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.     

The effect of ketamine hydrochloride anesthesia on basal and N-methyl-D,L-aspartate induced plasma prolactin secretion in the adult male rhesus monkey

The excitatory amino acids (EAAs), glutamate and aspartate, acting predominantly on N-methyl-D-aspartate (NMDA) receptor, have been shown to be involved in the central regulation of the secretion of several anterior pituitary hormones including prolactin (PRL), whereas ketamine hydrochloride (KH), a widely used anesthetic, has been reported to antagonize a variety of NMDA receptor mediated actions of these EAAs. In the present study, the effect of KH on basal PRL levels as well as on N-methyl-D,L-aspartate (NMA), an agonist of NMDA receptor, induced plasma PRL secretion was investigated in the adult male rhesus monkey. The values were compared to those obtained from the same animals restrained in primate chairs. The plasma PRL concentrations were higher in animals receiving KH administered either intramuscularly (2.5 mg/kg BW at 30 min intervals) or intravenously (10 mg/kg BW) as compared to those observed in the unanesthetized chair-restrained monkeys. NMA induced an unequivocal increase in plasma PRL concentrations in both conscious chair-restrained and KH anesthetized monkeys, but the response was greater in anesthetized animals than the conscious monkeys. The present findings suggest that KH has stimulatory effects on both basal and NMA induced plasma PRL secretion.     

NMDA受体通道参与大鼠脊髓背角C纤维诱发电位LTP的表达

以往研究表明,激动NMDA受体是引起海马长时程增强(LTP)的必备条件,而LTP的表达主要与AMPA受体的磷酸化及其受体组装到突触后膜有关.但是,近年来有研究表明NMDA受体通道也参与了LTP的表达.为探讨NMDA受体通道是否参与了脊髓背角C纤维诱发电位LTP的表达,诱导LTP后,分别静脉或脊髓局部给予NMDA受体拮抗剂MK801或APV,观察其作用.发现静脉注射非竞争性NMDA受体MK801(0.1mg/kg)对脊髓LTP无影响,注射0.5mg/kg显著抑制LTP,但是当剂量增高到1.0mg/kg时,抑制作用并未进一步增大.脊髓局部给予MK801也能抑制脊髓背角LTP.为验证上述结果,使用了竞争性NMDA受体拮抗剂APⅤ.结果显示,脊髓局部给予50μmol/LAPⅤ对LTP无影响,100μmol/L对LTP有显著的抑制作用,当浓度升至200μmol/L时,抑制作用并未见进一步增强.因此认为,NMDA受体通道部分地参与了脊髓背角C纤维诱发电位LTP的表达.     

Modulation of growth hormone, luteinizing hormone, and testosterone secretion by excitatory amino acids in boars

Ketamine hydrochloride, an n-methyl-d-aspartate (NMDA) receptor antagonist was used in an experiment that tested the hypothesis that fasting-induced increases in growth hormone (GH) secretion is mediated by excitatory amino acid (EAA) neurotransmission in boars. The effects of the drug on circulating concentrations of luteinizing hormone (LH) and testosterone were also evaluated. Blood was sampled at 15-min intervals for 8 h from 12 boars fitted with jugular vein catheters. At Hours 4 and 6, fasted boars (feed was withdrawn 48 h before the start of blood sampling) received i.m. injections of ketamine (19.9 mg/kg body weight; n=4) or .9% saline (n=4). Boars allowed feed on an ad libitum basis (n=4) received i.v. injections of n-methyl-d,l-aspartate (NMA; 2.5 mg/kg body weight), an NMDA receptor agonist, at Hours 4 and 6. Secretion of GH increased after NMA injections but was unaffected by treatment with ketamine or saline. Circulating concentrations of LH and testosterone were increased by injections of ketamine but were unaffected by injections of NMA or saline. Our results suggest that NMA is a potent GH secretagogue, but do not support the hypothesis that EAA neurotransmission drives the increased GH secretion displayed in fasted boars. Our finding that ketamine increased LH and testosterone release supports the notion that EAA have inhibitory effects on gonadotropin secretion in acutely fasted swine.     

