Responses of pineal photoreceptors in the brook and rainbow trout

Summary Electron microscopy of the pineal receptor cells in light- and dark-adapted brook trout, Salvelinus fontinalis and the rainbow trout, Salmo gairdneri, revealed no significant differences in the tubular and filamentous elements of the inner segment, neck and supranuclear regions. However, changes in synaptic relations between the photoreceptor and nerve cell were induced by light and darkness. In the light-adapted state, the synaptic relationship between axon terminals and photoreceptor basal processes predominates, while in darkness the synapses between photoreceptor basal processes and ganglion cell dendrites are more prominent. Further, in darkness, the photoreceptor basal processes show a number of synaptic vesicles and synaptic ribbons. These findings suggest that the sensory function of the fish pineal is enhanced during darkness but inhibited by light, and that the synaptic relationships are involved in the control of sensory activity in the pineal photoreceptor and ganglion cells. These results corroborate those of electrophysiological studies in that the maximal spontaneous discharge frequency of the ganglion cells occurs in the dark, and it also shows a burst when light is removed. The typical chemical synapse between the axon terminal and the photoreceptor basal process in light seems to function as an inhibitor.The authors thank Dr. Mary Ann Klyne for her assistance in several aspects of this work. Financial assistance was provided by the NSERC of Canada and the Ministry of Education of Québec     

Rhythm and soul in the avian pineal

The avian pineal gland, like that of mammals, displays a striking circadian rhythm in the synthesis and release of the hormone melatonin. However, the pineal gland plays a more prominent role in avian circadian organization and differs from that in mammals in several ways. One important difference is that the pineal gland in birds is relatively autonomous. In addition to making melatonin, the avian pineal contains photoreceptors and a circadian clock (thus, an entire circadian system) within itself. Furthermore, avian pineals retain their circadian properties in organ or dispersed cell culture, making biochemical components of regulatory pathways accessible. Avian pinealocytes are directly photosensitive, and novel candidates for the unidentified photopigments involved in the regulation of clock function and melatonin production, including melanopsin, pinopsin, iodopsin, and the cryptochromes, are being evaluated. Transduction pathways and second messengers that may be involved in acute and entraining effects, including cyclic nucleotides, calcium fluxes, and protein kinases, have been, and continue to be, examined. Moreover, several clock genes similar to those found in Drosophila and mouse are expressed, and their dynamics and interactions are being studied. Finally, the bases for acute and clock regulation of the key enzyme in melatonin synthesis, arylalkylamine N-acetyltransferase (AA-NAT), are described. The ability to study entrainment, the oscillator itself, and a physiological output in the same tissue at the same time makes the avian pineal gland an excellent model to study the bases and regulation of circadian rhythms.     

Taurine deficiency damages retinal neurones: cone photoreceptors and retinal ganglion cells

In 1970s, taurine deficiency was reported to induce photoreceptor degeneration in cats and rats. Recently, we found that taurine deficiency contributes to the retinal toxicity of vigabatrin, an antiepileptic drug. However, in this toxicity, retinal ganglion cells were degenerating in parallel to cone photoreceptors. The aim of this study was to re-assess a classic mouse model of taurine deficiency following a treatment with guanidoethane sulfonate (GES), a taurine transporter inhibitor to determine whether retinal ganglion cells are also affected. GES treatment induced a significant reduction in the taurine plasma levels and a lower weight increase. At the functional level, photopic electroretinograms were reduced indicating a dysfunction in the cone pathway. A change in the autofluorescence appearance of the eye fundus was explained on histological sections by an increased autofluorescence of the retinal pigment epithelium. Although the general morphology of the retina was not affected, cell damages were indicated by the general increase in glial fibrillary acidic protein expression. When cell quantification was achieved on retinal sections, the number of outer/inner segments of cone photoreceptors was reduced (20?%) as the number of retinal ganglion cells (19?%). An abnormal synaptic plasticity of rod bipolar cell dendrites was also observed in GES-treated mice. These results indicate that taurine deficiency can not only lead to photoreceptor degeneration but also to retinal ganglion cell loss. Cone photoreceptors and retinal ganglion cells appear as the most sensitive cells to taurine deficiency. These results may explain the recent therapeutic interest of taurine in retinal degenerative pathologies.     

