Blood Prolactin Concentrations Research Articles

Melatonin receptors in the brain and pituitary gland of hypothalamo-pituitary disconnected Soay rams.

In Soay rams in which the pituitary gland has been surgically separated from the hypothalamus, blood prolactin concentrations vary in response to changes in photoperiod and the administration of melatonin, as in intact animals, providing evidence that melatonin acts within the pituitary gland to control prolactin secretion. In this study the presence of potentially functional melatonin receptors in the pars tuberalis and zona tuberalis (PT/ZT) of hypothalamo-pituitary disconnected (HPD) Soay rams is confirmed using both in vitro autoradiography with the ligand 2-(125I)-iodomelatonin and in situ hybridization for the melatonin receptor. There was no effect of the HPD operation on the pattern and quantity of 2-(125I)iodomelatonin binding in the brain demonstrating that this binding is independent of hypothalamic regulation. The possibility that melatonin may control prolactin secretion directly via specific receptors on lactotrophs was investigated using dual in situ hybridization with a (35S) labelled probe for the ovine melatonin receptor (Mel 1a(b)) and a Digoxigenin labelled probe for ovine prolactin. Melatonin receptor gene expression was observed in the PT/ZT in both intact and HPD rams, however, there was no colocalization with prolactin gene expression; only in the ZT was there a close association between cells expressing the melatonin receptor and lactotrophs. The results provide strong support for the view that melatonin acts via the PT/ZT to mediate the effects of photoperiod on the seasonal cycle in prolactin secretion.

Refractoriness to a static melatonin signal develops in the pituitary gland for the control of prolactin secretion in the ram.

Hypothalamo-pituitary disconnected (HPD) and control Soay rams were treated chronically for 48 wk with s.c., continuous-release implants of melatonin while under long days (16L:8D). The implants produced continuously elevated blood concentrations of melatonin 2-3 times higher than the normal nocturnal maximum. The long-term treatment induced a biphasic effect on prolactin secretion in both the HPD and control rams, with a marked decrease in the blood prolactin concentrations for 10 wk followed by a gradual increase. The introduction of a second melatonin implant after 20 wk failed to affect prolactin secretion. The treatment with melatonin also caused a dynamic effect on FSH secretion, but this occurred in the control rams only. Blood concentrations of FSH in the HPD rams were very low throughout, but there were minor changes in testicular diameter that were correlated with variations in prolactin. Overall, the results support the conclusion that 1) melatonin acts primarily in the pituitary gland to affect prolactin secretion, and partial refractoriness develops at this level for control of prolactin; and 2) melatonin acts most probably in the hypothalamus to affect gonadotropin secretion, and refractoriness develops at the level of the neural tissue regulating GnRH release for control of gonadotropins.

Blood prolactin concentrations affect prolactin transfer into goat milk: implications for maintenance of lactation.

125I-Labelled ovine prolactin was infused for 15 min into a pudic artery supplying one mammary gland of lactating goats (n = 17). Between 0 and 4.25 h significantly more total (P < 0.01) and trichloroacetic acid (TCA)-precipitable (P < 0.001) radioactivity appeared in the milk of the infused compared with the non-infused gland. Gel chromatography and antibody precipitation indicated the presence of undegraded 125I-labelled prolactin in milk whey. Maximum transfer occurred 60-80 min after the end of infusion suggesting passage via a transcellular route. High plasma prolactin concentrations, resulting from infusion of cold prolactin with labelled prolactin in late lactation or from seasonally elevated prolactin at peak lactation, reduced the specific activity of infused prolactin and depressed the difference in secretion of 125I-labelled prolactin into milk of infused and non-infused glands. This suggests the operation of a competitive and saturable mechanism. Together with the increase in the milk to blood ratio of prolactin in goats given long-term (3 week) bromocriptine treatment, the results suggest that the goat mammary gland has a high avidity for prolactin especially when circulating prolactin is low. There was also evidence from TCA precipitation that prolactin may be protected from degradation in these circumstances. These mechanisms may contribute to the resistance of ruminant lactation to reduction in plasma prolactin and protect lactation from seasonal prolactin fluctuations.

