Modulation of presynaptic calcium transients by metabotropic glutamate receptor activation: a differential role in acute depression of synaptic transmission and long-term depression

Abstract

Activation of group I metabotropic glutamate receptors (mGluRs) can induce acute depression of excitatory synaptic transmission and long-term depression (LTD) in area CA1 of the rat hippocampus. The underlying mechanisms for both forms of depression are unknown. By measuring presynaptic calcium transients, we show that a reduction in the stimulation-induced presynaptic calcium rise that triggers vesicular release causes the acute depression of transmission by group I mGluRs. In contrast, the mechanism underlying mGluR-induced LTD does not involve a persistent change in stimulation-induced calcium influx. However, analysis of paired-pulse facilitation experiments suggests a presynaptic location for expression of this form of LTD. Furthermore, we show that mGluR-induced LTD can be completely blocked by a specific mGluR5 antagonist, whereas mGluR1 antagonists strongly attenuate the acute depression of transmission. These results support the hypothesis that the acute depression of transmission caused by activation of group I mGluRs involves regulation of stimulation-induced presynaptic calcium transients, whereas mGluR-induced LTD involves a distinct presynaptic modulation downstream of calcium influx.

Fig. 1.

Field potentials and corresponding calcium signals. a, Membrane-permeant fura-2 AM was pressure injected into the SCC tract and taken up by axons, in which intracellular esterases cleaved the AM form to the membrane-impermeant calcium indicator, which filled the presynaptic terminals. Fluorescence was recorded from a small spot as shown. CA3 axons were stimulated with a bipolar electrode placed in the SCC tract, and field recordings were made in area CA1. b, Bath application of ionotropic GluR antagonists abolished fEPSPs, although presynaptic volleys persisted. Simultaneously measured Ca²⁺ signals remained unchanged, indicating their presynaptic origin.

Fig. 2.

mGluR5 activation is required for mGluR-LTD.a, Evoked fEPSPs measured in stratum radiatum; stimulation artifacts are truncated. The amplitude and slope of the fEPSPs were reduced during application of DHPG (100 μm) and recovered partially during washout, revealing a DHPG-induced LTD. b, Slope of fEPSPs evoked at 1 min interstimulus interval, as percentage of baseline values. In control experiments (filled circles), DHPG caused a strong acute depression of the fEPSPs followed by LTD. The total response can be separated into two parts, a reversible suppression by DHPG and the LTD. Application of the mGluR5 antagonist MPEP at 1 μm (open circles) or 10 μm(open squares) did not abolish the acute depression but prevented the induction of LTD. c, Application of the mGluR1 antagonists LY367385 (10 μm; open circles) or 10 μm CPCCOEt (open squares) strongly attenuated the acute depression but not LTD.

Fig. 3.

Acute depression of transmission by DHPG but not mGluR-LTD involves a reduction in stimulation-induced calcium influx.a, Simultaneously recorded fEPSPs and presynaptic calcium signals. Fura-2 was excited at 380 nm; thus, a decrease in ΔF/F indicates an increase in calcium concentration. In the presence of DHPG (100 μm), the calcium signal was attenuated but returned to control values after washout. b, The slope of the fEPSPs (dots) and the ΔF/Fchanges (triangles), measured every minute, are shown before, during, and after DHPG application, revealing both acute depression and mGluR-LTD of transmission. c, The mGluR-LTD (filled diamonds) was isolated by subtracting the predicted reduction of the fEPSP, calculated as [Ca²⁺]⁴ (open squares), from the measured fEPSP (open diamonds). For clarity, error bars are only shown for the calculated mGluR-LTD.

Fig. 4.

Paired-pulse low-frequency stimulation induces NMDA-independent LTD. Long-lasting (15 min) PP-LFS (50 to 1 Hz) during application of the NMDA antagonist d-APV (25 μm) induces NMDA-independent LTD of synaptic transmission (filled circles in a andb). a, PP-LFS-LTD was completely blocked by 10 μm MPEP (open circles), indicating an essential role for mGluR5 activation. b, Stimulus-induced presynaptic calcium (open circles) rise was not changed during PP-LFS-LTD.

Fig. 5.

mGluR-LTD and PP-LFS-LTD both modulate paired-pulse facilitation. a, fEPSPs at control, during DHPG, and after washout. The traces are normalized to the first fEPSP at control (calibration applies to control), revealing the change in PPF during DHPG and after washout. b, DHPG increased PPF, which partially returned to control after washout.c, Histogram summarizing the PPF increase during DHPG (n = 8) and after induction of mGluR-LTD, as well as in PP-LFS-LTD (n = 4). *p < 0.05; **p < 0.01.