Behavioral depression: thyroid interactions with norepinephrine-depleting drugs

Temporary suppression of rats' bar pressing, activity, and feeding by the dopamine beta-hydroxylase inhibitor FLA-63 was synergistically potentiated by triiodothyronine (T3) treatment. The increased severity of duration of behavioral depression from the combination of mildly-depressing doses of FLA-63 (10 mg/kg, SC) and T3 (200 mug/kg, SC, 4X) was most marked 36-72 hr after FLA-63 and closely resembled the depressive syndrome produced by higher (30-90 mg/kg) doses of FLA-63 alone in timing and specific behavioral features; these depressions were more likely due to toxicity than to depletion of brain norepinephrine. T3 did not potentiate behavioral depression induced by diethyldithiocarbamate (25 mg/kg, SC). This pattern of findings suggested an interpretation of the T3-FLA-63 synergism in terms of increased FLA-63 toxicity in hyperthyroidism.