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 circa dian system, and antidepressant drug response, the antidepressant drug clorgyline, a monoamine oxidase inhibitor (MAOI), was administered ch ronically to sham-operated or thyroparathyroidectomized rats. Wheel-ru nning was monitored continuously in a light-dark (LD) cycle, and then in constant dim light. In LD, MAOI treatment increased levels of runni ng. This effect was delayed in hypothyroid rats relative to euthyroid rats. In constant light, the MAOI-induced increase in running was dimi nished in euthyroid but not hypothyroid animals. Hypothyroid animals w ere less responsive to the change in lighting than were euthyroid anim als, and this was more apparent in hypothyroid rats given MAOI. The da ily pattern of running differed with lighting condition as well as wit h treatment group. MAOI-treatment of hypothyroid animals phase-advance d the pattern of wheel-running. MAOI-treatment of control animals incr eased the amplitude of wheel-running particularly in the LD cycle. The se results indicate that thyroid status, lighting, and MAOI treatment interact to alter the behavioral response to chronic drug treatment.