Chronic estrogen treatment promotes a functional uncoupling of the D2 dopamine receptor in rat anterior pituitary gland

Anastrozole as add-on therapy for cabergoline-resistant prolactin-secreting pituitary adenomas: real-life experience in male patients

INTRODUCTION

Prolactin-secreting adenoma (PRLoma) can present as large and invasive neoplasm, with increased markers of cellular proliferation. First-line approach is Dopamine Agonists (DAs) treatment; however, DA-resistance has been reported, especially in male patients. Estrogens induce lactotroph cell replication and PRL secretion: the use of anti-estrogen treatment in patients with PRLoma have been described in few cases. We reported our experience regarding treatment with the aromatase inhibitor anastrozole (ANA) as add-on therapy for male patients with DA resistant PRLoma.

MATERIALS AND METHODS

We describe four male patients (26, 38, 29 and 19 years old at diagnosis), with PRLoma (median diameter 26 mm, PRL 7730 μg/L). They were resistant to cabergoline (CAB, > 2 mg/week) in terms of PRL secretion and tumor size reduction. ANA 1 mg/day was added to the maximum tolerated dose of CAB for at least 1 year. Magnetic Resonance was performed at baseline, after 6 months of CAB + ANA combination and every 12 months afterward.

RESULTS

PRL levels decreased in all patients after CAB + ANA (mean - 70%, range - 44/- 97%), achieving a normalization of PRL levels in one case. Tumor size decreased in all cases (mean - 47%, range - 24.5/- 68%). No severe adverse effects have been reported, a moderate weight gain has been observed in two cases.

CONCLUSIONS

Addition of an aromatase inhibitor (ANA) to the dopamine agonist therapy improved the control of prolactin levels and induced tumour regression.

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.

Effect of oestradiol on dopamine receptors and protein kinase C activity in the rat pituitary: binding of oestradiol to pituitary membranes

Oestradiol exerts an important modulatory influence on the release of prolactin which is accomplished partly through disruption of the inhibitory influence of dopamine. We have focused on the status of the anterior pituitary D2 dopamine receptor in female rats treated chronically with oestradiol or progesterone. A direct membrane effect of these steroids on the dopamine system was also investigated in vitro. Both steroids affected the status of the D2 receptor, oestradiol decreasing the number of sites in vitro and progesterone increasing it both in vitro and in vivo. The in vitro studies demonstrated that these steroids exert a direct membrane effect on the D2 receptor. These results correlated with an in vitro short-term physiological effect of oestradiol and progesterone on the dopaminergic inhibition of prolactin release, oestradiol decreasing it while progesterone had the opposite effect. Binding studies with [3H] oestradiol on pituitary membranes revealed a site for oestradiol of high affinity and low capacity, indicating that oestradiol's membrane effects could be mediated by a specific receptor. In vivo treatment with oestradiol also induces proliferation of prolactin-secreting cells (lactotrophs). We focused on the effect of oestradiol on protein kinase C activity, which is involved in both secretion and proliferation. In female rats treated with oestradiol total protein kinase C activity was increased by 74% (particulate 90%, soluble 71%) in comparison with controls. This effect was reversed by concomitant treatment with a dopamine agonist. Thus in the pituitary oestradiol and progesterone affect the characteristics of membrane components that are implicated in the physiological control of the cell. Whether these effects are post-transcriptional only or are also mediated through direct membrane mechanisms needs further investigation.

Prolactinoma and estrogens: pregnancy, contraception and hormonal replacement therapy

The stimulatory role of estrogen on prolactin secretion and on proliferation of lactotropic cells is well established in terms of physiology but could this phenomenon be extended to include harmful effects of estrogens on prolactinoma? The aim of this review is to provide an up-to-date assessment of this subject with regard to pregnancy, use of contraceptive pills and postmenopausal hormone replacement therapy. Dopamine agonists allow women presenting prolactinoma to recover their ovulation cycles and become pregnant. There is no adverse data concerning the safety of dopamine agonists such as bromocriptine, if the woman is treated during the first trimester of pregnancy but there is little information regarding the most recent treatments such as cabergoline or quinagolide. In women with microadenomas, pregnancy generally has little impact on their adenoma, delivery is normal and breast-feeding is allowed. Concerning macroprolactinomas, tumor progression during pregnancy is possible and endocrine follow-up remains necessary. Contraceptive pills containing estrogen and progestins are currently the best-tolerated and the most effective contraception. This type of contraceptive has long been avoided in patients presenting prolactinoma. While the literature has little to say on this subject and provides no adverse information, professional experience suggests that this attitude should be amended and that women presenting microprolactinoma should be allowed to use current contraceptive pills (containing 30 microg or less of ethinyl estradiol). The most important problem to overcome with this type of prescription, which masks the clinical consequences of hyperprolactinemia, is the possibility of overlooking hypophyseal disease that could result from this approach. The problem of macroprolactinoma is different; the possibility of prescribing contraceptive pills must be evaluated on a case-by-case basis and the impact of the drug on the adenoma must be very closely monitored. Estrogen replacement therapy in patients presenting hypogonadism should be attempted in patients with a history of prolactinoma and standard-monitoring precautions should be taken. In menopausal women, when replacement therapy is desirable, the presence of a microprolactinoma should not by itself avoid this prescription.

