Resumo

OBJECTIVES: Cannabidiol (CBD), a non-psychotropic component of Cannabis sativa, has been associated to antagonism of CB1 and CB2 receptors, activation of TRPV1 vanilloid receptors, activation of 5HT1A receptors and allosteric modulation of mu and kappa opioid receptors. These receptors are found in important structures involved in processing of aversive memories, such as the medial prefrontal cortex (mPFC). Despite the relevance of endocannabinoid modulation of cognitive/emotional processes, the behavioral effects of CBD are poorly understood. Therefore, in the present work we investigated the effects of direct administration of CBD in mPFC on contextual aversive memory of rats. METHODS: Adult male Wistar rats were subjected to stereotactic surgery under ketamine/xylazine anesthesia (100 mg/kg i.p. / 15 mg/kg i.p.) for bilateral implantation of cannulae in the mPFC. Three days after recovery, animals were habituated (one exposure to a footshock chamber; 4 min; with no shock) and conditioned to sound-shock pairings (cued fear conditioning) in the same environment. The conditioning session consisted of five 10-s sound stimuli followed by a footshock (1 mA; 2 s) every 75 s. Freezing behavior, as characterized by the absence of movements except for breathing, was scored every 15 s. Five hours after conditioning, the animals were divided in two groups: one group received a bilateral mPFC microinjection of CBD (1.27 nmol/side, 0.2 µl; in 5% DMSO or grape oil) and the other group received vehicle (control animals). All animals were subjected to two test sessions, 24h and 5 d after conditioning, with re-exposure to the sound but without footshock. Freezing behavior was scored every 15 s for 8 min. Within five minutes after the test at 5 d, the animals were decapitated and their brains removed. The mPFC (PL/IL; +3.7-2.7 mm; bregma), dorsal posterior hippocampus (CA1; -4.8-5.8 mm; bregma) and barrel cortex (S1BF; -1.8-2.8 mm; bregma) were dissected (frozen punches) and processed for monoamine quantification by high-performance liquid chromatography (HPLC-ED).RESULTS: Intra-group comparisons (Conditioning x 24 h Test; Conditioning x 5 d Test; 24 h Test x 5 d Test) of experimental groups (CBD in DMSO 5%; CBD in grape oil) and controls (DMSO 5%; grape oil) indicated a statistically significant effect of treatment only for CBD groups: CBD in DMSO 5% [n=6, F(2,10)=12.3, p=0.002] and CBD in grape oil [n=8, F(2,14)=5.2, p=0.021] (one-way ANOVA RM). The complement to ANOVA test (Bonferroni test) showed that CBD produces a significant decrease in freezing behavior at 5 d compared to conditioning day (CBD in grape oil: -24.7% and CBD in DMSO 5%: 38.9% p<0.05). Monoamine levels in the mPFC showed a decrease in dopamine release (-34.8%, p<0.05, Student's t-test) and a trend toward to decrease of serotonin release (-38.4, p≈0.09, Student's t-test).CONCLUSION: CBD modulation of mPFC activity 5 h after conditioning impairs aversive memory consolidation in rats. Beside, CBD promotes significant changes in the prefrontal cortex monoaminergic neurotransmission. These results, together with data from the other laboratories, may provide further evidence on the possible clinical application of CBD in some cases of posttraumatic stress disorder.