Acute glutamate-induced elevations in serum testosterone and luteinizing hormone

Influence of sexual behavior of Dorper rams treated with glutamate and/or testosterone on reproductive performance of anovulatory ewes

The aim of this study was to determine if exogenous administration of glutamate and (or) testosterone to male rams during the season of reproductive arrest is able to re-activate male sexual behavior and, later on, to promote through the male effect, both sexual and reproductive competence of anovulatory nulliparous ewes. Therefore, an experiment was performed under long-day photoperiods (spring; photo-reproductive arrest, 26°N). Dorper rams were randomly divided into four homogeneous experimental groups (n = 5 males each) regarding live weight (LW), body condition score (BCS), scrotal circumference (SC) and odor intensity (OI). Then, groups were treated with: i) GG (7 mg kg^-1 LW of glutamate, every 4d × 30d, im.), ii) GGT (7 mg kg^-1 LW of glutamate every 4d × 30d im + 25 mg of testosterone propionate, every 3d × 15d, im.), iii) GT (25 mg of testosterone propionate every 3d × 15d, im, and iv) GC (1 mL of saline, every 4d × 30d, im.). Thereafter, Dorper rams, (n = 4 per group) were selected and exposed to Dorper anovulatory-nulliparous ewes divided in four groups (n = 14 ewes each), and all the appetitive (ASB) and consummatory (CSB) sexual behaviors and indicators of sexual rest (ISR) were registered during the first 48 h of this male-to-females contact. Thereafter, males continued the male-to-female contact for another 8 d, in order to quantify the ewe's sexual and reproductive response through the male effect. During the sexual behavior tests, the GGT rams showed the highest ASB + CSB frequencies (P < 0.05) followed by the GG-rams with the lowest frequencies showed by the GC and GT groups. While the highest ISR behaviors were shown by the GG and GGT groups (P < 0.05) followed by the GC and GT-rams, no differences occurred regarding LW, BCS, and SC along the experimental breeding, with the largest (P < 0.05) OI shown by the GGT-rams and the lowest value observed in the control rams. Regarding the reproductive response of the nulliparous-anovulatory ewes exposed to treated rams, the glutamate + testosterone treated males induced not only an increased (P < 0.05) ovulatory activity and faster (P < 0.05) estrus response but an augmented (P < 0.05) pregnancy rate of the previously anovulatory ewes. To conclude, whereas the GG and GGT treatments generated encouraging sexual and reproductive outcomes, our results are also thought-provoking from a comparative sexual behavior perspective while may embrace potential applications to other animal industries.

Lack of pulse and surge modes and glutamatergic stimulation of luteinising hormone release in Kiss1 knockout rats

Kisspeptin, encoded by the Kiss1 gene, has attracted attention as a key candidate neuropeptide in controlling puberty and reproduction via regulation of gonadotrophin-releasing hormone (GnRH) secretion in mammals. Pioneer studies with Kiss1 or its cognate receptor Gpr54 knockout (KO) mice showed the indispensable role of kisspeptin-GPR54 signalling in the control of animal reproduction, although detailed analyses of gonadotrophin secretion, especially pulsatile and surge-mode of luteinising hormone (LH) secretion, were limited. Thus, in the present study, we have generated Kiss1 KO rats aiming to evaluate a key role of kisspeptin in governing reproduction via pulse and surge modes of GnRH/LH secretion. Kiss1 KO male and female rats showed a complete suppression of pulsatile LH secretion, which is responsible for folliculogenesis and spermatogenesis, and an absence of puberty and atrophic gonads. Kiss1 KO female rats showed no spontaneous LH/follicle-stimulating hormone surge and an oestrogen-induced LH surge, suggesting that the GnRH surge generation system, which is responsible for ovulation, does not function without kisspeptin. Furthermore, challenge of major stimulatory neurotransmitters, such as monosodium glutamate, NMDA and norepinephrine, failed to stimulate LH secretion in Kiss1 KO rats, albeit they stimulated LH release in wild-type controls. Taken together, the results of the present study confirm that kisspeptin plays an indispensable role in generating two modes (pulse and surge) of GnRH/gonadotrophin secretion to regulate puberty onset and normal reproductive performance. In addition, the present study suggests that kisspeptin neurones play a critical role as a hub integrating major stimulatory neural inputs to GnRH neurones, using newly established Kiss1 KO rats, which serve as a useful model for detailed analysis of hormonal profiles.

The effects of glutamate and citrate on absorption and distribution of aluminum in rats

The objective of this study was to evaluate the effect of glutamate (Glu) and citrate (Cit) on the absorption and distribution of aluminum in rats. In the in vitro experiment, 18 adult male Sprague-Dawley rats (average weight of 250 ± 15 g) were randomly divided into three groups. The entire intestine was rapidly removed and cultured in prediction samples of 20 mmol AlCl(3), 20 mmol AlCl(3)+20 mmol Cit, and 20 mmol AlCl(3)+20 mmol Glu, respectively. Liquid in different intestines and the intestines were obtained for Al determination. In the in vivo chronic study, 24 adult male Sprague-Dawley rats (average weight of 127 ± 10 g) were divided into four groups fed with the following diets: no Al and Glu added (control), AlCl(3) (1.2 mmol), AlCl(3) (1.2 mmol) + Cit (1.2 mmol), and AlCl(3) (1.2 mmol) + Glu (1.2 mmol) daily for 50 days, respectively. After rat sacrifice, blood samples were obtained for biochemical analyses, and organ samples like the brain, kidney, liver, and bone were rapidly taken for Al determination. The results showed that the absorption rate of Al with the following order: duodenum > jejunum > ileum in the in vitro study and the administration of AlCl(3)+Cit or AlCl(3)+Glu resulted in significant increases in Al absorption in the three parts of the gut (duodenum, jejunum, and ileum) compared to the AlCl(3) alone group based on wet weight (P < 0.05). There were no differences between the AlCl(3)+Cit and AlCl(3)+Glu groups. In the in vivo chronic study, supplementing either AlCl(3) alone or AlCl(3)+Glu decreased food consumption significantly (P < 0.05) compared with the control group. Compared with the control group, animals fed with the AlCl(3) diet monitored for red blood cell, kidney, and liver showed a higher level (P < 0.05), but did not significantly increase Al retention in the brain and bone (P > 0.05); animals fed with AlCl(3)+Cit diets were monitored for higher Al retention in the brain, kidney, bone, and liver (P < 0.05), while animals fed with AlCl(3)+Glu diets were monitored for red blood cell, brain, and kidney (P < 0.05). Compared with the AlCl(3) group, simultaneous administration of AlCl(3) and Glu led to a significant increase in Al retention in red blood cell, brain, and kidney (P < 0.01) while AlCl(3) and Cit in the kidney and bone (P < 0.01). Simultaneous administration of AlCl(3) and Cit significantly increases plasma malondialdehyde level (P < 0.05); both simultaneous administration of AlCl(3) and Glu or AlCl(3) and Cit led to significant decreases in superoxide dismutase level in the plasma (P < 0.05), while AlCl3 alone did not. The results indicated that both Cit and Glu enhanced Al absorption in the intestine in vitro, and Glu increased Al deposition in red blood cell, brain, and kidney in vivo.

