Acute stimulation of prolactin release by estradiol: mediation by the posterior pituitary

We have previously shown that the posterior pituitary contains a potent PRL-releasing factor (PRF). Estradiol stimulates PRL release by acting at three possible sites: the hypothalamus, the anterior pituitary, and the posterior pituitary. The objectives were 1) to document the profiles of PRL and LH release in response to an acute administration of estradiol, and 2) to identify the site of action of estradiol by employing two surgical approaches, pituitary stalk section (SS) and posterior pituitary lobectomy (LOBEX). Ovariectomized rats were used throughout. In Exp 1, rats were injected iv with 5 micrograms/kg 17 beta-estradiol, and blood was collected at 30-min intervals for 4 h. Estradiol induced a rapid and profound decline in plasma LH levels and a delayed, 5- to 6-fold rise in PRL. The purpose of the second experiment was to determine whether estradiol stimulates PRL release by acting at the anterior pituitary. Injection of estradiol to SS rats failed to stimulate a rise in PRL. We have previously reported that lactotrophs of SS rats are responsive to PRL secretagogues such as TRH. The objective of the third experiment was to differentiate between hypothalamic and posterior pituitary sites of estradiol action. Estradiol induced only a small rise in PRL when injected into LOBEX rats. However, LOBEX and control rats showed similar large rises in PRL in response to injection of alpha-methyl-para-tyrosine, an inhibitor of tyrosine hydroxylase. The latter indicates that the hypothalamic dopaminergic system as well as anterior pituitary lactotrophs are functionally intact in LOBEX rats. We conclude that estradiol administration to ovariectomized rats induces a rapid decline in LH and a delayed marked increase in PRL. The posterior pituitary, probably via PRF, is the primary site that mediates the acute effects of estradiol on PRL release. Estradiol does not stimulate PRL release directly from the anterior pituitary. The role of the hypothalamus is unclear. Estradiol could act directly on PRF-containing cells in the posterior pituitary or indirectly, via hypothalamic neurons terminating in the posterior pituitary. The hypothalamus also has a minor component that responds to estradiol and is independent of the posterior pituitary.     

Effects of starvation in rats on serum levels of follicle stimulating hormone, luteinizing hormone, thyrotropin, growth hormone and prolactin; response to LH-releasing hormone and thyrotropin-releasing hormone.

Adult male Sprague-Dawley rats averaging 300 g each were subjected to complete food removal for 7 days (acutely starved), 7 days complete food removal followed by 2 weeks of 1/4 ad libitum food intake (chronically strved), 7 days complete food removal and 2 weeks of 1/4 ad libitum intake followed by ad libitum feeding for 7 days (refed), or fed ad libitum throughout (controls). Serum LH, FSH, TSH, PRL, and GH levels were measured by radioimmunoassays for each group of rats. The in vivo response to the combination of synthetic LHRH and TRH also was tested in each group of rats. Circulating LH, TSH, GH, and PRL were significantly depressed in acutely and chronically starved rats, and FSH was lowered only in acutely starved rats. After 7 days of refeeding, serum levels of LH and FSH were significantly greater than in ad libitum fed controls, PRL returned to control levels, and TSH and GH increased but were still below control levels. After LHRH + TRH injection serum LH and TSH were increased significantly in all groups of rats, FSH and PRL rose in acutely but not in chronically starved rats, and GH was not elevated in any group. The increases in serum LH, FSH, TSH and prolactin in response to LHRH + TRH injection in acutely or chronically starved rats were equal to or greater than in the ad libitum fed controls. These data indicate that severe reductions in food intake result in decreased release of at least 5 anterior pituitary hormones, and this is due primarily to reduced hypothalamic stimulation rather than to inability of the pituitary to secrete hormones.     

