Pigment-dispersing factor is involved in age-dependent rhythm changes in Drosophila melanogaster

Most animals show rest/activity rhythms that are regulated by an endogenous timing mechanism, the so-called circadian system. The rhythm becomes weaker with age, but the mechanism underlying the age-associated rhythm change remains to be elucidated. Here we employed Drosophila melanogaster as a model organism to study the aging effects on the rhythm. We first investigated activity rhythms under light-dark (LD) cycles and constant darkness (DD) in young (1-day-old) and middle-aged (30-, 40-, and 50-day-old) wild-type male flies. The middle-aged flies showed a reduced activity level in comparison with young flies. Additionally, the free-running period significantly lengthened in DD, and the rhythm strength was diminished. Immunohistochemistry against pigment-dispersing factor (PDF), a principal neurotransmitter of the Drosophila clock, revealed that PDF levels declined with age. We also found an attenuation of TIMELESS (TIM) oscillation in the cerebral clock neurons in elder flies. Intriguingly, overexpression of PDF suppressed age-associated changes not only in the period and strength of free-running locomotor rhythms but also in the amplitude of TIM oscillations in many pacemaker neurons in the elder flies, suggesting that the age-dependent PDF decline is responsible for the rhythm attenuation. These results suggest that the age-associated reduction of PDF may cause attenuation of intercellular communication in the circadian neuronal network and of TIM cycling, which may result in the age-related rhythm decay.