Adrenalectomy prevents changes in rat pineal melatonin content and N-acetyltransferase activity induced by acute insulin stress.

The activity of N-acetyltransferase (NAT) and the content of melatonin (MEL) in the rat pineal have been shown to be sensitive to several types of stressors. This study was designed to assess the role of the adrenals in mediating the effect of one such stressor, insulin-induced hypoglycemia, on pineal synthetic activity. Intact and bilaterally adrenalectomized (ADX) adult male rats were kept under light:dark cycles of 14:10 (lights on 0600 h) and injected intraperitoneally with 10 IU insulin at 1300 h, and groups (n = 8) were killed 2, 3, or 4 h postinjection. Plasma catecholamines were assayed by means of high performance liquid chromatography and radioimmunoassay was used to assess pineal NAT activity and MEL content. All injected groups were rendered hypoglycemic by insulin administration. Compared to uninjected controls, plasma epinephrine in hypoglycemic intact rats rose after 2 h, whereas epinephrine did not change in hypoglycemic ADX animals. The increase in epinephrine in intact animals was correlated with a rise in NAT activity at 2 h. Moreover, pineal MEL content at 2, 3, and 4 h was significantly greater than control values. In contrast, no changes in pineal biosynthetic function were found in ADX rats. This differential response by intact and ADX rats suggests that an adrenal product (possibly epinephrine) is responsible for mediating the stimulatory effects of acute insulin-induced hypoglycemic stress on the rat pineal.