Quantitative measurement of changes in calcium channel activity in vivo utilizing dynamic manganese-enhanced MRI (dMEMRI)

The ability of manganese ions (Mn(2+)) to enter cells through calcium ion (Ca(2+)) channels has been used for depolarization dependent brain functional imaging with manganese-enhanced MRI (MEMRI). The purpose of this study was to quantify changes to Mn(2+) uptake in rat brain using a dynamic manganese-enhanced MRI (dMEMRI) scanning protocol with the Patlak and Logan graphical analysis methods. The graphical analysis was based on a three-compartment model describing the tissue and plasma concentration of Mn. Mn(2+) uptake was characterized by the total distribution volume of manganese (Mn) inside tissue (V(T)) and the unidirectional influx constant of Mn(2+) from plasma to tissue (K(i)). The measurements were performed on the anterior (APit) and posterior (PPit) parts of the pituitary gland, a region with an incomplete blood brain barrier. Modulation of Ca(2+) channel activity was performed by administration of the stimulant glutamate and the inhibitor verapamil. It was found that the APit and PPit showed different Mn(2+) uptake characteristics. While the influx of Mn(2+) into the PPit was reversible, Mn(2+) was found to be irreversibly trapped in the APit during the course of the experiment. In the PPit, an increase of Mn(2+) uptake led to an increase in V(T) (from 2.8±0.3 ml/cm(3) to 4.6±1.2 ml/cm(3)) while a decrease of Mn(2+) uptake corresponded to a decrease in V(T) (from 2.8±0.3 ml/cm(3) to 1.4±0.3 ml/cm(3)). In the APit, an increase of Mn(2+) uptake led to an increase in K(i) (from 0.034±0.009 min(-1) to 0.049±0.012 min(-1)) while a decrease of Mn(2+) uptake corresponded to a decrease in K(i) (from 0.034±0.009 min(-1) to 0.019±0.003 min(-1)). This work demonstrates that graphical analysis applied to dMEMRI data can quantitatively measure changes to Mn(2+) uptake following modulation of neural activity.

Kisspeptin signaling is required for peripheral but not central stimulation of gonadotropin-releasing hormone neurons by NMDA

NMDA and kisspeptins can stimulate gonadotropin-releasing hormone (GnRH) release after peripheral or central administration in mice. To determine whether these agonists act independently or through a common pathway, we have examined their ability to stimulate GnRH/luteinizing hormone (LH) release after peripheral or central administration in Kiss1- or Gpr54 (Kiss1r)-null mutant mice. Peripheral injection of NMDA failed to stimulate GnRH/LH release in prepubertal or gonadally intact mutant male mice. Dual-labeling experiments indicated a direct activation of Kiss1-expressing neurons in the arcuate nucleus. In contrast, central injection of NMDA into the lateral ventricle increased plasma LH levels in both Kiss1 and Gpr54 mutant male mice similar to the responses in wild-type mice. Central injection of NMDA stimulated c-Fos expression throughout the hypothalamus but not in GnRH neurons, suggesting an action at the nerve terminals only. In contrast, kisspeptin-10 stimulated LH release after both central and peripheral injection but induced c-Fos expression in GnRH neurons only after central administration. Finally, central injection of NMDA induces c-Fos expression in catecholamine- and nitric oxide-producing neurons in the hypothalamus of mutant mice, indicating a possible kisspeptin-independent GnRH/LH release by NMDA through activation of these neurons. Thus, NMDA may act at both GnRH cell bodies (kisspeptin-independent) and nerve terminals (kisspeptin-dependent) in a dual way to participate in the GnRH/LH secretion in the male mouse.

Novel aspects of glutamatergic signalling in the neuroendocrine system

L-glutamate, the main excitatory neurotransmitter, influences virtually all neurones of the neuroendocrine hypothalamus via synaptic mechanisms. Vesicular glutamate transporters (VGLUT1-3), which selectively accumulate L-glutamate into synaptic vesicles, provide markers with which to visualise glutamatergic neurones in histological preparations; excitatory neurones in the endocrine hypothalamus synthesise the VGLUT2 isoform. Results of recent dual-label in situ hybridisation studies indicate that glutamatergic neurones in the preoptic area and the hypothalamic paraventricular, supraoptic and periventricular nuclei include parvocellular and magnocellular neurosecretory neurones which secrete peptide neurohormones into the bloodstream to regulate endocrine functions. Neurosecretory terminals of GnRH, TRH, CRF-, somatostatin-, oxytocin- and vasopressin-secreting neurones contain VGLUT2 immunoreactivity, suggesting the co-release of glutamate with hypophysiotrophic peptides. The presence of VGLUT2 also indicates glutamate secretion from non-neuronal endocrine cells, including gonadotrophs and thyrotrophs of the anterior pituitary. Results of in vitro studies show that ionotropic glutamate receptor analogues can elicit hormone secretion at neuroendocrine/endocrine release sites. Structural constituents of the median eminence, adenohypophysis and neurohypophysis contain elements of glutamatergic transmission, including glutamate receptors and enzymes of the glutamate/glutamine cycle. The synthesis of VGLUT2 exhibits robust up-regulation in response to certain endocrine challenges, indicating that altered glutamatergic signalling may represent an important adaptive mechanism. This review article discusses the newly emerged non-synaptic role of glutamate in neuroendocrine and endocrine communication.

Expression of glutamate receptor subunits in the hypothalamus of the female rat during the afternoon of the proestrous luteinizing hormone surge and effects of antiprogestin treatment and aging

The excitatory transmitter, glutamate has been implicated in the control of reproduction, hormone secretion and neuroendocrine regulation. The present study examined whether the hypothalamic expression of three key ionotropic glutamate receptor subunits (NMDAR1, GluR1 and GluR6) fluctuates significantly on proestrus in the rat, and whether treatment with the antiprogestin, RU486 affected glutamate receptor subunit expression. The studies revealed that NMDAR1, GluR1 and GluR6 mRNA levels in the mediobasal hypothalamus (MBH) and preoptic area (POA) fluctuate little throughout the day of proestrus. However, treatment with the antiprogestin, RU486 induced a significant elevation of GluR6 mRNA levels at 14.00 and 16.00 h on proestrus in the MBH, suggesting that endogenous progesterone (P4) may act to inhibit hypothalamic GluR6 levels. In support of this suggestion, exogenous P4 treatment to estrogen (E2)-primed ovariectomized (ovx) rats significantly suppressed GluR6 mRNA levels in the afternoon (12.00-16.00 h) in the MBH, and at 12.00 h in the POA, which preceded LH surge induction. Likewise, temporal examination of hypothalamic GluR6 protein levels in E2 + P4-treated young and middle-aged ovx rats revealed an early elevation from 12.00 to 14.00 h, which was followed by a fall from 16.00 to 20.00 h. The early elevation of GluR6 protein levels was most pronounced in the POA of the young rat, and this elevation was markedly attenuated in the middle-aged rat. As a whole, the studies suggest that glutamate receptor expression fluctuates little on proestrus in the hypothalamus, but that expression of the kainate GluR6 receptor subunit may be modulated by progesterone and aging.

