Immunocytochemical localization of serotonin and photoreceptor-specific proteins (rod-opsin, S-antigen) in the pineal complex of the river lamprey, Lampetra japonica, with special reference to photoneuroendocrine cells

The stepwise development of the lamprey visual system and its evolutionary implications

Lampreys, which represent the oldest group of living vertebrates (cyclostomes), show unique eye development. The lamprey larva has only eyespot-like immature eyes beneath a non-transparent skin, whereas after metamorphosis, the adult has well-developed image-forming camera eyes. To establish a functional visual system, well-organised visual centres as well as motor components (e.g. trunk muscles for locomotion) and interactions between them are needed. Here we review the available knowledge concerning the structure, function and development of the different parts of the lamprey visual system. The lamprey exhibits stepwise development of the visual system during its life cycle. In prolarvae and early larvae, the 'primary' retina does not have horizontal and amacrine cells, but does have photoreceptors, bipolar cells and ganglion cells. At this stage, the optic nerve projects mostly to the pretectum, where the dendrites of neurons in the nucleus of the medial longitudinal fasciculus (nMLF) appear to receive direct visual information and send motor outputs to the neck and trunk muscles. This simple neural circuit may generate negative phototaxis. Through the larval period, the lateral region of the retina grows again to form the 'secondary' retina and the topographic retinotectal projection of the optic nerve is formed, and at the same time, the extra-ocular muscles progressively develop. During metamorphosis, horizontal and amacrine cells differentiate for the first time, and the optic tectum expands and becomes laminated. The adult lamprey then has a sophisticated visual system for image-forming and visual decision-making. In the adult lamprey, the thalamic pathway (retina-thalamus-cortex/pallium) also transmits visual stimuli. Because the primary, simple light-detecting circuit in larval lamprey shares functional and developmental similarities with that of protochordates (amphioxus and tunicates), the visual development of the lamprey provides information regarding the evolutionary transition of the vertebrate visual system from the protochordate-type to the vertebrate-type.

β-arrestin functionally regulates the non-bleaching pigment parapinopsin in lamprey pineal

The light response of vertebrate visual cells is achieved by light-sensing proteins such as opsin-based pigments as well as signal transduction proteins, including visual arrestin. Previous studies have indicated that the pineal pigment parapinopsin has evolutionally and physiologically important characteristics. Parapinopsin is phylogenetically related to vertebrate visual pigments. However, unlike the photoproduct of the visual pigment rhodopsin, which is unstable, dissociating from its chromophore and bleaching, the parapinopsin photoproduct is stable and does not release its chromophore. Here, we investigated arrestin, which regulates parapinopsin signaling, in the lamprey pineal organ, where parapinopsin and rhodopsin are localized to distinct photoreceptor cells. We found that beta-arrestin, which binds to stimulated G protein-coupled receptors (GPCRs) other than opsin-based pigments, was localized to parapinopsin-containing cells. This result stands in contrast to the localization of visual arrestin in rhodopsin-containing cells. Beta-arrestin bound to cultured cell membranes containing parapinopsin light-dependently and translocated to the outer segments of pineal parapinopsin-containing cells, suggesting that beta-arrestin binds to parapinopsin to arrest parapinopsin signaling. Interestingly, beta-arrestin colocalized with parapinopsin in the granules of the parapinopsin-expressing cell bodies under light illumination. Because beta-arrestin, which is a mediator of clathrin-mediated GPCR internalization, also served as a mediator of parapinopsin internalization in cultured cells, these results suggest that the granules were generated light-dependently by beta-arrestin-mediated internalization of parapinopsins from the outer segments. Therefore, our findings imply that beta-arrestin-mediated internalization is responsible for eliminating the stable photoproduct and restoring cell conditions to the original dark state. Taken together with a previous finding that the bleaching pigment evolved from a non-bleaching pigment, vertebrate visual arrestin may have evolved from a “beta-like” arrestin by losing its clathrin-binding domain and its function as an internalization mediator. Such changes would have followed the evolution of vertebrate visual pigments, which generate unstable photoproducts that independently decay by chromophore dissociation.

