Comparison of primary and secondary stimulation of male rats by estradiol in terms of prolactin synthesis and mRNA accumulation in the pituitary

Male rats received acute or chronic primary or acute secondary stimulation with estradiol, and the effects on pituitary prolactin synthesis and its mRNA accumulation were examined. Prolactin synthesis was determined by the in vitro incorporation of [(3)H]leucine into prolactin over a period of 1 hr. Prolactin mRNA was measured both by cell-free translation in a nuclease-treated rabbit reticulocyte lysate and by hybridization to the complementary DNA. The latter two methods gave similar results under all experimental conditions. Acute primary stimulation with estradiol produced a significant increase in pituitary prolactin mRNA accumulation at 12 hr, which further increased by 2- to 3-fold over the next 48 hr. In contrast, no increase in prolactin synthesis was observed during the first 24 hr. Chronic stimulation with estradiol induced increases of both prolactin synthesis and prolactin mRNA that were quantitatively indistinguishable over the period of 1-4 weeks, reaching a plateau at 5-fold the basal values. By the 13th day after withdrawal of therapy both prolactin synthesis and mRNA had returned to the prestimulation levels. When the effects of estradiol on previously unexposed and estrogen withdrawn animals were compared, it was found that secondary stimulation not only produced a more rapid accumulation of the prolactin mRNA but also abolished the lag period of prolactin synthesis observed during the primary estrogen stimulation. These data demonstrate a lag in the endogenous translation of newly accumulated pituitary prolactin mRNA translatable in vitro after primary estrogen stimulation of male rats. The mechanism for the abolition of this lag during the secondary stimulation is now known.     

Growth hormone-releasing factor regulates growth hormone mRNA in primary cultures of rat pituitary cells.

A peptide with high intrinsic activity for specifically stimulating the secretion of immunoreactive growth hormone (GH; somatotropin) has been characterized and reproduced by total synthesis. This peptide, human pancreatic growth hormone-releasing factor, 44-amino-acid form (hpGRF1-44-NH2), was isolated from a tumor localized in the pancreas of a patient with acromegaly. We report here the effect of this growth hormone-releasing factor (GRF) on GH release and the GH mRNA levels in monolayer cultures of rat pituitary. The cytoplasmic dot hybridization technique was used to examine the effect of GRF on GH mRNA levels. Incubation of rat pituitary cultures with GRF for 72 hr resulted in a 2- to 2.5-fold increase in GH mRNA levels, and the maximal levels of stimulation were achieved at GRF concentrations as low as 1 fM. GRF did not stimulate prolactin release, nor did it affect specific prolactin mRNA levels in the pituitary cultures. We conclude that GRF is a potent and specific GH secretagogue that also affects specifically GH mRNA levels in normal pituitary cells.     

Biphasic effect of estradiol on luteinzing hormone response to gonadotropin-releasing hormone in castrated men

This study was designed to investigate the possibility that in men estradiol (E₂) has a stimulatory effect on the gonadotropin response to GnRH. Nine castrated adult men, who presented extremely low testosterone (T) concentrations, received 5 mg/day estradiol benzoate (E₂B) i.m. every 24 hr for several days, starting 5 days after orchidectomy. During E₂B treatment the pituitary responsiveness to GnRH (100 μg given as an iv bolus) was tested after 24, 48, 72, 96, 120, and 144 hrs of E₂B administration. The pituitary responsiveness to GnRH was also tested in untreated men from day 5 to day 10 following bilateral orchidectomy. In the E₂B-treated subjects the increased serum estradiol concentrations induced an initial decrease and a subsequent increase of the LH response to GnRH. The responses were decreased after 24 hr of treatment; thereafter, the LH responses were progressively increased and were markedly augmented after 120 hr of E₂B treatment. On the contrary, during treatment the FSH response to GnRH was preferentially blunted. In the untreated castrated men the LH and FSH responses to GnRH increased progressively from day 5 to day 10 after orchidectomy, but decreased responses were never observed during this period of observation. The maximum LH concentrations, which occurred at 30–60 min following GnRH in untreated castrated men, did not occur until 120–150 min in the E₂B treated men.     

