Dispersion of latencies in photoreceptors of Limulus and the adapting- bump model

To light stimuli of very low intensity, Limulus photoreceptors give a voltage response with a fluctuating delay. This phenomenon has been called "latency dispersion." If the generator potential is the superposition of discrete voltage events ("bumps"), and if the effect of light upon bump size is negligible, then the latency dispersion and the bump shape completely characterize the frequency response to sinusoidal flicker. For very low light intensities, the latency dispersion of the bumps, the bump shape, and the frequency response are measured. It is found that for data obtained at 20 degrees C, the frequency response can be accounted for completely by the latency dispersion and by the bump shape derived from steady-state noise characteristics. At 10 degrees C, the time scale of the response of the photoreceptor is lengthened. The dispersion of latencies and the bump shape are found not to have the same temperature dependence. However, just as those measured at 20 degrees C, the bump shape and the dispersion of latencies measured at 10 degrees C can predict the frequency response measured under the same conditions. These results strongly suggest that the major mechanisms involved in the generator potential are the latency process and the bump process. At high light intensities, the time scale of the generator potential shortens. The decrease in time scale of the generator potential can be attributed to the decreases in time scales of the bumps and of the latency dispersion process.     

Light-induced pigment granule migration in the retinular cells of Drosophila melanogaster. Comparison of wild type with ERG-defective mutants

The dependence of pigment granule migration (PGM) upon the receptor potential was examined using several strains of electroretinogram (ERG)- defective mutants of Drosophila melanogaster. The mutants that have a defective lamina component but a normal receptor component of the ERG (no on-transient A [nonA] and tan) exhibited normal pigment granule migration. The mutants that have very small or no receptor potentials (certain no receptor potential A [norpA] alleles), on the other hand, exhibited no PGM. In the case of the temperature-sensitive norpA mutant, norpAH52, normal PGM was present at 17 degrees but not at 32 degrees C or above, corresponding to its electrophysiological phenotype. In the transient receptor potential (trp) mutant, whose receptor potential decays to the baseline within a few seconds during a sustained light stimulus, the pigment granules initially moved close to the rhabdomere when light was turned on but moved away after about 5 s during a sustained light stimulus. All these results lend strong support to the notion that PGM is initiated by a light-evoked depolarization of the receptor membrane, i.e., the receptor potential. However, under certain experimental conditions, the receptor potentials failed to induce PGM in the trp mutant. The depolarization of the receptor, thus, appears to be closely associated with PGM but is not a sufficient condition for PGM.     

Atypical Granules in the Eyes of the White Mutant of Drosophila melanogaster Are Lysosome-Related Organelles

In the pigment cells of the white mutant of Drosophila melanogaster, as described earlier, two types of abnormal granules are found by conventional electron microscopy. However, both types of abnormal granules, in addition to those in pigment cell invaginations, are also present in the cytoplasm of the photoreceptor cells. Three enzymes (acid phosphatase, peroxidase, and tyrosinase) are localized within the eyes of wild type and white mutant Drosophila melanogaster by electron microscopy. Peroxidase activity is present in lamellar bodies close to the rhabdomeral microvilli of both fly types. However the organelles containing peroxidase activity are 6-fold more frequent in the wild type than in the mutant. Acid phosphatase is present in lamellar bodies between and at the bases of the rhabdomeral microvilli of the wild type, as well as in ommochrome granules of the photoreceptor cells. In the white mutant, however, acid phosphatase was located in electron lucent vacuoles in the cytoplasm of the receptor cells. These acid phosphatase-positive vacuoles also contained both types of abnormal granules. The latter result indicates that abnormal granules in the receptor cells originate from lysosomal degradation and that targeting of lysosomal enzymes is altered in the white mutant. Due to the tyrosinase activity in the hemolymph of flies, the extracellular spaces are electron dense after DOPA incubation. Since some abnormal granules within the photoreceptor cells are not surrounded by an extracellular space, they can be assumed to originate within the photoreceptor cells.     

