Circadian Structural Changes in the Retina of Lycosa tarentula (Araneae: Lycosidae)

Daily structural changes of the rhabdoms are described in the four eye types of a lycosid spider submitted to laboratory conditions: LD 12:12 (light on at 08.00, 100 lux). From a study of animals submitted to constant darkness during fourteen days, rhabdom turnover was shown to be circadian. Each type of retina exhibits its own rhythmic turnover of rhabdoms. Anterior-lateral, posterior-median and lateral eyes are mostly diurnal; anterior-median retinae comprise 300 receptors with mainly nocturnal functioning and 150 receptors with mainly diurnal functioning. A correlation was found between rhythms of locomotor activity, other activity, and rhabdom turnover in L. tarentula suggesting that the same pacemaker controls these different rhythms.     

Diminution and enlargement of the mosquito rhabdom in light and darkness

The rhabdoms of the larval ocelli of the mosquito Aedes aegypti undergo morphological light and dark adaptation over periods of hours. The rhabdom enlarges during dark adaptation and grows smaller during light adaptation. Diminution is exponential, enlargement linear, and rates of change are proportional to log light intensity. Rhabdoms maintained at a constant intensity level off at a constant volume proportional to log intensity. We argue that changes in rhabdom volume after changes in light intensity reflect an influence of light on the turnover of photoreceptro membrane, and that the volumes at which rhabdoms level off represent equilibria between opposed processes of membrane loss and renewal.     

The diurnal pattern of protein and photopigment synthesis in the retina of the crayfish,Procambarus clarkii

Summary The interrelationship between the diurnal cycle of membrane loss and synthesis of new rhabdom components remains a key element in forming a complete picture of the turnover of photopigment-containing membrane in the crayfish photoreceptor cell. In order to examine this aspect of the turnover process, the diurnal pattern of photopigment synthesis was examined using an in vitro incubation system for incorporation of³H-leucine into photoreceptor protein. The incorporation of³H-leucine into total protein and photopigment specifically was measured in photoreceptors isolated from incubated retinas. The results indicate that for both total protein and photopigment there is no significant variation in the rate of synthesis during the 12-12 light-dark cycle. These data combined with earlier data on diurnal membrane loss from the rhabdom suggest that light-stimulated rhabdom membrane loss is superimposed on a diurnally constant level of synthesis and assembly of new rhabdom constituents.Abbreviations dpm disintegration per minute - LRB lysosome related body - TCA trichloroacetic acid     

Fundamental differences in the optical structure of the eyes of nocturnal and diurnal mosquitoes

We have studied the anatomy and optics of the eyes of a range of mosquito species from the wholly dark-active blood-feeding Anopheles gambiae to the diurnal plant-feeder Toxorhynchites brevipalpis. Consistent with studies by Satô in the 1950s, we find that dark-active and crepuscular species have short fused rhabdoms with a conical construction. This maximises the amount of light the rhabdoms receive from the almost hemispherical wide-aperture lenses. Toxorhynchites, on the other hand, has long narrow rhabdomeres that are separated from each other over their entire length, and so resemble the open rhabdoms of advanced flies (Brachycera and Cyclorrhapha). These findings are confirmed by studies of the pseudopupil, whose form indicates the layout of the rhabdomere tips in the focal plane of each ommatidial lens. In anopheline species the pseudopupil is a single undivided ellipse, indicating a fused rhabdom structure, whereas in Toxorhynchites there is a ring of six outer elements surrounding a central one. This means that each rhabdomere views a separate direction in space, and our measurements indicate that, as in higher Diptera, adjacent rhabdomeres share their fields of view with one of the rhabdomeres in the immediately adjacent ommatidia. This in turn means that in the diurnal type of mosquito eye there is a basis for neural superposition, but the fused construction of anopheline rhabdoms precludes this. The Aedes species studied were similar to Anopheles but with lenses of less extreme aperture, and Sabethes cyaneus, a diurnal blood-feeder, was intermediate in structure, with fused conical rhabdoms in the centre of the eye and unfused rhabdomeres around the periphery.     

