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PubMed1976

Failure to demonstrate transfer of milk prolactin into blood of milk-fed rats and calves.

Possible absorption of ingested prolactin into blood of milk-fed neonates was studied in two species by different experimental approaches. In calves, secretion of endogenous prolactin was suppressed by administration of ergocryptine. Prolactin was measured in blood plasma at 20-min intervals before and after ingestion by the calves of bovine milk or colostrum which contained prolactin. Prolactin in plasma during the 2-h period after feeding was the same as that during the 1-h period before feeding. therefore, these data failed to demonstrate any absorption of ingested prolactin into the blood of neonatal calves. Previous research showed that bovine prolactin could be transferred into rat milk by injecting it into lactating rats. Blood serum was collected from rat pups that were suckling dams previously injected with bovine prolactin. Several dosages and times of administration were used in the experimental design, and blood serum also was collected from rat pups nursing control dams. All samples were immunoassayed for bovine prolactin with reagents that crossreacted only slightly with endogenous rat prolactin. Injection of lactating dams with bovine prolactin did not increase the amount of immunoreactive bovine prolactin in the sera of suckling rat pups. These results agree with those from the calf study and indicate that ingested bovine prolactin was not absorbed intact into the blood of neonatal rats.

Absorption↗

PubMed2001

Molecular heterogeneity of prolactin in the plasma of patients with systemic lupus erythematosus.

OBJECTIVE: Systemic lupus erythematosus (SLE) has been associated with high levels of prolactin in the circulation of some patients. Although prolactin stimulates immune responses, the relationship between hyperprolactinemia and the pathophysiology of SLE remains controversial. This study was undertaken to investigate whether circulating bioactive prolactin isoforms are associated with the activity of SLE. METHODS: The molecular heterogeneity of prolactin was studied in the plasma of patients with active and inactive SLE and in healthy volunteers by radioimmunoassay (RIA), enzyme-linked immunosorbent assay (ELISA), Nb2-cell bioassay, and immunoprecipitation-Western blots. The specificity of the bioassay determinations was assessed by neutralization of growth-promoting effects with antiserum to human prolactin. RESULTS: Significantly higher prolactin levels were detected by bioassay and by ELISA than by RIA in both subsets of SLE patients and in normal individuals. Plasma prolactin levels in the SLE patients were significantly greater than those in the normal controls when measured by ELISA, but not by RIA or bioassay. The bioassay:ELISA and bioassay:RIA ratios were similar between SLE patients and controls, suggesting that prolactin biopotency was not altered with the disease, and none of the 3 assays detected a difference in prolactin levels between patients with active SLE and those with inactive SLE. However, the prolactin detected in plasma was associated with immunoreactive proteins of 130 kd and 23 kd, and the concentration of the 130-kd prolactin-like species was 10-fold higher in inactive SLE versus active SLE patients. CONCLUSION: Discrepancies among assays substantiate the molecular heterogeneity of circulating prolactin. The prolactin isotype that is found in association with inactive SLE could be of potential use as a marker for the inactive form of the disease and as an index for the efficacy of treatment.

Animals↗

PubMed2005

Molecular evolution of prolactin in primates.

