Electrophysiological manifestation of luteinizing hormone-releasing hormone pulse generator activity in the rhesus monkey: influence of alpha-adrenergic and dopaminergic blocking agents

Characteristic increases in neuronal electrical activity associated with the initiation of each LH pulse were recorded from ovariectomized rhesus monkeys bearing multiple chronic electrodes in the medial basal hypothalamus. These electrophysiological manifestations of hypothalamic LHRH pulse generator activity were inhibited by the alpha-adrenergic blocker phentolamine or the alpha 1-adrenoceptor blockers phenoxybenzamine and prazosin. At the dosages used, the effects of single injections of these drugs ranged from a reduction in the frequency of LHRH pulse generator activity to its complete arrest. This was faithfully reflected in the pattern of pulsatile LH discharges. The dopaminergic blocking agent metaclopramide similarly reduced the frequency of the pulse generator or arrested its activity altogether. The alpha 2-adrenoceptor blocker yohimbine had no demonstrable effect on hypothalamic electrical activity at the doses studied. These findings support the view of a central action of alpha 1-adrenergic and dopaminergic blockade on LHRH pulse generator activity and the concept that central adrenergic and dopaminergic inputs can modulate the frequency of the LHRH pulse generator.     

Norepinephrine is a possible neurotransmitter stimulating pulsatile release of luteinizing hormone-releasing hormone in the rhesus monkey

The hypothesis that norepinephrine (NE) plays a facilitatory role in controlling the pulsatile release of LHRH was tested with a modified push-pull perfusion technique in conscious rhesus monkeys. The in vivo LHRH release in perfusate samples collected from the stalk-median eminence of ovariectomized females was pulsatile and synchronous with pulsatile LH release. Catecholamines measured in aliquots of perfusate samples revealed that in vivo NE release was also pulsatile and was synchronous with LHRH release. Local infusion of NE or methoxamine (an alpha 1-adrenergic stimulant) through a push cannula stimulated LHRH release, while iv injection of prazosin (an alpha 1-adrenergic blocker) suppressed LHRH release. It is concluded that NE is a possible neurotransmitter stimulating pulsatile LHRH release.     

Alpha 2-adrenergic receptors are involved in the suppression of luteinizing hormone release during acute fasting in the ovariectomized estradiol-primed rats.

It has been previously reported that the adrenergic system is involved in the control of feeding behavior and LH release. In the present study, the role of the adrenergic receptors in the suppression of LH release during acute fasting are examined by injecting the alpha 1-antagonist (prazosin), alpha 2-antagonists (idazoxan, SKF 86466-A, piperoxan), or beta-antagonist (propranolol) into the third ventricle of unfasted and 48 h fasted ovariectomized estradiol-treated rats. Blood samples were collected every 6 min for 3 h and the drugs were administered after the first hour of the sampling period. Prazosin caused a significant suppression of LH release in the unfasted animals while idazoxan and propranolol had no significant effects. In contrast, all alpha 2-antagonists blocked the inhibitory effect of fasting on LH release and significantly reinstated the suppressed LH release while prazosin and propranolol had no significant effects. We conclude from these results that the suppression of LH release during acute fasting is mediated by alpha 2-adrenergic receptors but not alpha 1- or beta-adrenergic receptors.     

The effect of morphine on the electrophysiological activity of the hypothalamic luteinizing hormone-releasing hormone pulse generator in the rhesus monkey

In ovariectomized rhesus monkeys, the electrophysiological manifestation of luteinizing hormone-releasing hormone (LHRH) pulse generator activity was arrested by morphine and reinitiated by naloxone. These responses were noted within 1 and 2 min, respectively, after the intravenous injection of the drugs. Naloxone given alone had no effect. These results support the view that opioids modulate pulsatile gonadotropin release by an action on the hypothalamic LHRH pulse generator.     

Adrenergic and dopaminergic regulation of gonadotropin-releasing hormone release from goldfish preoptic-anterior hypothalamus and pituitary in vitro.

