Involvement of H1 histamine receptor in basal and estrogen-stimulated luteinizing hormone-releasing hormone secretion in rats in vitro

For determination of the roles of histamine and its receptors, H1 and H2, in the control of basal and estrogen-induced LH-RH secretion, the mediobasal hypothalamus (MBH) and/or pituitary was excised from normally cycling female rats and perifused in an in vitro sequential double chamber perifusion system. Administration of 10(-7) M histamine caused significant release (90-170% increase, p less than 0.05) of LH from the pituitary in sequence with the MBH, whereas 10(-7) M histamine had no effect on LH release from the pituitary perifused alone; administration of 10(-5) M 2-methylhistamine, an H1 agonist, induced significant release (50-120% increase, p less than 0.05) of LH-RH from the MBH, and addition of 10(-5) M mepyramine, and H1 antagonist, abolished this LH-RH release. The LH concentrations in the efflux were not affected by the administration of the H2 agonist 4-methylhistamine. Estradiol caused significant release of LH-RH from the MBH and LH from the pituitary in sequence with the MBH. The estradiol-induced release of LH-RH and LH were completely abolished by perifusion with medium containing 10(-5) M mepyramine. The H2 receptor antagonist ranitidine did not affect estradiol-induced LH release. Histamine did not change the LH release induced by 20 ng LH-RH. These findings suggest that histamine induces release of hypothalamic LH-RH, and that histamine H1 receptors in the hypothalamus are involved in the basal and estradiol-induced LH-RH release.     

LH-RH and dopamine levels in hypophysial stalk plasma and their relationship to plasma gonadotrophins and prolactin levels in male rats bearing a prolactin- and adrenocorticotrophin-secreting pituitary tumor

The present study was concerned with the effects of a transplantable pituitary tumor secreting prolactin (PRL) and adrenocorticotrophin (ACTH) on the levels of LH and FSH in peripheral plasma and on the hypothalamic release of LH-RH and dopamine in the male rat. Male rats of the same age not inoculated with the tumor served as controls. Hypophysial stalk blood was collected from urethane-anesthetized rats 4-5 weeks after tumor inoculation to measure their LH-RH and dopamine content. A peripheral blood sample was withdrawn from the animals just before sectioning the hypophysial stalk to measure their content of LH, FSH and PRL. It was found that in the tumor-bearing rats the levels of PRL increased 17-fold, whereas plasma levels of LH and FSH decreased by 45 and 70% respectively, when compared with the control rats. In the tumor-bearing rats, the secretion rate of dopamine in hypophysial stalk plasma increased from 1.4 to 4.1 ng/h, whereas the secretion rate of LH-RH decreased from 122 to 61 pg/h. However, when at the time of tumor inoculation adrenalectomy was performed, the tumor did not decrease plasma levels of LH and FSH and the secretion of LH-RH into hypophysial stalk blood any longer. The effect of the tumor on hypothalamic dopamine secretion was, however, still present in the adrenalectomized rats. It is concluded that the effect of the PRL- and ACTH-secreting pituitary tumor on plasma levels of LH and FSH requires the presence of the adrenal gland and that this effect is mediated through an inhibition of the hypothalamic release of LH-RH. Furthermore, this tumor increases the hypothalamic release of dopamine independent of the presence of the adrenal gland.     

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.     

Histaminergic regulation of prolactin secretion: involvement of serotoninergic neurons

