SENSITIVITY OF SYNAPTIC PLASTICITY TO THE CA-2- A MODEL OF LONG-TERM POTENTIATION IN HIPPOCAMPAL-NEURONS( PERMEABILITY OF NMDA CHANNELS )

We have examined a model by Holmes and Levy (1990) of the induction of associative long-term potentiation (LTP) by a rise in the free Ca2+ c oncentration ([Ca2+]) after synaptic activation of dendritic spines. T he previously reported amplification of the change in [Ca2+] caused by coactivation of several synapses was found to be quite sensitive to c hanges in the permeability of the N-methyl-D-aspartate (NMDA) receptor channels to Ca2+. Varying this parameter indicated that maximum ampli fication is obtained at values that are close to Ca2+ permeabilities r eported in the literature. However, amplification failed if permeabili ty is reduced by more than 50%. We also found that the maximum free [C a2+] reached in an individual spine during synaptic coactivation of se veral spines depended on the location of that spine on the dendritic t ree. Distal spines attained a higher [Ca2+] than proximal ones, with d ifferences of up to 80%. The implications of this result for the unifo rmity of induction of associative LTP in spines in different regions o f the dendrite are discussed.