Serum prolactin levels of rats under continuous estrogen stimulation and 2 Br-alpha-ergocryptine (CB-154) injection.

The ability of 2 Br-alpha-ergocryptine (CB-154) to suppress serum prolactin levels was examined in animals under the influence of continuous estrogen stimulation. A single injection of polyestradiol phosphate (Estradurin) once every 21 days produced a constant elevation of serum prolactin. The simultaneous administration of 1 mg/day of CB-154 to estrogenized animals suppressed serum prolactin levels below that of Estradurin alone but the levels were significantly greater than those of animals receiving CB-154 alone. It was suggested that, while CB-154 prevents the release of prolactin, estrogen stimulates prolactin synthesis despite the block of its release. Ether anesthesia may be capable of partially overriding the block of CB-154 and released the stored hormone from the gland.     

2-bromo-alpha-ergocryptine mesylate (CB-154) inhibits prolactin and luteinizing hormone secretion in the prepubertal female rat

Treatment of immature female rats with 100 micrograms 2-bromo-alpha-ergocryptine mesylate (CB-154) per ml drinking water beginning on Day 30 of age until vaginal opening delayed puberty by 6 days. Rats treated with CB-154 exhibited vaginal opening at 43.3 +/- 0.6 days whereas controls exhibited vaginal opening at 37.9 +/- 0.8 days. Most interestingly, serum levels of luteinizing hormone (LH) and prolactin (PRL) on Days 31-35, determined by a homologous radioimmunoassay were significantly lower in treated rats than in controls. The ovarian concentrations of progesterone (P) and androstenedione (A) were lower in rats treated with CB-154 than in controls; ovarian estradiol (E2) concentrations were low in both groups. Serum levels of P (but not A and E2) were reduced on Days 31-35 of the treatment period. Cessation of the CB-154 treatment on the morning of Day 35 returned the onset of puberty to normal values; steroid and gonadotropin levels also returned to normal values within 2 days after removal of the CB-154 from the drinking water. Near the time of onset of puberty, serum levels of LH in rats treated with CB-154 returned to control values. These data indicate that in the female rat the delay in puberty induced by CB-154 might be due to a reduction in the secretion of LH, especially since the onset of delayed puberty in rats treated with CB-154 correlates with an increase in the serum level of LH. Further studies are needed to elucidate the specific effects of hypoprolactinemia on ovarian function and the onset of puberty in the rat.     

Effects of CB-154 (2-Br-alpha-ergocryptine) on prolactin and growth hormone release in an acromegalic patient with galactorrhea.

An acromegalic patient with galactorrhea was treated with an ergot alkaloid, 2-Br-alpha-ergocryptine (CB-154). Serum prolactin decreased rapidly to normal level by CB-154 and the complete cessation of galactorrhea was noted. The inhibitory effect of CB-154 On growth hormone (GH) release was also noted, but slight. The mechanism of inhibitory action of CB-154 on both prolactin and GH secretion was discussed in connection with the experimental model of pituitary tumors, in which both hormones were produced by a single type of tumor cells. The discontinuation of CB-154 treatment was associated with the return of both prolactin and GH levels to the initial high values with resumption of galactorrhea.     

The effects of methoxyflurane and ether alone or in combination with apomorphine or 2 Br-α-ergocryptine (CB-154) on prolactin release in ovariectomized,estrogen-treated rats

Continuous methoxyflurane (MF) anesthesia caused a temporary increase in plasma prolactin but abolished the afternoon prolactin surge in ovariectomized, estrogenized rats with aortic catheters. In the morning hours (0900–1200), prior to the prolactin surge, administration of Apomorphine HCl (APO) or CB-154 lowered plasma prolactin and MF did not alter these responses. Similarly, continuous ether anesthesia prior to the afternoon surge elevated plasma prolactin but did not override the suppressive effects of APO and only partially overrode those of CB-154. It is suggested that the sharp and immediate increases in plasma prolactin produced by MF and ether are due to involvement of the dopamine/PIF system since two different dopaminergic receptor stimulators blocked the response.     

