《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:Structure,Function and Regulation of Secretion MARC E FREEMAN.BELA KANYICSKA.ANNA LERANT.AND GYORGY NAGY Se Brain creting cell lines rv. m of prol n s Prolac CNS an.Marc E.Bela Kanvicska.Anna Lerant,and Gyorgy Nagy.Prolactin:Structure Function.and ion of Secretion.Physio Rev 80: -1631,200 Prolactin is a protein hormone of the anterior pituitary within the central n rous system.the immune sv the mammary gland itsef Moreover,its biological actions are not limited solely to reproduction because t has been *The sequence of authorship does not imply importance of but s presented alphabetically http://physrev.physiology.org 31-33/00$the American Physiological Society

Prolactin: Structure, Function, and Regulation of Secretion MARC E. FREEMAN, BE´ LA KANYICSKA, ANNA LERANT, AND GYO¨ RGY NAGY* Department of Biological Science, Florida State University, Tallahassee, Florida; Department of Anatomy, The University of Mississippi Medical Center, Jackson, Mississippi; and Neuroendocrine Research Laboratory, Department of Human Morphology, Semmelweis University School of Medicine, Budapest, Hungary I. Introduction 1524 II. Prolactin Chemistry and Molecular Biology 1524 A. Prolactin: gene, primary structure, and species specificity 1524 B. Secondary and tertiary structure of prolactin 1524 C. Prolactin variants 1524 III. Sites of Synthesis and Secretion of Prolactin 1525 A. Anterior pituitary gland 1525 B. Brain 1526 C. Placenta, amnion, decidua, and uterus 1527 D. Mammary gland and milk 1527 E. The immune system 1528 F. Prolactin-secreting cell lines 1528 IV. Prolactin Receptors 1529 A. Prolactin receptor: gene, splicing variants, and isoforms 1529 B. Activation of prolactin-R and the associated signal transduction pathways 1529 C. Distribution of prolactin-R 1532 V. Biological Actions of Prolactin 1533 A. Reproduction 1533 B. Homeostasis 1536 VI. Patterns of Pituitary Prolactin Release 1537 A. Circadian rhythm of prolactin secretion 1537 B. Patterns of prolactin secretion in different reproductive states 1538 C. Prolactin release in response to exteroceptive stimuli 1540 VII. Regulation of Pituitary Prolactin Secretion 1542 A. CNS 1542 B. Intrapituitary regulation 1573 C. Peripheral organs 1579 VIII. Epilogue 1585 Freeman, Marc E., Be´la Kanyicska, Anna Lerant, and Gyo¨rgy Nagy. Prolactin: Structure, Function, and Regulation of Secretion. Physiol Rev 80: 1523–1631, 2000.—Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although * The sequence of authorship does not imply importance of contribution but is presented alphabetically. PHYSIOLOGICAL REVIEWS Vol. 80, No. 4, October 2000 Printed in U.S.A. http://physrev.physiology.org 0031-9333/00 $15.00 Copyright © 2000 the American Physiological Society 1523

1524 FREEMAN,KANYICSKA,LERANT,AND NAGY Volume 80 rigin ides inhibitor control ov peripheral organs the purbts I.INTRODUCTION a the tin prohormone of 227 amino acids.The signal peptide pituitary gland,the lactotrophs.The hormone was given contains 28 amin acids;thus the mature human prolactin its name based on the fact that an extract of bovine is co e growth crop sac an pr with thre a single chair of amin la mote lactation in rabbits (1477).However we now ap ciate that prolactin has over 300 separate biological ac- and Cys -Cys1 in humans)(357).The sequence homol- tivities(18)not represented by its name.Indeed,not only ogy can vary from the striking9 among pr 【0a does prolact erve multiple roles in static roles in the organism.Futher amino acids whereas in sheen (1036),pig(1035),cattle aware that synthesis and secretion of prolactin is not (1851),and humans(1624)it consists of 199 amino acids restricted to the anterior pituitary gland,but other organs with a molecular mass of ~23,000 Da and sues in t body have th Indeed,the B.Secondary and Tertiary Structure of Prolactin tilin." In this review we integrate the burgeoning informa- n the secondary str of prolactin hav shown that 50%6 of the amino acid chain is arr ed in onon prolactin structure (sect.)sy and re a-helices,while the rest of it forms loops(169).Although ism of th was predicted earlier (1311), there are still no direc functions(sect.v),and the patterns (sect.vi)and regula- tion of its secretion (sect.vu). mology modeling approach (635).based on the structura similarities between prolactin and other helix bundle pro IL. teins,especially growth hormone )According to the long a-helicmension pr s arranged lel A.Prolactin:Gene,Primary Structure,and 438). Species Specificity C.Prolactin Variants ased on its gen rowth hormone placental lact family ig roup I of the Although the major form of prolactin found in the is 23 kD helix bundle protein hormones (195 791)1 Genes encod. nts of prolactin have been ing prolactin,growth hormone,and placental lactogen lactin variants plicing of the 131 mmon ance al ge ot by gen primary transcript,proteolytic cleavage and other post- ages ooge translational modifications of the amino acid chain. 