A nongenomic action of estradiol as the mechanism underlying the acute suppression of secretion of luteinizing hormone in ovariectomized ewes

The objective of the present study was to determine how rapidly estradiol (E2) was able to suppress the secretion of LH in ovariectomized (OVX) ewes and to evaluate the ability of conjugated forms of E2 (E2 conjugated to BSA [1,3,5(10)-estratrien-3,17beta-diol-6-one-6-carboxymethyloxime:BSA [E2-BSA] and a novel conjugate, E2 conjugated to a small peptide [E2-PEP]) to mimic the actions of E2 on secretion of LH and FSH. Animals (n = 5-6 per group) were given infusions for 4 h of 50 microg of E2 or equimolar concentrations of E2-BSA or E2-PEP. Treatments with E2, E2-BSA, and E2-PEP each induced an acute suppression of LH secretion (<20 min, P < 0.01). In contrast, E2, but not E2-BSA or E2-PEP, induced the characteristic preovulatory-like surge of LH (at 10 h after priming treatment) and decreased secretion of FSH (at 4 h after priming treatment). In conclusion, the acute inhibition of LH secretion induced by E2 in OVX ewes supports the concept of a nongenomic action as the mechanism underlying the sudden suppression in secretion of LH. In addition, the fact that conjugated forms of E2 mimicked only the acute suppression of secretion of LH, without inducing the putative genomic actions on secretion of LH or FSH (i.e., a preovulatory-like surge), suggests that the acute effect of E2 may be mediated via the plasma membrane.

Estrogen mediated inhibition of dopamine transport in the striatum: regulation by G alpha i/o

In the current study, the interaction between estrogen priming and dopamine D2 receptor activation on dopamine uptake in the striatum of ovariectomized female rats was investigated. Basal ADP-[(32)P(i)]ribosylation of G(i/o) was examined in synaptosomal membranes prepared from ovariectomized, estrogen primed or N-p-(isothiocyanatophenethyl) spiperone (NIPS) treated rats. [(32)P(i)]-incorporation was significantly increased (141%) in tissue from NIPS treated animals but attenuated (57%) in tissue from estrogen primed animals. Dopamine uptake kinetics were measured in vivo following manipulation of the heterotrimeric G-protein by pertussis toxin (0.5 microg, 48 h). Pertussis toxin significantly inhibited dopamine uptake at all concentrations of dopamine examined. Co-treatment with estrogen and pertussis toxin resulted in a further attenuation of dopamine transport at high but not low dopamine concentrations. These data are consistent with an estrogen mediated alteration of G-protein activity and support the hypothesis that estrogen may alter transporter activity through a modulation of dopamine D2 autoreceptor/G alpha(i/o) protein coupling.

Reversible thioridazine-induced magnetic resonance imaging-documented pituitary enlargement associated with hyperprolactinemia

OBJECTIVE

To document a case of reversible thioridazine-induced pituitary enlargement associated with hyperprolactinemia in a patient with schizophrenia.

METHODS

We describe a 19-year-old woman with paranoid schizophrenia who was taking thioridazine (a phenothiazine), in whom hyperprolactinemia, galactorrhea, oligomenorrhea, and a reversible anatomic pituitary abnormality developed. Serial magnetic resonance imaging (MRI) studies were used to assess the status of the pituitary gland during various pharmaceutical therapies.

RESULTS

Laboratory evaluation revealed increased serum prolactin (PRL) levels that ranged from 76 to 135 mg/L. Results of thyroid function tests were normal, and gonadotropins and estradiol levels were low, consistent with hyperprolactinemia. MRI revealed asymmetric enlargement of the right side of the pituitary gland. Discontinuing the thioridazine therapy resulted in normalization of the serum PRL and resolution of the pituitary abnormality. Subsequent worsening of the patient's psychiatric condition necessitated a course of electroconvulsive therapy and initiation of treatment with clozapine, a D4 dopamine receptor antagonist. At 1-year follow-up, at which time the patient was maintained on clozapine and was not taking thioridazine, both serum PRL levels and MRI findings remained normal.

CONCLUSION

Our patient was shown to have asymmetric pituitary enlargement associated with thioridazine-induced hyperprolactinemia, which reversed when use of the drug was discontinued. In patients with serum PRL levels in excess of 100 mg/L during antipsychotic drug therapy, evaluation for a prolactinoma is warranted.

The regulatory action of estrogen and vasoactive intestinal peptide on prolactin secretion in sea bream (Sparus aurata, L.)

