Effect of chronic thyroxine treatment on catalepsy in rats.

The effect of chronic thyroid hormone (thyroxine, T4) administration on the duration of cataleptic freezing was studied in males of random-bred Wistar and genetic cataleptic (GC) rat strains. It was found that thyroidectomy brought about a sharp increase in immobility time in Wistar rats. Replacement with 0.015 mg/kg per day of T4 for 30 days from the day after thyroidectomy prevented the development of predisposition to catalepsy, whereas the same dose of T4 failed to attenuate the predisposition to catalepsy in the case of a month delay between the thyroidectomy and the beginning of treatment. A chronic administration of T4 at a dose of 0.1 mg/kg per day clearly decreased the genetically determined high expression of cataleptic reaction in GC rats. The results are evidence of the involvement of T4 in the regulation of cataleptic freezing and suggest that predisposition to catalepsy may be caused or enhanced by a deficit of thyroid hormone.