Immunohistochemical characterization of a parapinopsin-containing photoreceptor cell involved in the ultraviolet/green discrimination in the pineal organ of the river lamprey Lethenteron japonicum

In the pineal organ, two types of ganglion cell exhibit antagonistic chromatic responses to UV and green light, and achromatic responses to visible light. In this study, we histologically characterized UV-sensitive photoreceptor cells that contain a unique non-visual UV pigment, lamprey parapinopsin, in order to elucidate the neural network that is associated with antagonistic chromatic responses. These characteristics were compared with those of lamprey rhodopsin-containing cells, most of which are involved in achromatic responses. RT-PCR analysis revealed that lamprey parapinopsin was expressed in the pineal organ but not in the retina, unlike lamprey rhodopsin, which was expressed in both. Lamprey parapinopsin and lamprey rhodopsin were immunohistochemically localized in the dorsal and ventral regions of the pineal organ, respectively. The two pigments were localized in distinct photoreceptor cells throughout the pineal organ, namely the dorsal and ventral regions as well as the peripheral region, which corresponds to the dorso-ventral border region. The ratio of the number of lamprey parapinopsin-containing cells to lamprey rhodopsin-containing cells around the peripheral region was higher than in the central region. Electron-microscopic analysis revealed that lamprey parapinopsin-containing dorsal cells have outer segments and synaptic ribbons similar to those of ventral photoreceptor cells. However, unlike lamprey rhodopsin-containing cells, lamprey parapinopsin-containing cells connected with each other in a wide area of dorsal and peripheral portions and made direct contact with ganglion cells, mainly in the peripheral portion. These results suggest that UV light information captured by lamprey parapinopsin-containing photoreceptor cells is converged and directly transmitted to chromatic-type ganglion cells in the peripheral region to generate antagonistic chromatic responses.

Development of the serotonergic system in the central nervous system of the sea lamprey

Lampreys belong to the most primitive extant group of vertebrates, the Agnathans, which is considered the sister group of jawed vertebrates. Accordingly, characterization of neuronal groups and their development appears useful for understanding early evolution of the nervous system in vertebrates. Here, the development of the serotonergic system in the central nervous system of the sea lamprey, Petromyzon marinus, was investigated by immunohistochemical analysis of specimens ranging from embryos to adults. The different serotonin-immunoreactive (5-HT-ir) neuronal populations that are found in adults were observed between the embryonic and metamorphic stages. The earliest serotonergic neurons were observed in the basal plate of the isthmus region of late embryos. In prolarvae, progressive appearance of new serotonergic cell groups was observed: firstly in the spinal cord, then in the pineal organ, tuberal region, zona limitans intrathalamica, rostral isthmus, and the caudal part of the rhombencephalon. In early larvae a new group of serotonergic cells was observed in the mammillary region, whereas in the pretectal region and the parapineal organ the first serotonergic cells were seen in the middle and late larval stages, respectively. The first serotonergic fibres appeared in early prolarvae, with fibres that ascend and descend from the isthmic cell group, and the number of immunoreactive fibres increased progressively until the adult stage. The results reveal strong resemblances between lampreys and other vertebrates in the spatio-temporal pattern of development of brainstem populations. This study also reveals a shared pattern of early ascending and descending serotonergic pathways in lampreys and jawed vertebrates.

Bistable UV pigment in the lamprey pineal

Lower vertebrates can detect UV light with the pineal complex independently of eyes. Electrophysiological studies, together with chromophore extraction analysis, have suggested that the underlying pigment in the lamprey pineal exhibits a bistable nature, that is, reversible photoreaction by UV and visible light, which is never achieved by known UV pigments. Here we addressed the molecular identification of the pineal UV receptor. Our results showed that the long-hypothesized pigment is a lamprey homologue of parapinopsin, which exhibits an absorption maximum at 370 nm, in the UV region. UV light causes cis-trans isomerization of its retinal(2) chromophore, forming a stable photoproduct having an absorption maximum at 515 nm, in the green region. The photoproduct reverts to the original pigment upon visible light absorption, showing photoregeneration of the pigment. In situ hybridization showed that parapinopsin is selectively expressed in the cells located in the dorsal region of the pineal organ. We successfully obtained the hyperpolarizing responses with a maximum sensitivity of approximately 380 nm from the photoreceptor cells at the dorsal region, in which the outer segment was clearly stained with anti-parapinopsin antibody. These results demonstrated that parapinopsin is the pineal UV pigment having photointerconvertible two stable states. The bistable nature of the parapinopsin can account for the photorecovery of the pineal UV sensitivity by background green light in the lamprey. Furthermore, we isolated the parapinopsin homologues from fish and frog pineal complexes that exhibit UV sensitivity, suggesting that parapinopsin is a common molecular basis for pineal UV reception in the vertebrate.

