Plasma Prolactin Activity in Inappropriate Lactation

Using a bioassay dependent on the development of a lactogenic response in rabbit mammary tissue cultured in vitro, prolactin distinct from immunoreactive growth hormone has been found in the plasma of patients of both sexes with inappropriate lactation with and without evidence of pituitary tumours. It has also been found in one patient with primary hypothyroidism and galactorrhoea, and in another during chlorpromazine therapy, but not in nine patients with gynaecomastia without galactorrhoea. Plasma prolactin levels were examined in seven patients during oral glucose tolerance tests: no change occurred in the four patients with pituitary tumours, but the levels were suppressed in the three patients with normal pituitary fossae. Prolactin appears to be a distinct pituitary hormone in man, as in animals, and also to be aetiologically related to states of inappropriate lactation.     

Long-term Treatment of Galactorrhoea and Hypogonadism with Bromocriptine

Seventeen women and four men with galactorrhoea and associated hypogonadism have been treated with bromocriptine for 2 to 28 months. In 18 patients the gonadal status became normal as the galactorrhoea improved. The gonadally unresponsive patients had either pituitary tumours or premature menopause. Prolactin levels fell with treatment; withdrawal of the drug was associated with an increase in serum prolactin and a recurrence of the galactorrhoea and hypogonadism. Two patients tried to become pregnant on treatment and both succeeded. Raised prolactin levels appear to block the actions of the gonadotrophins at a gonadal level rather than prevent their synthesis or release; lowering prolactin secretion with bromocriptine allows resumption of normal gonadal function. Bromocriptine appears to be the treatment of choice for inappropriate lactation in association with hypogonadism on a long-term basis.     

Stress response patterns of plasma corticosterone, prolactin, and growth hormone in the rat, following handling or exposure to novel environment.

Corticosterone, prolactin, and growth hormone responses to 5 s of handling or 3 min of novel environment were compared in rats at crest and trough of the diurnal adrenal rhythm 0, 5, 15, 30, and 60 min after stimulation. All hormones responded to stimulation, corticosterone and prolactin with a dramatic rise, and growth hormone with a precipitous fall. Resting corticosterone levels evidenced the expected diurnal variation, and prolactin but not growth hormone also showed a baseline diurnal variation of small magnitude at the times studied. Growth hormone response characteristics were unaffected by time of day or type of stimulation. Both corticosterone and prolactin response profiles differed at both times of day and following both types of stimulation. Corticosterone and prolactin levels were highly correlated and each was negatively correlated with growth hormone levels. This study confirms that hormone responses to stress are complex and depend not only on the stimulus but the context of stimulation.     

Comparative immunoenzymatic localization of prolactin and growth hormone in human and rat pituitaries.

Twelve human and twelve rat pituitaries were stained by an immunohistochemical method using a rabbit anti-ovine prolactin serum, a rabbit anti-human growth hormone serum and a sheep anti-rabbit immunoglobulin serum conjugated with horseradish peroxidase. On the same pituitary section, growth hormone cells were stained brown by using 3-3'-diaminobenzidine as peroxidase substrate, and prolactin cells were stained purplish blue by using 4-chloro-1-naphtol. Growth hormone cells outnumbered prolactin cells, especially in human pituitaries where the proportion is at least 10:1. No cells containing both brown granules stained for growth hormone and blue granules stained for prolactin were found in any of the sections examined. In the fetal pituitaries, there was no apparent hypertrophy of the prolactin cells, although the circulating levels of the hromone are known to be as high in the fetus at term as in the mother and much higher than in nonpregnant women.     

Effects of hypersecretion of growth hormone and prolactin on plasma levels of glucagon and insulin in GH3-cell-tumor-bearing rats, and the influence of bromocriptine treatment

Plasma levels of prolactin, growth hormone, glucagon insulin and glucose were measured in non-treated control rats, bromocriptine-treated control rats and GH3-cell-tumor-bearing rats with and without bromocriptine treatment. Bromocriptine treatment increased plasma levels of glucagon, insulin and glucose in control rats. Tumor-bearing rats had increased body weight and increased plasma levels of prolactin, growth hormone, glucagon, insulin and glucose. Bromocriptine treatment reduced body weight and decreased the plasma levels of prolactin, glucagon and insulin, as compared to non-treated tumor-bearing rats. The drug had no effect on plasma levels of growth hormone and glucose. These results indicate that, in GH3-cell-tumor-bearing rats, prolactin, glucagon and insulin are more sensitive to the action of bromocriptine than growth hormone.     