Neurobiological mechanisms of the onset of puberty in primates

An increase in pulsatile release of LHRH is essential for the onset of puberty. However, the mechanism controlling the pubertal increase in LHRH release is still unclear. In primates the LHRH neurosecretory system is already active during the neonatal period but subsequently enters a dormant state in the juvenile/prepubertal period. Neither gonadal steroid hormones nor the absence of facilitatory neuronal inputs to LHRH neurons is responsible for the low levels of LHRH release before the onset of puberty in primates. Recent studies suggest that during the prepubertal period an inhibitory neuronal system suppresses LHRH release and that during the subsequent maturation of the hypothalamus this prepubertal inhibition is removed, allowing the adult pattern of pulsatile LHRH release. In fact, y-aminobutyric acid (GABA) appears to be an inhibitory neurotransmitter responsible for restricting LHRH release before the onset of puberty in female rhesus monkeys. In addition, it appears that the reduction in tonic GABA inhibition allows an increase in the release of glutamate as well as other neurotransmitters, which contributes to the increase in pubertal LHRH release. In this review, developmental changes in several neurotransmitter systems controlling pulsatile LHRH release are extensively reviewed.

The role of glutamate and nitric oxide in the reproductive neuroendocrine system

The preovulatory surge of gonadotropin releasing hormone (GnRH) is essential for mammalian reproduction. Recent work has implicated the neurotransmitters glutamate and nitric oxide as having a key role in this process. Large concentrations of glutamate are found in several hypothalamic nuclei known to be important for GnRH release and glutamate receptors are also located in these key hypothalamic nuclei. Administration of glutamate agonists stimulate GnRH and LH release, while glutamate receptor antagonists attenuate the steroid-induced and preovulatory LH surge. Glutamate has also been implicated in the critical processes of puberty, hormone pulsatility, and sexual behavior. Glutamate is believed to elicit many of these effects by activating the release of the gaseous neurotransmitter, nitric oxide (NO). NO potently stimulates GnRH by activating a heme containing enzyme, guanylate cyclase, which in turn leads to increased production of cGMP and GnRH release. Recent work has focused on identifying anchoring and (or) clustering proteins that target glutamate receptors to the synapse and couple the glutamate-NO neurotransmission system. The present review will discuss these new findings, as well as the role of glutamate and nitric oxide in important mammalian reproductive events, with a focus on the hypothalamic control of preovulatory GnRH release.

Key words: glutamate, nitric oxide, GnRH, postsynaptic density, hypothalamus.

Pituitary hormone secretion in normal male humans: acute responses to a large, oral dose of monosodium glutamate

Numerous studies have shown that the administration of a glutamate receptor agonist or a high dose of glutamate stimulates pituitary hormone secretion in animals. However, only a single human study has reported that an oral load of glutamic acid induced the secretion of prolactin and probably adrenocorticotropic hormone (ACTH) (but not other pituitary hormones). Because of glutamate's use in foods as monosodium glutamate (MSG), a flavoring agent, and the limited amount of human data, we studied the effect of a large oral dose of MSG in humans on the secretion of prolactin and other pituitary hormones. Fasting male subjects bearing venous catheters received on separate days each of the following four treatments: a vehicle, MSG (12.7 g), a high protein meal (a physiologic stimulus of prolactin secretion) by mouth, or an intravenous infusion of thyrotropin-releasing hormone (TRH, a pharmacologic stimulus of prolactin secretion). Plasma hormone responses were quantitated by RIA at 20-min intervals for 4 h. The protein meal induced a modest increase and TRH infusion a substantial increase in plasma prolactin, whereas MSG ingestion did not. MSG ingestion also did not raise the plasma concentrations of any of the other pituitary hormones measured (luteinizing hormone, follicle-stimulating hormone, thyroid-stimulating hormone, growth hormone) or of cortisol. Ingestion of MSG raised plasma glutamate concentrations 11-fold; the protein meal did not raise plasma glutamate. The results demonstrate that MSG ingestion in humans does not modify anterior pituitary hormone secretion. One implication is that diet-derived glutamate may not penetrate into hypothalamic regions controlling anterior pituitary function.

Effect of excitatory amino acids on serum TSH and thyroid hormone levels in freely moving rats

The actions of glutamate (L-Glu), and glutamate receptor agonists on serum thyroid hormones (T4 and T3) and TSH levels have been studied in conscious and freely moving adult male rats. The excitatory amino acids (EAA), L-Glu, N-methyl-D-aspartate (NMDA), kainic acid (KA) and domoic acid (Dom) were administered intraperitoneally. Blood samples were collected through a cannula implanted in the rats jugular 0--60 min after injection. Thyroid hormone concentrations were measured by enzyme immunoassay, and thyrotrophin (TSH) concentrations were determined by radioimmunoassay. The results showed that L-Glu (20 and 25 mg/kg) and NMDA (25 mg/kg) increased serum thyroxine (T4), triiodothyronine (T3) and TSH concentrations. Serum thyroid hormone levels increased 30 min after treatment, while serum TSH levels increased 5 min after i.p. administration, in both cases serum levels remained elevated during one hour. Injection of the non-NMDA glutamatergic agonists KA (30 mg/kg) and Dom (1 mg/kg) produced an increase in serum thyroid hormones and TSH levels. These results suggest the importance of EAAs in the regulation of hormone secretion from the pituitary-thyroid axis, as well as the importance of the NMDA and non-NMDA receptors in this stimulatory effect.

