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Effect of Chronic Prolactin Infusion on Pituitary Prolactin and Hypothalamic Proopiomelanocortin
Author(s) -
Wardlaw Sharon L.,
Kim Jennie,
Matera Cristina
Publication year - 1997
Publication title -
journal of neuroendocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.062
H-Index - 116
eISSN - 1365-2826
pISSN - 0953-8194
DOI - 10.1046/j.1365-2826.1997.00549.x
Subject(s) - medicine , endocrinology , proopiomelanocortin , prolactin , hypothalamus , endogeny , pituitary gland , beta endorphin , peptide hormone , chemistry , biology , hormone
Although there is evidence that endogenous opioids, and in particular β ‐endorphin ( β ‐EP), may mediate some of the suppressive effects of hyperprolactinemia on the hypothalamic‐pituitary‐gonadal (HPG) axis, there is controversy about the effects of prolactin (PRL) on β ‐EP and its precursor, proopiomelanocortin (POMC), in the hypothalamus. In this study we have therefore examined the effects of chronic peripheral and intracerebroventricular (i.c.v.) infusion of ovine PRL on POMC gene expression and β ‐EP levels in the medial basal hypothalamus (MBH) of castrated male and female rats. Endogenous pituitary and plasma PRL levels were determined by RIA with an antiserum to rat PRL which does not crossreact with oPRL. Suppression of endogenous rPRL levels was used as a confirmation of the biological effectiveness of the infused oPRL. POMC mRNA was measured in the MBH by solution hybridization assay. In the first experiment oPRL (5 μg/μl/h) or vehicle was infused for 2 weeks by osmotic minipump into the right lateral ventricle of ovariectomized rats. The mean plasma concentration of rPRL declined from 3.7±1.0 ng/ml in the controls to 1.4±0.13 ng/ml in the oPRL infused animals (P<0.05); pituitary rPRL content similarly decreased from 39.1±4.6 μg to 20.4±3.7 μg (P<0.02). There was no significant change in the concentration of POMC mRNA or β ‐EP in the MBH of the oPRL treated animals. In the second experiment oPRL was infused for 1 week into the third ventricle of orchiectomized rats. Again despite a fall in endogenous PRL levels, there was no significant change in POMC or β ‐EP in the MBH. In the third experiment oPRL was infused subcutaneously into orchiectomized rats for 2 weeks. Mean plasma oPRL levels were 150±7.3 ng/ml after 1 week and 58±7.5 ng/ml after 2 weeks. Pituitary rPRL content was again suppressed in the oPRL treated animals but no change in POMC or β ‐EP was detected in the MBH. We conclude that oPRL can be infused both peripherally and centrally for up to 2 weeks with resulting suppression of endogenous pituitary PRL content and release. Under these conditions no effects on the concentrations of POMC mRNA or β ‐EP could be demonstrated in the hypothalamus. These results suggest that either PRL has nongenomic effects on hypothalamic β ‐EP or that endogenous opioids other than β ‐EP mediate the suppressive effects of PRL on the HPG axis.