Non-ocular dermal photoreception in the pond snail Lymnaea stagnalis

The expression of opsins in the human skin and its implications for photobiomodulation: A Systematic Review

BACKGROUND

Skin is the organ most extensively exposed to light of a broad range of wavelengths. Several studies have reported that skin expresses photoreceptive molecules called opsins. However, the identity and functional role of opsins in the human skin remain elusive. We aim to summarize current scientific evidence on the types of opsins expressed in the skin and their biological functions.

METHODS

A primary literature search was conducted using PubMed to identify articles on dermal opsins found in nonhuman animals and humans.

RESULTS

Twenty-two articles, representing, however, a non-exhaustive selection of the scientific papers published in this specific field, met the inclusion criteria. In nonhuman animals, opsins and opsin-like structures have been detected in the skin of fruit fly, zebrafish, frog, octopus, sea urchin, hogfish, and mouse, and they mediate skin color change, light avoidance, shadow reflex, and circadian photoentrainment. In humans, opsins are present in various skin cell types, including keratinocytes, melanocytes, dermal fibroblasts, and hair follicle cells. They have been shown to mediate wound healing, melanogenesis, hair growth, and skin photoaging.

CONCLUSION

Dermal opsins have been identified across many nonhuman animals and humans. Current evidence suggests that opsins have biological significance beyond light reception. In nonhuman animals, opsins are involved in behaviors that are critical for survival. In humans, opsins are involved in various functions of the skin although the underlying molecular mechanisms remain unclear. Future investigation on elucidating the mechanism of dermal opsins will be crucial to expand the therapeutic benefits of photobiomodulation for various skin disorders.

Adaptive colour change and background choice behaviour in peppered moth caterpillars is mediated by extraocular photoreception

Light sensing by tissues distinct from the eye occurs in diverse animal groups, enabling circadian control and phototactic behaviour. Extraocular photoreceptors may also facilitate rapid colour change in cephalopods and lizards, but little is known about the sensory system that mediates slow colour change in arthropods. We previously reported that slow colour change in twig-mimicking caterpillars of the peppered moth (Biston betularia) is a response to achromatic and chromatic visual cues. Here we show that the perception of these cues, and the resulting phenotypic responses, does not require ocular vision. Caterpillars with completely obscured ocelli remained capable of enhancing their crypsis by changing colour and choosing to rest on colour-matching twigs. A suite of visual genes, expressed across the larval integument, likely plays a key role in the mechanism. To our knowledge, this is the first evidence that extraocular colour sensing can mediate pigment-based colour change and behaviour in an arthropod.

Light avoidance by a non-ocular photosensing system in the terrestrial slug Limax valentianus

Although the eye is the best-studied photoreceptive organ in animals, the presence of non-ocular photosensing systems has been reported in numerous animal species. However, most of the roles that non-ocular photosensory systems play remain elusive. We found that the terrestrial slug Limax valentianus avoids light and escapes into dark areas even if it is blinded by the removal of the bilateral superior tentacle. The escape behaviour was more evident for short-wavelength light. Illumination to the head with blue but not red light elicited avoidance behaviour in the blinded slugs. Illumination to the tail was ineffective. The light-avoidance behaviour of the blinded slugs was not affected by the removal of the penis, which lies on the brain in the head, suggesting that the penis is dispensable for sensing light in the blinded slug. mRNA of Opn5A, xenopsin, retinochrome and, to a lesser extent, rhodopsin was expressed in the brain according to RT-PCR. Light-evoked neural responses were recorded from the left cerebro-pleural connective of the isolated suboesophageal ganglia of the brain, revealing that the brain is sensitive to short wavelengths of light (400-480 nm). This result is largely consistent with the wavelength dependency of the light-avoidance behaviour of the blinded slugs that we observed in the present study. Our results strongly support that the terrestrial slug L. valentianus detects and avoids light by using its brain as a light-sensing organ in the absence of eyes.

