Effect of chronic Δ9-THC alone and in combination with fluoxetine on the regulation of CB1 signalling pathways in bulbectomized

Abstract

Several reports indicate that brain endocannabinoid (EC) system may be involved in depression as well as in antidepressant-like activity as demonstrated in some experimental models. In addition, recent data indicate that EC activity might be modulated by different antidepressant paradigms. To extend our knowledge on the crosstalk mechanisms between brain EC and serotonergic systems and their implication in depression, we have evaluated the effects of chronic in vivo exposure to the SSRI drug fluoxetine alone or in combination with D9-tetrahydrocannabinol (D9-THC) on CB1 signalling pathway in an animal model of depression such as olfactory bulbectomy (OB). Bulbectomized animals (male Sprague-Dawley rats) were treated for 21 days with vehicle (saline), D9-THC (10 mg/kg/day, i.p.), fluoxetine (10 mg/kg/day, p.o.) and D9-THC plus fluoxetine and sacrified 24 hours later. Olfactory bulbectomy resulted in a significant motor hyperactivity as measured by open field test. This behavioural modification was reversed by chronic fluoxetine. Olfactory bulbectomy induced a hipersensitization at the level of CB1 receptor density, without a significant change in postreceptorial pathways. In OB rats, CB1 receptor density ([3H]CP55,940 saturation binding) in the rat prefrontal cortex remained unaltered after chronic fluoxetine, however a decrease in CB1 receptor density was observed in OB-D9-THC treated rats. These changes in receptor density after D9-THC administration were similar to those observed by the concomitant administration of both drugs. In addition, the maximal effect of the cannabinoid agonist WIN55,212-2 to stimulate [35S]GTPgS binding was attenuated by chronic D9- THC alone (% red=26.7± 4.9 ; p< 0.05) or in combination with fluoxetine (% red= 27.1±6.1; p< 0.05); in contrast, chronic fluoxetine did not significantly alter CB1 agonist-stimulated [35S]GTPgS binding. Finally, the maximal ability of WIN55,212-2 to inhibit AC (forskolin-stimulated-cAMP accumulation) was significantly enhanced (EmaxOB-vehicle= 69.7 ± 3.5 vs EmaxOB-fluoxetine= 51.0 ± 3.5; p < 0.05) and decreased (ImaxOBD9-THC = 84.9 ± 2.8; %; p < 0.05 vs vehicle; one-way ANOVA) in the prefrontal cortex of OB-rats treated with fluoxetine and D9-THC respectively. In contrast, in the D9-THC plus fluoxetine treated group the inhibition of AC induced by WIN55,212-2 was not significantly different from the vehicle group. Our data indicate that chronic fluoxetine normalizes the behavioural modifications induced by olfactory bulbectomy, without reversing the increase in the density of CB1 receptors. They also suggest that the regulatory changes induced by chronic D9-THC in prefrontal cortex in this model do not represent an >antidepressant> effect, but they are the consequence of a receptor desensitization due to chronic stimulation.