358).In the human genome,a single found on e 6,encodes prolactin (1363) The prolactin 1.Alternative splicing chromosom and is compe d or exons and Altemative splicing of prolactin mRN regulated by two inder The evidence si proximal 5,000-bp region directs pituitary-specific expres- spliced prolactin variant of 137 amino acids has been sion (160),while a more upstream promoter region is described in the anterior pituitary(501,1882).In addition

it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin’s function and its regulation and to expose some of the controversies still existing. I. INTRODUCTION Prolactin is a polypeptide hormone that is synthe￾sized in and secreted from specialized cells of the anterior pituitary gland, the lactotrophs. The hormone was given its name based on the fact that an extract of bovine pituitary gland would cause growth of the crop sac and stimulate the elaboration of crop milk in pigeons or pro￾mote lactation in rabbits (1477). However, we now appre￾ciate that prolactin has over 300 separate biological ac￾tivities (184) not represented by its name. Indeed, not only does prolactin subserve multiple roles in reproduction other than lactation, but it also plays multiple homeo￾static roles in the organism. Furthermore, we are now aware that synthesis and secretion of prolactin is not restricted to the anterior pituitary gland, but other organs and tissues in the body have this capability. Indeed, the multiple roles and sources of prolactin had led Bern and Nicoll (154) to suggest renaming it “omnipotin” or “versa￾tilin.” In this review we integrate the burgeoning informa￾tion on prolactin’s structure (sect. II), synthesis and re￾lease from varying sources (sect. III), the intracellular mechanism of its action (sect. IV), its major biological functions (sect. V), and the patterns (sect. VI) and regula￾tion of its secretion (sect. VII). II. PROLACTIN CHEMISTRY AND MOLECULAR BIOLOGY A. Prolactin: Gene, Primary Structure, and Species Specificity Based on its genetic, structural, binding and func￾tional properties, prolactin belongs to the prolactin/ growth hormone/placental lactogen family [group I of the helix bundle protein hormones (195, 791)]. Genes encod￾ing prolactin, growth hormone, and placental lactogen evolved from a common ancestral gene by gene duplica￾tion (1311). The divergence of the prolactin and growth hormone lineages occurred ;400 million years ago (357, 358). In the human genome, a single gene, found on chromosome 6, encodes prolactin (1363). The prolactin gene is 10 kb in size and is composed of 5 exons and 4 introns (357, 1772). Transcription of the prolactin gene is regulated by two independent promoter regions. The proximal 5,000-bp region directs pituitary-specific expres￾sion (160), while a more upstream promoter region is responsible for extrapituitary expression (159). The hu￾man prolactin cDNA is 914 nucleotides long and contains a 681-nucleotide open reading frame encoding the prolac￾tin prohormone of 227 amino acids. The signal peptide contains 28 amino acids; thus the mature human prolactin is composed of 199 amino acids (1640). The prolactin molecule is arranged in a single chain of amino acids with three intramolecular disulfide bonds between six cysteine residues (Cys4 -Cys11, Cys58-Cys174, and Cys191-Cys199 in humans) (357). The sequence homol￾ogy can vary from the striking 97% among primates to as low as 56% between primates and rodents (1640). In rats (358) and mice (968), pituitary prolactin consists of 197 amino acids, whereas in sheep (1036), pigs (1035), cattle (1851), and humans (1624) it consists of 199 amino acids with a molecular mass of ;23,000 Da. B. Secondary and Tertiary Structure of Prolactin Studies on the secondary structure of prolactin have shown that 50% of the amino acid chain is arranged in a-helices, while the rest of it forms loops (169). Although it was predicted earlier (1311), there are still no direct data about the three-dimensional structure of prolactin. The tertiary structure of prolactin was predicted by ho￾mology modeling approach (635), based on the structural similarities between prolactin and other helix bundle pro￾teins, especially growth hormone (2, 438). According to the current three-dimensional model, prolactin contains four long a-helices arranged in antiparallel fashion (2, 438). C. Prolactin Variants Although the major form of prolactin found in the pituitary gland is 23 kDa, variants of prolactin have been characterized in many mammals, including humans. Pro￾lactin variants can be results of alternative splicing of the primary transcript, proteolytic cleavage and other post￾translational modifications of the amino acid chain. 