A Comparative Study of Cell Specific Effects of Systemic and Volatile Anesthetics on Identified Motor Neurons and Interneurons of Lymnaea stagnalis (L.), Both in the Isolated Brain and in Single Cell Culture

1. A comparative descriptive analysis of systemic (sodium pentobarbital, sodium thiopentone, ketamine) and volatile (halothane, isoflurane, enflurane) general anesthetics revealed important differences in the neuronal responses of identified motor neurons and interneurons in the isolated central nervous system (CNS) and cultured identified neurons in single cell culture of Lymnaea stagnalis (L.). 2. At high enough concentrations all anesthetics eventually caused cessation of spontaneous or evoked action potentials, but volatile anesthetics were much faster acting. Halothane at low concentrations caused excitation, thought to be equivalent to the early excitatory phase of anesthesia. Strong synaptic inputs were not always abolished by pentobarbital. 3. There were cell specific concentration-dependent responses to halothane and pentobarbital in terms of membrane potential, action potential characteristics, the after hyperpolarization and patterned activity. Individual neurons generated specific responses to the applied anesthetics. 4. The inhalation anesthetics, enflurane, and isoflurane, showed little concentration dependence of effect, in contrast to results obtained with halothane. Enflurane was faster acting than halothane and isoflurane was particularly different, producing quiescence in all cells types studied at all concentrations studied. 5. Halothane, enflurane, the barbiturate general anesthetics, pentobarbital, and sodium thiopentone and the dissociative anesthetic ketamine, produced two distinctly different effects which could be correlated with cell type and their location in the isolated brain: either a decline in spontaneous and evoked activity prior to quiescence in interneurons or paroxysmal depolarizing shifts (PDS) in motor neurons, again prior to quiescence, which were reversed when the anesthetic was eliminated from the bath. In the strongly electrically coupled motor neurons, VD1 and RPD2, both types of response were observed, depending on the anesthetic used. Thus, with the exception isoflurane, all the motor neurons subjected to the anesthetic agents studied here were capable of generating PDS in situ, but the interneurons did not do so. 6. The effects of halothane on isolated cultured neurons indicates that PDS can be generated by single identified neurons in the absence of synaptic inputs. Further, many instances of PDS in neurons that do not generate it in situ have been found in cultured neurons. The nature of PDS is discussed.

Olfactory navigation in aquatic gastropods

Gastropod diversity is substantial in marine and freshwater habitats, and many aquatic slugs and snails use olfactory cues to guide their navigation behaviour. Examples include finding prey or avoiding predators based on kairomones, or finding potential mates using pheromones. Here, I review the diversity of navigational behaviours studied across the major aquatic taxa of gastropods. I then synthesize evidence for the different theoretical navigation strategies the animals may use. It is likely that gastropods regularly use either chemotaxis or odour-gated rheotaxis (or both) during olfactory-based navigation. Finally, I collate the patchwork of research conducted on relevant proximate mechanisms that could produce navigation behaviours. Although the tractability of several gastropod species for neurophysiological experimentation has generated some valuable insight into how turning behaviour is triggered by contact chemoreception, there remain many substantial gaps in our understanding for how navigation relative to more distant odour sources is controlled in gastropods. These gaps include little information on the chemoreceptors and mechanoreceptors (for detecting flow) found in the peripheral nervous system and the central (or peripheral) processing circuits that integrate that sensory input. In contrast, past studies do provide information on motor neurons that control the effectors that produce crawling (both forward locomotion and turning). Thus, there is plenty of scope for further research on olfactory-based navigation, exploiting the tractability of gastropods for neuroethology to better understand how the nervous system processes chemosensory input to generate movement towards or away from distant odour sources.

