Tonic GABAA conductance favors temporal over rate coding in the rat hippocampus

Synaptic plasticity is triggered by different patterns of neuronal network activity. Network activity leads to an increase in ambient GABA concentration and tonic activation of GABAA receptors. How tonic GABAA conductance affects synaptic plasticity during temporal and rate-based coding is poorly understood. Here, we show that tonic GABAA conductance differently affects long-term potentiation (LTP) induced by different stimulation patterns. The LTP based on a temporal spike - EPSP order (spike-timing-dependent [st] LTP) was not affected by exogenous GABA application. Backpropagating action potential, which enables Ca2+ entry through N-methyl-D-aspartate receptors (NMDARs) during stLTP induction, was only slightly reduced by the tonic conductance. In contrast, GABA application impeded LTP dependent on spiking rate (theta-burst-induced [tb] LTP) by reducing the EPSP bust response and, hence, NMDAR-mediated Ca2+ entry during tbLTP induction. Our results may explain the changes in different forms of memory under physiological and pathological conditions that affect tonic GABAA conductance.