Effects of D-aspartate on excitatory amino acid-induced release of [3H]GABA from goldfish retina

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.

Transporter mediated GABA release in the retina: Role of excitatory amino acids and dopamine

In general, the release of neurotransmitters in the central nervous system is accomplished by a calcium-dependent process which constitutes a common feature of exocytosis, a conserved mechanism for transmitter release in all species. However, neurotransmitters can also be released by the reversal of their transporters. In the retina, a large portion of GABA is released by this mechanism, which is under the control of neuroactive agents, such as excitatory amino acids and dopamine. In this review, we will focus on the transporter mediated GABA release and the role played by excitatory amino acids and dopamine in this process. First, we will discuss the works that used radiolabeled GABA to study the outflow of the neurotransmitter and then the works that took into consideration the endogenous pool of GABA and the topography of GABAergic circuits influenced by excitatory amino acids and dopamine.

D-aspartic acid in the nervous system of Aplysia limacina: possible role in neurotransmission

In the marine mollusk Aplysia limacina, a substantial amount of endogenous D-aspartic acid (D-Asp) was found following its synthesis from L-aspartate by an aspartate racemase. Concentrations of D-Asp between 3.9 and 4.6 micromol/g tissue were found in the cerebral, abdominal, buccal, pleural, and pedal ganglia. In non nervous tissues, D-Asp occurred at a very low concentration compared to the nervous system. Immunohistochemical studies conducted on cultured Aplysia neurons using an anti-D-aspartate antibody demonstrated that D-Asp occurs in the soma, dendrites, and in synaptic varicosities. Synaptosomes and synaptic vesicles from cerebral ganglia were prepared and characterized by electron microscopy. HPLC analysis revealed high concentrations of D-Asp together with L-aspartate and L-glutamate in isolated synaptosomes In addition, D-Asp was released from synaptosomes by K+ depolarization or by ionomycin. D-Asp was one of the principal amino acids present in synaptic vesicles representing about the 25% of total amino acids present in these cellular organelles. Injection of D-Asp into live animals or addition to the incubation media of cultured neurons, caused an increase in cAMP content. Taken as a whole, these findings suggest a possible role of D-Asp in neurotransmission in the nervous system of Aplysia limacina.

D-aspartate localization in the rat pituitary gland and retina

Rat pituitary gland and retina were probed with anti-D-aspartate (D-Asp) antibody previously prepared in this laboratory [Lee et al., Biochem. Biophys. Res. Commun., 231 (1997) 505-508]. D-Asp immunoreactivity (IR) was observed only in the posterior lobe of the pituitary gland of 3-day-old rats, whereas the anterior and posterior lobes were also positive in 3-week and 6-week-old rats, respectively. In the anterior lobe, intense IR was scattered throughout the lobe and the D-Asp-positive cells appeared to be prolactin-containing cells or some other very closely related type of cell. In the retina, D-Asp IR was observed only in the ganglion cell and nerve fiber region of 3-day-old rats. In contrast, during the transient increase in D-Asp levels in 7-day-old rats, D-Asp IR was additionally evident in regions where differentiating bipolar cells had begun to make contact with other types of cells. The functional relevance of D-Asp localization in these tissues is discussed.

Occurrence of D-aspartic acid in rat brain pineal gland

We found a high concentration (1030 pmol per pineal gland) of D-aspartic acid (D-Asp) in the pineal gland of 6-week-old Sprague-Dawley (SD) rats. The content of D-Asp decreased with age, being 210 and 33 pmol per pineal gland in 28- and 45-week-old rats respectively. The proportion of D-Asp [(D-Asp/total Asp) x 100] also decreased with age, declining from 66% to 10% between 6 and 45 weeks after birth. The proportion of D-Asp did not differ between the sexes. The concentration of D-Asp was higher at night (at 2.00 a.m. 2830 +/- 485 pmol per pineal gland) than during the day (at 10.00 a.m. 1030 +/- 200 and at 3:00 p.m. 682 +/- 194 pmol per pineal gland), suggesting that biosynthesis of D-Asp in the pineal gland occurs at night. D-Asp was found to be distributed in the cytosol of pinealocytes, but its biological role remains unclear.