Evidence that Melatonin Acts in the Pituitary Gland through a Dopamine‐independent Mechanism to Mediate Effects of Daylength on the Secretion of Prolactin in the Ram

A previous study provided evidence that melatonin acts in the pituitary gland to mediate the effects of daylength on the secretion of prolactin in sheep. This was based on the observation that hypothalamo-pituitary disconnected (HPD) Soay rams showed normal patterns in the changes in the peripheral blood concentrations of prolactin in response to alterations in photoperiod (10-fold higher concentrations under long than short days), and in response to exogenous melatonin (rapid decline following the administration of a constant-release implant of melatonin). The purpose of this study was to establish whether dopamine (DA) might be involved in mediating the effects of melatonin on the secretion of prolactin. Groups of HPD (n = 7) and control Soay rams (n = 8) were treated with vehicle (control, 2.0 ml 0.1 M tartaric acid/saline sc), bromocriptine (DA agonist, 0.06 mg/kg sc) or sulpiride (DA antagonist, 0.6 mg/kg sc), and the acute prolactin responses were measured over the next 4 h. Treatments were carried out under short days (8L: 16D, low prolactin), long days (16L: 8D), high prolactin), and under long days in the presence of a constant-release implant of melatonin (low prolactin). The prolactin response to TRH (1.25 micrograms/kg iv) was also measured. Bromocriptine caused a decrease in the plasma concentrations of prolactin in both HPD and control rams under short and long days. Sulpiride had no effect in the HPD rams on any occasion, but caused a very marked increase in the plasma concentrations of prolactin in the control rams under short days, long days, and under long days + melatonin. TRH caused an acute increase in the plasma concentrations of prolactin in the HPD rams under both long and short days although the responses were notably reduced compared with the controls especially under long days + melatonin. Overall, the inhibitory response to the DA agonist in HPD rams indicates the presence of DA D2 receptors linked to functional lactotrophs in the isolated pituitary gland. However, the total lack of a response to the DA antagonist indicates the absence of endogenous DA mechanisms regulating the secretion of prolactin in the HPD rams. The conclusion is that melatonin acts directly on the pituitary gland to mediate effects of photoperiod through a DA-independent mechanism.

Melatonin and fleece growth in Australian cashmere goats

Three groups each of eight Australian cashmere goats (four males, four females, 10 months old) were implanted at the base of the ear with melatonin pellets (18 mg) between July and October (July group), January and April (January group), or sham implanted (control). Cashmere and hair length changes were measured during the year following implantation. Rates of growth, maximum length, initiation and cessation dates of cashmere growth were calculated from cumulative cashmere length measurements. All goats were maintained on pasture under natural daylight. Control goats showed the normal circannual pattern of growth: no growth from July until the cycle was initiated, on average, on 9 November 1988 and ceased on 27 May 1989 (199 days). Implantation in July permitted the continuation of cashmere growth until 1 November 1988, and this extended cycle was followed by another cycle of short duration (146 days) with initiation (26 January 1989) and cessation (21 June 1989) dates later than those of controls. Hair growth patterns also changed during this period. Implantation with melatonin in January reduced maximum fleece length (59 mm) compared with controls (72 mm) by induction of an early cessation of cashmere growth (3 May 1989). There were no differences ( P < 0.05) between sexes in response to treatments. Melatonin implants in July (short days) partially suppressed a rise in blood prolactin concentrations in October, and implants in January (long days) resulted in prolactin levels similar to those found in short days. Melatonin implants had no effect on serum thyroxine concentrations. Results suggest that melatonin implants at strategic times of the year may be a practical method for increasing cashmere production from young goats.

