Strychnine-insensitive [3H] glycine binding to synaptosomal membranes from the chick retina

D-serine: a new word in the glutamatergic neuro-glial language

Gliotransmission is a process in which astrocytes are dynamic elements that influence synaptic transmission and synaptogenesis. The best-known gliotransmitters are glutamate and ATP. However, in the past decade, it has been demonstrated that D-serine, a D-amino acid, acts as a gliotransmitter in glutamatergic synapses. The physiological relevance of D-serine is sustained by the way in which it modulates the action of glutamatergic neurotransmission, neuronal migration and long-term potentiation (LTP). In addition, the synthesis and degradation mechanisms of D-serine have been proposed as potential therapeutic targets for the treatment of Alzheimer's disease, schizophrenia and related disorders. In the present review, detailed information is provided about the physiological and physiopathological relevance of D-serine, including metabolic and regulation aspects.

Cloning and characterization of the chick NMDA receptor subunit-1 gene

The N-methyl-D-aspartate family of glutamate receptors (NMDARs) are tetrameric cation channels including NR1, NR2, and possibly NR3 subunits. The physiological properties of the receptor are directly related to the subunit composition of the oligomer. Whereas NR1 is essential for the formation of functional channels, NR2 and NR3 play a modulatory role. This work reports, for the first time, the cloning of a non-mammalian NR1 gene, including the 5'-regulatory region. The chick gene spans 31 kb of genomic DNA sequence composed of 22 exons interrupted by 21 introns. The exon/intron organization and the deduced amino acid sequence up to the end of exon 19 showed 85% homology to mammalian NR1 cloned genes. Significant differences from mammals were found at the C-terminal region which in the chick gene, includes a novel exon (exon 20) previously identified at the mRNA level in the chick retina. The basal promoter activity was shown to reside within the proximal 377 bp of 5'-regulatory region. The transcriptional activity of the 5'-flanking region of the chick NR1 gene was shown to be higher in neuronally-differentiated PC12 cells and in chick retinal neurons, than in non-differentiated PC12 cells and Müller glia. Comparison of the 5'-flanking region of chick NR1 gene with mammalian NR1 genes suggests that, in spite of significant differences in the nucleotide sequence, they share common DNA binding sites such as RE1, SP1, AP2, CREB, NFkappaB, and MEF2; therefore, some of the molecular mechanisms involved in transcriptional regulation of NR1 gene expression could be conserved among species.

Developmental expression of N-methyl-D-aspartate glutamate receptor 1 splice variants in the chick retina

Glutamate is the major excitatory neurotransmitter in the vertebrate retina. The N-methyl-D-aspartate glutamate receptor (NMDAR) is assembled as a tetramer containing NR1 and NR2, and possibly NR3 subunits, NR1 being essential for the formation of the ion channel. The NMDAR1 (NR1) gene encodes for mRNAs that generate at least eight functional variants by alternative splicing of exon 5 (cassette N1), 21 (cassette C1), or 22 (cassettes C2 or C2'). NR1 splice variants were identified in the mature chick retina, and their variation during embryonic development (ED) was analyzed. NR1 was shown to lack N1 in early ED, shifting to N1-containing variants in the mature retina, which could contribute to explaining the distinct biochemical properties of retinal NMDARs compared with the CNS. Sequence analysis of C-terminal variants containing C1 and C2 cassettes suggests a membrane-targeting mechanism for avian NMDARs distinct from that in mammals. An NR1 variant containing a novel alternative C-terminal splice exon named C3 was found, which encodes six amino acids containing a predicted casein kinase II phosphorylation site. This new variant is expressed in the retina during a restricted period of ED, coincident with the generation of spontaneous calcium activity waves, which precedes synapse formation in the retina, suggesting its participation in this process.

Calcium-independent release of [3H]spermine from chick retina

Spermine has been shown to influence NMDA receptor function through an interaction at the coagonist site for glycine in the central nervous system (CNS) and the retina. In order to support a role for spermine as neurotransmitter or neuromodulator in the chick retina, specific stimulated-release of spermine should be demonstrated. Isolated chick retinas, preloaded with [3H]spermine, were stimulated with 1 mM NMDA and other glutamate agonists at ionotropic receptors, in a continuous superfusion system. [3H]spermine was released from the retina by depolarization with 50 mM KCl, in a Ca2+-independent manner. Inhibition of Na+/K+-ATPase by ouabain or digitoxigenin also induced spermine release following 36 min in the presence of the drugs; such effect seems unrelated to changes in Na+ electrochemical gradients, since nigericin and veratrine did not induce release in Na+ containing medium. The lack of effect of glutamate, NMDA and kainate at 1 mM concentration, suggests that release of spermine in the retina is mediated by the reversal of uptake and not necessarily linked to EAA-receptor activation.

[3H]Spermine binding to synaptosomal membranes from the chick retina

The binding of [3H]spermine to synaptosomal membranes from chick retina was examined. Saturable specific binding of [3H]spermine to synaptosomal membranes from plexiform layers of retina (P1 and P2) has been characterized, and found to concentrate in the inner plexiform layer compared to the outer plexiform layer (Bmax=9.3 and 37 pmol/mg protein for P1 and P2, respectively). Kinetics of specific [3H]spermine binding yield a sigmoidal saturation curve, indicating positive cooperativity (nH: 2.4 and 3.2 for P1 and P2, respectively) with high affinity: Kapp=61 and 67 nM for P1 and P2. The time required to attain equilibrium at room temperature was less than 5 min in both fractions. Dose-response curves for spermine, spermidine, and diethylene-triamine (DET) show different potencies for inhibiting [3HDET. Our results support a role for polyamines (PA) as neurotransmitters or neuromodulators in the vertebrate retina.