Catecholamine depletion modulates serum LH levels, GAD67 mRNA, and GABA synthesis in the goldfish

It is established that dopamine inhibits while GABA stimulates LH release in goldfish. In this study, we examine dopaminergic regulation of GABAergic activity in the hypothalamus of early recrudescent female goldfish (Carassius auratus). We utilize a unique technique that permits concomitant quantification and correlation of in vivo GAD65 and GAD67 mRNA with GABA synthesis rate in response to decreased dopamine levels. Catecholamine depletion was achieved by treatment with alpha-methyl-para-tyrosine methyl ester (alphaMPT; 240 microg/g body weight), an inhibitor of tyrosine hydroxylase. Endogenous GABA levels were increased by intraperitoneal administration of gamma-vinyl GABA (GVG; 300 microg/g body weight), an inhibitor of the GABA catabolic enzyme GABA transaminase. Dual treatment of GVG+alphaMPT increased serum LH levels 4-fold. However, LH mRNA levels in the pituitary remained stable, suggesting that treatments affected secretion and not synthesis. In the hypothalamus, GABA synthesis rates increased 30% in response to alphaMPT treatment. This was correlated (r=0.61; p<0.05) to increased levels of GAD67 mRNAs but not GAD65 (r=0.14; p>0.05). These observations suggest that catecholamines inhibit GABA synthesis in the goldfish hypothalamus through isoform specific regulation of GAD67.     

Hypothalamic and serum factors influence on prolactin and luteinizing hormone release by the pituitary gland of the young turkey (Meleagris gallopavo)

Intravenous administration of 1.0 or 3.0 eq hypothalamic extract (HE) to 8-week-old male and 7-week-old female turkeys, respectively, induced an increase in circulating prolactin (PRL) levels but had no effect on circulating luteinizing hormone (LH) levels. The incubation of dissociated anterior pituitary cells from 13-week-old female turkeys with HE induced a dose-related increase in PRL release; however, only the highest dose of HE induced an increase in LH release. Coincubation of a hypothalamic fragment with anterior pituitary cells from 9-week-old females induced a release of both PRL and LH. Dissociated pituitary cells from 11-week-old females initially incubated for 3 hr in medium containing charcoal-treated (stripped) turkey serum yielded a larger release of PRL and LH in the presence of HE than did cells initially incubated with turkey serum or no serum. Luteinizing hormone-releasing hormone (LHRH) induction of LH release was greatest from cells initially incubated with stripped serum for 3 or 24 hr. The LHRH-induced LH release was completely blocked in cells initially incubated for 24 hr with turkey serum. The initial incubation of cells for 24 hr with stripped serum yielded a larger release of PRL and LH in response to HE than did cells initially incubated with serum. The hypothalamus of the young turkey contains substantial PRL-releasing activity as well as LH-releasing activity. The ability of the releasing factors to stimulate pituitary hormone release is influenced by factors present in the blood of the young turkey. This is especially evident in the LHRH-induced LH release where serum factors inhibited the release.     

GABAergic biochemical parameters of the tuberoinfundibular neurons following chronic hyperprolactinemia

The effect of chronic hyperprolactinemia was studied on (a) GABA concentration in the pituitary anterior lobe; (b) GABA biosynthesis enzyme, glutamate decarboxylase (GAD) activity in the hypothalamic median eminence, and (c) GABA degradation enzyme GABA-transaminase (GABA-T) activity at both levels. In male rats bearing the prolactin-secreting tumor MtTF4 for 1 month or treated for 5 days with estradiol benzoate, the plasma prolactin concentration was markedly increased (between 4- and 10-fold basal values). In both cases, GABA concentration was significantly increased (40-60%) in the anterior pituitary lobe. A slight reduction (20-30%) in GABA-T activity was observed in the anterior lobe while no change in GAD or GABA-T activity was measured in the median eminence. These results are discussed in relationship to a possible feedback input of prolactin on the tuberoinfundibular GABAergic system.     

Modulation of pituitary gonadotropins and prolactin secretion by testosterone in vitro.