The involvement of adrenergic and dopaminergic receptor subtypes on in vitro release of radioimmunoassayable gonadotropin-releasing hormone (GnRH) from incubated preoptic-anterior hypothalamic (P-AH) slices and pituitary fragments of sexually mature male goldfish was studied. Norepinephrine (NE) produced a dose-related stimulation of GnRH from P-AH slices, but not from pituitary fragments. The effects of some adrenergic receptor agonists (1 microM) on GnRH release from P-AH slices were tested: phenylephrine (alpha 1-agonist) significantly stimulated GnRH release; clonidine (alpha 2-agonist) and isoproterenol (beta-agonist) were ineffective. Incubation of P-AH slices with phentolamine (alpha 1/alpha 2-antagonist) and prazosin (alpha 1-antagonist), at a concentration of 1 microM, inhibited the release of GnRH induced by NE (60 microM); the alpha 2-antagonist yombibin and the beta-antagonist propanolol were ineffective. None of the adrenergic antagonists (1 microM) tested produced significant effects on spontaneous release of GnRH from both tissue preparations. Spontaneous release of GnRH from both P-AH slices and pituitary fragments was reduced by dopamine (DA) in a dose-related manner. The effects of some DA agonists (1 microM) were tested: apomorphine (D1/D2-agonist) and SKF 38398 (D1-agonist), but not bromocriptine and LY-171555 (D2-agonists) significantly reduced spontaneous GnRH release from P-AH slices in vitro. On the other hand, D2-agonists, but not D1-agonists, significantly reduced GnRH release from pituitary fragments. The effects of DA antagonists (1 microM) were also tested: in P-AH slices, addition of SKF-83566 (D1-antagonist) significantly reduced spontaneous GnRH release; pimozide and domperidone (D2-antagonist) were ineffective when tested alone.(ABSTRACT TRUNCATED AT 250 WORDS)     

The role of neuropeptide Y in the progesterone-induced luteinizing hormone-releasing hormone surge in vivo in ovariectomized female rhesus monkeys

Progesterone induces a LHRH surge in estrogen-primed ovariectomized rhesus monkeys, with a concomitant increase in the pulse frequency of neuropeptide Y (NPY) release. However, the role for NPY in the positive feedback action of progesterone on LHRH release in primates is unknown. The present study examines the effect of an antisense oligodeoxynucleotide for NPY messenger RNA (AS NPY) on the progesterone-induced LHRH surge in vivo using push-pull perfusion. The AS NPY was directly infused into the stalk-median eminence (S-ME), whereas perfusates were collected for assessment of LHRH release. For a control, a scrambled oligodeoxynucleotide was infused. The results indicate that 1) the scrambled oligodeoxynucleotide did not interfere with the progesterone-induced LHRH surge, 2) whereas AS NPY blocked the progesterone-induced increase in LHRH release, and 3) no LHRH surges were induced by oil as a control for progesterone, but the AS NPY also reduced LHRH release in oil controls. These data suggest that 1) AS NPY infusion into the S-ME results in reduction in LHRH release; and 2) NPY release in the S-ME is important for the positive feedback effects of progesterone on LHRH release in estrogen-primed ovariectomized monkeys.     

Neuropharmacological analysis of the control of LH secretion in gonadectomized male and female rats: altered hypothalamic responses to inhibitory neurotransmitters in long-term castrated rats

The control of LHRH and LH by neurotransmitters and neuromodulators such as the endogenous opioid peptides is essentially the same in intact adult male and female rats: adrenergic and dopaminergic agonists stimulate LH release and opioid agonists inhibit it. Several weeks after gonadectomy, however, the contribution of the endogenous ligands of adrenergic, dopaminergic and opioidergic receptors to the control of LHRH is altered. A detailed pharmacological analysis in long-term ovariectomized females confirmed previous reports that adrenergic and dopaminergic agonists still enhance secretion of LHRH and LH and opioid receptor agonists still suppress it. A similar investigation in long-term castrated males also confirmed previous reports that opioid agonists fail to block LH secretion. In addition, we have found that while adrenergic and dopaminergic agonists cause increases in serum concentrations of LH, adrenoreceptor and dopamine receptor antagonists do not inhibit LH release in long-term castrates. Furthermore, the opioid antagonist naloxone does not raise serum LH levels in either sex after long-term gonadectomy. These observations therefore imply reduced opioidergic, dopaminergic and adrenergic transmission, in relation to LHRH release, after long-term castration. In addition, opioid receptor activity (assessed by responsiveness to an opioid receptor agonist) of female rats is maintained, whereas that of male rats is lost, after long-term gonadectomy.     

