Effect of N-methyl-d,l-aspartate on luteinizing hormone secretion in ovariectomized ewes in the absence and presence of estradiol

Kisspeptin, neurokinin B, and dynorphin act in the arcuate nucleus to control activity of the GnRH pulse generator in ewes

Recent work has led to the hypothesis that kisspeptin/neurokinin B/dynorphin (KNDy) neurons in the arcuate nucleus play a key role in GnRH pulse generation, with kisspeptin driving GnRH release and neurokinin B (NKB) and dynorphin acting as start and stop signals, respectively. In this study, we tested this hypothesis by determining the actions, if any, of four neurotransmitters found in KNDy neurons (kisspeptin, NKB, dynorphin, and glutamate) on episodic LH secretion using local administration of agonists and antagonists to receptors for these transmitters in ovariectomized ewes. We also obtained evidence that GnRH-containing afferents contact KNDy neurons, so we tested the role of two components of these afferents: GnRH and orphanin-FQ. Microimplants of a Kiss1r antagonist briefly inhibited LH pulses and microinjections of 2 nmol of this antagonist produced a modest transitory decrease in LH pulse frequency. An antagonist to the NKB receptor also decreased LH pulse frequency, whereas NKB and an antagonist to the receptor for dynorphin both increased pulse frequency. In contrast, antagonists to GnRH receptors, orphanin-FQ receptors, and the N-methyl-D-aspartate glutamate receptor had no effect on episodic LH secretion. We thus conclude that the KNDy neuropeptides act in the arcuate nucleus to control episodic GnRH secretion in the ewe, but afferent input from GnRH neurons to this area does not. These data support the proposed roles for NKB and dynorphin within the KNDy neural network and raise the possibility that kisspeptin contributes to the control of GnRH pulse frequency in addition to its established role as an output signal from KNDy neurons that drives GnRH pulses.

The effect of nutrition on the neural mechanisms potentially involved in melatonin-stimulated LH secretion in female Mediterranean goats

This research examines which neural mechanisms among the endogenous opioid, dopaminergic, serotonergic and excitatory amino acid systems are involved in the stimulation of LH secretion by melatonin implantation and their modulation by nutritional level. Female goats were distributed to two experimental groups that received either 1.1 (group H; n=24) or 0.7 (group L; n=24) times their nutritional maintenance requirements. Half of each group was implanted with melatonin after a long-day period. Plasma LH concentrations were measured twice per week. The effects of i.v. injections of naloxone, pimozide, cyproheptadine and N-methyl-d,l-aspartate (NMDA) on LH secretion were assessed the day before melatonin implantation and again on days 30 and 45. The functioning of all but the dopaminergic systems was clearly modified by the level of nutrition, melatonin implantation and time elapsed since implantation. Thirty days after implantation, naloxone increased LH concentrations irrespective of the level of nutrition (P<0.05), similar to NMDA in the melatonin-implanted H goats (HM; P<0.01). On day 45, naloxone increased LH concentrations in the HM animals (P<0.05), similar to cyproheptadine in both the non-implanted H (HC) and the HM animals (P<0.01). Finally, at 45 days, NMDA increased the LH concentration in all subgroups (P<0.01). These results provide evidence that the effects of different neural systems on LH secretion are modified by nutritional level and melatonin implantation. Endogenous opioids seem to be most strongly involved in the inhibition of LH secretion on days 30 and 45 after melatonin implantation. However, the serotonergic mechanism appears to be most influenced by nutritional level.

Seasonal differences of gene expression profiles in song sparrow (Melospiza melodia) hypothalamus in relation to territorial aggression

Background

Male song sparrows (Melospiza melodia) are territorial year-round; however, neuroendocrine responses to simulated territorial intrusion (STI) differ between breeding (spring) and non-breeding seasons (autumn). In spring, exposure to STI leads to increases in luteinizing hormone and testosterone, but not in autumn. These observations suggest that there are fundamental differences in the mechanisms driving neuroendocrine responses to STI between seasons. Microarrays, spotted with EST cDNA clones of zebra finch, were used to explore gene expression profiles in the hypothalamus after territorial aggression in two different seasons.

Methodology/Principal Findings

Free-living territorial male song sparrows were exposed to either conspecific or heterospecific (control) males in an STI in spring and autumn. Behavioral data were recorded, whole hypothalami were collected, and microarray hybridizations were performed. Quantitative PCR was performed for validation. Our results show 262 cDNAs were differentially expressed between spring and autumn in the control birds. There were 173 cDNAs significantly affected by STI in autumn; however, only 67 were significantly affected by STI in spring. There were 88 cDNAs that showed significant interactions in both season and STI.

Conclusions/Significance

Results suggest that STI drives differential genomic responses in the hypothalamus in the spring vs. autumn. The number of cDNAs differentially expressed in relation to season was greater than in relation to social interactions, suggesting major underlying seasonal effects in the hypothalamus which may determine the differential response upon social interaction. Functional pathway analyses implicated genes that regulate thyroid hormone action and neuroplasticity as targets of this neuroendocrine regulation.

