Pineal biosynthetic activity and neuroendocrine physiology in the aging hamster and gerbil

The effects of advancing age on the ability of the pineal gland to produce melatonin and on the neuroendocrine physiology of Syrian hamsters and Mongolian gerbils were investigated. In 2-month-old male and female hamsters the nocturnal rise in pineal melatonin levels is on the order of 8-fold. As hamsters age the nighttime increases become progressively more depressed until in 18-month-old animals the nighttime rise in pineal melatonin is only twice the levels measured during the daylight hours. In male gerbils, the normal nocturnal increase in pineal melatonin levels observed in 2-month-old animals has completely disappeared by the time gerbils reach 19 months of age. The results suggest a marked reduction in pineal biosynthetic activity as animals age. In hamsters, neither hypothalamic norepinephrine nor dopamine levels were altered in 18-month-old animals compared to those in hamsters killed 8 weeks after birth. The reproductive system of both male and female hamsters and male gerbils seem to function relatively normally until the animals are 1 ^{1 2} years old. Thus, the majority of the 18-month-old female hamsters exhibit normal vaginal cyclicity. Likewise, the patterns of gonadotrophin and prolactin secretion are quite similar to those in the young females. At the light microscope level, most of the ovaries of the old hamsters, like those of the young animals, were found to contain pre-antral and antral follicles and corpora lutea. The uteri of the old hamsters exhibited the most pronounced changes. There was an obvious paucity of endometrial glands and the submucosa appeared fibrous. The testes of neither the old (18 month) hamsters nor the old (19 month) gerbils appeared greatly different from those of the young animals. Likewise, in advancing age there were no marked effects on circulating levels of gonadotrophins and prolactin in male hamsters. Finally, old male gerbils had plasma levels of testosterone similar to those observed in much younger animals.