DIURNAL, PHOTOPERIODIC, AND AGE-RELATED-CHANGES IN PLASMA GROWTH-HORMONE LEVELS IN THE GOLDEN-HAMSTER

The golden hamster has been used extensively as an animal model for th e study of both circadian and seasonal rhythms, and their regulation b y the light-dark (LD) cycle. More recently, this species has been used to examine how the generation and entrainment of circadian rhythms ar e altered in advanced age. Recent studies in both humans and rodents i ndicate that age-related changes in the diurnal rhythm of pituitary gr owth hormone (GH) release may mediate some of the adverse effects of a ging on a variety of physiological systems. As a first step in determi ning whether or not age-related changes in circulating GH levels are a ssociated with changes in the regulation and/or expression of circadia n rhythms, the effects of age on both the ultradian and diurnal patter ns of plasma GH levels were determined in 3- to 22-month-old male hams ters that were bled every 15 min for a 24-hr period while entrained to an LD 14:10 light cycle. An additional study involving a similar bloo d collection protocol examined whether or not the length of the day is involved in the regulation of plasma GH levels. Although the frequenc y of pulsatile GH release did not change with advanced age, both the m ean levels of GH per sample and the mean amplitude per pulse of GH wer e significantly elevated in 3- to 4-month-old animals, compared to ani mals that were 12-13, 15-16, or 21-22 months of age. In hamsters aged 3-4 and 12-13 months, there was an increase in both mean levels and th e mean amplitude per pulse of GH, but not pulse frequency, during the night as compared to daytime values. No such diurnal rhythm was detect ed in the two groups of older animals. A clear diurnal rhythm in GH le vels was also detected in animals maintained in a short-day (LD 6:18) cycle, and the mean levels of GH per sample were greater in hamsters m aintained on short compared to long LD 14:10) days. These results indi cate that there are pronounced age-related changes in pituitary GH rel ease in the hamster, and that both the time of day and the length of t he day influence the pattern of GH release.