Thyrotropin releasing hormone: direct evidence for stimulation of prolactin production by pituitary cells in culture

Revealing Sexual Dimorphism in Prolactin Regulation From Early Postnatal Development to Adulthood in Murine Models

Serum prolactin (PRL) levels exhibit a gradual rise both in male and female rats from birth to adulthood, with females consistently displaying higher levels compared to age-matched males. This pattern has traditionally been attributed to the development and maturation of endocrine and neuroendocrine networks responsible for regulating PRL synthesis and secretion. However, the effect of dopamine (DA), which acts as an inhibitory factor on lactotroph function, also increases from birth to puberty, particularly in females. Nonetheless, the secretion of PRL remains higher in females compared to males. On the other hand, the observed sex differences in serum PRL levels during early postnatal development cannot be attributed to the influence of estradiol (E2). While serum E2 levels gradually increase after birth, only after 45 days of life do the disparities in E2 levels between females and males become evident. These observations collectively suggest that neither the maturation of hypothalamic DA regulation nor the rise in E2 levels can account for the progressive and sustained elevation in serum PRL levels and the observed sexual dimorphism during postnatal development. This review highlights the importance of recent discoveries in animal models that shed light on inhibitory mechanisms in the control of PRL secretion within the pituitary gland itself, that is intrapituitary mechanisms, with a specific emphasis on the role of transforming growth factor β1 and activins in PRL secretion.

The Thyroid Hormone Axis and Female Reproduction

Thyroid function affects multiple sites of the female hypothalamic-pituitary gonadal (HPG) axis. Disruption of thyroid function has been linked to reproductive dysfunction in women and is associated with menstrual irregularity, infertility, poor pregnancy outcomes, and gynecological conditions such as premature ovarian insufficiency and polycystic ovarian syndrome. Thus, the complex molecular interplay between hormones involved in thyroid and reproductive functions is further compounded by the association of certain common autoimmune states with disorders of the thyroid and the HPG axes. Furthermore, in prepartum and intrapartum states, even relatively minor disruptions have been shown to adversely impact maternal and fetal outcomes, with some differences of opinion in the management of these conditions. In this review, we provide readers with a foundational understanding of the physiology and pathophysiology of thyroid hormone interactions with the female HPG axis. We also share clinical insights into the management of thyroid dysfunction in reproductive-aged women.

A single nucleotide polymorphism of the thyrotropin releasing hormone degrading ectoenzyme (TRHDE) gene is associated with post-partum anestrus in Murrah buffalo

Thyrotropin releasing hormone degrading enzyme (TRHDE) gene is implicated in Thyrotropin releasing hormone (TRH) mediated prolactin secretion. It has been shown that the prolactin secretion alters the Gonadotropin-releasinghormone(GnRH) mediated estrous cycle. Therefore, TRHDE may also regulate postpartum anestrus. Earlier studies reported the role of non-synonymous single nucleotide polymorphism (SNPs) in various pathophysiological conditions by altering the structure and function of the proteins. Hence, in the present study, we identified SNPs in the putative promoter, first exon, middle exon and 3'-UTR containing the last exon of TRHDE gene and determined their association with postpartum anestrus (PPA) in Murrah buffaloes. We found one non synonymous SNP (G > C at 118095875 bp on chromosome 4) in the first exon of TRHDE and performed its association analysis in a population sample of 50 extreme PPA (residual PPAI: 123.06 ± 12.98 days) and 50 normal (residual PPAI: -80.46 ± 3.19 days) buffaloes. The residual PPAI value was the observed PPAI adjusted for the effect of 38 non-genetic factors. The analysis showed a significant (P < 0.004167) association of this SNP with PPA in buffaloes. Molecular dynamics simulations (MDS) also supported that the C allele altering Glutamine to Histidine at the amino acid 148 of TRHDE could enhance the stability and rigidity of TRHDE protein, which may lower its activity, increase TRH and prolactin, and reduce GnRH in PPA buffaloes. The MDS analysis further strengthens the association of the SNP (G > C) in the TRHDE gene with PPA condition in Murrah buffaloes. However, further investigation is needed to prove the MDS observations.

β-Arrestin2 Is Critically Involved in the Differential Regulation of Phosphosignaling Pathways by Thyrotropin-Releasing Hormone and Taltirelin

In recent years, thyrotropin-releasing hormone (TRH) and its analogs, including taltirelin (TAL), have demonstrated a range of effects on the central nervous system that represent potential therapeutic agents for the treatment of various neurological disorders, including neurodegenerative diseases. However, the molecular mechanisms of their actions remain poorly understood. In this study, we investigated phosphosignaling dynamics in pituitary GH1 cells affected by TRH and TAL and the putative role of β-arrestin2 in mediating these effects. Our results revealed widespread alterations in many phosphosignaling pathways involving signal transduction via small GTPases, MAP kinases, Ser/Thr- and Tyr-protein kinases, Wnt/β-catenin, and members of the Hippo pathway. The differential TRH- or TAL-induced phosphorylation of numerous proteins suggests that these ligands exhibit some degree of biased agonism at the TRH receptor. The different phosphorylation patterns induced by TRH or TAL in β-arrestin2-deficient cells suggest that the β-arrestin2 scaffold is a key factor determining phosphorylation events after TRH receptor activation. Our results suggest that compounds that modulate kinase and phosphatase activity can be considered as additional adjuvants to enhance the potential therapeutic value of TRH or TAL.

The effect of pergolide mesylate on adrenocorticotrophic hormone responses to exogenous thyrotropin releasing hormone in horses

Thyrotropin releasing hormone (TRH) stimulation testing is often used to support a diagnosis of pituitary pars intermedia dysfunction (PPID) in horses although it is unclear whether or not repeat TRH stimulation testing post-treatment is a valid means of assessing response to medical therapy. Laboratory submissions from 64 suspected equine PPID cases were examined including the initial pre-treatment TRH stimulation test and a follow up test within 100 days of starting medical therapy with pergolide. In a subset of cases, further follow-up tests were examined beyond 100 days of starting treatment. Results from tests conducted between 1 July and 30 November 30 were excluded. Significant improvements were seen in both the baseline and TRH-stimulated adrenocorticotrophic hormone (ACTH) concentrations within 100 days with no further improvements seen in the subset of cases examined thereafter. Although 88% (n=56/64) of all cases showed a decreased response to TRH post-treatment, only 24% (n=9/38) of horses with positive pre-treatment TRH stimulation tests normalised following treatment, with a further 34% (n=13/38) improving into an equivocal test outcome category. Most commonly (42%; n=16/38), horses with positive pre-treatment TRH stimulation tests remained positive following treatment, although 75% (n=12/16) of these showed a numerically lower post-treatment response to TRH. These results will help inform practitioners of expected changes in TRH stimulation test results when assessing response of horses with PPID to medical therapy with pergolide.

A New Perspective on Thyroid Hormones: Crosstalk with Reproductive Hormones in Females

Accumulating evidence has shown that thyroid hormones (THs) are vital for female reproductive system homeostasis. THs regulate the reproductive functions through thyroid hormone receptors (THRs)-mediated genomic- and integrin-receptor-associated nongenomic mechanisms, depending on TH ligand status and DNA level, as well as transcription and extra-nuclear signaling transduction activities. These processes involve the binding of THs to intracellular THRs and steroid hormone receptors or membrane receptors and the recruitment of hormone-response elements. In addition, THs and other reproductive hormones can activate common signaling pathways due to their structural similarity and shared DNA consensus sequences among thyroid, peptide, and protein hormones and their receptors, thus constituting a complex and reciprocal interaction network. Moreover, THs not only indirectly affect the synthesis, secretion, and action of reproductive hormones, but are also regulated by these hormones at the same time. This crosstalk may be one of the pivotal factors regulating female reproductive behavior and hormone-related diseases, including tumors. Elucidating the interaction mechanism among the aforementioned hormones will contribute to apprehending the etiology of female reproductive diseases, shedding new light on the treatment of gynecological disorders.

