Exposure to PM2.5, seminal plasma metabolome, and semen quality among Chinese adult men: Association and potential mediation analyses

Exposure to ambient fine particulate matter (PM2.5) has been linked to a decline in semen quality, but the underlying mechanisms for this association remain unclear. We aimed to examine whether specific metabolites act as mediators in the association between PM2.5 exposure and changes in semen quality. We conducted untargeted metabolomics analysis using LC-MS/MS platforms to identified seminal plasma metabolites associated with various semen quality parameters among 200 Chinese adult men. Additionally, we performed mediation analyses to examine the effects of the seminal plasma metabolites on the association between PM2.5 exposure and semen quality. We identified 140 differential metabolites between the normal and abnormal semen groups, involving two metabolic pathways: Alanine, aspartate and glutamate metabolism, and Aminoacyl-tRNA biosynthesis. We additionally identified 7 specific seminal plasma metabolites that were associated with discrepant metabolic networks related to semen quality. The mediation analysis revealed that D-Aspartate might play a mediating role in the adverse effects of ambient PM2.5 exposure on both total and progressive motility during spermatogenesis period (70-90 days before ejaculation), with a proportion of mediation up to 16% and 17%, respectively. Exposure to PM2.5 was associated with alterations in D-Aspartate levels, which might partially mediate the association between PM2.5 and reduced sperm motility.

Beneficial effects of Lactobacillus rhamnosus hsryfm 1301 fermented milk on rats with nonalcoholic fatty liver disease

A growing stream of research suggests that probiotic fermented milk has a good effect on nonalcoholic fatty liver disease. This work aimed to study the beneficial effects of Lactobacillus rhamnosus hsryfm 1301 fermented milk (fermented milk) on rats with nonalcoholic fatty liver disease induced by a high-fat diet. The results showed that the body weight and the serum levels of total cholesterol, total glyceride, low-density lipoprotein, alanine transaminase, aspartate aminotransferase, free fatty acid, and reactive oxygen species were significantly increased in rats fed a high-fat diet (M) for 8 wk, whereas high-density lipoprotein cholesterol and superoxide dismutase were significantly decreased. However, the body weight and the serum levels of total cholesterol, total glyceride, alanine transaminase, aspartate aminotransferase, free fatty acid, reactive oxygen species, interleukin-8, tumor necrosis factor-α, and interleukin-6 were significantly decreased with fermented milk (T) for 8 wk, and the number of fat vacuoles in hepatocytes was lower than that in the M group. There were significant differences in 19 metabolites in serum between the M group and the C group (administration of nonfermented milk) and in 17 metabolites between the T group and the M group. The contents of 7 different metabolites, glycine, glycerophosphocholine, 1,2-dioleoyl-sn-glycero-3-phosphocholine, thioetheramide-PC, d-aspartic acid, oleic acid, and l-glutamate, were significantly increased in the M group rat serum, and l-palmitoyl carnitine, N6-methyl-l-lysine, thymine, and 2-oxadipic acid were significantly decreased. In the T group rat serum, the contents of 8 different metabolites-1-O-(cis-9-octadecenyl)-2-O-acetyl-sn-glycero-3-phosphocholine, acetylcarnitine, glycine, glycerophosphocholine, 1,2-dioleoyl-sn-glycero-3-phosphocholine, d-aspartic acid, oleic acid, and l-glutamate were significantly decreased, whereas creatinine and thymine were significantly increased. Kyoto Encyclopedia of Genes and Genomes pathway analysis showed that 50 metabolic pathways were enriched in the M/C group and T/M group rat serum, of which 12 metabolic pathways were significantly different, mainly distributed in lipid metabolism, amino acid, and endocrine system metabolic pathways. Fermented milk ameliorated inflammation, oxygenation, and hepatocyte injury by regulating lipid metabolism, amino acid metabolic pathways, and related metabolites in the serum of rats with nonalcoholic fatty liver disease.

D-Aspartic Acid in Vertebrate Reproduction: Animal Models and Experimental Designs‡

This article reviews the animal models and experimental designs that have been used during the past twenty years to demonstrate the prominent role played by d-aspartate (d-Asp) in the reproduction of vertebrates, from amphibians to humans. We have tabulated the findings of in vivo and in vitro experiments that demonstrate the effects of d-Asp uptake on hormone production and gametogenesis in vertebrate animal models. The contribution of each animal model to the existing knowledge on the role of d-Asp in reproductive processes has been discussed. A critical analysis of experimental designs has also been carried out. Experiments performed on wild animal species suggest a role of d-Asp in the mechanisms that regulate the reproductive cycle. Several in vivo and in vitro studies carried out on mouse and rat models have facilitated an understanding of the molecular pathways activated by D-Asp in both steroidogenesis and spermatogenesis, with particular emphasis on testosterone biosynthesis. Some attempts using d-Asp for the improvement of reproductive activity in animals of commercial interest have yielded mixed results. The increased transcriptome activity of enzymes and receptors involved in the reproductive activity in d-Asp-treated broiler roosters revealed further details on the mechanism of action of d-Asp on the reproductive processes. The close relationship between d-Asp and reproductive activity has emerged, particularly in relation to its effects exerted on semen quality, proposing therapeutic applications of this amino acid in andrology and in medically-assisted procreation techniques.

Vibrio fischeri DarR Directs Responses to d-Aspartate and Represents a Group of Similar LysR-Type Transcriptional Regulators

