Glutamate metabolism by squid nerve fiber sheaths

A splice variant of glutamate transporter GLT1/EAAT2 expressed in neurons: cloning and localization in rat nervous system

Rapid uptake of synaptically released glutamate via the high affinity glutamate transporter 1 (GLT1; EAAT2) is important for limiting transmitter signalling and prevents a harmful receptor overstimulation. So far, in the adult brain GLT1 protein has only been detected in astrocytes. Here, we describe the cDNA cloning of a variant of GLT1 from rat brain which is generated by alternative splicing at the 3'-end of the GLT1 cDNA. Reverse transcription-polymerase chain reaction revealed that the GLT1 variant message was present not only in brain, but also in peripheral organs. Northern blot analysis showed that in brain the mRNA of GLT1 (approximately 11 kb) is predominant while in the retina the mRNA of GLT1 variant (approximately 12.5 kb) prevails. In situ hybridization using cRNA and oligonucleotide probes, and immunocytochemistry using an antibody against a synthetic GLT1v peptide were applied in order to identify the cell types expressing GLT1 variant in the adult rat nervous system. GLT1 variant is preferentially expressed in neurons of the CNS and PNS, but is also detected in glial cells (oligodendrocytes, ependymal cells, epithelial cells of the plexus choroideus, satellite cells, and Schwann cells). In contrast to GLT1, GLT1 variant was only occasionally detected in astrocytes. Immunolabelling revealed a preferentially cytoplasmic (frequently granular) staining of neurons and glial cells, suggesting a localization of GLT1 variant protein in vesicle membranes. The studies provide evidence that the cellular expression of the GLT1 variant in the CNS is almost complementary to that of GLT1 and that the GLT1 variant does not seem to be restricted to the CNS.

Uptake and metabolism of glutamate at non-synaptic regions of crayfish central nerve fibers: implications for axon-glia signaling

In crayfish and squid giant nerve fibers, glutamate appears to be an axon-glia signaling agent. We have investigated glutamate transport and metabolism by crayfish central nerve fibers in order to identify possible mechanisms by which glutamate could subserve this non-synaptic signaling function. Accumulation of radiolabeled L-glutamate by desheathed cephalothoracic nerve bundles was temperature and Na(+) dependent, linear with time for at least 8h and saturable at about 0.5-1mM L-glutamate. Most accumulated radiotracer was associated with the periaxonal glial sheath and remained as glutamate. Compounds known to block glutamate transport in invertebrate peripheral nerves or mammalian brain slices or cell cultures were also effective on crayfish central nerve fibers. Tissue radiotracer levels were only 3% of control levels when 1mM p-chloromercuriphenylsulfonate was present, and 13%, 20%, 26%, 38% and 42% of control levels, respectively, when L-cysteate, L-cysteine sulfinate, L-aspartate, D-aspartate or DL-threo-beta-hydroxyaspartate was present. L-Glutamine, GABA, N-methyl-DL-aspartate, alpha-aminoadipate and D-glutamate were without inhibitory effect on tissue tracer accumulation. Radiolabeled D-aspartate was an equivalent non-metabolized substitute for radiolabeled L-glutamate. D-Aspartate, p-chloromercuriphenylsulfonate and GABA had comparable effects on isolated medial giant nerve fibers.These studies indicate that L-glutamate is taken up primarily by the periaxonal glia of crayfish central nerve fibers by a low-affinity, saturable, Na(+)-dependent transport system and is retained by the fibers primarily in that form. Our results suggest that the glia are not only the target of the glutamate signal released from non-synaptic regions of the crayfish medial giant axon during high-frequency stimulation, but that they are also the primary site of its inactivation.