Fear memories results from the association of a neutral stimulus and an aversive experience, such as footshocks. Considerable evidences show that the encoding of these memories can be facilitated when stressful experiences occur before fear conditioning, resulting in strong and long-lasting fear memories. However, relative few studies investigated if a threatening experience can affect the recall and the persistence of a previously consolidated fear memory. The present study was undertaken to test the hypothesis that an emotionally negative experience can modify the expression of a contextual fear memory upon retrieval. In the current research, we use the chemical stimulation (microinjection of 100pmol NMDA) of the dorsolateral periaqueductal gray matter (dlPAG) of rats which has been considered to induce an aversive state biologically relevant. Such stimulation was performed one day after a weak contextual fear conditioning protocol and fear expression was assessed in subsequent re-exposures to the conditioned context. Enhanced fear was observed 24h and 6 days after dlPAG stimulation, and this potentiation was contingent to the associated context since it was not evident when NMDA-stimulated animals were subsequently placed in a non-associated environment. Rats re-exposed to the associated context 48h after the local infusion of NMDA into the dlPAG did not display enhanced freezing, suggesting that the promoting effect of such stimulation on fear memory is not evident after 24h with this particular NMDA dose. The present study suggest that the emotionally arousal induced by the chemical stimulation of dlPAG interacts with the original memory trace during retrieval , modifying the emotional relevance of the contextual cue and enhancing fear expression, which persists at least 6 days after dlPAG stimulation.
