Figure 2.

Glutamate release by brief stimulus trains to mossy fibers activates presynaptic mGluRs. Mossy fibers were stimulated with a brief train and 200 ms later a test pulse was applied to the same pathway (A). To prevent interference from other presynaptic inhibitory transmitters we used CGP35348 and naloxone in all of the glutamate spillover experiments. MCPG (thin line) significantly increased the response to the test pulse, whereas the response to the first stimulus in the train remained unchanged (B₁). The first pulse is sensitive to mGluR activation (thin line), as revealed by the application of 1 μM of the exogenous mGluR agonist L-AP4 (1 μM) (B₂). (C) The fEPSP amplitude for one example is plotted. (D) The ratio between test response and the first response is normalized and the increase in the ratio under MCPG (1.5 mM) is shown for five experiments.