Ovarian steroids modulate responsiveness to dopamine and expression of G-proteins in lactotropes

Prolactin and Other Pituitary Disorders in Kidney Disease

Prolactin levels are increased in chronic kidney disease (CKD) as a result of reduced clearance and increased secretion. Hyperprolactinemia manifests as galactorrhea and hypogonadism. Treatment of hyperprolactinemia should focus on improving bothersome galactorrhea or hypogonadism by using dopamine agonists and/or replacement of sex hormone(s). Changes in the hypothalamic-pituitary-adrenal axis in CKD are characterized by increases in adrenocorticotropic hormone (ACTH) and cortisol levels, largely preserved circadian rhythms of ACTH and cortisol, and a normal response of cortisol to ACTH, metyrapone, and insulin-induced hypoglycemia. However, the hypothalamic-pituitary-adrenal axis is less inhibited by 1 mg dexamethasone but retains normal suppression by higher-dose dexamethasone. Diagnosis of adrenal insufficiency in CKD patients, as in normal subjects, usually is made by finding a subnormal cortisol response to ACTH. The mainstay of treatment of adrenal insufficiency is to replace glucocorticoid hormone. Cushing's disease in CKD is difficult to diagnose and relies on the dexamethasone suppression test and the midnight salivary cortisol test because the 24-hour urine free cortisol test is not useful because it is increased already in CKD. Treatment of Cushing's disease involves surgery, complemented by radiation and/or medical therapy if necessary. Growth hormone levels are increased and insulin-like growth factor 1 levels are normal in patients with CKD. In a normal patient with CKD, as in one with acromegaly, there can be a paradoxic increase in growth hormone after an oral glucose load. Therefore, diagnosis of acromegaly in renal insufficiency is challenging. The treatment of choice for acromegaly is surgery, although data for medical treatment for acromegaly in CKD are rare. In patients with renal impairment, arginine vasopressin levels are increased as a result of decreased clearance, and there also is impairment of arginine vasopressin signaling in renal tubules. Diabetes insipidus can be masked in advanced kidney disease until kidney transplantation. Diagnosis of the syndrome of inappropriate antidiuretic hormone is similar in mild or moderate kidney disease as in normal subjects, but is challenging in patients with advanced kidney disease owing to the impairment in urine dilution.

Kisspeptin regulates tuberoinfundibular dopaminergic neurones and prolactin secretion in an oestradiol-dependent manner in male and female rats

Prolactin (PRL) secretion is inhibited by hypothalamic dopamine. Kisspeptin controls luteinising hormone (LH) secretion and is also involved in PRL regulation. We further investigated the effect of kisspeptin-10 (Kp-10) on the activity of tuberoinfundibular dopaminergic (TIDA) neurones and the role of oestradiol (E2 ) in this mechanism. Female and male rats were injected with i.c.v. Kp-10 and evaluated for PRL release and the activity of dopamine terminals in the median eminence (ME) and neurointermediate lobe of the pituitary (NIL). Kp-10 at the doses of 0.6 and 3 nmol increased plasma PRL and decreased 4-dihydroxyphenylacetic acid (DOPAC) levels in the ME and NIL of ovariectomised (OVX), E2 -treated rats but had no effect in OVX. In gonad-intact males, 3 nmol Kp-10 increased PRL secretion and decreased DOPAC levels in the ME but not in the NIL. Castrated males treated with either testosterone or E2 also displayed increased PRL secretion and reduced ME DOPAC in response to Kp-10, whereas castrated rats receiving oil or dihydrotestosterone were unresponsive. By contrast, the LH response to Kp-10 was not E2 -dependent in either females or males. Additionally, immunohistochemical double-labelling demonstrated that TIDA neurones of male rats contain oestrogen receptor (ER)-α, with a higher proportion of neurones expressing ERα than in dioestrous females. The dopaminergic neurones of periventricular hypothalamic nucleus displayed much lower ERα expression. Thus, TIDA neurones express ERα in male and female rats, and kisspeptin increases PRL secretion through inhibition of TIDA neurones in an E2 -dependent manner in both sexes. These findings provide new evidence about the role of kisspeptin in the regulation of dopamine and PRL.

Activation of D2 dopamine receptors inhibits estrogen response element-mediated estrogen receptor transactivation in rat pituitary lactotrophs

Estrogen and dopamine are major opposing regulators of the endocrine functions of pituitary lactotrophs. Dopamine inhibits estrogen-induced changes in the synthesis and secretion of prolactin, and lactotroph proliferation. We studied the mechanism of the inhibitory effects of dopaminergic stimulation on estrogen-induced functional changes of rat lactotrophs in primary culture. The dopaminergic agonist, bromocriptine (BC), suppressed 17β-estradiol-stimulated lactotroph proliferation, prolactin promoter activity, and mRNA expression of some estrogen-responsive genes. In lactotroph-enriched pituitary cells, BC treatment inhibited the estrogen response element (ERE) DNA sequence-mediated estrogen receptor (ER) transcriptional activity. Using a lactotroph-specific ERE transcriptional assay, we found that BC inhibition of the ERE-mediated ER transcriptional activity partly involved D2 dopamine receptor-mediated, pertussis toxin-sensitive G protein-coupled, cAMP/protein kinase A-dependent signaling. BC treatment had no effect on the cellular concentration of ERα or its phosphorylation status at Ser-118. Similar transcriptional inhibition by BC was also found in GH4ZR7 cells, a D2 dopamine receptor-expressing somatomammotrophic cell line. These results suggest that activation of the D2 dopamine receptors inhibits estrogen-dependent lactotroph functions in part via attenuation of ERE-mediated ER transactivation.

