Abstract
1. The properties of individual excitatory synaptic sites onto adult CA1 hippocampal neurons were investigated using paired pulse minimal stimulation and low noise whole-cell recordings. Non-NMDA receptor-mediated synaptic responses were isolated using a pharmacological blockade of NMDA and GABAA receptors. Amongst the twenty-five stationary ensembles there were twelve showing paired pulse potentiation, two showing paired pulse depression and eleven with no significant net change. The signal-to-noise ratio averaged 4.5:1. There was no correlation between the amplitude of the first and second responses after separation of failures: the percentage of failures averaged 33.6% for the conditioning pulse and 31.7% for the test pulse. 2. Site-directed Bayesian statistical analysis was developed to predict the likely number of activated synapses, synaptic response amplitudes, probability of release and intrinsic variation at each individual synaptic site. Extensive simulations showed the usefulness of this model and defined appropriate parameters. These simulations demonstrated only small errors in estimating parameters of data sets with a small number of sites (< 10) and similar characteristics to the physiological data sets. 3. Physiological ensembles showed between one and three synaptic sites, which exhibited a wide range of values for release probability (0.03-0.99), synaptic amplitudes (1.46-16.8 pA; approximately 62% coefficient of variation between sites) and intrinsic variation over time (approximately 36%). Paired pulse plasticity occurred primarily from alterations in the release probabilities but a few ensembles also showed small changes in site amplitude. Initial release probability correlated negatively with the degree of paired pulse potentiation. Whilst it was possible to use simple assumptions regarding site homogeneity (such as required for a binomial process) for 48% (12 out of 25) of the data sets, the Bayesian analysis was necessary to reveal the complex changes and heterogeneity that occurred in the other 52% of the data sets. The Bayesian site analysis robustly indicated the presence of considerable site heterogeneity, significant intrinsic site variation over time and changes in parameters at individual synaptic sites with plasticity.