Investigations were undertaken to study the differential modulation of LH, FSH and PRL secretion by testosterone (T) using whole pituitary (PI) or pituitary-hypothalamus coincubates (PHC) as in vitro constructs. PI and PHC from intact and castrated rats were incubated with or without T thrice, for 24 h each, (24 h x 3, total incubation period 72 h). The spent media was replenished every 24 h. At the end of 72 h, a few of the pituitary glands were challenged with 10 nM LHRH for 4 h. The spent media and pituitary glands were analyzed for LH, FSH and PRL using specific RIAs. Incubation of PI or PHC from intact rats with T stimulated the release of LH and FSH but inhibited the release of PRL. T had no effect on the intrapituitary contents of LH but inhibited intrapituitary contents of FSH and PRL, as compared to controls incubated without T. Castration increased intrapituitary contents of LH and FSH with concomitant decrease in PRL levels. Incubation of PI or PHC from castrated rats with T inhibited intrapituitary contents of LH to intact pituitary levels, while PRL levels were further reduced instead of being ameliorated. It is concluded that PI or PHC can be used as convenient in vitro models to monitor the effect of castration or of T modulation of pituitary and hypothalamus functions. T does not affect the synthesis of LH at the gonadotroph level but facilitates the regulation of intracellular LH and FSH levels. It is postulated that T inhibits the synthesis of FSH/PRL at the gonadotroph/lactotroph levels.     

Interactions of delta 9-tetrahydrocannabinol (THC) with hypothalamic neurotransmitters controlling luteinizing hormone and prolactin release

The effects of delta 9-tetrahydrocannabinol (THC) on hypothalamic norepinephrine (NE) and dopamine (DA) turnover and hypothalamic serotonin (5-HT), 5-hydroxyindole acetic acid (5-HIAA) and LHRH content preceding and during a progesterone- (P) induced LH and prolactin (PRL) surge were investigated in ovariectomized estrogen-primed rats. THC had no effect on basal LH levels, but it inhibited basal PRL levels and blocked the surges of both LH and PRL. The turnover of NE, as estimated by measuring NE depletion after inhibition of tyrosine hydroxylase with alpha-methyl tyrosine (250 mg/kg), in both the anterior (AH) and medial basal hypothalamus (MBH) was significantly inhibited by THC. THC did not significantly affect AH or MBH DA or 5-HT content nor MBH-DA-turnover. Hypothalamic LHRH levels were significantly elevated 4 h after THC administration as compared to the vehicle-injected controls, but pituitary response to exogenous LHRH was not affected. These data suggest that THC inhibits the steroid-induced positive feedback release of LH by reducing NE metabolism and the release of hypothalamic LHRH. Although the mechanism for the inhibition of PRL release by THC is not clear from these experiments, it does not appear that alterations in DA turnover are a contributing factor.     

A conditional tetracycline-regulated increase in Gamma amino butyric acid production near luteinizing hormone-releasing hormone nerve terminals disrupts estrous cyclicity in the rat

