EXCITATORY SYNAPTIC SITE HETEROGENEITY DURING PAIRED-PULSE PLASTICITYIN CA1 PYRAMIDAL CELLS IN RAT HIPPOCAMPUS IN-VITRO

1. The properties of individual excitatory synaptic sites onto adult C A1 hippocampal neurons were investigated using paired pulse minimal st imulation and low noise whole-cell recordings. Non-NMDA receptor-media ted synaptic responses were isolated using a pharmacological blockade of NMDA and GABA(A) receptors. Amongst the twenty-five stationary ense mbles there were twelve showing paired pulse potentiation, two showing paired pulse depression and eleven with no significant net change. Th e signal-to-noise ratio averaged 4.5:1. There was no correlation betwe en the amplitude of the first and second responses after separation of failures: the percentage of failures averaged 33.6% for the condition ing pulse and 31.7% for the test pulse. 2. Site-directed Bayesian stat istical analysis was developed to predict the likely number of activat ed synapses, synaptic response amplitudes, probability of release and intrinsic variation at each individual synaptic site. Extensive simula tions showed the usefulness of this model and defined appropriate para meters. These simulations demonstrated only small errors in estimating parameters of data sets with a small number of sites (< 10) and simil ar characteristics to the physiological data sets. 3. Physiological en sembles showed between one and three synaptic sites, which exhibited a wide range of values for release probability (0.03-0.99), synaptic am plitudes (1.46-16.8 pA; similar to 62% coefficient of variation betwee n sites) and intrinsic variation over time (similar to 36%). Paired pu lse plasticity occurred primarily from alterations in the release prob abilities but a few ensembles also showed small changes in site amplit ude. Initial release probability correlated negatively with the degree of paired pulse potentiation. Whilst it was possible to use simple as sumptions regarding site homogeneity (such as required for a binomial process) for 48% (12 out of 25) of the data sets, the Bayesian analysi s was necessary to reveal the complex changes and heterogeneity that o ccurred in the other 52% of the data sets. The Bayesian site analysis robustly indicated the presence of considerable site heterogeneity sig nificant intrinsic site variation over time and changes in parameters at individual synaptic sites with plasticity.