In vivo secretion of LHRH in ovariectomized rats is regulated by a possible autofeedback mechanism

An episodic secretion of LHRH in pituitary portal blood was observed in ovariectomized and hypophysectomized rats under Saffan anesthesia. This anesthetic did not effect the pulsatile release of LH in ovariectomized rats. Third-ventricular administration of an LHRH agonist, at a concentration which did not cross-react in the LHRH RIA, suppressed both the pulse amplitude and frequency of LHRH release. This inhibitory action of the LHRH agonist on LHRH release was blocked by an LHRH antagonist.     

A role for norepinephrine in the control of puberty in the female rhesus monkey, Macaca mulatta.

The onset of puberty in female rhesus monkeys is characterized by increases in pulsatile LHRH release. In this study we have tested the hypothesis that changes in input to the LHRH neurosecretory system from noradrenergic neurons contribute to this pubertal increase in LHRH release. In the first experiment, the ability of the LHRH neurosecretory system of prepubertal (12-20 months of age, no signs of puberty evident), early pubertal (24-30 months, premenarchial), and midpubertal (30-45 months, postmenarchial but prior to first ovulation) monkeys to respond to alpha 1-adrenergic stimulation was tested. LHRH release in the stalk-median eminence of conscious monkeys was measured using an in vivo push-pull perfusion method. During push-pull perfusion, perfusates were collected continuously in 10-min fractions, and the alpha 1-adrenergic stimulant methoxamine (MTX, 10(-8), 10(-5) M) or vehicle was infused through the push cannula for 10 min at 90 min intervals. LHRH levels in perfusates were estimated by RIA. Monkeys in all three age groups responded to MTX with significant increases in LHRH release, with the response of the prepubertal group being significantly greater than that of the older age groups. The results indicate that alpha 1-adrenergic receptors are present and functional prior to puberty. In the second experiment, norepinephrine (NE) release in perfusates collected from monkeys in the three age groups was measured by HPLC with electrochemical detection. NE release increased significantly from the pre- and early pubertal to the midpubertal stage. The enhanced sensitivity of prepubertal monkeys to MTX may be due to the absence of high levels of endogenous NE, which results in a situation similar to denervation hypersensitivity. During the early pubertal stage, increases in input from noradrenergic neurons to the LHRH neurosecretory system may occur, thereby resulting in increases in LHRH release, since early pubertal monkeys are highly sensitive to alpha-adrenergic input. Therefore, we propose that the increase in NE release during puberty contributes to the developmental increase in LHRH release.     

Alpha 2-adrenergic control of growth hormone (GH) secretion in conscious male rabbits: involvement of endogenous GH-releasing factor and somatostatin

