title: Entrainment of the circadian rhythm in the rat pineal N-acetyltransferase activity by melatonin is photoperiod dependent.
abstract: Entraining effect of melatonin on the circadian rhythm in rat pineal N-acetyltransferase (NAT) activity was studied under various photoperiods. Melatonin administration prior to dark onset for 5 successive days phase-advanced the evening NAT rise under the light:dark (LD) cycle of either LD 10:14 or LD 8:16, but not under LD 12:12. It is assumed that under the latter regime, the end of a light period exhibited a phase-delaying effect on the NAT rise. The light exposure appeared to be a stronger Zeitgeber than melatonin itself. Data show that melatonin applied in the late light period advances the evening NAT rise under a short photoperiod only; under a longer photoperiod, the phase-advancing effect of melatonin may conflict with a phase-delaying effect of the end of a light period, and the effect of light exposure overrides that of melatonin.
authors.affiliation: Institute of Physiology, Czechoslovak Academy of Sciences, Prague.
authors.forename: M
authors.initials: M
authors.lastname: Humlová
authors.forename: H
authors.initials: H
authors.lastname: Illnerová
mesh_terms: D000818:Animals; D001191:Arylamine N-Acetyltransferase / Q000378:metabolism*; D002940:Circadian Rhythm / Q000187:drug effects*; D008297:Male; D008550:Melatonin / Q000494:pharmacology*; D009043:Motor Activity / Q000187:drug effects; D017440:Photoperiod; D010870:Pineal Gland / Q000201:enzymology*; D051381:Rats; D017208:Rats, Wistar
chemical_list: D001191:Arylamine N-Acetyltransferase; D008550:Melatonin