Comparative ultrastructure and opsin immunocytochemistry of the retina and pineal organ in fish

Transducin duplicates in the zebrafish retina and pineal complex: differential specialisation after the teleost tetraploidisation

Gene duplications provide raw materials that can be selected for functional adaptations by evolutionary mechanisms. We describe here the results of 350 million years of evolution of three functionally related gene families: the alpha, beta and gamma subunits of transducins, the G protein involved in vision. Early vertebrate tetraploidisations resulted in separate transducin heterotrimers: gnat1/gnb1/gngt1 for rods, and gnat2/gnb3/gngt2 for cones. The teleost-specific tetraploidisation generated additional duplicates for gnb1, gnb3 and gngt2. We report here that the duplicates have undergone several types of subfunctionalisation or neofunctionalisation in the zebrafish. We have found that gnb1a and gnb1b are co-expressed at different levels in rods; gnb3a and gnb3b have undergone compartmentalisation restricting gnb3b to the dorsal and medial retina, however, gnb3a expression was detected only at very low levels in both larvae and adult retina; gngt2b expression is restricted to the dorsal and medial retina, whereas gngt2a is expressed ventrally. This dorsoventral distinction could be an adaptation to protect the lower part of the retina from intense light damage. The ontogenetic analysis shows earlier onset of expression in the pineal complex than in the retina, in accordance with its earlier maturation. Additionally, gnb1a but not gnb1b is expressed in the pineal complex, and gnb3b and gngt2b are transiently expressed in the pineal during ontogeny, thus showing partial temporal subfunctionalisation. These retina-pineal distinctions presumably reflect their distinct functional roles in vision and circadian rhythmicity. In summary, this study describes several functional differences between transducin gene duplicates resulting from the teleost-specific tetraploidisation.

Pineal organs of deep-sea fish: photopigments and structure

We have examined the morphology and photopigments of the pineal organs from a number of mesopelagic fish, including representatives of the hatchet fish (Sternoptychidae), scaly dragon-fish (Chauliodontidae) and bristlemouths (Gonostomidae). Although these fish were caught at depths of between 500 and 1000 m, the morphological organisation of their pineal organs is remarkably similar to that of surface-dwelling fish. Photoreceptor inner and outer segments protrude into the lumen of the pineal vesicle, and the outer segment is composed of a stack of up to 20 curved disks that form a cap-like cover over the inner segment. In all species, the pineal photopigment was spectrally distinct from the retinal rod pigment, with lambdamax displaced to longer wavelengths, between approximately 485 and 503 nm. We also investigated the pineal organ of the deep demersal eel, Synaphobranchus kaupi, caught at depths below 2000 m, which possesses a rod visual pigment with lambdamax at 478 nm, but the pineal pigment has lambdamax at approximately 515 nm. In one species of hatchet fish, Argyropelecus affinis, two spectral classes of pinealocyte were identified, both spectrally distinct from the retinal rod photopigment.

Molecular identification and developmental expression of UV and green opsin mRNAs in the pineal organ of the Atlantic halibut

The pineal organ is the only differentiated photoreceptor organ present in embryos and early larvae of the Atlantic halibut (Hippoglossus hippoglossus). We investigated the molecular identity of opsins in the pineal organ, and their expression during different life stages. Using RT-PCR we identified two 681-bp gene sequences, named HPO1 and HPO4, in cDNA from adult pineal and whole embryos. The predicted amino acid sequences showed highest identity to the transmembrane regions of teleostean RH2 green cone opsins (HPO1, 72-91%) and SWS-1 UV cone opsins (HPO4, 71-83%). In situ hybridization revealed expression of HPO1 and HPO4 mRNA transcripts in photoreceptors in the pineal organ of embryos, larvae and adults. HPO1 and HPO4 mRNA transcripts were also expressed in the larval retina. Our study provides molecular evidence for short and middle wavelength light sensitive photoreceptors in the pineal organ of Atlantic halibut throughout life, and suggests that pineal photoreception may play an important role during embryonic and larval life stages, especially at the time when the retina does not possesses corresponding photoreceptor capacity.

Immunocytochemistry and calcium cytochemistry of the mammalian pineal organ: A comparison with retina and submammalian pineal organs

Morphologically the mammalian pineal organ is a part of the diencephalon. It represents a neural tissue histologically ("pineal nervous tissue") and is dissimilar to endocrine glands. Submammalian pinealocytes resemble the photoreceptor cells of the retina, and some of their cytologic characteristics are preserved in the mammalian pinealocytes together with compounds demonstrable by cyto- and immunocytochemistry and participating in photochemical transduction. In our opinion, the main trend of today's literature on pineal functions--only considering the organ as a common endocrine gland--deviates from this structural and histochemical basis. In mammals, similar to the lower vertebrates, the pinealocytes have a sensory cilium developed to a different extent. The axonic processes of pinealocytes form ribbon-containing synapses on secondary pineal neurons, and/or neurohormonal terminals on the basal lamina of the surface of the pineal nervous tissue facing the perivascular spaces. Ribbon-containing axo-dendritic synapses were found in the rat, cat, guinea pig, ferret, and hedgehog. In the cat, we found GABA-immunoreactive interneurons, while the secondary nerve cells, whose axons enter the habenular commissure, were GABA-immunonegative. GABA-immunogold-labeled axons run between pinealocytes and form axo-dendritic synapses on intrapineal neurons. There is a similarity between the light and electron microscopic localization of Ca ions in the mammalian and submammalian pineal organs and retina of various vertebrates. Calcium pyroantimonate deposits--showing the presence of Ca ions--were found in the outer segments of the pineal and retinal photoreceptors of the frog. In the rat and human pineal organ, calcium accumulated on the plasmalemma of pinealocytes and intercellularly among pinealocytes. The formation of pineal concrements in mammals may be connected to the high need for Ca exchange of the pinealocytes for their supposed receptor and effector functions.