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NEUROPHARMACOLOGY

Methanandamide Allosterically Inhibits in Vivo the Function of Peripheral Nicotinic Acetylcholine Receptors Containing the 7-Subunit

Urszula Baranowska, Manfred Göthert, Radosaw Rud, and Barbara Malinowska

Department of Experimental Physiology, Medical University of Biaystok, Biaystok, Poland (U.B., M.G., R.R., B.M.); and Department of Pharmacology and Toxicology, University of Bonn, Bonn, Germany (M.G.)

Methanandamide (MAEA), the stable analog of the endocannabinoid anandamide, has been proven in Xenopus oocytes to allosterically inhibit the function of the 7-nicotinic acetylcholine receptors (nAChRs) in a cannabinoid (CB) receptor-independent manner. The present study aimed at demonstrating that this mechanism can be activated in vivo. In anesthetized and vagotomized pithed rats treated with atropine, we determined the tachycardic response to electrical stimulation of preganglionic sympathetic nerves via the pithing rod or to i.v. nicotine (0.7 µmol/kg) activating nAChRs on the cardiac postganglionic sympathetic neurons. MAEA (3 and 10 µmol/kg) inhibited the electrically induced tachycardia (maximally by 15–20%; abolished by the CB₁ receptor antagonist AM 251 [N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide]; 3 µmol/kg) in pentobarbitone-anesthetized pithed rats, but not in urethane-anesthetized pithed rats, which, thus, are suitable to study the CB₁ receptor-independent inhibition of nicotine-evoked tachycardia. The subunit-nonselective nAChR antagonist hexamethonium (100 µmol/kg) and the selective 7-subunit antagonist methyllycaconitine (MLA; 3 and 10 µmol/kg) decreased the nicotine-induced tachycardia by 100 and 40%, respectively (maximal effects), suggesting that nAChRs containing the 7-subunit account for 40% of the nicotine-induced tachycardia. MAEA (3 µmol/kg) produced an AM 251-insensitive inhibition (maximum again by 40%) of the nicotine-induced tachycardia. Simultaneous or sequential coadministration of MLA and MAEA inhibited the nicotine-induced tachycardia to the same extent (maximally by 40%) as each of the drugs alone. In conclusion, according to nonadditivity of the effects, MAEA mediates in vivo inhibition by the same receptors as MLA, namely 7-subunit-containing nAChRs, although at an allosteric instead of the orthosteric site.

Address correspondence to: Dr. Barbara Malinowska, Department of Experimental Physiology, Medical University of Biaystok, 2A Mickiewicz Str., 15-222 Biaystok, Poland. E-mail: bmalin{at}umwb.edu.pl