Decrease of LRH receptors in the pituitary gland in streptozotocin-induced diabetic rats

In studies into the mechanism of anovulation in the diabetic condition, the LRH receptor content of the pituitary gland of rats with diabetes mellitus was determined. Normal female rats weighing 180-200 g were injected iv with either streptozotocin (6 mg/100 g body weight) or vehicle in dioestrus of the oestrous cycle. The rats were sacrificed by decapitation 9 days after treatment, and serum LH concentrations and the LRH content of the medial basal hypothalamus (MBH) were measured by radioimmunoassay (RIA), and the LRH receptor content of the pituitary gland was determined. The serum concentration of LH and the LRH content of the MBH in diabetic rats were 34.4 +/- 4.8 ng/ml (mean +/- SEM) and 2.05 +/- 0.04 ng/MBH, respectively, which were similar to the respective values of normal rats in dioestrus. However, the LRH receptor content of diabetic rats (13.2 +/- 3.9 fmol/pituitary) was significantly (P less than 0.05) lower than that of normal rats in dioestrus (68.0 +/- 7.1 fmol/pituitary) and pro-oestrus (59.4 +/- 13.6 fmol/pituitary). These results suggest that anovulation in diabetic rats is at least partly attributable to a low content of LRH receptors in the pituitary gland.     

Physiological role of alpha-melanocyte-stimulating hormone in modulating the secretion of prolactin and luteinizing hormone in the female rat.

Long-term ovariectomized (OVX) rats were injected in the third cerebral ventricle with 5 microliter of the globulin fraction of an antiserum raised against alpha-melanocyte-stimulating hormone (alpha-MSH) or an equal volume of the globulin fraction of normal rabbit serum (NRS). Immunoneutralization of brain alpha-MSH produced an increase in the area under the secretion curve of prolactin (Prl), the amplitude of Prl pulses, and mean plasma Prl (P less than 0.01). In animals that had received two injections of NRS or anti-MSH and were subjected to a 2-min ether stress, Prl levels significantly increased within 5 minutes in the NRS-injected rats, whereas Prl levels in the antiserum-injected rats did not increase any further from the initially high baseline levels. The administration of antibodies against alpha-MSH produced a small increase (P less than 0.05) in the area under the secretion of luteinizing hormone (LH) and mean plasma LH; however, the number of LH pulses was unaffected. We conclude that endogenous alpha-MSH of central origin is a physiological neuromodulator of release of Prl and LH in the OVX rat and is involved in the stress-induced release of Prl.     

Effect of sulpiride-induced hyperprolactinaemia on catecholamine turnover and LRH concentration in the medial basal hypothalamus of rats

In order to clarify the mechanism of hyperprolactinaemic anovulation, the medial basal hypothalamic (MBH) catecholamine (CA) turnover and LRH concentration, and the serum levels and pituitary contents of gonadotrophins and prolactin (Prl) in hyperprolactinaemic female rats were examined. Hyperprolactinaemia (HPrl) was produced by oral administration of sulpiride for 10 consecutive days; each measurement made on the sulpiride-treated rats was compared with that of control dioestrus rats. Prl, LH, FSH and LRH were determined by radioimmunoassay; CA turnover, as assessed by the accumulation of CA following monoamine oxidase inhibition, was assayed by high performance liquid chromatography with electrochemical detection. Sulpiride treatment induced (1) an increase in the serum Prl and a decrease in the serum LH, (2) an increase in the pituitary FSH and LH contents, (3) an increase in the MBH LRH concentration, and (4) an increase in the MBH dopamine (DA) turnover. These results suggest that HPrl may induce anovulation by impaired LH secretion which was caused by the suppression of LRH release due to an increase in DA turnover in the MBH.     