The effect of estradiol-17beta (E(2)) implants on the in vitro secretion of prolactin (PRL) and its modulation by vasoactive intestinal peptide (VIP) in a marine teleost, sea bream (Sparus aurata L.), was determined. Experiments were conducted during winter and spring. During winter, fish (n=130, body weight 50-70 g) were randomly divided into 2 groups; control and E(2) treated (10 mg/kg, wet weight). Fish were sacrificed after 7 days treatment and in vitro pituitary cultures in Ringer bicarbonate supplemented with increasing doses (0-200 nM) of VIP were carried out for 18 h. Culture medium was analysed by PAGE and secreted PRL quantified by densitometry. Fish treated with E(2) secreted significantly more PRL (P<0.05) in vitro than control fish. In E(2) primed fish VIP caused a dose-dependent inhibition of PRL secretion in vitro. VIP had no detectable effect on the secretion of PRL from control pituitaries. Treatment with E(2) had a different effect during spring; PRL secretion was significantly decreased (P<0.01) compared with the control fish. Anatomical evidence of abundant VIP immunoreactive nerve fibres in neurohypophysial (NH) tissue penetrating the rostral pars distalis provide further evidence supporting an action for VIP in the regulation of PRL cells. In conclusion, the responsiveness of PRL in the pituitary gland varied with season. Moreover, in the sea bream VIP appears to modulate PRL secretion from E(2) primed pituitary glands.

The novel use of very high doses of cabergoline and a combination of testosterone and an aromatase inhibitor in the treatment of a giant prolactinoma

Most prolactinomas respond rapidly to low doses of dopamine agonists. Occasionally, stepwise increases in doses of these agents are needed to achieve gradual prolactin (PRL) reductions. Approximately 50% of treated men remain hypogonadal, yet testosterone replacement may stimulate hyperprolactinemia. A 34-yr-old male with a pituitary macroadenoma was found to have a PRL level of 10,362 micro g/liter and testosterone level of 3.5 nmol/liter. Eleven months of dopamine agonist therapy at standard doses lowered PRL levels to 299 micro g/liter. Subsequent stepwise increases in cabergoline (3 mg daily) further lowered PRL levels to 71 micro g/liter, but hypogonadism persisted. Initiation of testosterone replacement resulted in a rise and discontinuation in a fall of PRL levels. Aromatization of exogenous testosterone to estradiol and subsequent estrogen-stimulated PRL release was suspected. Concomitant use of cabergoline with the aromatase inhibitor anastrozole after resuming testosterone replacement resulted in the maintenance of testosterone levels and restoration of normal sexual function, without increasing PRL. Ultimately, further reduction in PRL on this therapy permitted endogenous testosterone production. Thus, novel pharmacological maneuvers may permit successful medical treatment of some patients with invasive macroprolactinomas.

A sexual arousability model involving steroid effects at the plasma membrane

This review will discuss the status of research related to sexual arousability. It will also present a model for sexual arousability based on current knowledge of steroids effects at the membranes of cells. Steroids have multiple rapid actions that are suggested to result from actions at membrane-associated receptors. When stimulated by steroids these receptors alter G-protein coupling in a manner unique to this complex. Initial stimulation of the receptors by steroids alters the coupling pattern of G-proteins and of other binding sites associated with the complex. This change in G-protein coupling is a stable alteration and thus may serve as a long-term change in the system, which is a requirement of sexual arousability. Stimulation of this receptor system by a surge of oxytocin at ejaculation or orgasm then decouples the G-protein and reduces arousability. Sex hormone binding globulin may be an important ligand at this complex. This model suggests completely new relationships among steroids and their receptors that may complement or diverge from actions at known intracellular receptors.

Alteration of dopamine transport in the striatum and nucleus accumbens of ovariectomized and estrogen-primed rats following N-(p-isothiocyanatophenethyl) spiperone (NIPS) treatment

The ability of N-(p-isothiocyanatophenethyl) spiperone (NIPS, 10 mg/kg, 24 h), a selective, irreversible alkylating agent of the dopamine D(2) receptor, to alter properties of dopamine uptake and clearance in the striatum and nucleus accumbens of ovariectomized and estrogen-primed (estradiol benzoate, 10 microg, 48 h, 24 h) rats was examined using voltammetry. The effectiveness of NIPS was evaluated independently by measuring agonist mediated potentiation of [35S]-guanosine 5'-(gamma-thiotriphosphate) ([35S]-GTPgammaS) binding and [3H]-dopamine uptake. A decrease in E(max) for ligand potentiated [35S]-GTPgammaS binding and a loss of quinpirole potentiated [3H]-dopamine uptake was observed consistent with a NIPS mediated alkylation and functional down-regulation of the dopamine D(2) receptor. This down-regulation was associated with an attenuation of the dose dependent uptake of dopamine in both the striatum and the accumbens. Co-administration of estrogen and NIPS resulted in a further attenuation of dopamine potentiated [35S]-GTPgammaS binding measured in vitro and dopamine uptake measured in vivo. Analysis of the voltammetric profile revealed that clearance and T(50) times were significantly prolonged in animals treated with estrogen and NIPS compared with those treated with NIPS alone. These data are consistent with both a steroid mediated impairment in dopamine autoreceptor/dopamine transporter coupling and an independent action of estrogen at the level of the dopamine transporter.

