Circannual oscillations of thyroxine and cholesterol levels after pinealectomy and ganglionectomy related to the weight changes of the pineal, pituitary, and thyroid glands.

Circannual rhythms of thyroxine (T4) and cholesterol blood levels, as well as of pineal, pituitary and thyroid weights were compared in intact, pinealectomized (Px), and ganglionectomized (Gx) rats. Special emphasis was put on the similarities and differences between the effect of Px and of Gx exerted on the neuroendocrine regulation of thyroid function. Animals were kept under changing lighting schedules simulating the natural seasonal conditions (December, L:D 8:16; June, L:D 16:8; March and September, L:D 12:12). Animals were killed 90 d after surgical interventions in the above-mentioned 4 months. In each season, animals were killed at four time points of the daily rhythm, and the means of the four data of a single day were used for further analysis evaluating circannual changes. All the investigated five parameters showed a monophasic circannual rhythm. The lowest T4 blood level was measured in summer; in contrast, cholesterol level and organ weights had a maximum in the same season of the year. The annual mean value of T4 level was diminished, while cholesterol level and pituitary and thyroid weights were increased in Px and Gx animals. Pineal weight showed a decrease in the Gx animals. Both Px and Gx failed to influence the circannual rhythms of the measured parameters. However, a significant T4-decreasing effect of Px and Gx was observed in the second half of the year, whereas a cholesterol-level-increasing effect, as well as thyroid hypertrophy, was registered in the first half of the year. These results suggest a prothyroid function of the pineal gland.