Brief constant light accelerates serotonergic re-entrainment to large shifts of the daily light/dark cycle.
Brief ( approximately 2 day) constant light exposure (LL(b)) 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 LL(b) 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 LL(b) 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 LL(b) (P>0.5 vs. vehicle); however in animals receiving LL(b,)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<0.035 vs. vehicle). Next, the effects of LL(b) combined with 8-OH-DPAT or L-tryptophan+fluoxetine on serotonergic re-entrainment to 10 h phase-advance and phase-delay shifts of the LD cycle were assessed. In groups not exposed to LL(b), vehicle controls re-entrained slowly to the advance and delay shifts (means=16+/-1 and 24+/-4 days, respectively), but those treated with 8-OH-DPAT re-entrained faster (means=11+/-2 and 9+/-2 days, respectively; both P<0.05 vs. vehicle). In groups exposed to LL(b), vehicle controls re-entrained slowly to the advance and delay shifts (means=15+/-2 and 25+/-3 days, respectively); however those receiving 8-OH-DPAT rapidly re-entrained to the delay and advance shifts, with the majority (75%) requiring only 1-2 days (means=2+/-1 and 4+/-2 days, respectively; both P<0.05 vs. vehicle). Animals exposed to LL(b) and treated with L-tryptophan+fluoxetine also exhibited accelerated re-entrainment to a 10 h advance shift (mean=5+/-2 days; P<0.05 vs. vehicle). Thus through enhancing serotonergic phase-resetting, LL(b) facilitates rapid re-entrainment to large shifts of the LD cycle which offers a potential approach for treating circadian-related desynchronies.
Kaur
G
G
Department of Biological Sciences, Kent State University, Kent, OH 44242, USA.
Thind
R
R
Glass
J D
JD
D017371:8-Hydroxy-2-(di-n-propylamino)tetralin / Q000008:administration & dosage; D000704:Analysis of Variance; D000818:Animals; D002940:Circadian Rhythm / Q000187:drug effects / Q000502:physiology*; D006224:Cricetinae; D005473:Fluoxetine / Q000008:administration & dosage; D008297:Male; D008647:Mesocricetus; D009043:Motor Activity / Q000187:drug effects; D010775:Photic Stimulation; D017440:Photoperiod; D012701:Serotonin / Q000378:metabolism*; D018490:Serotonin Agents / Q000008:administration & dosage; D017366:Serotonin Receptor Agonists / Q000008:administration & dosage; D017367:Selective Serotonin Reuptake Inhibitors / Q000008:administration & dosage; D013997:Time Factors; D014364:Tryptophan / Q000008:administration & dosage
D017371:8-Hydroxy-2-(di-n-propylamino)tetralin; D005473:Fluoxetine; D012701:Serotonin; D018490:Serotonin Agents; D017366:Serotonin Receptor Agonists; D017367:Selective Serotonin Reuptake Inhibitors; D014364:Tryptophan