The morphology of the limulus visual system. IV. The lateral optic nerve

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.

Fine structural correlates of sensitivity in the eyes of the ctenid spider, Cupiennius salei Keys

We studied fine structural correlates of sensitivity in the principal and secondary eyes of the nocturnal hunting spider Cupiennius salei. In night-adapted eyes the four rhabdomeres of the principal eye photoreceptors are 58 microm long and occupy together 234 microm(2) in cross-section (average), whereas the two rhabdomeres of the secondary eye photoreceptors are about 49 microm long and measure 135-183 microm(2) in cross-section (average). The rhabdoms (photosensitive structures) consist of tightly packed microvilli (diameter 0.1 microm, maximum length 3.5 microm) and occupy up to 63% of the cross-sectional area of the retina. When calculating the amount of light the eyes of Cupiennius are able to capture according to their morphological characteristics, the values for sensitivity S(see Land, 1981, 1985) are between 78 and 109 microm(2). Cupiennius is more sensitive than any other hunting spider examined except Dinopis whose posterior median eyes are the most sensitive ones of all terrestrial arthropod eyes studied. In day-adapted eyes the rhabdomeral microvilli are almost completely degraded. The remaining microvillar surface amounts to only about one-tenth compared with the night-adapted state. Efferent synaptoid terminals have been found to contact the photoreceptors in all eyes of C. salei. The present fine structural data are compared to previous electrophysiological research and underline the significance of vision in Cupiennius.

The eyes of Limulus polyphemus (Xiphosura, Chelicerata) and their afferent and efferent projections

The visual system of the American horseshoe crab Limulus polyphemus (L. polyphemus) is an important preparation for studying the photoresponse, the circadian modulation of the photoresponse and visual information processing. Given its unique position in phylogeny the structure of its visual system also informs studies of the relationships among arthropods and the characteristics of eurarthropods. Much has been learned about the organization of the relatively simple L. polyphemus visual system, but much remains to be discovered. This review summarizes current knowledge of the structure of L. polyphemus eyes and the organization of their afferent and efferent projections and points to important unanswered questions.

Circadian efferent input to Limulus eyes: anatomy, circuitry, and impact

Much is known about the anatomy of Limulus retinal efferent neurons and the structural and functional consequences of their activation. Retinal efferent axons arise from cell bodies located in the cheliceral ganglia of the brain, and they project out all of the optic nerves. Their unique neurosecretory-like terminals contact all cell types in lateral eye ommatidia, the retinular cells of the median eye, and the internal rhabdom of ventral photoreceptors. Lateral and median rudimentary photoreceptors are also innervated. The activity of the efferents is circadian. They are active during the subjective night and inactive during the subjective day. Activation of the efferents drives dramatic and diverse changes in the structure and function of Limulus eyes and causes the sensitivity and responsiveness of the eyes to light to increase at night. Relatively little is known about the molecular mechanisms that produce these structural and functional changes, but one efferent-activated biochemical cascade has been identified. The biogenic amine octopamine is released from efferent terminals, and an octopamine-stimulated rise in cAMP in photoreceptors, with a subsequent activation of cAMP-dependent protein kinase, mediates many of the known effects of efferent input. A photoreceptor-specific protein, myosin III, is phosphorylated in response to efferent input; this protein may play a role in the efferent stimulated changes in photoreceptor structure and function. Anatomical, biophysical, biochemical, and molecular approaches are now being effectively combined in studies of Limulus eyes; thus, this preparation should be particularly useful for further detailed investigations of mechanisms underlying the modulation of primary sensory cells by efferent input.