Studies on prolactin in human serum, urine and milk.

Prolactin activity was measured in serum, urine and milk using a specific human prolactin radioimmunoassay (RIA). Serum, urine and milk were parallel with the human prolactin standard in the RIA. There was no correlation between serum prolactin levels and urinary prolactin activity. Dialysis of urine samples resulted in complete loss of human prolactin activity while the addition of human prolactin to the urine resulted in the recovery of over 50% of the hormone after dialysis. Thus it was concluded that prolactin is not present in urine. In additional experiments it was observed that the RIA prolactin activity in urine was significantly correlated with the osmolality of the urine and that Na+ and K+ were contributory elements. On the other hand, prolactin was found in human milk and correlated well with the expected serum levels of this hormone. This latter finding is interesting because prolactin receptors have been shown to exist on the serosal side of the mammary epithelial cells. The presence of prolactin in milk suggests the possibility of other sites of action for this hormone in addition to the cell membrane.     

Studies on human prolactin physiology

Although the clinical and experimental data were in favour of the existence of prolactin in humans like other vertebrates, as a pituitary hormone distinct from growth hormone, its presence remained contested until recent years. The predominant influence of the human hypothalamus on prolactin secretion is inhibitory. Circulating prolactin shows diurnal variations, which are not synchronized with that of TSH or ACTH; the prolactin rhythm is abolished during the last trimester of pregnancy and in patients with prolactin secreting tumors. Estrogens appeared to be less marked stimulators of prolactin secretion in man than in animals, although serum prolactin levels follow a pattern similar to that of endogenous estrogens during the normal menstrual cycle and during pregnancy. After delivery, basal prolactin levels declined progressively. In women under long term medroxyprogesterone acetate treatment, the immunoreactive serum prolactin was within the normal range of cycling women. Prolactin is found in appreciable amounts in amniotic fluid and in the serum of newborn infants. Synthetic LH and FSH releasing hormone did not change circulating prolactin levels in normal humans. A possible luteotrophic action of human prolactin in synergism with LH cannot be excluded.     

The effects of partial opiate agonists on plasma prolactin and growth hormone levels in the rat.

Opiate agonists, partial agonists, and antagonists differed in their effects on release of prolactin and growth hormone. Agonists (morphine, methadone or meperidine) elevated plasma levels of both hormones. An antagonist (naloxone) lowered levels of prolactin but not growth hormone. All partial agonists studied raised growth hormone levels; among these, levallorphan, nalorphine, and ciramadol lowered prolactin levels while pentazocine and meptazinol did not. Naloxone blocked morphine-induced release of prolactin and growth hormone. The partial agonists suppressed morphine-induced prolactin release, and several suppressed the elevated growth hormone levels as well. Data from the opiate radioreceptor assay (displacement of ³H-naloxone) in the presence and absence of sodium agrees with the above placement of agents into three classes. These results suggest that classification of opioid compounds into agonists, partial agonists and antagonists may be made by their effects on prolactin and growth hormone release.     

Culture of human pituitary prolactin and growth hormone cells

Summary Fragments of pituitary tissue obtained from a total of 37 patients with either breast cancer, diabetic retinopathy, galactorrhea, or acromegaly were dissociated into single cell suspensions prior to cell culture. Release of human growth hormone (hGH) and human prolactin (hPRL) into the culture medium was measured by radioimmunoassay. During a 3-week culture period, prolactin cells released 9–13 times the intracellular levels of hPRL at the time of seeding, whereas hGH release from growth hormone cells was only 1–2 times that of their initial intracellular level during this same time. Both growth hormone and prolactin cells retained distinctive ultrastructural features during culture. The prolactin cells responded to TRH stimulation by elevated release of PRL into the medium. No evidence for mitotic division of prolactin cells in vitro was found.This work was supported by NCI Contract NO 1-CB-23863     

Purification, cloning, and expression of the prolactin receptor

The rat liver prolactin receptor has been purified to homogeneity, and partial amino acid sequences have been obtained. The structure of the receptor has been deduced from a single complementary DNA clone. The mature protein of 291 amino acids has a relatively long extracellular region, a single transmembrane segment, and a short (57 amino acids) cytoplasmic domain. With the rat cDNA used as a probe, the prolactin receptor in rabbit mammary gland and human hepatoma cells has also been isolated. These tissues contain a second, longer form of the receptor (592 and 598 amino acids, respectively). Both the short and long forms of the prolactin receptor show regions of strong sequence identity with the human and rabbit growth hormone receptors, suggesting that the prolactin and growth hormone receptors originate from a common ancestor.     

