On association between cortical 5-HT2A receptors and behavior in rats with experimental thyroid disturbances

Distinct behavioral phenotypes in male mice lacking the thyroid hormone receptor α1 or β isoforms

Thyroid hormones influence both neuronal development and anxiety via the thyroid hormone receptors (TRs). The TRs are encoded by two different genes, TRα and TRβ. The loss of TRα1 is implicated in increased anxiety in males, possibly via a hippocampal increase in GABAergic activity. We compared both social behaviors and two underlying and related non-social behaviors, state anxiety and responses to acoustic and tactile startle in the gonadally intact TRα1 knockout (α1KO) and TRβ (βKO) male mice to their wild-type counterparts. For the first time, we show an opposing effect of the two TR isoforms, TRα1 and TRβ, in the regulation of state anxiety, with α1 knockout animals (α1KO) showing higher levels of anxiety and βKO males showing less anxiety compared to respective wild-type mice. At odds with the increased anxiety in non-social environments, α1KO males also show lower levels of responsiveness to acoustic and tactile startle stimuli. Consistent with the data that T4 is inhibitory to lordosis in female mice, we show subtly increased sex behavior in α1KO male mice. These behaviors support the idea that TRα1 could be inhibitory to ERα driven transcription that ultimately impacts ERα driven behaviors such as lordosis. The behavioral phenotypes point to novel roles for the TRs, particularly in non-social behaviors such as state anxiety and startle.

Chronic actions of thyroxine on behavior and serotonin receptors in mouse strains with contrasting predispositions to catalepsy

The studies reported here addressed the effects of chronic administration of thyroxine (2 mg/liter for 60 days) on catalepsy and the functional activity and expression of the 5-HT(1A) and 5-HT(2A) receptor genes in the brains of adult male mice of the cataleptic ASC strain and the catalepsy-resistant AKR strain. Thyroxine induced cataleptics in AKR mice but had anticataleptic activity in ASC animals. Chronic thyroxine administration increased the functional activity and expression of 5-HT(2A) receptors in the frontal cortex in AKR mice but not in ASC mice. In ASC mice, the hormone significantly weakened the hypothermic effect of the 5-HT(1A) receptor agonist 8-OH-DPAT, though it did not alter the expression of these receptors. These results suggest that 5-HT(2A) receptors are involved in the cataleptogenic while 5-HT(1A) receptors are involved in the anticataleptic effects of the hormone in mice.

Chronic administration of imipramine normalizes decreased sexual motivation and increased predisposition to catalepsy induced by propylthiouracil in rats

Thyroxine synthesis inhibitors increase the predisposition to catalepsy and decrease sexual motivation and the amplitude of the acoustic startle reflex in rats. The sensitivity of these behavioral changes to antidepressants remains uncertain. Chronic administration of the classical tricyclic antidepressant imipramine (15 mg/kg, 21 days) prevented the appearance of high sensitivity to catalepsy and the decrease in sexual motivation in Wistar rats given propylthiouracil (50 mg/liter, 28 days), without altering the amplitude of the startle reflex. Normalization of behavior in response to imipramine was not associated with changes in movement activity in the open field test or the animals' body weight. There was also no change in the expression of the 5-HT2A serotonin receptor gene in the frontal cortex. The model of propylthiouracilinduced catalepsy with reduced sexual motivation in rats has potential for studying the role of thyroid abnormalities in the development of depression and the mechanisms of action of antidepressants.

Chronic thyroxine treatment activates the 5-HT2A serotonin receptor in the mouse brain

Chronic thyroxine administration (2.0 mg/l, 60 days) significantly increases the 5-HT2A serotonin receptor gene mRNA level in the frontal cortex and augments the frequency of head twitches induced by the receptor agonist DOI (1.0 mg/kg, i.p.) in AKR mice. The result indicates thyroid hormones involvement in 5-HT2A receptor regulation in adult brain.