A switch from cycloheximide-resistant consolidated memory to cycloheximide-sensitive reconsolidation and extinction in Drosophila

Abstract

It is generally accepted that, after learning, memories stabilize over time and integrate into long-term memory (LTM) through the process of consolidation, which depends on de novo protein synthesis. Besides, studies on several species have shown that reactivation of already stabilized LTM can either make this memory labile and then restabilize it (a process called reconsolidation) or inhibit it (extinction). However, the identity of both processes and their interactions with consolidation are still under debate. Regarding memory stabilization, Drosophila offers a striking exception since, in this species, LTM is not the sole stable form of memory. Under specific learning conditions, anesthesia-resistant memory (ARM) can be formed through processes yet unknown but that are resistant to cycloheximide, a classical protein synthesis inhibitor that impairs LTM. Here, we took advantage of this dichotomy to ask whether both ARM and LTM could be extinguished and/or reconsolidated. We also studied whether two forms of memory extinction and reconsolidation exist in flies, as for memory stabilization. We show that either reconsolidation or extinction can be induced after olfactory conditioning in Drosophila, depending on the number of reactivations as in other species. Furthermore, regarding the effect of cycloheximide, the ARM/LTM dichotomy for stabilization does not apply to extinction and reconsolidation. Blocking protein synthesis interfered with both processes regardless of whether initial stabilization was sensitive (LTM) or not (ARM) to cycloheximide. These results thus show that Drosophila is a useful model to tackle these questions and that reconsolidation is not necessarily a mere repetition of consolidation.

Figure 1.

CXM sensitivity following a massed or spaced training acquisition protocol. Spaced protocol: memory was tested 24 h after conditioning (CXM just before conditioning: n = 44 per group) or 48 h after conditioning (CXM 24 h after conditioning: n = 36 per group). Massed protocol: memory was tested 24 h after conditioning (n = 16 per group) or 48 h after conditioning (n = 16 per group) (**p < 0.01). Error bars show SEM.

Figure 2.

Effect of reactivation on stability of consolidated memory after a massed (A) or a spaced (B) protocol. A, Zero, one, three, or five cycles of CS+ re-exposure after massed conditioning (control group: n = 16 per group; CXM-treated group: n = 15–16 per group) B, Zero, one, three or five cycles of CS+ re-exposure 24 h after spaced conditioning (control group: 0 cycle, n = 36; 1 cycle, n = 12; 3 cycles, n = 12; 5 cycles, n = 20; CXM-treated group: 0 cycle, n = 36; 1 cycle, n = 12; 3 cycles, n = 12; 5 cycles, n = 19) (*p < 0.05; **p < 0.01). Error bars show SEM.

Figure 3.

Specificity of moderate reactivation. Effect of 3 CS+ or CS− exposure 24 h after a massed conditioning (ARM). CXM treatment is administrated just before reactivation (3CS+: n = 8 per group; 3CS−: n = 8 per group) (**p < 0.01). Error bars show SEM.

Figure 4.

Specificity of intense reactivation. Effect of 5 CS+ or CS− exposure 24 h after spaced (n = 8 per group) or massed conditioning (n = 8 per group) (**p < 0.01). Error bars show SEM.