History dependence of circadian pacemaker period in the cockroach

Breakdown of selection-mediated correlation between development time and clock period

Previously we have reported that selection for faster pre-adult development in fruit flies speeded-up development by ~29-h and shortened the clock period (τ) by ~0.5h, which suggests that development time and τ are correlated. Since it is known that τ is altered following exposure to light/dark (LD) cycles, we asked whether this correlation persists in the faster developing (FD) and control (BD) flies by examining the τ of the activity/rest rhythm and its difference between the two stocks following exposure to a variety of cyclic conditions. We assayed the activity/rest behavior of FD and BD flies under DD, following a week-long exposure to (a) LD cycles of 10:10h, 12:12h and 14:14h, or (b) LD12:12h with different light intensities (10, 100 and 1000lx), or (c) 12:12h warm/cold (WC) cycles of 25:18°C (WC1) and 29:25°C (WC2), or (d) WC1 or WC2, in-phase or out-of-phase with LD. The results revealed that both LD and WC altered the τ of FD and BD flies, and considerably reduced the selection-mediated difference between the two stocks. LD10:10 caused more severe after-effects on τ compared to LD12:12 and LD14:14. Among the WC cycles, WC1 which had a higher contrast caused period shortening. Irrespective of the phase relationship, imposition of LD cycles on WC cycles made no difference to the extent of after-effects; however, interestingly there was a reversal in the trend, in that, now WC2 with LD caused most drastic reduction in τ. These results suggest that cyclic environments modulate the circadian organization of Drosophila melanogaster altering the selection-mediated correlation between pre-adult development time and clock period.

Integration of biological clocks and rhythms

Animals, plants, and microorganisms exhibit numerous biological rhythms that are generated by numerous biological clocks. This article summarizes experimental data pertinent to the often-ignored issue of integration of multiple rhythms. Five contexts of integration are discussed: (i) integration of circadian rhythms of multiple processes within an individual organism, (ii) integration of biological rhythms operating in different time scales (such as tidal, daily, and seasonal), (iii) integration of rhythms across multiple species, (iv) integration of rhythms of different members of a species, and (v) integration of rhythmicity and physiological homeostasis. Understanding of these multiple rhythmic interactions is an important first step in the eventual thorough understanding of how organisms arrange their vital functions temporally within and without their bodies.

Influence of photoperiod in accelerating the reentrainment in Drosophila

Efficacy of the short photoperiod (Spp) and the long photoperiod (Lpp) in accelerating the reentrainment was assessed in Drosophila biarmipes. The Spp accelerated the reentrainment after the phase advance of light-dark (LD) cycles, which was associated with the early activity onset (Ψo) and the short period of free-running rhythm (τ). The Lpp accelerated the reentrainment after the phase delay of LD cycles, which was associated with the late Ψo and the long τ. This study indicates that the photoperiodic modulation of the circadian waveform of the underlying pacemaker that controls activity rhythm influenced the rate of reentrainment in D. biarmipes.

Isochron-based phase response analysis of circadian rhythms

Circadian rhythms possess the ability to robustly entrain to the environmental cycles. This ability relies on the phase synchronization of circadian rhythm gene regulation to different environmental cues, of which light is the most obvious and important. The elucidation of the mechanism of circadian entrainment requires an understanding of circadian phase behavior. This article presents two phase analyses of oscillatory systems for infinitesimal and finite perturbations based on isochrons as a phase metric of a limit cycle. The phase response curve of circadian rhythm can be computed from the results of the analyses. The application to a mechanistic Drosophila circadian rhythm model gives experimentally testable hypotheses for the control mechanisms of circadian phase responses and evidence for the role of phase and period modulations in circadian photic entrainment.

Photoperiodic modulation of circadian rhythms in the cricket Gryllus bimaculatus

The waveform and the free-running period of circadian rhythms in constant conditions are often modulated by preceding lighting conditions. We have examined the modulatory effect of variable length of light phase of a 24h light cycle on the ratio of activity (alpha) and rest phase (rho) as well as on the free-running period of the locomotor rhythm in the cricket Gryllus bimaculatus. When experienced the longer light phases, the alpha/rho-ratio was smaller and the free-running period was shorter. The magnitude of changes in alpha/rho-ratio was dependent on the number of cycles exposed, while the free-running period was changed by a single exposure, suggesting that there are separate regulatory mechanisms for the waveform and the free-running period. The neuronal activity of the optic lobe showed the alpha/rho-ratio changing with the preceding photoperiod. When different photoperiodic conditions were given to each of the two optic lobe pacemakers, the alpha/rho-ratio of a single pacemaker was rather intermediate between those of animals treated with either of the two conditions. These results suggest that the storage of the photoperiodic information occurs at least in part in the optic lobe pacemaker, and that the mutual interaction between the bilateral optic lobe pacemakers is involved in the photoperiodic modulation.

