Quantitative immunocytochemical studies on the gonadotrophs isolated from the pituitary of the male rat

Estradiol suppresses phosphorylation of cyclic adenosine 3',5'-monophosphate response element binding protein (CREB) in the pituitary: evidence for indirect action via gonadotropin-releasing hormone

Estradiol acts on the hypothalamus and pituitary gland to modulate the synthesis and secretion of gonadotropins. We recently reported that GnRH-induced transcription of the human gonadotropin alpha-gene promoter is increased markedly in transfected pituitary cells derived from animals treated with estradiol. Because the cAMP response element binding (CREB) protein plays an important role in the transcriptional regulation of this promoter and is highly regulated by posttranslational phosphorylation, we hypothesized that it might serve as a target for estradiol-induced sensitivity to GnRH. In this study, we assessed the roles of estradiol and GnRH in the regulation of CREB phosphorylation in the rat pituitary. Using an antibody that specifically recognizes phosphorylated CREB (pCREB), we found that the pituitary content of pCREB was inversely related to the level of estradiol during the estrous cycle. Ovariectomy increased the level of pCREB, and treatment with estradiol for 10 days decreased the content of pCREB dramatically (93% inhibition). A similar reduction of pCREB was seen when ovariectomized rats were treated with a GnRH receptor antagonist for 10 days. This result indicates that the ovariectomy-induced increase in pCREB is GnRH-dependent. In alphaT3 gonadotrope cells, estradiol had no direct effect on CREB phosphorylation, whereas GnRH increased CREB phosphorylation 4- to 5-fold within 5 min. We conclude that estradiol inhibits CREB phosphorylation in the gonadotrope, probably by inhibiting GnRH production. The estradiol-induced decrease in CREB phosphorylation is proposed to lower basal alpha-promoter activity and increase its responsiveness to GnRH.

How the fixation-embedding protocol affects the specificity and efficiency of immunocytochemical stains for gonadotropin subunits

This report describes a study designed to test factors that may affect the efficiency and specificity of stains for gonadotropins. These include chemical or freeze-fixation and dehydration, heat polymerization of the plastic embedding compound, dehydration in organic solvents, and etching. Specifically, postembedding stains for LH or FSH subunits were applied to 1-micron sections of 1) Araldite-embedded pituitaries that were either chemically fixed and dehydrated or freeze-fixed and freeze-dried; 2) Aldehyde-fixed pituitaries that were dehydrated in water-soluble glycol methacrylate (GMA) and embedded in GMA at 4 degrees C; and 3) p-formaldehyde-fixed pituitaries that were embedded in paraffin. A fourth group of pituitaries was dispersed and grown in monolayers for 1-3 days. These were stained following glutaraldehyde fixation. The optimal dilution of the primary antisera varied with the protocol; however, the percentage of cells staining for beta subunits did not change. In contrast, postembedding stains showed that alpha subunit reactivity is masked or destroyed in pituitaries that are fixed in glutaraldehyde and embedded in Araldite. Alpha chain reactivity was detected (in 14% of cells) either after freeze-fixation and freeze-drying followed by Araldite embedding, or after 4% paraformaldehyde fixation and GMA embedding (in 17% of cells). Staining in paraffin-embedded pituitaries was seen in only 10% of the cells. Preembedding stains for alpha chains were strikingly sensitive, however, and immunoreactivity was seen in 18-26% of the population of monolayer cells. Thus, whereas the percentages of cells staining for beta subunits do not change following the use of most of the fixation and embedding protocols, alpha chain reactivity is destroyed by all but the mildest. These findings show that one can control or improve the specificity of the stains for LH and FSH by the fixation-embedding protocol.

Seasonal variations in pituitary LH-gonadotropes of the hibernating bat Myotis lucifugus lucifugus: an immunohistochemical study

Pituitary gonadotropes were identified throughout the year in the seasonally breeding, hibernating bat Myotis lucifugus lucifugus by means of light microscopic immunohistochemistry. In both male and female bats, these cells were immunoreactive with an antiserum directed to the beta subunit of luteinizing hormone. Some gonadotropes were aggregated near a portion of the infundibular stalk which crosses the anterior lobe, while most were scattered singly in a uniform manner throughout the rest of the pars distalis. This cell population exhibited seasonal variations in both sexes. In males, the proportional volume of the pars distalis occupied by immunoreactive gonadotropes (volume fraction) was significantly reduced in late July, when plasma testosterone levels were approaching their seasonal peak. In females, the volume fraction declined in April, following ovulation, and remained low during pregnancy and lactation. The size and shape of gonadotropes appeared relatively constant throughout the annual reproductive cycle in male bats; the immunoreactive cells were irregular in shape, with cytoplasmic extensions insinuating between and often "cupping" other secretory cell types. In females, the gonadotropes resembled those of males throughout most of the year, except during pregnancy, when these cells became enlarged and ovoid. No evidence of involution was observed in these anterior pituitary cells in either males or females during hibernation.