Origin of variability in quantal size in cultured hippocampal neurons and hippocampal slices

The size of synaptic quanta has been found to display considerable variation in cultured hippocampal neurons, but the source of this variability was previously unknown. We have now compared the properties of locally evoked miniature excitatory postsynaptic currents in cultured hippocampal neurons and in thin hippocampal slices using whole-cell patch-clamp recordings. The variability in miniature excitatory postsynaptic current size was similar in both preparations and occurred in cultured neurons when only one or a few synaptic boutons were stimulated. Thus, the variability in miniature excitatory postsynaptic current amplitude is not an artifact of cultured neurons and arises predominantly from variability within a single bouton. Possible origins of this variability are discussed.