Triiodothyronine increases desipramine with changing the concentrations of monoamines, in the brain of rat given imipramine

Thyroid hormones, serotonin and mood: of synergy and significance in the adult brain

The use of thyroid hormones as an effective adjunct treatment for affective disorders has been studied over the past three decades and has been confirmed repeatedly. Interaction of the thyroid and monoamine neurotransmitter systems has been suggested as a potential underlying mechanism of action. While catecholamine and thyroid interrelationships have been reviewed in detail, the serotonin system has been relatively neglected. Thus, the goal of this article is to review the literature on the relationships between thyroid hormones and the brain serotonin (5-HT) system, limited to studies in adult humans and adult animals. In humans, neuroendocrine challenge studies in hypothyroid patients have shown a reduced 5-HT responsiveness that is reversible with thyroid replacement therapy. In adult animals with experimentally-induced hypothyroid states, increased 5-HT turnover in the brainstem is consistently reported while decreased cortical 5-HT concentrations and 5-HT2A receptor density are less frequently observed. In the majority of studies, the effects of thyroid hormone administration in animals with experimentally-induced hypothyroid states include an increase in cortical 5-HT concentrations and a desensitization of autoinhibitory 5-HT1A receptors in the raphe area, resulting in disinhibition of cortical and hippocampal 5-HT release. Furthermore, there is some indication that thyroid hormones may increase cortical 5-HT2 receptor sensitivity. In conclusion, there is robust evidence, particularly from animal studies, that the thyroid economy has a modulating impact on the brain serotonin system. Thus it is postulated that one mechanism, among others, through which exogenous thyroid hormones may exert their modulatory effects in affective illness is via an increase in serotonergic neurotransmission, specifically by reducing the sensitivity of 5-HT1A autoreceptors in the raphe area, and by increasing 5-HT2 receptor sensitivity.

Influence of imipramine on the hypothalamic/pituitary/thyroid axis of the rat

The effects of the tricyclic antidepressant drug imipramine at different levels of the hypothalamic/pituitary/thyroid axis were investigated in the rat. Intraperitoneal (IP) treatment for 14 days with imipramine at 10 mg/kg, but not 2 mg/kg, reduced serum total thyroxine (T4) and triiodothyronine (T3). A similar decrease in serum total T4 was observed in thyroidectomized T4-treated rats, suggesting that imipramine treatment enhances T4 clearance instead of reducing T4 secretion. There were no parallel decreases in serum free T4 and T3 concentrations, due to the simultaneous increase in the free fractions of both T4 and T3 following imipramine treatment. In vitro experiments using equilibrium dialysis indicated that neither imipramine nor its metabolite desipramine directly influenced the binding of T4 or T3 to their transport proteins following addition to normal serum, suggesting an indirect effect of imipramine or desipramine on free hormone concentrations in vivo. Concentrations of T4 and T3 in the brain, liver, and heart were unaffected by imipramine treatment, suggesting that the drug did not affect cellular uptake and metabolism of T4 and T3. Serum concentrations of thyrotropin (TSH) were unaffected by imipramine pretreatment at either dose level, compatible with the fact that serum free T4 and T3 concentrations were not reduced. Moreover, there was no difference in thyrotrope responsiveness to stimulation by TSH-releasing hormone (TRH) and to inhibition by T4 and T3 in rat anterior pituitary cells cultured ex vivo for 18 hours from control and imipramine-treated rats. Furthermore, in vitro exposure of cultured rat anterior pituitary cells to imipramine and desipramine indicated that both agents decreased TSH secretion only at concentrations greater than 10(-6) mol/L. These concentrations of imipramine and desipramine in the culture medium would exceed the free concentrations of these drugs seen in vivo therapeutically. In addition, no direct effects of 10(-6) mol/L imipramine or desipramine on the TSH response to TRH or to T3 were observed in vitro in cultured pituitary cells. A potential indirect effect of imipramine or desipramine on TSH secretion via altered hypothalamic control of thyrotropes does not seem likely, due to the lack of effect of imipramine treatment on serum TSH concentrations in imipramine-treated rats. In conclusion, imipramine treatment reduces serum total T4 and T3 in the rat, with enhanced clearance being the most likely explanation for the effect on T4. There was no evidence for altered tissue T4 or T3 concentrations or for altered thyrotrope function. The enhanced T4 clearance may explain the reduction in total T4 reported for imipramine-treated depressed patients. However, the effects of imipramine treatment on the transport of thyroid hormones in plasma need to be examined in more detail in patients, since interspecies differences in the nature of the transport proteins preclude extrapolation of the present results from the rat.

Effect of acute and chronic treatment with triiodothyronine on serotonin levels and serotonergic receptor subtypes in the rat brain

Hyperthyroidism is often associated with behavioral disorders, and thyroid hormones modify receptor sensitivity as well as the synthesis and/or turnover rate of many neurotransmitters. We evaluated the influence in adult rats of triiodothyronine (T3), administered s.c. (100 micrograms/kg) acutely (once only) or chronically (once a day for 3 or 7 consecutive days), on brain serotonin concentration and on the density and affinity of two brain serotonin (5-HT) receptor subtypes mainly involved in behavioral effects. After both acute and chronic T3 treatment, serotonin levels increased in the cerebral cortex but not in the hippocampus. The density and affinity of 5-HT1A receptors (using [3H]-8-OH-DPAT as ligand) were not affected, while there was a significant decrease in the number of 5-HT2 receptors in the cerebral cortex (using [3H]ketanserin as ligand). This observation might indicate that thyroid hormones enhance 5-HT concentration in certain brain areas, thus causing a down-regulation of 5-HT2 receptors. The serotonergic system could be involved in the complex brain-neurotransmitter imbalance underlying hyperthyroidism-linked behavioral changes.