Environmental effects on circadian rhythms in photosynthesis and stomatal opening

Persistent circadian rhythms in photosynthesis and stomatal opening occurred in bean (Phaseolus vulgaris L.) plants transferred from a natural photoperiod to a variety of constant conditions. Photosynthesis, measured as carbon assimilation, and stomatal opening, as conductance to water vapor, oscillated with a freerunning period close to 24 h under constant moderate light, as well as under light-limiting and CO₂-limiting conditions. The rhythms damped under constant conditions conducive to high photosynthetic rates, as did rates of carbon assimilation and stomatal conductance, and this damping correlated with the accumulation of carbohydrate. No rhythm in respiration occurred in plants transferred to constant darkness, and the rhythm in stomatal opening damped rapidly in constant darkness. Damping of rhythms also occurred in leaflets exposed to constant light and CO₂-free air, demonstrating that active photosynthesis and not simply light was necessary for sustained expression of these rhythms. This is CIWDPB Publication No. 1142 This research was supported by National Science Foundation grant BSR 8717422 (C.B.F.) and a U.S. Department of Agriculture training grant to Stanford University (T.L.H.).     

Circadian rhythms

The neurobiological substratum of circadian rhythmicity encompasses three levels of integration: firstly, generation of time signals by circadian pacemakers; secondly, entrainment of pacemakers by environmental influences; thirdly, coupling of circadian pacemakers among themselves and with target systems responsible for the expression of overt rhythms. From recent contributions, the notion that circadian organization results from the interaction of independent oscillators and pathways has been strengthened. In addition, recent evidence supports the existence of circadian rhythmicity in single isolated neurons. New information was produced on the gene control of circadian rhythm generation in Drosophila, as well as interesting advances in the understanding of neuronal mechanisms involved in the generation, entrainment and coupling of circadian rhythms in various species.     

Different circadian pacemakers control feeding and locomotor activity rhythms in European starlings

Summary In higher organisms, many physiological and behavioral functions exhibit daily variations, generated by endogenous circadian oscillators. It is not yet clear whether all the various rhythms that occur within an individual depend on one and the same pacemaker or whether different pacemakers are involved. To examine this question, the feeding and perch-hopping rhythms were measured in European starlings (Sturnus vulgaris) under light-dark cycles and continuous dim light. In dim light, the internal phase relationship between the feeding and perch-hopping rhythms changed systematically as a function of the circadian period, and the two rhythms could even dissociate and show different circadian periods in individuals with extremely long or extremely short circadian periods. Moreover, in some birds kept on lowamplitude light-dark cycles, the rhythm of feeding was synchronized 180° out of phase with the rhythm of locomotor activity. These results strongly suggest that in the European starling the feeding and locomotor activity rhythms are controlled by separate circadian pacemakers.     

Isolation of an insect circadian clock

Summary Under constant conditions the compound eyes of the ground beetleAnthia sexguttata exhibit sensitivity changes in a very clear circadian rhythm. Usually the rhythms in both eyes in constant darkness are mutually coupled. After transection of the optic tract between the lobula and the supraesophageal ganglion the circadian rhythms of the two eyes continue without interruption, but coupling between them is abolished. Even if the entire supraesophageal ganglion is removed, leaving the optic ganglia intact, the circadian rhythms in the eyes continue without interruption independently. But the rhythm is abolished if the region of the lobula is damaged.The experiments show thatAnthia has circadian pacemakers in the left and right optic ganglia in or close to the lobula. These pacemakers can function independently from the rest of the brain and control circadian rhythms of physiological events.Supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 45 Vergleichende Neurobiologie des Verhaltens E1     

Cylindrical structures in the chloroplasts of Sirogonium melanosporum

Summary Cylindrical structures with outside diameters of 390 to 410 Å have been observed in the chloroplasts of mature vegetative cells of the filamentous, green alga Sirogonium melanosporum. These cylindrical structures are either parallel to one another or randomly oriented in the stroma matrix of the chloroplast. Some of the outer cylindrical structures of the complex appear to be continuous with thylakoid membranes, suggesting a relationship between these two structures.This work was supported by an Institutional Grant from the American Cancer Society to the University of Arizona and by grant GB2440 from the National Science Foundation to R. W. Hoshaw. The authors thank Dr. Wayneferris for the use of the electron microscope supported by NSF grant GB3330.     

