N-Methyl-D-aspartic Acid (NMDA) in the nervous system of the amphioxus Branchiostoma lanceolatum
- Salvatore D'Aniello1Email author,
- George H Fisher2,
- Enza Topo3,
- Gabriele Ferrandino3,
- Jordi Garcia-Fernàndez†1 and
- Antimo D'Aniello†3
© D'Aniello et al; licensee BioMed Central Ltd. 2007
Received: 05 April 2007
Accepted: 20 December 2007
Published: 20 December 2007
NMDA (N-methyl-D-aspartic acid) is a widely known agonist for a class of glutamate receptors, the NMDA type. Synthetic NMDA elicits very strong activity for the induction of hypothalamic factors and hypophyseal hormones in mammals. Moreover, endogenous NMDA has been found in rat, where it has a role in the induction of GnRH (Gonadotropin Releasing Hormone) in the hypothalamus, and of LH (Luteinizing Hormone) and PRL (Prolactin) in the pituitary gland.
In this study we show evidence for the occurrence of endogenous NMDA in the amphioxus Branchiostoma lanceolatum. A relatively high concentration of NMDA occurs in the nervous system of this species (3.08 ± 0.37 nmol/g tissue in the nerve cord and 10.52 ± 1.41 nmol/g tissue in the cephalic vesicle). As in rat, in amphioxus NMDA is also biosynthesized from D-aspartic acid (D-Asp) by a NMDA synthase (also called D-aspartate methyl transferase).
Given the simplicity of the amphioxus nervous and endocrine systems compared to mammalian, the discovery of NMDA in this protochordate is important to gain insights into the role of endogenous NMDA in the nervous and endocrine systems of metazoans and particularly in the chordate lineage.
Amphioxus is an emerging animal model, situated in a key phylogenetic position, as a descent of a chordate ancestor. With vertebrates, amphioxus shares many body plan characteristics such as a dorsal nerve cord, notochord, endostyle, segmental muscles, pharyngeal gill slits and a post-anal tail. The genetic programs patterning the amphioxus embryo are also similar to those of vertebrate embryos, even if they are supported from a limited set of genes. In fact, cephalochordates did not undergo the extensive genome duplications that gave raise to more complex pathways and the gene redundancy typical of polyploid organisms, such as vertebrates.
Noticeably, in the last few years the position of cephalochordates and tunicates has been reversed. In fact, an accurate phylogenetic analysis of a large amount of molecular data indicated that urochordates are the closest relatives to vertebrates, while cephalochordates branched earlier . This change in the chordate phylogeny has profound implications for the understanding of the evolution of body plans, and it may imply that the chordate ancestor had a body plan organization similar to contemporary amphioxus.
Taking this into consideration, cephalochordates represent a valuable model organism in understanding chordate's evolution, and in particular to distinguish between conserved (potentially ancestral) and derived (potentially novel) characteristics of vertebrates.
Recently, we reported that the amphioxus nervous system contains considerable amounts of free D-aspartic acid . Since in rat and in Ciona this amino acid constitutes the precursor for the in vivo biosynthesis of NMDA [9–11], in the present study we have designed approaches to reveal if NMDA is also present in the amphioxus nervous system and to know if this excitatory amino acid is biosynthesized from D-aspartic acid.
Purification of NMDA and HPLC determination
Comparative concentration of NMDA between amphioxus and other animal phyla
Concentration of N-Methyl-D-Aspartic acid (NMDA) in the nervous system of amphioxus Branchiostoma lanceolatum.
nmol/g wet tissue
Mean ± S.D.
3.08 ± 0.37
10.52 ± 1.41
0.55 ± 0.051
Comparative study on the occurrence of NMDA in the nervous system of various animal species.
Mammals (Rattus norvegicus)#
1.40 ± 0.30
Birds (Gallus gallus)*
2.20 ± 0.20
Amphibians (Rana esculenta)*
2.24 ± 0.38
Fishes (Torpedo ocellata)*
7.76 ± 0.85
Tunicates (Ciona intestinalis)*
13.8 ± 2.65
3.08 ± 0.37
10.52 ± 1.41
Crustaceans (Carcinus maenas)*
4.46 ± 0.73
Cephalopod (Octopus vulgaris)*
2.20 ± 0.48
The concentration of NMDA in the nervous tissue of several animals ranges normally between 1.4 and 7.6 nmol/g of tissue, except for the cerebral ganglion of Ciona intestinalis, where NMDA occurs at considerably higher concentrations, 13.8 nmol/g of tissue (Table 2). Interestingly also the amphioxus brain contains a very high NMDA concentration (10.52 ± 1.41 nmol/g).
