Efferent control of temporal response properties of the Limulus lateral eye

The sensitivity of the Limulus lateral eye exhibits a pronounced circadian rhythm. At night a circadian oscillator in the brain activates efferent fibers in the optic nerve, inducing multiple changes in the physiological and anatomical characteristics of retinal cells. These changes increase the sensitivity of the retina by about five orders of magnitude. We investigated whether this increase in retinal sensitivity is accompanied by changes in the ability of the retina to process temporal information. We measured the frequency transfer characteristic (FTC) of single receptors (ommatidia) by recording the response of their optic nerve fibers to sinusoidally modulated light. We first measured the FTC in the less sensitive daytime state and then after converting the retina to the more sensitive nighttime state by electrical stimulation of the efferent fibers. The activation of these fibers shifted the peak of the FTC to lower frequencies and reduced the slope of the low-frequency limb. These changes reduce the eye's ability to detect rapid changes in light intensity but enhance its ability to detect dim flashes of light. Apparently Limulus sacrifices temporal resolution for increased visual sensitivity at night.     

Cellular analysis of ocular circadian pacemaker coupling in Bulla: role of efferent impulses in phase shifting

The eyes of Bulla gouldiana, a marine snail, contain circadian oscillators that are coupled to each other. Obvious candidates for the coupling signals are the optic nerve compound action potentials (CAPs) that express the circadian rhythm and lead to efferent impulses in the contralateral optic nerve. In the present experiments, the role of the CAPs as coupling signals was evaluated. We found that, following desynchronization of the two ocular oscillators by phase-delaying one eye with manganese, subsequent phase shifts in the initially unshifted ocular rhythm only occurred during the time that efferent optic nerve signals were present. In addition, in the absence of ocular desynchrony, phase shifts of the ocular rhythm could still be effected by activation of the efferent pathway. The influence of efferent impulses on identified retinal cells was also evaluated. No effect of efferent signals on receptor layer cells was detected, while it was found that efferent impulses generated depolarizations in basal retinal neurons (BRNs), the putative circadian oscillator cells. Depolarization of the BRNs has been shown previously to be involved in the light entrainment pathway. Depolarization appears to be similarly involved in the coupling pathway, since membrane depolarizations that mimicked the efferent-induced postsynaptic potentials likewise generated phase shifts of the ocular rhythm.     

Limulus vision in the marine environment

Horseshoe crabs use vision to find mates. They can reliably detect objects resembling potential mates under a variety of lighting conditions. To understand how they achieve this remarkable performance, we constructed a cell based realistic model of the lateral eye to compute the ensembles of optic nerve activity ("neural images") it transmits to the brain. The neural images reveal a robust encocding of mate-like objects that move underwater during the day. The neural images are much less clear at night, even though the eyes undergo large circadian increases of sensitivity that nearly compensate for the millionfold decreasein underwater lighting after sundown. At night the neurral images are noisy, dominated by bursts of nerve impulses from random photon events that occur at low nighttime levels of illumination. Deciphering the eye's input to the brain begins at the first synaptic level with lowpass temporal and spatial filtering. Both neural filtering mechanisms improve the signal-to-noise properties of the eye's input, yielding clearer neural images of potential mates, especiallyat night. Insights about visual processing by the relatively simple visual system of Limulus may aid in the designof robotic sensors for the marine environment.     

Circadian rhythms in Limulus photoreceptors. I. Intracellular studies

The sensitivity of the lateral eye of the horseshoe crab, Limulus polyphemus, is modulated by efferent optic nerve impulses transmitted from a circadian clock located in the brain (Barlow, R. B., Jr., S. J. Bolanowski, and M. L. Brachman. 1977. Science. 197:86-89). At night, the efferent impulses invade the retinular, eccentric, and pigment cells of every ommatidium, inducing multiple anatomical and physiological changes that combine to increase retinal sensitivity as much as 100,000 times. We developed techniques for recording transmembrane potentials from a single cell in situ for several days to determine what circadian changes in retinal sensitivity originate in the primary phototransducing cell, the retinular cell. We found that the direct efferent input to the photoreceptor cell decreases its noise and increases its response. Noise is decreased by reducing the rate of spontaneous bumps by up to 100%. The response is increased by elevating photon catch (photons absorbed per flash) as much as 30 times, and increasing gain (response per absorbed photon) as much as 40%. The cellular mechanism for reducing the rate of spontaneous quantum bumps is not known. The mechanism for increasing gain appears to be the modulation of ionic conductances in the photoreceptor cell membrane. The mechanism for increasing photon catch is multiple changes in the anatomy of retinal cells. We combine these cellular events in a proposed scheme for the circadian rhythm in the intensity coding of single photoreceptors.     

