Going into more and more detail, here is a February 11, 2005 post about the current knowledge about the circadian organization in my favourite animal - the Japanese quail.
Japanese quail (Coturnix coturnix japonica), also known as the Asian Migratory Quail, are gallinaceous birds from the family Phasianidae, until 1960s thought to be a subspecies of European migratory quail (Coturnix coturnix coturnix), but now considered to be a separate species, designated as Coturnix japonica. The breeding range of the wild population encompasses Siberia, Mongolia, northeastern China and Japan, while the wintering range is over south and southeastern Asia.
These birds were domesticated in Japan for song contests. During World War II most of the domesticated stock was lost and the remaining birds were crossed with imports from Korea and China and selected for egg and meat productivity. Wild and feral quail in Japan also migrate from island to island. Attempts to release Japanese quail in North America for hunting were not successful. Very fast maturation, prolific reproduction, and ease of husbandry, made the Japanese quail a popular laboratory animal in the fields of developmental, neuroendocrine and behavioral biology. As with most, if not all, temperate zone birds the reproductive system of Japanese quail is under the control of photoperiod: long photoperiods (above about 12.5 hrs) stimulate the development and maturation of the gonads.
Quail's circadian system
The circadian system of Japanese quail is composed of several components including the pineal, the eyes and sites in the hypothalamus. Light can influence the circadian system of quail via several routes including the eyes, the pineal, and extraretinal-extrapineal photoreceptors in the brain.
Pineal
In contrast to some other avian species, the pineal organ is not endogenously rhythmic in Japanese quail but pineal rhythmicity in constant conditions is driven by neural inputs from the central pacemakers. This neural input may involve adrenergic inputs from the superior cervical ganglia, but central control of pineal rhythmicity can persist even after removal of the superior cervical ganglia. Under rhythmic neural stimulation, the pineal gland secretes melatonin in a rhythmic fashion. A rhythm of pineal melatonin secretion can also be directly driven by 24 hr light-dark cycles because the pineal is directly photosensitive. Accordingly, in vitro, pineal melatonin rhythmicity can be directly driven by light-dark cycles but rhythmicity cannot be sustained in constant conditions. In addition to direct light perception, pineal melatonin rhythmicity may also be influenced by light perceived by the eyes and, possibly, by extraretinal photoreceptors in the brain. Retinal input to the pineal includes the optic tract because cutting the optic nerve abolishes the ability of retinally perceived light to influence the pineal melatonin rhythmicity.
Eyes
By alternately patching each eye for a period of 12 hours in quail held in LL it is possible to entrain the melatonin rhythm of one eye 12 hours out of phase with the melatonin rhythm expressed by the other eye. Significantly, sectioning the optic nerves does not abolish the ability of an alternating-patch protocol to entrain the melatonin rhythm in the eyes 12 hours out of phase. Since cutting the optic nerves would deprive any extraocular clocks of entraining inputs, the fact that ocular rhythmicity is entrainable following optic nerve section proves that the clocks driving melatonin rhythmicity are located in the eyes themselves. Because blinding by complete eye removal causes quail to become arrhythmic in DD, it is likely that the eyes are not only biological clocks but they have an important role in helping to control the rest of the circadian system as well, that is, they are circadian pacemakers. The pineal and the hypothalamic oscillators are unable to sustain circadian oscillations in constant conditions in the absence of the ocular pacemakers. The Japanese quail are therefore unique among the vertebrates examined to date insofar as the eyes are essential for maintaining rhythmicity. The eyes of the pigeon are also important components of the circadian system but arrhythmicity in constant conditions requires removal of both the eyes and pineal.
The ocular pacemakers communicate with the pacemakers located in the hypothalamus neurally, presumably via the retino-hypothalamic tracts, as well as chemically, probably via the rhythmic synthesis and release of melatonin. The quail's eye shows a robust rhythm of melatonin synthesis and secretion in vivo and in vitro. Under a LD12:12 light cycle the blood shows a robust rhythm of melatonin content; about one third of the blood melatonin content is due to melatonin secretion from the eyes and the remaining two-thirds are secreted by the pineal. In continuous darkness (DD) the eyes contribute about half of the melatonin found in the blood. All normal quail exhibit a robust rhythm of body temperature in DD. Complete removal of the eyes, however, abolishes the rhythms of activity and body temperature in DD, while sectioning of optic nerves and leaving the eyes in situ renders about 25% of quail arrhythmic in DD. This implies that the eyes are coupled to the rest of the circadian system by both neural and hormonal inputs. The hormonal input is likely melatonin because optic nerve section does not affect the eye's ability to synthesize and secrete melatonin, and daily exogenous melatonin administration can entrain the circadian system of quail.