Effects of gonadal steroids on gonadotropin secretion in males: studies with perifused rat pituitary cells

The feedback effects of testosterone (T) and estradiol (E2) on FSH and LH secretion were compared in dispersed pituitary cells from adult male rats perifused with pulses of GnRH. Cells were stimulated with 10 nM GnRH for 2 min every 1 h. T (10 nM) pretreatment for 24 h reduced the amplitude of FSH and LH pulses to 77 +/- 4% (mean +/- SE) and 47 +/- 3% of control values, respectively (P less than 0.01), whereas 6-h T treatment was without effect. By contrast, interpulse secretion of FSH was increased after 24 h T to 184 +/- 7% of the control value (P less than 0.01), but interpulse LH release was unchanged (104 +/- 5%). E2 (0.075 nM) treatment of pituitary cells reduced GnRH-stimulated FSH and LH release within 2 h to 75 +/- 2% and 73 +/- 3% of control values, respectively (P less than 0.01). E2 pretreatment for 24 h stimulated (P less than 0.025) GnRH-induced FSH (136 +/- 10%) and LH (145 +/- 8%) release and also increased (P less than 0.01) interpulse FSH (127 +/- 5%) and LH (145 +/- 8%) secretion. These data indicate that the suppression of FSH and LH secretion by T in males is due in part to a direct effect on the pituitary. The findings that T suppresses GnRH-stimulated FSH less than LH, and that T stimulates interpulse FSH, but not LH, provide evidence for differential regulation of FSH and LH secretion by T. The dissimilar actions of T on GnRH-stimulated pulses and interpulse gonadotropin secretion suggest that interpulse secretion is unrelated to stimulation by GnRH, although its physiological significance is unknown. Since E2, in physiological levels for males, increased pituitary FSH and LH secretion, the suppression of gonadotropin secretion by E2 in vivo in males may result from an effect on the hypothalamic pulse generator; however, additional studies are needed before extending these conclusions to higher mammals and men.     

Cyclic and diurnal alterations in the response of luteinizing hormone and prolactin to prostaglandin E2 during the rat oestrous cycle

Two micrograms of prostaglandin E2 (PGE2) was infused into a lateral ventricle of the brain of female rats at 09.00 or 13.00 h on the different days of the oestrous cycle and the effect on luteinizing hormone (LH) and prolactin (Prl) release was determined. At 09.00 h PGE2 caused a pronounced release of LH in pro-oestrous, oestrous and metoestrous rats whereas the LH response in dioestrous rats was moderate. The secretion of Prl was only stimulated in rats from the pro-oestrous phase. When infused at 13.00 h PGE2 had a marked stimulatory effect on the release of LH in all groups of rats. The response was almost the same in oestrous, metoestrous and dioestrous rats but pro-oestrous rats a 2-fold higher LH response was observed. On each day of the oestrous cycle it was found that the LH-releasing activity of PGE2 was greater at 13.00 h than at 09.00 h. Thus, the overall greatest responsiveness of LH to PGE2 was noted at 13.00 h on pro-oestrus i.e. at a time which was prior to the onset of the spontaneous LH surge. At 13.00 h - as at 09.00 h - PGE2 was only capable of stimulating Prl release in pro-oestrous rats. Resembling the LH response it was found that PGE2-induced Prl release was greater at 13.00 h than at 09.00 h. In adult male rats the stimulatory effect of PGE2 on LH and Prl release was independent of the time of administration. It is concluded that the neuroendocrine elements of the hypothalamo-pituitary unit in mature female rats exhibit cyclic as well as diurnal alterations in the responsiveness to PGE2.     

Regulation and localization of estrogen and progestin receptors in the pituitary of steroid-treated monkeys