Effects of Ionotropic Excitatory Amino Acid Receptor Antagonists on Glutamate Transport and Transport-Mediated Changes in Extracellular Excitatory Amino Acids in the Rat Striatum

Abstract: This study examined the effects of intrastriatal administration of ionotropic excitatory amino acid receptor antagonists on biochemical markers of excitatory amino acid transmission in the rat striatum. High-affinity glutamate uptake was measured ex vivo on striatal homogenates 15 min after the local administration of either 6,7-dinitroquinoxaline-2,3-dione (DNQX), a non-NMDA receptor antagonist, or dl -2-amino-5-phosphonopentanoic acid (AP5), a competitive NMDA antagonist, at various doses (10–500 pmol injected). DNQX induced a dose-dependent increase in glutamate uptake rate, related to an increase in the V _max of the transport process, whereas no significant change in glutamate uptake was detected after AP5 administration. Similar results were obtained from animals subjected to excitotoxic lesion of striatal neurons by kainate administration 15 days before the injection of DNQX or AP5. In a parallel series of experiments using in vivo microdialysis we showed that DNQX (10⁻⁵ M ) in the dialysis probe diminished by ∼30–40% the increases in the concentrations of glutamate and aspartate elicited by l - trans -pyrrolidine-2,4-dicarboxylic acid (1 m M ). These data suggest that presynaptic glutamate transmission in the rat striatum may undergo facilitatory autoregulatory processes involving ionotropic non-NMDA receptors and highlight the view that transporters for glutamate may be potent regulatory sites for glutamatergic transmission.     

Effects of intracerebroventricular administration of D,L-2-amino-5-phosphonovaleric acid, an N-methyl-D-aspartate receptor antagonist, on luteininizing hormone release in ovariectomized lambs.

To determine whether endogenous glutamate and aspartate control LH secretion via N-methyl-D-aspartate (NMDA) receptors in the sheep, we evaluated the effects of the NMDA receptor antagonist D,L-2-amino-5-phosphonovaleric acid (AP5) on secretion of LH in ovariectomized lambs. Twelve lambs were ovariectomized and surgically implanted with lateral cerebroventricular cannulae. At the time of the experiment, (38 wk of age) they received intracerebrally 4 injections of either 50 (n = 4), 100 (n = 4), or 200 micrograms (n = 4) of AP5. Blood samples were collected every 10 min for 8 h with animals receiving AP5 at hours 4, 5, 6, and 7. Patterns of LH during the preinjection period were compared to those during the period encompassing AP5 injections. Mean concentrations of LH were lower during AP5 injections than during the preinjection periods, a response that was not influenced by dose (0.87 +/- 0.08 vs. 0.69 +/- .07 ng/ml; p < 0.01). LH pulse amplitude decreased during AP5 treatment relative to the preinjection periods, but this difference was not statistically significant (0.79 +/- 0.11 vs. 0.68 +/- 0.10 ng/ml; p = 0.09). There were no effects of AP5 on LH pulse frequency (1.00 +/- 0.10 vs. 0.83 +/- 0.15 pulses/h for injection and preinjection periods; p > 0.10). A second experiment was done to evaluate a higher dose of AP5. Four animals were chosen to receive 4 injections of 2 mg of AP5 in a design identical to that used in the first experiment.     