Immunocytochemical demonstration of visual pigments in the degenerate retinal and pineal photoreceptors of the blind cave salamander (Proteus anguinus)

Visual pigments in the regressed eye and pineal of the depigmented neotenic urodele, the blind cave salamander (Proteus anguinus anguinus), were studied by immunocytochemistry with anti-opsin antibodies. The study included light- and electron-microscopic investigations of both the eye and the pineal organ. A comparison was made with the black pigmented subspecies Proteus anguinus parkelj (black proteus), which has a normal eye structure. In the retina of the black proteus, we found principal rods, red-sensitive cones and a third photoreceptor type, which might represent a blue- or UV-sensitive cone. Photoreceptors in the regressed eye of the blind cave salamanders from the Planina cave contained degenerate outer segments, consisting of a few whorled discs and irregular clumps of membranes. The great majority of these outer segments showed immunolabelling for the red-sensitive cone opsin and only a few of them were found to be positive for rhodopsin. An even more pronounced degeneration was observed in the photoreceptors of the animals derived from the Otovec doline, which are completely devoid of an outer segment, most of them not even possessing an inner segment. Even in some of these highly degenerate cells, the presence of rhodopsin could be detected in the plasma membrane; however, immunoreactions with antibodies recognizing cone visual pigment were negative. In the pineals of all studied animals, the degenerate photoreceptor outer segments were recognized exclusively by the antibody against the red-sensitive cone opsin. The presence of immunopositive visual pigments indicates the possibility of a retained light sensitivity in the blind cave salamander photoreceptors.     

Regulation of cyclic nucleotides in retinal photoreceptors

Summary The distribution of cyclases in retinal photoreceptors of dark- and light-adapted brook trout was studied by means of a cytochemical method (lead precipitation). It confirms earlier reports that retinal photoreceptors contain high levels of cyclic nucleotides, and that cAMP predominates in cones and cGMP in rods. There is an apparent difference in the level of the cyclases with the adaptive states. In addition, the catalytic unit of cyclase is interlamellar in cones. In rods, adenylate cyclase is intradiscal, while the location of guanylate cyclase varies with the adaptive state. The variation of cyclase with adaptation indicates that this enzyme has a role in the process of visual transduction.     

Calcium and light adaptation in retinal photoreceptors.

In the past several years there has been great progress in the understanding of the phototransduction process in retinal photoreceptors. Recently, this knowledge has expanded and we now understand the mechanism of background light adaptation in these cells and the role that calcium has to play.     

Circadian biology: the physiology of inner retinal photoreceptors

Non-traditional photoreceptors detect overall irradiance in the vertebrate retina. Such cells in the mouse inner retina show increased intracellular Ca(2+) levels following illumination. Neurons in the outer retina of fish also display characteristics appropriate for an irradiance detector.     

Ontogenetic development of S-antigen- and rodopsin immunoreactions in retinal and pineal photoreceptors of Xenopus laevis in relation to the onset of melatonin-dependent color-change mechanisms

Summary In Xenopus laevis Daud., the ontogenetic occurrence of two photoreceptor-specific proteins, S-antigen and rod-opsin, was investigated and correlated to the maturation of the neurohormonal effector system involved in melatonin-dependent color-change mechanisms. Tadpoles ranging from stage 12 to 57 (Nieuwkoop and Faber 1956) were fixed in Zamboni's or Bouin's solution. Frozen or paraffin sections of either total heads or dissected brains and eyes were prepared and treated with highly specific antisera against S-antigen and rod-opsin. In the retina, immunoreactive S-antigen and rod-opsin were first demonstrated in a few centrally located photoreceptors at stage 37/38. Photoreceptors of the peripheral (iridical) portions of the retina gradually became immunoreactive during further development. As in the retina, the first S-antigen-immunoreactive photoreceptors in the pineal complex appeared at stage 37/ 38. At this and all later stages investigated rod-opsin immunoreactivity was restricted to a few dot-like structures resembling developing pineal outer and inner segments. In most animals rod-opsin immunoreactivity was completely absent from the pineal complex. The analysis of retinal proteins with the immunoblotting technique (Western blot) revealed that the S-antigen antibody bound to a 48-kDa protein and the rod-opsin antibody to a 38-kDa protein. The body lightening reaction was determined with the aid of the melanophore index in larvae fixed in light or darkness, respectively. Aggregation of melanophore melanosomes in darkness (the melatonin-dependent primary chromatic response) first occurred at stage 37/38 when melanophores started to differentiate and became pigmented. These results indicate that in Xenopus laevis (i) the molecular mechanisms of photoreception develop simultaneously in retina and pineal complex; (ii) most pineal photoreceptors differ from retinal rods in that they contain immunoreactive S-antigen but essentially no immunoreactive rod-opsin; and (iii) the differentiation of phototransduction processes coincides with the onset of melatonin-dependent photoneuroendocrine regulation of color-change mechanisms.Supported by USUHS protocol C07049 (MDR) and the Deutsche Forschungsgemeinschaft (HWK)     

