Thyroid hormone-responsive elements of the prolactin gene: evidence for both positive and negative regulation

T3 is linked to stress-associated reduction of prolactin in lactating women

The relationship between stress responses and lactation is bidirectional. Breastfeeding confers many benefits to maternal health, including attenuated hypothalamic-pituitary-adrenal axis responsiveness to stress. However, increased stress burden can impair lactation. The mechanisms that underlie these relationships are poorly understood. The present study aimed to compare breastfeeding habits, as well as subjective and objective measures of stress, in employed and non-employed lactating women and assess the relationships between these measures and prolactin (PRL), thyroid hormones (thyroid-stimulating hormone, triiodothyronine [T3] and thyroxine), vasopressin and cortisol levels. A dexamethasone suppression test was also administered to determine the sensitivity of the hypothalamic-pituitary-adrenal axis to negative-feedback. We report that lactating employed women had lower breastfeeding rates and lower PRL than lactating non-employed women. They also had a significantly higher stress burden, indicated by elevations in blood pressure and evening cortisol, relative to lactating non-employed women. In regression analyses that controlled for feeding modality and breastfeeding duration, we found these factors differentially affected PRL in the two groups and there were significant differences in PRL across groups that were not accounted for by these factors. A mediation regression analysis suggested that group differences in PRL were best explained by differences in T3 and income levels, rather than breastfeeding duration or other variables. Our data fit a speculative model in which elevated maternal stress increases cortisol, which suppresses T3, leading to decreased PRL. The decreases in PRL are associated with higher rates of bottlefeeding, which may further contribute to decreased PRL.

A permissive retinoid X receptor/thyroid hormone receptor heterodimer allows stimulation of prolactin gene transcription by thyroid hormone and 9-cis-retinoic acid

Heterodimers of the retinoid X receptor (RXR) with the thyroid hormone receptor (TR) are considered to be nonpermissive. It is believed that within these complexes RXR acts as a "silent partner." We demonstrate here that a permissive heterodimer mediates stimulation of prolactin expression by the thyroid hormone T3 and by 9-cis retinoic acid (9-cis-RA). A response element located in the prolactin distal enhancer mediates transactivation by both ligands in pituitary cells, and RXR recruits coactivators when bound to this element as a heterodimer with TR. Furthermore, transcription by the RXR agonist can be obtained in CV-1 cells only after overexpression of coactivators, and overexpression of corepressors inhibits the response in pituitary cells. Thus, cell type-specific differences in coregulator recruitment can determine the cellular response to both ligands. Coactivator recruitment by 9-cis-RA requires the ligand-dependent transactivation domains (AF-2) of both heterodimeric partners. Interestingly, the presence of the RXR ligand can overcome the deleterious effect of the AF-2 mutation E401Q on association with coactivators and transactivation. These results demonstrate an unexpected role for RXR in TR signaling and show that in particular cellular environments this receptor can act as a "nonsilent" partner of TR, allowing stimulation by RXR agonists.

Differential transcriptional regulation by the alpha- and gamma-catalytic subunit isoforms of cAMP-dependent protein kinase

The C gamma and C alpha isoforms of the cAMP-dependent protein kinase (PKA) share 83% identity including all critical catalytic and substrate-binding residues defined to date. Compared to C alpha, C gamma has a different substrate specificity and a selective pseudosubstrate specificity, exhibiting inhibition by regulatory subunits, but not by the protein kinase inhibitor. In these studies, C gamma-mediated gene transcription regulation was compared with that of C alpha in four cell lines using transient transfection/dual luciferase assays. As compared to C gamma, C alpha more efficiently activated a cAMP-response element (CRE)-regulated fragment of the human alpha-glycoprotein hormone promoter which was coupled to a firefly luciferase reporter gene (pGH alpha-fluc). This occurred in Cos7, Y1, and Kin8 adrenal cells by 23-, 6.5-, and 1.4-fold, respectively. In contrast, C gamma, but not C alpha, activated the Sp1RE-regulated herpes simplex virus thymidine kinase promoter which was coupled to a Renilla luciferase reporter (pTK-rluc). In Sp1-deficient Sf9 cells, pGH alpha-fluc expression was maintained for both isoforms, but cotransfection with an Sp1 expression plasmid was necessary and sufficient for activation of pTK-rluc expression by C gamma. In all cell lines, cotransfection with a PDK1 expression plasmid enhanced the transcriptional activation of both C alpha and C gamma (1.5- to 3-fold), while a catalytically inactive PDK1 mutant (PDK.KD) did not. These results suggest that both C alpha and C gamma can activate CRE-responsive genes; however, C alpha does so with better efficiency than C gamma. In contrast to C alpha, C gamma activates transcription of genes containing pTK-like Sp1RE sites. Activation of different C subunit isoforms can provide a means to diversify cAMP-mediated transcription, possibly affecting cell phenotype.