Excitatory amino acid-induced toxicity in chick retina: amino acid release, histology, and effects of chloride channel blockers

Acute excitotoxicity in embryonic chick retina and the ability of Cl- channel blockers to prevent toxicity were evaluated by measurement of endogenous amino acid release and histology. Treatment of retina with kainate, quisqualate, or N-methyl-D-aspartate resulted in a large dose-dependent release of gamma-aminobutyric acid and taurine, moderate release of glutamine and alanine, and no measurable release of glutamate or aspartate. Concentrations inducing maximal gamma-aminobutyric acid release were 50 microM quisquaalate, 100 microM kainate, and 100 microM N-methyl-D-aspartate. Treatment with 1 mM glutamate resulted in significant gamma-aminobutyric acid release, as well as an elevation in medium aspartate levels. Typical excitotoxic retinal lesions were produced by the agonists and, at the lower concentrations tested, revealed a regional sensitivity. There was a positive correlation between the amount of gamma-aminobutyric acid release and the extent of tissue swelling, suggesting that release may be secondary to toxic cellular events. Omission of Cl- completely blocked cytotoxic effects due to kainate or glutamate. Likewise, addition of the Cl-/bicarbonate anion channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonate at 600 microM protected retina from cytotoxic damage from all excitotoxic analogs and restored amino acid levels to baseline values. Furosemide, which blocks Na+/K+/2Cl- cotransport, was only minimally effective in reducing amino acid release induced by the agonists. Consistent with the latter, histological examination showed the continued presence of the lesion but with general reduction of cellular edema. These results indicate that although influx of Cl- is a central component of the acute excitotoxic phenomenon, mechanisms other than passive Cl- flux may be involved.

Pharmacological characterization of voltage-clamped catfish rod horizontal cell responses to kainic acid

Excitatory amino acid-induced currents were examined in voltage-clamped rod horizontal cells dissociated from the catfish retina. The cells responded to glutamate (GLU) and the GLU analogues kainate (KA), quisqualate (QA), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), while N-methyl-D-aspartate (NMDA) produced inconsistent responses. Of the effective agonists, only KA produced large, concentration-dependent current responses. While QA, AMPA, GLU, and NMDA were poor agonists, these compounds were able to block rod horizontal cell responses to KA. The rank order potency for this inhibition was: QA greater than AMPA greater than or equal to L-GLU much greater than D-GLU = NMDA. Several excitatory amino acid receptor antagonists were also able to inhibit rod horizontal cell responses to KA. The rank order potency for the inhibition by the compounds tested was: kynurenate greater than cis-piperidine-dicarboxylic acid much greater than D,L-alpha-amino-adipate. Comparison of the potency of several ligands to inhibit rod and cone horizontal cell responses to KA suggested similarities in the KA binding sites of both cell types.

D-aspartate potentiates the effects of both L-aspartate and L-glutamate on carp horizontal cells

L-Aspartate, L-glutamate and D-aspartate, alone or in combination, were applied by superfusion or by atomization to the isolated carp retina while recording from cone horizontal cells. Each of these agents when applied alone depolarized horizontal cells and reduced the size of their light responses, an action which mimics the effect of the endogenous photoreceptor transmitter. Application of D-aspartate in conjunction with either of the L-amino acids potentiated the effects the L-amino acids so that the threshold concentration was reduced by about five-fold, compared to when the L-amino acids were applied alone. The potentiating effect of D-aspartate occurred with all types of cone horizontal cells--both L- and C-types. Furthermore, the potentiating effect of D-aspartate occurred not only in the dark but also in the presence of bright light background or Co2+ ions, conditions during which the release of photoreceptor transmitter is reduced or blocked. D-Aspartate also potentiated the depolarizing effects of the acidic amino acid cysteine sulfinate. The potentiating effect of D-aspartate can be attributed to its action as an amino acid uptake blocker in the outer retina. Thus, these findings, in themselves, cannot eliminate L-aspartate, L-glutamate or cysteine sulfinate as candidates for the carp cone transmitter. However, other evidence, previously reported, strongly suggests that L-glutamate and not L-aspartate is the cone transmitter.