Influence of single or repeated experience of rats with forced swimming on behavioural and physiological responses to the stressor.

In the present work behavioural (struggling and immobility), physiological (hypothermia, glycaemia) and endocrine (hypothalamo-pituitary-adrenal (HPA) hormones) response to repeated forced swimming (FS) for 15 days was studied in adult male rats and compared with the response of rats having only one single experience with FS either 1 or 14 days before the last exposure to the stressor. Repeated experiences with FS reduced struggling and increased immobility as compared with stress-naïve rats, whereas a single previous exposure to FS, regardless of the time elapsed, had the same, but less marked, effect. Hypothermia followed the same trend. FS-induced hyperglycaemia was not sensitive to a previous single experience, but rather it was totally abolished in chronically stressed rats. Neither a single nor chronic exposure to FS modified the secretion of ACTH in response to the last FS session. However, repeated FS enhanced the speed of recovery of plasma corticosterone as compared to control rats, suggesting a dissociation between the two hormones. The present results revealed great differences in the sensitivity of various behavioural and physiological responses to repeated FS stress and suggest that reduced response to repeated FS, when found, is not a consequence of the time elapsed between exposures but to the repetition of the stressful situation.