Gamma amino butyric acid (GABA) is the main inhibitory neurotransmitter controlling LH-releasing hormone (LHRH) secretion in the mammalian hypothalamus. Whether alterations in GABA homeostasis within discrete regions of the neuroendocrine brain known to be targets of GABA action, such as the median eminence, can disrupt the ability of the LHRH releasing system to maintain reproductive cyclicity is not known but amenable to experimental scrutiny. The present experiments were undertaken to examine this issue. Immortalized BAS-8.1 astroglial cells were genetically modified by infection with a regulatable retroviral vector to express the gene encoding the GABA synthesizing enzyme glutamic acid decarboxylase-67 (GAD-67) under the control of a tetracycline (tet) controlled gene expression system. In this system, expression of the gene of interest is repressed by tet and activated in the absence of the antibiotic. BAS-8.1 cells carrying this regulatory cassette, and cultured in the absence of tet ("GAD on"), expressed abundant levels of GAD-67 messenger RNA and GAD enzymatic activity, and released GABA when challenged with glutamate. All of these responses were inhibited within 24 h of exposure to tet ("GAD off"). Grafting "GAD on" cells into the median eminence of late juvenile female rats, near LHRH nerve terminals, did not affect the age at vaginal opening, but greatly disrupted subsequent estrous cyclicity. These animals exhibiting long periods of persistent estrus, interrupted by occasional days in proestrus and diestrus, suggesting the occurrence of irregular ovulatory episodes. Administration of the tetracycline analog doxycycline (DOXY) in the drinking water inhibited GAD-67synthesis and restored estrous cyclicity to a pattern indistinguishable from that of control rats grafted with native BAS-8.1 cells. Animals carrying "GAD on" cells showed a small increase in serum LH and estradiol levels, and a marked elevation in serum androstenedione, all of which were obliterated by turning GAD-67 synthesis off in the grafted cells. Morphometric analysis of the ovaries revealed that both groups grafted with GABA-producing cells had an increased incidence of large antral follicles (>500 micrometer) compared with animals grafted with native BAS-8.1 cells, but that within this category the incidence of steroidogenically more active follicles (i.e. larger than 600 micrometer) was greater in "GAD on" than in "GAD off" rats. These results indicate that a regionally discrete, temporally controlled increase in GABA availability to LHRH nerve terminals in the median eminence of the hypothalamus suffices to disrupt estrous cyclicity in the rat, and raise the possibility that similar local alterations in GABA homeostasis may contribute to the pathology of hypothalamic amenorrhea/oligomenorrhea in humans.     

Hypothalamic prolactin stimulates the release of luteinizing hormone-releasing hormone from male rat hypothalamus

Previous works from our laboratory and others have shown that there is a PRL-like immunoreactive protein with immunological, chromatographic, and biological characteristics identical to those of pituitary PRL, and this is widely distributed in the rat central nervous system. Since pituitary PRL is important in controlling hypothalamic LHRH release, we have hypothesized that hypothalamic PRL-like immunoreactive protein might serve a similar role, that of an endogenous neuromodulator influencing hypothalamic LHRH release. To this end, we have examined the effect of PRL antiserum and normal rabbit serum on the release of immunoreactive LHRH from rat hypothalamic fragments cultured in vitro. In the first experiment, LHRH release from hypothalami of intact rats, bathed in PRL antiserum (1:200 in Krebs-Ringer bicarbonate buffer), was significantly lower than that from hypothalami bathed in normal rabbit serum (1:200 in Krebs-Ringer bicarbonate buffer) for 90 min of incubation. It was, however, possible that the PRL, immunoneutralized in the first experiment, was material that represented contamination from pituitary PRL. Therefore, we repeated the experiment using hypothalami from animals that had been hypophysectomized 2 weeks before death. Again, PRL antibody significantly inhibited the release of LHRH compared with that by hypothalami incubated in normal rabbit serum. Since testosterone is important to LHRH synthesis, a third experiment was carried out using hypothalami from hypophysectomized male rats that had been implanted sc with testosterone-containing capsules 72 h before death. By 72 h serum testosterone levels had normalized. PRL antibody added to medium containing hypothalamic explants from these animals substantially inhibited in vitro LHRH release, a pattern essentially similar to that seen in intact and hypophysectomized animals without testosterone replacement. From these studies we have concluded that hypothalamic PRL is an important neuromodulator that promotes the release of LHRH from the hypothalamus. Testosterone, at least under the experimental conditions employed, appears not to be essential in this hypothalamic PRL-LHRH interaction.     

Evidence for gamma-aminobutyric acid modulation of ovarian hormonal effects on luteinizing hormone secretion and hypothalamic catecholamine activity in the female rat

