Depression-like behavior in subclinical hypothyroidism rat induced by hemi-thyroid electrocauterization.

The purpose of this study was to investigate the depression-like behavior performances of subclinical hypothyroidism (SCH) rat. SCH rat model was induced by hemi-thyroid electrocauterization, and the behavior performances were measured by sucrose preference test, force swimming test (FST), and tail suspension test (TST). SCH rat model was established successfully by hemi-thyroid electrocauterization. In the behavior tasks, SCH rats displayed depression-like behavior were indicated as a significant elevation of immobility time in both the TST and FST, though the sucrose preference was not significantly decreased. The index of left adrenal cortex in both SCH and clinical hypothyroidism (CH) group significantly increased, and many large lipid vacuoles were observed in the zona fasciculata cells. The serum corticosterone concentration and hypothalamic corticotropin-releasing hormone mRNA expression 2 h after behavior test was markedly up-regulated in CH rats, but not SCH rats, indicated that SCH induced a less impairment of HPA axis than CH did. The important finding of this study was that the concentration of hippocampal T3 was lower in SCH group than that of the sham group. Furthermore, the results of Pearson correlation test showed that the immobility behaviors in TST and FST were both negatively correlated with hippocampal T3 concentration. Taking together, our results indicated that SCH could result in depression-like behavior, accompanied with subtle hyperactivity of HPA axis. The reduced hippocampal T3 prior to the reduction of thyroid hormone in serum might be taken as an early sign of hippocampus impairment in the progression from SCH to CH.