Michael G. Garelick and Daniel R. Storm* Department of Pharmacology, University of Washington, Seattle, WA 98195
A
memory must be acquired, stored, and accessible to be successful. Although memory is achieved through multiple phases, memory retrieval is the only way a memory can be measured. Beyond utilization of a stored memory, memory retrieval is also a dynamic process that triggers additional memory processing. In this issue of PNAS, Ouyang and Thomas (1) present evidence that memory retrieval, and not the cue that generates retrieval, is necessary to initiate memory extinction. Classical fear conditioning in rodents is a useful tool with which to study the memory processing at molecular and systems levels (2–4). One example, contextual fear conditioning (Fig. 1), is a hippocampus-dependent form of memory (5, 6). In this paradigm, a mouse is placed in a training cage (the context). The mouse receives a mild foot shock and learns to associate the context with the shock. When the animal is returned to the training context, memory for context is manifested as increased freezing in the training context, a characteristic fear response. The context is a cue for fear retrieval; if the mouse cannot retrieve the memory for context, it will not freeze. Animal models of classical fear conditioning have illustrated that different signaling components contribute to distinct phases of memory processing (2–4). In contextual fear conditioning, acquisition refers to the pairing of the context to the shock. The stabilization of the fear association into long-term memory is referred to as consolidation (2–4). Retrieval is the utilization of the fear memory. Retrieval is an important aspect of memory processing because it is the only way a memory can be measured. However, retrieval is also a dynamic aspect of memory that can trigger further rounds of processing. One such process is reconsolidation, where retrieval triggers a time interval when the original memory becomes labile and is restabilized (7, 8). Another process that occurs during retrieval is extinction (9). Extinction is evoked by the same presentation that triggers contextual memory retrieval (Fig. 1 A) (8–10). Memory extinction is a process in which a conditioned response gradually diminishes over time as an animal learns to uncouple a response from a stimulus (9). With contextual fear, www.pnas.org兾cgi兾doi兾10.1073兾pnas.0504017102
extinction occurs when the mouse is placed into the context without shock after training. With exposure to the context in the absence of additional shocks, the fear response diminishes. Extinction is an active process; contextual fear memory remains robust for months (11), whereas extinction occurs over days, hours, or even minutes (8–10). Extinction is not accelerated forgetting; the original association remains at least partially intact. Years of study have established extinction as a distinct learning process (9). After fear memory retrieval, two seemingly opposing processes take place. Reconsolidation serves to preserve memory for context, whereas extinction serves to reduce contextual memory. Although reconsolidation and extinction are separate processes relying on distinct signaling mechanisms (10), both rely on protein synthesis (7–10). Studies using protein synthesis inhibitors have shown that the nature of the retrieval session can determine which process will be dominant. Extinction dominates over reconsolidation when retrieval sessions are longer and spaced close together (8–10). Does extinction learning occur because the animal recalls the fear association? Because extinction learning modifies the fear response, it may seem obvious that memory retrieval triggers extinction learning. But it is conceivable that extinction learning can occur independently. In this case, it would be similar to a phenomenon known as latent inhibition (Fig. 1B) (9, 12). Latent inhibition occurs when the animal is exposed to the context before training. This exposure reduces the fear response to the context after training. If extinction and latent inhibition are manifestations of the same mechanism, then exposure to context without a shock will reduce fear response, regardless of whether the animal experiences contextual fear retrieval. In this case, the reduced fear response seen after extinction learning could be a result of weighing memory where context is aversive against memory where context is neutral (Fig. 1 C and D). It has been difficult to address the interaction between retrieval and extinction because it is hard to dissociate the two experimentally. For example, administration of inhibitors of phosphatidylinositol 3 (PI3)-kinase to a trained mouse before placing it into context blocks fear retrieval (13). If a PI3-kinase inhibitor is adminis-
Fig. 1. Contextual fear conditioning pairs context (rectangles) with shock (bolts). (A) The mouse is placed into context and receives a shock. Retrieval occurs when the mouse is again placed in context and exhibits fear response (!!!). Extinction reduces the response (!). (B) Latent inhibition also reduces fear response. (C) If extinction and latent inhibition share the same mechanism, then context triggers extinction independent of retrieval. (D) Model supported by Ouyang and Thomas (1), where extinction is triggered by memory retrieval.