Spike firing pattern of output neurons of the Limulus circadian clock

The lateral eyes of the horseshoe crab (Limulus polyphemus) show a daily rhythm in visual sensitivity that is mediated by efferent nerve signals from a circadian clock in the crab's brain. How these signals communicate circadian messages is not known for this or other animals. Here the authors describe in quantitative detail the spike firing pattern of clock output neurons in living horseshoe crabs and discuss its possible significance to clock organization and function. Efferent fiber spike trains were recorded extracellularly for several hours to days, and in some cases, the electroretinogram was simultaneously acquired to monitor eye sensitivity. Statistical features of single- and multifiber recordings were characterized via interval distribution, serial correlation, and power spectral analysis. The authors report that efferent feedback to the eyes has several scales of temporal structure, consisting of multicellular bursts of spikes that group into clusters and packets of clusters that repeat throughout the night and disappear during the day. Except near dusk and dawn, the bursts occur every 1 to 2 sec in clusters of 10 to 30 bursts separated by a minute or two of silence. Within a burst, each output neuron typically fires a single spike with a preferred order, and intervals between bursts and clusters are positively correlated in length. The authors also report that efferent activity is strongly modulated by light at night and that just a brief flash has lasting impact on clock output. The multilayered firing pattern is likely important for driving circadian rhythms in the eye and other target organs.     

Circadian rhythms in Limulus photoreceptors. II. Quantum bumps

The light response of the lateral eye of the horseshoe crab, Limulus polyphemus, increases at night, while the frequency of spontaneous discrete fluctuations of its photoreceptor membrane potential (quantum bumps) decreases. These changes are controlled by a circadian clock in the brain, which transmits activity to the eye via efferent optic nerve fibers (Barlow, R. B., S. J. Bolanski, and M. L Brachman. 1977. Science. 197:86-89). Here we report the results of experiments in which we recorded from single Limulus photoreceptors in vivo for several days and studied in detail changes in their physiological and membrane properties. We found that: (a) The shape of (voltage) quantum bumps changes with the time of day. At night, spontaneous bumps and bumps evoked by dim light are prolonged. The return of the membrane potential to its resting level is delayed, but the rise time of the bump is unaffected. On average, the area under a bump is 2.4 times greater at night than during the day. (b) The rate of spontaneous bumps decreases at night by roughly a factor of 3, but their amplitude distribution remains unchanged. (c) The resting potential and resistance of the photoreceptor membrane do not change with the time of day. (d) the relationship between injected current and impulse rate of the second order neuron, the eccentric cell, also remains unchanged with the time of day. Thus the efferent input from the brain to the retina modulates some of the membrane properties of photoreceptor cells. Our findings suggest that the efferent input acts on ionic channels in the membrane to increase the sensitivity of the photoreceptor to light.     

The role of ocelli in circadian singing rhythms of crickets

ABSTRACT. . There is a direct quantitative relationship between the free-running period (r) of the circadian stridulation rhythm of male Australian field crickets, Teleogryllus commodus (Walker), and the intensity of the constant light conditions. Both T. commodus and the house cricket, Acheta domesticus (L.), show free-running periods of the singing rhythm of c. 24 h when the light intensity is 0.00025 lux. In both species the severance of the three ocellar nerves significantly slows the circadian period which is indicative of a reduced perception of the available light intensity. To test whether this period reduction is a peripheral or a central effect, electro-retinograms (ERGs) were recorded from compound eyes of male T. commodus with the ocelli fully functional, then occluded, and then uncovered. The size of the compound eye ERG is reduced by 20% with ocellar occlusion and can subsequently be fully restored to the intact level, which indicates that the ocellar effect is a peripheral one. Intensified CoCl₂ fills reveal one neurone in A. domesticus and two in T. commodus which travel from the lateral ocellar nerve out into the ipsilateral optic lobe of the compound eye. These neurones all terminate in or distal to the lobular neuropile. The data are interpreted to indicate a role for ocelli in modulating the light intensity perception of the compound eye. The final effect of the ocellar afferents is at a peripheral level prior to the input of the visual information to the optic lobe circadian pacemaker. Hence ocelli play an indirect role in circadian rhythmicity, augmenting the sensitivity of the primary photoreceptors to better perceive photic entrainment signals.     