Growth hormone and prolactin secretion in water-deprived chickens

The deprivation of water for 12 or 24 hr increased the prolactin concentration in the plasma of immature chickens but had no effect on the circulating growth hormone (GH) level. The increase in plasma prolactin level reflected an increase in the basal rate of prolactin release from incubated hemipituitary glands and an increase in the responsiveness of the pituitary gland to hypothalamic releasing factors. The deprivation of water had no effect on basal level of pituitary GH release in vitro but abolished the stimulatory effect of the hypothalamus on in vitro GH secretion.     

Hypoglycemia as a provocative test of prolactin release

Insulin-induced hypoglycemia has been traditionally used to test growth hormone (GH) and cortisol reserve. In order to determine its usefulness as a provocative test for prolactin (PRL) release, 31 healthy men and women, 38 patients with definite pituitary abnormalities (pituitary tumors, 17; hypopituitarism [other causes]—complete, 4, or partial, 17), and 17 patients with suspected pituitary dysfunction (delayed puberty, 5; short stature, 4; secondary amenorrhea, 6; empty sella, 2, received regular i.v. insulin (0.05–0.15 U/kg), and the plasma was assayed serially for PRL, GH, cortisol, and glucose. In the 31 healthy subjects, PRL increased from 16.3 ± 1.8 ng/ml (mean ± SEM) to 45.5 ± 7.9 (p < 0.001) at 60 min and was still elevated at 120 min (25.8 ± 3.1 ng/ml). The maximal rise to 52.2 ± 8.0 ng/ml occurred between 40 and 90 min. There was no significant sex difference in the maximal PRL increase, maximal increment, or concentration at any time. In 21 of the 31 subjects, PRL increased at least 10 ng/ml with a doubling of baseline levels—criteria for a positive response. In addition, 12 of the healthy subjects received thyrotropin-releasing hormone (TRH) (500 μg i.v.) while 5 received chlorpromazine (50 mg i.m.). There was no significant difference among the maximal prolactin increments following insulin (36.7 ± 7.9 ng/ml), TRH (46.4 ± 6.3 ng/ml), or chlorpromazine (63.4 ± 21.9 ng/ml). In patients with definite pituitary abnormalities, 28 of 38 had diminished PRL release after insulin. Of these 28, 23 also had inadequate GH and 13 impaired cortisol release. In the 10 partially hypopituitary subjects with normal PRL responses, GH increased normally in 7 and cortisol in all. Thirteen of the 17 patients with suspected pituitary dysfunction had adequate PRL increases, while the GH and cortisol responses were intact in 16 and 17 subjects, respectively. Overall, the PRL response was concordant with changes in GH in 44 of 55 patients and in cortisol in 32 of 55 patients. It is concluded that insulin-induced hypoglycemia (1) releases PRL in most normal subjects and (2) is useful in determining the integrity of the hypothalamic pituitary axis for PRL release in patients with suspected abnormalities of pituitary function. Moreover, in combination with TRH, it may aid in localizing the site of abnormality in patients with these disorders.     

Characterization of antiestrogen stimulation of cell number and prolactin production

The antiestrogens LY117018 and tamoxifen increased prolactin production about 4-fold and cell number about 2.5-fold in the pituitary tumor cell line, GH4C1; these increases were 30-40% of the maximal effects of estradiol. The antiestrogens competed with binding of [3H]estradiol, and LY117018 was more active than tamoxifen in biological activities and binding activity. The antiestrogens inhibited stimulation caused by 10(-10) M estradiol; the inhibition could be overcome by increased estradiol concentrations. Tamoxifen and LY117018 increased the amount of prolactin mRNA per cell. These antiestrogens behave as partial agonists in the GH4C1 cells, but have two unusual features. Estrogens are approximately 10-fold more potent in stimulating cell number than in stimulating prolactin production, but the antiestrogens showed the same dose-response for both effects. The partial agonist activity was biphasic and at higher concentrations the antiestrogens showed more antagonist activity (GH4C1 cells, 17 beta-estradiol, tamoxifen).     

Estrogen control of prolactin synthesis in vitro.

Primary cultures of rat pituitary cells respond to estradiol-17beta by increased incorporation of radiolabeled precursors into prolactin but not into the bulk of other cellular proteins. The rate of increase in prolactin synthesis is dose dependent, reaching maximal levels in the physiological range of estradiol. At a concentration of 1 nM, estradiol, diethylstilbestrol, and estriol are stimulatory whereas androgens, progesterone, and corticosterone are without significant effect. Exposure of pituitary cells to 10 nM estradiol resulted in a 500% increase in prolactin synthesis after 7 days of culture. The results indicate that estradiol can stimulate prolactin synthesis through direct action on the pituitary.     

