Actions of endogenous vasopressin and oxytocin on anterior pituitary hormone secretion

To evaluate the significance of endogenous vasopressin and oxytocin in control of anterior pituitary hormone release, antiserum against vasopressin (AB-VP) or oxytocin (AB-OT) were microinjected into the third ventricle (3V) of conscious, ovariectomized rats to immunoneutralize endogenous VP or OT, respectively. Blood samples were collected just before and at different times after the microinjections. There were no differences in the plasma LH, FSH, PRL and TSH concentrations between control groups injected into the 3V with normal rabbit serum (NRS) and groups submitted to the intraventricular injection of AB-OT or AB-VP for 24 h after the injections. Plasma growth hormone (GH) declined significantly by 4 h after NRS injection, remained low at 6 h and had rebounded to nearly initial levels at 24 h. This pattern was not changed by microinjection of AB-VP, but plasma GH increased significantly compared to initial values in the period from 1 to 24 h after intraventricular microinjection of AB-OT. The intraventricular injection of AB-VP or AB-OT significantly decreased plasma ACTH; however, the effect of AB-VP was more prolonged and persisted for 6 rather than 4 h after injection. Thus, endogenous oxytocin may play a role in the control of basal GH release probably by stimulating somatostatin secretion and/or inhibiting GH-releasing hormone secretion or by both actions. On the other hand, both endogenous vasopressin and oxytocin play a physiologically significant stimulatory role in the control of basal ACTH release.