The nicotinic acetylcholine receptors of zebrafish and an evaluation of pharmacological tools used for their study

Zebrafish (Danio rerio) have been used to study multiple effects of nicotine, for example on cognition, locomotion, and stress responses, relying on the assumption that pharmacological tools will operate similarly upon molecular substrates in the fish and mammalian systems. We have cloned the zebrafish nicotinic acetylcholine receptor (nAChR) subunits and expressed key nAChR subtypes in Xenopus oocytes including neuronal (alpha 4 beta 2, alpha 2 beta 2, alpha 3 beta 4, and alpha 7) and muscle (alpha 1 beta 1(b)epsilon delta) nAChR. Consistent with studies of mammalian nAChR, nicotine was relatively inactive on muscle-type receptors, having both low potency and efficacy. It had high efficacy but low potency for alpha 7 receptors, and the best potency and good efficacy for alpha 4 beta 2 receptors. Cytisine, a key lead compound for the development of smoking cessation agents, is a full agonist for both mammalian alpha 7 and alpha 3 beta 4 receptors, but a full agonist only for the fish alpha 7, with surprisingly low efficacy for alpha 3 beta 4. The efficacy of cytisine for alpha 4 beta 2 was somewhat greater than typically reported for mammalian alpha 4 beta 2. The ganglionic blocker mecamylamine was most potent for blocking alpha 3 beta 4 receptors, least potent for alpha 7, and roughly equipotent for the muscle receptors and the beta 2-containing nAChR. However, the block of beta 2-containing receptors was slowly reversible, consistent with effective targeting of these CNS-type receptors in vivo. Three prototypical alpha 7-selective agonists, choline, tropane, and 4OH-GTS-21, were tested, and these agents were observed to activate both fish alpha 7 and alpha 4 beta 2 nAChR. Our data therefore indicate that while some pharmacological tools used in zebrafish may function as expected, others will not. (C) 2012 Elsevier Inc. All rights reserved.