Neural control of circadian rhythmicity in the crayfish: II. The erg amplitude rhythm

Reciprocal Coupling of Circadian Clocks in the Compound Eye and Optic Lobe in the Cricket Gryllus bimaculatus

The circadian system comprises multiple clocks, including central and peripheral clocks. The central clock generally governs peripheral clocks to synchronize circadian rhythms throughout the animal body. However, whether the peripheral clock influences the central clock is unclear. This issue can be addressed through a system comprising a peripheral clock (compound eye clock [CE clock]) and central clock (the optic lobe [OL] clock) in the cricket Gryllus bimaculatus. We previously found that the compound eye regulates the free-running period (τ) and the stability of locomotor rhythms driven by the OL clock, as measured by the daily deviation of τ at 30°C. However, the role of the CE clock in this regulation remains unexplored. In this study, we investigated the importance of the CE clock in this regulation using RNA interference (RNAi) of the period (per) gene localized to the compound eye (perCE-RNAi). The perCE-RNAi abolished the compound eye rhythms of the electroretinogram (ERG) amplitude and clock gene expression but the locomotor rhythm driven by the OL clock was maintained. The locomotor rhythm of the tested crickets showed a significantly longer τ and greater daily variation of τ than those of control crickets treated with dsDsRed2. The variation of τ was comparable with that of crickets with the optic nerve severed. The τ was considerably longer but was comparable with that of crickets with the optic nerve severed. These results suggest that the CE clock regulates the OL clock to maintain and stabilize τ.

A Circadian Rhythm of Visual Sensitivity in the American Lobster, Homarus americanus

AbstractTo determine whether eyes of American lobsters (Homarus americanus) are more sensitive to light at night than during the day, electroretinograms were continuously recorded from 23 adult lobsters for at least 3 days (range: 3 to 9 days) in constant darkness. A green light-emitting diode, mounted 10 cm away from the eyes, was briefly flashed every 2 minutes to evoke the electroretinogram. The average increase in the response to a light flash, between the minimum during the subjective day and the maximum during the subjective night, was 105.6% ± 38.8%; and there was a statistically significant difference between day and night responses. This change in visual sensitivity took place while lobsters were held in constant darkness, suggesting that it was due to the influence of a circadian clock. The average period (tau) for the 10 animals that expressed significant circadian rhythms was 23.4 ± 0.8 hours. Previous studies have demonstrated that lobsters have circadian clocks that influence their locomotor activity; and the present data suggest that this is also true for their eyes, leading to an increase in their visual sensitivity at night, when they are typically most active.

Putative pacemakers of crayfish show clock proteins interlocked with circadian oscillations

Although the molecular mechanisms that control circadian rhythms in many animals, particularly in the fly, are well known, molecular and biochemical studies addressing the location and function of the proteins and genes contributing to the cycling of the clock in crayfish Procambarus clarkii are scarce. In this study, we investigated whether three proteins that interact in the feedback loop of the molecular clock described for Drosophila are expressed in the putative circadian pacemakers of crayfish retina, eyestalk and brain and whether their expression cycles in a manner consistent with elements of the circadian clock. Here we identified PER, TIM and CLK immunoreactivity in the cytoplasm and nucleus of cells located in the retina as well as in clusters of cells and neuropils of the optic ganglia, lateral protocerebrum and brain. Brain clusters 6, 10, 9 and 11, in particular, showed Per, Tim and Clk-like immunoreactivity at the perikarya and nucleus, and these antigens colocalized at Zeitgeber time (ZT) 0 and/or ZT 12. A biochemical assay demonstrated circadian functionality of Per, Tim and Clk proteins. Both in the eyestalk and in the brain, these proteins demonstrated apparent daily and circadian rhythms. The presence and colocalization of these clock proteins in the cytoplasm and/or nucleus of several cells of retina, optic lobe and brain, depending on time, as well as their circadian oscillations, suggest interactions between positive and negative transcription factors and clock proteins similar to those forming the feedback loop of the canonical model proposed for different animals.