Dopamine and alcohol relapse: D1 and D2 antagonists increase relapse rates in animal studies and in clinical trials

A considerable number of animal studies on the effects of dopaminergic agents on alcohol intake behavior have been performed. Acute alcohol administration in rats induces dopamine release in the caudate nucleus and in the nucleus accumbens, an effect related among others to reinforcement. It has been repeatedly suggested that D1 and D2 receptor activation mediates reward. As alcohol consumption and dopaminergic transmission seem to have a close relationship, all kinds of dopaminergic agents may be regarded as putative therapeutics for preventing relapse. In a prospective European double-blind multicenter clinical trial, comparing the D1, D2, D3 antagonist flupenthixol and placebo in 281 chronic alcohol-dependent patients (27.4% women), the application of the Lesch typology made an outcome differentiation possible. It could be shown in which patients flupenthixol administration was followed by a significantly higher relapse rate and in which patient groups no differences were found when compared to placebo.

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.

Physiological regulation of G protein-linked signaling

Heterotrimeric G proteins in vertebrates constitute a family molecular switches that transduce the activation of a populous group of cell-surface receptors to a group of diverse effector units. The receptors include the photopigments such as rhodopsin and prominent families such as the adrenergic, muscarinic acetylcholine, and chemokine receptors involved in regulating a broad spectrum of responses in humans. Signals from receptors are sensed by heterotrimeric G proteins and transduced to effectors such as adenylyl cyclases, phospholipases, and various ion channels. Physiological regulation of G protein-linked receptors allows for integration of signals that directly or indirectly effect the signaling from receptor-->G protein-->effector(s). Steroid hormones can regulate signaling via transcriptional control of the activities of the genes encoding members of G protein-linked pathways. Posttranscriptional mechanisms are under physiological control, altering the stability of preexisting mRNA and affording an additional level for regulation. Protein phosphorylation, protein prenylation, and proteolysis constitute major posttranslational mechanisms employed in the physiological regulation of G protein-linked signaling. Drawing upon mechanisms at all three levels, physiological regulation permits integration of demands placed on G protein-linked signaling.

Suppression of luteal phase, but not midcycle, prolactin levels by chronic follicular phase opiate antagonism

OBJECTIVE

To investigate whether establishment and maintenance of chronic opioid blockade throughout the follicular phase of the menstrual cycle influences midcycle and luteal phase prolactin levels.

DESIGN

Randomized, double-blind, crossover study.

SETTING

Academic research environment.

PATIENT(S)

Volunteers, aged 21-35 years, with regular menstrual cycles.

INTERVENTION(S)

Naltrexone (50 mg) or placebo were administered on cycle days 2-14. Blood samples were obtained in the early follicular phase and in the periovulatory and midluteal phases of the menstrual cycle.

MAIN OUTCOME MEASURE(S)

Serum prolactin levels.

RESULT(S)

In the early follicular phase, serum prolactin levels were equivalent in naltrexone (12.0 +/- 2.7 microgram/L; mean +/- SE) and placebo (12.1 +/- 2.9 micrograms/L) cycles. A statistically significant increase in serum prolactin was observed on the day of the LH surge (naltrexone: 22.6 +/- 3.7 micrograms/L; placebo: 21.7 +/- 2.7 micrograms/L; P < 0.05 versus early follicular phase), but no difference between treatments was observed. However, midluteal prolactin levels were statistically significantly lower in naltrexone cycles compared with placebo cycles (12.6 +/- 3.3 versus 15.4 +/- 3.0 micrograms/L; P < 0.05).

CONCLUSION(S)

Chronic blockade of opioid activities during the follicular phase does not affect midcycle prolactin increments, but withdrawal of opioid blockade may enhance opioid effects on prolactin levels in the luteal phase.

Triiodothyronine attenuates estradiol-induced increases in dopamine D-2 receptor number in rat anterior pituitary

Estrogens promote adenohypophyseal enlargement and tumor transformation, and thyroid hormones antagonize these effects. Hormone-induced pituitary enlargement may be mediated by alterations in pituitary dopaminergic function. The present study examined the effects of chronic (20 days) administration of estradiol benzoate (EB), triiodothyronine (T3), or EB and T3 (T3 + EB) on dopamine (D-2) receptors in rat anterior pituitary. D-2 receptor number increased after EB without altered receptor affinity. T3 alone did not affect D-2 receptor number in the anterior pituitary but significantly attenuated the effect of EB. T3 administration also inhibited EB-induced anterior pituitary hyperplasia. D-2 receptor upregulation by EB more likely could reflect a compensatory response to decreased receptor occupation. The present results suggest that D-2 receptors could play an important role in estrogen-induced adenohypophyseal tumor formation and hyperprolactinemia and that thyroid hormones may inhibit estrogen-induced pituitary tumor development via adenohypophyseal D-2 receptors.

