Zebrafish Circadian Clock Entrainment and the Importance of Broad Spectral Light Sensitivity

One of the key defining features of an endogenous circadian clock is that it can be entrained or set to local time. Though a number of cues can perform this role, light is the predominant environmental signal that acts to entrain circadian pacemakers in most species. For the past 20 years, a great deal of work has been performed on the light input pathway in mammals and the role of intrinsically photosensitive retinal ganglion cells (ipRGCs)/melanopsin in detecting and sending light information to the suprachiasmatic nucleus (SCN). In teleost fishes, reptiles and birds, the biology of light sensitivity is more complicated as cells and tissues can be directly light responsive. Non-visual light signalling was described many years ago in the context of seasonal, photoperiodic responses in birds and lizards. In the case of teleosts, in particular the zebrafish model system, not only do peripheral tissues have a circadian pacemaker, but possess clear, direct light sensitivity. A surprisingly wide number of opsin photopigments have been described within these tissues, which may underpin this fundamental ability to respond to light, though no specific functional link for any given opsin yet exists. In this study, we show that zebrafish cells show wide spectral sensitivities, as well as express a number of opsin photopigments – several of which are under direct clock control. Furthermore, we also show that light outside the visual range, both ultraviolet and infrared light, can induce clock genes in zebrafish cells. These same wavelengths can phase shift the clock, except infrared light, which generates no shift even though genes such as per2 and cry1a are induced.

A Comparative Perspective on Extra-retinal Photoreception

Ubiquitous in non-mammalian vertebrates, extra-retinal photoreceptors (ERPs) have been linked to an array of physiological, metabolic, behavioral, and morphological changes. However, the mechanisms and functional roles of ERPs remain one of the enduring questions of modern biology. In this review article, we use a comparative framework to identify conserved roles and distributions of ERPs, highlighting knowledge gaps. We conclude that ERP research can be divided into two largely unconnected categories: (i) identification and localization of photoreceptors and (ii) linkage of non-retinal light reception to behavioral and physiological processes, particularly endocrine systems. However, the emergence of novel gene editing and silencing techniques is enabling the unification of ERP research by allowing the bridging of this divide.

Behavioral thermoregulation and the role of melatonin in a nocturnal snake

Daily and seasonal variations in hormone levels influence the complex interactions between behavior and physiology. Ectothermic animals possess the unique ability behaviorally to adjust body temperature (T(b)) to control physiological rate processes. Thus, a hormone may indirectly influence a physiological rate by directly influencing the behaviors that adjust or control that rate process. Although many hormonal influences on behavioral regulation of T(b) remain uninvestigated, melatonin (MEL) generally is considered a hormone that decreases mean preferred T(b). Many ectotherms demonstrate the selection of lower T(b)'s in response to increased MEL concentrations. Here, we examined the influence of MEL on the behavioral regulation of T(b) in the nocturnal African house snake Lamprophis fuliginosus. A series of experiments with two injection regimes of MEL had no significant effect on the mean preferred T(b) of L. fuliginosus. In addition, mean preferred T(b)'s during the photophase did not differ significantly from those during scotophase. Our findings suggest that L. fuliginosus does not respond to elevated concentrations of either endogenous or exogenous MEL. To verify that the African house snake is nocturnal, we investigated activity patterns of L. fuliginosus throughout the photoperiod. The activity period of L. fuliginosus occurs in the scotophase of the photoperiod, a pattern consistent with that of nocturnal species. This suggests that nocturnal organisms such as L. fuliginosus may not respond to MEL in the same manner as many diurnal species. Our results support the hypothesis that some animals, particularly nocturnal species, may have developed alternative responses to increased plasma concentrations of MEL.

