Opioid modulation of FSH, growth hormone and prolactin secretion in the prepuberal gilt.

The role of endogenous opioid peptides (EOP) in modulating GH, prolactin (PRL) and FSH secretion was evaluated in prepuberal (P) gilts. In experiment I, P gilts received 1 (n = 2), 3 (n = 3) or 6 (n = 3) mg naloxone (NAL)/kg body weight i.v. Blood was collected every 15 min for 2 h prior to and 2 h after NAL and an additional 1 h after 100 micrograms gonadotrophin-releasing hormone (GnRH) i.v. In experiment II, P and mature (M) gilts were ovariectomized. Three weeks after ovariectomy, P and M gilts were injected twice a day for 10 days with either 0.85 mg progesterone (P4)/kg body weight or oil vehicle (V), resulting in the following groups: PP4 (n = 11), PV (n = 10), MP4 (n = 11) and MV (n = 10). All gilts received 1 mg NAL/kg body weight on the last day of treatment. Blood samples were collected every 15 min for 4 h before and 2 h after NAL and an additional 1 h after 100 micrograms GnRH i.v. In experiment III, six P and five M gilts were ovariectomized and surgically implanted with intracerebroventricular (i.c.v.) cannulae. Blood was collected every 15 min for 3 h before and 3 h after i.c.v. injection of 500 micrograms morphine in artificial cerebrospinal fluid (CSF) or 250 microliters CSF. In experiment I, all doses of NAL failed to alter PRL secretion, while NAL increased (P less than 0.05) GH secretion in three out of eight gilts. However, NAL suppressed (P less than 0.05) FSH concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)     

EFFECT OF FENOLDOPAM, A DOPAMINE D-1 RECEPTOR AGONIST, ON PITUITARY, GONADAL AND THYROID HORMONE SECRETION

The influence of fenoldopam, a dopamine (DA) D-1 receptor agonist, on basal and GnRH/TRH stimulated PRL, GH, LH, TSH, testosterone and thyroid hormone secretion was studied in nine normal men. All men received 4-h infusions of either 0.9% saline or fenoldopam at an infusion rate of 0.5 microgram/kg min, 12-16 ml/h, adjusted according to weight. After 3 h of infusion, 50 micrograms GnRH and 100 micrograms TRH was given i.v. Blood samples were collected every 15 min from 1 h before to 1 h after the infusion for a total of 6 h for measurements of PRL, LH, FSH, GH, TSH, testosterone, T4 and T3. The median PRL concentration increased significantly (P less than 0.01) to 128%, range 87-287, of preinfusion levels, compared to the decline during control infusion (85%, 78-114). Basal TSH levels declined significantly to 71% (60-91) during fenoldopam compared with 82% (65-115) during control infusion (P less than 0.05). Basal LH, FSH, GH and thyroid hormones were similar during fenoldopam and control infusions (P greater than 0.05). The LH response to GnRH/TRH was significantly (P less than 0.02) increased by fenoldopam infusion. Basal and stimulated testosterone concentration was lower during fenoldopam (P less than 0.01) infusion compared with control. Other hormones were similar after GnRH/TRH stimulation during fenoldopam and saline infusions. These results suggest that DA D-1 receptors are involved in the modulation of pituitary hormone secretion. We suggest that the effect of fenoldopam on PRL and TSH is mainly at the hypothalamic level. Regarding the effect on LH concentrations, an additional direct effect of fenoldopam on testosterone regulation can not be excluded.     

Evidence for temporal coupling of growth hormone, prolactin, LH and FSH pulsatility overnight during normal puberty

The patterns of secretion of GH, LH, FSH and prolactin were determined over a single night (20.00-08.00 h; 15-min sampling) in 34 normal subjects (17 male, 17 female, aged 9.1-20.9 years). Plasma GH was measured by an immunoradiometric assay and LH, FSH and prolactin by radioimmunoassay in all samples. Data were analysed by Fourier transformation and cross-correlation after stationarization. The highest mean GH levels were noted in girls at Tanner stage 2/3 and in boys at stages 4/5. Prolactin levels were highest in girls at stage 4/5 and in boys at stage 2/3. LH and FSH showed a progressive rise by puberty stage in both sexes. The dominant pulse periodicities of GH and prolactin were 150-180 min in girls and 180 min in boys. LH and FSH pulse periodicity was around 90 min in early puberty and 180 min in later puberty in both sexes. LH and prolactin pulses showed a phase relationship with GH with a lag of 30-75 min (r = 0.32; P less than 0.001) and 30 min (r = 0.47; P less than 0.0001) respectively. Generally, LH and prolactin pulses were in phase (r = 0.42; P less than 0.0001) and there was a highly significant correlation (r = 0.64; P less than 0.0001) between FSH and LH pulsatility. Whereas mean overnight concentrations and pulse periodicity of the principal pituitary hormones varied between the sexes during early puberty, by the end of puberty a dominant pulse periodicity of around 150-180 min was established and there was remarkable temporal coupling of pulsatility.     