Estradiol negative feedback regulation by glutamatergic afferents to A15 dopaminergic neurons: variation with season

It is now clear that seasonal breeding in ewes is due to an increase in response to estradiol (E(2)) negative feedback in the nonbreeding season (anestrus) that is mediated by the A15 group of dopaminergic (DA) neurons. Because A15 cells do not contain estrogen receptors, we have postulated the presence of estrogen-responsive afferents and recently reported evidence that input from neurons containing gamma-aminobutyric acid (GABA) contribute to the control of A15 activity by E(2). However, GABAergic afferents account for only a fraction of A15 synaptic input and do not appear to vary with season. We therefore investigated the possible role of stimulatory glutamatergic input to A15 neurons. In experiments 1 and 2, local administration into the A15 of either a N-methyl-D-aspartate (NMDA) receptor or a kainate/alpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate (AMPA) receptor antagonist stimulated episodic LH secretion in a dose-dependent manner in ovary-intact anestrous ewes. In experiment 3, we examined the number of glutamatergic close contacts onto A15 neurons using dual immunocytochemistry in tissue from E(2)-treated ovariectomized anestrous and breeding season ewes. All A15 DA neurons were contacted by glutamatergic vesicles, and the number of close contacts was significantly higher in anestrus than the breeding season. Finally, using a triple-label immunocytochemistry procedure, we did not observe any colocalization of markers for GABA and glutamate in vesicles contacting A15 neurons. These results support the hypothesis that glutamatergic afferents actively stimulate A15 DA neurons in ovary-intact anestrous ewes and raise the possibility that alterations in this input may contribute to increased A15 neural activity during anestrus.

Neuroanatomical plasticity in the gonadotropin-releasing hormone system of the ewe: seasonal variation in glutamatergic and gamma-aminobutyric acidergic afferents

Temperate zone animals time the onset of reproductive events to coincide with specific portions of the sidereal year. Although the neural mechanisms involved remain poorly understood, a marked annual variation in the brain's sensitivity to estradiol negative feedback is thought to mediate many of the changes in neuroendocrine hormone secretion, especially that of the gonadotropin-releasing hormone (GnRH) neurons, via neural afferents. The aim of the present study was to determine whether glutamatergic inputs to GnRH neurons in sheep vary seasonally and to expand our previous observations of seasonal changes in gamma-aminobutyric acid (GABA)-ergic inputs. Brains from adult sheep were collected during the breeding season (N = 8) or the nonbreeding season (anestrus; N = 7). Confocal microscopy and optical sectioning were used to quantify the density of labeled VGLUT2 and VGAT immunoreactivity onto GnRH neurons. The results reveal a significantly greater number of VGLUT2-ir inputs to GnRH dendrites during the breeding season vs. the nonbreeding season but no seasonal changes on GnRH cell somas. The number of VGAT-ir terminals onto GnRH dendrites was reduced in the breeding season compared with the nonbreeding season. GnRH neurons were also found to receive dual-phenotype (VGLUT + VGAT) inputs; these varied with season in a manner similar to VGAT inputs. Morphologically, the numbers of branches of proximal dendrites increased significantly in a subset of GnRH neurons located near the midline. Together these results reveal a dynamic seasonal reorganization of identified inputs onto GnRH neurons and lend additional support to the overall hypothesis that seasonal modulation of GnRH neurons involves glutamatergic and GABAergic neural plasticity.

d-Aspartate and reproductive activity in sheep

D-aspartic acid (D-Asp) has been isolated from neuroendocrine tissues of many invertebrates and vertebrates. Recently, it has been demonstrated that this D-amino acid may be converted to N-methyl-D-aspartic acid (NMDA), a neuromodulator associated with sexual activity. In this study, we determined D-Asp and NMDA concentrations in endocrine glands and other tissues in ewes after D-Asp administration and in controls. We also evaluated the effects of d-Asp administration on the reproductive activity of ewes by determining either progesterone concentrations or LH pulses in the presence or absence of estradiol benzoate. The pineal gland showed the highest natural content of D-Asp (1.47+/-0.22 micromol/g tissue), whereas the pituitary gland had the highest capability to store d-Asp, with a peak value (9.7+/-0.81 micromol/g tissue) 6 h after its administration. NMDA increased sharply 12 h following D-Asp administration, reaching values three times higher than the baseline in both the pituitary and brain. D-Asp was quickly adsorbed after subcutaneous administration, with a peak in plasma levels 2 h after administration and a return to baseline values after 6 h. D-Asp administration achieved a significant (P < 0.001) increase in LH values with respect to estradiol or estradiol + D-Asp treatments. d-Asp treatment once or twice a week did not successfully drive acyclic ewes into reproductive activity. In conclusion, the results obtained in this study demonstrated that D-Asp is endogenously present in sheep tissues and electively stored in endocrine glands and brain after its administration. NMDA and LH increase following D-Asp administration suggesting a role of this D-amino acid in the reproductive activity of sheep.

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.

Seasonal changes in the inputs to gonadotropin-releasing hormone neurones in the ewe brain: an assessment by conventional fluorescence and confocal microscopy

The seasonal pattern of breeding in sheep offers an opportunity to examine plasticity of neuronal inputs to gonadotropin-releasing hormone (GnRH) neurones. We used conventional fluorescence microscopy and confocal microscopy to compare the extent of input to GnRH neurones from various neuropeptide/neurotransmitter systems in ewes during the breeding and anestrous seasons. Using double-labelling immunohistochemistry, we counted close appositions between GnRH cells and varicosities that were immunoreactive for either glutamic acid decarboxylase (GAD; for gamma-amino butyric acid-GABA-neurones), dopamine beta hydroxylase (DBH; for noradrenergic neurones), vesicular glutamate transporter-1 (VGluT-1, for glutamatergic neurones), neuropeptide Y (NPY) and tyrosine hydroxylase (TH; for dopaminergic/noradrenergic neurones). The percentage of GnRH cells displaying close appositions to GABA-ergic varicosities was higher (P < 0.02) in anestrus than in the breeding season. The percentage of GnRH cells receiving input from varicosities that were positive for TH, DBH and VGluT-1 was similar in both seasons. Approximately 26-49% of GnRH neurones were seen to receive inputs from NPY, TH, GABAergic or noradrenergic neurones, while a larger number of GnRH cells (72-75%) received input from glutamatergic neurones. Conventional microscopy consistently overestimated the number of close contacts on GnRH neurones compared to confocal microscopy. For TH-immunoreactive varicosities in the preoptic area, only 16-35% were also immunoreactive for DBH, suggesting that the remainder are dopaminergic. Approximately half of the noradrenergic inputs in the preoptic area were also immunoreactive for NPY. In conclusion, we present numerical data on the consensus between light and confocal microscopy and the level of input of various neuronal systems to GnRH cells; the data indicate a seasonal change in the GABAergic input to GnRH neurones.

