EFFECT OF MELATONIN ON SERUM-CHOLESTEROL AND PHOSPHOLIPID LEVELS, ANDON PROLACTIN THYROID-STIMULATING HORMONE AND THYROID-HORMONE LEVELS, IN HYPERPROLACTINEMIC RATS

The effects of melatonin treatment and pituitary transplants on serum total and free cholesterol levels, cholesterol esterification index, p hospholipid levels and prolactin, thyroid-stimulating hormone (TSH), t hyroxine (T4) and triiodothyronine (T3) levels were examined in rats. Male rats were grafted an anterior pituitary under the kidney capsule or were sham-operated on day 30th of life. Thirty days later, the rats received 4 daily s.c. injections of melatonin (25, 50 or 100 mu g/rat ) or vehicle, 2 h before lights off and were killed 15 h after the las t injection, and after a 24-hour fasting period. In pituitary-grafted rats, a decrease in serum free cholesterol with unmodified total chole sterol levels, and thus an augmented cholesterol esterification index, occurred. Pituitary-grafted rats showed also an increase in serum pho spholipids. In control, but not in pituitary-grafted rats, melatonin i njection decreased free cholesterol without modifying total cholestero l levels. Melatonin treatment (50 mu g/day or greater) normalized the augmented serum phospholipid levels found in pituitary-grafted rats an d increased serum phospholipids in control rats. Melatonin injection a lso reduced the high serum prolactin and T3 levels found in pituitary- grafted rats, and decreased T4 concentration in control rats. Neither melatonin nor pituitary grafts modified serum TSH concentration. The r esults demonstrate that melatonin counteracts in part lipid disturbanc es of hyperprolactinemic rats and lowers free plasma cholesterol and a ugmented serum phospholipids in control rats.