Effects of bromocriptine and naloxone on plasma levels of prolactin, LH and FSH during suckling in the female rat: responses to gonadotrophin releasing hormone

The roles of dopamine and the endogenous opiate peptides in the mediation of the inverse relationship between prolactin and gonadotrophin secretion during lactation were studied by comparing the effects of bromocriptine and naloxone on plasma levels of prolactin, LH and FSH during suckling in the female rat. The effects of exogenous gonadotrophin releasing hormone (GnRH) on the LH and FSH responses to bromocriptine and naloxone were also assessed. In control animals (saline), there was a marked fall in LH levels and a large increase in prolactin levels within 15 min of suckling. In response to GnRH (25 ng) there was a small progressive increase in LH levels reaching a maximum at 45 min. Both bromocriptine (500 micrograms) and naloxone (500 micrograms) markedly suppressed the suckling-induced prolactin surge when administered in two separate groups of animals. However, despite the bromocriptine-induced suppression of prolactin levels, there was no increase in LH levels which remained low throughout the suckling period. Naloxone (500 micrograms), however, induced a twofold increase in LH levels within 15 min suggesting that an enhanced opiate rather than dopaminergic activity may be responsible for the suppression of GnRH and hence gonadotrophin secretion during suckling. This is supported by the finding that whereas combined bromocriptine (500 micrograms) and GnRH (25 ng) treatment suppressed the suckling-induced prolactin rise and also induced only a small progressive increase in LH (similar to GnRH alone), combined naloxone (500 micrograms) and GnRH (25 ng) treatment induced a sharp sixfold increase in LH levels within 15 min while at the same time markedly suppressing prolactin levels. None of these drug treatments affected the levels of FSH.(ABSTRACT TRUNCATED AT 250 WORDS)     

Acylated ghrelin inhibits spontaneous luteinizing hormone pulsatility and responsiveness to naloxone but not that to gonadotropin-releasing hormone in young men: evidence for a central inhibitory action of ghrelin on the gonadal axis

Context: Recent evidence suggests that ghrelin exerts a negative modulation on the gonadal axis. Ghrelin was reported to suppress LH secretion in both animal and human models. Moreover, acylated ghrelin (AG) also decreases the LH responsiveness to GnRH in vitro. Objective: The objective of the study was to evaluate the effects of AG infusion on spontaneous and stimulated gonadotropin secretion. Design, Participants, and Intervention: In seven young healthy male volunteers (age mean +/- sem 26.4 +/- 2.6 yr), we evaluated LH and FSH levels every 15 min during: 1) iv isotonic saline infusion; 2) iv saline followed by AG; LH and FSH response to GnRH (100 mug iv as a bolus), 3) alone and 4) during AG infusion; LH and FSH response to naloxone (0.1 mg/kg iv as a slow bolus), 5) alone and 6) during AG infusion. Results: Significant LH but not FSH pulses were recorded in all subjects under saline infusion. AG infusion inhibited LH levels [area under the curve((240-480)): 415.8 +/- 69.7 mIU/ml.min during AG vs. 744.6 +/- 120.0 mIU/ml.min during saline, P < 0.02] and abolished LH pulsatility. No change in FSH secretion was recorded. The LH and FSH responses to GnRH during saline were not affected by AG administration. However, AG inhibited the LH response to naloxone [area under the curve ((120-210)): 229.9 +/- 39.3 mIU/ml.min during AG vs. 401.1 +/- 44.6 mIU/ml.min during saline, P < 0.01]. FSH levels were not modified by naloxone alone or in combination with AG. Conclusions: AG inhibits both spontaneous LH pulsatility and the LH response to naloxone. Because AG does not affect the LH response to GnRH, these findings indicate that the ghrelin system mediates central inhibition of the gonadal axis.     

