Why is it hard to induce long-term depression?

Most of the interest in computational modelling is devoted to the phenomenon of long-term potentiation (LTP) of synaptic efficacy. Long-term depression (LTD) of synaptic connections is often overlooked. We have found that it is in fact very difficult to induce LTD experimentally and the conditions for LTD induction are much less clear than those for induction of LTP. We concentrate on granule cells in the hippocampal dentate gyrus and using the spike-timing-dependent plasticity (STDP) rule with metaplasticity we show that to evoke homosynaptic LTD or depotentiation of previously potentiated synapses in the spiking model of granule cell is not easy similarly like in a number of experimental studies. On the other hand, heterosynaptic LTD, which accompanies homosynaptic LTP is induced readily both in the model as well as in experiments. We offer possible explanation of these phenomena from STDP, metaplasticity and spontaneous activity.