Evidence for prolonged recovery of dopaminergic transmission after detoxification in alcoholics with poor treatment outcome

It has been hypothesized that dysfunction of dopaminergic neurotransmission is involved in the pathogenesis of alcohol addiction. Therefore, peripheral dopamine levels, sensitivity of central dopamine receptors (apomorphine-induced Growth Hormone (GH) secretion), and the inhibitory efficacy of G-proteins on adenylyl cyclase activity (as an indicator for dopamine D2-receptor coupled second messenger mechanisms) were measured in 45 alcohol-dependent patients before and after detoxification and in 10 healthy controls. The time needed to adjust to abstinence conditions differed between patients with good and poor treatment outcome. In subsequent abstainers, effects of alcohol withdrawal were already found during the first 24 hours of abstinence (normalisation of GH response, increases in dopamine levels and the inhibitory efficacy of G-proteins). During the next 7 days of abstinence, no more significant changes were observed in the assessed variables. In subsequent relapsers, no significant effect of acute ethanol withdrawal on the same measures was found. However, at day 8 of abstinence, increases in apomorphine-induced GH secretion (towards normalisation), in dopamine plasma levels, and in the inhibitory efficacy of G-proteins (towards above-normal levels) were observed. This retarded adjustment of dopaminergic signal transduction seems to reflect the relapse risk of treatment nonresponders.

Specific [3H]spiperone binding sites in the pituitary of rainbow trout (Oncorhynchus mykiss) and goldfish (Carassius auratus)

Dopamine, a catecholamine neurohormone, modulates pituitary hormone release in teleost fishes and other vertebrates. The existence and binding parameters of a pituitary dopamine-neuroleptic receptor from trout were examined and compared with those from goldfish. Pituitary homogenate was incubated with [3H]spiperone (D2 antagonist) under several experimental paradigms; incubations were terminated by filtration and bound 3H radioactivity was assessed by liquid scintillation spectroscopy. Specific binding of [3H]spiperone was tissue dependent. Equilibrium displacement analyses using domperidone (D2 antagonist) indicated a single class of binding site (LIGAND) with Kd = 2.49 +/- 0.89 microM and a capacity of 3.10 +/- 0.45 nmol/mg protein; the goldfish Kd and capacity were both significantly (p < 0.05) larger: Kd = 4.63 +/- 0.30 microM and capacity = 20.66 +/- 2.03 nmol/mg protein. The Kd and capacity for the trout pars distalis (2.45 +/- 0.33 microM and 3.27 +/- 0.24 nmol/mg protein, respectively) did not differ significantly (p < 0.05) from that of the neurointermediate lobe (2.50 +/- 0.08 microM and 3.58 +/- 0.56 nmol/mg protein, respectively). Dopamine D2 receptor ligands differentially displaced [3H]spiperone from the trout pituitary, while D1 ligands, a D4 ligand, and a 5-hydroxytryptamine (5HT2) receptor antagonist had only small nonspecific effects. Comparison of the trout and goldfish pituitary dopamine-neuroleptic receptor indicates conservation of receptor affinity (Kd); however, differences in receptor numbers and in the distribution of receptors between the pars distalis and neurointermediate lobe in the two species may be due in part to species or developmental differences, and may reflect differences in the role(s) and degrees of influence of dopamine in these fishes.

Mechanism of the prolactin rebound after dopamine withdrawal in rat pituitary cells

To study the mechanism underlying the effect of dopamine withdrawal on prolactin release, continuous perfusion experiments were performed on rat lactotroph-enriched primary cultures. Removal of dopamine (10(-7) M) after a short-term application (15 min) produced a rebound of prolactin secretion, which was enhanced by pretreatment of the cell culture with 17 beta-estradiol (10(-8) M for 48 h). Ca2+ channel blockade by Co2+ (1 mM) abolished the rebound in prolactin release. An increase in intracellular adenosine 3',5'-cyclic monophosphate by either forskolin (5 microM) or 3-isobutyl-1-methylxanthine (100 microM) enhanced the prolactin rebound after dopamine withdrawal. Application of thyrotropin-releasing hormone (10(-7) M) increased the prolactin rebound after dopamine withdrawal with a maximum effect obtained by commencing treatment immediately after removal of dopamine. Pretreatment of cell cultures with pertussis toxin (100 ng/ml, for 10 h) totally abolished the effects of dopamine on prolactin secretion. The dopamine agonist bromocriptine (10(-9) M) significantly decreased prolactin secretion, but no rebound effect was observed after its removal. We conclude that the rebound of prolactin release after dopamine treatment involves the influx of Ca2+.