Effects of naloxone and morphine on the proestrous surge of prolactin and gonadotropins in the rat

The effects of naloxone hydrochloride and morphine sulfate on the proestrous surge of PRL and gonadotropins (LH and FSH) were investigated in normal cycling Sprague-Dawley rats. Blood samples (0.45-0.50 ml) were withdrawn without anesthesia every 20 min from 1400-2000 h through an atrial cannula implanted the same morning. RIA revealed that a single iv injection of naloxone (0.2 mg/kg) at 1400 h completely suppressed the surge of PRL, and this was reversed by a concomitant injection of morphine (10 mg/kg). Morphine itself did not alter the peak of the PRL surge. Morphine suppressed only the early phase of the LH surge, and this was reversed by naloxone. Naloxone alone did not change the peak of the LH surge but maintained higher levels than controls during the declining phase. The FSH surge was not altered by either morphine or naloxone. These results suggest that endogenous opioid peptides may have a role in regulating the PRL and LH surges during proestrus in the rat.     

Naloxone increases the frequency of pulsatile luteinizing hormone secretion in women with hyperprolactinemia

The ability to change the frequency and amplitude of pulsatile GnRH secretion may be an important mechanism in maintaining regular ovulatory cycles. Hyperprolactinemia is associated with anovulation and slow frequency LH (GnRH) secretion in women. To assess whether the slow frequency of LH (GnRH) secretion is due to increased opioid activity, we examined the effect of naloxone infusions in eight amenorrheic hyperprolactinemic women (mean +/- SE, serum PRL, 160 +/- 59 micrograms/L). After a baseline period, either saline or naloxone was infused for 8 h on separate days, and LH was measured in blood obtained at 15-min intervals. Additional samples were obtained for plasma FSH, PRL, estradiol, and progesterone. Responses to exogenous GnRH were assessed at the end of the infusions. LH pulse frequency increased in all subjects from a mean of 4.0 +/- 0.5 pulses/10 h (mean +/- SE) during saline infusion to 8.0 +/- 1.0 pulses/10 h during naloxone infusion (P less than 0.01). LH pulse amplitude did not change, and mean plasma LH increased from 7.4 +/- 0.8 IU/L (+/- SE) to 11.2 +/- 1.5 IU/L during naloxone (P less than 0.01). A small but significant increase was seen in mean plasma FSH. Plasma PRL, estradiol, and progesterone were unchanged by naloxone infusion. These data suggest that elevated serum PRL reduces the frequency of LH (GnRH) secretion by increasing hypothalamic opioid activity and suggest that the anovulation in hyperprolactinemia is consequent upon persistent slow frequency LH (GnRH) secretion.     

Differential effects of opiate peptides and alkaloids on anterior pituitary hormone secretion

In order to investigate the opiate receptors involved in the control of anterior pituitary hormone secretion, five different opioid drugs were administered intravenously to groups of 6 normal male subjects. Morphine (10 mg), methadone (10 mg), pentazocine (30 mg), nalorphine (10 mg) and 0.25 mg of the met-enkephalin analogue, DAMME, all caused similar increases in circulating prolactin with falls in serum LH and cortisol. Methadone and DAMME also elevated GH and TSH; morphine elevated TSH but not GH, nalorphine GH but not TSH. After pentazocine neither GH nor TSH changed. FSH failed to change significantly after any drug. All these changes, except serum cortisol, were antagonised by 4 mg naloxone. Taking into account the known receptor subtypes preferentially activated by each opiate, it is suggested that prolactin secretion is modulated by epsilon-receptors and TSH by mu-receptors. The control of ACTH probably involves delta-or kappa-receptors, that for LH kappa-or epsilon-receptors. It is not possible on present data to allocate a specific receptor mediating the opioid control of GH.     

