PDZ protein mediated activity-dependent LTP/LTD developmental switch at rat retinocollicular synapses

Abstract

The insertion of amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors into the plasma membrane and removal via internalization are essential for regulating synaptic strength, which underlies the basic mechanism of learning and memory. The retinocollicular pathway undergoes synaptic refinement during development and shows a wide variety of long-term synaptic changes; however, still little is known about its underlying molecular regulation. Here we report a rapid developmental long-term potentiation (LTP)/long-term depression (LTD) switch and its intracellular mechanism at the rat retinocollicular pathway from postnatal day 5 (P5) to P14. Before P9, neurons always exhibited LTP, whereas LTD was observed only after P10. Blockade of GluR2/3-glutamate receptor-interacting protein (GRIP)/AMPA-receptor-binding protein (ABP)/protein interacting with C kinase 1 (PICK1) interactions with pep2-SVKI could sustain the LTP after P10. This suggests that the LTP/LTD switch relied on PDZ protein activities. Selective interruption of GluR2/3-PICK1 binding by pep2-EVKI blocked the long-lasting effects of both LTP and LTD, suggesting a role for PICK1 in the maintenance of long-term synaptic plasticity. Interestingly, synaptic expression of GRIP increased more than twofold from P7 to P11, whereas ABP and PICK1 expression levels remained stable. Blockade of spontaneous retinal input suppressed this increase and abolished the LTP/LTD switch. These results suggest that the increased GRIP synaptic expression may be a key regulatory factor in mediating the activity-dependent developmental LTP/LTD switch, whereas PICK1 may be required for both LTP and LTD to maintain their long-term effects.

Fig. 1.

A developmental switch from long-term potentiation (LTP) to long-term depression (LTD) at rat retinocollicular synapses. A: averaged excitatory postsynaptic current (EPSC) amplitude change (normalized) induced by a 2-Hz train (arrow) for 250 s (means ± SE, n = 7) from control (circle) and d-2-amino-5-phosphonopentanoic acid (APV, 50 μM, triangle) group at P7. Inset: sampled EPSCs taken at times labeled. Dotted line shows the 100% level. Stim, stimulation. Only half means ± SE was shown to make the figure clear. B: averaged EPSC amplitude change induced by a 50-Hz train for 10 s (n = 6) at P7. Others were the same as A. C and D: similar to A and B, but with animals from P11. E: scatter-plot of EPSC amplitudes change (30 min after stimulation, n = 126) aged from P5 to P14 (2 Hz: circle, 50 Hz: triangle). F: summary of averaged EPSC amplitudes changed from P5 to P14, showing rapid LTP/LTD switch occurred around P9/10, which was independent of stimulation frequency.

Fig. 2.

Developmental LTP/LTD switch relies on the retinal activities. A: averaged EPSC amplitude change (normalized) induced by a 2-Hz train (arrow) for 250 s (means ± SE, n = 7) from tetradotoxin (TTX)-treated rats at P7. Inset: sampled EPSCs taken at times labeled. Dotted line shows the 100% level. Stim, stimulation. B: EPSC amplitude change induced by a 50-Hz train for 10 s (n = 6) at P7 from TTX-treated group. Others were the same as A. C and D: similar to A and B, but with animals from P11. E: scatter plot of EPSC amplitudes change (30 min after stimulation, n = 116) aged from P5 to P14 (2 Hz: circle, 50 Hz: triangle). Dotted line shows the 100% level. F: summary of EPSC amplitudes changed from P5 to P14. Notice that the LTP/LTD switch was suppressed. Data from normal animals were added for comparison (filled circle, data from 2 and 50 Hz were mixed because of no significant difference).

Fig. 3.

LTP/LTD switch is not caused by shortening of N-methyl-d-aspartate receptor (NMDAR) kinetics. A: sampled traces of spontaneous recording of NMDAR currents with 0 Mg²⁺, 25 μM 6,7-dinitroquinoxaline-2,3-dione (DNQX), and 10 μM bicuculline methiodide (BMI) (top). NMDAR currents could be completely blocked by 50 μM APV (middle) and recovered after washing out the APV (bottom). B: averaged NMDAR currents of P9 and P11 rats from control and TTX-treated group (n = 4 in each case). The decay time shortened from P9 to P11 in both groups. C and D: comparison of amplitude and time constant of NMDAR currents of control (n = 26 in total) and TTX-treated animals (n = 28 in total) from P7 to P12. Although the mean amplitude and time constant of NMDAR currents of TTX-treated group were smaller than those from control group in each individual day, the abrupt decrease in the time constant of NMDAR currents still occurred in the TTX-treated group.

Fig. 4.

3-Glutamate receptor-interacting protein (GRIP)/AMPA-receptor-binding protein (ABP)/protein interacting with C kinase 1 (PICK1) (GRIP/ABP/PICK1) activity associated with the LTP/LTD switch. A: averaged EPSC amplitude change (normalized) induced by both 2- and 50-Hz trains at P7 and P11 with intracellular application of pep2-SVKI (100 μM). The LTP/LTD switch was blocked and only STP was observed in both P7 and P11. B: similar to A, but with intracellular application of pep2-EVKI (100 μM). STP was observed in P7 and STD was observed in P11. C: summary results of all three peptides from P5 to P14. Data from different frequency were mixed.

Fig. 5.

Synaptic expressions of GRIP, ABP, and PICK1. A: synaptic expression of GRIP (top), PICK1 (middle), and ABP (bottom) at P7 and P11 from control group and binocular enucleated (BE) group. β-Actin was used as a nonspecific control. B: comparison of the expression level of GRIP, PICK1, and ABP from A. The GRIP expression increased dramatically from P7 to P11 in the control group, whereas no significant increase was observed in the BE group. PICK1 and ABP expression kept relative constant from P7 to P11 in both groups. *Significant difference level of < 0.05 (ANOVA test).