Temporal Expression of c-fos and Genes Coding for Neuropeptides and Enzymes of Amino Acid and Amine Neurotransmitter Biosynthesis in Retina, Pineal and Hypothalamus of a Migratory Songbird: Evidence for Circadian Rhythm-Dependent Seasonal Responses

This study investigated whether, in photoperiodic songbirds, the circadian pacemaker system (CPS) connects to the seasonal photoperiodic responses, by changes at transcriptional level in the level and 24-h rhythm of its constituent neurotransmitters. We used black-headed buntings (Emberiza melanocephala), which exhibit distinct seasonal states in captivity under appropriate photoperiods and hence served as a useful model system. Under short days, buntings remain in the photosensitive state (Pse) (winter phenotype: non-migratory, non-breeding). Under long days, however, buntings undergo through early-photostimulated (spring phenotype: pre-migratory, pre-breeding), late photostimulated (summer phenotype: migratory, breeding) and photorefractory (autumn phenotype: post-breeding) states. During all four seasonal states, we measured in the retina, pineal and hypothalamus, which together form avian CPS, 4-hourly mRNA expression of c-fos (a neuronal-activity marker) and of genes coding for neuropeptides (vasoactive intestinal peptide, vip; somatostatin, sst; neuropeptide Y, npy) and for intermediary enzymes of amino acid (glutamate: glutaminase, gls and glutamic-oxaloacetic transaminase 2, got2; GABA: glutamic acid decarboxylase, gad65) and amine (dopamine: tyrosine hydroxylase, th) neurotransmitters biosynthetic pathway. There was a significant alteration in level and 24-h pattern of mRNA expression, albeit with seasonal differences in presence, waveform parameters and phase relationship of 24-h rhythm, of different genes. Particularly, mRNA expression of all candidate genes (except hypothalamic vip, pineal gls and retinal th) was arrhythmic in late photostimulated state. These results underscore that circadian rhythm of peptide, amino acid and amine neurotransmitter biosynthesis in CPS plays a critical role in the photoperiodic regulation of seasonal states in birds.