Proper function of the Drosophila trp gene product during pupal development is important for normal visual transduction in the adult

The response of invertebrate photoreceptors consists of the summation of quantum bumps, each representing the response to a single photon. The bumps adapt depending on the intensity of the stimulus: their average size is relatively large in dim light and small in bright light. The rate of occurrence of the bumps varies proportionally with light intensity. In the Drosophila mutant trp, unlike in the wild type, the rate does not increase with increasing light intensity and the bumps do not adapt. Here we report an analysis of the trp gene and its expression in normal and mutant flies. Our results suggest that the trp protein is a novel photoreceptor membrane-associated protein, that this protein is not required for the occurrence of bumps but is necessary for adaptation, and that proper function of the trp gene product during pupal development is important for normal visual transduction in the adult.     

Morphological Characterization of Three Phenotypes of the Isopod Armadillidium vulgare

The morphological characteristics and ommochrome quantity in the integument of red, white, and wild type (black-grey) Armadillidium vulgare were studied. The red phenotype was found to possess two kinds of immature ommochrome pigment granules within its pigment cells, in addition to mature pigment granules. The immature granules seemed to contain uniformly distributed fibrilles, or to have an electron-dense central region surrounded by an electron-lucent outer edge. Since these immature pigment granules were typically observed to be distributed along with the mature ones, and were also more easily extractable than the wild type's, it is hypothesized that ommochrome granule maturation in the red phenotype may occur slowly due to a defect in the pigment granule internal process which combines pigments with matrix proteins. Regarding the white phenotype, although its pigment cells were undeveloped, several large-sized vesicles containing a small amount of electron-dense material appeared in the pigment cell cytoplasm. The wild and red type males of A. vulgare were found to have an ommochrome content twice as large as that of the corresponding females, with no ommochrome pigment being detected in the white phenotype. The genetic relationship between the white and red phenotypes was discussed using as a basis the observed pigment granule structure.     

Seasonal changes in retinal function of the frog Rana Ridibunda

The electroretinogram (ERG) of Rana ridibunda was recorded at different times of the year. The frogs exhibited distinct seasonal changes in various parameters of the ERG.The amplitudes of both the a- and the b-waves were greater in summer than in winter. With a luminance of 3000 cd/m² the mean amplitude of the a-wave ranged from minimal 30 μ V (February) to maximal 60 μ V (July), and that of the b-wave from 190 μ V (February) to maximal 550 μ V (June). Average latency was about 28 ms in winter and 14 ms in summer. The greatest difference was found in flicker-fusion frequency which averaged 29 Hz in July and 9 Hz in January. There was also a significant change in the average threshold for response to white light (0.011 cd/m² in March).     

The flicker fusion frequencies of six laboratory insects, and the response of the compound eye to mains fluorescent 'ripple'

ABSTRACT. The ERG response of the compound eye to single, brief, light pulses, to sustained stimulation for 2 s, and the dark adapted flicker-fusion frequency (FFF) under stroboscopic light was measured in six species: Locusta migratoria (FFF range: 40–90 Hz), Periplaneta americana (25–60 Hz), Saturnia pavonia (65–85 Hz), Antheraea pernyi (25–70 Hz), Glossina morsitans (85–205 Hz) and Drosophila hydei (60–100 Hz). The first four species have typical 'slow-eyed', monophasic ERG responses; the two flies typical 'fast-eyed', biphasic responses. The FFF proved to be dependent on the state of light adaptation, being 40–70% higher than the above figures after only 2 min exposure to as little as 300 lx. Adult male Glossina , but not Locusta nymphs, showed a clear 100 Hz ERG ripple in response to single-phase, mains fluorescent lighting. To three-phase fluorescent lighting no 300 Hz ERG ripple was detected, but the 100 Hz component was still evident.     