Three-dimensional reconstruction of the axon extending from the dermal photoreceptor cell in the extraocular photoreception system of a marine gastropod, onchidium

The marine gastropod Onchidium has a multiple photoreceptive system consisting of stalk eyes, dorsal eyes, photosensitive neurons, and extraocular dermal photoreceptor cells (DPCs). The DPCs were widespread all over the dorsal mantle and distributed singly or in groups in the dermis, but were not discernible by the naked eye. The DPC was oval in shape and large in size, and characterized by features specific to gastropod photoreceptor cells such as massive microvilli, photic vesicles, and a depolarized response. DPC-17, one of a group of 19 DPCs, was examined on serial semi-thin sections of 0.4 µm in thickness with a high-voltage transmission electron microscope (HVTEM). The axon emerged specifically from the lateral side between the distal microvillous portion and proximal cytoplasm, travelled through the connective tissue, and joined a small nerve bundle (NB). Two types of supportive cells were found along the length of the axon. The first type was a covering cell (CC) surrounding the surface of the DPC body and continuing onward to the axon sheath. DPC-17 was covered by 11 CCs, while the larger DPC-6 was only covered by four CCs. The second type was a sheath cell (ShC) wrapping the surface of the small NB where the axon of the DPC merged with undefined nerve fibers. The axon extending directly from DPC-17 was reconstructed three-dimensionally (3D) using DeltaViewer software. The 3D-reconstructed image of the sheath of the axon and the CC demonstrated the continuity between the two structures, especially when the image was rotated using DeltaViewer.

Morphological and physiological characteristics of dermal photoreceptors in Lymnaea stagnalis

Dermal photoreceptors located in the mantle of Lymnaea stagnalis were histologically and physiologically characterized. Our previous study demonstrated that the shadow response from dermal photoreceptors induces the whole-body withdrawal response. Through the interneuron, RPeD11, we detected that the light-off response indirectly originated from a dermal photoreceptor. Previous observations, based on behavioral pharmacology, revealed that cyclic guanosine monophosphate acts as a second messenger in the dermal photoreceptor. Furthermore, gastropods possess dermal photoreceptors containing rhodopsin, as a photopigment, and another photo-sensitive protein, arrestin, responsible for terminating the light response. Thus, we chose three antibodies, anti-cGMP, anti-rhodopsin, and anti-β-arrestin, to identify the dermal photoreceptor molecules in Lymnaea mantle. Extracellular recording, using a suction electrode on the mantle, revealed a light off-response from the right parietal nerve. Overlapping structures, positive against each of the antibodies, were also observed. Numerous round, granular particles of 3-47 μm in diameter with one nucleus were distributed around pneumostome and/or inside the mantle. The cells surrounding the pneumostome area, located 10 μm beneath the surface, tended to have smaller cell soma ranging from 3 to 25 μm in diameter, while cells located in other areas were distributed uniformly inside the mantle, with a larger diameter ranging from 12 to 47 μm. The histological examination using back-filing Lucifer Yellow staining of the right parietal nerve with the three dermal photoreceptor antibodies confirmed that these overlapping-stained structures were dermal photoreceptors in Lymnaea.

Understanding the dermal light sense in the context of integrative photoreceptor cell biology

While the concept of a dermal light sense has existed for over a century, little progress has been made in our understanding of the mechanisms underlying dispersed photoreception and the evolutionary histories of dispersed photoreceptor cells. These cells historically have been difficult to locate and positively identify, but modern molecular techniques, integrated with existing behavioral, morphological, and physiological data, will make cell identification easier and allow us to address questions of mechanism and evolution. With this in mind, we propose a new classification scheme for all photoreceptor cell types based on two axes, cell distribution (aggregated vs. dispersed) and position within neural networks (first order vs. high order). All photoreceptor cells fall within one of four quadrants created by these axes: aggregated/high order, dispersed/high order, aggregated/first order, or dispersed/first order. This new method of organization will help researchers make objective comparisons between different photoreceptor cell types. Using integrative data from four major phyla (Mollusca, Cnidaria, Echinodermata, and Arthropoda), we also provide evidence for three hypotheses for dispersed photoreceptor cell function and evolution. First, aside from echinoderms, we find that animals often use dispersed photoreceptor cells for tasks that do not require spatial vision. Second, although there are both echinoderm and arthropod exceptions, we find that dispersed photoreceptor cells generally lack morphological specializations that either enhance light gathering or aid in the collection of directional information about light. Third, we find that dispersed photoreceptor cells have evolved a number of times in Metazoa and that most dispersed photoreceptor cells have likely evolved through the co-option of existing phototransduction cascades. Our new classification scheme, combined with modern investigative techniques, will help us address these hypotheses in great detail and generate new hypothesis regarding the function and evolution of dispersed photoreceptor cells.