Recent evidence suggests that gamma-aminobutyric acid (GABA)-containing neurons may inhibit LH release under certain circumstances. The present experiments tested whether GABA agonists block the LH surge induced in ovariectomized rats by estradiol benzoate (EB) plus progesterone (P) treatment and whether these agents affect the concentration and turnover of hypothalamic catecholamines, assessed from the depletion that occurs after synthesis inhibition. Ovariectomized rats received EB, followed 2 days later by P. Simultaneously with P, rats received either saline or one of the GABA agonists, baclofen or muscimol. Other agonist-treated rats received a second injection 4 h later or were additionally treated with the postsynaptic GABA antagonist bicuculline. Additional experiments tested the effects of these agents on LH release in response to exogenous LHRH. The LH surge induced by EB plus P was blocked by administration of either baclofen or muscimol in a dose-dependent manner. Bicuculline did not prevent the effect of baclofen, but partially prevented the effect of muscimol. Neither baclofen nor muscimol significantly affected LH release in rats receiving LHRH. In a second set of studies in EB plus P-treated rats, baclofen and muscimol decreased the steady state concentrations of norepinephrine in the medial preoptic area and medial basal hypothalamus for several hours and markedly decreased the turnover rate of norepinephrine in these areas. The concentrations and turnover of epinephrine were also decreased by these GABA agonists in the medial basal hypothalamus. The drugs had no effect on dopamine levels or turnover in either structure. These results support the hypothesis that a GABAergic system regulates LH release via modulation of noradrenergic and adrenergic systems that control LHRH secretion.     

THE INTERACTION OF GROWTH HORMONE RELEASING HORMONE WITH OTHER HYPOTHALAMIC HORMONES ON THE RELEASE OF ANTERIOR PITUITARY HORMONES

To determine whether the 29 amino-acid fragment of growth hormone releasing hormone (GHRH) can be combined with other hypothalamic releasing hormones in a single test of anterior pituitary reserve, the responses of anterior pituitary hormones to combinations of an i.v. bolus of GHRH(1-29)NH2 or saline with an i.v. bolus of either LH releasing hormone (LHRH) plus TRH, ovine CRH(oCRH) or saline were studied. Each infusion of GHRH(1-29)NH2 resulted in a rapid increment of the plasma GH value. Infusion of GHRH(1-29)NH2 also caused a small and transient rise in plasma PRL, but no change in the integrated PRL response. The combination of GHRH(1-29)NH2 with LHRH plus TRH caused a larger increment of peak and integrated plasma TSH levels than LHRH plus TRH alone. GHRH(1-29)NH2 did not affect the release of other anterior pituitary hormones after infusion with oCRH or LHRH plus TRH. Because of the finding of potentiation of the TSH-releasing activity of LHRH plus TRH by GHRH(1-29)NH2, the study was extended to the investigation of TSH release after infusion of TRH in combination with either GHRH(1-29)NH2 or GHRH(1-40). In this study the combination of TRH with both GHRH preparations also caused a larger increment of the peak and integrated plasma TSH levels than TRH alone. It is concluded that GHRH(1-29)NH2 possesses moderate PRL-releasing activity apart from GH-releasing activity. In addition, GHRH potentiates the TSH-releasing activity of TRH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of Nitric Oxide Synthase Inhibitors on Preventing Ethanol-Induced Suppression of the Hypothalamic-Pituitary-Gonadal Axis in the Male Rat

Ethanol (EtOH) suppression of the hypothalamic-pituitary-gonadal (HPG) axis results in broad reproductive malfunction. In the HPG axis, the suppressive effects of EtOH are manifested by decreased serum testosterone, reduced testicular luteinizing hormone (LH) receptor numbers, lowered serum LH and pituitary β-LH mRNA levels (in castrated animals), and impaired luteiniring hormone releasing hormone (LHRH) release from the hypothalamus. Increasing evidence has suggested that nitric oxide (NO) plays a role in regulation of the HPG axis. NO was shown to stimulate LHRH secretion from the hypothalamus and to have variable effects on LH release from the pituitary. At the gonadal level, NO is inhibitory to testosterone production. NO may directly inhibit some testicular steroidogenic enzymes. To investigate the effect of EtOH, NO, and their interaction on the male HPG axis, three NO synthase (NOS) inhibitors, N^Q-nitro- l -arginine methyl ester, N^Q-nitro- l -arginine, and 7-nitro indazole were used to study overall HPG function in the presence and absence of EtOH. Animals were given intraperitoneal injections of saline, EtOH, various NOS inhibitors, or EtOH, along with NOS inhibitors 2 hr before sacrifice. Serum testosterone and LH concentrations, pituiiry βLH mRNA levels, hypothalamic LHRH mRNA levels, and LHRH content were determined. It was found that blocking NOS by these NOS inhibitors prevented EtOH-induced suppression of testosterone and, in some cases, serum LH. However, this was not accompanied by concurrent changes with NOS blockade on LHRH mRNA, hypothalamic pro-LHRH or LHRH content or pituitary LH β mRNA levels. It appears that the protective effect of NOS blockade was largely, although not completely, due to a direct effect at the gonadal level.     