Differential effects of N-methyl-D-aspartate receptor stimulation on growth hormone secretion at specific stages of postnatal development of the male rhesus monkey

The present study attempts to examine the role of N-methyl-D-aspartate (NMDA) receptor in the central regulation of growth hormone (GH) secretion during specific stages of pubertal development of the male rhesus monkey (Macaca mulatta). Infantile (n=4), prepubertal (n=5), peripubertal (n=5) and adult (n=5) intact male rhesus monkeys were given an agonist of NMDA receptor, N-methyl-D,L-aspartate (NMA) (15 mg/kg BW) through a teflon cannula implanted in the saphenous vein. Blood samples were collected 20-60 min before and 40-80 min after the injection of the drug at 10-20 min intervals. NMA was dissolved in normal saline immediately before use and passed through a 0.22 microm filter at the time of injection. All bleedings were carried out under ketamine hydrochloride anesthesia (initial dose 5 mg/kg BW, im followed by 2.5 mg/kg at 30 min intervals). The plasma levels of GH and testosterone (T) were determined by using specific assay systems. The hypothalamic-somatotrope activity under basal conditions was studied by averaging all the GH concentrations obtained before NMA injection, whereas the sensitivity of NMDA receptor to NMA stimulation was determined by comparing basal GH levels immediately before NMA injection at 0 min and GH concentrations obtained 10 min after the injection. The mean basal plasma concentrations of GH in the four groups of animals showed marked age-related differences. The levels of GH were found to be higher in infantile and peripubertal monkeys as compared to those of prepubertal and adult animals. A single iv injection of NMA produced differential effects on GH secretion during specific stages of postnatal development depending upon the level of GH secretion under basal conditions. Whereas NMA had no demonstrable effect on GH secretion in infantile and peripubertal animals in which the basal GH levels were high, it produced pronounced effects on GH secretion in prepubertal and adult monkeys wherein baseline GH concentrations were low. In conclusion, the present study suggests that the glutamatergic component of the control system that governs GH secretion by utilizing NMDA receptor may participate in regulation of age-related changes in the secretion of GH in the male rhesus monkey.     

A comparison of the effects of suckling or transient dopamine antagonism on thyrotropin-releasing hormone and suckling induced prolactin release in lactating rats

Prolactin (PRL) release was studied in mid-lactational female rats by comparing the stimulatory influence of suckling to a drug protocol that mimics the effect of suckling on the anterior pituitary (AP). Animals that nursed pups for 15 minutes and were allowed to suckle again 60 minutes later for 10 minutes, released PRL effectively during both nursing episodes; however, in animals that received the dopamine (DA) agonist 2-Br-alpha-ergocryptine maleate (CB-154, 0.5 mg/rat i.v.) at the end of the first nursing period did not show an increase in plasma PRL to a second suckling stimulation by the pups. When thyrotropin releasing hormone (TRH) was substituted for the second suckling period in CB-154 treated rats, a slight increase in plasma PRL occurred 5 minutes after the injection. In a third study we transiently blocked the action of DA at the AP by injecting the DA antagonist domperidone (0.01 mg/rat i.v.), followed 5 minutes later by the administration of CB-154. One hour later animals were either allowed to suckle pups for 10 minutes or were injected with TRH. Treatment with TRH resulted in an 11 fold increase in plasma PRL but suckling was completely ineffective in inducing PRL release. These data suggest that the lack of PRL release to suckling in CB-154 treated rats was due to inhibitory effects of CB-154 on neural mechanisms which link nursing to PRL release. In addition, the data show that pharmacologic DA antagonism affects TRH releasable PRL more than does suckling. This may be due to a reduction, by suckling, of the pool of PRL that is available to be released by TRH administration.     

Occurrence of D-aspartic acid and N-methyl-D-aspartic acid in rat neuroendocrine tissues and their role in the modulation of luteinizing hormone and growth hormone release.

