D. J. Haisenleder

Gonadotropin-Releasing Hormone Pulses: Regulators of Gonadotropin Synthesis and Ovulatory Cycles

The data reviewed present evidence that the pattern of GnRH secretion is an important factor in the regulation of gonadotropin subunit gene expression, gonadotropin synthesis, and secretion. The information on regulation of mRNA expression by GnRH pulses should be considered with some caution, as the experiments were performed in male rats and may not accurately reflect events in female primates or humans. However, an overall pattern emerges which suggests that common factors may be involved in all mammalian species. If current evidence is correct, and only a single gonadotropin-releasing hormone exists, then mechanisms to differentially regulate the three gonadotropin genes may involve changes in GnRH secretion. Alterations in GnRH pulse frequency and amplitude are recognized by the pituitary gonadotrope cell and could be the mechanism used to effect differential expression of the gonadotropin subunit genes. Differential regulation of subunit gene expression would be expected to be critically important in the establishment of pubertal maturation, and subsequently in the maintenance of ovulatory cycles in women. Our hypotheses, proposing a major role of pulsatile GnRH secretion in the regulation of human reproduction, are summarized in schematic form in Fig. 14 for men and Fig. 15 for women. In utero and during the first few months of life, GnRH is secreted at a relatively fast frequency (approximately 1 pulse/hour). During the first year, GnRH secretion is inhibited and both the amplitude and apparent frequency of pulsatile release is markedly reduced. The mechanisms involved in inhibiting GnRH release remain unclear in humans. Similarly, the mechanisms involved in the disinhibition of GnRH secretion, which first occurs during sleep at the initiation of puberty, are unclear, but in humans do not appear to involve opiates. In males, the increased frequency and amplitude of GnRH secretion favor LH synthesis and release, which in turn stimulates testosterone secretion (Fig. 14). Testosterone acts at the hypothalamus, perhaps through opioid mechanisms, to inhibit GnRH pulse frequency and to maintain a regular pattern of pulses occurring approximately every 90-110 min in adult males. In females, the mechanisms involving alterations in the patterns of GnRH secretion to regulate reproduction appear more complex. This may reflect the need to differentially synthesize and secrete FSH and LH at different times during reproductive cycles to allow orderly follicular maturation and ovulation. As shown in Fig. 15, we hypothesize that the events during the first decade of life and through the initiation of nocturnal GnRH secretion at puberty are similar in both sexes.(ABSTRACT TRUNCATED AT 400 WORDS)

FOLLICLE-STIMULATING HORMONE BETA SUBUNIT MESSENGER RIBONUCLEIC ACID CONCENTRATIONS DURING THE RAT ESTROUS CYCLE

Serum follicle-stimulating hormone (FSH), pituitary FSH content and FSH beta subunit mRNA concentrations were measured at 1 to 3h intervals throughout the 4 day estrous cycle in rats. Serum FSH was stable (range 200-320 ng/ml) apart from the biphasic proestrus surge (5 fold elevation) which was present from 1800 h of proestrus through 0800 h on estrus. Basal FSH beta mRNA concentrations from late metestrus through the afternoon of proestrus were 0.10 +/- 0.04 f mol cDNA bound/100 micrograms pituitary DNA. The major increase in FSH beta mRNA began at 2000 h on proestrus, 2 h after the initial rise in serum FSH and peak mRNA concentrations (0.43 +/- 0.08 f mol cDNA bound) occurred at 0200 h on estrus. FSH beta subunit mRNA concentrations were again increased at 2300 h on estrus (peak 0.24 f mol cDNA bound) and remained elevated through 1700 h on metestrus. Pituitary FSH content was transiently increased during metestrus and diestrus, but was elevated at 1000 h through 1900 h on proestrus (peak 5-fold increase). FSH content fell rapidly at 2000 h and remained low until 1400 h on estrus when values again rose. These data show that FSH beta mRNA is increased 4-5 fold during the proestrus FSH surge, and a smaller increase occurs on metestrus in the absence of elevated FSH secretion. The increased concentrations of FSH beta mRNA occurred at different times to the previously reported changes in alpha and LH beta mRNAs. Therefore, the data suggest that different mechanisms are involved in the regulation of LH and FSH beta subunit gene expression during the 4-day estrous cycle in rats.