Stimulatory and inhibitory effects of the opioids on gonadotropin secretion

In order to gain additional information on the role played by the opioids in the control of the secretion of anterior pituitary gonadotropins, morphine (an opioid agonist) and naloxone (an opioid antagonist) have been injected intraventricularly (i.v.t.) into normal or castrated male rats. The animals were killed by decapitation at different time intervals after treatment and serum luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin were measured by radioimmunoassay. Animals injected i.v.t. with 0.9% saline solution and sacrificed at the same time intervals served as controls. When morphine (at the dose of 200 and 400 micrograms/rat) and naloxone (at the dose of 7.5 and 15 micrograms/rat) were injected i.v.t. into normal male rats, a significant increase of serum levels of LH was observed 10 and 20 min after injection. There was no effect at 5, 40 and 60 min. Lower doses of morphine (6.25, 12.5, 25, 50 and 100 micrograms/rat) given i.v.t. were ineffective. When morphine (200 micrograms/rat) and naloxone (either in the dose of 7.5 micrograms/rat or of 15 micrograms/rat) were given simultaneously, serum LH was significantly higher than in the saline-treated controls both at 10 and 20 min. However, the increases of serum LH levels induced by the combined treatment were in both instances lower than those produced by the administration of either drug alone. Morphine (200 micrograms/rat) when administered i.v.t. to normal male rats significantly enhanced prolactin release at 10 and 20 min, and this effect of morphine was blunted by the concomitant i.v.t. administration of naloxone (7.5 and 15 micrograms/rat).(ABSTRACT TRUNCATED AT 250 WORDS)     

Action kinetics of inhibin in superfused pituitary cells depend on gonadotropin-releasing hormone treatment

We examined the effects of partly purified inhibin from porcine follicular fluid on FSH and LH release in superfused rat pituitary cell cultures exposed to different GnRH stimuli. Pituitary cells from immature male rats were cultured in chemically defined medium. After 4 days of static culture in the absence of inhibin preparation and GnRH, the cell monolayers were superfused for approximately 10 h at a constant speed (0.15 or 0.25 ml/min) with medium with or without inhibin preparation (1 micrograms/ml). During the superfusion, some cultures were stimulated with GnRH (10 nM) continuously or intermittently (1 min/0.5 h or 6 min/1 h). In the basal condition (no GnRH), inhibin suppressed FSH release after 5 h of exposure (P less than 0.01), whereas LH secretion was not affected. In cultures treated with GnRH pulses (of either frequency), the inhibitory effects on the GnRH-stimulated FSH and LH release were statistically significant (P less than 0.01) after 2 h of exposure, became more pronounced in the next several hours, then remained stable until the end of the experiment. In cultures exposed to GnRH continuously, the suppressing effects of inhibin preparation became significant (P less than 0.01) after 3 h of exposure and were maximal at 4 h (52% and 61% of control values for FSH and LH, respectively). Later, the suppressing effect became less pronounce due to the decreasing rate of gonadotropin secretion in control (no inhibin) cultures exposed continuously to GnRH. The magnitude of FSH and LH suppression after 9 h of exposure to the inhibin preparation was statistically different (P less than 0.05) for different GnRH treatments and was more pronounced with GnRH pulses (24-27% and 54-57% of control values for FSH and LH, respectively) than with cultures exposed to GnRH continuously (77% and 89% of control values for FSH and LH, respectively) or in the absence of GnRH (50% and 92% of control values for FSH and LH, respectively). We conclude that both the kinetics and magnitude of action of the inhibin preparation on FSH and LH release can differ significantly depending on the presence or absence of GnRH as well as on the mode of GnRH stimulation. Of particular importance is the observation that suppressive effects of inhibin preparation decline in cultures that have been desensitized to GnRH after prolonged continuous GnRH exposures. These differences stress the role of GnRH-inhibin interactions in the regulation of gonadotropin secretion and emphasize the importance of the mode of GnRH stimulation in studies concerning inhibin action on pituitary cells in vitro.     

Effect of short-term starvation on reproductive hormone gene expression, secretion and receptor levels in male rats

The effects of 4-6 days of food deprivation on the pituitary-testicular function of adult male rats were studied. Fasting decreased body weights on average by 23% (P less than 0.01) and those of seminal vesicles by 55% (P less than 0.01) in 4 days. No consistent changes were found in testicular and ventral prostate weights. The pituitary levels of gonadotrophin-releasing hormone (GnRH) receptors decreased by 50% (P less than 0.01). Serum and pituitary levels of LH, FSH and prolactin decreased by 25-50% (P less than 0.01 for all). Testicular and serum levels of testosterone decreased by 70-80%, testicular LH receptors by 26%, those of prolactin by 50% (P less than 0.01 for all), but those of FSH remained unaffected. Acute (2 h) stimulation by a GnRH agonist (buserelin, 10 micrograms/kg i.m.) resulted in similar LH, FSH and testosterone responses in the fasted and control animals, and human chorionic gonadotrophin (hCG) stimulation (30 IU/kg i.m.) in similar increases in testosterone. A 42% decrease was found in pituitary content of mRNA of the common alpha subunit (P less than 0.05), but the mRNAs of the LH- and FSH-beta chains and prolactin were unaffected by fasting for 4 days. When the same mRNAs were measured after 6 days of fasting, the decrease of the mRNA of FSH-beta also became significant (50%, P less than 0.01). In contrast, the mRNA of LH-beta was increased twofold (P less than 0.01) at this time and serum LH levels were similar in control and starved animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of morphine and naloxone on plasma prolactin concentrations in the fetal sheep and pregnant ewe during late gestation

