DYNAMIC STORAGE OF GLUTAMATE IS RAT-BRAIN SYNAPTIC VESICLES

Storage of [H-3]glutamate accumulated by highly purified synaptic vesicles from brain was characterized. [H-3]Glutamate was lost with single exponential kinetics with a time constant of minutes after synaptic vesicles were diluted into medium that allowed uptake to continue but that contained unlabeled glutamate in place of [H-3]glutamate. This [H-3]glutamate efflux occurred at similar rates in media containing 50 and 500 mu M glutamate, which suggests that it did not depend on the rate of glutamate transport and was independent of the external and internal glutamate concentrations. All efflux was blocked at 0 degrees C. These results imply that glutamate stored in synaptic vesicles turns over with a half-time of minutes, even during active uptake under physiological conditions.