Estrogen inhibits D2S receptor-regulated Gi3 and Gs protein interactions to stimulate prolactin production and cell proliferation in lactotropic cells

The neurotransmitter dopamine (DA) is known to inhibit prolactin (PRL) secretion and the proliferation of lactotropes in the pituitary gland. Dopamine-2 (D2) receptor short (D2S) isoform is expressed in a reduced level while the D2 receptor long (D2L) isoform is expressed in an elevated level during estradiol (E(2))-induced PRL production and cell proliferation in lactotropes. To evaluate the role of these D2 receptor isoforms in E(2)-regulated lactotropic cell function, we compared E(2) effects on the level of PRL, cell proliferation, and G proteins in enriched lactotropes and lactotrope-derived PR1 cells containing only D2S isoform (D2S cells), D2L isoform (D2L cells), or no D2 receptor (V cells). Additionally, we determined the effects of G protein blockade on the E(2)-induced PRL production and cell proliferation in these cells. We here show that E(2) actions on G proteins, PRL production, and cell proliferation were maximally achieved in D2S cells, oppositely or marginally achieved in D2L cells, and absent in V cells. We also show that the DA and pertussis toxin modulations of E(2) actions on PRL, G proteins, and cell proliferation were maximally achieved in D2S cells compared with in D2L or V cells. Furthermore, we provide evidence for the existence of an inhibitory action of Gi3 on Gs that is under the control of the D2S receptor and is inhibited by E(2). These results suggest that the suppression of D2S-regulated Gi3 inhibition of Gs protein may be one of the mechanisms controlling E(2)-activated PRL synthesis and cell proliferation in lactotropes.

Gαo potentiates estrogen receptor α activity via the ERK signaling pathway

The estrogen receptor α (ERα) is a transcription factor that mediates the biological effects of 17β-estradiol (E(2)). ERα transcriptional activity is also regulated by cytoplasmic signaling cascades. Here, several Gα protein subunits were tested for their ability to regulate ERα activity. Reporter assays revealed that overexpression of a constitutively active Gα(o) protein subunit potentiated ERα activity in the absence and presence of E(2). Transient transfection of the human breast cancer cell line MCF-7 showed that Gα(o) augments the transcription of several ERα-regulated genes. Western blots of HEK293T cells transfected with ER±Gα(o) revealed that Gα(o) stimulated phosphorylation of ERK 1/2 and subsequently increased the phosphorylation of ERα on serine 118. In summary, our results show that Gα(o), through activation of the MAPK pathway, plays a role in the regulation of ERα activity.

Estrogen regulation of the dopamine-activated GIRK channel in pituitary lactotrophs: implications for regulation of prolactin release during the estrous cycle

Prolactin (PRL), synthesized and secreted from lactotrophs of the anterior pituitary gland, is tonically inhibited by hypothalamic dopamine (DA) throughout the female reproductive (estrous) cycle. Our laboratory has shown that DA hyperpolarizes these cells by activating G protein-coupled inwardly rectifying K(+) (GIRK) channels; however, this response is only observed on proestrus. While the cellular mechanisms that allow for functional expression of this unique DA-signaling pathway are unclear, we hypothesized that activation of the DA-GIRK effector pathway is due to the rise in circulating estrogen (E₂) during the preceding day of diestrus. Thus, we examined the effects of E₂ on primary lactotrophs isolated from female rats. Treatment with a physiological concentration of E₂ (40-80 pg/ml, in vivo or in vitro) induced a proestrous phenotype in diestrous lactotrophs. These cells exhibited a DA-induced membrane hyperpolarization, as well as a secretory rebound of PRL following DA withdrawal (characteristic of proestrous cells). Internal dialysis of GTPγS demonstrated that E₂ exposure enabled functional expression of GIRK channels, and this regulation by E₂ did not involve the D₂R. The effect of E₂ was blocked by the receptor antagonist, ICI 182,780, and by the protein synthesis inhibitor, cycloheximide. Single-cell analysis revealed increased mRNA expression of GIRK channel subunits in E₂-treated lactotrophs. While E₂ is known to have multiple actions on the lactotroph, the present findings illuminate a novel action of E₂ in lactotrophs-regulation of the expression of a DA effector, the GIRK channel.

Ion channels and signaling in the pituitary gland

Endocrine pituitary cells are neuronlike; they express numerous voltage-gated sodium, calcium, potassium, and chloride channels and fire action potentials spontaneously, accompanied by a rise in intracellular calcium. In some cells, spontaneous electrical activity is sufficient to drive the intracellular calcium concentration above the threshold for stimulus-secretion and stimulus-transcription coupling. In others, the function of these action potentials is to maintain the cells in a responsive state with cytosolic calcium near, but below, the threshold level. Some pituitary cells also express gap junction channels, which could be used for intercellular Ca(2+) signaling in these cells. Endocrine cells also express extracellular ligand-gated ion channels, and their activation by hypothalamic and intrapituitary hormones leads to amplification of the pacemaking activity and facilitation of calcium influx and hormone release. These cells also express numerous G protein-coupled receptors, which can stimulate or silence electrical activity and action potential-dependent calcium influx and hormone release. Other members of this receptor family can activate calcium channels in the endoplasmic reticulum, leading to a cell type-specific modulation of electrical activity. This review summarizes recent findings in this field and our current understanding of the complex relationship between voltage-gated ion channels, ligand-gated ion channels, gap junction channels, and G protein-coupled receptors in pituitary cells.