Immunocytochemical localization of opsin, visual arrestin, myosin III, and calmodulin in Limulus lateral eye retinular cells and ventral photoreceptors

The photoreceptors of the horseshoe crab Limulus polyphemus are classical preparations for studies of the photoresponse and its modulation by circadian clocks. An extensive literature details their physiology and ultrastructure, but relatively little is known about their biochemical organization largely because of a lack of antibodies specific for Limulus photoreceptor proteins. We developed antibodies directed against Limulus opsin, visual arrestin, and myosin III, and we have used them to examine the distributions of these proteins in the Limulus visual system. We also used a commercial antibody to examine the distribution of calmodulin in Limulus photoreceptors. Fixed frozen sections of lateral eye were examined with conventional fluorescence microscopy; ventral photoreceptors were studied with confocal microscopy. Opsin, visual arrestin, myosin III, and calmodulin are all concentrated at the photosensitive rhabdomeral membrane, which is consistent with their participation in the photoresponse. Opsin and visual arrestin, but not myosin III or calmodulin, are also concentrated in extra-rhabdomeral vesicles thought to contain internalized rhabdomeral membrane. In addition, visual arrestin and myosin III were found widely distributed in the cytosol of photoreceptors, suggesting that they have functions in addition to their roles in phototransduction. Our results both clarify and raise new questions about the functions of opsin, visual arrestin, myosin III, and calmodulin in photoreceptors and set the stage for future studies of the impact of light and clock signals on the structure and function of photoreceptors.

A myosin III from Limulus eyes is a clock-regulated phosphoprotein

The lateral eyes of the horseshoe crab Limulus polyphemus undergo dramatic daily changes in structure and function that lead to enhanced retinal sensitivity and responsiveness to light at night. These changes are controlled by a circadian neural input that alters photoreceptor and pigment cell shape, pigment migration, and phototransduction. Clock input to the eyes also regulates photomechanical movements within photoreceptors, including membrane shedding. The biochemical mechanisms underlying these diverse effects of the clock on the retina are unknown, but a major biochemical consequence of activating clock input to the eyes is a rise in the concentration of cAMP in photoreceptors and the phosphorylation of a 122 kDa visual system-specific protein. We have cloned and sequenced cDNA encoding the clock-regulated 122 kDa phosphoprotein and show here that it is a new member of the myosin III family. We report that Limulus myosin III is similar to other unconventional myosins in that it binds to calmodulin in the absence of Ca2+; it is novel in that it is phosphorylated within its myosin globular head, probably by cAMP-dependent protein kinase. The protein is present throughout the photoreceptor, including the region occupied by the photosensitive rhabdom. We propose that the phosphorylation of Limulus myosin III is involved in one or more of the structural and functional changes that occur in Limulus eyes in response to clock input.

A circadian clock in the Limulus brain transmits synchronous efferent signals to all eyes

A circadian clock in the brain of the horseshoe crab, Limulus polyphemus, has an important role in the function of the peripheral visual system. At night, the clock transmits neural activity to the lateral, ventral, and median eyes via efferent optic nerve fibers. The activity occurs in synchronous bursts (maximum rate of 2 bursts/s) with individual efferent fibers contributing a single spike in each burst. The circadian efferent activity originates in the protocerebrum. Lateral connections synchronize the efferent activity recorded from the two halves of the protocerebrum, suggesting the existence of bilateral circadian oscillators. Circadian efferent activity survives excision of the brain and isolation of the protocerebrum. We conclude that circadian clock and its complex neural circuitry are fundamental components of the Limulus visual system.

Central projections of Limulus photoreceptor cells revealed by a photoreceptor-specific monoclonal antibody

Studies of lateral, median, and ventral eyes of the chelicerate arthropod Limulus polyphemus (the common American horseshoe crab) are providing important basic information about mechanisms for information processing in the peripheral visual system and for the modulation of visual responses by light and circadian rhythms. The processing of visual information in Limulus brain is less well understood in part because the specific central projections of the various classes of visual neurons are not known. This study describes a mouse monoclonal antibody, 3C6A3, which binds to Limulus photoreceptor cell bodies, their axons, and terminals, but not to any other cell type in the central nervous system. This antibody, and intracellular injection of biocytin, are used to demonstrate the central projections of each type of photoreceptor. Our main conclusions are that: 1) the photoreceptors (retinular cells) of the lateral eye project only to the lamina; 2) the photoreceptors of the lateral rudimentary eye project to both the lamina and medulla; 3) the photoreceptors of the median ocellus project only to the ocellar ganglion; and 4) the photoreceptors of the rudimentary median (endoparietal) eye project to the ocellar ganglion and also into the optic tract. These results, along with previous studies, allow us to infer the projections of the secondary cells. The eccentric cells of the lateral eye project to the lamina, medulla, optic tract, and ocellar ganglion. The arhabdomeral cells of the median ocellus project through the ocellar ganglion and to optic tract to the medulla.