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'.

Early development of the retina and pineal complex in the sea lamprey: comparative immunocytochemical study

Lampreys have a complex life cycle, with largely differentiated larval and adult periods. Despite the considerable interest of lampreys for understanding vertebrate evolution, knowledge of the early development of their eye and pineal complex is very scarce. Here, the early immunocytochemical organization of the pineal complex and retina of the sea lamprey was studied by use of antibodies against proliferating cell nuclear antigen (PCNA), opsin, serotonin, and gamma-aminobutyric acid (GABA). Cell differentiation in the retina, pineal organ, and habenula begins in prolarvae, as shown by the appearance of PCNA-negative cells, whereas differentiation of the parapineal vesicle was delayed until the larval period. In medium-sized to large larvae, PCNA-immunoreactive (-ir) cells were numerous in regions of the lateral retina near the differentiated part of the larval retina (central retina). A late-proliferating region was observed in the right habenula. Opsin immunoreactivity appears in the pineal vesicle of early prolarvae and 3 or 4 days later in the retina. In the parapineal organ, opsin immunoreactivity was observed only in large larvae. In the pineal organ, serotonin immunoreactivity was first observed in late prolarvae in photoreceptive (photoneuroendocrine) cells, whereas only a few of these cells appeared in the parapineal organ of large larvae. No serotonin immunoreactivity was observed in the larval retina. GABA immunoreactivity appeared earlier in the retina than in the pineal complex. No GABA-ir perikaryon was observed in the retina of larval lampreys, although a few GABA-ir centrifugal fibers innervate the inner retina in late prolarvae. First GABA-ir ganglion cells occur in the pineal organ of 15-17 mm larvae, and their number increases during the larval period. The only GABA-ir structures observed in the parapineal ganglion of larvae were afferent fibers, which appeared rather late in development. The time sequence of development in these photoreceptive structures is rather different from that observed in teleosts and other vertebrates. This suggests that the unusual development of the three photoreceptive organs in lampreys reflects specialization for their different functions during the larval and adult periods.

WILSON STEPHEN. Asymmetry in the epithalamus of vertebrates

The epithalamus is a major subdivision of the diencephalon constituted by the habenular nuclei and pineal complex. Structural asymmetries in this region are widespread amongst vertebrates and involve differences in size. neuronal organisation, neurochemistry and connectivity. In species that possess a photoreceptive parapineal organ, this structure projects asymmetrically to the left habenula, and in teleosts it is also situated on the left side of the brain. Asymmetries in size between the left and right sides of the habenula are often associated with asymmetries in neuronal organisation, although these two types of asymmetry follow different evolutionary courses. While the former is more conspicuous in fishes (with the exception of teleosts), asymmetries in neuronal organisation are more robust in amphibia and reptiles. Connectivity of the parapineal organ with the left habenula is not always coupled with asymmetries in habenular size and/or neuronal organisation suggesting that, at least in some species, assignment of parapineal and habenular asymmetries may be independent events. The evolutionary origins of epithalamic structures are uncertain but asymmetry in this region is likely to have existed at the origin of the vertebrate, perhaps even the chordate, lineage. In at least some extant vertebrate species, epithalamic asymmetries are established early in development, suggesting a genetic regulation of asymmetry. In some cases, epigenetic factors such as hormones also influence the development of sexually dimorphic habenular asymmetries. Although the genetic and developmental mechanisms by which neuroanatomical asymmetries are established remain obscure, some clues regarding the mechanisms underlying laterality decisions have recently come from studies in zebrafish. The Nodal signalling pathway regulates laterality by biasing an otherwise stochastic laterality decision to the left side of the epithalamus. This genetic mechanism ensures a consistency of epithalamic laterality within the population. Between species, the laterality of asymmetry is variable and a clear evolutionary picture is missing. We propose that epithalamic structural asymmetries per se and not the laterality of these asymmetries are important for the behaviour of individuals within a species. A consistency of the laterality within a population may play a role in social behaviours between individuals of the species.

