Rapid elevation of rat serum prolactin concentration by cyclosporine, a novel immunosuppressive drug

Within one hr of the administration of cyclosporine to rats, there was a 4-fold elevation in the serum prolactin concentration. Doses of 0.12, 1.2, and 12 micrograms/100 g body weight cyclosporine significantly elevated the serum prolactin level. Higher doses, 120 or 1200 micrograms/100 g body weight cyclosporine resulted in small but insignificant elevations of the serum prolactin concentration. Bromocriptine, a dopamine agonist which inhibits prolactin release from the anterior pituitary, completely blocked the elevation in serum prolactin in response to cyclosporine alone. These data suggest that the ability of cyclosporine to suppress immune function may involve its ability to rapidly produce hyperprolactinemia.     

Plasma gonadotropin,prolactin levels and hypothalamic tyrosine hydroxylase activity in rats during estrous cycle,after overiectomy and after blockade of catecholamine biosynthesis

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase activity were evaluated at 0900, 1200 and 1700 h during diestrus, proestrus and estrus, ovariectomized and after systemic administration of reserpine or α-methyl p-tyrosine, which interfere with catecholamine biosynthesis, in rats. Gonadotropin and prolactin levels showed peak values during the afternoon of proestrus, while hypothalamic tyrosine hydroxylase activity was markedly lowered at 1200 on proestrus. Gonadotropin levels were slightly lowered whereas prolactin concentrations and hypothalamic tyrosine hydroxylase activity were significantly increased by reserpine. Depletion of hypothalamic dopamine by reserpine apparently resulted in significant elevation of prolactin levels which inturn induce tyrosine hydroxylase. Gonadotropin levels and hypothalamic tyrosine hydroxylase activity were significantly suppressed after the administration of α-methyl p-tyrosine. Prolactin levels, however, were elevated significantly. These results indicate that catecholamines are involved in the control of gonadotropin and prolactin release during estrous cycle and inhibition of catecholamines biosynthesis by α-methyl p-tyrosine could result in suppression of gonadotropin levels, whereas removal of tonic inhibition of hypothalamic dopamine by α-methyl-p-tyrosine elevate prolactin levels.     

Tachykinins and the control of prolactin secretion

Tachykinins are present in the pituitary gland and in brain areas involved in the control of the secretion of pituitary hormones. Tachykinins have been demonstrated to stimulate prolactin release acting directly on the anterior pituitary gland. These peptides have also been revealed to be able to act at the hypothalamic level, interacting with neurotransmitters and neuropeptides that have the potential to affect prolactin secretion. Tachykinins seem to act by stimulating or inhibiting the release of the factors that affect prolactin secretion. Among them, tachykinins have been demonstrated to stimulate oxytocin and vasopressin release, which in turn results in prolactin release. Tachykinins also potentiated the response to vasoactive intestinal peptide (VIP) and reinforced the action of glutamate, which in turn result in prolactin release. They have also been shown to interact with serotonin, a neurotransmitter involved in the control of prolactin secretion. In addition, tachykinins have been shown to inhibit GABA release, a neurotransmitter with prolactin-release inhibiting effect. This inhibition may result in an increased prolactin secretion by removal of the GABA inhibition. On the other hand, tachykinins have also been shown to stimulate dopamine release by the hypothalamus, an action that results in an inhibition of prolactin release. Dopamine is a well known inhibitor of prolactin secretion. In conclusion, although tachykinins have been shown to have a predominantly stimulatory effect on prolactin secretion, especially at the pituitary level, under some circumstances they may also exert an inhibitory influence on prolactin release, by stimulating dopamine release at the hypothalamic level.     