Pituitary prolactin, like growth hormone (GH) and several other protein hormones, shows an episodic pattern of molecular evolution in which sustained bursts of rapid change contrast with long periods of slow evolution. A period of rapid change occurred in the evolution of prolactin in primates, leading to marked sequence differences between human prolactin and that of nonprimate mammals. We have defined this burst more precisely by sequencing the coding regions of prolactin genes for a prosimian, the slow loris (Nycticebus pygmaeus), and a New World monkey, the marmoset (Callithrix jacchus). Slow loris prolactin is very similar in sequence to pig prolactin, so the episode of rapid change occurred during primate evolution, after the separation of lines leading to prosimians and higher primates. Marmoset prolactin is similar in sequence to human prolactin, so the accelerated evolution occurred before divergence of New World monkeys and Old World monkeys/apes. The burst of change was confined largely to coding sequence (nonsynonymous sites) for mature prolactin and is not marked in other components of the gene sequence. This and the observations that (1) there was no apparent loss of function during the episode of rapid evolution, (2) the rate of evolution slowed toward the basal rate after this burst, and (3) the distribution of substitutions in the prolactin molecule is very uneven support the idea that this episode of rapid change was due to positive adaptive selection. In the slow loris and marmoset there is no evidence for duplication of the prolactin gene, and evidence from another New World monkey (Cebus albifrons) and from the chimpanzee and human genome sequences, suggests that this is the general position in primates, contrasting with the situation for GH genes. The chimpanzee prolactin sequence differs from that of human at two residues and comparison of human and chimpanzee prolactin gene sequences suggests that noncoding regions associated with regulating expression may be evolving differently from other noncoding regions.

Amino Acid Sequence↗

PubMed1986

Evidence of short-loop inhibition of decidual prolactin synthesis by decidual proteins. Part I.

Induction of uterine endometrial prolactin synthesis is dependent on progesterone-induced decidualization of stromal cells. These decidual cells are not dependent on progesterone for continued prolactin synthesis. The factors modifying decidual prolactin synthesis remain largely unknown. To test the hypothesis that a decidual protein is the major modulator of new prolactin synthesis, decidua were cultured within dialysis membranes allowing the accumulation of proteins greater than 12,000 molecular weight in a metabolically neutral environment, and the rate of new synthesis was compared with prolactin synthesis from samples cultured in 10 times the available volume for protein distribution. The rate of new prolactin synthesis at 48-hour intervals up to 144 hours was compared. Initial and postculture decidual prolactin content was obtained and was found not to vary significantly between groups (0.05 less than p less than 0.10). At 48 hours significant suppression of decidual prolactin synthesis was apparent (p less than 0.05) within the dialysis membranes. As prolactin concentration increased during in vitro culture this suppression was enhanced (p less than 0.005). Gel chromatography and immunoprecipitation of iodine 125-labeled prolactin added at time 0 revealed no significant degradation of the 125I-labeled prolactin and maintenance of its immunoactivity even at 144 hours. This confirms that the plateauing of prolactin concentration within the dialysis membranes is due to suppression of new synthesis rather than metabolism of previously synthesized prolactin.

Cells, Cultured↗

PubMed1986

Nocturnal surges and reflexive release of prolactin in parentally behaving virgin female and male rats.

Maternally behaving virgin rats are capable of releasing prolactin reflexively in response to stimulation by pups, especially during the proestrous/estrous phase of the cycle. When such rats are chronically exposed to pups they usually undergo a state of pseudopregnancy during which prolactin is secreted in a pattern of nocturnal surges. The present series of experiments evaluated the initiation of nocturnal prolactin surges in maternally behaving virgins, the role of estrogen in the reflexive release of prolactin, and the influence of gender on these two modes of prolactin secretion. It was found that the nocturnal surges of prolactin are already present on Days 1 and 2 of pup-induced pseudopregnancy. At this stage, however, the surges are not yet autonomous, seeing that pseudopregnancy is interrupted shortly after removal of the pups on Day 2. Activation by vaginocervical stimulation of the "mnemonic" neurogenic system that controls the autonomous nocturnal prolactin surges did not interfere with the reflexive pup-induced release of prolactin in maternally behaving virgins. The capacity of reflexive prolactin release in the virgin rat was abolished by ovariectomy, restored by estrogen replacement, and persisted for only 24 hr following estrogen removal. Paternally behaving males subjected to chronic exposure to pups were incapable of secreting nocturnal surges of prolactin characteristic of the pseudopregnant female. Such males were also incapable of releasing prolactin reflexively in response to stimulation by pups, even when supplemented with exogenous estrogen. These results indicate that the two modes of prolactin secretion are sex dependent, and that the maternally behaving virgin, unlike the postpartum rat, requires concurrent estrogenic facilitation for releasing prolactin in response to stimulation by young.