Photoperiodically-induced cycles in the secretion of prolactin in hypothalamo-pituitary disconnected rams: evidence for translation of the melatonin signal in the pituitary gland.

Long term changes in the secretion of prolactin were monitored in groups of hypothalamo-pituitary disconnected rams (HPD rams, n = 8) and control rams (HPD sham-operated and unoperated, n = 8) while exposed to an artificial lighting regimen of alternating 16-weekly periods of long days (16L:8D) and short days (8L:16D) for 72 weeks, and during a treatment with subcutaneous constant-release implants of melatonin under long days. The HPD rams showed all the clinical characteristics of complete pituitary disconnection (diabetes insipidus, gonadal regression and slight obesity), and were unresponsive to a range of provocation tests (exposure to a barking sheep dog, cannulation of the jugular vein, injection of serotonin and NMDA) which caused acute changes in the blood plasma concentrations of prolactin in the controls. Nevertheless, there was a clearly defined cycle in the blood concentrations of prolactin in the HPD rams related to the imposed lighting regimen with values 10-fold higher under long days compared to short days (HPD mean +/- SEM: 90.1 +/- 24.7 vs 9.4 +/- 2.0 micrograms/l, long vs short day respectively, P < 0.001). The temporal pattern was very similar to that observed in the controls, although the concentrations of prolactin were higher in the HPD rams and more variable (control mean +/- SEM: 55.6 +/- 3.6 vs 3.0 +/- 0.5 micrograms/l, long vs short day, P < 0.001). There was a corresponding cycle in the growth and moulting of the wool in the HPD rams consistent with a biological response to the photoperiodically-induced changes in the secretion of prolactin. The diurnal rhythm in the blood concentrations of prolactin was absent in the HPD rams, but there was a normal rhythm in the secretion of melatonin. The treatment of the animals with constant-release implants of melatonin under long days caused a marked decrease in the blood concentrations of prolactin in both the HPD and control rams. The overall conclusion is that the endogenously generated daily melatonin signal which encodes daylength acts directly in the pituitary gland to mediate the effects of photo-period on the secretion of prolactin. The photo-period transduction pathway thus by-passes the hypothalamus.

Fleece growth in Australian cashmere goats. III. The seasonal patterns of cashmere and hair growth, and association with growth hormone, prolactin and thyroxine in blood

The seasonal patterns of cashmere and hair growth, and prolactin, thyroxine and growth hormone concentration in blood were investigated in male, female and castrated male 6- month-old Australian cashmere goats for 27 months. Seasonal patterns of cashmere growth were distinct with length increasing from a minimum in December to maximum (50-60 mm) in July. Initiation of cashmere growth in female goats was consistently earlier than that of males, with that of castrates being similar, later and earlier than males in 1987, 1988 and 1989 respectively. The initiation of cashmere growth progressed in a wave from the hip to the shoulder (6 weeks). There were no significant effects of sex on maximum length of cashmere grown. Prolactin secretion was also seasonal, increasing from minimum values in July-August (20 ng/mL) to peak levels around December (50-350 ng/mL). Thyroxine concentrations were higher in summer than winter, but seasonal changes were not as distinct as those of prolactin. Growth hormone secretion was aseasonal, and declined with age. It has been proposed that peak levels of prolactin in December were associated with the initiation of cashmere growth, with active growth occurring as levels declined. Neither thyroxine nor growth hormone appear to have a regulatory influence on the cashmere growth cycle.

