Role of prolactin in the gonadotroph responsiveness to gonadotrophin-releasing hormone during the equine annual reproductive cycle

A New Perspective on Regulation of Pituitary Plasticity: The Network of SOX2-Positive Cells May Coordinate Responses to Challenge

Plasticity of function is required for each of the anterior pituitary endocrine axes to support alterations in the demand for hormone with physiological status and in response to environmental challenge. This plasticity is mediated at the pituitary level by a change in functional cell mass resulting from a combination of alteration in the proportion of responding cells, the amount of hormone secreted from each cell, and the total number of cells within an endocrine cell population. The functional cell mass also depends on its organization into structural and functional networks. The mechanisms underlying alteration in gland output depend on the strength of the stimulus and are axis dependent but in all cases rely on sensing of output of the functional cell mass and its regulation. Here, we present evidence that the size of pituitary cell populations is constrained and suggest this is mediated by a form of quorum sensing. We propose that pituitary cell quorum sensing is mediated by interactions between the networks of endocrine cells and hormone-negative SOX2-positive (SOX2+ve) cells and speculate that the latter act as both a sentinel and actuator of cell number. Evidence for a role of the network of SOX2+ve cells in directly regulating secretion from multiple endocrine cell networks suggests that it also regulates other aspects of the endocrine cell functional mass. A decision-making role of SOX2+ve cells would allow precise coordination of pituitary axes, essential for their appropriate response to physiological status and challenge, as well as prioritization of axis modification.

Age related and seasonal changes of plasma concentrations of insulin-like peptide 3 and testosterone from birth to early-puberty in Thoroughbred male horses

The peripheral blood concentrations of insulin-like peptide 3 (INSL3) have been detected in many mammalian species, but the level of INSL3 in horse remains unknown. The objectives were to develop a time-resolved fluorescence immunoassay (TRFIA) to detect INSL3 concentrations from horse blood as well as to determine the age-related and seasonal changes of plasma concentrations of INSL3 and testosterone from birth to early-puberty in Thoroughbred male horse (n = 11). Monthly blood sample and measurement of body weight, height, chest and cannon bone size were done from birth until 16 mo. The TRFIA and EIA were used to measure plasma concentrations of INSL3 and testosterone, respectively. An increase in mean body weight, height, chest and cannon bone size was observed throughout the study. The monthly blood sampling revealed an increase in mean plasma INSL3 concentrations up to 2 mo, followed by a decreasing and increasing pattern until the end of experiment at 16 mo. A high testosterone level was detected at birth followed by a sharp decrease to basal level within 1 mo, maintained low level up to10 mo before a gradual rise until 16 mo. In case of seasonality, there was no difference in mean plasma INSL3 concentrations between breeding (March to September) and non-breeding (October to February) seasons, whereas a higher (P < 0.001) mean plasma testosterone concentrations in the second breeding season compared to non-breeding season was observed. In age categorized group, an increase (P < 0.01) in mean plasma INSL3 concentrations was noticed at pre-puberty (1-12 mo) and early-puberty (13-16 mo) compared to birth, but a lower (P < 0.001) mean plasma testosterone concentrations was observed at pre-puberty compared to birth and early-puberty. In conclusion, a TRFIA was developed to measure INSL3 levels in horse. An increase in plasma concentrations of INSL3 and testosterone were observed with the advancement of age, whereas for testosterone a very lower level was detected at the non-breeding season than in the second breeding season after birth in Thoroughbred male horse. The INSL3 secretions seemed independent of seasonal influence, at least before puberty.

The anterior pituitary gap junctions: potential targets for toxicants

The anterior pituitary regulates endocrine organs and physiological activities in the body. Environmental pollutants and drugs deleterious to the endocrine system may affect anterior pituitary activity through direct action on anterior pituitary cells. Within the gland, endocrine and folliculostellate cells are organized into and function as individual tridimensional networks, each network regulating its activity by coordinating the connected cells' responses to physiological or pathological cues. The gap junctions connecting endocrine cells and/or folliculostellate cells allow transmission of information among cells that is necessary for adequate network function. Toxicants may affect gap junctions as well as the physiology of the anterior pituitary. However, whether toxicants effects on anterior pituitary hormone secretion involve gap junctions is unknown. The folliculostellate cell gap junctions are sensitive to hormones, cytokines and growth factors. These cells may be an interesting experimental model for evaluating whether toxicants target anterior pituitary gap junctions.

