RESUMO
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 (α4ß2, α2ß2, α3ß4, and α7) and muscle (α1ß1(b)Éδ) 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 α7 receptors, and the best potency and good efficacy for α4ß2 receptors. Cytisine, a key lead compound for the development of smoking cessation agents, is a full agonist for both mammalian α7 and α3ß4 receptors, but a full agonist only for the fish α7, with surprisingly low efficacy for α3ß4. The efficacy of cytisine for α4ß2 was somewhat greater than typically reported for mammalian α4ß2. The ganglionic blocker mecamylamine was most potent for blocking α3ß4 receptors, least potent for α7, and roughly equipotent for the muscle receptors and the ß2-containing nAChR. However, the block of ß2-containing receptors was slowly reversible, consistent with effective targeting of these CNS-type receptors in vivo. Three prototypical α7-selective agonists, choline, tropane, and 4OH-GTS-21, were tested, and these agents were observed to activate both fish α7 and α4ß2 nAChR. Our data therefore indicate that while some pharmacological tools used in zebrafish may function as expected, others will not.