Temporal expression of clock genes in central and peripheral tissues of spotted munia under varying light conditions: Evidence for circadian regulation of daily physiology in a non-photoperiodic circannual songbird species.

We investigated if the duration and/or frequency of the light period affect 24-h rhythm of circadian dock genes in central and peripheral tissues of a non-photoperiodic songbird, the spotted munia (Lonchura punctulata), in which a circannual rhythm regulates the reproductive cycle. We monitored activity-rest pattern and measured 24-h mRNA oscillation of core clock (Bmal1, Clock, Per2, Cryl and Cry2) and dock-controlled (E4bp4, Rora and Rev-erba) genes in the hypothalamus, retina, liver and gut of spotted munia subjected to an aberrant light-dark (LD) cycle (3.5L:3.5D; 17, T = period length of LD cycle) and continuous light (LL, 24L:OD), with controls on 24-h LD cycle (T24, 12L:12D). Munia exhibited rhythmic activity-rest pattern with period matched to T7 or T24 under an LD cycle and were arrhythmic with a scattered activity pattern and higher activity duration under LL. At the transcriptional level, both dock and clock-controlled genes showed a significant 24-h rhythm in all four tissues (except Clock in the liver) under 12L:12D, suggesting a conserved tissue-level circadian time generation in spotted munia. An exposure to 3.5L:3.5D or LL induced arrhythmicity in transcriptional oscillation of all eight genes in the hypothalamus (except Rev-erba) and liver (except Bmall and Rev-erba under T7 and Cryl under LL). In the retina, however, all genes showed arrhythmic 24-h mRNA expression under LL, but not under T7 (except in E4bp4 and Rora). Interestingly, unlike in the liver, Small, Per2, Cryl, Rora and Rev-erba mRNA expressions were rhythmic in the gut under both 17 (except Rora) and LL conditions. These results showed variable relationship of internal circadian clocks with the external light environment and suggested a weak coupling of circadian docks between the central (hypothalamus and retina) and peripheral (liver and gut) tissues. We suggest tissue-level circadian dock regulation of daily physiology and behavior in the spotted munia.