Circadian organization and the role of the pineal in birds

Abstract

All organisms exhibit significant daily rhythms in a myriad of functions from molecular levels to the level of the whole organism. Significantly, most of these rhythms will persist under constant conditions, showing that they are driven by an internal circadian clock. In birds the circadian system is composed of several interacting sites, each of which may contain a circadian clock. These sites include the pineal organ, the suprachiasmatic nucleus (SCN) of the hypothalamus, and, in some species, the eyes. Light is the most powerful entraining stimulus for circadian rhythms and, in birds, light can affect the system via three different pathways: the eyes, the pineal, and extraretinal photoreceptors located in the deep brain. Circadian pacemakers in the pineal and in the eyes of some avian species communicate with the hypothalamic pacemakers via the rhythmic synthesis and release of the hormone melatonin. Often the hypothalamic pacemakers are unable to sustain persistent rhythmicity in constant conditions in the absence of periodic melatonin input from the pineal (or eyes). It has also been proposed that pineal pacemakers may be unable to sustain rhythmicity in constant conditions without periodic neural input from the SCN. Significant variation can occur among birds in the relative roles that the pineal, the SCN, and the eyes play within the circadian system; for example, in the house sparrow pacemakers in the pineal play the predominant role, in the pigeon circadian pacemakers in both the pineal and eyes play a significant role, and in Japanese quail ocular pacemakers play the predominant role.

Avian central clocking system

“…Ptaki posiadają więc trzy oscylatory w CCS, które in vitro działają niezależnie od siebie, generując własny rytm uwalniania różnych związków chemicznych. W warunkach in vivo wypadkowa działań wszystkich oscylatorów skutkuje wspólnym rytmem biologicznym wielu procesów życiowych organizmu (14,21,33,56,66,90). Mechanizm współdziałania trzech oscylatorów CCS ptaków na poziomie molekularnym nie jest jeszcze, niestety, poznany.…”

“…Szyszynka wydaje się mieć największe znaczenie w utrzymywaniu rytmu bezsenności podczas nocnej wędrówki u ptaków migrujących (87). U przepiórki japońskiej dominującym oscylatorem w CCS jest prawdopodobnie siatków-ka, gdyż enukleacja powodowała zaburzenie rytmu termoregulacji i aktywności lokomotorycznej (90). Enukleacja znosiła rytm aktywności ruchowej także u gołębiowatych (27), natomiast nie znosiła u wró-blowatych (88).…”

Circadian control of nocturnal songbird migration

“…Evidence suggest that the timing of Zugunruhe is controlled by an endogenous pacemaker system, located predominantly in the pineal gland, which rhythmically synthesizes and secretes melatonin (Gwinner and Brandstätter 2001, Underwood 2001, Kumar et al 2010. Pinealectomy has been shown to abolish circadian rhythms of Zugunruhe in white-throated sparrows (McMillan 1972).…”

Dynamics of three coupled van der Pol oscillators with application to circadian rhythms

“…The circadian system of all avian species consists of three primary components (see Figure 1.1): the pineal gland which rhythmicly produces and secretes melatonin, a hypothalamic oscillator (presumably located in the suprachiasmatic nucleus, or SCN) that most likely acts through neural signals, and the retinae of the eyes, which act through hormonal (melatonin) and/or neural outputs (see [13] for a review). Experimental evidence shows the relative importance of these components in generating circadian rhythms varies from species to species.…”