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

Abstract

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.

Abnormalities of hypothalamic-pituitary-adrenal and hypothalamic-somatotrophic axes in Fawn-Hooded rats

“…First, true basal activity of the HPA axis does not appear to be supranormal in FH rats, as the same laboratory that reported basal hypercorticosteronaemia subsequently showed normal corticosterone concentrations (11) -a finding corroborated by our own results (12). Secondly, FH rats showed lower relative adrenal weight and greater relative thymus weight as compared with both Wistar and SD rats (11,12). Thirdly, activity of FH rats in the forced swimming test, which is presumed to evaluate depression-like behaviour (13,14), is dependent on the particular substrain used (15).…”

“…However, there are several findings arguing against this hypothesis. First, true basal activity of the HPA axis does not appear to be supranormal in FH rats, as the same laboratory that reported basal hypercorticosteronaemia subsequently showed normal corticosterone concentrations (11) -a finding corroborated by our own results (12). Secondly, FH rats showed lower relative adrenal weight and greater relative thymus weight as compared with both Wistar and SD rats (11,12).…”

“…Although high basal corticosterone concentrations had previously been found in FH rats compared with Wistar rats (10), more recent papers, including one from the same laboratory that originally reported hypercorticosteronaemia, have reported normal basal concentrations of ACTH and corticosterone in FH rats, as compared with either Wistar or SD rats (11,12). Therefore, earlier results showing higher than usual concentrations of serum/plasma corticosterone in FH rats could have been due to stress associated with blood sampling.…”

“…In fact, there are no signs of glucocorticoids hyperactivity in FH rats. Rather, their greater thymus weight (12) and their tendency to develop inflammatory diseases (11) might be indicative of HPA hypoactivity. At higher levels of the HPA axis (mRNA for GR in the hippocampus, mRNA for CRF in the PVN, and serum ACTH concentrations), no differences were observed between the strains.…”

“…Total serum corticosterone was measured by radioimmunoassay (RIA) using a procedure based on one described previously (28), with small modifications (12). Serum ACTH was assayed immunoradiometrically, using a commercial kit (Nichols, San Juan Capistrano, CA, USA).…”