Kisspeptin regulates prolactin release through hypothalamic dopaminergic neurons

Prolactin (PRL) is tonically inhibited by dopamine (DA) released from neurons in the arcuate and periventricular nuclei. Kisspeptin plays a pivotal role in LH regulation. In rodents, kisspeptin neurons are found mostly in the anteroventral periventricular and arcuate nuclei, but the physiology of arcuate kisspeptin neurons is not completely understood. We investigated the role of kisspeptin in the control of hypothalamic DA and pituitary PRL secretion in adult rats. Intracerebroventricular kisspeptin-10 (Kp-10) elicited PRL release in a dose-dependent manner in estradiol (E2)-treated ovariectomized rats (OVX+E2), whereas no effect was found in oil-treated ovariectomized rats (OVX). Kp-10 increased PRL release in males and proestrous but not diestrous females. Associated with the increase in PRL release, intracerebroventricular Kp-10 reduced Fos-related antigen expression in tyrosine hydroxylase-immunoreactive (ir) neurons of arcuate and periventricular nuclei in OVX+E2 rats, with no effect in OVX rats. Kp-10 also decreased 3,4-dihydroxyphenylacetic acid concentration and 3,4-dihydroxyphenylacetic acid-DA ratio in the median eminence but not striatum in OVX+E2 rats. Double-label immunofluorescence combined with confocal microscopy revealed kisspeptin-ir fibers in close apposition to and in contact with tyrosine hydroxylase-ir perikarya in the arcuate. In addition, Kp-10 was not found to alter PRL release from anterior pituitary cell cultures regardless of E2 treatment. We provide herein evidence that kisspeptin regulates PRL release through inhibition of hypothalamic dopaminergic neurons, and that this mechanism is E2 dependent in females. These findings suggest a new role for central kisspeptin with possible implications for reproductive physiology.

Variations in the response of pituitary lactotrophs to oxytocin during the rat estrous cycle

Although removal of dopamine inhibition is established as a major factor in prolactin (PRL) release, a large body of evidence suggests that hypothalamic oxytocin (OT) may serve as a PRL-releasing hormone in the rat. PRL release is modulated by estradiol (E2), which rises between diestrus and proestrus of the estrous cycle, causing a PRL surge in the afternoon of proestrus. Given that E2 strongly modulates OT actions in both central and peripheral tissues, OT action on lactotrophs might also be modulated by the stage of the estrous cycle. To test this hypothesis, we have monitored PRL release and intracellular calcium levels ([Ca(2+)](i)) induced by OT in pituitary lactotrophs obtained from female rats in either diestrus 1 or proestrus. We found that both secretory and [Ca(2+)](i) responses to OT are significantly increased in lactotrophs obtained on proestrus. Moreover, we show that these differences are due to an increase in both the number of OT-responding lactotrophs and the magnitude of their individual [Ca(2+)](i) responses. Both secretory and [Ca(2+)](i) responses were abolished by a specific OT antagonist. Finally, dose-dependent studies show that the increased PRL-releasing effect of OT on proestrus is significant over a wide range of concentrations, particularly those observed in hypophyseal portal plasma. These results suggest that the rising E2 titers that culminate on proestrus facilitate the stimulatory action of OT on lactotrophs and support the notion that OT is a PRL-releasing hormone with an important role in the production of the proestrous surge of PRL.

Correlation of alternative splicing of the D2 dopamine receptor mRNA and estrogen receptor mRNA in the prolactinomas and gonadotrope tumors

OBJECTIVE

Estradiol (E2) acts to modulate the ratio of two dopamine D2 receptor isoforms (D2L/D2S) by the nuclear estrogen receptor (ER) and to reduce dopamine's inhibitory action on PRL secretion. Here we demonstrate the correlation between the expression of ER mRNA and D2R mRNA isoforms in pituitary neoplasms cells.

METHODS

Twenty-four human pituitary adenomas (14 prolactinomas and 10 gonadotrope tumors) were examined for the expression of both ER mRNA and D2R mRNA by means of semi-quantitative RT-PCR analysis.

RESULTS

No significant difference was found in ERbeta mRNA expression levels between prolactinomas and gonadotrope tumors (P = 0.871), but there was a significant difference in the expression of ERalpha mRNA (P = 0.003). The significant difference was found between the two pituitary adenomas types in both levels of D2S and D2L mRNA expression (P = 0.036 and 0.007 respectively). Furthermore, both levels of expression in prolactinomas were significantly higher than that in gonadotrope tumors. Additionally, a negative correlation between D2S and ERalpha mRNA expression and a positive correlation between D2L and ERalpha mRNA expression were found in these tumors.

