Propranolol-induced Impairment of Contextual Fear Memory Reconsolidation in Rats: A similar Effect on Weak and Strong Recent and Remote Memories

Abstract

Introduction: Previous studies have demonstrated that the β-adrenergic receptor antagonist propranolol impairs fear memory reconsolidation in experimental animals. There are experimental parameters such as the age and the strength of memory that can interact with pharmacological manipulations of memory reconsolidation. In this study, we investigated the ability of the age and the strength of memory to influence the disrupting effects of propranolol on fear memory reconsolidation in rats.

Methods: The rats were trained in a contextual fear conditioning using two (weak training) or five (strong training) footshocks (1mA). Propranolol (10mg/kg) injection was immediately followed retrieval of either a one-day recent (weak or strong) or 36-day remote (weak or strong) contextual fear memories.

Results: We found that propranolol induced a long-lasting impairment of subsequent expression of recent and remote memories with either weak or strong strength. We also found no memory recovery after a weak reminder shock. Furthermore, no significant differences were found on the amount of memory deficit induced by propranolol among memories with different age and strength.

Discussion: Our data suggest that the efficacy of propranolol in impairing fear memory reconsolidation is not limited to the age or strength of the memory.

Keywords: Fear Conditioning; Memory Age; Memory Reconsolidation; Memory Strength; Propranolol.

Figure 1

The strength of contextual fear memories acquired with two or five footshocks (FS). A: Schematic of the experimental design. Separate groups of rats received 2FS or 5FS. Two days after training, they received extinction sessions. At the end of the extinction session, rats trained with 5FS had significantly more freezing (B). *P < 0.05, *P < 0.01 as compared with weak training at the same test.

Figure 2

Post-reactivation administration of propranolol impairs reconsolidation of recent contextual fear memories with either weak or strong strength. A: Schematic of the experimental design. Rats received either two or five footshocks. One day after training, the memory was reactivated with exposure of rats into the same conditioning box for 90s and immediately followed by saline or propranolol injections. Long-term memory was tested one (Test1), two (Test2), and three (Test3) days after memory reactivation. Propranolol impaired long-term memory with either weak (B) or strong strength (C). Application of a weak reminder shock did not recover the original memory.*P < 0.05, **P < 0.01 as compared with saline-treated animals at the same test.

Figure 3

Post-reactivation administration of propranolol impairs reconsolidation of remote contextual fear memories with either weak or strong strength. A: Schematic of the experimental design. Rats received either two or five footshocks. 36 days after training, the memory was reactivated with exposure of rats into the same conditioning box for 90s and immediately followed by saline or propranolol injections. Long-term memory was tested one (Test1), two (Test2), and three (Test3) days after memory reactivation. Propranolol impaired long-term memory with either weak (B) or strong (C) strength. Application of a weak reminder shock did not recover the original memory.*P < 0.05, **P < 0.01 as compared with saline-treated animals at the same test.

Figure 4

Propranolol does not impair retention of non-reactivated fear memories. A: Schematic of the experimental design. Rats received two or five footshocks. 1 or 36 days after training, saline or propranolol was injected in the absence of memory reactivation and tested one day later. Propranolol did not impair long-term memory in any experimental groups (B). NR: Noreactivation.