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

A lamprey neural cell type atlas illuminates the origins of the vertebrate brain

The vertebrate brain emerged more than ~500 million years ago in common evolutionary ancestors. To systematically trace its cellular and molecular origins, we established a spatially resolved cell type atlas of the entire brain of the sea lamprey-a jawless species whose phylogenetic position affords the reconstruction of ancestral vertebrate traits-based on extensive single-cell RNA-seq and in situ sequencing data. Comparisons of this atlas to neural data from the mouse and other jawed vertebrates unveiled various shared features that enabled the reconstruction of cell types, tissue structures and gene expression programs of the ancestral vertebrate brain. However, our analyses also revealed key tissues and cell types that arose later in evolution. For example, the ancestral brain was probably devoid of cerebellar cell types and oligodendrocytes (myelinating cells); our data suggest that the latter emerged from astrocyte-like evolutionary precursors in the jawed vertebrate lineage. Altogether, our work illuminates the cellular and molecular architecture of the ancestral vertebrate brain and provides a foundation for exploring its diversification during evolution.

Galanin in an Agnathan: Precursor Identification and Localisation of Expression in the Brain of the Sea Lamprey Petromyzon marinus

Galanin is a neuropeptide that is widely expressed in the mammalian brain, where it regulates many physiological processes, including feeding and nociception. Galanin has been characterized extensively in jawed vertebrates (gnathostomes), but little is known about the galanin system in the most ancient extant vertebrate class, the jawless vertebrates or agnathans. Here, we identified and cloned a cDNA encoding the sea lamprey (Petromyzon marinus) galanin precursor (PmGalP). Sequence analysis revealed that PmGalP gives rise to two neuropeptides that are similar to gnathostome galanins and galanin message-associated peptides. Using mRNA in situ hybridization, the distribution of PmGalP-expressing neurons was mapped in the brain of larval and adult sea lampreys. This revealed PmGalP-expressing neurons in the septum, preoptic region, striatum, hypothalamus, prethalamus, and displaced cells in lateral areas of the telencephalon and diencephalon. In adults, the laterally migrated PmGalP-expressing neurons are observed in an area that extends from the ventral pallium to the lateral hypothalamus and prethalamus. The striatal and laterally migrated PmGalP-expressing cells of the telencephalon were not observed in larvae. Comparison with studies on jawed vertebrates reveals that the presence of septal and hypothalamic galanin-expressing neuronal populations is highly conserved in vertebrates. However, compared to mammals, there is a more restricted pattern of expression of the galanin transcript in the brain of lampreys. This work provides important new information on the early evolution of the galanin system in vertebrates and provides a genetic and neuroanatomical basis for functional analyses of the galanin system in lampreys.

Sox1a mediates the ability of the parapineal to impart habenular left-right asymmetry

Left-right asymmetries in the zebrafish habenular nuclei are dependent upon the formation of the parapineal, a unilateral group of neurons that arise from the medially positioned pineal complex. In this study, we show that both the left and right habenula are competent to adopt left-type molecular character and efferent connectivity upon the presence of only a few parapineal cells. This ability to impart left-sided character is lost in parapineal cells lacking Sox1a function, despite the normal specification of the parapineal itself. Precisely timed laser ablation experiments demonstrate that the parapineal influences neurogenesis in the left habenula at early developmental stages as well as neurotransmitter phenotype and efferent connectivity during subsequent stages of habenular differentiation. These results reveal a tight coordination between the formation of the unilateral parapineal nucleus and emergence of asymmetric habenulae, ensuring that appropriate lateralised character is propagated within left and right-sided circuitry.

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.

Neural connections of the pretectum in zebrafish (Danio rerio)

The pretectum is a complex region of the caudal diencephalon which in adult zebrafish comprises both retinorecipient (parvocellular superficial, central, intercalated, paracommissural, and periventricular) and non-retinorecipient (magnocellular superficial, posterior, and accessory) pretectal nuclei distributed from periventricular to superficial regions. We conducted a comprehensive study of the connections of pretectal nuclei by using neuronal tracing with fluorescent carbocyanine dyes. This study reveals specialization of efferent connections of the various pretectal nuclei, with nuclei projecting to the optic tectum (paracommissural, central, and periventricular pretectal nuclei), the torus longitudinalis and the cerebellar corpus (paracommissural, central, and intercalated pretectal nuclei), the lateral hypothalamus (magnocellular superficial, posterior, and central pretectal nuclei), and the tegmental regions (accessory and superficial pretectal nuclei). With regard to major central afferents to the pretectum, we observed projections from the telencephalon to the paracommissural and central pretectal nuclei, from the optic tectum to the paracommissural, central, accessory and parvocellular superficial pretectal nuclei, from the cerebellum to the paracommissural and periventricular pretectal nuclei and from the nucleus isthmi to the parvocellular superficial and accessory pretectal nuclei. The parvocellular superficial pretectal nucleus sends conspicuous projections to the contralateral magnocellular superficial pretectal nucleus. The composite figure of results reveals large differences in connections of neighbor pretectal nuclei, indicating high degree of nuclear specialization. Our results will have important bearings in functional studies that analyze the relationship between specific circuits and behaviors in zebrafish. Comparison with results available in other species also reveals differences in the organization and connections of the pretectum in vertebrates.

