, stable analogs for each form were prepared and we investigated which of the forms of anabaseine is pharmacologically active.

Technical Abstract: Three major forms of the nicotinic agonist toxin anabaseine (cyclic iminium, cyclic imine and the monocationic open-chain ammonium-ketone) co-exist in almost equal concentrations at physiological pH. We asked the question: Which of these forms is pharmacologically active? First, we investigated the pH dependence of anabaseine inhibition of [3H]-methylcarbamylcholine binding at rat brain a4ß2 nicotinic acetylcholine receptors (nAChRs). These experiments indicated that one or both monocationic forms interact with the orthosteric binding site for ACh. However, since they occur at equal concentrations near physiological pH, we employed another approach, preparing a stable analog of each form and examining its agonist activities and binding affinities at several vertebrate brain and neuromuscular nAChRs. Only 2-(3-pyridyl)-1,4,5,6-tetrahydropyrimidine monohydrogen chloride (PTHP), the cyclic iminium analog, displayed nAChR potencies and binding affinities similar to anabaseine. The cyclic imine analog 2,3'-bipyridyl and the open-chain ammonium-ketone analog 5-methylamino-1-(3-pyridyl)-1-pentanone (MAPP), displayed =1% of the activity predicted if the one form was solely active. The lower potency of weakly basic 2,3'-bipyridyl can be explained by the presence of a small concentration of its monocationic form. Since the open chain ammonium-ketone monocationic form of anabaseine has some structural similarity to the neurotransmitter GABA, we also tested the ability of anabaseine and its 1,2-dehydropyrrolidinyl analog myosmine to activate a mammalian GABAA receptor, but no activity was detected. We conclude that the monocationic cyclic iminium is the form which avidly binds and activates vertebrate nAChRs.