INTERACTION BETWEEN PAIRED-PULSE FACILITATION AND LONG-TERM POTENTIATION IN AREA CA1 OF GUINEA-PIG HIPPOCAMPAL SLICES - APPLICATION OF QUANTAL ANALYSIS

The aim of the study was to further specify mechanisms of maintenance of hippocampal long-term potentiation. Previous analysis of excitatory postsynaptic potentials showed increases in quantal content (mean num ber of neurotransmitter quanta released by every testing pulse) with s maller increases in quantal size (effect of one transmitter quantum) f ollowing long-term potentiation induction. Here we recorded intracellu larly excitatory postsynaptic potentials from CA1 pyramidal neurons of guinea-pig hippocampal slices after minimal paired-pulse stimulation of monosynaptic inputs. Statistical parameters underlying excitatory p ostsynaptic potential fluctuations were estimated by a deconvolution p rocedure using a quantal model. The parameters of excitatory postsynap tic potentials following paired-pulse stimulation were studied before and after induction of long-term potentiation. Under both conditions, paired-pulse facilitation was found to be accompanied by increases in quantal content and quantal size. During long-term potentiation, paire d-pulse facilitation of amplitude and quantal content was lower. The r espective changes in the paired-pulse facilitation ratios correlated w ith long-term potentiation magnitude. In contrast, the paired-pulse fa cilitation of quantal size did not change significantly following long -term potentiation induction. The results are compatible with the exis tence of two separate mechanisms of long-term potentiation maintenance . They support the suggestion that changes in quantal content are main ly due to presynaptic mechanisms which are shared by long-term potenti ation and paired-pulse facilitation. The mechanisms underlying changes in quantal size are of a different nature for long-term potentiation and paired-pulse facilitation. For long-term potentiation they might b e located postsynaptically.