tered after placement into the context, retrieval is spared, and extinction is blocked. With such a manipulation, there is no way to disrupt retrieval while leaving extinction intact. Similarly, disruption of PKA or extracellular signal-regulated kiSee companion article on page 9347. *To whom correspondence should be addressed. E-mail:
[email protected]. © 2005 by The National Academy of Sciences of the USA
PNAS 兩 June 28, 2005 兩 vol. 102 兩 no. 26 兩 9091–9092
COMMENTARY
The relationship between memory retrieval and memory extinction
nase (ERK)兾mitogen-activated protein kinase (MAPK) signaling disrupts both retrieval and extinction learning (14, 15). Ouyang and Thomas (1) devised a clever way around this problem by disrupting -adrenergic signaling in the brain. Previous work from the Thomas laboratory demonstrated that -adrenergic signaling in the hippocampus is required for contextual memory retrieval (16). The disruption of the dopamine hydroxylase (dbh) gene prevents the synthesis of norepinephrine (NE) (17). NE can be restored in dbh⫺/⫺ mice by treatment with L-threo-3,4-dihydroxyphenyl serine (L-DOPS) (18). Although dbh⫺/⫺ mice lack NE, heterozygous littermates (dbh⫹/⫺) have normal NE production. Thus, the dbh⫺/⫺ mouse provides a system where NE signaling can be specifically manipulated. The dbh⫺/⫺ mice were trained for contextual and cued fear conditioning (16). When trained for cued memory, the dbh⫺/⫺ mice display short- and long-term fear memory that is similar to that of their dbh⫹/⫺ littermates. With respect to hippocampus-dependent contextual fear memory, dbh⫺/⫺ mice have a specific deficit. Short-term contextual fear memory is intact up to 2 h after training, and longterm memory beyond 5 days is intact. Between 2 h and 5 days after training, however, the dbh⫺/⫺ mice display impaired contextual memory compared with their dbh⫹/⫺ littermates. Although restoration of NE with L-DOPS during contextual training partially improves memory during this period in dbh⫺/⫺ mice, treatment with L-DOPS before testing retrieval completely rescues contextual memory during this period. The fact that short-term and longerterm memories are spared in dbh⫺/⫺ mice suggests that acquisition and consolidation are not affected by lack of NE signaling. The transient nature of the deficit, along with the ability of NE restoration during context exposure to rescue memory, suggests that NE is required for retrieval. Because contextual but not cued fear conditioning is affected in the mice, the role for NE seems limited to retrieval of hippocampus-dependent memory. Further experiments demonstrated that NE signals through 1-adrenergic receptors in the brain during retrieval.
Pharmacological blockade of -adrenergic receptors in heterozygous dbh⫹/⫺ contextually trained mice disrupts retrieval, and 1 receptor agonists rescue contextual retrieval in dbh⫺/⫺ mice. Cannulation experiments demonstrated that 1 activity in the dorsal hippocampus is required for contextual retrieval and that a 1 agonist applied to the dorsal hippocampus can rescue retrieval in dbh⫺/⫺ mice. In addition, 1 receptor knockout mice display similar deficits in contextual fear retrieval. Similarly, 1 signaling is required for memory retrieval in the Morris water maze, a hippocampus-dependent spatial memory task, implying a general requirement of 1 signaling in retrieval of hippocampus-dependent memory. Ouyang and Thomas (1) used this system to demonstrate that extinction processing relies on the retrieval of a previously formed memory. First, they demonstrated that mice experiencing contextual fear retrieval undergo extinction, which is true for both dbh⫹/⫺ mice and dbh⫺/⫺ mice treated with a 1 agonist when exposed to context to rescue retrieval. When retrieval is blocked in context, either by treatment of a  receptor antagonist to dbh⫹/⫺ mice or lack of treatment with dbh⫺/⫺ mice, no extinction learning is apparent during subsequent context retrieval. Ouyang and Thomas (1) also demonstrated that extinction learning is independent of NE signaling. In cued fear conditioning, a hippocampus-independent learning task (6), untreated dbh⫺/⫺ mice undergo retrieval and extinction similar to their dbh⫹/⫺ littermates. Furthermore, untreated dbh⫺/⫺ mice show normal contextual extinction when exposed to context 5 days after training, the period where contextual retrieval no longer requires NE. Although the treatment of  receptor antagonists up to 3 h after retrieval impairs extinction in dbh⫹/⫺ mice, 1 activation only rescues extinction in dbh⫺/⫺ mice during context exposure, when retrieval is rescued as well. These data support the notion that extinction depends on memory retrieval. Extinction occurs without NE signaling when NE is not critical for retrieval. During the time period where NE is critical for contextual retrieval, extinction is only rescued by  receptor activation when
retrieval also is rescued. The alternative interpretation that NE is required for acquisition of contextual extinction is less likely, given that the transient requirement of NE for extinction overlaps the requirement for NE in retrieval. Even taken conservatively, these results demonstrate that extinction is distinct from latent inhibition. The signaling events necessary for extinction depend on the temporal relationship between context training and context reexposure. The requirement of NE for extinction is transient, lasting only 2 h to 5 days after training. Because the mechanism of extinction depends on how recently training occurred, it is not completely independent of the original fear association. In contrast, latent inhibition takes place independent of training, because training occurs after latent inhibition is acquired. Similarly, it has been demonstrated that trained mice show greater activation of the ERK兾MAPK and PI3-kinase pathways in the hippocampus after exposure to context than mice that did not receive a shock, suggesting that hippocampal processing in context can be affected by the original fear memory (13). Because 1 agonists applied to the dorsal hippocampus rescue contextual fear retrieval in dbh⫺/⫺ mice, Ouyang and Thomas (1) tested to see whether this treatment rescues extinction during the NE-dependent period. Surprisingly, it did not. Furthermore, infusion of 1 agonist into the central hippocampus rescued extinction but did not rescue the freezing response in dbh⫺/⫺ mice. Because this dissociation occurs when retrieval is sensitive to NE, the researchers favor the interpretation that retrieval can trigger extinction independently of the behavioral response. These results have important implications for the study of memory in animal models. Because retrieval is the only way to measure memory, it is important to consider the experimental definition of retrieval. In animal models, retrieval is almost always assessed by the behavioral response. The idea that retrieval may occur independently from behavior suggests that retrieval may be considered not only in terms of the behavioral response but also in its relationship to extinction and reconsolidation.
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