《动物生理学》课程教学资源（文献资料）催乳素综述 Prolactin - Structure, Function, and Regulation of Secretion

PROLACTIN 1543 Circadian input Environmental stimuli Progesteror pgcocortic Sound Stress Reproductive stimuli alamteregulgtoycthreu PIF DA SST.GABA TRH.NT.OT. Long portal vein orlobeothepn gu PRL with various r family (279,280,1171).Recent evi 1.Biogenic amines ve anterior A)DOPAMINE.I Dopamine is the major PIF.Observa- 9251526e nyperp. totroph adenoma in both male and female D.knockout in n(79.3 eminence (593)and the hypophysial stalk blood (140,142. endocrine dopaminergi 621,1422)led several investigators to conclude that do- method (5).pahlstrom and Fuxe (379)mapped the pamine is the major physiological hypothalamic PIF. Am- catecholaminergic neuron populations and classified ple experim them as Al to A. ng to their rostrocaudal distri and in vitrg (096-108s subsea tly donamine re ceptors have been detected on pituitary membranes hypothalamus (termed A14and A12,respectively)provide (226,369,373,639).Dopamine receptors located on lactotroph membranes belong to the D.subclass of the

estrous cycle (e.g., ovariectomized females with various ovarian steroid replacement) are also considered. 1. Biogenic amines A) DOPAMINE. I) Dopamine is the major PIF. Observa￾tions that drugs affecting catecholamine metabolism also alter prolactin secretion (79, 361) and the fact that dopa￾mine is present in high concentration in both the median eminence (593) and the hypophysial stalk blood (140, 142, 621, 1422) led several investigators to conclude that do￾pamine is the major physiological hypothalamic PIF. Am￾ple experimental evidence shows that dopamine inhibits prolactin release from pituitary lactotrophs both in vivo and in vitro (1096–1098). Subsequently, dopamine re￾ceptors have been detected on pituitary membranes (226, 369, 373, 639). Dopamine receptors located on lactotroph membranes belong to the D2 subclass of the dopamine receptor family (279, 280, 1171). Recent evi￾dence emphasizes the physiological importance of hy￾pothalamic dopamine in regulating lactotroph function. Mice with a disrupted D2 receptor gene have anterior lobe lactotroph hyperplasia and hyperprolactinemia (925, 1526). The hyperplasia ultimately leads to lac￾totroph adenoma in both male and female D2 knockout mice (92). II) Anatomy of the neuroendocrine dopaminergic neurons. Using the Falk and Hillarp amine-fluorescence method (533), Dahlstro¨ m and Fuxe (379) mapped the catecholaminergic neuron populations and classified them as A1 to A15 according to their rostrocaudal distri￾bution in the CNS (379). The dopaminergic neurons of the periventricular and arcuate nuclei of the medial-basal hypothalamus (termed A14 and A12, respectively) provide dopamine to the pituitary gland (655, 656, 783, 922). The A14 and A12 dopaminergic neuron populations FIG. 3. An overview of the regulation of prolactin secretion. Prolactin secre￾tion is paced by a light-entrained circa￾dian rhythm, which is modified by envi￾ronmental input, with the internal milieu and reproductive stimuli affecting the in￾hibitory or stimulatory elements of the hypothalamic regulatory circuit. The final common pathways of the central stimu￾latory and inhibitory control of prolactin secretion are the neuroendocrine neu￾rons producing prolactin inhibiting fac￾tors (PIF), such as dopamine (DA), so￾matostatin (SST), and g-aminobutyric acid (GABA), or prolactin releasing fac￾tors (PRF), such as thyrotropin releasing hormone (TRH), oxytocin (OT), and neu￾rotensin (NT). PIF and PRF from the neu￾roendocrine neurons can be released ei￾ther at the median eminence into the long portal veins or at the neurointermediate lobe, which is connected to the anterior lobe of the pituitary gland by the short portal vessels. Thus lactotrophs are reg￾ulated by blood-borne agents of central nervous system or pituitary origin (a-me￾lanocyte stimulating hormone) delivered to the anterior lobe by the long or short portal veins. Lactotrophs are also influ￾enced by PRF and PIF released from neighboring cells (paracrine regulation) or from the lactotrophs themselves (au￾tocrine regulation). October 2000 PROLACTIN 1543