A phospholipase inhibitor, manoalide, and a G-protein activator, Mas-7, both affect the turnover of phototransductive membranes by crab retinas in darkness With a model for the regulation of rhabdomeral membrane turnover

(1) In vitro retinas of a crab, Leptograpsus, were treated with a phospholipase inhibitor, manoalide, or a G-protein activator, Mas-7. Both drugs address early stages of the phototransduction cascade. (2) Manoalide inhibited the light-dependent reduction of rhabdoms during the `day' phase of the light cycle, but did not induce rhabdom overgrowth. Following a period of darkness manoalide failed to affect the diminution of illuminated rhabdoms. (3) The diminution of rhabdoms that follows photoreceptor depolarisation induced by 100 mmol · l<sup>−1</sup> K<sup>+</sup> in darkness was not affected by 2␣μmol · l<sup>−1</sup> manoalide. (4) When retinas in the `night' phase were treated with Mas-7 in darkness, rhabdom diameters were augmented, concurrently with endocytosis of photoreceptor plasma membranes. (5) The results of combining manoalide and Mas-7 with actinomycin D, U-57908 or okadaic acid, drugs used in previous studies to manipulate steps notionally lower in the transduction cascade, lead to a hypothetical model for the regulation of phototransductive membrane turnover by arthropods. Accepted: 3 October 1996     

Interspecific variations in the morphology and ultrastructure of the rhabdoms of oplophorid shrimps

Interspecific variations in rhabdom structure between various oplophorid shrimps are described and the differences are related to the light environment at different depths within the mesopelagic zone. The ultrastructure of the distal rhabdom in these species is described for the first time. Quantitative measurements show that the proportion of the rhabdom layer occupied by the distal rhabdom varies from 3.5-25% in the dorsoventral plane of the eye of Systellaspis debilis. The distal rhabdom occupies less than 1% of the rhabdoms in the eye of Acanthephyra pelagica, where it can only be seen by using the electron microscope. It is suggested that the rhabdoms of those species that remain within the photic zone (such as S. debilis) are adapted to maximize contrast, whereas in those whose depth ranges extend into the aphotic zone (such as A. pelagica) they are adapted for maximum sensitivity.     

Systematic variations in microvilli banding patterns along fiddler crab rhabdoms

Polarisation sensitivity is based on the regular alignment of dichroic photopigment molecules within photoreceptor cells. In crustaceans, this is achieved by regularly stacking photopigment-rich microvilli in alternating orthogonal bands within fused rhabdoms. Despite being critical for the efficient detection of polarised light, very little research has focused on the detailed arrangement of these microvilli bands. We report here a number of hitherto undescribed, but functionally relevant changes in the organisation of microvilli banding patterns, both within receptors, and across the compound eye of fiddler crabs. In all ommatidia, microvilli bands increase in length from the distal to the proximal ends of the rhabdom. In equatorial rhabdoms, horizontal bands increase gradually from 3 rows of microvilli distally to 20 rows proximally. In contrast, vertical equatorial microvilli bands contain 15–20 rows of microvilli in the distal 30 µm of the rhabdom, shortening to 10 rows over the next 30 µm and then increase in length to 20 rows in parallel with horizontal bands. In the dorsal eye, horizontal microvilli occupy only half the cross-sectional area as vertical microvilli bands. Modelling absorption along the length of fiddler crab rhabdoms suggests that (1) increasing band length assures that photon absorption probability per band remains constant along the length of photoreceptors, indicating that individual bands may act as units of transduction or adaptation; (2) the different organisation of microvilli bands in equatorial and dorsal rhabdoms tune receptors to the degree and the information content of polarised light in the environment.     