PRL increases during pregnancy in primates with rising levels of placental estradiol (E) and progesterone (P). However, while E will increase PRL secretion in monkey pituitary cell cultures, P has no effect. We recently localized progestin receptors (PR) to gonadotropes, but not lactotropes, with an immunocytochemical technique to double stain monkey pituitary cell cultures. The following studies were performed to confirm the immunocytochemical localization of PR in intact pituitary tissue and to determine the effect of E and P on the levels of estrogen receptors (ER) and PR in the pituitary. ER and PR levels were determined in the endometrium of the same animals for an internal comparison. Thirteen adult cycling female cynomolgus monkeys were ovariectomized and treated for 28 days with 1) an empty Silastic capsule (Spay), 2) a 2-cm E-filled capsule (E), or 3) a 2-cm E-filled capsule for 14 days plus a 6-cm P-filled capsule implanted for an additional 14 days (E + P). Blood samples were drawn daily for assay of serum E, P, and PRL levels. Serum PRL was not significantly affected by E, but the sequential addition of P significantly increased serum PRL levels over those observed in Spray animals. The anterior pituitary and endometrium were removed for measurement of ER and PR levels by a sucrose gradient shift assay incorporating monoclonal antibodies against ER and PR. Pituitary ER levels did not vary significantly with steroid treatment (158.2 +/- 33.6, 135.5 +/- 24.9, 104.3 +/- 13.4 fmol/mg DNA in Spay, E, and E + P animals, respectively). Pituitary PR levels were undetectable in Spay animals, were induced by E (393.3 +/- 53.4 fmol/mg DNA), and were suppressed to undetectable levels by the addition of P. A portion of the pituitary was frozen for immunocytochemical single staining for ER, PR, PRL, and LH and double staining for PRL + PR and LH + PR. ER staining was observed in many parenchymal cells, but there was no apparent change with steroid treatment. PR staining was absent in the Spay animals; many PR-positive cells were observed in E-treated females, and only a small number of faintly staining cells were detected in the E + P animals. Double staining for PRL + PR and LH + PR revealed PR in gonadotropes, but not lactotropes. In conclusion, PR, but not ER, are regulated by E and P in the monkey pituitary. Importantly, PR is regulated within gonadotropes, but not lactotropes. Therefore, P probably increases PRL secretion through a hypothalamic action.     

Enhanced hypothalamic dopaminergic inhibition of LH, TSH and GH release in patients with pathological hyperprolactinaemia

Recent studies have suggested that patients with prolactinomas have a defect in the central regulation of prolactin (Prl) release but it is not clear whether the defect results from a true loss of hypothalamic dopamine activity or from a functional inability of inherent dopaminergic inhibition to be mediated effectively. We have studied this question by the use of monoiodtyrosine (MIT, 1 g orally), a specific inhibitor of central dopamine synthesis to remove dopaminergic inhibitory control of Prl release in 10 normal ovulating women, 8 women on oral contraceptive steroids (OC) and 8 patients with pathological hyperprolactinaemia (PHP). LH, TSH and GH were also measured during the study in view of recent reports suggesting that dopaminergic mechanisms may be involved in modulating their secretion. Subjects on OC had a significantly higher (P less than 0.05) mean basal Prl (353 +/- 34 vs 280 +/- 26 mIU/l) and a significantly greater (P less than 0.05) peak response (incremental change 2270 +/- 300 mIU/l to MIT than normal controls (1320 +/- 220 mIU/l). Patients with PHP had a highly significantly blunted (P less than 0.001). Prl response (incremental chane 290 +/- 95 mIU/l) compared to controls. MIT administration caused a significant increase in LH (P less than 0.05), TSH (P less than 0.01) and GH (P less than 0.01) in patients with PHP but not in normal or OC-treated subjects. The augmented Prl response of subjects on OC is consistent with an increase in dopaminergic inhibitory control of Prl release. The lack of Prl response in subjects with PHP is indicative of a functional loss of dopaminergic control.(ABSTRACT TRUNCATED AT 250 WORDS)     

Epidermal growth factor stimulates secretion of rat pituitary luteinizing hormone in vitro

The effects of epidermal growth factor (EGF) on pituitary luteinizing hormone (LH) release and on the releases induced by oestradiol (E2) and LH-releasing hormone (LRH) were examined in a sequential double chamber perifusion system. In this system the mediobasal hypothalami (MBH) and/or pituitaries excised from normally cycling female rats in dioestrus were perifused with test media. Perifusion with EGF at 1 ng/ml for 30 min induced significant release (80-100% increase, P less than 0.05) of LH from hypothalamo-pituitary pairs, but not from the pituitary alone. Perifusion of the pituitary alone with medium containing 1 ng/ml EGF, resulted in significant release of LH (70-140% increase, P less than 0.05) after administration of 10(-7) M E2, but did not significantly influence LH release in response to 20 ng/ml LRH. These findings suggest that EGF may be involved in the regulation of pituitary gonadotrophin secretion by a direct effect on the hypothalamus and indirectly by increasing the pituitary responsiveness to E2.     