1. Alternative splicing Alternative splicing of prolactin mRNA has been pro￾posed as one source of the variants (1639, 1640). Indeed, evidence suggestive of the existence of an alternatively spliced prolactin variant of 137 amino acids has been described in the anterior pituitary (501, 1882). In addition, 1524 FREEMAN, KANYICSKA, LERANT, AND NAGY Volume 80

PROLACTIN 1525 alterative splicing involving retention of introns is also fore exocytosis and involves esterification of hydroxvl possible.However,alternative splicing is not considered a groups of serine and threonine residues(670).Phosphor- major source of prolactin variants. ylated prolactin isoforms have beer e(1337)pituitary gla 2.Proteolytic cleavage sphorylae Of the cleaved forms that have been characterized. total pituitary prolactin in catle( 14- 16 and 2 2-kDa prolactin variants have been most prolactin has been shown to be secreted in vitro,it is not The sig widely s known if it is secreted into the plasma in vivo. posttra pro in th tactins has (736.hC ated pre ctin has much lower hiological activity than non tivity with the 16-kDa fragment (335 1765)Both s m to phosphorylated prolactin (1859).However. DOS lated prolactin may subserve a unique role as an autocrine ae虹 prolacun-(I oI p. tin se (a642207 an supp (1643)pituitary glands as well as in human plasma (643) (768).Phosphor rvlation of pr The 16-kDa prolactin is a product of kallikrein enzymatic ratio of phosphorvlated to nonphosphorylated isoforms activity. ced seems to be regulated throughout the estrous cycle (769) an estrogen-indu trypsin-lik ugn the ce of th f 1433 cleave rat prolactin in a thiol-dependent manner ist to the simal transduction pathways (362)and proliferative activities alters the conformation of prolactin such that kallikrein init iated by unmodified prolactin on Nb2 lymphoma cells 81 ves xypep of nl /11780 spl cells agment can be detected in pituitary and tis blot using an antiserum produced specifically against the c)GLYCOSYLATON.Glycosylated prolactin has been -kDa prolactin fragment(45).Its ems that the produc found in the pituitary glands of a wide variety of mamma ti lian,amphibian,and avia ,spee (1640 The degree o a. inhibition by dopa mine (45)Althe an fragments have been found in pituitary gland and serum (1640).The linkage of the earbobydrate moiety may be more work is required to de ermine their physiologica ility rem they may be harie2atioDmeCa either through nitrogen (N-glycosylation)or oxygen (O ohydrate residue s of th varyt diffe b ecies physiological and pathologieal states 3.Other posttranslational modifieations 1640).Lk othe prolactin variants,glycosylation also Besides prote tin olhtedeavagethenaioiotpmolhg ogical activity (1127 of the itary gland or the plasma.These include dimerization and poly merization,phosphorylation,glycosylation,sulfation, and deamidation (1702) activity or clearance of the molecule A) tion with binding proteins.such as immund SITES OF SYNTHESIS AND SECRETION covalent and noncovalent bonds may result in high-mo OF PROLACTIN lecular-weigh forms cular A.Anterior Pituitary Gland detection and difrerential diagno sis of different 1.Morpholog of lactotrophs lactinemias is targeted primarily in clinical studies(299). B).Phosphorylation of prolactin oc The cells of the anterior pituitary gland which syn- curs within the secretory vesicle of lactotrophs just be- thesize and secrete prolactin were initially described by