Nodal signalling and asymmetry of the nervous system

The role of Nodal signalling in nervous system asymmetry is still poorly understood. Here, we review and discuss how asymmetric Nodal signalling controls the ontogeny of nervous system asymmetry using a comparative developmental perspective. A detailed analysis of asymmetry in ascidians and fishes reveals a critical context-dependency of Nodal function and emphasizes that bilaterally paired and midline-unpaired structures/organs behave as different entities. We propose a conceptual framework to dissect the developmental function of Nodal as asymmetry inducer and laterality modulator in the nervous system, which can be used to study other types of body and visceral organ asymmetries. Using insights from developmental biology, we also present novel evolutionary hypotheses on how Nodal led the evolution of directional asymmetry in the brain, with a particular focus on the epithalamus. We intend this paper to provide a synthesis on how Nodal signalling controls left-right asymmetry of the nervous system.This article is part of the themed issue 'Provocative questions in left-right asymmetry'.

Information processing in the vertebrate habenula

The habenula is a brain region that has gained increasing popularity over the recent years due to its role in processing value-related and experience-dependent information with a strong link to depression, addiction, sleep and social interactions. This small diencephalic nucleus is proposed to act as a multimodal hub or a switchboard, where inputs from different brain regions converge. These diverse inputs to the habenula carry information about the sensory world and the animal's internal state, such as reward expectation or mood. However, it is not clear how these diverse habenular inputs interact with each other and how such interactions contribute to the function of habenular circuits in regulating behavioral responses in various tasks and contexts. In this review, we aim to discuss how information processing in habenular circuits, can contribute to specific behavioral programs that are attributed to the habenula.

Heterochrony and Morphological Variation of Epithalamic Asymmetry

Heterochrony is one proposed mechanism to explain how morphological variation and novelty arise during evolution. To experimentally approach heterochrony in a comprehensive manner, we must consider all three aspects of developmental time (sequence, timing, duration). This task is only possible in developmental models that allow the acquisition of high-quality temporal data in the context of normalized developmental time. Here we propose that epithalamic asymmetry of teleosts is one such model. Comparative studies among related teleost species have revealed heterochronic shifts in the timing of ontogenic events leading to the development of epithalamic asymmetry. Such temporal changes involve neural structures critical for tissue-tissue interactions underlying the generation of asymmetry and are concurrent with the appearance of morphological differences in the pattern of asymmetry between species. Based on these findings, we hypothesize that interspecies variation of epithalamic asymmetry results from changes in the timing of tissue-tissue interactions critical for the establishment of asymmetry during ontogeny. Importantly, this hypothesis can be tested by systematic comparative approaches among teleosts species based on normalized developmental time, combined with experimental manipulation of epithalamic asymmetry development.

Afferent Connectivity of the Zebrafish Habenulae

The habenulae are bilateral nuclei located in the dorsal diencephalon that are conserved across vertebrates. Here we describe the main afferents to the habenulae in larval and adult zebrafish. We observe afferents from the subpallium, nucleus rostrolateralis, posterior tuberculum, posterior hypothalamic lobe, median raphe; we also see asymmetric afferents from olfactory bulb to the right habenula, and from the parapineal to the left habenula. In addition, we find afferents from a ventrolateral telencephalic nucleus that neurochemical and hodological data identify as the ventral entopeduncular nucleus (vENT), confirming and extending observations of Amo et al. (2014). Fate map and marker studies suggest that vENT originates from the diencephalic prethalamic eminence and extends into the lateral telencephalon from 48 to 120 hour post-fertilization (hpf). No afferents to the habenula were observed from the dorsal entopeduncular nucleus (dENT). Consequently, we confirm that the vENT (and not the dENT) should be considered as the entopeduncular nucleus "proper" in zebrafish. Furthermore, comparison with data in other vertebrates suggests that the vENT is a conserved basal ganglia nucleus, being homologous to the entopeduncular nucleus of mammals (internal segment of the globus pallidus of primates) by both embryonic origin and projections, as previously suggested by Amo et al. (2014).

