Activin-inhibitory action on lactotrophs is decreased in lactotroph hyperplasia.

Among all the hormone-secreting pituitary tumours, prolactinomas are the most frequently found in the clinic. Since dopamine is the primary inhibitor of lactotroph function, dopamine agonists represent the first-line therapy. However, a subset of patients exhibits resistance to these drugs, and therefore, alternative treatments are desired. As activins inhibit prolactin gene expression through the inhibition of Pit-1 involving the p38MAPK pathway, in the present work, we studied the local activin system as an alternative inhibitory system for lactotroph hyperplasia treatment. We used two different mouse models of prolactinoma: transgenic mice with overexpression of the human chorionic gonadotropin ß-subunit (hCGß) and mice lacking dopamine receptor type 2. In both models, females, but not males, develop lactotroph hyperplasia from the fourth month of life. We found reduced expression of pituitary activin subunits and activin receptors in hyperplastic pituitaries from both models compared with wild-type counterparts. Consequently, hyperplastic pituitaries presented a reduced activin-inhibitory action on prolactin secretion. Additionally, while female wild-type lactotrophs presented high levels of phospho-p38MAPK, it was lost in prolactinomas, concomitant with decreased activin expression, increased Pit-1 expression and tumour development. In contrast, male pituitaries express higher mRNA levels of activin subunits ßA and ßB, which would suggest a stronger activin inhibitory function on lactotrophs, protecting this sex from tumour development, despite genotype. The present results highlight the importance of the activin inhibitory action on lactotroph function and place the local activin system as a new target for the treatment of dopamine agonist-resistant prolactinomas.

Participation of membrane progesterone receptor α in the inhibitory effect of progesterone on prolactin secretion.

The membrane progesterone receptors (mPRα, mPRß, mPRγ, mPRδ and mPRε) are known to mediate rapid nongenomic progesterone functions in different cell types. However, the functions of these receptors in the pituitary have not been reported to date. In the present study, we show that the expression of mPRα was the highest among the mPRs in the rat anterior pituitary gland. Immunostaining of mPRα was detected in somatotrophs, gonadotrophs and lactotrophs. Interestingly, 63% of mPRα-positive cells within the pituitary were lactotrophs, suggesting that mPRα is involved in controlling prolactin (PRL) secretion in the pituitary. To test this hypothesis, rat pituitaries were incubated (1 hour) with either progesterone (P4) or the mPRα-specific agonist Org OD 02-0. PRL secretion was then measured by radioimmunoassay. The results of this experiment revealed that both P4 and Org OD 02-0 decreased PRL secretion. Moreover, the results from the GH3 cell line (CCL-82.1) showed that P4 and Org OD 02-0 inhibited PRL release, although the nuclear PR agonist R5020 was ineffective. Our investigation of the cellular mechanisms behind mPRα activity indicated that both P4 and Org OD 02-0 decreased cAMP accumulation, whereas R5020 was ineffective. In addition, the Org OD 02-0-effect on PRL release was blocked by pretreatment with pertussis toxin, an inhibitor of Go/Gi proteins. Because transforming growth factor (TGF)ß1 is a potent inhibitor of PRL secretion in lactotrophs, we lastly evaluated whether TGFß1 was activated by progesterone and whether this effect was mediated by mPRα. Our results showed that P4 and Org OD 02-0, but not R5020, increased active TGFß1 levels. This effect was not observed when cells were transfected with mPRα-small interfering RNA. Taken together, these data provide new evidence suggesting that mPRα mediates the progesterone inhibitory effect on PRL secretion through both decreases in cAMP levels and activation of TGFß1 in the lactotroph population.

Hypothalamic orexin, OX1, alphaMSH, NPY and MCRs expression in dopaminergic D2R knockout mice.