CONCLUSION

This study for the first time shows a good correlation between expression of ER and D2R isoforms in prolactinomas and gonadotrope tumors. Reducing the amount of the ERalpha in neoplasm cells can alter the ratio of D2L/D2S, which may increase the drug sensitivity of pituitary adenomas.

Alteration in G proteins and prolactin levels in pituitary after ethanol and estrogen treatment

BACKGROUND

Chronic administration of ethanol increases plasma prolactin levels and enhances estradiol's mitogenic action on the lactotropes of the pituitary gland. The present study was conducted to determine the changes in the pituitary levels of G proteins during the tumor development following alcohol and ethanol treatments.

METHODS

Using ovariectomized Fischer-344 female rats, we have determined ethanol and estradiol actions at 2 and 4 weeks on pituitary weight and pituitary cell contents of prolactin, Gs. Gq11, Gi1, Gi2, and Gi3 proteins. Western blots were employed to measure protein contents.

RESULTS

Ethanol increased basal and estradiol-enhanced wet weight and the prolactin content in the pituitary in a time-dependent manner. Chronic exposure of estradiol increased the levels of Gs protein in the pituitary. Unlike estradiol, ethanol exposure did not show significant effect on the basal level of Gs protein, but moderately increased the estradiol-induced levels of this protein. Estradiol exposure enhanced Gq11 protein levels in the pituitary after 2 and 4 weeks, while ethanol treatment failed to alter these protein levels in the pituitary in control-treated or estradiol-treated ovariectomized rats. In the case of Gi1, estradiol but not ethanol increased the level of this protein at 4 weeks of treatment. However, estradiol and ethanol alone reduced the levels of both Gi2 and Gi3 proteins at 2 and 4 weeks of treatment. Ethanol also significantly reduced the estradiol-induced Gi2 levels at 4 weeks and Gi3 level at 2 and 4 weeks.

CONCLUSIONS

These results confirm ethanol's and estradiol's growth-promoting and prolactin stimulating actions on lactotropes of the pituitary and further provide evidence that ethanol and estradiol may control lactotropic cell functions by altering expression of specific group of G proteins in the pituitary.

Adenohypophyseal and hypothalamic GABA B receptor subunits are downregulated by estradiol in adult female rats

Gamma-aminobutyric acid (GABA) participates in neuroendocrine regulation. Since steroid hormones have been shown to modulate the GABAergic system, here we evaluated the effect of chronic in vivo estradiol administration on GABA B receptor (GABA(B)R) expression. GABA(B1) and GABA(B2) subunits were analyzed by Western Blot and RT-PCR, in hypothalami and anterior pituitaries of adult female rats: a) treated for 1 week with estradiol-valerate (a single dose of 100 mug /kg: E1), b) implanted with a 10 mg pellet of estradiol-benzoate for 5 weeks (E5) or c) on proestrous (P), d) ovariectomized (OVX). Pituitary GABA(B)R levels were correlated to a biological effect: baclofen, a GABA(B)R agonist, action on intracellular calcium titers ([Ca(2+)](i)) in pituitary cells. E5 pituitaries showed a significant decrease in the expression of GABA(B1) and GABA(B2) mRNAs compared to P. The GABA(B1a) splice variant of GABA(B1) was always more abundant than GABA(B1b) in this tissue. Similar to the pituitary, hypothalamic GABA(B1) and GABA(B2) mRNAs decreased in E5; this was confirmed at the protein level. In the hypothalamus GABA(B1b) was the main variant expressed in P rats, and was the one significantly sensitive to estradiol-induced decrease, as determined by Western Blots. Castration did not modify GABA(B)R expression with regards to P in either tissue. In P pituitary cells baclofen induced a decrease in [Ca(2+)](i), in contrast this effect was lost in E5 cells. We conclude that chronic estradiol treatment negatively regulates the expression of the GABA(B)R subunits in the pituitary and the hypothalamus. This effect is coupled to a loss of baclofen action on intracellular calcium in pituitary cells.

Genesis of prolactinomas: studies using estrogen-treated animals

Prolactin-secreting adenomas (prolactinomas) are the most prevalent form of pituitary tumors in humans. Our knowledge of the formation of these tumors is limited. Experimental work in animal has uncovered that estradiol exposure leads to prolactinoma formation via orchestrated events involving dopamine D2 receptors, transforming growth factor-beta(TGF-beta) isoforms and their receptors, as well as factors secondary to TGF-beta action. Additionally, these studies determined that TGF-beta and b-FGF interact to facilitate the communication between lactotropes and folliculo-stellate cells that is necessary for the mitogenic action of estradiol. The downstream signaling that governs lactotropic cell proliferation involves activation of the MAP kinase p44/42-dependent pathway.

Neuroendocrine regulation of prolactin secretion during late pregnancy: easing the transition into lactation

Prolactin is an anterior pituitary hormone critical for maintaining pregnancy and lactation. Under normal conditions, prolactin secretion is tightly regulated by inhibitory dopaminergic neuronal systems within the mediobasal hypothalamus in a process known as short-loop negative feedback. This review focuses on neuroendocrine adaptations to prolactin negative feedback during late pregnancy. It is suggested that, in terms of prolactin regulation, late pregnancy is a transition period into lactation because many of the neuroendocrine adaptations promoting hyperprolactinemia in lactation develop during late pregnancy. As a consequence, the maternal brain is geared to provide unrestrained prolactin release critical for milk production, maternal care and thus survival of the offspring before parturition. The mechanisms responsible for these changes are discussed.

