Post-retrieval propranolol treatment does not modulate reconsolidation or extinction of ethanol-induced conditioned place preference

Abstract

The reconsolidation hypothesis posits that established emotional memories, when reactivated, become labile and susceptible to disruption. Post-retrieval injection of propranolol (PRO), a nonspecific β-adrenergic receptor antagonist, impairs subsequent retention performance of a cocaine- and a morphine-induced conditioned place preference (CPP), implicating the noradrenergic system in the reconsolidation processes of drug-seeking behavior. An important question is whether post-retrieval PRO disrupts memory for the drug-cue associations, or instead interferes with extinction. In the present study, we evaluated the role of the β-adrenergic system on the reconsolidation and extinction of ethanol-induced CPP. Male DBA/2J mice were trained using a weak or a strong conditioning procedure, achieved by varying the ethanol conditioning dose (1 or 2 g/kg) and the number of ethanol trials (2 or 4). After acquisition of ethanol CPP, animals were given a single post-retrieval injection of PRO (0, 10 or 30 mg/kg) and tested for memory reconsolidation 24 h later. Also, after the first reconsolidation test, mice received 18 additional 15-min choice extinction tests in which PRO was injected immediately after every test. Contrary to the prediction of the reconsolidation hypothesis, a single PRO injection after the retrieval test did not modify subsequent memory retention. In addition, repeated post-retrieval administration of PRO did not interfere with extinction of CPP in mice. Overall, our data suggest that the β-adrenergic receptor does not modulate the associative processes underlying ethanol CPP.

Figure 1. Post-retrieval PRO treatment did not affect reconsolidation of a weak ethanol-induced conditioned place preference (CPP) in mice

Mean ± SEM time spent on the grid floor for the grid+ (G+) and grid− (G−) subgroups during the 15-min drug-free test (n=72). Following 2 conditioning trials with 1 g/kg of ethanol all mice expressed weak but reliable place preference (Fig. 1a; T1=retrieval test; first CPP test after conditioning). However, post-retrieval PRO treatment (10 or 30 mg/kg) did not affect the reconsolidation of a weak ethanol-induced CPP (Fig. 1b; T2=reconsolidation test; CPP test given after the retrieval test). The results of 4 days of consecutive preference tests (T2–T5) after 1 single PRO injection are presented in Figure 1c (Mean ± SEM percent time spent for all groups on ethanol-paired floor). These results suggest that PRO did not modulate the retrieval of ethanol-induced CPP despite repeated testing over several days. * Denotes a significant main effect of conditioning subgroup but no effect of treatment, p < 0.01).

Figure 2. Post-retrieval PRO treatment did not affect reconsolidation of a moderate ethanol-induced CPP in mice

Mean ± SEM time spent on the grid floor for the grid+ (G+) and grid− (G−) subgroups during the 15-min drug-free test (n=72). All groups showed preference following 2 ethanol (2 g/kg) conditioning trials (Fig. 2a; T1=retrieval test; first CPP test after conditioning). The effect of PRO after memory reactivation is depicted in Figure 2b (T2=reconsolidation test; CPP test given after the retrieval test). As is shown, PRO (10 or 30 mg/kg) did not block reconsolidation at either dose. This experiment also evaluated the effect of a single post-retrieval PRO injection during 4 subsequent preference tests. Performance on all 5 post-conditioning tests is depicted in Figure 2c (T2–T5) as mean ± SEM percent time spent on the ethanol-paired floor. As is shown, all groups (saline, 10 and 30 mg/kg) expressed conditioned preference over the course of 4 post-retrieval tests, after 1 single PRO injection. * Denotes a significant main effect of conditioning subgroup but no effect of treatment, p < 0.01).

Figure 3. Post-retrieval PRO treatment did not affect reconsolidation of a strong ethanol-induced CPP in mice

Data represent mean ± SEM time spent on the grid floor for the grid+ (G+) and grid− (G−) subgroups during the 15-min drug-free test (n=48). Both groups expressed a robust CPP after 4 conditioned trials with 2 g/kg of ethanol (Fig. 3a and Fig. 3c). Post-retrieval treatment of PRO (10 mg/kg) immediately after the first CPP test after conditioning (T1=retrieval test) did not disrupt the reconsolidation of ethanol-induced CPP (Fig. 3b and 3d; T2=reconsolidation test; CPP test given after the retrieval test). (HC): Refers to injections (saline or PRO) administered 3 h after T1 in their home cages. * Denotes a significant main effect of conditioning subgroup but no effect of treatment, p < 0.01).

Figure 4. Repeated post-retrieval PRO treatment did not interfere with extinction of ethanol-induced CPP in mice

Performance on 19 post-conditioning tests as percent time spent on the ethanol-paired floor is depicted in Figure 4 as mean ± SEM (n=48). Mice received repeated post-retrieval injections of saline immediately after memory reactivation and PRO 3 h later in their home cages (HC) (Saline-PRO 10 mg/kg) or an immediate injection of PRO and saline 3 hours later (PRO 10 mg/kg-Saline). As it can be seen, both groups expressed long lasting preference for the ethanol-paired compartment that progressively decreased over the course of 19 post-conditioning tests (T1–T19).