Mechanism of serotonin effects on prolactin and growth hormone secretion in domestic fowl

Brain serotonin levels were increased in immature chickens by ip injection of pargyline (75 mg/kg) and clorgyline (5 mg/kg) and by L-tryptophan (100 mg/kg) and imipramine (10 mg/kg) treatment. These treatments increased the circulating prolactin level and reduced the concentration of plasma growth hormone (GH). Treatment with para-chlorophenylalanine (PCPA, 100 mg/kg) reduced the brain serotonin content and the level of plasma prolactin. Treatment with these drugs in vivo similarly affected the basal level of prolactin release from pituitary glands in vitro, although it did not affect the basal level of GH release. The in vitro responsiveness of the pituitary gland to hypothalamic stimuli eliciting prolactin secretion was increased by in vivo pargyline and combined tryptophan: imipramine treatment but reduced by PCPA administration. The in vitro GH response to hypothalamic stimulation was reduced after the in vivo injection of pargyline, clorgyline and tryptophan: imipramine. The hypothalami from clorgyline and tryptophan: imipramine treated birds induced a greater stimulation of in vitro prolactin secretion from control pituitary glands than hypothalami from controls birds, whereas the GH releasing activity was reduced. These results suggest that serotonin stimulates prolactin secretion in chickens by increasing pituitary responsiveness to hypothalamic releasing factors and by increasing the prolactin releasing activity of the hypothalamus. Serotonin appears to suppress GH secretion by reducing pituitary sensitivity to releasing factors and by reducing hypothalamic GH releasing activity.     

Newly synthesized prolactin is preferentially secreted by the adenohypophysis in a primary cell culture system

A relationship between prolactin synthesis and release was studied using a dispersed adenohypophysial cell primary-culture system. Newly synthesized prolactin was identified by the amount of prolactin-associated tritium activity. No new prolactin was synthesized during a 1-h incubation with ³H-Leu. After 4 h a significant amount of newly synthesized prolactin was released into the medium but no labelled prolactin was detected in the cells. Radioimmuno-assayable prolactin, however, was equally divided between the cells and the medium indicating that the newly synthesized prolactin was preferentially secreted. In contrast, approx. 2 times more newly labelled GH was found in the cell extract than in the medium while by GH radioimmunoassay, the cells contained twice as much GH as the medium in the control group. During the 1-h incubation, estradiol (1.0 ng/ml) increased prolactin content in the medium, although no newly synthesized prolactin appeared in the medium or the cells. During the 4-h incubation period significantly more labelled prolactin was released into the medium in the presence of estradiol (1.0 ng/ml) than in the control. These results indicate that (1) estradiol has a stimulatory effect on prolactin secretion, (2) estradiol activates a prolactin-synthesis mechanism, (3) newly synthesized prolactin is preferentially released, and (4) the rate of release of newly synthesized prolactin is different from that of newly synthesized GH. This result is consistent with prolactin and GH having different mechanisms of hypothalamic control, an inhibiting factor for the prolactin and a releasing factor for GH.     

The effect of altering the ambient salinity of the freshwater teleost Xiphophorus maculatus on the histophysiology of its prolactin cells: I. Progressive changes in one-third sea water

The platyfish, Xiphophorus maculatus, requires prolactin for survival in its natural fresh-water habitat but can be maintained in dilute sea water if deprived of the hormone. In the current study, males were transferred from fresh water to one-third sea water for periods ranging from 2 hr to 30 days, and several were returned from 30 days in the saline medium to fresh water for up to 72 hr. Progressive cytometric and cytological changes of the prolactin cells were compared with autoradiographic labeling after injections of tritiated leucine. Reduced synthetic activity, as judged by cytologic and autoradiographic evidence, was seen as early as 12 hr after the fish were placed into dilute sea water. A marked reduction in cytoplasmic granulation and cell size was first noted at 48 hr, suggesting that hormonal release was continuing independently of the rate of synthesis during this period. After 30 days, cytometry, cytology, and labeling of the prolactin cells were similar to that observed at 48 hr. There was cytologic and autoradiographic evidence of increased synthetic activity in the prolactin cells by 72 hr after the fish were returned to fresh water from 30 days in one-third sea water. Our observations suggest that there is a reduction in synthesis when platyfish are placed in sea water which can be increased when they are returned to fresh water. These changes in synthetic activity presumably reflect requirements for prolactin in these two environments.     

