Excitatory synaptic transmission persists independently of the glutamate-glutamine cycle

The glutamate-glutamine cycle is thought to be integral in continuously replenishing the neurotransmitter pool of glutamate. Inhibiting glial transfer of glutamine to neurons leads to rapid impairment in physiological and behavioral function; however, the degree to which excitatory synaptic transmission relies on the normal operation of this cycle is unknown. In slices and cultured neurons from rat hippocampus, we enhanced the transfer of glutamine to neurons, a fundamental step in this cycle, by adding exogenous glutamine. Although raising glutamine augments synaptic transmission by increasing vesicular glutamate, access to this synthetic pathway by exogenously applied glutamine to neurons is delayed and slow, challenging mechanisms linking the rapid effects of pharmacological inhibitors to decreased vesicular glutamate. We find that pharmacological inhibitors of glutamine synthetase or system A transporters cause an acute depression of basal synaptic transmission that is rapidly reversible, which is unlikely to be attributable to the rapid loss of vesicular glutamate. Furthermore, release of vesicular glutamate remains robust even during the prolonged removal of glutamine from pure neuronal cultures. We conclude that neurons have the capacity to store or produce glutamate for long periods of time, independently of glia and the glutamate-glutamine cycle.