In an attempt to clarify the regulatory role in GH secretion of central alpha-adrenergic and dopaminergic mechanisms, the effects of iv administration of alpha 1- and alpha 2-adrenergic antagonists and dopaminergic antagonists were investigated in undisturbed conscious male rabbits. During a 6-h observation period (1030-1630 h), control animals demonstrated spontaneous pulsatile GH secretion with mean (+/- SEM) 6-h GH levels of 6.49 +/- 0.54 ng/ml (n = 16). Intravenous injection of yohimbine (YOM; an alpha 2-antagonist), chlorpromazine (CPZ), and haloperidol (HAL; dopamine and alpha-adrenergic antagonists) completely suppressed this pulsatile GH secretion (mean 6-h GH levels, 2.98 +/- 0.24, 3.48 +/- 0.24, and 2.91 +/- 0.29 ng/ml, respectively; P less than 0.001), whereas prazosin (an alpha 1-antagonist), pimozide (a selective dopamine antagonist), sulpiride, and YM-09151-2 (YM; D2-specific antagonists) failed to affect the GH secretory pattern (mean 6-h GH levels, 6.61 +/- 0.73, 6.71 +/- 0.56, 5.44 +/- 0.44, and 6.87 +/- 1.44 ng/ml, respectively). While an iv injection of 2 micrograms synthetic human GH-releasing factor-(1-44)-NH2 (hGRF) induced GH rises in prazosin-, HAL-, pimozide-, sulpiride-, and YM-treated rabbits as well as control rabbits, YOM and CPZ completely abolished these GH responses to hGRF injection. An iv injection of 10 ml antisomatostatin gamma-globulin caused a prompt and transient GH rise, followed by a sustained elevation of GH trough levels in normal control rabbits. YOM treatment completely abolished this highly oscillated GH release. However, suppression by YOM or CPZ of hGRF-induced GH rises was significantly reversed by iv administration of 10 ml antisomatostatin gamma-globulin. Therefore, the inhibitory effect of YOM and CPZ on both episodic GH release and hGRF-induced GH rises is due to the enhanced release of somatostatin, with a simultaneous suppression of endogenous GRF. On the other hand, HAL, possessing a weaker alpha-blocking action than CPZ, blunted pulsatile GH secretion, but only modestly suppressed hGRF-induced GH rises. These results suggest the following. 1) Central alpha 2-adrenergic mechanisms play a more important role in the regulation of GH secretion than alpha 1-adrenergic mechanisms in the rabbit as well as in other species. 2) The alpha 2-adrenergic blockade causes suppression of the release of hypothalamic GRF and enhanced release of endogenous somatostatin, thereby suppressing GH secretion.(ABSTRACT TRUNCATED AT 400 WORDS)     

Differential regulation of pulsatile luteinizing hormone (LH) and follicle-stimulating hormone secretion in ovariectomized rats disclosed by treatment with a LH-releasing hormone antagonist and phenobarbital

Although pulsatile LH release in ovariectomized (OVX) rats appears to be controlled by pulsatile discharges of LHRH, the neuroendocrine regulation of episodic FSH release remains to be explored. The main objective of the present study is to compare and contrast the effects of a potent LHRH antagonist (ALHRH) and a central nervous system depressant, phenobarbital (PhB), on pulsatile LH and FSH release in OVX rats. Three to 4 weeks after ovariectomy, blood samples were obtained at 10-min intervals for 3 h, after which LHRH was injected and sampling continued for an additional hour. In control OVX rats, periodic increases in plasma LH and FSH levels occurred approximately every 30 to 60 min, respectively. Treatment of OVX rats with PhB several hours earlier resulted in a suppression of mean plasma levels and pulse frequencies of both LH and FSH. Interestingly, PhB suppressed the pulse amplitude of LH, but not of FSH. Phenobarbital increased pituitary LH responses to LHRH, but did not alter the FSH responses. When ALHRH was given to OVX rats 24 h before blood sampling, mean plasma LH levels as well as LH pulse frequency and amplitude were severely diminished. In striking contrast, ALHRH did not affect the frequency or amplitude of FSH pulses. However, mean plasma FSH levels were suppressed to 31% of levels measured in control OVX rats. These results demonstrate that in contrast to LH secretion, FSH secretion in OVX rats appears to be regulated by two distinct neuroendocrine mechanisms: an LHRH-dependent mechanism controlling the nonepisodic component of FSH secretion (baseline secretion) and a LHRH-independent mechanism controlling pulsatile FSH release.     

Interleukin 1 alpha inhibits prostaglandin E2 release to suppress pulsatile release of luteinizing hormone but not follicle-stimulating hormone.