Modulation by estradiol and progesterone of the GTP effect on striatal D-2 dopamine receptors

Agonist binding properties of rat striatal D-2 dopamine (DA) receptors were investigated after in vivo or in vitro estradiol or progesterone exposures in order to elucidate the mechanism of action of steroid hormones on DA receptors. Chronic estradiol treatment of ovariectomized rats (10 micrograms, twice each day, for 2 weeks) increased lateral striatum total receptor density and left unchanged the proportion and affinity of the agonist high- and low-affinity states of this receptor in the striatum. In addition, when GTP was added in DA competition for [3H]spiperone binding experiments, D-2 receptors in the medial part of the striatum from estrogen-treated animals were more sensitive to GTP than those in the lateral part, whereas GTP had equal activity in both parts of the striatum in vehicle-treated rats. With apomorphine, but not with DA competition for [3H]spiperone binding, addition of estradiol (1 nM) to striatal homogenates of intact male rats prevented the expected shift of the high- to the low-affinity state of D-2 receptors, normally induced by GTP (100 microM) under these conditions. This effect of estradiol was not observed in the presence of 4 mM MgCl2, while in vitro progesterone (100 nM) had no effect in either the absence or presence of MgCl2. In addition, in vivo chronic progesterone treatment of ovariectomized rats left striatal [3H]spiperone density and affinity unchanged. Moreover, 1 nM estradiol increased the IC50 of GTP for inhibition of [3H]N-propylnorapomorphine binding to the high-affinity state of striatal D-2 receptors. This effect was also observed but decreased by 2-fold in the presence of MgCl2. Our data suggest that estradiol in vivo and in vitro interferes with the effect of GTP on striatal D-2 DA receptors.

G protein modulation by estrogens

Levels of various G protein subunits were assayed by immunoblot and densitometry, using specific antibodies, in anterior pituitaries and striata of female rats exposed to physiological or pharmacological modifications of ovarian hormone levels and, for comparison, in the same tissues of coeval male rats. Treatment of ovariectomized rats with 17 beta-estradiol 10 micrograms/rat/day for 5, 10 or 20 days induced a time-dependent rise in plasma prolactin (PRL) levels. While no change in G protein levels was observed in the striatum, estrogen treatment induced a significant reduction of all pituitary G protein levels except those of alpha i1, which remained unchanged, and of alpha s42, which increased in a time-dependent manner. A highly significant correlation was observed between pituitary alpha s42 values and plasma PRL levels. During the estrous cycle, pituitary values of alpha o, alpha i3 and alpha s47 were generally lower than those of ovariectomized rats, suggesting the existence of tonic inhibitory influence of circulating ovarian hormones. Pituitary levels of alpha o, alpha i1 and alpha s42 also showed a significant modulation during the various phases of the estrous cycle, and those of alpha o, alpha i3, alpha s47 and beta were significantly lower in female than in male rats. No significant effects of estrous cycle hormone variations or sex differences were observed in the values of striatum G proteins. In conclusion, these data clearly indicate that ovarian hormones, and particularly estrogens, have a significant and specific effect on pituitary G protein levels which may modulate the secretion of pituitary hormones such as PRL.

G proteins in normal rat pituitaries and in prolactin-secreting rat pituitary tumors

It is still undetermined which GTP-binding (G) protein is involved in the regulation of prolactin (PRL) release and through which effector. This study shows that, when compared to normal pituitary tissue, the levels of alpha o protein were very low in dopamine (DA)-resistant, PRL-secreting pituitary tumors 7315a and MtTW15, while alpha o mRNA was present in the two tumors. In the MtTW15 tumor alpha i1, alpha i2 and alpha i3 levels were decreased while those of alpha s42 and alpha s47 were increased, and in the 7315a tumor alpha i2, alpha i3 and beta levels were decreased and those of alpha s47 increased. In an estrone-induced, DA-sensitive prolactinoma the levels of alpha i3 were greatly reduced. DA was unable to inhibit basal PRL release by 7315a and MtTW15 and basal cAMP accumulation by adenomatous and MtTW15 cells. Vasoactive intestinal peptide (VIP) increased both cAMP accumulation and PRL release by all cell preparations which could be suppressed by DA with adenomatous and 7315a but not with MtTW15 cells. These and previously published results provide circumstantial evidence that alpha o, alpha i1 and alpha i3 are all involved in the transduction of the DA inhibitory message while alpha s47 transduces cAMP activating messages and alpha s42 is responsible for the constitutive activation of L-type Ca2+ channels, adenylate cyclase and baseline PRL release.