Some calcium and lysosome antagonists inhibit 3H-spirerone binding to the porcine anterior pituitary

The dopamine antagonist ³H-spiperone binds to dopamine receptors in crude membrane preparations of the porcine anterior pituitary. In competition studies, several calcium channel antagonists (bepridil, D600 and verapamil) and a lysosomal enzyme inhibitor (chloroquine) displaced spiperone binding in a dose related manner. Because dopamine receptor activity modulates prolactin release from the anterior pituitary, it is suggested that the previously observed effects of some of these agents on prolactin release or storage may have been initiated at the level of the dopamine receptor or through physical modifications of the plasma membrane rather than the calcium channel or lysosome.     

Increased ability of apomorphine to reduce serum concentrations of prolactin in rats treated chronically with alpha-methyltyrosine

Serum concentrations of prolactin were significantly increased in rats for up to 9 hours after a single i.p. injection of α-methyltyrosine (50 mg/kg); apomorphine caused a dose-dependent reduction of the elevated prolactin concentrations. Doses of apomorphine required to reduce serum prolactin concentrations.were lower in animals previously injected with α-methyltyrosine three times a day for 10 days than in animals which received a single injection of α-methyltyrosine. These results suggest that chronic disruption of the normal release of dopamine from tuberoinfundibular neurons leads to the development of increased sensitivity of dopamine receptors involved with the inhibition of prolactin release from the anterior pituitary.     

Effects of dopamine on prolactin secretion and cyclic AMP accumulation in the rat anterior pituitary gland

The effects of dopamine on pituitary prolactin secretion and pituitary cyclic AMP accumulation were studied by using anterior pituitary glands from adult female rats, incubated in vitro. During 2h incubations, significant inhibition of prolactin secretion was achieved at concentrations between 1 and 10nm-dopamine. However, 0.1–1μm-dopamine was required before a significant decrease in pituitary cyclic AMP content was observed. In the presence of 1μm-dopamine, pituitary cyclic AMP content decreased rapidly to reach about 75% of the control value within 20min and there was no further decrease for at least 2h. Incubation with the phosphodiesterase inhibitors theophylline (8mm) or isobutylmethylxanthine (2mm) increased pituitary cyclic AMP concentrations 3- and 6-fold respectively. Dopamine (1μm) had no effect on the cyclic AMP accumulation measured in the presence of theophylline, but inhibited the isobutylmethylxanthine-induced increase by 50%. The dopamine inhibition of prolactin secretion was not affected by either inhibitor. Two derivatives of cyclic AMP (dibutyryl cyclic AMP and 8-bromo cyclic AMP) were unable to block the dopamine (1μm) inhibition of prolactin secretion, although 8-bromo cyclic AMP (2mm) significantly stimulated prolactin secretion and both compounds increased somatotropin (growth hormone) release. Cholera toxin (3μg/ml for 4h) increased pituitary cyclic AMP concentrations 4–5-fold, but had no effect on prolactin secretion. The inhibition of prolactin secretion by dopamine was unaffected by cholera toxin, despite the fact that dopamine had no effect on the raised pituitary cyclic AMP concentration caused by this factor. Dopamine had no significant effect on either basal or stimulated somatotropin secretion under any of the conditions tested. We conclude that the inhibitory effects of dopamine on prolactin secretion are probably not mediated by lowering of cyclic AMP concentration, although modulation of the concentration of this nucleotide in some other circumstances may alter the secretion of the hormone.     

