Transient striatal GLT-1 blockade increases EAAC1 expression, glutamate reuptake, and decreases tyrosine hydroxylase phosphorylation at ser19

article i nfo Three glutamate transporters, GLT-1, GLAST, and EAAC1, are expressed in striatum. GLT-1 and, to a lesser ex- tent, GLAST are thought to play a primary role in glutamate reuptake and mitigate excitoxicity. Progressive tyrosine hydroxylase (TH) loss seen in Parkinson's disease (PD) is associated with increased extracellular glu- tamate. Glutamate receptor antagonists reduce nigrostriatal loss in PD models. These observations suggest that excess synaptic glutamate contributes to nigrostriatal neuron loss seen in PD. Decreased GLT-1 expres- sion occurs in neurodegenerative disease and PD models, suggesting decreased GLT-1-mediated glutamate reuptake contributes to excitotoxicity. To determine how transient GLT-1 blockade affects glutamate reup- take dynamics and a Ca 2+ -dependent process in nigrostriatal terminals, ser 19 phosphorylation of TH, the GLT-1 inhibitor dihydrokainic acid (DHK) was delivered unilaterally to striatum in vivo and glutamate reup- take was quantified ex vivo in crude synaptosomes 3 h later. Ca 2+ -influx is associated with excitotoxic con- ditions. The phosphorylation of TH at ser 19 is Ca 2+ -dependent, and a change resulting from GLT-1 blockade may signify the potential for excitotoxicity to nigrostriatal neurons. Synaptosomes from DHK infused stria- tum had a 43% increase in glutamate reuptake in conjunction with decreased ser 19 TH phosphorylation. Using a novel GLAST inhibitor and DHK, we determined that the GLAST-mediated component of increased glutamate reuptake increased 3-fold with no change in GLAST or GLT-1 protein expression. However, GLT- 1 blockade increased EAAC1 protein expression ~20%. Taken together, these results suggest that GLT-1 block- ade produces a transient increase in GLAST-mediated reuptake and EAAC1 expression coupled with reduced ser 19 TH phosphorylation. These responses could represent an endogenous defense against excitotoxicity to the nigrostriatal pathway.