Effect of estrogen on thyrotropin-releasing hormone-induced release of prolactin in intact, ovariectomized, and stalk-sectioned female rhesus monkeys

The effects of estrogen treatment on basal and TRH-induced serum PRL concentrations were studied in three groups of four female rhesus monkeys; intact monkeys, ovariectomized animals, and monkeys in which the pituitary gland had been isolated from direct hypothalamic influences by pituitary stalk section. The TRH tests (50 microgram, iv) were performed before and 7 and 21 days after the sc implantation of one or two 3-cm long silastic capsules containing 17 beta-estradiol. Treatment with estradiol significantly increased the PRL response to TRH in the three groups of animals. The highest PRL response to TRH was observed after stalk section. The estrogen treatment also increased basal PRL concentrations in stalk-sectioned monkeys but no statistically significant increase was observed in intact or ovariectomized animals. These results indicate that physiological amounts of estradiol increase the magnitude of TRH-induced PRL release in rhesus monkeys, and that this estrogen effect is probably enacted directly at the level of the anterior pituitary gland.     

The impact of euglycemia and hyperglycemia on stimulated pituitary hormone release in insulin-dependent diabetics

To study the influence of different blood glucose (BG) concentrations on the release of pituitary hormones, the effect of the simultaneous iv administration of LRH (200 micrograms), TRH (400 micrograms), and arginine (30 g/30 min) upon the serum concentrations of LH, FSH, TSH, PRL, and GH was determined in six male insulin-dependent diabetics. BG concentration was clamped by feedback control and an automated glucose-controlled insulin infusion system at euglycemic (BG 4-5 mmol/liter) or hyperglycemic (BG, 14-18 mmol/liter) levels. Increments in serum concentrations of LH, FSH, TSH, and PRL were similar in the euglycemic and hyperglycemic steady states, whereas the GH response to arginine was suppressed during the hyperglycemic clamp (P less than 0.01). Omission of exogenous insulin during hyperglycemia did not modify the observed hormonal responses. Thus, the release of LH, FSH, TSH, and PRL in response to adequate acute stimuli at the pituitary level is not modulated by hyperglycemia in insulin-dependent diabetes, while arginine-induced GH release is suppressed. Since the effect of arginine on GH is most likely mediated by an action on the hypothalamus, the data suggest that elevated glucose concentrations may exert their modulatory influence on GH secretion at the hypothalamic rather than at the pituitary level.     

Effects of delta-9-tetrahydrocannabinol on the hypothalamic-pituitary control of luteinizing hormone and follicle-stimulating hormone secretion in adult male rats

The main psychoactive component of marihuana, delta-9-tetrahydrocannabinol (THC) was injected into the 3rd cerebral ventricle. A single dose of THC (2 microliter of 10(-6) M) decreased serum LH temporarily but did not alter serum follicle-stimulating hormone (FSH) levels. The mediobasal hypothalamic (MBH) luteinizing hormone-releasing hormone (LHRH) content was elevated by 30 min after the injection. The elevation persisted for 1 h. Then, the LHRH content returned towards the preinjection level. In contrast, the LHRH in the organum vasculosum of the lamina terminalis did not change after a single dose of THC. The results indicate that THC alters pituitary LH release by inhibiting the release of LHRH which then increases in the MBH by continued synthesis or transport from rostral areas. In addition, the data support the existence of an FSH releasing factor, the release of which is not suppressed by this dose of THC. THC did not alter the release, storage or responsiveness to LHRH of cultured anterior pituitary cells, which further supports the view that its principal site of action is on the hypothalamus.     