Behavioural evidence for a sleep-like quiescent state in a pulmonate mollusc, Lymnaea stagnalis (Linnaeus)

The objective of this study was to determine whether the great pond snail, Lymnaea stagnalis, expresses a sleep-like behavioural state. We found that snails spontaneously enter a relatively brief (22±1 min) quiescent state characterized by postural relaxation of the foot, mantle and tentacles, and cessation of radula rasping. Quiescence was reversed ('aroused') by appetitive (sucrose solution) and aversive (tactile) stimuli. Responsiveness to both stimuli was significantly lower in quiescent snails than in active snails. However, tactile stimuli evoked a more sustained defensive response in quiescent snails. Quiescence bouts were consolidated into 'clusters' over an infradian timescale and were only weakly affected by time of day. Clusters contained 7±0.5 bouts, lasted 13±1 h and were separated by long (37±4 h) intervals of almost continuous activity. Analysis of Kaplan-Meier survival curves revealed that the quiescent bout duration was described by an exponential probability distribution (time constant 15±1 min). Active bout duration was described by a bi-exponential probability distribution (time constants 62±4 and 592±48 min). We found no evidence for a 'sleep rebound' mechanism and quiescence expression appeared to be regulated through stochastic processes causing state transitions to resemble a Markovian random walk. We conclude that Lymnaea is a potentially valuable model system for studies of cellular function in sleep.

The shadow-induced withdrawal response, dermal photoreceptors, and their input to the higher-order interneuron RPeD11 in the pond snailLymnaea stagnalis

The shadow-induced withdrawal response in Lymnaea stagnalis is mediated by dermal photoreceptors located on the foot, mantle cavity, and skin around the pneumostome area. Here, we determined whether we could obtain a neural correlate of the withdrawal response elicited by a shadow in a higher-order central neuron that mediates withdrawal behavior. We measured the electrophysiological properties of the higher-order interneuron Right Pedal Dorsal 11 (RPeD11), which has a major role in Lymnaea withdrawal behavior. In semi-intact preparations comprising the circumesophageal ganglia, the mantle cavity and the pneumostome, but not the foot and eyes, a light-on stimulus elicited a small short-lasting hyperpolarization and a light-off stimulus elicited a depolarization of RPeD11. We also determined that dermal photoreceptors make a monosynaptic contact with RPeD11. The dermal photoreceptor afferents course to the circumesophageal ganglia via the anal and genital nerves to the visceral ganglion, and/or via the right internal and external parietal nerves to the parietal ganglion. Finally, in addition to responding to photic stimuli, RPeD11 responds to both mechanical and chemical stimuli delivered to the pneumostome.

Cyclic nucleotide-gated channels are involved in phototransduction of dermal photoreceptors in Lymnaea stagnalis

Dermal photoreceptors in the pond snail Lymnaea stagnalis mediate the whole-body withdrawal response, including pneumostome closure, elicited by a shadow passing over the pneumostome area. The pneumostome closure response is part of the defense reaction in Lymnaea. The shadow or 'light-off' stimulus elicits activity in a higher order interneuron, RPeD11, which has a major role in mediating defensive withdrawal behavior elicited by noxious or threatening stimuli. Here, we tested our hypothesis that cyclic nucleotide-gated (CNG) channels are involved in the dermal photoreceptor-mediated transduction of the shadow stimulus. The response to the shadow stimulus recorded in RPeD11 was abolished by 500 μM cis-diltiazem, which blocks cGMP-activated conductance of CNG channels. On the other hand, the shadow response elicited in RPeD11 was not blocked by 2-amino ethyldiphenyl borate (2-APB), a transient receptor potential (TRP) channel blocker. Consistent with the electrophysiologic data, cis-diltiazem blocked the shadow-evoked withdrawal response, whereas 2-APB did not block the withdrawal response evoked by the shadow stimulus in intact freely behaving Lymnaea. Together, these findings support the hypothesis that the second messenger in dermal photoreceptors involves CNG and not TRP channels.