Calcitonin inhibition of physiological and stimulated prolactin secretion in rats

The administration of salmon Calcitonin (sCT) intravenously (2.5 or 10 μg/kg) or into the lateral cerebral ventricles (2.5 or 25 ng/rat, i.c.v.) of unanaestized male rats induced clearcut decreases in plasma prolactin(PRL) levels. The i.c.v. injection of one of these doses of sCT (25 ng/rat) into rats with median eminence lesions was completely ineffective, while it induced a dramatic decrease in plasma PRL levels of sham-operated rats. Morphine- and heat stress-stimulated PRL levels were also abolished by sCT injection (250 ng/rat i.c.v.). The sCT-induced decrease in PRL levels was completely overcome by haloperidol, a dopamine-receptor blocker. We conclude that sCT may affect PRL secretion via an hypothalamic system, probably involving dopaminergic neurons. The present results indicate that CT, like many others peptides, may affect PRL secretion, directly or indirectly, even though further research is necessary to determine whether this effect has pharmacological or physiological importance.     

Prolactin, progesterone and luteinizing hormone secretion after bromocriptine (CB-154) treatment in cyclic sows

The effect of bromocriptine (CB-154) on prolactin, progesterone and luteinizing hormone (LH) secretion was studied in cyclic sows. Four sows were given subcutaneous injections of bromocriptine on Day 14 of the estrous cycle (70 mg CB-154) and again on Day 16 of the cycle (50 mg CB-154). Two control sows were injected with vehicle at similar time intervals. Blood samples were taken four times daily (0700, 1100, 1500 and 1900 h) from Day 11 of the estrous cycle to Day 2 of the following estrous cycle. Prolactin peaks during the estrous cycle were not observed after CB-154 treatment. CB-154 treatment had no effect on plasma LH concentration, but plasma progesterone concentrations appeared to fluctuate more and slowly decreased.     

Immunocytochemical and ultrastructural correlates of the pineal inhibition of prolactin cell activity in blind-anosmic female rats

Summary The effects of the pineal gland on the light microscopic-immunocytochemical and ultrastructural appearance of pituitary mammotrophs were studied in female rats eight weeks after prepubertal blinding and olfactory bulbectomy.Blinding and anosmia resulted in a marked decrease in the size of the pars distalis concomitant with a reduction in the apparent number and size of PRL cells as compared with intact animals. Ultrastructurally, these cells appeared much less active than those of intact rats. The small and angular-shaped mammotrophs of blind-anosmic rats characteristically exhibited scant arrays of rough endoplasmic reticulum, small Golgi complexes with few immature secretory granules, few mature secretory granules and rare exocytosis patterns.Pinealectomy tended to reverse the effects of blinding and anosmia on pars distalis size and PRL cell size, apparent number and ultrastructure. In fact, the mammotrophs of blind-anosmic-pinealectomized rats were quite similar in ultrastructural appearance to those of intact rats.From these data we conclude that the pineal causes mammotroph hypotrophy and hypoplasia in blind-anosmic female rats.Supported by USPHS Biomedical Research Support Grant # RR 05675. The authors thank Dr. Bruce A. Richardson for his kind help with the immunocytochemistry     

Naloxone inhibition of stress-induced increase in prolactin secretion

Naloxone, an opiate antagonist that acts by binding to opiate receptors in the brain, was given to rats stressed by immobilization or heat in an attempt to inhibit stress-induced release of prolactin. Both stresses resulted in approximately a 5-fold increase in serum prolactin concentration. Naloxone, at a dose of 0.2 mg/kg b.w. completely or partially inhibited the stress-induced rises in serum prolactin, and reduced serum prolactin concentrations in unstressed rats to below control values. It is concluded that endorphins may be responsible for increased release of PRL during stressful conditions.     

2-(Allylthio)pyrazine inhibition of aflatoxin B1-induced hepatocarcinogenesis in rats.