Mounting evidence suggests that d-amino acids play previously underappreciated roles in diverse organisms. In bacteria, even d-amino acids that are absent from canonical peptidoglycan (PG) may act as growth substrates, as signals, or in other functions. Given these proposed roles and the ubiquity of d-amino acids, the paucity of known d-amino-acid-responsive transcriptional control mechanisms in bacteria suggests that such regulation awaits discovery. We found that DarR, a LysR-type transcriptional regulator (LTTR), activates transcription in response to d-Asp. The d-Glu auxotrophy of a Vibrio fischerimurI::Tn mutant was suppressed, with the wild-type PG structure maintained, by a point mutation in darR This darR mutation resulted in the overexpression of an adjacent operon encoding a putative aspartate racemase, RacD, which compensated for the loss of the glutamate racemase encoded by murI Using transcriptional reporters, we found that wild-type DarR activated racD transcription in response to exogenous d-Asp but not upon the addition of l-Asp, l-Glu, or d-Glu. A DNA sequence typical of LTTR-binding sites was identified between darR and the divergently oriented racD operon, and scrambling this sequence eliminated activation of the reporter in response to d-Asp. In several proteobacteria, genes encoding LTTRs similar to DarR are linked to genes with predicted roles in d- and/or l-Asp metabolism. To test the functional similarities in another bacterium, darR and racD mutants were also generated in Acinetobacter baylyi In V. fischeri and A. baylyi, growth on d-Asp required the presence of both darR and racD Our results suggest that multiple bacteria have the ability to sense and respond to d-Asp.IMPORTANCE d-Amino acids are prevalent in the environment and are generated by organisms from all domains of life. Although some biological roles for d-amino acids are understood, in other cases, their functions remain uncertain. Given the ubiquity of d-amino acids, it seems likely that bacteria will initiate transcriptional responses to them. Elucidating d-amino acid-responsive regulators along with the genes they control will help uncover bacterial uses of d-amino acids. Here, we report the discovery of DarR, a novel LTTR in V. fischeri that mediates a transcriptional response to environmental d-Asp and underpins the catabolism of d-Asp. DarR represents the founding member of a group of bacterial homologs that we hypothesize control aspects of aspartate metabolism in response to d-Asp and/or to d-Asp-containing peptides.

In vitro effects of zinc, D-aspartic acid, and coenzyme-Q10 on sperm function

Reactive oxygen species favor reproductive processes at low concentrations, but damage spermatozoa and decrease their fertilizing capacity at high concentrations. During infection and/or inflammation of the accessory sex glands reactive oxygen species overproduction may occur which, in turn, may negatively impact on sperm motility, sperm DNA fragmentation, and lipid peroxidation. A number of nutraceutical formulations containing antioxidant molecules have been developed to counteract the deleterious effects of the oxidative stress. A recent formulation containing zinc, D-aspartic acid, and coenzyme-Q10 is present in the pharmaceutical market. Based on these premises, the aim of the present study was to evaluate the effects of this combination on spermatozoa in vitro. The study was conducted on 24 men (32.2 ± 5.5 years): 12 normozoospermic men and 12 asthenozoospermic patients. Spermatozoa from each sample were divided into two control aliquots (aliquot A and B) and an aliquot incubated with zinc, D-aspartic acid, and coenzyme-Q10 (aliquot C). After 3 h of incubation, the following parameters were evaluated: progressive motility, number of spermatozoa with progressive motility recovered after swim-up, lipid peroxidation, and DNA fragmentation. Incubation with zinc, D-aspartic acid, and coenzyme-Q10 maintained sperm motility in normozoospermic men (37.7 ± 1.2 % vs. 35.8 ± 2.3 % at time zero) and improved it significantly in asthenozoospermic patients (26.5 ± 1.9 % vs. 18.8 ± 2.0 % at time zero) (p < 0.01). This resulted in a significantly higher (p < 0.01) number of spermatozoa with progressive motility recovered after swim-up in both normozospermic men (4.1 ± 0.9 vs. 3.3 ± 1.0 millions) and asthenozooseprmic patients (3.2 ± 0.8 vs. 1.6 ± 0.5 millions). Finally, a statistically significant lower sperm lipid peroxidation was found after incubation with zinc, D-aspartic acid, and coenzyme-Q10 (p < 0.05) in both normozospermic men (1.0 ± 0.4 % vs. 2.4 ± 0.9 %) and asthenozooseprmic patients (0.2 ± 0.1 % vs. 0.6 ± 0.2 %). No statistically significant effect was observed on sperm DNA fragmentation. This nutraceutical formulation may be indicated in vitro during the separation of the spermatozoa in the assisted reproduction techniques, during which the spermatozoa undergo an increased oxidative stress.

D-Aspartic acid stimulates steroidogenesis through the delay of LH receptor internalization in a mammalian Leydig cell line

PURPOSE

Recent experimental evidence on non-mammalian animal models showed that D-Aspartic acid (d-Asp) administration increases testosterone levels through upregulation of StAR in Leydig cells. In this study, we aimed to investigate in vitro the signaling pathway associated with d-Asp stimulation in MA-10 murine Leydig cells.

METHODS

MA-10 cells were stimulated with different concentrations of d-Asp, in presence or absence of hCG. Then total testosterone (T) levels in the culture medium were evaluated by electrochemiluminescence immunoassay, and StAR and LHR protein expressions were quantified by the means of Western blotting. LHR cellular localization after hormonal stimulation was assessed by immunofluorescence.

RESULTS

Stimulation with the sole d-Asp did not induce any relevant increase of T release from cultured cells. On the other hand, stimulation with hCG induced significant increase of T (P = 0.045). Concomitant stimulation with hCG and d-Asp, at the concentration of 0.1 and 1 nM, induced additional and significant increase of released T (P = 0.03 and P = 0.04, respectively). StAR protein levels increased after concomitant stimulation with hCG and d-Asp 0.1 nM, compared with stimulation with the sole hCG (P = 0.02), whereas no variation in LHR protein expression was observed. Finally, d-Asp attenuated displacement of LHR staining, from cell membrane to cytoplasm, subsequent to hCG stimulation.

CONCLUSIONS

In this study, we confirmed a steroidogenic role for d-Asp, in concert with hCG, on murine Leydig cells, which is mediated by an increase in StAR protein levels. In addition, we showed that the possible mechanism subtending the effect of d-Asp could rely on the modulation of LHR exposure on the cell membrane.

Peptidyl prolyl isomerase Pin1-inhibitory activity of D-glutamic and D-aspartic acid derivatives bearing a cyclic aliphatic amine moiety

Pin1 is a peptidyl prolyl isomerase that specifically catalyzes cis-trans isomerization of phosphorylated Thr/Ser-Pro peptide bonds in substrate proteins and peptides. Pin1 is involved in many important cellular processes, including cancer progression, so it is a potential target of cancer therapy. We designed and synthesized a novel series of Pin1 inhibitors based on a glutamic acid or aspartic acid scaffold bearing an aromatic moiety to provide a hydrophobic surface and a cyclic aliphatic amine moiety with affinity for the proline-binding site of Pin1. Glutamic acid derivatives bearing cycloalkylamino and phenylthiazole groups showed potent Pin1-inhibitory activity comparable with that of known inhibitor VER-1. The results indicate that steric interaction of the cyclic alkyl amine moiety with binding site residues plays a key role in enhancing Pin1-inhibitory activity.