alternative splicing involving retention of introns is also possible. However, alternative splicing is not considered a major source of prolactin variants. 2. Proteolytic cleavage Of the cleaved forms that have been characterized, 14-, 16-, and 22-kDa prolactin variants have been most widely studied. The 14-kDa NH2-terminal fragment is a posttranslational product of the prolactin gene that is processed in the hypothalamus and shares biological ac￾tivity with the 16-kDa fragment (335, 1765). Both seem to possess a unique biological activity, which will be de￾scribed later. The 16-kDa fragment [prolactin-(1O148)] was first described in rat pituitary extracts (1207) and has subsequently been found in mouse (1642) and human (1643) pituitary glands as well as in human plasma (1643). The 16-kDa prolactin is a product of kallikrein enzymatic activity. Kallikrein is an estrogen-induced, trypsin-like serine protease that is found in the Golgi cisternae and secretory granules of lactotrophs (1433). This enzyme will cleave rat prolactin in a thiol-dependent manner. Thiol alters the conformation of prolactin such that kallikrein recognizes it as a substrate. Treatment of native prolactin with carboxypeptidase-B results in a 22-kDa prolactin fragment [prolactin-(1O173)]. Surprisingly, this synthetic fragment can be detected in pituitary extracts by Western blot using an antiserum produced specifically against the 22-kDa prolactin fragment (45). It seems that the produc￾tion and release of these proteolytic fragments from the pituitary gland is specific to female rats and sensitive to inhibition by dopamine (45). Although these and other fragments have been found in pituitary gland and serum, more work is required to determine their physiological significance since the possibility remains that they may be preparative artifacts (1640). 3. Other posttranslational modifications Besides proteolytic cleavage, the majority of prolac￾tin variants can be the result of other posttranslational processing of the mature molecule in the anterior pitu￾itary gland or the plasma. These include dimerization and polymerization, phosphorylation, glycosylation, sulfation, and deamidation (1702). A) DIMERIZATION AND POLYMERIZATION: MACROPROLACTINS. Dimerization and polymerization of prolactin or aggrega￾tion with binding proteins, such as immunoglobulins, by covalent and noncovalent bonds may result in high-mo￾lecular-weight forms. In general, the high-molecular￾weight forms have reduced biological activity (1640). The role of prolactin-IgG macromolecular complexes in the detection and differential diagnosis of different pro￾lactinemias is targeted primarily in clinical studies (299). B) PHOSPHORYLATION. Phosphorylation of prolactin oc￾curs within the secretory vesicle of lactotrophs just be￾fore exocytosis and involves esterification of hydroxyl groups of serine and threonine residues (670). Phosphor￾ylated prolactin isoforms have been isolated from bovine (224) and murine (1337) pituitary glands. Phosphorylated isoforms of prolactin may constitute as much as 80% of total pituitary prolactin in cattle (938). Although phospho￾prolactin has been shown to be secreted in vitro, it is not known if it is secreted into the plasma in vivo. The sig￾nificance of phosphorylated and nonphosphorylated pro￾lactins has been reviewed in detail (736). Phosphorylated prolactin has much lower biological activity than non￾phosphorylated prolactin (1859). However, phosphory￾lated prolactin may subserve a unique role as an autocrine regulator of prolactin secretion since it suppresses the release of nonphosphorylated prolactin from GH3 cells (768). Phosphorylation of prolactin as well as the relative ratio of phosphorylated to nonphosphorylated isoforms seems to be regulated throughout the estrous cycle (769), although the physiological relevance of this finding is not yet appreciated. However, novel data indicate that phos￾phorylated prolactin acts as an antagonist to the signal transduction pathways (362) and proliferative activities initiated by unmodified prolactin on Nb2 lymphoma cells (315). Further investigation is needed to determine the significance of phosphorylated prolactin in primary cells and tissues. C) GLYCOSYLATION. Glycosylated prolactin has been found in the pituitary glands of a wide variety of mamma￾lian, amphibian, and avian species (1640). The degree of glycosylation varies from 1 to 60% among species and may also vary between reproductive states within species (1640). The linkage of the carbohydrate moiety may be either through nitrogen (N-glycosylation) or oxygen (O￾glycosylation). The carbohydrate residues of the oligosac￾charide chain may contain varying ratios of sialic acid, fucose, mannose, and galactose that differ considerably between species, physiological, and pathological states (1640). Like other prolactin variants, glycosylation also lowers biological activity (1127, 1641) as well as receptor binding and immunologic reactivity of prolactins (740). Glycosylation also alters the metabolic clearance rate of prolactin (1641). Taken together, glycosylation of prolac￾tin may play a role either in regulation of the biological activity or clearance of the molecule. III. SITES OF SYNTHESIS AND SECRETION OF PROLACTIN A. Anterior Pituitary Gland 1. Morphology of lactotrophs The cells of the anterior pituitary gland which syn￾thesize and secrete prolactin were initially described by October 2000 PROLACTIN 1525

