Fluoxetine prevents the cognitive and cellular effects of chemotherapy in the adult hippocampus

Rationale: CMF (cyclophosphamide: CP; methotrexate: MTX; 5-fluorouracil: 5-FU) is a chemotherapy combination associated with the cognitive impairments which many cancer patients experience after treatment. A reduction in hippocampal neurogenesis is a known means by which cytotoxic drugs alter cognition and is the mechanism investigated in the present study. There is currently no way of treating or preventing the cognitive deficits produced by chemotherapy and a simple pharmacological approach to achieving this could potentially have significant benefits for patients. Objectives: The studies in the present thesis use an animal model to investigate the effects of the individual agents in the CMF combination on spatial working memory and the proliferation and survival of neural precursors involved in hippocampal neurogenesis. It was also investigated whether the cognitive impairment produced by chemotherapy could be reversed or prevented by the antidepressant fluoxetine. Methods: In 4 separate experiments, adult male Lister-hooded rats were chronically administered with CP (30mg/kg, 4 or 7 i.v. doses), MTX (75mg/kg, 2 i.v. doses) or 5-FU (25mg/kg, 5 i.p. doses). Some rats were co-administered with fluoxetine (10mg/kg/day, in drinking water) for different time periods. Spatial memory was tested using the novel location recognition (NLR) task and the spontaneous alternation in the T-maze memory tasks. Proliferation and survival of hippocampal cells was quantified using immunohistochemistry and the levels of doublecortin (DCX) and brain-derived neurotrophic factor (BDNF) were quantified in the hippocampus and frontal cortex using Western blotting. Neural stem cells (NSC) were also isolated from the adult mouse hippocampus, to examine the direct effects of 5-FU, fluoxetine and its active metabolite, norfluoxetine (0.01-100µM) in vitro. Results: Rats treated with 5-FU and MTX showed impairment in the NLR task but not the spontaneous alternation in the T-maze task. They also exhibited a reduction in cell proliferation (Ki67-positive cells) and survival (BrdU-positive cells) in the dentate gyrus, compared to saline treated controls, but no difference was seen in the levels of DCX of BDNF. The induced cognitive and cellular impairments were not seen when fluoxetine was co-administered with the chemotherapy. The impairments caused by 5-FU were counteracted when fluoxetine was co-administered before and during 5-FU treatment but not when it was only administered after treatment. CP did not impair performance in the NLR task or hippocampal cell proliferation; however it significantly reduced cell survival. 5-FU, fluoxetine and norfluoxetine all decreased cell viability in vitro. Conclusions: These results demonstrate that MTX and 5-FU have more pronounced effects on spatial memory and hippocampal cell proliferation than CP in the CMF combination. Furthermore these impairments can be reversed by fluoxetine in a mechanism of prevention but not recovery. Although further work is required, it would be beneficial to establish an in vitro model of chemotherapy-induced cognitive impairment to justify this conclusion and to investigate the potential benefits of fluoxetine in cancer patients.