Interleukin 1 alpha (IL-1 alpha), a powerful endogenous pyrogen released from monocytes and macrophages by bacterial endotoxin, stimulates corticotropin, prolactin, and somatotropin release and inhibits thyrotropin release by hypothalamic action. We injected recombinant human IL-1 alpha into the third cerebral ventricle, to study its effect on the pulsatile release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in conscious, freely moving, ovariectomized rats. Intraventricular injection of 0.25 pmol of IL-1 alpha caused an almost immediate reduction of plasma LH concentration; this decrease was statistically significant 20 min after injection and occurred through a highly significant reduction in the number of LH pulses, with no effect on pulse amplitude. In contrast, there was no change in pulse frequency but a small significant elevation in amplitude of FSH pulses. Intraventricular injection of the diluent had no effect on gonadotropin release. The results provide further evidence for separate hypothalamic control mechanisms for FSH and LH release. To determine the mechanism of the suppression of LH release, mediobasal hypothalamic fragments were incubated in vitro with IL-1 alpha (10 pM) and the release of LH-releasing hormone (LHRH) and prostaglandin E2 into the medium was measured by RIA in the presence or absence of norepinephrine (50 microM). IL-1 alpha reduced basal LHRH release and blocked LHRH release induced by norepinephrine. It had no effect on the basal release of prostaglandin E2; however, it completely inhibited the release of PGE2 evoked by norepinephrine. To evaluate the possibility that IL-1 alpha might also interfere with the epoxygenase pathway of arachidonic acid metabolism, epoxyeicosatrienoic acids were also measured. IL-1 alpha had no effect on the content of epoxyeicosatrienoic acids in the hypothalamic fragments as measured by gas chromatography and mass spectrometry. In conclusion, IL-1 alpha suppresses LH but not FSH release by an almost complete cessation of pulsatile release of LH in the castrated rat. The mechanism of this effect appears to be by inhibition of prostaglandin E2-mediated release of LHRH.     

A norepinephrine-dependent mechanism in the preoptic/anterior hypothalamic area but not in the mediobasal hypothalamus is involved in the regulation of the gonadotropin-releasing hormone pulse generator in ovariectomized rats.

Pulsatile gonadotropin secretion from the pituitary is dependent upon the gonadotropin-releasing hormone (Gn-RH) pulse generator producing intermittent release of the neuropeptide into the portal vessels. Various neurotransmitters seem to be involved in the regulation of pulsatile Gn-RH release. The present study was an attempt to determine in vivo the temporal relation of preoptic/anterior hypothalamic area (PO/AH) norepinephrine (NE) release and pulsatile luteinizing hormone (LH) secretion in ovariectomized rats. To assess whether NE acts in the PO/AH to maintain pulsatile Gn-RH release, we applied locally an alpha 1-receptor antagonist into this structure. Push-pull cannulae (PPC) were implanted into the PO/AH of ovariectomized rats. The contralateral, not PPC-implanted PO/AH was lesioned electrochemically. Another group of ovariectomized rats was implanted with a PPC into the mediobasal hypothalamus. Two experiments were performed: (1) To determine whether the PO/AH or the mediobasal hypothalamus is the site where NE exerts its stimulatory effect on LH secretion, we applied doxazosine, a new specific alpha 1-receptor antagonist, locally into these structures by means of PPC. The effect of this adrenergic drug on the Gn-RH pulse generator was examined by measuring blood LH levels. (2) To study the temporal relation between in vivo release rates of NE and amine metabolites in the preoptic area and pulsatile pituitary LH secretion, preoptic perfusates and blood samples were collected at 5-min intervals. Brain perfusates were subjected to high-performance liquid chromatography-electrochemical analysis. In blood samples LH concentrations were determined.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interactions between neuropeptide Y and adrenergic systems in the stimulation of luteinizing hormone release in steroid-primed ovariectomized rats

