Forgetting of long-term memory requires activation of NMDA receptors, L-type voltage-dependent Ca2+ channels, and calcineurin

Abstract

In the past decades, the cellular and molecular mechanisms underlying memory consolidation, reconsolidation, and extinction have been well characterized. However, the neurobiological underpinnings of forgetting processes remain to be elucidated. Here we used behavioral, pharmacological and electrophysiological approaches to explore mechanisms controlling forgetting. We found that post-acquisition chronic inhibition of the N-methyl-D-aspartate receptor (NMDAR), L-type voltage-dependent Ca(2+) channel (LVDCC), and protein phosphatase calcineurin (CaN), maintains long-term object location memory that otherwise would have been forgotten. We further show that NMDAR activation is necessary to induce forgetting of object recognition memory. Studying the role of NMDAR activation in the decay of the early phase of long-term potentiation (E-LTP) in the hippocampus, we found that ifenprodil infused 30 min after LTP induction in vivo blocks the decay of CA1-evoked postsynaptic plasticity, suggesting that GluN2B-containing NMDARs activation are critical to promote LTP decay. Taken together, these findings indicate that a well-regulated forgetting process, initiated by Ca(2+) influx through LVDCCs and GluN2B-NMDARs followed by CaN activation, controls the maintenance of hippocampal LTP and long-term memories over time.

Figure 1. Rats maintain the object location memory in 1 and 3, but not in 5 or 7 days after learning.

(A) Experimental design of the OL paradigm (top panel), and the exploration ratios during the training (white bars) and retention tests (blue bars) performed on days 1, 3, 5, or 7 after learning. Rats keep their memory until day 3, since they spent more time exploring the object that was switched into a new position. (B) The overall exploration activity in the test does not differ among all groups. Dotted line indicates similar exploration activity between both novel and familiar objects. Data expressed as mean ± SEM. ^*p < 0.5. All other comparisons were not significant (n = 4 to 6 per group).

Figure 2. Long-term memory forgetting is regulated through NMDAR-dependent signaling.

(A) Dose-dependent effect of the NMDAR antagonist memantine upon the test conducted 7 days after training in the OL task. Rats chronically treated with the high dose of memantine explored significantly more the displaced object compared to other groups. (B) The NMDAR antagonist MK801 prevent memory forgetting for either 7 or 10 days after learning in the same paradigm. (C) NMDAR inhibition also regulates the decay of the OR memory. White bars represent the performance in the training, and blue bars in the tests. (D) The overall exploration does not differ among the groups, indicating that memantine or MK801 have no effect on basal exploratory activity. Data expressed as mean ± SEM. *p < 0.5, **p < 0.01, ***p < 0.001 (n = 6 to 9 per group).

Figure 3. Hippocampal LTP decay in vivo is regulated by GluN2B-NMDAR.

Corresponding representative traces of anesthetized rats infused with vehicle or ifenprodil (GluN2B-NMDAR antagonist) 30 min after LTP induction. The plot presents CA1-evoked synaptic potentials recorded for 180 min (left panel). In this protocol, hippocampal LTP in the control group start to decay around 90 min after HFS, whereas the application of the GluN2B-NMDAR antagonist prevents the natural decay of LTP. Right panel shown the% of change of normalized amplitude in three distinct intervals (1 = baseline response; 2 = 50–60 min after HFS; 3 = 170–180 min after HFS) (n = 4 per group).

Figure 4. Ca2⁺ -dependent signaling through LVDCCs regulates long-term memory maintenance.

(A) Rats treated chronically with nimodipine, a LVDCC blocker, significantly spent more time exploring the new object location than the control group during the test conducted 7 d after training. White bars represent the performance in the training, and blue bars in the test. (B) The overall exploration does not differ between the groups. Data expressed as mean ± SEM. *p < 0.5, **p < 0.01 (n = 6 per group).

Figure 5. Calcineurin activation induces forgetting of the OL memory.

(A) Compared to saline-treated group, rats treated with FK506, an inhibitor of the protein phosphatase calcineurin, significantly spent more time exploring the new object location during the drug-free retention test conducted 7 d after learning. White bars represent the performance in the training, and blue bars in the test. (B) The overall exploratory activity indicates no differences among groups. Data expressed as mean ± SEM. *p < 0.5, **p < 0.01 (n = 8–9 per group).