Effect of prolactin and estradiol on rat striatal dopamine receptors

Chronic estrogen treatment has been found to increase the level of rat striatal dopamine receptors. Since it is well known that estrogen treatment increases circulating prolactin levels, we have investigated the possibility that the stimulatory effect of estrogens on dopamine receptors is exerted via prolactin. Ovariectomized female or intact male rats were implanted with three adenohypophyses under the kidney capsule or treated with 17 β-estradiol (10 μg, twice daily) for 2 weeks. In animals of both sexes, the pituitary-implanted and estradiol-treated rats showed higher levels of [³H]spiperone binding to striatal dopamine receptors. This effect of estradiol or pituitary implants on dopamine receptors was further investigated in ovariectomized rats. The pituitary-implanted and estradiol-treated rats had elevated plasma prolactin levels and an increased density of striatal dopamine receptors without alteration of their affinity. The role of the pituitary in the effect of estradiol was next investigated using hypophysectomized female rats treated with 17 β-estradiol (10 μg, twice daily), o-prolactin (500 μg, twice daily) or bearing three anterior pituitary implants. The implants as well as the treatment with estradiol or prolactin increased the level of striatal dopamine receptors in hypophysectomized rats while, as expected, the estradiol-treated animals did not have elevated plasma prolactin levels. The present data indicate that high prolactin levels lead, as observed with chronic estradiol treatment, to an increased density of striatal dopamine receptors. However, the effect of estradiol may not be explained exclusively by increased prolactin levels since a similar stimulatory effect is observed in hypophysectomized animals.     

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.     

Regulation of prolactin in mice with altered hypothalamic melanocortin activity

This study used two mouse models with genetic manipulation of the melanocortin system to investigate prolactin regulation. Mice with overexpression of the melanocortin receptor (MC-R) agonist, α-melanocyte-stimulating hormone (Tg-MSH) or deletion of the MC-R antagonist agouti-related protein (AgRP KO) were studied. Male Tg-MSH mice had lower blood prolactin levels at baseline (2.9±0.3 vs. 4.7±0.7ng/ml) and after restraint stress (68±6.5 vs. 117±22ng/ml) vs. WT (p<0.05); however, pituitary prolactin content was not different. Blood prolactin was also decreased in male AgRP KO mice at baseline (4.2±0.5 vs. 7.6±1.3ng/ml) and after stress (60±4.5 vs. 86.1±5.7ng/ml) vs. WT (p<0.001). Pituitary prolactin content was lower in male AgRP KO mice (4.3±0.3 vs. 6.7±0.5μg/pituitary, p<0.001) vs. WT. No differences in blood or pituitary prolactin levels were observed in female AgRP KO mice vs. WT. Hypothalamic dopamine activity was assessed as the potential mechanism responsible for changes in prolactin levels. Hypothalamic tyrosine hydroxylase mRNA was measured in both genetic models vs. WT mice and hypothalamic dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) content were measured in male AgRP KO and WT mice but neither were significantly different. However, these results do not preclude changes in dopamine activity as dopamine turnover was not directly investigated. This is the first study to show that baseline and stress-induced prolactin release and pituitary prolactin content are reduced in mice with genetic alterations of the melanocortin system and suggests that changes in hypothalamic melanocortin activity may be reflected in measurements of serum prolactin levels.     

In vivo and in vitro effects of cholecystokinin octapeptide on the release of prolactin in rats

The effect of cholecystokinin octapeptide (CCK-8) on the release of prolactin (PRL) in rats was studied in vivo and in vitro. Intravenous injection of 5 micrograms/100 g BW of CCK-8 resulted in significant increase in the plasma PRL level after 10 and 20 min. CCK-8 at concentrations of 10(-11) M to 10(-7) M also caused dose-dependent stimulation of PRL release from dispersed cells of rat anterior pituitary. On the other hand, dopamine inhibited PRL release from dispersed cells of rat anterior pituitary in a dose-related manner at concentrations of 10(-8) M to 10(-6) M. Release of PRL from the cells was increased by addition of K+ at high concentration (53 mM) in a Ca++-dependent manner. Addition of 10(-3) M verapamil to the incubation medium inhibited CCK-8-induced PRL release from the cells. Addition of dopamine (10(-7) M) to the incubation medium inhibited PRL release from the cells induced by CCK-8 or high K+ (53 mM). These results indicate that CCK-8 acts directly on the anterior pituitary cells to stimulate PRL release and that calcium ion is involved in the mechanism of this effect.     

