Effect of bombesin on basal and stimulated secretion of some pituitary hormones in humans

The effect of bombesin (5 ng/kg/min X 2.5 h) on basal pituitary secretion as well as on the response to thyrotropin releasing hormone (TRH; 200 micrograms) plus luteinizing hormone releasing hormone (LHRH; 100 micrograms) was studied in healthy male volunteers. The peptide did not change the basal level of growth hormone (GH), prolactin, thyroid-stimulating hormone (TSH), luteinizing hormone (LH) and follicle-stimulating hormone (FSH). On the contrary, the pituitary response to releasing hormones was modified by bombesin administration. When compared with control (saline) values, prolactin and TSH levels after TRH were lower during bombesin infusion, whereas LH and FSH levels after LHRH were higher. Thus bombesin affects in man, as in experimental animals, the secretion of some pituitary hormones.     

Interactions between neuropeptide Y, luteinizing hormone-releasing hormone and estradiol in the control of luteinizing hormone release from cultured ovine pituitary cells

This study used pituitary cells in culture firstly to test the hypothesis that NPY may augment the pituitary LH response to LHRH and secondly to determine whether this interaction is dependent on the presence of estradiol. LHRH (10(-10)-10(-6) M) caused a significant increase in LH secretion from dispersed ovine pituitary cells maintained in culture for six days, a response which was enhanced when cells were pretreated for three days with 4 x 10(-11) M estradiol. NPY 10(-10)-10(-6) M) had no effect on basal LH release from ovine pituitary cells maintained either in the presence or absence of estradiol. NPY (10(-10) and 10(-8) M) also had no effect on LHRH-stimulated LH release either in the presence or absence of estradiol. These results substantiate previous observations that physiologically relevant concentrations of estradiol enhance the LH response to LHRH in cultured ovine pituitary cells. However, in contrast to experiments carried out using rat pituitary cells in culture, the present data provide no evidence to support the hypothesis that NPY alone interacts with LHRH in the control of LH secretion from the ovine pituitary gland.     

Catecholamine mechanisms in the feedback effects of estradiol benzoate on the release of LH and prolactin

The role of hypothalamic catecholamines and luteinizing hormone releasing hormone (LHRH) in the negative feedback effect of estradiol benzoate (EB) on luteinizing hormone (LH) release was studied in chronic ovariectomized rats. Administration of 10 micrograms EB decreased plasma LH levels and increased LHRH content in the medial basal hypothalamus (MBH) 1 day after injection. Inhibition of dopamine and norepinephrine synthesis with alpha-methyl-p-tyrosine (alpha-MT) reduced the LHRH content in the MBH in both oil- and EB-treated animals and partially reversed the decrease in plasma LH levels. Inhibition of norepinephrine synthesis with fusaric acid decreased LHRH content in both oil- and EB-treated rats but had no effect on plasma LH levels. The results suggest that at least a portion of the inhibitory effect of EB on LH release is due to the stimulation of an inhibitory dopaminergic mechanism which reduces LHRH release from the MBH. This feedback mechanism is apparently not susceptible to dopaminergic receptor blockade since administration of pimozide had no effect on LH levels. The stimulatory feedback effect of EB on prolactin release was studied in the same animals. alpha-MT and EB produced additive effects on plasma prolactin levels whereas fusaric acid blocked the EB-induced increase in plasma prolactin levels. Pimozide appeared to potentiate the effect of EB on prolactin release. The results reconfirm the possible role of noradrenergic neurons in the release of prolactin induced by EB and also suggest that EB stimulates a dopaminergic mechanism which is inhibitory to prolactin release but is normally masked by increased noradrenergic activity.     

