Variability factors in the expression of stress-induced behavioural sensitisation.

Altered behavioural and physiological responsivity following a short session of foot shocks in the rat has proven to be a stable and clinically relevant model of stress-induced sensitisation. However, a number of key factors influencing effect size or direction have not previously been reported. Rats underwent a single, 15-min session of foot shocks and were exposed to a variety of novel stressful challenges 1 or 2 weeks later. Sensitised behavioural responses (increased immobility) in preshocked rats remained present over 3 days of repeated exposure to noise stress. In mild novel challenges (open field, empty cage), behavioural sensitisation and defecation was most clearly expressed at the beginning of the dark phase (evening). Higher-arousal challenges (prod, noise) caused increased behavioural inhibition in preshocked rats at all three time points (morning, afternoon, evening). Female preshocked rats showed a different pattern of behavioural and defecation sensitisation than preshocked males. The robustness of the model makes it suitable for further investigations into the mechanisms and vulnerability factors involved in the long-term consequences of stress.