Characterization of ionotropic glutamate receptors in rat hypothalamus, pituitary and immortalized gonadotropin-releasing hormone (GnRH) neurons (GT1-7 cells)

Evidence from various sources suggested that the Gonadotropin-Releasing Hormone (GnRH) neuron does not contain glutamate receptors. Northern analysis of the hypothalamus showed the presence of NMDAR1, GluR1, GluR4 and GluR6 mRNA, while the pituitary showed the presence of NMDAR1, GluR1 and GluR6 mRNA. Western blot analysis also showed the presence of NMDAR1 and GluR1 protein. Since there are relatively few GnRH neurons in the hypothalamus, and GT1-7 cells have been considered to be a GnRH neuronal cell line, GT1-7 cells were studied in detail. GT1-7 cells contained NMDAR1 mRNA levels as shown by Northern analysis but did not contain GluR1, GluR4, or GluR6 mRNA. They did not show the presence of NMDAR1 and GluR1 protein by Western analysis. In addition, GT1-7 cells showed no NMDA receptor binding using the competitive inhibitor CGP-39563 and the noncompetitive inhibitor MK-801. Likewise, no binding was detected for kainate receptors. However, a small amount of binding for AMPA receptors was found in GT1-7 cells. GT1-7 cells did not exhibit glutamate toxicity and NMDA failed to elicit inward currents using patch-clamp techniques, although GABA did induce currents in the cells. As a whole, these studies suggest that GT1-7 cells lack or possess only low levels of ionotropic glutamate receptors.

Glucocorticoids induce glutamine synthetase in folliculostellate cells of rat pituitary glands in vivo and in vitro

Glutamine synthetase (GS) is a glucocorticoid-inducible enzyme that has a key role for glutamate metabolism in the central and peripheral nervous system. In this study GS activity was measured and the amount of immunoreactive GS (ir-GS) cells in the rat anterior pituitary gland was quantified as a function of age. In addition, the effects of GS inhibitors, glucocorticoid administration, and adrenalectomy on GS activity were examined. Some of the ir-GS cells were also immunoreactive for S100 protein (ir-S100) which is a known marker for folliculostellate cells (FS) in the anterior pituitary. FS cells expressing GS were first detected in 3-d-old rats, and this cell population, expressed as the immunostained cell area divided by a standard unit area, increased as a function of age. The percentages of FS cells also expressing GS were 0.2, 6.4, 25 and 74% at 3 d, 30 d, 60 d and 2 y of age, respectively. GS enzyme activity also increased in parallel with the increase of ir-GS cell population maturation. The subcutaneous injection of methionine sulphoximine, a GS and gamma-glutamylcysteine synthetase inhibitor, reduced pituitary GS activity by 83%, but increased the population of ir-GS cells 3.5-fold in 30-d-old rats. Buthionine sulphoximine, a specific inhibitor of y-glutamylcysteine synthetase, had little effect on GS activity or the ir-GS cell population. Neither methionine sulphoximine nor buthionine sulphoximine changed the population of ir-S100 protein cells (FS cells). Dexamethasone and hydrocortisone increased the population of ir-GS cells by 3.1 and 4.2-fold, respectively, within 12 h after administration. A significant increase of GS activity due to the injection of glucocorticoids was observed in the anterior pituitary, but not in the brain, retina or liver of immature rats. Adrenalectomy did not cause decrease of pituitary GS activity, and dexamethasone administration increased GS activity in both adrenalectomised and intact rats. In the monolayer culture of anterior pituitary cells, glucocorticoids increased GS activity by x 1.5, and methionine sulphoximine reduced the activity by over 94%. These results demonstrate that GS in folliculostellate cells is a glucocorticoid-inducible enzyme in vivo and in vitro, and that the age-dependent increase of GS activity is independent of endogenous adrenal glucocorticoids.

Regulation of the preovulatory gonadotropin surge by endogenous steroids

Estradiol secreted by growing ovarian follicle(s) has been considered classically to be the neural trigger for the preovulatory surge of gonadotropins. The observation that the estradiol-induced gonadotropin surge in ovariectomized rats is of lesser magnitude and duration than that found in the cycling rat at proestrus has resulted in a search for other steroid regulators. Progesterone is a major regulator of the preovulatory gonadotropin surge. It can only act in the presence of an estrogen background, which is necessary for the synthesis of progesterone receptors. In the estrogen-primed ovariectomized rat, progesterone is able to initiate and enhance the gonadotropin surge to the magnitude observed on the day of proestrus and limit it to 1 day. The physiological role of progresterone in the induction of the preovulatory gonadotropin surge has been demonstrated by the attenuation of the progesterone-induced surge and the endogenous proestrus surge by progesterone receptor antagonist RU486 and the progesterone synthesis inhibitor trilostane. The promoter region of the follicle-stimulating hormone (FHS)-beta gene contains multiple progesterone response elements and progesterone brings about FSH release as well. The reduction of progesterone in the 5 alpha-position appears to be important for the regulation of progesterone secretion. Corticosteroids appear to play a significant role in the secondary FSH surge on late proestrus and early estrus.

Neuroendocrine mechanisms underlying the control of gonadotropin secretion by steroids

There is considerable evidence that although estradiol may trigger the preovulatory surge of gonadotropins, progesterone is required for its full magnitude and duration and that glucocorticoids bring about selective follicle-stimulating hormone release. The luteinizing hormone-releasing hormone (LHRH) neuron does not have steroid receptors and is regulated by excitatory amino acid neurotransmission. Steroids do not appear to modulate excitatory amino acid receptors directly but increase release of glutamate in the preoptic area. This may be due to the suppression by steroids of the enzyme glutamatic acid decarboxylase67 that converts glutamate into GABA. NMDA receptors colocalize with nitric oxide synthase-containing neurons that surround the LHRH neurons in the preoptic area and intersect the LHRH fibers in the median eminence. Other potential novel pathways of LHRH release that are currently being explored include carbon monoxide generated by the action of heme oxygenase-2 on heme molecules and bradykinin acting via bradykinin B2 receptors.

Opioid-glutamate-nitric oxide connection in the regulation of luteinizing hormone secretion in the rat

Opioid neurons are recognized to be an important component of the inhibitory "brake" in the CNS that restrains LHRH secretion. Opioid inhibition could be exerted directly on LHRH neurons, or it could be achieved via indirect mechanisms involving restrainment of excitatory "accelerator" neurons that facilitate LHRH release. The purpose of the present study was to explore the second hypothesis by investigating whether removal of opioid inhibition by administering the opioid antagonist, naloxone leads to enhanced activation of glutamate and nitric oxide (NO) neurons, which are known to be important excitatory "accelerator" components for the control of LHRH secretion. Naloxone administration (2.5 mg/kg) to adult male rats induced a significant elevation of serum LH levels at 20 min post injection. NOS activity in preoptic area (POA) and medial basal hypothalamic (MBH) fragments was demonstrated to be significantly elevated 20 min post naloxone injection. Administration of a glutamate (NMDA) receptor antagonist (MK-801, 0.2 mg/kg) abolished the naloxone-induced increase in NOS activity in the POA and MBH, with a corresponding block of the naloxone-induced LH release. Glutamate appears to only be involved in LH surge generation and not to regulate basal LH levels, as MK-801 had no effect on basal LH release. Because previous work by our laboratory and others have provided evidence that NO is a mediator of glutamate effects in the hypothalamus, these findings are interpreted to mean that opioid inhibition is mediated on glutamate neurons that are upstream of NO neurons. In support of this contention, we found that NMDA treatment enhanced NOS activity in the male rat POA and MBH fragments in vitro, an effect that was specific as it was completely blocked by the NMDA receptor antagonist, MK-801. Additionally, in vivo microdialysis studies revealed that naloxone treatment significantly enhances glutamate release in the preoptic area (POA) at 15 min post injection in conscious, unanesthetized, freely moving male rats. Release rates of the control amino acid, serine did not change significantly following naloxone injection. Taken as a whole, these findings provide evidence for an opioid-glutamate-NO pathway in the control of LHRH secretion, and they demonstrate the importance of "brake-accelerator" interactions in the control of LHRH and LH secretion.