Effects of N-methyl-D-aspartate on luteinizing hormone release and Fos-like immunoreactivity in the male White-crowned sparrow (Zonotrichia leucophrys gambelii)

Seasonal breeding is terminated in the White-crowned sparrow by the onset of absolute photorefractoriness, a condition in which the reproductive system is switched off indefinitely until it is dissipated by short day lengths. Absolute photorefractoriness is controlled by the central nervous system; however, the mechanisms underlying GnRH quiescence in photorefractory birds have yet to be elucidated. Using the excitatory amino acid glutamate agonist N-methyl-D-aspartate (NMDA), plasma LH levels in White-crowned sparrows were significantly elevated regardless of the reproductive or photoperiodic condition. NMDA also significantly induced Fos-like immunoreactivity (FLI) within the infundibular nucleus and median eminence, regions previously shown to express FLI after a photoperiodically driven LH rise. NMDA did not induce FLI within GnRH I neurons; instead, it significantly activated cells within the organum vasculosum of the lamina terminalis in close proximity to GnRH I perikarya. These findings provide the first evidence that photorefractoriness is not due to depletion of GnRH stores, as LH and presumably GnRH were secreted in response to excitatory amino acid stimulation. NMDA activation of FLI in the region of the organum vasculosum of the lamina terminalis and the basal tuberal hypothalamus suggests that seasonal reproductive neuroendocrine control may be mediated via cells in the region of the GnRH I perikarya and terminals.

Hormonal regulation of glutamate receptor gene expression in the anteroventral periventricular nucleus of the hypothalamus

Glutamate plays an important role in mediating the positive feedback effects of ovarian steroids on gonadotropin secretion, and the preoptic region of the hypothalamus is a likely site of action of glutamate. The anteroventral periventricular nucleus (AVPV) of the preoptic region is an essential part of neural pathways mediating hormonal feedback on gonadotropin secretion, and it appears to provide direct inputs to gonadotropin releasing hormone (GnRH)-containing neurons. Immunohistochemistry and in situ hybridization were used in this study to define the distribution and hormonal regulation of glutamate receptor subtypes in the AVPV of juvenile female rats. Neurons that express the NMDAR1 receptor subtype are abundant in the AVPV, as are cells that express AMPA receptor subtypes (GluR1, GluR2, and GluR3 but not GluR4), and the AVPV appears to contain a dense plexus of NMDAR1-immunoreactive presynaptic terminals. However, AVPV neurons do not seem to express detectable levels of kainate receptor (GluR5, GluR6, and GluR7) or metabotropic receptor (mGluR1-6) subtypes. Treatment of ovariectomized juvenile rats with estradiol induced expression of GluR1 mRNA but did not alter levels of GluR2 or GluR3 mRNA. Treatment of estrogen-primed ovariectomized juvenile rats with progesterone caused an initial increase in GluR1 mRNA expression, followed by a small decrease 24 hr after treatment. In contrast, estrogen appears to suppress levels of NMDAR1 mRNA in the AVPV, which remained unchanged after progesterone treatment. Thus, one mechanism whereby ovarian steroids may provide positive feedback to GnRH neurons is by altering the sensitivity of AVPV neurons to glutamatergic activation.

Effect of glucose availability on pulsatile luteinizing hormone release in rams before and after castration

The hypothesis tested was that availability of glucose modulates the control of luteinizing hormone (LH) release. A second objective was to determine the role of testicular hormones in the control of pulsatile LH secretion during depressed blood glucose. Serial blood samples were collected at 15 min intervals for 8 h from intact pubertal Suffolk rams (n = 8; 7 months old) on consecutive days (Days 1, 2 and 3). Rams were castrated after sampling on Day 3 and samples were collected 3 weeks later on consecutive days (Days 4, 5 and 6). Insulin (120 units, iv) was given at Hour 4 of each of the six days to lower blood glucose. On Days 1 and 4, no other treatments were given (Control). On Days 2 and 5, LH releasing hormone (LHRH; 5 ng/kg, iv) was given at Hours 5, 6 and 7 to assess the ability of the pituitary to release LH. On Days 3 and 6, N-methyl-D,L-aspartate (NMA; 5 mg/kg, iv) was given at Hours 5, 6 and 7 to assess the ability of the hypothalamus to release LHRH. Insulin reduced plasma glucose by 52% for at least 3 h (P < 0.001). Insulin reduced the mean LH concentration (P < 0.05) and tended to reduce the LH response area (P < 0.10) in castrated animals during the control period. LHRH increased LH pulse number (P < 0.001) in intact rams and increased mean LH concentration (P < 0.01), LH pulse amplitude (P < 0.05) and LH response area (P < 0.01) in castrated animals compared to respective control periods. NMA increased mean LH concentration in intact rams (P < 0.0001) but did not affect mean LH in castrates. NMA increased LH pulse number in rams (P < 0.0001) but decreased number of pulses in castrates (P < 0.0001) compared to control periods. NMA increased LH pulse amplitude in both intact (P < 0.001) and castrated animals (P < 0.05). In conclusion, these results support the hypothesis that blood glucose concentrations influence the control of LH release in sheep. In addition, LH release in response to the LHRH secretagogue, NMA, is positively influenced by testicular hormones.