Effect of dietary protein level and yeast culture on growth, blood prolactin and mohair fibre characteristics of British Angora goats

Twenty-four male castrate British Angora goats aged about 10 months and weighing 18.5 kg live weight initially were studied in an 84-day experiment. They were allocated in a 2 × 2 factorial design to receive diets containing, kg −1 dry matter (DM), concentrations of crude protein (N × 6.25) of 110 g low protein (LP) or 180 g high protein (HP) per kilogram and supplemented (Y) or not (O) with 2 g per head per day of the yeast culture product ‘Yea-Sacc’. The animals were offered the diets at approximately 0.95 × the weight of voluntary food intake. In comparison with the LP diets, the HP diets on average increased liveweight gain, raw mohair fibre yield, fibre diameter measured on a 100 cm 2 midside sample, improved food conversion efficiency, and variably increased concentrations of glucose, urea and prolactin in blood. There was evidence of significant increases in liveweight gain attributable to the yeast culture supplement during the early part of the study on Diet HP only, and variable reductions in the yield of raw fibre, but the blood characteristics measured were not affected. Attention is directed particularly towards the positive association of elevated dietary protein, fibre yields and plasma prolactin concentrations in the context of the biological control of fibre growth.

Repeated ECT and prolactin release in depressed patients

Whereas sham electroconvulsive therapy (ECT) increases the blood concentration of prolactin (PRL) up to two-fold after 15 m~n, real ECT increases PRL concentration several fold (Deakin et al 1983). Sequential study of the effects of electrode placement has established that bilateral ECT releases 25% (Swartz and Abrams 1984)-50% (Papakostos et al 1986) more PRL than does unilateral ECT which may demonstrate a greater hypothalan,ic-stimulating effect of bilateral ECT (Swartz and Abrams 1984). Electroencephalographic (EEG) study of the relationship between the length of cerebral seizure activity and PRL release has been inconclusive (Abrams and Swartz 198~b; Robin et at .~985), as has been the study of the relationship betw~,~ :, PRL release and recovery from depression (Deakin et al 1983; Abrams and Swartz 1985a). It has been suggested that the study of pituitary hormone release over a course of ECT may provide important information about alterations in central monoaminergic transmission in depressed patients. (Whalley et al 1982; Aperia et al 1985). The major role of the hypothalamus in the regulation of PRL release is inhibitory through the tuberoinfundibular dopaminergic system. A PRL releasing factor, such as thyrotropin releasing hormor ~ or vasoactive intestinal l~eptide, may be involved in stimulating PRL release and this may involve serotonin (5HT) (Leong et al 1983). After ECT, PRL release has been reported to be significantly reduced later on in a course of treatment for depression as compared to the first treatment (Deakin et al 1983; Abrams and Swartz 1985a; Aperia et al 1985; Cooper et al 1989), suggesting an increase in the responsiveness of post-synapuc dopamine receptors (Abrams and Swartz 1985a; Aperia et al 1985). This finding has not always been confirmed (Whalley et al 1982; Linnoila et al 1984; Haskett et al 1985). The intake of neuroleptic drugs increases both basal and ECl'-induced PRL release (Swartz 1985a; Aperia et a! 1985), but only one report concerned depressed patients free of neuroleptic drugs in the 2 weeks before ECT (Abrams and Swartz 1985a). O~:r aim was to study PRL release over a course of ECT in depressed patients who had not takeL', neuroleptic drugs during their index illness. ECT was monitored by 6-channel EEG because the ~gpatial distribution of epileptic seizures in hum,ms influences PRL release in that generalized se~ :ures release more PRL than focal seizures (Spc.ding et al 1986).

Relationships among LH, FSH and prolactin secretion, storage and response to secretagogue and hypothalamic GNRH content in ovariectomized pony mares administered testosterone, dihydrotestosterone, estradiol, progesterone, dexamethasone or follicular fluid

