Sex differences in the effects of neonatal handling on the animal's response to stress and the vulnerability for depressive behaviour.

Neonatal handling is known to affect the programming of the hypothalamic-pituitary-adrenal axis and, as a result, the ability of the organism to respond to stress. We determined the effect of neonatal handling on the animal's response in three animal models of depression, as well as to either (a) acute or (b) chronic forced swimming stress. Neonatal handling resulted in a significant increase in the immobility time in the Porsolt forced swimming test in both sexes, and in the 8-hydroxy-2-(di-n-propylamino) tetralin-induced hypothermia in the males. On the other hand, handling had sex-dependent effects when animals were exposed to a chronic stressor. After exposure to chronic restraint stress, statistically more handled than non-handled females failed to adapt, while no such difference was found in the males. In the chronic forced swimming stress, handled males had shorter immobility times, and higher plasma corticosterone levels, while the opposite held true in the females. Furthermore, neonatal handling significantly decreased basal plasma corticosterone levels in both pre- and post-pubertal animals. Thus, the early experience of handling provides males with a greater capacity to actively face chronic stressors, while in the females it increases their susceptibility to express 'depressive' behaviour since they are unable to cope and adopt a 'passive, despaired' behaviour.