Calcineurin plays different roles in group II metabotropic glutamate receptor- and NMDA receptor-dependent long-term depression

Abstract

We investigated metabotropic glutamate receptor (mGluR)-dependent long-term depression (LTD) in hippocampal CA1 pyramidal neurons of 6- to 8-d-old [postnatal days 6-8 (P6-P8)] and 21- to 25-d-old (P21-P25) rats. In P6-P8 rats, induction of LTD depended on the activity of group II mGluRs. In P21-P25 rats, however, this LTD disappeared, and instead, NMDA receptor (NMDAR)-dependent LTD appeared. A bath containing a specific calcineurin (CaN) inhibitor restored the group II mGluR-dependent LTD in the neurons of the P21-P25 rats. Although postsynaptic injection of CaN inhibitors suppressed NMDAR-dependent LTD, it did not affect induction of group II mGluR-dependent LTD. These results demonstrate that CaN plays different roles in the induction of two forms of LTD: presynaptic CaN inhibits group II mGluR-dependent LTD, whereas postsynaptic CaN facilitates NMDAR-dependent LTD. These findings are the first demonstration in vitro of group II mGluR-dependent LTD that is negatively regulated by CaN via an age-dependent mechanism.

Fig. 1.

In P6–P8 rats, LTD induction is fully dependent on group II mGluR activity. The figure shows averaged field potential recordings. The initial slopes of field EPSPs were normalized to the baseline value preceding the induction of LTD. Each data point represents mean ± SEM. Open barsindicate LFS of 1 Hz for 15 min. A, LFS given in the presence of d-AP-5 evoked LTD (n = 5).Inset, Representative field EPSPs before and 50 min after LFS. Calibration: 20 msec, 1 mV. B–D, This induction was blocked by bath application of MCPG (n = 7), EGLU (n = 6), and MSOPPE (n = 6) but not by AIDA (n = 7) or MPEP (n = 5).

Fig. 2.

In P6–P8 rats, bath application of mGluR agonists evoked LTD. The figure shows averaged field potential recordings. The initial slopes of field EPSPs were normalized to the baseline value preceding the induction of LTD. Each data pointrepresents mean ± SEM. A, Bath application oftrans-ACPD induced LTD (n = 6).B, Bath application of l-CCG-1 induced LTD (n = 6).

Fig. 3.

In P21–P25 rats, NMDAR-dependent, but not mGluR-dependent, LTD was induced. The figure shows averaged field potential recordings. The initial slopes of field EPSPs were normalized to the baseline value preceding the induction of LTD. Each data point represents mean ± SEM. Open barsindicate the LFS of 1 Hz for 15 min. A, A typical LTD was induced by LFS (n = 6). B, Bath application of d-AP-5 (filled circles), but not MCPG (open triangles), blocked LTD induction. C, Bath application oftrans-ACPD failed to induce LTD. D, Bath application of l-CCG-1 failed to induce LTD.

Fig. 4.

In P21–P25 rats, bath application of FK506 restored group II mGluR-dependent LTD. The figure shows field potential recordings. The initial slopes of field EPSPs were normalized to the baseline value preceding the induction of LTD. Each data point represents mean ± SEM. Open barsindicate the LFS of 1 Hz for 15 min. A–E, LFS applied in the presence of d-AP-5. A, LFS did not induce LTD. In the same slice, however, after application of FK506 for 30 min (black bar), the same stimulation elicited LTD (n = 7). B–D, This LTD was blocked by MCPG (n = 6), EGLU (n = 7), and MSOPPE (n = 7) but not by AIDA (n = 8) or MPEP (n = 7).E, LFS given after bath application of rapamycin (black bar) for 30 min failed to induce LTD (n = 5). F, Applying LFS together with kynurenic acid (hatched bar) failed to induce LTD. In the same slices, however, this induction protocol elicited LTD when given after bath application of FK506 (black bar) for 20 min (n = 5).

Fig. 5.

In P21–P25 rats, bath application of FK506 restored agonists-induced group II mGluR-dependent LTD. The figure shows averaged field potential recordings. The initial slopes of field EPSPs were normalized to the baseline value preceding the induction of LTD. Each data point represents mean ± SEM.A, Bath application of trans-ACPD (hatched bars) resulted in transient depression of field EPSPs but not in LTD. In the same slices, however, the same concentration of trans-ACPD in the presence of FK506 (black bar) resulted in LTD (n = 6).B, Bath application of l-CCG-1 (hatched bars) resulted in transient depression of field EPSPs but not in LTD. In the same slices, however, the same concentration of l-CCG-1 in the presence of FK506 (black bar) induced LTD (n = 6).

Fig. 6.

Presynaptic CaN contributes to inhibition of group II mGluR-dependent LTD, and postsynaptic CaN activity is required for NMDAR-dependent LTD. The figure shows averaged whole-cell EPSC recordings in slices from P21–P25 rats. The initial slopes of field EPSPs were normalized to the baseline value preceding the induction of LTD. Each data point represents mean ± SEM.Open bars indicate LFS of 1 Hz for 10 min.A, Applying LFS led to LTD in control slices (n = 6). This LTD was blocked by bath application of d-AP-5 (n = 5).Insets, Sample waveforms taken at the times indicated, from a typical experiment. B, FS given in the presence ofd-AP-5 paired with FK506 in extracellular solution induced LTD (n = 5). This LTD was blocked by EGLU (n = 6). C, During postsynaptic diffusion of FK506 (open triangles;n = 5) or CaN-AIP (filled circles; n = 5) through the patch pipette, LFS given in the presence of d-AP-5 failed to induce LTD.D, In the presence of MCPG, applying LFS evoked LTD (n = 6). Postsynaptic diffusion of FK506 blocked this LTD (n = 6).

Fig. 7.

Schematic diagram showing that CaN developmentally regulates group II mGluR- and NMDAR-dependent LTD. In P6–P8 rats, CaN does not inhibit induction of group II mGluR-dependent LTD, because it is only weakly expressed in very young rats. In P21–P25 rats, however, CaN was expressed strongly enough to inhibit this LTD via a presynaptic mechanism. On the other hand, postsynaptic CaN facilitates the induction of NMDAR-dependent LTD.