In this study the authors compare the effects of acute and chronic administration of the tricyclic antidepressants imipramine and protriptyline on the turnover and matabolism of norepinephrine in rat brain. The turnover of norepinephrine was decreased after acute administration but increased during chronic administration of these drugs. This increase in norepinephrine turnover occurred sooner when thyroxine was administered with the imipramine. This may help to explain why clinical antidepressant effects are generally observed only after chronic administration of imipramine or protriptyline and why thyroid hormone may accelerate and enhance the clinical antidepressant effects of imipramine.
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