On the mechanism by which dopamine inhibits prolactin release in the anterior pituitary

An $\text{in}$$\text{vitro}$ perfusion system was used to assess the effects of chloride channel blockers, dopamine (DA) receptor agonists and antagonists, and GABA receptor agonists and antagonists on prolactin release from the mouse anterior pituitary. Dopamine and muscimol inhibited prolactin release ($\text{IC}{}_{50}{}^1\text{= 6 × 10}{}^{\mathrm{?8}}\text{M and}\text{10}{}^{\mathrm{?5}}\text{M}$ respectively). The GABA receptor antagonist bicuculline blocked the inhibition of prolactin release by muscimol but not dopamine. The dopamine receptor antagonist chlorpromazine blocked the dopamine- but not muscimol-induced inhibition of prolactin release. Haloperidol, however, reversed both the muscimol and dopamine induced inhibition of prolactin release. Furthermore, the chloride channel blocker picrotoxinin blocked the inhibition of prolactin release elicited by both dopamine and muscimol. These later results suggest that the anterior pituitary dopamine receptor which mediates the inhibition of prolactin release may be coupled to a picrotoxinin sensitive chloride ionophore and that haloperidol may affect the function of both DA and GABA receptors in the anterior pituitary.     

Purification of a non-dopaminergic and non-GABAergic prolactin release-inhibiting factor (PIF) in sheep stalk-median eminence

Prolactin release-inhibiting factor (PIF) extracted from 1200 sheep stalk-median eminences was purified by gel filtration on a Sephadex G-25 column (4.5 X 150 cm). PIF activity was determined by measuring the inhibition of prolactin release from dispersed anterior pituitary cells of adult male or estrogen-primed, ovariectomized rats. Using this system, PIF was detected in tube fractions 122-127 (volume = 20 ml/tube). These fractions also contained LHRH and somatostatin; however, these peptides had no prolactin-inhibiting activity in the quantities present. No dopamine or gamma-aminobutyric acid (GABA) was detected in the active fractions by radioenzymatic assay and fluorophotoenzymatic assay, respectively. Furthermore, receptor blockers for dopamine or GABA did not interfere with the PIF activity. These findings indicate that the PIF activity cannot be attributed to either dopamine or GABA, both of which are known to inhibit prolactin release, and provide evidence for the presence of a non-dopaminergic and non-GABAergic PIF within the hypothalamus.     

Effects of arginine vasotocin on levels of plasma gonadotropins and prolactin in ovariectomized conscious rats

Arginine vasotocin was injected into the third ventricle or intravenously in conscious, ovariectomized rats and its effect on gonadotropin and prolactin release evaluated. The peptide lowered plasma levels of both LH and prolactin in doses of 40 or 100 ng given intraventricularly. The higher dose was slightly more effective than the lower dose. Intravenous injection of a 1-microgram dose of vasotocin failed to alter plasma LH in the ovariectomized animals; however, a 5-micrograms dose induced a slight depression apparent at only 60 min following injection. Intravenous injection of 1 microgram produced a significant lowering of plasma prolactin, whereas a dramatic lowering followed the injection of the higher dose. Plasma FSH was unaffected in these experiments. Incubation of dispersed anterior pituitary cells from ovariectomized rats with various doses of vasotocin revealed no effect of the peptide on the release of FSH, LH, or prolactin. It also did not alter the response to LHRH, but it partially blocked the action of dopamine to inhibit prolactin release. The data indicate that quite low doses of arginine vasotocin act within the brain to inhibit LH and prolactin secretion in ovariectomized, conscious animals.     

