Expression and function of α2-adrenoceptors in zebrafish:: drug effects, mRNA and receptor distributions

The alpha(2)-adrenoceptors are G-protein-coupled receptors that mediate many of the physiological effects of norepinephrine and epinephrine. Mammals have three subtypes of alpha(2)-adrenoceptors, alpha(2A), alpha(2B) and alpha(2C). Zebrafish, a teleost fish used widely as a model organism, has five distinct alpha(2)-adrenoceptor genes. The zebrafish has emerged as a powerful tool to study development and genetics, with many mutations causing diseases reminiscent of human diseases. Three of the zebrafish adra2 genes code for orthologues of the mammalian alpha(2)-adrenoceptors, while two genes code for alpha(2Da)- and alpha(2Db)- adrenoceptors, representing a duplicated, fourth alpha(2)-adrenoceptor subtype. The three different mammalian alpha(2)-adrenoceptor subtypes have distinct expression patterns in different organs and tissues, and mediate different physiological functions. The zebrafish alpha(2)-adrenergic system, with five different alpha(2)-adrenoceptors, appears more complicated. In order to deduce the physiological functions of the zebrafish alpha(2)-adrenoceptors, we localized the expression of the five different alpha(2)-adrenoceptor subtypes using RT-PCR, mRNA in situ hybridization, and receptor autoradiography using the radiolabelled alpha(2)-adrenoceptor antagonist [ethyl-H-3]RS-79948-197. Localization of the alpha(2A)-, alpha(2B)- and alpha(2C)-adrenoceptors in zebrafish shows marked conservation when compared with mammals. The zebrafish alpha(2A), alpha(2Da), and alpha(2Db) each partially follow the distribution pattern of the mammalian alpha(2A): a possible indication of subfunction partitioning between these subtypes. The alpha(2)-adrenergic system is functional in zebrafish also in vivo, as demonstrated by marked locomotor inhibition, similarly to mammals, and lightening of skin colour induced by the specific alpha(2)-adrenoceptor agonist, dexmedetomidine. Both effects were antagonized by the specific alpha(2)-adrenoceptor antagonist atipamezole.