Novel 8 alpha-ergolines with inhibitory and stimulatory effects on prolactin secretion in rats

Four derivatives of the ergot dopamine (DA) agonist lisuride (LIS), namely 6-n-propyl-lisuride (6-n-propyl-LIS), transdihydrolisuride (TDHL), 6-n-propyl-transdi-hydrolisuride (6-n-propyl-TDHL) and 2-bromolisuride (2-Br-LIS) were investigated in female rats with regard to their influence on hyperprolactinaemia induced by pretreatment with reserpine (2 mg/kg i.p., 24 h) at various intervals following their subcutaneous or oral administration (0.05 mg/kg). Two hours after administration, LIS, 6-n-propyl-LIS, and 6-n-propyl-TDHL caused a statistically significant inhibition of reserpine-induced hyperprolactinaemia of about the same extent. Eight hours after administration 6-n-propyl-LIS and 6-n-propyl-TDHL were as active as after 2 h in inhibiting prolactin (PRL) secretion whereas LIS was almost ineffective in this respect. TDHL caused a statistically significant inhibition of PRL secretion at 2 and 8 h after oral administration; this effect was less pronounced after s.c. administration. In contrast to the aforementioned derivatives 2-Br-LIS further increased the reserpine-induced hyperprolactinaemia. In normal male rats pretreatment with 2-Br-LIS (0.025-6.25 mg/kg s.c., 2 h) dose-dependently stimulated PRL secretion. The present data support the assumption of the longlasting DA agonistic action of 6-n-propyl-LIS and 6-n-propyl-TDHL and of the antidopaminergic properties of 2-Br-LIS recently derived from behavioural studies.     

The effects of serotonergic and adrenergic receptor antagonist on prolactin release in the monkey.

The influence of serotonergic and adrenergic antagonists on serum prolactin levels was studied in ketamine anesthetized monkeys. Methysergide, a serotonergic receptor blocker, at 0.035, 0.1 and 1 mg/kg body weight induced a rapid and transient increase in serum prolactin. Cyproheptadine, another serotonergic receptor blocker, at 0.05, 0.5 and 1 mg/kg induced a rapid and sustained increase in serum prolactin. SQ 10631, a third serotonergic receptor blocker, had a minimal effect on increasing basal prolactin levels even at doses as high as 10 mg/kg. Propranolol, a β adrenergic blocker, at a dose of 5 mg/kg induced a small sustained increase in serum prolactin, while a lower dose (1 mg/kg) had a slight but significant effect. Phentolamine, an α adrenergic receptor blocker, at a dose of 5 mg/kg induced a rapid and transient increase in plasma prolactin while a lower dose (1 mg/kg) had no effect. Phenoxybenzamine, a potent α adrenergic receptor blocker, had only a minimal effect on prolactin release even at doses of 3 and 5 mg/kg. It appears that the time course and extent of prolactin release differs among neural antagonists even within the same biogenic amine system.     

Gravitational effects on the response to different stimulatory signals in T cells

The purpose of this paper is to present new data on the in vitro activation of purified T cells with three different mitogenic agents in conditions simulating low-gravity.     

Thyroidectomy blocks stress-induced prolactin rise in rats: role of the central serotonergic system.

Thyroidectomized (TX) adult Wistar male rats and their sham-operated controls were submitted to immobilization stress during forty minutes. Thyroidectomy partially blocks stress-induced prolactin (PRL) secretion. Previous administration of MK 212, a serotonin agonist, reverts this picture. The effect of MK 212 is specifically due to its interaction with 5HT2 receptors, since the injection of LY 53857, a selective blocker of these receptors, 30 min before MK 212, prevents the effect of this serotonin agonist. LY 53857, injected alone, yields a partial blockade of PRL secretion during stress in sham-operated rats. TX rats receiving LY 53857 or saline have comparable low values of plasma PRL during stress. It is suggested that thyroidectomy disrupts the functional integrity of the central serotonergic pathways involved in the stress-induced PRL rise.     

Effects of serotonergic agents on plasma prolactin levels in pyridoxine-deficient adult male rats

Plasma concentration of prolactin was significantly reduced in pyridoxine-deficient as compared to control (pyridoxine-supplemented) adult male rats. Administration of pyridoxine to deficient rats resulted in a significant increase in plasma prolactin. The reduction in plasma prolactin in pyridoxine-deficient rats corresponded with the significantly reduced hypothalamic contents of pyridoxal phosphate and serotonin in pyridoxine-deficient rats. Plasma prolactin concentrations were also measured in response to serotonergic agents in both groups of rats. The administration of the 5HT_1A agonist (8-hydroxy 2-n-dipropylamino tetralin) resulted in a significant increase in plasma prolactin and that of the specific 5HT_1A antagonist spiroxatrine had the opposite effect. The results suggest that the hypothalamic serotonergic regulation of prolactin release is impaired in pyridoxine deficiency.     