Gap junction proteins are key drivers of endocrine function

It has long been known that the main secretory cells of exocrine and endocrine glands are connected by gap junctions, made by a variety of connexin species that ensure their electrical and metabolic coupling. Experiments in culture systems and animal models have since provided increasing evidence that connexin signaling contributes to control the biosynthesis and release of secretory products, as well as to the life and death of secretory cells. More recently, genetic studies have further provided the first lines of evidence that connexins also control the function of human glands, which are central to the pathogenesis of major endocrine diseases. Here, we summarize the recent information gathered on connexin signaling in these systems, since the last reviews on the topic, with particular regard to the pancreatic beta cells which produce insulin, and the renal cells which produce renin. These cells are keys to the development of various forms of diabetes and hypertension, respectively, and combine to account for the exploding, worldwide prevalence of the metabolic syndrome. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Mechanisms regulating angiogenesis underlie seasonal control of pituitary function

Seasonal changes in mammalian physiology, such as those affecting reproduction, hibernation, and metabolism, are controlled by pituitary hormones released in response to annual environmental changes. In temperate zones, the primary environmental cue driving seasonal reproductive cycles is the change in day length (i.e., photoperiod), encoded by the pattern of melatonin secretion from the pineal gland. However, although reproduction relies on hypothalamic gonadotrophin-releasing hormone output, and most cells producing reproductive hormones are in the pars distalis (PD) of the pituitary, melatonin receptors are localized in the pars tuberalis (PT), a physically and functionally separate part of the gland. How melatonin in the PT controls the PD is not understood. Here we show that melatonin time-dependently acts on its receptors in the PT to alter splicing of vascular endothelial growth factor (VEGF). Outside the breeding season (BS), angiogenic VEGF-A stimulates vessel growth in the infundibulum, aiding vascular communication among the PT, PD, and brain. This also acts on VEGF receptor 2 (VEGFR2) expressed in PD prolactin-producing cells known to impair gonadotrophin secretion. In contrast, in the BS, melatonin releases antiangiogenic VEGF-A_xxxb from the PT, inhibiting infundibular angiogenesis and diminishing lactotroph (LT) VEGFR2 expression, lifting reproductive axis repression in response to shorter day lengths. The time-dependent, melatonin-induced differential expression of VEGF-A isoforms culminates in alterations in gonadotroph function opposite to those of LTs, with up-regulation and down-regulation of gonadotrophin gene expression during the breeding and nonbreeding seasons, respectively. These results provide a mechanism by which melatonin can control pituitary function in a seasonal manner.

Intrapituitary mechanisms underlying the control of fertility: key players in seasonal breeding

Recent studies have shown that, in conjunction with dynamic changes in the secretion of GnRH from the hypothalamus, paracrine interactions within the pituitary gland play an important role in the regulation of fertility during the annual reproductive cycle. Morphological studies have provided evidence for close associations between gonadotropes and lactotropes and gap junction coupling between these cells in a variety of species. The physiological significance of this cellular interaction was supported by subsequent studies revealing the expression of prolactin receptors in both the pars distalis and pars tuberalis regions of the pituitary. This cellular interaction is critical for adequate gonadotropin output because, in the presence of dopamine, prolactin can negatively regulate the LH response to GnRH. Receptor signaling studies showed that signal convergence at the level of protein kinase C and phospholipase C within the gonadotrope underlies the resulting inhibition of LH secretion. Although this is a conserved mechanism present in all species studied so far, in seasonal breeders such as the sheep and the horse, this mechanism is regulated by photoperiod, as it is only apparent during the long days of spring and summer. At this time of year, the nonbreeding season of the sheep coincides with the breeding season of the horse, indicating that this inhibitory system plays different roles in short- and long-day breeders. Although in the sheep, it contributes to the complete suppression of the reproductive axis, in the horse, it is likely to participate in the fine-tuning of gonadotropin output by preventing gonadotrope desensitization. The photoperiodic regulation of this inhibitory mechanism appears to rely on alterations in the folliculostellate cell population. Indeed, electron microscopic studies have recently shown increased folliculostellate cell area together with upregulation of their adherens junctions during the spring and summer. The association between gonadotropes and lactotropes could also underlie an interaction between the gonadotropic and prolactin axes in the opposite direction. In support of this alternative, a series of studies have demonstrated that GnRH stimulates prolactin secretion in sheep through a mechanism that does not involve the mediatory actions of LH or FSH and that this stimulatory effect of GnRH on the prolactin axis is seasonally regulated. Collectively, these findings highlight the importance of intercellular communications within the pituitary in the control of gonadotropin and prolactin secretion during the annual reproductive cycle in seasonal breeders.