Quinolinic acid: effect on the nucleus arcuatus of the hypothalamus in the rat (ultrastructural evidence)

Quinolinic acid was administered intraperitoneally to male Wistar rats in a dose of 60 mmol, once daily for 8 days. By electron microscopy, in quinolinic acid-treated rats, the neuronal cell bodies in the arcuate nucleus had features of increased cellular activity, but some damage of neuronal cell bodies was also evident.

Control of gonadotropin secretion in prepubertal male rats by excitatory amino acids

The role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in the control of FSH and LH secretion was analysed in prepubertal male rats. In the first experiment 4, 8, 12, 16, 20 and 30-day-old males were decapitated 15 min after vehicle, NMDA or kainic acid (KA) administration. In the second experiment, 23-day-old males were sham-orchidectomized or orchidectomized. Orchidectomized males were or were not implanted with silastic capsules containing testosterone and were sacrificed on day 30 after injection with vehicle, NMDA (15 mg kg-1), LHRH (100 ng rat-1), NMDA plus LHRH, or MK801, a non-competitive NMDA antagonist, (1 mg kg-1). In the third experiment, 30-day-old intact males pretreated with Nw-nitro-L-arginine methyl ester (NAME) or NG-methyl-L-arginine (MA), blockers of nitric oxide (NO) synthase, were sacrificed after NMDA or KA administration. In the fourth experiment, the effects of NMDA, KA, LHRH and NAME were analysed in monolayer cultures of dispersed adenohypophyseal cells. We found that: (i) NMDA and KA stimulated LH and FSH secretion in intact males; (ii) the NMDA effect on LH secretion remained after orchidectomy; (iii) FSH and LH responses to LHRH were not affected by simultaneous NMDA administration; (iv) antagonization of NMDA receptors with MK801 reduced the LH secretion in intact and orchidectomized males and blunted the FSH response to orchidectomy; (v) the stimulatory effect of NMDA or KA on LH secretion was not affected by pretreatment with blockers of NO synthase; (vi) the in vitro LH secretion remained unchanged after administration of NMDA, KA or NAME. We conclude that NMDA and non-NMDA receptors play a physiological role in the control of the basal and post-orchidectomy secretion of gonadotropins, and that this effects is independent of changes in NO generation and does not include changes in pituitary responsiveness to LHRH.

Immunohistochemical evidence of neurons with GHRH or LHRH in the arcuate nucleus of male mice and their possible role in the postnatal development of adenohypophysial cells

BACKGROUND

The neonatal administration of monosodium L-glutamate (MSG) has been used in investigations of the possible role of the arcuate nucleus in neuroendocrine regulation during postnatal development. We used this method to examine whether the mouse arcuate contained cell bodies immunoreactive with antisera to growth hormone releasing hormone (GHRH) or luteinizing hormone releasing hormone (LHRH), and whether these hypothalamic peptides affect synthesis and secretion of growth hormone and gonadotropin and the testis.

METHODS

The hypothalamus, pituitary, and testes of adult male mice treated with MSG during the neonatal period were fixed in Bouin's fluid or 10% neutral formalin. The hypothalamus was used in immune staining, the pituitary was used in both morphometry and immune staining, and the testis was stained with hematoxylin and eosin.

RESULTS

Body weights in control and treated mice were not different. The treated mice had more subcutaneous adipose tissue and a shorter body than the control mice. The testes were heavier in the controls. Many perikarya immunoreactive with antisera to GHRH or LHRH were found in the arcuate nucleus in control mice, but few such perikarya were found in this nucleus in treated mice. The size of the anterior lobe and the number and size of GH cells, follicle stimulating hormone (FSH) cells, and prolactin (PRL) cells in treated mice were less than those of control mice.

CONCLUSIONS

GHRH and LHRH neurons in the arcuate nucleus in male mice may cause body and testis weight to increase via GH and LH cells, respectively, in the adenohypophysis during postnatal development. There are some differences in the hypothalamo-pituitary-testis axis of mice and rats.

Ontogenic changes in the hypothalamic levels of amino acid neurotransmitters in the female rat

In order to evaluate the participation of several amino acid neurotransmitters (AANT) in sexual maturation we measured the hypothalamic concentrations of aspartate (Asp), glutamate (Glu), homocysteic acid (HCA), glycine (Gly), taurine (Tau) and gamma-aminobutyric acid (GABA) in female rats at different ages of sexual development. Animals (15-, 20-, 25-, 30-, 35- and 40-day-old female rats, as well as adult diestrous rats, n = 8-10/group) were decapitated at noon; each brain was rapidly removed and frozen on dry ice. Preoptic area (POA) and mediobasal hypothalamic (MBH) needle punch samples were obtained from 500-600 microns thick coronal slices, homogenized and centrifuged. AANT concentrations were measured in the supernatants following derivatization with phenyl isothiocyanate and reverse-phase HPLC separation by UV detection. Significant (P < 0.05) changes in hypothalamic AANT concentrations can be summarized as follows: POA Glu, HCA and Gly levels rose transiently at day 20 and then increased steadily in 35- and 40-day-old rats. Asp concentrations rose in 35- and 40-day-old rats. Tau concentrations decreased markedly from day 30 onwards. GABA levels were lowest in 15-day-old rats, as compared to the other age groups. Asp, Glu, Gly and GABA concentrations were highest in adult diestrous rats. Changes in MBH AANT levels were not as relevant: only Gly concentrations rose significantly at day 40; Asp, Glu, HCA, Gly and GABA concentrations increased in adult diestrous rats and Tau concentrations were lower from day 35 onwards.(ABSTRACT TRUNCATED AT 250 WORDS)

A report of the proceedings of an MSG workshop held August 1991

This report of the proceedings of a workshop on monosodium glutamate (MSG) represents the output of an exchange of scientific information, discussed and debated, by a group of experts representing a variety of disciplines. Experts in the areas of food science, potential adverse reactions to foods, pharmacology, neuroscience, biochemistry, nutrition, pediatrics, and anatomy reviewed the current scientific literature relative to the safe use of MSG in foods. These proceedings supplement the extensive literature compiled by various prestigious international expert committees since the flavor-enhancing properties of MSG were identified around the turn of the century,

Excitotoxic neuronal damage and neuropsychiatric disorders

Excitatory amino acids (EAA) serve important physiological functions in the vertebrate CNS, including participation in fast excitatory synaptic transmission, modulation of synaptic plasticity and regulation of neuronal morphology during development. However, paradoxically they also harbor neurotoxic (excitotoxic) potential, which, if unleashed, can cause widespread degeneration of CNS neurons. Accumulating evidence suggests a role for excitotoxins in a variety of human neuropsychiatric disorders. This paper reviews the classes of EAA receptors in the CNS, the mechanisms underlying EAA-mediated neuronal damage and the role of EAA in specific human disorders.