This study investigated the effects of intrafetal morphine or naloxone administration on fetal and maternal plasma prolactin concentrations in the sheep during late pregnancy (117-143 days gestation). After intravenous morphine (3 mg/kg) there was a significant increase (p less than 0.05) in fetal plasma prolactin concentrations which was sustained for 180 min post-injection. There was no significant effect of either gestational age (125-133 days compared to 134-143 days gestation) or repeated administration (up to 3 treatments) of morphine on the fetal prolactin response to morphine. Intrafetal administration of naloxone (3.8 mg/kg bolus + 9.9 mg/kg/60 min), blocked the fetal prolactin response to morphine. Maternal plasma concentrations of prolactin were significantly increased (p less than 0.05) at 180 min after the intrafetal morphine bolus. When naloxone alone was infused, there was no change in fetal plasma prolactin concentrations, but there was a significant fall (p less than 0.05) in maternal plasma prolactin from 25 min after the start of the naloxone infusion. Thus, acute administration of morphine is associated with fetal and maternal hyperprolactinaemia. Although the endogenous opioids do not appear to mediate basal prolactin secretion in the fetus, they may have a role in the control of prolactin release in the pregnant ewe during late gestation.     

Dopamine affects basal and augmented pituitary hormone secretion.

Although the role of the neurotransmitter, dopamine (DA), in the regulation of PRL has been well documented, controversy exists regarding its participation in the regulation of the other pituitary hormones. Consequently, we infused DA into six healthy male subjects (ages 19-32) and studied its effects on both basal pituitary hormone levels and augmented hormonal release induced by insulin hypoglycemia (ITT), TRH, and gonadotropin-releasing hormone (GnRH). DA alone produced a modest though significant increase in GH concentration from 2.2 +/- 0.5 to 11.9 +/- 3.7 ng/ml (P less than 0.05) by 60 min, but the peak incremental GH response to ITT was significantly inhibited by DA (43.5 +/- 5.0 vs. 16.3 +/- 3.3 ng/ml; P less than 0.01). PRL concentrations fell during the DA infusion (20.4 +/- 3.0 to 10.6 +/- 1.5 ng/ml; P less than 0.02) at 235 min, and the PRL responses to both ITT and TRH were completely abolished. Although the basal LH and FSH concentrations were unaffected by DA, the incremental LH response to GnRH was inhibited (45.5 +/- 10.6 to 24.4 +/- 5.4 mIU/ml; P less than 0.05), while the FSH response was unchanged. DA significantly reduced the basal TSH concentration from 3.9 +/- 0.2 to 2.5 +/- 0.2 micro U/ml (P less than 0.01) at 230 min and blunted the peak incremental TSH response to TRH (6.0 +/- 1.5 vs. 2.9 +/- 0.9 microU/ml; P less than 0.01). DA had no effect on basal cortisol levels, the cortisol response to ITT, basal plasma glucose, or the degree of hypoglycemia after ITT. Our data provide new evidence that DA has an inhibitory as well as a stimulatory role in the regulation of GH secretion in normal humans. It inhibits centrally as well as peripherally mediated PRL secretion and blunts the LH response to GnRH. In addition, DA lowers both basal and TRH-mediated TSH release, confirming the reports of other investigators.     

Hormone ontogeny in the ovine fetus. XVIII. The effect of an opioid antagonist on luteinizing hormone secretion