Thirty-five ovariectomized pony mares were used to study the relationships among luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) concentrations in blood (secretion), in pituitary (storage) and in blood after secretagogue administration, as well as the content of gonadotropin releasing hormone (GnRH) in hypothalamic areas, under various conditions of steroidal and nonsteroidal treatment. Five mares each were treated daily for 21 d with vegetable shortening (controls), testosterone (T; 150 μg/kg of body weight, BW), dihydrotestosterone (DHT; 150 μg/kg BW), estradiol (E 2; 35 μg/kg BW), progesterone (P 4; 500 μg/kg BW), dexamethasone (DEX; 125 μg/kg BW) or charcoal-stripped equine follicular fluid (FF; 10 ml). Secretagogue injections (GnRH and thyrotropin releasing hormone, TRH, at 1 and 4 μg/kg of BW, respectively) were given one d prior to treatment and again after 15 d of treatment. Relative to controls, treatment with T, DHT and DEX reduced (P <. 05) LH secretion, storage and response to exogenous GnRH, whereas treatment with E 2 increased (P < .05) these same characteristics. Treatment with P 4 reduced (P < .05) only LH secretion. Treatment with T, DHT, E 2 and DEX reduced (P < .05) FSH secretion, whereas treatment with P 4 increased (P < .05) it and FF had no effect (P > .1). All treatments increased (P < .05) FSH storage, whereas only treatment with T and DHT increased (P < .05) the FSH response to exogenous GnRH. Other than a brief increase (P < .05) in PRL secretion in mares treated with E 2, secretion of PRL did not differ (P > .1) among groups. Only treatment with E 2 increased (P < .01) PRL storage, yet treatment with T or DHT (but not E 2) increased (P < .05) the PRL response to exogenous TRH. Content of GnRH in the body and pre-optic area of the hypothalamus was not affected (P > .1) by treatment, whereas treatment with T, E 2 and DEX increased (P < .1) GnRH content in the median eminence. For LH, secretion, storage and response to exogenous GnRH were all highly correlated (r ≥ .77; P < .01). For FSH, only storage and response to exogenous GnRH were related (r = .62; P < .01). PRL characteristics were not significantly related to one another. Moreover, the amount of GnRH in the median eminence was not related (P > .1) to any LH or FSH characteristic.

Possible oncostatic action of cysteamine on the pituitary glands of oestrogen-primed hyperprolactinaemic rats

Cysteamine was investigated for its potential to reduce the size and secretion of oestrogen-primed hyperprolactinaemic rat pituitary glands. Subcutaneous administration of 80 and 120 mg cysteamine/kg significantly reduced plasma prolactin concentrations by 58 and 91% respectively, after 4h. Administration of cysteamine (60 mg s.c./kg body weight per day) for 10 days, to rats which had received an injection of 2 mg oestradiol benzoate on day 1, resulted in a significant reduction in pituitary mass (19%) and GH concentration (21%). Oral administration of 60 mg cysteamine/kg body weight to hyperprolactinaemic rats also produced a significant reduction in plasma prolactin of 94% after 2h. Oral administration of 60 mg cysteamine/kg body weight per day to rats for a 20-day period, during which they had received two injections of 2 mg oestradiol benzoate on day 1 and day 14 of treatment, resulted in a significant reduction in pituitary mass (29%) and the concentration of trunk blood prolactin concentration (35%). However, when oral cysteamine (60 mg cysteamine/kg body weight per day) was given for 20 days to rats which had been treated with 2 mg oestradiol benzoate once every 14 days over a 90-day period, it caused no change in pituitary weight, prolactin or GH concentration, or the concentration of prolactin in trunk blood.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma levels of prolactin and gonadal steroids in relation to multiple-brooding and renesting in free-living populations of the song sparrow, Melospiza melodia

