Dopamine agonists both stimulate and inhibit prolactin ...prolactin-release inhibiting factors...

EXPERIMENTAL STUDY

Dopamine agonists both stimulate and inhibit prolactinrelease in GH4ZR7 cellsAnnisa Chang and Seon H Shin

Department of Physiology, Queen’s University, Kingston, Ontario, Canada, K7L 3N6

(Correspondence should be addressed to S H Shin, Department of Physiology, Queen’s University, Kingston, Ontario, Canada, K7L 3N6;Email: [email protected])

AbstractProlactin secretion from the anterior pituitary gland is regulated by multiple factors includingprolactin-release inhibiting factors (PIFs) and prolactin releasing factors. PIFs, however, usuallydominate to exert a tonic inhibition in the biological system, and the physiological PIF is believed tobe dopamine. However, there is accumulating evidence that dopamine can not only inhibit but alsostimulate prolactin release. Many investigators believe that this is achieved by activating inhibitory andstimulatory subtypes of dopamine receptors. We tried to demonstrate that one subtype of dopaminereceptors is capable of both inhibiting or stimulating prolactin release using GH4ZR7 cells. GH4ZR7 cellsexpress only a short form of dopamine D2 receptors (D2s). Low concentrations of three well-establishedD2 receptor agonists (dopamine, apomorphine and bromocriptine) stimulated prolactin release fromGH4ZR7 cells while high concentrations inhibited the release. Haloperidol, a D2 receptor antagonist,blocked the inhibitory action, but was unable to block the dopamine-induced stimulatory action.Pretreatment of cells with phenoxybenzamine, a receptor alkylating agent, abolished both thedopamine-induced stimulatory and inhibitory actions. Our results support the thesis that the stimulationof prolactin release induced by dopamine is mediated through dopamine D2s receptors since the GH4ZR7cells have only D2s receptors among dopamine receptors. We have concluded that the D2s receptor iscapable of both stimulating and inhibiting prolactin release, probably via the activation of a Gs proteinby low concentrations and a Gi protein by high concentrations of dopaminergic agents.

European Journal of Endocrinology 141 387–395

Introduction

The neuroendocrine control of prolactin secretionfrom the anterior pituitary gland involves multiplefactors including prolactin releasing factors (PRFs) andprolactin-release inhibiting factors (PIFs); however, thelatter usually predominate to exert a tonic inhibitorycontrol (1–4). The concept of inhibitory regulation issupported by evidence that hypothalamic lesions (5)and transplantation of the anterior pituitary to extra-cranial sites such as the kidney capsule (5) resulted in amarked increase in prolactin secretion. Convincingevidence has been given that dopamine (DA), secretedby the tuberfundibular dopaminergic (TIDA) neurons ofthe hypothalamus, is the main physiological PIF (4, 6–9), but DA alone cannot satisfy all the criteria to be thePIF and we have recently concluded that the physiolo-gical PIF is DA plus ascorbate (10). PRFs, on theother hand, are important in mediating the rise inprolactin release under a variety of physiologicaland experimental conditions. A number of hypothala-mic stimulating factors such as thyrotropin-releasing

hormone (TRH), cholecystokinin, vasoactive intestinalpolypeptide (VIP), and neurotensin have been identified,but their role in the physiological regulation of prolactinsecretion is not yet certain (11). Under several of thestimulatory conditions studied, for example duringsuckling, the level of DA in portal blood decreases, andthere has been a suggestion that the fall of DA in portalblood, although very transient, is a pre-requisite for afull prolactin response to PRF (2, 12).

Although DA is established as the PIF, there issubstantial evidence showing that DA, at much lowerconcentrations than those required for inhibition ofprolactin secretion, stimulates prolactin release bothin vitro (13–20) and in vivo (21). DA exerts its actionthrough the binding and activation of specific receptorswhich belong to the G protein-coupled receptor super-family. Five distinct DA receptor subtypes (D1-D5), whichare encoded by separate genes, have been identified(22–25), yet it is widely believed that only the D2receptors are responsible for the inhibitory actions of DAin the anterior pituitary (26). The genomic sequence ofthe dopamine D2 receptor is alternatively spliced to

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generate two isoforms (a short isoform (D2s) and a longisoform (D2l)) which differ by a 29 amino acid insert inthe third cytoplasmic domain (27–30). D2s and D2lreceptors are structurally very similar, and they are alsopharmacologically indistinguishable.

