Prolactin release, oestrogens and proliferation of prolactin-secreting cells in the anterior pituitary gland of adult male rats

Relevance of pituitary aromatase and estradiol on the maintenance of the population of prolactin-positive cells in male mice

In previous studies we demonstrated the expression of aromatase in pituitary cells. This expression is gender related, and is also associated with the presence of prolactinomas. To ascertain the relevance of aromatase in modulating the populations of prolactin-positive pituitary cells an immunocytochemical and morphometric study of prolactin-positive pituitary cells was carried out using the pituitary glands of adult male and female aromatase-knockout (ArKO) mice. Additionally has been determined if pituitary aromatase is involved in a gender-linked differentiated regulation of the prolactin-producing pituitary cells. Compared to wild-type mice, the knockout animals of both genders showed a significant decrease (p<0.01) in the cellular and nuclear areas of their prolactin cells, as well as in the percentages of the prolactin-positive cells and the proliferating prolactin cells. Our results suggest that estradiol is responsible for the maintenance of the population of prolactin cell in males and, so as not to disturb the endocrine reproductive environment, estradiol is synthesized inside the pituitary by circulating testosterone via means of aromatase P450, which acts in paracrine way. This new role for pituitary aromatase may well explain the previous findings establishing that the pituitary expression of aromatase is higher in males than in females, and the association between the development of prolactinomas and the increased expression of aromatase in tumours.

Local transformations of androgens into estradiol by aromatase P450 is involved in the regulation of prolactin and the proliferation of pituitary prolactin-positive cells

In previous studies we demonstrated the immunohistochemical expression of aromatase in pituitary cells. In order to determine whether pituitary aromatase is involved in the paracrine regulation of prolactin-producing pituitary cells and the physiological relevance of pituitary aromatase in the control of these cells, an in vivo and in vitro immunocytochemical and morphometric study of prolactin-positive pituitary cells was carried out on the pituitary glands of adult male rats treated with the aromatase antagonist fadrozole. Moreover, we analyzed the expression of mRNA for the enzyme in pituitary cells of male adult rats by in situ hybridization. The aromatase-mRNA was seen to be located in the cytoplasm of 41% of pituitary cells and was well correlated with the immunocytochemical staining. After in vivo treatment with fadrozole, the size (cellular and nuclear areas) of prolactin cells, as well as the percentage of prolactin-positive cells and the percentage of proliferating-prolactin cells, was significantly decreased. Moreover, fadrozole decreased serum prolactin levels. In vitro, treatment with fadrozole plus testosterone induced similar effects on prolactin-positive cells, inhibiting their cellular proliferation. Our results suggest that under physiological conditions aromatase P450 exerts a relevant control over male pituitary prolactin-cells, probably transforming testosterone to estradiol in the pituitary gland.

The expression of AIB1 correlates with cellular proliferation in human prolactinomas

Estrogens as well as certain growth factors strongly influence the development and growth of prolactinomas. However, the molecular mechanisms by which extracellular factors trigger prolactinomas are not well known. Amplified in breast cancer 1 (AIB1), also known as steroid receptor co-activator 3 (SRC-3), belongs to the p160/SRC family of nuclear receptor co-activators and is a major co-activator of the estrogen receptor. Here, we report that the estrogen receptor coactivator AIB1 is overexpressed in human prolactinomas and correlates with the detection of aromatase and estrogen receptor α (ERα). Of the 87 pituitary tumors evaluated in women, 56%, corresponding to hyperoprolactinemic women, contained an enriched population of prolactin-positive cells and hence were further classified as prolactinomas. All prolactinomas stained positive for both ERα and AIB1. Moreover, AIB1 sub-cellular distribution was indicative of the cell-cycle status of tumors; the nuclear expression of AIB1 was correlated with proliferative markers whereas the cytoplasmic localization of AIB1 coincided with active caspase-3. Thus, our results demonstrate for the first time that AIB1 is expressed in prolactinomas and suggest its participation in the regulation of proliferation and apoptosis of tumoral cells. Because aromatase expression is also enhanced in these prolactinomas and it is involved in the local production of estradiol, both mechanisms, ER-AIB1 and aromatase could be related.

