Memory retrieval, a rapid reconstructive process involving a recapitulation of the learned information (Sara, 2000
; Wagner et al., 2005
), has not been explored with the same intensity as has memory acquisition and consolidation or reconsolidation. However, pioneering studies have shown that memory retrieval can be enhanced by the activation of β-adrenergic receptors (Sara and Devauges, 1988
; Devauges and Sara, 1991
). It has also been shown that learning and memory can be genetically enhanced (Tang et al., 1999
; Tang et al., 2001
; White and Youngentob, 2004
). Thus, it would be important to explore the means by which memories (e.g., traumatic war memories and unwanted fear memories) might be inducibly and selectively erased.
In the current study, we have chosen three different behavioral paradigms, namely novel object recognition memory, contextual fear memory, and cued fear memory, for the following reasons. First, the behavioral expression of novel object recognition memory (via active exploration) is quite opposite to the behavioral expression of fear memories (via freezing); therefore, the observed retrieval deficits were unlikely due to the deficits in motor performance of memory tasks. Second, all of these behavioral tests involve a single training session, and acquisitions of those memories are known to be dependent on NMDA receptor activation (Brown and Aggleton, 2001
; Falls et al., 1992
; Maren et al., 1996
; Rampon et al., 2000
). Third, the novel object recognition memory tends to only last for several days (Tang et al., 1999
), whereas contextual and cued fear memories in mice can last for at least 10 months (Cui et al., 2004
). This information provides an opportunity to compare these memory processes that are known to have distinct temporal durations. Finally, novel object recognition memory is known to engage in different neuronal circuits compared to the fear memories, whereas the contextual fear memory and cued fear memory share many of the fundamental circuits (Davis et al., 1987
), thereby allowing us to measure and directly compare whether the retrieval of those short-term and long-term memories shares any commonalities.
Our behavioral analyses illustrate that rapid memory retrieval can be unexpectedly manipulated by altering CaMKII activity using our chemical genetic method. Our previous biochemical studies show that NM-PP1 is highly specific to the genetically sensitized CaMKII-F89G and was completely ineffective against wild-type kinases, including αCaMKII (IC50
> 800 μM), CaMKIV (IC50
> 800 μM), PKC (IC50
> 800 μM), and CDK5 (IC50
> 300 μM) (Wang et al., 2003
; Cho et al., 2007
). Thus, this chemical genetic method has integrated the molecular and regional specificity of genetics with the high temporal resolution of chemical inhibition and is highly suitable for the analysis of memory retrieval.
The high-resolution, temporally controlled expression of αCaMKII-F89G activity during recall impairs the retrievals of both short-term and long-term memories, suggesting a common molecular process despite obvious differences in their synaptic and neuronal status (Gerard, 1949
; Squire, 1987
). Moreover, the retrieval deficits have been observed in several different memory tests, such as the novel object memory and fear memory tests, which are also likely to use somewhat different neural circuits to store and retrieve memory (Brown and Aggleton, 2001
; Rudy et al., 2005
). This further indicates a shared molecular process that is sensitive to the disruption by excessive CaMKII activity. More importantly, despite overlapping circuits involved in encoding contextual and cued fear memories, the selective erasure to contextual, but not cued, memory (or vice versa) strongly suggests that the erasure occurs at a highly specific set of synapses and circuits that define the specific features for contextual or cued memories.
The observed retrieval deficits are unlikely attributed to the potential locomotor-related performance problem caused by elevated CaMKII activity for the following reasons. First, the transgenic mice performed normally in the open-field measurement and rotarod tests. Second, the retrieval deficits in the transgenic mice have been assessed by two completely different behavioral measurements (e.g., novel object recognition memory, which involves the measurement of the preference in exploring the new or old objects, versus fear conditioning memories, which are measured by the amount of freezing time during recall). Third, in the case of fear conditioning, the freezing behaviors in the transgenic mice seem to be the same as that of the wild-type controls since the amount of immediate freezing is indistinguishable between the transgenic and control mice. Fourth, as shown in the cat odor exposure experiments, the transgenic mice indeed exhibit the same amount of fear freezing in comparison to that of their wild-type littermates. Finally, as shown in , Tg mice still had exhibited performance deficits in the second recall tests even when the transgenic CaMKII level was suppressed by NM-PP1 injection. This again suggests that retrieval impairment is not due to the motor/performance problems caused by CaMKII overexpression. Therefore, the above evidence collectively supports the interpretation that the presence of transgenic CaMKII-F89G activity is likely caused by the recall-induced erasure/degradation of the memory traces that are being retrieved.
In the literature, some memory deficits (e.g., induced by protein synthesis inhibitors) prove to be only temporary, and spontaneous recovery from initial memory deficits has been reported over the time course of several days or week(s) (Miller and Springer, 1974
; Lattal and Abel, 2004
). Interestingly, the memory degradation described here seems to be long lasting since the poor performances were still observed even when the interval between the first and second recall tests were increased to the 2 week duration ().