Steroidogenic gene expression following D-aspartate treatment in frog testis

Previous studies have provided evidence that D-Asp plays a role in steroid-mediated reproductive biology in amphibians, reptiles, birds and mammals. To examine the molecular involvement of D-Asp on steroidogenic pathway regulation, we analysed the expression of StAR, P450 aromatase and 5αRed2 mRNAs in Pelophylax esculentus testis, either in relation to the reproductive cycle or D-Asp treatment. Basal StAR mRNA levels, as well as D-Asp and testosterone concentrations, were higher in reproductive than in post-reproductive frogs. D-Asp treatment increased StAR mRNA expression and immunolocalisation in both the reproductive and post-reproductive periods. In control testis, aromatase mRNA levels were higher in the post-reproductive period, but following D-Asp administration, they increased only in the reproductive period. The level of 5αRed2 mRNA was higher in reproductive frogs than in post-reproductive frogs, and it increased after D-Asp treatment only in the post-reproductive phase. Our results suggest that, in P. esculentus testis, D-Asp increases StAR mRNA in both periods, and P450 aromatase and 5αRed2 mRNAs at different points during the reproductive cycle.

Unique ionotropic receptors for D-aspartate are a target for serotonin-induced synaptic plasticity in Aplysia californica

The non-L-glutamate (L-Glu) receptor component of D-aspartate (D-Asp) currents in Aplysia californica buccal S cluster (BSC) neurons was studied with whole cell voltage clamp to differentiate it from receptors activated by other well-known agonists of the Aplysia nervous system and investigate modulatory mechanisms of D-Asp currents associated with synaptic plasticity. Acetylcholine (ACh) and serotonin (5-HT) activated whole cell excitatory currents with similar current voltage relationships to D-Asp. These currents, however, were pharmacologically distinct from D-Asp. ACh currents were blocked by hexamethonium (C6) and tubocurarine (D-TC), while D-Asp currents were unaffected. 5-HT currents were blocked by granisetron and methysergide (MES), while D-Asp currents were unaffected. Conversely, while (2S,3R)-1-(Phenanthren-2-carbonyl)piperazine-2,3-dicarboxylic acid(PPDA) blocked D-Asp currents, it had no effect on ACh or 5-HT currents. Comparison of the charge area described by currents induced by ACh or 5-HT separately from, or with, D-Asp suggests activation of distinct receptors by all 3 agonists. Charge area comparisons with L-Glu, however, suggested some overlap between L-Glu and D-Asp receptors. Ten minute exposure to 5-HT induced facilitation of D-Asp-evoked responses in BSC neurons. This effect was mimicked by phorbol ester, suggesting that protein kinase C (PKC) was involved.

D-Asp: a new player in reproductive endocrinology of the amphibian Rana esculenta

We investigated the involvement of D-Aspartic acid (D-Asp) on ovarian and testicular morphology of the green frog, Rana esculenta, and its effect on the testosterone production. The study has been performed throughout the reproductive cycle. In both ovary and testis a substantial amount of D-Asp is endogenously present and its concentration varies as function of reproduction. In the frog, D-Asp content is differently correlated with gonadal and plasmatic levels of testosterone, depending on the sex. In fact, the amount of the D-Asp is inversely linked with that of the testosterone in the ovary, while this correlation directly matched in the testis. In vivo short-term experiments, consisting of a single intra-peritoneal injection of D-Asp (2.0 μmol/g body weight), demonstrated that the enantiomer is significantly accumulated by both the ovary and testis, reaching after 3 h the highest uptake and thereafter decreasing to baseline values within 24 h. Furthermore, D-Asp influences the synthesis and/or the release of testosterone, causing a decrease of its level in the female, and an increase in the male, respectively. In vivo long-term experiments, D-Asp, chronically administered to the frogs of both sexes, enhances the maturation of both gonads, determining in the oocytes an higher accumulation of carbohydrate yolk plates in the ooplasm, and stimulating the spermatogenesis in the testis. Taken altogether, our results show that D-Asp operates differently in female and male frog gonads, indicating that it has different targets in the reproductive machinery depending on the sex.

Protective effects of the pyrolyzates derived from bamboo against neuronal damage and hematoaggregation

ETHNOPHARMACOLOGICAL RELEVANCE

Bamboo species are thought to be originally from Central China, but are now found in many temperate and semi-tropical regions around the world. Although the extracts from bamboo may have antioxidant activities and anti-inflammatory effects, their exact biological activities have not been elucidated.

AIM OF THE STUDY

Two biological activities of bamboo-derived pyrolyzates were investigated; the protective effects against N-methyl-d-aspartate (NMDA)-induced cell death in primary cultured cortical neuron and the anti-plasmin effects determined by using fibrin and fibrinogen degradation products (FDPs) assay.

RESULTS

Treatment of neuronal cells with pyrolyzates of Phyllostachys pubescens, Phyllostachys nigra and Phyllostachys bambusoides resulted in restored cell viability when compared to untreated cells in an NMDA-induced neuronal cell death assay. In addition, cortical neurons treated with Phyllostachys pubescens and Phyllostachys nigra showed a reduction of apoptosis following exposure to NMDA, as determined by Hoechst 33342 staining. In addition, Phyllostachys nigra pyrolyzates also exhibited anti-plasmin action in a FDP assay. It is of interest to note that pyrolyzates exhibited activities of NMDA-receptor antagonist and antifebrin (ogen), since a combination of NMDA receptor antagonists, glucocorticosteroids, GABAergic drugs and heparin are useful for treatment in delayed postischemic injury.

CONCLUSION

Our results indicate that the pyrolyzates derived from bamboo may have anti-apoptotic effects, and can be useful as a supplement for ischemic injury treatment.

The role and molecular mechanism of D-aspartic acid in the release and synthesis of LH and testosterone in humans and rats

BACKGROUND

D-aspartic acid is an amino acid present in neuroendocrine tissues of invertebrates and vertebrates, including rats and humans. Here we investigated the effect of this amino acid on the release of LH and testosterone in the serum of humans and rats. Furthermore, we investigated the role of D-aspartate in the synthesis of LH and testosterone in the pituitary and testes of rats, and the molecular mechanisms by which this amino acid triggers its action.

METHODS

For humans: A group of 23 men were given a daily dose of D-aspartate (DADAVIT) for 12 days, whereas another group of 20 men were given a placebo. For rats: A group of 10 rats drank a solution of either 20 mM D-aspartate or a placebo for 12 days. Then LH and testosterone accumulation was determined in the serum and D-aspartate accumulation in tissues. The effects of D-aspartate on the synthesis of LH and testosterone were gauged on isolated rat pituitary and Leydig cells. Tissues were incubated with D-aspartate, and then the concentration (synthesis) of LH and cGMP in the pituitary and of testosterone and cAMP in the Leydig cells was determined.

