Circadian change in function of Limulus ventral photoreceptors

Opsins and Their Expression Patterns in the Xiphosuran Limulus polyphemus

The American horseshoe crab Limulus polyphemus (Linnaeus, 1758) is one of four extant species of xiphosuran chelicerates, the sister group to arachnids. Because of their position in the arthropod family tree and because they exhibit many plesiomorphic characteristics, Xiphosura are considered a proxy for the euchelicerate ancestor and therefore important for understanding the evolution and diversification of chelicerates and arthropods. Limulus polyphemus is the most extensively studied xiphosuran, and its visual system has long been a focus of studies critical for our understanding of basic mechanisms of vision and the evolution of visual systems in arthropods. Building upon a wealth of information about the anatomy and physiology of its visual system, advances in genetic approaches have greatly expanded possibilities for understanding its biochemistry. This review focuses on studies of opsin expression in L. polyphemus, which have been significantly advanced by the availability of transcriptomes and a recent high-quality assembly of its genome. These studies show that the repertoire of expressed opsins in L. polyphemus is far larger than anticipated, that the regulation of their expression in rhabdoms is far more complex than anticipated, and that photosensitivity may be distributed widely throughout the L. polyphemus central nervous system. The visual system of L. polyphemus is now arguably the best understood among chelicerates, and as such, it is a critical resource for furthering our understanding of the evolution and diversification of visual systems in arthropods.

What the clock tells the eye: lessons from an ancient arthropod

Circadian changes in visual sensitivity have been observed in a wide range of species, vertebrates, and invertebrates, but the processes impacted and the underlying mechanisms largely are unexplored. Among arthropods, effects of circadian signals on vision have been examined in most detail in the lateral compound eye (LE) of the American horseshoe crab, Limulus polyphemus, a chelicerate arthropod. As a consequence of processes influenced by a central circadian clock, Limulus can see at night nearly as well as they do during the day. The effects of the clock on horseshoe crab LE retinas are diverse and include changes in structure, gene expression, and rhabdom biochemistry. An examination of the known effects of circadian rhythms on LEs shows that the effects have three important outcomes: an increase in visual sensitivity at night, a rapid decrease in visual sensitivity at dawn, and maintenance of eyes in a relatively low state of sensitivity during the day, even in the dark. All three outcomes may be critically important for species' survival. Specific effects of circadian rhythms on vision will certainly vary with species and according to life styles. Studies of the circadian regulation of Limulus vision have revealed that these effects can be extremely diverse and profound and suggest that circadian clocks can play a critical role in the ability of animals to adapt to the dramatic daily changes in ambient illumination.

Loop 2 of limulus myosin III is phosphorylated by protein kinase A and autophosphorylation

Little is known about the functions of class III unconventional myosins although, with an N-terminal kinase domain, they are potentially both signaling and motor proteins. Limulus myosin III is particularly interesting because it is a phosphoprotein abundant in photoreceptors that becomes more heavily phosphorylated at night by protein kinase A. This enhanced nighttime phosphorylation occurs in response to signals from an endogenous circadian clock and correlates with dramatic changes in photoreceptor structure and function. We seek to understand the role of Limulus myosin III and its phosphorylation in photoreceptors. Here we determined the sites that become phosphorylated in Limulus myosin III and investigated its kinase, actin binding, and myosin ATPase activities. We show that Limulus myosin III exhibits kinase activity and that a major site for both protein kinase A and autophosphorylation is located within loop 2 of the myosin domain, an important actin binding region. We also identify the phosphorylation of an additional protein kinase A and autophosphorylation site near loop 2, and a predicted phosphorylation site within loop 2. We show that the kinase domain of Limulus myosin III shares some pharmacological properties with protein kinase A, and that it is a potential opsin kinase. Finally, we demonstrate that Limulus myosin III binds actin but lacks ATPase activity. We conclude that Limulus myosin III is an actin-binding and signaling protein and speculate that interactions between actin and Limulus myosin III are regulated by both second messenger mediated phosphorylation and autophosphorylation of its myosin domain within and near loop 2.

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.