Light reduces the excitation efficiency in the nss mutant of the sheep blowfly Lucilia

The nss (no steady state) phototransduction mutant of the sheep blowfly Lucilia was studied electrophysiologically using intracellular recordings. The effects of the nss mutation on the receptor potential are manifested in the following features of the light response. (a) The responses to a flash or to dim lights are close to normal, but the receptor potential decays close to the baseline level during prolonged illumination after a critical level of light intensity is reached. (b) The decline of the response is accompanied by a large reduction in responsiveness to light that recovers within 20 s in the dark. (c) The full reduction in responsiveness to light is reached when approximately 13% of the photopigment molecules are converted from rhodopsin (R) to metarhodopsin (M). (d) A maximal net pigment conversion from R to M by blue light induces persistent inactivation in the dark, without an apparent voltage response. This inactivation could be abolished at any time by M-to-R conversion with orange light. The above features of the mutant indicate that the effect of the nss mutation on the light response of Lucilia is very similar to the effects of the transient receptor potential (trp) mutation on the photoreceptor potential of Drosophila. Noise analysis and voltage measurements indicate that the decay of the receptor potential is due to a severe reduction in the rate of occurrence of the elementary voltage responses (bumps). The bumps are only slightly modified in shape and amplitude during the decline of the response to light of medium intensity. There is also a large increase in response latency during intense background illumination. These results are consistent with the hypothesis that separate, independent mechanisms determine bump triggering and bump shape and amplitude. The nss mutation affects the triggering mechanism of the bump.     

Histochemical localization of Mg2+-activated ATPases on microtubule bundles in epidermal cells of Carausius morosus: Possible significance to pigment migration

We have demonstrated the presence of Mg²⁺-stimulated ATPases on microtubule bundles in the epidermal cells at the light microscope level, using specific histochemical techniques. This method provides an alternative method to immunohistochemistry for identifying microtubule bundles in the epidermal cells of Carausius morosus using the light microscope. The close association between ommochrome granules and the microtubule bundles support the hypothesis, that these ATPases play an important role in force generation, required to move ommochrome granules during physiological colour change.     

Adapting bump model for ventral photoreceptors of Limulus

Light-evoked current fluctuations have been recorded from ventral photoreceptors of Limulus for light intensity from threshold up to 10(5) times threshold. These data are analyzed in terms of the adapting bump noise model, which postulates that (a) the response to light is a summation of bumps; and (b) the average size of bump decreases with light intensity, and this is the major mechanism of light adaptation. It is shown here that this model can account for the data well. Furthermore, the model provides a convenient framework to characterize, in terms of bump parameters, the effects of calcium ions, which are known to affect photoreceptor functions. From responses to very dim light, it is found that the average impulse response (average of a large number of responses to dim flashes) can be predicted from knowledge of both the noise characteristics under steady light and the dispersion of latencies of individual bumps. Over the range of light intensities studied, it is shown that (a) the bump rate increases in strict proportionality to light intensity, up to approximately 10(5) bumps per second; and (b) the bump height decreases approximately as the -0.7 power of light intensity; at rates greater than 10(5) bumps per second, the conductance change associated with the single bump seems to reach a minimum value of approximately 10(-11) reciprocal ohms; (c) from the lowest to the highest light intensity, the bump duration decreases approximately by a factor of 2, and the time scale of the dispersion of latencies of individual bumps decreases approximately by a factor of 3; (d) removal of calcium ions from the bath lengthens the latency process and causes an increase in bump height but appears to have no effect on either the bump rate or the bump duration.     