Immunocytochemical localization of octopamine in the central nervous system of Limulus polyphemus: a light and electron microscopic study

We have determined the distribution and localization of the monoamine octopamine in the prosomal central nervous system of the horseshoe crab, Limulus polyphemus, by light and electron microscopic immunocytochemistry. Sixteen discrete clusters of octopamine-like immunoreactive neurons are situated bilaterally in the tritocerebrum and circumesophageal ring of fused thoracic ganglia. Two pairs of anterior clusters are located laterally in the cheliceral and first pedal ganglia; the remaining six pairs of clusters are located ventromedially in the second through fifth pedal ganglia, chilarial ganglia, and opercular ganglia. The immunoreactive somata range from about 40 to 100 microns in diameter and occur in clusters of 12-24 cells. There is extensive distribution of octopamine-immunoreactive nerve fibers in Limulus; dense fiber tracts course anteroposteriorly through the central nervous system, and most neuropil regions are innervated by immunoreactive processes and terminals. This wide distribution of octopamine-like immunoreactivity provides an anatomical basis for the several effects of octopamine in Limulus. We determined the subcellular localization of octopamine by postembedding immunoelectron microscopy. The immunogold-labelled terminals are morphologically unique; they contain large, distinctively shaped dense-core granules, typically cylindrical with a prominent indentation in one end. These large granules are 100-150 nm in diameter and range from 150-400 nm in length. The dense labelling of these unusual granules with immunogold particles indicates that octopamine is sequestered in or associated with the granules.

Two classes of Limulus ventral photoreceptors

Ventral photoreceptors of the horseshoe crab, Limulus polyphemus, have been important in the study of visual transduction, due to their large size and hardiness in vitro. This study shows that there are two classes of ventral photoreceptors that can be distinguished on the basis of differences in cellular and nuclear dimensions, soma and rhabdom morphology, and axon size. Large protoreceptors, which have been the subject of many physiological studies, have an extensive superficial rhabdom, a nuclear diameter of 20-24 microns, and measure 100-150 microns in length. In contrast, small photoreceptors measure 45-65 microns in length and have a nucleus 13-16 microns across. Small photoreceptors are found singly or in association with large photoreceptors. The rhabdom of isolated small photoreceptors is surrounded by a calyx originating from the soma, so that it appears to be located internally. The rhabdomeral lobe of small photoreceptors associated with large photoreceptors characteristically is divided into several segments, each of which invaginates the rhabdomeral lobe of the adjacent large photoreceptor. The entire external rhabdom of the associated small photoreceptor abuts the rhabdom of the large photoreceptor. Morphometric analysis of the ventral nerves shows that there are two size classes of photoreceptor axons, corresponding to the two classes of photoreceptors. The numbers of axons in each size class are nearly equal. Unlike the ventral eye, none of the other eyes of Limulus have been reported to have more than one morphological class of photoreceptor. Functional differences between the two classes of ventral photoreceptors are suggested by experiments, reported in the accompanying paper (Herman (1991), J. Comp. Neurol. 303:11-21), showing that the large photoreceptors exhibit light-stimulated rhabdom turnover while the small ones do not.

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.