Effects of estrogen on 3H-leucine uptake by the hypothalamus and pituitary: correlation with hypothalamic and serum LHRH and LH.

An injection of estradiol benzoate (EB, 5 microgram/rat) to ovariectomized rats suppressed serum LH levels 2, 4, 8 and 24 h later; serum LHRH levels were unaltered but the LHRH content in the medial basal hypothalamus (MBH) was significantly elevated at 24 h. EB treatment also modified the sensitivity of the pituitary to LHRH administration: a rapid decrease at 4 h followed by an augmentation in LH release at 8 and 24 h. 3H-Leucine incorporation into proteins of the pituitary and neural tissues was unaffected for 4 h following EB treatment at the time when serum LH and the responsiveness of the pituitary to LHRH were clearly depressed. However, a marked increase in 3H-leucine incorporation into the proteins of the pituitary alone occurred at 8, 16 and 24 h after EB treatment, coincident with the enhanced pituitary sensitivity to LHRH. These studies showed that estrogen promoted storage of LHRH in the MBH and alteration in protein metabolism may be an important initial step in the estrogen-induced augmentation of the pituitary responsiveness to LHRH.     

The effect of neoplasia-induced hypercalcemia on the neuroendocrine axis of the Fischer rat

The effects of neoplasia-induced hypercalcemia on the hypothalamic-pituitary axis of the Fischer rat were determined by measuring the responses of TSH to TRH, PRL to TRH and haloperidol, and LH to LHRH in 3 groups of 8 rats prior to and at 4 and 8 days after transplantation of a Leydig cell tumor. The mean serum calcium was 8.2 ± 0.2 mg/dl in 8 Fischer rats pretransplantation; had risen at 4 days posttransplantation to 9.8 ± 0.2 mg/dl; and at 8 days was 11.2 ± 0.1 mg/dl. TSH response to TRH was greater in the pretransplant rats than in the groups studied at 4 and 8 days posttransplant. Basal PRL levels and PRL responses to TRH and haloperidol were less in the groups studied at 4 and 8 days posttransplant. Basal LH values were similar in all 3 groups, but the LH response to LHRH was less for the posttransplant groups. The TSH response to TRH, PRL responses to TRH and haloperidol, and LH responses to LHRH were less in the 8 days posttransplants than in the groups studied at 4 days posttransplantation. There were significant negative correlations between TSH (r = ?0.79) and PRL (r = ?0.80) responses to TRH, PRL responses (r = ?0.94) to haloperidol and LH responses (r = ?0.91) to LHRH and the serum calcium. The results indicate that increasing levels of serum calcium following Leydig cell tumor transplantation are associated with suppression of adenohypophysial release of TSH, PRL, and LH; the degree of suppression is related to the magnitude of hypercalcemia achieved in this model.     

Early effects of cranial irradiation on hypothalamic-pituitary function

Hypothalamic-pituitary function was studied in 31 patients before and after cranial irradiation for nasopharyngeal carcinoma. The estimated radiotherapy (RT) doses to the hypothalamus and pituitary were 3979 +/- 78 (+/- SD) and 6167 +/- 122 centiGrays, respectively. All patients had normal pituitary function before RT. One year after RT, there was a significant decrease in the integrated serum GH response to insulin-induced hypoglycemia. In the male patients, basal serum FSH significantly increased, while basal serum LH and testosterone did not change. Moreover, in response to LHRH, the integrated FSH response was increased while that of LH was decreased. Such discordant changes in FSH and LH may be explained by a defect in LHRH pulsatile release involving predominantly a decrease in pulse frequency. The peak serum TSH response to TRH became delayed in 28 patients, suggesting a defect in TRH release. Twenty-one patients were reassessed 2 yr after RT. Their mean basal serum T4 and plasma cortisol levels had significantly decreased. Hyperprolactinemia associated with oligomenorrhoea was found in 3 women. Further impairment in the secretion of GH, FSH, LH, TSH, and ACTH had occurred, and 4 patients had hypopituitarism. Thus, progressive impairment in hypothalamic-pituitary function occurs after cranial irradiation and can be demonstrated as early as 1 yr after RT.     