We have previously shown that DA at very lowconcentrations, stimulates, and at high concentrations,inhibits prolactin secretion from cloned GH4ZR7 cells(20). The GH4ZR7 cells express only the short isoform ofthe rat dopamine D2s receptor by transfecting the D2sreceptors into GH4C1 cells (31). The GH4C1 cells do notexpress any catecholamine receptors (32). In this study,we further explored the nature of the stimulatoryaction of DA on prolactin release using different DAagonists and antagonists. We selected three well-established D2 receptor agonists (dopamine, apomor-phine and bromocriptine) to investigate whether theDA-induced stimulatory effect on prolactin secretion inGH4ZR7 cells is unique to DA, or if it is a generalphenomenon of other DA agonists. In addition, weused a dopamine D2 receptor antagonist (haloperidol)and an alkylating agent (phenoxybenzamine) of a-adrenergic and DA receptors (33) to confirm that allthe observed dopaminergic actions were indeedmediated via activation of dopamine D2 receptors onthe GH4ZR7 cells.

Materials and methods

Cell culture

The GH4ZR7 cell line was kindly provided by Dr HElsholtz, Clinical Biochemistry, University of Toronto,Canada. The cells were cultured in a mixture (3:4) ofHam’s Nutrient Mixture F-12 (Connaught LaboratoriesLtd, Willowdale, Canada) and Dulbecco’s modifiedEagle’s medium (DMEM) (Gibco Lab., Grand Island,NY, USA) supplemented with 2.5% fetal calf serum,15% horse serum (Gibco Lab.) and penicillin 50 IU/ml(Sigma Chemical Co., St Louis, MO, USA) (culturemedium). The cells were grown in tissue culture flasks(75 cm2 style, Becton Dickinson Labware, Lincoln Park,NJ, USA) which were incubated at 37 8C under ahumidified atmosphere of 5% CO2:95% air in a water-jacketed incubator (Forma Scientific, Marietta, OH, USA),and were passaged once per week into new flasks. Forcollecting cells from the monolayer in the flask, we usedjet streams of medium (obtained by squeezing medium inand out of a Pasteur pipette) to lift up cells from thesurface without any treatment with a proteinase, andthen we harvested the lifted cells by centrifugation(700×g for 5 min). This method eliminates the possibilitythat receptors may be damaged by enzyme treatment.

ChemicalsAll experimental solutions were prepared immediatelyprior to use. Dopamine (dopamine-HCl, Sigma Chemical

Co.) was dissolved in appropriate volumes of DMEMcontaining bovine serum albumin (1 mg/ml) (DMEM-BSA) to make solutions of different concentrations.Bromocriptine and haloperidol (Sigma Chemical Co.)were first dissolved in absolute alcohol (10 mmol/l) andthen diluted with DMEM-BSA to the appropriateconcentrations.

Incubation experimentsThe collected cells were resuspended in culture mediumat a concentration of 105 cells/ml and 0.5 ml cellsuspension was distributed to each well of a 24-wellmicrotiter plate (Corning Glass Work, Corning, NY,USA). Cultures were returned to the humidifiedincubator and maintained at 37 8C in a 5% CO2:95%air atmosphere for 24 to 48 h.

On the day of experiment, plated cells were rinsedtwice with DMEM-BSA. All the experimental solutionswere prepared in DMEM-BSA, and 1 ml experimentalsolution was applied to each well. After the cellswere incubated with the experimental solutions inthe incubator for 4 h, the supernatant from each wellwas gently pipetted from the wells and storedin disposable cups (24×14 mm, Sarstedt CanadaInc., V-St Laurent, Quebec, Canada) at ¹20 8C untilassay.

Experiments were run at least twice and they werehighly reproducible. Quadruplicate tests were normallyperformed in each experiment using 24-well plates.When we repeated experiments with the wider ranges ofdoses, additional data were added to the originalobservations.