1526 FREEMAN,KANYICSKA,LERANT,AND NAGY Volume 80 light mic lation phs are not hor orphology motrophs,comprise 20-50%of the cellular population of hormonal phenotype,distribution,or function. physiol wer B.Brain e(109 (110.127.and human111.725.1387 using so The first observation that prolactin is produced in the brain was by Fuxe et al.(594)who found prolactin im- specific prolactin antibodies.Ontogenetically,lactotrophs Pitl-depend y was in the te n th 643.1382.1599) tum (433),caudate putamen (502,737),brain stem The morphology and distribution of lactotrophs have (433.737.cerebellum (1589).spinal cord (737.1630) been best described in the rat (1768) where prolactin- containing ce distributed in the choroid plexi,and the circumventricular organs (1741). are 127 heterogeneous a aring as either nolyhe dral or a Prolactin immunoreactivity is found within numer but at times rounde ed or oval (429).With the use of either shypothalamic areas in a variety of mammals(29,677 velo se me 67 132 .1630.1741) un th rat hypo their dial vor ia【676. anule size and content (1650)as well as on the (735)nuclei Several an ches have been taken to amount of prolactin and prolactin mRNA present (1813) 2.functional heterogeneity of lactotrophs nt ofi ie hy Aside from morphological heteroge heity.lactotrophs display functional heterogeneity as well.Development of the reverse hemolytic plaque assay (572,576,1300)led t hypoth nd( ith the and tide mapping eted from a distinct cell type in the pituitar othalamic eDNA from inta and hy gland,the lactotroph,both prolactin and growth hormone ysectomized rats (1882)it has been established that the can also be secreted from the interm primary structure of prolactin of hypothalamic and pitu opns (272 Ines rigin i the neonatal rats (770)differentiate into lactotr nhs in the olactin gene of the anter r pituitary (501 1882) presence of estrogen (191).Mammosomatotrophs also Although the role of prolactin of hypothalamic origin differentiate into lactotrophs in pups in the pre nce of a in the central nervous system (CNS)is not apparent,it appears 1429 nyp ap There also an ars to he functional het into 16 and 14-kDa fragments (435).We do not know if among lactotrophs with regard to their regio nal distrib ctin of neural origin exerts its central effect as a tion within the anterior lobe (1246)as well as to the neuromodula of tue ond to thy releasing horme in art bec e it is difficult to differ ntiate h reen the (TRH than those of the inner zone adia ent to the inter. effects of p olactin of pituitary versus hypothalamic ori mediate lobe of the pituitary gland (188) On the other gin in the CNS One cause of thes difficulties is tha and, dopami ary p bypa oity is alsc roid reflected in the discordance between prolactin gene tran- plexi have a very high density of prolactin receptors(pro scription and prolactin release in some lactotroph popu- lactin-Rs)as demonstrated by autoradiography (1113

light microscopy using conventional staining techniques (753). These cells, designated lactotrophs or mam￾motrophs, comprise 20–50% of the cellular population of the anterior pituitary gland depending on the sex and physiological status of the animal. Lactotrophs were sub￾sequently identified unequivocally by immunocytochem￾istry in the anterior pituitary gland of the mouse (109), rat (110, 1287), and human (111, 725, 1387) using species￾specific prolactin antibodies. Ontogenetically, lactotrophs descend from the Pit-1-dependent lineage of pituitary cells, together with somatotrophs and thyrotrophs (348, 643, 1382, 1599). The morphology and distribution of lactotrophs have been best described in the rat (1768), where prolactin￾containing cells are sparsely distributed in the lateroven￾tral portion of the anterior lobe and are present as a band adjacent to the intermediate lobe (1287). Their shapes are heterogeneous, appearing as either polyhedral or angular but at times rounded or oval (429). With the use of either velocity sedimentation at unit gravity (1650) or discontin￾uous Percoll gradients (1813) to separate cell populations, it has been shown that lactotrophs vary based on their secretory granule size and content (1650) as well as on the amount of prolactin and prolactin mRNA present (1813). 