Indomethacin blocks the increased conditioned rewarding effects of cocaine induced by repeated social defeat

Abstract

It is well established that repeated social defeat stress can induce negative long-term consequences such as increased anxiety-like behavior and enhances the reinforcing effect of psychostimulants in rodents. In the current study, we evaluated how the immune system may play a role in these long-term effects of stress. A total of 148 OF1 mice were divided into different experimental groups according to stress condition (exploration or social defeat) and pre-treatment (saline, 5 or 10 mg/kg of the anti-inflammatory indomethacin) before each social defeat or exploration episode. Three weeks after the last social defeat, anxiety was evaluated using an elevated plus maze paradigm. After this test, conditioned place preference (CPP) was induced by a subthreshold dose of cocaine (1 mg/kg). Biological samples were taken four hours after the first and the fourth social defeat, 3 weeks after the last defeat episode, and after the CPP procedure. Plasma and brain tissue (prefrontal cortex, striatum and hippocampus) were used to determine the levels of the pro-inflammatory cytokine interleukin 6 (IL-6). Results showed an increase of peripheral and brain IL-6 levels after the first and fourth social defeat that was reverted three weeks later. Intraperitoneal administration of the anti-inflammatory drug indomethacin before each episode of stress prevented this enhancement of IL-6 levels and also reversed the increase in the rewarding effects of cocaine in defeated mice. Conversely, this protective effect was not observed with respect to the anxiogenic consequences of social stress. Our results confirm the hypothesis of a modulatory proinflammatory contribution to stress-induced vulnerability to drug abuse disorders and highlight anti-inflammatory interventions as a potential therapeutic tool to treat stress-related addiction disorders.

Fig 1. Administration of the highest indomethacin dose before each social defeat blocked acquisition of the CPP induced by 1 mg/kg of cocaine in defeated mice.

Before the social stress protocol animals were randomly assigned to the following groups according to the pre-treatment they received: saline (EXP-SAL n = 13; RSD-SAL n = 14); 5 (RSD-INDO5 n = 16) or 10 mg/kg (EXP-INDO10 n = 13; RSD-INDO10 n = 17) of indomethacin. Bars represent the time (s) spent in the drug-paired compartment before conditioning sessions in the PRE-C test (white bars), after conditioning sessions in the POST-C test (dark grey bars), in the last extinction (EXTINCT) session (light gray bars), and in the reinstatement (REINST 0.5) test (black bars). Data presented as mean values ± SEM *p <0.05, ***p <0.001 significant difference in the time spent in the drug-paired compartment versus PRE-C.

Fig 2. Social defeat increases IL-6 levels in the plasma, STR, PFC and hippocampus.

Groups defined by stress condition and social defeat episode: (a) Plasma (EXP n = 11, 1RSD n = 11, 4RSD n = 11, 3WEEKS n = 8); (b) Striatum (EXP n = 12, 1RSD n = 12, 4RSD n = 12; 3WEEKS n = 8); (c) PFC (EXP n = 7, 1RSD n = 7, 4RSD n = 7; 3WEEKS n = 7); (d) Hippocampus (EXP n = 8, 1RSD n = 8, 4RSD n = 8; 3WEEKS n = 8). Data are presented as mean values ± SEM (pg/ml in plasma and pg/mg in brain tissue) *p <0.05; **p <0.01; ***p < 0.001 vs. exploration (EXP) group.

Fig 3. Indomethacin decreases plasma and striatum (STR) IL-6 levels in defeated mice.

Groups defined by stress condition and social defeat episode, all pretreated with indomethacin 10 mg/kg: (a) Plasma (EXP n = 7, 1RSD n = 8, 4RSD n = 8, 3WEEKS = 8); (b) Striatum (EXP n = 6, 1RSD n = 7, 4RSD n = 8, 3WEEKS n = 8). Data are presented as mean values ± SEM (pg/mg) *p <0.05 vs. exploration (EXP) group.

Fig 4. Social defeat increases plasma and striatal IL-6 levels after cocaine-induced CPP.

Groups defined by stress condition and pre-treatment: (a) Plasma (EXP-SAL n = 8, RSD-SAL n = 8, RSD-INDO10 n = 8); (b) Striatum (EXP-SAL n = 6, RSD-SAL n = 6, RSD-INDO10 n = 10). Data are presented as mean values ± SEM (pg/mg); ***p < 0.001 vs. exploration (EXP) group.; +++p< 0.001; +p <0.05 vs. pretreated defeated (RSD-INDO10) group.