Fawn-hooded rats show enhanced active behaviour in the forced swimming test, with no evidence for pituitary-adrenal axis hyperactivity

Fawn-hooded (FH) rats have been reported to have high basal corticosterone levels that can be normalized by antidepressant administration. In the present work, some behavioural and endocrine aspects were compared in FH and Sprague-Dawley (SD) rats. No interstrain differences in basal corticosterone levels or response to acute tail-cut stress were observed either in the morning or in the evening. Relative adrenal weight was lower in FH than SD, and relative thymus weight was, accordingly, higher in FH than SD rats. FH rats were hyperactive in the holeboard and showed behaviour similar to that of SD in the plus-maze. In contrast, FH rats showed greater levels of active behaviour (struggling) and lower levels of immobility than SD rats in the forced swimming test (FST). After desipramine (DMI) administration the differences between the two strains in the FST were more marked than in vehicle-treated animals. These results indicate that FH rats show no signs of pituitary-adrenal (PA) hyperactivity or depression-like behaviour, and therefore they are not an appropriate animal model of depression.