On the transition from reconsolidation to extinction of contextual fear memories

Retrieval of an associative memory can lead to different phenomena. Brief re-exposure sessions tend to trigger reconsolidation, whereas more extended ones trigger extinction. In appetitive and fear cued pavlovian memories, an intermediate “null point” period has been observed where neither process seems to be engaged. Here we investigated whether this phenomenon extends to contextual fear memory. Adult rats were subjected to a contextual fear conditioning paradigm, re-exposed to the context two days later for 3, 5, 10, 20 or 30 min, with immediate injections of MK-801 or saline following re-exposure, and tested on the following day. We observed a significant effect of MK-801 with the 3-min and 30-min sessions, impairing reconsolidation and extinction, respectively. However, it did not have significant effects with 5-, 10- or 20-min sessions, even though freezing decreased from re-exposure to test. Further analyses indicated that this is not likely to be due to a variable transition point at the population level. In conclusion, the results show that in contextual fear memories there is a genuine “null point” between the parameters that induce reconsolidation and extinction, as defined by the effects of MK-801, although NMDA receptor-independent decreases in freezing can still occur in these conditions.