Seasonality and role of SCN in entrainment of lizard circadian rhythms to daily melatonin injections

To establish whether the capability of daily melatonin injections to entrain circadian rhythms varies with season, we examined in constant conditions the locomotor behavior of lizards Podarcis sicula collected and subjected to daily melatonin injections at different times of the year. Although in summer locomotor rhythms of both pineal-intact and pinealectomized lizards became entrained to the 24-h injection period, in the other seasons their rhythms did not entrain to the injection period. To establish whether the suprachiasmatic nuclei (SCN) mediate summer entrainment of locomotor rhythms to melatonin, we examined the behavioral effects of daily melatonin injections in lizards subjected to either bilateral (SCN-X) or unilateral (USCN-X) ablation of the SCN. SCN-X lizards became behaviorally arrhythmic, and daily melatonin injections did not restore rhythmicity. USCN-X lizards remained rhythmic, and their locomotor rhythms did entrain to the injections. Besides demonstrating for the first time in a vertebrate that daily melatonin injections are capable of entraining circadian rhythmicity in only one season (summer), the present results support the view that the SCN (and not the pineal gland) are the primary target sites of melatonin in the circadian system of P. sicula.

Light perception in the vertebrate brain: an ultrastructural analysis of opsin- and vasoactive intestinal polypeptide-immunoreactive neurons in iguanid lizards

Recent biochemical and immunocytochemical evidence indicates that a population of circadian and reproductive rhythm-entraining photoreceptors lies in the basal diencephalon of iguanid lizards. Here, we report the results of correlated light and electron microscopy of opsin-immunoreactive cells in the basal brain, and we discuss their ultrastructural relationship to known photoreceptors. Cerebrospinal fluid (CSF)-contacting bipolar neurons in the lizards Anolis carolinensis and Iguana iguana were immunolabeled with antisera generated against vertebrate retinal opsins and vasoactive intestinal polypeptide (VIP). Within the brain, opsin-immunoreactive cells were found exclusively in the ependyma of the basal region of the lateral ventricles (adjacent to nucleus paraolfactorius/nucleus ventromedialis and neostriatum/paleostriatum). Cells in the same anatomical location and with the same morphology were labeled with anti-VIP antisera. These cells possessed a dendritic process that extended toward the lateral ventricle, ending in a bulbous terminal that protruded into the ventricle. Axonal processes travelled ventrally and caudally. The entire cell, including the axonal process, exhibited opsin-like and VIP-like immunoreactivity. By light microscopy, opsin-like immunostaining appeared punctate, with immunoreactivity greatest in the bulbous terminal. Opsin- and VIP-immunostained thick sections were resectioned, and individual cells observed by light microscopy were then characterized using electron microscopy. We found that all immunostained cells were morphologically similar and that they were morphologically distinct from neighboring nonimmunoreactive cells. CSF-contacting opsin- and VIP-immunoreactive cells lacked the membranous stacks characteristic of retinal photoreceptors but were ciliated and contained numerous large electron-dense vesicles. Multiple synaptic contacts were made on the soma and putative dendritic processes of opsin- and VIP-immunoreactive CSF-contacting neurons. Our results provide the first ultrastructural characterization of opsin-immunostained encephalic CSF-contacting neurons in a vertebrate animal, and they indicate that these putative photoreceptors share structural features with pineal photoreceptors and with certain invertebrate extraretinal photoreceptors, but they are morphologically and biochemically distinct from visual photoreceptors of the retina.

Dermal photoreceptors regulate basking behavior in the lizard Podarcis muralis

There is evidence that dermal photic responsiveness can be found in a wide range of animals. Behavioral responses to dermal stimulation by light have been observed in pigeon squabs and new-born rats, and more recently in a sea snake. Here we report that painting the dorsal surface of the lizard (Podarcis muralis) with opaque black paint impairs the animal's ability to position itself beneath a light source containing negligible heat. Experiments using light of different spectra and intensities show that the effect is due to light of wavelengths shorter than 600 nm and of intensity higher than 2.5 mW cm-2. These experiments demonstrate for the first time that overt behavior in a terrestrial vertebrate can be mediated by a dermal light sense.

