| Intra-ocular circadian clocks can influence extra-ocular rhythms in mammalian and non-mammalian vertebrates. Perhaps the most salient example is seen in Japanese quail where ocular clocks are circadian pacemakers since eye removal causes all quail tested to become arrhythmic in constant darkness (DD). Furthermore, the eyes communicate with the remainder of the circadian system via neural and hormonal mechanisms. This investigation was designed to define further the role of the eye in the quail 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. In this study we demonstrate that: (1) the ocular pacemakers remain coupled in prolonged DD; (2) the ocular clocks can be entrained 180° out of phase, and each clock can be shown to drive a discrete component of the body temperature rhythm; (3) out-of-phase clocks will rapidly regain their normal phase relationship within 5-6 days when exposed to DD.; Since the eyes are tightly coupled pacemakers, the mechanisms of pacemaker coupling were also investigated. Both neural (optic nerve) and hormonal (melatonin) coupling mechanisms contribute to ocular pacemaker coupling; however, neither the superior cervical ganglia nor the ciliary ganglia appear to transmit circadian information between ocular pacemakers.; Our model of the quail circadian system also proposes that the pineal-SCN complex is incapable of sustained rhythmicity in the absence of daily neural and hormonal input from the eyes. We demonstrate that, in the absence of ocular pacemakers, the pineal-SCN complex cannot drive persistent rhythmicity of blood melatonin profiles in DD.; Finally, our description of the quail pacemaker proposes that the eye is the site of an autonomous oscillator that can communicate via cyclic synthesis and release of melatonin. We demonstrate that quail retinal explants possess an autonomous circadian clock that can synthesize and secrete melatonin rhythmically in vitro. The Japanese quail is an excellent model for the study of vertebrate circadian organization. |