RadioimmunoassayOne hundred microliters of the medium were assayed intriplicate using a radioimmunoassay kit for rat prolactinkindly supplied by Drs A F Parlow and P F Smiththrough the Rat Pituitary Hormone Distribution Pro-gram. The quantity of prolactin was expressed in termsof National Institute of Diabetes and Digestive andKidney Diseases (NIADDK) rat prolactin PR-3. Coeffi-cients of variation for inter- and intra-assay variabilitywere 14.5 and 7.2% respectively. The sensitivity was0.03 ng/tube.

Data analysisThe prolactin level in a well was expressed as apercentage of the mean prolactin level of the controlwells (receiving vehicle only) on the same plate becauseprolactin levels often showed some inter-plate varia-bility. Differences between groups were evaluated byone-way ANOVA with Bonferroni post-tests in Graph-Pad Prism 2.0 (GraphPad, San Diego, CA, USA). A Pvalue <0.05 was considered to be statistically signifi-cant. Data were expressed as the mean6standard error

388 A Chang and S H Shin EUROPEAN JOURNAL OF ENDOCRINOLOGY (1999) 141


of the mean (S.E.M.) with the indicated number ofreplicates performed.

Results

Effects of dopamineThe dose–response relationships between the DAconcentrations and the amounts of prolactin releasedinto media were tested in a wide range of DAconcentrations between 10¹10 and 10¹3 mol/l after4 h incubation. Mean values of prolactin concentrationsof DA (10¹10 and 10¹9 mol/l)-treated groups were 1046 22% and 112 6 26% respectively, relative to controlvalues (10062%) but were not statistically significant.However, the prolactin concentration was graduallyelevated by increasing concentrations of DA peaking at10¹8 mol/l DA (139 6 14%) which was significantlyhigher than the control group (10062%) (Fig. 1). Asexpected, higher concentrations (10¹7-10¹5 mol/l) ofDA progressively lowered prolactin concentrations to126 6 10%, 114 6 9% and 104 6 5% respectively of

the control level as compared with the peak level of 1396 14%. When even higher concentrations of DA (10¹4

and 10¹3 mol/l) were used, prolactin concentrationsdropped further to below the control level (87 6 7% and64 6 7% respectively). However, only the highestconcentration (10¹3 mol/l) of DA induced a statisticallysignificant decrease in prolactin secretion from the cells(Fig. 1). The inhibitory action of DA on GH4ZR7 cells ismuch less sensitive than on primary cultured ratpituitary cells.

Effects of apomorphine

Apomorphine effects on prolactin release were tested ina wide range of concentrations between 10¹12 and10¹4 mol/l after 4 h incubation, and a biphasic effectwas observed. Apomorphine at a very low concentra-tion (10¹10 mol/l) significantly elevated prolactin con-centrations in the medium to 138 6 13% of the controllevel (10063%). On the other hand, higher concentra-tions of apopmorphine progressively decreased prolactinconcentrations, and the reduction reached a statistically

The D2 receptor stimulates prolactin release 389EUROPEAN JOURNAL OF ENDOCRINOLOGY (1999) 141

Figure 1 Dose–response relationship between DA concentrations and prolactin release. Plated cells were incubated with DMEMcontaining 0.1% BSA plus various concentrations of DA for 4 h at 37 8C. Results are expressed as a percentage of control prolactin (PRL)concentration (10062% when DA=0 mol/l). The numbers in parentheses represent the number of repetitions of the particular experiment.Vertical bars=S.E.M. *P<0.05.


significant level only at 10¹4 mol/l apomorphine whichlowered the level of prolactin secretion to only 5169%of the control level (Fig. 2). In our experimentalconditions, the stimulatory action of apomorphinewas more potent (peak concentration 10¹10 mol/l)than that of DA (peak concentration 10¹8 mol/l) whilethe inhibitory actions of DA and apomorphine wereapproximately equal.