Biphasic Effect of Basic Fibroblast Growth Factor on Anterior Pituitary Folliculostellate TtT/GF Cell Coupling, and Connexin 43 Expression and Phosphorylation

Basic fibroblast growth factor (bFGF) is a mitogenic and differentiating cytokine. In the anterior pituitary, folliculostellate (FS) cells constitute the major source of bFGF. bFGF affects endocrine cell proliferation and secretion in the anterior pituitary. In addition, bFGF increases its own expression by acting directly on FS cells. FS cell Cx43-mediated gap junction intercellular communication allows the establishment of an intrapituitary network for the transmission of information. In the present study, we assessed how bFGF regulates FS cell coupling. Time course studies were carried out on the FS cell line TtT/GF. Short-term bFGF treatment induced a transient cell uncoupling and the phosphorylation in Ser368 of membrane-bound Cx43 without modifying Cx43 levels. We demonstrated the involvement of the protein kinase C (PKC) isoform α in the phosphorylation of Cx43 in S368. Moreover, we showed that bFGF induced PKCα activation by stimulating its expression, phosphorylation and association with the plasma membrane. The long-term incubation with bFGF increased TtT/GF cell coupling, total Cx43 levels and Cx43 accumulation at the cell membrane of cytoplasmic projections. The Cx43 level increase was a result of the stimulation of Cx43 gene transcription as mediated by the extracellular-regulated kinase 1/2 signalling pathway. Taken together, the data show that bFGF modulates TtT/GF cell coupling by activating different pathways that lead to opposite effects on Cx43 phosphorylation and expression depending on the duration of the exposure of the cells to bFGF. A short-term bFGF exposure reduces cell-to-cell communication as a mean of desynchronising FS cells. By contrast, long-term exposure to bFGF enhances cell-to-cell communication and facilitates coordination among FS cells.

Grass carp prolactin: molecular cloning, tissue expression, intrapituitary autoregulation by prolactin and paracrine regulation by growth hormone and luteinizing hormone

Prolactin (PRL), a pituitary hormone with diverse functions, is well-documented to be under the control of both hypothalamic and peripheral signals. Intrapituitary modulation of PRL expression via autocrine/paracrine mechanisms has also been reported, but similar information is still lacking in lower vertebrates. To shed light on autocrine/paracrine regulation of PRL in fish model, grass carp PRL was cloned and its expression in the carp pituitary has been confirmed. In grass carp pituitary cells, local secretion of PRL could suppress PRL release with concurrent rises in PRL production and mRNA levels. Paracrine stimulation by growth hormone (GH) was found to up- regulate PRL secretion, PRL production and PRL transcript expression, whereas the opposite was true for the local actions of luteinizing hormone (LH). Apparently, local interactions of PRL, GH and LH via autocrine/paracrine mechanisms could modify PRL production in carp pituitary cells through differential regulation of PRL mRNA stability and gene transcription.

Anterior pituitary cell networks

Both endocrine and non-endocrine cells of the pituitary gland are organized into structural and functional networks which are formed during embryonic development but which may be modified throughout life. Structural mapping of the various endocrine cell types has highlighted the existence of distinct network motifs and relationships with the vasculature which may relate to temporal differences in their output. Functional characterization of the network activity of growth hormone and prolactin cells has revealed a role for cell organization in gene regulation, the plasticity of pituitary hormone output and remarkably the ability to memorize altered demand. As such, the description of these endocrine cell networks alters the concept of the pituitary from a gland which simply responds to external regulation to that of an oscillator which may memorize information and constantly adapt its coordinated networks' responses to the flow of hypothalamic inputs.

Photoperiodic modulation of the suppressive actions of prolactin and dopamine on the pituitary gonadotropin responses to gonadotropin-releasing hormone in sheep