1544 FREEMAN,KANYICSKA,LERANT,AND NAGY Vobme 80 MR TIDA THDA 17 ME LP of the he se to s oI th SP ine of TiDA THDA A.and PHDA origin cone of the r can be divided of theent systems s136s nd me sig the distinct lobes of the pituitary gland (Fig.4).TIDA icant differences hetween the regulator neurons are located mostly in the dorsomedial part of the ( basa al TIDA activity (412,414)and r ponsiveness to pro 4 in male rats hy endog ous opioids,which is not neural lobes of the pituitary gland are innervated by two in female rats(1123).TIDA neuronal activity is decreased t groups of hypothala mic dopaminer by ovariectomy and increased by orchidectomy (708) () The perA14 ere 1 hypothalamic periventricular nucleus and terminate in to str which decreases TIDA activity in females.but the intermediate lobe (655).On the other hand,THDA not in male rats (415).Because all of the above experi- neurons are found in the rostral arcuate nucleus A12 ments were performed in vivo,it cannot be firmly con- en the t the sexual specificity was directly at the do (783 784)The microanatomy and hiocher as well Due to the preponderant influence of dopamine on as the distinct physiological functions of the TIDA and prolactin secretion at the pituitary level and the well- THDA neurons were first described by Holzbauer feedback action of prolactin on these neu Rack o 1 ent studies provid rons,It Is d late the three neu ndocrine dopaminerg ms of the Therefore until rec ently the role of dor amine as neuro thalamus(419.657.759.92210761279.12811373.1414 transmitter in regulation of prolactin secretion was poorly UD TIDA n trons and their regulatory propertie documented.Nevertheless,dopamine can affect prolactir Dopamin hough long porta comsideroihem4orplysOiogCalPegaiorotpoOh 156.4 male rats ific D.antagonists elevate,whereas D secretion(1024). agonists (88)decrease dihydroxyphenylacetic acid TIDA neurons have unique regulatory properties (DOPAC)content in the median eminence (155,475)

can be divided into three anatomically and functionally different systems based on the rostrocaudal distribution of the dopaminergic perikarya and their terminal fields in the distinct lobes of the pituitary gland (Fig. 4). TIDA neurons are located mostly in the dorsomedial part of the arcuate nucleus (A12) and project to the external zone of the median eminence (922) where dopamine is released into the perivascular space surrounding the capillary loops of the pituitary portal system. The intermediate and neural lobes of the pituitary gland are innervated by two virtually independent groups of hypothalamic dopaminer￾gic neurons (130, 656). The periventricular hypophysial dopaminergic (PHDA) neurons (A14) are located in the hypothalamic periventricular nucleus and terminate in the intermediate lobe (655). On the other hand, THDA neurons are found in the rostral arcuate nucleus (A12) between the previous two cell groups and project to both the intermediate and the neural lobe of the pituitary gland (783, 784). The microanatomy and biochemistry, as well as the distinct physiological functions of the TIDA and THDA neurons, were first described by Holzbauer and Racke (783). Subsequent studies provided more detailed morphological and functional characterization of the three neuroendocrine dopaminergic systems of the hypo￾thalamus (419, 657, 759, 922, 1076, 1279, 1281, 1373, 1414). III) TIDA neurons and their regulatory properties. Dopamine of TIDA origin, delivered though long portal vessels into the sinusoid capillaries of the anterior lobe, is considered the major physiological regulator of prolactin secretion (1024). TIDA neurons have unique regulatory properties compared with other dopaminergic neurons of the CNS, like the nigrostriatal (NSDA) and mesolimbic (MLDA) dopaminergic neurons (1368). Moreover, there are signif￾icant differences between the regulatory properties of TIDA neurons in males and females (1368). In females, basal TIDA activity (412, 414) and responsiveness to pro￾lactin are higher than in males (414). This may be ex￾plained by tonic inhibition of the activity of TIDA neurons in male rats by endogenous opioids, which is not present in female rats (1123). TIDA neuronal activity is decreased by ovariectomy and increased by orchidectomy (708). These effects were reversed by appropriate steroid treat￾ment (709). There are sexual differences in the response to stress, which decreases TIDA activity in females, but not in male rats (415). Because all of the above experi￾ments were performed in vivo, it cannot be firmly con￾cluded that the sexual specificity was directly at the do￾paminergic neuron. Due to the preponderant influence of dopamine on prolactin secretion at the pituitary level and the well￾established feedback action of prolactin on these neu￾rons, it is difficult to separate/isolate the central effects of dopamine from its direct influence on prolactin secretion. Therefore, until recently, the role of dopamine as neuro￾transmitter in regulation of prolactin secretion was poorly documented. Nevertheless, dopamine can affect prolactin secretion by acting centrally, in addition to its direct action on the lactotroph (155, 156, 474, 475, 486, 487). In male rats, specific D1 antagonists elevate, whereas D1 agonists (88) decrease dihydroxyphenylacetic acid (DOPAC) content in the median eminence (155, 475), FIG. 4. Neuroendocrine dopaminergic neuron populations in the rat hypothalamus. Perikarya of the periventricular hypothalamic dopaminergic (PHDA) neurons (A14 cell group) are located in the periventricular nucleus, and their axons terminate in the intermediate lobe