TEMPORAL CONSTRAINTS IN ASSOCIATIVE SYNAPTIC PLASTICITY IN HIPPOCAMPUS AND NEOCORTEX

We present comparative experimental evidence for the induction of syna ptic potentiation and depression in organotypic cultures of hippocampu s and in visual cortex in vitro and in vivo. The effects of associativ e pairings on the efficacy of synaptic transmission are analyzed as a function of the temporal delay between presynaptic activity and post-s ynaptic changes imposed in membrane potential. Synchronous association at a low temporal frequency (<0.5 Hz) between presynaptic input and p ostsynaptic depolarization resulted in homosynaptic potentiation of fu nctionally identified postsynaptic potentials in the three types of pr eparation. Synchronous pairing of afferent activity with hyperpolariza tion of the postsynaptic cell resulted in homosynaptic depression in v isual cortex in vivo and in vitro. An associative form of depression w as induced in hippocampus when the test input was followed repeatedly with a fixed-delay postsynaptic depolarization imposed either by intra cellular current injection or synaptically. The fatter process might p lay a significant role in heterosynaptic plasticity in visual cortex i n vivo and in vitro, if it is assumed that associative depression stil l operates in visual cortex a few seconds after the initial surge of c alcium in the postsynaptic cell. We conclude that the precise timing b etween presynaptic activity and polarization changes in postsynaptic m embrane potential up- and down-regulates the efficacy of active pathwa ys.