Brief constant light accelerates serotonergic re-entrainment to large shifts of the daily light/dark cycle

Abstract Brief (∼2 day) constant light exposure (LLb ) in hamsters dramatically enhances circadian phase-resetting induced by the 5-HT receptor agonist, (±)-2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT) and other nonphotic stimuli. The present study was undertaken to determine if LLb can also amplify phase-resetting responses to endogenous 5-HT and accelerate re-entrainment to large-magnitude advance and delay shifts of the light/dark (LD) cycle. First, central serotonergic activity was increased by i.p. injection of l -tryptophan±the 5-HT reuptake inhibitor fluoxetine. Hamsters under LD or exposed to LLb received vehicle or drugs during the early morning, and phase-shifts of the locomotor activity rhythm were measured after release to constant darkness. Neither drug phase-shifted animals not exposed to LLb ( P >0.5 vs. vehicle); however in animals receiving LLb, l -tryptophan with and without fluoxetine produced large phase-advance shifts (means=2.5±0.4 h and 2.6±0.2 h, respectively; both P