Intraventricular injection of neuropeptide Y (NPY) stimulates LH release in estradiol benzoate- and progesterone-primed (EBP) ovariectomized rats. Because adrenergic neurotransmitters, norepinephrine (NE) and epinephrine (E), show intraneuronal coexistence with NPY in certain brain regions of the rat and there are similarities in the effects of NPY and NE/E on LH release, we investigated the possible interaction of NPY and adrenergic receptor systems in the stimulation of LH release in EBP-treated ovariectomized rats. The experiments were designed to determine whether NPY exerted its effects via adrenergic receptors and whether combined administrations of NPY and NE can act synergistically or in an additive manner to enhance the LH release response. Permanent stainless steel cannulae were placed in the third ventricle of the brain, and the rats were ovariectomized. Two weeks after surgery, rats were injected with EB (30 micrograms/rat) and P (15 mg/rat). Two days later, the effects of either vehicle alone (control) or various adrenergic and dopaminergic receptor antagonists and an opiate receptor agonist on stimulation of LH release by NPY were assessed. Intraventricular injection of 0.47 nmol NPY increased plasma LH levels at 10, 20, and 30 min in control rats. The NPY-induced LH response was not blocked by pretreatment with any of the following drugs: the alpha-adrenoreceptor antagonist phenoxybenzamine, the alpha 1-adrenoreceptor antagonist prazosin, beta-adrenoreceptor antagonist propranolol, the dopamine receptor antagonist pimozide, or the opiate receptor agonist morphine. All of these drugs affect LH release in other circumstances. On the other hand, the alpha 2-adrenoreceptor antagonist yohimbine significantly attenuated the NPY-induced LH increments. In the second study dealing with the possible synergistic or additive interactions between NPY and NE, we observed that when doses of NPY and NE that separately were only minimally effective in stimulating LH release were administered together, the amounts of LH secreted were greater than the sum of the individual responses. However, when NE and NPY were given together in doses that alone had either no stimulatory or maximal stimulatory effects, there were no additive or synergistic effects on LH release.(ABSTRACT TRUNCATED AT 400 WORDS)     

Absence of steroid-dependent, endogenous opioid peptide suppression of pulsatile luteinizing hormone release between diestrus 1 and diestrus 2 in the rat estrous cycle

The objective of this study was to determine whether the negative feedback action of ovarian steroids on pulsatile luteinizing hormone (LH) release in the diestrous 1 (D1)-diestrous 2 (D2) interval of the rat estrous cycle is mediated by endogenous opioid peptides (EOPs), by examining the pulsatile LH release response to naloxone infusions in the presence or absence of D1-D2 levels of estradiol (E2) and progesterone (P). As plasma E2 and P levels increased between D1 and D2, mean blood LH levels decreased due solely to a decrease in LH pulse amplitude as frequency remained stable. However, ovariectomy increased both parameters of pulsatile LH release, indicating the effect of loss of ovarian steroid-negative feedback in this interval. Replacement of D1-D2 plasma levels of E2 and P restored D2 values for both parameters of pulsatile LH release, and E2 + P did not alter in vivo pituitary responsiveness to LH-releasing hormone (LHRH). In ovariectomized rats lacking the negative feedback provided by E2 + P in this cycle interval, continuous infusion of naloxone caused a further dose-dependent augmentation in both LH pulse amplitude and frequency. This stimulatory action of naloxone was prevented by simultaneous infusion with morphine, and was not associated with any change in in vivo pituitary responsiveness to LHRH, indicating that this was an action exerted through centrally located EOP receptors. Naloxone also increased both parameters of pulsatile LH release in E2 + P-treated rats. However, the magnitudes of the naloxone-induced increments in LH pulse amplitude and frequency in ovariectomized, steroid-treated rats were not greater than those seen in ovariectomized, nonsteroid-treated rats given naloxone versus saline. In addition, mean values for both parameters of pulsatile LH secretion during EOP receptor blockade in steroid-treated rats were reduced when compared to values in ovariectomized, nonsteroid-treated rats infused with naloxone. Thus the stimulatory effect of naloxone on pulsatile LH release was similar in the presence or absence of the negative feedback action of D1-D2 plasma levels of E2 + P. This indicates that the negative feedback effect of E2 + P on pulsatile LH release in this interval is not mediated by EOPs whose actions are blocked by naloxone.     

Failure to monoaminergic and cholinergic receptor blockers to prevent prostaglandin E2-induced luteinizing hormone release.