The effect of excitatory aminoacids on GABA release from mediobasal hypothalamus of female rats

The purpose of the present study was to examine the in vitro effect of L-glutamate and its agonists on basal and potassium-evoked GABA release from incubated mediobasal hypothalamus (MBH) of intact, ovariectomized (OVX) and OVX-estrogenized female rats. L-glutamate (100 microM) decreased evoked GABA release from MBH of intact female rats in diestrus. NMDA and quisqualate (10 and 100 microM) modified neither basal nor evoked hypothalamic GABA release of intact rats. However, kainate (10 and 100 microM) decreased hypothalamic basal and evoked GABA release of intact rats. Kainate induced no changes in basal or in evoked GABA release from hypothalami of OVX rats, but decreased GABA release in chronically estrogenized rats. DNQX (6,7-dinitroquinoxaline-2,3-dione), a non-NMDA receptor antagonist, failed to affect GABA release but blocked the inhibitory effect of kainate. The kainate effect was not Mg2+-sensitive and was not inhibited by D-AP5 (D(-)-2-amino-5-phosphonopentanoic acid), an NMDA-specific receptor antagonist. Kainate induced no changes in nitric oxide synthase activity in MBH of either intact or estrogenized rats. These data indicate that kainate decreases GABA release from MBH of female rats through a non-NMDA receptor subtype, and provide evidence to support the view that kainate-mediated decrease of the hypothalamic GABAergic tone is affected by estrogens.

Effects of administration of N-methyl-D,L-aspartate (NMA) on gonadotropin secretion in untreated and steroid-treated ovariectomized mares during the breeding season and in intact and ovariectomized mares during anestrus

The purpose of this experiment was to investigate whether N-methyl-D,L-aspartate stimulated gonadotropin secretion in mares and to determine the response in two experimental paradigms where gonadotropin secretion is low or elevated. In Experiment 1, conducted during the breeding season (summer), eight long-term ovariectomized mares were treated daily for 21 d with progesterone plus estradiol (n = 4) or oil vehicle. Beginning on Day 14, each mare received, in a randomized design on alternate days, an intravenous injection of either 0, 0.5, 1.0, or 5.0 mg/kg NMA. Treatment with NMA was not accompanied by a change in gonadotropin release in vehicle-treated ovariectomized mares. In steroid-treated mares, gonadotropin values were suppressed compared with vehicle controls, but NMA only induced a significant increase in follicle-stimulating hormone (FSH) and only at the highest dose. In Experiment 2, the effect of the administration of NMA was compared in intact anestrous mares (n = 4) and long-term ovariectomized mares (n = 4) during the nonbreeding season. At the highest doses (1.0 and 5.0 mg/kg), NMA induced a significant increase in luteinizing hormone (LH) and FSH in intact mares. In contrast, pretreatment concentrations of LH and FSH were higher in long-term ovariectomized mares and NMA only increased LH, but not FSH, at the highest dose. The results demonstrate that the administration of NMA stimulates the release of LH and FSH in mares. During anestrus, ovarian intact mares appear more responsive than long-term ovariectomized mares. At high doses, NMA may induce differential release of the gonadotropins in long-term ovariectomized mares.

Aspartate and glutamate modulation of growth hormone secretion in the pig: possible site of action

The influence of excitatory amino acids (EAA) on growth hormone (GH) secretion and the possible site of action was investigated in the pig. In Experiment (Exp) I three replicates were conducted with 30 prepuberal gilts, 130 d of age and averaging 70.6 +/- 1.3 kg body weight (BW). Six gilts each received intravenously (i.v.) 0, 50, 100, or 150 mg/kg BW of aspartate (ASP) or glutamate (GLU) in saline. Blood samples were collected every 15 min for 2 hr before and 3 hr after treatment. In Exp II, mature ovariectomized gilts (163 +/- 10 kg BW) that had been immunized against growth hormone releasing factor (GRF) conjugated to human serum albumin (GRFi; n = 4) or against human serum albumin alone (HSAi; n = 5) received 150 mg/kg BW ASP or GLU i.v. in a 2 x 2 factorial arrangement of treatments, which was then repeated in a crossover design. One week later, all animals received 10 mg/kg N-methyl-D,L-aspartate (NMA; EAA agonist) in saline i.v. Blood samples were collected as described above. In Exp III, cultures of anterior pituitary cells from market-weight (averaging 105 kg BW) gilts were studied. On Day 4 of culture, cells (10(5) seeded/well) were challenged with 10(-8), 10(-6), or 10(-4) M ASP or GLU, 10(-6) M [Ala15]-human GRF (1-29)-NH2, or the EAA antagonist, 2-amino-5-phosphonopentanoic acid (10(-4) M; AP5), alone or in combination with ASP or GLU. In Exp I, all doses of ASP and the 100- and 150-mg doses of GLU increased (P < 0.05) GH secretion when compared with Time 0. However, serum GH concentrations were higher (P < 0.01) after 150 mg/kg of ASP when compared with those after 150 mg/kg of GLU. In Exp II, serum GH concentrations increased (P < 0.05) in HSAi but not in GRFi pigs (averaging 1.2 +/- 0.2 ng/ml before and 8.2 +/- 0.7, 6.3 +/- 0.5, and 9.2 +/- 0.5 ng/ml by 15 min after ASP, GLU, and NMA, respectively). In Exp III, relative to controls (40 +/- 6 ng/ml), GH increased (P < 0.05)1.6-, 1.9-, and 1.9-fold and 1.7-, 1.8-, and 2.0-fold after 10(-8), 10(-6), and 10(-4) M ASP and GLU, respectively. The EAA receptor antagonist AP5 failed to prevent the GH response to ASP or GLU, except for 10(-8) M ASP. In summary, ASP is a more potent secretagogue of GH secretion than is GLU in vivo, whereas each is equipotent in vitro. Because no stimulation of GH by EAA was observed in GRFi pigs and no specific dose-response effect of EAA was found in vitro, it may be concluded that modulation by EAA is mediated primarily at the level of the hypothalamus or higher brain centers.

