Extinction of contextual fear memory is facilitated in TRPM2 knockout mice

Transient receptor potential melastatin type 2 (TRPM2) is a nonselective cation channel involved in synaptic plasticity. We investigated its role in contextual fear conditioning and extinction of conditioned fear using Trpm2-deficient (Trpm2^-/-) mice. Trpm2^-/- mice exhibited reduced acquisition of contextual fear memory during conditioning but had an intact freezing response to conditioning context 24 h after conditioning. They also showed a reduced freezing response to extinction training, indicating facilitated extinction. Consistent with this, infusion of flufenamic acid (FFA), a TRPM2 antagonist, into the dentate gyrus (DG) of the hippocampus in fear-conditioned mice facilitated extinction of contextual fear. The enhanced extinction in Trpm2^-/- and FFA-treated mice was associated with down-regulation of immediate-early genes (IEGs) including Npas4, c-Fos, Arc and Egr1 in the hippocampus after extinction training. Our results indicate that TRPM2 plays a positive role in retention of contextual fear memory by modulating neuronal activity in the hippocampus, and suggest that TRPM2 activity could potentially be targeted to strengthen extinction-based exposure therapies for post-traumatic stress disorder (PTSD).