GABA-ergic and dopaminergic mechanisms in gonadotrophin secretion in males--effects of baclofen and metoclopramide

The GABA analogue, baclofen, and the dopamine antagonist, metoclopramide, were studied with respect to their effects on basal and LRH-induced LH and FSH release in 6 normal male volunteers. Basal gonadotrophin secretion was unchanged following the administration of baclofen or metoclopramide given alone or in combination. LRH-stimulated LH release was significantly blunted after metoclopramide administration in the baclofen pre-treated volunteers. Serum LH concentration (mean +/- SD) in the control phase was 30.1 +/- 17.2 mIU/ml and was 19.4 +/- 9.6 mIU/ml after baclofen plus metoclopramide (P less than 0.02). LRH-stimulated values, however, were unaffected by baclofen or metoclopramide when the drugs were given alone. LRH-stimulated FSH release was not significantly influenced by baclofen or metoclopramide given alone or in combination. Basal Prl secretion increased significantly when baclofen and metoclopramide were given separately and in combination. Basal Prl concentration (mean +/- SD) increased from 14 +/- 2 ng/ml to 18.7 +/- 4.8 ng/ml after baclofen and to 111.5 +/- 31.9 ng/ml after metoclopramide (P less than 0.01). The rise in serum Prl concentration, however, was not significantly different when measured after metoclopramide alone (111 +/- 31.9 ng/ml) or after metoclopramide and baclofen (112 +/- 33.3 ng/ml). It is proposed that GABA and dopamine exert opposing effects on LH secretion in normal men.     

SECRETORY DYNAMICS OF OESTRADIOL (E2) AND PROGESTERONE (P4) DURING PERIODS OF RELATIVE PITUITARY LH QUIESCENCE IN THE MIDLUTEAL PHASE OF THE MENSTRUAL CYCLE

Although the temporal relationship between pulsatile pituitary luteinizing hormone (LH) secretion and steroid hormone release from the corpus luteum has been investigated, the secretory profiles of oestradiol (E2) and progesterone (P4) during periods without any discernible LH pulsatile activity remain unknown. Consequently, blood was sampled at 15-min intervals for 24 h from 16 women during the midluteal phases (6-8 days after midcycle LH surge) of their cycles. LH was measured in all samples and analysed for significant pulses by the Cluster pulse algorithm. Nine studies showing the lowest LH pulse frequencies and large LH pulse amplitudes were also assessed for E2 and P4 in all samples. All three hormones were released in pulsatile fashions. Pulses of E2 and P4 were found to be synchronous. While the release frequencies for E2 (mean +/- SEM: 8.9 +/- 0.7 pulses/24 h) and P4 (8.5 +/- 0.7 pulses/24 h) were comparable, the LH pulse frequency (4.6 +/- 0.4 pulses/24 h) was found to be significantly (P less than 0.001) lower than the ovarian steroid pulse frequencies. Maximum (P less than 0.01) cross-correlation coefficients were determined at positive time lags of 28.1 +/- 7.7 min for LH/E2 and 31.7 +/- 5.8 min for LH/P4, indicating that changes in E2 or P4 levels tended to occur within approximately 30 min following LH concentration changes. Further, the degree of concomitance between a steroid pulse and an LH peak was much higher (P less than 0.001) than by chance. Maximum (P less than 0.01) cross-correlation coefficients between E2 and P4 hormonal data series were found at zero time lag, suggesting that these sex steroids were secreted simultaneously. The pulse amplitudes, pulse durations and areas under the peaks of those E2 or P4 pulses preceded by large (greater than 5 IU/l) amplitude LH pulse were significantly greater (P less than 0.05 or less for all comparisons) than for steroid pulses not associated with preceding LH pulses. Thus, two populations of steroid pulses were observed; one associated with preceding LH pulses and having greater magnitude of all pulse attributes (duration, amplitude, area under the peaks), and another, not associated with preceding LH pulses and having pulse characteristics of lower magnitude. This observation suggests that the pulsatile release of ovarian steroids is a result of the episodic modulating influence of LH and that pulsatile steroid hormone secretion pertains with smaller magnitude during periods of relative pituitary quiescence of LH pulsatility.     