Evidence for paracrine interaction between gonadotrophs and lactotrophs in pituitary cell aggregates

Pituitary cell aggregates prepared from 14-day-old male or female rats and maintained for 4-5 days in culture were superfused with LHRH during periods of 20 or 90 min. LHRH provoked a rapid and sustained rise of PRL release at concentrations similar to those stimulating LH release (10(-11)-10(-8) M). Dopamine, at a concentration inhibiting PRL release for 90%, weakened but did not prevent this stimulation. LHRH also stimulated PRL release in aggregates prepared from adult male rat pituitary cells, but the effect was weaker and seen only after a more prolonged period in culture. There was no PRL response to LHRH in aggregates of lactotroph-enriched populations, obtained by gradient sedimentation at unit gravity, in which only few and small gonadotrophs are present. When a lactotroph-enriched/gonadotroph-poor population was coaggregated with a highly enriched population of large gonadotrophs, LHRH very effectively stimulated PRL release, the extent of stimulation being dependent on the proportional number of gonadotrophs in the coculture. Superfusion of lactotroph-enriched/gonadotroph-poor aggregates with medium in which the gonadotroph-enriched aggregates had previously been incubated for 3 h with 1 nM LHRH (gonadotroph-conditioned medium) also provoked a clear-cut rise in PRL release. This effect was not due to LH, FSH, or the small amounts of PRL present in the gonadotroph-conditioned medium. The LHRH antagonist [D-Phe2-D-Ala6]LHRH was capable of blocking the PRL response to LHRH but not that to the gonadotroph-conditioned medium. In the lactotroph-gonadotroph coaggregates TRH stimulated PRL release but had no effect on LH release. TRH was also ineffective in releasing LH or FSH in populations containing both gonadotrophs and thyrotrophs. The present data suggest that gonadotrophs can activate the secretory activity of the lacotrophs through the release of a paracrine humoral factor.     

D-Trp-6-luteinizing hormone-releasing hormone inhibits hyperprolactinemia in female rats

The effect of a potent agonistic LHRH analog D-Trp-6-LHRH on the hyperprolactinemia induced by haloperidol was tested in intact and ovariectomized female rats. The administration of D-Trp-6-LHRH at two dose levels (5 and 50 micrograms/day) for 20 days blocked the increase in serum PRL induced by haloperidol in intact as well as ovariectomized rats. The pituitary PRL concentration was also decreased by the administration of the analog in intact, but not ovariectomized, rats. Serum LH levels were significantly increased and the pituitary LH concentration was reduced by D-Trp-6-LHRH in intact rats. In ovariectomized rats, D-Trp-6-LHRH decreased serum as well as pituitary LH levels compared with levels in control rats. Another in vivo model to induce hyperprolactinemia consisted of grafting anterior pituitary glands under the kidney capsule in intact female rats. The administration of D-Trp-6-LHRH for 20 days (50 micrograms/day, sc) to rats bearing pituitary grafts blocked the hyperprolactinemia observed in similar animals injected with the vehicle only. Serum LH levels were increased after the administration of D-Trp-6-LHRH, whereas pituitary LH concentrations were significantly decreased in the rats treated with the analog. These results demonstrate that the LHRH agonist D-Trp-6-LHRH can counteract the hyperprolactinemic effect of haloperidol, and that this effect is not mediated by suppression of ovarian estrogens. The treatment with the analog blocked the hypersecretion of PRL by pituitary grafts, suggesting a direct effect of the analog on the pituitary gland to modulate PRL secretion.