The Hypothalamic Paraventricular Nucleus Is the Center of the Hypothalamic-Pituitary-Thyroid Axis for Regulating Thyroid Hormone Levels

Background: Thyrotropin-releasing hormone (TRH) was the first hypothalamic hormone isolated that stimulates pituitary thyrotropin (TSH) secretion. TRH was also later found to be a stimulator of pituitary prolactin and distributed throughout the brain, gastrointestinal tract, and pancreatic β cells. We previously reported the development of TRH null mice (conventional TRHKO), which exhibit characteristic tertiary hypothyroidism and impaired glucose tolerance due to insufficient insulin secretion. Although in the past five decades many investigators, us included, have attempted to determine the hypothalamic nucleus responsible for the hypothalamic-pituitary-thyroid (HPT) axis, it remained obscure because of the broad expression of TRH. Methods: To determine the hypothalamic region functionally responsible for the HPT axis, we established paraventricular nucleus (PVN)-specific TRH knockout (PVN-TRHKO) mice by mating Trh floxed mice and single-minded homolog 1 (Sim1)-Cre transgenic mice. We originally confirmed that most Sim1 was expressed in the PVN using Sim1-Cre/tdTomato mice. Results: These PVN-TRHKO mice exhibited tertiary hypothyroidism similar to conventional TRHKO mice; however, they did not show the impaired glucose tolerance observed in the latter, suggesting that TRH from non-PVN sources is essential for glucose regulation. In addition, a severe reduction in prolactin expression was observed in the pituitary of PVN-TRHKO mice compared with that in TRHKO mice. Conclusions: These findings are conclusive evidence that the PVN is the center of the HPT axis for regulation of serum levels of thyroid hormones and that the serum TSH levels are not decreased in tertiary hypothyroidism. We also noted that TRH from the PVN regulated prolactin, whereas TRH from non-PVN sources regulated glucose metabolism.

Thyrotropin-releasing hormone axonal varicosities appear to innervate dopaminergic neurons in the human hypothalamus

Thyrotropin-releasing hormone (TRH) has a critical role in the central regulation of thyroid-stimulating hormone (TSH) from the anterior pituitary, and subsequently, thyroid hormone secretion from the thyroid gland. In addition to its role in the regulation of HPT axis, TRH is a potent regulator of prolactin (PRL) secretion by stimulating PRL secretion either directly from lactotrophs or indirectly via its action on the tuberoinfundibular dopamine (TIDA) neurons. In rodents, the TRH neurons which regulate TSH and thyroid hormone secretion, called hypophysiotropic TRH neurons, are in the medial subdivision of the parvicellular paraventricular nucleus (PVN). In humans, the PVN also contains a large population of TRH neurons, especially in its medial part, but the location of hypophysiotropic TRH neurons is not yet known. In addition to regulating TSH and PRL secretion, TRH also functions as a neurotransmitter/neuromodulator. In rodents and teleosts, TRH axons densely innervate TIDA neurons to inhibit tyrosine hydroxylase (TH) biosynthesis, neuronal firing, and dopamine turnover which may contribute to increasing PRL secretion. No such connections have been reported in humans, although dopaminergic neurons express TRH receptors and TRH also regulates PRL secretion. The objectives of this study were to map TRH-IR and TH-IR structures in the human hypothalamus with single-label light microscopic immunocytochemistry and study their interaction with double-label light microscopic immunocytochemistry. We show that TRH-IR nerve terminals densely surround TH-IR neurons (perikarya and dendrites) in the infundibulum of the human hypothalamus. The micrographs illustrating these juxtapositions were taken by Olympus BX45 microscope equipped with a digital camera and with 100X oil immersion objective. Composite images were created from the consecutive micrographs if the neurons were larger than the frame of the camera, using Adobe Photoshop software. As no gaps between TRH-IR and TH-IR elements were seen, these contacts may be functional synapses by which TRH regulates the activity of dopaminergic neurons and subsequently TSH and PRL secretion.

Interrelationships Between Pituitary Hormones as Assessed From 24-hour Serum Concentrations in Healthy Older Subjects

CONTEXT

Hormones of the hypothalamic-pituitary-target gland axes are mostly investigated separately, whereas the interplay between hormones might be as important as each separate hormonal axis.

OBJECTIVE

Our aim is to determine the interrelationships between GH, TSH, ACTH, and cortisol in healthy older individuals.

DESIGN

We made use of 24-hour hormone serum concentrations assessed with intervals of 10 minutes from 38 healthy older individuals with a mean age (SD) of 65.1 (5.1) years from the Leiden Longevity Study. Cross-correlation analyses were performed to assess the relative strength between 2 24-hour hormone serum concentration series for all possible time shifts. Cross-approximate entropy was used to assess pattern synchronicity between 2 24-hour hormone serum concentration series.

RESULTS

Within an interlinked hormonal axis, ACTH and cortisol were positively correlated with a mean (95% confidence interval) correlation coefficient of 0.78 (0.74-0.81) with cortisol following ACTH concentrations with a delay of 10 minutes. Between different hormonal axes, we observed a negative correlation coefficient between cortisol and TSH of -0.30 (-0.36 to -0.25) with TSH following cortisol concentrations with a delay of 170 minutes. Furthermore, a positive mean (95% confidence interval) correlation coefficient of 0.29 (0.22-0.37) was found between TSH and GH concentrations without any delay. Moreover, cross-approximate entropy analyses showed that GH and cortisol exhibit synchronous serum concentration patterns.

CONCLUSIONS

This study demonstrates that interrelations between hormones from interlinked as well as different hypothalamic-pituitary-target gland axes are observed in healthy older individuals. More research is needed to determine the biological meaning and clinical consequences of these observations.

Effect of thyroxin on cell morphology and hormone secretion of pituitary grafts in rats

INTRODUCTION

Growth hormone and prolactin secretion is affected by thyroid hormones. To see if this influence is subsidiary to the hyptothalamus, we investigated the effects of thyroxin (T4) on hormone secretion and histology of sellar pituitaries and pituitary grafts detached from the hypothalamus (autografted or allografted under the kidney capsule).

MATERIALS AND METHODS

Male Wistar rats were divided into eight groups: control, thyroidectomised, pituitary autografted, pituitary allografted, and four additional groups that were injected with T4 for two weeks, starting four weeks after surgery. At sacrifice, adenohypophysial hormone blood levels were assessed, and tissue from sellar and grafted pituitaries were investigated by histology and electron microscopy.

RESULTS

Growth hormone and prolactin blood levels, as well as the number of growth hormone immunopositive cells increased in T4-treated groups. Both pituitary auto- and allo-grafts showed lactotroph hyperplasia and displayed spongiform areas containing cells with vesicles in their cytoplasm resembling thyroidectomy cells. This phenomenon was minimized in their respective T4-treated group. Thyroidectomy cells were identified in pituitary grafts, indicating that hypothalamic control was not essential to induce them.