Gonadotropin-releasing hormone-stimulated luteinizing hormone secretion by perifused pituitary cells from normal, gonadectomized, and testicular feminized rats

To elucidate further the manner in which gonadal steroids influence the secretion of LH, we examined the effects of gonadectomy and the absence of functional androgen receptors on GnRH-induced LH release from dispersed rat anterior pituitary cells. Intact and gonadectomized (GNX) normal rats and androgen-resistant, testicular feminized (Tfm) animals from the King x Holtzman strain (a mutant strain that possesses defective androgen receptors) were used. Dispersed pituitary cells were perifused with Medium 199 during a 4-h equilibration period and then subjected to eight 2.5-min pulses of GnRH introduced at 30-min intervals at concentrations ranging from 0.03-100 nM. Basal LH secretion by cells from intact male and female rats was indistinguishable (P = 0.79) and was substantially lower (P less than 0.0001) than that by cells from GNX male and female animals. Basal LH secretion by cells from Tfm rats was significantly higher (P less than 0.01) than that by cells from intact animals, but lower (P less than 0.005) than that by cells from GNX animals. In response to GnRH, perifused pituitary cells from animals representing all experimental groups demonstrated concentration-dependent LH release. Pituitary cells from intact female rats showed an overall greater (P less than 0.05) response to GnRH than cells from intact male rats. Pituitary cells from Tfm rats demonstrated a greater GnRH-stimulated LH mean response than cells from intact male (P less than 0.0001) or intact female (P less than 0.0001) rats. Gonadectomy of male rats resulted in an overall GnRH-stimulated LH release similar to that exhibited by cells from gonadectomized female rats (P = 0.61). Cells from Tfm animals released more LH in response to GnRH than those from gonadectomized male and female rats (P less than 0.001). These data demonstrate that the release of LH in response to GnRH by pituitary cells from intact male rats (i.e. in the presence of androgen and functional androgen receptors) is less than that seen by cells from intact females rats. Since circulating levels of testosterone and estradiol are known to be elevated in the testicular feminized rat, the heightened GnRH-stimulated LH release by cells from such animals may reflect either the long term lack of androgenic influence and/or the combined effects of androgen resistance and elevated levels of circulating estrogens.     

Effects of progesterone on the estradiol-induced follicle-stimulating hormone (FSH) surge and FSH beta messenger ribonucleic acid in the rat

This study was designed to investigate the effects of progesterone on the estradiol (E2)-induced FSH surge and FSH beta messenger RNA (mRNA) using immature rat models developed previously to demonstrate inhibition or facilitation of the LH surge by progesterone. Twenty-eight day-old rats that received E2 implants at 0900 h had FSH surges about 1700 h on day 29 (32 h). In rats treated with E2 alone, serum FSH was 15.1 +/- 1.6 ng/ml at this time, while in those animals treated concurrently with E2 and progesterone, serum FSH was significantly suppressed (8.3 +/- 0.7 ng/ml, P less than 0.001). For demonstration of progesterone facilitation, rats were primed for 24 h with E2 before progesterone treatment. This led to premature and enhanced FSH secretion: at 1400 h on day 29 serum FSH was 45.5 +/- 2.7 ng/ml compared to 6.4 +/- 0.5 ng/ml in rats treated with E2 alone. To examine the effects of these dual actions of progesterone on FSH synthesis, steady state concentrations of FSH beta mRNA were measured by Northern analysis. FSH beta mRNA generally increased in parallel with FSH release. Levels of this mRNA were about 1.5-fold higher in rats undergoing E2-induced FSH surges than in rats in which the surge was blocked by progesterone. Also, at the onset of the progesterone-facilitated FSH surge, FSH beta mRNA was about 5-fold higher in animals treated with E2 and progesterone than in those treated with E2 only. On the morning after the FSH surge (48 h after E2 treatment) FSH beta mRNA was low to undetectable. In contrast, levels of FSH beta mRNA were 7- to 8-fold higher at this time in rats in which the surge was blocked by progesterone. Serum inhibin concentrations were significantly elevated (P less than 0.05) in animals treated with E2 alone for 32 h (3077 +/- 260 fmol/ml) or 48 h (2344 +/- 148 fmol/ml) compared to those treated with E2 and progesterone in the inhibition paradigm (2469 +/- 106, 1896 +/- 114 fmol/ml, respectively). After 32 h of E2 treatment in the facilitation paradigm, serum inhibin was comparable (P greater than 0.2) in rats treated for 8 h with blank implants (2592 +/- 168 fmol/ml) and those treated for 8 h with progesterone (2720 +/- 188 fmol/ml).(ABSTRACT TRUNCATED AT 400 WORDS)     

