Non-linear Computation Between Neighboring Dendritic Hotspots in Rat CA1 Hippocampal Interneurons

Dendritic hotspots have been shown to play a determining role as local activity foci during sensory information coding and memory formation in cortical neurons in vivo. Here we aim to reveal the characteristics of co-activating more dendritic hotspots themselves within a dynamic time window. Using electrical stimulation and patterned two-photon uncaging for the coincident activation of two neighboring dendritic hotspots in rat hippocampal CA1 interneurons, the evoked local dendritic Ca2+ and the associated excitatory postsynaptic potentials (EPSPs) summed supralinearly. Between active hotspots, supralinearity was present in a relatively broad temporal (∼10 ms) and spatial (∼30 µm) window, mediated by NMDA receptors. Though at larger inter-hotspot distances (>30 µm) supralinear summation of EPSPs was replaced by sublinear integration, the summation of dendritic Ca2+ responses remained supralinear. The inter-hotspot integration represents a novel signal integration state, extending the interaction distance between dendritic inputs, and allowing long-range coincidence detection.