Avian central clocking system

Abstract

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 the 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 own sensory input system (photopigments) and effective output system (products for direct biological effects). So far in the CCS in birds 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. Photopigments are placed in all layers of the retina; in 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 nucleotide phototransduction path, typical for vertebrates, but in avian CCS also phosphoinositol phototransduction path, characteristic for invertebrates, is existing 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 supposing, that all nonvisual photopigments have a direct role in the synchronization of the oscillator activity with the environmental light, but molecular mechanisms of the interaction between photopigments and the oscillator remain unknown. The impact of each of the three oscillators of CCS in generating of biological rhythms in birds show great species differentiation. The differences concern both the domination of one of oscillators over the others and the assignation of biological processes which individual oscillator synchronizes rhythmically with the environmental light..