The thyroid hormone, triiodothyronine, enhances fluoxetine-induced neurogenesis in rats: Possible role in antidepressant-augmenting properties

The thyroid hormone triiodothyronine (T3) may accelerate and augment the action of antidepressants. Antidepressants up-regulate neurogenesis in adult rodent hippocampus. We studied the effect of T3 and T3+fluoxetine in enhancement of hippocampal neurogenesis beyond that induced by fluoxetine alone and the correlation with antidepressant behaviour in the novelty suppressed feeding test (NSFT). Rats were administered fluoxetine (5 mg/kg.d), T3 (50 g/kg.d), fluoxetine (5 mg/kg.d)+T3 (50 g/kg.d) or saline, for 21 d. Neurogenesis was studied by doublecortin (DCX) immunohistochemistry in the subgranular zone (SGZ) of the hippocampus and the subventricular zone (SVZ). In the NSFT, latency to feeding in animals deprived of food was measured. Fluoxetine and fluoxetine+T3 increased the number of doublecortin-positive (DCX⁺) cells in the SGZ compared to saline (p=0.00005, p=0.008, respectively). There was a trend towards an increased number of DCX⁺ cells by T3 compared to saline (p=0.06). Combined treatment with fluoxetine+T3 further increased the number of DCX⁺ cells compared to T3 or fluoxetine alone (p=0.001, p=0.014, respectively). There was no effect of any of the treatments on number of DCX⁺ cells in the SVZ. In the NSFT, all treatments (T3, fluoxetine+T3 and fluoxetine) reduced latency to feeding compared to saline (p=0.0004, p=0.00001, p=0.00009, respectively). Fluoxetine+T3 further reduced latency to feeding compared to T3 alone (p=0.05). The results suggest that enhancement of antidepressant action by T3 may be related to its effect of increasing hippocampal neurogenesis and that the antidepressant effect of these treatments is specific to the hippocampus and does not represent a general effect on cell proliferation.