Horizontal cell spinule dynamics in fish are affected by rearing in monochromatic light

Structure, Distribution, and Function of Neuronal/Synaptic Spinules and Related Invaginating Projections

Neurons and especially their synapses often project long thin processes that can invaginate neighboring neuronal or glial cells. These "invaginating projections" can occur in almost any combination of postsynaptic, presynaptic, and glial processes. Invaginating projections provide a precise mechanism for one neuron to communicate or exchange material exclusively at a highly localized site on another neuron, e.g., to regulate synaptic plasticity. The best-known types are postsynaptic projections called "spinules" that invaginate into presynaptic terminals. Spinules seem to be most prevalent at large very active synapses. Here, we present a comprehensive review of all kinds of invaginating projections associated with both neurons in general and more specifically with synapses; we describe them in all animals including simple, basal metazoans. These structures may have evolved into more elaborate structures in some higher animal groups exhibiting greater synaptic plasticity. In addition to classic spinules and filopodial invaginations, we describe a variety of lesser-known structures such as amphid microvilli, spinules in giant mossy terminals and en marron/brush synapses, the highly specialized fish retinal spinules, the trophospongium, capitate projections, and fly gnarls, as well as examples in which the entire presynaptic or postsynaptic process is invaginated. These various invaginating projections have evolved to modify the function of a particular synapse, or to channel an effect to one specific synapse or neuron, without affecting those nearby. We discuss how they function in membrane recycling, nourishment, and cell signaling and explore how they might change in aging and disease.

Conical tomography of a ribbon synapse: structural evidence for vesicle fusion

To characterize the sites of synaptic vesicle fusion in photoreceptors, we evaluated the three-dimensional structure of rod spherules from mice exposed to steady bright light or dark-adapted for periods ranging from 3 to 180 minutes using conical electron tomography. Conical tilt series from mice retinas were reconstructed using the weighted back projection algorithm, refined by projection matching and analyzed using semiautomatic density segmentation. In the light, rod spherules contained ∼470 vesicles that were hemi-fused and ∼187 vesicles that were fully fused (omega figures) with the plasma membrane. Active zones, defined by the presence of fully fused vesicles, extended along the entire area of contact between the rod spherule and the horizontal cell ending, and included the base of the ribbon, the slope of the synaptic ridge and ribbon-free regions apposed to horizontal cell axonal endings. There were transient changes of the rod spherules during dark adaptation. At early periods in the dark (3–15 minutes), there was a) an increase in the number of fully fused synaptic vesicles, b) a decrease in rod spherule volume, and c) an increase in the surface area of the contact between the rod spherule and horizontal cell endings. These changes partially compensate for the increase in the rod spherule plasma membrane following vesicle fusion. After 30 minutes of dark-adaptation, the rod spherules returned to dimensions similar to those measured in the light. These findings show that vesicle fusion occurs at both ribbon-associated and ribbon-free regions, and that transient changes in rod spherules and horizontal cell endings occur shortly after dark onset.