DISCUSSION AND CONCLUSION

It is intriguing that the pituitary allografted group, even maintaining normal T4 blood levels, developed thyroidectomy cells in their grafts, suggesting that a long- term deficit of vascularization (>4 weeks) prevented T4 from reaching the graft. After 6 weeks, post T4 treatment of two weeks seemed to be the determining factor to minimize thyroidectomy cells in both pituitary autografted + T4 and pituitary allografted + T4 grafts compared to the untreated groups, although more time and/or higher T4 doses may be required to fully restore the euthyroid morphology.

Characterization of a novel thyrotropin-releasing hormone receptor, TRHR3, in chickens

The physiological roles of thyrotropin-releasing hormone (TRH) are proposed to be mediated by TRH receptors (TRHR), which have been divided into 3 subtypes, namely, TRHR1, TRHR2, and TRHR3, in vertebrates. Although 2 TRH receptors (TRHR1 and TRHR3) have been predicted to exist in birds, it remains unclear whether TRHR3 is a functional TRH receptor similar to TRHR1. Here, we reported the functionality and tissue expression of TRHR3 in chickens. The cloned chicken TRHR3 (cTRHR3) encodes a receptor of 387 amino acids, which shares high-amino-acid identities (63–80%) to TRHR3 of parrots, lizards, Xenopus tropicalis, and tilapia and comparatively lower sequence identities to chicken TRHR1 or mouse TRHR2. Using cell-based luciferase reporter assays and Western blot, we demonstrated that similar to chicken TRHR1 (cTRHR1), cTRHR3 expressed in HEK 293 cells can be potently activated by TRH and that its activation stimulates multiple signaling pathways, indicating both TRH receptors are functional. Quantitative real-time PCR revealed that cTRHR1 and cTRHR3 are widely, but differentially, expressed in chicken tissues, and their expression is likely controlled by promoters located upstream of exon 1, which display strong promoter activities in cultured DF-1 cells. cTRHR1 is highly expressed in the anterior pituitary and testes, while cTRHR3 is highly expressed in the muscle, testes, fat, pituitary, spinal cord, and many brain regions (including hypothalamus). These findings indicate that TRH actions are likely mediated by 2 TRH receptors in chickens. In conclusion, our data provide the first piece of evidence that both cTRHR3 and cTRHR1 are functional TRH receptors, which helps to elucidate the physiological roles of TRH in birds.

Physiological and Pathological Role of Circadian Hormones in Osteoarthritis: Dose-Dependent or Time-Dependent?

Osteoarthritis (OA), the most common form of arthritis, may be triggered by improper secretion of circadian clock-regulated hormones, such as melatonin, thyroid-stimulating hormone (TSH), or cortisol. The imbalance of these hormones alters the expression of pro-inflammatory cytokines and cartilage degenerative enzymes in articular cartilage, resulting in cartilage erosion, synovial inflammation, and osteophyte formation, the major hallmarks of OA. In this review, we summarize the effects of circadian melatonin, TSH, and cortisol on OA, focusing on how different levels of these hormones affect OA pathogenesis and recovery with respect to the circadian clock. We also highlight the effects of melatonin, TSH, and cortisol at different concentrations both in vivo and in vitro, which may help to elucidate the relationship between circadian hormones and OA.

Forms of selenium in vitamin-mineral mixes differentially affect the expression of genes responsible for prolactin, ACTH, and α-MSH synthesis and mitochondrial dysfunction in pituitaries of steers grazing endophyte-infected tall fescue

The goal of this study was to test the hypothesis that sodium selenite (inorganic Se, ISe), SEL-PLEX (organic forms of Se, OSe), vs. a 1:1 blend (MIX) of ISe and OSe in a basal vitamin-mineral (VM) mix would differentially alter pituitary transcriptome profiles in growing beef steers grazing an endophyte-infected tall fescue (E+) pasture. Predominately Angus steers (BW = 183 ± 34 kg) were randomly selected from fall-calving cows grazing E+ pasture and consuming VM mixes that contained 35 ppm Se as ISe, OSe, or MIX forms. Steers were weaned, depleted of Se for 98 d, and subjected to summer-long common grazing of a 10.1 ha E+ pasture containing 0.51 ppm ergot alkaloids. Steers were assigned (n = 8 per treatment) to the same Se-form treatments on which they were raised. Selenium treatments were administered by daily top-dressing 85 g of VM mix onto 0.23 kg soyhulls, using in-pasture Calan gates. As previously reported, serum prolactin was greater for MIX (52%) and OSe (59%) steers vs. ISe. Pituitaries were collected at slaughter and changes in global and selected mRNA expression patterns determined by microarray and real-time reverse transcription PCR analyses, respectively. The effects of Se treatment on relative gene expression were subjected to one-way ANOVA. The form of Se affected the expression of 542 annotated genes (P < 0.005). Integrated pathway analysis found a canonical pathway network between prolactin and pro-opiomelanocortin (POMC)/ACTH/α-melanocyte-stimulating hormone (α-MSH) synthesis-related proteins and that mitochondrial dysfunction was a top-affected canonical pathway. Targeted reverse transcription-PCR analysis found that the relative abundance of mRNA encoding prolactin and POMC/ACTH/α-MSH synthesis-related proteins was affected (P < 0.05) by the form of Se, as were (P ≤ 0.05) mitochondrial dysfunction-related proteins (CYB5A, FURIN, GPX4, and PSENEN). OSe steers appeared to have a greater prolactin synthesis capacity (more PRL mRNA) vs. ISe steers through decreased dopamine type two receptor signaling (more DRD2 mRNA), whereas MIX steers had a greater prolactin synthesis capacity (more PRL mRNA) and release potential by increasing thyrotropin-releasing hormone concentrations (less TRH receptor mRNA) than ISe steers. OSe steers also had a greater ACTH and α-MSH synthesis potential (more POMC, PCSK2, CPE, and PAM mRNA) than ISe steers. We conclude that form of Se in VM mixes altered expression of genes responsible for prolactin and POMC/ACTH/α-MSH synthesis, and mitochondrial function, in pituitaries of growing beef steers subjected to summer-long grazing an E+ pasture.

Possible involvement of thyrotropin-releasing hormone receptor 3 in the release of prolactin in the metamorphosing bullfrog larvae

In amphibians, thyrotropin (TSH), corticotropin (ACTH) and prolactin (PRL) are regarded as the major pituitary hormones involved in metamorphosis, their releasing factors being corticotropin-releasing factor (CRF), arginine vasotocin (AVT), and thyrotropin-releasing hormone (TRH), respectively. It is also known that thyrotropes and corticotropes are equipped with CRF type-2 receptor and AVT V1b receptor, respectively. As for PRL cells, information about the type of receptor for TRH (TRHR) through which the action of TRH is mediated to induce the release of PRL is lacking. In order to fill this gap, an attempt was made to characterize the TRHR subtype existing in the PRL cells of the anterior pituitary gland of the bullfrog, Rana catesbeiana. We cloned cDNAs for three types of bullfrog TRHRs, namely TRHR1, TRHR2 and TRHR3, and confirmed that all of them are functional receptors for TRH by means of reporter gene assay. Analyses with semi-quantitative reverse transcription-PCR and in situ hybridization revealed that TRHR3 mRNA is expressed in the anterior lobe and that the signals reside mostly in the PRL cells. It was also noted that the expression levels of TRHR3 mRNA in the anterior pituitary as well as in the PRL cells of metamorphosing tadpoles elevate as metamorphosis progresses. Since the pattern of changes in TRHR3 mRNA levels in the larval pituitary is almost similar to that previously observed in the pituitary PRL mRNA and plasma PRL levels, we provide a view that TRHR3 mediates the action of TRH on the PRL cells to induce the release of PRL that is prerequisite for growth and metamorphosis in amphibians.