Biosynthesis of NMDA
Biosynthesis of NMDA from D-Asp and other amino acids in the nervous tissues of the amphioxus Branchiostoma lanceolatum.
Amino acid *
NMDA synthesized (nmol/g tissues)
33.4 ± 4.3
45.5 ± 5.4
1.5 ± 0.4
3.4 ± 0.4
3.2 ± 0.6
0.8 ± 0.2
3.8 ± 0.5
2.3 ± 0.4
0.7 ± 0.1
3.5 ± 0.4
2.1 ± 0.4
0.6 ± 0.1
3.2 ± 0.3
2.2 ± 0.3
0.5 ± 0.2
3.3 ± 0.4
3.2 ± 0.6
0.8 ± 0.2
2.9 ± 0.4
2.6 ± 0.4
0.6 ± 0.2
3.5 ± 0.6
1.7 ± 0.2
0.7 ± 0.2
3.6 ± 0.4
1.9 ± 0.5
0.6 ± 0.2
3.1 ± 0.4
2.3 ± 0.4
0.5 ± 0.1
In order to know if only D-Asp constitutes the NMDA precursor molecule, several different amino acids, in both D- and L-conformation, were used as potential substrate in the same reaction conditions as previously described for D-Asp (Table 3). The results obtained demonstrate that, except for D-Asp, no other amino acid is able to act as precursor for the synthesis of NMDA.
We recently found that the neural system of amphioxus contains a considerable amount of D-aspartic acid, approximately 280 nmol/g of tissue . In the present work we found in the nervous system of B. lanceolatum a considerable quantity of NMDA in different regions (nerve cord and brain) similar to that found in the cerebral ganglion of Ciona intestinalis (Tables 1 and 2). In addition, as occurs in rat and in urochordates, NMDA is synthesized also in amphioxus from D-Asp by a SAM-dependent NMDA synthase, which transports a methyl group from SAM to D-Asp, transforming it into NMDA (Table 3). Thus, as in other animals, a biological pathway for the endogenous production of NMDA seems to exist in amphioxus.
Recently, we also demonstrated that NMDA is present in the rat neuroendocrine system, in C. intestinalis and in other animal phyla [9–12]. Synthetic NMDA is well known for its stimulating action on the NMDA-type L-glutamate receptors. In addition to having this effect, NMDA is also known for inducing hormone release in the hypothalamus and hypophysis. In fact, already back in 1978, Price and collaborators found that a very low amount of NMDA injected into the rat increased LH levels . After that, Gay and colleagues  found that NMD/LA (N-methyl-D/L-aspartic acid) elicits hypothalamic gonadotropic release in the Rhesus monkey (Macaca mulatta), and Downing and colleagues  observed that a minute quantity of NMD/LA injected into the ewe induced the release of gonadotropin releasing hormone (GnRH) from the hypothalamus, and growth hormone (GH) and prolactin (PRL) from the pituitary gland. It has also been demonstrated that in rat synthetic NMDA is able to elicit the release of LH and PRL [9, 10].
In addition, an elevation of testosterone and progesterone concentration in rat has been observed as a consequence of the LH increase . The same phenomenon has been shown to occur in C. intestinalis . In tunicates, interestingly, D-Asp is present in the cerebral ganglion while NMDA is present in the neural gland. NMDA is synthesized in the neural gland from D-Asp as precursor and in turn it induces the release of the GnRH. Then, GnRH reaches the gonads where it induces the synthesis and release of testosterone and progesterone. Thus, in both mammals and in the protochordate Ciona intestinalis, NMDA is involved in hormone production and activity . Although our data do not allow to ascertain the functional effects of NMDA in amphioxus physiology or endocrine system, its high concentration in the brain area is suggestive of it, and indicates that NMDA-driven actions most probably predated chordate evolution.