Endogenous opioid-like peptides influence gonadotropin release in adult animals and man; however, the role of these peptides in the regulation of fetal LH secretion is not known. We administered naloxone hydrochloride (1.3 mg/kg iv), a specific opioid receptor antagonist, to 22 chronically catheterized ovine fetuses of gestational ages 94-143 days (term = 147 days). As a control, each fetus also received the vehicle on a separate occasion, the sequence of the studies being randomized. After the administration of naloxone, LH secretion increased from 38.6 +/- 5.8 to 114 +/- 21 ng/h ml-1 (P less than 0.001); LH release was not affected by administration of the vehicle. Morphine (13 mg/kg) and naloxone (1.3 mg/kg) were administered together to three fetuses (gestational age 94-105 days); LH secretion was sharply reduced from 411 +/- 14.3 ng/h ml-1 after naloxone alone to 53 +/- 17.5 ng/h ml-1 after the administration of both naloxone and morphine (P less than 0.01). The response to naloxone varied with gestational age. Fetuses of 94-115 days showed a significantly higher increment in LH secretion when given naloxone (112.3 +/- 30.7 ng/h ml-1) than did older fetuses of gestational age 126-143 days (64.8 +/- 20.8 ng/h ml-1) (P less than 0.02). These findings indicate that, in the ovine fetus endogenous opioid-like peptides exert a tonic suppressive effect on LH secretion at least as early as 94 days gestation. Moreover, the effectiveness of naloxone in augmenting LH release decreases with advancing gestational age. This latter observation supports the concept that, in the ovine fetus, endogenous opioid tone is not the sole factor involved in the dampened fetal LH secretion which is characteristic of late gestation.     

Differential regulation of luteinizing hormone, follicle-stimulating hormone, and free alpha-subunit secretion from the gonadotrope by gonadotropin-releasing hormone (GnRH): evidence from the use of two GnRH antagonists

To examine the differential regulation of glycoprotein hormone secretion from the gonadotrope by GnRH, the Nal-Glu GnRH antagonist was administered to euthyroid women in the early follicular phase (days 1-5) of the menstrual cycle, and the results compared to previous studies with the Nal-Arg GnRH antagonist. After a 4-h period of baseline sampling at a frequency of every 10 min, a single sc dose of the GnRH antagonist was administered to each subject. Frequent sampling continued for 8 h, followed by hourly sampling for a further 16 h. LH, FSH, and free alpha-subunit were measured serially in assays with high specificity. There was a 90% concordance of LH and free alpha-subunit pulses during the baseline sampling period. Pulsatile secretion of LH and free alpha-subunit was immediately abolished at the highest dose of the Nal-Glu antagonist for at least 8 h. The maximum percent suppression of LH after administration of the Nal-Glu GnRH antagonist was 70 +/- 4%, 80 +/- 4%, and 83 +/- 1% at doses of 15, 50, and 150 micrograms/kg, respectively, compared to 51 +/- 10%, 70 +/- 5%, and 69 +/- 5% at doses of 50, 150, and 500 micrograms/kg Nal-Arg antagonist. Decreases in FSH were 28 +/- 2%, 32 +/- 7%, and 39 +/- 2%, with increasing doses of the Nal-Glu antagonist compared with 25 +/- 6%, 17 +/- 6%, and 28 +/- 4% reductions at increasing doses of the Nal-Arg antagonist. Free alpha-subunit decreased 22 +/- 4%, 23 +/- 4%, and 28 +/- 3% at increasing doses of the Nal-Glu antagonist and 12 +/- 4%, 27 +/- 4%, and 30 +/- 7% with increasing doses of the Nal-Arg antagonist. For the Nal-Glu antagonist, suppression of LH was greater than that of FSH and free alpha-subunit at all doses (P less than 0.001), while FSH suppression was greater than that of free alpha-subunit at the highest dose only (P less than 0.05). For the Nal-Arg antagonist, LH suppression was greater than that of FSH or free alpha-subunit at all doses (P greater than 0.01), and FSH suppression exceeded that of free alpha-subunit at the 50 micrograms/kg dose. Suppression of LH was greater with the Nal-Glu antagonist than with the Nal-Arg antagonist at doses of 50 and 150 micrograms/kg (P less than 0.05), and FSH suppression was greater with the Nal-Glu antagonist at 150 micrograms/kg (P less than 0.01), while the degrees of maximum suppression were similar for the two different GnRH antagonists for free alpha-subunit.(ABSTRACT TRUNCATED AT 400 WORDS)     

Evidence of differential control of FSH and LH secretion by gonadotropin-releasing hormone (GnRH) from the use of a GnRH antagonist