Suprachiasmatic Nuclei
The exact anatomical locus of the central pacemaker in quail is not known, but it is likely to reside in an equivalent of the mammalian suprachiasmatic nucleus. Lesions of the suprachiasmatic area that likely encompassed both the MHN and the LHRN rendered quail arrhythmic in DD. Recent studies in Japanese quail, utilizing molecular techniques point to the medial SCN (MHN) as the site of the biological clock, as the mRNA of the clock genes Per, Clk and Cry cycle in this region but clock gene expression is absent in the LHRN. However, either immature males or animals of undeclared sex and age were used in these studies. It is possible that clock genes may be expressed in the LHRN of mature birds under the control of reproductive hormones. In chicken, for example, estradiol had been reported to elicit the expression of the clock gene cry1 in the MHN. Interestingly, in house sparrows, a small subset of individuals exhibited expression of clock gene RNAs in the LHRN, but the age and sex of these birds was not reported. A recent study describes a neural connection between the two sets of nuclei, suggesting that both pairs may contain circadian pacemakers.
Retinal and Extraretinal photoreception
The circadian systems of non-mammalian vertebrates are characterized by multiple photic inputs: the eyes, extraretinal receptors in the brain, and pineal photoreceptors. Entrainment to 24 hr LD cycles is possible after removal of the eyes and/or pineal in many species of fish, amphibians, reptiles and birds. Significant amounts of light can penetrate the skull and reach the brain of vertebrates. The extraretinal photoreceptors are sensitive to intensities of light as low as the equivalent of moonlight. The exact location of extraretinal photoreceptors is not clearly established. That these receptors are located in the brain is shown by blocking light to the head which abolishes entrainment in blinded house sparrows, frogs and lizards. Also, localized illumination of the brain via fiber optics can entrain the activity rhythm of the lizard Sceloporus olivaceus.
Extraretinal photoreceptors are also involved in the photoperiodic response of non-mammalian vertebrates. Russell Foster and Sir Brian Follett showed that the extraretinal photoreceptors mediating the reproductive response to photoperiod in the Japanese quail likely employ a photopigment with wavelength sensitivity similar to that of rhodopsin. A number of studies in birds have localized the extraretinal photoreceptors mediating the photoperiodic response to the medial basal hypothalamus, although other brain sites have occasionally been implicated as well. However, it is not known if the extraretinal photoreceptors mediating entrainment are identical to the extraretinal photoreceptors mediating the photoperiodic response. Studies suggest multiple types of brain photopigments and at least two types of photoreceptors. Foster and colleagues suggest that CSF-contacting neurons are the strongest candidates for deep brain photoreceptors in lampreys, reptiles and birds, while classical neurosecretory neurons (NMPO cells) are photosensory in fish and amphibians.
That the eyes contribute to entrainment has been demonstrated in some fish, lizards and birds. The relative contribution of retinal and extraretinal photoreceptors to entrainment has not been established. Because the extraretinal photoreceptors have not been definitively localized, it has not been possible to determine if entrainment persists in animals with intact eyes but without extraretinal and pineal photoreceptors. Recently, molecular techniques were used to localize opsin reactivity to the nucleus ventromedialis of the forebrain in a lizard, Podarcis sicula. Inhibition of the opsin in this area by anti-sense RNA abolished extraretinally mediated circadian photoentrainment.
In the quail, entrainment of the pineal melatonin rhythm occurred if the eyes were "patched" for 12 out of every 24 hours in birds otherwise held in LL. The entrainment pathway from the eyes to the pineal involved the optic nerve because optic nerve section abolished entrainment in response to the patching regimen. Interestingly, a similar experiment performed in pigeons failed to elicit entrainment of the pineal melatonin rhythm.
