Role of the ERK pathway in psychostimulant-induced locomotor sensitization

Abstract

Background: Repeated exposure to psychostimulants results in a progressive and long-lasting facilitation of the locomotor response that is thought to have implications for addiction. Psychostimulants and other drugs of abuse activate in specific brain areas extracellular signal-regulated kinase (ERK), an essential component of a signaling pathway involved in synaptic plasticity and long-term effects of drugs of abuse. Here we have investigated the role of ERK activation in the behavioral sensitization induced by repeated administration of psychostimulants in mice, using SL327, a brain-penetrating selective inhibitor of MAP-kinase/ERK kinase (MEK), the enzyme that selectively activates ERK.

Results: A dose of SL327 (30 mg/kg) that reduced the number of activated ERK-positive neurons by 62 to 89% in various brain areas, had virtually no effect on the spontaneous locomotor activity or the acute hyperlocomotion induced by cocaine or D-amphetamine. Pre-treatment with SL327 (30 mg/kg) prior to each drug administration prevented the locomotor sensitization induced by repeated injections of D-amphetamine or cocaine. The SL327 pre-treatment abolished also conditioned locomotor response of mice placed in the context previously paired with cocaine or D-amphetamine. In contrast, SL327 did not alter the expression of sensitized response to D-amphetamine or cocaine.

Conclusion: Altogether these results show that ERK has a minor contribution to the acute locomotor effects of psychostimulants or to the expression of sensitized responses, whereas it is crucial for the acquisition of locomotor sensitization and psychostimulant-conditioned locomotor response. This study supports the important role of the ERK pathway in long-lasting behavioral alterations induced by drugs of abuse.

Figure 1

Effects of SL327 on spontaneous locomotor activity. Locomotor activity was measured during 90 min after acute administration of vehicle or the MEK inhibitor SL327 (15, 30 and 50 mg/kg). Data were analyzed with one-way ANOVA (F_(3,92) = 10.57, P < 0.001) followed by Bonferroni test (** P < 0.01, SL327 vs vehicle). 24 mice per group.

Figure 2

Effects of SL327 on the locomotor activity induced by injection of D-amph or cocaine. Mice pre-treated with vehicle or SL327 (15, 30 and 50 mg/kg) were injected 30 min later with D-amph (2 mg/kg i.p., A, B) or cocaine (20 mg/kg i.p., C, D). A) Time course of D-amph-induced locomotor activity measured every 5 min is illustrated for mice pre-treated by Vehicle (Veh) or the highest doses of SL327 (30 mg/kg or 50 mg/kg) (10 mice per group). Time course obtained with 15 mg/kg SL327 that was similar to that obtained with 30 mg/kg SL327 is not shown for improving legibility of the figure. Data (30 and 50 mg/kg SL327 and vehicle) were analyzed with a mixed factor ANOVA (repeated measure over time) (effect of SL327 F_(2,216) = 43.55, P < 0.01). B) Total locomotor activity during the first hour after D-amph administration was analyzed with one-way ANOVA (F_(3,36) = 5.277, P < 0.01). * P < 0.01 Vehicle (Veh) vs SL327 (50 mg/kg). C) Time course of cocaine-induced locomotor activity measured every 5 min for mice pre-treated by Vehicle (Veh) or SL327 (30 mg/kg or 50 mg/kg) (10 mice per group). Time course obtained with 15 mg/kg SL327 was similar to that obtained with 30 mg/kg SL327 (not shown in the figure). Data (30 and 50 mg/kg SL327 and vehicle) were analyzed with a mixed factor ANOVA (repeated measure over time) (effect of SL327 F_(2,216) = 12.745, P < 0.05). D) Total locomotor activity during the first hour after cocaine administration was analyzed with one-way ANOVA (F_(3,36) = 1.058, NS).

Figure 3

Effects of SL327 on the induction and expression of locomotor sensitization by repeated administration of D-amph. A) Locomotor responses were measured during 5 consecutive days after administration of D-amph (2 mg/kg, i.p.) 30 min after pre-treatment with vehicle (Veh) or SL327 (30 mg/kg), as indicated (10 mice per group). After 10 days of withdrawal (day 15), the Veh/D-amph and SL327/D-amph groups received an additional challenge with Veh/D-amph or SL327/D-amph, respectively. The following day (day 16, Additional Challenge), mice treated previously with Veh/D-amph were challenged with D-amph in the presence of SL327 whereas those treated previously with SL327/D-amph received an injection of D-amph alone. B) Locomotor responses measured during 5 consecutive days after administration of saline 30 min after pre-treatment with vehicle (Veh) or SL327 (30 mg/kg), as indicated (10 mice per group). After 10 days of withdrawal (day 15), the Veh/saline and SL327/saline groups received an additional treatment with Veh/D-amph. Data were analyzed with a mixed factor ANOVA (repeated measure over time): for saline group (Days 1–5: effect of time: F_(4,90) = 2.15, NS; effect of SL327: F_(1,90) = 1.07, NS); for D-amph group (Days 1–15: effect of time: F_(6,126) = 13.48, P < 0.01; effect of SL327: F_(1,126) = 26.21, P < 0.01). Bonferroni test: * P < 0.01 compared to day 1, ° P < 0.01 Veh/D-amph vs SL327/D-amph.

Figure 4

Effects of SL327 on the induction and expression of locomotor sensitization by repeated administration of cocaine. A) Locomotor responses measured during 5 consecutive days after administration of cocaine (20 mg/kg, i.p.) 30 min after pre-treatment with vehicle (Veh) or SL327 (30 mg/kg), as indicated (10 mice per group). After 10 days of withdrawal (day 15), the Veh/cocaine and SL327/cocaine groups received an additional treatment with Veh/cocaine and SL327/cocaine, respectively. B) Locomotor responses measured during 5 consecutive days after administration of saline 30 min after pre-treatment with vehicle (Veh) or SL327 (30 mg/kg), as indicated (10 mice per group). After 10 days of withdrawal (day 15), the Veh/saline and SL327/saline groups received an additional treatment with Veh/cocaine. Data were analyzed with a mixed factor ANOVA (repeated measure over time): for saline group (effect of time: F_(4,90) = 1.66, NS; effect of SL327: F_(1,90) = 1.29, NS); for cocaine group (effect of time: F_(6,126) = 11.27, P < 0.01; effect of SL327: F_(1,126) = 18.27, P < 0.01). Bonferroni test: * P < 0.01 compared to day 1, ° P < 0.01 Veh/D-amph vs SL327/D-amph.

Figure 5

Effects of SL327 on the induction of a conditioned locomotor response to saline administration. A) Locomotor responses measured during the first 30 min after injection of saline in mice which have received repeated injections of saline or D-amph, preceded by pre-treatment by vehicle (Veh) or SL327 (30 mg/kg) 30 min before (see Fig 3). Saline injection was done in the same actimeter in which the mice had received the repeated injections. B). Analysis of conditioned response in mice chronically treated with cocaine (same protocol as in A except that mice had received repeated injections of cocaine, 20 mg/kg, instead of D-amph). Data were analyzed with one-way ANOVA (A): F_(3,36) = 8.235, P < 0.001; (B) F_(3,36) = 8.585, P < 0.001, followed by Bonferroni test: * P < 0.05, ** P < 0.01 saline vs D-amph or cocaine; ° P < 0.01 vehicle vs SL327. 10 mice per group.