In 5-month-old male and female dopamine receptor 2 (D2R) knockout mice food intake per animal was unaltered while food per g BW was increased. We wished to evaluate the effect of D2R disruption on different components of energy balance and food intake regulation. We determined hypothalamic orexin precursor (PPO) expression, its receptor OX1, serum leptin levels, hypothalamic leptin receptor (OBR), circulating and pituitary alpha MSH levels, as well as central MC3 and MC4 receptors and NPY mRNA in wildtype and D2R knockout mice (KO). Loss of D2R caused a marked increase in serum prolactin levels, to higher levels in females compared to male KO mice. On the other hand, it produced a female-specific increase in circulating alphaMSH, and hypothalamic alphaMSH content, while neurointermediate alphaMSH content was decreased in both sexes. No differences were found in hypothalamic NPY, MC3R or MC4R concentration. Hypothalamic PPO mRNA expression was significantly decreased only in female KOs, while OX1 mRNA was not different between genotypes. Serum leptin levels were also similar in both genotypes. Our results show that in female and not in male mice disruption of the D2R produces two potentially anorexigenic events: an increase in serum and hypothalamic alphaMSH, and a decrease in hypothalamic orexin expression. Very high prolactin levels, which are orexigenic, probably counterbalance these effects, so that food intake is slightly altered. In males, on the other hand, hypothalamic PPO, and serum or hypothalamic alphaMSH are not modified, and increased prolactin levels may account for increased food intake per g BW. These results suggest a sexually dimorphic participation of the D2R in food intake regulation.

Increased pituitary vascular endothelial growth factor-a in dopaminergic D2 receptor knockout female mice.

Vascular endothelial growth factor (VEGF)-A is an important angiogenic cytokine in cancer and pathological angiogenesis and has been related to the antiangiogenic activity of dopamine in endothelial cells. We investigated VEGF expression, localization, and function in pituitary hyperplasia of dopamine D2 receptor (D2R)-knockout female mice. Pituitaries from knockout mice showed increased protein and mRNA VEGF-A expression when compared with wild-type mice. In wild-type mice, prolonged treatment with the D2R antagonist, haloperidol, enhanced pituitary VEGF expression and prolactin release, suggesting that dopamine inhibits pituitary VEGF expression. VEGF expression was also increased in pituitary cells from knockout mice, even though these cells proliferated less in vitro when compared with wild-type cells, as determined by 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium proliferation assay, proliferating cell nuclear antigen expression, and [(3)H]thymidine incorporation. In contrast to other animal models, estrogen did not increase pituitary VEGF protein and mRNA expression and lowered serum prolactin secretion in vivo and in vitro in both genotypes. VEGF (10 and 30 ng/ml) did not modify pituitary cell proliferation in either genotype and increased prolactin secretion in vitro in estrogen-pretreated cells of both genotypes. But conditioned media from D2R(-/-) cells enhanced human umbilical vein cell proliferation, and this effect could be partially inhibited by an anti-VEGF antiserum. Finally, using dual-labeling immunofluorescence and confocal laser microscopy, we found that in the hyperplastic pituitaries, VEGF-A was mostly present in follicle-stellate cells. In conclusion, pituitary VEGF expression is under dopaminergic control, and even though VEGF does not promote pituitary cellular proliferation in vitro, it may be critical for pituitary angiogenesis through paracrine actions in the D2R knockout female mice.

Disruption of the D2 dopamine receptor alters GH and IGF-I secretion and causes dwarfism in male mice.

We determined the consequences of the loss of D2 receptors (D2R) on the GH-IGF-I axis using mice deficient in functional dopamine D2 receptors by targeted mutagenesis (D2R(-/-)). Body weights were similar at birth, but somatic growth was less in male D2R(-/-) mice from 1-8 months of age and in D2R(-/-) females during the first 2 months. The rate of skeletal maturation, as indexed by femur length, and the weight of the liver and white adipose tissue were decreased in knockout male mice even though food intake was not altered. The serum GH concentration was significantly decreased during the first 2 months in knockout female and male mice, and IGF-I and IGF-binding protein-3 levels were lower in knockout mice. PRL was significantly higher in knockout mice, and females attained higher levels than males. Pituitaries from adult knockout mice had impaired basal GH release and a lower response to GHRH in vitro. We propose that the D2R participates in GHRH/GH release in the first month of life. In accordance, the D2R antagonist sulpiride lowered GH levels in 1-month-old wild-type mice. Our results indicate that lack of D2R alters the GHRH-GH-IGF-I axis, and impairs body growth and the somatotrope population.

Metabolic cues for puberty onset in free grazing Holstein heifers naturally infected with nematodes.