In another recent report, Shema et al. described a very interesting phenomenon in which posttraining injection of an inhibitor of the protein kinase M zeta into the insular cortex caused retrieval deficits of one-month conditioned taste aversion memory (Shema et al., 2007
). In that study, the authors show that application of the inhibitor at various time points (e.g., 3 days, 7 days, or 25 days after learning) is all capable of causing performance deficits in one-month retention tests (Shema et al., 2007
). In our case, memory retrieval deficit only occurs if transgenic αCaM-KII-F89G is expressed at the time of recall. On the other hand, the transient presence of transgenic αCaMKII-F89G enzyme at second, third, or fourth posttraining weeks had no effect on the recall of one-month contextual and cued fear memories as long as retrieval took place within the suppression of transgenic αCaMKII-F89G activity (Wang et al., 2003
). Furthermore, the inhibitor of protein kinase M zeta seems to need at least 2 hr to exert its effects on the stored memory, whereas the memory erasure described here is an extremely rapid process (within 1–3 min) at the time of memory recall. Finally, unlike the general effects of the protein kinase M zeta inhibitor on the multiple memories stored in the same region (two conditioned tests were used), the effects we observed here are highly restricted to the one that is undergoing active retrieval while leaving the unretrieved memory intact. Therefore, transgenic αCaMKII-F89G overexpression-mediated memory erasure takes place only when that memory is undergoing active recall.
What might be the cellular and physiological explanations that can account for the observed recall-induced retrieval deficits? It is interesting to note that the transgenic overexpression of αCaMKKII-F89G can alter the bidirectional synaptic plasticity response curve (Wang et al., 2003
). This is consistent with the role of the CaMKII in regulating LTP (Malinow et al., 1989
; Silva et al., 1992
; Barria et al., 1997
; Malenka and Nicoll, 1999
; Mayford et al., 1995
). Our previous measurement of synaptic physiology has revealed that, in addition to the larger LTP in response to 10 or 100 Hz stimulation, the transgenic hippocampus can produce a profound shift of LTD peak from 1 to 3 Hz. This shift in LTD peak response can be readily reversed by the addition of 5 μM NM-PP1 in the recording solution (Wang et al., 2003
It is tempting to speculate that the relation between the rapid induction of LTD response by 3 Hz frequency may play a role in memory erasure observed in the transgenic mice. The 3 Hz frequency stimulation usually does not produce changes in synaptic efficacy in the wild-type brain (Bear and Malenka, 1994
; Korz and Frey, 2005
). Many in vivo studies report that neurons often fire in the range of 3–12 Hz during recognition of spatial and nonspatial cues (O’Keefe and Dostrovsky, 1971
; Deadwyler et al., 1996
; Wood et al., 1999
; Lin et al., 2005
; Lin et al., 2006
; Quirk et al., 1992
; Sharp, 1999
). Therefore, it would be interesting to entertain the idea that, in the presence of the elevated CaMKII activity, 3 Hz-like firing patterns embedded in the retrieval neural process might have induced abnormal LTD-like responses in those memory circuits of the transgenic brains, leading to aberrant and selective degradation of the recalled memory traces. While this explanation is attractive, we currently do not know whether such altered LTD responses have also occurred in other brain regions.
It is also prudent to consider the theoretical possibility that increasing αCaMKII activity at the time of encoding may produce some sort of neurotoxicity to the cells involved in forming the memory trace. Our data seem to argue against this scenario: in both one-hour and one-month memory tests (see , ), the constitutive presence of transgenic αCaMKII-F89G at the time of learning and postlearning periods did not impair memory performance in the transgenic mice.
One future effort might be to define further the critical subregions in the forebrain where those memories might be stored and retrieved (Cui et al., 2004
; Wagner et al., 2005
; Frankland and Bontempi, 2005
; Rudy et al., 2005
). For example, it would be most ideal if our manipulation could be further restricted to a set of specific subsites, say, the lateral amygdala versus central amygdala, the anterior cingular cortex versus perirhinal cortex, etc. However, it is worthy to note that memory may require multiple regions to participate various aspects of memory components (Ribeiro et al., 2004
; Euston et al., 2007
; Tsien, 2007
), thereby requiring simultaneous manipulation of the transgenic activity in those subregions.
Given the fact that so many war veterans coming back from war zones often suffer from reoccurring traumatic memory replays, our report of selective erasure of fear memories in an inducible and rapid way suggests the existence of molecular paradigm(s) under which some traumatic memories can be erased or degraded while preserving other memories in the brain. However, we do not think that our chemical genetic approach in its current form can be applied directly as a clinical strategy since it would require the development of some sorts of αCaMKII-specific activators. On the other hand, it might be useful to further identify the downstream drugable targets through which overexpressed αCaMKII produces such an effect. In conclusion, we have shown that transient elevation of transgenic αCaMKII activity at the time of memory recall can cause rapid erasure of memory being retrieved. Our experiments have illustrated a molecular genetic paradigm through which a selected set of memories, such as new and old fear memories, can be rapidly and specifically erased in a controlled and inducible manner while leaving other memories intact in the brain.