Ethanol induces hyperprolactinemia by increasing prolactin release and lactotrope growth in female rats

BACKGROUND: Alcohol drinking is known to cause hyperprolactinemia in both humans and laboratory animals. The mechanism by which alcoholism causes hyperprolactinemia is not known. This study investigated whether increased pituitary production of prolactin, which leads to alcohol-induced hyperprolactinemia, results from an increase in cell number and/or cell production of prolactin in the pituitary. METHODS: The effects of ethanol on lactotropes were determined in vivo using female rats as an animal model and in vitro using primary cultures of mixed rat anterior pituitary cells and enriched lactotropes. In vivo experiments involved administration of ethanol for 2 and 4 weeks using a liquid diet containing 8.7% ethanol (v/v), which provides 37% of the calories in cyclic, ovariectomized, and estradiol-17beta-treated ovariectomized Fischer-344 rats. The control group was pair-fed an isocaloric diet minus the ethanol or fed a normal diet ad libitum. These animals were used to determine ethanol's effects on plasma prolactin levels, pituitary wet weights, pituitary total protein levels, and the number of mitotic lactotropes. In vitro studies determined ethanol's effects in the presence and absence of estradiol on prolactin release and lactotropic cell proliferation. Prolactin levels in plasma and media samples were measured using radioimmunoassay. Mitotic lactotropes were determined using bromodeoxyuridine incorporation assay. RESULTS: Ethanol treatment increased in a time-dependent manner the plasma levels of prolactin in cyclic, ovariectomized, and estradiol-treated ovariectomized rats. Ethanol treatment also increased pituitary wet weight and/or pituitary total protein levels and DNA synthesis in lactotropes. Determination of ethanol's action on lactotropic cell proliferation and hormone secretion in vitro using primary cultures of mixed pituitary cells revealed that ethanol stimulated both basal and estradiol-induced prolactin secretion and lactotropic cell proliferation. When ethanol's actions were studied in isolated lactotropes, ethanol alone or in combination with estradiol stimulated prolactin secretion but failed to increase lactotropic cell proliferation. CONCLUSIONS: These results suggest that ethanol causes hyperprolactinemia by elevating prolactin release from lactotropes and by increasing the number of lactotropes in the anterior pituitary gland. The mitotic action of ethanol requires cell-cell communication between lactotropes and other pituitary cells. Furthermore, ethanol's mode of action on prolactin release and lactotrope growth is similar to that observed for estradiol.     

Comparison of the regulation of carboxypeptidase E and prolactin in GH4C1 cells, a rat pituitary cell line

The treatment of GH4C1 cells, a prolactin-producing rat anterior pituitary cell line, with estradiol (1 nM), insulin (300 nM) and epidermal growth factor (10 nM) has been previously shown to substantially increase both the intracellular level of prolactin, as well as the number of secretory granules. In this study, we examined the effect of this treatment on levels of carboxypeptidase E (CPE), a prohormone-processing enzyme. GH4C1 cells contain CPE mRNA and enzymatic activity. The secretion of both prolactin and CPE activity from GH4C1 cells is stimulated 10-fold by 50 mM KCl and 2- to 3-fold by 100 nM thyrotropin-releasing hormone, suggesting that these two proteins are contained in secretory granules. Treatment of GH4C1 cells with estradiol, insulin, and epidermal growth factor causes an increase in the intracellular level of CPE to approximately 2-fold of control values. This change is much smaller than the change in the level of prolactin: intracellular prolactin is increased 140-fold by the treatment. Kinetic analysis of the CPE activity indicates that the treatment does not alter the Km of substrate hydrolysis, with the change in activity the result of an increase in apparent Vmax. Northern blot analysis indicates that the level of CPE mRNA is not influenced (less than 10%) by the treatment, whereas the level of prolactin mRNA is increased 9-fold. These results indicate that CPE is not coordinately regulated with prolactin in the GH4C1 cell line, although some regulation of CPE activity does occur.     