Analysis of androgen action on pituitary gonadotropin and prolactin secretion in ewes

To study the role of androgens in the control of gonadotropin and prolactin secretion in ther ewe, we have characterized androgen receptors in pituitary cytosol, and investigated the effect of androgens on pituitary hormone release in vivo and in vitro. High affinity, low capacity receptors, with an affinity for methyltrienolone (R1881) greater than 5 alpha-dihydrotestosterone (5 alpha-DHT) greater than testosterone (T) much greater than androstenedione (A4), estradiol-17 beta (E2) and progesterone (P), were identified in pituitary cytosol. Addition of 1 nM 5 alpha-DHT, but not A4, inhibited luteinizing hormone (LH) release from pituitary cells in vitro, induced by 10(10) to 10(-7) M luteinizing hormone releasing hormone (LHRH). The release of follicle-stimulating hormone (FSH) with 10(-9) M LHRH was inhibited when cells were incubated with 1 nM 5 alpha-DHT. 5 alpha-DHT had no effect when higher or lower doses of LHRH were used. In ovariectomized ewes, neither an i.v. injection of 1 mg, nor intracarotid injections of up to 1 mg, 5 alpha-DHT affected plasma LH, FSH or prolactin levels, despite dose-related increases in plasma 5 alpha-DHT levels. Daily or twice daily i.m. injections of 5 mg 5 alpha-DHT in oil did not affect LH or FSH levels, but daily injections of 20 mg significantly reduced plasma LH levels within 4 days and plasma FSH levels within 6 days. Thus, despite the presence of androgen receptors in the ewe pituitary, we conclude that androgens per se are of minimal importance in the regulation of pituitary LH, FSH and prolactin secretion in the ewe. The low binding affinity of A4 and the lack of its effect on hormone secretion in vitro suggests that A4 may act as an estrogen precursor rather than an androgenic hormone. The function of the pituitary androgen receptor remains to be established.     

Galanin regulation of LH release in male rats

The present study examines the role of cerebroventricular administered (IIIrd ventricle) galanin on LHRH and LH release in adult and immature male rats. In both age groups, galanin stimulated LHRH synthesis and release from the hypothalamus, leading to a higher release of pituitary LH which in turn increased plasma LH levels. Galantide, a galanin receptor blocker, on the other hand, drastically reduced hypothalamic LHRH and plasma LH while increasing pituitary LH. In vitro incubation of anterior pituitary cells with galanin followed by LHRH resulted in increased release of pituitary LH but not by galanin alone. Galantide exhibited no such effect either alone or with LHRH. These results indicate that galanin is an important regulator for both hypothalamic LHRH and hypophysial LH and its role is independent of age in the case of male rats.     

Oestrogen and progesterone interactions in the control of gonadotrophin and prolactin secretion

Oestrogen and progesterone have marked effects on the secretion of the gonadotrophins and prolactin. During most of the oestrous or menstrual cycle the secretion of gonadotrophin is maintained at a relatively low level by the negative feedback of oestrogen and progesterone on the hypothalamic-pituitary system. The spontaneous ovulatory surge of gonadotrophin is produced by a positive feedback cascade. The cascade is initiated by an increase in the plasma concentration of oestradiol-17 beta which triggers a surge of luteinizing hormone releasing hormone (LHRH) and an increase in pituitary responsiveness to LHRH. The facilitatory action of oestrogen on pituitary responsiveness is reinforced by progesterone and the priming effect of LHRH. How oestrogen and progesterone exert their effects is not clear but the facilitatory effects of oestrogen take about 24 h, and the stimulation of LHRH release is produced by an indirect effect of oestradiol on neurons which are possibly opioid, dopaminergic or noradrenergic and which modulate the activity of LHRH neurons. In the rat, a spontaneous prolactin surge occurs at the same time as the spontaneous ovulatory gonadotrophin surge. The prolactin surge also appears to involve a positive feedback between the brain-pituitary system and the ovary. However, the mechanism of the prolactin surge is poorly understood mainly because the neural control of prolactin release appears to be mediated by prolactin inhibiting as well as releasing factors, and the precise role of these factors has not been established. The control of prolactin release is further complicated by the fact that oestradiol stimulates prolactin synthesis and release by a direct action on the prolactotrophes. Prolactin and gonadotrophin surges also occur simultaneously in several experimental steroid models. A theoretical model is proposed which could explain how oestrogen and progesterone trigger the simultaneous surge of LH and prolactin.     