The rhabdom structure in the ommatidia of the Heteroptera (Insecta), and its phylogenetic significance

In its plesiomorphic state the insect ommatidium consists of eight retinula cells forming a fused rhabdom. It has long been observed that, in contrast to this pattern, Heteroptera have open rhabdoms. However, there has so far been no comprehensive and comparative study of heteropteran ommatidia. For this reason, we investigated the rhabdom structure in 36 species from all higher groups of Heteroptera, as well as from Coleorrhyncha and Auchenorrhyncha as outgroup representatives. In addition we surveyed the data of earlier authors, which brings the number of examined species to a total of more than 70. All examined Heteroptera do have open rhabdoms, with a system of six peripheral and two central rhabdomeres. Outgroup comparison shows that the open rhabdom is an autapomorphy of the Heteroptera. As for the rhabdom structure within the Heteroptera, we found further autapomorphic patterns in Corixidae (Nepomorpha), Gerromorpha, and Leptopodomorpha. Finally, the Cimicomorpha and Pentatomomorpha share a special pattern of the two central rhabdomeres, which we call V-pattern. This is a new synapomorphy of these two taxa. Accepted: 22 November 1999     

Functional morphology of the ommatidia in the compound eye of the moth,Antheraea polyphemus (Insecta,Saturniidae)

Summary The fine structure of the superposition eye of the Saturniid moth Antheraea polyphemus Cramer was investigated by electron microscopy. Each of the approximately 10000 ommatidia consists of the same structural components, but regarding the arrangement of the ommatidia and the rhabdom structure therein, two regions of the eye have to be distinguished. In a small dorsal rim area, the ommatidia are characterized by rectangularly shaped rhabdoms containing parallel microvilli arranged in groups that are oriented perpendicular to each other. In all other ommatidia, the proximal parts of the rhabdoms show radially arranged microvilli, whereas the distal parts may reveal different patterns, frequently with microvilli in two directions or sometimes even in one direction. Moreover, the microvilli of all distal cells are arranged in parallel to meridians of the eyes. By virtue of these structural features the eyes should enable this moth not only discrimination of the plane of polarized light but also skylight-orientation via the polarization pattern, depending on moon position. The receptor cells exhibit only small alterations during daylight within the natural diurnal cycle. However, under illumination with different monochromatic lights of physiological intensity, receptor cells can be unbalanced: Changes in ultrastructure of the rhabdomeres and the cytoplasm of such cells are evident. The effects are different in the daytime and at night. These findings are discussed in relation to the breakdown and regeneration of microvilli and the influence of the diurnal cycle. They are compared with results on photoreceptor membrane turnover in eyes of other arthropod species.     

黑翅土白蚁有翅成虫复眼的形态结构

采用组织切片法光镜下观察黑翅土白蚁Odontotermes formosanus(Shiraki)有翅成虫的复眼形态结构及光、暗适应条件下色素颗粒移动的规律。结果如下:(1)头正前方观,复眼外部形态略呈圆形。(2)有翅成虫复眼类型属于并列像眼,每只复眼约由360个小眼组成。(3)每个小眼是由1套屈光器(1个角膜和1个晶锥)、小网膜色素细胞、视杆和基细胞等几部分组成。小网膜色素细胞内均含有丰富的色素颗粒。(4)在光适应条件状态下,屈光器及视杆周围的色素颗粒主要分布在视杆部位的上侧,暗度适应条件状态时则较均匀地分布于视杆两侧上下;性别对色素颗粒分布无明显影响。     

Photosensitivity spectrum of crayfish rhodopsin measured using fluorescence of metarhodopsin

Discrepancies exist among spectral measurements of sensitivity of crayfish photoreceptors, their absorption in situ, and the number and absorption spectra of crayfish photopigments that are extracted by digitonin solutions. We have determined the photosensitivity spectrum of crayfish rhodopsin in isolated rhabdoms using long wavelength fluorescence emission from crayfish metarhodopsin as an intrinsic probe. There is no measurable metarhodopsin in the dark-adapted receptor, so changes in the emission level are directly proportional to metarhodopsin concentration. We therefore used changes in metarhodopsin fluorescence to construct relaxation and saturation ("photoequilibrium") spectra, from which the photosensitivity spectrum of crayfish rhodopsin was calculated. This spectrum peaks at or approximately 530 nm and closely resembles the previously measured difference spectrum for total bleaches of dark-adapted rhabdoms. Measurements of the kinetics of changes in rhabdom fluorescence and in transmittance at 580 nm were compared with predictions derived from several model systems containing one or two photopigments. The comparison shows that only a single rhodopsin and its metarhodopsin are present in the main rhabdom of crayfish, and that other explanations must be sought for the multiple pigments seen in digitonin solution. The same analysis shows that there is no detectable formation of isorhodopsin in the rhabdom.     