Luteinizing hormone and growth hormone concentrations in serum of prepubertal gilts treated with N-methyl-D,L-aspartate

Prepubertal Yorkshire gilts, 189 d of age and 96 kg body weight (BW), were used to determine the effects of intravenous injections of N-methyl-D,L-aspartate (NMA) on circulating concentrations of luteinizing hormone (LH) and growth hormone (GH). In Experiment 1, blood was sampled from four gilts every 15 min for 2 hr on four consecutive days. One hour after the initiation of sampling on each day, gilts received either vehicle (0.9% saline solution) or 1.25, 2.5, or 5 mg of NMA/kg BW as per a 4 x 4 latin square arrangement of treatments. There was no effect of treatment (P > 0.1) on concentrations of LH in serum. However, relative to vehicle-treated animals, gilts administered 2.5 mg of NMA/kg BW had elevated (1,120%; P < 0.05) circulating levels of GH. In Experiment 2, samples were collected from 15 gilts at 15-min intervals for 1 hr before and 1 hr after NMA (10 mg/kg BW; n = 8) or vehicle (n = 7). NMA increased serum concentrations of both LH (by 80%; P < 0.05) and GH (by 700%; P < 0.01). We suggest that LH and GH secretion in gilts was enhanced by treatment with NMA.

Effects of excitatory amino acid receptor agonists and antagonists on the secretion of melatonin, luteinizing hormone and prolactin in the ram

To assess the role of excitatory amino acids (EAA) as neurotransmitters in the transmission of light information from the retina to the pineal gland, we have determined whether the systemic injection of EAA agonists in Soay rams will mimic the suppressive effect of light on the secretion of melatonin, and whether pretreatment of rams with EAA antagonists will block this effect. In addition, the efficacy of the drugs in affecting neuroendocrine systems was investigated by measuring the changes in the secretion of luteinizing hormone (LH) and prolactin. Injections fo the EAA receptor agonist, NMDA (N-methyl-D,L-aspartate: 4.0 mg/kg iv), and the non-NMDA type EAA receptor agonist, AMPA (DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid: 0.2 mg/kg iv) given at night to rams exposed to long days (16 h light: 8 h darkness), caused no change in the blood plasma concentrations of melatonin. The treatments induced an acute increase in the concentrations of LH, and NMDA, but not AMPA, caused a sustained increase in the concentrations of prolactin. Injections of the specific NMDA-type receptor antagonist, CGP (CGP 37849: 1.0 mg/kg iv) and the non-NMDA-type receptor antagonist, DNQX (6,7 Dinitroquinoxaline-2,3-dione: 0.5 mg/kg iv), given prior to a 1-h light period at night, in rams under long days, caused no change in the light-induced decrease in blood plasma concentrations of melatonin. The drug treatments had no effect on the plasma concentrations of LH, but CGP, and not DNQX, stimulated an acute increase in the plasma concentrations of prolactin. These results provide support for the hypothesis that EAA mechanisms operate in the hypothalamus to regulate the release of peptides and catecholamines which control the secretion of LH and prolactin from the pituitary gland; different sub-types of EAA receptors are involved in the control of the two pituitary hormones. The failure of the treatments to affect the secretion of melatonin may indicate that EAA receptor activation is not involved in the photic relay to the pineal gland, or may merely reflect the inability of the drugs to penetrate into the retina/SCN/pineal neural circuits to produce a response.

Effect of a single intravenous injection of N-methyl-D,L-aspartic acid on secretion of luteinizing hormone and growth hormone in Holstein bull calves

The role of N-methyl-D-aspartate (NMDA) receptor activation in the central regulation of luteinizing hormone (LH) and growth hormone (GH) was tested by administering a bolus intravenous dose of N-methyl-D,L-aspartic acid (NMA), a NMDA receptor agonist, to 24-week-old intact (n=5), estradiol-treated intact (n=3) and castrated (n=3) Holstein bull calves. The calves were bled for 12h pre- and 100 min post-NMA injection (1.75 mg-/kgBW) periods at 10 min intervals. Concentrations of LH and GH in plasma were measured by specific RIA. Prior to administration of NMA, the average concentration of LH, but not GH, differed significantly among the 3 groups. As expected, administration of estradiol prevented the normal ontogeny of pulsatile LH secretion, while castration resulted in an increased frequency of LH discharges. Injection of NMA resulted in an acute (P<0.001) release of LH in 3 of 5 intact and 3 of 3 estradiol-treated intact calves with the peak response being observed at 20 min (3.18 +/- 1.3 and 5.58 +/- 1.3 ng/ml, respectively) following the challenge. Treatment with NMA did not alter the release of LH in castrate calves. Concentrations of GH in plasma increased (P<0.001) within 20 to 30 min after administration of NMA in intact, estradiol-treated intact and castrate calves with a similar response being observed in each group. Based on these findings, we suggest an involvement of glutamatergic neurotransmission in the hypothalamic or supra-hypothalamic control of LH and GH secretion, and that the excitatory effects of NMDA receptor activation on LH release are overtly influenced by gonadal steroids in bull calves.