2-(Allylthio)pyrazine (2-AP), a synthetic pyrazine derivative with an allylsulfur moiety, has hepatoprotective effects against toxicants. Effect of 2-AP on hepatic tumorigenesis in association with glutathione S-transferase (GST) induction was examined in rats exposed to aflatoxin B1 (AFB1). Both AFB1-DNA adduct formation in the liver and urinary elimination of 8,9-dihydro-8-(N7-guanyl)-9-hydroxy-aflatoxin B1 (AFB1-N7-guanine) adduct were also determined. Male Sprague Dawley rats were treated with 2-AP at the daily oral doses of 10, 25 and 50 mg/kg for 16 consecutive days, during which four repeated doses of AFB1 (1.0 mg/kg) were given to the animals. Rats were then subjected to two-thirds of hepatectomy, followed by administration of phenobarbital (PB). Focal areas of hepatocellular alteration were identified after 44 days and preneoplastic foci expressing the placental form of glutathione S-transferase P (GST-P) were quantified by immunostaining of liver sections. 2-AP reduced the volume of liver occupied by GST-P foci by 65-96%. Under these experimental conditions, 2-AP treatment resulted in significant elevations in GST activity in the liver. Levels of radiolabeled AFB1 covalently bound to hepatic DNA, RNA and proteins were significantly reduced in rats treated with 2-AP for 5 days. 2-AP pretreatment also caused a 45% reduction in the urinary elimination of AFB1-N7-guanine adduct over the 24-h postdosing period. The present findings demonstrated that 2-AP exhibited protective effects against AFB1-induced hepatocarcinogenesis in rats with a marked decrease in the level of AFB1-DNA adduct. Reduction of hepatic DNA adducts might result from elevations of activity of GST, which catalyzes detoxification of the carcinogen.     

Pituitary prolactin concentrations associated with daily prolactin surges in pseudopregnant rats

During pseudopregnancy (PSP) two surges of prolactin (PRL) secretion from the pituitary are observed, the nocturnal surge at dawn and the diurnal surge in the evening. An attempt was made to clarify the correlation between changes in serum and pituitary PRL concentrations on day 5-6 of PSP. During the nocturnal surge, pituitary PRL concentration decreased significantly from 0000 hr to 0300-0600 hr. On the other hand, the high pituitary PRL concentration remained unchanged during the diurnal surge from 1200 hr to 1800 hr. These findings suggest that the nocturnal and diurnal PRL surges are regulated by separate controlling mechanisms.     

In vitro studies of prolactin inhibition of luteinizing hormone action on Leydig cells of rats and mice

Previous in vivo studies have shown that in male rabbits prolactin inhibits the testosterone production stimulated by luteinizing hormone (LH) or human chorionic gonadotropin (hCG). This inhibition has now been studied in vitro using both mouse and rat testicular interstitial cells. First, the dose response of human LH (hLH) stimulation of testosterone was studied in detail using testicular interstitial cells from both species. Next, a small but stimulatory dose of hLH was selected and extensive prolactin doses were studied in vitro. NIH B-6 (bovine) prolactin in varying doses was added to the interstitial cells 30 min prior to the addition of a constant dose of hLH. Under these circumstances prolactin inhibited LH action over a wide range of doses. In both species a biphasic dose-response curve existed: large doses of 100 to 1000 ng/ml produced less inhibition or augmented LH action, compared to smaller doses. Next, entire hLH dose-response curves were produced in the presence of three doses of prolactin (0.33, 33, and 1000 ng/ml) as well as in the absence of prolactin. The addition of prolactin shifted the hLH dose-response curve to the right and depressed the maximal response in comparison to the curve without prolactin. Finally, inhibitory doses of prolactin resulted in no detectable change in LH receptor number as estimated from Scatchard plots. It is concluded that prolactin inhibits LH action on interstitial cells as determined by rate of testosterone production except at very large doses of prolactin where LH action is less inhibited or augmented. The inhibitory action of prolactin in this in vitro interstitial cell assay was not accompanied by a decrease in LH receptor number. Thus, a postreceptor action is likely to be involved.     

