Lack of cyclin D2 impairing adult brain neurogenesis alters hippocampal-dependent behavioral tasks without reducing learning ability

The exact function of the adult brain neurogenesis remains elusive, although it has been suggested to play a role in learning and memory processes. In our studies, we employed cyclin D2 gene knockout (cD2 KO) mice showing impaired neurogenesis as well as decreased hippocampal size. However, irrespectively of the genetic background of cD2 KO mice, this phenotype resulted in neither deficits in the hippocampal-dependent learning ability nor the memory formation. In the present study, cD2 KO mice and control littermates were subjected to hippocampal-dependent behavioral tests with little or no learning component. The knockout mice showed significant impairment in such species-typical behaviors as nest construction, digging, and marble burying. They were building none or poorer nests, digging less robustly, and burying fewer marbles than control mice. Such impairments were previously described, e.g., in animals with hippocampal lesions. Moreover, cD2 KO animals were also more active in the open field and automated motility chamber as well as showed increased explorative behavior in IntelliCage. Both increased motility and explorative behaviors were previously observed in hippocampally lesioned animals. Finally, cD2 KO mice showed normal sucrose preference, however starting from the second exposure to the sweetened solution, while control animals displayed a strong preference immediately. Presented results suggest that either morphological abnormalities of the hippocampal formation or adult brain neurogenesis impairment (or both) alter hippocampal-dependent behaviors of mutant mice without influencing learning abilities. These results may also suggest that adult brain neurogenesis is involved in species-typical behaviors.