No pituitary gland volume change in medication-free depressed patients

Increased serum cortisol levels and a hyperactive hypothalamo-pituitary-adrenal (HPA) axis have been proposed to play an important role in the pathophysiology of Major Depressive Disorder (MDD). However, there are inconsistent results regarding pituitary gland volume (PGV), which is one of the key elements of the HPA axis evaluated by MRI in depressed patients. In this study, we analyzed the PGV of medication-free moderately depressed MDD patients (N=34) and age and sex matched healthy controls (N=39). PGV did not differ between MDD patients and healthy controls [mean volume+/-S.D.; 0.76+/-0.17 cm3 and 0.75+/-0.14 cm3; ANCOVA, F1,69=1.25 p>0.05; respectively]. Our results confirm that volumetric PGV changes are not crucial for depression pathophysiology among unmedicated, moderately depressed adults.

Pituitary volume in unaffected relatives of patients with schizophrenia and bipolar disorder

BACKGROUND

Hypothalamic-pituitary-adrenal (HPA) axis hyperactivity has been demonstrated in both schizophrenia and bipolar disorder, but the mechanisms underlying this abnormality are still unclear. Enlarged pituitary volume has been recently reported in patients with first episode psychosis and been interpreted as a consequence of an increased activation of the HPA axis. The aim of this study was to assess the contribution of familial liability to pituitary volume in schizophrenia and bipolar disorder. Pituitary volume may be an indirect measure of HPA axis activity.

METHODS

MRI brain scans and measurements of pituitary volumes were obtained for 183 subjects: 26 patients with established schizophrenia or schizoaffective disorder, 44 of their unaffected first-degree relatives (22 familial schizophrenia, 22 non-familial schizophrenia), 29 patients with established bipolar disorder, 38 of their unaffected first-degree relatives, and 46 healthy comparison subjects.

RESULTS

We found a significantly larger pituitary volume (effect size=0.7) in unaffected relatives of patients with schizophrenia compared with controls (p=0.002); the pituitary was even larger in relatives of patients with familial schizophrenia (effect size=0.8, p=0.005). We did not find a significant difference in pituitary volume when comparing the relatives of bipolar patients with controls. Among patients, those with schizophrenia who were receiving prolactin-elevating antipsychotics had an increased pituitary volume compared with controls (effect size=1.0, p=0.006).

CONCLUSIONS

These results suggest that the larger pituitary volume previously reported in first episode schizophrenia could be partly due to a genetic susceptibility to over-activate the HPA axis.

Pituitary volume in psychosis: the first review of the evidence

The pituitary gland regulates hypothalamic-pituitary-adrenal (HPA) axis activity by secreting adrenocorticotropic hormone (ACTH), and HPA axis abnormalities have been described in psychosis. Moreover, the pituitary gland secretes prolactin, and some antipsychotics increase the secretion of this hormone. Therefore, it is possible that psychosis is associated with an abnormal volume of the pituitary, as a consequence of a dysfunction in either or both these hormonal systems. The present review of the studies conducted so far clearly indicates that the pituitary is a dynamic organ, which changes differently at different stages of the psychotic disorder, in response to both the disorder itself and the treatment with antipsychotics. Specifically, the pituitary is larger in the months immediately preceding or following the psychosis onset, independently from antipsychotic treatments. However, following this initial enlargement, the pituitary tends to become smaller, as suggested by studies in patients with psychosis of at least two years of duration. On top of these dynamic changes that are linked to the course of the disorder, antipsychotics, and especially antipsychotics inducing hyperprolactinaemia, exert additional enlarging effects on pituitary volume. We suggest that the increased pituitary volume associated with the development of psychosis is due to activation of the hormonal stress response and, specifically, to an increase in the size and number of corticotroph cells producing ACTH, while the increased pituitary volume induced by antipsychotics is linked to the stimulating effects of these drugs on lactotroph cells producing prolactin. Future studies should address these issues that are relevant in improving the care of patients with psychosis.