Leptin is a new plausible candidate for the molecular link between nutritional status and the reproductive axis. In previous studies we described that continuous natural nematode infections in heifers retarded growth and delayed the onset of puberty, and that the insulin-like growth factor I (IGF-I) was involved. In the present study we monitored the leptin levels during development in heifers naturally parasitized versus those chronically treated with ivermectin and we investigated whether growth hormone (GH) accounted for the differences in IGF-I previously noted. Insulin levels were also measured. Prolactin hormone was recorded as an indicator of immune system activation. We found a direct correlation between leptin and body weight during development and a prepubertal surge of the hormone 2 weeks before the first progesterone peak that indicates the onset of puberty. This suggests that leptin may act as a signal for this event. Insulin did not vary during growth and prepuberty. On the other hand, GH as not responsible for diminished IGF-I levels in parasitized animals as levels were similar in both groups. The GH levels were high at birth and then diminished rapidly and remained constant during development and puberty. The last hormone studied, prolactin, followed seasonal changes of sunlight duration and presented sporadic bursts in infected animals. These were related to high nematode infection and are probably involved in the immune response of the host.

Desensitization of angiotensin II: effect on [Ca2+]i, inositol triphosphate, and prolactin in pituitary cells.

Activation of pituitary angiotensin (ANG II) type 1 receptors (AT1) mobilizes intracellular Ca2+, resulting in increased prolactin secretion. We first assessed desensitization of AT1 receptors by testing ANG II-induced intracellular Ca2+ concentration ([Ca2+](i)) response in rat anterior pituitary cells. A period as short as 1 min with 10(-7) M ANG II was effective in producing desensitization (remaining response was 66.8 +/- 2.1% of nondesensitized cells). Desensitization was a concentration-related event (EC(50): 1.1 nM). Although partial recovery was obtained 15 min after removal of ANG II, full response could not be achieved even after 4 h (77.6 +/- 2.4%). Experiments with 5 x 10(-7) M ionomycin indicated that intracellular Ca2+ stores of desensitized cells had already recovered when desensitization was still significant. The thyrotropin-releasing hormone (TRH)-induced intracellular Ca2+ peak was attenuated in the ANG II-pretreated group. ANG II pretreatment also desensitized ANG II- and TRH-induced inositol phosphate generation (72.8 +/- 3.5 and 69.6 +/- 6.1%, respectively, for inositol triphosphate) and prolactin secretion (53.4 +/- 2.3 and 65.1 +/- 7.2%), effects independent of PKC activation. We conclude that, in pituitary cells, inositol triphosphate formation, [Ca2+](i) mobilization, and prolactin release in response to ANG II undergo rapid, long-lasting, homologous and heterologous desensitization.

Bromocriptine restores angiotensin II response in pituitary hyperplasia.

In estrogen-induced pituitary hyperplasia AII-evoked prolactin release is decreased and the octapeptide does not generate a spike elevation in [Ca(2+)](i) in vitro. We studied whether or not bromocriptine could restore AII response in diethylstilbestrol treated rats. Co-administration of bromocriptine resulted in involution of pituitary size and lowering of serum prolactin. In vitro, prolactin release per cell was reduced in the hyperplastic group, and levels were not significantly increased by in vivo bromocriptine treatment. Immunocytochemical analysis revealed that hyperplastic pituitaries contained fewer prolactin granules than control pituitaries, and that bromocriptine, did not increase prolactin storage. Nevertheless, in this group, prolactin response to AII increased, and AII evoked a consistent spike in [Ca(2+)](i), albeit lower than in the control group. Such spike was abolished by thapsigargin, and not by removal of extracellular calcium or by K(+), indicating that it was mainly dependent on intracellular calcium stores, as in normal cells. We conclude that bromocriptine treatment partially restores AII response in the hyperplastic pituitary.

Endocrine studies in ivermectin-treated heifers from birth to puberty.

