Molecular Brain Research 84 (2000) 1–6 www.elsevier.com / locate / bres Research report ZFOR2, a new opioid receptor-like gene from the teleost zebrafish ( Danio rerio) a,c b,c b,c ´ Alejandro Barrallo , Rogelio Gonzalez-Sarmiento , Francisco Alvar , a,c, * ´ Raquel E. Rodrıguez a Department of Biochemistry and Molecular Biology, Department of Medicine, Faculty of Medicine, University of Salamanca, Campus Unamuno, Salamanca 37007, Spain b Molecular Medicine Unit, Department of Medicine, Faculty of Medicine, University of Salamanca, Campus Unamuno, Salamanca 37007, Spain c ´ ´ Institute of Neuroscience of Castilla y Leon, Castilla y Leon, Spain Accepted 13 June 2000 Abstract A new opioid receptor-like (ZFOR2) has been cloned and characterized in an anamniote vertebrate, the teleost zebrafish ( Danio rerio). ZFOR2 encodes a 384-amino-acid protein with seven potential transmembrane domains, and its predicted amino acid sequence presents an overall 74% degree of identity to mammalian m opioid receptors. Its inclusion in a dendrogram generated from the alignment of the opioid receptor’s protein sequences, confirms its classification as a m opioid receptor. Divergences in sequence are greater in the regions corresponding to extracellular loops, suggesting possible differences in ligand selectivity with respect to the classical m opioid receptors. The genomic structure of ZFOR2 is also highly conserved throughout the phylogenetic scale, supporting the origin of opioid receptors early in evolution. Nevertheless, ZFOR2 lacks the fourth exon found in human and rodent m opioid receptors, that is known to be involved in desensibilization and internalization processes.  2000 Elsevier Science B.V. All rights reserved. Theme: Neurotransmitters, modulators, transporters, and receptors Topic: Opioid receptors Keywords: m Opioid receptor; Genomic structure; Anamniote vertebrate; Molecular evolution 1. Introduction with seven potential transmembrane domains. Upon their activation, opioid receptors inhibit adenylate cyclase and 1 21 Opioid receptors are the targets of the endogenous modulate K and Ca conductance [11]. An additional opioid peptides and opiate drugs, and represent one of the receptor with high homology to the opioid receptors called main mechanisms of pain control in humans; they also ORL (opioid receptor like) has been also cloned; the play a role in other systems, like the immune or neuroen- sequence of this new receptor is closely related to that of docrine systems [23]. Opioid receptors have been classical- the opioid receptors [34]. ly classified following pharmacological criteria into three Although the main body of research on opioid receptors types: m, d and k [16], although subtypes for each one of has been performed on mammalian models, its presence in these receptors have been proposed to exist [4,22,26]. other phyla was suggested shortly after their discovery Nevertheless, only three different opioid receptors, corre- [24,25]. Pharmacological and immunohistochemical data sponding to the three main types, have been recently point towards the existence of an opioid system in birds, cloned in mammals [9,13,29]. All these receptors are teleosts [3,7] and even in invertebrates [10], but the role members of the G protein-coupled receptor superfamily, that opioid receptors play in the biology of such organisms remains obscure. It has been proposed that the opioid system arose as an immunomodulatory system in inverte- *Corresponding author. Tel.: 134-923-294-553; fax: 134-923-294- brates and that the analgesic properties were developed 626. ´ E-mail address: requelmi@gugu.usal.es (R.E. Rodrıguez). later in evolution, when pain appeared as an alerting 0169-328X / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0169-328X(00)00152-2