Activation of AMPA, kainate, and metabotropic receptors at hippocampal mossy fiber synapses: Role of glutamate diffusion

Glutamatergic transmission at mossy fiber (MF) synapses on CA3 pyramidal neurons in the hippocampus is mediated by AMPA, kainate, and NMDA receptors and undergoes presynaptic modulation by metabo-tropic glutamate receptors. The recruitment of different receptors has thus far been studied by altering presynaptic stimulation to modulate glutamate release and interfering pharmacologically with receptors and transporters. Here, we introduce two novel experimental manipulations that alter the fate of glutamate molecules following release. First, an enzymatic glutamate scavenger reduces the postsynaptic response as well as presynaptic modulation by metabotropic receptors. At physiological temperature, however, the scavenger is effective only when glutamate uptake is blocked, revealing a role of active transport in both synaptic and extrasynaptic communication. Second, AMPA and kainate receptor-mediated postsynaptic signals are enhanced when extracellular diffusion is retarded by adding dextran to the perfusion solution, as is feedback modulation by metabotropic receptors, suggesting that the receptors are not saturated under baseline conditions. These results show that manipulating the spatiotemporal profile of glutamate following exocytosis can alter the involvement of different receptors in synaptic transmission.