The possible involvement of the serotoninergic system in histamine-induced PRL secretion was studied in urethane anesthetized male rats. Intracerebroventricular infusion of histamine (30 micrograms) stimulated PRL secretion 10-fold. This effect was mimicked by the H2-receptor agonist dimaprit (300 micrograms), while the H1-receptor agonist 2-thiazolylethylamine (140 micrograms) had no effect. Pretreatment with the serotonin receptor blockers methysergide (2.5 mg/kg i.p.) or ketanserin (2.5 or 10.0 mg/kg i.p.) reduced the PRL peak response to histamine 75, 54, or 58%, respectively. During serotonin receptor blockade, dimaprit had a stimulatory effect similar to that of histamine, while 2-thiazolylethylamine had no effect. Intraarterial infusion of histamine (420 micrograms) stimulated PRL secretion 6-fold. This effect was mimicked by the H1-receptor agonist 2-thiazolylethylamine (1,900 micrograms), while the H2-receptor agonist dimaprit (3,000 micrograms) had no effect. Pretreatment with methysergide (2.5 mg/kg i.p.) or ketanserin (2.5 or 10.0 mg/kg i.p.) reduced the peak response to histamine 54, 54, or 51% respectively. The effect of histamine was mimicked by 2-thiazolylethylamine, while dimaprit slightly inhibited the PRL secretion. The antiserotoninergic activity of methysergide and ketanserin was demonstrated by their ability to prevent the PRL-releasing effect to serotonin. The effects of methysergide and ketanserin were not due to dopamine-like activity, since none of the drugs affected basal PRL secretion and since the dopamine receptor antagonist pimozide did not prevent the inhibitory effect of methysergide on the histamine-induced PRL release. The findings indicate that histamine-stimulated PRL secretion is mediated in part by serotoninergic neurons.     

In vitro, nitric oxide (NO) stimulates LH secretion and partially prevents the inhibitory effect of dopamine on PRL release

In recent years nitric oxide (NO) has emerged as an important intra- and intercellular transmitter involved in the control of hypothalamic-pituitary axis. In order to discriminate the potential actions of NO at hypothalamic or pituitary level in the control of PRL and LH release, we have studied PRL and LH secretion by dispersed pituitary cells obtained from males, cycling and lactating females in the presence of 1) sodium nitroprusside (SNP), a NO donor; 2) cyclic guanosine monophosphate (cGMP), the second messenger for a wide range of NO actions; 3) Nw-nitro-L-arginine methyl ester (NAME), a competitive inhibitor of NO synthase (NOS) and 4) oxadialoquinoxalione (OQD) and LY 83,583, antagonists of guanylyl cyclases. We found that SNP (at doses of 100 and 500 micromol) stimulated LH and FSH release and partially blocked the inhibitory action of dopamine (50 and 100 nmol) on prolactin secretion. These effects were not mimicked by cGMP and remained in the presence of OQD and LY 83,583. NAME alone had no significant effect on hormone secretion. These results suggest that NO plays a role in the control of gonadotropins and prolactin secretion acting directly at the pituitary level and that these effects are mediated by mechanisms other than changes in cGMP levels.     

Hyperprolactinemia decreases naloxone binding in the arcuate nucleus of ovariectomized rats

Hyperprolactinemia suppresses endogenous prolactin (PRL) secretion and inhibits LH release in ovariectomized rats. Opiate peptides appear to mediate the suppressive effects of hyperprolactinemia on both endogenous PRL and LH secretion. A mechanism by which hyperprolactinemia may change the ability of opiates to influence PRL and LH is by altering the density of opiate receptors. We, therefore, examined the effect of hyperprolactinemia on the density of naloxone binding sites in hypothalamic regions that are important in the regulation of PRL and LH secretion. Female rats, ovariectomized for 4 days, were treated with ovine prolactin (oPRL) every 8 h for 2 days, and naloxone binding sites were measured using autoradiographic procedures. oPRL treatment suppressed the concentration of naloxone binding sites throughout the arcuate nucleus but had no effect in the median eminence, suprachiasmatic nucleus, and medial preoptic nucleus. There is evidence that the tuberoinfundibular dopaminergic neurons of the arcuate nucleus are directly influenced through opiate receptors. We propose that the observed decrease in the density of opiate receptors may occur on dopaminergic neurons. This theory provides an explanation for a mechanism for the suppression of endogenous PRL and LH by hyperprolactinemia: a decrease in opiate receptors will decrease opiate suppression of dopamine neurons allowing dopamine activity to increase. Increase in dopamine release are known to decrease PRL and LH secretion in ovariectomized rats. Alternatively, decreased naloxone binding may result from homologous down-regulation of receptors due to increased opiate activity. If opiate activity increases, it may directly inhibit LHRH neurons and may suppress the activity of inhibitory neurons leading to increased dopamine activity.     

