Thyroid state: effects on pre- and postsynaptic central noradrenergic mechanisms.

For hypothyroid rats, spontaneous motor activity was less than that in matched normal controls, and the specific activity of tyrosine hydroxylase in the midbrain was significantly greater than that in controls. Rats made hyperthyroid with thyroxine became hyperactive and showed increased sensitivity to the behaviorally activating effects of norepinephrine administered intraventricularly. In hyperthyroid rats, the specific activity of tyrosine hydroxylase in the midbrain remained within the normal range. These results are consonant with studies that suggested both receptor "tuning" and feedback regulation of activity of enzymes involved in biosynthesis of presynaptic neurotransmitter as methods of regulation of the central catecholamine synapse. These results may also help explain the reported potentiation by thyroid hormone of the antidepressant effects of imipramine.