Immunological demonstration of Gq-protein in Limulus photoreceptors

The phototransduction cascade in invertebrates involves the coupling of rhodopsin activation to the action of the enzyme phospholipase C. This step is performed by G-proteins. An antibody against the alpha-subunit of a mouse Gq type G-protein recognized protein bands in Western blots of lateral eye and ventral nerve photoreceptors of Limulus. The protein bands had an apparent molecular mass of about 42 kDa. The antibody also recognized protein bands of a similar molecular mass in immunoblots of brain and intestine tissue. Immunoreactivity was found in lateral eye frozen sections where it was confined to the rhabdom region. When the antibody was applied to ultrathin sections of ventral nerve photoreceptors, the highest density of labeling was found on the rhabdomeral microvilli, but gold particles were also scattered in the cytoplasm. We conclude that a G-protein of the type Gq participates in the phototransduction of Limulus.

Characterization of protein phosphatases type 1 and type 2A in Limulus nervous tissue: their light regulation in the lateral eye and evidence of involvement in the photoresponse

The activities of both protein phosphatases and protein kinases are responsible for the transient changes in the levels of phosphorylation and probably the functions of protein intermediates involved in the biochemical and physiological mechanisms underlying the photoresponse in photoreceptor cells from both vertebrate and invertebrate organisms. Of the known protein serine/threonine phosphatases, various forms of type 1 (PP 1) and type 2A (PP 2A) protein phosphatases are responsible for dephosphorylating many of the known phosphoproteins including those involved in photoreceptor cell function. In this report, we provide biochemical evidence for both PP 1- and PP 2A-like activities in the visual and nonvisual tissue of the horseshoe crab, Limulus polyphemus, that membrane and soluble forms of both enzymes are present, and that the activities of both enzymes are greater in light- than in dark-adapted lateral eyes. These activities were characterized using glycogen phosphorylase a, a substrate for both PP 1 and PP 2A, and various protein phosphatase inhibitors, including okadaic acid. We also report that okadaic acid, at concentrations required to inhibit PP 1, inhibited physiological functions of photoreceptor cells from the ventral eye, causing a delayed reduction of the resting membrane, and slowing and reducing light responses.

An enzymatically enhanced recording technique for Limulus ventral photoreceptors: physiology, biochemistry, and morphology

Enzymatic treatments that facilitated whole-cell electrophysiological recordings were used on Limulus ventral photoreceptor cells. Ventral optic nerves were treated with either collagenase or collagenase, papain, and trypsin. Either treatment greatly increased the ease of making whole-cell recordings of transmembrane potentials. Light responses obtained from enzyme-treated photoreceptor cells were nearly identical to results obtained without enzyme treatment and compared favorably to in vivo recordings of light responses from the compound lateral eye. Enzyme-treated cells also responded to applied octopamine, as do untreated cells, with an increased phosphorylation of a 122-kD protein. This suggests that the external receptors and internal biochemical machinery required for at least one second-messenger cascade are present after enzyme treatment. The morphological integrity of enzyme-treated photoreceptor cells was examined with light microscopy as well as with scanning and transmission electron microscopy. In general, we found that each enzyme treatment greatly reduced the integrity of the layers of glial cells that surround the photoreceptor cells thereby making these cells easily accessible for whole-cell recordings of transmembrane potentials. The morphology of the rhabdomere was normal after enzymatic degradation of the adjacent glial covering.

Photoreceptor cells dissociated from the compound lateral eye of the horseshoe crab, Limulus polyphemus, II: Function

