In birds many life processes runs in diurnal (e.g. locomotor activity, feeding, melatonin secretion) and seasonal rhythms (e.g. reproduction, song, feathering, migration) depending on the environmental light and the activity of their central clock system (CCS). The structure and mechanisms of the activity of the avian CCS are the most complex among vertebrates. CCS consists of three oscillators (in the retina, SCN and pineal gland) possessing their own sensory input system (photopigments) and effective output system (products for direct biological effects). So far, 14 forms of photopigments (Opn1, Opn2, Opn3, TMT, Opn4x, Opn4m, Opn5, RGR, RRH, VA-opsins, pinopsin, Cry1, Cry2 i Cry4) and 12 clock genes making up oscillators (Bmal1, Bmal2, Clock, NPas2 called also Mop4 and Rorα – positive genes and Cry1, Cry2, Cry4, Per2, Per3, E4bp4 and Rev-erbα – negative genes) have been described in the CCS in birds. Photopigments are placed in all layers of the retina; in the brain – mainly in regions of nuclei: septalis lateralis, premammillaris, habenularis and paraventricularis; in the pineal gland – in all kinds of pinealocytes. Most photopigments belonging to the opsin family are linked with the nucleotide phototransduction path, typical for vertebrates, but, in avian CCS, also the phosphoinositol phototransduction path, characteristic for invertebrates, exists and concerns Opn4x and Opn5. Oscilators are placed in nuclei of cells of all layers of the retina, in mSCN and vSCN (with great species variability) and in pinealocytes. It is supposed that all nonvisual photopigments have a direct role in the synchronization of the oscillator activity with the environmental light, but molecular the mechanisms of the interaction between photopigments and the oscillator remain unknown. The impact of each of the three oscillators of the CCS in the generation of biological rhythms in birds show great species differentiation. The differences concern both the domination of one of the oscillators over the others and the assignation of biological processes which the individual oscillator synchronizes rhythmically with the environmental light.
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