Add time:09/03/2019         Source:sciencedirect.com

The potency of several oxytocin-related peptides in inducing penile erection and yawning after injection into a lateral ventricle of male rats was compared. Substitution of two amino acids in the oxytocin molecule or deletion of the C-terminal glycinamide as in des-GlyNH2-oxytocin [oxytocin(1–8)] reduced oxytocin potency in inducing both effects, the rank order being: oxytocin > [Thr4,Gly7]-oxytocin ⋍ isotocin ([Ser4,Ile8]-oxytocin) > vasopressin ([Phe3,Arg8]-oxytocin) > des-GlyNH2-oxytocin. Oxytocin's ability to induce penile erection and yawning was abolished by permanent opening of the disulfide bridge by reduction and carboxymethylation. Oxytocin(1–6) and oxytocin(7–9) were also inactive. Penile erection and yawning induced by oxytocin-related peptides were antagonized in a dose-dependent manner by nonapeptide antagonists with a rank order of potency that follows their antioxytocic activity (d[(CH2)5Tyr(Me)Orn8]-vasotocin ⋍ [Pen1,Phe(Me)2,Thr4,Orn8]-oxytocin>d[(CH2)5Tyr(Me)Arg8]-vasopressin). Carboxymethylated oxytocin, oxytocin(1–6), and oxytocin(7–9) were devoid of antagonistic activity. The present results suggest that central oxytocin receptors mediating the expression of penile erection and yawning are structurally related to those present in the uterus and in the mammary gland.

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