Control of bursting by local inhibition in the rat subiculum in vitro

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Abstract
The subiculum, which provides the major hippocampal output, contains different cell types including weak/strong bursting and regular-spiking cells, and fast-spiking interneurons. These cellular populations play different roles in the generation of physiological rhythms and epileptiform activity. However, their intrinsic connectivity and the synaptic regulation of their discharge patterns remain unknown. In the present study, the local synaptic responses of subicular cell types were examined in vitro. To this purpose, slices were prepared at a specific orientation that permitted the antidromic activation of projection cells as a tool to examine local circuits. Patch recordings in cell-attached and whole-cell configurations were combined with neurobiotin labelling to classify cell types. Strong (≈75 %), but not weak (≈22 %), bursting cells typically fired bursts in response to local synaptic excitation, whereas the majority of regular-spiking cells (≈87 %) remained silent. Local excitation evoked single spikes in more than 70 % of fast-spiking interneurons. This different responsiveness was determined by intrinsic membrane properties and not by the amplitude and pharmacology of synaptic currents. Inhibitory GABAergic responses were also detected in some cells, typically as a component of an excitatory/inhibitory sequence. A positive correlation between the latency of the excitatory and inhibitory responses, together with the glutamatergic control (via non-NMDA receptors) of inhibition, suggested a local mechanism. The effect of local inhibition on synaptically activated firing of different cell types was evaluated. It is shown that projection bursting cells of the subiculum are strongly controlled by local inhibitory circuits.