RESULTS

In humans and rats, sodium D-aspartate induces an enhancement of LH and testosterone release. In the rat pituitary, sodium D-aspartate increases the release and synthesis of LH through the involvement of cGMP as a second messenger, whereas in rat testis Leydig cells, it increases the synthesis and release of testosterone and cAMP is implicated as second messenger. In the pituitary and in testes D-Asp is synthesized by a D-aspartate racemase which convert L-Asp into D-Asp. The pituitary and testes possesses a high capacity to trapping circulating D-Asp from hexogen or endogen sources.

CONCLUSION

D-aspartic acid is a physiological amino acid occurring principally in the pituitary gland and testes and has a role in the regulation of the release and synthesis of LH and testosterone in humans and rats.

Opposing effects of D-aspartic acid and nitric oxide on tuning of testosterone production in mallard testis during the reproductive cycle

D-Aspartic acid (D-Asp) and nitric oxide (NO) play an important role in tuning testosterone production in the gonads of male vertebrates. In particular, D-Asp promotes either the synthesis or the release of testosterone, whereas NO inhibits it. In this study, we have investigated for the first time in birds the putative effects of D-Asp and NO on testicular testosterone production in relation to two phases of the reproductive cycle of the adult captive wild-strain mallard (Anas platyrhynchos) drake. It is a typical seasonal breeder and its cycle consists of a short reproductive period (RP) in the spring (April-May) and a non reproductive period (NRP) in the summer (July), a time when the gonads are quiescent. The presence and the localization of D-Asp and NO in the testis and the trends of D-Asp, NO and testosterone levels were assessed during the main phases of the bird's reproductive cycle. Furthermore, in vitro experiments revealed the direct effect of exogenously administered D-Asp and NO on testosterone steroidogenesis.

METHODS

By using immunohistochemical (IHC) techniques, we studied the presence and the distributional pattern of D-Asp and NO in the testes of RP and NRP drakes. D-Asp levels were evaluated by an enzymatic method, whereas NO content, via nitrite, was assessed using biochemical measurements. Finally, immunoenzymatic techniques determined testicular testosterone levels.

RESULTS

IHC analyses revealed the presence of D-Asp and NO in Leydig cells. The distributional pattern of both molecules was in some way correlated to the steroidogenic pathway, which is involved in autocrine testosterone production. Indeed, whereas NO was present only during the NRP, D-Asp was almost exclusively present during the RP. Consistently, the high testosterone testicular content occurring during RP was coupled to a high D-Asp level and a low NO content in the gonad. By contrast, in sexually inactive drakes (NRP), the low testosterone content in the gonad was coupled to a low D-Asp content and to a relatively high NO level. Consequently, to determine the exogenous effects of the two amino acids on testosterone synthesis, we carried out in vitro experiments using testis sections deriving from both the RP and NRP. When testis slices were incubated for 60 or 120 min with D-Asp, testosterone was enhanced, whereas in the presence of L-Arg, a precursor of NO, it was inhibited.

CONCLUSION

Our results provide new insights into the involvement of D-Asp and NO in testicular testosterone production in the adult captive wild-strain mallard drake. The localization of these two molecules in the Leydig cells in different periods of the reproductive cycle demonstrates that they play a potential role in regulating local testosterone production.

d-Aspartic acid: An endogenous amino acid with an important neuroendocrine role

D-Aspartic acid (d-Asp), an endogenous amino acid present in vertebrates and invertebrates, plays an important role in the neuroendocrine system, as well as in the development of the nervous system. During the embryonic stage of birds and the early postnatal life of mammals, a transient high concentration of d-Asp takes place in the brain and in the retina. d-Asp also acts as a neurotransmitter/neuromodulator. Indeed, this amino acid has been detected in synaptosomes and in synaptic vesicles, where it is released after chemical (K(+) ion, ionomycin) or electric stimuli. Furthermore, d-Asp increases cAMP in neuronal cells and is transported from the synaptic clefts to presynaptic nerve cells through a specific transporter. In the endocrine system, instead, d-Asp is involved in the regulation of hormone synthesis and release. For example, in the rat hypothalamus, it enhances gonadotropin-releasing hormone (GnRH) release and induces oxytocin and vasopressin mRNA synthesis. In the pituitary gland, it stimulates the secretion of the following hormones: prolactin (PRL), luteinizing hormone (LH), and growth hormone (GH) In the testes, it is present in Leydig cells and is involved in testosterone and progesterone release. Thus, a hypothalamus-pituitary-gonads pathway, in which d-Asp is involved, has been formulated. In conclusion, the present work is a summary of previous and current research done on the role of d-Asp in the nervous and endocrine systems of invertebrates and vertebrates, including mammals.

d-Aspartic acid and nitric oxide as regulators of androgen production in boar testis

D-Aspartic acid (D-Asp) and nitric oxide (NO) are two biologically active molecules playing important functions as neurotransmitters and neuromodulators of nerve impulse and as regulators of hormone production by endocrine organs. We studied the occurrence of D-Asp and NO as well as their effects on testosterone synthesis in the testis of boar. This model was chosen for our investigations because it contains more Leydig cells than other mammals. Indirect immunofluorescence applied to cryostat sections was used to evaluate the co-localization of D-Asp and of the enzyme nitric oxide synthase (NOS) in the same Leydig cells. D-Asp and NOS often co-existed in the same Leydig cells and were found, separately, in many other testicular cytotypes. D-Asp level was dosed by an enzymatic method performed on boar testis extracts and was 40+/-3.6 nmol/g of fresh tissue. NO measurement was carried out using a biochemical method by NOS activity determination and expressed as quantity of nitrites produced: it was 155.25+/-21.9 nmol/mg of tissue. The effects of the two molecules on steroid hormone production were evaluated by incubating testis homogenates, respectively with or without D-Asp and/or the NO-donor L-arginine (L-Arg). After incubation, the testosterone presence was measured by immunoenzymatic assay (EIA). These in vitro experiments showed that the addition of D-Asp to incubated testicular homogenates significantly increased testosterone concentration, whereas the addition of L-Arg decreased the hormone production. Moreover, the inclusion of L-Arg to an incubation medium of testicular homogenates with added D-Asp, completely inhibited the stimulating effects of this enantiomer. Our results suggest an autocrine action of both D-Asp and NO on the steroidogenetic activity of the Leydig cell.