Daily and circadian expression of cryptochrome during the ontogeny of crayfish

Cryptochromes (CRY) are proteins with a dual role in the circadian function of different animals, participating in phototransduction and light signaling to the clock and as a transcriptional repressor that provides negative feedback in the clock feedback loop. Here we characterize functional expression of CRY as a marker of the functionality of the circadian pacemaker of crayfish (Procambarus clarkii) throughout post-embryonic development. Using different experimental light protocols and by means of immunofluorescence and biochemical methods, we report that, as in the adult, in young crayfish from the first embryonic stage CRY is present in cells adjacent to the eyestalk hemiellipsoidal body and the anterior margin of the brain protocerebrum. In the brain, CRY cycles after 72 h darkness, entraining to LD cycles. Meanwhile, as in the adult eye, in juveniles CRY is driven by light, showing an arrhythmic pattern in DD and cycling under LD. These results, as well as the completely different period length found in the brain circadian oscillations of 2nd post-embryonic stage and juvenile animals, suggest important changes in the properties of the crayfish pacemaker through the development. Therefore these data support a previous idea about the functionality of the circadian system from hatching.

The circadian system of crayfish: a developmental approach

Adult crayfish exhibit a variety of overt circadian rhythms. However, the physiological mechanisms underlying the overt rhythms are controversial. Research has centered on two overt rhythms: the motor activity and the retinal sensitivity rhythms of the genus Procambarus. The present work reviews various studies undertaken to localize pacemakers and mechanisms of entrainment responsible for these two rhythms in adult organisms of this crustacean decapod. It also describes an ontogenetic approach to the problem by means of behavioral, electrophysiological, and neurochemical experiments. The results of this approach confirm previous models proposed for adult crayfish, based on a number of circadian pacemakers distributed in the central nervous system. However, the coupling of rhythmicity between these independent oscillators might be complex and dependent on the interaction between serotonin (5-HT), light, and the crustacean hyperglycemic hormone (CHH). The latter compound has, up until now, not been considered as an agent in the genesis and synchronization of the retinal sensitivity rhythm.

Circadian rhythm of ERG in Iguana iguana: role of the pineal

In green iguanas, the pineal controls the circadian rhythm of body temperature but not the rhythm of locomotor activity. As part of a program to investigate the characteristics of this multioscillator circadian system, the authors studied the circadian rhythms of the electroretinographic response (ERG) and asked whether the pineal gland is necessary for the expression of this rhythm. ERGs from a total of 24 anesthetized juvenile iguanas were recorded under four different conditions: (a) complete darkness (DD), (b) dim light-dark cycles (dLD), (c) constant dim light (dLL), and (d) pinealectomized in DD. Results demonstrate that the b-wave component of the ERG shows a very clear circadian rhythm in DD and that this rhythm persists in dLL and entrains to dLD cycles. The ERG response is maximally sensitive during the subjective day. Pinealectomy does not abolish the circadian rhythm in ERG, demonstrating that the oscillator responsible for the ERG rhythm is located elsewhere.

Nightly increase in visual sensitivity correlated with bioluminescent flashing activity in the firefly Photuris versicolor (Coleoptera:Lampyridae)

Changes in visual sensitivity as a function of time-of-day were studied in the firefly Photuris versicolor. The electroretinograms (ERGs) elicited by photic stimuli of constant intensity and duration were recorded over long periods (18-52 h) while the insect was in constant darkness. The amplitudes of the ERGs elicited by the standard flash were small and remained constant during the daytime hours and increased on average sixfold during the evening hours, reaching a maximum at the onset of darkness and remaining at that high level until the morning hours. The amplitude of the standard response again was reduced in the morning hours to a daytime low value which again persisted until the following evening (Fig. 1). In addition, a series of flashes of varying intensity were administered periodically and logV/logI curves as a function of time-of-day were obtained. Compared to the daytime logV/logI curves, the nighttime curves were shifted about 4 log units towards the left on the intensity axis (Fig. 2). This period of high visual sensitivity was correlated with the species' flashing activity period, which occurs during the night hours (Fig. 1).

