Relationships among release of prolactin, synthesis of DNA and growth of the anterior pituitary gland of the rat: effects of oestrogen and sulpiride

Strain difference in regulation of pituitary tumor transforming gene (PTTG) in estrogen-induced pituitary tumorigenesis in rats

Recently a novel oncogene, PTTG (pituitary tumor transforming gene) was isolated from a rat pituitary tumor cell line whose expression is apparently correlated with pituitary tumorigenesis. In the rat, estradiol (E(2)) is known to induce anterior pituitary hyperplasia. The effects of E(2), however, vary greatly among rat strains. Therefore we examined the expression of PTTG and its regulation by E(2) in F344, Wistar, Brown-Norway and Donryu rats. Four-week-old females were ovariectomized and a pellet containing 10 mg of E(2) was given s.c. Total RNA was isolated from the pituitary gland and PTTG mRNA was measured with a competitive RT-PCR technique. The F344 strain was the most susceptible to E(2) induction of pituitary tumorigenesis, followed by Wistar and Brown-Norway, while no increase in pituitary weight was noted in Donryu rats. PTTG mRNA in the gland was induced by E(2) within 48 - 72 h in F344 and Wistar, but not in Brown-Norway or Donryu strains. These data suggest that PTTG expression may at least in part be responsible for strain differences in E(2)-induced pituitary tumorigenesis.

Targeted disruption of the murine galanin gene

The 29 amino acid neuropeptide galanin is widely distributed in the nervous and endocrine systems; highest levels of galanin synthesis and storage occur within the hypothalamus in the median eminence, but it is also abundantly expressed in the basal forebrain, the peripheral nervous system, and gut. To further define the role played by galanin in the peripheral nervous and endocrine systems, a mouse strain carrying a loss-of-function germ-line mutation of the galanin locus, engineered by targeted mutagenesis in embryonic stem cells, has been generated. The mutation removes the first five exons containing the entire coding region for the galanin peptide. Germ-line transmission of the disrupted galanin locus has been obtained, and the mutation has been bred to homozygosity on the inbred 129O1aHsd background. Phenotypic analysis of mice lacking a functional galanin gene demonstrate that these animals are viable, grow normally, and can reproduce. A marked reduction in both the anterior pituitary prolactin content and in circulating plasma levels of the hormone is evident. Lactation is abolished along with abrogation of the proliferative response of the lactotroph to estrogen. The responses of sensory neurons to injury in the mutants are markedly impaired. Peripheral nerve regeneration is reduced with associated long-term functional deficits. There is a striking reduction in the development of chronic neuropathic pain. These two phenotypic changes may be explained, in part, by the observation that a subset of dorsal root ganglion neurons is lost in the mutant animals, implying a role for galanin as a trophic cell survival factor. These initial findings have important implications for our understanding and potential therapeutic treatment of (a) sensory nerve regeneration and neuropathic pain and (b) disordered pituitary proliferation and the development of prolactinoma.

Age-related changes in the numbers of mammotrophs, somatotrophs and mammosomatotrophs in the anterior pituitary gland of female rats: a flow cytometric study

Age-related changes in the numbers of mammotrophs, somatotrophs, mammosomatotrophs, and the cells of other types in the anterior pituitary of female Wistar rat were measured by flow cytometry. The mammotrophs increased with age, and the somatotrophs decreased with senescence. The mammosomatotrophs increased remarkably in senescent rats, and these cells of the rats older than 21 months were about 10 times more than those of 3, and 12-13 months old rats. This result indicates that the stability of gene expression in cell differentiation is reduced in the aging process of the anterior pituitary.

Development and heterogeneity of prolactin cells

Prolactin (PRL) is synthesized in pituitary cells called mammotrophs (PRL cells). Ample evidence demonstrates that the PRL cell population consists of structurally and functionally heterogeneous PRL cells. Multiple variants of PRL molecules are found in various species. Prolactin cells may be divided into various subtypes in the rat and mouse. Secretory activities differ among the PRL cell population. These heterogeneities may reflect various phases of the maturation process of PRL cells, or the integrated outcome of various functional differences in PRL cells. To clarify the significance of heterogeneities among PRL cells, we present updated reports on the differentiation, proliferation, and development of PRL cells, and discuss factors responsible for the functional differences in PRL cell population. The age-related alteration in PRL secretion in the rat is summarized, because it is one of the most important aspects of the developmental changes in PRL cells. A mammosomatotroph, which secretes growth hormone and PRL, is found in various species. Prolactin cells and somatotrophs are derived from the same lineage. The possible relationship among PRL cells, somatotrophs, and mammosomatotrophs is discussed.

The mammosomatotroph: a transitional cell between growth hormone and prolactin producing cells? An immunocytochemical study

In this report the occurrence of mammosomatotroph (MS) cells was correlated with changes in the somatotroph population of adult rat pituitary gland submitted to various experimental conditions (ovariectomized, orchidectomized and intact males, and after treatment with oestradiol benzoate). Cell and volume density of somatotrophs were assessed in sections stained with the immunogold-silver enhancement technique. Mammosomatotrophs were identified by double immunogold labelling at the electron microscopic level. Colocalization of prolactin (PRL) and growth hormone (GH) in the same cell was rarely observed. Only a few MS cells (0.1-0.2% of all parenchymal cells) were found in some experimental models. Oestrogen treatment decreased both cell and volume density of somatotrophs in ovariectomized rats. In this model, serum GH increased significantly but no changes in the pituitary content of the hormone were observed. Our results demonstrate that MS cells are an uncommon cell type in the pituitary of adult ovariectomized, orchidectomized and intact male rats. The oestrogen treatment, which is well known to induce proliferation of lactotrophs, has no effects on the MS population. Data presented in this report do not support the suggested role for mammosomatotrophs as transitional cells in the presumptive interconversion of PRL and GH producing cells.