In a variety of species, the LH-secretory response to gonadotropin-releasing hormone (GnRH) is completely suppressed by the combined actions of prolactin (PRL) and dopamine (DA). In sheep, this effect is only observed under long days (nonbreeding season [NBS]). To investigate the level at which these mechanisms operate, we assessed the effects of PRL and bromocriptine (Br), a DA agonist, on the gonadotropin-secretory and mRNA responses to GnRH in pituitary cell cultures throughout the ovine annual reproductive cycle. As expected, the LH-secretory response to GnRH was only abolished during the NBS following combined PRL and Br application. Conversely, the LHB subunit response to GnRH was reduced during both the BS and NBS by the combined treatment and Br alone. Similar results were obtained in pars distalis-only cultures, indicating that the effects are pars tuberalis (PT)- independent. Further signaling studies revealed that PRL and Br alter the LH response to GnRH via convergence at the level of PLC and PKC. Results for FSH generally reflected those for LH, except during the BS where removal of the PT allowed PRL and Br to suppress the FSH-secretory response to GnRH. These data show that suppression of the LH-secretory response to GnRH by PRL and DA is accompanied by changes in mRNA synthesis, and that the photoperiodic modulation of this inhibition operates primarily at the level of LH release through alterations in PKC and PLC. Furthermore, the suppressive effects of PRL and DA on the secretion of FSH are photoperiodically regulated in a PT-dependent manner.

Coordination of calcium signals by pituitary endocrine cells in situ

The pulsatile secretion of hormones from the mammalian pituitary gland drives a wide range of homeostatic responses by dynamically altering the functional set-point of effector tissues. To accomplish this, endocrine cell populations residing within the intact pituitary display large-scale changes in coordinated calcium-spiking activity in response to various hypothalamic and peripheral inputs. Although the pituitary gland is structurally compartmentalized into specific and intermingled endocrine cell networks, providing a clear morphological basis for such coordinated activity, the mechanisms which facilitate the timely propagation of information between cells in situ remain largely unexplored. Therefore, the aim of the current review is to highlight the range of signalling modalities known to be employed by endocrine cells to coordinate intracellular calcium rises, and discuss how these mechanisms are integrated at the population level to orchestrate cell function and tissue output.

Connexin-dependent signaling in neuro-hormonal systems

The advent of multicellular organisms was accompanied by the development of short- and long-range chemical signalling systems, including those provided by the nervous and endocrine systems. In turn, the cells of these two systems have developed mechanisms for interacting with both adjacent and distant cells. With evolution, such mechanisms have diversified to become integrated in a complex regulatory network, whereby individual endocrine and neuro-endocrine cells sense the state of activity of their neighbors and, accordingly, regulate their own level of functioning. A consistent feature of this network is the expression of connexin-made channels between the (neuro)hormone-producing cells of all endocrine glands and secretory regions of the central nervous system so far investigated in vertebrates. This review summarizes the distribution of connexins in the mammalian (neuro)endocrine systems, and what we know about the participation of these proteins on hormone secretion, the life of the producing cells, and the action of (neuro)hormones on specific targets. The data gathered since the last reviews on the topic are summarized, with particular emphasis on the roles of Cx36 in the function of the insulin-producing beta cells of the endocrine pancreas, and of Cx40 in that of the renin-producing juxta-glomerular epithelioid cells of the kidney cortex. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Identification of estradiol/ERα-regulated genes in the mouse pituitary

Estrogen acts to prime the pituitary prior to the GnRH-induced LH surge by undiscovered mechanisms. This study aimed to identify the key components that mediate estrogen action in priming the pituitary. RNA extracted from the pituitaries of metestrous (low estrogen) and proestrus (high estrogen) stage mice, as well as from ovariectomized wild-type and estrogen receptor α (ERα) knockout mice treated with 17β-estradiol (E(2)) or vehicle, was used for gene expression microarray. Microarray data were then aggregated, built into a functional electronic database, and used for further characterization of E(2)/ERα-regulated genes. These data were used to compile a list of genes representing diverse biological pathways that are regulated by E(2) via an ERα-mediated pathway in the pituitary. This approach substantiates ERα regulation of membrane potential regulators and intracellular vesicle transporters, among others, but not the basic components of secretory machinery. Subsequent characterization of six selected genes (Cacna1a, Cacna1g, Cited1, Abep1, Opn3, and Kcne2) confirmed not only ERα dependency for their pituitary expression but also the significance of their expression in regulating GnRH-induced LH secretion. In conclusion, findings from this study suggest that estrogen primes the pituitary via ERα by equipping pituitary cells with critical cellular components that potentiate LH release on subsequent GnRH stimulations.

