CYTOCHEMICAL DETECTION OF GONADOTROPIN-RELEASING HORMONE-BINDING SITES ON RAT PITUITARY-CELLS WITH LUTEINIZING-HORMONE, FOLLICLE-STIMULATING-HORMONE, AND GROWTH-HORMONE ANTIGENS DURING DIESTRUS UP-REGULATION

Pituitary cells with GnRH receptors increase over 2-fold during diestr us to reach a peak during the morning of proestrus. This is followed b y a rapid fall during the afternoon of proestrus to reach a nadir by e strus. The objective of this study was to learn the identity of the ne w target cells added during diestrus. This was particularly important in view of recent evidence showing that gonadotropes with LHbeta and F SHbeta mRNA have GH antigens. Pituitary cells from diestrous and proes trous rats were exposed to biotinylated GnRH (Bio-GnRH) for 10 min. Bi o-GnRH was detected by avidin peroxidase, and then the cells were immu nolabeled for pituitary hormones. The percentages of cells labeled for Bio-GnRH rose during diestrus from 6.6 +/-0.8% in the morning to 11.9 +/- 0.7% by evening (mean +/- SD). By the morning of proestrus, the p ercentages of Bio-GnRH target cells increased further to 16 +/- 0.7%. The percentages of pituitary cells dual labeled for LHbeta antigens an d Bio-GnRH rose from 4.3 +/- 0.6% to 9% +/- 1% during diestrus and ave raged 13 +/- 0.7% by the morning of proestrus. At this time, 90% of ce lls with LH antigens bound BioGnRH. When percentages of pituitary cell s with FSHbeta antigens and Bio-GnRH-binding sites were analyzed, ther e was an increase during diestrus from 4 +/- 0.4% to 9.7 +/- 0.7%; a p eak level of 14 +/- 0.9% wa reached by the morning of proestrus. Bio-G nRH binding was expressed by 86% of FSH cells during this peak. Finall y, GH antigens were also detected in GnRH target cells. The percentage of cells dual labeled for Bio-GnRH and GH increased from 4 +/- 0.8% t o 8 +/- 1% during diestrus and the morning of proestrus. During the di estrous and proestrous peak periods of expression, Bio-GnRH binding wa s seen in 32% of GH cells. None of the other pituitary cell types show ed significant GnRH binding. These studies showed that most of the new GnRH-receptive cells stem from maturing gonadotropes. Half of the GnR H-receptive cells also contain GH antigens, which correlated with resu lts from previous studies that showed GH antigens in cells with gonado tropin mRNAs. This might reflect expression of gonadotrope functions b y a subset of GH cells. Alternatively, the GH antigens may be bound to GH receptors in gonadotropes. This latter possibility may signify a p aracrine regulation of gonadotrope function by GH.