Absence of an Opioid Stimulatory Tone on Growth Hormone Secretion in Women With Microprolactinoma

Abstract

Introduction: Literature data suggest that prolactinoma is a tumor with a complex pathogenesis and that its growth is the result of changes at the pituitary and/or hypothalamic level. Abnormal release of hypothalamic factors (including endogenous opioid peptides) may contribute to prolactinoma development. An increased endogenous opioid tone (EOT) occurs in patients with prolactinoma, and seems to play an important role in pituitary secretion, as suggested by the ability of the opiate receptor antagonist naloxone to stimulate luteinizing hormone pulsatile secretion in these patients. Objective: To investigate the effect of the EOT on growth hormone (GH) secretion in women with prolactinoma. Design: Eleven women aged 30.4 ± 6.7 years (range 20–41), with an established diagnosis of microprolactinoma, were studied. GH-releasing hormone (GHRH), 100 μg as an intravenous (IV) bolus, was administered with and without preadministration of IV naloxone, an opioid receptor antagonist, 2 mg as a bolus followed by a constant infusion of 1.6 mg/h. Blood samples were taken for 120 min after GHRH administration. Results: Naloxone did not affect the GH response to GHRH, measured as single times, mean peak values, or as integrated concentrations. Conclusion: Our data suggest that an opioid stimulatory tone on GH secretion in women with prolactinoma is absent.