Effects of the selective M1 muscarinic receptor antagonist dicyclomine on emotional memory

Abstract

The nonselective muscarinic antagonist scopolamine is known to impair the acquisition of some learning tasks such as inhibitory avoidance. There has been recent research into the effects of this drug in contextual fear conditioning and tone fear conditioning paradigms. The purpose of the present study was to assess the role of the selective M1 muscarinic antagonist dicyclomine in these paradigms and in the inhibitory avoidance test. Rats were administered different doses of dicyclomine or saline 30 min before acquisition training. The animals were tested 24 hr later, and it was observed that 16 mg/kg of dicyclomine impaired both contextual fear conditioning and inhibitory avoidance. However, dicyclomine (up to 64 mg/kg) did not affect tone fear conditioning. These results suggest that the selective M1 muscarinic antagonist dicyclomine differentially affects aversively motivated tasks known to be dependent on hippocampal integrity (such as contextual fear conditioning and inhibitory avoidance) but does not affect similar hippocampus-independent tasks.

Figure 1

Effects of dicyclomine (2, 8, and 16 mg/kg) on the freezing response of rats in the contextual conditioning test. DIC2, dicyclomine 2 mg/kg; DIC8, dicyclomine 8 mg/kg; DIC16, dicyclomine 16 mg/kg; SAL, saline. The number of animals per group is shown in parentheses after the group names; *p < .05 compared to SAL.

Figure 2

Effects of dicyclomine (8 and 16 mg/kg) on the latency to enter the black compartment during one trial training and retention test of the inhibitory avoidance task. DIC8, dicyclomine 8 mg/kg; DIC16, dicyclomine 16 mg/kg; SAL, saline. The number of animals per group is shown in parentheses after the group names; *p < .05 compared to SAL.

Figure 3

Effects of dicyclomine (16 and 64 mg/kg) on the freezing response of rats in the tone conditioning test before and after tone. DIC16, dicyclomine 16 mg/kg; DIC64, dicyclomine 64 mg/kg; SAL, saline. The number of animals per group is shown in parentheses after the group names.