Influence of reconsolidation in maintenance of cocaine-associated contextual memories formed during adolescence or adulthood

Abstract

Adolescents are at increased risk to develop substance use disorders and suffer from relapse throughout life. Targeted weakening of drug-associated memories has been shown to reduce relapse-like behavior in adult rats, however this process has been understudied in adolescents. We aimed to examine whether adolescent-formed, cocaine-associated memories could be manipulated via reconsolidation mechanisms. To accomplish this objective, we used an abbreviated operant cocaine self-administration paradigm (ABRV Coc-SA). Adult and adolescent rats received jugular catheterization surgery followed by ABRV Coc-SA in a distinct context for 2 h, 2×/day over 5 days. Extinction training (EXT) occurred in a second context for 2 h, 2×/day over 4 days. To retrieve cocaine-context memories, rats were exposed to the cocaine-paired context for 15 min, followed by subcutaneous injection of vehicle or the protein synthesis inhibitor cycloheximide (2.5 mg/kg). Two additional EXT sessions were conducted before a 2 h reinstatement test in the cocaine-paired context to assess cocaine-seeking behavior. We find that both adult and adolescent cocaine-exposed rats show similar levels of cocaine-seeking behavior regardless of post-reactivation treatment. Our results suggest that systemic treatment with the protein synthesis inhibitor cycloheximide does not impair reconsolidation of cocaine-context memories and subsequent relapse during adulthood or adolescence.

Figure 1

Schematic and timeline of behavioral experiments to examine reconsolidation of cocaine-context memories in adult and adolescent cocaine-exposed rats. All experiments included abbreviated cocaine self-administration training (Coc-SA, 2 h sessions, 2×/day over 5 days, minimum of 10 sessions) in a distinct context, followed by extinction training (EXT, 2 h sessions, 2×/day over 4 days, minimum of 8 sessions). On average, adolescent rats started Coc-SA at P38 and adults at P70. The average age at the start of EXT was P45 for adolescent rats and P76 for adult rats, due to differences in the number of sessions needed to acquire Coc-SA criteria. To retrieve cocaine-associated contextual memories, rats were returned to the previous cocaine-paired context for memory reactivation (MR, 15 min session), followed by a subcutaneous injection of vehicle or 2.5 mg/kg cycloheximide (VEH, CHX) immediately after the MR session. Two additional sessions of EXT were given 1 day after MR (post-MR EXT), followed by a reinstatement test in the previous cocaine-paired context (Test, 2 h).

Figure 2

Effect of age on behavioral phases. Mean ± SEM of active and inactive lever responses for adult and adolescent rats during (A) cocaine self-administration (Coc-SA, 2 h sessions, 2×/day over 5 days), (B) extinction (EXT, 2 h sessions, 2×/day over 4 days), (C) memory reactivation (MR, 15 min session) in the previous cocaine-paired context to retrieve cocaine-associated memories. Rats received subcutaneous injection of vehicle or 2.5 mg/kg cycloheximide (VEH, CHX) immediately after the MR session. (D) Two additional sessions of EXT were given 1 day after MR (post-MR EXT), followed by a reinstatement test in the previous cocaine-paired context (Test, 2 h). (E) Active and inactive lever responses during 20 min Bins of the 2 h Test. Symbols indicate significant within-subject differences revealed by Tukey’s test (B) ^#p < 0.01: EXT session 1 > 8, (D) final post-MR EXT session < Test, (E) 20 min Test Bin 1 > 6. Groups denoted by: Blue = adult (n = 16), Orange = adolescent (n = 14), Light blue and Light orange individual points = VEH, Black individual points = CHX.

Figure 3

Effect of cycloheximide on reconsolidation of cocaine-context memories in adult cocaine-exposed rats. Mean ± SEM of active and inactive lever responses for adult rats during (A) cocaine self-administration (Coc-SA, 2 h sessions, 2×/day over 5 days), (B) extinction (EXT, 2 h sessions, 2×/day over 4 days), (C) memory reactivation (MR, 15 min session) in the previous cocaine-paired context to retrieve cocaine-associated memories. Adult rats received subcutaneous injection of vehicle or 2.5 mg/kg cycloheximide (VEH, CHX) immediately after the MR session. (D) Two additional sessions of EXT were given 1 day after MR (post-MR EXT), followed by a reinstatement test in the previous cocaine-paired context (Test, 2 h). (E) Active and inactive lever responses during 20 min Bins of the 2 h Test. Symbols indicate significant within-subject differences revealed by Tukey’s test (B) ^#p < 0.01: EXT session 1 > 8, (D) final post-MR EXT session < Test, (E) 20 min Test Bin 1 > 6. Groups denoted by: Light blue = adult VEH (n = 8), Black = adult CHX (n = 8).

Figure 4

Effect of cycloheximide on reconsolidation of cocaine-context memories in adolescent cocaine-exposed rats. Mean ± SEM of active and inactive lever responses for adolescent rats during (A) cocaine self-administration (Coc-SA, 2 h sessions, 2×/day over 5 days), (B) extinction (EXT, 2 h sessions, 2×/day over 4 days), (C) memory reactivation (MR, 15 min session) in the previous cocaine-paired context to retrieve cocaine-associated memories. Adolescent rats received subcutaneous injection of vehicle or 2.5 mg/kg cycloheximide (VEH, CHX) immediately after the MR session. (D) Two additional sessions of EXT were given 1 day after MR (post-MR EXT), followed by a reinstatement test in the previous cocaine-paired context (Test, 2 h). (E) Active and inactive lever responses during 20 min Bins of the 2 h Test. Symbols indicate significant within-subject differences revealed by Tukey’s test (B) ^#p < 0.01: EXT session 1 > 8, (D) ^#p < 0.01, active; ^#p < 0.05, inactive: final post-MR EXT session < Test (E) ^#p < 0.01, active; ^#p < 0.05, inactive: 20 min Test Bin 1 > 6. Groups denoted by: Light orange = adolescent VEH (n = 6), Black = adolescent CHX (n = 8).