233. Elevated thyroid hormone levels in brains of congenital learned helpless rats

Abstract

Learned helplessness has been well characterized as a model of depression. Since alterations in brain thyroid hormone levels have been hypothesized to occur in depressive illness, the present study examined brain thyroid hormone levels in rats selectively bred (42 generations) for increased susceptibility to learned helpless behavior (congenital helpless rats, cLH) and in their non-helpless (cNLH) counterparts. Brains from eight stress-naive female rats from each group were examined. A small, but significant, elevation in whole-brain levels of thyroxine was found in cLH rats as compared to cNLH rats (12%, p < 0.04). Congenital helpless rats also showed a 32% increase in whole brain triiodothyronine levels compared to cNLH rats, but this difference was not statistically significant. Activity of type II deiodinase (which converts thyroxine to triiodothyronine in brain) was not significantly different between the two groups. These results provide the first indication that brain thyroid hormone levels are elevated in congenital helpless rats and the absence of any exposure to stressful stimulation. The data further suggests that increased brain thyroid hormone levels may contribute to depressive-like disorders.