of the pituitary gland (IL). The arcuate nucleus (A12 cell group) contains the perikarya of two distinct neuroendocrine dopaminergic neuron populations. The tuberohypophysial dopaminergic (THDA) neu￾rons project from the rostral arcuate nucleus both to the neural (NL) and intermediate (IL) lobes of the pituitary gland. From the dorsomedial part of the arcuate nucleus the tuberoinfundibular (TIDA) neurons project to the external zone (EZ) of the median eminence (ME). TIDA terminals release dopamine into the perivascular spaces of the fenestrated capillary loops of the EZ, giving rise to the long portal veins (LP). The long portal veins empty into the sinusoids of the anterior lobe (AL) of pituitary gland. Small short portal (SP) veins connect the fenestrated capillaries of the neural and intermediate lobes with the anterior lobe sinusoids. Thus dopamine of TIDA, THDA, and PHDA origin can reach lactotrophs, located in the anterior lobe of the pituitary gland. III.v, 3rd ventricle; OC, optic chiasma; MB, mammillary body; IZ, internal zone of the median eminence; PS, pituitary stalk. [From Lerant et al. (1029). Copyright The Endocrine Society.] 1544 FREEMAN, KANYICSKA, LERANT, AND NAGY Volume 80

PROLACTIN 1545 ntormediated inactivation of TIDA neu cyte stimulating hormone (o-MSH)from melanotrophs of rons.Because D receptors have been shown to be cou- the intermediate lobe(201,1054,1747).Therefore,assum- pled to GorG.proteins (1401)that stimulate adenylat ing that PHDA neurons participate in the regulation o teH du ges山 neuron innervating IDA neurons stimulus like suckling (1811).However,it has Other pharmacological experiments suggest that shown that there is no change in plasma a-MSH in re TIDA neurons receive stimulatory input thro ugh D2 recep sponse to nursing (933).Therefore,an acute diminution in the ac I d- d h adence is t [Io gic)interneurons (1124).In earlier pharmacological ex. vations that the dopamine concentration in the periments acute administration of less selective dopa termediate lobe is lower and the basal level of plasma mine agonists eg apomorphine)or anta agonists (e.g higher in lactating than in cycling female rats the act 11)ind 413.539 the using pharmacological probes mo tive at different V)Is dope mine the sole PIf?The auestion whethe dopamine receptor subtypes,it has been concluded that a dopamine is the sole PIF mediating tonic hypothalamic simultaneous activation or inhibition of D and Da recep- inhibition is still unsettled.In early studies of this issue TIDA cancels th eceptors on alk rep the IV)THDA and PHDA ds of the prolactin inhibition ormally obse /402 released from PHDA and THDA axon terminals at the 1299).This conclusion was based on quantitative studies neurointermediate lobe has attracted m e attention dur ted of en the past n repo nit wi e ty is ind and the do mine infusion was set to mimic the levels measured in ences sbetween male and female rats in the activity of stalk blood of intact animals (621,1024). Although the THD d that TIDA inhibitor on pit prolacti the f ion of response to the suckling stimulus (1281 Surgical re- donamine and pituitary secretion of prolactin does not moval of the neurointermediate lobe results in a three-to always exist.For instance,the dopamine level in hypo incre e in b I plasma pr mal physial sta time lowe in male a th t chemically detectable dopamine in the anterior pituitary age relationshin hetween don mine coneentrations of gland is reduced after surgical removal of the the median eminence or the portal blood and plasma 1obe(1248) Mo and suck na not been demonstrated in lactation 49 afte 1279.Be the THDA electively activate (30. 785 1443 esolved bys suming that additional pie ce g GAba 1764),it is conceivable that dopamine,released by r nerve and somatostatin)may also contribute to the negative terminals in the neur of the may 2)are idol (a D.dor mine VD Prolactin secretion due to dopamine with can block drawal.The most plausible mechanism for an increase in non or an elevati on of dopamine prolactin release is d sinhibitio n,i.e.,that a given stimulus porta u tion phs can be afrected during lactation ntaneously at a very hial It is well known that dopamine,released from termi- rate (1024,1299,1301).Indeed,treatment of rats with nals of PHDA neurons in the intermediate lobe (19,19, sufficient amounts of a-MpT to comp ely suppress do 493,1711),tonically inhibits the secretion of a-melano- pamine secretion into hypophysial stalk blood results in

