Stimulation of prolactin secretion by oestradiol in the rat is associated with increased hypothalamic release of thyrotrophin-releasing hormone

17β-Estradiol modulates the prolactin secretion induced by TRH through membrane estrogen receptors via PI3K/Akt in female rat anterior pituitary cell culture

Considering that estradiol is a major modulator of prolactin (PRL) secretion, the aim of the present study was to analyze the role of membrane estradiol receptor-α (mERα) in the regulatory effect of this hormone on the PRL secretion induced by thyrotropin-releasing hormone (TRH) by focusing on the phosphatidylinositol 3-kinase (PI3K)/protein kinase B (Akt) pathway activation. Anterior pituitary cell cultures from female rats were treated with 17β-estradiol (E(2), 10 nM) and its membrane-impermeable conjugated estradiol (E(2)-BSA, 10 nM) alone or coincubated with TRH (10 nM) for 30 min, with PRL levels being determined by RIA. Although E(2), E(2)-BSA, TRH, and E(2)/TRH differentially increased the PRL secretion, the highest levels were achieved with E(2)-BSA/TRH. ICI-182,780 did not modify the TRH-induced PRL release but significantly inhibited the PRL secretion promoted by E(2) or E(2)-BSA alone or in coincubation with TRH. The PI3K inhibitors LY-294002 and wortmannin partially inhibited the PRL release induced by E(2)-BSA, TRH, and E(2)/TRH and totally inhibited the PRL levels stimulated by E(2)-BSA/TRH, suggesting that the mER mediated the cooperative effect of E(2) on TRH-induced PRL release through the PI3K pathway. Also, the involvement of this kinase was supported by the translocation of its regulatory subunit p85α from the cytoplasm to the plasma membrane in the lactotroph cells treated with E(2)-BSA and TRH alone or in coincubation. A significant increase of phosphorylated Akt was induced by E(2)-BSA/TRH. Finally, the changes of ERα expression in the plasmalemma of pituitary cells were examined by confocal microscopy and flow cytometry, which revealed that the mobilization of intracellular ERα to the plasma membrane of lactotroph cells was only induced by E(2). These finding showed that E(2) may act as a modulator of the secretory response of lactotrophs induced by TRH through mER, with the contribution by PI3K/Akt pathway activation providing a new insight into the mechanisms underlying the nongenomic action of E(2) in the pituitary.

Reversible thioridazine-induced magnetic resonance imaging-documented pituitary enlargement associated with hyperprolactinemia

OBJECTIVE

To document a case of reversible thioridazine-induced pituitary enlargement associated with hyperprolactinemia in a patient with schizophrenia.

METHODS

We describe a 19-year-old woman with paranoid schizophrenia who was taking thioridazine (a phenothiazine), in whom hyperprolactinemia, galactorrhea, oligomenorrhea, and a reversible anatomic pituitary abnormality developed. Serial magnetic resonance imaging (MRI) studies were used to assess the status of the pituitary gland during various pharmaceutical therapies.

RESULTS

Laboratory evaluation revealed increased serum prolactin (PRL) levels that ranged from 76 to 135 mg/L. Results of thyroid function tests were normal, and gonadotropins and estradiol levels were low, consistent with hyperprolactinemia. MRI revealed asymmetric enlargement of the right side of the pituitary gland. Discontinuing the thioridazine therapy resulted in normalization of the serum PRL and resolution of the pituitary abnormality. Subsequent worsening of the patient's psychiatric condition necessitated a course of electroconvulsive therapy and initiation of treatment with clozapine, a D4 dopamine receptor antagonist. At 1-year follow-up, at which time the patient was maintained on clozapine and was not taking thioridazine, both serum PRL levels and MRI findings remained normal.

CONCLUSION

Our patient was shown to have asymmetric pituitary enlargement associated with thioridazine-induced hyperprolactinemia, which reversed when use of the drug was discontinued. In patients with serum PRL levels in excess of 100 mg/L during antipsychotic drug therapy, evaluation for a prolactinoma is warranted.

The thyroid C cells of ovariectomized rats treated with estradiol

The structure and function of thyroid C cells were studied in ovariectomized (Ovx) adult female rats without and after chronic treatment with estradiol dipropionate (EDP). A peroxidase-antiperoxidase method was applied for localization of calcitonin (CT) in the C cells. Morphometric changes in their volume, nuclei, and relative volume density were evaluated in comparison with sham-operated control rats using a stereological method. The number of C cells was calculated. CT content in the sera was determined by radioimmunoassay. Ovariectomy (Ovx) led to a 21% increase in body weight ( P<0.005), while treatment of Ovx rats with EDP decreased body weight by 25% ( P<0.01). The immunoreactivity for CT in C cells of the Ovx rats was markedly increased. Significant decreases in the volume of C cells (by 13%; P<0.05) and serum CT (by 45%) were recorded, while the C cell number increased by 59% ( P<0.05) in relation to the corresponding controls. The treatment of Ovx rats with EDP caused conspicuous degranulation of the C cells. The cellular volume was increased by 11% and serum CT by 36% in comparison with Ovx animals. At the same time a decrease in C cell number by 29% ( P<0.05) was evident. It may be concluded that estradiol deficiency after Ovx reduced the synthesis and release of CT, while chronic treatment of these animals with EDP had a positive effect on the secretory activity of thyroid C cells.