Effect of somatomedin and growth hormone on bone collagen synthesis in vitro

Although pituitary hormones, particularly growth hormone (GH), are known to influence skeletal growth, there is no evidence for a direct effect of GH or GH-dependent factors (somatomedins) on bone as opposed to cartilage. We have examined the effects of GH, a somatomedin (Sm) preparation, and serum from intact and hypophysectomized rats on bone collagen synthesis in cultures of 21-day fetal rat calvaria. Collagen synthesis and non-collagen protein synthesis were measured by the incorporation of ³H-proline into collagenase-digestible (CDP) and noncollagen protein (NCP). Bovine and rat GH caused a small inhibition in the incorporation of labeled proline into CDP which was not dose related. Sm in doses of 18–540 mU/ml increased the incorporation of proline into CDP up to three-fold after 24 hr in culture. Sm also had a smaller and more variable stimulatory effect on the labeling of NCP. The effects of Sm were maximal after 3 hr of treatment and were maintained for 96 hr. Sm (60 mU/ml) and insulin (10^?8 M) had effects of similar magnitude and were not additive. The addition of 10% serum from hypophysectomized rats stimulated the labeling of both CDP and NCP, but serum from rats with intact pituitaries had a greater effect. Treatment of hypophysectomized rats with thyroxine, corticosterone, and GH. did not increase the bone collagen synthesis stimulating activity of the serum, although GH treatment did increase serum Sm activity by a pig cartilage assay. The results indicate that GH dose not stimulate bone collagen synthesis directly. However, they suggest that the pituitary gland either releases or stimulates the production of factors which stimulate bone collagen synthesis. Sm may be such a factor, but sulfation activity and bone collagen synthesis stimulating activity may be dissociable.     

Stimulatory effect of pituitary adenylate-cyclase activating polypeptide (PACAP) and its PACAP type I receptor (PAC1R) on prolactin synthesis in rat pituitary somatolactotroph GH3 cells

In this present study, we investigated the role of pituitary adenylate cyclase-activating polypeptide (PACAP) and its receptor, PACAP type I receptor (PAC1R) on prolactin synthesis in pituitary somatolactotroph GH3 cells. PACAP increased prolactin promoter activity up to 1.3 ± 0.1-fold. This increase, while significant, was less than the increase resulting from thyrotropin-releasing hormone (TRH) stimulation. By transfection of a PAC1R expression vector to the cells, the response to PACAP on prolactin promoter activity was dramatically potentiated to a degree proportional to the amount of PAC1R transfected. In the PAC1R expressing GH3 cells, TRH and PACAP alone increased prolactin promoter up to 3.3 ± 0.3-fold and 4.9 ± 0.2-fold, respectively, and combined treatment with TRH and PACAP further increased prolactin promoters up to 6.8 ± 0.6-fold. PACAP binds both Gs- and Gq-coupled receptors and stimulates adenylate cyclase/cAMP and protein kinase C/extracellular signal-regulated kinase (ERK) signaling pathways. PACAP increased ERK phosphorylation in PAC1R expressing cells to the same degree as TRH. Combined treatment with TRH and PACAP had a synergistic effect on ERK activation. GH3 cells produce both prolactin and growth hormone. Stimulation of GH3 cells with TRH significantly increased the mRNA level of prolactin and attenuated growth hormone mRNA expression. PACAP increased both prolactin and growth hormone mRNA levels, particularly in PAC1R expressing cells. In addition, increasing amount of PAC1R in GH3 cells potentiated the action of TRH on prolactin promoter activity, as well as on ERK phosphorylation. PAC1R was induced by PACAP itself, but not by TRH. Our current study demonstrates that PACAP and its PAC1R, functions as a stimulator of prolactin alone or with TRH in prolactin producing cells.     

Acute and chronic estrogen effects upon serum somatomedin activity, growth hormone, and prolactin in man.