A combination of enzymatic digestions and mechanical disruption was used to isolate photoreceptor cells from the compound lateral eye of the horseshoe crab, Limulus polyphemus. The cells were maintained in a culture medium and tested for function using whole-cell and cell-attached patch configurations of the gigaseal technique. The cells dissociated from the eye generated spontaneous voltage and current bumps in the dark, and depolarized in a graded fashion to increasing intensities of light over several decades, producing responses similar to those of cells in vivo. Currents evoked during voltage clamp were similar to those in ventral photoreceptor cells of Limulus, although transient currents in the dark- and light-activated currents were smaller in isolated lateral eye cells, perhaps because of the slow speed and spatial nonuniformity of the clamp in these large cells. In addition to isolated cells, dissociation of the compound eye produced small clusters of cells and isolated ommatidia which were also tested for function. Comparison of the electrical characteristics of isolated cells with those of cells in small clusters and in their ommatidial matrix suggests that the electrical junctions normally connecting photoreceptor cells within an ommatidium are functional in the latter groups, but not in isolated cells. Cell-attached patches of rhabdomeral membrane of isolated cells contained light-activated channels, resembling those observed in ventral photoreceptor cells, but no voltage-activated channels. Similar patches of arhabdomeral membrane contained voltage-activated channels, but no light-activated channels. We conclude that this preparation is suitable for studies of processes involved in generating the light response in invertebrate photoreceptor cells.

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.

The light-induced rise in cytosolic calcium starts later than the receptor current of the Limulus ventral photoreceptor

The intracellular arsenazo signal indicating the transient light-evoked change in cytosolic Ca2+ (or Sr2+) concentration was measured in Limulus ventral photoreceptors simultaneously with the receptor current under voltage clamp conditions at 15 degrees C. The latency of the light-evoked arsenazo response was consistently more than 25 msec longer than the latency of the electrical light response (receptor current or -potential). Replacing calcium by strontium in the superfusate caused, within 30-40 min, reversible changes: an enlargement of the arsenazo response and a considerable prolongation of both latencies, that of the electrical and that of the arsenazo response; the difference between the two latencies, however, stayed essentially constant.

Central origin of the efferent neurons projecting to the eyes of Limulus polyphemus

Circadian rhythms affect the anatomy, physiology, and biochemistry of the visual cells in the eyes of the horseshoe crab (Limulus polyphemus). These rhythms are mediated by the activity of efferent neurons that project from the central nervous system to all of the eyes. In this study, the optic nerves of Limulus were backfilled with Neurobiotin revealing the location of efferent cell bodies and their projections through the central nervous system. We propose that this efferent system mediates the circadian changes in visual functions in Limulus. Whether these cells are the circadian pacemaker neurons is unknown. The cell bodies of the efferent neurons are ovoid and have a diameter of 40-80 microns. They lie within the cheliceral ganglion of the tritocerebrum, just posterior to the protocerebrum. This ganglion is on the lateral edge of the circumesophageal ring, near the middle of the dorsal-ventral axis of the ring. Each optic nerve contains axons from both ipsilateral and contralateral efferent cells, and some, possibly all, of them project bilaterally and to more than one type of optic nerve. The efferent axons form a tract that projects anteriorly from the cell bodies to the protocerebrum, and bifurcates just lateral to the protocerebral bridge. One branch crosses the midline and projects anteriorly to the optic tract and medulla on the side contralateral to the cell of origin; the other branch follows a symmetric pathway on the ipsilateral side. Small branches arising from the major efferent axons in the optic tract project through the ocellar ganglia to the median optic nerves. The efferent axons branch again in the medulla, and some of these branches innervate the ventral optic nerves. The major branches of the efferent axons continue through the lamina and enter the lateral optic nerve.

Histamine: a putative afferent neurotransmitter in Limulus eyes

Histamine has been proposed as a photoreceptor neurotransmitter in two major groups of arthropods, the insects and the crustacea. In this study biochemical and immunocytochemical approaches were used to examine the synthesis, endogenous content, and cellular distribution of histamine in the visual system of the horseshoe crab Limulus polyphemus, an ancient chelicerate arthropod. Studies with this animal have been critical to our understanding of the basic processes of vision. High-voltage paper electrophoresis was used to assay for histamine synthesis in Limulus tissues incubated with radiolabeled histidine; histamine synthesis was detected in the lateral, median, and ventral eyes and optic nerves and in the visual centers in the brain. Endogenous histamine, assayed as its orthophthalaldehyde derivative by high-performance liquid chromatography and electrochemical detection, was also detected in these tissues. Immunocytochemical analyses, with an antiserum directed against a protein conjugate of histamine, revealed histamine-like immunoreactivity in the somata of photoreceptors in each of the eyes and in the regions of the brain where the photoreceptors terminate. Histamine-like immunoreactivity was also intense in the cell bodies and axon collaterals of eccentric cells in the lateral eye and in eccentric cell projections in the brain. These results show that histamine is a major biogenic amine in the Limulus visual system, and they suggest that this amine is involved in transmitting visual information from the eyes to the brain and in lateral inhibition, a fundamental mechanism for processing visual information in the lateral eye.