Adaptive plasticity during the development of colour vision

Colour vision greatly enhances the discriminatory and cognitive capabilities of visual systems and is found in a great majority of vertebrates and many invertebrates. However, colour coding visual systems are confronted with the fact that the external stimuli are ambiguous because they are subject to constant variations of luminance and spectral composition. Furthermore, the transmittance of the ocular media, the spectral sensitivity of visual pigments and the ratio of spectral cone types are also variable. This results in a situation where there is no fixed relationship between a stimulus and a colour percept. Colour constancy has been identified as a powerful mechanism to deal with this set of problems; however, it is active only in a short-term time range. Changes covering longer periods of time require additional tuning mechanisms at the photoreceptor level or at postreceptoral stages of chromatic processing. We have used the trichromatic blue acara (Aequidens pulcher, Cichlidae) as a model system and studied retinal morphology and physiology, and visually evoked behaviour after rearing fish for 1-2 years under various conditions including near monochromatic lights (spectral deprivation) and two intensities of white light (controls). In general, long-term exposure to long wavelengths light had lesser effects than light of middle and short wavelengths. Within the cone photoreceptors, spectral deprivation did not change the absorption characteristics of the visual pigments. By contrast, the outer segment length of middle and long-wave-sensitive cones was markedly increased in the blue rearing group. Furthermore, in the same group, we observed a loss of 65% short-wave-sensitive cones after 2 years. These changes may be interpreted as manifestations of compensatory mechanisms aimed at restoring the balance between the chromatic channels. At the horizontal cell level, the connectivity between short-wave-sensitive cones and the H2 cone horizontal cells, and the spinule dynamics were both affected in the blue light group. This observation rules out the role of spinules as sites of chromatic feedback synapses. The light-evoked responses of H2 horizontal cells were also sensitive to spectral deprivation showing a shift of the neutral point towards short wavelengths in the blue rearing group. Interestingly, we also found an intensity effect because in the group reared in bright white light the neutral point was more towards longer wavelength than in the dim light group. Like the changes in the cones, the reactions of horizontal cells to spectral deprivation in the long wave domain can be characterised as compensatory. We also tested the spectral sensitivity of the various experimental groups of blue acara in visually evoked behaviour using the optomotor response paradigm. In this case, the changes in the relative spectral sensitivity were more complex and could not be explained by a simple extrapolation of the adaptive and compensatory processes in the outer retina. We conclude that the inner retina, and/or the optic tectum are also involved and react to the changes of the spectral environment. In summary, we have shown a considerable developmental plasticity in the colour vision system of the blue acara, where epigenetic adaptive processes at various levels of the visual system respond to the specific spectral composition of the surroundings and provide a powerful mechanism to ensure functional colour vision in different visual environments. We suggest that processes involving an active fine-tuning of the photoreceptors and the postreceptoral processing of chromatic information during ontogenetic development are a general feature of all colour vision systems. Such mechanisms would establish a functional balance between the various chromatic channels. This appears to be an essential condition for the cognitive systems to extract the relevant and stable information from the unstable and changing stimulus situation.

Developmental plasticity of NMDA receptor function in the retina and the influence of light

Despite the early expression of NMDA receptors (NMDARs) in the retina, not much is known about their regulation and involvement in plasticity processes during retinal development and synapse formation. Here we report that NMDAR function in the inner retina is developmentally regulated and controlled by ambient light condition. A prominent down-regulation after eye opening of NMDAR function was observed in rat retinal ganglion cells (RGCs), which was prevented by dark rearing the animals for 1 month but was again induced by subsequent light exposure. As shown by molecular analysis of single RGCs, alterations in the subunit composition of NMDAR did not account for the light-dependent regulation of NMDAR function. Immunocytochemistry showed no differences in the NMDAR protein expression pattern between normal and dark-reared animals. In conclusion, our data clearly demonstrate that NMDAR function is modulated during periods of retinal plasticity independent of structural alterations in its subunit composition and thus different from mechanisms observed in higher visual centers.

Effects of restricted spectral rearing on the development of zebrafish retinal physiology

Research has shown that rearing in abnormal lighting environments affects both visual behavior and retinal physiology in zebrafish larvae. These studies, however, used only constant dark and constant white light as the experimental rearing conditions. This study assessed the effects of rearing larvae in restricted spectral lighting environments on zebrafish retinal physiology. Larvae were reared in one of seven different lighting environments: cyclic white light (control group), constant blue light, constant green light, constant orange light, cyclic blue light, cyclic green light, and cyclic orange light. Assessment of retinal physiology was done using the electroretinogram (ERG). Results showed that rearing larvae in constant light conditions caused deficits in sensitivity to ultraviolet- and short-wavelength stimuli, but had little effect on sensitivity to middle- and long-wavelength stimuli. Rearing larvae in cyclic light did not cause differences in sensitivity to middle- and long-wavelength stimuli, but did cause extreme deficits in sensitivity to ultraviolet- and short-wavelength stimuli in the cyclic green and orange light-rearing conditions. Sensitivity of the cyclic blue light-rearing group was similar to the control group to stimuli of all wavelengths. The results support the notion that the light-rearing environment impacts the development of the ultraviolet- and short-wavelength cone mechanisms but has little impact on the development of the middle- and long-wavelength cone mechanisms; these effects coincide with the development of the various cone types. This study supports the notion that the zebrafish is a viable model for studying the effects of the lighting environment on visual development.