Using two specific and sensitive fluorometric/HPLC methods and a GC-MS method, alone and in combination with D-aspartate oxidase, we have demonstrated for the first time that N-methyl-D-aspartate (NMDA), in addition to D-aspartate (D-Asp), is endogenously present as a natural molecule in rat nervous system and endocrine glands. Both of these amino acids are mostly concentrated at nmol/g levels in the adenohypophysis, hypothalamus, brain, and testis. The adenohypophysis maximally showed the ability to accumulate D-Asp when the latter is exogenously administered. In vivo experiments, consisting of the i.p. injection of D-Asp, showed that D-Asp induced both growth hormone and luteinizing hormone (LH) release. However, in vitro experiments showed that D-Asp was able to induce LH release from adenohypophysis only when this gland was co-incubated with the hypothalamus. This is because D-Asp also induces the release of GnRH from the hypothalamus, which in turn is directly responsible for the D-Asp-induced LH secretion from the pituitary gland. Compared to D-Asp, NMDA elicits its hormone release action at concentrations approximately 100-fold lower than D-Asp. D-AP5, a specific NMDA receptor antagonist, inhibited D-Asp and NMDA hormonal activity, demonstrating that these actions are mediated by NMDA receptors. NMDA is biosynthesized from D-Asp by an S-adenosylmethionine-dependent enzyme, which we tentatively denominated as NMDA synthase.     

Glycine modulates N-methyl-D-aspartic acid induced learning facilitation in rats

Summary Pretraining i.p. administration of N-methyl-D-aspartic acid (NMDA) at doses of 10 and 20mg/kg dose-dependently facilitated performance in a water T-maze learning task in rats. The effect of NMDA was inhibited by the competitive NMDA receptor antagonist CGP37849 [(DL)-E(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid] (CGP) at a dose of 6mg/kg, and by the NMDA receptor complex glycine site antagonist 1-hydroxy-3-amino-2-pyrrolidone (HA-966) at a dose of 10mg/kg. The NMDA site antagonist, when given alone, did not impair learning. The glycine precursor milacemide (2-N-pentylaminoacetamide HCl), at doses of 5 and 10mg/kg accelearted learning acquisition and its effect was antagonized by HA-966. The learning rate was impaired following the administration of NMDA 10mg/kg together with milacemide 5mg/kg when compared with the effect of 10mg/kg NMDA alone.The administration of 5mg/kg NMDA was associated with an elevated tissue concentration of aspartate in the hippocampus, an effect which was antagonized by 6mg/kg of CGP. NMDA at doses of 10 and 20mg/kg elevated the concentration of glycine but decreased the concentration of aspartate, glutamate and glutamine in the cortex and aspartate in the hippocampus. The cortical effects of NMDA 10mg/kg were antagonized by 6mg/kg of CGP. Milacemide at the dose of 10mg/kg elevated glycine, aspartate, glutamate and taurine concentrations. The coadministration of 5 mg/kg NMDA with 5mg/kg milacemide elevated the concentrations of glycine, glutamate and glutamine in the cortex and taurine in the hippocampus. These amino acid levels were higher than after administration of 5mg/kg either agent alone. The results demonstrate a dose-dependent facilitation effect on learning performance by NMDA and glycine receptor agonists. Antagonists at the NMDA and glycine sites counteracted the learning improvement of NMDA, and the glycine site antagonist the effect of milacemide.     

Naloxone injected into the preoptic region has hypophysiotropic and seizurogenic actions in rats.

The effects of microinjection of naloxone, an opiate receptor antagonist, into the medial preoptic area (MPO) and diagonal band of Broca (DBB) on luteinizing hormone (LH) and prolactin (PRL) secretion were examined in the intact male rat and female rat in diestrus 1. In both the male and female rats, the injection of 50 micrograms naloxone at 1300 h produced an acute, two- to three-fold increase in serum LH, attaining the peak at 20 min. The PRL concentration in the female 20 min-2 h after the injection was significantly lower than in the saline-injected rat. In the male rat, naloxone caused a decrease in the PRL concentration in the late afternoon when a small rise occurred in the saline-injected rat, although it caused no immediate changes. In addition to these hypophysiotropic effects, naloxone injected in the MPO and DBB unexpectedly had seizurogenic actions. More than 40% of the animals of both sexes given an injection of naloxone had behavioral seizures, which began after about 20 min and were repeated intermittently at 15-20 min intervals through the sampling period of 6 h. In the LH and PRL response to naloxone, there was no significant difference between animals with and without seizure response in both sexes. The results suggest that in the preoptic opioid system there is no difference according to sex in the control of LH, and only a small one, if any, in the control of PRL. Further, on the basis of previous reports, there is a GABAergic system in the preoptic region, that is antagonized by naloxone and causes the activation of cortical neuronal activity.     

