Circadian organization in Japanese quail: Ocular clocks are pacemakers

Abstract

A growing body of evidence supports the idea that circadian clocks in the eye can influence extra-ocular rhythms in both mammalian and non-mammalian vertebrates. Perhaps the most salient example is seen in Japanese quail where ocular clocks are pacemakers in the circadian system since eye removal causes all quail tested to become arrhythmic in constant darkness (DD). Moreover, the eyes play an endocrine role in the quail circadian system since one-third to one-half of the blood melatonin is of ocular origin and rhythmic blood-borne melatonin can affect circadian body temperature and activity rhythms. This investigation was designed to define further the role of the eye as a pacemaker in the quail?s circadian system.

Given that quail maintain robust circadian rhythms of body temperature in prolonged DD, the two putative ocular pacemakers in an individual bird must maintain the same phase; otherwise, a consolidated circadian output could not be generated. Furthermore, if ocular clocks are indeed pacemakers, the two putative pacemakers should rapidly regain coupling after being forced out of phase. These predictions were confirmed by demonstrating that 1) the ocular melatonin rhythms of the two eyes maintained phase for at least 57 days in DD and 2) after ocular pacemakers were forced out of phase by alternately patching the eyes (AP) in constant light, two components of body temperature were observed that fused into a consolidated rhythm after 5?6 days in DD, demonstrating pacemaker recoupling. The ability to maintain phase in DD and rapidly recouple after out-of phase entrainment demonstrates that the eyes are strongly coupled pacemakers that work in synchrony to drive circadian rhythmicity in Japanese quail.

Since the eyes are indeed coupled pacemakers, the mechanisms of pacemaker coupling were also investigated. Quail were subjected to one of several surgical procedures including optic-nerve section, superior cervical ganglionectomy, ciliary ganglionectomy, and melatonin implantation. Birds were then subjected to AP protocols to determine if these manipulations would affect pacemaker recoupling. Optic-nerve section and melatonin implantation lengthened the recoupling time of ocular pacemakers to 8 and 7 days, respectively, and each were significantly different than control groups. However, neither the superior cervical ganglia nor the ciliary ganglia appear to transmit circadian information between ocular pacemakers. Therefore, both neural (optic nerve) and hormonal (melatonin) coupling mechanisms contribute to ocular pacemaker coupling.

It is known that eye-removal abolishes rhythmicity of body temperature and activity rhythms in quail. We hypothesized that the pineal-SCN complex in quail is incapable of maintaining rhythmicity in the absence of ocular pacemakers. This hypothesis was confirmed since birds blinded by complete eye-removal displayed arrhythmic blood melatonin profiles in DD further supporting the idea that ocular clocks are pacemakers in quail.

Based on current and previous data, the eye plays a major role within the quail circadian system that can, at least in part, exert its control via the cyclic synthesis and release of melatonin. If this view of the quail circadian system is correct, one must assume that the ocular pacemaker is an autonomous oscillator. In the present study we demonstrate that quail retinal explants possess a light-entrainable circadian clock that can synthesize and secrete melatonin rhythmically in vitro for at least five days in DD.

Taken together, the present experiments fully support the hypotheses that 1) the eyes are the loci of tightly coupled circadian pacemakers in Japanese quail, 2) ocular pacemakers are coupled via neural and hormonal mechanisms, 3) in the absence of ocular pacemakers, the pineal-SCN complex is incapable of maintaining rhythmic blood melatonin profiles in DD, and 4) the quail's eye possesses an autonomous, light-entrainable circadian clock capable of rhythmic production and secretion of melatonin.