Mediation of the stress-induced prolactin release by hypothalamic histaminergic neurons and the possible involvement of vasopressin in this response

We investigated the role of histamine (HA) in the neuroendocrine regulation of PRL secretion in conscious male rats. Blood samples were obtained by decapitation of the animals at 0 min. Intracerebroventricular infusion of HA (34-540 nmol at -15 min) stimulated PRL secretion dose dependently with an ED50 of approximately 135 nmol. Inhibition of neuronal HA synthesis by the specific histidine decarboxylase-inhibitor (S)alpha-fluoro-methylhistidine (alpha FMH; 100 mumol/kg ip at -6 h or 400 mumol/kg ip at -20 h and -6 h) caused a 50% reduction (P less than 0.01) in the PRL response to 5 min of restraint stress. Inhibition of neuronal HA metabolism by the specific histamine-methyltransferase-inhibitor SKF-91488 (400 and 800 nmol icv at -20 min) augmented the restraint stress-induced PRL release 26% and 37%, respectively (P less than 0.05). The two enzyme inhibitors had no or only modest effect on the basal PRL secretion. Pretreatment with a specific antiserum (0.5 ml) to arginine vasopressin (AVP) or an AVP antagonist (25 nmol) administered iv at -20 min inhibited the PRL response to HA (270 nmol icv) 80% and 45%, respectively (P less than 0.01) and inhibited the PRL response to restraint stress 70% (P less than 0.01). In contrast, pretreatment with a specific oxytocin (OT) antagonist (50 nmol) had no effect on the HA- or stress-induced PRL release. The AVP antiserum showed less than 0.0003% cross-reactivity with OT, and radiolabeled OT did not bind to serial dilutions of plasma from rats treated with the AVP antiserum. The AVP antiserum or the AVP antagonist almost prevented the PRL release induced by iv infusion of 800 pmol AVP or a posterior pituitary extract. Infusion of AVP (24-800 pmol iv at -15 min) stimulated PRL secretion dose dependently. However, the dose of AVP (800 pmol) required to induce an increase in plasma PRL similar to that obtained by HA-stimulation, led to a rise in plasma AVP which was approximately 1000-fold higher than that induced by HA, which increased plasma AVP 2-fold. Restraint stress had no effect on the plasma AVP level. We conclude that neuronal HA participates in the mediation of the PRL response to stress and that the stress- and HA-induced release of PRL may involve AVP, whereas an involvement of OT seems unlikely.(ABSTRACT TRUNCATED AT 400 WORDS)     

Transmitter mediated arginine vasopressin release from superfused hypothalamus and pituitary gland.

The study was designed to investigate the effect of various neurotransmitters on the hypothalamus and pituitary gland to determine the sites of their action. Superfused isolated rat hypothalami and pituitary glands demonstrated basal secretion of arginine vasopressin (AVP) and repeated response to stimulation thus showing the viability of the preparation. Acetylcholine and histamine stimulated the release of AVP at the hypothalamic and pituitary levels; dopamine and norepinephrine released AVP in a dose related manner only from the hypothalamus; angiotensin II released AVP in the same fashion only from the pituitary gland. AVP secretion stimulated by dopamine and norepinephrine may represent synaptic inputs which are localized at the hypothalamus and must be distinguished from the site of action at the pituitary gland of angiotensin II.     

Histaminergic regulation of prolactin secretion: involvement of tuberoinfundibular dopaminergic neurons