Effects of subchronic alternating cadmium exposure on dopamine turnover and plasma levels of prolactin, GH and ACTH

This study was undertaken to analyze if the effects of subchronic alternating cadmium exposure on pituitary hormone secretion are mediated by changes in dopamine turnover in an age dependent way or are directly correlated to cadmium accumulation at the hypothalamic-pituitary axis. Male rats were treated sc. from day 30 to 60 (prepubertal period) or from day 60 to 90 (adult age) of life, with cadmium chloride (CdCl₂) at a dose of 0.5 and 1.0 mg kg^–1 bw, every 4th day in an alternate schedule, starting with the smaller dose. Dopamine (DA) turnover, expressed as the ratio of acid 3,3-dihidroxifenil acetic (DOPAC)/DA in various hypothalamic areas, the plasma levels of prolactin, growth hormone (GH) and adrenocorticotropic hormone (ACTH), and cadmium accumulation in the hypothalamus and pituitary were studied. Prepubertal cadmium exposure decreased DA content in all hypothalamic areas studied, although its turnover was not modified. A decrease in plasma ACTH levels with no changes in plasma prolactin and GH levels were found. Cadmium did not accumulate in pituitary while it increased in the hypothalamus. Metal exposure during adulthood decreased DA content in mediobasal and posterior hypothalamus, and its turnover in posterior hypothalamus and median eminence. It decreased plasma prolactin and ACTH levels but not those of GH. Cadmium concentration increased in both hypothalamus and pituitary. These results suggest that cadmium exposure produces age dependent changes on the secretory mechanisms of the pituitary hormones studied, related to the selective accumulation of the metal at both hypothalamic and hypophyseal level changes. However the effects of the metal are not mediated by dopamine.     

Temporal dynamics of pituitary prolactin depletion and release in three strains of rats.

Previous reports have shown that pituitary prolactin is rapidly transformed to a less soluble, but much more releasable, form prior to release from the lactotroph. One manifestation of this transformation is that pituitary prolactin depletion is significantly greater than concurrent release both in vivo and in vitro. The objective of this study was to compare the magnitude and temporal dynamics of depletion and release from pituitaries of ovariectomized estrogen-treated rats of three different strains in vitro to assess the effect of strain on the transformation process. Mature ovariectomized Wistar-Furth (WF), Sprague-Dawley (SD) and Long-Evans (LE) rats (7-10/group) were killed by decapitation 7 days after a single s.c. injection of 100 micrograms of polyestradiol phosphate. The anterior pituitaries were quickly removed and cut into quarters which were incubated for up to 4 hrs in the absence of dopamine or other prolactin secretagogues. Representative fragments from each strain were not incubated but were snap frozen to measure pre-incubation content. Fragments from each strain were removed from incubation at 30, 60, 120, 180 and 240 min for prolactin content measurement. Medium was collected at 30 min intervals and replaced with fresh medium. The experiments were repeated twice. Prolactin in medium and pituitary homogenates was measured by radioimmunoassays using NIAMDD-RP-1 as standard. In all three stains release of prolactin was approximately 30-50% of the prolactin depleted from the pituitary in 4 hrs. Strains varied in the magnitude of this difference and the time course over which it occurred. WF and SD rats showed significantly greater depletion and release of prolactin than did LE rats when the data were expressed as micrograms prolactin/mg pituitary. When the data were expressed as a percentage of prolactin available for release, the differences in depletion between strains disappeared and the LE rats released a significantly greater percentage of the prolactin available for release than did the other two strains. We conclude that pituitary prolactin undergoes a process of transformation prior to release which causes it to disappear from the pituitary but not appear in culture medium. We further conclude that the magnitude and temporal dynamics of this process are not equivalent across all strains of rats.     

Changes in pituitary hormone levels induced by met-enkephalin in man—The role of dopamine

The interaction of dopamine with the effects of the opiate agonist peptide D-Ala²-MePhe⁴-met-enkephalin-O-o1 (DAMME) on anterior pituitary hormone secretion was investigated in normal male subjects. DAMME produced clear elevations in prolactin, growth hormone and thyroid-stimulating hormone, while inhibiting the release of luteinising hormone and cortisol. There was no change in follicle stimulating hormone. The elevations in prolactin and TSH were enhanced by the dopamine antagonist, domperidone, and blocked by an infusion of dopamine. Neither dopamine nor domperidone modulated the changes in growth hormone, luteinising hormone or cortisol. The data are comptible with the association of the release of prolactin and TSH by opiate peptides with decreased hypothalamic dopaminergic activity; changes in the other anterior pituitary hormones seem to involve different mechanisms.     