2. Functional heterogeneity of lactotrophs Aside from morphological heterogeneity, lactotrophs display functional heterogeneity as well. Development of the reverse hemolytic plaque assay (572, 576, 1300) led to a more precise description of functional heterogeneity in lactotrophs (1090). Although prolactin is largely found and secreted from a distinct cell type in the pituitary gland, the lactotroph, both prolactin and growth hormone can also be secreted from the intermediate cell population called mammosomatotrophs (572, 574, 576, 1300). These bifunctional cells, which predominate in the pituitary of neonatal rats (770), differentiate into lactotrophs in the presence of estrogen (191). Mammosomatotrophs also differentiate into lactotrophs in pups in the presence of a maternal signal that appears in early lactation (1427) and is delivered to the pups through the mother’s milk (1429). There also appears to be functional heterogeneity among lactotrophs with regard to their regional distribu￾tion within the anterior lobe (1246) as well as to the nature of their responsiveness to secretagogues (188); that is, lactotrophs from the outer zone of the anterior lobe respond greater to thyrotrophin releasing hormone (TRH) than those of the inner zone, adjacent to the inter￾mediate lobe of the pituitary gland (188). On the other hand, dopamine-responsive lactotrophs (84) are more abundant in the inner than the outer zone of the anterior pituitary. Surprisingly, functional heterogeneity is also reflected in the discordance between prolactin gene tran￾scription and prolactin release in some lactotroph popu￾lations (296, 1562). Taken together, it is clear that lac￾totrophs are not homogeneous in their morphology, hormonal phenotype, distribution, or function. B. Brain The first observation that prolactin is produced in the brain was by Fuxe et al. (594) who found prolactin im￾munoreactivity in hypothalamic axon terminals. Prolactin immunoreactivity was subsequently found in the telen￾cephalon in the cerebral cortex, hippocampus, amygdala, septum (433), caudate putamen (502, 737), brain stem (433, 737), cerebellum (1589), spinal cord (737, 1630), choroid plexi, and the circumventricular organs (1741). 1. Hypothalamus Prolactin immunoreactivity is found within numer￾ous hypothalamic areas in a variety of mammals (29, 677, 678, 737, 1321, 1630, 1741). Within the rat hypothalamus, prolactin immunoreactivity is detectable in the dorsome￾dial, ventromedial (676), supraoptic, and paraventricular (735) nuclei. Several approaches have been taken to prove that prolactin found in the hypothalamus is synthe￾sized locally, independent of prolactin synthesis in the pituitary gland. Indeed, hypophysectomy has no effect on the amount of immunoreactive prolactin in the male hy￾pothalamus and only diminishes but does not abolish the quantity of immunoreactive prolactin in the female rat hypothalamus (433). With the use of conventional peptide mapping (434) and sequencing of a polymerase chain reaction (PCR) product of hypothalamic cDNA from intact and hypoph￾ysectomized rats (1882), it has been established that the primary structure of prolactin of hypothalamic and pitu￾itary origin is identical. Thus it seems that the prolactin gene expressed in the rat hypothalamus is identical to the prolactin gene of the anterior pituitary (501, 1882). Although the role of prolactin of hypothalamic origin in the central nervous system (CNS) is not apparent, it should be noted that the hypothalamus contains the ap￾propriate proteolytic enzymes to cleave 23-kDa prolactin into 16- and 14-kDa fragments (435). We do not know if prolactin of neural origin exerts its central effect as a neurotransmitter, neuromodulator, or a central cytokine regulating vascular growth and/or glial functions. To as￾cribe a role for prolactin of neural origin is troublesome, in part, because it is difficult to differentiate between the effects of prolactin of pituitary versus hypothalamic ori￾gin in the CNS. One cause of these difficulties is that pituitary prolactin from the circulation bypasses the blood-brain barrier and enters the CNS through the cho￾roid plexi of the brain ventricles. Coincidentally, choroid plexi have a very high density of prolactin receptors (pro￾lactin-Rs) as demonstrated by autoradiography (1113, 1526 FREEMAN, KANYICSKA, LERANT, AND NAGY Volume 80