Continuous treatment with ivermectin from birth to puberty advanced sexual maturation by 3.7 wk in Holstein heifers grazing pastures naturally infected with nematodes. Every 14 d jugular blood samples were taken from birth to 45 wk of age from all heifers. No differences in serum FSH, estradiol, or thyroxine levels were observed during the trial between the treated and untreated group. Mean LH levels were diminished in untreated heifers 4 wk before the first estrus and the amplitude of LH pulses was augmented in treated heifers when puberty was reached. Serum IGF-I levels increased from birth to 22 wk of age and then reached a plateau in both groups, but levels were consistently higher in treated heifers from 26 wk of age onward. Body weight gain was retarded in parasitized heifers and IGF-I values were positively correlated with body weight only during the first 20 wk of life. We suggest that enhanced prepubertal IGF-I levels in conjunction with increased prepubertal LH levels and pubertal LH pulse amplitude might be involved in the accelerated somatic maturation and in puberty advancement observed in ivermectin-treated heifers.

Calcium influx and intracellular stores in angiotensin II stimulation of normal and hyperplastic pituitary cells.

In rat pituitary cells from estrogen-induced hyperplasia, angiotensin II (ANG II) does not evoke a clear spike elevation of intracellular Ca2+ concentration ([Ca2+]i) but induces a plateau increase. The present work was undertaken to establish whether this difference was related to a differential participation of intracellular and/or plasma membrane Ca2+ channels. We first tested the effect of 10 nM ANG II on [Ca2+]i in the absence of extracellular Ca2+ in cells depolarized with 25 mM K+ or in the presence of blockers of L-type voltage-sensitive Ca2+ channels (VSCC). These treatments did not alter spike elevation in [Ca2+]i in controls but reduced plateau levels in hyperplastic cells. Intracellular Ca2+ stores were similar in both groups, as assessed by thapsigargin treatment, but this drug abolished spike increase in controls and scarcely modified plateau levels in hyperplastic cells. Finally, inositol trisphosphate (InsP3) production in response to ANG II was significantly higher in control cells. We conclude that the observed plateau rise in hyperplastic cells results mainly from Ca2+ influx through VSCC. In contrast, in control cells, the ANG II-induced spike increase in [Ca2+]i results from mobilization of Ca2+ from thapsigargin-sensitive internal channels, activated by higher inositol 1,4,5-trisphosphate generation.

Effects of continuous ivermectin treatment from birth to puberty on growth and reproduction in dairy heifers.

The effect of continuous ivermectin treatment from birth to puberty on growth and reproductive performance was studied in Holstein heifer calves grown on pastures in comparison to naturally nematode-infected, untreated animals. Ivermectin effectively abated the presence of nematode eggs in feces. Eggs per gram (EPG) in parasitized animals increased rapidly from wk 12 to 18 of age and then decreased. Animals treated with ivermectin grew faster than untreated ones, and differences in body weight became significant at 6 wk of life, even before eggs appeared in the feces of either treatment group. Ivermectin-treated heifers reached puberty 3 wk earlier than infected ones as assessed with serum progesterone concentrations (ivermectin, 30.4 +/- .8 vs untreated, 33.7 +/- 1.3 wk of age). This delay was not directly related to body weight. In addition, pelvic area at 39 wk and at 15 mo of age was increased in treated heifers (8 and 11%, respectively) compared with parasitized animals. No differences in the wither heights were observed. We conclude that ivermectin treatment in dairy heifers may increase growth rate during development, advance the onset of ovarian function, and positively affect yearling pelvic area.

Effect of stage of development and sex on gonadotropin-releasing hormone secretion in in vitro hypothalamic perifusion.

Marked sexual and ontogenic differences have been described in gonadotropin regulation in the rat. These could arise from events occurring both at the hypothalamic or hypophyseal levels. The present experiments were designed to evaluate the capacity of the hypothalamus in releasing GnRH in vitro, basally and in response to depolarization with KCl, during ontogeny in the rat. To that end we chose two well-defined developmental ages that differ markedly in sexual and ontogenic characteristics of gonadotropin regulation, 15 and 30 days. We compared GnRH release from hypothalami of females, neonatal androgenized females and males. Mediobasal hypothalami were perifused in vitro, and GnRH measured in the effluent. Basal secretion of the decapeptide increased with age in the three groups with no sexual differences encountered. When studying GnRH release induced by membrane depolarization, no differences within sex or age were encountered. On the other hand FSH serum levels decreased with age in females and increased in males, and in neonatal androgenized females followed a similar pattern to that of females. LH levels were higher in infantile females than in age-matched males or androgenized females. Such patterns of gonadotropin release were therefore not correlated to either basal or K+-induced GnRH release from the hypothalamus. We conclude that sexual and ontogenic differences in gonadotropin secretion in the developing rat are not dependent on the intrinsic capability of the hypothalamus to release GnRH in response to membrane depolarization. The hormonal differences observed during development and between sexes are probably related to differences in the sensitivity of the GnRH neuron to specific secretagogue and neurotransmitter regulation, and/or to differences in hypophyseal GnRH receptors and gonadotrope sensitivity.