Neuroendocrine control of gonadotropin secretion

Luteinizing hormone releasing hormone (LHRH), a hypothalmic peptide that is concentrated in granules of neurons, has the capacity to release gonadotropins (luteinizing hormone (LH) and follicle stimulating hormone) from the pituitary gland. LHRH has been found in hypophysial portal blood of rats, monkeys, and rabbits. Antibodies to LHRH depress plasma LH concentrations in castrated animals and evoke testicular atrophy, but passive immunization against LHRH does not block the LH surge induced by estrogen in monkeys. Estrogens, progestin, prolactin, and dopamine have marked effects on LH secretion, yet an association between these effects and altered hypophysial portal blood concentrations of LHRH is not established. In view of the paucity of evidence demonstrating such a cause and effect relationship, two alternative proposals have become tenable. One, hormones and neurotransmitters may not alter the levels of portal blood LHRH, but rather alter the frequency of pulsatile LHRH secretion. Two, hormones, such as estrogens, progesterone, and prolactin, may alter the responsiveness of the gonadotropin-secreting cells to LHRH by affecting the secretion of dopamine.     

Enhanced luteinizing hormone release by luteinizing hormone-releasing hormone in incubating female turkeys (Meleagris gallopavo)

To determine what role pituitary responsiveness plays in the suppression of gonadotropin level during incubation in the turkey, the ability of the pituitary to release luteinizing hormone (LH) in response to luteinizing hormone-releasing hormone (LHRH) was compared in incubating, laying, and photorefractory birds. In all three groups, the i.m. injection of LHRH (4 micrograms/kg) increased serum LH levels; however, the LH response was markedly enhanced in the incubating turkeys as compared with the laying (6.6-fold increase over preinjection levels vs. 1.9-fold; p less than 0.05) or the photorefractory birds (9.7-fold vs. 3.1-fold; p less than 0.05). The LHRH-induced LH release was also determined in turkeys as they shifted from the laying to the incubating phase of the reproductive cycle. This response increased (p less than 0.05) in magnitude as the birds started to incubate. The high prolactin level of incubating turkeys does not have a depressing effect on LHRH-stimulated LH release; thus, impaired LH response to LHRH is not a mechanism involved in the diminished gonadotropin secretion of incubating turkeys.     

Gonadal steroids and neuropeptide Y-opioid-LHRH axis: interactions and diversities

We report that the two classes of regulatory neuropeptides, neuropeptide Y (NPY) and endogenous opioid peptides (EOP), modulate luteinizing hormone (LH) release in diverse fashion in gonad-intact rats. Each neuropeptide acts at two loci, the hypothalamus and pituitary, to excite (NPY) or inhibit (EOP) LH release. At the hypothalamic level, NPY stimulates luteinizing hormone releasing hormone (LHRH) release, a response mediated by alpha 2-adrenoreceptors and amplified in the presence of adrenergic agonists. At the pituitary level, NPY acts in concert with LHRH to amplify the LH response. In contrast, EOP inhibit LHRH release by decreasing the supply of excitatory adrenergic signals in the vicinity of LHRH neurons in the preoptic-tuberal pathway, and at the pituitary level, they decrease LH release in response to LHRH. Further, the gonadal steroidal milieu facilitates NPY neurosecretion and postsynaptic expression of NPY in concert with adrenergic system; a similar clear-cut facilitatory effect of gonadal steroids on EOP secretion is not yet obvious. Our additional studies imply that the EOP system has the potential to increase sensitivity towards gonadal steroids and that to induce the preovulatory LH surge the neural clock may decrease the inhibitory EOP tone prior to the critical period on proestrus. This antecedent neural event allows the excitatory adrenergic and NPY signals to evoke LHRH secretion at a higher frequency approximating that seen in ovariectomized rats. Further studies are under way to delineate the steroid-induced subcellular events that integrate the action of these regulatory peptides in the control of the episodic LHRH secretion pattern which sustains basal and cyclic gonadotropin release in the rat.     

Sex steroids and the control of LHRH secretion

Gonadal steroids are important hormonal signals that regulate the activity of LHRH synthesizing and releasing neurons. Aside from a direct effect through the feedback mechanisms exerted at hypothalamic and/or anterior pituitary level, gonadal steroids may modify the rhythmic LHRH release by modulating other systems affecting LHRH neurons. 1. In ovariectomized E2-treated female rats, progesterone is able to evoke LHRH release from the perifused hypothalamus without affecting LH and FSH release. 2. Excitatory amino acids (EAA) and their related analogs (NMDA and kainate) are known to stimulate LH release in young rats. When tested in a perifusion system on hypothalamic and anterior pituitary tissues, they differentially stimulate the release of LHRH (NMDA) and of LH (KA); their effect on both structures is markedly reduced following orchidectomy. It appears that gonadal steroids might exert a facilitatory action on the neurosecretory activity of LHRH neurons as well as a modulatory influence on the effect of EAA.     

