Abstract

First, in mice implanted with tetrodes and optic fibres in the dentate gyrus (DG) we will use a closed-loop brain-machine interface to detect dentate spikes and silence dentate granule cells (DGCs), virally transfected with the light-sensitive rapid neuronal silencer ArchT. We will use a transgenic mouse line (expressing Cre recombinase under the metabotropic glutamate receptor-2 promoter) which shows exquisite specificity for DGCs, with no observable expression in CA1-3 or the hilus. We will silence DGCs during sleep after learning events to determine the contribution of dentate spikes to the neuronal and behavioural correlates of memory. Next we will investigate the neuronal inputs to DGCs that drive dentate spikes. We will use two transgenic mouse lines (Ai32 and Ai40D) that express the light-driven neuronal activator channelrhodopsin-2 (ChR2) and silencer ArchT, respectively, only in Cre-expressing cells. We will inject a retrograde adeno-associated viral vector into the DG to transduce DGC-projecting medial entorhinal cortex (MEC) cells with Cre recombinase. The presence of Cre in these MEC cells will drive ChR2 in Ai32 mice and ArchT in Ai40D mice. We will then test how stimulating DGC-projecting MEC cells during sleep alters the probability or amplitude of dentate spikes. Finally, we will use ‘TetTag’ mice in which ArchT expression is driven by the immediate-early gene cFos. ArchT expression is controlled by an experimenter-defined window because it is suppressed by dietary doxycycline (dox). When dox is removed from the diet, neuronal activation induces cFos which then drives ArchT expression in those neurons. Thus we can ‘tag’ DGCs active during a specific memory episode. We will not interfere with the initial memory consolidation but will later ‘reactivate’ this memory by presenting retrieval cues which renders the memory labile. We will test how inhibiting the tagged DGCs during dentate spikes following this reactivation affects subsequent memory.