Giant siliceous spicules from the deep-sea glass sponge Monorhaphis chuni

Only 13 years after realizing, during a repair of a telegraph cable pulled out from the deep sea, that the depth of the ocean is plentifully populated with a highly diverse fauna and flora, the Challenger expedition (1873-1876) treasured up a rich collection of vitreous sponges (Hexactinellida). They had been described by Schulze and represent the phylogenetically oldest class of siliceous sponges (phylum Porifera); they are eye-catching because of their distinct body plan, which relies on a filigree skeleton. It is constructed by an array of morphologically determined elements, the spicules. Soon after, during the German Deep Sea Expedition "Valdivia" (1898-1899), Schulze could describe the largest siliceous hexactinellid sponge on Earth, the up to 3-m high Monorhaphis chuni, which develops the equally largest bio-silica structure, the giant basal spicules (3 mx10 mm). Using these spicules as a model, basic knowledge on the morphology, formation, and development of the skeletal elements could be achieved. They are formed by a proteinaceous scaffold (composed of a 27-kDa protein), which mediates the formation of the siliceous lamellae, into which the proteins are encased. The high number of 800 of 5-10 microm thick lamellae is concentrically arranged around the axial canal. The silica matrix is composed of almost pure silicon oxide, providing it with unusually optophysical properties, which are superior to those of man-made waveguides. Experiments might suggest that the spicules function in vivo as a nonocular photoreception system. In addition, the spicules have exceptional mechanical properties, combining mechanical stability with strength and stiffness. Like demosponges, also the hexactinellids synthesize their silica enzymatically, via the enzyme silicatein (27-kDa protein). It is suggested that these basic insights will surely contribute to a further applied utilization and exploration of silica in bio-material/biomedical science.

Upside-down gliding of Lymnaea

The pond snail Lymnaea stagnalis can often be observed moving upside down on its back just below the surface of the water. We have termed this form of movement "upside-down gliding." To elucidate the mechanism of this locomotion, we performed a series of experiments involving behavioral analyses and microscopic observations. These experiments were designed (1) to measure the speed of this locomotion; (2) to determine whether the mucus secreted from the foot of Lymnaea repels water, thereby allowing the snail to exploit the surface tension of the water for upside-down gliding; and (3) to observe the beating of foot cilia in this behavior. The beating of these cilia is thought to be the primary driving force for upside-down gliding. Our results demonstrate that upside-down gliding is an efficient active process involving the secretion of mucus that floats up to the water surface to serve as a substrate upon which cilia beat to cause locomotion at the underside of the water surface.

Comparative study of visuo-vestibular conditioning in Lymnaea stagnalis

In this review, we compare the current understanding of visuo-vestibular conditioning in Hermissenda crassicornis and Lymnaea stagnalis on the basis of behavioral, electrophysiologic, and morphologic studies. Paired presentation of a photic conditioned stimulus (CS) and an orbital rotation unconditioned stimulus (US) results in conditioned escape behavior in both species. In Hermissenda, changes in excitability of type B photoreceptors and morphologic modifications at the axon terminals follow conditioning. Caudal hair cells, which detect mechanical turbulence, have reciprocal inhibition with type B photoreceptors. In Lymnaea, the interaction between photoreceptors and hair cells is dependent on statocyst location. Furthermore, the organization of the Lymnaea eye is complex, with more than 100 photoreceptors distributed in a uniquely folded retina. Although the optimal conditions to produce long-term memory (memory persistent for >1 week) are almost identical in Hermissenda and Lymnaea, physiologic and morphologic differences suggest that the neuronal mechanisms underlying learning and memory are distinct.

