Quantal analysis of EPSCs recorded from small numbers of synapses in hippocampal cultures

1. We have studied the statistical properties of excitatory post-synaptic currents (EPSCs) measured at small numbers of synaptic contacts between pairs of hippocampal neurons maintained in dissociated cell culture. Synaptic transmission at few synapses was enabled by microperfusion of a small region of the postsynaptic cell with Ca-containing solution, while blocking transmission at all other synaptic boutons by bathing them in low-Ca solution. Frequency histograms of the amplitudes of EPSCs recorded in this way showed no clear quantization. Numbers of active synapses, estimated immunohistochemically with the use of light microscopy, ranged from 4 to 14 in different experiments. 2. Miniature EPSCs (mEPSCs), originating in the same small population of synapses as produced the evoked EPSCs, were elicited by microperfusion of bath solution made hypertonic by the addition of sucrose. These "sucrose-evoked" mEPSCs appeared to be identical to "spontaneous" mEPSCs in every respect except control over their frequency and site of origin. Sucrose-evoked mEPSCs originating in few synapses still exhibited a broad amplitude distribution. Thus, if mEPSCs constitute the postsynaptic response to a single quantum of neurotransmitter (the "quantal amplitude"), their broad amplitude distribution would tend to obliterate evidence of quantization in evoked EPSC amplitudes, even if evoked release was, indeed, quantal. 3. This idea, which is a corollary of the Katz model of quantal transmission, was tested quantitatively by assuming 1) neurotransmitter release obeys uniform binomial statistics, and 2) the quantal amplitude has a distribution given by the observed distribution of sucrose-evoked mEPSCs. The expected distribution, calculated on the basis of these two assumptions, was fitted to the observed distribution of evoked EPSC amplitudes by varying two free parameters, the binomial parameters N and p. In five cells out of six that were fully analyzed, the Poisson limit of the binomial model (N large, p small) provided a very good fit to the data. This and other evidence suggests that the release probability at a single presynaptic terminal is low. In two out of the six cells, the binomial model, with N constrained to the histochemically determined bouton count, yielded acceptable fits; for the remaining cells the constrained binomial model could be rejected. 4. It is concluded that the Katz model of quantized release of neurotransmitter gives an adequate description of excitatory synaptic transmission in hippocampal cultures, when one assumes the broad distribution of mEPSC amplitudes reflects the distribution of the postsynaptic effect of a single quantum of transmitter.