Enhanced self-administration of the CB1 receptor agonist WIN55,212-2 in olfactory bulbectomized rats: evaluation of possible serotonergic and dopaminergic underlying mechanisms

Abstract

Depression has been associated with drug consumption, including heavy or problematic cannabis use. According to an animal model of depression and substance use disorder comorbidity, we combined the olfactory bulbectomy (OBX) model of depression with intravenous drug self-administration procedure to verify whether depressive-like rats displayed altered voluntary intake of the CB1 receptor agonist WIN55,212-2 (WIN, 12.5 μg/kg/infusion). To this aim, olfactory-bulbectomized (OBX) and sham-operated (SHAM) Lister Hooded rats were allowed to self-administer WIN by lever-pressing under a continuous [fixed ratio 1 (FR-1)] schedule of reinforcement in 2 h daily sessions. Data showed that both OBX and SHAM rats developed stable WIN intake; yet, responses in OBX were constantly higher than in SHAM rats soon after the first week of training. In addition, OBX rats took significantly longer to extinguish the drug-seeking behavior after vehicle substitution. Acute pre-treatment with serotonin 5HT1B receptor agonist, CGS-12066B (2.5-10 mg/kg), did not significantly modify WIN intake in OBX and SHAM Lister Hooded rats. Furthermore, acute pre-treatment with CGS-12066B (10 and 15 mg/kg) did not alter responses in parallel groups of OBX and SHAM Sprague Dawley rats self-administering methamphetamine under higher (FR-2) reinforcement schedule with nose-poking as operandum. Finally, dopamine levels in the nucleus accumbens (NAc) of OBX rats did not increase in response to a WIN challenge, as in SHAM rats, indicating a dopaminergic dysfunction in bulbectomized rats. Altogether, our findings suggest that a depressive-like state may alter cannabinoid CB1 receptor agonist-induced brain reward function and that a dopaminergic rather than a 5-HT1B mechanism is likely to underlie enhanced WIN self-administration in OBX rats.

Keywords: WIN55212-2; cannabinoid; depression; dopamine; drug dependence; methamphetamine; olfactory bulbectomy; serotonin.

Figure 1

OBX rats display anhedonia. The sucrose preference test in SHAM (n = 6) and OBX (n = 7) Lister Hooded rats. Data are percentages (±s.e.m.) of sucrose solution consumed at two time-points, i.e., after 4 and 24 h from the beginning of the test. Student t-test, ^***p < 0.001.

Figure 2

OBX rats display hyperactivity in a novel aversive environment. Spontaneous horizontal and vertical activity, expressed as mean counts of photobeam interruptions, and total distance travelled (in centimeters) in SHAM (n = 6) and OBX (n = 7) Lister Hooded rats. Data are shown as means (±s.e.m.) at two time-points (5 and 10 min of measurement). Student t-test, ^*p < 0.05, ^**p < 0.01, and ^***p < 0.001.

Figure 3

Cannabinoid self-administration training in OBX and SHAM rats. Data are shown as means (±s.e.m.). (A) Acquisition phase: mean active lever presses in SHAM (n = 6) and OBX (n = 7) Lister Hooded rats during WIN55,212-2 self-administration. ^***α < 0.001, repeated measures ANOVA. (B) Maintenance phase: mean WIN intake over the last seven training sessions in SHAM (n = 6) and OBX (n = 7) Lister Hooded rats during WIN self-administration. ^***α < 0.001, repeated measures ANOVA. (C) Extinction phase: mean active lever presses in SHAM (n = 6) and OBX (n = 7) Lister Hooded rats during extinction of WIN self-administration. ^*α < 0.05, ^**α < 0.01, and ^***α < 0.001, repeated measures ANOVA.

Figure 4

Latency to the first active response (A) and patterns of responding (B) in SHAM and OBX Lister Hooded rats during WIN self-administration. (A) Latency (seconds) to the first active lever presses in SHAM (n = 6) and OBX (n = 7) rats. Data are shown as means (±s.e.m.). ^*α < 0.05 and ^***α < 0.001, repeated measures ANOVA. (B) Quantitative, but not qualitative, differences in the active responding patterns between OBX and SHAM rats. Individual representative records illustrating responding patterns of OBX (first three patterns) and SHAM (last three patterns) rats on the active lever on the last day of the self-administration training (day 30). Each tick denotes the time of every response on the active lever. Cumulative numbers of active responses made over the 2 h test sessions are illustrated on the right side of each pattern.

Figure 5

Acute pre-treatment with CGS does not affect WIN self-administration. Effect of acute pre-treatment with CGS-12066B on WIN self-administration in SHAM (n = 6) and OBX (n = 7) Lister Hooded rats. Data are expressed as percentage changes of active lever pressing compared to 6-day baseline (assumed as 100%). The repeated measures ANOVA did not detect a significant effect of drug treatment.

Figure 6

Acute pre-treatment with CGS does not affect METH self-administration. (A) METH self-administration in OBX and SHAM rats. Data expressed as daily means (±s.e.m.) of active nose-pokes in SHAM (n = 7) and OBX (n = 7) Sprague Dawley rats during methamphetamine self-administration training. Significant difference was recorded from the day 7 onwards, ^*α < 0.05 (day 7–12: ^*α = 0.041, day 13–18: ^*α = 0.027, repeated measures ANOVA). (B) Effect of acute pre-treatment with CGS-12066B on METH self-administration. Data are expressed as percentage changes of active lever pressing compared to six-day baseline (assumed as 100%). The repeated measures ANOVA did not detect a significant effect of drug treatment.

Figure 7

OBX rats do not show enhanced dopamine level following acute WIN challenge. (A) Basal extracellular levels (fmol/μl of dialysate, mean ±s.e.m.) of DA in the NAc shell of SHAM (n = 4) and OBX (n = 4) Lister Hooded rats. No significant difference was found between the two groups (One-Way ANOVA, p = 0.45). (B) Effect of an intravenous administration of WIN 0.3 mg/kg on DA release in the nucleus accumbens shell of SHAM (n = 4) and OBX (n = 4) Lister Hooded rats. ^*p < 0.05 and ^*p < 0.01, Two-Way ANOVA followed by Bonferroni's post-hoc test.