Adrenalectomy or superior cervical ganglionectomy modifies the nocturnal increase in rat pineal type II thyroxine 5'deiodinase

We studied the response of type II thyroxine 5'deiodinase (5'D) activity to superior cervical ganglionectomy (SCGX) or adrenalectomy (ADX) in the rat pineal gland and other tissues. The results show that no difference was found between controls and SCGX animals during the day, but at night, SCGX modified the day-night cycle of 5'D activity in the pineal gland. In the same way, ADX did not modify the enzyme activity during the day in pineal gland, harderian gland, hypophysis, or brain frontal cortex (BFC). However, in brown adipose tissue (BAT), where thyroid hormone metabolism is extremely dependent on α₁-adren-ergic stimulation by blood circulating catecholamines, 5'D activity is significantly decreased. At the time point of maximal pineal 5'D activity in controls (02:00 h), ADX animals did not exhibit the nocturnal increase of the enzyme activity that occurs with control rats. Moreover, at 04:00 h ADX did not show any effect on pineal 5'D activity. These results seem to suggest that the presence of catecholamines in blood is necessary for the pineal 5'D activity nocturnal increase, although it does not participate in regulating the basal enzyme activity during the day. Key Words: Deiodinase - Thyroxine - Pineal gland - Adrenalectomy - Superior cervical ganglionectomy - Circadian rhythm.