Influence of serotonergic and adrenergic antagonists on TRH-induced prolactin release in the monkey.

The influence of methysergide, cyproheptadine and SQ 10,631 (serotonergic receptor blockers) at the dose of 35 μg/kg, 50 μg/kg and 5 mg/kg, respectively, and propranolol, phentolamine and phenoxybenzamine (adrenergic receptor blockers) at the dose of 1 mg/kg on TRH-induced prolactin release was studied in sexually mature female monkeys. The serotonergic antagonists had no effect on TRH-induced prolactin release. Both β and α adrenergic antagonist gave a similar potentiation of the TRH-induced prolactin response but only phenoxybenzamine plus TRH was statistically different (P < 0.05) from TRH alone. The effect of the adrenergic receptor blockers is believed to be due to actions on dopamine receptors.     

Acute stimulatory effects of prostaglandin F2 alpha on serum progesterone concentrations in pregnant and pseudopregnant pigs.

The objective of this study was to investigate whether PGF2 alpha, administered to pregnant and pseudopregnant gilts in vivo, would cause an acute increase in serum progesterone concentrations prior to luteolysis. Pregnant (n = 9) and pseudopregnant (n = 4) gilts were fitted with a jugular vein cannula on day 40, were treated with 3 ml vehicle (control) i.m. on day 42 and with 15 mg PGF2 alpha on day 45. Blood samples were collected at frequent (5 and 15 min) intervals from 1 h before until 1 h after control and PGF2 alpha injections, at 15 min intervals for 4 h, and then at 5, 6, 9, 21, 33, 45 and 57 h post injection. Progesterone was measured by radioimmunoassay (RIA) in all samples. Porcine LH was measured by RIA in samples collected frequently in the 1 h pre- and 1 h post-injection periods. Serum progesterone concentrations were unchanged in both pregnant and pseudopregnant animals in response to control injection on day 42. However, in both pregnant and pseudopregnant gilts, PGF2 alpha injection on day 45 resulted in an acute increase (approximately 75-80% above pre-treatment levels; p less than 0.05) in serum progesterone lasting approximately 1 h, followed by a return to pre-treatment levels by 2 h, and then a decline to 1 ng/ml or less by 45-57 h (pregnant) or 21-57 h (pseudopregnant), associated with luteolysis. Serum LH concentrations were unchanged between 1 h pre- and post-treatment periods in response to either control or PGF2 alpha-treatment, in both pregnant and pseuodpregnant gilts. These results indicate that PGF2 alpha-injection produces a rapid and transient increase in serum progesterone concentrations which may result from a rapid and direct stimulatory action of PGF2 alpha on porcine luteal cell progesterone synthesis/secretion in vivo.     

Evidence that physiological levels of circulating leptin exert a stimulatory effect on luteinizing hormone and prolactin surges in rats.

Increasing evidence suggests that leptin, an adipocyte-derived hormone, may positively regulate the reproductive axis, and serve as a critical metabolic signal linking nutrition and the reproductive function. However, along this line there remains an as-of-yet unresolved important issue whether physiological levels of circulating leptin exert a stimulatory effect on the reproductive axis. It is also unknown whether hyperleptinemia affects the reproductive function. In this study, we attempted to examine these unexplored issues, employing as an indicator the estradiol/progesterone-induced luteinizing hormone (LH) and prolactin (PRL) surges in ovariectomized female rats. Experiments were performed on normally fed, 3-day starved, 3-day starved + murine leptin (100 microg/kg/day), and normally fed + murine leptin (300 microg/kg/day) groups. Leptin was administered utilizing osmotic minipumps during 3 days immediately before experimentation. From 11:00 to 18:00 h, blood was collected every 30 min to measure LH and PRL. The 3-day starvation completely abolished both LH and PRL surges, but 3-day starved + leptin (100 microg/kg/day) group, whose plasma leptin levels (3.7 +/- 0.4 ng/ml) were similar to those in normally fed group (3.4 +/- 0.5 ng/ml), showed a significant recovery of the hormonal surges. On the other hand, the magnitudes of LH and PRL surges in normally fed + leptin (300 microg/kg/day) group, whose leptin levels were 10.8 +/- 1.5 ng/ml, were statistically the same as those in normally fed group. These results indicate for the first time that physiological concentrations of circulating leptin exert a stimulatory effect on the steroid-induced LH and PRL surges in the rat. It was also suggested that mild hyperleptinemia of 3 days' duration may not significantly affect the hormonal surges.     

