Effect of benzodiazepines on the epithelial and neuronal high-affinity glutamate transporter EAAC1

EAAC1-mediated glutamate transport concentrates glutamate across plasma mem branes of brain neurons and epithelia. In brain, EAAC1 provides a presynapt ic uptake mechanism to terminate the excitatory action of released glutamat e and to keep its extracellular concentration below toxic levels. Here we r eport the effect of well known anxiolytic compounds, benzodiazepines, on gl utamate transport in EAAC1-stably transfected Chinese hamster ovary (CHO) c ells and in EAAC1-expressing Xenopus laevis oocytes. Functional properties of EAAC1 agreed well with already reported characteristics of the neuronal high-affinity glutamate transporter (K-m D-Asp, CHO cells: 2.23 +/- 0.15 mu M; K-m D-Asp,oocytes: 17.01 +/- 3.42 mu M). In both expression systems, lo w drug concentrations (10-100 mu M) activated substrate uptake (up to 200% of control), whereas concentrations in the millimolar range inhibited (up t o 50%). Furthermore, the activation was more pronounced at low substrate co ncentrations (1 mu M), and the inhibition was attenuated. The activity of o ther sodium cotransporters such as the sodium/D-glucose cotransporter SGLT1 , stably transfected in CHO cells, was not affected by benzodiazepines. In electrophysiological studies, these drugs also failed to change the membran e potential of EAAC1-expressing Xenopus laevis oocytes. These results sugge st a direct action on the glutamate transporter itself without modifying th e general driving forces. Thus, in vivo low concentrations of benzodiazepin es may reduce synaptic glutamate concentrations by increased uptake, provid ing an additional mechanism to modulate neuronal excitability.