Animals↗

PubMed1982

Non-opiate beta-endorphin fragments and dopamine-V. gamma-Type endorphins and prolactin secretion in rats.

The effects on prolactin secretion of three peptide-derivatives of beta-endorphin which show neuroleptic-like activities in rats were studied. Intravenous administration of gamma-endorphin (beta-endorphin (beta E) 1-17) enhanced plasma prolactin levels. Gamma-Endorphin did not affect the prolactin secretion by hemipituitary glands incubated in vitro and by cultured pituitary cells, but it reversed, in a dose-dependent way, the dopamine-induced inhibition of prolactin release. This effect of gamma-endorphin could be prevented by co-incubation with the opiate-antagonist naloxone. In vivo studies with non-opiate-like fragments of gamma-endorphin, des-Tyr1-gamma-endorphin (DT gamma E, beta E 2-17) and des-enkephalin-gamma-endorphin (DE gamma E, beta E 6-17), showed that these peptides suppressed the plasma prolactin levels, when prolactin release was slightly stimulated. However, DE gamma E dose-dependently enhanced plasma prolactin levels, when prolactin release was low. Subchronic treatment with DE gamma E for 4 days made prolactin release more sensitive to the inhibitory action of small doses of apomorphine. Neither DT gamma E nor DE gamma E affected prolactin release by hemipituitary glands and cultured pituitary cells. They did also not affect the dopamine-induced inhibition of prolactin release in vitro. It is suggested that gamma-endorphin increases prolactin secretion by interfering with dopaminergic systems in the pituitary and that DT gamma E and DE gamma E have a modulatory action on prolactin secretion.

Animals↗

PubMed1982

Relationship between prolactin in the serum and cerebrospinal fluid of ovariectomized female rhesus monkeys.

The entry of prolactin into the cerebrospinal fluid from the blood, and the relation between levels in the two compartments were studied under a variety of conditions in ovariectomized rhesus monkeys. Prolonged treatment with either domperidone or sulpiride, both dopamine-receptor blockers, elevated prolactin levels in serum and cerebrospinal fluid proportionally equally, so that the cerebrospinal fluid serum ratio was unchanged from controls (circa 12-20%). Gel filtration showed that only 'little' (monomeric) prolactin entered the cerebrospinal fluid in such monkeys. Following acute elevations of blood prolactin after a single injection of either sulpiride or ovine prolactin, cerebrospinal fluid levels increased linearly over a 90 min sampling period, despite falling serum levels. The rate of entry of prolactin into the cerebrospinal fluid was similar after either procedure, and was independent of absolute serum or cerebrospinal fluid levels, suggesting a rate-limiting mechanism. Furthermore, retrograde portal blood flow from the pituitary is not necessary to account for these results. Clearance of prolactin from the third ventricle was studied following intraventricular injection of prolactin. Prolactin was removed from the cerebrospinal fluid by a mechanism whose efficiency compares with that in serum, so that the half-life of prolactin in either compartment is about the same. Measurement of sodium, potassium and calcium in the cerebrospinal fluid during prolonged hyperprolactinaemia showed no change, indicating that the central effects of prolactin are not due to alteration of these electrolytes. These results show that cerebrospinal fluid levels of prolactin, and hence, those surrounding the brain, can be inferred accurately from those in the blood, and suggest that there may be a selective mechanism regulating the entry of prolactin into the cerebral compartment.

Animals↗

PubMedDate not supplied

Review of evidence that posttransplantation psychiatric treatment commonly affects prolactin levels and thereby influences graft fate.