It has been shown that histamine (HA) stimulates prolactin (PRL) secretion via H2 receptors following intra-cerebroventricular infusion and via H1 receptors following systemic (intra-arterial) infusion. Since the effect of HA appears to be exerted at a suprapituitary level, we investigated the involvement of the tuberoinfundibular dopaminergic (TIDA) system in HA-induced PRL secretion in urethane-anesthetized male rats. HA infused intracerebroventricularly (30 micrograms) or intra-arterially (420 micrograms) decreased the dopamine (DA) concentration in pituitary portal blood by 30 and 23%, respectively. Blockade of DA receptors by pimozide did not prevent the stimulation of PRL secretion induced by intracerebroventricular infusion of HA or the H2 receptor agonist dimaprit. Furthermore, during DA receptor blockade intracerebroventricular infusion of the H1 receptor agonist 2-thiazolylethylamine inhibited PRL secretion. In contrast, pimozide prevented the stimulation of PRL secretion induced by intra-arterial infusion of HA and the H1 receptor agonist 2-thiazolylethylamine. In fact, under these conditions intra-arterial infusion of HA or the H2-receptor agonist dimaprit inhibited PRL secretion. During treatment with alpha-methyl-p-tyrosine, which reduced the hypothalamic DA content by 50%, HA infused intracerebroventricularly stimulated PRL secretion, while HA infused intra-arterially inhibited the secretion, which is in accordance with the results obtained during pimozide treatment. Cholinergic blockade by atropine did not prevent the HA-induced PRL release, excluding the possibility that the observed effect of pimozide is due to its anticholinergic property. We suggest that intracerebroventricular infusion of HA by activation of H2 receptors may stimulate PRL secretion partly via inhibition of the TIDA system and partly via other mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of naloxone and morphine on LH and prolactin release in androgen-sterilized rats

Many studies suggest that the hypothalamic opiate system modulates the secretion of LH and prolactin (PRL) by its effects on catecholamine release. We previously provided evidence that the LH response to the opiate receptor antagonist naloxone (NAL) may depend upon spontaneous activity in the hypothalamic noradrenergic system at the time the drug is administered. Thus, when NAL is given to rats which have low turnovers of hypothalamic norepinephrine (NE), only small transient rises in LH occur. This is contrasted to the effects of NAL on the LH responses of animals with high rates of NE turnover where marked amplification of phasic LH release is observed. In the present studies, we examined the effects of NAL on LH and morphine on PRL responses in androgen-sterilized rats (ASR). These animals do not respond to the positive feedback actions of estrogen by having LH surges, and hypothalamic NE turnovers do not increase during the afternoon as they do in normal rats. Female rats were given a single injection of testosterone propionate (50 micrograms s.c.) at 5 days of life and ovariectomized (OVX) at 100 days of age. Seven days later (day 0), estrogen capsules were inserted subcutaneously, and on day 2, their responses to NAL or morphine were examined. Comparable estrogen-treated gonadectomized controls also were studied. In control rats, NAL (10 mg/kg s.c.) markedly amplified the phasic secretion of plasma LH. In contrast, NAL had no effect on the basal afternoon secretion of LH in ASR. To determine if neonatal androgen treatment deleteriously affected opiate-tuberoinfundibular dopamine (TIDA)-serotonergic interactions, a second series of studies was performed.(ABSTRACT TRUNCATED AT 250 WORDS)     

THE ACUTE EFFECTS OF A DOPAMINE ANTAGONIST (DOMPERIDONE) ON LUTEINISING HORMONE, FOLLICULE STIMULATING HORMONE, PROLACTIN AND THYROTROPHIN SECRETION IN POLYCYSTIC OVARIAN SYNDROME: DIFFERENTIAL EFFECT OF OVULATION

An alteration of the hypothalamic dopaminergic regulation of LH secretion has been implicated in the abnormal LH secretion of polycystic ovarian syndrome (PCOS). To investigate this, the acute effect of dopamine (DA) receptor blockade on LH, FSH, PRL and TSH secretion was determined. No effect was observed on either LH or FSH secretion in the PCOS patients or in normal women and LH pulsatility appeared to be maintained. When the PCOS patients were divided into those who ovulated in the preceding cycle and those who were anovulatory, it was observed that the ovulatory patients had a normal PRL and TSH response; whereas in the anovulatory patients, the PRL rise was blunted and the TSH response was absent. We conclude that this study gives no evidence to support the hypothesis of altered hypothalamic DA regulation of LH secretion in PCOS. The lack of LH response to DA blockade suggests that a physiological role of DA in the control of LH secretion seems unlikely as determined under these experimental conditions. The differences in PRL and TRH response may reflect anovulation rather than a fundamental abnormality in PCOS.     

Effects of gamma-aminobutyric acid and histamine on prolactin secretion in the rat.