Plasticity of circadian behavior and the suprachiasmatic nucleus following exposure to non-24-hour light cycles

Period aftereffects are a form of behavioral plasticity in which the free-running period of circadian behavior undergoes experience-dependent changes. It is unclear whether this plasticity is age dependent and whether the changes in behavioral period relate to changes in the SCN or the retina, 2 known circadian pacemakers in mammals. To determine whether these changes vary with age, Per1-luc transgenic mice (in which the luciferase gene is driven by the Period1 promoter) of different ages were exposed to short (10 h light: 10 h dark, T20) or long (14 h light: 14 h dark, T28) light cycles (T cycles). Recordings of running-wheel activity in constant darkness (DD) revealed that the intrinsic periods of T20 mice were significantly shorter than of T28 mice at all ages. Aftereffects following the shorter light cycle were significantly smaller in mice older than 3 months, corresponding with a decreased ability to entrain to T20. Age did not diminish entrainment or aftereffects in the 28-h light schedule. The behavioral period of pups born in DD depended on the T cycle experienced in utero, showing maternal transference of aftereffects. Recordings of Per1-luc activity from the isolated SCN in vitro revealed that the SCN of young mice expressed aftereffects, but the periods of behavior and SCN were negatively correlated. Enucleation in DD had no effect on behavioral aftereffects, indicating the eyes are not required for aftereffects expression. These data show that circadian aftereffects are an age-dependent form of plasticity mediated by stable changes in the SCN and, importantly, extra-SCN tissues.

Genetics and molecular biology of rhythms in Drosophila and other insects

Application of generic variants (Sections II-IV, VI, and IX) and molecular manipulations of rhythm-related genes (Sections V-X) have been used extensively to investigate features of insect chronobiology that might not have been experimentally accessible otherwise. Most such tests of mutants and molecular-genetic xperiments have been performed in Drosophila melanogaster. Results from applying visual-system variants have revealed that environmental inputs to the circadian clock in adult flies are mediated by external photoreceptive structures (Section II) and also by direct light reception chat occurs in certain brain neurons (Section IX). The relevant light-absorbing molecuLes are rhodopsins and "blue-receptive" cryptochrome (Sections II and IX). Variations in temperature are another clock input (Section IV), as has been analyzed in part by use of molecular techniques and transgenes involving factors functioning near the heart of the circadian clock (Section VIII). At that location within the fly's chronobiological system, approximately a half-dozen-perhaps up to as many as 10-clock genes encode functions that act and interact to form the circadian pacemaker (Sections III and V). This entity functions in part by transcriptional control of certain clock genes' expressions, which result in the production of key proteins that feed back negatively to regulate their own mRNA production. This occurs in part by interactions of such proteins with others that function as transcriptional activators (Section V). The implied feedback loop operates such that there are daily variations in the abundances of products put out by about one-half of the core clock genes. Thus, the normal expression of these genes defines circadian rhythms of their own, paralleling the effects of mutations at the corresponding genetic loci (Section III), which are to disrupt or apparently eliminate clock functioning. The fluctuations in the abundance of gene products are controlled transciptionally and posttranscriptionally. These clock mechanisms are being analyzed in ways that are increasingly complex and occasionally obscure; not all panels of this picture are comprehensive or clear, including problems revolving round the biological meaning or a given features of all this molecular cycling (Section V). Among the complexities and puzzles that have recently arisen, phenomena that stand out are posttranslational modifications of certain proteins that are circadianly regulated and regulating; these biochemical events form an ancillary component of the clock mechanism, as revealed in part by genetic identification of Factors (Section III) that turned out to encode protein kinases whose substrates include other pacemaking polypeptides (Section V). Outputs from insect circadian clocks have been long defined on formalistic and in some cases concrete criteria, related to revealed rhythms such as periodic eclosion and daily fluctuations of locomotion (Sections II and III). Based on the reasoning that if clock genes can regulate circadian cyclings of their own products, they can do the same for genes that function along output pathways; thus clock-regulated genes have been identified in part by virtue of their products' oscillations (Section X). Those studied most intensively have their expression influenced by circadian-pacemaker mutations. The clock-regulated genes discovered on molecular criteria have in some instances been analyzed further in their mutant forms and found to affect certain features of overt whole-organismal rhythmicity (Sections IV and X). Insect chronogenetics touches in part on naturally occurring gene variations that affect biological rhythmicity or (in some cases) have otherwise informed investigators about certain features of the organism's rhythm system (Section VII). Such animals include at least a dozen insect species other than D. melanogaster in which rhythm variants have been encountered (although usually not looked for systematically). The chronobiological "system" in the fruit fly might better be graced with a plural appellation because there is a myriad of temporally related phenomena that have come under the sway of one kind of putative rhythm variant or the other (Section IV). These phenotypes, which range well beyond the bedrock eclosion and locomotor circadian rhythms, unfortunately lead to the creation of a laundry list of underanalyzed or occult phenomena that may or may not be inherently real, whether or not they might be meaningfully defective under the influence of a given chronogenetic variant. However, such mutants seem to lend themselves to the interrogation of a wide variety of time-based attributes-those that fall within the experimental confines of conventionally appreciated circadian rhythms (Sections II, III, VI, and X); and others that consist of 24-hr or nondaily cycles defined by many kinds of biological, physiological, or biochemical parameters (Section IV).

Life before the clock: modeling circadian evolution

A feedback loop that functions via transcription and translation is thought to be the mechanistic core of circadian rhythmicity. Numerous modeling efforts incorporate the identified components and their modifications to recreate the circadian clock in computer simulations. Several issues remain problematic, including the lack of precise quantitative kinetics and the likely existence of additional, as-yet-undiscovered components. Even without these complications, models and flow charts of the circadian system have reached high complexity. They attempt to reconcile all observations without violating current views and concepts. In this article, the authors consider the mechanisms that may have preceded the circadian system in evolution. Given that cellular metabolism and biochemistry were presumably already interconnected in cascading feedback reactions prior to the appendage of the transcription/translation feedback loop, a coordinated response to exogenous changes would be advantageous over unsystematic responses. The authors hypothesize that those mechanisms that allowed synchronization in spite of metabolic complexity form the basis for the evolution of circadian properties and are as fundamental to the circadian system as the transcriptional/translational feedback loop.