Reconsolidation and Extinction of Conditioned Fear: Inhibition and Potentiation

Discussion

The present results demonstrate that blockade of NMDA receptors by the noncompetitive antagonist MK-801 impairs both the reconsolidation and extinction of conditioned fear memories, depending on the number of CS re-exposures presented during the memory reactivation/extinction session. In contrast, enhancement of NMDA receptor-mediated glutamatergic transmission by DCS potentiates the extinction of conditioned freezing when the CS is re-exposed many times, and also enhances reconsolidation when administered before a single CS presentation. Furthermore, we show that a primary locus of action of DCS is the BLA, because intra-BLA infusions of DCS also potentiate extinction and reconsolidation under different experimental conditions.

Studies of the competition between extinction and reconsolidation have demonstrated that the extent to which memory reactivation also induces extinction has a significant impact on the behavioral effect of amnestic treatment (Eisenberg et al., 2003; Pedreira and Maldonado, 2003; Suzuki et al., 2004). When stimulus re-exposure during memory reactivation is brief compared with the strength of conditioning, little extinction is induced and so protein synthesis inhibition disrupts the dominant reconsolidation process, resulting in reduced memory expression (Eisenberg et al., 2003; Pedreira and Maldonado, 2003; Suzuki et al., 2004). In contrast, when the reactivation session induces significant extinction, protein synthesis inhibition impairs the now dominant extinction process, resulting in preserved memory expression. Thus, the effect of protein synthesis inhibition depends critically on the dominant memory process that is engaged at the time of treatment (Eisenberg et al., 2003). The present experiments revealed an identical pattern of results using systemically administered MK-801 to block both fear memory reconsolidation and extinction. Furthermore, systemic or intra-amygdala DCS, which enhances NMDA receptor-mediated glutamatergic transmission (Rouaud and Billard, 2003), resulted in the opposite pattern of results, reducing memory expression when infused before a long extinction session, and preserving/elevating memory expression when infused before a short reactivation session. Therefore, DCS potentiated fear memory extinction, as has been demonstrated previously (Walker et al., 2002; Ledgerwood et al., 2003). Moreover, given that it is highly unlikely that the same infusion of DCS could both enhance and prevent extinction, and that DCS potentiates a range of learning processes (Monahan et al., 1989; Quartermain et al., 1994; Land and Riccio, 1999; Walker et al., 2002; Ledgerwood et al., 2003), the elevated freezing and lack of extinction after DCS infusion before short memory reactivation likely reflects a potentiation of conditioned fear memory reconsolidation. Reconsolidation and extinction have opposing effects on behavioral output, and so an enhancement of reconsolidation would act against the behavioral impact of extinction. When baseline levels of freezing averaged 65%, DCS protected against extinction, maintaining the already high levels of freezing. However, when a weaker fear conditioning protocol was used, resulting in baseline freezing levels of <40%, DCS potentiated subsequent freezing to 60%. Thus, the failure to observe an increase in freezing levels in Figure 1a likely reflects a “ceiling effect” as a result of the high baseline levels of freezing.

Reconsolidation enhancement has been observed previously with the use of systemic treatments such as glucose (Rodriguez et al., 1999) and strychnine (Gordon, 1977), as well as more recently after protein kinase A (PKA) activation in the amygdala (Tronson et al., 2006). NMDA receptor antagonists impair both extinction (Falls et al., 1992; Lee and Kim, 1998) and reconsolidation (Suzuki et al., 2004; Torras-Garcia et al., 2005), and so it is not unexpected that DCS potentiates the extinction and reconsolidation of reactivated memories under different circumstances. That the direction of the effects of DCS, as well as MK-801, changes depending on the degree of CS re-exposure is particularly important. Usually, nonreactivated and nonextinguished controls would be necessary to rule out the possibility that agents affect memory directly in a manner that is not dependent on the memory reactivation or extinction session. However, given that MK-801 does not directly affect the expression of conditioned fear 3 h later (PR-STM) (Fig. 1), and that MK-801 can elevate conditioned freezing when administered before extinction training, its effects to reduce freezing must be critically dependent on the single CS re-exposure during memory reactivation. In the present study, the nonreactivated/nonextinguished controls further confirm that the administration of MK-801 and DCS does not have any nonspecific effects on subsequent freezing, demonstrating that the fear memory is fully consolidated 1 d after acquisition. Thus, MK-801 disrupted fear memory reconsolidation, and the same logic can be applied to demonstrate that MK-801 impaired extinction. Similarly, because DCS either reduced or enhanced subsequent conditioned freezing when administered before long or short CS re-exposure sessions, and DCS alone has been shown previously not to affect conditioned fear 24 h later (Walker et al., 2002; Ledgerwood et al., 2003), we can conclude that DCS potentiates both the extinction and reconsolidation of conditioned fear memories.