Receptor blocking drugs were used to determine whether adrenergic, dopaminergic, serotoninergic, or cholinergic synapses are involved in mediating the LH release induced by intraventricularly injected PGE2. Prostaglandin E2 (5mug) was injected into the 3rd ventricle (3rd V) of ovariectomized rats, and plasma LH concentrations before and after treatment were determined by radioimmunoassay. Phentolamine, 20 or 30 mug, or pronethalol, 20 mug (alpha and beta adrenergic receptor blockers, respectively) injected into the 3rd V failed to alter the elevation of plasma LH evoked by PGE2 injected into the ventricle 10 min later. Likewise, LH release following PGE2 was not changed when a dopaminergic blocker, pimozide (0.63 mg/kg, SC), was injected 2 h prior to PGE2. Two antagonists of serotonin, methysergide maleate (3 mg/kg ip) or cinanserin HC1 (1 mg/kg iv) given 2 h or 45 min before PGE2, respectively, failed to alter the action of PGE2. Atropine (100 or 250 mug) injected into the 3rd V 10 min prior to PGE2 was also ineffective in blocking the increase in plasma LH following PGE2. The results of this study indicate that the effect of PGE2 on LH release is not mediated by adrenergic, dopaminergic, serotoninergic, or cholinergic receptors. They also suggest that PGE2 is not acting trans-synaptically but probably directly on the LHRH neuron to induce the discharge of LHRH into the hypophysial portal vessels which then evokes release of LH from the adenohypophysis.     

Noradrenergic regulation of growth hormone secretion in the baboon

We have investigated the effects of iv administered noradrenergic agonists and antagonists on plasma GH concentration in the adolescent baboon, Papio papio, with the aim of defining the relative roles of adrenergic receptor subtypes (alpha 1, alpha 2, beta 1, and beta 2) in the regulation of GH release. Clonidine (0.02 mg/kg) or UK-14,304 (0.02 mg/kg), potent centrally acting alpha 2 noradrenergic agonists, were infused into 24 animals pretreated with either saline, or selective alpha 1 and alpha 2 noradrenergic antagonists. Both agonists potently augment plasma GH, producing peak levels of 30-60 ng/ml 15 min post infusion. These responses can be prevented by the prior infusion of the alpha 2 antagonist, piperoxane (1.0 mg/kg), but not by the alpha 1 antagonist, prazosin (2.0 mg/kg). Log dose response curves of the 2 agonists demonstrate a greater potency for UK-14,304 vs. clonidine on a molar basis. In animals pretreated with monoamine depleting agents (reserpine and alpha-methyl paratyrosine) the plasma GH response to an infusion of clonidine (0.02 mg/kg) is significantly enhanced (P less than 0.001). Beta-Adrenoreceptor antagonism by propranolol (0.02 or 1.0 mg/kg) or the more selective beta 2-adrenoreceptor antagonist, ICI 118,551 (0.02-1.0 mg/kg), results in a rapid and significant (P less than 0.01) increase in plasma GH. The beta 1-antagonist, practolol (0.2-2.0 mg/kg), does not alter plasma GH levels. It is proposed that in the baboon, noradrenaline acts on alpha 2-noradrenergic receptors to stimulate GH release and on beta 2-noradrenergic receptors to inhibit GH release.     

Neuropeptide Y is a neuromodulator of pulsatile luteinizing hormone-releasing hormone release in the gonadectomized rhesus monkey.