Plasma levels of testosterone (T), estradiol (E2), and prolactin were measured in samples collected from free-living song sparrows, Melospiza melodia. In males, plasma levels of T were elevated early in the season when territories were established and when females laid the first clutch of eggs. Thereafter, T levels declined and remained low throughout the remainder of the breeding cycle. However, if the first brood was lost to a predator, or by experimental removal of the nest, plasma levels of T increased as renesting, to replace the clutch, occurred. Circulating levels of prolactin in males began to rise during the egg-laying stage of the first brood, reached a maximum toward the end of the incubation stage, remained elevated until breeding was terminated, and then declined throughout the moulting stage to basal values in October. Prolactin levels remained high throughout the breeding season irrespective of whether a brood was raised successfully or whether the nest was lost and renesting occurred. In females, plasma levels of E2 were elevated prior to the egg-laying stage for each brood as is typical of multiple-brooded species. However, prolactin titers rose dramatically during egg-laying for the first clutch (slightly higher than in males) and were maximal by onset of incubation. Only females of this species incubate, although males do feed young. As in males, plasma prolactin in females remained high between broods and during experimentally induced renesting, and then declined to basal by the end of the moult stage in October. These data suggest that there are no differences in the temporal patterns of prolactin concentrations in blood between multiple-brooding and renesting. In a separate experiment, captive male song sparrows were transferred from a short day to artificial long days (18L 6D) and a control group was maintained on 9L 15D. In the long-day group, prolactin levels rose abruptly over the first 20 days, as the testes developed, and remained high well into postnuptial moult after the gonads had regressed. Prolactin remained basal in the control group. These data suggest that the temporal pattern of circulating prolactin levels throughout the breeding season is regulated at least partly by changing photoperiod. However, nonphotoperiodic factors are also important since photoperiodically induced increases in prolactin are significantly less than those seen in free-living individuals. These differences may be related to parental behavior.

Prolactin Regulation of Dopaminergic Neurons in the Infundibulum Pituitary Stalk of Bull Calves

The effects of elevated circulating concentrations of prolactin were examined on neurochemical estimates of activity of dopaminergic neurons in the infundibulum/pituitary stalk of Holstein bull calves (8-10 weeks of age). Activity of these neurons was estimated by measuring the accumulation of dihydroxyphenylalanine, the immediate precursor of dopamine, 15 min after an intravenous injection of the aromatic L-amino acid decarboxylase inhibitor, 3-hydroxybenzylhydrazine. Subcutaneous injections of the dopamine antagonist haloperidol every 6 hr for 1 day increased serum concentrations of prolactin and accumulation of dihydroxyphenylalanine in the infundibulum/stalk. Intravenous infusions of prolactin for 1 or 9 days increased accumulation of dihydroxyphenylalanine in the infundibulum/stalk, indicating that these neurons remain responsive to elevated prolactin for at least 9 days. It is concluded that elevated concentrations of prolactin in blood stimulate dopaminergic neurons in the infundibulum/pituitary stalk of bull calves. We speculate that these neurons may be analogous to the tuberoinfundibular dopaminergic neurons that regulate prolactin in rats.

Effects of infusions of various doses of bovine growth hormone-releasing factor on blood hormones and metabolites in lactating Holstein cows

In two experiments, the effects of i.v. infusions of various doses of bovine GH-releasing factor (GRF) on blood hormones and metabolites in lactating Holstein cows were determined. In experiment 1, cows were infused with GRF (0, 3.125, 6.25, 12.5, 25.0 or 50.0 mg/cow per 24 h) for 24 h. Blood was sampled at -1, 5, 11, 15 and 23 h relative to the start of the infusion. The serum concentration of somatomedin C (SM-C) before infusion was 303 +/- 8 (S.E.M.) micrograms/l. Doses of GRF of between 3.125 and 50.0 mg were equipotent in stimulating (P less than 0.05) SM-C by 1.5- to 2.5-fold. GRF-induced increases in SM-C occurred by 11 h from the start of the infusion. In experiment 2, primiparous cows were infused with GRF (0, 1 or 3 mg/24 h) for 20 days. Blood was sampled for 12 h on days 1, 10 and 19. The 1 mg dose of GRF increased (P less than 0.05) blood concentrations of SM-C (on days 10 and 19) and glucose (on day 19), but did not affect blood concentrations of prolactin, insulin, cortisol, tri-iodothyronine (T3), thyroxine (T4), non-esterified fatty acids (NEFA) or glucose. The 3 mg dose of GRF increased (P less than 0.05) blood concentrations of SM-C (on days 10 and 19), T3 (on days 10 and 19), insulin (on day 19), NEFA (on days 1, 10 and 19) and glucose (on day 19), but did not affect blood concentrations of prolactin, cortisol or T4. We conclude that these data are consistent with the hypothesis that the galactopoietic effect of exogenous GRF in dairy cattle is mediated by increased secretion of GH.

