Letrozole delays acquisition of water maze task in female BALB/c mice: possible involvement of anxiety

Abstract

Letrozole, an aromatase inhibitor (AI), is used as an adjuvant therapy in estrogen receptor-positive (ER+) breast cancer patients. Similar to other AIs, it induces many side effects, including impaired cognition. Despite its negative effect in humans, results from animal models are inconsistent and suggest that letrozole can either impair or improve cognition. Here we studied effects of letrozole on cognitive behavior of adult female BALB/c mice, a relevant animal model for breast cancer studies. Mice were continuously treated with once-a-day subcutaneous (s.c.) injections of letrozole (0.1 or 0.3 mg/kg/day) or vehicle and subjected to behavioral testing starting on day 21 after treatment initiation. During the treatments, vaginal smears were taken from the mice to evaluate estrous cyclicity. Both doses of letrozole suspended cyclicity and the smears showed that the mice were in constant metestrus. Exposure to letrozole did not significantly affect response to novelty measured as a locomotor activity in open field. However, repeated testing in open field (4 days x 15 min) revealed that letrozole 0.3 mg/kg facilitated locomotor habituation (a form of non-associative learning), significantly reducing locomotor activity on 3rd and 4th day of testing. These findings suggest that certain doses of letrozole may have positive effects on cognitive behavior. Training to find a hidden platform in the Morris water maze (15 days x 4 trials), however, indicated that letrozole 0.1 mg/kg-treated mice had significant learning impairment, as, throughout the training, they swam longer times than vehicle-treated mice to reach the hidden platform. Similarly, in a probe test performed 72 h after the last day of the training, letrozole 0.1 mg/kg-treated mice did not show preference for the training platform zones. These results indicate that cognitive impairments reported by women treated with letrozole can be captured in BALB/c mice treated with clinically relevant doses of the drug. Interestingly, most of the letrozole 0.1 mg/kg-treated mice were able to learn the new platform position in reversal training and performed similar to control mice in a reversal probe test. Results of the reversal test suggest that letrozole did not completely disrupt spatial navigation but rather delayed acquisition of spatial information. The current study shows that letrozole dose dependently modulates behavioral response and that its effects are task dependent.