Role of pituitary adenylate cyclase-activating polypeptide in modulating hypothalamic-pituitary system

BACKGROUND

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a multifunctional peptide that is isolated and identified from the ovine hypothalamus, whose effects and mechanisms have been elucidated in numerous studies. The PACAP and its receptor are widely expressed, not only in the hypothalamus but also in peripheral organs.

METHODS

The studies on the role of PACAP in the hypothalamic-pituitary system, including those by the authors, were summarized.

RESULTS

In the pituitary gonadotrophs, PACAP increases the gonadotrophin α-, luteinizing hormoneβ-, and follicle-stimulating hormone β-subunit expression and the expression of gonadotropin-releasing hormone (GnRH) receptor and its own receptor, PAC1R. Moreover, a low-frequency GnRH pulse increases the expression of PACAP and PAC1R more than a high-frequency GnRH pulse in the gonadotrophs. The PACAP stimulates prolactin synthesis and secretion and increases PAC1R in the lactotrophs. In the hypothalamus, PACAP increases the expression of the GnRH receptors, although it is unable to increase the expression of GnRH in the GnRH-producing neurons.

CONCLUSION

The PACAP not only acts directly in each hormone-producing cell, it possibly might regulate hormone synthesis via the expression of its own receptors or those of other hormones.

Exploring the Sea Urchin Neuropeptide Landscape by Mass Spectrometry

Neuropeptides are essential cell-to-cell signaling messengers and serve important regulatory roles in animals. Although remarkable progress has been made in peptide identification across the Metazoa, for some phyla such as Echinodermata, limited neuropeptides are known and even fewer have been verified on the protein level. We employed peptidomic approaches using bioinformatics and mass spectrometry (MS) to experimentally confirm 23 prohormones and to characterize a new prohormone in nervous system tissue from Strongylocentrotus purpuratus, the purple sea urchin. Ninety-three distinct peptides from known and novel prohormones were detected with MS from extracts of the radial nerves, many of which are reported or experimentally confirmed here for the first time, representing a large-scale study of neuropeptides from the phylum Echinodermata. Many of the identified peptides and their precursor proteins have low homology to known prohormones from other species/phyla and are unique to the sea urchin. By pairing bioinformatics with MS, the capacity to characterize novel peptides and annotate prohormone genes is enhanced. Graphical Abstract.

Cyclic 3',5'-adenosine monophosphate (cAMP) signaling in the anterior pituitary gland in health and disease

The cyclic 3',5'-adenosine monophosphate (cAMP) was the first among the so-called "second messengers" to be described. It is conserved in most organisms and functions as a signal transducer by mediating the intracellular effects of multiple hormones and neurotransmitters. In this review, we first delineate how different members of the cAMP pathway ensure its correct compartmentalization and activity, mediate the terminal intracellular effects, and allow the crosstalk with other signaling pathways. We then focus on the pituitary gland, where cAMP exerts a crucial function by controlling the responsiveness of the cells to hypothalamic hormones, neurotransmitters and peripheral factors. We discuss the most relevant physiological functions mediated by cAMP in the different pituitary cell types, and summarize the defects affecting this pathway that have been reported in the literature. We finally discuss how a deregulated cAMP pathway is involved in the pathogenesis of pituitary disorders and how it affects the response to therapy.

Interactions between Two Different G Protein-Coupled Receptors in Reproductive Hormone-Producing Cells: The Role of PACAP and Its Receptor PAC1R

Gonadotropin-releasing hormone (GnRH) and gonadotropins are indispensable hormones for maintaining female reproductive functions. In a similar manner to other endocrine hormones, GnRH and gonadotropins are controlled by their principle regulators. Although it has been previously established that GnRH regulates the synthesis and secretion of luteinizing hormone (LH) and follicle-stimulating hormone (FSH)—both gonadotropins—from pituitary gonadotrophs, it has recently become clear that hypothalamic GnRH is under the control of hypothalamic kisspeptin. Prolactin, which is also known as luteotropic hormone and is released from pituitary lactotrophs, stimulates milk production in mammals. Prolactin is also regulated by hypothalamic factors, and it is thought that prolactin synthesis and release are principally under inhibitory control by dopamine through the dopamine D2 receptor. In addition, although it remains unknown whether it is a physiological regulator, thyrotropin-releasing hormone (TRH) is a strong secretagogue for prolactin. Thus, GnRH, LH and FSH, and prolactin are mainly regulated by hypothalamic kisspeptin, GnRH, and TRH, respectively. However, the synthesis and release of these hormones is also modulated by other neuropeptides in the hypothalamus. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a hypothalamic peptide that was first isolated from sheep hypothalamic extracts based on its ability to stimulate cAMP production in anterior pituitary cells. PACAP acts on GnRH neurons and pituitary gonadotrophs and lactotrophs, resulting in the modulation of their hormone producing/secreting functions. Furthermore, the presence of the PACAP type 1 receptor (PAC1R) has been demonstrated in these cells. We have examined how PACAP and PAC1R affect GnRH- and pituitary hormone-secreting cells and interact with their principle regulators. In this review, we describe our understanding of the role of PACAP and PAC1R in the regulation of GnRH neurons, gonadotrophs, and lactotrophs, which are regulated mainly by kisspeptin, GnRH, and TRH, respectively.

Role of thyrotropin-releasing hormone in prolactin-producing cell models

Thyrotropin-releasing hormone (TRH) is a hypothalamic hypophysiotropic neuropeptide that was named for its ability to stimulate the release of thyroid-stimulating hormone in mammals. It later became apparent that it exerts a number of species-dependent hypophysiotropic activities that regulate other pituitary hormones. TRH also regulates the synthesis and release of prolactin, although whether it is a physiological regulator of prolactin that remains unclear. Occupation of the Gq protein-coupled TRH receptor in the prolactin-producing lactotroph increases the turnover of inositol, which in turn activates the protein kinase C pathway and the release of Ca(2+) from storage sites. TRH-induced signaling events also include the activation of extracellular signal-regulated kinase (ERK) and induction of MAP kinase phosphatase, an inactivator of activated ERK. TRH stimulates prolactin synthesis through the activation of ERK, whereas prolactin release occurs via elevation of intracellular Ca(2+). We have been investigating the role of TRH in a pituitary prolactin-producing cell model. Rat pituitary somatolactotroph GH3 cells, which produce and release both prolactin and growth hormone (GH), are widely used as a model for the study of prolactin- and GH-secreting cells. In this review, we describe the general action of TRH as a hypophysiotropic factor in vertebrates and focus on the role of TRH in prolactin synthesis using GH3 cells.

60 YEARS OF NEUROENDOCRINOLOGY: TRH, the first hypophysiotropic releasing hormone isolated: control of the pituitary-thyroid axis

This review presents the findings that led to the discovery of TRH and the understanding of the central mechanisms which control hypothalamus-pituitary-thyroid axis (HPT) activity. The earliest studies on thyroid physiology are now dated a century ago when basal metabolic rate was associated with thyroid status. It took over 50 years to identify the key elements involved in the HPT axis. Thyroid hormones (TH: T4 and T3) were characterized first, followed by the semi-purification of TSH whose later characterization paralleled that of TRH. Studies on the effects of TH became possible with the availability of synthetic hormones. DNA recombinant techniques facilitated the identification of all the elements involved in the HPT axis, including their mode of regulation. Hypophysiotropic TRH neurons, which control the pituitary-thyroid axis, were identified among other hypothalamic neurons which express TRH. Three different deiodinases were recognized in various tissues, as well as their involvement in cell-specific modulation of T3 concentration. The role of tanycytes in setting TRH levels due to the activity of deiodinase type 2 and the TRH-degrading ectoenzyme was unraveled. TH-feedback effects occur at different levels, including TRH and TSH synthesis and release, deiodinase activity, pituitary TRH-receptor and TRH degradation. The activity of TRH neurons is regulated by nutritional status through neurons of the arcuate nucleus, which sense metabolic signals such as circulating leptin levels. Trh expression and the HPT axis are activated by energy demanding situations, such as cold and exercise, whereas it is inhibited by negative energy balance situations such as fasting, inflammation or chronic stress. New approaches are being used to understand the activity of TRHergic neurons within metabolic circuits.