Cellular circadian clocks in the pineal

Daily rhythms are a fundamental feature of all living organisms; most are synchronized by the 24 hr light/dark (LD) cycle. In most species, these rhythms are generated by a circadian system, and free run under constant conditions with a period close to 24 hr. To function properly the system needs a pacemaker or clock, an entrainment pathway to the clock, and one or more output signals. In vertebrates, the pineal hormone melatonin is one of these signals which functions as an internal time-keeping molecule. Its production is high at night and low during day. Evidence indicates that each melatonin producing cell of the pineal constitutes a circadian system per se in non-mammalian vertebrates. In addition to the melatonin generating system, they contain the clock as well as the photoreceptive unit. This is despite the fact that these cells have been profoundly modified from fish to birds. Modifications include a regression of the photoreceptive capacities, and of the ability to transmit a nervous message to the brain. The ultimate stage of this evolutionary process leads to the definitive loss of both the direct photosensitivity and the clock, as observed in the pineal of mammals. This review focuses on the functional properties of the cellular circadian clocks of non-mammalian vertebrates. How functions the clock? How is the photoreceptive unit linked to it and how is the clock linked to its output signal? These questions are addressed in light of past and recent data obtained in vertebrates, as well as invertebrates and unicellulars.

Distribution of serotonin (5HT)-immunoreactive structures in the central nervous system of two chondrostean species (Acipenser baeri and Huso huso)

The distribution of serotonin-immunoreactive (5HT-ir) elements was studied in the brain and rostral spinal cord of two chondrosteans, Acipenser baeri and Huso huso, by using an antibody against serotonin. The distribution of these elements was similar in both sturgeon species. In the telencephalon, 5HT-ir cells were found in the olfactory bulb and in the medioventral wall of the telencephalic ventricle, rostral to the anterior commissure, the latter being cerebrospinal fluid-contacting (CSF-C) neurons. The diencephalon contained the highest number of 5HT-ir cell bodies, most of them of CSF-C type, located in the preoptic recess organ, paraventricular organ, posterior recess nucleus, and in the ventromedial thalamus. 5HT-ir non-CSF-C neurons appeared in the dorsal thalamic nucleus. In the brainstem, 5HT-ir neurons were located in four raphe nuclei (dorsal, superior, medial and inferior raphe nuclei) and four lateral reticular nuclei. The dorsal raphe nucleus contained 5HT-ir CSF-C cells, a type of serotoninergic cell that has not been described before in raphe nuclei of fishes or of other vertebrates. CSF-C and non-CSF-C 5HT-ir cells were observed in the spinal cord. 5HT-ir fibers were also widely distributed in the central nervous system of both sturgeon species. Comparison of these results with the distribution of serotoninergic systems in lampreys and other vertebrates suggests that widespread distribution of 5HT-ir cells is a feature of early vertebrate lines.

Afferent and efferent connections of the parapineal organ in lampreys: a tract tracing and immunocytochemical study