Delirium, depression and other psychiatric difficulties are commonly encountered by posttransplantation patients, and antipsychotic medicines are frequently used to treat these difficulties. This article reviews previous research data concerning the immunological effects of these medicines, with particular focus on the consequences of prolactin elevation. Unproven but of concern is that these effects may influence graft fate. Older antipsychotic medicines such as haloperidol and chlorpromazine have a high likelihood of elevating prolactin. Prolactin is an immunologically active molecule generally promoting bone marrow function. This may be of benefit post-stem-cell transplant, helping engraftment, but could further rejection of solid-organ transplants. Elevated prolactin is implicated in the facilitation of graft-versus-host disease. Aripiprazole is the antipsychotic medicine least likely to increase prolactin (and may actually decrease prolactin); risperidone, the most likely to increase prolactin. Olanzapine, quetiapine and ziprazadone are antipsychotic medicines with a lower likelihood of elevating prolactin. Older ("neuroleptic") antipsychotics, such as chlorpromazine, droperidol and haloperidol, perphenazine and many others, are likely to elevate serum prolactin. Among antidepressants, most serotonin reuptake inhibitors, with the exception of sertraline, can slightly elevate prolactin. The atypical (i.e., alone in their class) antidepressants bupropion and mirtazapine are prolactin neutral. The immunological consequences of psychiatric medicines should be considered when treating transplant patients for delirium, depression and thought disorders; in addition, if elevation of prolactin is thought to be of immunological importance during psychiatric treatment, then it should be monitored and treated. The dopamine agonists used to treat Parkinson's disease--bromocriptine, pergolide, pramipexole, ropinerole--usually reverse antipsychotic-induced prolactin increases without compromising psychiatric effectiveness.

Antipsychotic Agents↗

PubMed1998

NMDA receptor antagonists block stress-induced prolactin release in female rats at estrus.

In order to evaluate the role of glutamate in prolactin secretion, we examined the effects of N-methyl-D,L-aspartic acid (NMDA) receptor antagonists on serum prolactin levels at both resting and restraint-stress conditions in female rats at estrus. NMDA increased basal serum prolactin levels. Administration of the selective NMDA receptor antagonist, cis-4-phosphonomethyl-2-piperidine carboxylic acid (CGS 19755) (5 and 10 mg/kg i.p.), to rats under resting conditions enhanced basal prolactin levels. A low dose of CGS 19755 (3 mg/kg) was unable to modify the hormone serum level. Under stress conditions the pretreatment with CGS 19755 (3 and 5 mg/kg) prevented the increase in serum prolactin levels. This effect was reversed by NMDA (60 mg/kg s.c.). The NMDA receptor antagonist (5 mg/kg) decreased the median eminence concentration of the dopamine metabolite, 3,4-dihydroxyphenylacetic acid (DOPAC), without modifying dopamine content. To examine the probable link between serotonin (5-HT) and glutamate in prolactin release, the 5-HT2A/5-HT2C receptor antagonist, ritanserin, was used. Under resting conditions, a dose of 5 mg/kg s.c. blocked the NMDA-induced prolactin release. In rats submitted to restraint, ritanserin decreased the prolactin response and NMDA was unable to correct the stress serum prolactin levels. The 5-HT1A receptor agonist, 8-hidroxypropyl-amino tetralin (8-OH-DPAT) (3 mg/kg s.c.), increased basal serum prolactin levels and restored serum prolactin in stressed animals pretreated with CGS 19755 (5 mg/kg). The present data strongly suggest that the glutamatergic system participates in the regulation of prolactin secretion. A stimulation tone seems to be exerted via the tuberoinfundibular dopaminergic system, and the prolactin release evoked by restraint apparently involves glutamate/NMDA receptors linked to a serotoninergic pathway.

3,4-Dihydroxyphenylacetic Acid↗

PubMed1981

Hydrophobic-interaction chromatography and anion-exchange chromatography in the presence of acetonitrile. A two-step purification method for human prolactin.

