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

<p>We investigated if the duration and/or frequency of the light period affect 24-h rhythm of circadian clock genes in central and peripheral tissues of a non-photoperiodic songbird, the spotted munia (<i>Lonchura punctulata</i>), in which a circannual rhythm regulates the reproductive cycle. We monitored activity–rest pattern and measured 24-h mRNA oscillation of core clock (<i>Bmal1, Clock, Per2, Cry1</i> and <i>Cry2</i>) and clock-controlled (<i>E4bp4, Rorα</i> and <i>Rev-erbα</i>) genes in the hypothalamus, retina, liver and gut of spotted munia subjected to an aberrant light–dark (LD) cycle (3.5L:3.5D; T7, T = period length of LD cycle) and continuous light (LL, 24L:0D), 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 clock and clock-controlled genes showed a significant 24-h rhythm in all four tissues (except <i>Clock</i> 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 <i>Rev-erbα</i>) and liver (except <i>Bmal1</i> and <i>Rev-erbα</i> under T7 and <i>Cry1</i> under LL). In the retina, however, all genes showed arrhythmic 24-h mRNA expression under LL, but not under T7 (except in <i>E4bp4</i> and <i>Rorα</i>). Interestingly, unlike in the liver, <i>Bmal1, Per2, Cry1, Rorα</i> and <i>Rev-erbα</i> mRNA expressions were rhythmic in the gut under both T7 (except <i>Rorα</i>) and LL conditions. These results showed variable relationship of internal circadian clocks with the external light environment and suggested a weak coupling of circadian clocks between the central (hypothalamus and retina) and peripheral (liver and gut) tissues. We suggest tissue-level circadian clock regulation of daily physiology and behavior in the spotted munia.</p>