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 pin-pointing 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 supraesophageal 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.

An appraisal of tidal activity rhythms

Organisms of the marine littoral zone experience a much wider range of periodicities in their environment than do their terrestrial counterparts. Tidal cycles of semidiurnal, diurnal, lunar, and semilunar frequencies may all recur at the same locality, in addition to the diel cycle of light and darkness. The relationship of endogenous activity patterns to the prevailing geophysical variables thus poses problems for the temporal organization of the organism. The way in which intertidal animals synchronize their behaviour and physiology to such a diversely fluctuating environment, and the efficacy of different environmental factors as entraining agents is considered. Evidence pertaining to the endogenous control mechanisms, both physiological and behavioural, is reviewed, and the organization of the endogenous time-keeping system discussed in terms of identifiable oscillators of different frequencies.

Protocerebral circadian pacemakers in crayfish: evidence for mutually coupled pacemakers

Similar to intact crayfish, animals with an isolated protocerebrum-eyestalk complex, exhibit competent circadian rhythms in the electroretinogram (ERG). The ERG rhythms of the two eyes remain in phase after isolation of the protocerebrum but can be desynchronized after surgical bisection of the protocerebrum. The desynchrony of the two ERG rhythms reveals the existence of at least two circadian pacemakers in the eyestalk-protocerebrum complex. In addition, the fact that desynchrony of the ERG rhythms only occurs in bisected preparations suggests that pathways between the protocerebral lobes normally couple the two pacemakers.

Dual controls for screening pigment movement in photoreceptors of the Limulus lateral eye: circadian efferent input and light

The radial and longitudinal distribution of retinular screening pigment in the lateral eye of the horseshoe crab Limulus polyphemus was quantified under a variety of experimental conditions. Pigment position was characterized by the center and width of the radial distribution at four levels in the ommatidium. Under diurnal lighting, intact animals show movement of pigment granules from the periphery of the retinular cell at night towards the junction of the arhabdomeral and rhabdomeral segments of the retinular cell in the day. In constant darkness, intact animals exhibit the same circadian rhythm in pigment migration. Animals with bilaterally cut optic nerves do not receive circadian efferent input from the brain and show little pigment movement in diurnal lighting. In all of these cases, pigment was either aggregated in a band just peripheral to the rays of the rhabdom or dispersed to the periphery of the retinular cell. When dark-adapted animals are exposed to a sudden large light increment, pigment moves inward between the rays of the rhabdom. During the day, this inward response begins immediately and reverses as the ommatidial aperture begins to close. At night, the onset of the inward movement is delayed, but then occurs more rapidly than during the day. No significant longitudinal movement of photoreceptor screening pigment was detected under any of these experimental conditions. Two opposing mechanisms control the movement of screening pigment in these cells. Release of neurotransmitters from the circadian efferents causes outward movement; large increments of light cause inward movement. In the absence of sudden changes in light intensity, circadian efferent input, not cyclic lighting, appears to be the major determinant of screening pigment position. A sudden and large increment of light triggers the rapid inward movement which appears to be a protective mechanism optimized for daytime performance.

Masking in invertebrates

Masking effects are a common feature of daily rhythmicity in invertebrates; and, particularly with respect to activity/rest cycles in arthropods and mollusks, there are numerous examples of masking in response to external environmental stimuli. Internal masking, in which endogenous processes modulate circadian patterns, has also been documented in a few species. In general, however, because of the absence of appropriate experimental investigations on masking, the functional significance (in an ecological sense) of masking effects is not understood.

The electroretinogram of the cockroach Leucophaea maderae

1. We have previously found that two components, the off-transient and the sustained component, of the electroretinogram (ERG) recorded from the eye of the cockroach Leucophaea maderae exhibit circadian rhythms in amplitude (Wills et al., 1985). 2. Analysis of the intensity-response functions of the two rhythmic components reveals that the circadian modulation of this visual system involves at least two independent regulatory processes. 3. Surgical and pharmacological manipulations suggest anatomically distinct sites of origin for the off-transient and the sustained component. 4. The results suggest that in this system the circadian pacemaker regulates visual processing both peripherally, at the level of the photoreceptors, and centrally, at the source of the off-transient.