The neural connections of the parapineal organ of two species of lampreys were studied with the fluorescent dye 1,1'-dioctadecyl-3,3,3'3'-tetramethylindocarbocyanine perchlorate (DiI) and with immunocytochemistry. The lamprey parapineal organ consists of a vesicle and a ganglion that are connected to the left habenula. Labeling experiments included the application of DiI to the parapineal organ, left and right fasciculus retroflexus, left habenula, and the left pretectal region. Afferent parapineal fibers run in the left fasciculus retroflexus to the interpeduncular nucleus. The parapineal fibers of this fascicle arose from parapineal ganglion cells, whereas DiI application to the left habenula labeled both neurons of this ganglion and bipolar cells in the parapineal vesicle. Efferent neurons were observed in the left habenula, and bilaterally in the subhippocampal nucleus and the dorsal pretectum. Labeling with DiI also revealed a hippocampal projection. Immunocytochemical study of the parapineal vesicle revealed serotonin-immunoreactive cells in both species of lamprey, as well as substance P-immunoreactive (SP-ir) cells in sea lamprey and choline acetyltransferase-immunoreactive (ChAT-ir) cells in the river lamprey. The SP-ir cells and ChAT-ir cells formed a rich neuropil in the parapineal ganglion. Calretinin-ir cells were numerous in the ganglion. Neuropeptide Y-immunoreactive and gamma-aminobutyric acid-immunoreactive efferent fibers were observed in the parapineal organ. Neuropeptide Y-immunoreactive fibers originate in the subhippocampal nucleus, whereas gamma-aminobutyric acid-immunoreactive fibers might also arise in the pretectal nucleus. A few galanin-ir fibers were observed. These results indicate that the parapineal connections are completely different from those of the pineal organ. The possible homology between parapineal organs of vertebrates is discussed.

Parapinopsin, a novel catfish opsin localized to the parapineal organ, defines a new gene family

Multiple sites of extraretinal photoreception are present in vertebrates, but the molecular basis of extraretinal phototransduction is poorly understood. This study reports the cloning of the first opsin specifically expressed in the directly photosensitive pineal and parapineal of cold-blooded vertebrates. This opsin, identified in channel catfish and termed parapinopsin, defines a new gene family of vertebrate photopigments and is expressed in a majority of parapinealocytes and a subset of pineal photoreceptor cells. Parapinopsin shows a caudal-rostral gradient of expression within the pineal organ. This study also reports the cloning of partial cDNAs encoding the channel catfish orthologues of rhodopsin and the red cone pigment-the full complement of retinal opsins in the species. In situ hybridization studies using probes derived from these retinal opsins, together with parapinopsin, reveal no expression of retinal opsins in pineal and parapineal organ and no expression of any opsin tested in the "deep brain," iris, or dermal melanophores. These data imply that phototransduction in these sites of extraretinal photoreception must be mediated by novel opsins.

Review of larval and postlarval eye ultrastructure in the lamprey (Cyclostomata) with special emphasis on Geotria australis (Gray)

The literature dealing with the lateral eye in lampreys is briefly reviewed here. While there appears to be no longer much doubt that the short and long photoreceptor cells in the lamprey eye correspond to rods and cones, questions of dark/light adaptational changes, the nature of visual pigments, and the roles of retinal serotonin and melatonin need to be re-addressed. Eyes of the larval and postlarval ("macrophthalmia") stages of the lamprey Geotria australis were examined by electron microscopy and it was found that the larval retina is largely undifferentiated except for a small central zone surrounding the optic nerve head. The retina of the postlarval stage is fully differentiated and the photoreceptor outer segments undergo renewal, which appears to involve the retinal pigment epithelium (RPE). The distribution of larval RPE and choroidal pigments, postlarval ganglion cells, and the orientation of scleral collagen are unusual for vertebrates. No obvious positional or size differences of any retinal cell type were apparent between day- and night-fixed specimens.

Spectral sensitivity and mechanism of interaction between inhibitory and excitatory responses of photosensory pineal neurons

The characteristics and distribution of chromatic-type neurons in the photosensory pineal organ of the river lamprey, Lampetra japonica, were investigated electrophysiologically. Neuronal activity was inhibited by light of short wavelengths and excited by middle to long wavelengths. The maximum sensitivities of the inhibitory and excitatory responses were at about 380 nm and 540 nm respectively. The spike activity of the neurons during steady illumination for a 10-min period was measured. Although a flash of short-wavelength light caused a strong inhibition in the neuron, this effect was not sustained during 10 min of photic stimuli. It was found that the inhibitory effect continued when excitatory (middle-wavelength) light was delivered together with inhibitory (short-wavelength) light. The result supports the hypothesis of photoregeneration in the pineal photoreceptor, which occurs when photoreceptors having high sensitivity to short wavelengths receive middle-wavelength light. Contrary to the inhibitory response, the excitatory one caused by middle wavelengths continued during stimulation. Spike frequency of the neuron was determined by the spectral composition of the light. Since environmental light contains both inhibitory and excitatory components, the neuron would keep both sensitivities during the daytime and could measure the variation in the spectral composition. Judging from the recording sites, the chromatic-type neurons are distributed in the peripheral part of the pineal organ.