Identification of thyroid regulatory elements in the Na-K-ATPase alpha3 gene promoter

A -1027 bp to + 108 bp region of Na-K-ATPase alpha3 gene promoter has been searched for the presence of thyroid response elements (TRE). Computer analysis of this sequence using a consensus TRE sequence revealed the presence of four putative TRE rich regions referred to as regions I (-636 to -457 bp), II (-218 to -106 bp), III (-106 to -6 bp) and IV (-6 to +108 bp). Cotransfection of the luciferase linked full length construct as well as constructs progressively devoid of the TRE rich regions in Cos1 cells revealed that regions I and III are positively regulated by T3 whereas there are some sequences in region II which can suppress the positive regulatory effect of region III but not of region I. TRE IV seems to have no functional role. EMSA of the three functional TRE rich regions (I, II and III) showed strong and specific interaction with thyroid hormone receptor (TR) cloned and expressed in baculovirus. The overall results suggest the regulation of Na-K-ATPase alpha3 gene by T3 is complex involving several thyroidal regulatory elements.

Upregulation of pancreatic polypeptide-sensitive neuropeptide Y (NPY) receptors in estrogen-induced hypertrophy of the anterior pituitary gland in the Fischer-344 rat

Implants of diethylstilbestrol inducing anterior pituitary prolactinomas in female Fischer-344 rats produced a considerable elevation of high-affinity binding of either rat or human pancreatic polypeptide (hPP). No comparable upregulation of high-affinity binding sites was noted for the ligand [125I](Leu31,Pro34)hPYY (LP-PYY) (masked by 2 nM hPP to force selectivity for the Y1 sites), or of the Y2-selective ligand [125I]hPYY(3-36). The Y5-like sites displayed the rank order of potency of hPP = rPP, hNPY, LP-PYY > pPYY(1-36) > hNPY(2-36) > hPYY(3-36) >> aPP, similar to the previously described rabbit kidney or hypothalamic Y5-like receptors. The PP binding in the anterior pituitary was not sensitive to the Y1-selective non-peptidic antagonist BIBP-3226. The PP binding was highly sensitive to alkali metal cations, and to a N5-substituted amiloride antagonist of sodium transport, but not to a C2-guanido substituted amiloride antagonist of sodium channels. The binding was also sensitive to phospholipase C antagonist U-73122, and to alkylating alpha-adrenergic agonist chloroethylclonidine. Lactotrophs isolated in Percoll gradients after enzymic dispersion of the anterior pituitary gland retained the high-affinity PP binding. Following removal of estrogen implants, the hPP binding sites decreased to very low levels within 3 days, in parallel to the decrease of plasma prolactin.

Synergistic activation of the prolactin promoter by vitamin D receptor and GHF-1: role of the coactivators, CREB-binding protein and steroid hormone receptor coactivator-1 (SRC-1)

PRL gene expression is dependent on the presence of the pituitary-specific transcription factor GHF-1/Pit-1, which is transcribed in a highly restricted manner in cells of the anterior pituitary. In pituitary GH3 cells, vitamin D increases the levels of PRL transcripts and stimulates the PRL promoter. We have analyzed the role of GHF-1 and of the vitamin D receptor (VDR) to confer vitamin D responsiveness to the PRL promoter. For this purpose we have used nonpituitary HeLa cells, which do not express GHF-1. We found that VDR activates the PRL promoter both in a ligand-dependent and -independent manner through a sequence located between positions -45/-27 in the proximal 5'-flanking region. This sequence also confers VDR and vitamin D responsiveness to a heterologous promoter. In the context of the PRL gene, VDR requires the presence of GHF-1 to activate the promoter. Truncation of the last 12 C-terminal amino acids of VDR, which contain the ligand-dependent activation function (AF2), abolishes regulation by vitamin D, suggesting that binding of coactivators to this region mediates ligand-dependent stimulation of the PRL promoter by the receptor. Indeed, expression of the coactivators, steroid hormone receptor coactivator-1 (SRC-1) and CREB-binding protein (CBP), significantly enhances the stimulatory effect of vitamin D mediated by the wild-type VDR but not by the AF2 mutant receptor. Furthermore, CBP also increases the activation of the PRL promoter by GHF-1 and the ligand-independent activation by both wild-type and mutant VDR.

