Extra-hypothalamic brain clocks in songbirds: Photoperiodic state dependent clock gene oscillations in night-migratory blackheaded buntings, Emberiza melanocephala

Abstract

The avian circadian pacemaker system is comprised of independent clocks in the retina, pineal and hypothalamus, as shown by daily and circadian oscillations of core clock genes (Per2, Cry1, Bmal1 and Clock) in several birds including migratory blackheaded buntings (Emberiza melanocephala). This study investigated the extra-hypothalamic brain circadian clocks in blackheaded buntings, and measured Per2, Cry1, Cry2, Bmal1 and Clock mRNA expressions at 4h intervals over 24h beginning 1h after light-on in the left and right telencephalon, optic tectum and cerebellum, the brain regions involved in several physiological and cognitive functions. Because of seasonal alterations in the circadian clock dependent brain functions, we measured daily clock gene oscillations in buntings photoperiod-induced with the non-migratory state under short days (SDnM), and the pre-migratory (LDpM), migratory (LDM) and post-migratory (refractory, LDR) states under long days. Daily Per2 oscillations were not altered with changes in the photoperiodic states, except for about 2–3h phase difference in the optic tectum between the SDnM and LDpM states. However, there were about 3–5h differences in the phase and 2 to 4 fold change in the amplitude of daily Bmal1 and Cry1 mRNA oscillations between the photoperiod-induced states. Further, Cry2 and Clock genes lacked a significant oscillation, except in Cb (Cry2) and TeO and Rt (Clock) under LDR state. Overall, these results show the presence of circadian clocks in extra-hypothalamic brain regions of blackheaded buntings, and suggest tissue-dependent alterations in the waveforms of mRNA oscillations with transitions in the photoperiod-induced seasonal states in a long-day species.