Circadian rhythms in the electroretinogram of the cockroach

Circadian regulation of the amplitude of the electroretinogram (ERG) of the cockroach Leucophaea maderae was investigated. Two components of the ERG exhibited circadian rhythms in amplitude. Interestingly, the peak amplitudes for the two rhythms were approximately 12 hr out of phase. The dominant corneal negative potential (the "sustained component") exhibited maximum amplitude during the subjective night. A second corneal negative potential (the "off-transient") was at a maximum during the subjective day. Intensity-response curves of the sustained component were measured at both the peak and trough of the rhythm. The results showed that the circadian rhythm in amplitude reflected a sensitivity change equivalent to 0.2-0.6 log unit of intensity. An effort was also made to identify the anatomical locus of the pacemaking oscillator for the ERG rhythm in a series of lesion experiments. Neural isolation of the optic lobe from the midbrain by bisection of the optic lobe proximal to the distal edge of the lobula had no effect on the circadian rhythm of ERG amplitude. Bisection of the optic lobe distal to the lobula abolished the ERG amplitude rhythm. These results suggest that the pacemaker is located in the optic lobe near the lobula; that its motion continues in the absence of neural connections with the rest of the nervous system; and that its regulation of ERG amplitude depends on neural pathways in the optic lobe.

Cyclic motor activity; migrating motor complex: 1985

Most of the gastrointestinal tract and the biliary tract have a cyclic motor activity. The electric counterpart of this motor activity is called cyclic myoelectric activity. A typical motor cycle in the LES, stomach, and small intestine is composed of a quiescent state, followed by progressively increasing amplitude and frequency of contractions culminating in a state of maximal contractile activity. The colonic motor cycle has only the quiescent and the contractile states. In the small intestine, these motor complexes migrate in an aborad direction, and in the colon in both orad and aborad directions. The mechanisms of initiation and migration of these complexes are best understood in the small intestine. Both the initiation and migration of these complexes seem to be controlled by enteric neural mechanisms. The functions of the enteric mechanisms may be modulated by the central nervous system and by circulating endogenous substances. The mechanisms of initiation of these complexes are not completely understood in the rest of the gastrointestinal tract and in the biliary tract. The physiologic function of these motor complexes that occur only after several hours of fast in the upper gastrointestinal tract of nonruminants may be to clean the digestive tract of residual food, secretions, and cellular debris. This function is aided by a coordinated secretion of enzymes, acid, and bicarbonate. In ruminants, phase III activity is associated with the distal propulsion of ingested food. The function of colonic motor complexes that are not coordinated with the cyclic motor activities of the rest of the gastrointestinal tract may be only to move contents back and forth for optimal absorption.

Visual circadian rhythmicity in splitbrain crayfish: a plastic behavioral expression of symmetric circadian pacemakers

Electroretinographic evoked potentials (ERG's) were continuously recorded in dark-adapted, splitbrain crayfishes Procambarus bouvieri. Pulses of light (0.95 cd/m2) illuminating the left or the right eyes were alternatively applied every 15 or 30 min. As compared to intact crayfish, uni or bilateral damping or suppression of circadian retinal sensitivity rhythm could be caused by surgical bisection of cerebral ganglion in these crustaceans. The damped ERG circadian rhythm was rapidly reversed by reduction or elimination of the test light stimulus to the contralateral eye. Given the redundant processing of pacemaking information coming from bilaterally positioned cephalic circadian pacemakers to the central nervous system in splitbrain crayfish, photodependent damping of ERG rhythm revealed a plastic potential of central circadian pacemakers. The possibility that a strong but reversible inhibitory influence acts simultaneously upon the left and right protocerebral circadian pacemakers while receiving bilateral photic stimulation is considered.