The presence of NMDA primarily in the cephalic vesicle of the amphioxus B. lanceolatum led us to argue that in cephalochordates NMDA may have a role in both nervous and endocrine systems. Thus, as in mammals and in tunicates, where D-Asp and NMDA are implicated in the development and function of the neuroendocrine system, it is reasonable to suggest that the presence of D-Asp and NMDA in the nervous system of the amphioxus gives support to the concept that modern vertebrates and cephalochordates share a common chordate ancestor, in which D-Asp and NMDA neurotransmission could play a relevant function in the neuroendocrine system. Although data in other deuterostomes (echinoderms and hemichordates) are not available, the presence of NMDA in the nervous system of several mollusks may indicate that NMDA-derived neurotransmission could even be a more general character of Bilateria.
Animals collection and chemicals
Adult amphioxus Branchiostoma lanceolatum were collected in the bay of Argelès-sur-Mer (southern coast of France) in January 2007 as described in Fuentes et al., 2007 . Adult animals were kept for a week before dissection in glass tanks in the laboratory at a water temperature of 15°C and a 10/14 day-night light cycle (corresponding to natural conditions). Filtered seawater was changed daily and kanamycin antibiotic (10 mg/l sea water) was added to prevent bacterial contamination. Specimens were dissected under an optic microscope using a micro-fine knife with a 4 mm cutting edge (F.S.T.) on ice. No anaesthetic was used to immobilize the animals, in order to avoid drug-induced alterations in the measurement of NMDA. All salts, amino acids, OPA (o-phthaldialdehyde), NAC (N-acetyl-L-cysteine) and other chemicals were purchased from Sigma Chemical Company. The Sep-Pak® ODS C18 (octadecylsilyl-C18) cartridges were purchased from Waters Corporation, Milford, MA, USA.
Tissue homogenization and purification of NMDA from other amino acids and amino compounds
Nerve cord, brain and muscle tissues were obtained from adults' animals and divided in five pools (each pool containing tissue from 20 animals). Samples were homogenized in 0.1 M TCA (trichloroacetic acid) in a ratio of 1:10 and centrifuged at 13,000 g for 5 minutes. The supernatants were purified to separate NMDA from other free amino acids and amino compounds. The sample purification was necessary to avoid interference of the NMDA peak by the peaks of other amino acids or amino compounds at HPLC. The principle of purification was based on the fact that free amino acids and amino compounds react with OPA (o-phthaldialdehyde) to give a complex "amino acid-OPA" which, if passed on an ODS-C18 Sep-Pak cartridge, binds strongly to the resin. NMDA instead, since it does not possess a primary amino group (NH2), does not react with OPA, therefore it does not form a complex with OPA and passes through the ODS-C18 column in purified form [9–12]. Briefly, the procedure is the following: 0.3 ml of TCA supernatant of tissue homogenate, as obtained above, is brought to pH 9.5 with 0.1 M NaOH and then mixed with 100 μl of 0.1 M sodium borate-boric acid buffer, pH 9.5 (to stabilize the pH), 20 μl of 1 M OPA and 2 μl of mercaptoethanol and incubated for 30 min at 37°C to complete the formation of the "OPA-amino acids" complex. After incubation, the mixture is brought to pH 8.2 with 0.1 M HCl and passed through an ODS-C18 cartridge containing about 800 mg of resin (the Sep-Pak had been previously regenerated by treatment with 10 ml of methanol and washed with 10 ml of distilled water). After the sample has been loaded on the cartridge, the eluent is collected. The cartridge is washed with 5 ml distilled water and this last eluent is pooled with the previous eluent and dried in a Petri dish, placed on a warm plate at 40–50°C and under aspiration in a hood until dried. The residue is dissolved in 0.3 ml distilled water and used for the HPLC determination of NMDA.