Prolactin receptor regulation by LH in the rat mammary gland

The aim of this study was to investigate hormonal factors responsible for the huge increase in PRL receptors on the day of estrus in the rat mammary gland. For this purpose, ovariectomized rats were primed with E2 so as to reach a physiological serum concentration of E2 (21.5 +/- 1.2 pg/ml) and high PRL serum values (72.8 +/- 21.9 ng/ml). In these conditions, PRL specific binding and capacity were respectively 22.8 +/- 8.3%/mg protein and 96 +/- 29 fm/mg protein. An injection of either LHRH (500 ng/rat) or LH (60 micrograms LH-RP1/rat) was capable of increasing significantly both PRL specific binding and capacity. Capacity reached the values of 498 +/- 103 and 507 +/- 240 fm/mg protein for LHRH and LH respectively. LHRH action appeared to be mainly mediated through LH secretion, since no difference was found between LHRH and LH. LHRH and LH injections alone were unable to modify PRL binding, suggesting that they only potentiate E2 and PRL action. These results show for the first time that LH is involved in the regulation of PRL receptors in the rat mammary gland.     

Effect of caerulein on decreased latency of passive avoidance response in rats treated with NMDA receptor antagonists

The effect of subcutaneous injection of caerulein on memory impairment induced by intracerebroventricular administration of NMDA receptor antagonists was examined in the passive avoidance response of the rat. When rats were treated with AP5, AP7, CPP or MK-801, the retention latencies decreased markedly. However, in rats that received caerulein immediately after the training trials, the latency increased to some extent. Pretreatment with caerulein and subsequent injection of the competitive NMDA receptor antagonists AP5, AP7 and CPP caused a more apparent increase in the latency. The noncompetitive NMDA receptor antagonist MK-801 was not affected by pretreatment with caerulein. The difference might be, at least in part, due to the sites of action of these NMDA receptor antagonists.     

N-methyl-d, l-aspartate stimulates growth hormone but not luteinizing hormone secretion in the sheep

The objective of this experiment was to determine the effects of N-methyl-d, l-aspartate (NMA) on luteinizing hormone (LH) and growth hormone (GH) secretion in castrated male sheep. Blood was sampled from Hampshire wethers every 15 min for 8 hr on day 1. At 4 and 6 hr after the initiation of the experiment, wethers were treated i.v. with NMA at a dose of 12 mg/kg body weight (n = 5) or .9% saline (n = 5). The dosage of NMA was within the range of doses that was previously demonstrated to stimulate LH secretion in monkeys. Blood samples were also collected every 15 min for 1 hr on day 2, beginning 24 hr after the first injection of NMA or saline. Treatment with NMA had no effect on mean LH concentrations, LH pulse frequency or LH pulse amplitude during the 4 hr period following the first injection on day 1. On day 2, however, mean LH concentrations were lower (p less than .01) in NMA versus saline-treated wethers. Conversely, administration of NMA evoked a dramatic increase (p less than .02) in mean GH concentrations on day 1. The mechanisms responsible for the effects of NMA described herein and whether or not these effects are relevant to the physiological control of LH and GH release in the sheep warrants further scrutiny.     