The circadian rhythm of a behavioral photoresponse in the dinoflagellate Gyrodinium dorsum

Summary The circadian rhythm of the photoresponse to blue light in the dinoflagellate Gyrodinium dorsum Kofoid was investigated by the use of a closed circuit television system. The initial cessation of movement upon stimulation (stop-response) was used as the index of light reception. Under constant dark conditions cells grown on a 12L:12D regime show an endogenous circadian rhythm in their stop-response with maximum responsiveness occurring approximately one hour before the beginning of the expected light phase. This rhythmic response was only observed if the cells were irradiated with red light (620 nm) prior to stimulation with blue light. After preirradiation both far-red reversibility and the shift in the stop-response action spectrum from 470 nm to 490 nm could also be demonstrated. These findings may be related to the diurnal migration of marine dinoflagellates.This study was supported by National Science Foundation grant GB 5137.     

Clock control of foraging in the isopod Scyphax ornatus Dana

Abstract

The terrestrial sand beach isopod Scyphax ornatus Dana exhibits circadian and circa-semilunar activity rhythms when kept in constant conditions in the laboratory. The circadian rhythm restricts activity to the subjective night and can be synchronised to artificial light cycles. The circa-semilunar rhythm is expressed as a cyclic variation in overall activity level, with isopods alternately active on 6–8 consecutive nights and then inactive on the next 6–8 nights. The circa-semilunar rhythm matches a fortnightly cycle in food availability on the isopods’ home beach, but does not appear to be causally related to the lunar cycle, the spring/neap tidal cycle, or to moulting. This and other evidence suggests that the ecologicalrole of the endogenous circa-semilunar rhythm is to allow Scyphax, within the protection of their burrows, to predict nightly foraging opportunities; a new role for such a long-period rhythm.     

The Circadian System of Crustaceans

Crustaceans exhibit a variety of overt circadian rhythms. Observations on intact animals suggest the existence of more than one circadian pacemaker in the nervous system. Ablation experiments so far have been inconclusive in pinpointing the location of putative pacemakers. However, various structures, most notably the optic peduncle, have been shown to sustain circadian rhythmicity in vitro. Retinal sensitivity and neurosecretory activity display circadian rhythms in the isolated optic peduncle, but they are also responsive to synchronizing influences from other regions of the central nervous system, most notably the supra-esophageal ganglion. A model based on a number of circadian pacemakers distributed in the central nervous system best fits the experimental results at present. Coupling of rhythmicity between independent circadian pacemakers is likely to occur, and a neuroendocrine stage of integration has been proposed for several rhythms. Various entraining agents have been identified, and more than one may play a part in the synchronization of a given rhythm.     

Period shortening and phase shifting effects of ethanol on theGonyaulax glow rhythm

Summary In a number of organisms which exhibit circadian rhythmicity, a continuous exposure to ethanol at moderate (0.1%) concentrations is known to cause period lengthening. In studies of the effects of ethanol on the circadian luminescence glow rhythm of the marine dinoflagellateGonyaulax, we observed that 0.1% ethanol causes instead a period shortening. We have also found that ethanol pulses cause phase shifts, with little or no after-effects on the period of the circadian rhythm which continues thereafter.Abbreviation CT circadian time This work has been supported in part by a grant from the National Institutes of Health GM-19536 to J.W. Hastings, and by a Rackham Block Grant to Walter Taylor from the University of Michigan     

Persistance of phase-coupling between the circadian rhythms of hypophyseal hormones in free-running rats

The circadian rhythms in plasma ACTH, TSH, LH and PRL were explored in sighted or blind, spayed and estrogen-implanted rats. A marked endogenous circadian rhythmicity was shown to persist in the blind animals for the 4 endocrine rhythms. The endogenous rhythms also kept very close reciprocal phase relationship as in the synchronized state, and they were peaking almost simultaneously, after 60 d. of free-running. Finally the endogenous hormonal rhythm maintained their usual phase relationships with the endogenous activity rhythm, so that the circadian phase of increased hormonal secretion coincided with the circadian resting phase of the sleep/wake rhythm. These results are discussed in the light of the alternate theory of one vs multiple but phase-locked circadian pacemakers driving endocrine and behavioral circadian rhythms.     

Kinetics in metabolic control of time measurement in photoperiodism

Abstract

The period length of the locomotor activity rhythm of Drosophila melanogaster wild form is under conditions of continuous weak red light 23.38 h, whereas die eye mutants Ly3 with a 23.71 h mean period and JK 84 with 23.14 h differ significantly. This might be due to a changed perception of light and not the result of a change in the circadian pacemaker by the mutation.

The mutant sine oculis exhibits a normal activity rhythm if the complex eyes are not completely reduced. If this is the case, the activity pattern is either less rhythmic, composed of several rhythms with different periods or truely arrhythmic depending on the individual fly.

