Efeito do 2-Araquidonoilglicerol nas respostas defensivas mediadas pela substância cinzenta periaquedutal dorsolateral de ratos em modelos animais de ansiedade

Abstract

The endocannabinoids have an important role on the regulation of fear expression in response to aversive stimuli, in several brain regions, including the periaqueductal gray (PAG). The 2-arachidonoylglycerol (2-AG) and anandamide (AEA) are the main endocannabinoids and exerts its effects through activation of cannabinoid receptors type 1 (rCB1) and type 2 (rCB2). Previous works have been show the anti-aversive effects induced by augmentation of AEA signalling in different animal models of anxiety. However, the role of 2-AG on the aversive reactions modulated by dlPAG is not completely understood. Thus, we verified whether the administration of 2AG or URB602, an inhibitor of MGL (enzyme responsible for 2-AG hydrolysis), will induce anxiolytic-like effect in two animal´s model of anxiety, the Contextual Fear Conditioned test and Vogel Conflict test. Male Wistar rats with unilateral cannulae aimed at the dlPAG, were submitted to Contextual Fear Conditioning (CFC) test and VCT test. After a 7 days recovery period, the animals were submitted to behavioral tests. In the Contextual Fear Conditioned Test (CFC), the animals were exposed to an acrylic chamber where they received electrical-foot shocks (6 cycles of 1.5mA randomized with duration of 2s) in an interval of ten minutes. Twenty-four hours later the animals received intra-SCPdl injection of 2-AG5, 50 and 500 pmol/0.2 L, URB602 30, 100, 300 pmol/0.2L or vehicle (0.2L), followed by re-exposure to acrcylic chamber10 minutes after injection. The time expent in freezing behavior was evaluated during ten minutes. In the Vogel conflict test (VCT), the animal was initially deprived of water for 24 hours, after this period they were exposed to an acrylic chamber containing a bottle of water. The animals had free access to water during 3 minutes. After 24 hours, the animals received intra-SCPdl administration of 2-AG5, 50, 500 and 5000/0.2Lpmol or URB 602 30, 100, 300 pmol/L or vehicle (0.2 L). Ten minutes later they were placed in the chamber for 3 minutes, where it received punished licks (1 shock of 0.5 mA, every 20 licks). Our results showed that the administration of 2-AG (50pmol and 500pmol) significantly decreased the freezing time (s) observed during the re-exposure to the aversive conditioned context , and only the dose of 5000 pmol increased the number of punished licks suggesting an anxiolytic-like effect of 2-AG. These injection of URB 602 did not induced these effects in both models. In order to verify which cannabiboid receptor was involved in 2-AG effects, we performed experiments where the animals received the pre-treatment of CB1 receptor antagonist, AM 251 100pmol/0.2L, or CB2 antagonist, AM 630 1000/0.2L before 2-AG injection. The anxiolytic-like effects of 2-AG were dependent upon activation of rCB1 and rCB2 since their antagonistsprevented the effects induced by 2-AG in the CFC and VCT. Our results suggest that facilitation of 2-AG signaling in the dlPAG, through MGL inhibitor, may not be essential to inhibit aversive stimuli in the dlPAG. However, we cannot exclude the role of this endocannabinoid in aversive responses modulated by this region, since the 2-AG injected directly induced anxiolytic-like effects.