Histochemical demonstration of a circadian rhythm of succinate dehydrogenase in rat pineal gland. Influence of coenzyme Q10 addition.

Succinate dehydrogenase activity was investigated histochemically in the rat pineal gland. The influence of fixation on the activity pattern, the possible diffusion of enzyme, the nothing dehydrogenase reaction, and the substantivity of the tetrazolium salts and formazans were investigated in control experiments. In rats maintained on a 17/7 h light/dark schedule a distinct circadian rhythm of the succinate dehydrogenase was demonstrated in the pineal gland. Activity was lowest during the day and highest during the night. The dorsocaudal part of the gland showed the highest activity and within the same part of the gland the activity varied between individual pinealocytes. A relative lack of endogenous coenzyme Q, as well as a circadian rhythm of this coenzyme, highly influenced the activity of succinate dehydrogenase. It is concluded that succinate dehydrogenase activity in the pineal gland of the rat is regulated by changing the concentration of the active enzyme itself as well as the level of the endogenous coenzyme Q. Whether this is caused by a circadian rhythm in the synthesis or in the catabolism of the enzyme and the coenzyme was not revealed by the present study .