Do biological clocks age like their owners?

Abstract

Aging of biological clocks that time various biochemical, physiological and behavioural functions in organisms, may be reflected in its modified expression in aged animals compared to the young ones. Traditionally sleep disorders have been considered to be the major symptoms of aging of biological clocks, which in turn cause disruption in other functions such as digestion, mood, fatigue, and decrease in alertness. These disorders are treated using hypnotics, which are well known for their side effects. Although the aging of biological clocks has been studied to some details, our understanding about the mechanism remains in its infancy. It has been reported that circadian rhythms in aged individuals are less precise, shorter in period, smaller in amplitude, slow in resynchronization to shifted light/dark cycle, advanced in phase compared to the younger individuals and that finally, just before death the rhythms disappear altogether. There is hardly any evidence as to whether these overt changes in the biological rhythms are due to impairment of the biological clock itself, and/ or the entrainment pathways, and/or in the pathways downstream to the biological clock. At the molecular level, it is now known that the levels of some clock genes and its products in mice (mPer1, mCryl and mClk) do not change with age. The level of the Period gene mPer2, however, was demonstrated to change with age. Therefore, the mPer2 gene and its protein product should be studied in great details both in young and old animals to understand the molecular mechanisms behind aging of biological clocks in mice. Some studies also indicated that the undesirable modifications of biological rhythms at old age might be caused mainly due to modifications of the processes downstream to the central clock mechanisms in the suprachiasmatic nucleus (SCN). In order to decelerate the aging of the biological clocks, several non-pharmacological treatments like administration of bright light of appropriate strength at appropriate phases of the circadian cycle have been recommended in the place of the several hypnotics which are associated with severe side effects.