The effects of spaceflight on mammary metabolism in pregnant rats.

The effects of spaceflight on mammary metabolism of 10 pregnant rats was measured on Day 20 of pregnancy and after parturition. Rats were flown on the space shuttle from Day 11 through Day 20 of pregnancy. After their return to earth, glucose oxidation to carbon dioxide increased 43% (P < 0.05), and incorporation into fatty acids increased 300% (P < 0.005) compared to controls. It is unclear whether the enhanced glucose use is due to spaceflight or a response to landing. Casein mRNA and gross histology were not altered at Day 20 of pregnancy. Six rats gave birth (on Day 22 to 23 of pregnancy) and mammary metabolic activity was measured immediately postpartum. The earlier effects of spaceflight were no longer apparent. There was also no difference in expression of beta-casein mRNA. It is clear from these studies that spaceflight does not impair the normal development of the mammary gland, its ability to use glucose, nor the ability to express mRNA for a major milk protein.     

Acute stimulatory effects of an LH-RH agonist on the ovary in pseudopregnant rats

Pseudopregnant rats were anaesthetized on day 5 of pseudopregnancy with pentobarbital and hypophysectomy or sham operation was performed. Polyethylene cannulas were inserted into one of the femoral arteries and utero-ovarian veins. Five-minute blood samples were collected from the ovary for 40 minutes. Following the first five minute blood fraction LH-RH, an agonistic analogue (D-Phe6, des Gly-NH10(2)-LH-RH-ethylamide) or an antagonistic analogue (Formyl-D-Tryp1,3, D-pCl-Ph2, D-Phe6LH-RH) were superfused into the ovarian bursa for 5 minutes in a volume of 50 microliters. Blood pressure, ovarian venous outflow and haematocrit were continuously measured. From the blood samples progesterone and oestradiol-17 beta were determined by RIA and the secretion rate was calculated. It was found, that LH-RH and its antagonistic analogue has no effect on the blood flow, progesterone (P) and oestradiol-17 beta (E2) secretion of ovary. However, an agonistic analogue of LH-RH induced a rapid elevation of blood flow, diminished vascular resistance in the ovary, and in a dose of 50 ng increased the secretion rate of E2. In sham operated animals the effects of agonistic analogue was similar to the effects of 1.0 IU of hCG.     

The stimulatory effect of beta-endorphin on the plasma prolactin levels was diminished in the rats treated with 6-hydroxydopamine

Intraventicular injection of beta-endorphin (beta LPH_61^?91) in urethane anesthetized male rats led to a dose dependent increase of plasma prolactin levels. Intravenous injection of apomorphine completely abolished the stimulatory effect of beta-endorphin. Animals treated with 6-hydroxydopamine (6-OHDA) and 6-OHDA plus desmethylimipramine showed inhibition of beta-endorphin induced prolactin release. These results suggest that beta-endorphin presynaptically inhibits the activity of dopaminergic neurones, leading to the stimulation of plasma prolactin levels.     

Release of endogenous danger signals from HIFU-treated tumor cells and their stimulatory effects on APCs

The effects of high-intensity focused ultrasound (HIFU) on the release of endogenous danger signals from tumor cells and subsequent activation of antigen-presenting cells (APCs) were evaluated in vitro. MC-38 mouse tumor cells were treated using a 1.1 MHz HIFU transducer under two different protocols (P-=6.7 MPa, 30% duty cycle, 5 s vs. P-=10.7 MPa, 3% duty cycle, 30 s) to produce either thermal necrosis or mechanical lysis of the tumor cells. Here, we report that HIFU treatment can cause the release of endogenous danger signals (ATP and hsp60) and exposure of dendritic cells (DCs) and macrophages to the supernatants of HIFU-treated tumor cells leads to an increased expression of co-stimulatory molecules (CD80 and CD86) with enhanced secretion of IL-12 by the DCs and elevated secretion of TNF-alpha by the macrophages. The potency in APC activation produced by mechanical lysis is much stronger than thermal necrosis of the tumor cells. These findings suggest that optimization of treatment strategy may help to enhance HIFU-elicited anti-tumor immunity.     

Influence of adrenergic antagonists and apomorphine on prolactin release induced by serotonergic antagonists in the monkey.