The effect of neurotensin on pituitary secretion of thyrotrophin and prolactin in vitro

The effects of neurotensin on thyrotrophin (TSH) and prolactin (Prl) release were studied in two in vitro systems - anterior pituitary cells in culture and perifused anterior pituitary fragments. Neurotensin significantly reduced basal secretion of both TSH and Prl (P less than 0.001) from cultured pituitary cells, and abolished thyrotrophin releasing hormone (TRH)-stimulated TSH thyrotrophin releasing hormone (TRH)-stimulated TSH TSH and Prl release (P less than 0.02) from perifused pituitary fragments. These data indicate that neurotensin has a direct inhibitory effect on TSH and Prl secretion by the anterior pituitary.     

Clomiphene citrate induces pituitary GnRH receptors in ovariectomized rats: its possible role in induction of ovulation

Since our previous studies have shown that clomiphene citrate (clomiphene) acts directly on the pituitary gland and exerts a facilitatory role on oestradiol-17 beta (E2)-induced LH surge in chronically ovariectomized rats, the effect of clomiphene on pituitary GnRH receptors was investigated. A single ip injection of either 5 micrograms E2 or 200 micrograms clomiphene did not induce LH release in adult rats ovariectomized 1-2 weeks before the injection. However, a significant increase in serum LH was noted 24 h after a single injection of E2 in the ovariectomized rats, if clomiphene was pre-injected 48 h before the E2 injection. The content of pituitary GnRH receptors in the ovariectomized rats (62 +/- 9 fmol/pituitary) remained almost unchanged until 24 h after a single injection of clomiphene but significantly increased 48 h after the injection (105 +/- 13 fmol/pituitary) without any alterations in the affinity for GnRH. To determine steroid specificity for the increase in pituitary GnRH receptors, other classes of steroids were injected in the ovariectomized rats. A single dose of E2 increased GnRH receptors, but either progesterone or 5 alpha-dihydrotestosterone failed to show any effect on the level of GnRH receptors. These results suggest that clomiphene may augment oestrogen-induced pre-ovulatory LH surge in anovulatory women, at least in part by increasing the number of pituitary GnRH receptors.     

Ca+2 dependence of gonadotropin-releasing hormone-stimulated luteinizing hormone secretion: in vitro studies using continuously perifused dispersed rat anterior pituitary cells

A continuously perifused dispersed rat anterior pituitary cell system was used to determine the importance of calcium (Ca+2) on the release of LH by GnRH. In response to continuous exposure to 10 nM GnRH, LH was released in a biphasic fashion; arbitrarily, phase I was defined as being the LH released during the initial 40 min and phase II as the subsequent release. Withdrawal of Ca+2 from the perifusion medium during phases I or II of LH release led to a rapid inhibition of the LH secretion. Cells were exposed to GnRH for 2.5 min, washed with medium for 30 min, and then reexposed to GnRH for 30 min. This sequence was repeated 1 h later under identical conditions in the presence of a Ca+2 blocking agent; D600 (20 or 100 microM). D600 inhibited both the 2.5- and the 30-min GnRH-stimulated LH release. The results were expressed as the ratio obtained by dividing the total LH released during the second GnRH exposure (either 2.5 or 30 min) by the total LH released during the respective initial GnRH exposure of same duration. For the cells perifused with 20 microM D600 the ratios +/- SE (D600 vs. control) were 0.48 +/- 0.06 vs. 1.28 +/- 0.13 (P = 0.0001) and 0.29 +/- 0.05 vs. 1.01 +/- 0.08 (P less than or equal to 0.0001) for the 2.5- and 30-min exposures, respectively. For the cells perifused with 100 microM D600 the ratios +/- SE (D600 vs. control) were 0.18 +/- 0.05 vs. 1.28 +/- 0.13 (P less than or equal to 0.00001) and 0.12 +/- 0.03 vs. 1.01 +/- 0.08 (P = 0.002) for the 2.5- and 30-min exposures, respectively, revealing an even more profound inhibitory effect of D600 on GnRH stimulated LH secretion. Our data both confirm previous reports that Ca+2 is involved in LH release and demonstrate that Ca+2 is an essential requirement during both phases of GnRH-stimulated LH release in perifused dispersed rat anterior pituitary cells.     