indicating D1 receptor-mediated inactivation of TIDA neu￾rons. Because D1 receptors have been shown to be cou￾pled to Go or Gs proteins (1401) that stimulate adenylate cyclase activity, these data suggest D1-mediated decrease in TIDA activity is mediated by activation of an inhibitory neuron innervating TIDA neurons. Other pharmacological experiments suggest that TIDA neurons receive stimulatory input through D2 recep￾tors (156, 474, 486). The latter influence is thought to be mediated by inhibiting inhibitory (possibly dynorphiner￾gic) interneurons (1124). In earlier pharmacological ex￾periments, acute administration of less selective dopa￾mine agonists (e.g., apomorphine) or antagonists (e.g., haloperidol) failed to alter the activity of TIDA neurons (413, 539). On the basis of recent observations made by using pharmacological probes more selective at different dopamine receptor subtypes, it has been concluded that a simultaneous activation or inhibition of D1 and D2 recep￾tors cancels the actions mediated by these receptors on TIDA neurons (475). IV) THDA and PHDA neurons. The role of dopamine released from PHDA and THDA axon terminals at the neurointermediate lobe has attracted more attention dur￾ing the past few years. For example, it has been reported that the activity of both PHDA and THDA neurons, unlike the TIDA system, is independent of circulating gonadal steroids (1279). Moreover, there are no marked differ￾ences between male and female rats in the activity of THDA neurons (759). In addition, it has recently been found that TIDA and THDA neurons, but not PHDA neu￾rons, regulate the control of the secretion of prolactin in response to the suckling stimulus (1281). Surgical re￾moval of the neurointermediate lobe results in a three- to fourfold increase in basal plasma prolactin levels in male, as well as cycling and lactating female rats (143, 1406). Consistent with this finding is the report that electro￾chemically detectable dopamine in the anterior pituitary gland is reduced after surgical removal of the posterior lobe (1248). Moreover, in lactating rats, basal- and suck￾ling-induced pituitary prolactin secretion is suppressed after water deprivation (1279). Because the THDA system is selectively activated by dehydration (30, 785, 1443, 1764), it is conceivable that dopamine, released by nerve terminals in the neurointermediate lobe of the pituitary gland, may travel to the anterior lobe through the short portal vessels to affect prolactin secretion. Indeed, halo￾peridol (a D2 dopamine receptor antagonist) pretreatment can block dehydration-induced plasma prolactin deple￾tion (1279). Thus a reduction or an elevation of dopamine level in blood carried by the short portal vessels may provide a mean by which prolactin secretion of lac￾totrophs can be affected during lactation. It is well known that dopamine, released from termi￾nals of PHDA neurons in the intermediate lobe (19, 19, 493, 1711), tonically inhibits the secretion of a-melano￾cyte stimulating hormone (a-MSH) from melanotrophs of the intermediate lobe (201, 1054, 1747). Therefore, assum￾ing that PHDA neurons participate in the regulation of prolactin secretion, one can expect parallel changes in plasma levels of prolactin and a-MSH during an acute stimulus like suckling (1811). However, it has been clearly shown that there is no change in plasma a-MSH in re￾sponse to nursing (933). Therefore, an acute diminution in the activity of PHDA-regulated a-MSH secretion does not occur during the suckling stimulus. However, the obser￾vations that the dopamine concentration in the neuroin￾termediate lobe is lower and the basal level of plasma a-MSH is higher in lactating than in cycling female rats (1811) indicate some supporting role for PHDA neurons in the regulation of prolactin secretion during lactation. V) Is dopamine the sole PIF? The question whether dopamine is the sole PIF mediating tonic hypothalamic inhibition is still unsettled. In early studies of this issue, investigators reported that the amount of dopamine in stalk blood is sufficient to account for only about two￾thirds of the prolactin inhibition normally observed (403, 1299). This conclusion was based on quantitative studies in which dopamine was replaced in rats depleted of en￾dogenous dopamine with the tyrosine hydroxylase (TH, the rate-limiting enzyme of dopamine synthesis) inhibitor a-methyl-para-tyrosine (a-MpT), and the rate of dopa￾mine infusion was set to mimic the levels measured in stalk blood of intact animals (621, 1024). Although the inhibitory influence of dopamine on pituitary prolactin secretion is established beyond a reasonable doubt, an inverse relationship between hypothalamic secretion of dopamine and pituitary secretion of prolactin does not always exist. For instance, the dopamine level in hypo￾physial stalk plasma is five to seven times lower in males than in females (140, 142, 696), but plasma levels of prolactin are not much different. Moreover, a mirror￾image relationship between dopamine concentrations of the median eminence or the portal blood and plasma prolactin has also not been demonstrated in lactation (404, 1421, 1423). The apparent inconsistency between dopaminergic activity and prolactin secretion could easily be resolved by assuming that additional PIF (e.g., GABA and somatostatin) may also contribute to the negative control of prolactin secretion. These alternative PIF can￾didates (listed in Table 2) are discussed later in this section. VI) Prolactin secretion due to dopamine with￾drawal. The most plausible mechanism for an increase in prolactin release is disinhibition, i.e., that a given stimulus reduces the tonic inhibitory effect of the hypothalamus thus freeing the pituitary gland to express its inherent capacity to secrete prolactin spontaneously at a very high rate (1024, 1299, 1301). Indeed, treatment of rats with sufficient amounts of a-MpT to completely suppress do￾pamine secretion into hypophysial stalk blood results in October 2000 PROLACTIN 1545

