J. EndocrinoLlnvest. 14.·463-468,1991 Oxytocin does not modify the prolactin response to metoclopramide in normal women P. Chiodera*, R. Volpi**, D. Gramellini***, C. Cigarini****, A. Corradi**, U. Cavazzini**, L. Bianconi*, C. Papadia*, A. Caiazza*****, L. Capretti******, G. Rossi*******, and V. Coi ro** *Cattedra di Endocrinologia e Patologia Costituzionale, **Cattedra di Clinica Medica Generale, ** *Istituto di Ostetricia e Ginecologia, Universita di Parma, Parma; ****Divisione di Ostetricia e Ginecologia, Ospedale di Reggio Emilia, Reggio Emilia; *****Divisione di Medicina Generale, Ospedale di Fidenza, Fidenza; ******Divisione di Medicina Generale, Ospedale di Codogno, Codogno; *******Laboratorio RIA , Ospedale di Guastalla, Guastalla, Italy ABSTRACT. The present study was undertaken in order to establish whether oxytocin (OT) affects the dopaminergic control of PRL secretion in normal women during follicular, periovulatory and luteal phase of their menstrual cycle. For this purpose, 22 normal women were tested with a lower (1 mg) or higher (10 mg) dose of the dopaminergic antago- nist metociopramide (MCP) with or without the con- current treatment with OT (2 IU injected plus 0.033 INTRODUCTION The nonapeptide hormone oxytocin (OT) is synthe- sized within cell bodies of the hypothalamic par- aventricular and supraoptic nuclei, transported through neuroaxons to the posterior pituitary and se- creted into the blood stream (1). In addition, OT con- taining nerve terminals have been found in the exter- nal zone of the median eminence near the primary portal capillaries (2 , 3) and hence are probably se- creted into the hypophyseal portal circulation, where elevated OT concentrations have been found (3 , 4) . Recent studies suggest a role of OT in the control of anterior pituitary function (5). In the present study, we focused our attention on the effect of OT on PRL secretion. Previous studies failed to provide evidence of OT effects in modulating PRL response to insulin induced hypoglycemia (6), exercise (7) or TRH (5) in male human subjects. In contrast, OT has been reported to stimulate PRL secretion from the female rat pituitary both in vivo and in vitro (8- Key-words: Oxytoc in , dopamine, metoclopramide. prolact in . Correspondence: Dr. Paolo Chi odera, Cattedra di Endocrinologia e Patolog ia Costituzionale. Universita di Parma. Via Gramsci 1 4, 43100 Parma, I ta l y. Received July 12. 1990; accepted March 1 8. 1 99 1 463 IU/min infused for 2 h). Since OT was found unable to modify the effect of either 1 or 10 mg MCP, in ad- ditional experiments the same doses of MCP and OT were administered after dopamine (0.04 Ilg/kg/min for 2 h) infusion. Also in these experi- mental conditions OT failed to modify the PRL re- sponse to MCP. These data argue against a role of OT in modulation of the dopaminergic control of PRL secretion in normal women. 10); furthermore, OT has been found able to en- hance the PRL response to TRH in normal men- struating women (11). In the present study, we examined whether OT ad- ministration in normal women modulates the PRL response to the antidopaminergic agent metocio- pram ide (MCP), another potent stimulator of PRL secretion (12, 13). For this purpose MCP was giv- en to normally menstruating women with or without the simultaneous treatment with OT. In additional experiments we evaluated the effect of dopamin- ergic activity on basal as well as MCP-stimulated PRL release during infusion of dopamine. MATERIALS AND METHODS Sixty-two normal women, aged 22-32 yr, partici- pated in this study after giving informed consent. None of them presented clinical or laboratory evi- dence of endocrine-metabolic, renal, hepatic or other intercurrent diseases. All women were within 10% of their ideal body weight and had a history of regular menstrual cycles of normal duration (26-30 days). Basal body temperature and circulating con- centrations of ovarian steroids were measured dai- ly and were utilized as criteria to determine the pre- cise period of the cycle.