Regulation of lutenizing hormone secretion and subunit messenger ribonucleic acid expression by gonadal steroids in perifused pituitary cells from male monkeys and rats.

The mechanisms by which gonadal steroids regulate gonadotropin secretion remain incompletely understood. As previous studies suggest that the pituitary actions of testosterone (T) and estradiol (E) differ in male primates and rodents, we compared the effects of 10 nM T, 0.1 nM E, and 10 nM dihydrotestosterone (DHT) on the LH response to hourly pulses of GnRH as well as the GnRH receptor (GnRH-R) and LH subunit messenger RNA (mRNA) levels in dispersed pituitary cells from intact male monkeys and rats. T suppressed (P < 0.01) and E increased (P < 0.05) GnRH-stimulated LH secretion by rat pituitary cells. With monkey pituitary cells, on the other hand, there was no significant effect of either T or DHT on GnRH-stimulated LH secretion. In E-treated monkey cells, a period of initial enhancement (P < 0.05) was followed by significant suppression (P < 0.05) of LH secretion. GnRH-R mRNA was unchanged by T or E in either rat or monkey cells. T suppressed LHbeta (P < 0.01) and alpha-subunit (P < 0.01) mRNAs, whereas E increased alpha-subunit (P < 0.01), but did not alter LHbeta mRNA levels in rat cells. In monkey cells, however, neither T nor E affected LHbeta or alpha-subunit mRNA levels significantly. Our results identify different regulatory mechanisms by which testicular steroid hormones control LH secretion by the pituitary in male primates and rodents. We propose that the primary site of androgen negative feedback in the male primate is to restrain GnRH pulsatile secretion, whereas in the male rat T also decreases gonadotropin synthesis and secretion by directly affecting the pituitary. E suppresses GnRH-stimulated LH secretion in the primate pituitary, but amplifies the action of GnRH in the rat. Our data also reveal that the action of T to suppress LH secretion and subunit mRNA in male rats is not through decreased GnRH-R gene expression.     

Inhibition by prolactin of post-castration rise in LH.

Castration of male and female rats resulted in a marked rise in serum LH. The rise in serum LH was partially or completely prevented by injection of prolactin (Prl), by implantation of a small amount of Prl in the median eminence (ME), by grafting 2 anterior pituitaries (APs) underneath the kidney capsule, or by transplantation of a Prl-secreting pituitary tumor underneath the skin. The larger pituitary tumor transplants secreted more Prl and were more effective in reducing LH release than the smaller tumors which secreted less Prl. Suppression of LH release generally was greater during the earlier than in the later phases of the different treatments. The pituitary LH response to synthetic LH-RH was the same in ovariectomized rats with or without pituitary grafts, and the decrease in hypothalamic LH-RH after orchidectomy was prevented by pituitary grafts. These results indicate that Prl can depress LH release after castration and that these effects are mediated via the hypothalamus.     

Suppressive effect of prolactin on oestrogen-induced secretion of LH by sequentially perifused rat hypothalamus-pituitary

The effect of prolactin (Prl) on oestrogen-induced gonadotrophin secretion was examined in vitro in a sequential double chamber perfusion system. As control groups, mediobasal hypothalamus (MBH)-pituitary pairs or pituitaries without the MBHs were perifused with Medium 199. As an experimental group, MBH-pituitary pairs were perifused with Medium 199 containing 1 micrograms/ml of rat Prl. These groups were stimulated with 10(-7) M oestradiol-17 beta (E2) for 30 min, and luteinizing hormone (LH) in the serial fractions of effluent was measured. In the control group of MBH-pituitary pairs perifused with medium without Prl, secretion of LH began to rise within 30 min after the beginning of stimulation, reached a peak 30 min after the end of stimulation and then remained at a plateau for the rest of the experimental period, whereas in the control group of pituitaries alone no significant response was observed. In the experimental group perifused with medium containing Prl, LH-secretion showed peaks 20 and 80 min after the end of E2-stimulation, respectively, and the first peak was significantly (P less than 0.01) less than the level in the control group. These data demonstrate that Prl at this concentration suppressed the rapid LH release induced by E2. Its site of action is suggested to be at the hypothalamic level, and its possible mechanism of action is discussed.     