1546 FREEMAN,KANYICSKA,LERANT,AND NAGY Volume 80 tion uantitativ apid reduction in do ation fro ose that results can be obtained in vitro when removal of dona tion)to 10- 0r10- Mcaused a greater stimulation o mine infusion n a rapid of prola prolactin release entia ethan that evoked by complete removal of rele D).Thus s a p nech 128 ublishe 2) co-worker hich gy and d a sible r al rol change in dopaminergic neuronal activity in respons to these in vitro data.Arey et al(72)have demonstrated that the suckling stimulus.Dopaminergic neuror infusion of 10 ngkg "min dopamine to freely moving rats In the 1240. 322,1181,1219 1585 ready elevate MD The n those in which the mammary nerve was stimulated groun has cells obtained from electrically to s imulate suckling and dop ne release nor ckle d (sepa rated from their litters for 4 h)or suck was dete ted in hypophys 404, or d(for 10 min)lact nng rats that were ar 42 el his d h 3-5min)60 mine rel ingly,pituitary cells from nonsuckled rats exhibited only served(404,1421 the prolactin-inhibitory response to dopamine but never of dopamine above the ly st prolac ove Da as has /14231T1 e led to th hat decrease in dopamine outflow from the othalamus stimulus applied immediately before death re itself is insufficient to account for the uckling induced dered the prolactin cells responsive to stimulation by prolact releas recen exp hav 10 on o dopan Th f prolacti of the or lob dopam d ir eed after a 10min suekdling stimulus (1281) This the physiological relevance of this phen non Have finding seems to explain the previous controversial re. lactotrophs ir situ ever been exposed to dopamine levels sults,since inner zone has low d on m in lond TRH ANG I 1499)Donamine tration in the nortal circulatio forskolin)have been observed in lactotrophs of the inner of cycling rats is the lowest during the zone. but not th oute he ant lobe of the wever.it Is dou tful that a 5 70%d crease i pitu 1280 tion Several observations on nituitary ells in vitro indi- On the other hand,dopamine concentration in stalk blood cate that dopamine is also capable of stimulating prolac (140,142,621)is in the low nanomolar range (10- especially at lov (DM concentra ion (25 is either ineffective or has only a w inhibitor app old in vitro for exa nle lactot s that the diminution in dopamine arriving at tained from suckled lactating rats (761)or estradiol-and the anterior pituitary through the long portal vessels may progesterone-tre rats (346) not t the total amount of dopamine the a prope ed,a signific ef et al.（42 then Shin (1614)first re rted tha nal in the neurointermediate lobe (1248)and is deliverec very low concentration of dopamine (1000-fold lower through short portal vessels (420 422) than those required for maximal inhibition)could actually It has been argued that dynamic release of prolactin rat pitu ary ce 117 Burris and oo workors (251 259)have the rents mediating dop ric inhibition of pr studies using both static and dynamic cultures of pituitary lactin secretion are completely reversed when dopamine cells from cycling female rats.This latter group has found is acutely withdrawn (674.675.767.Though there is no

an increase in prolactin secretion (621, 694) quantitatively similar to those observed after suckling or stress. Similar results can be obtained in vitro when removal of dopa￾mine infusion results in a rapid increase of prolactin release (530). Thus disinhibition is a potential mechanism by which neurogenic stimuli induce release of prolactin. However, conflicting results have been reported about the change in dopaminergic neuronal activity in response to the suckling stimulus. Dopaminergic neuronal activity has been described to increase (595), remain unchanged (1240, 1835), or slightly decrease (322, 1181, 1219, 1585) during a single bout of suckling. Most direct studies have been those in which the mammary nerve was stimulated electrically to simulate suckling and dopamine release was detected in hypophysial stalk blood (404, 1423) or in the median eminence with an electrochemical probe (1421). Using this experimental paradigm, only a brief (3–5 min) 60–70% decline in dopamine