Selective release of follicle-stimulating hormone by 5 alpha-dihydroprogesterone in immature ovariectomized estrogen-primed rats

The effect of 5 alpha-dihydroprogesterone (5 alpha-DHP) on gonadotropin release was examined in the immature acutely ovariectomized (OVX) rat primed with a low dose of estradiol (E2). Treatment with various doses of 5 alpha-DHP given in combination with E2 increased levels of follicle-stimulating hormone (FSH) but had no effect on serum luteinizing hormone (LH). A single injection of a maximally stimulating dose of 5 alpha-DHP (0.4 mg/kg) stimulated increases in serum FSH at 1200 h and, 6 h later, at 1800 h. Pituitary LH and FSH content was dramatically enhanced by 1600 h and levels remained elevated at 1800 h. The administration of pentobarbital at 1200 h, versus 1400 h or 1600 h, prevented the increase in basal serum FSH levels at 1800 h, implying that the release of hypothalamic LH releasing hormone (LHRH) is modulated by 5 alpha-DHP. In addition, changes in pituitary sensitivity to LHRH as a result of 5 alpha-DHP were measured and a significant increase in the magnitude of FSH release was observed at 1200 h and 1800 h. Although the LH response to LHRH in 5 alpha-DHP-treated rats was not different from controls, the duration of LH release was lengthened. These results suggest that 5 alpha-DHP may stimulate FSH release by a direct action at the pituitary level. Together, these observations support the theory that 5 alpha-DHP mediates the facilitative effect of progesterone on FSH secretion and further suggests an action of 5 alpha-DHP in this phenomenon at both pituitary and hypothalamic sites.     

Effects of naloxone,morphine and methionine enkephalin on serum prolactin,luteinizing hormone,follicle stimulating hormone,thyroid stimulating hormone and growth hormone

The response of 5 anterior pituitary hormones to single injections of naloxone, morphine and metenkephalin administration was measured in male rats. Morphine and met-enkephalin significantly increased serum prolactin and GH concentrations, and significantly decreased serum LH and TSH concentrations. Naloxone reduced serum prolactin and GH concentrations, increased serum LH and FSH, but had little effect on serum TSH concentrations. Concurrent injections of naloxone with morphine or met-enkephalin reduced the response to each of the drugs given separtely. These results suggest that endogenous morphinomimetic substances may participate in regulating secretion of anterior pituitary hormones.     

Effects of Neuropeptide Y (NPY) on the release of anterior pituitary hormones in the rat

Neuropeptide Y (NPY) has been recently localized in several hypothalamic nuclei in the mammalian brain. In order to investigate the possible role of NPY on neuroendocrine function, we have investigated the effects of the peptide on the release of anterior pituitary hormones in the rat. Both intravenous (300 μg) or intraventricular (2 to 15 μg) injection of NPY produced in gonadectomized male rats a significant and long-lasting decrease of plasma LH levels. A short duration stimulating effect on prolactin plasma levels was also observed after the intravenous but not after the intraventricular injection of NPY. Plasma levels of the other pituitary hormones were not significantly modified after NPY injection. When incubated in vitro with anterior pituitary cells in monolayer culture, NPY produced no significant change in release of pituitary hormones. Thus NPY seems to exert a selective effect on LH release. Since this effect can be observed after both intravenous and intraventricular injection, it might be hypothesized that NPY could affect LHRH release in two areas which lack blood-brain barrier: the organum vasculosum of the lamina terminalis (OVLT) which contains LHRH cell bodies and NPY fibers and the median eminence which contains both LHRH and NPY fibers. The effect on prolactin release needs to be carefully evaluated in different experimental conditions.     