In a previous study, we have demonstrated that infusion of neuropeptide Y (NPY) into the stalk-median eminence (S-ME) of gonadectomized rhesus monkeys stimulated LHRH in a dose-dependent manner. This finding led us to address the following questions: 1) What are the characteristics of NPY release in vivo? 2) How does NPY release relate to LHRH release? 3) Is endogenous NPY essential to pulsatile LHRH release? To answer these questions, three experiments using push-pull perfusion were performed in adult gonadectomized rhesus monkeys. Perfusate samples from the S-ME were collected at 10-min intervals for 6 to 12-h periods, and the concentrations of LHRH and NPY in perfusates were determined by RIA. In Exp I, the release pattern of NPY and LHRH in the S-ME was independently determined in a group of 11 conscious monkeys: NPY release in the S-ME was pulsatile with an interpulse interval of 44.9 +/- 3.3 min (n = 11). This interpulse interval was similar to that seen for LHRH release (43.8 +/- 1.1 min, n = 7). Exp II was designed to determine whether NPY pulses and LHRH pulses occur synchronously and to examine whether NPY release in the S-ME is correlated with circulating LH pulses. NPY and LHRH concentrations in aliquots of the same perfusate sample from the S-ME and circulating LH levels were concurrently measured in 8 monkeys sedated with Saffan. It was found that NPY pulses were temporally correlated (P less than 0.001) with LHRH pulses, which were also temporally correlated (P less than 0.001) with LH pulses. Moreover, NPY pulses were correlated (P less than 0.05) with LH pulses. NPY peaks preceded LHRH peaks by 4.5 +/- 0.6 min, LHRH peaks preceded LH peaks by 5.5 +/- 0.6 min, and NPY peaks preceded LH peaks by 9.7 +/- 0.8 min. In Exp III, the role of endogenous NPY in LHRH release was evaluated by infusing a specific antiserum to NPY into the S-ME during push-pull perfusion in 8 conscious monkeys. Infusion of a specific antiserum to NPY into the S-ME at 1:100 and 1:1000 dilutions suppressed pulsatile LHRH release significantly (P less than 0.05). Infusion of nonimmune serum as a control was without effect. These results are summarized as follows: 1) NPY release in the S-ME is pulsatile, 2) NPY pulses occur synchronously with LHRH and LH pulses, and 3) immunoneutralization of endogenous NPY in the S-ME suppresses pulsatile LHRH release.(ABSTRACT TRUNCATED AT 400 WORDS)     

Prazosin, an alpha 1-adrenergic receptor antagonist, suppresses experimental autoimmune encephalomyelitis in the Lewis rat.

Prazosin, an antagonist of alpha 1-adrenergic receptors, has been found to suppress the clinical and histological expression of experimental autoimmune encephalomyelitis (EAE) in the Lewis rat. Suppression was more significant in females than in males and was a dose-dependent phenomenon. Analysis of the effect of other adrenergic receptor antagonists supports the conclusion that the suppressive effect of prazosin is a consequence of blockade of the alpha 1-receptor since treatment with either the alpha 2-antagonist yohimbine or the beta-antagonist propranolol exacerbated the disease, whereas treatment with the long-acting mixed alpha 1/alpha 2-antagonist phenoxybenzamine had some suppressive activity. Treatment with prazosin was also able to suppress clinical and histological signs of EAE in animals sensitized by adoptive transfer with activated spleen or lymph node cells. Whether prazosin acts through altering vascular permeability or the immune response, or both, remains to be determined.     

Inhibitory effect of central LHRH on LH secretion in the ovariectomized ewe

The role of central luteinizing hormone releasing hormone (LHRH) in the control of pulsatile LHRH and luteinizing hormone (LH) secretion was investigated in ovariectomized adult ewes. Injection of LHRH (2.1-21 pmol) into the third cerebral ventricle caused a delayed but sustained inhibition of LH secretion. Pulse frequency, pulse amplitude and mean LH levels were reduced significantly when compared with the responses to the control injection of saline (50 microliters). The inhibitory effect of centrally administered LHRH was not accompanied by a reduction in the pituitary responsiveness to intravenous LHRH. In contrast to the effect on LH, plasma levels of follicle-stimulating hormone (FSH) and prolactin were unaffected by central LHRH. The inhibitory action of LHRH was antagonized by prior injection of an LHRH antagonist ([N-Ac-D-Nal(2)1, D-p-Cl-Phe2, D-Trp3, D-hArg (Et2)6, D-Ala10] LHRH, 69 pmol) into the third ventricle. Central injection of the LHRH antagonist alone (at the same concentration) did not influence any characteristic of pulsatile LH secretion. In conclusion, these data indicate that exogenous administration of LHRH into the brain exerts a dose-related and receptor-mediated inhibition of LHRH pulse generator activity. However, the physiological significance of endogenous LHRH in the regulation of the LHRH pulse generator remains unresolved.