Described is a two-chromatographic-step preparative-scale technique for the purification of human prolactin from a frozen pituitary homogenate. The method utilizes hydrophobic interaction chromatography on the mildly hydrophobic adsorbent phenyl-Sepharose CL-4B and anion-exchange chromatography on DEAE-cellulose in the presence of acetonitrile. Human prolactin was solubilized at pH10.0 after a prior extraction of pituitaries at pH4.0, the acid pH being ineffective at solubilizing human prolactin but capable of solubilizing large amounts of interfering protein. An 11-fold increase in the potency of the solubilized human prolactin was achieved in this manner. Prolactin could be adsorbed to phenyl-Sepharose at low ionic strengths (I<0.01); few other proteins were adsorbed under these conditions. This is a demonstration of the hydrophobic nature of human prolactin. The amount of phenyl-Sepharose was limited to the minimum (35mg of protein/g of phenyl-Sepharose) necessary to adsorb human prolactin, further reducing the uptake of other pituitary protein. Desorption was achieved by using an acetonitrile gradient (0-30%, v/v), resulting in a purification of human prolactin of 85-fold and recovery of 78%. Acetonitrile (20%, v/v) was also included in all buffers for DEAE-cellulose chromatography, increasing the resolution and recovery of human prolactin, apparently by minimizing non-ionic interactions with the matrix. Prolactin (10mg) was recovered from 63g if pituitaries, an overall recovery of 58%. It was homogeneous by gel filtration and sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, contained less than 0.1% somatotropin (growth hormone), on iodination demonstrated more than 95% binding to excess anti-(human prolactin) serum and could be displaced from anti-(human prolactin) serum in a manner indistinguishable from the serum of a patient with a human prolactin-secreting adenoma.

Acetonitriles↗

PubMed1983

Adenosine 3',5'-cyclic monophosphate-dependent release of prolactin from GH3 pituitary tumour cells. A quantitative analysis.

The involvement of cyclic AMP in mediating regulatory peptide-controlled prolactin release from GH3 pituitary tumour cells was investigated. Cholera toxin and forskolin elicited concentration-dependent increases in both GH3 cell cyclic AMP content and prolactin release. The maximum rise in prolactin release with these agents was 2-fold over basal. 8-Bromo-cyclic AMP produced a similar stimulation of prolactin release. The phosphodiesterase inhibitor isobutylmethylxanthine also produced an increase in prolactin release and GH3 cell cyclic AMP content. However, the magnitude of the stimulated prolactin release exceeded that obtained with any other agent. Thyrotropin-releasing hormone (thyroliberin) and vasoactive intestinal polypeptide produced a concentration-dependent rise in both cell cyclic AMP content and prolactin release. However, only vasoactive intestinal polypeptide elicited an increase in cell cyclic AMP content at concentrations relevant to the stimulation of prolactin release. Vasoactive intestinal polypeptide and thyrotropin-releasing hormone, when used in combination, were additive with respect to prolactin release. Vasoactive intestinal polypeptide and forskolin, at concentrations that were maximal upon prolactin release, were, when used in combination, synergistic upon GH3 cell cyclic AMP content but were not additive upon prolactin release. In conclusion the evidence supports a role for cyclic AMP in the mediation of vasoactive intestinal polypeptide- but not thyrotropin-releasing hormone-stimulated prolactin release from GH3 cells. A quantitative analysis indicates that a 50-100% rise in cyclic AMP suffices to stimulate cyclic AMP-dependent prolactin release fully.

1-Methyl-3-isobutylxanthine↗

PubMed1998

Effect of photostimulation on concentrations of plasma prolactin in castrated bantams (Gallus domesticus).