Light-stimulated rhabdom turnover in Limulus ventral photoreceptors maintained in vitro

The role of light in turnover of photosensitive membranes was studied in isolated photoreceptors maintained in vitro. Ventral photoreceptors of the horseshoe crab, Limulus polyphemus, were used since they have been the subjects of many in vitro physiological studies. This study shows that the two classes of ventral photoreceptors, the large and small photoreceptors (Herman: companion paper), differ in their morphological response to light. The rhabdom of small photoreceptors is remarkable for its regularity, independent of lighting condition. The photosensitive microvilli of the rhabdom of small photoreceptors are narrow and almost always tightly packed in a hexagonal arrangement. In contrast, the morphology of the rhabdom of the large ventral photoreceptors is different in the dark and in the light, and the rhabdom undergoes turnover during lighting transitions. When fully dark-adapted, the photosensitive microvilli of large photoreceptors are narrow and well organized, sometimes in a crystalline array. However, in the light-adapted state, the microvilli are much thicker and very irregular. The transitions between the dark and light-adapted states, examined at midday, are rapid. After 5 minutes light exposure, the microvilli are dilated at their bases and shed membranes are present in the cytoplasm. By 30 minutes after light onset, the appearance of the rhabdom of large photoreceptors is indistinguishable from fully light-adapted cells. The transition to the dark-adapted state is equally rapid. Even at 5 or 12 minutes after light offset, most microvilli are narrow and quite regular, and by 30 minutes, the rhabdom usually appears to be fully dark-adapted. These experiments show that both the synthetic and degradative phases of rhabdom renewal take place in isolated photoreceptors. No efferent neural activity is required to initiate turnover; rather, changes in illumination alone are sufficient to generate rhabdom turnover in large ventral photoreceptors in vitro.

Light-regulated proteins in Limulus ventral photoreceptor cells

The protein intermediates of the photoresponse and the modulation of this response in invertebrate photoreceptors are largely unknown. As a first step toward identifying these proteins, we have examined light-stimulated changes in protein phosphorylation in preparations of Limulus photoreceptors. Here we show that light modulates the level of phosphorylation of three proteins associated with Limulus ventral photoreceptors: the upper band of a 46-kD protein doublet (46A) and a 122-kD protein, which become more heavily phosphorylated in response to light, and the lower component of the 46-kD doublet (46B), which is phosphorylated in dark-adapted cells, but not in cells maintained in the light. In dark-adapted preparations, 46A is phosphorylated within 30 s after a flash of light and dephosphorylates over a period of many minutes. It is also a major substrate for calcium/calmodulin-dependent protein kinase (Wiebe et al., 1989); therefore, we speculate that 46A is involved in some aspect of dark adaptation. Interestingly, the level of phosphorylation of 46A is the same when measured from preparations maintained in complete darkness or ambient light for at least 1.5 h. The 122-kD phosphoprotein is the same protein which becomes phosphorylated in response to efferent innervation to Limulus eyes (Edwards et al., 1988) and the efferent neurotransmitter, octopamine (Edwards and Battelle, 1987). It may be involved in the increase in retinal sensitivity and the enhanced response of photoreceptors to light that is initiated by efferent innervation. Its role in light-stimulated processes is not clear. The level of phosphorylation of 46B may be most relevant to the long-term state of adaptation of the photoreceptor cell to light and dark.