Efferent neurotransmission of circadian rhythms in Limulus lateral eye. II. Intracellular recordings in vitro

We investigated efferent neurotransmission in the Limulus lateral eye by studying the action of pharmacological agents on responses of photoreceptor cells in vitro. We recorded transmembrane potentials from single cells in slices of retina that were excised during the day and maintained for several days in a culture medium. Potentials recorded in the absence of pharmacological agents resemble those recorded from cells in vivo during the day. Octopamine, a putative efferent neurotransmitter, induced changes in photoreceptor potentials that mimicked in part those generated at night by a circadian clock located in the brain. Specifically, octopamine (100 to 500 microM) decreased the frequency of occurrence of quantum bumps in the dark and increased the amplitude of photoreceptor responses to intermediate and high light intensities. Similar actions were produced by naphazoline (25 to 100 microM, potent agonist of octopamine), forskolin (8 to 400 microM, activator of adenylate cyclase), IBMX (1 mM, inhibitor of phosphodiesterase), and 8-bromo-cAMP (500 microM, analogue of cAMP). 8-bromo-cGMP (500 microM, analogue of cGMP) decreased the rate of spontaneous quantum bumps only. Our results support the hypothesis that (1) octopamine is an efferent neurotransmitter of circadian rhythms in the Limulus eye and that (2) it activates adenylate cyclase to increase levels of the second messenger, cAMP, in photoreceptor cells. Circadian changes in photoreceptor responses to moderate intensities may be a specific action of cAMP, since cGMP has no effect. Circadian changes in the rate of spontaneous quantum bumps may involve a less specific intermediate, since both cAMP and cGMP reduce bump rate. Characteristics of the retinal slice preparation precluded a detailed study of the effects of pharmacological agents on retinal morphology.

Circadian change in function of Limulus ventral photoreceptors

Efferent fibers from a central circadian clock innervate photoreceptors along the ventral nerve of Limulus and release octopamine when active. We have recorded ERG-like responses from the ventral eye in vivo over several day periods. We have also used intracellular microelectrodes to study changes in ventral photoreceptor function during exogenous applications of octopamine (the putative efferent neurotransmitter), IBMX (a phosphodiesterase inhibitor), and forskolin (an adenylate cyclase activator): (1) Responses to light measured at night from ventral photoreceptors in vivo are greater in amplitude than those recorded during the day; (2) Octopamine and agents that increase intracellular levels of cAMP in ventral photoreceptors decrease the rate of spontaneous (dark) bumps, increase photoreceptor response to light without changing threshold, and often increase the bump duration; and (3) These changes in function of ventral photoreceptors are similar to those that have been observed in the photoreceptor of the lateral eye during circadian clock activity at night, and in vitro in the presence of those same pharmacological agents.

Circadian rhythms in adaptation to light of Limulus photoreception

1. The lateral eye of Limulus responds differently to light-adaptation depending upon whether the eye is tested during the day, or at night when a central circadian clock is known to activate efferent fibers that terminate in the retina. 2. At night, the decrement in retinal sensitivity due to light-adaptation is less pronounced immediately following a light-flash but is greater at longer times after the flash when compared with responses recorded during the day. 3. In our quantitative description of dark adaptation we find that at least two time constants are needed to describe the time course, and that the two decaying exponentials are differentially altered by circadian clock action. 4. We relate these day-to-night alterations in light-adaptive properties to circadian changes in physiology and morphology studied previously in Limulus photoreceptors.

An atlas of the brain of the horseshoe crab Limulus polyphemus

An atlas of the brain of the horseshoe crab Limulus polyphemus is developed. All of the neuronal groups are identified and named, and regions of neuropil are segregated and named where possible. The nomenclature incorporates functionally neutral earlier names and assigns geographical names to newly distinguished structures. The atlas provides a basis for correlating the results of neuroanatomical, neurophysiological, and neurochemical studies, which yield information about individual neurons or groups of neurons in this species.

Octopamine release from centrifugal fibers of the Limulus peripheral visual system

Octopamine, a biogenic amine, is synthesized and stored within centrifugal (efferent) fibers that project from the brain to the lateral and ventral eyes of the horseshoe crab, Limulus polyphemus. The experiments described here show that depolarization of Limulus lateral and ventral eyes, produced by elevating the concentration of extracellular K+, causes the selective release of newly synthesized octopamine from centrifugal fibers in a manner that requires the influx of extracellular Ca2+. Conjugates of octopamine and tyramine that are also stored within centrifugal fibers are not released in response to K+-induced depolarization. These findings add further support to the hypothesis that octopamine is a neurotransmitter synthesized by and released from centrifugal fibers in Limulus eyes. This amine may be responsible for many of the alterations in lateral eye structure and function that are mediated by centrifugal innervation.

