Effects of the M1 muscarinic antagonist dicyclomine on emotional memory retrieval.

Extensive research has shown the involvement of the central cholinergic system in the acquisition and consolidation of tasks involving conditioned fear responses, such as those observed in contextual fear conditioning (CFC), tone fear conditioning (TFC) and inhibitory avoidance (IA). However, there are few data concerning the role of this system in the memory retrieval process. Therefore, the present study aimed to compare the effects of the administration of an M1 antagonist on retrieval during these tasks. For each behavioral procedure, groups of male Wistar rats were trained. Twenty-four hr later, they were treated with different doses of dicyclomine (16, 32, or 64 mg/kg, i.p.) or with saline 30 min before the test session. The results showed that dicyclomine at doses of 16 and 32 mg/kg impaired CFC without interfering with IA performance. Moreover, only 64 mg/kg impaired TFC. These data suggest that M1 muscarinic receptors contribute to memory retrieval in CFC and TFC but are not essential for retrieval in IA.