Since the mutation in sine oculis affects in addition to the complex eye the distal part of die medulla and the lamina of the optic lobe, it is suggested that the circadian pacemakers for the locomotor activity rhythm is localized in these parts.     

Egg-laying rhythm in <Emphasis Type="Italic">Drosophila melanogaster</Emphasis>

Extensive research has been carried out to understand how circadian clocks regulate various physiological processes in organisms. The discovery of clock genes and the molecular clockwork has helped researchers to understand the possible role of these genes in regulating various metabolic processes. In Drosophila melanogaster, many studies have shown that the basic architecture of circadian clocks is multi-oscillatory. In nature, different neuronal subgroups in the brain of D. melanogaster have been demonstrated to control different circadian behavioural rhythms or different aspects of the same circadian rhythm. Among the circadian phenomena that have been studied so far in Drosophila, the egg-laying rhythm is unique, and relatively less explored. Unlike most other circadian rhythms, the egg-laying rhythm is rhythmic under constant light conditions, and the endogenous or free-running period of the rhythm is greater than those of most other rhythms. Although the clock genes and neurons required for the persistence of adult emergence and activity/rest rhythms have been studied extensively, those underlying the circadian egg-laying rhythm still remain largely unknown. In this review, we discuss our current understanding of the circadian egg-laying rhythm in D. melanogaster, and the possible molecular and physiological mechanisms that control the rhythmic output of the egg-laying process.     

PDFR and CRY signaling converge in a subset of clock neurons to modulate the amplitude and phase of circadian behavior in Drosophila

Background

To synchronize their molecular rhythms, circadian pacemaker neurons must input both external and internal timing cues and, therefore, signal integration between sensory information and internal clock status is fundamental to normal circadian physiology.

Methodology/Principal Findings

We demonstrate the specific convergence of clock-derived neuropeptide signaling with that of a deep brain photoreceptor. We report that the neuropeptide PDF receptor and the circadian photoreceptor CRYPTOCROME (CRY) are precisely co-expressed in a subset of pacemakers, and that these pathways together provide a requisite drive for circadian control of daily locomotor rhythms. These convergent signaling pathways influence the phase of rhythm generation, but also its amplitude. In the absence of both pathways, PER rhythms were greatly reduced in only those specific pacemakers that receive convergent inputs and PER levels remained high in the nucleus throughout the day. This suggested a large-scale dis-regulation of the pacemaking machinery. Behavioral rhythms were likewise disrupted: in light∶dark conditions they were aberrant, and under constant dark conditions, they were lost.

Conclusions/Significance

We speculate that the convergence of environmental and clock-derived signals may produce a coincident detection of light, synergistic responses to it, and thus more accurate and more efficient re-setting properties.     

INTERNAL DESYNCHRONIZATION OF THE CIRCADIAN ACTIVITY AND FEEDING RHYTHM IN AN OWL MONKEY (AOTUS LEMURINUS GRISEIMEMBRA): A CASE STUDY

In the free-running circadian locomotor activity rhythm of a 7-year-old male owl monkey (Aotus lemurinus griseimembra) kept under constant light and climatic conditions (LL 0.2 lux, 25°C ± 1°C, 60 ± 5% relative humidity [RH]), a second rhythm component developed that showed strong relative coordination with the free-running activity rhythm of 24.4h and a 24h rhythm. The simultaneously recorded feeding activity rhythm strongly resembled this rhythm component. Therefore, it seems justified to infer that there was an internal desynchronization between the two behavioral rhythms or their circadian pacemakers, that is, between the light-entrainable oscillator located in the suprachiasmatic nuclei (SCN) and a food-entrainable oscillator located outside the SCN. This internal desynchronization may have been induced and/or maintained by a zeitgeber effect of the (irregular) 24h feeding schedule on the food-entrainable oscillator. The weak relative coordination shown by the activity rhythm indicates a much weaker coupling of the light-entrainable oscillator to the food-entrainable oscillator than vice versa. (Chronobiology International, 17(2), 147–153, 2000)     

Chlamydomonas reinhardtii as a unicellular model for circadian rhythm analysis

Chlamydomonas reinhardtii has been used as an experimental model organism for circadian rhythm research for more than 30 yr. Some of the physiological rhythms of this alga are well established, and several clock mutants have been isolated. The cloning of clock genes from these mutant strains by positional cloning is under way and should give new insights into the mechanism of the circadian clock. In a spectacular space experiment, the question of the existence of an endogenous clock vs. an exogenous mechanism has been studied in this organism. With the emergence of molecular analysis of circadian rhythms in plants in 1985, a circadian gene expression pattern of several nuclear and chloroplast genes was detected. Evidence is now accumulating that shows circadian control at the translational level. In addition, the gating of the cell cycle by the circadian clock has been analyzed. This review focuses on the different aspects of circadian rhythm research in C. reinhardtii over the past 30 yr. The suitability of Chlamydomonas as a model system in chronobiology research and the adaptive significance of the observed rhythms will be discussed.     