Growth hormone-releasing factor (GRF) suppresses the in vitro proliferation of mammotrophs from the adult rat

In order to examine the hypothalamic control of acidophilic proliferation, anterior pituitary cells from adult female rats were cultured with or without rat growth hormone-releasing factor fragment 1-29 (GRF-29). Changes in the numbers of mammotrophs and somatotrophs during culture were measured by immunocytochemical staining. The addition of GRF suppressed the increase in the number of mammotrophs even at the very low concentration of 10(-12) M. The number of somatotrophs increased in the medium containing GRF. The increase in mammotroph number was blocked by cytosine arabinoside, a mitotic inhibitor. GRF had no effect on the in vitro proliferation of fibroblasts. These results indicate the important role of hypothalamic GRF in the differential growth and secretion of the acidophils in vivo.

Effects of estrogen on the human pituitary: a clinicopathologic study

Pituitary glands obtained at autopsy of 67 men treated with diethylstilbestrol were examined for diffuse and nodular lactotrophic hyperplasia as well as prolactin cell tumorlets or adenomas. A control group consisted of 42 untreated patients with prostatic carcinoma and 209 other elderly men. Diffuse and nodular lactotrophic hyperplasia and the percentage of prolactin cells were greater in treated patients, but these differences were not statistically significant. The higher frequency of prolactin cell adenomas among treated patients (19%) than among control subjects (11%) also lacked statistical significance. An apparent low frequency of occurrence of adenoma in control patients with prostatic carcinoma remains unexplained. No correlation was noted between tumor number, size, morphologic features, or immunoreactivity and such factors as dose of estrogen therapy, associated diseases, ultimate cause of death, or patient age. A correlation was noted, however, between duration of estrogen therapy and the total number of pituitary adenomas, including those composed of prolactin cells. Relative proportions of other types of adenoma were similar within the study and control groups. We conclude that estrogen medication cannot be considered a major risk factor in the cause of prolactin-producing adenomas in older men.

Bromocriptine inhibits incorporation of [3H]thymidine into rat pituitary tumor cells

The effects of bromocriptine on GH3 pituitary tumor cell [3H]thymidine incorporation were studied. Cells were grown in the presence of bromocriptine, then exposed to a short-term pulse of [3H]thymidine in serum-free medium containing deoxycytidine (10 microM) to prevent deoxythymidine triphosphate (dTTP) pooling. After 48 h exposure to bromocriptine, basal prolactin (PRL)-secretion during 45 min was inhibited by 50% by 10 microM bromocriptine and thyroid releasing hormone-induced PRL stimulation was suppressed. Incorporation of radiolabelled thymidine into acid-precipitable DNA increased progressively from 15 to 60 min and was abolished by simultaneous incubation with excess unlabelled thymidine (100 microM). Bromocriptine (10 microM) inhibited incorporation of 5-50 microM [3H]thymidine, but this was not reversed by simultaneous incubation with metoclopramide (10 microM). Aminopterin, an inhibitor of endogenous de novo DNA synthesis, stimulated [3H]thymidine incorporation twofold and this increased DNA salvage pathway activity was also blocked by bromocriptine. As incorporation of [3H]thymidine into acid-soluble cell nucleotides was also inhibited by bromocriptine, the data suggest that in these cells the drug inhibits thymidine kinase activity, a salvage pathway of DNA synthesis.

Regulation of pituitary DNA synthesis during different reproductive states in the female rat: role of estrogens and prolactin

We studied DNA synthesis in the rat adenohypophysis during the estrous cycle, pregnancy and lactation. During the estrous cycle, DNA synthesis was 3 times higher on the morning of estrus than on the other days. This peak was abolished completely by ovariectomy or pentobarbital, which also blocked the preovulatory surges of LH and prolactin. Methallibure , which blocked the LH but not the prolaction surge, had a partial effect on DNA synthesis. An acute and significant decrease in pituitary DNA synthesis occurred between days 0 (estrus) and 1 of pregnancy, followed by a less pronounced diminution until parturition. After delivery, DNA synthesis increased steeply on day 1 of lactation, returning to low values by day 3, under normal suckling conditions. Thelectomy , which blocked suckling-induced prolactin release, or antiestrogen treatment, which did not decrease prolactin secretion, diminished pituitary DNA synthesis on day 1 of lactation. Estrogen administration to intact or ovariectomized rats on days 9-11 of lactation stimulated (100%) DNA synthesis. Ovariectomy had no effect. In conclusion, in the different reproductive states studied, pituitary DNA synthesis is related to prolactin release in the presence of estrogens.

The influence of estrogen administration on the subcellular receptors distribution and its biological effect in the rat anterior pituitary gland

We studied the effects of estrogen administration in the anterior pituitary gland (APG) of male rats. The subcellular distribution of estradiol receptors, and some biological responses, such as prolactin secretion and progestine receptors were examined. In the presence of circulating estrogens, the binding of [3H]-estradiol decreased in the cytosol and increased in nuclei. The total binding augmented significantly (P less than 0.05). These changes were dose and time dependent. So, in long chronic treatment, the changes in subcellular receptor distribution were greater, in accordance with higher serum estrogen level. Serum prolactin concentration and progestin receptor in the APG increased continuously as a function of time of the estrogen treatment. They were related to the presence of nuclear estradiol receptors, which remained elevated throughout the entire experimental period.