Asynchrony in the Diurnal Rhythms of Dopaminergic D2 Receptors in Different Brain Regions of the Rat

Diurnal variations of dopaminergic D₂ receptors have been described in the striatum of rats, while other dopaminergic regions remain unstudied. Diurnal variations of dopamine D₂ receptors in the striatum, frontal cortex, and amygdala of the rat, were characterized by the stereospecific binding of [³H]-spiperone. Clear rhythms were found in all these areas, but asynchronous to each other. Striatal receptors had diurnal variations with a single peak at 00:00 hours. Frontal cortex receptors showed two peaks at 00:00 and 12:00 hours. Amygdaline complex receptors had two peaks at 18:00 and 06:00 hours. Saturation binding curves and their Scatchard analysis indicated that the diurnal variations in [³H]-spiperone binding are related to changes in receptor density rather than its affinity. The diurnal variations asynchrony in [³H]-spiperone binding to dopaminergic D₂ receptors from different neural regions, suggest different regulation in each area. Other functional implications of these rhythms remains to be established.     

The Rhythmic GABAergic System

GABA is the major inhibitory neurotransmitter in the mammalian brain, and has been implicated in the regulation of a variety of behavioral functions, including biological rhythms. The focus of this minireview is the rhythmic variation of the central GABAergic system, comprising fluctuations of GABA levels and turnover, GABA receptor affinity and postsynaptic activity on the chloride ionophore in rodent's brain. Neurochemical rhythms correlated with diurnal and circadian changes in several behaviors associated with the GABA_A receptor, e.g., anxiolysis-related behavior. GABA is considered to be the principal neurotransmitter of the mammalian circadian system, being present in the suprachiasmatic nuclei and the intergeniculate leaflet. Pharmacological manipulations of GABA_A receptors phase shift circadian rhythms and alter circadian responses to light. Administration of putative modulators of GABA function, like melatonin or neuroactive steroids, affects the timing of biological rhythms. Therefore, not only does the GABAergic system exhibit strong diurnal and circadian variations, but it also serves as one of the key modulators of the circadian apparatus.     

Spatial control of photomechanical movements in the lateral eye of the American horseshoe crab, Limulus polyphemus

This study asks whether photomechanical movements in the retinal cells of the lateral eye of the American horseshoe crab, Limulus polyphemus, are controlled locally within each ommatidium, or whether they are a retinal array property involving lateral communication between ommatidia. Three experiments were performed. A small spot, a vertical slit down the center, or the anterior third of an otherwise masked eye was illuminated. The contralateral eye was fully illuminated in each experiment and served as a light-adapted control. Morphometric analyses of aperture length and rhabdom dimensions were made from serial 1-microm plastic sections. The results suggest there is a different spatial threshold for photomechanical movement for aperture lengthening than for rhabdom lengthening. When only a few ommatidia are illuminated, the aperture does not change. When about 10% are illuminated, they lengthen, but the masked ommatidia do not. When about a third are illuminated, all the ommatidia in the eye lengthen together, including the two thirds that were masked. When either only a few ommatidia or about 10% of the ommatidia are illuminated, rhabdom shape is unchanged. When a third of the eye is illuminated, the illuminated rhabdoms lengthen, but the masked rhabdoms do not.     