The differential regulation of immunoactive FSH and LH secretion by endogenous GnRH was studied using a GnRH antagonist, [Ac-D2Nal1,D4FPhe2,DTrp3,DArg6]GnRH (the NAL-ARG antagonist), in normal women in the early follicular phase of the menstrual cycle, and their responses were compared to those in two groups of control women. Pulsatile LH secretion was examined as an index of the completeness of blockade of endogenous GnRH secretion. There was a dose-dependent decrease in both the frequency and amplitude of LH pulses. At the highest dose, LH pulses were completely abolished within 20 min after sc administration of the GnRH antagonist and for a minimum of 8 h in all women. The mean plasma LH levels were reduced within the first 4 h after antagonist administration at all doses (P less than 0.001). The duration of LH suppression was influenced by antagonist dose, with a continued effect 24 h after administration of the 500 micrograms/kg dose only. The maximum degree of LH suppression was 40% after 50 micrograms/kg (n = 6), 60% after 150 micrograms/kg (n = 6), and 59% after 500 micrograms/kg (n = 5). In contrast, plasma immunoreactive FSH levels did not change after these doses of the NAL-ARG GnRH antagonist. The maximum degree of FSH suppression was 16%, and the changes in plasma FSH concentrations were not dose dependent. Serum antagonist concentrations rose within 30 min after its administration to mean peak levels of 7.5 +/- 2.1 (+/- SE), 20.4 +/- 6.1, and 151 +/- 21 ng/mL after the 50, 150, and 500 micrograms/kg doses, respectively. The half-time of the disappearance of the NAL-ARG GnRH antagonist from plasma was 8.8 +/- 1.5 h. While there were no effects of antagonist administration on hematological, hepatic, or renal function, three women developed urticaria distant from the site of injection when administered the highest dose. We conclude that blockade of GnRH receptors by a GnRH antagonist 1) effectively antagonizes the action of GnRH, as assessed by its ability to block pulsatile LH secretion and reduce mean plasma LH levels; and 2) inhibits LH release to a considerably greater degree than FSH release, providing further evidence of possible GnRH-independent FSH secretion.     

Gonadotropin-releasing hormone increases the amount of messenger ribonucleic acid for gonadotropins in ovariectomized ewes after hypothalamic-pituitary disconnection

To investigate the role of GnRH in regulating the synthesis and secretion of gonadotropins, GnRH (250 ng/6 min every other hour for 7 days) or saline was administered to ovariectomized (OVX) ewes after hypothalamic-pituitary disconnection (HPD). Blood samples were collected from all HPD ewes on the day before and the day after HPD and on days 1 and 7 of GnRH or saline. At the end of day 7, anterior pituitary glands were removed for analysis of hormone, receptor, and mRNA content. The amount of mRNA for gonadotropins was lower (P less than 0.05) in saline-treated HPD ewes than in GnRH-treated HPD or OVX ewes. Administration of GnRH restored the amount of mRNA for FSH beta and alpha-subunits to levels similar (P greater than 0.05) to those measured in OVX ewes. The amount of mRNA for LH beta was higher (P less than 0.05) in GnRH-treated HPD ewes than in saline-treated HPD ewes, but lower (P less than 0.05) than that in OVX ewes. The pituitary content of LH and FSH was lower (P less than 0.05) in saline-treated HPD ewes than in OVX ewes. Administration of GnRH to HPD ewes maintained the ewes. Administration of GnRH to HPD ewes maintained the pituitary content of LH, but not FSH, compared to the pituitary gonadotropin content in OVX ewes. There were no differences (P greater than 0.05) in the amount of mRNA for GH or PRL or the pituitary content of these hormones among treatments. The number of hypophyseal receptors for GnRH was reduced in saline-treated HPD ewes (P less than 0.05) compared to that in OVX ewes and GnRH-treated HPD ewes. The number of hypophyseal receptors for 17 beta-estradiol was lower (P less than 0.05) in GnRH- and saline-treated HPD ewes than in OVX ewes. Serum LH concentrations were lower (P less than 0.05) after HPD than before HPD, but were restored to normal (P greater than 0.05) by GnRH replacement. Serum concentrations of FSH were lower (P less than 0.05) after HPD and were not affected by GnRH replacement. Serum PRL concentrations in all ewes were higher (P less than 0.05) after HPD than before HPD. Serum GH concentrations in all ewes were similar (P greater than 0.05) before and after HPD. Since synthesis and secretion of GH and PRL were not diminished after HPD, it was considered that the pituitary gland remained viable and functioned independently of hypothalamic input in OVX ewes after HPD.(ABSTRACT TRUNCATED AT 400 WORDS)     

Opiate inhibition of growth hormone secretion in young chickens

Plasma concentrations of growth hormone (GH) were decreased following the intravenous administration of morphine sulfate. Maximum inhibition of GH secretion was observed 40 min after morphine sulfate challenge. At this time, doses of morphine sulfate (at 5 mg and 50 mg/kg) reduced the GH concentrations by 86 and 90%, respectively, in comparison with those in the vehicle-injected controls. An opiate antagonist, naloxone, had no stimulatory effect on basal GH concentrations, but attenuated the GH response to morphine. Neither morphine nor naloxone had any significant effect on circulating luteinizing hormone (LH) levels. These results indicate an inhibitory opiate pathway in the control of GH release and demonstrate effects on GH and LH secretion contrary to those observed in mammalian species.     