Foster and colleagues have proposed a theoretical explanation for why both retinal and extraretinal photoreceptors may coexist in non-mammalian vertebrates. The optical nature of the eye allows a focused representation of the environment: large numbers of photons must be sampled quickly to build a spatial image of the world. The eye measures brightness in a particular point in space (radiance) but not from the whole field of view (irradiance). On the other hand, entrainment of circadian pacemakers (or perception of daylength) requires irradiance detection, but not a spatial imaging capability. Extraretinal photoreceptors are well-suited for irradiance detection because overlaying tissues scatter light. Because mammals lack extraretinal photoreceptors, Foster and colleagues hypothesize that the random projections of retinal ganglion cells to the location of the mammalian circadian pacemaker (the suprachiasmatic nucleus) could result in a form of irradiance detection in mammals. Behavioral studies in mammals, however, have failed to detect extraretinal photoreception.
Role of the reproductive system
In some birds, steroid hormones can have an effect on the circadian system. For instance, injections of testosterone induce splitting of the activity rhythm in male starlings (Sturnus vulgaris) and an increase in the duration of the active phase of the locomotor rhythm. Also, sexually mature male Japanese quail show a longer freerunning period than immature birds and exogenous administration of testosterone increases the freerunning period in DD of castrated male quail. An important role for the reproductive hormones in controlling the circadian system of female Japanese quail is seen because: (1) the freerunning activity, body temperature, and oviposition rhythms show an average periodicity of 26.7 hours in birds held in LL whereas the average freerunning period of activity and body temperature of non-ovulating birds in DD is 22.5 hours, (2) castration abolishes rhythmicity in female quail exposed to LL, and (3) normal birds are arrhythmic in LL until the onset of oviposition, whereupon the birds begin showing rhythms of activity and body temperature with the same periodicity as the rhythm of oviposition.
Placing permanent patches over both eyes of female quail in LL results in "splitting" of the body temperature rhythm into two circadian components: a "short" component driven by the pacemaker in the eyes which are experiencing DD, and a "long" component driven by the ovulatory cycle. The reproductive system remains active in eye-patched birds in LL because the photoperiodic response is stimulated directly by light via extraretinal photoreceptors in the brain. The two components in eye-patched birds in LL show constantly changing phase-relationships with each other. Significantly, at some phases ovulation is either delayed or inhibited, suggesting a phase-dependent effect of the oscillators on reproductive function.
Light can influence the system via three routes: the eyes, the pineal and the deep brain extraretinal photoreceptors. Extraretinal photoreceptors are involved both in the transduction of photoperiodic information and in entrainment. The pineal is not autonomously rhythmic but its rhythm of melatonin synthesis and secretion is driven by the rest of the system through a neural pathway that may, or may not, involve the superior cervical ganglia. The central pacemakers (SCN?) are considered to be "complex" pacemakers; that is, each SCN is, itself, composed of a set of coupled circadian clocks; that is, many, if not most, of the neurons comprising the SCN are competent clocks. The eyes are the sites of pacemakers that can drive a rhythm of melatonin synthesis and secretion. The ocular pacemakers are coupled to the rest of the system via the cyclic synthesis and release of melatonin and via a neural pathway, possibly the retino-hypothalamic (RH) tract.
Because the hypothalamic pacemakers (SCN) are, themselves, composed of multiple cellular clocks, under certain experimental conditions it is possible to cause these complex pacemakers to split into two circadian components: a short-period component driven by the ocular pacemakers and a long-period component driven by feedback from gonadal steroids. The precise location of the hypothalamic pacemaker is unknown but it is likely localized in a structure homologous to the mammalian SCN (either the MHN and/or LHRN). The central pacemakers, or perhaps some portions of them, may be directly sensitive to the steroid hormones. Mammalian SCN expresses estrogen receptors. Similar studies in birds did not reveal steroid receptor expression in the putative SCN, yet none of these studies were conducted in reproductively mature females. Alternatively, the effects of steroids on the SCN may be indirect, via centers in the preoptic area, which are known to contain neurons that express steroid receptors.
Sex differences and the role of reproductive system in circadian function are much more dramatic in the Japanese quail than in any other organism studied to date. Thus, the Japanese quail is an ideal laboratory model for studies of interactions between circadian and reproductive systems and of sex differences in such interactions. Interactions between the circadian and reproductive systems seem to act in both directions. In one direction, the circadian system controls daily rhythms of egg-laying and it is also crucially involved in daylength measurement used for seasonal timing of reproduction. On the other hand, the reproductive state affects properties of the circadian clock.
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