Hyperprolactinemia (hyperPRL)frequently suppresses luteinizing hormone (LII) and endogenous rat prolactin (rPRL) secretion under a variety of e.zperimental circumstances. Several lines of evidence suggest that elevatedprolactin (PRL) may act at the hypothalamic-pitwtary axis to inhibit pituitary hormone secretion. The goal of this study was to detennine whether hyperPRL, achieved by administration of ovine PRL (oPRL), influences LII and rPRL secretion as assessed by the reverse hemolytic plaque

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... assay. Young Sprague-Dawley rats were ovariecto.nized on Day 0 and were treated with oPRL (4mg/kg body weight, 3 times/day) beginning at 0900 h on Day 4. They were killed at 1000 h on Day 6, anterior piluitaries were collected, and cells were dispersed and prepared for the reverse hemolytic plaque assay. We analyzed mean plaque area by using a computerized image analysis system and determined the percentage of plaque-forming cells by counting the number of plaques compared to the total number of cells. HyperPRL decreases the percentage of LH plaque-forming cells under basal conditions. Although the mean I.Rplaque area was the swne in vehicle-treated and oPRL -treated rats under basal and gonadotropin-releasing hormone-stimulated conditions, hyperPRL altered the frequency distribution of d erent-sized plaques under basal conditions. It appears that hyperPRL shifts the distribution of dfferent-sized plaques such that there are more small plaques and no plaques of the Largest size classes. Basal and thyrotropin-releasing hormone-induced rPRL releasefrom single lactotropes, as measured by mean plaque area and the percentage of plaque-forming cells, is lower in lactotropes from hyperPRL rats than in controls after I h, but not 2 h, of incubation. HyperPRL alters the frequency distribution of differe nt-sized plaques after both I hand 2 h of secretion. These data suggest that hyperPRL suppresses LII and rPRL secretion by several mechanisms. In the case of its effect on gonadotropes, hyperPRL appears to decrease LII secretion primarily by decreasing the proportion of pituitary cells that secrete LII, whereas in the case of its effect on lactotropes, hyperPRL influences the secretion per cell and its responsiveness to secretagogues.