Multiple fluorescent ligands for dopamine receptors. I. Pharmacological characterization and receptor selectivity

We report the synthesis and pharmacological characterization of novel fluorescently labeled ligands with high affinity and specificity for D1 and D2 dopamine receptors. D1-selective antagonist probes have been synthesized using (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4,5-tetrahydro-3-methyl-[1H]-3- benzazepin-7-ol, the 4'-amino derivative of the high affinity D1-selective antagonist, SCH-23390, while D2-selective antagonist probes were synthesized using the high affinity, D2-selective agonist, N-(p-aminophenethyl)spiperone (NAPS). In addition, we have synthesized fluorescent probes using an amino-derivative of the high affinity, D2-selective agonist, 2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT or N-0434). These ligands were coupled to the fluorescent moieties, fluorescein, rhodamine, coumarin, Texas red, Cascade blue, or Bodipy. This resulted in a wide variety of dopaminergic ligands which fluoresce at different wavelengths: Cascade blue and coumarin are blue fluorophores, fluorescein and Bodipy, are yellow-green, and Texas red and rhodamine are red. The interaction of these fluorescent ligands with dopamine and serotonin receptors was evaluated by examining their ability to compete for radioligand binding to D1 and D2 dopamine receptors and 5-HT1A, 5-HT1C and 5-HT2 serotonin receptors. We report here that these novel fluorescent ligands exhibit high affinity and, in general, selectivity for either D1 or D2 dopamine receptors. In addition, we demonstrate that the fluorescent derivatives of PPHT retain the full agonist efficacy exhibited by the parent compound.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional Lactotroph Heterogeneity in Lactating Rats and in vitro Modification by 17Beta-Estradiol

Abstract Lactotrophs from lactating rats were separated by unit gravity sedimentation on a continuous density gradient of bovine serum albumin and were identified in two populations located in the light fractions (fractions 3-5) and in the heavy fractions (fractions 7-9) of the gradient. After 7 days in vitro, the effects on prolactin release of thyrotropin-releasing hormone (TRH) and dopamine before and after pretreatment with 17beta-estradiol were studied by a continuous perifusion system and reverse hemolytic plaque assay. Light fraction lactotrophs spontaneously released large quantities of prolactin (22 ng/ml/2 min/10(6) cells) and this basal release was markedly elevated (51 ng/ml/2 min/10(6) cells) by pretreatment with 17beta-estradiol (10(-8) M, 48 h), while the amount of intracellular prolactin remained stable. Mean hemolytic plaque area was increased in the same manner by 17beta-estradiol pretreatment but the number of cells and the percentage of plaque-forming cells were not changed. Perifusion of dopamine-containing medium (10(-7) M) almost completely blocked basal prolactin release from light fraction cells and this inhibition was markedly reduced by 17beta-estradiol pretreatment. TRH-containing medium (10(-7) M) weakly stimulated basal prolactin release (about 190% from basal) and this response was significantly enhanced (to about 300% of basal release) by 17beta-estradiol pretreatment. Both dopaminergic inhibition and TRH-stimulatory effects were dose-dependent and their half maximal effect values were not changed by 17beta-estradiol pretreatment. Secretion of prolactin evaluated at the single cell level by the reverse hemolytic plaque assay corroborated the results obtained from perifusion experiments. Lactotrophs from heavy fractions released small amounts of prolactin (12 ng/ml/2 min/10(6) cells) and neither this basal release nor the amount of intracellular prolactin were markedly modified by 17beta-estradiol pretreatment. As opposed to the light fraction cells, lactotrophs found in heavy fractions were very sensitive to TRH (10(-7) M) stimulation with maximal stimulation reaching ten times basal release, but were less sensitive to dopamine (10(-7) M), with an inhibition of only 40% basal prolactin liberation. Pretreatment of heavy fraction lactotrophs with 17beta-estradiol induced similar effects to those observed after pretreatment of light fraction cells: the stimulation by TRH was increased (from 11 times to 16 times) whereas the inhibition by dopamine was diminished (from 35% to 60%), but cell number and the percentage of prolactin-secreting cells remained unchanged. From the above results, we suggest that: 1) lactotrophs in the lactating rat pituitary can be divided into two major subpopulations with regard to cellular size and density, prolactin production and responsiveness to TRH and dopamine; 2) 17beta-estradiol pretreatment increases basal prolactin release from light fraction cells but does not affect basal prolactin release from heavy fraction cells in this way; 3) pretreatment with 17beta-estradiol enhances TRH stimulation and reduces dopaminergic inhibition of prolactin release from lactotrophs.

