@article{steele_zivkovic_siopes_underwood_2003, title={Ocular clocks are tightly coupled and act as pacemakers in the circadian system of Japanese quail}, volume={284}, ISSN={["0363-6119"]}, DOI={10.1152/ajpregu.00447.2002}, abstractNote={Our previous studies showed that the eyes of Japanese quail contain a biological clock that drives a daily rhythm of melatonin synthesis. Furthermore, we hypothesized that these ocular clocks are pacemakers because eye removal abolishes freerunning rhythms in constant darkness (DD). If the eyes are indeed acting as pacemakers, we predicted that the two ocular pacemakers in an individual bird must remain in phase in DD and, furthermore, the two ocular pacemakers would 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 in constant light, two components of body temperature were observed that fused into a consolidated rhythm after 5-6 days in DD, showing 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.}, number={1}, journal={AMERICAN JOURNAL OF PHYSIOLOGY-REGULATORY INTEGRATIVE AND COMPARATIVE PHYSIOLOGY}, author={Steele, CT and Zivkovic, BD and Siopes, T and Underwood, H}, year={2003}, month={Jan}, pages={R208–R218} }
 @article{underwood_steele_zivkovic_2001, title={Circadian organization and the role of the pineal in birds}, volume={53}, ISSN={["1097-0029"]}, DOI={10.1002/jemt.1068}, abstractNote={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. Microsc. Res. Tech. 53:48–62, 2001. © 2001 Wiley‐Liss, Inc.}, number={1}, journal={MICROSCOPY RESEARCH AND TECHNIQUE}, author={Underwood, H and Steele, CT and Zivkovic, B}, year={2001}, month={Apr}, pages={48–62} }
 @article{zivkovic_underwood_siopes_2000, title={Circadian ovulatory rhythms in Japanese quail: Role of ocular and extraocular pacemakers}, volume={15}, ISSN={["0748-7304"]}, DOI={10.1177/074873040001500211}, abstractNote={Previous studies have shown that the circadian system of Japanese quail is composed of multiple photic inputs and multiple oscillators. Among these are extraretinal photoreceptors that mediate both circadian and photoperiodic responses and circadian pacemakers in the eyes that, via neural and hormonal outputs, help to maintain rhythmicity of central circadian clocks (presumably located in the suprachiasmatic area of the hypothalamus). Furthermore, a component of the central circadian system is influenced by reproductive hormones. Under certain conditions, the circadian system of female quail can be induced to split into two circadian components: one driven by ocular pacemakers and one driven by feedback from reproductive hormones. Importantly, ovulation is either inhibited or permitted as these two oscillators (or sets of oscillators) constantly change internal phase relationships with each other, suggesting an “internal coincidence” mechanism in the control of ovulation. The oviposition patterns of quail in light-dark (LD) cycles also support an internal coincidence mechanism. The authors tested the hypothesis that the ocular pacemakers are an important component of an internal coincidence mechanism controlling ovulation by examining the effects of blinding by complete eye removal (EX), and the effects of eye-patching, on the body temperature and oviposition patterns of quail exposed to 24-h LD cycles. They also examined the effects of EX on quail exposed to continuous light (LL) and to continuous darkness (DD). Neither EX nor eyepatching affected the oviposition patterns of birds in LD. Furthermore, robust body temperature and oviposition rhythms continued in EX birds in LL, but body temperature became arrhythmic in DD with the cessation of ovulation. The results do not show a role for ocular pacemakers in the control of ovulation, but they do support the hypotheses that (1) entrainment of the central oscillators by extraretinally perceived light is sufficient to preserve a normal ovulatory pattern in LD in the absence of the ocular pacemakers, and (2) in LL, feedback of reproductive hormones onto the central oscillators is sufficient to organize the circadian system even in the absence of the ocular pacemakers. Whether or not the ocular pacemakers are normally involved in the control of ovulation is still an open question.}, number={2}, journal={JOURNAL OF BIOLOGICAL RHYTHMS}, author={Zivkovic, BD and Underwood, H and Siopes, T}, year={2000}, month={Apr}, pages={172–183} }
 @article{underwood_steele_zivkovic_1999, title={Effects of fasting on the circadian body temperature rhythm of Japanese quail}, volume={66}, ISSN={["0031-9384"]}, DOI={10.1016/S0031-9384(98)00287-X}, abstractNote={The effect of food deprivation on the body temperature and activity rhythms of quail was assessed in birds exposed to both light–dark (LD) cycles and to continuous darkness (DD). Quail normally exhibit a daily rhythm of body temperature in LD that will persist in DD (that is, the rhythm is circadian). In LD, 3 days’ food deprivation caused the body temperature to drop below its normal nighttime levels, whereas daytime body temperature was unaffected. In DD, food deprivation caused the body temperature to drop below normal at all phases of the circadian rhythm of body temperature. Accordingly, the lack of hypothermia during the light phase of the LD cycle following food deprivation must represent a direct exogenous or “masking” effect of light, and is not an endogenous property of the circadian system. Blind birds exposed to LD 12:12 exhibited an entrained body temperature rhythm, and food deprivation caused a drop in body temperature below normal levels during both the light and dark phases of the LD cycle. Accordingly, the masking effects of light observed in normal birds on LD cycles is mediated via retinal photoreceptors and not via extraretinal photoreceptors. Measurements of activity levels before and during fasting indicate that fasting-induced hypothermia cannot be explained simply as a consequence of decreases in activity levels. Food deprivation was also observed to cause significant phase shifts in the endogenous rhythm of body temperature.}, number={1}, journal={PHYSIOLOGY & BEHAVIOR}, author={Underwood, H and Steele, CT and Zivkovic, B}, year={1999}, month={Mar}, pages={137–143} }
 @article{zivkovic_underwood_steele_edmonds_1999, title={Formal properties of the circadian and photoperiodic systems of Japanese quail: Phase response curve and effects of T-cycles}, volume={14}, ISSN={["0748-7304"]}, DOI={10.1177/074873099129000786}, abstractNote={A role for the circadian system in photoperiodic time measurement in Japanese quail is controversial. The authors undertook studies of the circadian and photoperiodic system of Japanese quail to try to identify a role for the circadian system in photoperiodic time measurement. The circadian studies showed that the circadian system acts like a low-amplitude oscillator: It is readily reset by light without significant transients, has a Type 0 phase response curve (PRC), and has a large range of entrainment. In fact, a cycle length that is often used in resonance protocols (LD 6:30) is within the range of entrainment. The authors employed T-cycle experiments; that is, LD cycles with 6-and 14-h photoperiods and period lengths ranging from 18 to 36 h to test for circadian involvement in photoperiodic time measurement. The results did not give evidence for circadian involvement in photoperiodic time measurement: T-cycles utilizing 6-h photoperiods were uniformly noninductive (that is, did not stimulate the reproductive system), whereas T-cycles utilizing 14-h photoperiods were inductive (stimulatory). A good match was observed between the phase-angles exhibited on the T-cycles employing 6-h photoperiods and the predicted phase-angles calculated from a PRC generated from 6-h light pulses.}, number={5}, journal={JOURNAL OF BIOLOGICAL RHYTHMS}, author={Zivkovic, BD and Underwood, H and Steele, CT and Edmonds, K}, year={1999}, month={Oct}, pages={378–390} }