Behavioural and hormonal seasonal changes are well documented in various vertebrate species living in their natural environment but circannual variations that may occur in laboratory animals reared in standard conditions are poorly investigated. This study shows that, in laboratory mice, the effects of stress on behavioural inhibition, investigatory behaviour and blood concentration of corticosterone are seasonally dependent. No consistency was observed between the reactivity of biological structures controlling the hormonal response to stress and the behavioural activities investigated at every period of the year. During the springtime, stress, which elicited a decrease of investigatory behaviour (estimated by the walking time in an open field), increased behavioural inhibition (estimated by the percentage of walking in the central area of the open field) as well as the blood corticosterone concentration in laboratory mice. In autumn, stress had no significant effect on behaviour despite the great hormonal concentration increase. The results reveal that, at certain period of the year, a stressful procedure is unable to affect behavioural parameters in laboratory mice which were maintained in constant 12-h dark/12-h light cycle. The report constitutes a novel piece of information suggesting a potential role of the endogenous biological clock in the modulation of stress response in mammals.