The annual breeding cycle of 'unimproved' breeds of domestic chicken, including the bantam, at temperate latitudes, is terminated by decreasing daylength in autumn and is initiated in late winter, while daylengths are still short. Observations on photoperiodic birds that terminate seasonal breeding by the development of long day photorefractoriness suggest that the photoinduced pattern of prolactin secretion is associated with the pattern of gonadal growth and regression. It was predicted that, if there is a causal relationship between photoinduced changes in prolactin secretion and gonadal function in birds then, in the bantam, the pattern of prolactin secretion observed after photostimulation would not be the same as in birds terminating breeding by the development of long day photorefractoriness. Experiments were carried out on surgically castrated bantams to avoid confounding the effects of photostimulation and the stimulatory actions of testicular hormones on prolactin secretion. Transfer of photosensitive castrated bantams from 8 to 14, 16, 18 or 20 h light/day initially stimulated prolactin release and, subsequently, after 20-30 days, concentrations of plasma prolactin progressively decreased. After 148 days of photostimulation, concentrations of plasma prolactin approached but were still higher than short day controls. Transfer of photosensitive castrated bantam cockerels from 8 to 12 h light/day stimulated a slower increase in plasma prolactin that subsequently remained higher than in other photostimulated groups. A further 4 h increase in photoperiod in the birds exposed for 148 days to 12 or 16 h light/day resulted, respectively, in a transitory increase and no increase in prolactin secretion. Recovery of photosensitivity for prolactin release was observed in the birds transferred to 18 or 20 h light/day for 148 days after treatment with 8 h light/day for 35 days. Attempts to obtain an independent hormonal correlate of the prolactin responses to photostimulation by measurement of plasma luteinizing hormone (LH) were unsuccessful. The concentration of plasma LH in castrated bantams did not change in response to a change in photoperiod. These observations show that the photoinduced pattern of prolactin release in the bantam, a species which terminates seasonal breeding in response to decreasing daylength, is the same as that in birds which terminate seasonal breeding by the development of long day photorefractoriness. It is concluded that the photoinduced pattern of prolactin secretion in birds can be dissociated from the neuroendocrine mechanisms controlling the termination of seasonal breeding.

Animals↗

PubMed1999

Rat decidual prolactin. Identification, molecular cloning, and characterization.

Establishment and maintenance of pregnancy require the activity of a highly specialized maternal tissue, the decidua. It is well established that the human decidua synthesizes and releases prolactin. However, in the rat, no study has been able to demonstrate the production of prolactin by the decidua. In this report, we established for the first time using Northern blot analysis and reverse transcription-polymerase chain reaction, Western blot analysis, immunocytochemistry, and enzyme-linked immunosorbent assay, that a defined cell population located in the rat antimesometrial decidua expresses prolactin mRNA, as well as synthesizes and secretes this hormone. By reverse transcription-polymerase chain reaction and rapid amplification of cDNA ends, we cloned a full-length cDNA for rat decidua prolactin, whose sequence was identical to that of pituitary prolactin. Our results also showed that pituitary prolactin appeared to down-regulate decidual prolactin levels. Under these circumstances, inhibition of pituitary prolactin secretion led to a rise in both decidual prolactin mRNA and protein expression. Moreover, addition of exogenous prolactin to primary decidual cells in culture also caused a marked decrease in decidual prolactin mRNA expression. Finally, treatment of primary decidual cells with steroid hormones or 8-bromo-cAMP revealed a differential regulation of decidual prolactin expression from that of pituitary suggesting a tissue-specific regulation of prolactin gene expression, possibly through the use of an alternative promoter in rat decidua.

Amino Acid Sequence↗

PubMed2004

Maternal zinc deficiency raises plasma prolactin levels in lactating rats.