N-methyl-d,l-aspartate modulation of pituitary hormone secretion in the pig: role of opioid peptides

Sixteen ovariectomized (OVX) mature gilts, averaging 139.6 +/- 3.1 kg body weight (BW) were assigned randomly to receive either progesterone (P, 0.85 mg/kg BW, n = 8), or corn oil vehicle (OIL, n = 8) injections im twice daily for 10 d. On the day of experiment, all gilts received either the EAA agonist, N-methyl-d,l-aspartate (NMA; 10 mg/kg BW, iv) alone or NMA plus the EOP antagonist, naloxone (NAL, 1 mg/kg BW, iv), resulting in the following groups of 4 gilts each: OIL-NMA, OIL-NMA-NAL, P-NMA and P-NMA-NAL. Blood samples were collected via jugular cannula every 15 min for 6 hr. All pigs received NMA 5 min following pretreatment with either 0.9% saline or NAL 2 hr after blood collection began and a GnRH challenge 3 hr after NMA. Administration of NMA suppressed (P < 0.03) LH secretion in OIL-NMA gilts and treatment with NAL failed to reverse the suppressive effect of NMA on LH secretion in OIL-NMA-NAL gilts. Similar to OIL-NMA gilts, NMA decreased (P < 0.03) mean serum LH concentrations in P-NMA gilts. However, in P-NMA-NAL gilts, serum LH concentrations were not changed following treatment. All gilts responded to GnRH with increased (P < 0.01) LH secretion. Additionally, administration of NMA increased (P < 0.01) growth hormone (GH) and prolactin (PRL) secretion in both OIL-NMA and P-NMA gilts, but this increase in GH and PRL secretion was attenuated (P < 0.01) by pretreatment with NAL in OIL-NMA-NAL and P-NMA-NAL gilts.(ABSTRACT TRUNCATED AT 250 WORDS)

Sexual differences in N-methyl-D-aspartate receptor-mediated regulation of tuberoinfundibular dopaminergic neurons in the rat

The purpose of the present study was to examine the effects of N-methyl-D-aspartate (NMDA) receptor blockade on the activity of tuberoinfundibular dopaminergic (TIDA) neurons in male and female rats. TIDA neuronal activity was estimated by measuring either the accumulation of 3,4-dihydroxyphenylalanine (DOPA) 30 min after the administration of the decarboxylase inhibitor NSD-1015 or the concentration of the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) in the median eminence. The non-competitive NMDA receptor antagonist MK-801 markedly reduced prolactin secretion in both male and female rats. MK-801 also produced a dose- and time-dependent decrease in the activity of TIDA neurons in female rats, but had no effect on the activity of TIDA neurons in either intact, orchidectomized or orchidectomized, testosterone-treated male rats. Removal of the tonic stimulatory effects of prolactin on TIDA neurons in female rats by immunoneutralization of endogenous prolactin failed to alter the responsiveness of TIDA neurons to the inhibitory effects of MK-801. On the other hand, MK-801 was unable to inhibit TIDA neurons in ovariectomized female rats, but the responsiveness of TIDA neurons to MK-801 in ovariectomized female rats was restored following estrogen replacement, even in the absence of prolactin. Like MK-801, the competitive NMDA receptor antagonist CGS-19755 produced a dose-dependent decrease in TIDA neuronal activity in female rats, which was prevented in a dose-dependent manner by the NMDA receptor agonist D,L-(tetrazol-5-yl) glycine. Taken together, these results reveal a sexual difference in the responsiveness of TIDA neurons to NMDA receptor antagonists, and suggest that estrogen positively modulates NMDA receptor-mediated, tonic stimulation of TIDA neurons in female rats by a prolactin-independent mechanism.

N-methyl-D,L-aspartate modulation of luteinizing hormone and growth hormone secretion from pig pituitary cells in culture

Administration of n-methyl-d, l-aspartate (NMA) to pigs in vivo increased GH and suppressed LH secretion. Cultures of anterior pituitary cells from pigs in the follicular phase (FOL; n = 3) and luteal phase (LUT; n = 3) of the estrous cycle, and ovariectomized (OVX; n = 10) pigs were treated with NMA (10(-4), 10(-6) or 10(-8) M) or the NMA antagonist, 2-amino-5-phosphonopentanoic acid (AP5; 10(-4), 10(-6) or 10(-8) M), to determine if NMA affects the pituitary directly. Secreted LH and GH were measured at 4 h after treatment. Basal LH and GH secretion (control; C) were 1.1 +/- 0.6, 4.4 +/- 2.1 and 5.6 +/- 1.3 ng/well and 5.2 +/- 1.2, 7.5 +/- 1.2 and 5.2 +/- 1.7 ng/well for FOL, LUT and OVX, respectively. Relative to C, 10(-4) M NMA increased (P < 0.001) LH secretion 2.4-, 2.2- and 5.1-fold in FOL, LUT and OVX cultures, respectively. The effect of 10(-4) M NMA was inhibited by 10(-4) M AP5 (P < 0.05) in FOL cultures, but not in OVX cultures. GnRH increased (P < 0.001) LH levels 3.1-, 2.3- and 3.8-fold in FOL, LUT and OVX cultures, respectively. Relative to C, 10(-4), 10(-6) and 10(-8) M NMA increased (P < 0.03) GH secretion 1.5-, 1.5- and 2.3-fold in LUT and 1.7-, 2.3- 2.0-fold in OVX cultures, respectively. AP5 alone or in combination with NMA failed to alter basal GH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)

N-methyl-d,l-aspartate stimulates growth hormone and prolactin but inhibits luteinizing hormone secretion in the pig