Estrogen (E) reduces bioassayable GH-dependent serum somatomedin (SM) activity in acromegalics without affecting plasma growth hormone (GH) levels and inhibits the rise of SM activity normally produced by GH administration in GH-deficient subjects. We have now investigated the effect of E administration on serum SM activity and on plasma GH and prolactin (PRL) in 6 adult male subjects without pituitary pathology. Chronic E administration (ethinyl estradiol 0.5 mg/day for 7 to 70 days) reduced serum SM activity by 40 to 62% in each of 4 subjects (P less than 0.02 to less than 0.001). In 3 of the subjects, basal GH levels increased by 75 to 300% (P less than 0.05 to less than 0.001) and basal PRL levels increased by 90 to 200% (P less than 0.01 to less than 0.001). While iv administration of normal saline did not significantly affect either SM or GH, iv administration of E (bolus injection of 25 mg conjugated estrogens, USP) to 5 subjects resulted in: a) a 46 to 80% decrease in serum SM activity in all subjects, proceeding with an apparent half-life of 2 hours, becoming significant (P less than 0.05) at 2 hours (1 subject) to 3 hours (4 subjects), maximal at 6 hours, and persisting for 12 to 24 hours; b) GH elevation to 3 to 16 times baseline level (P less than 0.01) at 2 to 3 hours in 4 subjects; and c) no significant change of PRL levels in any subject. The mean GH response to iv E was maximal at a time (2 hours) when the mean SM activity had decreased only 20% and subsided well before the nadir of SM activity. The one patient without GH response to chronic or acute E administration may have been affected by absorption of triamcinolone being applied topically during the study. These results demonstrate that in males with normal pituitary function, E reduces serum SM activity, enhances basal GH and PRL secretion, and, upon iv injection, stimulates acute GH release. Although opposite chronic E effects upon GH and SM activity support a putative negative SM-GH feed-back mechanism, iv E administration apparently provokes acute GH release by a different mechanism. The half-life of serum SM activity in the human is probably much shorter than previously estimated.     

Chronic treatment by the calcium channel blocker +/- PN 200-110 in the rat counteracts the stimulations of pituitary weight, prolactin release and pituitary C-kinase activity induced by a chronic estradiol treatment, in vivo

In order to investigate whether a calcium channel blocker could modulate the protein kinase C activity in normal and estradiol pretreated rat pituitary, female Wistar rats were treated or not (controls) with +/- PN 200-110 (3 mg.kg-1.day-1, sc) for 8 days or with estradiol cervical implants for 8 or 15 days, alone or in combination with PN 200-110 the last 8 days. Estradiol treatment induced a significant increase in plasma prolactin levels and pituitary weight. PN 200-110 administered to normal rats did not modify these parameters, whereas it reduced the effects of the 15 days estradiol treatment on prolactin levels (53.1 +/- 4.9 vs 95.0 +/- 9.1 micrograms/l, p less than 0.0001) and pituitary weight (19.9 +/- 0.4 vs 23.0 +/- 0.6 mg, p less than 0.001), to values statistically comparable to those measured after 8 days of estradiol treatment. PN 200-110 alone did not induce any change in protein kinase C activity as compared with controls. In contrast, PN 200-110 treatment significantly counteracted the large increase in soluble activity and the decrease in the particulate one induced by estradiol between day 8 and day 15. We conclude that PN 200-110 opposed the stimulatory effects of chronic in vivo estradiol treatment on plasma prolactin levels and pituitary weight and that this regulation was related to a concomitant modulation of the protein kinase C activity.     

Effect of 1-5 hydroxytryptophan infusion on growth hormone and prolactin secretion in man.

The effect of a new soluble ester of 1-5 hydroxytryptophan (1-5 HTP, Ro 3-5940, 200 mg infusion) on prolactin (PRL) and growth hormone (GH) release was tested in 11 young, healthy subjects (6 men, 5 women). To minimize side-effects, peripheral decarboxylase inhibition was achieved with benserazide (Ro 4-4602.) PRL increased significantly (P less than 0.01) after benserazide alone in all subjects. A further significant increase (P less than 0.01) of PRL plasma levels occurred only in women up to 90 min after the infusion of 1-5 HTP was discontinued. Benserazide administration had no effect of basal GH levels, but a significant increase of GH release (P less than 0.01) was noticed 30-120 min after the end of 1-5 HTP infusion in both men and women. The mean peak value of GH plasma levels after 1-5 HTP administration was 32.0 +/- 8.8 ng/ml. It was postulated that benserazide penetrated at the level of the pituitary, decreasing the synthesis of dopamine and consequently reducing its known inhibitory effect on PRL release. The PRL increase (statistically significant only in women), as well as the release of GH after 1-5 HTP infusion, was considered as further evidence for stimulatory serotoninergic control of both PRL and GH secretion.     

Estradiol-17 beta stimulates basal and thyrotropin releasing hormone induced prolactin secretion by bovine pituitary cells in primary culture.

