Effect of baclofen on morphine-induced conditioned place preference, extinction, and stress-induced reinstatement in chronically stressed mice

Abstract

Rationale and objective: A stress-induced increase in excitability can result from a reduction in inhibitory neurotransmission. Modulation of gamma-aminobutyric acid (GABA)ergic transmission is an effective treatment for drug seeking and relapse. This study investigated whether baclofen, a GABA(B) receptor agonist, had an impact on morphine-induced conditioned place preference (CPP), extinction, and stress-induced relapse in chronically stressed mice.

Methods: Chronic stress was induced by restraining mice for 2 h for seven consecutive days. We first investigated whether chronic stress influenced morphine-induced CPP, extinction, and stress-induced relapse in the stressed mice. Next, we investigated whether three different doses of baclofen influenced chronic stress as measured by the expression of morphine-induced CPP. We chose the most effective dose for subsequent extinction and reinstatement experiments. Reinstatement of morphine-induced CPP was induced by a 6-min forced swim stress. Locomotor activity was also measured for each test.

Results: Chronic stress facilitated the expression of morphine-induced CPP and prolonged extinction time. Forced swim stress primed the reinstatement of morphine-induced CPP in mice. Baclofen treatment affected the impact of chronic stress on different phases of morphine-induced CPP.

Conclusions: Our results showed that baclofen antagonized the effects of chronic stress on morphine-induced CPP. These findings suggest the potential clinical utility of GABA(B) receptor-positive modulators as an anti-addiction agent in people suffering from chronic stress.

Fig. 1

Experimental design. a This schedule was used for experiments 1–2. On days 1–7, stressed mice underwent 2 h of restraint daily, and non-stressed mice experienced a similar degree of daily handling but were not restrained; then in experiment 1, all the mice were tested on days 8 (Pre-C), 12 (Post-C 4d), 16 (Post-C 8d), 17 (Extinction 1), 20 (Extinction 4), 23 (Extinction 7), and 24 (Reinstatement); in experiment 2, all the mice were tested on days 8 (Pre-C), 12 (Post-C 4d), and 16 (Post-C 8d). b This schedule was used for the stressed mice in experiments 3–5. Stressed mice underwent 2 h of restraint daily on days 1–7, and then in experiment 2 were tested on days 8 (Pre-C) and 12 (Post-C 4d). In experiment 3, stressed mice were tested on days 8 (Pre-C), 12 (Post-C 4d), and 16 (Extinction 4). In experiment 4, stressed mice were tested on days 8 (Pre-C), 12 (Post-C 4d), 22 (Extinction 10), and 23 (Reinstatement). c This schedule was used for the non-stressed mice in experiment 4. After 7 days handling, mice were tested on days 8 (Pre-C), 15 (Post-C 7d), 22 (Extinction 7), and 23 (Reinstatement). During all test periods, doors were raised, and mice were allowed to explore compartments. Time spent on each side was measured. C, conditioning; d, day

Fig. 2

Effect of stress on expression, extinction, and reinstatement of morphine-induced conditioned place preference. Animals were conditioned with 10 mg/kg morphine, and all groups were tested on the schedule described in Fig. 1. On reinstatement day, mice underwent 6 min of forced-swim stress and were returned to home cages for 20 min before testing. Data are means ± SEM and are as follows: a time in seconds spent in the drug-paired compartment; b distance traveled in millimeters during the test period. N = 10 mice per group. *P < 0.05, **P < 0.01 vs. pre-conditioning test (Pre-C). ^## P < 0.01 vs. the last extinction test. ^{^} P < 0.05 vs. the corresponding test in non-stressed mice. Sa, saline; Mor, morphine

Fig. 3

Effect of two lower doses of morphine on conditioned place preference in the stressed mice. Morphine (1 and 3 mg/kg) was chosen here to induce morphine preference, all the mice were tested only on days 8 (Pre-C), 12 (Post-C 4d), and 16 (Post-C 8d) on the schedule of Fig. 1a. Data are means ± SEM and are as follows: a time in seconds spent in the drug-paired compartment; b distance traveled in millimeters during the test period. N = 10 mice per group. *P < 0.05, **P < 0.01 vs. pre-conditioning test (Pre-C). ^# P < 0.05 vs. the corresponding test in non-stressed mice. Mor, morphine

Fig. 4

Effects of baclofen on morphine-induced conditioned place preference. All mice underwent restraint stress for 1 week and were tested on days 8 and 12. Three doses of baclofen were used: 0.612, 1.25, and 2.5 mg/kg. +, mice treated with drug or vehicle; − not treated. Data are means ± SEM and are as follows: a chronic stress as time in seconds spent in the drug-paired compartment; b locomotor activity as distance traveled in millimeters during the test period. N = 10 mice per group. **P < 0.01 vs. pre-conditioning test (Pre-C). ^# P < 0.05, ^## P < 0.01 vs. group injected with morphine only. BL, baclofen; Sa, saline; Mor, morphine

Fig. 5

Effects of baclofen on extinction of morphine-induced CPP and locomotor activity in stressed mice. After completing CPP training, extinction was from days 13 to 16. Baclofen was injected (2.5 mg/kg, s.c.) immediately after each extinction training session. Data are means ± SEM and are as follows: a time (in seconds) spent in the drug-paired compartment; b distance traveled (in millimeters) during the test period. N = 10 mice per group. **P < 0.01 vs. pre-conditioning test (Pre-C). ^## P < 0.01 vs. post-conditioning test (Pre-C 4d). BL, baclofen; Sa, saline; Mor, morphine

Fig. 6

Effects of baclofen on forced-swim-induced reinstatement of morphine place preference in stressed and non-stressed mice. a, b Non-stressed mice were tested on days 8, 16, 23, and 24 (non-stressed schedule). c, d Stressed mice were tested on days 8, 12, 22, and 23 (stressed schedule). Baclofen was injected (2.5 mg/kg, s.c.) 30 min before forced swim stress. Data are means ± SEM and are as follows: a, c time (in seconds) spent in the drug-paired compartment; b, d. distance traveled (in millimeters) during the test period. N = 10 mice per group. **P < 0.01 vs. pre-conditioning test (Pre-C). ^^P < 0.01 vs. the last extinction test. ^## P < 0.01 vs. the group injected only with morphine. BL, baclofen; Sa, saline; Mor, morphine