Histamine releases prolactin (PRL) in estrogen-progesterone primed male rats after both iv and intracisternal (ic) injection, but does not affect in vitro PRL secretion. This stimulatory effect is reversed by the alpha-adrenergic antagonist phenoxybenzamine and the histamine antagonist diphenhydramine, gamma-Amino butyric acid (GABA), which has little in vivo effect of its own, interferes with the activity of histamine and a variety of other stimulators of PRL secretion. These results suggest the involvement of the naturally-occurring substances histamine and GABA in the central regulation of PRL secretion.     

On the role of dopamine receptors in the naloxone-induced hormonal changes in man

To assess the role of dopamine receptors in naloxone-induced hormonal changes, the effects of dopamine and metoclopramide on anterior pituitary hormone secretion were studied during the infusion of the opiate blocker in normal men. Naloxone stimulated LH and cortisol secretion in all subjects, whereas FSH, TSH, PRL, and GH did not change. The infusion of dopamine completely suppressed the naloxone-induced LH rise; on the contrary, metoclopramide failed to alter the magnitude of the increments in LH observed during the infusion of the opiate blocker. The cortisol response to naloxone remained unchanged during dopamine and metoclopramide infusion. Metoclopramide stimulated PRL and TSH release during naloxone treatment, whereas dopamine suppressed PRL and TSH secretion. The data do not suggest a participation of dopamine receptors in the naloxone-induced hormonal changes in man and confirm a suppressive effect of dopamine infusion on LH release in humans.     

Direct effects of prolactin and dopamine on the gonadotroph response to GnRH

The intrapituitary mechanisms underlying the inhibitory actions of hyperprolactinaemia on the reproductive axis remain unclear. Previous work on primary pituitary cultures revealed combined suppressive effects of prolactin (PRL) and dopamine on the gonadotrophin response to GnRH. However, whether these effects occur directly at the level of the gonadotroph and are accompanied by changes in gene expression is still unresolved. Here, alphaT(3)-1 and LbetaT2 cells were used to investigate the effects of PRL and dopamine on gonadotrophin synthesis and release in gonadotroph monocultures under basal and GnRH-stimulated conditions. PRL receptor and dopamine receptor mRNA expressions were first determined by RT-PCR in both cell lines. Then, PRL and the dopamine agonist bromocriptine (Br), alone or in combination, were shown to block the maximal alpha-subunit and LHbeta-subunit mRNA responses to a dose-range of GnRH. The LH secretory response was differentially affected by treatments. GnRH dose-dependently stimulated LH release, with a 4-5 fold increase at 10(-8) M GnRH. Unexpectedly, PRL or Br stimulated basal LH release, with PRL, but not Br, enhancing the LH secretory response to GnRH. This effect was, however, completely blocked by Br. These results reveal direct effects of PRL and dopamine at the level of the gonadotroph cell, and interactions between these two hormones in the regulation of gonadotrophin secretion. Moreover, uncoupling between LH synthesis and release in both the basal and the GnRH-stimulated responses to PRL and dopamine was clearly apparent.     

Effects of specific activation of mu-, delta- and kappa-opioid receptors on the secretion of luteinizing hormone and prolactin in the ovariectomized rat

With the recent development of highly specific ligands for the mu, delta and kappa opioid receptors it was of interest to define the effects of activation of each of these receptor types on LH and prolactin (PRL) secretion. The compounds were infused (10 microliters/h) at various concentrations into the third cerebroventricle of unanesthetized, ovariectomized rats. The mu agonist, DAGO, at both 1 and 10 micrograms/h caused a significant suppression of LH secretion and a significant stimulation of PRL release. DPDPE, the delta agonist, had no effect on either hormone at 1 microgram/h but inhibited LH secretion at 10 micrograms/h. There was still no effect of this high dose of DPDPE on PRL release. The kappa agonist, U50,488H, had no effect on either hormone at 10 micrograms/h, but at 100 micrograms/h produced a significant suppression of LH release and a highly variable increase in PRL. Coinfusion of 100 micrograms/h of naloxone with the high dose of each of the agonists completely blocked the responses of both hormones to each of the agonists with one exception: the highly variable stimulation of PRL by U50,488H was not affected, thus indicating a nonspecific effect of U50,488H on PRL secretion. These results demonstrate that: activation of the mu receptors produces an inhibition of LH secretion and a stimulation of PRL release; activation of the delta receptors produces an inhibition of LH secretion but has no effect on PRL release, and activation of the kappa receptors produces an inhibition of LH release and a variable stimulation of PRL secretion.     