Effects of arginine vasotocin on levels of plasma gonadotropins and prolactin in ovariectomized conscious rats

Arginine vasotocin was injected into the third ventricle or intravenously in conscious, ovariectomized rats and its effect on gonadotropin and prolactin release evaluated. The peptide lowered plasma levels of both LH and prolactin in doses of 40 or 100 ng given intraventricularly. The higher dose was slightly more effective than the lower dose. Intravenous injection of a 1-microgram dose of vasotocin failed to alter plasma LH in the ovariectomized animals; however, a 5-micrograms dose induced a slight depression apparent at only 60 min following injection. Intravenous injection of 1 microgram produced a significant lowering of plasma prolactin, whereas a dramatic lowering followed the injection of the higher dose. Plasma FSH was unaffected in these experiments. Incubation of dispersed anterior pituitary cells from ovariectomized rats with various doses of vasotocin revealed no effect of the peptide on the release of FSH, LH, or prolactin. It also did not alter the response to LHRH, but it partially blocked the action of dopamine to inhibit prolactin release. The data indicate that quite low doses of arginine vasotocin act within the brain to inhibit LH and prolactin secretion in ovariectomized, conscious animals.     

Studies on a possible pituitary effect of monoamines on luteinizing hormone release in ovariectomized rats

Incubation of anterior pituitaries from ovariectomized rats with LHRH and various concentrations of dopamine, norepinephrine or serotonin indicated that none of these neurotransmitters could decrease pituitary LH secretion in response to the releasing hormone. This indicated that the inhibitions of pulsatile LH release previously observed in our laboratory in ovariectomized rats in response to intraventricularly administered catecholamines or stimulation of brain serotoninergic neurons are due to central rather than pituitary effects of these transmitters.     

The rapid effects of luteinizing hormone releasing hormone agonists and antagonists on the mouse pituitary in vitro

The effect of luteinizing hormone releasing hormone (LHRH) and its analogs on the release of FSH and LH by 20 day old whole mouse pituitary incubated in vitro for 3-4 hrs was investigated. Three agonistic analogs (AY 25650, 25205 and Buserelin) all of which are reported to be superactive in vivo showed approximately the same potency in this in vitro test system. Preincubation of the pituitaries for 1 h with the antagonistic analogs [Ac Dp Cl Phe1,2, D Trp3, D Phe6, D Ala10] LHRH and [Ac Dp Cl Phe1,2, D Trp3, D Arg6, D Ala10] LHRH inhibited the secretion of LH and FSH induced by 2.5 x 10(-9)M LHRH. The inhibitory response was dose dependent. The continued presence of the antagonists was not required for effective suppression of the LHRH effect. Experiments designed to find out the minimum time required for eliciting suppression of LHRH revealed that preincubation of the pituitary with the second antagonist for 5 mins followed by removal was adequate to produce effective inhibition of gonadotropin release. At lower doses of the antagonist, LH release was more effectively inhibited than FSH release. The results suggest that antagonistic analogs can effectively bind to LHRH receptors in the whole pituitary incubation preventing the subsequent action of LHRH. With the present incubation system assessment of bioactive LH and FSH release is possible within 24 hrs.     

The time interval effect on two consecutive LHRH-TRH tests on adult female baboons

To Clarify the relationship between the time interval and pituitary Luteinizing hormone (LH), Follicular stimulation hormone (FSH) and prolactin (PRL) secretion function under LHRH-TRH stimulation, 4 mature female baboons were used. Two consecutive LHRH (100 micrograms)-TRH (250 micrograms) stimulations with a 60 min interval between them was carried out in the early follicular phase, late follicular phase and mid luteal phase in the same baboon in the first menstrual cycle, then carried out with a 120 min interval between tests in the third menstrual cycle. The LH, FSH and PRL were measured by specific radioimmunoassay. The PRL maximum response to the first bolus of TRH was higher than maximum response to the second bolus of TRH. The PRL maximum response to the second TRH at a 120 min interval was higher than the maximum response to the second TRH at a 60 min interval. It seems that the TRH had the dominant effect on PRL releasing but not on PRL Priming. The maximum LH response to the second bolus of LHRH was higher than the maximum response to the first bolus of LHRH. The LH maximum response to the second bolus of LHRH at a 60 min interval was greater than the maximum response at a 120 min interval in the follicular phase but it was the reverse in the luteal phase. The FSH response to the second LHRH was different from the LH response.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of prolonged incubation of male rat whole pituitary or pituitary-hypothalamus complex with testosterone on release of gonadotrophin and prolactin in vitro.