Ionotropic glutamate receptor activation increases intracellular calcium in prolactin-releasing cells of the adenohypophysis

Endocrine cells of the anterior pituitary are controlled by the central nervous system through hormonal interactions and are not believed to receive direct synaptic connections from the brain. Studies suggest that some pituitary cells may be modulated by the neurotransmitter glutamate. We investigated prolactin (PRL)-releasing cells of the anterior pituitary of a euryhaline fish, the tilapia (Oreochromis mossambicus), for the presence of possible glutamate receptors (GluRs). Fura-2 imaging addressed the ability of glutamate to increase intracellular calcium. We observed a dose-dependent increase in intracellular calcium with transient perfusion (1-2 min) of glutamate (10 nM to 1 mM) in two-thirds of imaged cells. This increase was attenuated by the ionotropic GluR antagonist kynurenic acid (0.5-1.0 mM). The increase was also blocked or attenuated by antagonists of L-type voltage-gated calcium channels. The GluR agonist alpha-amino-3-hydroxy-5-methylisoxazole propionic acid (AMPA; 100 microM) produced intracellular calcium increases that were reversibly blocked by the selective AMPA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). In contrast, the selective agonist N-methyl-D-aspartate (NMDA; 100 microM to 1 mM in magnesium-free solution with 10 microM glycine) had no effect on intracellular calcium. Radioimmunoassays demonstrated that glutamate stimulated PRL release. CNQX but not the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid blocked this release. Antibodies for mammalian AMPA- and NMDA-type GluR produced a similar punctate immunoreactivity in the periphery of PRL cells. However, the NMDA antibody recognized a protein of a different molecular mass in PRL cells compared with brain cells. These results clearly indicate the presence of GluRs on tilapia PRL cells that can stimulate PRL release.     

2-amino-7-phosphonoheptanoic acid (AP7) produces discriminative stimuli and anticonflict effects similar to diazepam

The N-methyl-D-aspartate (NMDA) receptor antagonist, AP7, was evaluated in two animal test procedures known to be sensitive to the effects of diazepam. In rats trained to discriminate diazepam from vehicle, AP7 produced dose-dependent generalization to the diazepam interoceptive stimuli. This NMDA antagonist also increased the rates of conflict responding in a chronic test procedure used to identify compounds with potential anxiolytic effects. A comparison of AP7 with diazepam and two muscle relaxants (methocarbamol and baclofen) showed that excitatory amino acid antagonists (of the receptor site stimulated by NMDA) produce a muscle relaxant effect (drug discrimination) and may represent a new class of compounds for the treatment of anxiety-related disorders (conflict test).     

Estrogen modulates the action of nitric oxide in the medial preoptic area on luteinizing hormone and prolactin secretion

Several substances work as neuromediators of the estrogen direct and indirect (through glial cells or interneurons) action on luteinizing hormone- releasing hormone (LH-RH) neurons in medial basal hypothalamus and medial preoptic area (MPOA).Angiotensin II (AII) in the MPOA stimulates the LH and it inhibits PRL secretion in some situations. On the other hand, the effect of excitatory amino acids on LH and PRL surges during proestrus as well LH surge induced by steroids depend on nitric oxide (NO). In the present study we investigated the participation of MPOA endogenous NO on gonadotropin and PRL secretion mediated by estrogen and AII. Plasma LH, FSH and PRL was determinated in estrogen primed and unprimed ovariectomized Wistar rats that received microinjection of AII or saline into the MPOA, associated or not with a previous microinjection of an inhibitor for NOS. Our results show the following: 1 - there was no change in plasma FSH in estrogen- primed or unprimed ovarictomized related with microinjections of AII or NO antagonist in the MPOA; 2- the increase in LH secretion after ovariectomy depends on, at least in part, NO activity in the MPOA; 3- estrogen may have an indirect negative feedback action on LH-RH neurons in the MPOA through NO; 4- the stimulatory action of AII in the MPOA on LH secretion in ovariectomized rats treated with estrogen depends on NO; 5 - NO in the MPOA stimulates or inhibits PRL secretion depending on the absence or presence of estrogen, respectively; 6- the inhibitory action of AII into the MPOA on PRL secretion does not seem to depend on NO.     