Hypothalamic catecholamine biosynthesis and pituitary gonadotropin secretion in vitro: Effect of hyperprolactinemia

The effect of hyperprolactinemia on central catecholamine biosynthesis and anterior pituitary hormone release was studied using an in vitro methodology. The incorporation of [³H]tyrosine into hypothalamic and neurohypophyseal catecholamines was determined using a new method which combines high performance liquid chromatography (HPLC) with amperometric detection (LCEC). Elevated plasma prolactin levels, induced by pituitary transplants, resulted in increased in vitro biosynthesis of medial basal hypothalamic (MBH) dopamine (DA), but not norepinephrine (NE). Neurohypophyseal DA biosynthesis (including the intermediate lobe) was not affected. Plasma LH levels were depressed by hyperprolactinemia although the content of hypothalamic luteinizing hormone-releasing hormone (LHRH) was not changed. In parallel studies, the anterior pituitaries from these animals were incubated in vitro using a paired-half technique and LH and PRL release measured. While the basal release of prolactin was not altered by hyperprolactinemia, LH release was significantly decreased. Gonadotroph responsiveness to LHRH was significantly increased, while the inhibition of prolactin by dopamine was not altered. There was a decrease in pituitary prolactin content with normal LH levels. These experiments confirm several in vivo reports which show that hypothalamic dopaminergic but not noradrenergic activity is increased by prolactin. This action is specifically localized in the tuberoinfundibular dopaminergic neurons. Furthermore, these experiments suggest that these central changes result in alterations in both gonadotroph and mammotroph function.     

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

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

Effects of cimetidine on prolactin, LH, and sex steroid secretion in male and female volunteers.

The histamine H2-receptor antagonist cimetidine, which has recently been introduced for the treatment of gastric and duodenal ulcers and haemorrhage, respectively, stimulates prolactin (PRL) secretion. The release of PRL after a bolus injection of the compound is significantly higher in female than in male volunteers, and is more pronounced during the luteal than during the follicular phase of the menstrual cycle. Oral administration of the drug stimulates PRL to a similar extent as parenteral administration. Treatment of male and female volunteers with cimetidine for 20 days (1 g/day orally) resulted in elevated serum PRL concentrations in both sexes. Basal LH levels as well as the response to 25 microgram LH-RH were not significantly changed after treatment. Testosterone levels in males and oestradiol levels in females were not altered. Luteal progesterone, however, was significantly diminished in the menstrual cycles when cimetidine was given. The mechanism underlying the stimulatory effect of cimetidine on PRL secretion is not clear, but it does not seem that this ability is exerted via an inhibition of pituitary dopamine receptors. The dopaminergic effect of lisuride, an ergot alkaloid, could not be reversed by cimetidine.     

Role of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors in the control of prolactin, growth hormone and gonadotropin secretion in prepubertal rats.