Circadian rhythms in synaptic excitability of the dorsal lateral geniculate nucleus in the rat

Excitability changes of the dorsal lateral geniculate nucleus were recorded in male albino rats (Wistar strain) with chronically implanted electrodes. Both averaged postsynaptic and presynaptic evoked responses showed clear circadian rhythms under a constant dark condition. The circadian rhythmicity of the evoked responses was not as a result of motor activity or due to the sleep-wakefulness changes which might show circadian rhythms independently. Circadian rhythms of antidromic and orthodromic evoked responses were abolished after bilateral suprachiasmatic nucleus lesions. These results suggest that the circadian rhythm generated in the suprachiasmatic nucleus significantly affects visual information transmission in the lateral geniculate nucleus.

Autoradiographic localization of newly synthesized octopamine to retinal efferents in the Limulus visual system

The biogenic amine octopamine is synthesized from both tyrosine and tyramine in the lateral, median, and ventral eyes of Limulus. The autoradiographic studies presented here were designed to locate the sites of octopamine synthesis in the ventral and lateral eyes. We found that efferent fibers, which project to ventral and lateral eyes from the central nervous system, became intensely and selectively labeled during in vitro incubations with 3H-tyramine. In the ventral eye, more than 95% of the efferent fibers were labeled. Results of biochemical analyses suggested that most of the radioactive substance within these efferent fibers was newly synthesized octopamine. The selective labeling of efferent fibers during incubation with 3H-tyramine was used as an anatomical tool to study the number and distribution of efferent fibers within the ventral eye. Light microscopic (LM) reconstructions of the distribution of label in serial longitudinal sections through ventral optic nerves together with electron microscopic (EM) autoradiographic analyses revealed between 70 and 200 efferent axons. The results of these studies and of reconstructions of efferent innervation to photoreceptor somata suggest that each ventral photoreceptor cell or each small cluster of cells is innervated by a separate efferent fiber. Both LM reconstructions and EM analyses showed that efferent fibers ramify extensively and specifically in and near the internal rhabdom of ventral photoreceptor cells. In EM autoradiographs of lateral eyes incubated with 3H- tyramine, the silver grains that were located over ommatidia were concentrated exclusively over efferent fibers. All of these efferent fibers, which lay near rhabdoms and in partitions between retinular cells, were labeled. The results of our present studies support our hypothesis that octopamine is a neurotransmitter in Limulus retinal efferent fibers. This amine may modulate the biochemistry and physiology of ventral photoreceptor cells and may mediate many of the known effects of circadian efferent innervation to the lateral eye.

Range of modulation of light sensitivity by accessory pigments in the crayfish compound eye

The compound eye of the crayfish during dark adaptation undergoes an enhancement of light sensitivity within a range of 3 log units. Only 1 log unit can be explained by the increase in responsiveness of the retinula cells. The rest can be accounted for by the migration of the proximal and distal accessory pigments. In isolated retinas, with the distal pigment paralysed in light-adapted position and the proximal pigment only partially responsive, the sensitivity enhancement in darkness is reduced in more than 1 log unit. By hormonally inducing the expansion of the distal pigment while the rest of the system remains dark-adapted, there is a shift of one log unit in the V-log 1 curve. In a crayfish mutant devoid of the two dark accessory pigments, the sensitivity enhancement in dark adaptation only covers one log unit.

Electrophysiological evidences of mutual modulatory influences on the retinal activity of the crayfish Procambarus bouvieri (O)

Electroretinographic evoked potentials (ERGs) were recorded in dark adapted crayfish by the application of pulses of light (0.09 Cd/ft2) presented every 2.5 min. Heterolateral illumination (HI) for sixty min (0.06-0.3 Cd/ft2) induced up to 50% decrease in ERG after a latency of 12-25 min. ERG depression was proportional to the intensity of HI and also showed a circadian rhythm. During the alpha phase the ERG recovery started 3-10 min after HI was turned off. In contrast it started only after 10-20 min during the rho phase. The time course of the ERG depression, which was abolished in splitbrain animals, strongly suggests that a mutual modulatory influence, probably of neuroendocrine nature, is present in the crayfish visual system.