The effects of n-methyl-d,l-aspartate (NMA), a neuroexcitatory amino acid agonist, on luteinizing hormone (LH), prolactin (PRL) and growth hormone (GH) secretion in gilts treated with ovarian steroids was studied. Mature gilts which had displayed one or more estrous cycles of 18 to 22 d were ovariectomized and assigned to one of three treatments administered i.m.: corn oil vehicle (V; n = 6); 10 micrograms estradiol-17 b/kg BW given 33 hr before NMA (E; n = 6); .85 mg progesterone/kg BW given twice daily for 6 d prior to NMA (P4; n = 6). Blood was collected via jugular cannulae every 15 min for 6 hr. Pigs received 10 mg NMA/kg BW i.v. 2 hr after blood collection began and a combined synthetic [Ala15]-h GH releasing factor (1-29)-NH2 (GRF; 1 micrograms/kg BW) and gonadotropin releasing hormone (GnRH; .2 micrograms/kg BW) challenge given i.v. 3 hr after NMA. NMA did not alter LH secretion in E gilts. However, NMA decreased (P < .02) serum LH concentrations in V and P4 gilts. Serum LH concentrations increased (P < .01) after GnRH in all gilts. NMA did not alter PRL secretion in P4 pigs, but increased (P < .01) serum PRL concentrations in V and E animals. Treatment with NMA increased (P < .01) GH secretion in all animals while the GRF challenge increased (P < .01) serum GH concentrations in all animals except in V treated pigs. NMA increased (P < .05) cortisol secretion in all treatment groups. These results indicate that NMA inhibits LH secretion and is a secretagogue of PRL, GH and cortisol secretion with ovarian steroids modulating the LH and PRL response to NMA.

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.

Elicitation of release of luteinizing hormone by N-methyl-d,l-aspartic acid during three paradigms of suppressed secretion of luteinizing hormone in the female pig

Two experiments were conducted to determine the minimal effective dose during lactation and site of action of N-methyl-d,l-aspartic acid (NMA) for elicitation of release of luteinizing hormone (LH) in female pigs. In the first experiment, three doses of NMA were given to lactating primiparous sows in which endogenous LH was suppressed by suckling of litters. In the second experiment, ovariectomized gilts were pretreated with estradiol benzoate or porcine antisera against GnRH to suppress LH and then given NMA to determine if it elicited secretion of LH directly at the anterior pituitary or through release of GnRH. In experiment 1, 3 lactating sows (17 +/- 1.5 d postpartum) were each given three doses of NMA (1.5, 3.0 and 5.0 mg/kg body weight [BW]; IV) on 3 consecutive days in a Latin Square design. Blood samples were collected every 10 min from -1 to 1 hr from injection of NMA. NMA at 1.5 and 3.0 mg/kg did not affect (p greater than .5) secretion of LH; however, 5 mg NMA/kg elicited a 114% increase (p less than .001) in circulating levels of LH during 1 hr after treatment. In experiment 2, 8 ovariectomized gilts were given either estradiol benzoate (EB; 10 micrograms/kg BW; IM n = 4) to suppress release of GnRH or porcine antiserum against GnRH (GnRH-Ab; titer 1:8,000; 1 ml/kg BW; IV; n = 4) to neutralize endogenous GnRH. Gilts infused with GnRH-Ab were given a second dose of antiserum 24 hr after the first. Gilts were then given NMA (10 mg/kg BW; IV) 33 hr after EB or initial GnRH-Ab. Blood samples were drawn every 6 hr from -12 to 24 hr from EB or GnRH-Ab treatments, and every 10 min from -2 to 2 hr from NMA. Serum LH declined (p less than .001) after EB (from 1.87 +/- .2 ng/ml at 12 hr before EB to 0.46 +/- .02 ng/ml during 24 hr after EB) and GnRH-Ab (from 1.97 +/- .1 to 0.59 +/- .02 ng/ml). In gilts treated with EB, the area under the curve (AUC) for the LH response (ng.ml-1.min) 1 hr after NMA (38.7 +/- 3) was significantly greater (p less than .01) than the 1 hr prior to NMA (21.3 +/- 1.5). Treatment with NMA had no effect (p greater than .5) on secretion of LH in gilts infused with GnRH-Ab.(ABSTRACT TRUNCATED AT 400 WORDS)

Excitatory amino acid regulation of gonadotropin secretion: modulation by steroid hormones

Excitatory amino acids (EAAs) can potently modulate gonadotropin secretion in the male rat and monkey. In the present study we examined the effect of EAAs on luteinizing hormone (LH) and follicle-stimulating hormone (FSH) in the female rat under low estrogen (ovariectomized) and high estrogen (proestrus) backgrounds. In ovariectomized immature female rats N-methyl-D-aspartate (NMDA) inhibited LH but not FSH secretion at 30 min post-injection. In contrast, NMDA potently stimulated LH but not FSH secretion when administered on proestrus to adult female rats. Both glutamate and kainate were also found to stimulate LH but not FSH secretion in estrogen-treated ovariectomized immature rats. This study suggests that EAA neurotransmission may be an important component in the expression of gonadotropin surges and that EAA effects appear to be subject to gonadal steroid regulation.