Angiotensin II-induced Ca2+ mobilization and prolactin release in normal and hyperplastic pituitary cells.

We evaluated the effects of angiotensin II (ANG II) and its antagonists on prolactin release, intracellular calcium ([Ca2+]i) mobilization, and [3H]thymidine uptake in cells from normal rat pituitaries and from estrogen-induced pituitary tumors. ANG II (10(-7) to 10(-9) M) increased prolactin release significantly in control and not in tumoral cells. In control cells, ANG II (10(-6) to 10(-9) M) produced an immediate spike of [Ca2+]i followed by a plateau. Spike levels rose significantly between 10(-10) and 10(-8) M ANG II, whereas the onset of the spike was retarded with decreasing concentrations. In tumoral cells, ANG II did not produce a spike phase even at 10(-6) M. ANG II-induced prolactin release and calcium mobilization were blocked by losartan (AT1 receptor antagonist) and not by PD-123319 (AT2 antagonist). Finally, [3H]thymidine uptake was not modified by ANG II (10(-7) to 10(-10) M) or its antagonists in either group. Our results suggest that chronic in vivo estrogenic treatment alters in vitro pituitary response to ANG II. Alterations might function to limit excessive prolactin secretion of hypersecreting tumors. Besides, ANG II does not modify DNA synthesis in vitro of cells from normal or tumor-derived hypophyses.

Brain sexual differentiation and gonadotropins secretion in the rat.

1. The present work deals with sexual differences in gonadotropin regulation in the rat and the role of sexual organization of the hypothalamus in determining such differences. 2. Sex differences between male and female rats, with regard to their control of gonadotropin secretion, go beyond whether or not gonadotropins are released cyclically. Rats show additional sex differences (a) in the response of gonadotropins to removal and imposition of negative feedback signals and (b) in the ontogeny of gonadotropin regulation from birth to puberty. 3. There is a sensitive developmental period during which sexual differentiation of neural substrates proceeds irreversibly under the influence of gonadal hormones. In the rat this period starts a few days before birth and ends approximately 10 days after birth. Female rats treated during this sensitive period with androgens or estrogens will permanently lose the capacity to release GnRH in response to estrogenic stimulation. 4. Nevertheless although sexual differentiation is dramatically affected by events during the neonatal period, recent data question the "critical" nature of this period, as it has been shown that testosterone can still act on neural substrates well beyond (15 to 30 days of age) the neonatal period to defeminize and masculinize endocrine and behavioral functions. 5. Furthermore, the capacity for the normal display of female sexual behavior and for the cyclic release of gonadotropins is not, as has been assumed, inherent to central nervous tissue but depends on active hormonal estrogenic induction during a sensitive period of development. 6. Besides, during differentiation of male sexual brain function estrogens may be supportive, rather than directive, to the primary action of androgens. 7. Serotonergic, noradrenergic, and opioid systems participate in the sexual dimorphism in gonadotropin control in adult rats. 8. The sex difference in the postcastration LH rise is dependent on the early sexual organization of the hypothalamus, even though in adulthood it can also be influenced by a variety of factors such as the stage of the estrous cycle, age of the animal, estradiol pretreatment, and history of release from feedback inhibition. 9. The characteristic pattern of gonadotropin secretion in the female infantile rat, which is sexually differentiated, can be related to an increase in hypophyseal receptors coupled to an increase in the intracellular calcium response to GnRH. Such events depend on the sexual organization of the hypothalamus. In males the greater sensitivity to GnRH at 30 days is reflected in an increase in pituitary GnRH receptors but not in an increase in the magnitude of Ca2+ mobilization induced by GnRH, therefore it is probable that in this situation alternative second messengers may modulate high sensitivity. Neonatal androgenization of the hypothalamus may decrease the hypophyseal response to GnRH by an alteration in receptor concentration and signal transduction during the infantile period. 10. Finally, serotonergic, dopaminergic, opioid, and noradrenergic regulation of GnRH varies with increasing age, and the sexual organization of the hypothalamus by testosterone or estrogens is a determinant in such regulation.

