THE EFFECTS OF SYNAPTIC NOISE ON MEASUREMENTS OF EVOKED EXCITATORY POSTSYNAPTIC RESPONSE AMPLITUDES

Spontaneously occurring synaptic events (synaptic noise) recorded intr acellularly are usually assumed to be independent of evoked postsynapt ic responses and to contaminate measures of postsynaptic response ampl itude in a roughly Gaussian manner. Here we derive analytically the ex pected noise distribution for excitatory synaptic noise and investigat e its effects on amplitude histograms. We propose that some fraction o f this excitatory noise is initiated at the same release sites that co ntribute to the evoked synaptic event and develop an analytical model of the interaction between this fraction of the noise and the evoked p ostsynaptic response amplitude. Recording intracellularly with sharp m icroelectrodes in the in vitro hippocampal slice preparation, we find that excitatory synaptic noise accounts for up to 70% of the intracell ular recording noise, when inhibition is blocked pharmacologically. Up to 20% of this noise shows a significant correlation with the evoked event amplitude, and the behavior of this component of the noise is co nsistent with a model which assumes that each release site experiences a refractory period of similar to 60 ms after release. In contrast wi th classical models of quantal variance, our models predict that excit atory synaptic noise can cause the apparent variance of successive pea ks in an excitatory synaptic amplitude histogram to decrease from left to right, and in some cases to be less than the variance of the measu red noise.