Excitatory inputs to CA1 interneurons show selective synaptic dynamics.

The dynamic properties of synapses between neurons in the hippocampal CA1 area are important for the frequency-dependent signal transfer of the network. We have examined the synaptic dynamics of excitatory inputs to CA1 interneurons and pyramidal cells using whole cell voltage-clamp recordings. The CA1 network was activated using extracellular stimulation electrodes at the Schaffer collaterals (feedforward activation) or at the Alveus (activation of the feedback loop). The dynamic properties of input from the Schaffer collaterals to CA1 interneurons (basket and bistratified cells) were different from the synaptic dynamics of input from the Alveus. Synaptic input from the Schaffer collaterals to CA1 interneurons showed facilitation for most frequencies. After 10 stimuli the synaptic response reached a plateau level that was approximately 150% of the first response in the train. In contrast, the plateau levels of Alveus inputs to interneurons were not different from the first responses for frequencies <or=40 Hz. Paired-pulse facilitation of Schaffer input was stronger than for Alveus input. Cells in stratum oriens with horizontal dendritic trees appeared to be a special group of interneurons because Alveus input to these cells showed strong facilitation with plateau levels of 200% of the first responses. Schaffer input to CA1 basket and bistratified cells showed similar synaptic dynamics compared with Schaffer input to pyramidal cells for frequencies <or=80 Hz. The synaptic dynamics of Schaffer and Alveus input depended only weakly on the stimulus intensity. The difference between the dynamics of Alveus and Schaffer input to CA1 interneurons implies that the relative contribution of feedforward and -back inhibition to network activity depends on the frequency of the input signal at the afferent fibers, adding a level of complexity to transient responses.