Long-term potentiation induces synaptic plasticity at nontetanized adjacent synapses

The two forms of long-term potentiation (LTP) and one form of long-term depression (LTD) in hippocampal area CA1 are induced by different afferent tetani: LTD is induced by a 3- to 5-Hz tetanus, whereas higher frequencies are necessary for LTP mediated by NMDA receptors (25- to 50-Hz tetanus) and LTP mediated by voltage-dependent calcium channels (200-Hz tetanus). It has been suggested that the three forms are induced by graded increases in postsynaptic calcium, with LTD being induced at the lowest calcium concentration. We hypothesized that synapses near a site of LTP induction would elicit LTD owing to diffusion of calcium or its binding proteins. This was tested using a fixed multistimulating electrode array spanning stratum radiatum in area CA1 of rat hippocampal slices. The tetanized sites all displayed LTP. Nontetanized sites displayed LTD following a 50-Hz tetanus, whereas LTD was often seen following a 200-Hz tetanus. In most instances pEPSP and population spike responses were similiar; however, EPSP/spike dissociations (LTD of EPSP, LTP of spike) were seen following activation of NMDA receptors by 50-Hz and 200-Hz tetani. The results are discussed with respect to mechanisms of action and functional significance.