Effects of bromocriptineFigure 3 shows the dose-dependent effects of bromo-criptine on prolactin release from the cultured cells.After a 4-h incubation period, low concentrations ofbromocriptine (between 10¹12 and 10¹7 mol/l) did notsignificantly change prolactin concentrations in themedium when compared with the control group, butthe prolactin concentration was progressively elevatedwith concentrations of bromocriptine from 10¹9 mol/land peaked at 155 6 14% of control level (10062%)with 10¹6 mol/l bromocriptine (Fig. 3). In contrast, ahigher concentration of bromocriptine (10¹4 mol/l)

significantly inhibited prolactin release to only 41 65% of the control level (Fig. 3). All three DA agonistsshowed biphasic responses.

Effects of haloperidol on dopamine-inducedprolactin release

Effects of different concentrations of DA were tested inmedium containing haloperidol (10¹4 mol/l). Thehaloperidol was unable to block the stimulatoryaction of 10¹8-10¹6 mol/l DA on prolactin release(Fig. 4). The level of prolactin release was elevated to143 6 18% of the control value when cells wereincubated with 10¹8 mol/l DA plus haloperidol(10¹4 mol/l), and this increase was comparable tothat seen when cells were incubated with 10¹8 mol/lDA alone (Fig. 1). On the other hand, the inhibitoryeffect of 10¹4-10¹3 mol/l DA was abolished in thepresence of this DA antagonist. There was no differencein the prolactin concentrations between the controland the treated group with either 10¹4 mol/l (98 69%) or 10¹3 mol/l (105 6 8%) DA plus haloperidol


Figure 2 Dose–response relationship between apomorphine (Apo) concentrations and prolactin release. Plated cells were incubated withDMEM containing 0.1% BSA plus various concentrations of Apo for 4 h at 37 8C. Results are expressed as a percentage of control prolactin(PRL) concentration (10063% when Apo=0 mol/l). The numbers in parentheses represent the number of repetitions of the particularexperiment. Vertical bars=S.E.M. *P <0.05.


(10¹4 mol/l) when compared with the control(10064%).

Effects of pretreatment of cells withphenoxybenzamine on dopamine-inducedprolactin releasePre-incubation of cells with phenoxybenzamine(10¹6 mol/l) for 1 h markedly reduced both thestimulatory and inhibitory effects of DA on prolactinrelease. Prolactin concentrations remained at about thebasal level when the cells were incubated with DA atconcentrations between 10¹8 and 10¹6 mol/l (Fig. 5).When cells were incubated with 10¹5 and 10¹4 mol/lDA prolactin concentrations were 115 6 14% and 1166 14% respectively, but neither increase was statisti-cally significant.

DiscussionIt is well established that DA inhibits prolactin release;however, we have observed that a low concentration of

DA stimulates prolactin release while a high concentra-tion inhibits prolactin release from primary cultured ratpituitary cells (13) and, more recently, in the culturedpituitary tumor cell line (GH4ZR7) (20). Several othergroups of workers (14–19) have also reported astimulatory effect of DA in primary cultured lactotrophs,and their observations, along with ours, stronglysupport the notion that DA has dual actions –stimulatory and inhibitory – on prolactin release.However, it is not established whether the stimulatoryaction originates from activation of a stimulatorysubtype of DA receptor, or whether a dopamine D2receptor subtype is responsible for both the stimulatoryand inhibitory actions.

Multiple types and subtypes of DA receptors areexpressed in pituitary cells. While the D2 receptor hasbeen widely accepted as the inhibitory receptor, thestimulatory receptor, which mediates the stimulatorysignals of dopaminergic agents, has not been clearlydefined as yet (34). Dopamine D5 receptors are presentin primary cultured pituitary cells, and stimulateprolactin release in vitro (17) suggesting that the


Figure 3 Dose–response relationship between bromocriptine (BRCT) concentrations and prolactin release. Plated cells were incubatedwith DMEM containing 0.1% BSA plus various concentrations of BRCT for 4 h at 37 8C. Results are expressed as a percentage of controlprolactin (PRL) concentration (10062% when BRCT=0 mol/l). The numbers in parentheses represent the number of repetitions of theparticular experiment. Vertical bars=S.E.M. *P<0.05.


