title: Revealing a circadian clock in captive arctic-breeding songbirds, lapland longspurs (Calcarius lapponicus), under constant illumination.
abstract: Most organisms in temperate or tropic regions employ the light-dark (LD) cycle as the primary Zeitgeber to synchronize circadian rhythms. At higher latitudes (>66°33'), continuous illumination during the summer presents a significant time-keeping dilemma for polar-adapted species. Lapland longspurs (Calcarius lapponicus), arctic-breeding migratory songbirds, are one of the few recorded species maintaining an intact diel rhythm in activity and plasma melatonin titers during polar summer. However, it is unknown whether rhythms are endogenous and entrain to low-amplitude polar Zeitgeber signals, such as daily variations in light intensity and the spectral composition of the sun (as measured by color temperature). Wild-caught male and female longspurs were brought into captivity, and locomotor activity was assessed using infrared detection. To examine if rhythms were endogenous, birds were exposed to constant bright light (LL; 1300 lux) or constant darkness (DD; 0.1 lux). All birds exhibited free-running activity rhythms in LL and DD, suggesting the presence of a functional circadian clock. Mean periods in LL (22.86 h) were significantly shorter than those in DD (23.5 h), in accordance with Aschoff's rule. No birds entrained to diel changes in light intensity, color temperature, or both. To examine endogenous molecular clock function, the Per2 gene was partially cloned in longspurs (llPer2) and transcripts were measured in hypothalamic tissue punches, eye, and liver using competitive polymerase chain reaction. Ocular llPer2 gene expression was periodic in LL and elevated at ZT24 (CT24) for LD or constant conditions (LL and DD), but llPer2 rhythmicity was not detected in hypothalamus or liver. Plasma melatonin was significantly lower in LL compared with LD or DD. In conclusion, rhythmic ocular Per2 expression and melatonin secretion may maintain the circadian activity rhythm across the polar day.
authors.affiliation: Department of Biology, Western Kentucky University, Bowling Green, Kentucky, USA noah.ashley@wku.edu.
authors.forename: Noah T
authors.initials: NT
authors.lastname: Ashley
authors.affiliation: Department of Biology, Waseda University, Shinjuku, Tokyo, Japan.
authors.forename: Takayoshi
authors.initials: T
authors.lastname: Ubuka
authors.affiliation: Abteilung für Verhaltensneurobiologie, Max-Planck-Institut für Ornithologie, Seewiesen, Germany.
authors.forename: Ingrid
authors.initials: I
authors.lastname: Schwabl
authors.affiliation: Abteilung für Verhaltensneurobiologie, Max-Planck-Institut für Ornithologie, Seewiesen, Germany.
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Goymann
authors.affiliation: Department of Biological Sciences, University of Alaska Anchorage, Alaska, USA.
authors.forename: Brady M
authors.initials: BM
authors.lastname: Salli
authors.affiliation: Department of Integrative Biology, Helen Wills Neuroscience Institute, University of California, Berkeley, California, USA.
authors.forename: George E
authors.initials: GE
authors.lastname: Bentley
authors.affiliation: Department of Biological Sciences, University of Alaska Anchorage, Alaska, USA.
authors.forename: C Loren
authors.initials: CL
authors.lastname: Buck
mesh_terms: D000818:Animals; D001110:Arctic Regions; D030161:Avian Proteins / Q000235:genetics / Q000378:metabolism; D057906:Circadian Clocks / Q000502:physiology*; D003624:Darkness; D005123:Eye; D005260:Female; D015870:Gene Expression; D007031:Hypothalamus / Q000502:physiology; D008027:Light; D008099:Liver / Q000502:physiology; D008297:Male; D008550:Melatonin / Q000097:blood; D009043:Motor Activity / Q000502:physiology; D056950:Period Circadian Proteins / Q000235:genetics / Q000378:metabolism; D020308:Songbirds / Q000502:physiology*
chemical_list: D030161:Avian Proteins; D056950:Period Circadian Proteins; D008550:Melatonin
keywords: Lapland longspur; Per2; circadian rhythm; clock genes; melatonin