The pineal organ as a component of the biological clock. Phylogenetic and ontogenetic considerations

In conclusion, several trends are observed in regard to the phylogenetic development of the pineal organ, which are relevant for our understanding of the evolution of biological clock mechanisms. 1. The pineal organ of all vertebrates investigated thus far is capable of producing and releasing melatonin. Melatonin is rhythmically produced and released during darkness and, thus, represents an important neuroendocrine information on the ambient photoperiod. 2. The rhythmic production of melatonin is under control of endogenous oscillators and photoreceptor cells. In several nonmammalian species, these endogenous oscillators and photoreceptors are located within the pineal organ itself. In some avian species, the inherent rhythmicity of the pineal organ appears to be influenced by pacemakers located in other parts of the central nervous system. Their information may be transmitted to the pineal organ via the sympathetic innervation. This innervation develops progressively in the course of phylogeny. In mammals certain pinealocytes express proteins which are specific of retinal and pineal photoreceptors, but these proteins are obviously not involved in photoreception and phototransduction. The mammalian pineal organ lacks not only functioning photoreceptors, but also endogenous oscillators. The photoreceptor cells involved in regulation of the melatonin biosynthesis are located in the retina; the major endogenous oscillator is the suprachiasmatic nucleus (SCN) of the hypothalamus. Information from the retina and the SCN is transmitted to the mammalian pineal organ via a complex neuronal chain, whose last member is the sympathetic innervation originating from the superior cervical ganglion. This innervation is mandatory to maintain the rhythm of the melatonin biosynthesis in the mammalian pineal organ. Interestingly, the effects of noradrenaline, the major neurotransmitter in the sympathetic nerve fibers, displays opposite effects on the melatonin biosynthesis in birds and mammals: it stimulates the melatonin biosynthesis in the mammalian pineal organ, but inhibits the melatonin formation in the chicken. This conversion occurs at the level of the adrenoreceptors. 3. The intrapineal nerve cells giving rise to pinealofugal neuronal projections are reduced in the course of phylogeny. Nevertheless, direct neuronlike connections appear to exist between the pineal organ and the central nervous system of mammals. These projections originate from a population of pinealocytes. Whether such projections are involved in biological clock mechanisms remains an issue not yet resolved. The ontogenetic data reviewed support the notion that, in lower vertebrates, melatonin biosynthesis is primarily controlled by intrapineal photoreceptors, whereas, in mammals, it depends on retinal photoreceptors and the sympathetic innervation of the pineal.(ABSTRACT TRUNCATED AT 400 WORDS)

Neural projections of the pineal organ in the larval sea lamprey (Petromyzon marinus L.) revealed by indocarbocyanine dye tracing

The distribution of the central neural connections of the pineal organ of the larval sea lamprey was investigated by means of anterograde and retrograde tracing with the fluorescent lipophilic dye, DiI (1,1'-dioctadecyl 3,3,3',3'-tetramethylindocarbocyanine perchlorate). Pinealofugal projections are well developed in larvae, extending from the posterior commissure into the diencephalon and mesencephalon. Small numbers of neurons were retrogradely labelled in the transition zone between the diencephalon and the mesencephalic tegmentum. These cells may constitute the first pinealopetal system described in anamniotes.

Evidence for a frontal-organ homologue in the pineal complex of the salamander, Hynobius dunni

The pineal complex of larval and adult salamanders, Hynobius dunni, was examined by light and scanning electron microscopy. This pineal complex displays an anterior and a posterior portion, both of which possess a lumen. The anterior lumen is small and closed, whereas the posterior lumen is in open communication with the third ventricle. Cell processes of the photoreceptor cells and microvilli of the supportive cells are visible in both lumina. The anterior part of the complex is formed by an independent, second evagination from the common pineal anlage; this process takes place immediately after hatching. The anterior body of the pineal complex of H. dunni appears to be homologous to the frontal organ of anurans.

