Investigation of the Possible Pharmacologically Active Forms of the Nicotinic Acetylcholine Receptor Agonist Anabaseine

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 α4β2 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.

A Pharmacological Comparison of Two Isomeric Nicotinic Receptor Agonists: The Marine Toxin Isoanatabine and the Tobacco Alkaloid Anatabine

“…Both compounds are isomers differing only in the position of a single double bond in an otherwise saturated piperidine ring in the "parent" compound anabasine, which is present in wild tobacco [22]. The pharmacological properties of anabasine [23,24] and the related anabaseine [25,26] have been investigated, but not as extensively as those of nicotine.…”

Discovery of the Nicotinic Receptor Toxin Anabaseine in a Polystiliferan Nemertean

“…The absorbance spectrum of the second peak contained two major peaks, at 230 and 260 nm with the absorbance of the 230 nm peak being ~1.5× larger than the 260 nm peak, similar to the spectrum for unionized anabaseine injected into the same column. The presumed anabaseine peak was collected, evaporated to dryness in a rotary evaporator and subjected to reverse phase C18 LC-Electrospray Ionization MS. Like synthetic anabaseine, the isolated compound produced LC-ESI-MS m/z 161 and m/z 179 ions which eluted simultaneously with the LC-UV absorbance peak, whose retention time was typical for anabaseine; the 161 peak corresponded to the cyclic iminium form of anabaseine and the 179 peak to the open-chain ammonium-ketone form of anabaseine [ 21 , 22 ].…”

Marine Origin Ligands of Nicotinic Receptors: Low Molecular Compounds, Peptides and Proteins for Fundamental Research and Practical Applications

“…Anabaseine is chemically similar to the tobacco alkaloid, anabasine, but possesses an imine double bond in the otherwise saturated piperidine ring and is classified as a non-selective nicotinic agonist, since it stimulates a broad spectrum of nAChRs [ 57 ]. Among three major forms of anabaseine (cyclic iminium, cyclic imine and the monocationic open-chain ammonium-ketone) co-existing in almost equal concentrations at physiological pH, the cyclic iminium is the form that avidly binds and activates vertebrate nAChRs [ 58 ].…”