Differential regulation of pituitary-specific gene expression by insulin-like growth factor 1 in rat pituitary GH4C1 and GH3 cells

We have compared the influence of insulin-like growth factor 1 (IGF-1) on pituitary gene expression in the rat cell lines GH4C1 and GH3. Incubation with IGF-1 increased PRL messenger RNA (mRNA) levels in GH4C1 cells by 4- to 5-fold but decreased the levels of PRL transcripts in GH3 cells. In addition, the levels of GH-mRNA that were not affected by IGF-1 in GH4C1 cells were significantly inhibited by the growth factor in GH3 cells. IGF-1 also decreased PRL and GH-mRNA response to T3, retinoic acid, and Fk in GH3 cells. Stability of PRL or GH transcripts was not altered by IGF-1 in GH3 cells, suggesting that the inhibitory effect is exerted at a transcriptional level. The pituitary-specific transcription factor GHF-1/Pit-1 activates both the GH and PRL promoters. As analyzed by Western blot, IGF-1 did not alter GHF-1/Pit-1 protein levels in GH4C1 cells but reduced the levels of the transcription factor in GH3 cells. This decrease is secondary to a reduction of GHF-1/Pit-1 transcripts in IGF-1-treated GH3 cells. Thus, a different effect of IGF-1 on the expression of GHF-1/Pit-1 in GH3 and GH4C1 cells is likely involved in the different regulation of GH and PRL gene in both cell types. IGF-1 increases the activity of the PRL promoter in transient transfection assays in GH4C1 cells by a Ras-dependent mechanism. Expression of oncogenic Ras(Val12) mimics the effect of IGF-1, and the dominant negative Ras(Asn17) blocks IGF-1-mediated stimulation of the PRL promoter in GH4C1 cells. Although IGF-1 did not stimulate the PRL promoter in GH3 cells, Ras(Val12) strongly activated the promoter in these cells. Hence, the machinery to activate Ras-dependent signaling is intact in GH3 cells. Moreover, IGF-1 stimulates the mitogen-activated protein kinase in GH3 cells, showing that the components linking the IGF-1 receptor to Ras are also active. These results suggest that, in addition to the Ras/mitogen-activated protein kinase pathway, IGF-1 could activate a different pathway and that the combination of both is required to elicit PRL gene expression by the growth factor. This second pathway may be defective in GH3 cells that respond to Ras but not to IGF-1.

Changes in prolactin secretion in postnatal rats and effect of neonatal thyroidectomy

The prolactin (PRL) gene expression and PRL accumulation in rat pituitaries during the neonatal period were measured by means of radioimmunoassay and Northern blot analyses. The effects of neonatal thyroidectomy (Tx) on PRL synthesis and release were also investigated. The pituitary PRL mRNA level at 20 fetal days had already reached levels sufficient to detect and was increased markedly in the early neonatal period. The PRL concentrations in sera were at almost the same level during the early neonatal period, but the increases of PRL contents and PRL mRNA levels were marked in this period. Neonatal Tx effectively decreased the PRL mRNA level and PRL contents in the pituitary after 10 postnatal days. A significant decrease in the serum PRL of the Tx group was first observed at 20 postnatal days. These data indicate that thyroid hormone dependent PRL secretion is established in the neonatal period.

Characterization of DNA regions mediating the ability of Ca2+/calmodulin dependent protein kinase II to stimulate prolactin promoter activity

The ability of Ca2+/calmodulin-dependent protein kinases (CaMKs) to regulate transcription of the rat prolactin (PRL) gene has been examined. We found that KN-62, a potent inhibitor of CaM kinases, blunted the ability of TRH to activate the prolactin promoter. Transfection experiments using expression plasmids for constitutively active forms of CaMKI, CaMKII, or CaMKIV show that CaMKII is the most effective activator of prolactin promoter expression. Deletion studies demonstrated that the upstream boundary of sequences necessary to respond to CaMKII is located within the distal enhancer of the prolactin gene. Neither the distal enhancer alone nor the proximal region of the prolactin gene are sufficient to mediate a response to CaMKII. Mutational analysis suggests that several Pit-1 binding sites contribute to CaMKII responsiveness. These findings suggest that CaMKII responsiveness of the prolactin promoter requires multiple factor binding sites in both the distal and proximal regions of the gene.