Effects of cyclic steroid hormone replacement on prolactin and luteinizing hormone surges in female rats

The ability of steroid hormones to produce an LH or prolactin (PRL) surge was determined in rats ovariectomized at 6, 9 or 13 weeks of age and subjected to one, three or six cycles of estrogen and progesterone replacement. Sensitivity to steroid replacement was dependent on the age of the animal at the time of ovariectomy. Repeated cyclic steroid hormone replacement significantly increased the magnitude of the PRL response, but not the LH response, in animals ovariectomized at 6 weeks. The LH response was significantly altered by cyclic steroid replacement only in animals ovariectomized at 13 weeks. These results indicate that the mechanisms involved in the regulation of PRL secretion are influenced by steroid hormone replacement and that cyclic steroid hormone exposure increases the magnitude of the PRL secretory response.

A secondary surge of prolactin on the estrus afternoon

It has been described that throughout the estrous cycle of the rat, plasma prolactin (PRL) is basal except on proestrus afternoon when a preovulatory surge occurs. However, there have been controversies about PRL levels on the estrus day. Thus, the aim of this study was to evaluate the existence of a secondary surge of PRL on estrus afternoon and correlate it with plasma estradiol levels. The jugular vein of cycling rats was cannulated at 14:00 h on proestrus and a blood sample was withdrawn at 17:00 h for plasma LH measurement and determination of the preovulatory LH surge occurrence. In order to exclude the regular cycling rats that do not present the gonadotropins preovulatory surge and do not ovulate, only rats showing the LH surge on proestrus were considered in this study. Blood samples were collected hourly during estrus from midnight to 9:00 h (group 1) and from 10:00 to 18:00 h (group 2). In group 1, PRL showed a descending profile from midnight to 9:00 h, whereas the estradiol concentrations were constant. In group 2, a secondary surge of PRL was observed in 20 of 25 (80%) rats and plasma estradiol remained constant, but was higher in animals with the PRL surge. Thus the present data suggest the occurrence of a secondary surge of PRL in the afternoon of estrus that seems to be related to plasma estradiol levels of estrus day, which might exert only a permissive role in this surge generation.

Rhythmic secretion of prolactin in rats: action of oxytocin coordinated by vasoactive intestinal polypeptide of suprachiasmatic nucleus origin

Prolactin (PRL) is secreted from lactotrophs of the anterior pituitary gland of rats in a unique pattern in response to uterine cervical stimulation (CS) during mating. Surges of PRL secretion occur in response to relief from hypothalamic dopaminergic inhibition and stimulation by hypothalamic releasing neurohormones. In this study, we characterized the role of oxytocin (OT) in this system and the involvement of vasoactive intestinal polypeptide (VIP) from the suprachiasmatic nucleus (SCN) in controlling OT and PRL secretion of CS rats. The effect of OT on PRL secretion was demonstrated in cultured lactotrophs showing simultaneous enhanced secretion rate and increased intracellular Ca(2+). Neurosecretory OT cells of the hypothalamic paraventricular nucleus that express VIP receptors were identified by using immunocytochemical techniques in combination with the retrogradely transported neuronal tracer Fluoro-Gold (iv injected). OT measurements of serial blood samples obtained from ovariectomized (OVX) CS rats displayed a prominent increase at the time of the afternoon PRL peak. The injection of VIP antisense oligonucleotides into the SCN abolished the afternoon increase of OT and PRL in CS-OVX animals. These findings suggest that VIP from the SCN contributes to the regulation of OT and PRL secretion in CS rats. We propose that in CS rats the regulatory mechanism(s) for PRL secretion comprise coordinated action of neuroendocrine dopaminergic and OT cells, both governed by the daily rhythm of VIP-ergic output from the SCN. This hypothesis is illustrated with a mathematical model.

Estrogen decreases the sensitivity of anterior pituitary to the inhibitory effect of nitric oxide on prolactin release

OBJECTIVE

We investigated the effect of chronic estrogen treatment on the inhibitory action of nitric oxide (NO) on prolactin release.

METHODS

The effect of NO on prolactin release was studied in anterior pituitaries of female Wistar rats, intact at random stages, ovariectomized (OVX), and OVX treated for 15 days with 17beta-estradiol (OVX-E(2)).

RESULTS

Sodium nitroprusside (NP, 0.5 mM), a NO donor, inhibited prolactin release from anterior pituitaries and was able to stimulate cGMP synthesis in intact and OVX rats. Only a high, supraphysiological concentration of NP (2 mM) inhibited prolactin release from anterior pituitaries of OVX-E(2) rats and increased cGMP synthesis in OVX-E(2) rats. 8-Br-cGMP, a cGMP analogue, decreased prolactin release from anterior pituitaries of OVX rats but did not affect it in OVX-E(2) rats.

CONCLUSION

Our results suggest that estrogen may modify the sensitivity of the anterior pituitary to the inhibitory effect of NO on prolactin release by affecting guanylyl cyclase activity and the cGMP pathway.