A single-base deletion in an ABC transporter gene causes white eyes, white eggs, and translucent larval skin in the silkworm w-3(oe) mutant

The w-3(oe) silkworm mutant has white eyes and eggs due to the absence of ommochrome pigments in the eye pigment cells and serosa cells. The mutant is also characterized by translucent larval skin resulting from a deficiency in the transportation of uric acid, which acts as a white pigment in larval epidermal cells. A silkworm homolog of the fruitfly white gene, Bmwh3, a member of ATP-binding cassette transporter superfamily, was mapped on the w-3 locus. The w-3(oe) mutant has a single-base deletion in exon 2 and a premature stop codon at the 5' end of exon 3. These results show that w-3 is equivalent to Bmwh3 and is responsible for the transportation of ommochrome precursors and uric acid into pigment granules and urate granules, respectively.     

Frequency dependent flicker response enhancement in the lamina ganglionaris ofDrosophila

Summary At low light intensity and within the narrow frequency range of 55 to 66 s^–1, the eye ofDrosophila will follow a flashing light source by enhancing it's flicker response to every other flash. By contrast, at lower and higher frequencies the eye will follow every cycle of a respective flash frequency upto a fusion point around 200 s^–1.While the receptor cells involved are retinula cells R1–6, the flicker response enhancement is established to originate postsynaptically in the Large Monopolar Cells of the lamina with which the peripheral retinula cells synapse, and which respond with the cornealpositive on-transient component of the ERG. Not only is a prescribed frequency required for the enhancement, but also continuity of cue — since brief periods of light flashes within the required frequency range are resolved at every cycle.The flicker response behaviour provides further credence to the existence of fine tuning mechanisms together with amplification within the lamina neuropile.We are grateful to the late Dr. Richard Wright for his comments, and to Professor Aubrey Manning for the hospitality his Department gave to N.L.     

Graded and discrete receptor potentials in the compound eye of the Australian Bulldog-ant (Myrmecia gulosa)

1. Graded and discrete receptor potentials are recorded from the visual cells of the Australian Bulldog-ant. The intensity dependence of the graded responses is described by a new formula [Eq. (3)]. While the frequency of the discrete potentials in relation to the number of light quanta fits best a Poisson distribution, the graded potentials are best described by a logarithmic Gaussian distribution. 2. It is shown that the non-linear summation of single bumps and the reduction of the bump amplitude lead to a logarithmic intensity dependence. 3. The frequency spectrum of single bumps is measured with a Fast-Fourier-Analysis. It is observed that the harmonic frequencies have a negative slope around 12 dB/octave. 4. A difference is found in the higher harmonics of bumps generated at lower light intensities from those generated at higher light intensities. It is shown that this difference becomes more obvious if the bumps are further divided into short and longer latency groups. 5. From these results, it is concluded, that there is a mutual influence between neighbouring visual cells. Using this influence as a basis, a model of the low electric coupling between the cells is discussed.     

Electrophysiological organization of the eye of Aplysia

The eye of Aplysia californica was studied by electrophysiological and histological methods. It has a central spheroidal lens which is surrounded by a retina composed of several thousand receptor cells which are replete with clear vesicles, pigmented support cells, neurons which contain secretory granules, and glial cells. The thin optic nerve that connects the eye to the cerebral ganglion gives a simple "on" response of synchronized action potentials. Tonic activity occurs in the optic nerve in the dark and is dependent on previous dark adaptation. Micropipette recordings indicate that the ERG is positive (relative to a bathelectrode) on the outer surface of the eye and negative in the region of the distal segments of the receptors. Intracellular recordings show that receptor cells have resting potentials of 40–50 mv and respond to illumination with graded potentials of up to 55 mv. Dark-adapted receptors exhibit discrete bumps on the graded response to brief light flashes. Other elements in the retina that do not give large graded responses fall into two classes. One class responds to illumination with action potentials that are in synchrony with the extracellularly recorded compound optic nerve potentials. The other class is tonically active and is depolarized or hyperpolarized and inhibited upon illumination. It is apparent that complex excitatory and lateral inhibitory interactions occur among the elements of the retina.     