Biochemical parameters in the anterior pituitary during the course of tumorigenesis induced by diethylstilbestrol treatment.

Treatment of F344 rats with diethylstilbestrol (DES) for 1-2 months induces a prolactin (PRL)-secreting pituitary adenoma. After 8 weeks of DES treatment, we have shown that the ratio of regulatory subunits of the cAMP-dependent protein kinase (RI/RII) increased in the tumors. Presently we report the variations in RI/RII ratio, pituitary weight, DNA content, serum PRL, nuclear estrogen receptor (E2R) and of ornithine decarboxylase (ODC) activity from the time of DES pellet implantation until 8 weeks. Pituitary weight, DNA content and serum PRL rose significantly at 4 weeks with a maximum at 6-8 weeks, and significantly correlated with each other. E2R and ODC activity increased from week 1 onwards, with a maximum at 2 weeks and decreased at 8 weeks. Both variables showed a positive correlation but neither E2R nor ODC activity correlated with pituitary weight, DNA or serum PRL. Values for RI remained stable with time, but RII decreased progressively. The RI/RII ratio was maintained around unity between 1-4 weeks, increasing to 1.6-2 thereafter. This ratio positively correlated with pituitary weight and DNA. It is suggested that during tumor induction by estrogen in a sensitive strain of rats, growth signals with different time-courses become activated. Increases in pituitary weight and DNA content, indicators of mammotroph hypertrophy and hyperplasia, were preceded by early rises in E2R and ODC activity. Increases in the RI/RII ratio accompanied the adenomatous change, suggesting their role in cell transformation after 6 weeks of estrogen exposure.

Effects of LHRH and ANG II on prolactin stimulation are mediated by hypophysial AT1 receptor subtype.

We have used the nonpeptide angiotensin II (ANG II) receptor antagonists losartan (receptor subtype AT1) and PD-123319 (AT2) to determine the participation of ANG II receptor subtypes in luteinizing hormone-releasing hormone (LHRH)-induced prolactin release in a perifusion study using intact pituitaries in vitro. LHRH (1.85 x 10(-7) M) released prolactin consistently, whereas losartan (10(-5) M) abolished prolactin response without modifying basal prolactin or luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release. PD-123319 (10(-5) M) had no effect on basal or LHRH-induced prolactin, LH, or FSH release. We also determined that the effect of ANG II on prolactin release was mediated by the same receptor subtype. In adenohypophysial cells dispersed in vitro ANG II (10(-8) M) released prolactin. Losartan (10(-7) and 10(-6) M), but not PD-123319, inhibited this effect. We conclude that in intact hypophyses of 15-day-old female rats the effect of LHRH on prolactin release is readily demonstrated. LHRH-induced prolactin release appears to be mediated by ANG II acting in a paracrine manner on AT1 receptors located on lactotrophs.

Ontogeny of angiotensin-II-induced prolactin release in vivo and in vitro in female and male rats.

The prolactin-releasing effect of angiotensin II (AII) was studied in the developing female and male rat in vivo and in vitro. AII (50 and 100 micrograms/100 g b.w.) was injected intraperitoneally to female and male rats aged 4, 12, 20 and 28 days and males aged 38 days. AII (10(-6) M) was also tested in pituitaries incubated in vitro from animals of both sexes aged 12, 20 and 28 days. In addition, as two subtypes of AII receptors have been characterized on the basis of displacement with specific AII antagonists, we used the nonpeptide AII receptor antagonists losartan (AT1 subtype) and PD 123319 (AT2 subtype) to determine the AII receptor subtype functionally involved in AII-induced prolactin secretion in vivo in 25-day-old male rats. The efficiency of the prolactin-releasing effect of AII in vivo increased with age, and first responses were observed at 20 days of age in both sexes. No sexual differences were encountered. On the other hand, AII-induced prolactin release from pituitaries incubated in vitro was first demonstrated at 12 days in females and at 20 days in males. The effect increased with age in both sexes, and, at 28 days, pituitaries from females released more prolactin in response to AII than those from males. Losartan (3 mg/kg) completely abolished AII (50 micrograms/100 g b.w.)-induced prolactin release in vivo, while PD 123319 (3 mg/kg) did not. This suggests that pituitary AT1 receptors are functionally involved in the prolactin release induced by AII in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Restoration by bromocriptine of glucocorticoid receptors and glucocorticoid negative feedback on prolactin secretion in estrogen-induced pituitary tumors.