Effects of acetazolamide and 4-aminopyridine on CO2-induced slowly adapting pulmonary stretch receptor inhibition in rats

Inhibitory responses of slowly adapting pulmonary stretch receptor (SAR) activity to CO(2) inhalation (maximal tracheal CO(2) concentration ranging from 9.5 to 12.5%) for approximately 60 s were examined before and after administration of acetazolamide (a carbonic anhydrase inhibitor) or 4-aminopyridine (4-AP, a K(+) channel blocker). The experiments were performed in 35 anesthetized, artificially ventilated rats after unilateral vagotomy. Sixty-eight of eighty-four SARs were inhibited by CO(2) inhalation. The SAR inhibition was attenuated by pretreatment with either acetazolamide (20 mg/kg, n = 10) or 4-AP (0.7 and 2.0 mg/kg, n = 10). In other series of experiments, stainings to show the existence of carbonic anhydrase (CA) enzymatic reaction were not found in the smooth muscle of either extrapulmonary or intrapulmonary bronchi. Protein gene product 9.5 (PGP 9.5)-immunoreactive SAR terminals to form leaflike extensions were found in the bronchioles at different diameters and were smooth-muscle-related receptors. But in the same sections, CA isozyme II-like (erythrocyte CA) immunoreactive SAR terminals were not identified. These results suggest that CO(2)-induced inhibition of SARs may be involved in the CA-dependent CO(2) hydration in addition to the activation of 4-AP sensitive K(+) currents.     

Pituitary site of action of endothelin: selective inhibition of prolactin release in vitro

Endothelin-3 (ET-3) inhibited in a dose-dependent, significant fashion prolactin release from cultured anterior pituitary cells (ovariectomized female and intact male rat donors, ED50 = 5 X 10(-9) M). ET-3 in log doses ranging from 10(-11) to 10(-6) M did not alter significantly the release of luteinizing hormone, growth hormone or thyroid stimulating hormone. The inhibitory effect of ET-3 (rat, human) was specific for that molecule since ET-1 (porcine, human) was ineffective and was not due to an action on the dopamine receptor since the inhibitory action was still expressed in the presence of 100 nM domperidone. These data further suggest a role for neuropeptides of the posterior lobe in the control of lactotroph function.     

Thyroid stimulating hormone and prolactin secretion: reduced sensitivity to TRH-stimulated prolactin release after midpregnancy in rats

Prolactin (PRL) and thyroid stimulating hormone (TSH) plasma concentrations were measured during the latter part of the dark period in early and mid-late pregnancy in the rat. On Days 4-5 and 7-8 of pregnancy, plasma PRL concentrations surged between 22:00 and 06:00 hr and TSH values increased between 22:00 and 02:00 hr. While the TSH pattern was maintained during the second-half of pregnancy, surges in PRL release ceased and PRL levels remained at less than 10 ng/ml. The effects of thyrotropin releasing hormone (TRH) administration on PRL and TSH secretion were then measured to determine whether the second-half of pregnancy is associated with a decrease in sensitivity to an agent that can stimulate PRL release. Injection (iv) of cannulated pregnant rats with a low dosage (20 ng) of TRH stimulated a twofold increase in plasma TSH during both early (Days 5-9) and later (Days 14-18) pregnancy but did not change plasma PRL levels. Treatment with a high dosage (2 micrograms) of TRH induced a sixfold rise in plasma TSH during both phases of gestation. The higher dose of TRH also stimulated elevations in plasma PRL during early and mid-late pregnancy; however, both the absolute increase in the amount of PRL in plasma and the percentage increase over baseline levels were greater from Days 5-9 than from Days 14-16 of gestation. These data indicate that the neuroendocrine sensitivity to factors that stimulate PRL secretion changes as pregnancy progresses, and suggest that nocturnal secretion of PRL and TSH during pregnancy may be regulated, in part, by a common trophic factor.