Substrate-induced up-regulation of Na(+)-dependent glutamate transport activity

Sodium-dependent transporters regulate extracellular glutamate in the CNS. Recent studies suggest that the activity of several different neurotransmitter transporters can be rapidly regulated by a variety of mechanisms. In the present study, we report that pre-incubation of primary 'astrocyte-poor' neuronal cultures with glutamate (100 microM) for 30 min nearly doubled the V(max) for Na(+)-dependent accumulation of L-[(3)H]-glutamate, but had no effect on Na(+)-dependent [(3)H]-glycine transport. Pre-incubation with glutamate also increased the net uptake of non-radioactive glutamate, providing evidence that the increase in accumulation of L-[(3)H]-glutamate was not related to an increase in intracellular glutamate and a subsequent increase in exchange of intracellular non-radioactive glutamate for extracellular radioactive glutamate. The glutamate receptor agonists, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate, and (1 S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid did not mimic the effect of pre-incubation with glutamate and the glutamate-induced increase was not blocked by receptor antagonists. However, compounds known to interact with the transporters, including L-aspartate, D-aspartate, L-(-)-threo-3-hydroxyaspartate (L-THA) and L-trans-pyrrolidine-2,4-dicarboxylate (L-trans-PDC), caused variable increases in transport activity and attenuated the increase induced by glutamate, suggesting that the increase is related to the interaction of glutamate with the transporters. Several studies were attempted to define the mechanism of this regulation. We found no evidence for increases in transporter synthesis or cell surface expression. Inhibitors of signaling molecules known to regulate other neurotransmitter transporters had no effect on this stimulation. Using a variety of cultures, evidence is provided to suggest that this substrate-induced up-regulation of glutamate transport is specific for the GLT-1 and GLAST subtypes and does not influence transport mediated by EAAC1. These studies suggest that the interaction of glutamate with some of the subtypes of glutamate transporters causes an increase in transport activity. Conceivably, this phenomenon provides an endogenous mechanism to increase the clearance of glutamate during periods of prolonged elevations in extracellular glutamate.