Characterization and ontogenesis of N-methyl-d-aspartate-evoked luteinizing hormone secretion in immature female rats

The excitatory amino-acid L-glutamate appears to be involved in the neural regulation of puberty. We have now characterized the stimulatory effect of the glutamate agonist N-methyl-D-aspartic acid (NMDA) on the secretion of luteinizing hormone (LH) in immature female rats. NMDA injections (subcutaneously) rapidly induce LH secretion (maximum at 8 min) with an ED(50) of 3.25 mg/kg determined at postnatal day 30. The LH response to NMDA develops between 10 and 15 days after birth, reaches a maximum at approximately day 27 and disappears again by day 32, an age at which 80% of the rats are still prepubertal. In confirmation of work by others, we also showed that postpubertal male rats are unresponsive to NMDA. The effect of NMDA on LH release is blocked by prior injection of the glutamate antagonist MK-801 (0.1 mg/kg). MK-801 alone is able to reduce LH secretion in acutely ovariectomized rats but has little effect in intact, prepubertal rats. NMDA may not act exclusively on the gonadotropin-releasing hormone neuron since inhibition of norepinephrine biosynthesis by the drug U-14,624 largely prevents NMDA-induced LH secretion. Interestingly, rats treated neonatally with the neurotoxin monosodium glutamate responded normally to NMDA when tested at postnatal day 25. Finally, removal of the ovaries 48 h previously reduced the LH response to NMDA. Full responsiveness was restored following estrogen priming. In summary, hypothalamic glutamate receptors of the NMDA subtype comprise a significant regulatory component of LH secretion in prepubertal female rats. LH responsiveness appears to be age-, dose-, estrogen- and norepinephrine-dependent.

Physiological and pathophysiological roles of excitatory amino acids during central nervous system development

Recent studies suggest that excitatory amino acids (EAAs) have a wide variety of physiological and pathophysiological roles during central nervous system (CNS) development. In addition to participating in neuronal signal transduction, EAAs also exert trophic influences affecting neuronal survival, growth and differentiation during restricted developmental periods. EAAs also participate in the development and maintenance of neuronal circuitry and regulate several forms of activity-dependent synaptic plasticity such as LTP and segregation of converging retinal inputs to tectum and visual cortex. Pre- and post-synaptic markers of EAA pathways in brain undergo marked ontogenic changes. These markers are commonly overexpressed during development; periods of overproduction often coincide with times when synaptic plasticity is great and when appropriate neuronal connections are consolidated. The electrophysiological and biochemical properties of EAA receptors also undergo marked ontogenic changes. In addition to these physiological roles of EAAs, overactivation of EAA receptors may initiate a cascade of cellular events which produce neuronal injury and death. There is a unique developmental profile of susceptibility of the brain to excitotoxic injury mediated by activation of each of the EAA receptor subtypes. Overactivation of EAA receptors is implicated in the pathophysiology of brain injury in several clinical disorders to which the developing brain is susceptible, including hypoxia-ischemia, epilepsy, physical trauma and some rare genetic abnormalities of amino acid metabolism. Potential therapeutic approaches may be rationally devised based on recent information about the developmental regulation of EAA receptors and their involvement in the pathogenesis of these disorders.

Delay of puberty and impairment of growth in female rats given a non competitive antagonist of NMDA receptors

Reportedly, excitatory amino acids are involved in the control of gonadotropin secretion of rats and non-human primates. The aim of this study was to investigate the effect of chronic blockade of NMDA (N-methyl-D-aspartic acid) receptors by the non competitive receptor antagonist MK-801 on gonadotropin secretion and the onset of puberty in female rats. Moreover, since in humans alterations of the timing of puberty frequently coexist with disturbances of body growth, suggesting a common etiology for both events, we evaluated the effect of MK-801 also on the neural mechanisms controlling growth hormone (GH) secretion. Twenty-one-day-old female rats were treated with MK-801 (0.2 mg/kg ip, bid) or placebo for 10 days and were killed after 7 days of withdrawal. Administration of MK-801 induced a significant impairment of growth rate without altering food intake, and a delay in vaginal opening. Pituitaries from rats treated with MK-801 had a reduced luteinizing hormone (LH) content, and secreted in vitro lower amounts of LH both under basal and LHRH-stimulated conditions. MK-801 treated rats had a lower pituitary GH content and basal and GHRH-stimulated GH release and reduced plasma insulin-like growth factor-I levels. These data indicate that blockade of NMDA receptors in a critical period of the female rat life-span: 1) delays puberty by reducing gonadotropin secretion; 2) impairs growth rate by reducing GH secretion, with a mechanism still to be clarified.

Stimulation of pituitary hormone secretion by neurotransmitter amino acids in humans

The effects of several neurotransmitter amino acids on pituitary hormone secretion were examined in normal humans. Oral administration of 10 g of glutamic acid stimulated the secretion of prolactin (PRL) and cortisol to approximately twice baseline values, with no effect on GH, TSH or LH. Aspartic acid (10 g), taurine (5 g), and cysteine (5 or 10 g) had no consistent effect on any hormone measured, although the lack of effect of aspartic acid may relate to the modest increments in serum concentration achieved. Glutamic acid may be an important modulator of PRL and ACTH secretion in humans.

Age- and dose-dependent effects of neonatal monosodium glutamate (MSG) administration to female rats

The age- and dose-dependent effects of neonatal MSG were evaluated in pre- and postpubertal female rats. The neurotoxic action of MSG was assessed by examining monoamine content in microdissected regions of the mediobasal hypothalamus. MSG was administered at a dose of 4 mg/g on postnatal days 2 and 4 (MSG-Lo) or on postnatal days 2, 4, 6 and 8 (MSG-Hi). MSG-Hi treatment significantly reduced dopamine (DA) content in the arcuate nucleus (ANH) and lateral median eminence (LME) on postnatal day 21 when compared to NaCl-injected controls. DA content relative to controls was not altered in the ANH or LME postnatal or postnatal day 60 in MSG-Hi, however, norepinephrine (NE) was significantly (p less than 0.05) decreased on both postnatal day 21 and 60 in the LME. MSG-Lo treatment significantly (p less than 0.05) reduced ANH NE content on postnatal day 60 compared to controls. Both MSG-Hi and MSG-Lo treatment increased 5-hydroxyindoleacetic acid content in the preoptic area (POA) on postnatal day 60 relative to the controls. Developmental changes independent of MSG treatment were noted in the hypothalamus. DA and 3,4-dihydroxyphenylacetic acid (DOPAC) content in the POA were 2-3-fold higher on postnatal day 21 compared to postnatal day 60. In contrast, DA content increased with age in the ANH, LME and medial ME. NE content in the ANH increased as a function of age in controls, but not in MSG-treated rats. The effects of MSG treatment on the postnatal development and maturation of neurons in the mediobasal hypothalamus were discussed in relation to the direct neurotoxicity of MSG.