TIDAL WAVES: Network mechanisms in the neuroendocrine control of prolactin release

Neuroendocrine tuberoinfundibular dopamine (TIDA) neurons tonically inhibit pituitary release of the hormone, prolactin. Through the powerful actions of prolactin in promoting lactation and maternal behaviour while suppressing sexual drive and fertility, TIDA neurons play a key role in reproduction. We summarize insights from recent in vitro studies into the membrane properties and network behaviour of TIDA neurons including the observations that TIDA neurons exhibit a robust oscillation that is synchronized between cells and depends on intact gap junction communication. Comparisons are made with phasic firing patterns in other neuronal populations. Modulators involved in the control of lactation - including serotonin, thyrotropin-releasing hormone and prolactin itself - have been shown to change the electrical behaviour of TIDA cells. We propose that TIDA discharge mode may play a central role in tuning the amount of dopamine delivered to the pituitary and hence circulating prolactin concentrations in different reproductive states and pathological conditions.

Pharmacological overview of galactogogues

Galactogogues are substances used to induce, maintain, and increase milk production, both in human clinical conditions (like noninfectious agalactias and hypogalactias) and in massification of production in the animal dairy industry. This paper aims to report the state of the art on the possible mechanisms of action, effectiveness, and side effects of galactogogues, including potential uses in veterinary and human medicine. The knowledge gaps in veterinary clinical practice use of galactogogues, especially in the standardization of the lactogenic dose in some pure drugs and herbal preparations, are reviewed.

Tissue-specific alterations in thyroid hormone homeostasis in combined Mct10 and Mct8 deficiency

The monocarboxylate transporter Mct10 (Slc16a10; T-type amino acid transporter) facilitates the cellular transport of thyroid hormone (TH) and shows an overlapping expression with the well-established TH transporter Mct8. Because Mct8 deficiency is associated with distinct tissue-specific alterations in TH transport and metabolism, we speculated that Mct10 inactivation may compromise the tissue-specific TH homeostasis as well. However, analysis of Mct10 knockout (ko) mice revealed normal serum TH levels and tissue TH content in contrast to Mct8 ko mice that are characterized by high serum T3, low serum T4, decreased brain TH content, and increased tissue TH concentrations in the liver, kidneys, and thyroid gland. Surprisingly, mice deficient in both TH transporters (Mct10/Mct8 double knockout [dko] mice) showed normal serum T4 levels in the presence of elevated serum T3, indicating that the additional inactivation of Mct10 partially rescues the phenotype of Mct8 ko mice. As a consequence of the normal serum T4, brain T4 content and hypothalamic TRH expression were found to be normalized in the Mct10/Mct8 dko mice. In contrast, the hyperthyroid situation in liver, kidneys, and thyroid gland of Mct8 ko mice was even more severe in Mct10/Mct8 dko animals, suggesting that in these organs, both transporters contribute to the TH efflux. In summary, our data indicate that Mct10 indeed participates in tissue-specific TH transport and also contributes to the generation of the unusual serum TH profile characteristic for Mct8 deficiency.

Evidence that hyperprolactinaemia is associated with ovarian acyclicity in female zoo African elephants

African elephants of reproductive age in zoos are experiencing high rates of ovarian cycle problems (>40%) and low reproductive success. Previously, our laboratory found that 1/3 of acyclic females exhibit hyperprolactinaemia, a likely cause of ovarian dysfunction. This follow-up study re-examined hyperprolactinaemia in African elephants and found the problem has increased significantly to 71% of acyclic females. Circulating serum progestagens and prolactin were analysed in 31 normal cycling, 13 irregular cycling and 31 acyclic elephants for 12 months. In acyclic females, overall mean prolactin concentrations differed from cycling females (P < 0.05), with concentrations being either higher (n = 22; 54.90 ± 13.31 ng mL–1) or lower (n = 9; 6.47 ± 1.73 ng mL–1) than normal. No temporal patterns of prolactin secretion were evident in elephants that lacked progestagen cycles. In cycling females, prolactin was secreted in a cyclical manner, with higher concentrations observed during nonluteal (34.38 ± 1.77 and 32.75 ± 2.61 ng mL–1) than luteal (10.51 ± 0.30 and 9.67 ± 0.42 ng mL–1) phases for normal and irregular females, respectively. Of most concern was that over two-thirds of acyclic females now are hyperprolactinemic, a dramatic increase over that observed 7 years earlier. Furthermore, females of reproductive age constituted 45% of elephants with hyperprolactinaemia. Until the cause of this problem is identified and a treatment is developed, reproductive rates will remain suboptimal and the population nonsustaining.

Secondary amenorrhea in a woman with spinocerebellar degeneration treated with thyrotropin-releasing hormone: a case report and in vitro analysis

Introduction

While thyrotropin-releasing hormone is known to be a prolactin-release stimulating factor, thyrotropin-releasing hormone-tartrate and its derivative, taltirelin hydrate, are used for the treatment of spinocerebellar degeneration, a degenerative disease characterized mainly by motor ataxia. We report the case of a patient being treated with a thyrotropin-releasing hormone preparation for spinocerebellar degeneration who developed amenorrhea after a dose increase. Her hormonal background was analyzed and the effect of prolonged exposure to thyrotropin-releasing hormone on pituitary prolactin-producing cells was examined in vitro.

Case presentation

Our patient was a 36-year-old Japanese woman who experienced worsening of gait disturbance at around 23 years of age, and was subsequently diagnosed as having spinocerebellar degeneration. She had been treated with thyrotropin-releasing hormone-tartrate for four years. Taltirelin hydrate was added to the treatment seven months prior to her presentation, followed by an improvement in gait disturbance. Around the same period, she started lactating and subsequently developed amenorrhea three months later. Taltirelin hydrate was discontinued and she was referred to our hospital. She was found to have normal sex hormone levels. A thyrotropin-releasing hormone provocation test showed a normal response of thyroid-stimulating hormone level and an over-response of prolactin at 30 minutes (142.7 ng/mL). Resumption of menstruation was noted three months after dose reduction of thyrotropin-releasing hormone. In our in vitro study, following long-term exposure to thyrotropin-releasing hormone, cells from the rat pituitary prolactin-producing cell line GH3 exhibited an increased basal prolactin promoter activity but showed a marked decrease in responsiveness to thyrotropin-releasing hormone.

Conclusions

Physicians should be aware of hyperprolactinemia-associated side effects in patients receiving thyrotropin-releasing hormone treatment. Long-term treatment with a thyrotropin-releasing hormone preparation might cause a large amount of prolactin to accumulate in prolactin-producing cells and be released in response to exogenous thyrotropin-releasing hormone stimulation.

Elevated prolactin serum levels and history of alcohol withdrawal seizures

BACKGROUND

Prolactin has been discussed to be useful for differential diagnosis in epilepsia. Aim of the present study was to investigate the association between prolactin serum levels and previous alcohol withdrawal seizures.