Development of pigment-dispersing hormone-immunoreactive neurons in the American lobster: homology to the insect circadian pacemaker system?

We have examined the development of pigment-dispersing hormone (PDH)-immunoreactive neurons in embryos of the American lobster Homarus americanus Milne Edwards, 1837 (Decapoda, Reptantia, Homarida) by using an antiserum against β-PDH. This peptide is detectable in the terminal medulla of the eyestalks and the protocerebrum where PDH immunoreactivity is present as early as 20% of embryonic development. During ontogenesis, an elaborate system of PDH-immunoreactive neurons and fibres develops in the eyestalks and the protocerebrum, whereas less labelling is present in the deuto- and tritocerebrum and the ventral nerve cord. The sinus gland is innervated by PDH neurites at hatching. This pattern of PDH immunoreactivity has been compared with that found in various insect species. Neurons immunoreactive to pigment-dispersing factor in the medulla have been shown to be a central component of the system that generates the circadian rhythm in insects. Our results indicate that, in view of the position of the neuronal somata and projection patterns of their neurites, the immunolabelled medulla neurons in insects have homologous counterparts in the crustacean eyestalk. Since locomotory and other activities in crustaceans follow distinct circadian rhythms comparable with those observed in insects, we suggest that PDH-immunoreactive medulla neurons in crustaceans are involved in the generation of these rhythms. This study was supported by Deutsche Forschungsgemeinschaft (DFG) grant Ha 2540 and National Science Foundation grant IBN 0344448. S.H. was a Heisenberg Fellow of the DFG during the experimental part of this study. Bill Hansson and the Max Planck Society provided support during the final period of work reported in this paper.     

Locomotor activity rhythm in four wrasse species under varying light conditions

Under controlled laboratory conditions, the locomotor activity rhythms of four species of wrasses (Suezichthys gracilis, Thalassoma cupido, Labroides dimidiatus andCirrhilabrus temminckii) were individually examined using an actograph with infra-red photo-electric switches in a dark room at temperatures of 21.3–24.3°C, for 7 to 14 days. The locomotor activity ofS. gracilis occurred mostly during the light period under a light-dark cycle regimen (LD 12:12; 06:00-18:00 light, 18:00-06:00 dark). The locomotor activity commenced at the beginning of the light period and continued until a little before the beginning of dark period. The diel activity rhythm of this species synchronizes with LD. Under constant illumination (LL) this species shows distinct free-running activity rhythms varying in length from 23 hrs. 39 min. to 23 hrs. 47 min. Therefore,S. gracilis appears to have a circadian rhythm under LL. However, in constant darkness (DD), the activity of this species was greatly suppressed. All the fish showed no activity rhythms in DD conditions. After DD, the fish showed the diel activity rhythm with the resumption of LD, but this activity began shortly after the beginning of light period. The fish required several days to synchronize with the activity in the light period. Therefore,S. gracilis appeared to continue the circadian rhythm under DD. InT. cupido, the locomotor activity commenced somewhat earlier than the beginning of the light period and continued until the beginning of the dark period under LD. The diel activity rhythm of this species synchronizes with LD. Under LL, four of the five specimens of this species tested showed free-running activity rhythms for the first 5 days or longer varying in length from 22 hrs. 54 min. to 23 hrs. 39 min. Although the activity of this species was suppressed under DD, two of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 38 min. to 23 hrs. 50 min. under DD. Therefore, it was ascertained thatT. cupido has a circadian rhythm. InL. dimidiatus, the locomotor activity rhythm under LD resembled that observed inT. cupido. The diel activity rhythm of this species synchronizes with LD. Under LL, four of seven of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 07 min. to 25 hrs. 48 min. Although the activity of this species was suppressed under DD, three of five fish showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 36 min. to 23 hrs. 41 min. under DD. Therefore, it was ascertained thatL. dimidiatus has a circadian rhythm. Almost all locomotor activity of C.temminckii occurred during the light period under LD. The diel activity rhythm of this species coincides with LD. Under LL, two of four of this species showed free-running activity rhythms throughout the experimental period. The lengths of such free-running periods were from 23 hrs. 32 min. to 23 hrs. 45 min. Although the activity of this species was suppressed under DD, one of the four fish showed free-running activity rhythms throughout the experimental period. The length of the free-running period was 23 hrs. 21 min. under DD. Therefore,C. temminckii appeared to have a circadian rhythm. According to field observations,S. gracilis burrows and lies in the sandy bottom whileT. cupido, L. dimidiatus, andC. temminckii hide and rest in spaces among piles of boulders or in crevices of rocks during the night. It seems that the differences in nocturnal behavior among the four species of wrasses mentioned above are closely related to the intensity of endogenous factors in their locomotor activity rhythms.     