Calcitonin inhibits anterior pituitary cell proliferation in the adult female rats

Previous studies have shown that CT-like immunoreactive peptide(s) (pit-CT) is synthesized by the anterior pituitary (AP) gland, and exogenously added salmon(s) CT inhibits PRL release and PRL gene transcription in cultured AP cells. Anti-sCT serum, which immunoreacts with pit-CT, stimulates PRL secretion, suggesting pit-CT is a physiologically relevant PRL-inhibiting hormone. Using proliferating cell nuclear antigen (PCNA) staining and 5-bromo-2'deoxyuridine (BrdU) incorporation into newly replicated DNA, the effect of calcitonin (CT) on cellular proliferation in the rat anterior pituitary gland (AP) was examined. CT significantly attenuated PCNA-immunopositive as well as BrdU-positive AP cell populations in dispersed rat AP cells. A second series of experiments tested the effects of CT on AP cell proliferation in vivo. OVX + E2 rats were injected with 200 microg CT (iv), the rats killed at various time points, and the APs were processed for BrdU staining. CT inhibited BrdU incorporation at all time points up to 15 h after the injection, and this inhibitory effect was reversed at later time points. The effect of CT was concentration dependent, and a maximal inhibition was observed 10 h after the CT injection. Subsequent experiments identified CT-responsive AP cell populations using double immunofluorescence for BrdU and either PRL or FSH. The number of BrdU-labeled lactotropes in the AP gland declined by 74% in the CT-treated rats. Neutralization of endogenous pit-CT by passive immunization with anti-sCT serum caused a 2-fold increase in AP cell proliferation. These results suggest an important role for the endogenous pit-CT in regulation of lactotrope population of the AP gland.

Multifactorial modulation of TRH metabolism

1. Thyrotropin releasing hormone (TRH), synthesized in the paraventricular nucleus of the hypothalamus (PVN), is released in response to physiological stimuli through median eminence nerve terminals to control thyrotropin or prolactin secretion from the pituitary. 2. Several events participate in the metabolism of this neuropeptide: regulation of TRH biosynthesis and release as well as modulation of its inactivation by the target cell. 3. Upon a physiological stimulus such as cold stress or suckling, TRH is released and levels of TRH mRNA increase in a fast and transient manner in the PVN; a concomitant increase in cfos is observed only with cold exposure. 4. Hypothalamic cell cultures incubated with cAMP or phorbol esters show a rise in TRH mRNA levels; dexamethasone produces a further increase at short incubation times. TRH mRNA are thus controlled by transsynaptic and hormonal influences. 5. Once TRH is released, it is inactivated by a narrow specificity ectoenzyme, pyroglutamyl peptidase II (PPII). 6. In adenohypophysis, PPII is subject to stringent control: positive by thyroid hormones and negative by TRH; other hypothalamic factors such as dopamine and somatostatin also influence its activity. 7. These combined approaches suggest that TRH action is modulated in a coordinate fashion.

Estrogen attenuates expression of calcitonin-like immunoreactivity in the anterior pituitary gland

Estrogens increase prolactin (PRL) synthesis and release in rats and humans, whereas pituitary-derived calcitonin-like immunoreactive peptide (pit-CT) inhibits PRL gene expression and release. To test the hypothesis that estrogens stimulate lactotrophs by diminishing pit-CT expression, the present studies examined effects of ovariectomy (ovx) and estradiol (E2) administration on (1) pit-CT IR cell population; (2) pit-CT IR content and (3) release of pit-CT IR by cultured anterior pituitary (AP) cells. Ability of anti-calcitonin immunoglobulins (anti-CT IgG) to stimulate PRL release from cultured AP cells was also examined. The results suggest that ovariectomy induced a large increase in pit-CT IR cell population in the AP gland and E2-treatment dramatically reversed this increase. Similar changes were observed in pit-CT IR content of AP extracts. Cultured AP cells from ovx rats released significantly higher amounts of pit-CT IR, and anti-CT IgG induced a significant increase in basal PRL release. AP cells from E2-treated rats secreted lower amounts of pit-CT IR and this was associated with significantly higher PRL release. These results suggest that estrogens may stimulate lactotrophs, at least in part, by removing inhibitory influence of endogenous pit-CT.

Thyrotropin-releasing hormone (TRH) and metoclopramide testing in infertile women

Thyroid stimulating hormone (TSH) determinations before and after thyrotropin-releasing hormone (TRH) stimulation were obtained in 834 infertile women, from 1982 until 1985. Thyroid function disturbances were seen in 20% of the women, in accordance with the prevalence in South Germany. Postcoital tests were significantly poorer in women with subclinical hypothyroidism than in euthyroid patients. Spontaneous conception was more frequent in euthyroid (16%) than in hypothyroid (6%) women. During the same period, prolactin was determined after TRH stimulation in the early follicular phase and after metoclopramide stimulation in the luteal phase, in 759 women. The pregnancy rate was not improved by administration of dopamine agonists in women with an exaggerated response to TRH or metoclopramide. Our results suggest that subclinical hypothyroidism as well as disorders of prolactin secretion may play a role in infertility. The TRH test is proposed to rule out thyroid dysfunction. Neither the TRH nor the metoclopramide test was useful for the prognostic differentiation of prolactin secretion disorders.

Hyperprolactinaemia in congenital hypothyroidism

Serum prolactin concentrations were measured in nine untreated hypothyroid infants and 23 euthyroid controls. In the former group the serum prolactin concentrations were tenfold higher than the controls (352 vs 34 micrograms/l), declining gradually upon treatment. Although serum prolactin concentrations can be mildly elevated in adults with hypothyroidism, congenital hypothyroidism causes a profound and probably uniform hyperprolactinaemia; furthermore, the hormone concentration correlates directly with the simultaneous TSH concentration. The physiological importance, if any, of the hyperprolactinaemia in the fetus or neonate is uncertain.