The influence of adrenergic receptor blockers on the prolactin releasing effect of methysergide and cyproheptadine was examined in sexually mature female monkeys under ketamine anesthesia. Propranolol, a β-adrenergic blocker, at a dose of 1 mg/kg did not alter the prolactin releasing action of 0.1 mg/kg of methysergide but significantly potentiated (P < 0.025) the prolactin releasing action of 0.5 mg/kg of cyproheptadine. Phentolamine and phenoxybenzamine, both α-adrenergic blockers, at 1 mg/kg blunted the prolactin releasing effect of methysergide and cyproheptadine, but the pattern of prolactin blockade was different between the two putative antiserotonergic drugs. The prior administration of apomorphine, 4 mg/kg, a dopamine receptor stimulator, blocked the prolactin releasing effect of methysergide and cyproheptadine. Evidence presented here and from the literature indicate that the prolactin releasing action of methysergide and cyproheptadine is mediated by an antidopaminergic action directly on the pituitary.     

Sex-specific effects of prolactin on food intake by rats

While prolactin (PRL) has been reported to increase food intake by virgin female rats, its effects on food intake by male rats are relatively unexplored. The present studies examined the possibility that PRL has sex-specific effects on food intake by rats. In the first study, intact female and male rats were given subcutaneous injections of saline vehicle or ovine (o) PRL (1.0 mg/kg) twice daily at 08:00 and 20:00 h for 10 days. Food intake, body weight, and water intake were measured daily. Results indicate that oPRL administration increased food intake by an average of 4.5 g per day in female subjects, but did not significantly alter body weight or water intake. Male rats treated with oPRL did not significantly alter their food intake, even after an additional five days of treatment. In the second study, a wide range of oPRL doses (vehicle, 0.02, 0.2, 2.0, and 20.0 mg/kg/day) were tested in gonadectomized female and male rats. The results indicate that female rats responded to increasingly larger doses of oPRL with greater increases in food intake, with a maximum increase of approximately 6. 1 g per day at a dose of 20.0 mg/kg. In contrast, male rats maintained baseline levels of intake across all oPRL doses tested. These data suggest that PRL has sex-specific effects on food intake.     

Comparison of effects of prolactin and growth hormone on adrenal 5alpha-reductase in hypophysectomized rats.

Adrenal 5alpha-reductase activity was measured in female rats 0, 2, 5, and 6 days after hypophysectomy. Enzyme activity increased progressively exhibing a 35-fold elevation at 6 days. The effects of high (250 mug/100 g of body wt), intermediate (25 mug/100 g of body wt), and low (2.5 mug/100 of body wt) daily doses of bovine prolactin and bovine growth hormone were compared at 2 and 5 days posthypophysectomy. At 2 days, enzyme activity was partially inhibited by the high and intermediate doses of prolactin and not affected by growth hormone. At 5 days all doses of prolactin were inhibitory, whereas enzyme activity was suppressed only by the high dose of growth hormone. With a given dose of hormone, the amount of suppression of enzyme activity is greater at 5 days than at 2 days posthypophysectomy. In 5-day hypophysectomized rats the inhibitory effects of prolactin and growth hormone were additive. It is concluded that: (i) hormonal sensitivity and responsiveness of the adrenal reductase pathway increases with duration of pituitary ablation; (ii) the reductive pathway is more sensitive to the effects of prolactin than growth hormone; and (iii) the effects of growth hormone and prolactin on reductase activity are mediated via different mechanisms, as suggested by the additive effects of individual hormones.     

The effects of opiate agonists on growth hormone and prolactin release in rats

Various opioid receptor agonists, including Met5-enkephalin amide, Leu5-enkephalin amide, [D-Ala]2-Met5-enkephalin amide, [D-Ala]2-Leu5-enkephalin amide, morphine sulfate, d-methadone hydrochloride, and l-methadone hydrochloride were administered to adult male rats by subcutaneous injection. All opioid receptor agonists except Leu5-enkephalin amide significantly stimulated growth hormone and prolactin release. Naloxone and naltrexone blocked the hormone stimulatory effects of the opioids and both naloxone and naltrexone, when administered alone, significantly reduced serum growth hormone and prolactin concentrations. The dopaminergic agonist apomorphine, but not the alpha-adrenergic agonist clonidine, blocked opiate stimulation of prolactin. Morphine sulfate caused growth hormone release in rats pretreated with alpha-methyl-p-tryosine, a catecholamine synthesis inhibitor. Cholinergic agonists, physostigmine and pilocarpine, antagonized the growth hormone and prolactin release induced by morphine sulfate. The data suggest that the opiates stimulate prolactin via an interaction with catecholaminergic neurons controlling prolactin release and stimulate growth hormone via a mechanism independent of alpha-adrenergic or general catecholaminergic influence. The mechanism through which cholinergic agonists act to inhibit opiate agonist stimulation of growth hormone is presently unknown.