Tuning afferent synapses of hippocampal interneurons by neuropeptide Y.

Cholecystokinin (CCK)-expressing basket cells encompass a subclass of inhibitory GABAergic interneurons that regulate memory-forming oscillatory network activity of the hippocampal formation in accordance to the emotional and motivational state of the animal, conveyed onto these cells by respective extrahippocampal afferents. Various excitatory and inhibitory afferent and efferent synapses of the hippocampal CCK basket cells express serotoninergic, cholinergic, cannabinoid, and benzodiazepine sensitive receptors, all contributing to their functional plasticity. We explored whether CCK basket cells are modulated by neuropeptide Y (NPY), one of the major local neuropeptides that strongly inhibits hippocampal excitability and has significant effect on its memory function. Here, using GAD65-GFP transgenic mice for prospective identification of CCK basket cells and whole-cell patch-clamp recordings, we show for the first time that excitatory and inhibitory inputs onto CCK basket cells in the dentate gyrus of the hippocampus are modulated by NPY through activation of NPY Y2 receptors. The frequency of spontaneous and miniature EPSCs, as well as the amplitudes of stimulation-evoked EPSCs were decreased. Similarly, the frequency of both spontaneous and miniature IPSCs, and the amplitudes of stimulation-evoked IPSCs were decreased after NPY application. Most of the effects of NPY could be attributed to a presynaptic site of action. Our data provide the first evidence that the excitatory and inhibitory inputs onto the CCK basket cells could be modulated by local levels of NPY, and may change the way these cells process extrahippocampal afferent information, influencing hippocampal function and its network excitability during normal and pathological oscillatory activities.