Activities in Pit-1 determine whether receptor interacting protein 140 activates or inhibits Pit-1/nuclear receptor transcriptional synergy

Pituitary-specific transcription of the evolutionarily related rat (r) GH and PRL genes involves synergistic interactions between Pit-1 and other promoter-binding factors including nuclear receptors. We show that Pit-1/thyroid hormone receptor (TR) and Pit-1/estrogen receptor (ER) synergistic activation of the rGH and rPRL promoters are globally similar. Both synergies depend upon the same activation functions in Pit-1 and also require activation function-2 conserved in TR and ER. The activation function-2 binding protein, RIP140, previously thought to be a nuclear receptor coactivator, strongly inhibits both Pit-1/TR and Pit-1/ER synergy. RIP140 inhibition is profoundly influenced, in a promoter-specific fashion, by a synergism-selective function in Pit-1: deletion of Pit-1 amino acids 72-100 switches RIP140 to an activator of Pit-1/ER and Pit-1/TR synergy at the rPRL promoter but not at the rGH promoter. Pit-1 amino acids 101-125 are required for RIP140 inhibition or activation again only at the rPRL promoter. Therefore, functions within one factor can determine the activity of a coactivator binding to its synergistic partner. This promoter context-specific synergistic interplay between transcription factors and coactivators is likely an essential determinant of cell-specific transcriptional regulation.

Thyroid hormone inhibits the human prolactin gene promoter by interfering with activating protein-1 and estrogen stimulations

Transcription of the human PRL (hPRL) gene in the pituitary is subject to tissue-specific and multihormonal regulation involving two main regulatory regions, a proximal promoter and a distal enhancer. In this report we show that thyroid hormone inhibits the expression of the hPRL gene in rat pituitary cells. Transient expression experiments show that thyroid hormone regulation involves a strong inhibitory element, located in the proximal (-164/-35) promoter, which is modulated by a more distal stimulatory response control region. Gel retardation experiments reveal that the thyroid hormone receptor does not bind to the proximal negative element. We show the existence of an activating protein-1 (AP-1) response element located at positions -61 to -54 of the proximal promoter, conferring AP-1 stimulation to the hPRL promoter. This AP-1 induction is abolished when hormone-bound thyroid hormone receptor is present, indicating that there is an interference between the thyroid hormone receptor and AP-1 regulatory pathways. Furthermore, using the complete hPRL upstream region, we show that estrogen induction is abolished by simultaneous thyroid hormone treatment.

Isolation of a thyroid hormone-responsive gene by immunoprecipitation of thyroid hormone receptor-DNA complexes

Thyroid hormone (T3) receptor (TR) is a ligand-dependent transcription factor that acts through specific binding sites in the promoter region of target genes. In order to identify new genes that are regulated by T3, we used anti-TR antiserum to immunoprecipitate TR-DNA complexes from GH4 cell nuclei that had previously been treated with a restriction enzyme. Screening of the immunopurified, cloned DNA for TR binding sites by electrophoretic mobility shift assay yielded 53 positive clones. A subset of these clones was specifically immunoprecipitated with anti-TR antiserum and may therefore represent biologically significant binding sites. One of these clones, clone 122, was characterized in detail. It includes sequences highly related to the NICER long terminal repeat-like element and contains three TR binding sites as determined by DNase I footprinting. Two of the clone 122 TR binding sites are located upstream of the TATA box, and one is located downstream. The TR binding site downstream from the promoter was necessary and sufficient to confer T3-dependent regulation in transient transfection experiments. Expression of a reporter construct under the control of the clone 122 promoter region was activated by TR in the absence of ligand and returned to basal levels after T3 addition. Clone 122 sequences hybridize to at least two different mRNAs of approximately 6 and 10 kb from GH4 cells. The levels of both of these mRNAs increased upon removal of T3. Our studies suggest that specific immunoprecipitation of chromatin allows identification of binding sites and target genes for transcription factors.