stimulatory action of DA on prolactin release could bemediated by the stimulatory DA receptor subtype.Nevertheless, the existence of a stimulatory subtype inthe primary cultured cells does not exclude thepossibility that one DA receptor subtype could mediatethe two different signals of DA via different signalpathways. Therefore, we chose GH4ZR7 cells in thisstudy. GH4ZR7 cells are derived from GH4C1 cells bytransfecting D2s receptors (31), and the GH4C1 cellshave many different receptors such as TRH and VIPreceptors, but not DA receptors (32). Dopamine inhibitsprolactin release via the inhibition of cyclic AMPproduction. When GH4C1 and GH4ZR7 cells weretreated with dopamine, cyclic AMP production wasnot inhibited in GH4C1 cells, but it was effectivelyinhibited in GH4ZR7 cells (35) indicating that GH4C1cells do not have dopamine receptors. The use ofGH4ZR7 cells is ideal to clarify whether one subtype ofD2 receptors (D2s) can stimulate both a stimulatory andan inhibitory action on prolactin release. In fact, wehave recently reported that a low concentration of DA(10¹7 mol/l) stimulates while a high concentration of

DA (10¹4 mol/l) inhibits prolactin release in GH4ZR7cells using a perifusion system (20). In the present study,we continued to clarify the nature of the dopaminergiceffects, focusing on the stimulatory action on prolactinrelease. Since the studied cells express only one subtypeof DA receptor, the responses induced by dopaminergicagonists should be the result of activating dopamine D2sreceptors, supporting the concept that the D2s receptorsalone can mediate both stimulatory and inhibitorysignals of DA in lactotrophs.

Three well-established dopaminergic agonists (dopa-mine, apomorphine and bromocriptine) were tested atvarious concentrations under specified conditions.These dopaminergic agonists inhibit prolactin releasebut their inhibitory characteristics are very different(36). All three dopaminergic agonists have stimulatoryas well as inhibitory actions on prolactin release inGH4ZR7 cells, but their potencies regarding the stimu-latory action are different. We concluded that thestimulatory effects on prolactin release are not limitedonly to DA but are common to other dopaminergicagents.


Figure 4 Dose–response relationship between DA concentrations and prolactin release. Plated cells were incubated with DMEMcontaining 0.1% BSA and 10¹4 mol/l haloperidol plus various concentrations of DA for 4 h at 37 8C. Results are expressed as a percentageof control prolactin (PRL) concentration (10064% when DA=0 mol/l). The numbers in parentheses represent the number of repetitions ofthe particular experiment. Vertical bars=S.E.M. *P<0.05.


Prolactin release is usually inhibited by approxi-mately 10¹8 mol/l DA in primary cultured rat pituitarycells (36), but it is inhibited by about 10¹4 mol/l DA inGH4ZR7 cells, indicating that GH4ZR7 cells are muchless sensitive to DA than primary cultured lactotrophs.The difference in the potencies is probably due todifferent numbers of active D2 receptors (10). Incontrast to our finding that DA (10¹6 mol/l) stimulatesprolactin release from GH4ZR7 cells, Burris and Free-man (18), and Porter and coworkers (17) observed that10¹6 mol/l DA inhibited prolactin release and nostimulatory action was shown. The DA concentration(10¹6 mol/l) which inhibited prolactin release in theother studies is much lower than that which weobserved (10¹4-10¹3 mol/l) to be the inhibitoryconcentration. Differences in experimental conditions,such as different number of passages of cloned cells, thepresence or the absence of a DA stabilizer (ascorbic acidand sodium metabisulfite), and different incubationmedia, can produce different results. Under our

experimental conditions, the stimulatory actions wereshown in both static monolayer culture (Figs 1–3) andperifusion systems (20). The significance of our study isnot the different potencies of DA agonists but thedemonstration that one subtype of D2 receptors canboth stimulate and inhibit prolactin release dependingon the concentrations of agonists.

