Effects of triiodothyronine (T3) on the potentiation by antidepressants of L-5-hydroxytryptophan-induced head-twitches in mice.

1. Daily injection of T3 during three consecutive days dose-dependently enhanced L-5-HTP (4 mg/kg)-induced head-twitches in mice. 2. Pretreatment with a sub-effective dose of T3 (0.06 mg/kg) markedly enhanced the ability of drugs acting mainly on noradrenergic systems (clenbuterol, desipramine, maprotiline and nomifensine) to increase the response to L-5-HTP. 3. In contrast the potentiating effects of T3 were moderate on drugs with mixed action on serotonergic and noradrenergic systems (amitriptyline, clomipramine and imipramine) and minimal on drugs acting mainly on serotonergic systems (citalopram, fluvoxamine and indalpine). 4. These data suggest that the increased responsiveness to L-5-HTP caused by T3 involves an indirect (norepinephrine-mediated) rather than a direct effect on serotonergic processes.