Intra-pituitary administration revisited: development of a novel in vivo approach to investigate the ovine hypophysis

The anterior pituitary gland regulates physiological processes via the secretion of hormones, which are under the control of factors produced either in the hypothalamus or the pituitary gland itself. Studies investigating how the pituitary gland functions have employed both in vitro and in vivo approaches. Although in vitro analysis has the advantage that it is pituitary specific, the results may be incomplete because the tissue is isolated from other physiological inputs that could affect function under natural conditions. Without vascular input, such studies are inherently of short duration. Conversely, in vivo experiments that rely upon systemic hormone injections require high doses, are non-target specific and the precise hormone concentrations reaching the pituitary gland are difficult to control. Intracerebroventricular hormone infusions are reliant on assumptions that factors are transported to the pituitary gland from the cerebrospinal fluid and are without cerebral effects. Here we describe an innovative method to investigate anterior pituitary function in conscious sheep by direct infusion of peptides into the pituitary tissue surrounding the hypophyseal portal blood vessels. This approach is an adaptation of the hypophyseal portal cannulation technique whereby an indwelling cannula provides direct access to the rostral aspect of the adenohypophysis. Peptide infusions were achieved by insertion of a needle through the implanted cannula such that it penetrated the pituitary. Using this technique, infusion of TRH (17 ng/1 μl/min for up to 6h) induced a sustained rise in systemic prolactin levels that lasted for the duration of the infusion.

Encoding and decoding photoperiod in the mammalian pars tuberalis

In mammals, the nocturnal melatonin signal is well established as a key hormonal indicator of seasonal changes in day-length, providing the brain with an internal representation of the external photoperiod. The pars tuberalis (PT) of the pituitary gland is the major site of expression of the G-coupled receptor MT1 in the brain and is considered as the main site of integration of the photoperiodic melatonin signal. Recent studies have revealed how the photoperiodic melatonin signal is encoded and conveyed by the PT to the brain and the pituitary, but much remains to be resolved. The development of new animal models and techniques such as cDNA arrays or high throughput sequencing has recently shed the light onto the regulatory networks that might be involved. This review considers the current understanding of the mechanisms driving photoperiodism in the mammalian PT with a particular focus on the seasonal prolactin secretion.

Pituitary network connectivity as a mechanism for the luteinising hormone surge

Ovulation in vertebrates is caused by a surge of luteinising hormone (LH) from the pituitary. The LH surge is initiated by rising oestradiol concentration, although the precise mechanism of oestradiol action in humans and primates is not yet understood. Recent advances in labelling and three-dimensional imaging have revealed a rich pituitary structure of interwoven networks of different cell types. In the present study, we develop a mathematical model to test the hypothesis that oestradiol modulation of connectivity between pituitary cells can underlie the LH surge. In the model, gonadotrophin-releasing hormone (GnRH) pulses stimulate LH secretion by two independent mechanisms. The first mechanism corresponds to the well known direct action of GnRH on gonadotrophs, which is inhibited by the rising oestradiol concentration. The second mechanism of GnRH action is to stimulate a recurrent network of pituitary cells; in this case, the folliculostellate cells, which in turn stimulate LH secretion from the gonadotrophs. The network activity is modelled by a one-dimensional ordinary differential equation. The key to the LH surge in the model lies in the assumption that oestradiol modulates network connectivity. When the circulating oestradiol concentration is low, the network is barely connected, and cannot maintain a recurrent signal. When the oestradiol concentration is high, the network is highly connected, and maintains a high level of activity even after GnRH stimulation, thereby leading to a surge of LH secretion.

Renal blood flow and function in the rabbit after surgical trauma. IV. Effects of ureteral ligation

After 1, 2, 4, 7, 14 and 21 days of ureteral occlusion, renal blood flow was measured on both sides by means of a dye-dilution method and the glomerular and tubular functions were evaluated by measuring the extraction of 51-Cr-EDTA and 125-I-Hippuran. The results showed an increase in the weight of the obstructed kidney parenchyma and indicated an incipient compensatory hypertrophy of the contralateral kidney after 14 days of occlusion. The blood flow to the occluded kidney was reduced on the first day after ligation and ranged in all experiments between 24% and 44% of the total renal blood flow, without correlation to the duration of stasis. The blood flow to the contralateral kidney was mostly above the average in the control group. A small glomerular and tubular extraction was noted in more than 50% of the ligated kidneys. The glomerular filtration rate and the clearance of Hippuran by the contralateral kidney were increased after one day of occlusion, but no relationship between the changes in renal function and the duration of ligation was revealed. It is concluded that ureteral ligation causes a decrease in the blood flow to the occluded kidney and an increase in that to the contralateral kidney. The function of the contralateral kidney increases initially, but thereafter only as a consequence of the compensatory hypertrophy which is evident around the 14th day post occlusion.