It is interesting to point out that the three dopamineD2 receptor agonists seem to have very differentpotencies regarding their stimulatory actions inGH4ZR7 cells. Dopamine, apomorphine and bromocrip-tine produced their peak stimulatory effects at 10¹8,10¹10 and 10¹6 mol/l respectively. Regarding theinhibitory actions of the three DA agonists, on theother hand, our laboratory previously demonstratedthat bromocriptine inhibited prolactin synthesis andrelease more potently than DA in primary cultured ratpituitary cells (36, 37), and Denef et al. (14) demon-strated earlier that apomorphine is more potent thanDA in suppressing prolactin release in primary rat


Figure 5 Dose–response relationship between DA concentrations and prolactin release in phenoxybenzamine-pretreated cells. Platedcells were pretreated with 10¹6 mol/l phenoxybenzamine for 1 h and then were incubated with DMEM containing 0.1% BSA plus variousconcentrations of DA for 4 h at 37 8C. Results are expressed as a percentage of control prolactin (PRL) concentration (10063% whendopamine=0mol/l). The numbers in parentheses represent the number of repetitions of the particular experiment. Vertical bars=S.E.M.


pituitary cultures. However, in this study we observedthat their peak inhibitory actions were at similarconcentrations, between 10¹4 and 10¹3 mol/l.

In the latter part of this study, we pre-treated GH4ZR7cells with phenoxybenzamine before we introducedthem to a dopaminergic agent. Exposure of D2 receptor-expressing cells (e.g. GH4C1-hD2s) to the receptoralkylating compound, phenoxybenzamine, for 1 hcould abolish D2 receptors (38), and we showed thatthe treatment markedly reduced the DA-mediatedinhibition of prolactin release from primary culturedrat pituitary cells while phenoxybenzamine pretreat-ment was ineffective on somatostatin-, TRH-, andangiotensin-II-induced prolactin release (33). Ourobservation that phenoxybenzamine inactivates DA-induced prolactin release, is consistent with our prevousstudies (33). Taken together, our observations furthersupport the concept that DA-induced prolactin releasein the GH4ZR7 cells is mediated by dopamine D2sreceptors.

D2 receptor agonists and antagonists act on the D2receptor, but they have different potencies and affinitiesto the receptor. In addition, the patterns of prolactinrelease induced by these agonists and antagonists canalso be different. For example, both DA and bromocrip-tine inhibit prolactin release via activating dopamine D2receptors, yet only withdrawal of DA infusion, but notwithdrawal of bromocriptine infusion, causes rebound-ing of prolactin release over the basal rate in a perifusionsystem of primary cultured rat pituitary cells (36). Bothpimozide and d-butaclamol are well-established D2receptor antagonists, but temporal patterns of prolactinrelease in male rats after injection of pimozide ord-butaclamol are substantially different (39). Moreover,we observed that haloperidol was unable to block thestimulatory action on prolactin release. These observa-tions infer that these agonists and antagonists act on D2receptors but that there are subtle differences in theirmodes of action.

It is possible that a broken-down product of dopaminemay show stimulatory action by stimulating otherreceptors, since dopamine can easily be oxidized.However, no qualitative difference between freshlymade dopamine and 1-h old dopamine was observed(10). Bromocriptine, a stable compound, also showedstimulatory action suggesting that no degradationproduct of dopamine generated the stimulatory action.

Dopamine should act on D2s receptors to inhibit andstimulate prolactin release, and haloperidol should alsoact on the same D2s receptors to block the dopamineactions. However, the haloperidol was unable to blockthe stimulatory action. We have tentatively proposedthe following hypothesis to explain the inability of anantagonistic action of haloperidol on the stimulatoryaction. Dopamine binding to D2s receptors is largelyblocked by haloperidol but haloperidol cannot comple-tely block the receptor binding of dopamine sincedopamine and haloperidol are in an equilibrium state

on dopamine D2s receptors. High concentrations ofdopamine–receptor complex are required to inhibitprolactin release and thus the inhibitory action wasblocked by haloperidol. However, the reduced portion ofdopamine–receptor complex may be sufficient tostimulate prolactin release.

Finally, the chemical nature of the physiological PRFis not yet established; several biologically activechemicals are considered to be PRFs (40). DA is themajor PIF, but it may also play a role as a PRF since lowconcentrations of DA stimulate prolactin release.

