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

Abstract

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 8 h and saturable at about 0.5–1 mM 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 1 mM 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-β-hydroxyaspartate was present. l-Glutamine, GABA, N-methyl- dl-aspartate, α-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.