Unitary synaptic currents between lacunosum-moleculare interneurones and pyramidal cells in rat hippocampus

1. Unitary inhibitory postsynaptic currents (uIPSCs) were characterised bet ween 23 synaptically coupled interneurones at the border of stratum radiatu m and lacunosum-moleclare (LM) and CA1 pyramidal cells (PYR) using dual who le-cell recordings and morphological identification in rat hippocampal slic es. 2. LM interneurones presented a morphology typical of stellate cells, with a fusiform soma as well as dendritic and axonal arborisations in stratum ra diatum and lacunosum-moleculare. 3. Single spikes in interneurones triggered uIPSCs in pyramidal cells that were blocked by the GABAA antagonist bicuculline and mediated by a chloride conductance. The latency, rise time, duration and decay time constant of u IPSCs were a function of amplitude in all pairs, suggesting a homogeneity i n the population sampled. 4. During paired pulse stimulation, individual LM-PYR connections exhibited facilitation or depression. The paired pulse ratio was inversely related t o the amplitude of the first response. The transition from facilitation to depression occurred at 26 % of the maximal amplitude of the first uIPSC. Pa ired pulse depression was not modified by CGP 55845 and thus was GABA(B) re ceptor independent. 5. CGP 55845 failed to modify the amplitude of uIPSCs, suggesting an absenc e of tonic presynaptic GABA(B) inhibition at LM-PYR connections. B. Increas ing G;ABA release by repetitive activation of interneurones failed to induc e GABAB IPSCs. With extracellular minimal stimulation, increasing stimulati on intensity above threshold, or repetitive activation, evoked GABA(B) IPSC s, probably as a result of coactivation of several GABAergic fibres. 7. Thu s, dendritic inhibition by LM interneurones involves GABA(A) uIPSCs with ki netics dependent on response amplitude and subject to GABA(B)-independent p aired pulse plasticity.