Circadian organization inAplysia: internal desynchronization and amplitude of locomotor rhythm

Summary We have tested the hypothesis that the circadian oscillators in the eyes ofAplysia are coequal driver oscillators for the circadian locomotor rhythm. Three predictions based on this hypothesis were tested. Prediction 1: at a time when the phase difference between the eye rhythms is small, the amplitude of the locomotor rhythm in two eyed animals will be as great or greater than the amplitude in one eyed animals. Prediction 2: the amplitude of the locomotor rhythm of two eyed animals will decline under conditions in which the two eye rhythms become out of phase with each other. Prediction 3: the form of the locomotor rhythm will broaden or become biphasic in two eyed animals when the two eye rhythms become out of phase with each other.None of the predictions was confirmed. One eyedAplysia had higher amplitude locomotor rhythms than two eyedAplysia, even under conditions in which the two eye rhythms were probably not far out of phase with each other. There was no tendency for the amplitude of the locomotor rhythm of two eyed animals to decline under circumstances in which the phase difference between the two eye rhythms changes from less than 4 h to as much as 11.5 h. There was no tendency in two eyed animals for the locomotor rhythm to broaden or become biphasic as the eye rhythms became more out of phase with each other.The results led us to reject the hypothesis that the eyes are co-equal drivers for the locomotor rhythm. The ocular influence on locomotion is more likely to be mediated via mechanisms in the central nervous system that do not faithfully conserve the phase of the eye rhythms. One possibility is that the driver is a third circadian oscillator that interacts with the two eye oscillators.Abbreviations CAP compound action potentials - CC constant conditions - CT circadian time - DO driver oscillator - EO eye oscillator - RSD relative standard deviations (see Methods)     

Development of Circadian Oscillators in Neurosphere Cultures during Adult Neurogenesis

Circadian rhythms are common in many cell types but are reported to be lacking in embryonic stem cells. Recent studies have described possible interactions between the molecular mechanism of circadian clocks and the signaling pathways that regulate stem cell differentiation. Circadian rhythms have not been examined well in neural stem cells and progenitor cells that produce new neurons and glial cells during adult neurogenesis. To evaluate circadian timing abilities of cells undergoing neural differentiation, neurospheres were prepared from the mouse subventricular zone (SVZ), a rich source of adult neural stem cells. Circadian rhythms in mPer1 gene expression were recorded in individual spheres, and cell types were characterized by confocal immunofluorescence microscopy at early and late developmental stages in vitro. Circadian rhythms were observed in neurospheres induced to differentiate into neurons or glia, and rhythms emerged within 3–4 days as differentiation proceeded, suggesting that the neural stem cell state suppresses the functioning of the circadian clock. Evidence was also provided that neural stem progenitor cells derived from the SVZ of adult mice are self-sufficient clock cells capable of producing a circadian rhythm without input from known circadian pacemakers of the organism. Expression of mPer1 occurred in high frequency oscillations before circadian rhythms were detected, which may represent a role for this circadian clock gene in the fast cycling of gene expression responsible for early cell differentiation.     

Circadian rhythm mutants of the prokaryoticSynechococcus RF-1

Synechococcus RF-1 established circadian rhythms in nitrogen fixation and leucine uptake when growing in a diurnal light/dark regimen. The rhythms persisted in subsequent uniform light/light conditions. In order to analyze the circadian rhythm at the genetic level, mutants were induced by N-methyl-N-nitro-N-nitrosoguanidine and then isolated by procedures with the circadian nitrogen-fixing rhythm as a selecton marker. Characterization of the mutants with respect to the circadian rhythm indicated that some mutants were abnormal only in the nitrogen-fixing rhythm, while some simultaneously lost the ability to establish the nitrogen-fixing and leucine-uptake rhythms. The physiological properties of the circadian rhythm were compared. The genetic potential of the mutants that were abnormal in both rhythms is emphasized.