Fig. 3: Chemogenetic reactivation of hippocampal fear ensembles drives relapse of extinguished fear.

A Experimental design of adult male and female rats that were placed on doxycycline (DOX) diet and received cFos-tTA and TRE-hM3Dq viruses aimed at the dorsal hippocampus (DH). After recovery, all animals were removed from the DOX diet. Forty-eight hours later, two groups of animals underwent auditory fear conditioning; a third group was simply exposed to the conditioning chamber (VEH, n = 9; CNO, n = 7; CNO-NoCon, n = 8). After conditioning, animals were returned to DOX diet, and 48 h later underwent extinction (45 CS-alone trials). Twenty-four hours after the final extinction session, animals received the designer drug clozapine-N-oxide (CNO), immediately prior to an extinction retrieval test. B Left panel Schematic representation of viral expression within the DH for all the animals (viral expression common to all rats is indicated by the most darkly shaded areas). Right panel Representative immunofluorescent image of cFos-tTA and TRE-hM3Dq mCherry expression in the DH. Atlas figures are adapted from Swanson (2018). C Left panel displays percentage of freezing [mean ± standard error of the mean (SEM)] during the baseline (BL) and final conditioning CS. Middle panel displays percentage of freezing (mean ± SEM) during the first extinction block and the final extinction block. Right panel displays percentage of freezing (mean + SEM) with animals either receiving intra-RE VEH or MUS prior to extinction retrieval. Female animals are represented by white-filled, light gray-filled, and light blue-filled triangles, while male animals are represented by dark gray-filled, light blue-filled, and dark blue-filled diamonds in the bar graphs. *p < 0.05; **p < 0.01; ****p < 0.0001.