Developmental expression of growth hormone and prolactin genes in the bovine pituitary

Cell-free translation was used to initially characterize the major mRNA species present in the bovine anterior pituitary as a function of development. The only detectable change in translation products, which occurred during the transition from fetus to adult, was a reversal in the relative ratio of pituitary growth hormone and prolactin. Subsequent hybridization analysis with cloned growth hormone and prolactin cDNA probes indicated that growth hormone mRNA comprised over 40% of the total fetal mRNA and was 50- to 100-fold higher than prolactin mRNA. The steady state levels of growth hormone mRNA remained relatively constant throughout gestation. In contrast, prolactin mRNA levels, which were low early in gestation, increased during development to become the principal mRNA in the adult pituitary. Since growth hormone and prolactin are synthesized and secreted by specialized cells (somatotrophs and mammotrophs, respectively) immunochemical staining was used to determine whether the changes in the mRNA levels for these two hormones were a reflection of specific cell proliferation. For growth hormone, there was a close correlation between the number of somatotrophs and the relative levels of growth hormone mRNA. In contrast, the increase in prolactin mRNA exceeded the increase in the number of mammotrophs. Thus, the cellular concentration of growth hormone mRNA remains relatively constant during development, while the cellular concentration of prolactin mRNA increases by more than an order of magnitude.     

Enhancement of insulin secretion by human chorionic somatomammotropin and related hormones.

Hypophysectomized rats were treated for 6 days with 200 mug per day of either human chorionic somatomammotropin, human pituitary growth hormone, plasmin-modified human pituitary growth hormone, or ovine prolactin. All hormone preparations except ovine prolactin enhanced the ability of the pancreases of hypophysectomized rats to secrete insulin in the isolated pancreas perfusion system.     

Acquisition of Lubrol insolubility, a common step for growth hormone and prolactin in the secretory pathway of neuroendocrine cells

Rat prolactin in the dense cores of secretory granules of the pituitary gland is a Lubrol-insoluble aggregate. In GH(4)C(1) cells, newly synthesized rat prolactin and growth hormone were soluble, but after 30 min about 40% converted to a Lubrol-insoluble form. Transport from the endoplasmic reticulum is necessary for conversion to Lubrol insolubility, since incubating cells with brefeldin A or at 15 degrees C reduced formation of insoluble rat (35)S-prolactin. Formation of Lubrol-insoluble aggregates has protein and cell specificity; newly synthesized human growth hormone expressed in AtT20 cells underwent a 40% conversion to Lubrol insolubility with time, but albumin did not, and human growth hormone expressed in COS cells underwent less than 10% conversion to Lubrol insolubility. del32-46 growth hormone, a naturally occurring form of growth hormone, and P89L growth hormone underwent conversion, although they were secreted more slowly, indicating that there is some tolerance in structural requirements for aggregation. An intracellular compartment with an acidic pH is not necessary for conversion to Lubrol insolubility, because incubation with chloroquine or bafilomycin slowed, but did not prevent, the conversion. GH(4)C(1) cells treated with estradiol, insulin, and epidermal growth factor accumulate more secretory granules and store more prolactin, but not more growth hormone, than untreated cells; Lubrol-insoluble aggregates of prolactin and growth hormone formed to the same extent in hormone-treated or untreated GH(4)C(1) cells, but prolactin was retained longer in hormone-treated cells. These findings indicate that aggregation alone is not sufficient to cause retention of secretory granule proteins, and there is an additional selective process.     