AcknowledgementsThis work was supported by the Medical ResearchCouncil of Canada. Appreciation is expressed to Dr A FParlow and Dr S Raiti for supplying the rat prolactinradioimmunoassay kit through the NIADDK RatPituitary Hormone Distribution Program.

References1 MacLeod RM. Regulation of prolactin secretion. In Frontiers in

Neuroendocrinology, vol 4, pp 169–194. Eds L Martini & WFGanong. New York: Raven Press, 1976.

2 Leong DA, Frawley LS & Neill JD. Neuroendocrine control ofprolactin secretion. Annual Review of Physiology 1983 45 109–127.

3 Melmed S. Control of prolactin synthesis and secretion. Seminarsin Reproductive Endocrinology 1984 2 1–8.

4 Neill JD. Prolactin secretion and its control. In The Physiology ofReproduction, pp 1379–1390. Eds E Knobil & JD Neill. New York:Raven Press, 1988.

5 Chen CL, Amenomori Y, Lu KH, Voogt JL & Meites J. Serumprolactin levels in rats with pituitary transplants or hypothalamiclesions. Neuroendocrinology 1970 6 220–227.

6 Reymond MJ & Porter JC. Involvement of hypothalamic dopaminein the regulation of prolactin secretion. Hormone Research 198522 142–152.

7 Ben-Jonathan N, Arbogast LA & Hyde JF. Neuroendocrineregulation of prolactin release. Progress in Neurobiology 1989 33399–447.

8 Ben-Jonathan N. Regulation of prolactin secretion. In ThePituitary Gland, pp 261–283. Ed H Imura. New York: RavenPress, 1994.

9 Lamberts SWJ & MacLeod RM. Regulation of prolactin secretionat the level of the lactotroph. Physiological Review 1990 70 279–318.

10 Shin SH, Si F, Chang A & Ross GM. Dopamine requires ascorbicacid to be the prolactin release-inhibitory factor. Journal ofAmerican Physiology 1997 273 E593–E598.

11 Daughaday WH. The anterior pituitary. In Williams Textbook ofEndocrinology, 7th edn, pp 568–613. Eds JD Wilson & DW Foster.Philadelphia: Saunders, 1985.

12 Martinez de la Escalera G & Weiner RI. Transient removal ofdopamine potentiates the stimulation of prolactin release by TRHbut not VIP: stimulation by Ca2+/protein kinase C pathway.Neuroendocrinology 1988 47 38–45.

13 Shin SH. Dopamine-induced inhibition of prolactin release fromcultured adenohypophysial cells: spare receptors for dopamine.Life Sciences 1978 22 67–73.

14 Denef C, Manet D & Dewals R. Dopaminergic stimulation ofprolactin release. Nature 1980 285 243–246.

15 Kramer IM & Hopkins CR. Studies on the kinetics of dopamine-regulated prolactin secretion. Molecular and Cellular Endocrinology1982 28 191–198.



16 Burris TP, Nguyen DN, Smith SG & Freeman ME. The stimulatoryand inhibitory effects of dopamine on prolactin secretion involvedifferent G-proteins. Endocrinology 1992 130 926–932.

17 Porter TE, Grandy D, Bunzow J, Wiles CD, Civelli O & Frawley LS.Evidence that stimulatory dopamine receptors may be involved inthe regulation of prolactin secretion. Endocrinology 1994 1341263–1268.

18 Burris TP & Freeman ME. Comparison of the forms of thedopamine D2 receptor expressed in GH4C1 cells. Proceedings ofthe Society for Experimental Biology and Medicine 1994 205 226–235.

19 Kineman RD, Gettys TW & Frawley LS. Paradoxical effects ofdopamine (DA): Gi alpha 3 mediates DA inhibition of PRL releasewhile masking its PRL-releasing activity. Endocrinology 1994 135790–793.

20 Chang A, Shin SH & Pang SC. Dopamine D2 receptor mediatesboth inhibitory and stimulatory actions on prolactin release.Endocrine 1997 7 177–182.

21 Arey BJ, Burris TP, Basco P & Freeman ME. Infusion of dopamineat low concentrations stimulates the release of prolactin fromalpha-methyl-p-tyrosine-treated rats. Proceedings of the Society forExperimental Biology and Medicine 1993 203 60–63.

