The role of protein synthesis during the labile phases of memory: revisiting the skepticism

Abstract

Despite the fact that extensive evidence supports the view that phases of de novo protein synthesis are necessary for memory formation and maintenance, doubts are still raised. Skeptics generally argue that amnesia and the disruption of long-term synaptic plasticity are caused by "non-specific effects" of the reagents or approaches used to disrupt protein synthesis. This paper attempts to clarify some of these issues by reviewing, discussing and providing results addressing some of the major critiques that argue against the idea that de novo protein synthesis is necessary for the stabilization of long-term memory.

Figure 1

Time course of the effect of the ERE decoy following 5-HT treatment. Bar graph representing the percentage of change ±b SEM in EPSP amplitude recorded 24 hr after 5 pulses of 5-HT from cocultures injected with the C/EBP binding site ERE oligonucleotide at the indicated times after the end of 5-HT applications (From Alberini et al., 1994).

Figure 2

C/EBPbeta protein is selectively induced after inhibitory avoidance training. Western blot analyses were performed with anti-C/EBP antiserum. A, Western blot immunostaining of hippocampal extracts from rats killed immediately (0 h) and 9, 20, 28, 48, and 72 hr after training. Groups of animals either (1) underwent full training (+) or (2) entered the IA apparatus but received no shock (−). B, Densitometric analysis of C/EBPbeta Western blot depicted in A revealed a significant increase in C/EBPbeta protein at 9 hr (p < 0.05), 20 hr (p < 0.01), and 28 hr (p < 0.01) compared with 0 h- control rats. There were no significant differences in any of the time-paired no shock control groups. Data are expressed as mean percentage ± SEM of the 0 h (100%) control mean values (From Taubenfeld et al., 2001a).

Figure 3

C/EBPbeta antisense blocks long-term memory consolidation. (a) Injection, training and testing time points. (b) IA acquisition (Acq) and memory retention of unoperated rats. Retention was assessed 48 h after training by measuring the latency to reenter a context previously paired with a foot-shock. A single training event leads to a significant increase in latency (n = 16, *** p < 0.0001). (c) Hippocampal injection of C/EBPbeta antisense (-ODN) 5 h (n = 12) or 24 h (n = 8) after IA training significantly blocks memory retention at 48 h compared to SC-ODN-injected or unoperated groups (*** p < 0.0001). Injection of SC-ODN sequence or vehicle (PBS) at same time points has no effect. Because both controls produced similar retention latencies, the behavioral data of the two groups have been combined (SC/PBS) (5 h, n = 9, SC-ODN, n = 5 and PBS, n = 4; 24 h, n = 8, SC-ODN, n = 4 and PBS, n = 4). Injection of -ODN 1 h before (n = 9) or 46 h after (n = 7) training does not produce any significant memory impairment (From Taubenfeld et al., 2001b).

Figure 4

Protein synthesis is required for the induction of mCPP. A–C, Values are expressed as mean ± SEM time spent in the drug-conditioned chamber. *p < 0.05; **p < 0.01. A, Groups of rats were conditioned for 4 d to either vehicle (n = 8; white bars) or morphine (n = 15). At the end of conditioning, the morphine-conditioned rats were injected subcutaneously with either vehicle (n = 8; gray bars) or inhibitor (n = 7, of which n = 4 with cycloheximide and n = 3 with anisomycin; black bars). All rats were tested 24 h and 1 week later. B, Rats received daily administration of either cycloheximide (n = 8; black bars) or vehicle (n = 8; white bars) immediately after each conditioning session. CPP was tested 24 h, 1 week, and 4 weeks later. C, Rats were conditioned to vehicle instead of morphine and administered cycloheximide (n = 9; black bars) or vehicle (n = 11; white bars) immediately after each conditioning session. CPP was tested after 24 h. VE or Ve, Vehicle; MORP or morp, morphine; w, week; Post-cond., postconditioning; Cy, cycloheximide (From Milekic et al., 2006).

Figure 5

An established mCPP is disrupted by protein synthesis inhibitors administered after a single conditioning session. A–D, Values are expressed as mean ± SEM time spent in the drug-conditioned chamber. **p < 0.01; ***p < 0.001. A, Rats were conditioned to morphine and 1 week later received one additional conditioning session (1X CONDITIONING). Immediately after, one-half of the animals were injected with cycloheximide or anisomycin (inhibitors; n = 10, of which n = 6 with cycloheximide and n = 4 with anisomycin; black bars), and the other half were injected with vehicle (n = 8; white bars). Rats were tested 24 h and 1 week later (Post-1X cond.). B, Animals were conditioned and 1 week later received one additional conditioning (1X CONDITIONING) as described in A, followed by two injections of cycloheximide (CyX2; n = 10; n = 7 in 4 week test; black bars) or vehicle (VeX2; n = 11; white bars) 5 h apart. Rats were tested 24 h and 1, 2, and 4 weeks later. C, Rats received one morphine conditioning in the spontaneously preferred context followed by two injections of either cycloheximide (CyX2; black bars; n = 8) or vehicle (VeX2; white bars; n = 8) and were tested 24 h later. D, The conditioning context was omitted. Rats were conditioned for 4 d and 1 week later received one morphine treatment in their home cage. Thirty minutes later, they received two injections of either cycloheximide (CyX2; black bars; n = 13) or vehicle (VeX2; white bars; n = 12). CPP was tested 24 h after injection. MORP, Morphine; w, week; Ve, vehicle (From Milekic et al., 2006).