Differential properties of GABAergic synaptic connections in rat hippocampal cell cultures

Based on the effect of prolonged tetanic stimulation (30 Hz, 4 sec), we divided GABAergic synaptic connections in hippocampal cell cultures into two groups: connections facilitated ( approximately 45%) and connections depressed ( approximately 55%) by the tetanic stimulation. In order to reveal possible reasons for the differential effect of the tetanization, we compared several properties of the connections belonging to both groups. We found that, on average, evoked IPSCs in the connections facilitated by the tetanization have a smaller amplitude and larger coefficient of variation (CV) of IPSC amplitude compared to connections depressed by the tetanization. We also estimated quantal parameters for both groups of connections assuming that transmitter release is reasonably described by a binomial distribution. We found that a background release probability (P) is substantially lower in the connections facilitated by the tetanization (P approximately 0.5) than in the connections depressed by the tetanization (P approximately 0.9) and suggest that this difference may underlie the differential effect of the tetanization. We also found that the tetanization induces the opposite effect on connections made by distinct presynaptic neurons with the same postsynaptic cell (convergent connections) in a fraction of postsynaptic neurons studied (3 out of 9). These results support the idea that properties of the presynaptic neuron are of primary importance for the observed differential effect of the tetanization, but they do not exclude a role of the postsynaptic neuron in this effect.