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.

Neuroanatomy of the visual afferents in the horseshoe crab (Limulus polyphemus)

The central nervous system of Limulus consists of a circumesophageal ring of fused ganglia and a paired ventral nerve cord. The anterior portion, the protocerebrum, receives sensory inputs including visual information. Three optic nerves, one each from the lateral eye, median ocellus, and ventral eye enter each side of the protocerebrum. The central connections of each optic nerve were determined by staining cut nerve trunks with cobalt chloride. The lateral optic nerve innervates the lamina, medulla, optic tract, ventral central body, and ocellar ganglion. The branching patterns of single axons, probably those of eccentric cells in the lateral eye retina, were observed. Single, large-diameter axons in the lateral optic nerve ramify at seven loci including sites in each of the structures innervated by the lateral optic nerve as a whole. The median optic nerve innervates the ocellar ganglion, central body, optic tract, and medulla. Three types of branching patterns were observed for single, large-diameter fibers in the median optic nerve. One type bypasses the ocellar ganglion and innervates the central body. A second type passes through the ocellar ganglion and optic tract without branching and innervates the posterior medulla. A third type innervates the ocellar ganglion, ventral central body, optic tract, and medulla. The ventral optic nerve is composed of large-diameter axons of ventral photoreceptors. Each axon enters the ganglion cell layer of the medulla and branches over a planar area less than 150 micrograms in diameter. We also observed that axons from mechanoreceptors on the anterior carapace innervate the posterior neuropil of the medulla, and that about 5% of the neurons in the medullar ganglion cell layer send axons to the ipsilateral circumesophageal connective.

Efferent optic nerve fibers mediate circadian rhythms in the Limulus eye

When the horseshoe crab is kept in constant darkness, the lateral eye produces larger electroretinographic and optic nerve responses at night than during the day. These circadian rhythms are mediated by synchronous bursts of efferent impulses in the optic nerve trunk. The endogenous efferent activity appears to increase both the gain and the quantum catch of the photoreceptors.

The brain of the horseshoe crab (Limulus polyphemus). I. Neuroglia

The brain of the horseshoe crab, Limulus polyphemus, harbors three populations of neuroglial cells, whose distribution and cellular details are best appreciated by a combination of silver impregnation, scanning, and transmission electron microscopy. Stellate astrocytes envelop neurons as satellite cells, permeate the neuropile, and secrete a framework of sustentacular trabeculae throughout the brain. Velate astrocytes are restricted to Kenyon cells, i.e. small association neurons, of which they harbor up to 150 per neuroglial cell. Vascular neuroglia is composed of glycogen and mitochondria-laden, interlocked cells that form an open meshwork in the hemocoelic spaces of the brain. Aside from supportive functions of neuroglia, the vascular neuroglial cells in particular seem to subserve the role of a metabolic reserve cell for the central nervous system.

Fine structure of the compound eye of Porcellio scaber in light and dark adaption

The compound eyes of the terrestrial isopod Porcellio scaber comprises about 20 ommatidia. The dioptric apparatus of each ommatidia includes a biconvex corneal lens and a spherical crystalline cone that is secreted by two cone cells. The closed rhabdom is formed by the microvillar extensions of seven pigmented retinula cells and one apical eccentric cell. All retinular axons exit the eye in one bundle. During dark-adaption pigment granules in the retinula cells rapidly withdrew from around the rhabdom and the cell periphery, and migrated basally. Rhabdoms thickened because of movement of the microvilli, and mitochondria moved medially and basally. During light adaption these processes were reversed. Multivesicular bodies became less numerous and rough endoplasmic reticulum and ribosomes proliferated during the initial stages of light adaption.