Acute N-methyl-D,L-aspartate administration stimulates the luteinizing hormone releasing hormone pulse generator in the ovine fetus

To assess whether fetal luteinizing hormone releasing hormone (LH-RH) neurosecretory neurons have the capacity to respond to an exogenous stimulus, a synthetic excitatory amino acid analogue, N-methyl-D-L-aspartate (NMDA; 15 mg/kg), was given rapidly intravenously to 8 chronically catheterized fetuses (130-142 days of gestation; term 147 +/- 3 days). All 8 fetuses exhibited a rise in plasma ovine luteinizing hormone (oLH) and ovine follicle-stimulating hormone (oFSH) within 5 min. The mean maximal increments of oLH (2.25 +/- 0.36 ng/ml) and oFSH (1.21 +/- 0.32 ng/ml) were significantly greater than in 6 normal saline-injected controls (oLH p < 0.0002; oFSH p < 0.03). The secretion of ovine prolactin (oPRL) and ovine growth hormone (oGH) was unaffected. LH-RH (5 microg) evoked a greater oLH response (p < 0.0009) and a greater oFSH response (p < 0.03) than NMDA (n = 6). Desensitization of the fetal gonadotrope by a potent LH-RH agonist, D-Trp6Pro9NEt-LH-RH (10 microg/day i.v. x 4 days), abolished the fetal oLH and the oFSH response to NMDA (n = 5). Moreover, D, L-2-amino-5-phosphonovalerate, a specific competitive antagonist for the NMDA receptor, completely inhibited the fetal oLH and oFSH response to NMDA, whereas D-L-2-amino-5-phosphonovalerate alone did not affect the plasma oLH or oFSH levels, the gonadotropin response to LH-RH, or the release of oGH or oPRL (n = 3). In primary ovine fetal pituitary cell cultures, NMDA (10(-10) to 10(-6) M) had no effect on oLH, oFSH, oGH, or oPRL secretion, whereas LH-RH stimulated oLH (10(-8) M; p < 0.0004) and oFSH (10(-8) M; p < 0. 0001) release, evidence that NMDA did not have a direct pituitary effect. The results suggest that NMDA induces oLH and oFSH secretion by stimulation of the fetal LH-RH pulse generator and is mediated by central NMDA receptors. Fetal LH and FSH secretion and the response to LH-RH decrease in late gestation in the ovine and human fetus. The relative importance of sex steroid dependent and sex steroid independent central nervous system inhibition in this developmental change is unclear. It appears that central neural inhibition in addition to sex steroid negative feedback contributes to the decrease in fetal gonadotropin concentrations in late gestation. NMDA did not affect fetal oGH or oPRL secretion.     

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

N-methyl-d,l-aspartate (NMA), a potent agonist of the neuroexcitatory amino acids aspartate and glutamate, stimulates release of luteinizing hormone (LH) in rats and nonhuman primates. The objective of the experiments described here was to determine the effect of NMA on LH secretion in ovariectomized ewes, in both the absence and presence of estradiol. In Experiment 1, blood samples were collected from 16 ewes every 12 min for 4 h. At Hour 2, ewes received i.v. injections of either 0, 6, 12, or 24 mg NMA/kg body weight dissolved in 0.9% saline (n = 4 per treatment). Mean LH concentrations were unaltered by any dose of NMA (p greater than 0.3). Immediately after completion of Experiment 1, each ewe received an s.c. Silastic implant designed to maintain circulating concentrations of estradiol of approximately 1 pg/ml. Three weeks later, Experiment 2 was conducted, using the same blood sampling regimen and doses of NMA as Experiment 1. The estradiol implants decreased serum LH concentrations in all animals. Treatment with saline failed to alter mean LH concentrations (p greater than 0.3). In contrast, 6, 12, and 24 mg NMA/kg body weight increased mean LH concentrations by 326% (p less than 0.03), 1125% (p less than 0.02), and 441% (p less than 0.0001), respectively. These results demonstrate that exogenous estradiol suppresses LH release in sheep in a manner antagonized by NMA.