release was ob￾served (404, 1421, 1423). This decline was followed by a series of rapid pulses of dopamine above the baseline which lasted for the duration of mammary nerve stimula￾tion (1423). These results have led to the conclusion that a decrease in dopamine outflow from the hypothalamus itself is insufficient to account for the suckling-induced prolactin release. However, recent experiments have clearly demonstrated that dopamine content of the inner zone of the anterior lobe obtained from lactating rats is reduced after a 10-min suckling stimulus (1281). This finding seems to explain the previous controversial re￾sults, since the inner zone of the anterior lobe has been shown to be the most responsive to the inhibitory action of dopamine (188, 1280). Moreover, increased responsive￾ness to prolactin secretagogues (like TRH, ANG II, or forskolin) have been observed in lactotrophs of the inner zone, but not the outer zone of the anterior lobe of the pituitary gland after a 10 min suckling stimulus (1280). VII) Dopamine as a stimulator of prolactin secre￾tion. Several observations on pituitary cells in vitro indi￾cate that dopamine is also capable of stimulating prolac￾tin secretion, especially at low (pM) concentration (252, 427, 979, 1614). It appears that the in vivo status of the donor animals determines the lactotrophs’ responsive￾ness to dopamine in vitro. For example, lactotrophs ob￾tained from suckled lactating rats (761), or estradiol- and progesterone-treated ovariectomized female rats (346), have a propensity to respond by stimulation when chal￾lenged with dopamine in vitro. More than a decade ago, Denef et al. (427) then Shin (1614) first reported that a very low concentration of dopamine (1,000-fold lower than those required for maximal inhibition) could actually stimulate prolactin secretion from male rat pituitary cells in vitro. Kramer and Hopkins (979) and more recently Burris and co-workers (251, 252) have extended these studies using both static and dynamic cultures of pituitary cells from cycling female rats. This latter group has found that a rapid reduction in dopamine concentration from 1027 M (a dose that maximally inhibits prolactin secre￾tion) to 10210 or 10212 M caused a greater stimulation of prolactin release than that evoked by complete removal of dopamine (251). Arey et al. (72) and Nagy and co-workers (1280, 1282) have published the first reports, which have clearly suggested a possible physiological relevance of these in vitro data. Arey et al. (72) have demonstrated that infusion of 10 ngzkg21 zmin21 dopamine to freely moving rats results in a further increase in the already elevated plasma prolactin when synthesis of endogenous dopa￾mine is blocked by the TH inhibitor a-MpT. The other group has used anterior pituitary cells obtained from nonsuckled (separated from their litters for 4 h) or suck￾led (for 10 min) lactating rats that were exposed to vari￾ous concentrations of dopamine in vitro. Prolactin release was measured by reverse hemolytic plaque assay. Surpris￾ingly, pituitary cells from nonsuckled rats exhibited only the prolactin-inhibitory response to dopamine but never actually stimulated prolactin above basal values as has been found for pituitary cells derived from males or cy￾cling females (761, 1282). In striking contrast, a brief suckling stimulus applied immediately before death ren￾dered the prolactin cells responsive to stimulation by 10212 M concentration of dopamine (761). The case for dopamine enhancement of prolactin secretion both in vitro and in vivo raises the question of the physiological relevance of this phenomenon. Have lactotrophs in situ ever been exposed to dopamine levels low enough to be stimulatory? The suckling stimulus results in a brief and transient reduction in the level of dopamine in long hypophysial portal vessels (404, 1423, 1499). Dopamine concentration in the portal circulation of cycling rats is the lowest during the day of proestrus (140). However, it is doubtful that a 50–70% decrease in portal blood dopamine (140, 404) is sufficient to achieve concentrations capable of stimulating prolactin release. On the other hand, dopamine concentration in stalk blood (140, 142, 621) is in the low nanomolar range (1028 M), which is either ineffective or has only a weak inhibitory effect in vitro (1097), but 100- to 1,000-fold lower doses are required to stimulate prolactin release. One possible explanation is that the diminution in dopamine arriving at the anterior pituitary through the long portal vessels may not accurately reflect the total amount of dopamine the gland “sees.” Indeed, a significant portion of the dopamine arriving at the anterior lobe originates from axon termi￾nals in the neurointermediate lobe (1248) and is delivered through short portal vessels (420, 422). It has been argued that dynamic release of prolactin is partially the consequence of complete withdrawal of dopamine (1133, 1135, 1138). Indeed, many of the trans￾duction events mediating dopaminergic inhibition of pro￾lactin secretion are completely reversed when dopamine is acutely withdrawn (674, 675, 767). Though there is no 1546 FREEMAN, KANYICSKA, LERANT, AND NAGY Volume 80