Secretion rates and short-term patterns of gonadotrophin-releasing hormone, FSH and LH in the normal stallion in the breeding season

Pituitary venous blood was collected by a painless nonsurgical cannulation method from five ambulatory stallions at 5-min intervals for 5-6 h during the breeding season. In four adult stallions, statistical analysis showed that pulses of gonadotrophin-releasing hormone (GnRH) and LH were coincident (P less than 0.01), as were pulses of FSH and LH (P less than 0.05). Furthermore, the patterns of changes in concentration of FSH and LH were highly correlated in each of the four stallions. However, seemingly ineffective pulses of GnRH were also observed, with 28% of GnRH pulses failing to induce a significant gonadotrophin pulse. In the four adult stallions the amplitude of pituitary venous gonadotrophin pulses varied markedly but no correlation with GnRH pulse amplitude was observed. Peak secretion of FSH, but not LH, during pulses was correlated with the length of the interpulse interval. Consequently, the ratio of FSH to LH during peaks was least (P less than 0.02) when the interpulse interval was 30 min or less. Thus, differential FSH and LH secretion was achieved within a constant steroid milieu. Two stallions had regular contact with oestrous mares, and in these horses the secretion of GnRH and gonadotrophins occurred almost continuously with rapid, rhythmic pulses superimposed upon a tonic background. Mean (+/- S.D.) interval between GnRH pulses was 31.4 +/- 9.8 min and 27.7 +/- 10.1 min. This secretory pattern was not observed in the two stallions which had infrequent contact with oestrous mares, although the small numbers precluded statistical testing of this apparent difference. No GnRH pulses were observed in one of these stallions, while in the other mean (+/- S.D.) GnRH pulse interval was 45.0 +/- 48.7 min, the large variance being partly due to rapid pulses during a period in which the stallion teased mares. The fifth stallion was pubertal, and GnRH and LH secretion occurred in 15 and 0% of samples respectively, while low levels of FSH secretion were observed in 37% of samples and jugular testosterone levels were immeasurably low. We conclude that there is a statistically significant synchrony between pulses of GnRH, LH and FSH in the pituitary venous blood of stallions. Furthermore, decreasing intervals between gonadotrophin pulses result in a significant reduction in secretion of FSH but not LH.     

Free fatty acids suppress growth hormone, but not luteinizing hormone, secretion in sheep

An experiment was conducted to determine the effects of exogenously administered FFA on GH and LH secretion in sheep. Ovariectomized ewes received iv infusions of a mixture of FFA (166 mg/min; n = 5) or 0.9% saline (n = 4) for 10 h. Jugular blood was sampled every 15 min for 14 h, beginning 4 h before initiation of infusion. After 8 h of FFA or saline treatment, each ewe received a pituitary challenge of 10 micrograms GRF and 1 microgram GnRH, administered together as an iv bolus. Lipid infusion increased (P less than 0.01) serum FFA concentrations to levels characteristic of those in fasted sheep [23.0 +/- 0.8 mg/100 ml (mean +/- SE)]. Frequency of GH pulses (P less than 0.01) and the GH response to GRF (P less than 0.0001) were suppressed by FFA treatment. Mean serum GH concentrations increased gradually (P less than 0.01) during the 10-h infusion period in saline-treated but not lipid-treated, ewes. This finding may reflect diurnal changes in somatotrope secretory activity that are blocked by FFA. Mean serum LH concentrations, LH pulse frequency and amplitude, and the LH secretory response to GnRH were unaffected by FFA or saline infusion. In agreement with previous work in sheep and other species, these results provide evidence for an inhibitory effect of FFA on GH release. The exact mechanism responsible for this action, however, remains to be elucidated. Finally, acutely elevated FFA levels do not appear to influence LH secretion in the ovariectomized ewe.