Effects of D-amino acids on lipoperoxidation in rat liver and kidney mitochondria

The effects of the amino acids D-ser, D-asp, and D-ala on lipoperoxidation under conditions of hypertension, alcoholism, and ammonemia in rat liver and kidney mitochondria were studied. Under normal conditions, D-alanine increased in 54% free radicals production in liver mitochondria (p < 0.05). The D-amino acids had no effect on kidney mitochondria. D-ser and D-ala increased lipoperoxidation in spontaneously hypertensive rats (SHR) as compared with their normotensive genetic control Wistar-Kyoto (WKY) rats (p < 0.05). During hypertension and in oxidative stress in the presence of calcium, only D-ala produced 46% and 29% free radicals in liver and kidney mitochondria (p < 0.05), respectively. During chronic alcoholism, D-ser increased lipoperoxidation in 80% in kidney mitochondria (p < 0.05), as compared to control. During ammonemia, D-ser produced 41% free radicals.

A physiological mechanism to regulate d-aspartic acid and NMDA levels in mammals revealed by d-aspartate oxidase deficient mice

Free D-aspartic acid and NMDA are present in the mammalian central nervous system and endocrine glands at significant concentrations, but their physiological role is still matter of debate. The only enzyme known to metabolize in vitro selectively these D-amino acids is D-aspartate oxidase (DDO). To clarify the role in vivo of the enzyme, we generated mice with targeted deletion of Ddo gene by homologous recombination. Mutated animals showed increased amounts of both D-aspartic acid and NMDA in all tissues examined demonstrating a physiological role of DDO in the regulation of their endogenous levels.

Modulation of an AMPA-like glutamate receptor (SqGluR) gating by L- and D-aspartic acids

L- and D-aspartic acids (L-Asp and D-Asp) are present in the majority of nervous systems. In phylogeny, significant levels have been reported in mollusc brains, particularly cephalopods. To examine the role of L- and D-Asp on a cephalopod receptor, we studied ligand gating of a squid glutamate receptor (SqGluR) expressed in HEK 239 (human embryonic kidney) cells. Under voltage clamp, application of L-glutamate (L-Glu; 1-30 mM), but not D-glutamate (D-Glu), or L- or D-Asp, evoked an inward current of 0.1 nA. L- or D-Asp (200 microM) applied with 20 mM L-Glu, slowed the time course of activation and inactivation of the L-Glu gated current (time constant increased from 1 s (L-Glu alone) to 3 s (D-Asp and L-Glu) and to 19 s (L-Asp and L-Glu)). Our results suggest that in molluscan systems, aspartic acid could act as a neuromodulator during glutamatergic transmission and could significantly alter synaptic integration by slowing glutamate receptor gating.

D-aspartate regulates melanocortin formation and function: behavioral alterations in D-aspartate oxidase-deficient mice

D-aspartate, an abundant D-amino acid enriched in neuroendocrine tissues, can be degraded by D-aspartate oxidase (Ddo). To elucidate the function of D-aspartate, we generated mice with targeted deletion of Ddo (Ddo(-/-)) and observe massive but selective augmentations of D-aspartate in various tissues. The pituitary intermediate lobe, normally devoid of D-aspartate from endogenous Ddo expression, manifests pronounced increases of immunoreactive D-aspartate in Ddo(-/-) mice. Ddo(-/-) mice show markedly diminished synthesis and levels of pituitary proopiomelanocortin/alpha-MSH, associated with decreased melanocortin-dependent behaviors. Therefore, Ddo is the endogenous enzyme that degrades D-aspartate, and Ddo-enriched organs, low in D-aspartate, may represent areas of high turnover where D-aspartate may be physiologically important.

Oxidative metabolism in cultured rat astroglia: effects of reducing the glucose concentration in the culture medium and of D-aspartate or potassium stimulation

The glucose concentration in the culture medium may affect the energy metabolism of cultured cells. The oxidative metabolism of glucose in astrocytes might also be affected because the glucose concentration (25 mmol/L) of many culture media is higher than the physiological levels (approximately 3 mmol/L). In the present study, we assessed the effects of reducing the glucose concentration in the culture medium on the oxidative metabolism of glucose in cultured rat astroglia by measuring the oxidation rates of L-[U-14C]lactate or D-[U-14C]glucose to 14CO2. The effects of D-aspartate and elevated extracellular K+ levels on oxidative and glycolytic metabolism in astroglia were also investigated. The rates of [14C]lactate and [14C]glucose oxidation in astroglia cultured in a medium containing 2 mmol/L of glucose (astroglia2) were approximately twofold of those in astroglia cultured in a medium containing 22 mmol/L of glucose (astroglia22). D-Aspartate (500 micromol/L) significantly increased [14C]lactate oxidation by 156% in astroglia22 and by 83% in astroglia2. D-[U-14C]glucose oxidation in astroglia22 and astroglia2 was also increased by 94% and 76%, respectively. In contrast, an elevated extracellular K+ concentration (7.4 mmol/L) did not affect glucose and lactate oxidation, although it increased 2-deoxy-D-[1-14C]glucose phosphorylation. Astroglia grown in the physiological glucose concentration are more dependent on the oxidative metabolism of glucose than that in high-glucose concentration. Glucose concentration in culture medium has a strong influence on astrocytic oxidative capacity in vitro.

Ncm-D-aspartate: a novel caged D-aspartate suitable for activation of glutamate transporters and N-methyl-D-aspartate (NMDA) receptors in brain tissue

The D-isomer of aspartate is both a substrate for glutamate transporters and an agonist of N-methyl-D-aspartate (NMDA) receptors. To monitor the behavior of these receptors and transporters in intact tissue we synthesized a new photo-labile analogue of D-aspartate, N-[(6-nitrocoumarin-7-yl)methyl]-D-aspartic acid (Ncm-D-aspartate). This compound was photolyzed rapidly (t(1/2)=0.11 micros) by UV light with a quantum efficiency of 0.041 at pH 7.4. In acute hippocampal slices, photolysis of Ncm-D-aspartate by brief (1 ms) exposure to UV light elicited rapidly activating inward currents in astrocytes that were sensitive to inhibition by the glutamate transporter antagonist DL-threo-beta-benzyloxyaspartic acid (TBOA). Neither Ncm-D-aspartate nor the photo-released caging group exhibited agonist or antagonist activity at glutamate transporters, and Ncm-D-aspartate did not induce transporter currents prior to photolysis. Glutamate transporter currents were also elicited in cerebellar Purkinje cells in response to photolysis of Ncm-D-aspartate. Photo-release of D-aspartate from Ncm-D-aspartate did not induce alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor or metabotropic glutamate receptor (mGluR) currents, but triggered robust NMDA receptor currents in neurons; Ncm-D-aspartate and the photolzyed caging group were similarly inert at NMDA receptors. These results indicate that Ncm-D-aspartate can be used to study NMDA receptors at excitatory synapses and interactions between transporters and receptors in brain tissue.