Male gonadal function, prolactin secretion and lactotroph population in an experimental model of cirrhosis

Liver cirrhosis, a highly prevalent chronic disease, is frequently associated with endocrine dysfunctions, notably in the gonadal axis. We evaluated lactotroph population by immunohistochemistry, gonadotropins and prolactin by immunoradiometric assay and testosterone and estradiol by radioimmunoassay in adult male Wistar rats with cirrhosis induced by carbon tetrachloride. No significant difference in mean +/- SEM percentages of lactotrophs was found between cirrhotic animals and controls (N = 12, mean 18.95 +/- 1.29%). Although there was no significant difference between groups in mean serum levels of prolactin (control: 19.2 +/- 4 ng/mL), luteinizing hormone (control: 1.58 +/- 0.43 ng/mL), follicle-stimulating hormone (control: 19.11 +/- 2.28 ng/mL), estradiol (control: 14.65 +/- 3.22 pg/mL), and total testosterone (control: 138.41 +/- 20.07 ng/dL), 5 of the cirrhotic animals presented a hormonal profile consistent with hypogonadism, all of them pointing to a central origin of this dysfunction. Four of these animals presented high levels of estradiol and/or prolactin, with a significant correlation between these two hormones in both groups (r = 0.54; P = 0.013). It was possible to detect the presence of central hypogonadism in this model of cirrhotic animals. The hyperestrogenemia and hyperprolactinemia found in some hypogonadal animals suggest a role in the genesis of hypogonadism, and in the present study they were not associated with lactotroph hyperplasia.

Genetic disruption of dopamine production results in pituitary adenomas and severe prolactinemia

BACKGROUND

Dopamine release from tuberoinfundibular dopamine neurons into the median eminence activates dopamine-D2 receptors in the pituitary gland where it inhibits lactotroph function.

METHODS

We have previously described genetic dopamine-deficient mouse models which lack the ability to synthesize dopamine. Because these animals require daily treatment with 3,4-L-dihydroxyphenylalanine (L-dopa) to survive, it has not been possible to examine the consequences of chronic loss of dopamine on pituitary physiology. We use viral-mediated gene transfer to selectively restore dopamine to the dorsal striatum of dopamine-deficient mice which allows the mice to survive without L-dopa.

RESULTS

We find that mice chronically lacking tuberoinfundibular dopamine secrete large amounts of prolactin due to the development of severely enlarged pituitaries composed principally of hyperplastic hypertrophic lactotrophs and multifocal prolactinomas. In addition, these mice have elevated serum growth hormone levels and aged males develop hypertrophy of the seminal vesicles.

CONCLUSION

Our observations are consistent with the hypothesis that hypothalamic dopamine is a critical inhibitor of lactotroph proliferation and suggest additional roles for dopamine in the regulation of pituitary function.

Gene therapy for pituitary tumors

Pituitary tumors are the most common primary intracranial neoplasms. Although most pituitary tumors are considered typically benign, others can cause severe and progressive disease. The principal aims of pituitary tumor treatment are the elimination or reduction of the tumor mass, normalization of hormone secretion and preservation of remaining pituitary function. In spite of major advances in the therapy of pituitary tumors, for some of the most difficult tumors, current therapies that include medical, surgical and radiotherapeutic methods are often unsatisfactory and there is a need to develop new treatment strategies. Gene therapy, which uses nucleic acids as drugs, has emerged as an attractive therapeutic option for the treatment of pituitary tumors that do not respond to classical treatment strategies if the patients become intolerant to the therapy. The development of animal models for pituitary tumors and hormone hypersecretion has proven to be critical for the implementation of novel treatment strategies and gene therapy approaches. Preclinical trials using several gene therapy approaches for the treatment of anterior pituitary diseases have been successfully implemented. Several issues need to be addressed before clinical implementation becomes a reality, including the development of more effective and safer viral vectors, uncovering novel therapeutic targets and development of targeted expression of therapeutic transgenes. With the development of efficient gene delivery vectors allowing long-term transgene expression with minimal toxicity, gene therapy will become one of the most promising approaches for treating pituitary adenomas.

