Long-term synaptic plasticity accompanied by rearrangement of synaptic structures is assumed to be the cellular basis of long-term memory. Morphological changes associated with LTP are regarded as precursory phenomena leading to synaptogenesis. However, much less is known about the morphological consequences of LTD, another form of synaptic plasticity in mammalian brain. Recently we reported a long-lasting (i. e. lasting for ≧3 wk) decrease in synaptic strength accompanied by a reduction in the number of synaptic structures after the repetition of transient exposure to ACPD (an mGluR agonist) and APV (an NMDAR antagonist) in stable cultures of hippocampal slice, which treatment induced LTD in this specimen (cf. 2004 SfN Meeting abstract 856.2). This decrease was separate from LTD per se, since the LTD-coupled diminution of EPSP disappeared in <24 hr and another diminution developed gradually after that. Thus we proposed that this phenomenon should represent a new form of long-term synaptic plasticity. It i...