The effects of GnRH and adrenergic agents on PRL and beta-endorphin secretion by porcine pituitary cells in vitro

The direct effects of alpha- and beta-adrenergic agents on PRL and beta-endorphin (beta-END) secretion in vitro by porcine pituitary cells have been investigated. Pituitary glands were obtained from mature gilts, which were ovariectomised (OVX) one month before slaughter. Ovariectomised gilts, assigned to four groups, were primed with: (1) vehicle (OVX); (2) and (3) oestradiol benzoate (EB; 2.5 mg/100 kg b.w.) at 30-36 h (OVX+EB I) and 60-66 h (OVX+EB II) before slaughter, respectively; and (4) progesterone (P4; 120 mg/100 kg b.w.) for 5 consecutive days before slaughter (OVX+P4). Isolated anterior pituitary cells were submitted to 3.5 h incubation in the presence of GnRH, alpha- and beta-adrenergic agonists [phenylephrine (PHEN) and isoproterenol (ISOP), respectively], or alpha- and beta-adrenergic blockers [phentolamine (PHENT) and propranolol (PROP), respectively]. The culture media were assayed for PRL (exp. I) and beta-endorphin-like immunoreactivity (beta-END-LI) (experiment II). In experiment I, GnRH did not influence PRL release by pituitary cells in all experimental groups. Some of tested doses of adrenergic agonists, PHEN and ISOP, increased PRL release from pituitary cells of OVX gilts, but not from those of OVX+EB I animals. In the OVX+EB II group, PHEN alone, but ISOP with PROP, potentiated PRL secretion by the cells. In OVX+P4 animals, PHEN alone or in combination with PHENT and also ISOP alone or with PROP enhanced PRL output from the cells. In experiment II, addition of GnRH increased beta-END-LI release from pituitary cells only in the OVX+EB II group. PHEN and PHENT potentiated beta-END-LI secretion by pituitary cells in OVX+EB II and OVX+P4 groups, while ISOP and PROP increased beta-END-LI secretion by the cells of OVX and OVX+EB II animals. In turn, in the OVX+EB I group, effect of PHENT and PROP on PRL secretion by pituitary cells was inhibitory. In conclusion, our results suggest that adrenergic agents can modulate PRL and beta-END secretion by porcine pituitary cells in a manner dependent on the hormonal status of gilts.

Ethanol and estradiol modulate alternative splicing of dopamine D2 receptor messenger RNA and abolish the inhibitory action of bromocriptine on prolactin release from the pituitary gland

BACKGROUND

Several reports show evidence for the existence of high levels of prolactin (PRL) in alcoholic men and women. Previously we have shown that ethanol increases PRL release both in vivo and in vitro. How ethanol increases PRL release is not well understood.

METHODS

In this study, we determined the effects of ethanol in the presence and absence of estradiol-17 beta on PRL messenger RNA (mRNA) levels, dopamine D2 receptor mRNA splicing, and the PRL-inhibitory response of a dopaminergic agent, bromocriptine, in the pituitary of Fischer-344 rats and in primary cultures of anterior pituitary cells. Real-time reverse transcriptase-polymerase chain reaction was used for mRNA detection, and radioimmunoassay was used for hormone detection.

RESULTS

Estradiol and ethanol alone increased PRL mRNA expression in the pituitary gland. Ethanol also potentiated estradiol action on PRL mRNA expression in the pituitary. Determination of the D2 receptor splicing, by determining the changes in the percentage of D2 receptor mRNA expressed as its long form (D2L) and as its short form (D2S), revealed that both ethanol and estradiol altered D2 receptor splicing. Ethanol and estradiol, alone and together, increased the percentage of the D2L receptor but decreased the D2S receptor percentage. Similarly, ethanol and estradiol alone and in combination increased D2L, but decreased the D2S receptor percentage in primary cultures of pituitary cells. Evaluation of bromocriptine's inhibition of PRL release in primary cultures of pituitary cells indicated that ethanol reduced the ability of this D2 receptor agonist to inhibit PRL release.

CONCLUSIONS

These results confirm estradiol's inhibition of D2 function and provide novel evidence that ethanol, like estradiol, reduces dopamine's ability to inhibit PRL release by modifying alternative splicing of the dopamine D2 receptor in the pituitary.

Functional expression of the dopamine-activated K(+) current in lactotrophs during the estrous cycle in female rats: correlation with prolactin secretory responses

It is well established that hypothalamic dopamine (DA) is the major physiologic regulator of prolactin (PRL) secretion, exerting a tonic inhibition throughout most of the estrous cycle. A dramatic drop in the amount of DA perfusing the anterior pituitary occurs in the afternoon of proestrus and is critical for the production of the surge of PRL that occurs at that time. In my laboratory, we have identified and characterized a DA-activated K(+) channel (K(DA)) in lactotrophs derived from proestrous rats that underlies DA-induced membrane hyperpolarization of lactotrophs. We have also demonstrated that this hyperpolarization plays a critical role in both the inhibition of PRL release from proestrous cells and the PRL secretory rebound that occurs following DA withdrawal. We now report that the ability of DA to activate the K(DA) channel and elicit hyperpolarization in primary lactotrophs changes dramatically during the estrous cycle. Lactotrophs isolated from cycling female rats were studied using whole-cell voltage clamp. DA (1 microM) elicited a robust membrane K(+) current in the majority of proestrous lactotrophs (86%; 24.0 +/- 2.9 pA). By contrast, DA activated a considerably smaller membrane current (3.3 pA) in very few lactotrophs isolated from rats on either diestrus or estrus (8 and 0%, respectively). Using a perifusion system to examine temporal patterns of PRL release, we found that following application and withdrawal of DA, proestrous cells produced a robust secretory rebound, but diestrous and estrous cells did not. However, DA inhibited PRL release to the same extent regardless the stage of the cycle from which the cells were derived. These data are consistent with the presence of multiple DA effectors in lactotrophs and demonstrate that their relative importance shifts dramatically with changes in the endocrine status of the animal. We propose that the DA-activated K(+) channel (K(DA)) is a critical effector governing the unique secretory profile of PRL observed in proestrous animals.

