Extraocular photoreception of the pineal gland of the aquatic turtlePseudemys scripta elegans

Colored visual stimuli evoke spectrally tuned neuronal responses across the central nervous system of zebrafish larvae

BACKGROUND

Visually guided behaviors such as optomotor and optokinetic responses, phototaxis, and prey capture are crucial for survival in zebrafish and become apparent after just a few days of development. Color vision, which in zebrafish is based on a spatially anisotropic tetrachromatic retina, provides an additional important component of world representation driving fundamental larval behaviors. However, little is known about the central nervous system (CNS) circuitry underlying color vision processing downstream of the retina, and its activity correlates with behavior. Here, we used the transparent larva of zebrafish to image CNS neurons and their activity in response to colored visual stimuli.

RESULTS

To investigate the processing of chromatic information in the zebrafish larva brain, we mapped with cellular resolution, spectrally responsive neurons in the larva encephalon and spinal cord. We employed the genetically encoded calcium indicator GCaMP6s and two-photon microscopy to image the neuronal activity while performing visual stimulation with spectrally distinct stimuli at wavelengths matching the absorption peaks of the four zebrafish cone types. We observed the presence of a high number of wavelength-selective neurons not only in the optic tectum, but also in all other regions of the CNS, demonstrating that the circuitry involved in processing spectral information and producing color-selective responses extends to the whole CNS.

CONCLUSIONS

Our measurements provide a map of neurons involved in color-driven responses, revealing that spectral information spreads in all regions of the CNS. This suggests the underlying complexity of the circuits involved and opens the way to their detailed future investigation.

Functional heterogeneity in the pineal projection neurons of zebrafish

The zebrafish pineal organ is a photoreceptive structure containing two main neuronal populations (photoreceptors and projections neurons). Here we describe a subpopulation of projection neurons that expresses the melanopsin gene, opn4xa. This new pineal cell type, that displays characteristics of both projection neurons and photoreceptors, share a similar dependency for BMP and Notch signalling pathways with classical non-photosensitive projection neurons (PN). Functionally, however, whereas classical, opn4xa-negative PNs display an achromatic LIGHT OFF response, the novel cell type we describe exhibit a LIGHT ON character that is elicited by green and blue light. Taken together, our data suggest a previously unanticipated heterogeneity in the projection neuron population in the zebrafish pineal organ raising the question of the importance of these differences in pineal function.

The Only Known Jawed Vertebrate with Four Eyes and the Bauplan of the Pineal Complex

The pineal and parapineal organs are dorsal outpocketings of the vertebrate diencephalon that play key roles in orientation and in circadian and annual cycles. Lampreys are four eyed in that both the pineal and parapineal form eyelike photosensory structures, but the pineal is the dominant or sole median photosensory structure in most lower vertebrate clades. The pineal complex has been thought to evolve in a single direction by losing photosensory and augmenting secretory function in the transitions from three-eyed lower vertebrates to two-eyed mammals and archosaurs [1-3]. Yet the widely accepted elaboration of the parapineal instead of the pineal as the primary median photosensory organ [4] in Lepidosauria (lizards, snakes, and tuataras) hints at a more complex evolutionary history. Here we present evidence that a fourth eye re-evolved from the pineal organ at least once within vertebrates, specifically in an extinct monitor lizard, Saniwa ensidens, in which pineal and parapineal eyes were present simultaneously. The tandem midline location of these structures confirms in a striking fashion the proposed homology of the parietal eye with the parapineal organ and refutes the classical model of pineal bilaterality. It furthermore raises questions about the evolution and functional interpretation of the median photosensory organ in other tetrapod clades.

Functional characterisation of the chromatically antagonistic photosensitive mechanism of erythrophores in the tilapia Oreochromis niloticus

Non-visual photoreceptors with diverse photopigments allow organisms to adapt to changing light conditions. Whereas visual photoreceptors are involved in image formation, non-visual photoreceptors mainly undertake various non-image-forming tasks. They form specialised photosensory systems that measure the quality and quantity of light and enable appropriate behavioural and physiological responses. Chromatophores are dermal non-visual photoreceptors directly exposed to light and they not only receive ambient photic input but also respond to it. These specialised photosensitive pigment cells enable animals to adjust body coloration to fit environments, and play an important role in mate choice, camouflage and ultraviolet (UV) protection. However, the signalling pathway underlying chromatophore photoresponses and the physiological importance of chromatophore colour change remain under-investigated. Here, we characterised the intrinsic photosensitive system of red chromatophores (erythrophores) in tilapia. Like some non-visual photoreceptors, tilapia erythrophores showed wavelength-dependent photoresponses in two spectral regions: aggregations of inner pigment granules under UV and short-wavelengths and dispersions under middle- and long-wavelengths. The action spectra curve suggested that two primary photopigments exert opposite effects on these light-driven processes: SWS1 (short-wavelength sensitive 1) for aggregations and RH2b (rhodopsin-like) for dispersions. Both western blot and immunohistochemistry showed SWS1 expression in integumentary tissues and erythrophores. The membrane potential of erythrophores depolarised under UV illumination, suggesting that changes in membrane potential are required for photoresponses. These results suggest that SWS1 and RH2b play key roles in mediating intrinsic erythrophore photoresponses in different spectral ranges and this chromatically dependent antagonistic photosensitive mechanism may provide an advantage to detect subtle environmental photic change.

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'.