Rearing in different photic and spectral environments changes the optomotor response to chromatic stimuli in the cichlid fish Aequidens pulcher

Developmental plasticity of spectral processing in vertebrates was investigated in fish by using an innate behavior, the optomotor response. Rearing blue acara (Aequidens pulcher; Cichlidae) under white lights of different intensities as well as deprivation of long wavelengths induced significant changes in the animals' responses to chromatic stimuli. Deprivation of short wavelengths had no effect. With this and previous studies on animals reared under similar conditions, we have demonstrated that developmental plasticity in spectral processing is present at a wide range of neural levels, spanning from photoreceptors to behavior. We hypothesize that earlier studies did not reveal such effects because of the rearing and testing conditions used.

Effects of long-term spectral deprivation on the morphological organization of the outer retina of the blue acara (Aequidens pulcher)

To investigate the developmental plasticity of colour vision, we reared fish with a trichromatic cone system (Aequidens pulcher) under three near-monochromatic lights, differentially stimulating each spectral cone type from the larval stage to the age of at least one year. Control conditions comprised white lights of two intensities. The treatments did not affect the visual pigments, hut led to significant changes in cone outer segment lengths. Furthermore, in the blue-reared group the density of single cones within the retina was reduced by two-thirds after 18 months of exposure, while no changes were observed in the other groups. The connectivity of cone horizontal cells with the single cones was influenced by the intensity and spectral composition of the rearing lights: H1 cells were more sensitive to the spectral component, whereas H2 cells responded to intensity cues. In the blue-light group the dynamics of horizontal cell synaptic spinule formation and degradation were severely compromised. These observations show that long-term spectral deprivation leads to significant morphological changes at the level of photoreceptors and horizontal cells. While the reactions of photoreceptors may be interpreted mostly in terms of compensation, the functional consequences of the changes observed on the horizontal cell level remain to be determined electrophysiologically.

Morphological changes in the retina of Aequidens pulcher (Cichlidae) after rearing in monochromatic light

We investigate the processing of chromatic information in the outer retina of a cichlid fish, Aequidens pulcher. The colour opponent response characteristics of some classes of cone-specific horizontal cells in the fish retina are the result of feedforward-feedback loops with cone photoreceptors. To interfere with the reciprocal transmissions of signals, animals were reared in monochromatic lights which preferentially stimulated the spectrally different cone types. Here we report the effects on the cones. Their absorbance spectra were largely unaffected, indicating no change in photopigment gene expression. Significant changes were observed in the cone outer segment lengths and the frequencies of spectral cone types. Quantum catch efficiency and survival of cones appear to be controlled in a spectrally selective way. Our results suggest that the retina responds to spectral deprivation in a compensatory fashion aimed at balancing the input from the different cone types to second order neurons.

Connectivity patterns of cone horizontal cells in blue acara (Aequidens pulcher, Cichlidae) reared in different light regimes

Two types of cone horizontal cells were identified morphologically in the retina of a trichromatic fish by fluorescent labelling with Lucifer Yellow and confocal laser scanning microscopy. H1 cells are located adjacent to the outer plexiform layer, have large somata, small dendritic fields, and contact all cone types. H2 cells are positioned vitread to the H1 cells, have small somata, and large dendritic fields. Their dendrites invaginate the synaptic pedicles of short wavelength sensitive single cones and show a significant preference for one of the spectrally different members of the double cones, presumably the middle wavelength sensitive member. We tested the impacts of different visual environments on the development of these connectivity patterns and found minor changes induced by rearing in white light of different intensities or monochromatic blue light.