Prolactin and schizophrenia: clinical consequences of hyperprolactinaemia

Prolactin is a polypeptide hormone that is synthesized and secreted from specialised cells of the anterior pituitary gland, known as lactotrophs. The hormone was given it's name because extracts from the bovine pituitary gland caused growth of the crop sac and stimulated the elaboration of crop milk in pigeons, and promoted lactation in rabbits. Although prolactin is best known for the multiple effects it exerts on the mammary gland, it has over 300 separate biological activities not represented by its name. It sub serves multiple roles in reproduction other than lactation and is an important modulator of homeostasis in the mammalian organism. Hence Bern and Nicoll suggested renaming it "omnipotin or versatilin". Schizophrenia is a severe psychiatric disorder that affects approximately one percent of the population worldwide. It is well established that traditional typical anti-psychotics elevate prolactin levels. It is also agreed that the serum prolactin concentration is not elevated in patients with schizophrenia who are not receiving anti-psychotic medication. Hyperprolactinaemia has direct effects on the brain and on other organs. Direct consequences include galactorrhoea. Indirect consequences of hyperprolactinaemia include oligomenorrhoea and amenorrhoea, erratic or absent ovulation, sexual dysfunction, reduced bone mineral density and cardiovascular disease. With the advent of prolactin sparing anti-psychotics, ample consideration needs to be given to the physiological consequences of hyperprolactinaemia in schizophrenic patients. In this paper we will examine molecular biology, secretion and physiology of prolactin. The consequences of hyperprolactinaemia in humans including effects on fertility, sexual dysfunction, bone mineral density, cardiovascular disease, changes in psychopathology and movement disorders will be reviewed. The literature on the association between schizophrenia, anti-psychotic medication and hyperprolactinaemia and more specifically on the consequences of this hyperprolactinaemia in schizophrenic patients will also be reviewed.     

Features of structure-activity organization of prolactin

The secondary structure of seven hormones of the prolactin family was predicted by two known prediction algorithms with the following averaging of the results for the whole homologous group of proteins. It was shown that the mentioned hormones are related to the alpha-helical type of protein molecules. The comparative analysis of the prolactin family and the kindred growth hormone family has been carried out on different levels of their structural organization. A conclusion is drawn, that despite the significant differences in the primary structures and small differences in the secondary structures, the three-dimensional structure for the prolactin molecules and the growth hormone are very similar and repeat in the same way the packing of alpha-helices. The study of the relationship of the prolactin structure and function has been carried out. The regions of the amino acid sequence, able to form prolactin antigenic sites and conditionally incorporated into two highly specific spatial groups were revealed. The region of the primary structure 80-137 determining lactogenic and proliferational function of the molecule and forming the alpha-hairpin in the tertiary structure has been discovered. The structural particuliarity of one of the binding sites of prolactin and human growth hormone with lactogenic receptor was reveal. An explanation for the absence of lactogenic activity in all kinds of growth hormones except human ones has been proposed.     

Effect of season on milk temperature,milk growth hormone,prolactin, and somatic cell counts of lactating cattle

Monthly fluctuations in milk temperature, somatic cell counts, milk growth hormone and prolactin of lactating cows were measured in milk samples over a 1 year period. The seasonal patterns in milk temperature, somatic cell count and milk prolactin concentration showed a positive trend with increasing environmental temperatures. Milk growth hormone concentration increased with lactation level and declined significantly during summer heat. Milk temperature and the measured hormonal levels may serve as indicators of the impact of the climatic environment on lactating cattle.     

The effects of the acute administration of phencyclidine hydrochloride (PCP) on the release of corticosterone, growth hormone and prolactin in the rat

There is little information on the neuroendocrine effects of PCP. The present study examined the effects of the acute subcutaneous administration of PCP on serum levels of corticosterone, growth hormone and prolactin in the male rat. PCP increased serum levels of corticosterone, decreased serum levels of prolactin and failed to affect growth hormone levels. The results indicate that, like other drugs of abuse, PCP alters neuroendocrine function.     

Human growth hormone site 2 lactogenic activity requires a distant tyrosine164.

Comparison of crystallographic structures of human growth hormone, either bound to the prolactin receptor or free of receptors, reveals that human growth hormone binding to the prolactin receptor at site 1 is associated with a structural change in human growth hormone that influences the organization of residues which constitute site 2. We have identified Tyr164 as a residue that is critical for the propagation of this structural rearrangement. Tyr164 is a structural epitope for site 1 and is distal to site 2. Mutation of Tyr164 to glutamic acid (Y164E) does not affect the somatotrophic activity, absorption or fluorescence spectra or binding to the human prolactin receptor when compared to wild-type human growth hormone, indicating the subtle effects of the mutation. Lactogenic assays using extended concentrations of Y164E human growth hormone produce dose-response curves that are characterized by a right-shifted agonist phase and an unchanged antagonist phase when compared to wild-type human growth hormone. These results indicate that Tyr164 is required for the lactogenic activity of human growth hormone and that mutation to glutamic acid disrupts the lactogenic function of site 2.