Determination of NMDA by enzymatic HPLC method
The CH3NH2 formed is then treated with OPA-mercaptoethanol to generate a fluorescent compound and analyzed by HPLC. The method is specific for NMDA since the purified beef kidney D-AspO oxidizes only D-Asp and NMDA [16–18] and only NMDA can generate CH3NH2 when NMDA is oxidized by the D-AspO [7, 8]. Briefly, the procedure is as follows: 100 μl of purified sample is mixed with 50 μl of 0.2 M borate buffer, pH 8.2, and 5 μl of purified D-AspO (1–2 mg/ml; 200–300 U/ml) [16–18] and incubated at 37°C for 30 min. After incubation, 100 μl of 0.1 M sodium borate buffer, pH 10.0, and 5.0 μl of OPA-mercaptoethanol reagent (20 mg of OPA plus 40 μl of 2-mercaptoethanol in 2 ml of methanol) are added and mixed. After 2 min, 100 μl of this mixture is injected into an ODS-C18 Supelcosil column (μ ◂ 5 μm, 250 × 4.6 mm) connected to a Beckman-Gold HPLC system and to a fluorescence detector with excitation wavelength 330 nm and emission wavelength 450 nm. The column is eluted at 1.2 ml/min with a gradient consisting of 2 mobile phases, A and B. Solvent A consists of: 920 ml of double distilled water, 50 ml acetonitrile and 30 ml of 1.0 M citrate-phosphate buffer, pH 5.3 (final pH 5.6). Solvent B consists of: 900 ml acetonitrile and 100 ml double distilled water. The column is equilibrated with solvent A and then the following gradient program is carried out: 0–30% B over 3 min; 30–70% B over 11 min; 70–100% B over 4 min, 100% B for 4 min and return to 0% B in 1 min. The CH3-NH2 elutes as a sharp peak at the retention time of 11.8–12.0 min.
In the same way they are carried out a blank sample, a general standard and a general blank. The blank sample is prepared as the sample, but no D-AspO was added. The standard is prepared using 100 μl of NMDA at a concentration of 0.01 nmol/ml instead of the sample. The general blank is prepared using 100 μl of distilled water instead of the sample.
D-Aspartate oxidase (D-AspO: EC 126.96.36.199) is an oxidative enzyme, which specifically catalyzed the oxidation of only the dicarboxylic amino acids in D-form (D-Asp, D-Glu and NMDA). However, it has been shown that the purified enzyme obtained from beef kidney oxidizes primarily D-Asp and NMDA and only minimally D-Glu (D-Glu is oxidized in a ratio of 2–3% compared to D-Asp or NMDA). On the other hand, the purified D-AspO obtained from the hepatopancreas of the mollusc Octopus vulgaris oxidizes only D-Asp and D-Glu, and only minimally NMDA (4–6% compared to D-Asp or D-Glu [16–18]). In this study we have used purified beef kidney D-aspartate oxidase obtained by recombinant expression in E. coli according to the described procedure .
Biosynthesis of NMDA
Recently, we reported that the amphioxus nervous system contains considerable amounts of free D-aspartic acid . Since this amino acid constitutes the precursor for the in vivo biosynthesis of NMDA [9–11], in the present study we have designed experiments to reveal if NMDA is biosynthesized from D-Asp in the amphioxus nervous system. To verify this, we used the described procedure  modified as follows: pools of nerve cord, cephalic vesicle/hindbrain and muscle, obtained from 10 animals were homogenized with 200 μl of 0.05 M phosphate buffer, pH 7.5, and centrifuged at 13,000 rpm for 10 min. Then, 100 μl of the supernatant was incubated with 10 μl of 0.2 M D-Asp (used as precursor for the biosynthesis of NMDA) and with 10 μl of 10 mM SAM (S-AdenosylMethionine, used as methyl group donor) for 2 h at 37°C. A blank sample was carried out in the same way as the sample, but without D-Asp. After incubation, the sample was mixed with 200 μl of 0.2 M TCA and centrifuged at 13,000 rpm for 5 min. The supernatant was neutralized with 0.1 M NaOH and was subjected to the purification of NMDA by using the above method for the determination of NMDA by HPLC. In the same assay conditions other amino acids (L-Asp, D-Glu, L-Glu, D-Ala, L-Ala, D-Ser, L-Ser and Gly) instead of D-Asp were used.
Statistical analyses were performed using Statistical Pachage (Statsoft, 98th Edition, 1977).
We thank Hector Escrivà for providing us animals and fishing facilities at Laboratoire Arago, CNRS et Université Pierre et Marie Curie, Banyuls sur Mer (France) and the anonymous reviewers for their comments and suggestions. This work was funded by BFU2005-00252 grant (Spanish Ministry of Education and Science). SDA holds a "Juan de la Cierva" postdoctoral fellowship from the Ministerio de Educación y Ciencia, Spain.
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