In vitro immunoblockade of VIP inhibits the proliferation of pituitary prolactin cells

VIP is a peptide synthesised in the pituitary gland and is involved in the stimulation of prolactin secretion. However, to date it has not been determined whether VIP is able to regulate the proliferation of pituitary prolactin-producing cells, like other factors involved in the regulation of prolactin such as estradiol or dopamine. The aim of the present study was to address whether VIP is involved in regulating the proliferation of pituitary prolactin-secreting cells. Thus, we performed an in vitro study on monolayer cultures of rat pituitary cells, neutralising the possible paracrine effect of VIP by immunoblockade of the peptide and later determining the degree of proliferation of prolactin-secreting cells. The effects of immunoblockade were validated by determining the levels of VIP in the culture media, which were decreased (P < 0.01), and modifications in the patterns of the immunohistochemical reaction to prolactin-positive cells. Immunoblockade of VIP decreased the proliferation of pituitary prolactin-positive cells at all antibody concentrations analysed, mainly between 3 and 12 h (P < 0.01). Moreover, immunoblockade decreased the sizes of the cellular and nuclear areas, except at 1 h, at which point it only decreased the nuclear area of prolactin-positive cells. The results obtained suggest that-in the same way as it regulates the secretion of the hormone-VIP could be involved in regulating the proliferation of prolactin cells, like estradiol or dopamine.

Increased pituitary volume in antipsychotic-free and antipsychotic-treated patients of the AEsop first-onset psychosis study

Subjects at their first psychotic episode show an enlarged volume of the pituitary gland, but whether this is due to hypothalamic-pituitary-adrenal (HPA) axis hyperactivity, or to stimulation of the prolactin-secreting cells by antipsychotic treatment, is unclear. We measured pituitary volume, using 1.5-mm, coronal, 1.5 T, high-resolution MRI images, in 78 patients at the first psychotic episode and 78 age- and gender-matched healthy controls. In all, 18 patients were antipsychotic-free (12 of these were antipsychotic-naïve), 26 were receiving atypical antipsychotics, and 33 were receiving typical antipsychotics. As hypothesized, patients had a larger pituitary volume than controls (+22%, p< 0.001). When divided by antipsychotic treatment, and compared to controls, the pituitary volume was 15% larger in antipsychotic-free patients (p=0.028), 17% larger in patients receiving atypicals (p=0.01), and 30% larger in patients receiving typicals (p<0.001). Patients receiving typicals not only had the largest pituitary volume compared to controls but also showed a trend for a larger pituitary volume compared to the other patients grouped together (+11%, p=0.08). When divided by diagnosis, and compared to controls, the pituitary volume was 24% larger in patients with schizophrenia/schizophreniform disorder (n=40, p<0.001), 19% larger in depressed patients (n=13, p=0.022), 16% larger in bipolar patients (n=16, p=0.037), and 12% larger in those with other psychoses (n=9, p=0.2). In conclusion, the first-episode of a psychotic disorder is associated with a larger pituitary independently of the presence of antipsychotic treatment, and this could be due to activation of the HPA axis. Typical antipsychotics exert an additional enlarging effect on pituitary volume, likely to be related to activation of prolactin-secreting cells. This activation of the hormonal stress response could participate to the important metabolic abnormalities observed in patients with psychosis.

Antiproliferative action of calcitonin on lactotrophs of the rat anterior pituitary gland: evidence for the involvement of transforming growth factor beta 1 in calcitonin action