PROLACTIN 1527 1853.1854).immunocytochemistry (1495).standard re. prolactin-like protein J(1752).which is produced by the ceptor binding assays(1242),reverse transcriptase PCR, decidua during early pregnancy.Each of these prolactin- and ribonuclease protection assay (590).Interestingly. the 9731.1745 1746ae hlood to the cerebrosninal fluid by way of the choroid tin-rele asing factors (PRE).P esterone has also been plexus,pituitary prolactin may also reach the brain by identified as a potent stimulator of decidual prolactin retrograde blood flow from the anteror pituitary to the production (1143) In addition to stimulatory factors ypo in th5 the ac 731 decidual prolactin release and compe tes with the decid to its effects in the CNS without attributing a source. ual PRF (731).Recently,the N5 endometrial stromal cell line.which expresses the prolact gen driven by the 2.Regulation of hypothalamic prolactin synthesis d as a possibl Some well-established stimulat (210)Ample evidence indicates that decidual prolactin lactin se duction.For example,ovarian steroids modulate hypotha- diffuses into the amniotic fluid (1473,1474,1476,1501) release prolactin (43 437 Although the function of amniotic prolactin is uncertain n suggest al（S may serve ar e with Pestradiol (436).suggesting that these bryo/fetus 8o8uha have estrogen receptors.Ovariectomy lowers hypotha- Finally,the nonpregnant uterus has been shown to be lamic prola a source of prolactin as well.Indeed,a decidual-like pro conten n In rom pituitary prolactin (611 fact 15511mt o stim lates the production of decidual prolactin.it appears to be lar iniection of vasoactive intestinal peptide will i otent inhibitor of myometrial prolactin production (61 ogical role for myometrial prolactin has ever,other ously.much more work is needed to establish the conto of hypothalamic prolactin synthesis and release. D.Mar mary Gland and Milk C.Placenta,Amnion,Decidua,and Ute ru n can be det The placenta,in addition to its bidirectional fetoma ortion of the ternal metabolic transport functions,has a wide array of prolactin found in the milk originate in the pituitary endocrine functions as well.Among its ma gland and reaches the mammary gland through thec 253537 74,14sz 88,149 n h some or a mm 151 1605.1896.hamster874,1662-1664).cowC46.1612. Indeed a sic mincant amount of radiolabeled prolactin pig (568),and human (728).The rat placenta produces a introduced into the circulation appears in milk (685, bewildering array of prolactin-like molecule that bear 1253).Apparently,prolactin 058, reaches the milk by first PL mary eplt PLP)have been variously identined as PLL PLI PLIm within the mammary epithelial cell.and is ultimately (mosaic),PL-Iv (variant)(349,1487,1490),or PLP-A,-B. transported by exocytosis through the apical membrane -D-E.-F,and-G(356.392,851).n ddition into the alv lar lumen (1352,1583) a in (PLF in additi from the blood,th 1057 The decidua on the other hand prolactin.The s an of ce of nrolactir like molecule,that is indistinguishable from pituitary pro mRNA(992,1682)as well as synthe sis of immunoreactive lactin in human (35,342,147 1707),but is somewhat prolactin by mammary epithe al cells of lactating rats has dissimilar in rat (688).A novel member of this family is been described (1063 1064).It is possible that de novo

1853, 1854), immunocytochemistry (1495), standard re￾ceptor binding assays (1242), reverse transcriptase PCR, and ribonuclease protection assay (590). Interestingly, prolactin enhances the expression of its own receptors in the choroid plexus (1113). Aside from passage from the blood to the cerebrospinal fluid by way of the choroid plexus, pituitary prolactin may also reach the brain by retrograde blood flow from the anterior pituitary to the hypothalamus (1192, 1351). Therefore, because the ac￾tions of prolactin in the CNS can be due to the hormone of pituitary or hypothalamic origin, in this review we refer to its effects in the CNS without attributing a source. 