Release of big and small molecular forms of prolactin: dependence upon dynamic state of the lactotroph

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.

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.

Behaviour of a somatotroph population under a growth hormone releasing peptide treatment

In this investigation, we studied the effects of Momany peptide (GHRP-5), on somatotroph secretory activity. Acute and chronic administration of GHRP-5 provokes a significant release of growth hormone that can be closely correlated with ultrastructural changes in somatotroph populations. After 3,5 and 7 days of GHRP-5 treatment, two somatotroph cell subpopulations coexist. One of them has an enhanced secretory activity and the other presents a quiescent appearance. Therefore, pituitary growth hormone content was not affected in the first seven days of GHRP-5 treatment. After 14 days, there was a significant depletion of growth hormone pituitary content coincident with the highest levels of serum growth hormone. These results concur with the surge of a new hyperactive somatotroph subtype characterised by numerous immature secretory granules that are discharged bypassing the maturation step. Acute and chronic treatments caused no changes in somatotroph cell density, the area immunostained for growth hormone and the levels of total mRNA for transcription factor pit-1. The results of pituitary cell cultures incubated with specific blockers for different signalling pathways demonstrated an involvement of the phospholipase C-inositol phosphate system in GHRP-5 stimulated somatotroph secretion. GHRP-5 treatment enhanced significantly the release of growth hormone, thereby eliciting ultrastructural modifications in somatotrophs that can be correlated with an increased secretory activity devoid of cell density changes.

Prolactin secretory bypath exposed in cultured lactotrophs

In the present report, the prolactin secretory pathways were re-examined in cultured lactotrophs submitted to various experimental conditions of stimulation, inhibition and/or alteration of the intracellular flow of the synthesis and release of prolactin. Primary cultures of rat pituitary cells stimulated with thyrotropin-releasing hormone, or inhibited with either cycloheximide or dopamine in the presence or absence of 0.1 microg/ml brefeldin A, were used. The radioimmunoassay quantification of released and intracellular prolactin was correlated with ultrastructural and immunocytochemical studies. Brefeldin A diminished significantly the secretion and the intracellular content of prolactin 4 h after application, while morphological effects were seen starting from 30 min. The drug did not modify the response to thyrotropin-releasing hormone (120% increment). The simultaneous incubation of brefeldin A with cycloheximide or dopamine diminished the released prolactin concomitant with a lower (cycloheximide) or greater (dopamine) hormonal intracellular prolactin content with respect to brefeldin A. The combined treatment cycloheximide-dopamine inhibited prolactin secretion. The ultrastructural and immunocytochemical features of lactotrophs supported these radioimmunoassay data. These results revealed that prolactin release in vitro in the presence or not of brefeldin A is dependent on either: the neosynthesized hormone that can be inhibited by cycloheximide, and the hormone stored in granules, the exocytosis of which was blocked by dopamine, indicates the contribution of both constitutive and regulated pathways in the secretory process. The brefeldin A blockade of the intracellular transport also disclosed morphological evidence of an alternative pathway of prolactin secretion through vesicles originated in the rough endoplasmic reticulum bypassing the Golgi complex.

Heterogeneity of pituitary lactotrophs: immunocytochemical identification of functional subtypes

The existence of functional lactotroph subpopulations was confirmed in primary pituitary cell cultures of female rats submitted to estrogen treatment and stimulation with thyrotrophin releasing hormone (TRH) and angiotensin II (A-II). In cell cultures of pituitary tissue, prolactin (PRL) producing cells represent about 50% of the total cell count, most of which (90%) correspond to a typical lactotroph subpopulation characterized by large secretory granules, 500-900 nm in diameter, and well developed rough endoplasmic reticulum (RER) and Golgi complex. Few atypical lactotrophs were detected with a quiescent appearance and containing smaller secretory granules, often indistinguishable from granular content of other pituitary cells. Depletion of endogenous estrogen caused by ovariectomy (OVX) decreased the pituitary lactotroph population about 34%, with a relative increase of atypical forms (56%). Replacement therapy with benzoate estradiol (EB) to OVX rats did not reverse the proportion of typical and atypical lactotrophs gauged in control pituitary glands. The predominant lactotroph population of OVX rat was an atypical PRL producing cell which displayed a quiescent appearance compatible with a reduced secretory activity. By contrast, estrogen administration to OVX rats caused a striking development of the RER, a hypertrophy of the Golgi complex and an increased storage of mature and immature secretory granules in the majority of lactotrophs. These features are compatible with a reactivated protein synthesis. Estrogen also enhanced significantly (p < 0.05) the responsiveness of lactotrophs to A-II and the PRL secretion in both intact and OVX + EB treated rats increased by 40% and 30% respectively. By contrast, A-II did not produce any statistically significant response of lactotrophs from OVX female rats. At variance to this observation, in all models tested TRH increased significantly the PRL secretion (p < 0.05). The correlation of PRL secretion and morphology of different lactotroph subtypes authenticates the existence of a lactotroph subpopulation unresponsive to A-II in pituitary cell cultures from rats depleted of estrogen.

Fate of degenerating lactotrophs in rat pituitary gland after interruption of lactation: a histochemical and immunocytochemical study

In the pituitary gland of pregnant and lactating rats a striking proliferation of lactotrophs occurs to meet the increased demands for prolactin. Following interruption of lactation the redundant lactotrophs undergo a massive degeneration until pre-pregnant proportions are re-established. Immunocytochemical detection of prolactin allows the recognition of degenerating lactotrophs until advanced stages of degeneration and leads to the conclusion that this process is autolytic in nature. Histochemistry of acid phosphatase reveals a remarkable accumulation of this enzyme in Golgi cisternae and lysosomes. At later stages of degeneration the acid phosphatase spreads throughout the entire cell. The presence of increased numbers of necrotic cells appears to activate phagocytosis of stellate cells and, to a lesser extent, of follicular cells. Stellate cells responsible for the secondary processing of cell residues are isolated cells characterized by a prominent oval nucleus and an electron-lucent cytoplasm with scarce organelles and extensive cytoplasmic processes. They appear as scavenger cells engulfing cell remnants and debris. Immunocytochemistry of S-100 protein discloses differential staining of two types of cell, one forming clusters of 2-4 cells with faint immunoreactivity, while the other type consists of isolated cells with a stellate profile and stronger labelling to S-100 protein.

Morphometrical variations of prolactin cells in response to prolonged and systemic administration of Met-enkephalin in female rats

A stimulatory effect on prolactin secretion had been describe after acute and systemic administration of met-enkephalin, but the effects of this opioid after chronic administration has not been reported, and the response of mammotroph cells is not clear. As a complement to previous studies, a morphometric analysis (light and electron microscopy) was carried out on prolactin cells from female rats treated chronically with met-enkephalin. Clear features of cellular hyperactivity appeared after chronic and systemic administration of the opioid, and these persisted for two weeks. The changes consisted in increases of cellular, cytoplasmic and nuclear areas, volume and surface densities of the Golgi complex and rough endoplasmic reticulum, as well as the numbers of exocytotic figures. These morphological alterations were paralleled by an increase in serum prolactin levels as detected by RIA. It is concluded that the increase in the synthesis and secretory activity of prolactin cells following chronic and systemic administration of met-enkephalin is very similar to those observed after acute and intraventricular administration.

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.