Isolation of a thyroid hormone-responsive gene by immunoprecipitation of thyroid hormone receptor-DNA complexes

Thyroid hormone (T3) receptor (TR) is a ligand-dependent transcription factor that acts through specific binding sites in the promoter region of target genes. In order to identify new genes that are regulated by T3, we used anti-TR antiserum to immunoprecipitate TR-DNA complexes from GH4 cell nuclei that had previously been treated with a restriction enzyme. Screening of the immunopurified, cloned DNA for TR binding sites by electrophoretic mobility shift assay yielded 53 positive clones. A subset of these clones was specifically immunoprecipitated with anti-TR antiserum and may therefore represent biologically significant binding sites. One of these clones, clone 122, was characterized in detail. It includes sequences highly related to the NICER long terminal repeat-like element and contains three TR binding sites as determined by DNase I footprinting. Two of the clone 122 TR binding sites are located upstream of the TATA box, and one is located downstream. The TR binding site downstream from the promoter was necessary and sufficient to confer T3-dependent regulation in transient transfection experiments. Expression of a reporter construct under the control of the clone 122 promoter region was activated by TR in the absence of ligand and returned to basal levels after T3 addition. Clone 122 sequences hybridize to at least two different mRNAs of approximately 6 and 10 kb from GH4 cells. The levels of both of these mRNAs increased upon removal of T3. Our studies suggest that specific immunoprecipitation of chromatin allows identification of binding sites and target genes for transcription factors.

Impaired prolactin response to arginine in patients with hyperthyroidism

OBJECTIVE

Reduced PRL responses to TRH or dopamine antagonists have been described in hyperthyroid patients. Arginine stimulates PRL secretion through pathways other than the activation of TRH receptors or dopamine-dependent mechanisms. We therefore investigated PRL responses to arginine in patients with hyperthyroidism.

DESIGN

L-Arginine (30 g infused over 30 minutes) was administered at time zero.

SUBJECTS

Sixteen patients with untreated hyperthyroidism due to Graves' disease (8 female and 8 male), with a mean age (+/- SE) of 31.3 +/- 1.4 years (range 23-42), and 12 normal subjects (6 female and 6 male, ages 30.1 +/- 2.1 years, range 22-47) were studied.

MEASUREMENTS

Prolactin was measured by RIA between -30 and 120 minutes, at 15-minute intervals.

RESULTS

Basal PRL levels were similar in the hyperthyroid patients and normal control subjects. The hyperthyroid women showed blunted PRL responses compared to normal women (peak PRL levels, 364 +/- 44 mU/l, vs 760 +/- 156, P < 0.02). PRL responses to arginine, small but clearly detectable in normal men, were completely abolished in hyperthyroid men (peak PRL levels, 248 +/- 48 mU/l, vs 112 +/- 14, P < 0.01).

CONCLUSIONS

PRL responses to arginine are impaired in hyperthyroid patients. Therefore, arginine should be added to the list of PRL stimuli whose responses are blunted in hyperthyroidism. Inhibition of PRL gene expression, and thus reduced pituitary PRL synthesis and storage, may explain why PRL responses to all secretagogues are reduced in these patients.

Regulation of adenohypophyseal messenger RNAs in female rats by age, hypothyroidism, estradiol and neonatal androgenization

Hormonal regulation of adenohypophyseal messenger ribonucleic acids (mRNAs) encoding preprotachykinin (PPT), prolactin (PRL) and thyrotropin beta subunit (TSH beta) was examined in juvenile and pubertal female rats. Hypothyroidism, initiated on day 2 (d2) or 22 (d22) of life, increased PPT and TSH beta mRNAs but decreased PRL mRNA 17 days later. Exogenous estradiol given for 3 days reduced PPT mRNA in pubertal (d38) but not juvenile (d18) euthyroid females; conversely, estradiol increased PRL mRNA on d18 but not d38. In hypothyroid females however, estradiol decreased PPT and TSH beta mRNAs at both ages and increased PRL mRNA in pubertal but not juvenile females. Thus, regulation of adenohypophyseal mRNAs by estradiol varies with age and thyroid status. In previous studies, adenohypophyseal tachykinins increased in male, but not female rats at puberty. This sex difference was not reproduced here by neonatal androgenization of females, suggesting that it is not mediated by hypothalamic sexual differentiation. However, PRL mRNA increased in androgenized females; this increase was prevented by ovariectomy, suggesting its medication by estradiol.