METHODS

We assessed 118 male patients admitted for detoxification treatment. Previous withdrawal seizures were recorded and prolactin serum levels were measured using an enzymatic immunoassay.

RESULTS

Patients with a history of alcohol withdrawal seizures had significantly higher prolactin levels (17.8 ng/ml, SD=12.1) than patients without previous seizures (13.0 ng/ml, SD=8.1, p<0.05). Logistic regression revealed significant predictive qualities for prolactin serum levels (B=0.05, Wald=5.30, p=0.021, OR=1.06, 95%CI=1.01-1.11).

CONCLUSIONS

The present findings show an association between elevated prolactin serum levels and a history of withdrawal seizures. Hence, the results suggest that prolactin elevation at admission may be a clinical marker for an increased risk of withdrawal seizures.

Cocaine- and amphetamine-regulated transcript (CART) expression is differentially regulated in the hypothalamic paraventricular nucleus of lactating rats exposed to suckling or cold stimulation

Neural stimuli, such as suckling or cold exposure, increase TRH mRNA in the paraventricular nucleus (PVN) of the rat hypothalamus, yet only suckling induces prolactin secretion. As TRH co-localizes with cocaine- and amphetamine-regulated transcript (CART) in hypophysiotropic neurons of the PVN, and CART inhibits TRH-induced prolactin release but not TRH-induced TSH release in adenohypophyseal cell cultures, we raised the possibility that differential regulation of CART gene expression in the PVN may explain the differences in prolactin secretion following each of the two stimuli. Primiparous female rats were mated and handled daily during the pre- and postpartum periods. After delivery, the litter was adjusted to 8 pups and at mid-lactation, dams were separated from their pups for 8 h and exposed to either 1 h of cold or 30 min of suckling. Long-term effects of suckling were studied by separating pups from their mothers for 24 h, followed by a 12 h period of continuous suckling. Serum TSH levels increased in response to cold exposure, while prolactin levels were increased by suckling and diminished by cold exposure. CART mRNA levels increased in rostral and mid parts of the medial parvocellular PVN following cold exposure but not after suckling stimulation. These data demonstrate a differential regulation of CART gene expression in hypophysiotropic neurons in response to stimuli that increase TRH mRNA levels, and suggest that CART activation in the PVN may contribute to the decrease in PRL release when the thyroid axis is activated by cold exposure.

Prolactin serum levels and alcohol craving - an analysis using Lesch's typology

BACKGROUND

Prolactin secretion is closely connected to dopaminergic transmission that is known to play a crucial role in mediating reinforcement and craving in alcoholism.

OBJECTIVES

The study was performed to analyze the association between prolactin serum levels and alcohol craving during withdrawal differentiating alcohol-dependent patients using Lesch's typology.

METHODS

We assessed 115 male patients with the Obsessive Compulsive Drinking Scale at early alcohol withdrawal. In addition, serum was obtained to measure prolactin concentration and the patients were classified according to Lesch's typology into one of four subgroups.

RESULTS

Correlation analysis showed a significant association between prolactin serum levels and the extent of craving in Lesch's type 2 patients (r=0.32, p=0.015; n=57); however, no association was found for any other subgroup. The results were confirmed comparing patients with low and high craving (Mann-Whitney U test: Z=-2.805, p=0.005).

CONCLUSIONS

In patients of Lesch's type 2, who are characterized to suffer from anxiety and to use alcohol because of its anxiolytic effects, prolactin is associated with craving during early alcohol withdrawal.

Prolactin secretion in mice with thyrotropin-releasing hormone deficiency

The physiological roles of TRH in pituitary lactotrophs, particularly during lactation, remain unclear. We studied the prolactin (PRL) status, including serum PRL and PRL mRNA levels in the pituitary, in nonlactating and lactating TRH-deficient (TRH(-/-)) mice with a rescue study with thyroid hormone and TRH. We found that, as reported previously for male TRH(-/-) mice, neither the morphology of the lactotrophs, PRL content in the pituitary, nor the serum PRL concentration was changed in nonlactating female TRH(-/-) mice. However, concurrent hypothyroidism induced a mild decrease in the PRL mRNA level. In contrast, during lactation, the serum PRL level in TRH(-/-) mice was significantly reduced to about 60% of the level in wild-type mice, and this was reversed by prolonged TRH administration, but not by thyroid hormone replacement. The PRL content and PRL mRNA level in the mutant pituitary during lactation were significantly lower than those in wild-type mice, and these reductions were reversed completely by TRH administration, but only partially by thyroid hormone replacement. Despite the low PRL levels, TRH(-/-) dams were fertile, and the nourished pups exhibited normal growth. Furthermore, the morphology of the pituitary was normal, and high performance gel filtration chromatography analysis of the PRL molecule revealed no apparent changes. We concluded that 1) TRH is not essential for pregnancy and lactation, but is required for full function of the lactotrophs, particularly during lactation; and 2) the PRL mRNA level in the pituitary is regulated by TRH, both directly and indirectly via thyroid hormone.

A mathematical model for the mating-induced prolactin rhythm of female rats

For the first 10 days of pregnancy and the first 12 days of pseudopregnancy, the secretion of prolactin (PRL) from pituitary lactotrophs is rhythmic, with two surges/day. This rhythm can also be triggered by bolus injection of oxytocin (OT). We describe a mathematical model for the initiation, maintenance, and termination of the OT-induced PRL rhythm. In our model, the mechanism for this circadian rhythm is mutual interaction between lactotrophs and neuroendocrine dopamine (DA) neurons. This rhythm is, under normal lighting conditions, entrained by the suprachiasmatic nucleus (SCN) but persists in the absence of input from the SCN. We postulate that OT injection triggers the rhythm by activating a population of bistable hypothalamic neurons that innervate and inhibit DA neurons. The bistable nature of these neurons allows them to act as a memory device, maintaining the rhythm long after OT has been cleared from the blood. The mechanism for this memory device and the arguments supporting it are detailed with computer simulations. Finally, we consider potential targets for a rhythm-terminating factor and make predictions that may be used to determine which mechanism is operational in terminating the OT- or mating-induced PRL rhythm.

Alteration of prolactin serum levels during alcohol withdrawal correlates with craving in female patients

Dopaminergic transmission has been suggested to be a main mechanism mediating reinforcement, withdrawal and craving in alcohol dependency. Dopamine is associated with prolactin secretion, acting as a prolactin inhibitor. The aim of the present study was to investigate whether there is an association between altered prolactin levels and craving during early and late alcohol withdrawal. Therefore, we examined 145 patients suffering from alcohol dependency after admission to the detoxification unit, assessing craving with the Obsessive Compulsive Drinking Scale (OCDS) and measuring prolactin serum levels during early withdrawal (-EW: day 0 or day 1) and late withdrawal (-LW: day 7-day 10). We observed a significant influence of the alteration of prolactin during withdrawal on craving in female patients (Spearman's rho, OCDS-EW: r=-0.607, p=0.001; OCDS-LW: r=-0.730, p<0.001; n=26). The association between prolactin alteration in percentage and craving in females was confirmed with general linear models (OCDS-EW: F=15.819, p=0.001, r(2)=0.530; OCDS-LW: F=17.091, p<0.001, r(2)=0.535). In male patients we did not find any significant results. Our findings support the previously described role of the hypothalamic-pituitary-adrenal (HPA) axis in the neurobiology of alcohol craving and show evidence of an association between increased prolactin serum levels and lower craving during alcohol withdrawal in female patients.

