Figure 2.

Retrieval of hippocampus-dependent memory is impaired by xamoterol. Vehicle (Veh) or xamoterol (Xam, 6 mg/kg) was injected 30 min before testing 1 d after training. As expected, xamoterol impaired contextual fear memory relative to vehicle. The mice were tested again the next day without injection. Mice treated with vehicle on day 1 exhibited significantly lower freezing on day 2 compared to day 1, consistent with the extinction of contextual fear. Mice treated with xamoterol on day 1 exhibited significantly higher freezing on day 2 compared to day 1 and compared to vehicle-treated mice on day 2, indicating that extinction was minimal. Because it is not necessary to express contextual fear to obtain extinction (Ouyang and Thomas 2005), the results suggest that xamoterol impairs retrieval rather than the expression of contextual fear. There were 5 mice for Veh and 9 mice for Xam. F_(1,26) = 25 and P < 0.0001 for the interaction of treatment and day; the main effects of treatment and day were not significant. (* = P < 0.05, ^∧ = P < 0.01, and # = P < 0.001 for post-hoc comparisons.)