22 Sibley DR & Monsma FJ Jr. Molecular biology of dopaminereceptors. Trends in Pharmacological Sciences 1992 13 61–69.

23 Civelli O, Bunzow JR & Grandy DK. Molecular diversity of thedopamine receptors. Annual Reviews in Pharmacology and Toxicol-ogy 1993 32 281–307.

24 Gingrich JA & Caron MG. Recent advances in the molecularbiology of dopamine receptors. Annual Review in Neuroscience1993 16 299–321.

25 Sokoloff P & Schwartz J-C. Novel dopamine receptors half a decadelater. Trends in Pharmacological Sciences 1995 16 270–275.

26 Memo M, Castelletti L, Missale C, Valerio A, Carruba M & SpanoPF. Dopaminergic inhibition of prolactin release and calciuminflux induced by neurotensin in anterior pituitary is independentof cyclic AMP system. Journal of Neurochemistry 1986 47 1689–1695.

27 Dal Toso R, Sommer B, Ewert M, Herb A, Pritchett DB, Bach A,Shivers BD & Seeburg PH. The dopamine D2 receptor: twomolecular forms generated by alternative splicing. EMBO Journal1989 8 4025–4034.

28 Giros B, Sokoloff P, Martres MP, Riou JF, Emorine LJ & Schwartz JC.Alternative splicing direct the two D2 dopamine receptor isoforms.Nature 1989 342 923–926.

29 Monsma FJ Jr, McVittie LD, Gerfen CR, Mahan LC & Sibley DR.Multiple D2 dopamine receptors produced by alternative RNAsplicing. Nature 1989 342 926–929.

30 Falardeau P. Functional distinctions of dopamine D2long andD2short receptors. In Dopamine Receptors and Transporters, pp 323–342. Ed HB Niznik. New York: Marcel Dekker Inc., 1994.

31 Albert PR. Heterologous expression of G protein-linked receptorsin pituitary and fibroblast cell lines. Vitamins and Hormones 199448 59–109.

32 Tashjian AH. Clonal strains of hormone-producing pituitary cells.Methods in Enzymology 1979 58 527–535.

33 Shin SH, McAssey K, Heisler RL & Szabo MS. Phenoxybenzamineselectively and irreversibly inactivates dopaminergic D2 receptorson primary cultured rat lactotrophs. Neuroendocrinology 1992 56397–406.

34 Tagawa R, Takahara J, Sato M, Niimi M, Murao K & Ishida T.Stimulatory effects of quinpirole hydrochloride, D2-dopaminereceptor agonist, at low concentrations on prolactin release infemale rats in vitro. Life Science 1992 51 727–732.

35 Albert PR, Neve KA, Bunzow JR & Civelli O. Coupling of a clonedrat dopamine-D2 receptor in inhibition of adenylyl cyclase andprolactin secretion. Journal of Biological Chemistry 1992 2652029–2104.

36 Hanna S & Shin SH. Differential inhibition of dopamine andbromocriptine on induced prolactin release: multiple sites for theinhibition of dopamine. Neuroendocrinology 1992 55 591–599.

37 Barton RE & Shin SH. Bromocriptine inhibits prolactin synthesismore effectively than dopamine. Endocrine Journal 1994 2 639–643.

38 Nilsson CL, Hellstrand M, Ekman A & Eriksson E. Inverse agonismat dopamine D2 receptors: haloperidol-induced prolactin releasefrom GH4C1 cells transfected with the human D2 receptor isantagonized by R(-)-n-propylnorapomorphine, raclopride, andphenoxybenzamine. Neuropsychopharmacology 1996 15 53–61.

39 Shin SH. Pulsatile secretion of prolactin in the male rat afterpimozide administration is not due to pulsatile inhibition of PIFsecretion. Life Sciences 1979 24 1751–1756.

40 Shin SH, Papas S & Obonsawin MC. Current status of the ratprolactin releasing factor. Canadian Journal of Physiology andPharmacology 1987 65 2036–2043.

Received 4 February 1999Accepted 3 June 1999



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