The functional organization of the crayfish lamina ganglionaris

The functional properties of the multicolumnar interneurons of the crayfish lamina ganglionaris were examined by intracellular recording and the cell structures were revealed with the aid of Lucifer yellow or horseradish peroxidase iontophoresis. The multicolumnar monopolar cell M5 responds to a light pulse with a depolarizing compound EPSP and a burst of action potentials. Both the EPSP amplitude and the spike rate decay toward a lower level plateau in less than 200 ms after light onset. M5 is subject to surround inhibition, which is associated with a compound IPSP and net hyperpolarization of the membrane potential. Direct depolarization of M5 may provide a weak excitatory drive to medullary sustaining fibers (SF). Tangenital-cell type 1 (Tan1) has a broad expanse of neurites in the lamina (covering 10 to 15 cartridges) and a much narrower projection in the medulla (1 to 3 cartridges). The response to a light pulse has a long latency consistent with a polysynaptic receptor to Tan1 pathway. The response consists of a nearly rectangular hyperpolarization. Light 'off' elicits a depolarization and a burst of impulses. The polarity of the 'on' response can be reversed by hyperpolarizing the membrane by 23 mV. The receptive field is broad and the intensity-response function exceeds 4 log units. Direct hyperpolarization of Tan1 provides a strong excitatory signal to medullary SFs both in the dark and in the presence of illumination. We propose that Tan1 provides the principal steady-state excitatory drive to the SFs. Tangential-cell type 2 (Tan2) is distinguished from Tan1 by the extent and shape of the lamina process, which is a vertically oriented neurite spanning most of the lamina in a single plane. Functionally, Tan2 is similar in most respects to Tan1, but the response latency is much shorter, comparable to that of monopolar cells. T-cells may exhibit spontaneous impulse activity in the dark which is inhibited by a short latency hyperpolarizing light response. The receptive field, which is about 2 X larger than that of the columnar monopolar cells, is correlated with a small but multicolumnar dendritic arbor in the lamina. Since T-cells are aminergic, it is possible that the amines are normally released in the dark. A single amacrine cell was fully characterized. It exhibited a short latency hyperpolarizing response to light onset and a strong depolarizing 'off' response.(ABSTRACT TRUNCATED AT 400 WORDS)     

Circadian rhythms in screening pigment and invaginating organelles in photoreceptor terminals of the housefly's first optic neuropile

Screening pigment granules occur in the synaptic terminals of photoreceptors in the fly's (Musca domestica, L.) compound eye. The granules resemble ommochrome granules in the overlying photoreceptor cell body. There are also two types of invagination into receptor terminals: capitate projections (from glial cells) and invaginations from neighboring receptor terminals. The number of profiles of these organelles in the first optic neuropile, the lamina, have been counted using single-section quantitative electron microscopic methods. Pigment granules are concentrated proximally in the terminal, toward the brain. The numbers change, increasing during the night (1 h after lights off) up to values more than twice the number 1 h after lights on, apparently by longitudinal migration of granules from the cell body into the terminal. Flies entrained to day/night conditions and then held under constant darkness continue to exhibit changes in the numbers of profiles. Even though overall there were 80–90% fewer granule profiles than under day/night conditions, the numbers attained a peak many times higher at the end of the subjective day. Thus, the changes are endogenous, showing circadian rhythmicity. Although their significance is unknown, these changes parallel previously described circadian rhythms in the receptor terminals and their lamina monopolar-cell targets. The invaginations from receptor terminals were more numerous under day/night conditions than under constant darkness, and cycled in constant darkness, peaking at the end of subjective night. Capitate projections, by contrast, failed to change significantly under the experimental conditions analyzed, a lack of responsiveness they share with photoreceptor tetrad synapses. © 1997 John Wiley & Sons, Inc. J Neurobiol 32: 517–529, 1997     

Occurrence of ommochrome-containing pigment granules in the central nervous system of the silkworm, Bombyx mori