PROLACTIN 1547 doubt that these data support this contention.it is difficult G proteins (1065).Thus inhibition of prolactin secre to conceive that the dopaminergic neuron becomes com- tion in response to dopamine is a function of coupling of cretion.Indeed Da receptors to a Ga which inhibits adenylyl cyclase has ntly ex se to comnlete withdrawal (252) voltag sitive calcium channels Although the inbibi Another possible explanation for these apparent con- tion of cyelase activity may be unrelated to the inhibition troversies is the assumption that dopamine may require of exocytosis(1002),the net effect on prolactin secretion supplem d tary agent(s) by inhibition c calcium chan 1617.1620 app nd o Shin et al (1617)p In contrast sti of prolactin secretion bydo to protect dopamine from oxidation as a maior candidate pamine involves binding to a d.receptor (252)which is ortheapDlenmeagfaectorordoeanmlnetsatite lear functionally coupled to a G.(251)and i tum activates aS山 ve calciu d by 100 times.Therefore.ascorbic acid exocviosis (27 1740) agent for potentiating dona mine inhibi 1 Prolactin feedback on ne endocrine dopami tion of prolactin release. Frawley and co workers (761 nergic eurons.It is well established that prolactin af a-MSH fec sec by regula m contrast to ascorbic acid MSH dec in v siveness of lactotrophs to the inhibitory effect of a high lamic dopamine synthesis (412)and the concentration of dose of dopamine dopamine in hypothalamo-hypophysial portal blood(695) ory dose(761). Th either in the hypothalamus in the ctin E function as a lactotroph responsiveness factor RF tine (418)R factors can be defined as sub PHDA)of the neuroendocrine dopaminergic neurons(69 direc ice on gthe anatomical asis for short prolactir ctin (4).With the of an int ness to the classical hypothalamic releasing and/or inhib. model,the orolactin-defieient dwarf mouse it has beer iting factors shown that TIDA neurons do not develop in sufficient ductio pathways in lactot number in th e of prolactin(1413,1414).Prolactin ribed that rept (414) ng de minergic control of prolactin secretion.Inhibition ofp X The dopamine transp ter as remlator of mrolac lactin secretion by activation of D receptors has beer tin secretion Termination of dopamine action is primar linked to denylyl cye clas (6088 ily achieve by its reuptake by the dopamine transporter five different ion channels.Dopamine activa tes a notas sing their young and are significantly growth retarded sium current that induces plasma membrane hyperpolar- (626 These animals have a marked reduction in the size ization (847)and es two voltage ctivated potas of the ante nor and intermediate lobes but not the post he p )Ac (402 1067 1060 own that the nonhydrolyzable GTP anal og guanosine and growth hormone mes and an iner sed amount 5'-0-(3-thiotriphosphate)(GTPS)pote ntiates dopami- of extracellular dopamine in the anterior lobe (193.421 on of voltage-sensitive calcium chan It is not only the dopamine transporter of TIDA n eurons th aga HDAactin on excitation of Aetivity of the transnorter on TIDA THDA and PHD voltage-sensitive potassium channels through D2 dopa- neurons is required to clear dopamine from the respective mine receptors is a function of G,whereas the inhib perivascular spaces and thus allow prolactin secretior tion of voltage-activated calcium channels is mediated by B)NOREPINEPHRINE AND EPINEPHRINE.Early pharmacolog

doubt that these data support this contention, it is difficult to conceive that the dopaminergic neuron becomes com￾pletely quiescent to facilitate prolactin secretion. Indeed, it has been shown that a diminution of dopamine stimu￾lates a far greater amount of prolactin secreted than that in response to complete withdrawal (252). Another possible explanation for these apparent con￾troversies is the assumption that dopamine may require supplementary agent(s) to effectively inhibit prolactin re￾lease and thus properly function as the PIF (1617, 1620). Shin et al. (1617) proposed ascorbic acid, routinely used to protect dopamine from oxidation, as a major candidate for the supplementary factor of dopamine. It is quite clear that ascorbic acid is not a simple antioxidant and can truly potentiate the inhibitory effect of dopamine in vitro by 100 times. Therefore, ascorbic acid may serve as a “responsiveness” agent for potentiating dopamine inhibi￾tion of prolactin release. Frawley and co-workers (761) have provided evidence that a-MSH from the intermediate lobe can also function as a responsiveness factor in vitro. In contrast to ascorbic acid, a-MSH