Expression of functional D2 dopamine receptors following differentiation of Y-79 human retinoblastoma cells

Y-79 human retinoblastoma cells grown in serum-free medium in monolayer culture have previously been shown to undergo differentiation in response to dibutyryl cyclic AMP (Bt2cAMP). We report here that Y-79 cells treated in this manner also express very high levels of functional D2 dopamine receptors. In control Y-79 cells, cultured in suspension, D2 dopamine receptors, quantified via saturation analysis with the D2 antagonist [3H]methylspiperone, are expressed at a level of approximately 3 fmol/10(6) cells (approximately 1,800 receptor sites/cell). Differentiation is initiated by attachment of the cells to the culture dish with poly-D-lysine and fibronectin and continued culture in serum-free medium. After 8 days in serum-free culture, differentiation is further induced with continuous Bt2cAMP treatment. Using this differentiation protocol, D2 receptor levels increase up to a maximum of 30 fmol/10(6) cells (18,000 receptors/cell) on day 20, the limit of culture viability. Cultures of 15-17 days (7-9 days of Bt2cAMP treatment) expressing receptor levels of 15-20 fmol/10(6) cells are used for pharmacological and functional characterization of D2 dopamine receptors. The pharmacology of competition for [3H]methylspiperone binding to differentiated Y-79 (dY-79) cell membranes by a series of dopaminergic antagonists verifies the D2 receptor nature of this site, exhibiting appropriate affinities and the following rank order of potency: YM-09151-2 approximately spiperone greater than domperidone approximately (+)-butaclamol approximately fluphenazine greater than chlorpromazine greater than (-)-sulpiride greater than (+)-sulpiride greater than promethazine greater than (+)-SCH 23390 much greater than (-)-butaclamol.(ABSTRACT TRUNCATED AT 250 WORDS)

VIP: molecular biology and neurobiological function

In the mammalian brain, a major regulatory peptide is vasoactive intestinal peptide (VIP). This 28 amino acid peptide, originally isolated from the porcine duodenum, was later found in the central and peripheral nervous systems and in endocrine cells, where it exhibits neurotransmitter and hormonal roles. Increasing evidence points to VIP's importance as a mediator or a modulator of several basic functions. Thus, VIP is a major factor in brain activity, neuroendocrine functions, cardiac activity, respiration, digestion, and sexual potency. In view of this peptide's importance, the mechanisms controlling its production and the pathways regulating its functions have been reviewed. VIP is a member of a peptide family, including peptides such as glucagon, secretin, and growth hormone releasing hormone. These peptides may have evolved by exon duplication coupled with gene duplication. The human VIP gene contains seven exons, each encoding a distinct functional domain on the protein precursor or the mRNA. VIP gene transcripts are mainly found in neurons or neuron-related cells. VIP gene expression is regulated by neuronal and endocrine signals that contribute to its developmental control. VIP exerts its function via receptor-mediated systems, activating signal transduction pathways, including cAMP. It can act as a neurotransmitter, neuromodulator, and a secretagog. As a growth and developmental regulator, VIP may have a crucial effect as a neuronal survival factor. We shall proceed from the gene to its multiple functions.

Receptor-effector coupling by G-proteins implications for endocrinology

Discovered serendipitously in the course of studies on the mechanism of glucagon stimulation of hepatic cyclic AMP formation, G-proteins have emerged as an expanding family of signal transducers, coupling diverse receptors and effectors. Quantitative and/or qualitative changes in G-proteins may profoundly affect hormone action, and can lead to clinically apparent endocrine dysfunction.

Estradiol attenuates prolactin secretion and phosphoinositide hydrolysis in MMQ cells

We previously isolated a clonal cell line, designated MMQ, which only secretes prolactin (PRL) and whose secretory process is nonresponsive to thyrotropin releasing hormone (TRH) and angiotensin II (AII). In the present study, we injected MMQ cells into rats to determine whether the tumor cells would become responsive to secretagogues when subsequently propagated in vitro. We also investigated what effects in vivo administration of 17 beta-estradiol would have on secretagogue-induced PRL release and on intracellular biochemical mechanisms in these cells. MMQ cells were implanted subcutaneously in the backs of female rats. One group was injected with 100 micrograms polyestradiol phosphate (PEP) every 5 days, a second with saline. The inoculants grew into solid tumors within 3 weeks. The day after the tumors were removed and enzymatically dispersed, the cells, now designated MMQt cells, were perifused in vitro. Basal PRL released by MMQt cells was approximately 1 ng/min/10(7) cells and perifusions with 100 nM TRH or AII for 5 min significantly increased PRL release above baseline (integrated areas: 1.8 +/- 0.4 and 5.2 +/- 1.3 ng/10(7) cell, respectively; P less than 0.01). Two ng/ml maitotoxin (MTX), a calcium channel activator, increased PRL release (38.2 +/- 6.7 ng/10(7) cells; P less than 0.01). In PEP-treated perifused MMQt cells, basal in vitro PRL release was not different from that observed in the control group, but the responses to TRH, AII and MTX were greatly attenuated (TRH: 0.6 +/- 0.1, AII: 1.3 +/- 0.2 and MTX: 9.2 +/- 2.5 ng/10(7) cells).(ABSTRACT TRUNCATED AT 250 WORDS)