Physiological roles of prolactin-releasing peptide

Prolactin-releasing peptide (PrRP) was first isolated from bovine hypothalamus as an orphan G-protein-coupled receptor using the strategy of reverse pharmacology. The initial studies showed that PrRP was a potent and specific prolactin-releasing factor. Morphological and physiological studies, however, indicated that PrRP may play a wide range of roles in neuroendocrinology other than prolactin release, i.e., metabolic homeostasis, stress responses, cardiovascular regulation, gonadotropin secretion, GH secretion and sleep regulation. This review will provide the current knowledge of PrRP, especially its roles in energy metabolism and stress responses.

Effects of the cocaine- and amphetamine-regulated transcript peptide on the turnover of dopamine in tuberoinfundibular neurons and serum prolactin levels: studies using estrogen, melanin concentrating hormone, and melanocortin

Effects of the cocaine- and amphetamine-regulated transcript (CART) peptide on tuberoinfundibular dopaminergic (TIDA) neurons were examined in female and male Sprague-Dawley rats in the morning and afternoon. We also examined the blocking effects of melanin concentrating hormone (MCH) and the antagonists of alpha-melanocyte stimulating hormone (alpha-MSH), SHU9119 and HS014, on stimulation induced by the CART peptide in TIDA systems. Intracerebroventricular administration of 1 mug CART peptide (55-102) at 45 min, either in the morning or afternoon, produced an increase in the median eminence (ME) DOPAC (3,4-dihydroxyphenylacetic acid) level and a corresponding decrease in serum prolactin (PRL) levels. This resulted from stimulation of TIDA neurons regardless of castration, and whether or not male and female rats were estrogen-primed. The stimulatory effects of the CART peptide on ME DOPAC levels were similar in the morning and afternoon in both male and female rats. Central treatment with 1 microg SHU9119, HS014, or MCH significantly decreased the ME DOPAC levels and elevated serum PRL levels in female rats. However, only MCH prevented the stimulatory effect of the CART peptide on TIDA neurons. These results indicate that stimulation by the CART peptide on TIDA neurons is gender-independent; and this stimulatory effect can be blocked by MCH, but not the antagonists of alpha-MSH.

Generation of Thyrotropin-Releasing Hormone Receptor 1-Deficient Mice as an Animal Model of Central Hypothyroidism

To provide an animal model of central hypothyroidism, mice deficient in the TRH-receptor 1 (TRH-R1) gene were generated by homologous recombination. The pituitaries of TRH-R1−/− mice are devoid of any TRH-binding capacity, demonstrating that TRH-R1 is the only receptor localized on TRH target cells of the pituitary. With the exception of some retardation in growth rate, TRH-R1−/− mice appear normal, but compared with control animals they exhibit a considerable decrease in serum T3, T4, and prolactin (PRL) levels but not in serum TSH levels. In situ hybridization histochemistry and real-time RT-PCR analysis revealed that in adult TRH-R1−/− animals TSHβ-mRNA expression is not impaired whereas PRL mRNA and GH mRNA levels are considerably reduced compared with control mice. The numbers of thyrotropes, somatotropes, and lactotropes, however, are not affected by the deletion of the TRH-R1 gene. The mutant mice are fertile, and the dams nourish their pups well, indicating that TRH is not a decisive factor for suckling-induced PRL release. In situ hybridization and quantitative RT-PCR analysis, furthermore, revealed that, as in control animals, pituitary PRL-mRNA expression in TRH-R1−/− is considerably increased during lactation, albeit strongly reduced as compared with lactating control animals.

Cocaine- and amphetamine-regulated transcript co-contained in thyrotropin-releasing hormone (TRH) neurons of the hypothalamic paraventricular nucleus modulates TRH-induced prolactin secretion

TRH synthesized in hypophysiotropic neurons of the hypothalamic paraventricular nucleus (PVN) stimulates the release of TSH and prolactin from the anterior pituitary gland. Recent data from our laboratories have demonstrated that TRH and cocaine- and amphetamine-regulated transcript (CART) are co-contained only in hypophysiotropic neurons in the PVN. To determine whether CART and TRH interact in the regulation of anterior pituitary function, we have studied the effects of CART on TRH-induced release of TSH and prolactin in anterior pituitary cell cultures, and the effects of hypo- and hyperthyroidism on CART mRNA in the PVN. Dispersed anterior lobe cells from male rats were treated with CART (10(-6), 10(-8), 10(-10), and 10(-12) m) or TRH (10(-7) m) alone and TRH (10(-7) m) combined with various concentrations of CART for 4 h at 37 C. The medium was assayed for prolactin and TSH by RIA. TRH resulted in a marked increase of both prolactin and TSH release, whereas CART had no effect on prolactin or TSH secretion. When the two peptides were used in combination, CART dose-dependently inhibited TRH-induced prolactin release but had no significant effect on TRH-induced TSH release. By semiquantitative analysis of in situ hybridization autoradiographs, CART mRNA was significantly elevated in hypothyroid animals, whereas a reduction in CART mRNA was observed in hyperthyroid animals compared with euthyroid controls. These data raise the possibility that CART expressed in hypophysiotropic TRH neurons has an important role in the modulation of TRH-induced prolactin secretion. Increased secretion of CART may be responsible for the reduced TRH-induced prolactin response during hypothyroidism.

Fishy tales of prolactin-releasing peptide

Prolactin (PRL) is an important regulator of multiple biological functions, but a specific PRL-releasing factor, PRL-releasing peptide (PrRP), was isolated only recently from mammals and teleosts. Although this peptide seems to be a strong candidate for being a physiologically relevant stimulator of PRL expression and secretion in teleost pituitary and peripheral organs, it may not be a typical or classic hypothalamic releasing factor in rats. We now know that its biological actions are not limited solely to PRL stimulation, because it is also a neuromodulator of several hypothalamus-pituitary axes and is involved in some brain circuits with the regulation of food intake and cardiovascular functions. Moreover, it plays a direct role in hypertension and retinal information processing. It is the purpose of this review to provide a comprehensive survey of our current knowledge of PrRP and to provide a comparative point of view.

Effects of the cocaine- and amphetamine-regulated transcript peptide on the turnover of central dopaminergic neurons

The effects of the cocaine- and amphetamine-regulated transcript (CART) peptide on central dopaminergic (DA) neurons were examined in ovariectomized, estrogen-primed Sprague-Dawley rats in both the morning and afternoon. Intracerebroventricular administration of 1 microg, but not lower doses of the CART peptide (55-102), either in the morning or afternoon produced a prolonged increase in the 3,4-dihydroxyphenylacetic acid (DOPAC) level in the median eminence (ME) and a corresponding decrease of serum prolactin (PRL) levels, which resulted from stimulation of tuberoinfundibular dopaminergic neurons. The CART peptide stimulated DOPAC levels in the striatum (ST), nucleus accumbens (NA), hypothalamic paraventricular nucleus (PVN), and periventricular (A14), but had no effect in the medial prefrontal cortex (MPFC) or suprachiasmatic nucleus (SCN). These effects of the CART peptide on stimulation of central DA systems and inhibition of PRL levels are specific because the inactive form of the CART peptide (0.1 and 1 microg) could not induce a similar response. Stimulatory effects of the CART peptide on different central DA systems displayed differential time-response profiles in the NA and ST, ME, and PVN and A14. These findings indicate that the CART peptide may selectively regulate certain central DA neuronal activities.

