618. Modulation of hippocampal fear engrams reveals circuit mechanisms linking maladaptive fear, memory deficits, and depressive-like behaviour

Abstract

Abstract Background Classic PTSD symptoms have been linked to maladaptive fear memory processing, involving fear generalization and impaired extinction learning. In contrast, cognitive and mood symptoms remain understudied, and their link to fear-related mechanisms is unclear. Aims & Objectives Here, we investigate the mechanistic link between maladaptive fear, memory impairments and depressive-like behaviour. Method Nine-week-old male C57BL/6J mice received bilateral dorsal dentate gyrus (dDG) injections of AAV-Fos::CreERT2 and Cre-dependent DREADD hM4Di or mCherry. After 3-weeks, mice underwent contextual fear conditioning with low (3×0.45 mA) or high (3×0.8 mA) shocks. Two hours later, 4-hydroxytamoxifen (25 mg/kg, i.p.) was given to label active engram cells. In a subgroup of animals, the tagging was done in a neutral context prior to conditioning. Mice were later tested for fear retrieval, generalization, novel object recognition (NOR), and forced swim test (FST). Clozapine-N-oxide (5 mg/kg, i.p.) was administered 30 min before each test. DREADD expression and FOS activation were assessed via immunofluorescence. Data were analyzed using t-tests or ANOVA with Tukey post-hoc. Results First, we confirm that high- but not low-intensity-conditioning induced generalization and impaired extinction. Low-intensity did not induce memory deficits and inhibition of the engram had no effect on the NOR and FST. High-intensity-conditioning induced memory impairments that were prevented by inhibition of the engram which also led to reduced immobility in the FST. This was accompanied by increased CA1/CA3 activity. Inhibition of the engram in high-intensity-conditioned animals reduced generalization; following extinction, memory impairments were not observed, and engram inhibition had no further effect. The inhibition of a dDG random engram (pre-conditioning) failed to rescue memory deficits and induced an increase in immobility in the FST. Discussion & Conclusions Maladaptive, but not adaptive fear memory, induces generalization, memory impairments and mediates depressive-like behaviours. The inhibition of this engram rescues these effects likely by restoring hippocampal activity. Our data suggest a circuit-level mechanism linking maladaptive fear with cognitive and affective dysfunctions.