We previously reported a reduction of glucocorticoid receptors (GCR) in diethylstilbestrol-induced pituitary tumors (DES-T) in rats. Presently, we found that bromocriptine (BROM) treatment increased the levels of GCR in DES-T, demonstrated by steroid binding assays and immunocytochemistry using a monoclonal antibody against the type II GCR. We also found that the high content of nuclear estradiol receptors in the adenomata and the elevated levels of PRL in serum of DES-T were significantly reduced after BROM treatment. In parallel studies, PRL secretion was measured after administration of ether stress. In controls, serum PRL markedly increased after ether and this effect was blunted by prior dexamethasone (DEX) administration, due to the steroid negative feedback on PRL secretion. In animals with DES-T, ether stress had no effect on serum PRL, and the inhibition by DEX was lost unless they received BROM, which restored the negative feedback of DEX on serum PRL. Although increases of PRL titers in pituitary tumors may be due to estrogenic stimulation of lactotroph proliferation and function, coupled to absent dopaminergic inhibition on these cells, other mechanisms are possible. In this respect, inefficient steroid negative feedback on PRL synthesis due to down-regulation of GCR may contribute to hyperprolactinemia. This mechanism is supported from the restoration of GCR and steroid negative feedback on serum PRL by treatment of tumor-bearing rats with BROM.

Sexual and ontogenic differences in K(+)-induced gonadotropin and prolactin release in vitro.

Ontogenic and sexual differences have been described in the regulation of anterior pituitary hormone release. In the present experiments we studied basal release and the effect of a depolarizing concentration of K+ on in vitro gonadotropin and prolactin release from anterior pituitaries of male and female rats at 12, 20 and 28 days of age. Basal release of LH and FSH increased with age, values obtained from female glands being significantly higher than those obtained from male glands. K(+)-induced release of LH did not present differences among ages, although the response in females was always greater than that in age-matched males. If K(+)-induced release of LH was considered in relation to basal release, infantile 12-day-old rats of both sexes, had a significantly greater sensitivity to the effect of K+ in comparison to older ages, as has been described for the LH-releasing effect of LHRH and of other stimuli. K(+)-induced FSH release was maximal in females at 20 days of age, and in males at 28 days of age. Percentage increase relative to basal values, induced by K+ was also greatest at 12 days in both sexes, although values from female glands were significantly higher than those from males. Basal and K(+)-induced prolactin release increased significantly with age in both sexes. Basal prolactin release was greater in females than in males at 28 days of age, and no other sexual difference was evidenced.(ABSTRACT TRUNCATED AT 250 WORDS)

Ontogenic studies of the neural control of adenohypophyseal hormones in the rat. II. Prolactin.

1. Serum prolactin levels are low during the first 20 days of life and gradually increase toward puberty, in both male and female rats. 2. There is an age-related increase in the cell population engaged in prolactin secretion, as well as an increase in the synthesis of prolactin and of the amount of prolactin secreted from individual lactotropes. 3. The gradual increase in prolactin levels in the third week of life is not related to a decrease in dopaminergic inhibition but to an increase in the efficiency of prolactin releasing factors such as estrogen, serotonin, opiates, and posterior pituitary extracts. 4. Prolactin release induced by physiological factors, such as stress, cervical stimulation, or the expression of spontaneous diurnal and nocturnal surges, requires maturational events within the hypothalamic-pituitary axis which are evident at the end of the third week of life. 5. In the female rat the steadily increasing levels of prolactin are involved in the timing of puberty eclosion acting at the ovary and at the brain. 6. In the prepubertal male rat increasing titers of prolactin may be involved in testicular and accessory organ development and may facilitate the actions of luteinizing hormone, follicle stimulating hormone, and testosterone on male sexual organs.