Excitatory amino acids, such as glutamate, constitute a major transmitter system in the control of hypothalamic-pituitary secretion. Different subtypes of glutamate receptors, such as NMDA (N-methyl-d-aspartic acid) and KA (kainate) receptors, are involved in the control of anterior pituitary secretion. Other receptor subtypes, such as AMPA (activated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid) and metabotropic receptors, have been identified, although their role in the control of neuroendocrine function remains largely unknown. Recent reports have demonstrated the involvement of AMPA receptors in the control of the steroid-induced luteinizing hormone (LH) surge in female and growth hormone (GH) secretion in male rats. The aim of this study was to assess the potential role of AMPA receptors in the control of GH, prolactin (PRL), LH and follicle-stimulating hormone (FSH) secretion in prepubertal 23-day-old rats. To this end, prepubertal female rats were injected with AMPA (2.5 or 5 mg/kg i.p.) or the antagonist of AMPA receptors 1,2,3,4-tetrahydro-6-nitro-2, 3-dioxo-benzo (f) quinoxaline-7-sulfonamide (NBQX; 0.25 or 0.50 mg/kg i.p.). Serum LH and FSH concentrations and hypothalamic LH-releasing hormone (LHRH) content remained unchanged after AMPA or NBQX administration. In contrast, serum PRL levels significantly decreased 15, 30 and 60 min after i.p. administration of AMPA and increased 120 min after NBQX treatment, whereas serum GH levels increased after AMPA treatment and decreased after NBQX administration. Considering that AMPA has been shown to activate a subset of kainate receptors, its effects were compared with those elicited by 2.5 mg/kg KA in prepubertal female rats. At this age, however, KA was unable to reproduce the effects of AMPA on PRL and GH secretion, thus suggesting that the actions observed after AMPA administration were carried out specifically through AMPA receptors. In addition, as the effects of AMPA on LH secretion in adult females have been proved to be steroid-dependent, the effects of AMPA (2.5 mg/kg) and NBQX (0.5 mg/kg) were tested in prepubertal animals with different gonadal backgrounds, i.e. intact males, and intact and ovariectomized (OVX) females. The effects of AMPA in prepubertal females appeared to be modulated by ovarian secretion, as the inhibition of PRL secretion disappeared and LH secretion was partially suppressed by AMPA in OVX animals whereas the stimulatory effect on GH release was enhanced by ovariectomy. Furthermore, in male rats, AMPA administration significantly decreased PRL secretion and increased serum GH levels, the amplitude of the GH response being higher than in prepubertal females. To ascertain the pituitary component for the reported actions of AMPA, hemi-pituitaries of male rats were incubated in the presence of AMPA (10(-8)-10(-6) M). The results obtained showed no effect of AMPA on PRL, GH and gonadotropin secretion in vitro. Finally, we investigated the involvement of the dopaminergic (DA) system in the inhibitory action of AMPA on PRL secretion. Pre-treatment of prepubertal female rats with a dopamine receptor antagonist (domperidone: 1 mg/kg) resulted in the blockage of AMPA-mediated inhibition of PRL secretion, thus suggesting that this action is probably mediated by an increase in DA activity. In conclusion, we provide evidence for the physiological role of AMPA receptors in the control of PRL and GH secretion in prepubertal rats. In contrast, our data cast doubts on the involvement of AMPA receptors in the regulation of gonadotropin secretion at this age. The effects of AMPA reported herein were not mediated through activation of kainate receptors and were probably exerted at the hypothalamic or suprahypothalamic levels. In addition, we show that ovarian secretion actively modulates the effects of AMPA receptor activation on anterior pituitary secretion in prepubertal female rats.     

Effects of cupric acetate on hypothalamic gonadotropin-releasing hormone release in intact and ovariectomized rabbits

The effects of intravenous injection of cupric acetate (CuAc) on release of hypothalamic gonadotropin-releasing hormone (GnRH) and pituitary luteinizing hormone (LH), follicle-stimulating hormone (FSH), and prolactin (PRL) were explored in conscious, ovarian intact does and in ovariectomized (OVEX) does that received subcutaneous implants, either blank Silastic capsules or Silastic capsules containing crystalline estradiol-17 beta (E2). Animals were subjected to push-pull perfusion of the posterior median eminence for 6 h, and CuAc was intravenously injected (2.5 mg/kg body weight) at the end of the 2nd h of push-pull perfusion. Perfusate samples were collected continuously and pooled for assay at 10-min intervals. Peripheral blood samples were obtained at 10- to 30-min intervals. Levels of GnRH in push-pull perfusate and LH, FSH, and PRL in plasma were measured by specific radioimmunoassays. In intact does (n = 4), intravenous injection of CuAc stimulated the release of hypothalamic GnRH and pituitary LH, FSH, and PRL. The initial increase and subsequent decrease in hypothalamic GnRH after CuAc injection preceded those in plasma LH, FSH, and PRL. Injection of saline into intact does (n = 4) had no effect on any of these hormone levels. In OVEX does that received blank Silastic capsules (n = 4), CuAc failed to stimulate either hypothalamic GnRH release or pituitary LH, FSH, and PRL release. In contrast, CuAc stimulated the release of all four hormones in OVEX does that received E2 containing Silastic capsules (n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)