Long-term potentiation of synaptic responses in the rat dentate gyrus is due to increased quantal content

Long-term potentiation (LTP) of synaptic responses in the dentate gyrus neurons of the rat hippocampus was studied in in vitro slices with the use of intracellular recordings. The goal of the study was to determine if the expression of LTP is pre- or postsynaptic. LTP was induced by tetanic stimulation of the perforant pathway in the presence of bicuculline. The expression of potentiation was measured during low-intensity stimulation at 1-5 Hz. It was found that a 104% (S.E.M. +/- 35, n = 5) increase in the amplitude of evoked synaptic potentials was associated with a reduction in the number of transmission failures to 38% (S.E.M. +/- 15, n = 5) of the control values. The size of quantal responses was determined on the basis of asynchronous release from stimulated synapses. The average size of the quanta remained unchanged during LTP. The evident increase of quantal content suggests a presynaptic locus for expression of LTP.