The serotoninergic system of the brain of the lamprey, Lampetra fluviatilis: an evolutionary perspective

The distribution of serotonin(5HT)-immunoreactive cell bodies, nerve fibers and terminals was investigated by light microscopy in the lamprey Lampetra fluviatilis. Twenty-three distinct groups of 5HT neuronal somata were identified from diencephalic to rhombencephalic levels in the brain. The diencephalon contained a subependymal population of immunoreactive cells in contact with the cerebrospinal fluid (CSF), which could be subdivided into five separate groups situated in the hypothalamus and ventral thalamus; five additional groups of immunoreactive diencephalic neurons, situated in the dorsal thalamus and thalamo-pretectum, which were not in contact with the CSF, were also identified. In the midbrain, in addition to a few labelled neurons in the optic tectum, two structures containing immunoreactive cells were identified in the tegmentum mesencephali. None of these 5HT cells corresponded to the retinopetal neurons which are situated in the same region. A very large number of 5HT neurons were observed in the hindbrain which could be divided into seven groups in the isthmus rhombencephali and a further three in the rhombencephalon proper. Immunoreactive fibers and terminals were widely distributed throughout the neuraxis. In the telencephalon two 5HT fibers assemblies, lateral and medial, could be identified which terminated in both pallial and subpallial structures. The richest serotoninergic innervation in the telencephalon was found in the lateral portion of the primordium hippocampi and the medial part of the corpus striatum. In the diencephalon, the distribution of immunoreactive fibers and terminals was heterogeneous, being most pronounced in the lateral hypothalamic area and in the infundibulum. The densest arborization of fibers in the mesencephalon was found in the stratum fibrosum et cellulare externum of the optic tectum, a major site of retinal projection, and in the nucleus interpeduncularis mesencephali as well as in the oculomotor nuclei. The rhombencephalon is richly endowed with serotoninergic fibers and terminals, many labelled arborizations being found in the nuclei isthmi rhombencephali and around the nucleus motorius nervi trigemini. Comparative analysis of the serotoninergic systems of petromyzontiforms and gnathostomes indicates that the evolution of this system involves a progressive elimination of the rostral immunoreactive cells and an increasing complexity of the caudal population of serotoninergic neurons.

Signal transmission from pineal photoreceptors to luminosity-type ganglion cells in the lamprey, Lampetra japonica

In order to study the signal transmission from pineal photoreceptors to luminosity (achromatic)-type ganglion cells of the lamprey, Lampetra japonica, the electrical activity of these cell groups was investigated using intra- and extracellular electrodes. By intracellular recording, it was shown that the photoreceptor cells responded to flashes of light with hyperpolarizations, and the ganglion cells also hyperpolarized with concurrent suppression of spike discharges. Concerning the slow membrane potentials, the light intensity-response relationships of both cell groups followed the Naka-Rushton hyperbolic function. The intensity range over which the ganglion cells responded was broader than that of the photoreceptors. The spectral sensitivity curve of the luminosity-type ganglion cell coincided with that of the photoreceptor, showing a peak sensitivity at 525 nm. Membrane resistance of the ganglion cells increased during light stimulation. These results suggest that the luminosity-type ganglion cell receives and integrates signals from photoreceptors with various light sensitivities, having a peak spectral sensitivity at 525 nm. The synaptic mechanism from the photoreceptors to the ganglion cell is a type of disfacilitation.

[Pineal body in vertebrates: a model for investigations of receptor and effector mechanisms of neuronal systems]

Cell and molecular biological investigations have greatly contributed to our understanding of receptor and effector mechanisms in sensory, neuronal, and endocrine cells. A fascinating aspect of this line of research is how such mechanisms have evolved and how they interact with each other. As shown in this contribution, the vertebrate pineal organ is an interesting model to study these problems, because it undergoes a conspicuous transformation during phylogeny, comprises two well-characterized receptor mechanisms (photoreception and adrenoreception), and acts upon its targets via neuronal and neuroendocrine signals.