Inhibin secretion during the rat estrous cycle: Relationships to FSH secretion and FSH beta subunit mRNA concentrations

Serum inhibin and FSH and FSH beta subunit mRNA levels were measured at 3h intervals throughout the 4 day estrous cycle in female rats and hourly between 1000 and 2400 h of proestrus. On proestrus, serum inhibin concentrations fell during the late morning-early afternoon, then increased transiently during the late afternoon gonadotropin surges. Inhibin levels decreased during the late evening of proestrus, coincident with the FSH surge-related rise in FSH beta mRNA levels. Serum inhibin remained relatively stable during estrus and early metestrus, but rose during the late evening of metestrus and remained elevated until early diestrus. FSH beta mRNA levels were elevated on late estrus and early metestrus and declined during the evening of metestrus as serum inhibin levels increased. These data show that concentrations of serum inhibin change during the estrous cycle and that a general inverse relationship exists between serum inhibin and FSH levels and FSH beta mRNA concentrations in the pituitary. This suggests that inhibin may inhibit FSH beta gene expression and FSH secretion during the 4 day cycle in female rats.

The role of the suckling stimulus in regulating pituitary prolactin mRNA in the rat

Prolactin (PRL) gene expression and the synthesis and secretion of PRL were examined in ovarian-intact lactating rats suckling eight pups on 10 days postpartum. Plasma samples were assayed for PRL concentrations, and pituitary glands were analyzed for total PRL content and PRL mRNA levels. We found that suckling-induced hyperprolactinemia was associated with very high levels of plasma PRL and a doubling in pituitary PRL mRNA levels, whereas pituitary PRL content was not changed. Removal of the suckling pups decreased plasma PRL concentrations 15-fold within 24 h. This decrease in PRL secretion was not accompanied by any significant change in pituitary PRL content. Evidently, both synthesis and secretion of PRL were decreased in the pituitary gland within 24 h following cessation of suckling, as pituitary PRL mRNA content had returned to diestrous levels at this time. To determine whether or not ovarian steroids might have contributed to the changes in PRL synthesis and secretion during lactation and after withdrawal of the suckling stimulus, the experiments were repeated in lactating rats ovariectomized (OVX) on day 2 postpartum. The results in these OVX rats were qualitatively similar to those described in ovarian-intact rats. We concluded from these findings that the stimulus of suckling induces increases in PRL mRNA levels in the pituitary which provides for the increased PRL synthesis accompanying increased PRL secretion. The cessation of suckling led to prompt decreases in PRL synthesis and secretion within 24 h.

Editorial: The New Instructions to Authors for the Reporting of Steroid Hormone Measurements

The Endocrine Society Council recently established a “Sex Steroid Assays Reporting Task Force (SSARTF)” with the following charges: to perform a review of journal policies (our journals and sister journals) related to sex hormone measurementreportingrequirements;torecommendtoCouncilthroughthePublicationsCoreCommittee(PCC)asexhormone measurement reporting policy that ensures our journal policies are not contradictory; and to then make recommendations on how to implement and communicate the new policy to authors, reviewers and readers of our journals. The SSARTF met by video conferencing as well as email and phone communication to develop the new Instructions to Authors. First, our research revealed that few journals have specific policies relating to the reporting of sex hormones or more generallysteroidhormoneassaymeasurements.Althoughthescienceofoptimalassaymeasurementsformanyanalytes isalwaysinflux,allfeltthattheEndocrineSocietyshouldleadthefieldtowardsoptimizationofmeasurementofsteroid hormones in research publications to impact the validity of the science and ultimately the care of our patients. As the draftguidelinewasdeveloped,theSSARTFfeltthatthechargewasoverlynarrowlyrestrictedinspecifyingsexhormones and that the instructions could be widely applied to all steroid hormones. Our goal was to provide a set of instructions that were clear and concise. The new Instructions to Authors on the Reporting of Steroid Hormone Assay Measurements addresses important requirements for minimal analytical validity including standards of accuracy, precision, specificity, sensitivity, reproducibilityandstability.Werealizethatarequirementfortraceabilityofanassaytoacertifiedstandardisnotyetavailable for all steroid hormones, but is an important goal. Importantly, the instructions do not mandate a specific type of hormone assay, but clearly state the criteria for a valid assay for clinical and non-clinical studies and give examples of where some widely available steroid hormone assays are not sufficient to meet these criteria. Concerning implementation, the SSARTF suggests that a checklist for authors and reviewers be used to ensure that these criteria are fulfilled or justification for their omission be provided. Initially, the SSARTF suggests that specific “hormone assay expert reviewers” may be needed, similar to the use of “statistical experts”, to ensure optimal review ofmanuscriptswithneworcomplexsteroidhormonemethodology.Werealizethat,duetojournal-specificwordlimits, the details of assay methodology might appear in a supplementary data file. Finally, we appreciate that these new Instructions to Authors are only the first step in an ongoing process. With their implementation, the feedback from authors and reviewers, as well as changes in the scientific landscape of assay measurement, will drive the evolution of these instructions to remain consistent with the state of the science. In parallel, the Endocrine Society will lead the discussion in our scientific manuscripts and editorials as well as educational programing to engage industry, governmental and other stakeholders to support optimization of all hormone assay measurements to common standards. These steps will ensure the validity of our endocrine science and guide the care of patients with endocrine disorders.