Figure 4.

Switching functions of GABAergic synapses from excitation filter to amplifier by RyR activation. (a and b) Single-pulse stimulation of BAS-CA1 (at arrowhead) and SCH [at above-threshold intensities; truncated; stimulated at arrow (a)] evokes an IPSP and action potentials, respectively. The excitatory SCH (at the same above-threshold stimulation) input is filtered out by a costimulation of BAS-CA1 (b). (c–f) Single-pulse stimulation (c) of BAS-CA1 and of SCH at below-threshold intensities evokes an IPSP and an EPSP, respectively. The excitatory SCH (at the same below-threshold stimulation) input is below threshold as evoked by costimulation (single pulse) of BAS-CA1 and SCH inputs (d) before cADP-ribose application. cADP-ribose (30 min after the application) transforms BAS-CA1 IPSP and does not change much of the SCH-CA1 EPSP, evoked by single-pulse stimulation of BAS or SCH, respectively (e). The excitatory SCH (at the same below-threshold stimulation) input is amplified by the co-BAS stimulation after the cADP-ribose-induced synaptic transformation and induces action potentials (truncated; f). Traces were from the same cell. (g) Schematic diagram of GABAergic inputs functioning as either excitatory filter (Left) or amplifier (Right). Active GABAergic inputs, either through activation of SCH as feed-forward inputs (no. 1), feedback inputs (no. 2), or of other circuits (no. 3; such as from the septum), effectively filter excitatory signals so that only very strong excitatory inputs might evoke action potentials. The GABAergic synaptic transformation results in amplifying excitatory signals so that weaker inputs can pass through the neural circuits. BAS, basket GABAergic interneurons (in gray); Pyr, CA1 pyramidal cells; SP, stratum pyramidale.