Under periodic day-night environment, most circadian functions maintain a close phase relationship relative to each other, suggesting a common circadian pacemaker control of different overt rhythms. In birds, this seems highly unlikely, given multioscillatory nature of the circadian pacemaker and downstream generation of several circadian behaviors. We hypothesized the dissociation of overt rhythms from circadian gene oscillations, if the two were loosely coupled, under an aperiodic light condition. We tested this in daily rhythms in singing, activity and clock gene expressions in adult male zebra finches (Taeniopygia guttata) that were born and raised under the constant light (LL; 24L:0D), with controls on an LD cycle (12L: 12D). Particularly, we monitored daily pattern of singing and activity behavior, and measured 24 h mRNA expression of immediate early gene (c-Fos), clock genes (Bmal1, Per2 and Rev-erb β) and epigenetic marker genes (Dnmt3b and Tet2) in the hypothalamus, and of clock genes and genes coding for the aromatase (Arom), androgen receptor (Ar) and dopamine receptor (Drd2) in the song control nuclei (Area X and HVC) and cerebellum (motor control region). We found persistence of daily rhythms in activity and singing in all birds under LD, but in only 70% (14/20) birds under LL; thus, both behaviors were arrhythmic in 30% (6/20) birds) under LL. The overall song quality was also declined under LL. The clock genes showed daily rhythms in the hypothalamus, song control nuclei (except Per2 in Area X) and cerebellum under LD, although with differences in peak expression times; however, there was loss of rhythmicity in clock genes (except Bmal1 in Area X and HVC) under LL. We also found daily Ar mRNA rhythm in the Area X and cerebellum under LD. These results demonstrate for the first time the persistence of clock gene oscillations in the song control brain regions and show the dissociation of circadian behavior from genetic oscillations in relation to an imposed light environment.
Keywords: Circadian; clock genes; gene expression; hypothalamus; song; zebra finch.