3-Aminoglutarate is a "silent" false transmitter for glutamate neurons

Understanding the storage and release of the excitatory neurotransmitter, L-glutamate by synaptic vesicles has lagged behind receptor characterizations due to a lack of pharmacological agents. We report that the glutamate analog, 3-aminoglutarate (3-AG) is a "silent" false transmitter for glutamate neurons that may be a useful tool to study storage and release mechanisms. Like L-glutamate itself, 3-AG is a high-affinity substrate for both the plasma membrane (EAATs) and vesicular (vGLUT) glutamate transporters. As such, EAATs facilitate 3-AG entry into neuronal cytoplasm allowing 3-AG to compete with L-glutamate for transport into vesicles thus reducing glutamate content. In a synaptosomal preparation, 3-AG inhibited calcium-dependent endogenous L-glutamate release. Unlike L-glutamate, 3-AG had low affinity for both ionotropic (NMDA and AMPA) and G-protein coupled (mGlu1-8) receptors. Consequently, 3-AG behaves as a "silent" false transmitter that may be used in physiological experiments to probe synaptic vesicle storage and release mechanisms for L-glutamate. The companion paper by Wu et al. (2015) describes initial experiments that explore the effects of 3-AG on glutamate synaptic transmission under physiological and pathophysiological conditions.