The optic nerve mediates the circadian pigment migration in the median eyes of the scorpion

• 1.1. The peripheral visual pathway from the median eyes of the scorpion Androctonus australis was interrupted at different points and the effect on the circadian rhythm of median-eye sensitivity was examined.
• 2.2. Any interruption of the visual pathway distal to the supraesophageal ganglion abolishes the circadian sensitivity rhythm in the median eyes. This rhythm is thus controlled by efferents in the optic nerve (very probably via the neurosecretory axons) rather than by way of the hemolymph.
• 3.3. Following transection of the optic nerve, the sensitivity of the median eyes proceeds rapidly to the daytime state. This condition is associated with movement of the screening pigment into the distal ends of the visual cells.
• 4.4. The oscillator system controlling the circadian pigment migration in the median eye cannot be located in the eye itself, but must lie in the CNS, proximal to the first optic ganglion. The oscillator itself need not be connected to both median eyes in order to function normally, as revealed by the continued rhythm in the contralateral eye following unilateral optic nerve section.

     

Neurosecretory cells in the circadian-clock system of the scorpion,Androctonus australis

Summary The somata of the efferent neurosecretory fibers that control the circadian sensitivity rhythm in the median eyes of the scorpion, Androctonus australis, were detected in the brain by retrograde labeling with Lucifer Yellow CH. A total of 20–40 neurons are arranged in two groups displaying a bilaterally symmetrical, marginal position near the circumesophageal connectives. Half the cells in each group send fibers into the ipsilateral optic nerve; the fibers from the other half enter the contralateral optic nerve.     

Circadian rhythms in the green sunfish retina

We investigated the occurrence of circadian rhythms in retinomotor movements and retinal sensitivity in the green sunfish, Lepomis cyanellus. When green sunfish were kept in constant darkness, cone photoreceptors exhibited circadian retinomotor movements; rod photoreceptors and retinal pigment epithelium (RPE) pigment granules did not. Cones elongated during subjective night and contracted during subjective day. These results corroborate those of Burnside and Ackland (1984. Investigative Ophthalmology and Visual Science. 25:539-545). Electroretinograms (ERGs) recorded in constant darkness in response to dim flashes (lambda = 640 nm) exhibited a greater amplitude during subjective night than during subjective day. The nighttime increase in the ERG amplitude corresponded to a 3-10-fold increase in retinal sensitivity. The rhythmic changes in the ERG amplitude continued in constant darkness with a period of approximately 24 h, which indicates that the rhythm is generated by a circadian oscillator. The spectral sensitivity of the ERG recorded in constant darkness suggests that cones contribute to retinal responses during both day and night. Thus, the elongation of cone myoids during the night does not abolish the response of the cones. To examine the role of retinal efferents in generating retinal circadian rhythms, we cut the optic nerve. This procedure did not abolish the rhythms of retinomotor movement or of the ERG amplitude, but it did reduce the magnitude of the nighttime phases of both rhythms. Our results suggest that more than one endogenous oscillator regulates the retinal circadian rhythms in green sunfish. Circadian signals controlling the rhythms may be either generated within the eye or transferred to the eye via a humoral pathway.     

Analysis of mutual circadian pacemaker coupling between the two eyes of Bulla

The eyes of Bulla, a marine snail, express a circadian rhythm in the frequency of optic nerve compound action potentials (CAPs). The two ocular pacemakers are mutually coupled, and their interaction can be observed in vitro. The evidence for mutual coupling, as demonstrated in the present experiments, was as follows: (1) When intact Bulla were placed into darkness for up to 72 days, the two pacemakers did not desynchronize. (2) The free-running period of the ocular rhythm in the intact system (24.4 hr) was longer than the free-running period of the rhythm recorded from isolated eyes (23.7 hr). (3) When the two ocular pacemakers were experimentally desynchronized in vitro, resynchronization occurred if the pacemakers were allowed to interact for 48 hr. The coupling signals are most likely the CAPs. These impulses are conducted through the central ganglia and emerge as efferent impulses in the opposite optic nerve. Ocular-derived efferent impulse activity affects spontaneous impulse production in the target eye and alters the waveform of the circadian rhythm. The coupling pathway mediating syncrhonization consists of the two optic nerves, the cerebral ganglia, and the cerebral commissure. The demonstration of coupling in vitro provides a new opportunity for studying the cellular mechanisms underlying mutual pacemaker entrainment.     

