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-g lia signaling agent. We have investigated glutamate transport and metabolis m by crayfish central nerve fibers in order to identify possible mechanisms by which glutamate could subserve this non-synaptic signaling function. Ac cumulation 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 ne rves or mammalian brain slices or cell cultures were also effective on cray fish central nerve fibers. Tissue radiotracer levels were only 3% of contro l levels when 1 mM p-chloromercuriphenylsulfonate was present, and 13%, 20% , 26%, 38% and 42% of control levels, respectively, when L-cysteate, L-cyst eine sulfinate, L-aspartate, D-aspartate or DL-threo-beta-hydroxyaspartate was present. L-Glutamine, GABA, N-methyl-DL-aspartate, alpha-aminoadipate a nd 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-glutamare is taken up primarily by the periax onal glia of crayfish central nerve fibers by a low-affinity, saturable, Na +-dependent transport system and is retained by the fibers primarily in tha t form. Our results suggest that the glia are not only the target of the gl utamate signal released from non-synaptic regions of the crayfish medial gi ant axon during high-frequency stimulation, but that they are also the prim ary site of its inactivation. (C) 2000 IBRO. Published by Elsevier Science Ltd.