2. Regulation of hypothalamic prolactin synthesis Some well-established stimulators of pituitary pro￾lactin secretion also affect hypothalamic prolactin pro￾duction. For example, ovarian steroids modulate hypotha￾lamic synthesis and release of prolactin (436, 437). Approximately 33% of the prolactin immunoreactive neu￾rons in the medial basal hypothalamus can be labeled with [3 H]estradiol (436), suggesting that these neurons have estrogen receptors. Ovariectomy lowers hypotha￾lamic prolactin content, whereas estrogen replacement elevates it (436, 437). Of the known hypophysiotrophic factors, angiotensin II stimulates release of prolactin from hypothalamic fragments (437), and intracerebroventricu￾lar injection of vasoactive intestinal peptide will increase the amount of hypothalamic prolactin mRNA (212). How￾ever, other established stimulators of pituitary prolactin secretion such as TRH are without effect (437). Obvi￾ously, much more work is needed to establish the control of hypothalamic prolactin synthesis and release. C. Placenta, Amnion, Decidua, and Uterus The placenta, in addition to its bidirectional fetoma￾ternal metabolic transport functions, has a wide array of endocrine functions as well. Among its many secretory products are a family of placental lactogens found in the rat (354, 393, 537, 744, 1487–1489, 1491, 1651), mouse (1605, 1896), hamster (874, 1662–1664), cow (46, 1612), pig (568), and human (728). The rat placenta produces a bewildering array of prolactin-like molecules that bear structural similarity to pituitary prolactin (1058, 1652). These placental lactogens (PL) or prolactin-like proteins (PLP) have been variously identified as PL-I, PL-II, PL-Im (mosaic), PL-Iv (variant) (349, 1487, 1490), or PLP-A, -B, -C, -D, -E, -F, and -G (356, 392, 851). In addition, the placenta contains a lactogen known as proliferin (PLF) (1056) and proliferin-related protein (PRP) (1057). The decidua, on the other hand, produces a prolactin￾like molecule, that is indistinguishable from pituitary pro￾lactin in human (35, 342, 1475, 1707), but is somewhat dissimilar in rat (688). A novel member of this family is prolactin-like protein J (1752), which is produced by the decidua during early pregnancy. Each of these prolactin￾like molecules can bind to the prolactin-R (755, 860), and their secretion is regulated by local decidual (638, 689, 729–731, 1745, 1746), but not hypothalamic (637) prolac￾tin-releasing factors (PRF). Progesterone has also been identified as a potent stimulator of decidual prolactin production (1143). In addition to stimulatory factors, a substance with inhibitory activity is found in decidual conditioned media (731). This substance decreases basal decidual prolactin release and competes with the decid￾ual PRF (731). Recently, the N5 endometrial stromal cell line, which expresses the prolactin gene driven by the extrapituitary promoter, has been identified as a possible model system to study decidual prolactin gene expression (210). Ample evidence indicates that decidual prolactin diffuses into the amniotic fluid (1473, 1474, 1476, 1501). Although the function of amniotic prolactin is uncertain, it has been suggested that it may serve an osmoregulatory (1781), maturational (864), or immune (732) role in the embryo/fetus. Finally, the nonpregnant uterus has been shown to be a source of prolactin as well. Indeed, a decidual-like pro￾lactin, indistinguishable from pituitary prolactin (611), has been identified in the myometrium of nonpregnant rats (1855). Interestingly, although progesterone stimu￾lates the production of decidual prolactin, it appears to be a potent inhibitor of myometrial prolactin production (611). The physiological role for myometrial prolactin has yet to be identified. D. Mammary Gland and Milk Prolactin can be detected in epithelial cells of the lactating mammary gland (1326) as well as in the milk itself (680). There is little doubt that a portion of the prolactin found in the milk originates in the pituitary gland and reaches the mammary gland through the circu￾lation. Thus some of the prolactin found in milk is taken up rather than produced by the mammary epithelial cells. Indeed, a significant amount of radiolabeled prolactin introduced into the circulation appears in milk (685, 1253). Apparently, prolactin reaches the milk by first crossing the mammary epithelial cell basement mem￾brane, attaches to a specific prolactin binding protein within the mammary epithelial cell, and is ultimately transported by exocytosis through the apical membrane into the alveolar lumen (1352, 1583). In addition to uptake of prolactin from the blood, the mammary epithelial cells of lactating animals are capable of synthesizing prolactin. The presence of prolactin mRNA (992, 1682) as well as synthesis of immunoreactive prolactin by mammary epithelial cells of lactating rats has been described (1063, 1064). It is possible that de novo October 2000 PROLACTIN 1527