Dark-red pigment granules were found in the brain and ganglion of the normal strain of the silkworm, Bombyx mori, by light microscopy. No other pigmentation was seen in the brain or ganglia. Electron microscopy showed that the granules were electron-dense. The granules were similar to the ommochrome-containing pigment granules that are present in the epidermal cells of the quail mutant, as previously reported. The pigment in the larval central nervous system (CNS) of the normal silkworm was identical to the ommin standard with respect to the absorption spectrum, the infrared spectrum, and the Rf value in thin-layer chromatography (TLC). After acid hydrolysis of the pigment, 3-hydroxykynurenine was detected by TLC. The pigment granules in the CNS contained mainly ommin. An ommochrome-binding protein was also detected in the CNS by in vitro binding studies and Western blotting. The ommochrome granules may have an important function in the CNS of the silkworm.     

Dynamic characteristics of the receptive field of L-cells for monochromatic lights

We applied the Wiener theory to analyse receptive field responses of L-cells in the carp and studied some dynamic properties of the receptive field of L-cells for monochromatic light stimuli. The L-cells were stimulated by each monochromatic light modulated in white-noise fashion. They responded almost linearly to all the monochromatic light stimuli. The impulse responses of the L-cells became larger in amplitude and faster in latency, peak response time, and repolarising phase as a spot of monochromatic light was enlarged. The L-cells seem to respond like a lowpass filter and the cutoff frequency of their gain characteristics increases with the enlargement of the monochromatic light spot. The relation between shift of cutoff frequency and spot diameter was monotonic increasing for each monochromatic light.     

Light-induced reduction in excitation efficiency in the trp mutant of Drosophila

In the transient receptor potential (trp) mutant of Drosophila, the receptor potential appears almost normal in response to a flash but quickly decays to baseline during prolonged illumination. Photometric and early receptor potential measurements of the pigment suggest that the pigment is normal and that the decay of the trp response during illumination does not arise from a reduction in the available photopigment molecules. However, there is reduction in pigment concentration with age. Light adaptation cannot account for the decay of the trp response during illumination: in normal Drosophila a dim background light shortens the latency and rise time of the response and also shifts the intensity response function (V-log I curve) to higher levels of light intensity with relatively little reduction in the maximal amplitude (Vmax) of response. In the trp mutant, a dim background light or short, strong adapting light paradoxically lengthens the latency and rise time of the response and substantially reduces Vmax without a pronounced shift of the V-log I curve along the I axis. The effect of adapting light on the latency and V-log I curve seen in trp are associated with a reduction in effective stimulus intensity (reduction in excitation efficiency) rather than with light adaptation. Removing extracellular Ca+2 reduces light adaptation in normal Drosophila, as evidenced by the appearance of "square" responses to strong illumination. In the trp mutant, removing extracellular Ca+2 does not prevent the decay of the response during illumination.     

THE DEVELOPMENT OF PIGMENT GRANULES IN THE EYES OF WILD TYPE AND MUTANT DROSOPHILA MELANOGASTER

The eye pigment system in Drosophila melanogaster has been studied with the electron microscope. Details in the development of pigment granules in wild type flies and in three eye color mutants are described. Four different types of pigment granules have been found. Type I granules, which carry ommochrome pigment and occur in both primary and secondary pigment cells of ommatidia, are believed to develop as vesicular secretions by way of the Golgi apparatus. The formation of Type II granules, which are restricted to the secondary pigment cells and contain drosopterin pigments, involves accumulation of 60- to 80-A fibers producing an elliptical granule. Type III granules appear to be empty vesicles, except for small marginal areas of dense material; they are thought to be abnormal entities containing ommochrome pigment. Type IV granules are characteristic of colorless mutants regardless of genotype, and during the course of development they often contain glycogen, ribosomes, and show acid phosphatase activity; for these reasons and because of their bizarre and variable morphology, they are considered to be autophagic vacuoles. The 300-A particles commonly found in pigment cells are identified as glycogen on the basis of their morphology and their sensitivity to salivary digestion.