Visual behaviour and the structure of dark and light-adapted larval and adult eyes of the New Zealand glowworm Arachnocampa luminosa (Mycetophilidae: Diptera)

Both larval and adult New Zealand cave glowworms exhibit reactions to light; their photoreceptors must, therefore, be regarded as functional. The two principal stemmata of the larva possess large biconvex lenses and voluminous rhabdoms. Approximately 12 retinula cells are present. In light-adapted larvae the diameter of the rhabdom is 8 μm and that of an individual microvillus is 49.5 nm. Dark-adapted eyes have rhabdoms that measure 14 μm in cross section and microvilli with an average diameter of 54 nm. The compound eye of the adult comprises approximately 750 ommatidia, each with a facet diameter of 27–28 μm. A facet is surrounded by 1–6 interommatidial hairs which are up to 30 μm long. The interommatidial angle is 5.5°. Cones, consisting of 4 crystalline cone cells, are of the ‘acone’ type. Pigment granules in the primary pigment cells are twice as large as those of the retinula cells which measure 0.6–0.75 μm in diameter. The rhabdom is basically of the dipteran type, i.e. six open peripheral rhabdomeres surround 2 central rhabdomers arranged in a tandem position. The microvilli of cells 1–6 and cell 8 have diameters ranging from 68 to 73 nm, but those of the distally-located central rhabdomere 7 are 20% larger. This is irrespective of whether the eye is dark or light-adapted. In the latter the cones are long and narrow, the screening pigment granules closely surround the rhabdomeres, and the rhabdom is less voluminous than that of the dark-adapted eye.     

Ommatidial structure in relation to turnover of photoreceptor membrane in the locust

Summary In the compound eye of the locust, Locusta, the cross-sectional area of the rhabdoms increases at dusk by 4.7-fold due to the rapid assembly of new microvillar membrane, and decreases at dawn by a corresponding amount as a result of pinocytotic shedding from the microvilli. The rhabdoms at night have more and longer photoreceptor microvilli than rhabdoms during the day. The orientations of the six rhabdomeres that comprise the distal rhabdom also change. The density of intramembrane particles on the P-face of the microvillar membrane, putatively representing mostly rhodopsin molecules, or aggregates thereof, does not change.An alteration in the size of the ommatidial field-stop, produced by the primary pigment cells, is concomitant with the change in rhabdom size. At night the increase in size of the field-stop must widen the angular acceptance of a rhabdom, increasing the capture of photons from an extended field. Conversely, during the day, when photons are more abundant, its decrease must narrow the acceptance angle, increasing angular resolution. Because of the presence of this field-stop, the optics of the ommatidium would not be greatly affected if the rhabdom were to remain always at its night size. It is argued, therefore, that the variable-size rhabdom must have resulted from some demand other than that of light/dark adaptation.Changes in size and organisation of the rhabdoms in response to various light regimes indicate that: (1) Rapid shedding of photoreceptor membrane is induced by the onset of light, but shedding also occurs slowly in darkness during the day. (2) Microvillar assembly is initiated by the onset of darkness, but also occurs at the normal time of dusk without a change in ambient lighting, provided there has been some light during the day. Therefore, both shedding and assembly of microvillar membrane are affected by the state of illumination, but also appear to be under some endogenous control.     

Did neural pooling for night vision lead to the evolution of neural superposition eyes?

Observations of the infrared deep pseudopupil, optical determinations of the corneal nodal point, and histological methods were used to relate the visual fields of individual rhabdomeres to the array of ommatidial optical axes in four insects with open rhabdoms: the tenebrionid beetle Zophobas morio, the earwig Forficula auricularia, the crane fly Tipula pruinosa, and the backswimmer Notonecta glauca.The open rhabdoms of all four species have a central pair of rhabdomeres surrounded by six peripheral rhabdomeres. At night, a distal pigment aperture is fully open and the rhabdom receives light over an angle approximately six times the interommatidial angle. Different rhabdomeres within the same ommatidium do not share the same visual axis, and the visual fields of the peripheral rhabdomeres overlap the optical axes of several near-by ommatidia. During the day, the pigment aperture is considerably smaller, and all rhabdomeres share the same visual field of about two interommatidial angles, or less, depending on the degree of light adaptation. The pigment aperture serves two functions: (1) it allows the circadian rhythm to switch between the night and day sampling patterns, and (2) it works as a light driven pupil during the day.Theoretical considerations suggest that, in the night eye, the peripheral retinula cells are involved in neural pooling in the lamina, with asymmetric pooling fields matching the visual fields of the rhabdomeres. Such a system provides high sensitivity for nocturnal vision, and the open rhabdom has the potential of feeding information into parallel spatial channels with different tradeoffs between resolution and sensitivity. Modification of this operational principle to suit a strictly diurnal life, makes the contractile pigment aperture superfluous, and decreasing angular sensitivities together with decreasing pooling fields lead to a neural superposition eye.Abbreviations DPP deep pseudopupil - LMC large monopolar cell     