Diacylglycerol lipase and pituitary prolactin release in vitro: studies employing RHC 80267

We studied the possible involvement of diacylglycerol lipase in the regulatory mechanisms governing the release of prolactin by primary cultures of anterior pituitary cells. This was accomplished by studying the effect of a selective inhibitor of diacylglycerol lipase activity, RHC 80267, on basal prolactin release and that stimulated by TRH and elevated potassium concentrations. RHC 80267 produced a concentration-dependent reduction in basal prolactin release and abolished its increase produced by TRH and potassium. These results are consistent with the hypothesis that the production of arachidonate from lipids via the diacylglycerol lipase pathway is an important event in the governance of prolactin release.     

Stimulatory action of prolactin on gonadotropin secretion in vitro

The action of prolactin (PRL) on the secretion of gonadotropin was investigated by means of a cell culture system of rat anterior pituitary gland. Anterior pituitary glands were removed from Wistar male rats, enzymatically digested and cultured. Luteinizing hormone (LH) release into medium was increased by adding PRL dose-dependently in the range between 10 ng/ml and 1 microgram/ml. This effect of PRL was further augmented by the presence of either gonadotropin-releasing hormone or estradiol. The intracellular LH concentration was also increased by PRL. PRL also caused an increase in follicle-stimulating hormone release into medium dose-dependently. In conclusion, PRL was shown to stimulate the secretion of gonadotropin at the pituitary level, thus suggesting a paracrine mode of PRL action in the anterior pituitary gland.     

Increased synthesis of prolactin and growth hormone during incubation in the pituitary of broody Nagoya hens

Synthesis and release rates of prolactin and growth hormone (GH) in the anterior pituitary of laying and incubating broody chickens (Nagoya breed) were determined by a disc electrophoretic technique after in vitro incubation of anterior pituitaries with a labeled amino acid. Prolactin synthesis and release were two-fold higher in incubating than in laying hens, resulting in twofold increase in the concentration of prolactin in the gland. GH synthesis was three-fold higher in incubating than in laying hens but GH release was not affected by the incubation. GH concentration in the pituitary gland also increased in incubating hens. It is suggested that these changes in hormone synthesis, release, and concentrations are related to nesting behaviour and nutritional condition of incubating hens.     

Dopamine-inhibited adenylate cyclase in female rat adenohypophysis

A dopamine-inhibited adenylate cyclase has been demonstrated in anterior pituitary gland of adult female rats, lactating and not lactating. This inhibitory effect was completely GTP dependent. In contrast, in the adenohypophysis of male rats, dopamine had no detectable effect on adenylate cyclase activity. In female rats the inhibition of the enzyme appears mediated by specific dopaminergic receptors: the effect of dopamine was mimicked by the dopaminergic agonists apomorphine and the ergot derivative CH 29–717, while norepinephrine was much less potent. On the other hand, the dopaminergic antagonists trifluoperazine and sulpiride competitively antagonized the dopamine inhibition of the adenylate cyclase. The possibility that the dopamine-inhibited enzyme is located in mammotrophs appears supported 1) by its observation in the female rat pituitary, which contains this type of cells in much larger proportion than the male gland (33–38% vs. < 5%); 2) by the pharmacological similarity between the dopaminergic receptors mediating the adenylate cyclase inhibition (this work) and those regulating prolactin release (which have been characterized in previous studies). The well known inhibition of prolactin release brought about by dopamine could therefore be mediated, at least in part, by a decrease in the intracellular level of cAMP.