D-aspartate and NMDA, but not L-aspartate, block AMPA receptors in rat hippocampal neurons

1 The amino acid, D-aspartate, exists in the mammalian brain and is an agonist at the N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors. Here, for the first time, we studied the actions of D-aspartate on alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptors (AMPARs) in acutely isolated rat hippocampal neurons. 2 In the presence of the NMDA receptor channel blocker, MK801, D-aspartate inhibited kainate-induced AMPAR current in hippocampal neurons. The inhibitory action of D-aspartate on kainate-induced AMPAR current was concentration-dependent and was voltage-independent in the tested voltage range (-80 to +60 mV). 3 The estimated EC50 of the L-glutamate-induced AMPAR current was increased in the presence of D-aspartate, while the estimated maximum L-glutamate-induced AMPAR current was not changed. D-aspartate concentration-dependently shifted the dose-response curve of kainate to the right. Schild plot analysis indicated that D-aspartate acts competitively to block AMPARs. The K(b) for D-aspartate was estimated to be 0.93 mM. 4 D-aspartate also blocked L-glutamate-induced current in Xenopus laevis oocytes that expressed recombinant homomeric AMPARs. 5 NMDA possessed similar inhibitory action on AMPARs. However, L-aspartate had little inhibitory action on AMPARs. 6 D-Aspartate, but not L-aspartate, was found to reduce the amplitude of miniature excitatory postsynaptic current in cultured hippocampal neurons. 7 Our data are consistent with a model in which D-aspartate directly competes with kainate and L-glutamate in binding to the agonist binding site of AMPARs. The prevalence of D-aspartate in the brain suggests a possible role of D-aspartate in modulating AMPAR-mediated fast excitatory synaptic transmission.

Substrate-induced modulation of glutamate uptake in human platelets

In the central nervous system (CNS), glutamate rapidly upregulates the activities of different excitatory amino-acid transporter subtypes (EAATs) in order to help protect neurons from excitotoxicity. Since human platelets display a specific sodium-dependent glutamate uptake activity, and express the three major glutamate transporters, which may be affected in neurological disorders, we investigated whether platelets are subject to substrate-induced modulation as described for CNS. A time- and dose-dependent upregulation of [3H]-glutamate uptake (up to two-fold) was observed in platelets preincubated with glutamate. There was an increase in maximal velocity rate without affinity changes. Glutamate receptor agonists and antagonists did not modulate this upregulation and preincubation with glutamate analogues failed to mimic the glutamate effect. Only aspartate preincubation increased the uptake, albeit approximately 35% less with respect to glutamate. The effect of glutamate preincubation on the expression of the three major transporters was studied by Western blotting, showing an increase of approximately 70% in EAAT1 immunoreactivity that was completely blocked by cycloheximide (CEM). However, L-serine-O-sulphate, at a concentration (200 microM) known to block EAAT1/3 selectively, did not completely inhibit the effect of glutamate stimulation, indicating the possible involvement of EAAT2. In fact, glutamate stimulation was completely abolished only when, following CEM pre-incubation, the experiment was run in the presence of the selective EAAT2 inhibitor dihydrokainic acid. Since surface biotinylation experiments failed to show evidence of EAAT2 translocation, our results suggest the existence of a different way of regulating EAAT2 activity. These findings indicate that human platelets display a substrate-dependent modulation of glutamate uptake mediated by different molecular mechanisms and confirm that ex vivo platelets are a reliable model to investigate the dysfunction of glutamate uptake regulation in patients affected by neurological disorders.

Cloning and expression in Escherichia coli of the D-aspartate oxidase gene from the yeast Cryptococcus humicola and characterization of the recombinant enzyme

The D-aspartate oxidase (DDO) from the yeast Cryptococcus humicola UJ1 (ChDDO) is highly specific to D-aspartate. The gene encoding ChDDO was cloned and expressed in Escherichia coli. Sequence analysis of the ChDDO gene showed that an open reading frame of 1,110 bp interrupted by two introns encodes a protein of 370 amino acids. The deduced amino acid sequence showed an FAD-binding motif and a peroxisomal targeting signal 1 in the N-terminal region and at the C-terminus, respectively, and also the presence of certain catalytically important amino acid residues corresponding to those catalytically important in D-amino acid oxidase (DAO). The sequence exhibited only a moderate identity to human (27.4%) and bovine (28.0%) DDOs, and a rather higher identity to yeast and fungal DAOs (30.4-33.2%). Similarly, phylogenetic analysis showed that ChDDO is more closely related to yeast and fungal DAOs than to mammalian DDOs. The gene expression was regulated at the transcriptional level and specifically induced by the presence of D-aspartate as the sole nitrogen source. ChDDO was expressed in an active form in E. coli to an approximately 5-fold greater extent than in yeast. The purified recombinant enzyme was identical to the native enzyme in physicochemical and catalytic properties.

Retinal bipolar cell input mechanisms in giant danio. III. ON-OFF bipolar cells and their color-opponent mechanisms

Whole cell patch recording was performed from morphologically identified cone-driven on-off bipolar cells (Cabs) in giant danio retinal slices to study their glutamate receptors and light-evoked responses. Specific agonists were puffed in the presence of cobalt, picrotoxin, and strychnine to identify glutamate receptors on these cells. Most Cabs responded to both the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate receptor agonist kainate and the excitatory amino acid transporter (EAAT) substrate D-aspartate, and both responses were localized to the dendrites. Kainate generated depolarizations whereas D-aspartate had E(rev) close to E(Cl) and generated hyperpolarizations, indicating that the AMPA/kainate receptors are sign-preserving, whereas the EAATs are sign-inverting. In response to white light, some Cabs gave on bipolar cell-like responses whereas others gave off bipolar cell-like ones, but many cells' responses had both on and off bipolar cell components. In response to appropriately colored center-selective stimuli, many Cabs responded to short and long wavelengths with opposite polarities and were thus double color-opponent. The depolarizing components of the responses to white or colored stimuli were suppressed by the EAAT blocker DL-threo-beta-benzyloxyaspartate (TBOA), whereas the hyperpolarizing components were reduced by the AMPA/kainate receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). These results are consistent with the hypothesis that both EAATs and AMPA/kainate receptors are involved in the generation of light-evoked responses in Cabs and that they confer these cells with on and off bipolar cell properties, respectively. Cabs can generate double color-opponent center responses by receiving inputs from certain cones through EAATs and from other cones through AMPA/kainate receptors.