Calcitonin-like pituitary peptide, which is synthesized and secreted by gonadotrophs of the rat anterior pituitary (AP) gland, is a potent inhibitor of prolactin biosynthesis and lactotroph cell proliferation. Because TGF-beta 1 is an autocrine inhibitor of lactotroph cell proliferation, we investigated a possibility that calcitonin (CT) interacts with TGF-beta 1 to inhibit lactotroph cell proliferation. The actions of CT on GGH3 cell proliferation were examined in the absence or presence of anti-TGF-beta 1 serum. Subsequent experiments tested the effects of CT on TGF-beta 1 mRNA abundance as well as TGF-beta 1 synthesis. The studies also tested whether the stimulatory action of CT on TGF-beta 1 mRNA expression involves stabilization of TGF-beta 1 mRNA. Finally, the experiments investigated in vivo actions of CT on TGF-beta 1 synthesis in the AP gland. This was accomplished by studying the changes induced by i.v. administered CT in TGF-beta 1-immunopositive cell populations of adult female rat AP glands. The results have shown that the inhibitory action of CT on proliferation of GGH3 cells was attenuated by rabbit anti-TGF-beta 1 serum. Moreover, CT stimulated TGF-beta 1 mRNA expression, as well as TGF-beta 1 synthesis, in a dose-dependent fashion. Stimulatory action of CT on TGF-beta 1 expression may be posttranscriptional, because it significantly increased TGF-beta 1 mRNA stability. When administered in vivo, CT significantly increased TGF-beta 1-immunopositive cell populations of adult female rat AP gland. Colocalization studies for prolactin and TGF-beta 1 suggest that CT increased TGF-beta 1 synthesis in lactotrophs, and possibly in nonlactotroph cell populations. These results suggest that antiproliferative action of CT on lactotrophs may, at least in part, be mediated by CT-induced TGF-beta 1 expression.

Variations in the cellular proliferation of prolactin cells from late pregnancy to lactation in rats

Lactation is a physiological process associated with hyperactivity of hypophyseal prolactin-producing cells. It is known that the percentage of these cells is increased during lactation, although there are discrepancies in the reports regarding the mechanisms responsible for increasing the number of prolactin cells. In order to analyse whether this increase is a result of previous proliferation, variations in the proliferation rate of prolactin-positive cells were determined from late pregnancy to lactation in adult female rats by means of observation of the immunohistochemical expression of PCNA as a marker of cellular proliferation. During late pregnancy, a very significant increase in the percentage of proliferating prolactin cells was observed in comparison to non-pregnant females in the proestrus phase (p < 0.01). Although the percentage of prolactin-positive cells after one week of lactation was higher than in non-lactating or in pregnant females (p < 0.01), the proliferation rate was lower than in the other groups studied. In sum, our results suggest that late pregnancy constitutes a preliminary proliferative phase preparatory to the ensuing lactation phase and that endocrine changes in late pregnancy involve the cellular proliferation of hypophyseal prolactin cells in order to prepare the gland for later demands and to prevent proliferative changes from occurring during lactation.

Calcitonin expression in rat anterior pituitary gland is regulated by ovarian steroid hormones

Gonadotroph-derived calcitonin-like peptide (pit-CT) is a potent inhibitor of lactotroph function. We investigated the effect of ovarian hormones on pit-CT mRNA expression in the anterior pituitary (AP) gland of cycling female rats. Levels of mRNAs for pit-CT, CT receptor, prolactin (PRL), and beta-LH during 4-d estrous cycle were determined. In a second study, the effects of estrogens and progesterone on pit-CT and PRL mRNA levels were investigated. In a third group, the effect of estrogen or progesterone depletion on pit-CT mRNA expression was studied. In a fourth group, the effect of passive pit-CT immunization on PRL and LH mRNA expression was examined. Pit-CT mRNA levels varied during estrous cycle. They were highest in diestrus, but lowest in the evening of proestrus. CT-receptor mRNA levels displayed smaller fluctuations. Estrogen repletion caused a decline in pit-CT mRNA expression in ovariectomized rats, but progesterone produced a marked increase. ICI 182,780 prevented the decline of pit-CT mRNA levels during late proestrus-estrus, but RU 486 attenuated pit-CT mRNA levels. Passive CT immunization in diestrus altered PRL and LH mRNA expression, and advanced the estrus cycle. These results suggest that pit-CT mRNA expression is regulated by ovarian hormones, and depletion of pit-CT advances their estrous cycle.