There is an inverse relation between zinc (Zn) intake and plasma prolactin in men and nonpregnant women. Whether a relation exists in lactating women is unknown, despite the potential consequences of perturbations in prolactin regulation on lactation performance. We examined the effects of low Zn intake on prolactin concentration, the prolactin regulatory pathway in the pituitary gland, and lactation performance in lactating rats. Female rats were fed diets containing 7 (zinc deficient; ZD), 10 (marginally zinc deficient; MZD) or 25 mg Zn/kg (control) from 70 d preconception to lactation d 11. Rats were killed, pituitary glands dissected, and tissues and plasma collected and analyzed for prolactin concentration. Pituitary gland pituitary factor 1 (Pit-1), dopamine 2 receptor (D2R), and prolactin receptor mRNA expression were measured in the pituitary gland. Liver, mammary gland, plasma, and milk Zn were measured. Milk intake of the pups was also recorded. Plasma prolactin concentration was higher in rats fed the ZD (125.9 microg/L) diet compared with control rats (21.7 microg/L). Pituitary gland prolactin concentration was higher in rats fed the ZD diet (69.8 mg/g total protein) compared with controls (29.0 mg/g). Plasma Zn concentration was lower in rats fed the MZD and ZD diets, and mammary gland and milk Zn concentrations were lower in rats fed the ZD diet compared with control rats. Rats fed the ZD diet had lower D2R, prolactin receptor, and Pit-1 mRNA levels, whereas rats fed the MZD diet had lower prolactin receptor and Pit-1 mRNA levels compared with control rats. Milk intake was lower in pups of rats fed the MZD and ZD diets. Our results suggest that marginal Zn nutriture may compromise milk production despite increased prolactin levels. In addition, increased circulating prolactin concentration is not due to altered nursing behavior, but may be due to alterations in the prolactin regulatory pathway in the pituitary gland.

Animals↗

PubMed2002

Milk prolactin, feed volume and duration between feeds in women breastfeeding their full-term infants over a 24 h period.

Previous studies have suggested that the uptake of prolactin from the blood into the milk may be restricted when the alveolus is distended with milk. Therefore the aim of this study was to determine the relationship between prolactin in milk and milk production by measuring the concentration of prolactin in samples of fore- and hind-milk as well as the volume of milk removed for each breast, at each breastfeed over a 24 h period. The mean (+/- S.D.) concentration of prolactin in milk for all women (n = 15) over the 24 h period was 18.5 +/- 11.6 microg l(-1) (fore-milk) and 16.8 +/- 12.8 microg l(-1) (hind-milk). The variation between women masked small changes within women in the concentration of prolactin in milk over the 24 h period, therefore a prolactin ratio (individual fore- or hind-milk value divided by the mean for all fore- or hind-milk samples collected over a 24 h period) was determined. The concentration of prolactin was highest in milk between 02.01 and 06.00 h (prolactin ratio for fore- to hind-milk, 1.5), and lowest between 10.01 and 18.00 h (prolactin ratio for fore- to hind-milk, 0.8). Furthermore, we observed that the difference in prolactin concentration between the fore- and hind-milk (fore-hind gradient) was greatest between 06.01 and 10.00 h (4 microg l(-1)). To ensure that this effect was not due to permeability in the paracellular pathway, the concentrations of serum albumin and sodium in milk were measured. No significant (P > 0.05) changes over the 24 h period, or with increasing time since last feed were observed. We therefore concluded that when the breast is most drained of milk (in the late evening), and the rate of milk synthesis is greatest, the fore-hind prolactin gradient in the milk of the following feed (in the early morning) is highest. This is consistent with the observation that prolactin uptake from the blood by the lactocyte is dependent on the fullness of the breast, such that prolactin uptake may be inhibited in full alveoli.

Adult↗

PubMed1992

The effects of dopamine on prolactin mRNA levels in rat pituitary cells in culture.