Effect of furazolidone and nitrofurazone on brain gamma-amino butyric acid and glutamate concentrations in chickens

1. The concentrations of gamma-amino butyric acid (GABA) and glutamate were measured in the brains of chickens which were treated with furazolidone (FZ) or nitrofurazone (NF) at oral doses of 12.5, 25 or 50 mg/kg for 5 days. 2. At the end of the treatment, the birds lost about 9% of their bodyweight when on the small dose of FZ or NF and about 18% when on the high doses. 3. Both drugs produced dose-dependent increases in the concentrations of GABA and glutamate which were statistically significant at doses of 25 and 50 mg/kg of NF, and 50 mg/kg of FZ.

Effect of central administration of phencyclidine on plasma concentrations of luteinizing hormone

Previous research has demonstrated that excitatory amino acids acting at N-methyl-D-aspartate (NMDA) receptors stimulate the release of luteinizing hormone (LH). Castration also elevates LH, an effect that may also involve NMDA receptors since the specific NMDA antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), antagonizes this action. Since PCP antagonizes a variety of actions of NMDA agonists, we hypothesized that it would diminish the ability of glutamate, homocysteic acid and castration to elevate LH. Our data support this hypothesis.

Effect of furazolidone and nitrofurazone on brain gamma-amino butyric acid and glutamate concentrations in chickens

1. The concentrations of gamma-amino butyric acid (GABA) and glutamate were measured in the brains of chickens which were treated with furazolidone (FZ) or nitrofurazone (NF) at oral doses of 12.5, 25 or 50 mg/kg for 5 days. 2. At the end of the treatment, birds on the small dose of FZ or NF lost about 9% of their bodyweight, and those on the high doses lost about 18%. 3. Both drugs produced dose-dependent increases in the concentrations of GAA and glutamate which were statistically significant at doses of 25 and 50 mg/kg of NF and 50 mg/kg of FZ.

Hypothalamic site of action for N-methyl-D-aspartate (NMDA) on LH secretion

Excitatory amino acids have been shown to increase luteinizing hormone (LH) secretion following ventricular or systemic administration. In the present study we attempted to determine possible hypothalamic sites of action for the potent excitatory amino acid agonist, N-methyl-D-aspartate (NMDA). The ability of NMDA to enhance LH release was tested in male rats following infusion into the medial preoptic nucleus (MPO), anterior hypothalamic nucleus (AHY), ventromedial hypothalamic nucleus (VMH), and arcuate nucleus (ARC). In the MPO, infusion of 50 or 500 pmole NMDA increased pituitary LH secretion, resulting in a 2-7 fold increase in plasma LH. The 50 pmole dose was selected to test more caudal hypothalamic sites. Plasma LH levels were not affected following microinfusion of NMDA (50 pmole) into the AHY, VMH, and ARC. The present results indicate a regional specificity for NMDA in the enhancement of LH secretion. This regional specificity may reflect either a greater density of LHRH perikarya in the MPO or the presence of specific amino acid receptors on neuronal elements in the MPO, but not on neuronal elements in the other areas tested.

Acute effects of aspartic acid on ventilation of male and female rats

The effects of aspartic acid (aa) on ventilation were evaluated in awake male and female rats prior to and 15, 30, and 45 minutes after saline, 100 mg/kg or 580 mg/kg aa was injected subcutaneously. Subsequently, rats were exposed to hypoxic and hypercapnic gas challenges. In males, 100 mg/kg aa increased ventilation (VE) by increasing inspiratory flow rate (VT/TI), tidal volume (VT), and frequency of breathing (f) by 30 minutes, whereas in females VT was increased above saline levels only at 15 minutes. VE did not decrease over time. A dose of 580 mg/kg aa depressed ventilation in males for 2 hours by decreasing VT, VT/TI and f. In contrast, female rats exhibited a decreased ventilation only at 15 minutes which then began to return to saline levels by 45 minutes. Neither male nor female rats treated with either dose of aa showed a depressed response to hypoxia or hypercapnia. These data indicate that aa at two doses can affect the pattern of ventilation differently in male and female rats. One mechanism responsible for the differences noted between the two groups is the effect aspartic acid may have on testosterone production. An additional study comparing ventilatory responses of sham operated and castrated males to various doses of aa indicated that testosterone was not necessary to show the 'male' pattern of response.

Characterization and possible opioid modulation of N-methyl-D-aspartic acid induced increases in serum luteinizing hormone levels in the developing male rat

It has been previously reported that the excitatory amino acid, N-methyl-D-aspartic acid (NMDA), elicits prompt increases in serum luteinizing hormone (LH) levels in young male rats. The present studies were carried out to determine whether the effects of NMDA on LH were mediated by the release of LHRH from the hypothalamus. We also examined whether NMDA-sensitive neuronal pathways interacted with the endogenous opioid system regulating LHRH release and the ontogeny of NMDA-evoked increases in serum LH. We found that the age-response curve for NMDA-induced increases in LH was an inverted U; at early ages (10 and 15 days) the amino acid was marginally effective in increasing LH levels, it became maximally effective from post-natal days 20-40 and thereafter rapidly lost its efficacy such that it was virtually inactive in adult animals. Dose-response curves revealed that adult animals were more than 10-fold less sensitive to NMDA than their younger counterparts. Our studies also demonstrated that NMDA increased LH via a direct effect on the hypothalamic release of LHRH since a potent LHRH antagonist competitively inhibited the effects of NMDA. Finally, we observed that morphine competitively inhibited the effects of NMDA on LH release, suggesting a relationship between NMDA-sensitive neuronal pathways and those endogenous opioid-containing systems which are known to regulate LH release.

Blood pressure development in SHR and WKY rats: effects of neonatal monosodium glutamate treatment and evidence for transient hypertension in WKY rats

Neonatal administration of monosodium glutamate (MSG) failed to alter blood pressure (BP) development in either SHR or WKY rats. Hypothalamic disturbance relevant to hypertension in the SHR is not contributed to by neonatal MSG treatment. All normotensive WKY animals show a transient rise in BP corresponding to the time at which BP peaks in SHR.