Regulated, adenovirus-mediated delivery of tyrosine hydroxylase suppresses growth of estrogen-induced pituitary prolactinomas

Prolactin-secreting adenomas are one of the most common types of intracranial neoplasm found in humans. The modalities of clinical treatment currently in use include D(2)-dopamine receptor agonists, surgery, and radiotherapy, and the success rates for treatment are good. However, there are prolactinomas that are difficult to treat. As an alternative, we have developed a gene therapy strategy in which the rate-limiting enzyme in dopamine synthesis, tyrosine hydroxylase (TH), is overexpressed in the anterior pituitary (AP) gland. Because dopamine is known to have an inhibitory effect on lactotroph growth and prolactin secretion, we developed a system that would enable its local synthesis from freely available precursor amino acids. A dual adenovirus tetracycline-regulatable expression system was generated to control the production of TH. In the absence but not presence of the tetracycline analog doxycycline, TH expression was observed in AP tumor cell lines AtT20, GH3, and MMQ. In both primary AP cell cultures and the AP gland, in situ expression of TH was seen in lactotrophs, somatotrophs, corticotrophs, thyrotrophs, and gonadotrophs in the absence but not presence of doxycycline. The ability of this system to inhibit hyperprolactinemia and pituitary lactotroph hyperplasia was then assessed in a model of estrogen- or estrogen/sulpiride-induced pituitary tumors. In the absence but not presence of doxycycline, a 49% reduction in pituitary growth and 58% reduction in the increase of circulating prolactin levels were observed in estrogen, but not estrogen/sulpiride, treated rats. These results indicate that in situ dopamine enhancement gene therapy can be a useful tool for the treatment of prolactinoma. Dopamine synthesis can be tightly regulated and the therapeutic benefit of the system is only inhibited when local dopamine signaling is impaired.

Pituitary lactotroph adenomas develop after prolonged lactotroph hyperplasia in dopamine D2 receptor-deficient mice

Tuberoinfundibular dopamine tonically inhibits PRL expression and secretion from the pituitary gland by the activation of dopamine D2 receptors (D2R) localized on lactotrophs. Mutant female mice that lack D2Rs have persistent hyperprolactinemia but also develop extensive hyperplasia of pituitary lactotrophs and peliosis of the adenohypophysis at 9 to 12 months of age, while age-matched male D2R-deficient mice have no morphologic adenohypophysial lesion. We now report that both female and male D2R-deficient mice 17 to 20 months of age develop pituitary lactotroph adenomas. Of 12 aged female mice examined, all developed monohormonal PRL-immunoreactive neoplasms that had a characteristic juxtanuclear Golgi pattern of PRL staining and loss of the reticulin fiber network. Several of these adenomas were 50-fold larger than normal glands with marked suprasellar extension and invasion of brain but no gross evidence of distant metastases. They also had striking peliosis that was more marked than the lesion seen in the hyperplastic pituitaries of the younger females. These findings demonstrate that a chronic loss of neurohormonal dopamine inhibition promotes the hyperplasia-neoplasia sequence in adenohypophysial lactotrophs. Our results are analogous to previous data indicating that protracted stimulation of adenohypophysial cells by hormones or growth factors results in proliferation with initial hyperplasia followed by the development of neoplasia. Six aged male D2R-deficient mice had slightly enlarged anterior pituitaries similar in size to normal female glands. However, each case exhibited multifocal, microscopic lactotroph adenomas with strong nuclear immunoreactivity for estrogen receptors and Pit-1 transcription factor. The unexpected development of adenomas in males without preexisting or concomitant hyperplasia suggests that prolonged loss of dopamine inhibition may also cause neoplasia by distinct cellular mechanisms in male and female animals.

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.

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.

Galanin regulates prolactin release and lactotroph proliferation

The neuropeptide galanin is predominantly expressed by the lactotrophs (the prolactin secreting cell type) in the rodent anterior pituitary and in the median eminence and paraventricular nucleus of the hypothalamus. Prolactin and galanin colocalize in the same secretory granule, the expression of both proteins is extremely sensitive to the estrogen status of the animal. The administration of estradiol-17beta induces pituitary hyperplasia followed by adenoma formation and causes a 3,000-fold increase in the galanin mRNA content of the lactotroph. To further study the role of galanin in prolactin release and lactotroph growth we now report the generation of mice carrying a loss-of-function mutation of the endogenous galanin gene. There is no evidence of embryonic lethality and the mutant mice grow normally. The specific endocrine abnormalities identified to date, relate to the expression of prolactin. Pituitary prolactin message levels and protein content of adult female mutant mice are reduced by 30-40% compared with wild-type controls. Mutant females fail to lactate and pups die of starvation/dehydration unless fostered onto wild-type mothers. Prolactin secretion in mutant females is markedly reduced at 7 days postpartum compared with wild-type controls with an associated failure in mammary gland maturation. There is an almost complete abrogation of the proliferative response of the lactotroph to high doses of estrogen, with a failure to up-regulate prolactin release, STAT5 expression or to increase pituitary cell number. These data further support the hypothesis that galanin acts as a paracrine regulator of prolactin expression and as a growth factor to the lactotroph.