Evolution of photosensory pineal organs in new light: the fate of neuroendocrine photoreceptors

Pineal evolution is envisaged as a gradual transformation of pinealocytes (a gradual regression of pinealocyte sensory capacity within a particular cell line), the so-called sensory cell line of the pineal organ. In most non-mammals the pineal organ is a directly photosensory organ, while the pineal organ of mammals (epiphysis cerebri) is a non-sensory neuroendocrine organ under photoperiod control. The phylogenetic transformation of the pineal organ is reflected in the morphology and physiology of the main parenchymal cell type, the pinealocyte. In anamniotes, pinealocytes with retinal cone photoreceptor-like characteristics predominate, whereas in sauropsids so-called rudimentary photoreceptors predominate. These have well-developed secretory characteristics, and have been interpreted as intermediaries between the anamniote pineal photoreceptors and the mammalian non-sensory pinealocytes. We have re-examined the original studies on which the gradual transformation hypothesis of pineal evolution is based, and found that the evidence for this model of pineal evolution is ambiguous. In the light of recent advances in the understanding of neural development mechanisms, we propose a new hypothesis of pineal evolution, in which the old notion 'gradual regression within the sensory cell line' should be replaced with 'changes in fate restriction within the neural lineage of the pineal field'.     

Electrical coupling of photoreceptors in retinal network models

The spatial width of photoreceptor receptive fields affects the processing of signals in neural networks of the retina. This effect has been examined using the simple recurrent and non-recurrent network models, where lateral interaction strength was adjusted to approximate a prescribed receptive field profile. The results indicate that the optimal performance of the networks is obtained with photoreceptor receptive fields wider than the ganglion cell separation. It is thus concluded that while electrical coupling of photoreceptors in the retina reduces the intrinsic noise in the system, it also improves the sampling efficiency of the laterally coupled neural network of the retina.     

Nonvisual photoreceptors of the deep brain, pineal organs and retina

The role of the nonvisual photoreception is to synchronise periodic functions of living organisms to the environmental light periods in order to help survival of various species in different biotopes. In vertebrates, the so-called deep brain (septal and hypothalamic) photoreceptors, the pineal organs (pineal- and parapineal organs, frontal- and parietal eye) and the retina (of the "lateral" eye) are involved in the light-based entrain of endogenous circadian clocks present in various organs. In humans, photoperiodicity was studied in connection with sleep disturbances in shift work, seasonal depression, and in jet-lag of transmeridional travellers. In the present review, experimental and molecular aspects are discussed, focusing on the histological and histochemical basis of the function of nonvisual photoreceptors. We also offer a view about functional changes of these photoreceptors during pre- and postnatal development as well as about its possible evolution. Our scope in some points is different from the generally accepted views on the nonvisual photoreceptive systems. The deep brain photoreceptors are hypothalamic and septal nuclei of the periventricular cerebrospinal fluid (CSF)-contacting neuronal system. Already present in the lancelet and representing the most ancient type of vertebrate nerve cells ("protoneurons"), CSF-contacting neurons are sensory-type cells sitting in the wall of the brain ventricles that send a ciliated dendritic process into the CSF. Various opsins and other members of the phototransduction cascade have been demonstrated in telencephalic and hypothalamic groups of these neurons. In all species examined so far, deep brain photoreceptors play a role in the circadian and circannual regulation of periodic functions. Mainly called pineal "glands" in the last decades, the pineal organs actually represent a differentiated form of encephalic photoreceptors. Supposed to be intra- and extracranially outgrown groups of deep brain photoreceptors, pineal organs also contain neurons and glial elements. Extracranial pineal organs of submammalians are cone-dominated photoreceptors sensitive to different wavelengths of light, while intracranial pineal organs predominantly contain rod-like photoreceptor cells and thus scotopic light receptors. Vitamin B-based light-sensitive cryptochromes localized immunocytochemically in some pineal cells may take part in both the photoreception and the pacemaker function of the pineal organ. In spite of expressing phototransduction cascade molecules and forming outer segment-like cilia in some species, the mammalian pineal is considered by most of the authors as a light-insensitive organ. Expression of phototransduction cascade molecules, predominantly in young animals, is a photoreceptor-like characteristic of pinealocytes in higher vertebrates that may contribute to a light-percepting task in the perinatal entrainment of rhythmic functions. In adult mammals, adrenergic nerves--mediating daily fluctuation of sympathetic activity rather than retinal light information as generally supposed--may sustain circadian periodicity already entrained by light perinatally. Altogether three phases were supposed to exist in pineal entrainment of internal pacemakers: an embryological synchronization by light and in viviparous vertebrates by maternal effects (1); a light-based, postnatal entrainment (2); and in adults, a maintenance of periodicity by daily sympathetic rhythm of the hypothalamus. In addition to its visual function, the lateral eye retina performs a nonvisual task. Nonvisual retinal light perception primarily entrains genetically-determined periodicity, such as rod-cone dominance, EEG rhythms or retinomotor movements. It also influences the suprachiasmatic nucleus, the primary pacemaker of the brain. As neither rods nor cones seem to represent the nonvisual retinal photoreceptors, the presence of additional photoreceptors has been supposed. Cryptochrome 1, a photosensitive molecule identified in retinal nerve cells and in a subpopulation of retinal photoreceptors, is a good candidate for the nonvisual photoreceptor molecule as well as for a member of pacemaker molecules in the retina. When comparing various visual and nonvisual photoreceptors, transitory, "semi visual" (directional) light-perceptive cells can be detected among them, such as those in the parietal eye of reptiles. Measuring diffuse light intensity of the environment, semivisual photoreceptors also possess some directional light perceptive capacity aided by complementary lens-like structures, and screening pigment cells. Semivisual photoreception in aquatic animals may serve for identifying environmental areas of suitable illumination, or in poikilotermic terrestrial species for measuring direct solar irradiation for thermoregulation. As directional photoreceptors were identified among nonvisual light perceptive cells in the lancelet, but eyes are lacking, an early appearance of semivisual function, prior to a visual one (nonvisual --> semivisual --> visual?) in the vertebrate evolution was supposed.     

