Effect of intensity of unconditional stimulus on reconsolidation of contextual fear memory

Abstract

Memory reconsolidation is ubiquitous across species and various memory tasks. It is a dynamic process in which memory is modified and/or updated. In experimental conditions, memory reconsolidation is usually characterized by the fact that the consolidated memory is disrupted by a combination of memory reactivation and inhibition of protein synthesis. However, under some experimental conditions, the reactivated memory is not disrupted by inhibition of protein synthesis. This so called "boundary condition" of reconsolidation may be related to memory strength. In Pavlovian fear conditioning, the intensity of unconditional stimulus (US) determines the strength of the fear memory. In this study, we examined the effect of the intensity of US on the reconsolidation of contextual fear memory. Strong contextual fear memory, which is conditioned with strong US, is not disrupted by inhibition of protein synthesis after its reactivation; however, a weak fear memory is often disrupted. This suggests that a US of strong intensity can inhibit reconsolidation of contextual fear memory.

Keywords: Boundary condition; Contextual fear memory; Memory strength; Protein synthesis inhibitor; Reconsolidation.

Fig. 1

Administration of protein synthesis inhibitor blocks the consolidation of contextual fear memory. (A) Experimental design. (B) Effect of protein synthesis inhibito on consolidation of contextual fear memory. Bars represent the means±SEM of freezing levels assessed 24 h after conditioning. Compared to the vehicle group, the anisomycin group showed impaired freezing level (vehicle, n=9; anisomycin, n=8, unpaired t test, ^**p=0.0060).

Fig. 2

Protein synthesis inhibitor has differential effects on reconsolidation of contextual fear memory according to memory strength. (A) Experimental design. (B) Effect of protein synthesis inhibitor on reconsolidation of weak contextual fear memory (lest panel) and strong fear memory (right panel). Bars represent the means±SEM of freezing levels assessed at reactivation and 24 h after reactivation. Protein synthesis inhibitor blocks reconsolidation of weak fear memory, but not strong fear memory (left panel, vehicle, n=12; anisomycin, n=12; 2-way ANOVA test, ^*p<0.05) and strong contextual fear memory (vehicle, n=6; anisomycin, n=6; 2-way ANOVA test, p>0.05).