Cell proliferation and death in the irradiated pituitary gland and its modification by growth stimulants

PURPOSE

This study was undertaken to show whether the rate of expression of radiation injury in the rat pituitary gland could be accelerated by the use of growth stimulants.

METHODS AND MATERIALS

Rat pituitary glands were irradiated in situ with a range of single doses up to 20 Gy. The rats were then given subcutaneous slow-release implants containing 17beta-estradiol (E2) and sulpiride (S) to stimulate lactotroph proliferation. Two sequential cycles were used, each consisting of stimulation (3 weeks) and withdrawal (2 weeks). Measurements were made of gland weight; BrdU-labeled, giant, and apoptotic cells; lactotrophs; as well as pituitary prolactin content, in response to exogenous thyroid-releasing hormone (TRH).

RESULTS

The two cycles of stimulation/withdrawal resulted in marked changes in gland weight, BrdU-labeling index, and serum prolactin (PRL) levels in unirradiated rats. The proportion of immunopositive growth-hormone-producing (GH) cells increased after irradiation. Radiation inhibited the hypertrophic response to E2 + S and also inhibited increases in BrdU-labeling index and serum PRL levels. Also, giant lactotrophs were observed in the irradiated pituitaries. However, they were not seen in the unirradiated rats or in the irradiated rats treated with E2 + S. TRH promoted PRL secretion in the unirradiated rat. In contrast, TRH inhibited PRL secretion in the irradiated rat and in all treatment groups receiving E2 + S. Apoptosis was induced by irradiation and was substantially increased in lactotrophs and in other cell types by withdrawal of the E2 and S stimulus, although the highest observed incidence was only 7 per 10,000 cells.

CONCLUSION

Both irradiation and E2 + S treatment removed the hypothalamic control of PRL secretion, which reveals this important inhibitory action of TRH upon PRL secretion. This suggests that it is not suitable as a dynamic test of pituitary PRL reserves in such abnormal situations, where there may also be damage to the hypothalamic-pituitary vasculature. The increasing proportion of GH cells after irradiation indicates that lactotrophs respond more rapidly to irradiation. The stimulation by E2 + S somehow prevented the radiation-damaged lactotrophs from becoming giant cells. Also, the ratio of apoptotic cells to BrdU-labeled cells was increased by the E2 + S treatment, indicating that the E2 + S did enhance radiation-induced cell death relative to cell renewal. However, overall, the E2 + S stimulus protocol did not promote a dramatic increase in cell death (apoptosis) nor a marked decrease in residual gland weight after irradiation. Hence, its use would probably not be beneficial in the treatment of slow-responding prolactinomas, if malignant lactotrophs respond similarly to the normal pituitary lactotrophs. However, the observation of induced apoptosis after hormone and drug withdrawal suggests that agents which promote tumor shrinkage may be effective by causing rapid apoptosis of tumor cells in vivo.

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.

Hormonal regulation of androgen receptor mRNA in the brain and anterior pituitary gland of the male rat