D-aspartate modulates transcriptional activity in Harderian gland of frog,Rana esculenta: Morphological and molecular evidence

In the green frog, Rana esculenta, a substantial amount of D-aspartate (D-Asp) is found endogenously within the Harderian gland (HG) following its synthesis from L-aspartate (L-Asp) by an aspartate racemase. The frog HG is an orbital seromucoid gland that displays seasonal changes in secretory activity. Our in vivo experiments, consisting of i.p. injection of 2.0 mumol/g b.w. D-Asp in frogs collected during two periods of differing glandular activity (high or medium-low secretory activity), revealed that HG can to take up and accumulate D-Asp and that this amino acid may modulate the exocrine secretion through a kinase pathway. At a time when the gland shows relatively low secretory activity, i.p. administration of D-Asp rapidly induced activation of ERK1 and an increase in cells active in RNA synthesis. This increase in transcriptional activity was followed by a significant increase in mucous secretion. By contrast, administration of exogenous D-Asp when HG was showing high activity rapidly induced inhibition of both ERK1 and transcriptional activity. Since D-Asp is known to be recognized by receptors for N-methyl-D-aspartic acid (NMDA), it is possible that in the HG, D-Asp mediated NMDA activation may enhance the kinase pathway. The above activation of opposing stimulatory and inhibitory processes could reflect different levels of NMDA-receptor activity, which could vary as a function of the level of gland activity. This study provides the first evidence of a role for this excitatory amino acid in exocrine secretion. The effects of D-Asp in HG appear to be specific since they were not seen in frogs treated with other D- or L-amino acids with known excitatory effects on neurosecretion.

Retinal Bipolar Cell Input Mechanisms in Giant Danio. II. Patch-Clamp Analysis of on Bipolar Cells

Glutamate receptors on giant danio retinal on bipolar cells were studied with whole cell patch clamping using a slice preparation. Cone-driven on bipolars (Cbs) and mixed-input on bipolars (Mbs) were identified morphologically. Most Cbs responded to the excitatory amino acid transporter (EAAT) substrate d-aspartate but not to the group III metabotropic glutamate receptor (mGluR) agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4) or the AMPA/kainate receptor agonist kainate, suggesting EAATs are the primary glutamate receptors on Cbs. The EAAT inhibitor dl- threo-β-benzyloxyasparate (TBOA) blocked all light-evoked responses of Cbs, suggesting these responses are mediated exclusively by EAATs. Conversely, all Mbs responded to d-aspartate and l-AP4 but not to kainate, indicating they have both EAATs and group III mGluRs (presumably mGluR6). The light responses of Mbs involve both receptors because they could be blocked by TBOA plus (RS)-α-cyclopropyl-4-phosphonophenylglycine (CPPG, a group III mGluR antagonist) but not by either alone. Under dark-adapted conditions, the responses of Mbs to green (rod-selective) stimuli were reduced by CPPG but enhanced by TBOA. In contrast, both antagonists reduced the responses to red (cone-selective) stimuli, although TBOA was more effective. Furthermore, under photopic conditions, TBOA failed to eliminate light-evoked responses of Mbs. Thus on Mbs, rod inputs are mediated predominantly by mGluR6, whereas cone inputs are mediated mainly by EAATs but also by mGluR6 to some extent. Finally, we explored the interactions between EAATs and mGluR6 in Mbs. Responses to d-aspartate were reduced by l-AP4 and vice versa. Therefore mGluR6 and EAATs suppress each other, and this might underlie mutual suppression between rod and cone signals in Mbs.

Effects of L-glutamate/D-aspartate and monensin on lactic acid production in retina and cultured retinal Müller cells

We have investigated the dependence of the rate of lactic acid production on the rate of Na(+) entry in cultured transformed rat Müller cells and in normal and dystrophic (RCS) rat retinas that lack photoreceptors. To modulate the rate of Na(+) entry, two approaches were employed: (i) the addition of L-glutamate (D-aspartate) to stimulate coupled uptake of Na(+) and the amino acid; and (ii) the addition of monensin to enhance Na(+) exchange. Müller cells produced lactate aerobically and anaerobically at high rates. Incubation of the cells for 2-4 h with 0.1-1 mM L-glutamate or D-aspartate did not alter the rate of production of lactate. ATP content in the cells at the end of the incubation period was unchanged by addition of L-glutamate or D-aspartate to the incubation media. Na(+)-dependent L-glutamate uptake was observed in the Müller cells, but the rate of uptake was very low relative to the rate of lactic acid production. Ouabain (1 mM) decreased the rate of lactic acid production by 30-35% in Müller cells, indicating that energy demand is enhanced by the activity of the Na(+)-K(+) pump or depressed by its inhibition. Incubation of Müller cells with 0.01 mM monensin, a Na(+) ionophore, caused a twofold increase in aerobic lactic acid production, but monensin did not alter the rate of anaerobic lactic acid production. Aerobic ATP content in cells incubated with monensin was not different from that found in control cells, but anaerobic ATP content decreased by 40%. These results show that Na(+)-dependent L-glutamate/D-aspartate uptake by cultured retinal Müller cells causes negligible changes in lactic acid production, apparently because the rates of uptake are low relative to the basal rates of lactic acid production. In contrast, the marked stimulation of aerobic lactic acid production caused by monensin opening Na(+) channels shows that glycolysis is an effective source of ATP production for the Na(+)-K(+) ATPase. A previous report suggests that coupled Na(+)-L-glutamate transport stimulates glycolysis in freshly dissociated salamander Müller cells by activation of glutamine synthetase. The Müller cell line used in this study does not express glutamine synthetase; consequently these cells could only be used to examine the linkage between Na(+) entry and the Na(+) pump. As normal and RCS retinas express glutamine synthetase, the role of this enzyme was examined by coapplication of L-glutamate and NH(4) (+) in the presence and absence of methionine sulfoximine, an inhibitor of glutamine synthetase. In normal retinas, neither the addition of L-glutamate alone or together with NH(4) (+) caused a significant change in the glycolytic rate, an effect linked to the low rate of uptake of this amino acid relative to the basal rate of retinal glycolysis. However, incubation of the RCS retinas in media containing L-glutamate and NH(4)(+) did produce a small (15%) increase in the rate of glycolysis above the rate found with L-glutamate alone and controls. It is unlikely that this increase was the result of conversion of L-glutamate to L-glutamine, as it was not suppressed by inhibition of glutamine synthetase with 5 mm methionine sulfoximine. It appears that the magnitude of Müller cell glycolysis required to sustain the coupled transport of Na(+) and L-glutamate and synthesis of L-glutamine is small relative to the basal glycolytic activity in a rat retina.