Test of ML23 as an antagonist to the effects of melatonin in the ram

In Exp. 1, four groups of 8 yearling Soay rams were housed under long days (16L:8D) to induce reproductive quiescence and were treated daily for 12 weeks with: (I) vehicle (2 or 4 ml 50% ethanol/water), (II) ML23 (2 mg), (III) melatonin (2 mg) and (IV) melatonin and ML23 (2 mg of each). All treatments were given orally in the mid-light phase. In the rams receiving melatonin (Group III) there was an earlier increase in the plasma concentrations of FSH and testosterone and regrowth of the testes compared to the controls (time to maximum testicular diameter: 10.0 +/- 0.5 and 15.3 +/- 1.2 weeks). These differences were reversed after the end of the 12-week treatments when rapid testicular regression occurred in melatonin-treated rams but not in the controls. In the group receiving ML23 and melatonin (Group IV), there was early reactivation and regression of the reproductive axis as in the melatonin group (testis max. 9.9 +/- 0.7 and 10.0 +/- 0.5 weeks) while in the group receiving ML23 alone (Group II) there was a slower redevelopment and regression as in the controls (testis max. 15.7 +/- 1.1 and 15.3 +/- 1.2 weeks). The comparison between the 4 groups in the changes in the blood concentrations of prolactin, voluntary food intake and total body weight also indicated that the treatment with ML23 failed to modify the effect of melatonin (combined treatment vs melatonin) or the effect of the long day photoperiod (ML23 vs vehicle).(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma prolactin concentrations in mares and their neonates after oxytocin induction of parturition

Studies were undertaken to investigate the effects of oxytocin induction on prolactin release in term (Group II) and preterm (Group III) mares and to compare these effects to spontaneously foaling mares (Group I). Since physiological concentrations of prolactin in blood have not been measured in the neonatal foal, experiments were designed to monitor prolactin in the cord artery and jugular blood of the foals from all groups of mares. Although prolactin levels varied in term mares (Group I and II) during the last 11 days of pregnancy, an increase was observed between Day −6 and Day 0 (2.7 and 11.9 ng/ml respectively; P<0.1). The average concentration of prolactin over the last 4 days (Days −3 to 0) had increased by 40% when compared to the average concentration on Days −6, −5, and −4. These findings indicate a rising trend which appears to occur concomitantly with changes in concentrations of 2 mammary components tested, sodium and potassium. Prolactin concentrations did not significantly increase in term mares after oxytocin treatment or in spontaneously foaling mares. However, the preterm induced mares had higher prolactin concentrations during the first stage of labor (19.3 ± 7.2 ng/ml) than prior to treatment with oxytocin (4.7 ± 2.0 ng/ml; P<0.01). Levels of prolactin in all groups significantly declined by 20-min post-placental expulsion. For the first 30 min after birth, prolactin concentrations in foals from oxytocin-induced mares appeared to be 2-fold higher than those from spontaneously foaling mares. Thereafter, prolactin values declined to baseline values by 48 hrs. When comparing cord arterial plasma with cord venous plasma in each group, prolactin concentrations were similar. However, the average prolactin levels in both the cord artery and vein appeared higher (ave: 1.1 ng/ml) in Group II and III than in Group I (<0.5 ng/ml). From these results, the authors suggest that 1) prolactin may have a role in regulating mammary secretory products in mares just prior to parturition; 2) oxytocin may increase prolactin secretion in preterm induced mares; 3) oxytocin induction may have a short term effect to increase circulatory prolactin concentrations in neonates in utero regardless whether their dams were treated preterm or term.