Prolactin secretion and intracellular Ca(2+) change in rat lactotroph subpopulations stimulated by thyrotropin-releasing hormone

Thyrotropin-releasing hormone (TRH) may stimulate lactotrophs to increase intracellular Ca(2+) and to secrete prolactin (PRL). In this study, PRL contents in lactotrophs were determined by the sequential cell immunoblot assay (SCIBA) and their changes in intracellular Ca(2+) was analyzed by confocal microscopy. Significant correlations were found in the corresponding parameters between TRH treatments with a recovery interval of 2 h. Measuring the PRL contents after the first TRH treatment and then determining the intracellular Ca(2+) changes after the second TRH treatment revealed four lactotroph subpopulations. Type I cells (51%) showed significant responses of both PRL secretion and intracellular Ca(2+) concentration. Type II cells (22%) increased in PRL secretion, but without changes in intracellular Ca(2+). Type III cells (17%) have increased in intracellular Ca(2+), but without changes in PRL secretion. Type IV cells (10%) did not show changes in PRL secretion and intracellular Ca(2+).

Dysregulation of the hypothalamic-pituitary-thyroid axis in alcoholism

Thyroid dysfunction is a prominent finding in alcoholism. Subclinical and clinical hypothyroidism have been associated with clinical depression and cognitive impairment and may increase the relapse risk among alcoholics. In spite of these important clinical associations, there is no consensus on thyroid dysfunction in alcoholism in the literature. In this paper, we present a review of the literature and develop a hypothesis that may explain dysfunction of the hypothalamic-pituitary-thyroid axis in alcoholism. Based on a Medline research of the years 1980-2001 we found 33 empirical studies that assessed thyroid function in alcoholism. The most consistent findings were a reduction in total thyroxine and total and free triiodothyronine concentrations during early abstinence. About one-third of all alcoholics also displayed a blunted thyroid stimulation hormone (TSH) response in the thyrotrophin-releasing hormone test (TRH-test). Blunting was observed frequently during detoxification, but was also present in some alcoholics after several weeks of abstinence. We suggest that a reduction in peripheral thyroid hormones may be caused by a direct toxic effect of alcohol on the thyroid gland, which induces a central compensatory activation of the hypothalamic-pituitary axis with an increased TRH release. The TRH release induces a downregulation of pituitary TRH receptors, which manifest as a blunted TSH response to the TRH test. We discuss further additional effects of alcohol on thyroid-hormone metabolizing deiodinases and on monoaminergic systems, which may interact directly with mood states among abstinent alcoholics.

Involvement of angiotensin II, TRH and prolactin-releasing peptide in the estrogen-induced afternoon prolactin surge in female rats: studies using antisense technology

The roles of endogenous angiotensin II (AII), thyrotropin-releasing hormone (TRH) and prolactin-releasing peptide (PrRP) on the estrogen-induced prolactin (PRL) surge and the diurnal change of tuberoinfundibular dopaminergic (TIDA) neuronal activity were assessed in this study. Ovariectomized, estrogen-primed rats implanted with intracerebroventricular cannula received daily injection of antisense oligodeoxynucleotide (ODN, 10 microg/3 microl) against the mRNA of AII, TRH or PrRP for two days. Artificial cerebrospinal fluid or the sense ODN were used as the control. In the first experiment, serial blood samples (0.3 ml each) were obtained hourly from each rat through a pre-implanted intraatrial catheter from 1100 to 1700h. Half of the rats pretreated with respective antisense ODN received single injections of AII, TRH or PrRP (1 microg each, i.v.) at 1400h. In the second experiment, groups of rats were decapitated either at 1000 or 1500h. The hypothalamic median eminence tissue of each rat was dissected out and its DOPAC content was used as the index for TIDA neuronal activity. Plasma and serum PRL levels were determined by radioimmunoassay. Pretreatment of antisense ODN against the mRNA of either AII or TRH significantly attenuated the PRL surge; replacement injection of AII or TRH restored the surge. The effect of antisense ODN against PrRP was less significant. None of the treatments significantly affected the diurnal changes of TIDA neuronal activity. In summary, both AII and TRH may play an important role as the PRL-releasing hormone involved in the estrogen-induced afternoon PRL surge.

Effects of prolactin-releasing peptide on tuberoinfundibular dopaminergic neuronal activity and prolactin secretion in estrogen-treated female rats

Both systemic and central effects of a newly discovered prolactin (PRL)-releasing factor (PRF), prolactin-releasing peptide (PrRP), were determined in this study. Systemic injection of PrRP (1 and 10 microg/rat, i.v.) stimulated PRL secretion in ovariectomized, estrogen-treated rats similar to the effect of another PRF, thyrotropin-releasing hormone (TRH). Pretreatment with a dopamine D2 receptor antagonist, sulpiride (1 microg/rat, i.v.), potentiated the stimulatory effect of both PrRP and TRH on PRL secretion. Using the double-labeling immunohistochemical method, PrRP-immunoreactive terminals were found in close contact with tyrosine-hydroxylase-immunoreactive neurons in the hypothalamic arcuate nucleus. Central administration of PrRP (0.1-1,000 ng/rat, i.c.v.) stimulated tuberoinfundibular but not nigrostriatal dopaminergic neuronal activity in 15 min. Levels of 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence and striatum were used as indices for tuberoinfundibular dopaminergic (TIDA) and nigrostriatal dopaminergic neuronal activities, respectively. The serum PRL level, however, was not significantly changed. Similar treatment with TRH (10 ng/rat, i.c.v.) stimulated and inhibited TIDA neuronal activity and serum PRL, respectively, at 30 min. In summary, PrRP may play a role in both the central and peripheral control of PRL secretion.

d-Aspartate in a prolactin-secreting clonal strain of rat pituitary tumor cells (GH(3))

d-Aspartate (d-Asp) is found in prolactin (PRL)-containing cells of the rat anterior pituitary gland [Lee et al., Brain Res. 838, 193-199, 1999]. In order to determine whether d-Asp is actually produced by the anterior pituitary gland and whether it plays a physiological role in PRL function, a PRL-secreting clonal strain of rat pituitary tumor cells (GH(3)) was employed in this study. HPLC analysis and immunocytochemical staining detected the presence and synthesis of d-Asp in the cytoplasm of these cells. In addition, thyrotropin-releasing hormone-stimulated PRL secretion was increased in a dose-dependent fashion by d-Asp from these cells. These results suggest that the anterior pituitary gland synthesizes d-Asp and that d-Asp acts as a messenger in this gland.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Selective block by glyceryl nonivamide of inwardly rectifying K+ current in rat anterior pituitary GH3 cells

The effects of glyceryl nonivamide (GLNVA) on ionic currents were compared and examined in rat pituitary GH3 cells. Hyperpolarization-activated K+ currents in GH3 cells bathed in high-K+ Ca2+-free external solution were studied to assess effects of GLNVA on the an inwardly rectifying K+ current (I(K(IR))). GLNVA is very potent in blocking I(K(IR)) in a concentration-dependent manner, with a half maximal concentrations of 0.1 microM. The complete block of I(K(IR)) achieved with concentrations > or = 1 microM revealed the presence of a non-inactivating current. We also found that GLNVA at a concentration above 30 microM inhibited L-type voltage-dependent Ca2+ current and two components of K+ outward currents, while GLNVA (< or = 3 microM) did not have any effect on them. This study shows that GLNVA, in addition to retaining the capability of eliciting peptidergic neurons, is a selective block of I(K(IR)) in GH3 cells and will provide a useful tool for characterizing I(K(IR)) and understanding its physiological function. In addition, the carefulness should be taken about the interpretation of GLNVA-mediated responses in vivo or in vitro.