The interaction of thyroid state, MAOI drug treatment, and light on the level and circadian pattern of wheel-running in rats.

In order to examine the relationship between thyroid status, the circadian system, and antidepressant drug response, the antidepressant drug clorgyline, a monoamine oxidase inhibitor (MAOI), was administered chronically to sham-operated or thyroparathyroidectomized rats. Wheel-running was monitored continuously in a light-dark (LD) cycle, and then in constant dim light. In LD, MAOI treatment increased levels of running. This effect was delayed in hypothyroid rats relative to euthyroid rats. In constant light, the MAOI-induced increase in running was diminished in euthyroid but not hypothyroid animals. Hypothyroid animals were less responsive to the change in lighting than were euthyroid animals, and this was more apparent in hypothyroid rats given MAOI. The daily pattern of running differed with lighting condition as well as with treatment group. MAOI-treatment of hypothyroid animals phase-advanced the pattern of wheel-running. MAOI-treatment of control animals increased the amplitude of wheel-running particularly in the LD cycle. These results indicate that thyroid status, lighting, and MAOI treatment interact to alter the behavioral response to chronic drug treatment.