Pituitary cells with GnRH receptors increase over 2-fold during diestrus to reach a peak during the morning of proestrus. This is followed by a rapid fall during the afternoon of proestrus to reach a nadir by estrus. The objective of this study was to learn the identity of the new target cells added during diestrus. This was particularly important in view of recent evidence showing that gonadotropes with LH β and FSH β mRNA have GH antigens. Pituitary cells from diestrous and proestrous rats were exposed to biotinylated GnRH (Bio-GnRH) for 10 min. Bio-GnRH was detected by avidin peroxidase, and then the cells were immunolabeled 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 percentages of Bio-GnRH target cells increased further to 16 ± 0.7%. The percentages of pituitary cells dual labeled for LH β antigens and Bio-GnRH rose from 4.3 ± 0.6% to 9% ± 1% during diestrus and averaged 13 ± 0.7% by the morning of proestrus. At this time, 90% of cells with LH antigens bound Bio-GnRH. When percentages of pituitary cells with FSH β antigens and Bio-GnRH-binding sites were analyzed, there was an increase during diestrus from 4 ± 0.4% to 9.7 ± 0.7%; a peak level of 14 ± 0.9% was reached by the morning of proestrus. Bio-GnRH binding was expressed by 86% of FSH cells during this peak. Finally, 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% to 8 ± 1% during diestrus and the morning of proestrus. During the diestrous and proestrous peak periods of expression, Bio-GnRH binding was seen in 32% of GH cells. None of the other pituitary cell types showed significant GnRH binding. These studies showed that most of the new GnRH-receptive cells stem from maturing gonadotropes. Half of the GnRH-receptive cells also contain GH antigens, which correlated with results from previous studies that showed GH antigens in cells with gonadotropin mRNAs. This might reflect expression of gonadotrope functions by a subset of GH cells. Alternatively, the GH antigens may be bound to GH receptors in gonadotropes. This latter possibility may signify a paracrine regulation of gonadotrope function by GH.
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