AGING ALTERS THE PHASE-RESETTING PROPERTIES OF A SEROTONIN AGONIST ONHAMSTER CIRCADIAN RHYTHMICITY

Serotonergic mechanisms are believed to play a considerable role in me diating the effects of photic and nonphotic stimuli on circadian rhyth micity. Because aging is associated with significant changes in the re sponsiveness of the rodent circadian system to major synchronizing age nts in the environment, this study examined the phase-shifting effects of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetrali n [8-OH-DPAT; 2.0, 5.0, and 8.0 mg/kg ip at circadian time 8 (CT 8)], on the 24-h activity rhythm of young (3-4 mo old) and old (18-19 mo ol d) golden hamsters. Aging was associated with a dramatic attenuation o f the phase-shifting effects of 8-OH-DPAT in this species. The results suggest the existence of age-related deficits in the serotonergic con trol of mammalian circadian rhythmicity, which could interfere with th e temporal adaptation of the senescent organism to its environment.