Investigations were undertaken to study the effect of in vitro addition of testosterone (0.3 mM) on the release of luteinizing hormone (LH), follicle stimulating hormone (FSH) and prolactin (PRL) by pituitary-hypothalamus complex (PHC) or the whole pituitary (PI) incubated for 72 hr, with incubation media changed every 24 hr. PHC or PI were from adult intact or castrated (7 days post castration) rats. The tissues incubated with or without testosterone were further exposed to 0.1 nM luteinizing hormone-releasing hormone (LHRH) for 4 hr. Incubation media and the pituitary were analyzed for PRL and gonadotrophin content. While PHC from normal and castrated rats released increasing amounts of LH with diminishing amounts of FSH and PRL at different periods of incubation, PI showed a decrease in the amounts of gonadotrophin and PRL released. Co-incubation of PHC or PI of intact or castrated rats with testosterone stimulated the release of LH and FSH during the first or second-24 hr incubation but inhibited the release of PRL in all the three incubations of 24 hr each. The extent of PRL inhibition increased with increasing incubation period. Testosterone had no effect on LHRH induced release of PRL but inhibited LHRH induced release of LH and FSH by pituitaries from constructs of normal rats. Testosterone reduced intrapituitary contents of PRL and FSH of intact and castrated rats. The data are interpreted to suggest that hypothalamus is essential for the maintenance of functional pituitary in vitro and that intrinsic differences exist in mechanisms regulating the secretion of LH, FSH and PRL.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of a dopaminergic stimulant,amfonelic acid,on anterior pituitary hormone release in conscious rats

The response of plasma LH, Prolactin, GH and TSH levels to systematic administration of a specific central dopaminergic stimulant, amfonelic acid (AFA), by intravenous pulse injection in ovariectomized (OVX) and OVX estrogen-progesterone primed conscious rats has been evaluated. Intravenous injection of 0.2 mg/kg of AFA had no influence on plasma LH concentration until 60 min after injection when it was significantly elevated. Increasing the dose to 1 mg/kg reduced LH titers at 15 and 30 min with a return to preinjection levels by 60 min. AFA produced a dose-dependent decrease in plasma prolactin levels; the decrease occurred as early as 5 min after injection. AFA, both at 0.2 and 1 mg/kg doses, was effective in producing a sharp, dose-related rise in plasma GH levels. By contrast, TSH levels were significantly suppressed by both doses of AFA. Injection of the 1 mg/kg dose of AFA did not modify plasma LH levels in OVX-steroid-primed animals, white producing a comparable effect on plasma prolactin, GH and TSH levels to that observed in OVX animals. The present results indicate that endogenously released DA can have profound effects on pituitary hormone release, inhibiting PRL and TSH discharge, stimulating GH release and either inhibiting or stimulating LH release.     

Evaluation of in vitro incubation systems for the study of gonadotropin release.

A detailed study was undertaken in order to determine if a pituitary-half incubation system were a suitable model for the study of anterior pituitary response to estradiol and LHRH. Considerable variation in the gonadotropin content of randomized pituitary halves was observed. Much less variation was found in matched halves. During the initial thirty minutes incubation of pituitary halves, a large spontaneous release of gonadotropins was observed. Time course secretion studies indicated that by four hours incubation, in the presence of 50 ng/ml LHRH, cumulative secretion of LH and FSH had far exceeded that of controls. Elevations in both cumulative secretion and rate of secretion were evident within 15-30 minutes of incubation. Regardless of LHRH dose, only 2-4% of either gonadotropin was secreted. Estradiol in the range of 10, 100, 500, 1,000 and 50,000 pg/ml had no significant effect on pituitary response to LHRH or on basal release, tissue levels or total gonadotropin. Based on these results, it was concluded that while the pituitary-half incubation system may be suitable for studying LHRH induced gonadotropin secretion, it is apparently of insufficient sensitivity to allow the collection of meaningful data concerning the effects of estradiol alone on gonadotropin secretion or estradiol modulation of LHRH induced gonadotropin secretion.