Trajectories of brain development: point of vulnerability or window of opportunity?

Brain development is a remarkable process. Progenitor cells are born, differentiate, and migrate to their final locations. Axons and dendrites branch and form important synaptic connections that set the stage for encoding information potentially for the rest of life. In the mammalian brain, synapses and receptors within most regions are overproduced and eliminated by as much as 50% during two phases of life: immediately before birth and during the transitions from childhood, adolescence, to adulthood. This process results in different critical and sensitive periods of brain development. Since Hebb (1949) first postulated that the strengthening of synaptic elements occurs through functional validation, researchers have applied this approach to understanding the sculpting of the immature brain. In this manner, the brain becomes wired to match the needs of the environment. Extensions of this hypothesis posit that exposure to both positive and negative elements before adolescence can imprint on the final adult topography in a manner that differs from exposure to the same elements after adolescence. This review endeavors to provide an overview of key components of mammalian brain development while simultaneously providing a framework for how perturbations during these changes uniquely impinge on the final outcome.

Differential involvement of protein kinase C in basal versus acetylcholine-regulated prolactin secretion in rat anterior pituitary cells during aging

Although it is well known that plasma concentration of prolactin (PRL) increases during aging in rats, how the anterior pituitary (AP) aging per se affects PRL secretion remains obscure. The objectives of this study were to determine if changes in the pituitary PRL responsiveness to acetylcholine (ACh; a paracrine factor in the AP), as compared with that to other PRL stimulators or inhibitors, contribute to the known age-related increase in PRL secretion, and if protein kinase C (PKC) is involved. We also determined if replenishment with aging-declined hormones such as estrogen/thyroid hormone influences the aging-caused effects on pituitary PRL responses. AP cells were prepared from old (23-24-month-old) as well as young (2-3-month-old) ovariectomized rats. Cells were pretreated for 5 days with diluent or 17beta-estradiol (E(2); 0.6 nM) in combination with or without triiodothyronine (T(3); 10 nM). Then, cells were incubated for 20 min with thyrotropin-releasing hormone (TRH; 100 nM), angiotensin II (AII; 0.2-20 nM), vasoactive intestinal peptide (VIP; 10(-9)-10(-5) M), dopamine (DA; 10(-9)-10(-5) M), or ACh (10(-7)-10(-3) M). Cells were also challenged with ACh, TRH, or phorbol 12-myristate 13-acetate (PMA; 10(-6) M) following PKC depletion by prolonged PMA (10(-6) M for 24 h) pretreatment. We found that estrogen priming of AP cells could reverse the aging-caused effects on pituitary PRL responses to AII and DA. In hormone-replenished cells aging enhanced the stimulation of PRL secretion by TRH and PMA, but not by AII and VIP. Aging also reduced the responsiveness of cells to ACh and DA in suppressing basal PRL secretion, and attenuated ACh inhibition of TRH-induced PRL secretion. Furthermore, ACh suppressed TRH-induced PRL secretion mainly via the PMA-sensitive PKC in the old AP cells, but via additional mechanisms in young AP cells. On the contrary, basal PRL secretion was PKC (PMA-sensitive)-independent in the old AP cells, but dependent in the young AP cells. Taken together, these results suggest differential roles of PMA-sensitive PKC in regulating basal and ACh-regulated PRL responses in old versus young AP cells. The persistent aging-induced differences in AP cell responsiveness to ACh, DA, TRH, and PMA following hormone (E(2)/T(3)) replenishment suggest an intrinsic pituitary change that may contribute, in part, to the elevated in vivo PRL secretion observed in aged rats.

Gender differences in steroid modulation of angiotensin II-induced protein kinase C activity in anterior pituitary of the rat

To investigate whether the various steroid hormones can modulate the basal and angiotensin II-induced protein kinase C (PKC) activity in the anterior pituitary of the rat, female and male intact and ovariectomized female Wistar rats were treated in vivo with estradiol (E2), progesterone (P), dehydroepiandrostendione sulfate (DHEA-S), and pregnenolone sulfate (PREG-S). Estradiol caused the increase of basal PKC activity in intact and ovariectomized females, but did not change the enzyme activity in males. In ovariectomized animals the increase of PKC activity was lower than in intact females. Progesterone decreased PKC activity only in intact animals. DHEA-S strongly enhanced activity of PKC in ovariectomized females. Pregnenolone sulfate did not significantly change PKC function of all studied groups. Incubation with AngII enhanced the PKC activity in intact (without steroid treatment) animals of both genders. In females, AngII and estradiol together rise the PKC-stimulated phosphorylation in greater degree than used separately. Treatment with other investigated steroids reduced the effect of AngII. In intact males every examined hormone turned back the stimulatory effect of AngII on PKC activity. These data suggest that gender differences in PKC activity are likely related to hormonal milieu of experimental animals and may depend in part on the basic plasma level of estrogens.

