The procedure for acquisition and sub-optimal extinction of conditioned freezing are described in detail previously (Young et al. 2015 (link)). Briefly, mice were exposed to cued fear conditioning on Day 1, fear extinction training on Day 3, and extinction testing on Day 4. Cued fear conditioning consisted of a 4 pairings of a CS-tone (75-80dB, 6.0 kHz, 30 s) and a US-footshock (0.6 mA, 1 s). Extinction training was carried out 48 h after fear conditioning (‘Day 3’) in a different context from conditioning, where they were exposed to 16 CS tones separated by 45 s each. Saline or MDMA was administered 30 min prior to extinction training, and mice were individually-housed during the period between drug administration and the behavioral procedure. Freezing was scored by video recording and FreezeView software (Coulbourn Instruments, San Diego, CA, USA). For experiments exploring the effects of chronically-administered citalopram, mice were treated daily with 10 mg/kg (i.p.) of citalopram for 22 days between fear conditioning and extinction training. Mice were habituated to handling and i.p. injection for two days prior to experimentation.