Behavioral responses of Vipr2-/- mice to light

Vasoactive intestinal polypeptide and its receptor, VPAC2, play important roles in the functioning of the dominant circadian pacemaker, located in the hypothalamic suprachiasmatic nuclei (SCN). Mice lacking VPAC2 receptors (Vipr2-/-) show altered circadian rhythms and impaired synchronization to environmental lighting cues. However, light can increase phosphoprotein and immediate early gene expression in the Vipr2-/- SCN demonstrating that the circadian clock is readily responsive to light in these mice. It is not clear whether these neurochemical responses to light can be transduced to behavioral changes as seen in wild-type (WT) animals. In this study we investigated the diurnal and circadian wheel-running profile of WT (C57BL/6J) and Vipr2-/- mice under a 12-h light:12-h complete darkness (LD) lighting schedule and in constant darkness (DD) and used 1-h light pulses to shift the activity of mice in DD. Unlike WT mice, Vipr2-/- mice show grossly altered locomotor patterns making the analysis of behavioral responses to light problematic. However, analyses of both the onset and the offset of locomotor activity reveal that in a subset of these mice, light can reset the offset of behavioral rhythms during the subjective night. This suggests that the SCN clock of Vipr2-/- mice and the rhythms it generates are responsive to photic stimulation and that these responses can be integrated to whole animal behavioral changes.     

龟纹瓢虫成虫的复眼形态及其显微结构

利用光镜、组织切片法观察了龟纹瓢虫Propylaea japonica(Thunberg)成虫的复眼形态及其显微结构。结果如下:(1)头正前方观,复眼外形似半球,且后方稍向内合拢。每个复眼约包括630个小眼。(2)每个小眼是由1套屈光器(1个角膜和1个晶锥)、6至8个小网膜细胞及其特化产生的视杆和基细胞等几部分组成。晶体周围及小网膜色素细胞内均含有丰富的色素颗粒。(3)小眼整体纵切显示,其上、下段色素颗粒分布相对较多,中段分布较少。(4)明、暗适应状态对小眼的色素颗粒分布有影响,性别对其分布无明显影响。明适应状态下,其色素颗粒较均匀地分布于视杆两侧上下,暗适应状态时色素颗粒则主要分布在视杆部位的上侧,显示其具有一定的重叠眼性质;而在相同的明、暗适应状态下其雌、雄成虫复眼的色素颗粒分布间无明显差异。     

Asynchrony in the Diurnal Rhythms of Dopaminergic D2 Receptors in Different Brain Regions of the Rat

Diurnal variations of dopaminergic D₂ receptors have been described in the striatum of rats, while other dopaminergic regions remain unstudied. Diurnal variations of dopamine D₂ receptors in the striatum, frontal cortex, and amygdala of the rat, were characterized by the stereospecific binding of [³H]-spiperone. Clear rhythms were found in all these areas, but asynchronous to each other. Striatal receptors had diurnal variations with a single peak at 00:00 hours. Frontal cortex receptors showed two peaks at 00:00 and 12:00 hours. Amygdaline complex receptors had two peaks at 18:00 and 06:00 hours. Saturation binding curves and their Scatchard analysis indicated that the diurnal variations in [³H]-spiperone binding are related to changes in receptor density rather than its affinity. The diurnal variations asynchrony in [³H]-spiperone binding to dopaminergic D₂ receptors from different neural regions, suggest different regulation in each area. Other functional implications of these rhythms remains to be established.