Neuroendocrine regulation of the luteinizing hormone-releasing hormone pulse generator in the rat

We have analyzed the mechanisms by which several known regulators of the LHRH release process may exert their effects. For each, we have attempted to determine how and where the regulatory input is manifest and, according to our working premise, we have attempted to identify factors which specifically regulate the LHRH pulse generator. Of the five regulatory factors examined, we have identified two inputs whose primary locus of action is on the pulse-generating mechanism--one endocrine (gonadal negative feedback), and one synaptic (alpha 1-adrenergic inputs) (see Fig. 29). Other factors which regulate LHRH and LH release appear to do so in different ways. The endogenous opioid peptides, for example, primarily regulate LHRH pulse amplitude (Karahalios and Levine, 1988), a finding that is consistent with the idea that these peptides exert direct postsynaptic or presynaptic inhibition (Drouva et al., 1981). Gonadal steroids exert positive feedback actions which also result in an increase in the amplitude of LHRH release, and this action may be exerted through a combination of cellular mechanisms which culminate in the production of a unique, punctuated set of synaptic signals. Gonadal hormones and neurohormones such as NPY also exert complementary actions at the level of the pituitary gland, by modifying the responsiveness of the pituitary to the stimulatory actions of LHRH. The LHRH neurosecretory system thus appears to be regulated at many levels, and by a variety of neural and endocrine factors. We have found examples of (1) neural regulation of the pulse generator, (2) hormonal regulation of the pulse generator, (3) hormonal regulation of a neural circuit which produces a unique, punctuated synaptic signal, (4) hormonal regulation of pituitary responsiveness to LHRH, and (5) neuropeptidergic regulation of pituitary responsiveness to LHRH. While an attempt has been made to place some of these regulatory inputs into a physiological context, it is certainly recognized that the physiological significance of these mechanisms remains to be clarified. We also stress that these represent only a small subset of the neural and endocrine factors which regulate the secretion or actions of LHRH. A more comprehensive list would also include CRF, GABA, serotonin, and a variety of other important regulators. Through a combination of design and chance, however, we have been able to identify at least one major example of each type of regulatory mechanism.

N-methyl-D, L-aspartate induces a transient increase in LH secretion in the seasonally anestrous ewe

The primary objective of this study was to determine the LH response to an excitatory amino acid agonist, N-methyl-D, L-aspartate (NMA) in the seasonally anestrous ewe. In experiment 1, 3 i.v. injections of NMA were given; doses of 0.5, 1.5 and 4.5 mg/kg BW were tested. LH response to NMA depended on the dose. There was little response to the lowest dose. All animals responded to the first injection of the intermediate and the highest doses (mean pulse amplitude: 9.2 +/- 0.4 and 6.8 +/- 1.2 ng ml, respectively). The responses to the second or third injections of both doses were variable and were either absent or reduced compared to that of the first. In experiments 2 and 3, ewes were given 3 injections of normal saline (NS) followed by 3 injections of NMA (1.25 and 4.5 mg/kg BW, respectively) at 2 hr intervals. The last injection of NMA was followed 2 hr later by an injection of GnRH (3.0 ng/kg BW). In experiment 2, the first NMA injection induced an immediate LH pulse (mean pulse amplitude: 8.0 +/- 1.6 ng/ml) in all ewes, however, the second and third injections induced LH pulses in only 25% and 75% (mean pulse amplitude: 2.2 and 2.4 +/- 0.6 ng/ml) of the ewes, respectively. In experiment 3, NMA increased mean LH release (P less than 0.05) after all injections, but responsiveness to the third injection was reduced in some ewes. GnRH injections induced LH release in all ewes in experiments 2 and 3 (mean pulse amplitude: 6.9 +/- 1.8 and 6.4 +/- 2.2 ng/ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Possible modulation of N-methyl-D,L-aspartic acid induced prolactin release by testicular steroids in the adult male rhesus monkey

N-methyl-D,L-aspartic acid (NMA), an agonist of the neurotransmitter glutamate has been shown to acutely stimulate the release of prolactin (PRL) in intact rats and monkeys. To further investigate the role of neuroexcitatory amino acids in PRL secretion, the effects of NMA administration were examined on PRL release in long term orchidectomized adult rhesus monkeys, in both the absence and presence of testosterone. Intact and long term castrated adult male monkeys weighing between 8-13 kg, were implanted with a catheter via the saphenous vein for blood withdrawal and drug infusion. Blood samples were collected at 10 min intervals for 50 min before and 70 min after administration of the drug or vehicle. Plasma PRL concentrations were estimated using radioimmunoassay. Whereas a single iv injection of NMA (15 mg/kg BW) induced a prompt discharge of PRL in intact monkeys, an identical dose had surprisingly no effect on PRL secretion in orchidectomized animals. On the other hand, plasma PRL increases in response to a challenge dose of thyrotropin releasing hormone (TRH; 6 micrograms/kg BW, iv) were similar in magnitude in the two groups of monkeys. Testosterone replacement in orchidectomized animals by parenteral administration of testosterone enanthate (200 mg/wk) reinitiated the PRL responsiveness to acute NMA stimulation. These results indicate that N-methyl-D-aspartic acid (NMDA) dependent drive to PRL release in the adult male rhesus monkey may be overtly influenced by the sex steroid milieu.

Expression of c-fos protein by N-methyl-D-aspartic acid in hypothalamus of immature female rats: blockade by MK-801 or neonatal treatment with monosodium glutamate

Significant NMDA-induced cellular activity in the median eminence-arcuate region of immature female rats was immunocytochemically localized via c-fos protein-like immunoreactivity. The production of this protein was completely prevented by pre-injection of MK-801 or APV, antagonists of the NMDA receptor-coupled ion channel. Treatment of newborn rats with monosodium glutamate (MSG), which destroys glutamate-sensitive neurons, significantly attenuated NMDA-induced c-fos when tested in the peripubertal period. These results implicate NMDA receptors in the expression of c-fos protein-like immunoreactivity in cells of the arcuate nucleus-median eminence of immature female rats.