Spatial memory deficits in middle-aged mice correlate with lower exploratory activity and a subordinate status: role of hippocampal neurotrophins

The aim of the present work was to relate age-related individual differences in cognitive function with behavioural strategies employed in social and non-social challenges. To this purpose, the behaviour of adult (5-month-old) and middle-aged (13-month-old) CD-1 mice was scored in the social interaction, plus-maze, Morris water maze (MWM) and open-field tests. In addition, brain levels of nerve growth factor and brain-derived neurotrophic factor (BDNF) were analysed and correlated with the behaviours scored. Compared to adults, middle-aged mice showed greater anxiety in both non-social and social situations, spending less time in the open arms of the plus-maze and performing more freezing behaviour in response to aggression. Based upon their behaviour in the social interaction test, adult and middle-aged subjects were classified as dominant or subordinate and their behaviour in the open field, plus-maze and MWM tests subjected to factor analysis, taking into account age and social status. Results highlighted meaningful differences in exploratory strategies as a function of social status only in middle-aged subjects. In particular, middle-aged dominants were, overall, more explorative than same-aged subordinates, spending less time in peripheral areas and approaching more readily a novel object. Interestingly, in middle-aged mice, superior performance in the MWM task was associated with exploratory strategies exploited by dominants. At adulthood, BDNF hippocampal levels, but not specific behaviours, were positively correlated with the ability to learn a spatial task. Overall, data indicate that, in middle-aged subjects individual differences in exploratory strategies, rather than neurotrophin levels, are able to predict the degree of impairment in a spatial learning task.