Effect of melatonin on serum cholesterol and phospholipid levels, and on 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, phospholipid levels and prolactin, thyroid-stimulating hormone (TSH), thyroxine (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 microg/rat) or vehicle, 2 h before lights off, and were killed 15 h after the last injection, and after a 24-hour fasting period. In pituitary-grafted rats, a decrease in serum free cholesterol with unmodified total cholesterol levels, and thus an augmented cholesterol esterification index, occurred. Pituitary-grafted rats showed also an increase in serum phospholipids. In control, but not in pituitary-grafted rats, melatonin injection decreased free cholesterol without modifying total cholesterol levels. Melatonin treatment (50 microg/day or greater) normalized the augmented serum phospholipid levels found in pituitary-grafted rats and increased serum phospholipids in control rats. Melatonin injection also 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 results demonstrate that melatonin counteracts in part lipid disturbances of hyperprolactinemic rats and lowers free plasma cholesterol and augmented serum phospholipids in control rats.