Article ID Journal Published Year Pages File Type
2801048 General and Comparative Endocrinology 2011 13 Pages PDF
Abstract

The cDNA sequences encoding the mesotocin receptor (MTR) and vasotocin 1a receptor (VTR-1a) were identified in a urodele amphibian, the rough-skinned newt, Taricha granulosa. Saturation binding of [3H]oxytocin (OT) to the Taricha MTR (tMTR) was best fit by a two-state model; a high affinity-low abundance site and a lower affinity-high abundance site. Competition-binding studies found the following rank-order affinities for the tMTR: mesotocin (MT) > OT ≈ vasotocin (VT) > vasopressin (VP) > isotocin (IT). Inositol phosphate (IP) accumulation studies demonstrated functional activity of both the tMTR and Taricha VTR-1a (tVTR-1a) in a heterologous cell culture system. The rank-order potencies for the tMTR were MT > OT > VT ≈ VP > IT. The combined binding and IP results indicate that VT may act as a partial agonist of the tMTR. Rank-order potencies for the tVTR-1a were VT > VP > MT ≈ OT > IT. For both receptors, stimulation of IP accumulation was blocked by d(CH2)5[Tyr(Me)2]AVP (Manning compound) and d(CH2)5[Tyr(Me)2,Thr4,Tyr-NH2]OVT (OTA). OTA was a more potent antagonist for the transiently expressed tMTR while Manning compound was relatively more potent at inhibiting IP accumulation in tVTR-1a expressing cells. In contradiction to earlier assumptions, the absolute IC50 of Manning compound was lower for the tMTR (27 nM ± 13) than the tVTR-1a (586 nM ± 166) indicating its potential higher affinity for the tMTR, a finding with special relevance to interpretation of comparative studies investigating the behavioral and physiological actions of neurohypophysial peptides in non-mammalian species.

Research highlights► Both the mesotocin and V1a-like vasotocin receptors were cloned in Taricha granulosa. ► Manning compound, a frequently used antagonist of the vasotocin system in comparative studies, is a non-selective antagonist of both the mesotocin and V1a-like vasotocin receptor in T. granulosa. ► Vasotocin may act as a partial agonist of the mesotocin receptor in T. granulosa, similar to vasopressin and the oxytocin receptor in mammals. ► A phylogenetic analysis aligns each of the four identified T. granulosa neurohypophysial peptide receptors with each of the four subclasses in mammals.

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