PT  - JOURNAL ARTICLE
AU  - V. González-Nuñez
AU  - G. Arsequell
AU  - E. Szemenyei
AU  - G. Toth
AU  - G. Valencia
AU  - R. E. Rodriguez
TI  - Binding Profile of the Endogenous Novel Heptapeptide Met-Enkephalin-Gly-Tyr in Zebrafish and Rat Brain
AID  - 10.1124/jpet.105.084558
DP  - 2005 Aug 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 862--867
VI  - 314
IP  - 2
4099  - http://jpet.aspetjournals.org/content/314/2/862.short
4100  - http://jpet.aspetjournals.org/content/314/2/862.full
SO  - J Pharmacol Exp Ther2005 Aug 01; 314
AB  - Zebrafish is considered a model organism, not only for the study of the biological functions of vertebrates but also as a tool to analyze the effects of some drugs or toxic agents. Five opioid precursor genes homologous to the mammalian opioid propeptide genes have recently been identified; one of these, the zebrafish proenkephalin, codes a novel heptapeptide, the Met-enkephalin-Gly-Tyr (MEGY). To analyze the pharmacological properties of this novel ligand, we have labeled it with tritium ([3H]MEGY). In addition, we have also synthesized two analogs: (d-Ala2)-MEGY (Y-d-Ala-GFMGY) and (d-Ala2, Val5)-MEGY (Y-d-Ala-GFVGY). The binding profile of these three agents has been studied in zebrafish and rat brain membranes. [3H]MEGY presents one binding site in zebrafish, as well as in rat brain membranes, although it shows a slight higher affinity in zebrafish brain. The observed saturable binding is displaced by naloxone, thus confirming the opioid nature of the binding sites. Competition binding assays indicate that the methionine residue is essential for high-affinity binding of MEGY and probably of other peptidic agonists in zebrafish, whereas the change of a Gly for a d-Ala does not dramatically affect the ligand affinity. Our results show that the percentage of [3H]MEGY displaced by all the ligands studied is higher than 100%, thus inferring that naloxone (used to determine nonspecific binding) does not bind to all the sites labeled by [3H]MEGY. Therefore, we can deduct that some of the MEGY binding sites should not be considered classical opioid sites. The American Society for Pharmacology and Experimental Therapeutics