The effect of labour on prolactin and cortisol concentrations in the mother and the fetus

Concentrations of prolactin and cortisol were determined in maternal and umbilical cord serum of women delivered by elective Caesarian section and by vaginal delivery. There was no difference in the concentration of prolactin in the two groups of women. Similarly, cord blood prolactin concentrations were not significantly different in the two groups. Cortisol concentrations in the women undergoing Caesarian section were similar to the vaginal delivery group before the onset of labour. However, there was a significant increase in cortisol concentration at delivery following labour. The cord blood cortisol concentration was significantly higher in the neonates delivered vaginally, and it correlated with the maternal cortisol concentration at delivery. The relevance of these findings is discussed.

Effect of Hypo and Hyperprolactinemia on the Testicular Maturation of Wistar Rats During Puberal Transition

In Wistar rats, the structural and metabolic organization of the testis was influenced by the blood concentration of prolactin. The androgen dependent enzyme activities in plasma as well as in testis were higher under hyperprolactinemia and lower under hypoprolactinemia, as induced by bromocriptine. While prolactin had direct effect on the testicular functions, bromocriptine seemed to exert its influence through blocking hypophysial prolactin.

Concentration of growth hormone, prolactin and cortisol in fetal and maternal blood and amniotic fluid during late pregnancy and parturition in cows with cannulated fetuses

Growth hormone, prolactin and cortisol concentrations were measured by radioimmunoassay in samples of fetal and maternal plasma and amniotic fluid from 15 days before until 24 hours after spontaneous parturition in cows. Average gestation length in ten cows with chronically catheterized fetuses was 267 ± 2.7 days (SEM). Mean fetal plasma GH concentrations decreased gradually during the last 15 days before birth ( r = 0.925, P < 0.01) and were 4–6 times higher than mean concentrations in maternal plasma. The latter did not change during the period under observation. GH and PRL concentrations in amniotic fluid were low during late gestation and showed no changes around parturition. Prolactin concentrations changed only in maternal plasma and reached peak values just before birth of the calf. The fetal cortisol concentration was significantly ( P < 0.05) elevated at Days −9 to −7 compared with that in samples taken before Day −9. A further increase occurred during the last week before birth. It is concluded that prepartum changes in the concentrations of GH, PRL and cortisol in fetal and maternal plasma and amniotic fluid are not synchronous but take place according to a pattern which is rather specific for each hormone in each of the compartments.

The permanent effect of reproductive events on blood prolactin levels and its relation to breast cancer risk: a population study of postmenopausal women

In each of two population-based studies conducted on the Island of Guernsey between 1967–1976 and 1977–1984, respectively, single specimens of blood were taken from over 5000 normal women. From these two studies there were 1173 and 946 postmenopausal women in whom blood prolactin was determined and multivariate analysis was used to establish the association between blood prolactin concentration and possible determinants of risk of breast cancer. Since prolactin levels were log-normally distributed these analyses were done on log-transformed data. The age at menarche or menopause, age at first or last childbirth, length of reproductive life (i.e. time from menarche to menopause) or post-menopausal life (i.e. time from menopause to time of blood sampling), contraceptive use and history of breast cancer were not significantly associated with blood prolactin concentration. Of significance were age, parity, time of blood sampling and assay drift. Ponderosity (Quetelet's Index) was positively associated with prolactin concentration and this was significant using a one-tail criterion. Women with a mammographic pattern designated DY by Wolfe had significantly higher prolactin levels than those with N 1 patterns. However, the main finding to emerge was that after standardizing for all the other variables increasing parity was related to a step-wise reduction in blood prolactin levels. Since this had occurred in women who had had their last child up to 35 years previously it implies this effect is permanent. It could therefore be that the protective effect on breast cancer risk of multiparity and early first pregnancy could be mediated by such a life-long reduction in blood prolactin levels.