Reconsolidation of a long-term memory in Lymnaea requires new protein and RNA synthesis and the soma of right pedal dorsal 1

Abstract

Reconsolidation of a long-term memory (LTM) in the snail Lymnaea stagnalis can be disrupted by cooling, an RNA synthesis blocker (actinomycin D), and by specifically ablating the soma of a cell we know is a site of LTM consolidation (right pedal dorsal 1, RPeD1). Aerial respiratory behavior was conditioned operantly by applying a gentle tactile stimulus to the pneumostome area (the respiratory orifice) every time the snail began to open its pneumostome to perform aerial respiration. This resulted in a reduction of this behavior while leaving cutaneous respiration intact. One week after training one-half of the animals received a memory reactivation session, which was similar to the original training (i.e., animals received reinforcement). All animals then received 1 hr of cooling, an injection of actinomycin D or saline, or the soma ablation procedure. This was followed by a test for savings 4 hr or 4 d later, which was also similar to the original training. Only those animals that received both the memory reactivation session and the treatment showed memory impairment during the test for savings. That is, the impairment was contingent on memory reactivation. These data indicate that reconsolidation requires both new RNA and protein synthesis to stabilize a reactivated memory, and it demonstrates that the soma of RPeD1, a cell that we have shown previously to be required in the consolidation of an LTM, is necessary for reconsolidation. These data suggest that the critical molecular processes occurring during both consolidation and reconsolidation transpire in the same cell in Lymnaea.

Figure 1.

Four training sessions induce a 7 d memory. Operant training was administered (TR1-TR4) over the course of 2 d (n = 24). Animals demonstrated memory during the memory recall (MR) session 7 d later. Memory was assessed again 4 hr after MR (TS), and the animals still demonstrated memory. Animals that received yoked training did not demonstrate memory; ^*p < 0.01 as compared with MR.

Figure 2.

Cooling blocks the reconsolidation of a memory that has been reactivated. Operant training was administered (TR1-TR4) over the course of 2 d (n = 33). Then 7 d later 19 animals received a MR session (A), immediately followed by 1 hr of cooling (denoted by box), whereas the remaining animals (B; n = 14) received only the 1 hr of cooling. The group that received a MR session in conjunction with cooling did not demonstrate memory as tested 4 hr later. The remaining animals that did not receive a MR session but still received 1 hr of cooling still demonstrated memory during the test for savings; ^*p < 0.01 as compared with MR.

Figure 3.

Actinomycin D blocks the reconsolidation of a memory that has been reactivated. Operant training was administered (TR1-TR4) over the course of 2 d (n = 55). Then 7 d later 29 animals received a MR session (A), immediately followed by an injection of either ActD (n = 16) or saline (n = 13). The remaining animals (n = 26) did not receive a MR session (B) but still were administered either ActD (n = 12) or saline (n = 14). The group that received a MR session in conjunction with ActD did not demonstrate memory as tested 4 hr later. Saline-injected animals that also received a MR session did show memory 4 hr later. The remaining animals did not receive a MR session but still were injected with either ActD (n = 12) or saline (n = 14). Both groups showed memory 4 hr later during the test for savings; ^*p < 0.01 as compared with MR.

Figure 4.

Soma of RPeD1 is required for reconsolidation. Operant training was administered (TR1-TR4) over the course of 2 d (n = 19). Then 3 d later nine animals received a MR session (A), immediately followed by the ablation of the soma of RPeD1 (SoAb), whereas the remaining animals (B; n = 10) did not receive a MR session but still underwent the ablation procedure. Animals were tested for savings 4 d later. Memory was not observed in the group that received the MR session in conjunction with the ablation procedure. Memory was observed in the other soma-ablated group, the one that did not receive a MR session; ^*p < 0.01 as compared with MR.