Modulation of pituitary dopamine D1 or D2 receptors and secretion of follicle stimulating hormone and luteinizing hormone during the annual reproductive cycle of female rainbow trout

The two gonadotrophins follicle stimulating hormone (FSH) and luteinizing hormone (LH) have distinct temporal expression and release profiles in fish, but little is known regarding their neuroendocrine control, especially for FSH. The present experiments were performed on previtellogenic, mature and preovulatory female trout. The catecholamine synthesis inhibitor, alpha-methyl-p-tyrosine, increased plasma LH and FSH concentrations of mature fish. The dopamine agonist apomorphine decreased and the dopamine antagonist domperidone increased plasma LH concentration of preovulatory fish and delayed ovulation, but did not modify plasma FSH concentration. The dopamine D2 agonist bromocryptine inhibited LH release in cultured gonadotrophs from mature and preovulatory fish, but not from previtellogenic fish. Bromocryptine also significantly inhibited basal and salmon gonadotrophin releasing-hormone (sGnRH)-induced FSH release from cultured gonadotrophs of mature fish, but not of preovulatory fish, and increased FSH release from gonadotrophs of previtellogenic fish. The dopamine D1 agonist SKF 38393 had no observed effect on the release of FSH and LH, at any reproductive stage studied. The D1 agonist SKF 38393, the D2 agonist bromocriptine and sGnRH had no observed effects on cell contents of FSH and LH. Taken together, these data suggest that, at the level of the pituitary, dopamine inhibits LH release as vitellogenesis proceeds, via activation of dopamine D2 receptors. We demonstrate for the first time in fish a control of FSH release (a dopamine control), especially in mature fish which have low circulating concentrations of FSH.

Ovarian steroids influence the activity of neuroendocrine dopaminergic neurons

The secretion of prolactin (PRL) from the anterior lobe (AL) of the pituitary gland is tonically inhibited by dopamine (DA) of hypothalamic origin. While ovarian steroids play a role in the regulation of the secretion of PRL, their effect on all three populations of hypothalamic neuroendocrine dopaminergic neurons is not fully understood. In this study we describe the effects of ovarian steroids on regulation of the release of DA from tuberoinfundibular dopaminergic (TIDA), tuberohypophyseal dopaminergic (THDA) and periventricular-hypophyseal dopaminergic (PHDA) neurons. Adult female rats were bilaterally ovariectomized (OVX) and, 10 days following ovariectomy (day 0), injected with corn oil (vehicle), estrogen, or estrogen plus progesterone (day 1). Animals were sacrificed every 2 h from 09.00 to 21.00 h by rapid decapitation. Trunk blood was collected and the concentration of PRL in serum was determined by radioimmunoassay. The median eminence (ME) and the AL, intermediate (IL) and neural (NL) lobes of the pituitary gland were dissected and the concentration of DA and DOPAC in each was measured by HPLC-EC. OVX rats presented small but significant increases in the secretion of PRL at 15.00 and 17.00 h. Replacement of estrogen or estrogen plus progesterone increased the basal concentration of PRL. Moreover, injection of estrogen only, or estrogen plus progesterone increased the concentration of PRL in serum at 15.00 h through 19.00 h, respectively, followed by a decrease to baseline thereafter. The turnover of DA in the ME and NL of OVX rats increased at 13.00 and returned to low levels. Turnover of DA in the IL of OVX rats increased in the morning by 11.00 h and remained elevated before decreasing by 17.00 h. The turnover of DA in the ME, IL and NL of OVX rats increased by 19.00 h. Injection of estrogen advanced the increase of TIDA activity by 2 h in the ME compared to OVX rats. Moreover, administration of estrogen suppressed the activity of THDA and PHDA neurons in the afternoon compared to OVX rats. In estrogen plus progesterone-treated rats, the activity of hypothalamic neuroendocrine dopaminergic neurons terminating in the ME, IL, and NL was inhibited prior to the increase in the secretion of PRL. The concentration of DA in the AL diminished prior to the estrogen-induced increase of PRL. Administration of progesterone, in concert with estrogen, delayed the increase of PRL in serum and the decrease of DA in the AL, compared to estrogen-treated rats, by 4 h. These data suggest a major role for ovarian steroids in controlling increases in the secretion of PRL by not only stimulating PRL release from lactotrophs, but also by inhibiting the activity of all three populations of hypothalamic neuroendocrine DAergic neurons.

Estrogen Modulation of G-protein-coupled Receptors

Estrogen exerts long-term effects in almost every cell through regulation of gene transcription. However, it has been known for some time that estrogen can rapidly alter neuronal firing within seconds, indicating that some cellular effects of estrogen could occur via non-genomic mechanisms. G-protein-coupled receptors (GPCRs) are the largest class of membrane-bound receptors, and it appears that many of the rapid effects mediated by estrogen could involve changes in GPCR-effector system coupling in excitable cells within the reproductive axis.