Photic regulation of pineal function. Analogies between retinal and pineal photoreception

Light absorbed by a photopigment in a photoreceptor cell causes a photochemical reaction converting the 11-cis retinal chromophore into the all-trans configuration. These changes lead to a series of events that causes cGMP hydrolysis, a following decrease of cGMP in the cytoplasm of the photoreceptor outer segment and a closure of cGMP-gated cationic channels. As a consequence of these processes the membrane hyperpolarizes. In pineal photoreceptor cells of lower vertebrates these processes are only partly investigated. Molecules involved in the phototransduction process and the desensitization, like opsin, vitamin A, α-transducin and arrestin, have been immunocytochemically localized in pineal photoreceptors and also electrophysiological studies have shown that phototransduction mechanisms in pineal photoreceptors might be very similar to those found in retinal photoreceptors. This review will summarize some of the current knowledge on pineal photoreception and compare it with retinal processes.     

Metabolism of glucose and reduction of retinaldehyde in retinal photoreceptors

—The capacity of a variety of substrates to support reduction of retinaldehyde to retinol by preparations of outer segments from photoreceptor cells of bovine retina, the requirement for supplements, the effect of iodoacetate, and the stoichiometry of the metabolism of glucose and the reduction of retinaldehyde have been studied. Metabolism of glucose supported reduction of retinaldehyde by preparations of outer segments at approximately the rate observed for the intact retina. Throughout the physiological range of pH, the dehydrogenase reactions of the pentose cycle in outer segments provided virtually all of the reduced pyridine nucleotides that were utilized for reduction of retinaldehyde. It was concluded that reduction of retinaldehyde during light adaptation is exclusively dependent upon NADPH.     

Proteoglycan synthesis in cultures of murine retinal neurons and photoreceptors

1. In recent years, a number of histochemical and immunocytochemical studies have suggested that proteoglycans, particularly those in the interphotoreceptor matrix, exhibit altered distributions in several murine models for retinal degenerations. We are using a cell culture system to characterize the proteoglycans synthesized by neurons and photoreceptors derived from mouse retina, with the long-term goal of analyzing their role in retinal degenerations. 2. In this study we describe initial studies using cells derived from the retinas of normal mice. Cultures of retinal neurons and photoreceptors, which were free of glial, epithelia, or endothelial cells, were labeled with 3H-glucosamine and 35SO4. Proteoglycans isolated from the medium and cell layer were analyzed on the basis of charge, relative hydrodynamic size, and glycosaminoglycan content. 3. The studies indicate that the cultures actively synthesize proteoglycans. The medium contained predominantly chondroitin sulfate/dermatan sulfate, while the cell layer had a higher proportion of heparan sulfate, indicating a differential distribution between the two compartments.     

Cyclic GMP: control of concentration by light in retinal photoreceptors.

The cyclic GMP concentration of dark-adapted rod outer segments (ROS) is approximately 30 pmoles/mg protein and only 3 pmoles/mg protein in ROS light-adapted in vivo or in vitro. The cyclic AMP concentration of dark-adapted ROS is about 2 pmoles/mg protein with no difference observed upon bleaching. A role for cyclic GMP in the visual process may thus be indicated.