Multihormonal regulation of the human prolactin gene expression from 5000 bp of its upstream sequence

We have cloned DNA sequences extending up to 6000 bp upstream from the first exon of the human prolactin (hPRL) gene. 5000 bp of these upstream sequences were fused to a CAT reporter gene and shown to provide tissue-specific transient expression in rat pituitary GH3 cells. Multihormonal response was found in this transient expression assay, leading to significant 2- to 5-fold induction by addition of 8-chlorophenylthio-cyclic AMP, thyrotropin-releasing hormone, epidermal growth factor, basic fibroblast growth factor, phorbol myristate acetate, a calcium channel agonist (Bay K-8644) and triiodothyronine. A 3-fold inhibition was observed in the presence of the glucocorticoid agonist dexamethasone. The sequence of the hPRL promoter was determined up to coordinate -3470. Computer similarity search between the rat and human sequences showed two highly conserved regions corresponding to the proximal and distal tissue specific enhancers described in both PRL promoters.

Transfection of avian vitamin D-dependent calbindin-D28K 5' flanking promoter sequence in primary chick kidney cells

Expression of the vitamin D induced calbindin-D28K protein is transcriptionally controlled by the steroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in a tissue-specific manner in the intestine and kidney. In order to examine the cis-acting elements of the calbindin-D28K promoter and its modulation by 1,25-dihydroxyvitamin D3, chimeric plasmids containing 2.1 kb of 5' flanking region linked to the reporter gene chloramphenicol acetyl transferase (CAT) were transfected by lipofection into primary cultures of chick kidney cells. Transfected chick kidney cells exhibited a high basal expression of the chloramphenicol acetyl transferase gene, reflecting the strong activity of the calbindin-D28K promoter. Expression of the pCaBP2.1 reporter gene was increased 2-fold in the presence of the hormone 1,25(OH)2D3 in the primary kidney cells. Deletion of a 1.42 kb fragment ending -679 base pairs upstream from the transcription start site led to a 2-fold repression in the reporter gene activity by the hormone 1,25(OH)2D3 in primary chick kidney cultures. These preliminary results suggest that both positive and negative elements normally act to regulate the expression of the calbindin-D28K gene in primary chick kidney cells.

Control of prolactin secretion

1. Prolactin is a 21,500 Dalton single-chain polypeptide hormone but may occur in 50 kDa and 150 kDa molecular variants. 2. These large PRL variants may be secreted predominantly; this condition is termed "macroprolactinemia". It is characterized by high immunological and normal biological serum levels of prolactin, and lack of clinical symptoms of hyperprolactinemia. 3. The information on PRL is encoded on chromosome 6. Transcription can be enhanced and suppressed by a variety of hormonal factors. 4. PRL is secreted in a pulsatile fashion; it displays a circadian rhythm (with a maximum during sleep) and is stimulated by some amino acids. PRL also responds to mechanical stimulation of the breast. 5. PRL rises during pregnancy, and maintainance of hyperprolactinemia (and, thereby, physiological infertility) is dependent on the frequency and duration of breast feedings. 6. Hypothalamic regulation of prolactin mainly involves tonic inhibition via portal dopamine. The physiological importance of various stimulating factors present in the hypothalamus is still incompletely understood. In particular, there is still no place for TRH in PRL physiology. 7. PRL is released in response to stress; this response may be mediated by opioids. The low-estrogen, low-gonadotropin amenorrhea of endurance-training women is not mediated by prolactin, however. 8. Estrogens stimulate PRL gene transcription via at least two independent mechanisms. There are many clinical examples of this estrogen effect on prolactin serum levels, and also on the growth of prolactinomas. 9. Mild hyperprolactinemia remains an enigma which cannot satisfactorily be resolved by biochemical or radiological testing. The border between "normal" and "elevated" prolactin is ill-defined. The possibility of macroprolactinemia complicates this matter even further. 10. The number of drugs which suppress prolactin by acting on pituitary D2 receptors, and which are useful in the treatment of hyperprolactinemia, continues to increase. In the field of ergot alkaloids, parenteral application appears to be a logical solution to the problem of the high first-pass effect; in addition, this form of treatment is frequently better tolerated than the oral route. 11. Prolactinoma development is presently being studied employing molecular biological techniques; the question of whether tumorigenesis can be attributed to specific defects of gene regulation remains to be answered.