Circadian rhythm generation, expression and entrainment in a molluscan model system

The Bulla ocular pacemaker provides remarkable opportunities for cellular study of circadian pacemaker systems. The demonstration of circadian oscillations within individual neurons maintained in culture provides us with a first occasion to study the biophysical and biochemical properties of bona fide neuronal circadian pacemakers. The ocular clock is robust and shares formal similarity with other circadian systems. The development of molecular techniques that can be applied to single neurons should allow research on the Bulla retina to continue to progress towards a molecular analysis of circadian timekeeping.

Effects of green light on biological systems

Green light (510-565 nm) constitutes a significant portion of the visible spectrum impinging on biological systems. It plays many different roles in the biochemistry, physiology and structure of plants and animals. In only a relatively small number of responses to green light is the photoreceptor known with certainty or even provisionally and in even fewer systems has the chain of events leading from perception to response been examined experimentally. This review provides a detailed view of those biological systems shown to respond to green light, an evaluation of possible photoreceptors and a review of the known and postulated mechanisms leading to the responses.

The steroid hormone of sunlight soltriol (vitamin D) as a seasonal regulator of biological activities and photoperiodic rhythms

Neural and systemic somatotrophic effects of the ultraviolet component of sunlight through the skin-vitamin D endocrine system are considered as alternate or additional to the neuroendocrine effects of the visual component of light through the retino-diencephalic input. The extensive distribution of soltriol nuclear receptor cells, revealed by autoradiography with tritium-labeled 1,25 dihydroxycholecalciferol (vitamin D, soltriol) and related effects, indicate an involvement of vitamin D-soltriol in the actinic induction of seasonal biorhythms. This is considered to be independent of the traditionally assigned effects of vitamin D on systemic calcium regulation. Skin-soltriol mediated seasonal, and to a degree daily, genomic activation involves many target regions in the brain. These include neurons in the central nucleus of the amygdala, in the linked part of the bed nucleus of the stria terminalis, in periventricular hypothalamic neurons, dorsal raphe nucleus, reticular thalamic nucleus and autonomic, endocrine as well as sensory and motor components of the brainstem and spinal cord. Additional to the eye-regulated "suprachiasmatic clock", existence of a soltriol-vitamin D regulated neural "timing circuit(s)" is proposed. Both, activational and organizational effects of soltriol on mature and developing brain regions, respectively are likely to play a role in the regulation of neuronal functions that include the modulation and entrainment of biorhythms. Soltriol's central effects correlate with peripheral effects on elements in skin, bone, teeth, kidney, intestine, heart and blood vessels, endocrine organs, and tissues of the immune and reproductive system.

The pineal and melatonin: regulators of circadian function in lower vertebrates

The pineal has been identified as a major circadian pacemaker within the circadian system of a number of lower vertebrates although other pacemaking sites have been implicated as well. The rhythmic synthesis and secretion of the pineal hormone, melatonin, is suggested as the mechanism by which the pineal controls circadian oscillators located elsewhere. Both light and temperature cycles can entrain the pineal melatonin rhythm. The pineal, therefore, acts as a photo and thermoendocrine transducer which functions to synchronize internal cycle with cycles in the environment. A model is presented which portrays the pineal as a major component of a 'multioscillator' circadian system and which suggests how these multiple circadian clocks are coupled to each other and to cycles of light and temperature in the external world.

Circadian rhythms: influences of internal and external factors on the period measured in constant conditions

The article reviews most of the data available on the period tau of freerunning circadian rhythms, measured in constant conditions. Emphasis is placed on the effects of light intensity and ambient temperature on tau, with references to influences of other external as well as internal factors. In the introduction, examples are given of spontaneous and induced variations in tau and its dependence on the experimental history. The discussion concentrates largely on results obtained from arthropods and vertebrates.

Circadian organization in lizards: the role of the pineal organ

After pinealectomy, the iguanid lizard Sceloporus olivaceus exhibits a "splitting" of the circadian activity rhythm into two components, marked changes in the period of the rhythm, or arrhythmicity. These effects are consistent with either of two hypotheses: (i) The pineal organ is a coupling device between circadian oscillators in a multioscillator system, or (ii) the pineal organ is itself a master oscillator, which entrains other circadian oscillators and imposes its frequency upon them.