Submitted October 16, 2007
Returned for revision November 21, 2007
Accepted March 12, 2008
Testis
Age-Related Changes in Diurnal Rhythms and Levels of Gonadotropins, Testosterone, and Inhibin B in Male Rhesus Monkeys (Macaca mulatta)
Stefan Schlatt *,
Clifford R. Pohl ,
Jens Ehmcke ,
and
Suresh Ramaswamy
* To whom correspondence should be addressed. E-mail: schlatt{at}pitt.edu.
Abstract
Testosterone shows circadian rhythms in monkeys with low serum levels in the morning hours. The decline relies on a diminished frequency of LH pulses. Inhibin B shows no diurnal patterns. In aged men the diurnal rhythm of testosterone is blunted and inhibin levels fall. Here we explore whether aging exerts similar effects in the rhesus monkey. We collected blood samples from groups of young (6-9 years) and old (12-16 years) male rhesus monkeys at 20 minute intervals for a period of 24-hours under remote sampling via a venous catheter. We determined moment to moment changes in plasma levels of testosterone, FSH, LH by RIA and inhibin B by ELISA. We found significant diurnal patterns of testosterone in both groups. The circadian rhythm in testosterone was enhanced in older monkeys. Testosterone levels and pulse frequencies drop significantly below those of young monkeys during midday hours. Diminished pulse frequency of LH appeared to be responsible for the midday testosterone decrease in old monkeys while LH and testosterone pulse frequency did not change in young monkeys at corresponding time points. Old monkeys show extended periods of LH-pulse quiescence in the morning and midday hours. Inhibin B and FSH levels were generally lower in old monkeys compared to the young group but neither inhibin B nor FSH showed circadian rhythms. We conclude from these data that old rhesus monkeys have a more prominent circadian rhythm of LH and testosterone resulting from an extended midday period of quiescence in the hypothalamus-pituitary-gonadal axis.
Key words:
Aging •
Circadian rhythm •
Inhibin •
Luteinizing hormone •
Testosterone