Properties of visual cells in the lateral eye of Limulus in situ.

Excitatory properties of visual cells in the lateral eye of Limulus, investigated by optic nerve recordings in situ, differ significantly from the properties of cells in the classical, excised eye preparation. The differences suggest the possibility that two receptor mechanisms function in the eye in situ: one mechanism encodes low light intensities and the other responds to high intensities. The two mechanisms enable each ommatidium to respond over an intensity range of approximately 10 log units. This hypothesis was tested by measuring the increment threshold and the spectral sensitivity, by studying light and dark adaptation, and by analyzing the variability of the impulse discharge. Although the results do not conclusively identify two receptor mechanisms, they indicate that a process or a part of a process that functions in the eye in situ is abolished by excising the eye or cutting off its blood supply.     

Scorpion lateral eyes: Extremely sensitive receptors of Zeitgeber stimuli

Summary The circadian rhythm of sensitivity in the median eyes ofAndroctonus australis L. can be entrained by exposure of the lateral eyes to a 24-h light-dark rhythm. Presentation of the Zeitgeber to only the anteriormost one of the lateral eyes sufficed (Fig. 1). However, with illumination of an entire group of lateral eyes (Fig. 2), entrainment was obtained at extremely low light intensities — white light at luminance levels of 10^–4cd · m^–2 (=2.5 · 10^–4 lux, cf. Methods).The relatively less marked circadian rhythm of lateral-eye sensitivity is probably controlled via the optic nerve supplying these eyes (Fig. 4).Supported by the Deutsche Forschungsgemeinschaft (F1 77/5-6 and F1 77/7 Schwerpunktprogramm: Biologie der Zeitmessung)     

The effect of neuropeptides from Limulus on its circadian rhythm in retinal sensitivity

Retinal responses of the Limulus lateral eyes to light are greater at night than during the day. A circadian clock in the brain of the horseshoe crab controls these rhythmic changes of light sensitivity. The increase in sensitivity (as measured by the amplitude of the electroretinogram) is mediated at least in part by octopamine that is released from efferent axons terminating in the visual cells. Earlier studies indicate that certain factors in Limulus hemolymph can act in conjunction with octopamine. More recently, five neuropeptides (LP1-LP4 and Lip-HP) had been isolated from acetone extracts of the Limulus central nervous system using HPLC fractionation and radioimmunoassay with antisera against FMRFamide-like peptides for detection. Presently, we have injected into the Limulus lateral eye these five peptides and observed changes in retinal sensitivity. Injection during daytime had no immediate effect on that daytime electroretinogram but decreased the electroretinogram amplitude for the entire subsequent night (12 h). However, upon injection at night, we observed an immediate but only transitory decrease in electroretinogram amplitude for about 1 h without effect on the subsequent daytime electroretinogram. We suggest that the peptides act antagonistically to octopamine and are highly dependent upon the activity state of the efferent nerve terminals. Accepted: 25 August 1996     

A specific area of the compound eye in the cricket Gryllus bimaculatus sends photic information to the circadian pacemaker in the contralateral optic lobe

The circadian locomotor rhythm of the cricket Gryllus bimaculatus is primarily regulated by a pair of interacting optic lobe circadian pacemaker systems. The interaction involves phase-dependent modulation of the free-running period and phase-dependent suppression of activity. Since photic information has been shown to be involved in the interaction, we examined the regional difference in photoreception for the interaction within cricket compound eyes. The activity rhythm of animals receiving partial reduction of one compound eye combined with severance of the contralateral optic nerve split into entrained and free-running components under a 13-h light to 13-h dark cycle. All the animals operated on showed a phase-dependent suppression of activity, and most animals showed a phase-dependent modulation of the period of the free-running component. However, removal of the dorsocaudal area of the compound eye resulted in a severe reduction of the amplitude of the phase-dependent-period modulation. These results suggest that the dorsocaudal portion of the compound eye is a specific region receiving photic signals that are transmitted to the circadian pacemaker in the contralateral optic lobe and that the phase-dependent suppression of activity is caused by a mechanism separate from that for the period modulation.     