decreases the respon￾siveness of lactotrophs to the inhibitory effect of a high dose of dopamine and enhances their responsiveness to the stimulatory effect of a low dose (761). Ascorbic acid, a-MSH, and possibly other substances as well, produced either in the hypothalamus or in the pituitary gland, may function as a lactotroph responsiveness factor (LRF). These responsiveness factors can be defined as sub￾stances with little or no direct influence on prolactin release themselves while they can exert profound effects on prolactin secretion by altering lactotrophs’ responsive￾ness to the classical hypothalamic releasing and/or inhib￾iting factors. VIII) Signal transduction pathways in lactotrophs coupled to the dopamine receptor. A number of transduc￾tion mechanisms have been described that mediate dopa￾minergic control of prolactin secretion. Inhibition of pro￾lactin secretion by activation of D2 receptors has been linked to inhibition of adenylyl cyclase (508, 565) and inositol phosphate metabolism (272, 510, 513, 1635). Moreover, activation of the D2 receptor modifies at least five different ion channels. Dopamine activates a potas￾sium current that induces plasma membrane hyperpolar￾ization (847) and increases two voltage-activated potas￾sium currents while decreasing two voltage-activated calcium currents (492, 1067–1069, 1070, 1110). It has been shown that the nonhydrolyzable GTP analog guanosine 59-O-(3-thiotriphosphate) (GTPgS) potentiates dopami￾nergic inhibition of voltage-sensitive calcium channels (1066). With the use of varying antibodies raised against specific G proteins (1065) or antisense oligonucleotide technology (100), it has been shown that the excitation of voltage-sensitive potassium channels through D2 dopa￾mine receptors is a function of Gi-3a, whereas the inhibi￾tion of voltage-activated calcium channels is mediated by Goa proteins (1065). Thus inhibition of prolactin secre￾tion in response to dopamine is a function of coupling of D2 receptors to a Gi-3a which inhibits adenylyl cyclase activity and concomitantly excites voltage-sensitive po￾tassium channels while coupled to Goa and inhibiting voltage-sensitive calcium channels. Although the inhibi￾tion of cyclase activity may be unrelated to the inhibition of exocytosis (1002), the net effect on prolactin secretion appears to be mediated by inhibition of the calcium chan￾nels and excitation of potassium channels. In contrast, stimulation of prolactin secretion by do￾pamine involves binding to a D2 receptor (252) which is functionally coupled to a Gs (251) and in turn activates voltage-sensitive calcium channels (582) that subse￾quently increase intracellular calcium (248) to facilitate exocytosis (327, 1740). IX) Prolactin feedback on neuroendocrine dopami￾nergic neurons. It is well established that prolactin af￾fects its secretion by regulating its own hypothalamic control through a short-loop feedback mechanism (1194). Elevation of serum levels of prolactin increases hypotha￾lamic dopamine synthesis (412) and the concentration of dopamine in hypothalamo-hypophysial portal blood (695). The rate of dopamine synthesis is reduced by hypophy￾sectomy or lowering blood levels of prolactin with bro￾mocryptine (418). Recently, the presence of prolactin-R has been described in all subpopulations (TIDA, THDA, PHDA) of the neuroendocrine dopaminergic neurons (69, 1028), providing the anatomical basis for short prolactin feedback. All of these subpopulations are activated by prolactin (419). With the use of an interesting animal model, the prolactin-deficient dwarf mouse, it has been shown that TIDA neurons do not develop in sufficient number in the absence of prolactin (1413, 1414). Prolactin replacement during development (1498), but not when an adult (1414), reverses this deficit. X) The dopamine transporter as regulator of prolac￾tin secretion. Termination of dopamine action is primar￾ily achieved by its reuptake by the dopamine transporter located on the terminals of dopaminergic neurons. Mice lacking the dopamine transporter gene are incapable of nursing their young and are significantly growth retarded (626). These animals have a marked reduction in the size of the anterior and intermediate lobes but not the poste￾rior lobe of the pituitary gland (193). Accompanying the anterior pituitary hypoplasia is a diminution of prolactin and growth hormone message and an increased amount of extracellular dopamine in the anterior lobe (193, 421). It is not only the dopamine transporter of TIDA neurons that is effective in regulating prolactin secretion, but the transporter in THDA and PHDA neurons as well (421). Activity of the transporter on TIDA, THDA, and PHDA neurons is required to clear dopamine from the respective perivascular spaces and thus allow prolactin secretion. B) NOREPINEPHRINE AND EPINEPHRINE. Early pharmacolog￾October 2000 PROLACTIN 1547