NMDA receptors have been strongly implicated in mechanisms underlying learning and memory (Riedel et al., 2003). Not only does blockade of NMDA receptor transmission impair the acquisition of several forms of memory (Morris et al., 1986; Campeau et al., 1992; Burns et al., 1994; Maren et al., 1996; Ferry and Di Scala, 2000; Rodrigues et al., 2001; Goosens and Maren, 2004) and disrupt acutely the expression of previously acquired fear memories (Maren et al., 1996; Fendt, 2001; Lee et al., 2001) as observed here, but administration of partial NMDA receptor agonists also enhances learning (Monahan et al., 1989; Thompson et al., 1992; Quartermain et al., 1994; Land and Riccio, 1999). The extinction of conditioned fear memories, which involves new learning (Bouton and Bolles, 1979; Rescorla, 2001), has also been shown previously to be impaired by NMDA receptor antagonists (Falls et al., 1992; Baker and Azorlosa, 1996; Lee and Kim, 1998) and potentiated by the partial NMDA receptor agonist DCS (Walker et al., 2002; Ledgerwood et al., 2003), when these compounds are administered either systemically or directly into the BLA. The present results, using systemic MK-801 to block fear memory extinction and both systemic and intra-BLA DCS to potentiate extinction of conditioned freezing, both reinforce and extend these findings.

There is also previous evidence that NMDA receptors are involved in the reconsolidation of memories after CS re-exposure. Systemic MK-801 and CPP [d(−)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid] impaired the reconsolidation of a spatial memory (Przybyslawski and Sara, 1997) and a contextual fear memory (Suzuki et al., 2004). Furthermore, intracerebroventricular AP-5 disrupted the reconsolidation of an odor–reward association (Torras-Garcia et al., 2005). Here, we show further that systemic MK-801 also blocked the reconsolidation of a discrete conditioned fear memory. In contrast, systemic administration of DCS before brief CS re-exposure potentiated fear memory reconsolidation. This finding was replicated when DCS was infused directly into the BLA, indicating that the BLA is a primary locus of action of the effects of DCS to enhance reconsolidation. Thus, NMDA receptors in the BLA are important for the reconsolidation of fear memories conditioned to a discrete CS. It can further be hypothesized that the effects of NMDA receptor antagonism/partial agonism may be mediated by PKA, as the present effects on memory reconsolidation are similar to those observed after PKA inhibition and activation (Tronson et al., 2006).

Reactivation-related protein synthesis-dependent amnesia is generally considered to result from a disruption of reconsolidation (Nader, 2003; Alberini, 2005). However, it is also possible that the reduction in performance could be explained simply as a potentiation of extinction (Fischer et al., 2004). Such an explanation has been excluded previously in studies of conditioned fear through the use of reinforced reactivation procedures (Duvarci and Nader, 2004; Eisenberg and Dudai, 2004). The opposing effects of DCS and MK-801 in the present study after a short memory reactivation session further emphasize the distinction between these processes. If the MK-801 induced reduction in conditioned freezing is indeed a result of potentiated extinction, DCS treatment, which is known to enhance extinction processes (Walker et al., 2002; Ledgerwood et al., 2003), should have had the same memory-impairing effect, rather than the observed gain of function demonstrated here.

Both reconsolidation blockade and potentiation of extinction have been proposed as potential approaches to the treatment of maladaptive memory disorders such as post-traumatic stress, phobias, and drug addiction (Davis, 2002; Walker et al., 2002; Nader, 2003; Lee et al., 2004, 2005; Richardson et al., 2004). The present results indicate that the successful application of such approaches will depend critically on the parameters of re-exposure to the CS during memory reactivation or extinction training. When using reconsolidation blockade, if memory reactivation is too extensive, amnestic agents will impair extinction rather than reconsolidation. Furthermore, when attempting to potentiate extinction, if the extinction training is too limited, reconsolidation processes may be preferentially enhanced by drugs such as DCS. Therefore, if the parameters of CS re-exposure are not selected carefully, in accordance with the degree of conditioning, there is a great risk that the maladaptive memory will be preserved or even strengthened by attempts to block its reconsolidation or potentiate its extinction. Agents that dissociably potentiate extinction but not reconsolidation, or impair reconsolidation but not extinction may therefore be particularly useful in the treatment of maladaptive memory disorders.