Possible altered dopaminergic modulation of pituitary function in normal-menstruating women with insulin dependent diabetes mellitus (IDDM)

In order to assess whether a possible altered dopamine activity in normal-menstruating diabetic patients may influence the pituitary hormone secretion we have measured the basal serum concentrations of Prl, LH and FSH in 28 patients with insulin dependent diabetes mellitus (IDDM) and in 55 normal-menstruating women at day 3 to 6 of the menstrual cycle. In addition basal levels of oestradiol-17 beta, TSH, thyroxine (T4), triiodothyronine (T3) and resin-T3 uptake (RT3U) were determined in 17 patients with IDDM and in 17 controls. The responses of FSH, LH, Prl, GH and TSH to metoclopramide (MTC) administration (10 mg iv) were studied in 17 patients and 17 controls. In 10 patients with IDDM and 8 controls the short-term variations in pituitary hormones and blood glucose concentration were evaluated. Patients with IDDM had significantly lower basal levels of Prl (P less than 0.01) and TSH (P less than 0.05) and significantly (P less than 0.05) higher basal levels of GH than normal women. No significant (P greater than 0.05) differences were found regarding basal serum concentrations of FSH, LH, oestradiol, T4, T3 and RT3U. During the 3 h period the mean coefficient of variation of Prl, FSH, LH and GH was not significantly (P greater than 0.05) different between diabetic patients and controls. Both groups responded significantly (P less than 0.01) in Prl and TSH to MTC but the TSH response was significantly (P less than 0.05) lower in patients with IDDM. The Prl response to MTC was not significantly (P greater than 0.05) different within the two groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

DOPAMINERGIC CONTROL OF GONADOTROPHIN SECRETION IN NORMAL WOMEN AND IN PATIENTS WITH PATHOLOGICAL HYPERPROLACTINAEMIA

The role of dopaminergic mechanisms in the control of gonadotrophin secretion in normal and hyperprolactinaemic subjects is controversial. Whilst bromocriptine, a potent dopamine agonist, has been used to restore normal gonadotrophin secretion in subjects with pathological hyperprolactinaemia (PHP), dopamine and dopamine agonists have been reported to suppress basal and stimulated gonadotrophin release. We therefore investigated the importance of dopaminergic control of gonadotrophin secretion by studying LH, FSH and PRL responses in normal and PHP subjects to central dopamine synthesis inhibition using monoiodotyrosine (MIT) and to a 4 h dopamine infusion designed to elevate peripheral plasma dopamine concentration to levels reported for pituitary portal plasma (1-6 ng/ml). MIT administration resulted in a significant release of PRL (peak increment 520 +/- 84% above basal) but not of LH or FSH in normal subjects. In PHP subjects there was a blunted PRL response (peak 13.3 +/- 3.5%) to MIT administration and significant LH (P less than 0.05) but not FSH release. Dopamine infusion (0.5 microgram/kg/min) resulted in suppression of PRL (min 19 +/- 3% of basal) but not of LH or FSH. A rebound of PRL (peak 188 +/- 68% of basal) but not LH or FSH occurred on cessation of dopamine. There was an apparent rise in LH (P less than 0.02 vs. normals) but not FSH in PHP patients during dopamine infusion. Plateau dopamine levels achieved during the infusion were 2.9 +/- 0.3 ng/ml and 5.9 +/- 0.8 ng/ml in normal and PHP subjects respectively. The responses to MIT show that dopamine functions as an inhibitor of PRL but not of LH or FSH in normal subjects. In PHP patients the responses suggest increased dopaminergic inhibition of LH release but loss of inhibitory control of PRL release. Physiological concentrations of plasma dopamine do not significantly inhibit LH or FSH release in normal subjects but paradoxically results in an apparent release of LH in PHP patients. We conclude that dopamine mechanisms do not play a significant role in modulating gonadotrophin release in normal subjects. In PHP patients, PRL feedback results in increased hypothalamic dopamine activity which in turn inhibits LH release. We conclude that the inhibitory action of dopamine on PRL release restores LH secretion by removing central dopaminergic inhibition through hypothalamic feedback of PRL.