Binding and transport of [3H](2S,4R)- 4-methylglutamate, a new ligand for glutamate transporters, demonstrate labeling of EAAT1 in cultured murine astrocytes

Transporters for L-glutamate (excitatory amino acid transporters; EAATs), localized to astrocytes, are involved intimately in intermediary metabolism within the brain. Because (2S,4R)-4-methylglutamate (4MG) has affinity for glial EAATs, we employed [(3)H]4MG to define the characteristics of EAATs in cultured murine astrocytes and describe new approaches to analyze EAAT function. Specific binding of [(3)H]4MG in astrocytic membranes at 4 degrees C represented 90% of total binding. Binding was rapid (apparent t(1/2) approximately 7 min) and saturable. Saturation and Scatchard analyses indicated a single binding site (n(H) = 0.8) with a K(d) of 6.0 +/- 1.5 microM and B(max) = 9.7 +/- 2.9 pmol/mg protein. Binding of [(3)H]4MG to astrocytic homogenates was Na(+)-dependent and inhibited by K(+). Compounds acting at EAATs, such as L-glutamate (Glu), D-aspartate (D-Asp), L-(2S,3S,4R)-2-(carboxycyclopropyl)glycine and L-trans-pyrrolidine-2,4-dicarboxylate displaced binding to nonspecific levels. L-Serine-O-sulphate, an EAAT1-preferring ligand, fully displaced binding of [(3)H]4MG. In contrast, inhibitors having preferential affinity for EAAT2, L-threo-3-methylglutamate, dihydrokainate, and kainate, were relatively ineffective binding displacers. Agonists and antagonists for Glu receptors failed to significantly inhibit [(3)H]4MG binding. Studies with [(3)H]D-Asp reinforced evidence that [(3)H]4MG was binding to EAATs. These data were consistent with Western blot analyses, which indicated abundant expression of EAAT1 but not EAAT2. [(3)H]4MG was also accumulated rapidly (apparent t(1/2) approximately 4 min) into whole astrocytes by a sodium- and temperature-sensitive process (K(m) of 146 +/- 24 microM, V(max) = 336 +/- 27 nmol/mg protein/min), which possessed an EAAT1-like pharmacologic profile. These findings confirm that 4MG is a substrate for EAAT1 and that the binding assay developed using [(3)H]4MG can be utilized in various preparations including cultured astrocytes.

Activation of glutamate transporters in rods inhibits presynaptic calcium currents

We found that L-glutamate (L-Glu) inhibits L-type Ca2+currents (ICa) in rod photoreceptors. This inhibition was studied in isolated rods or rods in retinal slices from tiger salamander using perforated patch whole cell recordings and Cl−-imaging techniques. Application of L-Glu inhibitedICaby ∼20% at 0.1 mM and ∼35% at 1 mM. L-Glu also produced an inward current that reversed aroundECl. The metabotropic glutamate receptor (mGluR) agonists t-ADA (Group I), DCG-IV (Group II), and L-AP4 (Group III) had no effect onICa. However, the glutamate transport inhibitor, TBOA (0.1 mM), prevented L-Glu from inhibitingICa. D-aspartate (D-Asp), a glutamate transporter substrate, also inhibitedICawith significantly more inhibition at 1 mM than 0.1 mM. Using Cl−imaging, L-Glu (0.1–1 mM) and D-Asp (0.1–1 mM) were found to stimulate a Cl−efflux from terminals of isolated rods whereas the ionotropic glutamate receptor agonists NMDA, AMPA, and kainate and the mGluR agonist, 1S,3R-ACPD, did not. Glutamate-evoked Cl−effluxes were blocked by the glutamate transport inhibitors TBOA and DHKA. Cl−efflux inhibits Ca2+channel activity in rod terminals (Thoreson et al. (2000),Visual Neuroscience17, 197). Consistent with the possibility that glutamate-evoked Cl−efflux may play a role in the inhibition, reducing intraterminal Cl−prevented L-Glu from inhibitingICa. In summary, the results indicate that activation of glutamate transporters inhibitsICain rods possibly as a consequence of Cl−efflux. The neurotransmitter L-Glu released from rod terminals might thus provide a negative feedback signal to inhibit further L-Glu release.

Metabotropic glutamate receptor-mediated hetero-synaptic interaction of red- and green-cone inputs to LHC of carp retina

The role of presynaptic metabotropic glutamate receptor (mGluR) on the interaction of red- and green-cone signals was investigated in luminosity-type horizontal cell (LHC) of isolated carp retina. It was found that a dim red background could enhance LHC's light response to green stimulus, and a dim green background was also able to increase the cell's response to red flash. Such mutual color enhancement was eliminated by application of groups II and III mGluR antagonist (S)-methyl-4-carboxyphenyl-glycine (MCPG). Furthermore, inhibition of glutamate uptake by using D-aspartate (D-Asp) or DL-threo-beta-hydroxy-aspartic acid (THA) completely blocked the mutual enhancement of color signals in LHC. However, the GABAergic feedback pathway in the outer retina was unnecessarily involved.

The effect of D-aspartate on luteinizing hormone-releasing hormone, alpha-melanocyte-stimulating hormone, GABA and dopamine release

Since D-aspartate stimulates prolactin and LH release, our objective was to determine whether D-aspartate modifies the release of hypothalamic and posterior pituitary factors involved in the control of their secretion and whether its effects on these tissues are exerted through NMDA receptors and mediated by nitric oxide. In the hypothalamus, D-aspartate stimulated luteinizing hormone-releasing hormone (LHRH), alpha-melanocyte-stimulating hormone (alpha-MSH) and GABA release and inhibited dopamine release through interaction with NMDA receptors. It increased nitric oxide synthase (NOS) activity, and its effects on LHRH and hypothalamic GABA release were blunted when NOS was inhibited. In the posterior pituitary gland, D-aspartate inhibited GABA release but had no effect on dopamine or alpha-MSH release. We report that D-aspartate differentially affects the release of hypothalamic and posterior pituitary factors involved in the regulation of pituitary hormone secretion.