A series of experiments were conducted which demonstrate that estradiol-17β directly affects bovine pituitary cells in primary culture causing an increase in basal and thyrotropin releasing hormone (TRH)-induced prolactin secretion. Prolactin release by pituitary cells incubated with TRH at concentrations of 0.001, 0.01, 0.1 and 1 ng/ml increased linearly with increasing log concentrations. Exposure of pituitary cells to 5, 50 or 500 ng/ml estradiol for 4 h did not affect basal or TRH-induced prolactin release. However, when the period of exposure to estradiol was prolonged to 6, 12, or 24 h, 0.5, 5 or 50 ng estradiol/ml medium caused pituitary cells to release more prolactin and there was more total prolactin in the system (medium +cell content) than for comparable controls. These increases were linearly related to increasing log concentrations of estradiol used. To determine the chronic effect of estradiol on prolactin secretion, pituitary cells were incubated with estradiol-17β for 11 days during which medium was collected at 24 h intervals beginning on day 3. On day 3, prolactin accumulation in medium of control cultures averaged 2.5 ng/ml, and decreased gradually reaching relatively low levels by day 11 (100 ng/ml). Although prolactin secretion decreased during the culture period, stimulatory effects of estradiol were evident throughout. In addition, these cells still released prolactin in response to TRH (1 ng/ml) on day 11 and magnitude of TRH-induced prolactin release increased with increasing concentrations of estradiol-17β. We conclude that estradiol will increase basal and TRH-induced prolactin release by bovine lactotrophs. These results are consistent with the view that the increase in estradiol that occurs at the end of pregnancy in cattle, may participate in the prolactin surge that occurs at parturition in this species.     

Function of tuberoinfundibular dopamine neurons in pargyline- and reserpine-treated rats

The synthesis and release of dopamine within tuberoinfundibular neurons were studied in rats treated with reserpine and/or pargyline. The effect of reserpine to elevate the serum concentration of prolactin was accompanied by a 70-80% reduction in the concentration of dopamine in pituitary stalk plasma. When rats were given pargyline (75 mg/kg) prior to the administration of reserpine (2.5 mg/kg), the reserpine-induced alterations in serum prolactin and stalk plasma dopamine concentrations were completely prevented. Pargyline treatment alone resulted in a significant elevation of the concentration of dopamine in pituitary stalk plasma and a reduction in the serum concentration of prolactin. The effects of reserpine and pargyline on the synthesis of dopamine in the median eminence were found to be the opposite of their effects on the release of dopamine. Dopamine synthesis (as estimated by the accumulation of dihydroxyphenylalanine after inhibition of decarboxylase activity) was increased after reserpine and decreased after pargyline administration. Thus, these data serve to illustrate the point that, under certain conditions, the release of dopamine from tuberoinfundibular neurons can be dissociated from its rate of synthesis. More importantly, it appears the release of dopamine from these neurons is dependent upon intact dopamine storage function and monoamine oxidase activity, in addition to continued catecholamine synthesis.     

Estriol affects prolactin and LH secretion in rats

The effects of estriol on serum prolactin (PRL) and LH levels, on the pituitary response to TRH and LHRH and on the synthesis and release of PRL from the anterior pituitary gland were investigated in female rats. The increase of serum PRL levels after estradiol administration was found to be associated with an increase of glutamic acid decarboxylase (GAD) and GABA-transaminase (GABA-T) in the hypothalamus. Thus, a study was carried out on the effects of estradiol and estriol on PRL secretion and on GAD, GABA-T and gamma-amino butyric acid (GABA) in the hypothalamus and the anterior pituitary. Under basal and TRH-stimulated conditions, estriol increased serum PRL levels, decreased basal serum LH levels, and increased the response to LHRH, in terms of LH release. Estradiol and estriol increased the synthesis and release of 3H-PRL from hemipituitary glands in incubations of pretreated animals. Both estrogens induced hyperprolactinemia, concomitantly with an increase of hypothalamic GAD and GABA-T activity. Estriol increased hypothalamic GABA concentration, but did not modify GABA concentration in the pituitary glands. Our results show that estriol, at relatively high doses, seems to be active in increasing PRL synthesis and release and in decreasing serum LH levels; it can also modify pituitary response to TRH and LHRH stimulation.