Effect of Chronic Tibolone Administration on Memory and Choline Acetyltransferase and Tryptophan Hydroxylase Content in Aging Mice.

Gonadal steroids exert different effects on the central nervous system (CNS), such as preserving neuronal function and promoting neuronal survival. Estradiol, progesterone, and testosterone reduce neuronal loss in the CNS in animal models of neurodegeneration. However, hormone replacement therapy has been associated with higher rates of endometrial, prostate, and breast cancer. Tibolone (TIB), the metabolites of which show estrogenic and progestogenic effects, is an alternative to reduce this risk. However, the impact of TIB on memory and learning, as well as on choline acetyltransferase (ChAT) and tryptophan hydroxylase (TPH) levels in the hippocampus of aging males, is unknown. We administered TIB to aged C57BL/6J male mice at different doses (0.01 or 1.0 mg/kg per day for 12 weeks) and evaluated its effects on memory and learning and the content of ChAT and TPH. We assessed memory and learning with object recognition and elevated T-maze tasks. Additionally, we determined ChAT and TPH protein levels in the hippocampus by Western blotting. TIB administration increased the percentage of time spent on the novel object in the object recognition task. In addition, the latency of leaving the enclosed arm increased in both TIB groups, suggesting an improvement in fear-based learning. We also observed decreased ChAT content in the group treated with the 0.01 mg/kg TIB dose. In the case of TPH, no changes were observed with either TIB dose. These results show that long-term TIB administration improves memory without affecting locomotor activity and modulates cholinergic but not serotonergic systems in the hippocampus of aged male mice.