Figure 7.
Pharmacologically isolated stimulus-evoked mGluR-dependent LTD and NMDA receptor-dependent LTD are associated with reductions in both presynaptic release probability and postsynaptic reduction in quantal size, and the presynaptic component of changes associated with stimulus-evoked mGluR-dependent LTD require group II mGluR activation. A, VM relationships determined by altering [Ca2+]o to 1, 2, and 4 mm before (filled circles) and 30 min after (open circles) application of low-frequency Schaffer collateral stimulation (2 Hz, 10 min) in the presence of the NMDA receptor blocker AP-5 (50 μm; n = 7 cells). Both means and variances are normalized to the maximal peak amplitude recorded at 4 mm [Ca2+]o before LFS. After LFS, variances at all [Ca2+]o were shifted to the left, whereas data were fit by a lower-amplitude parabola, consistent with both a presynaptic reduction in Pr and postsynaptic reduction in quantal size. B, Calculation of Pr from the data in A confirms that mGluR-dependent LTD is composed of both presynaptic and postsynaptic alterations. C, VM relationships determined as in A before (filled circles) and 30 min after (open circles) application of a low-frequency train of paired-pulse stimuli (50 ms paired-pulse interval × 2 Hz, 10 min) in the presence of the mGluR receptor blocker MCPG (200 μm; n = 8 cells). D, Calculation of Pr from the data in C. In both C and D, the data collected before and after induction of mGluR-LTD fell on two different parabolas and showed reductions in Pr, indicating that mGluR-LTD is also a combination of presynaptic and postsynaptic alterations. E, VM relationship determined as in A before (filled circles) and 30 min after (open circles) application of a low-frequency train of paired-pulse stimuli (50 ms paired-pulse interval × 2 Hz, 10 min) in the presence of the NMDA receptor blocker AP-5 (50 μm) plus the group II mGluR blocker EGLU (250 μm; n = 7 cells). F, Calculation of Pr from the data in E. In both E and F, the data collected before and after induction of mGluR-LTD fell on two different parabolas with no differences in Pr, indicating that once group II mGluRs are blocked, group I-dependent mGluR-LTD is a purely postsynaptic phenomenon.