Developmental shift from long-term depression to long-term potentiation at the mossy fibre synapses in the rat hippocampus.

During development, in the CA1 hippocampal region, long-term potentiation (LTP) starts appearing at postnatal (P) day 7 and reaches its maximal expression towards the end of the second postnatal week. However, LTP is often preceded by long-term depression (LTD), an activity-dependent and long-lasting reduction of synaptic strength. LTD can be induced by sustained, low-frequency stimulation of the afferent pathway and is dependent on activation of N-methyl-D-aspartate (NMDA) receptors. We report here that, in the CA3 hippocampal region, during a critical period of postnatal development, between P6 and P14, a high-frequency stimulation train (100 Hz, 1 s) to the mossy fibres in the presence of the NMDA receptor antagonist (+)-3-(2-carboxy-piperazin-4-yl)-propyl-1-phosphonic acid (CPP; 20 microM) induced LTD. The depression of the amplitude of the field excitatory postsynaptic potential (EPSP) was 28 +/- 7% (n = 21). This form of LTD was NMDA-independent and synapse-specific. When a tetanus was applied in the presence of CPP and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 50 microM), which blocked the field EPSP, it failed to induce LTD upon washout of CNQX. LTD was probably postsynaptic in origin since it did not affect paired-pulse facilitation. A rise in extracellular calcium concentration (from 2 to 4 mM) produced LTP instead of LTD. At the end of the second postnatal week, the same high-frequency stimulation train to the mossy fibres induced LTP as in adult neurons.(ABSTRACT TRUNCATED AT 250 WORDS)