To determine possible cellular mechanisms governing androgen action in the brain, we examined the hormonal regulation of androgen receptor (AR) mRNA in neural tissues by Northern blot hybridization and RNase protection analysis. While a single hybridizable species of AR mRNA of approximately 11 kb was found in the anterior pituitary gland (AP) and ventral prostate gland (VP), an additional species of AR mRNA, approximately 2 kb smaller, was revealed in neural tissues. Furthermore, in these neural tissues, hormonal regulation of the two species of mRNA was coordinated; long-term castration increased levels of both forms, while testosterone replacement reduced them. The same pattern of regulation was observed for the single 11 kb form in the AP. An RNase protection assay was validated and utilized to quantitatively analyze the hormonal regulation of AR mRNA. Castration (4 days) resulted in significantly increased AR mRNA in the AP and hypothalamic-preoptic area, but not the amygdala, which subsequent administration of dihydrotestosterone (DHT; 1 day; 2 mg/animal) significantly decreased. In the AP, administration of estradiol benzoate (EB) for 1 or 5 days also reversed this effect. However, EB treatment increased the amount of total RNA isolated per gland. Consequently, when the data are normalized to RNA content per gland, 5 days of EB treatment resulted in a significant increase in AR mRNA content. These findings suggest that in contrast to the AP and VP, two forms of androgen receptor mRNA exist in the brain. In addition, there appears to be tissue and hormone specific regulation of AR mRNA.

In situ analysis of adenohypophysis proliferative activity in beagle dogs. Preliminary results

In order to establish control data on adenohypophysis cell cycle in beagle dogs, two different approaches were used. In the first approach, the adenohypophysis mitotic index (percentage of cells in mitosis: MI) was determined using pituitary glands from 50 control beagle dogs, 10 to 25 months old, from retrospective regulatory safety assessment studies. We found that the MI in males was 0.024 +/- 0.007% and in females 0.068 +/- 0.019%. The second approach involved the determination of the adenohypophysis cell fraction in S phase by means of bromodeoxyuridine (BrdU) anti-BrdU method in 16 control beagle dogs, 8 to 20 months old, from ongoing safety assessment studies. We found that the BrdU labeling index (percentage of BrdU-positive cells per total number of cells: BrdU LI) was 0.053 +/- 0.009% in males and 0.059 +/- 0.015% in females. Analysis of our data indicated that the results obtained from the BrdU LI were more reliable (coefficient of variation [CV] less than 0.07) compared to those obtained from the MI (CV greater than 1.4). No clear difference was found between male and female beagle dogs, regarding adenohypophysis cell proliferation. In both studies, adenohypophysis cell proliferation was found to be age-dependent, with the highest value in immature dogs. No correlation between estrus cycle in females and adenohypophysis cell proliferation was noted.

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.

Morphofunctional study of mammotropic cells following intraventricular administration of met-enkephalin

An ultrastructural and morphometric study was carried out on the adenohypophyseal mammotropic cells of rats treated intraventricularly with an acute dose (150 micrograms) of Met-enkephalin. In the female rats, clear features of cellular hyperactivity appeared after opioid administration. The changes affected the Golgi complex, the rough endoplasmic reticulum, the mature and immature secretory granules and the images of exocytosis. Such changes did not appear when naloxone was administered before the opioid, and naloxone induced an increase in the numerical density of lysosomal dense bodies with lipoid inclusions. In the male animals, administration of an identical dose of Met-enkephalin caused only a few significant changes, similar to those observed in the controls. It is concluded that Met-enkephalin administered intraventricularly causes evident modifications in the mammotropic cells of female rats whereas such changes in the male animals are not significant.

Prolactin levels and pituitary enlargement in hormone-treated male-to-female transsexuals

PRL levels were evaluated during long-term treatment with cyproterone acetate 100 mg and ethinyloestradiol 100 micrograms/day orally or depot-oestrogens in 214 male-to-female transsexuals. PRL levels increased above normal in all subjects (normal less than 300 mU/l). In 46 (21.4%) subjects PRL levels rose to greater than 1000 mU/l. The incidence of PRL levels greater than 1000 mU/l was 3.7-7.2% per treatment year. Grossly elevated PRL levels were associated with high doses of oestrogens (P less than 0.05) and advanced age at the start of treatment (P less than 0.05). In 23 subjects PRL levels greater than 1000 mU/l decreased by more than 50% spontaneously (n = 5) or after dose reduction (n = 18). In five of the subgroup of 15 subjects with persistent PRL levels greater than 1000 mU/l enlargement of the pituitary gland was shown by CT-scanning. These data suggest that the lowest possible oestrogen dose and lifelong follow-up of hormone-treated male-to-female transsexuals is essential.