Octopamine modulates photoreceptor function in the Limulus lateral eye

Activity at night in efferent nerve fibers from a central circadian clock produces changes in photoreceptor function in the lateral compound eye of Limulus: the response to light is increased; membrane potential fluctuations (bumps) occurring in the dark are suppressed; and the duration of bumps occurring both in the dark and under dim illumination is increased (Barlow et al., 1977; Kaplan & Barlow, 1980; Barlow, 1983; Barlow et al., 1985). Efferent nerve terminals release octopamine when activated (Battelle et al., 1982; Battelle & Evans, 1984, 1986); exogenous octopamine in vitro produces some of the changes resulting from efferent nerve activity in vivo (Kass et al., 1988). We report here that the increase in both on-transient and steady-state response to light induced by octopamine in the lateral eye in vitro are concentration dependent with threshold at or below 100 nM, saturation at or above 100 microM, and half-maximal increase in the range 1-10 microM. Octopamine also reduces bump activity in the dark in a concentration-dependent way. Unlike the increase in light response, the dependence of this effect on octopamine concentration is extremely variable from specimen to specimen. The effects of exogenous octopamine on light response and bump activity can sometimes be reversed by removing octopamine from the medium bathing the in vitro preparation. Octopamine also increases bump duration, apparently in a concentration-dependent manner. We have not succeeded in reversing this increase in bump duration. The concentration dependence of changes in photoreceptor response described here agrees qualitatively with the dependence of cAMP levels on octopamine in Limulus photoreceptors (Kaupp et al., 1982), lending further support to the idea that cAMP acts as a second messenger in the circadian control of photoreceptor function. Our results also suggest that the changes induced in the transient and steady-state response to light by both efferent nerve activity and exogenous octopamine have a common origin, which may differ from that responsible for the modulation of bump activity.

Primary afferent responses of a crustacean mechanoreceptor are modulated by proctolin, octopamine, and serotonin

Modulation of sensory responses recorded intracellularly in primary sensory afferents of a crustacean proprioceptor is described. The neuropeptide proctolin enhances the sensory response, whereas the bioamines octopamine and serotonin depress it. The lobster oval organ of the second maxilla, a simple stretch receptor lacking centrifugal control, provides a useful model for studies on nonsynaptic modulation at peripheral sensory loci. Its three large afferents, X, Y, and Z, were prepared for intracellular recording and tested under five experimental conditions: (1) when fully rested, (2) when adapted to maintained stretch and firing tonically, (3) when showing reduced responses after habituation to repetitive stimulation, (4) not stretched but depolarized with current injections, (5) after TTX blockade. The results, taken together, indicate that conductances contributing to the overall amplitude of the receptor potential are major targets for modulators. Thus proctolin increased receptor potential amplitudes with consequent augmentation of spiking, whereas serotonin and octopamine depressed the receptor potentials, often to subthreshold levels with loss of spiking. Octopamine was a less potent agent than serotonin and failed to act upon fibers under TTX blockade. Fibers Y and Z consistently showed sensitivity to the modulators tested. The largest fiber, X, typically was resistant to proctolin, octopamine, and serotonin. Threshold concentrations of 10(-10)-10(-11) M determined in vitro are well below the circulating levels for serotonin and octopamine found in vivo. Proctolin, however, is usually not detectable in the hemolymph, and it is suggested that a significant site of proctolin release may be the oval organ itself.

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.

Identification and function of octopamine and tyramine conjugates in the Limulus visual system

Major metabolites of octopamine and tyramine in the Limulus nervous system are identified here as gamma-glutamyl octopamine and gamma-glutamyl tyramine. We show that these conjugates are normal products of amine metabolism in Limulus, and that they are normally present in octopamine-rich Limulus tissues. The synthesis of these conjugates is not restricted to nervous tissue, but the highest activity of gamma-glutamyl amine synthetase was measured in the CNS. Our interest in these molecules stems from our previous observations which showed that they were synthesized and stored in, and released from, the efferent fibers to Limulus eyes which modulate the sensitivity of the eyes to light. Here we provide direct evidence for the release of the conjugates from Limulus eyes in response to depolarization, and that gamma-glutamyl octopamine can increase the sensitivity of the lateral eye to light. Our observations lend support to the hypothesis that gamma-glutamyl octopamine may serve as an intercellular messenger in the Limulus visual system.

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.