Electrophysiological Responses to Light of Neurons in the Eye and Statocyst of Lymnaea stagnalis

Lymnaea can be classically conditioned by pairing photic stimulation with a rotational stimulus. The electrophysiological properties of the Lymnaea photoreceptors and statocyst neurons are incompletely known. There are 2 types of ocular photoreceptors and 3 types of statocyst “hair cells.” Type A photoreceptors had a response latency from 200 to 400 ms, with a graded depolarizing response having maximum action spectra at 480–500 nm, corresponding to the βmax of rhodopsin. Additionally they extend their axons in the direction of the other type of photoreceptor neuron, the type T cell. These neurons have a 2-component response to light: a response reversibly reduced in Ca2+-free saline, and a component persisting in Ca2+-free saline. Type T cells send processes into the cerebral ganglion and terminate close to the ending of the statocyst hair cells. Hair cells send their terminal branches to the cerebral ganglia close to the terminations of the type T cells. Caudal hair cells respond to a light flash with a depolarization, whereas the rostral cells respond with a hyperpolarization. The response latency in all hair cells was dependent on the stimulus intensity; the brightest light tested had a latency of 200 ms. The photo-induced response was abolished in Ca2+-free saline, whereas it was still present in high Ca2+–high Mg2+ saline, consistent with the hypothesis that the connection between the photoreceptors and hair cells is monosynaptic. Thus the sensory information necessary for forming an association between photic and rotational stimuli converges on the statocyst neurons.

The early snail acquires the learning. Comparison of scores for conditioned taste aversion between morning and afternoon

The pond snail Lymnaea stagnalis acquires conditioned taste aversion (CTA) and maintains its memory for more than a month. Snails in our laboratory were cultured at 20 degrees C on a 12:12 light-dark cycle (light from 7 am to 7 pm). To examine the hours during which snails acquire CTA effectively, we trained some snails in the morning and others in the afternoon, and then compared their scores. CTA developed in both cases, but scores were significantly better in the morning than in the afternoon. To elucidate the cause of this difference in scores, we observed the voluntary activity of snails and found the circadian rhythm reflected in the snails' free-movement distances; distances at the circadian time 0-12 (daytime) were significantly longer than those at the circadian time 12-24 (nighttime). This rhythm was kept up for at least 3 days, even in constant darkness. In conclusion, L. stagnalis should be trained in the morning to acquire associative learning, possibly because of its greater propensity to roam about at that time as opposed to the afternoon.

CREB in the pond snail Lymnaea stagnalis: cloning, gene expression, and function in identifiable neurons of the central nervous system

The pond snail Lymnaea stagnalis is an excellent model system in which to study the neuronal and molecular substrates of associative learning and its consolidation into long-term memory. Until now, the presence of cyclic AMP (cAMP)-responsive element binding protein (CREB), which is believed to be a necessary component in the process of a learned behavior that is consolidated into long-term memory, has only been assumed in Lymnaea neurons. We therefore cloned and analyzed the cDNA sequences of homologues of CREB1 and CREB2 and determined the presence of these mRNAs in identifiable neurons of the central nervous system (CNS) of L. stagnalis. The deduced amino acid sequence of Lymnaea CREB1 is homologous to transcriptional activators, mammalian CREB1 and Aplysia CREB1a, in the C-terminal DNA binding (bZIP) and phosphorylation domains, whereas the deduced amino acid sequence of Lymnaea CREB2 is homologous to transcriptional repressors, human CREB2, mouse activating transcription factor-4, and Aplysia CREB2 in the bZIP domain. In situ hybridization revealed that only a relatively few neurons showed strongly positive signals for Lymnaea CREB1 mRNA, whereas all the neurons in the CNS contained Lymnaea CREB2 mRNA. Using one of the neurons (the cerebral giant cell) containing Lymnaea CREB1 mRNA, we showed that the injection of a CRE oligonucleotide inhibited a cAMP-induced, long-lasting synaptic plasticity. We therefore conclude that CREBs are present in Lymnaea neurons and may function as necessary players in behavioral plasticity.