Glutamate-induced asymmetry in the sexual and aggressive behavior of young chickens

A unilateral injection of monosodium glutamate (MSG) into the left hemisphere of the forebrain, at doses of either 50 nmol or 500 nmol per hemisphere on day 2 post-hatch, resulted in a marked and long lasting elevation of attack and copulatory behavior in cockerels. This was not observed when MSG was injected into the right hemisphere or both hemispheres, and further demonstrates functional brain asymmetry in lower vertebrates. A similar asymmetry was observed after administering the higher dose of MSG to females on day 2. A significant change in copulatory performance was observed when the higher dose was injected into the left hemisphere on day 11, compared to the controls, without affecting attack behavior, whereas, the lower dose failed to induce any changes. This may be due to the development of efficient brain uptake mechanisms for glutamate. The possibility that the behavioral changes were induced indirectly via increased secretion of plasma androgen, which then stimulates the appropriate brain centres, was found not to be the case.

Importance of amino acids on vasopressin-stimulated water flow

The presence of several naturally occurring amino acids in the serosal bath of toad urinary bladder significantly alters the hydrosmotic response of this tissue to vasopressin. We found that histidine, glutamate, and lysine increase vasopressin-stimulated water flow by 75%, 60%, and 43%, respectively. In contrast, alanine did not alter vasopressin-stimulated water flow, whereas glutamine decreased it by 25%. The effect of each amino acid represents intracellular events because their effects on theophylline-stimulated water flow were similar to those found with vasopressin. However, the site of action of amino acids varied, with some operating at steps before and others at steps after cyclic AMP generation. The fact that the metabolically inactive D-histidine and D-glutamate are as effective as their metabolically active L-counterparts suggests that the action of amino acids depends upon some physicochemical properties of their molecules. The ability of amino acids to influence the hydrosmotic effects of vasopressin was shown to be independent of prostaglandin generation, ionic composition, and molecular charge. In the case of histidine, we were able to obtain some understanding of the mechanism responsible for its action. We first showed that the effect of histidine does not depend upon its metabolism. In addition to D-histidine being as effective as the metabolically active L-histidine, we also showed that histidine is effective when its metabolism is abolished by low ambient temperature and also when its incorporation into proteins was prevented by cycloheximide. These findings suggest that histidine operates through some physicochemical property localized on its molecule. We were able to show that this property resides on the imidazole part of histidine. Imidazole, similar to histidine, increases vasopressin-stimulated water flow. Methylation of histidine on the imidazole ring completely abolished its effectiveness in increasing vasopressin-stimulated water flow. In contrast, methylation of histidine at the side chain increased vasopressin action similar to that found for histidine. We provide evidence that the physicochemical property of the imidazole ring of histidine is that of chelating Zn++ intracellularly, and that the intracellular site of action of histidine is closely linked to microtubules formation and/or action.

Quinolinic acid stimulates luteinizing hormone secretion in female rats: evidence for involvement of N-methyl-D-aspartate-preferring receptors

Pharmacological evidence suggests that endogenous excitatory amino acid neurotransmitters stimulate luteinizing hormone (LH) secretion in neonatal and adult rats. Recent studies have identified quinolinic acid (QUIN), an endogenous brain and peripheral metabolite of tryptophan, as a potent agonist at N-methyl-D-aspartate (NMDA)-preferring excitatory amino acid receptors. The present studies examined whether QUIN alters LH secretion in ovariectomized, estradiol-primed rats and whether such effects are mediated by specific amino acid receptor subtypes. In one experiment, animals received intracisternal injections of either quinolinic acid, N-methyl-DL-aspartate (NMA), aspartate (ASP), quisqualic acid (QA), or monosodium glutamate (GLU) five minutes prior to decapitation. In a second study, animals receiving central QUIN or NMA were treated simultaneously with either 2-amino-7-phosphonoheptanoic acid (APH) or kynurenic acid (KYA), both antagonists of NMDA-preferring receptors, or the quisqualate antagonist, glutamate diethyl ester (GDEE). Serum LH concentrations were measured by radioimmunoassay. Intracisternal administration of either QUIN or NMA resulted in an acute, dose-dependent increase of serum LH concentrations. Coadministration of APH blocked the effects of QUIN and NMA. QUIN stimulation of LH was also blocked by KYA, but not GDEE. Neither GLU nor ASP increased LH release, but QA did produce a small, significant elevation of LH. Light microscopic evaluation of brains showed no morphologic disturbance resulting from administration of these agents. The present results suggest that QUIN, or other endogenous ligands of NMDA-preferring receptors, may participate in the regulation of LH secretion in the adult female rat.

Effects of excitotoxic amino acids on pituitary hormone secretion in the rat

The acute effects of administration of 4 excitatory amino acids (n-methyl-DL-aspartic acid (NMA), kainic acid (KA), ibotenic acid (IA) and quinolinic acid (QA] on the serum concentrations of luteinizing hormone (LH), follicle-stimulating hormone (FSH), growth hormone (GH) and prolactin (PRL) were studied in the rat. NMA-treated rats exhibited increased serum LH and GH concentrations while KA-treated rats-showed increases only in serum GH concentrations. Neither IA nor QA altered adenohypophyseal hormone levels. These endocrine alterations induced by NMA and KA are different from those previously reported after administration of glutamate, another excitatory amino acid. The finding that all of the excitatory amino acids studied did not produce identical effects on anterior pituitary hormone secretion may be due to differential permeability of these substances into the central nervous system or because they act at different subtypes of excitatory amino acid receptors.

Neuroactive amino acids influence gonadotrophin output by a suprapituitary mechanism in either rodents or primates

Systemic administration of excitatory amino acids (Glu or NMA) rapidly enhances LH release (rodents or primates) and GABA blocks this action (rodents). In the present study rodent and primate pituitaries were incubated with NMA, Glu and/or GABA, with or without added LH-RH. Only LH-RH affected LH release. Therefore the probable site of NMA, Glu or GABA action on LH release in both rodents and primates is suprapituitary.

Glutamate in hypothalamic and limbic structures of diestrous, proestrous, ovariectomized and ovariectomized estrogen-treated rats

Concentrations of glutamic acid were determined in the nucleus accumbens (ACB), medial preoptic area (MPO), anterior and posterior mediobasal hypothalamus (AMBH, PMBH) and mediocortical amygdala (AMY) of diestrous (D), proestrous (P), ovariectomized (OVX) and OVX rats treated with estradiol-benzoate (EB) 12 h and 24 h before decapitation. Significant changes of glutamate concentrations were found in the ACV and PMBH. Glutamate is reduced in OVX when compared to D, P and OVX-EB rats. No significant changes of glutamate concentrations could be detected in the MPO, AMBH, and AMY.