Resumo

Dicyclomine, a muscarinic antagonist with high affinity for M1 receptors, impairs the acquisition of context fear conditioning. However, the role of the muscarinic M1 receptor in context-shock association during context fear conditioning may be obscured by the typical training paradigm, in which acquisition of context representation and context-shock association occurs almost simultaneously within a single training session. Using the paradigm of the context pre-exposure facilitation effect, in which the learning about the context is separated from the learning about the context-shock association, this study verified the effects of dicyclomine on the acquisition of the context representation and acquisition of context-shock association. Wistar male rats received intraperitoneal injection of salina or dicyclomine (8, 16 and 32 mg/kg) before pre-exposure (PE) or before context-shock association (CSA). The experimental groups were: saline-PE-saline-CSA; dicyclomine-PE-saline-CSA and saline-PE-dicyclomine-CSA. In the pre-exposure session, animals were placed in an acrylic box for five minutes. On the second day (CSA), animals returned to the same chamber were pre-exposure occurs and after fifteen seconds received a foot-shock. The context fear conditioning test was performed 48h after context-shock association phase. Animals were individually placed in the acrylic box and the freezing time was measured for 5 minutes, and used as an index of conditioning. The treatment with all dose of dicyclomine before the pre-exposure session did not impaired the conditioned freezing when compared with control group (SAL/SAL: 30.9 ± 4.15 (n=16); DICI08/SAL: 33.9 ± 6.13 (n=08); DICI16/SAL: 26.6 ± 5.21 (n=08); DICI32/SAL: 28.2 ± 6.70 (n=08), data represented as mean freezing time ± SE). However all doses of dicyclomine before the context-shock association session did impair the conditioned freezing response of animals (SAL/SAL: 30.9 ± 4.15 (n=16); SAL/DICI08: 18.1 ± 3.71 (n=08) p=0.05; SAL/DICI16: 14.7 ±3.36 (n=07) p=0.018; SAL/DICI32: 8.8 ± 3.53 (n=07), p=0.001). These results suggest that dicyclomine disrupted the learning of contextual fear conditioning by impairing the acquisition of context-shock association but not the acquisition of the context alone.