PHARMACOLOGICALLY DISTINCT SODIUM-DEPENDENT L-[H-3]GLUTAMATE TRANSPORT PROCESSES IN RAT-BRAIN

The transport of L-[H-3]glutamate into crude synaptosomal membrane fractions prepared from cerebellum, brainstem, hippocampus, cortex, striatum, and midbrain was characterized. In all brain regions, greater than 95% of the accumulation of radiolabel was sodium-dependent and the concentration-dependence was consistent with a single high affinity site. Dihydrokainate and L-alpha-aminoadipate were region specific inhibitors of uptake; this inhibition was consistent with a competitive mechanism. In the forebrain regions examined, dihydrokainate inhibited transport with IC50S of approx. 100-mu-M (range from 80 to 170-mu-M). Transport in cerebellum was essentially dihydrokainate-insensitive. L-alpha-Aminoadipate inhibited transport in forebrain regions with IC50S of approx. 700-mu-M (range from 590 to 800-mu-M) and inhibited transport in cerebellum with an IC50 of 40-mu-M. The inhibition data obtained with forebrain and cerebellar tissues were consistent with nearly homogeneous (> 80%) populations of non-interacting sites. Inhibition data obtained with tissue prepared from brainstem were best fit to a mixture of the two sites (35-50% of the type observed in cerebellum). Other previously identified uptake inhibitors, including DL-threo-hydroxyaspartate, L-aspartate-beta-hydroxamate, beta-glutamate, and L-cysteine sulfinate were not selective for the two types of transport. These data demonstrate that there are two pharmacologically distinct sodium-dependent high affinity transport systems with heterogeneous regional distributions.