Retina mediators in fresh-water mollusc Lymnaeae stagnalis

Retrograde staining of the Lymnaeae stagnalis retina with neurobiotin has shown that most photoreceptor cells send axons to optic nerve without intermediate contacts. A part of these photoreceptors have immunireactivity to glutamate that possibly provides synaptic transmission of visual signal to central neurons. Other photoreceptors stained through optic nerve seem to have different transmitter systems. In some retina cell, but not in optic nerve fibers, immunoreactivity to pigment-dispersing hormone has been revealed. In tissues surrounding the eye cup numerous serotonin-containing fibers are present, a part of them penetrating the retina basal layer. Some of them belong to central neurons responsible for efferent innervation of the pond snail eye. It is suggested that the serotoninergic innervation as well as the cell containing the pigment-dispersing hormone are included in the mechanism of regulation of light sensitivity of the mollusc eye.     

Circadian organization in Aplysia californica.

Substantial progress has been made in unraveling the organization of the circadian system of Aplysia californica. There are at least three circadian pacemakers in Aplysia. One has been localized in each eye and a third lies outside the eyes. Removal of the eyes disrupts the free-running locomotor activity rhythm; however, an extraocular oscillator can mediate a free-running rhythm in some eyeless animals. Although photoreceptors sufficient for entrainment of the ocular oscillator have been localized in the retina, photoreceptors outside the eyes are capable of "driving" a diurnal rhythm of locomotor activity and may also influence entrainment of ocular pacemakers. Finally, attention has been focused on the optic nerve as a coupling pathway between various parts of the system. The evidence suggests that information transmitted in the optic nerves is involved in entrainment of the ocular pacemaker by light, and in ocular control of the locomotor activity rhythm.     

Spectral sensitivities of jumping spider eyes

Summary Spectral sensitivities of the anterior lateral, posterior lateral and anterior median eyes of the jumping spider,Menemerus confusus Boes. et Str. have been studied by recording electroretinograms (ERGs) and receptor potentials. The anterior and posterior lateral eyes have a single type of visual cell with a maximum spectral sensitivity at about 535–540 nm. The anterior median eye has four types of visual cells with maximum sensitivities at about 360, 480–500, 520–540 and 580 nm, respectively. The ERGs recorded from the optic nerve side (posterior part of the retina) were affected greatly by long wave chromatic light and those on the corneal side (anterior part of the retina) by short wave chromatic light, suggesting that each receptor layer contains a different photopigment.     

Larval optic nerve and adult extra-retinal photoreceptors sequentially associate with clock neurons during Drosophila brain development

The visual system is one of the input pathways for light into the circadian clock of the Drosophila brain. In particular, extra-retinal visual structures have been proposed to play a role in both larval and adult circadian photoreception. We have analyzed the interactions between extra-retinal structures of the visual system and the clock neurons during brain development. We first show that the larval optic nerve, or Bolwig nerve, already contacts clock cells (the lateral neurons) in the embryonic brain. Analysis of visual system-defective genotypes showed that the absence of the afferent Bolwig nerve resulted in a severe reduction of the lateral neurons dendritic arborization, and that the inhibition of nerve activity induced alterations of the dendritic morphology. During wild-type development, the loss of a functional Bolwig nerve in the early pupa was also accompanied by remodeling of the arborization of the lateral neurons. Approximately 1.5 days later, visual fibers that came from the Hofbauer-Buchner eyelet, a putative photoreceptive organ for the adult circadian clock, were seen contacting the lateral neurons. Both types of extra-retinal photoreceptors expressed rhodopsins RH5 and RH6, as well as the norpA-encoded phospholipase C. These data strongly suggest a role for RH5 and RH6, as well as NORPA, signaling in both larval and adult extra-retinal circadian photoreception. The Hofbauer-Buchner eyelet therefore does not appear to account for the previously described norpA-independent light input to the adult clock. This supports the existence of yet uncharacterized photoreceptive structures in Drosophila.     

Effects of efferent activity on entrainment of the Aplysia eye

• 1.1. The effects of efferent optic nerve activity on the speed of entrainment of the ocular oscillators in Aplysia were investigated by severing one optic nerve in 21 animals and evaluating reentrainment rates to a light cycle phase advanced 10hr in relation to a prior light schedule.
• 2.2. Reentrainment to this phase advanced light cycle was essentially complete in both the intact and denervated eyes within 3 days; and there was no indication that the time course of entrainment was significantly different in the two eyes.
• 3.3. The phase angle of the rhythm of neurally isolated eyes, however, consistently phase led the intact eyes by approx 1 hr. Thus, the results indicate that while efferent activity does not significantly effect the speed of entrainment, it appears to influence steady-state phase angle.