Dopamine is known to be the prolactin-release inhibiting factor, but the effects of dopamine itself on regulation of prolactin messenger RNA have been little studied because of the instability of dopamine. We have compared the effects of dopamine and bromocriptine on the levels of prolactin mRNA and on the rates of synthesis, storage, and release of prolactin in primary cultured rat pituitary cells. The cells were incubated for 72 h with no secretagogue (control group) or in the presence of either dopamine (10 mumol/L) plus ascorbic acid (100 mumol/L) or bromocriptine (0.1 mumol/L). Prolactin mRNA was measured in cell extracts by means of slot blots, and newly synthesized prolactin was measured in similar incubations by the addition of [3H]leucine, followed by gel electrophoresis. The levels of total prolactin were measured by radioimmunoassay. Prolactin mRNA was reduced to 78 +/- 9% (mean +/- SEM) of control levels in bromocriptine-treated cells and to 59 +/- 7% in dopamine-treated cells, demonstrating that dopamine stabilized by ascorbic acid was able to reduce the levels of prolactin mRNA in rat pituitary cells in culture. Dopamine may act at sites in addition to the dopaminergic D2 receptor, since the level of prolactin mRNA was reduced more by a supramaximal dose of dopamine than by a supramaximal dose of bromocriptine. The results of the [3H]prolactin and prolactin measurements suggested that availability of mRNA was not a major factor in controlling the rate of prolactin synthesis.

Animals↗

PubMed1982

Hamster prolactin: physiological changes in blood and pituitary concentrations as measured by a homologous radioimmunoassay.

A specific and sensitive homologous radioimmunoassay (RIA) for determination of hamster prolactin is described and compared to heterologous hamster prolactin RIA and to the homologous and heterologous RIAs for rat prolactin. With this method, we have determined that, in the golden hamster, diurnal plasma prolactin fluctuations have a mean interpeak interval of 1.5 h, that serum prolactin concentrations are influenced by day length, by ether vapors, by animal's gender, and by refeeding following a fast but not by neurosurgical procedures which increase somatic growth and serum GH concentration in adult hamsters. Variations in day length influenced pituitary prolactin content and concentration, and pituitary removal abolished prolactin secretory responses to ether stress. A heterologous RIA for hamster prolactin which utilizes radioiodinated rat prolactin and rabbit antihamster prolactin serum (RK1-15) represents the most advantageous method for the measurement of hamster prolactin because of its high sensitivity (0.14 ng/ml), high antiserum binding to iodinated rat prolactin, and the ability to measure both hamster and rat prolactin.

Animals↗

PubMed1989

Presence of prolactin-like immunoreactivity and bioactivity in rat spinal cord.

Pursuant to our identification of prolactin-like immunoreactivity (PLI), widely distributed in rat brain, the spinal cord was examined for the presence of this pituitary-hormone-like protein. PLI was present in all spinal cord extracts examined and averaged 500 +/- 53 pg/mg protein. Hypophysectomy, causing a fall in serum prolactin to undetectable levels, was not associated with any change in levels of PLI in spinal cord. Recovery of rat prolactin standards added to spinal cord homogenates was 97.6 +/- 3.9%. When increasing concentrations of spinal cord extract were assayed in a prolactin radioimmunoassay, displacement of rat 125I-Prolactin from antiserum was parallel to that displacement produced by increasing concentrations of rat anterior pituitary standards. Upon subjection to gel permeation chromatography, the elution profiles of immunoreactive prolactin from spinal cord were different from the profiles of anterior pituitary prolactin. In addition to an immunoreactive prolactin peak eluting with pituitary prolactin, spinal cord extracts showed a large void volume peak and late eluting low-molecular-weight materials not seen with anterior pituitary. In the Nb2 lymphoma cell assay, all spinal cord extracts demonstrated prolactin-like bioactivity with a bioactivity/immunoreactivity ratio of 1.05 +/- 0.13. We conclude: (1) PLI, widely distributed in rat brain, is also present in spinal cord; (2) spinal cord prolactin levels are independent of levels in pituitary and peripheral circulation; (3) this immunoreactive prolactin is bioactive, and (4) differing gel permeation chromatographic elution profiles indicate that there may be some molecular differences between pituitary and spinal cord prolactin.

Animals↗