Differential Effects of Sex Pheromone Compounds on Adult Female Sea Lamprey (Petromyzon marinus) Locomotor Patterns

Synchronization of male and female locomotor activity plays a critical role in ensuring reproductive success, especially in semelparous species. The goal of this study was to elucidate the effects of individual chemical signals, or pheromones, on the locomotor activity in the sea lamprey (Petromyzon marinus). In their native habitat, adult preovulated females (POF) and ovulated females (OF) are exposed to sex pheromone compounds that are released from spermiated males and attract females to nests during their migration and spawning periods. In this study, locomotor activity of individual POF and OF was measured hourly in controlled laboratory conditions using an automated video-tracking system. Differences in the activity between a baseline day (no treatment exposure) and a treatment day (sex pheromone compound or control exposure) were examined for daytime and nighttime periods. Results showed that different pheromone compound treatments affected both POF and OF sea lamprey (p < 0.05) but in different ways. Spermiated male washings (SMW) and one of its main components, 7α,12α,24-trihydroxy-5α-cholan-3-one 24 sulfate (3kPZS), decreased activity of POF during the nighttime. SMW also reduced activity in POF during the daytime. In contrast, SMW increased activity of OF during the daytime, and an additional compound found in SMW, petromyzonol sulfate (PZS), decreased the activity during the nighttime. In addition, we examined factors that allowed us to infer the overall locomotor patterns. SMW increased the maximum hourly activity during the daytime, decreased the maximum hourly activity during the nighttime, and reduced the percentage of nocturnal activity in OF. Our findings suggest that adult females have evolved to respond to different male compounds in regards to their locomotor activity before and after final maturation. This is a rare example of how species-wide chemosensory stimuli can affect not only the amounts of activity but also the overall locomotor pattern in a vertebrate species.

Asymmetry of the Brain: Development and Implications

Although the left and right hemispheres of our brains develop with a high degree of symmetry at both the anatomical and functional levels, it has become clear that subtle structural differences exist between the two sides and that each is dominant in processing specific cognitive tasks. As the result of evolutionary conservation or convergence, lateralization of the brain is found in both vertebrates and invertebrates, suggesting that it provides significant fitness for animal life. This widespread feature of hemispheric specialization has allowed the emergence of model systems to study its development and, in some cases, to link anatomical asymmetries to brain function and behavior. Here, we present some of what is known about brain asymmetry in humans and model organisms as well as what is known about the impact of environmental and genetic factors on brain asymmetry development. We specifically highlight the progress made in understanding the development of epithalamic asymmetries in zebrafish and how this model provides an exciting opportunity to address brain asymmetry at different levels of complexity.

Ontogenetic shifts in brain scaling reflect behavioral changes in the life cycle of the pouched lamprey Geotria australis

Very few studies have described brain scaling in vertebrates throughout ontogeny and none in lampreys, one of the two surviving groups of the early agnathan (jawless) stage in vertebrate evolution. The life cycle of anadromous parasitic lampreys comprises two divergent trophic phases, firstly filter-feeding as larvae in freshwater and secondly parasitism as adults in the sea, with the transition marked by a radical metamorphosis. We characterized the growth of the brain during the life cycle of the pouched lamprey Geotria australis, an anadromous parasitic lamprey, focusing on the scaling between brain and body during ontogeny and testing the hypothesis that the vast transitions in behavior and environment are reflected in differences in the scaling and relative size of the major brain subdivisions throughout life. The body and brain mass and the volume of six brain structures of G. australis, representing six points of the life cycle, were recorded, ranging from the early larval stage to the final stage of spawning and death. Brain mass does not increase linearly with body mass during the ontogeny of G. australis. During metamorphosis, brain mass increases markedly, even though the body mass does not increase, reflecting an overall growth of the brain, with particularly large increases in the volume of the optic tectum and other visual areas of the brain and, to a lesser extent, the olfactory bulbs. These results are consistent with the conclusions that ammocoetes rely predominantly on non-visual and chemosensory signals, while adults rely on both visual and olfactory cues.

Immunohistochemical distribution of calretinin and calbindin (D-28k) in the brain of the cladistian Polypterus senegalus

Polypteriform fishes are believed to be basal to other living ray-finned bony fishes, and they may be useful for providing information of the neural organization that existed in the brain of the earliest ray-finned fishes. The calcium-binding proteins calretinin (CR) and calbindin-D28k (CB) have been widely used to characterize neuronal populations in vertebrate brains. Here, the distribution of the immunoreactivity against CR and CB was investigated in the olfactory organ and brain of Polypterus senegalus and compared to the distribution of these molecules in other ray-finned fishes. In general, CB-immunoreactive (ir) neurons were less abundant than CR-ir cells. CR immunohistochemistry revealed segregation of CR-ir olfactory receptor neurons in the olfactory mucosa and their bulbar projections. Our results confirmed important differences between pallial regions in terms of CR immunoreactivity of cell populations and afferent fibers. In the habenula, these calcium-binding proteins revealed right-left asymmetry of habenular subpopulations and segregation of their interpeduncular projections. CR immunohistochemistry distinguished among some thalamic, pretectal, and posterior tubercle-derived populations. Abundant CR-ir populations were observed in the midbrain, including the tectum. CR immunoreactivity was also useful for characterizing a putative secondary gustatory/visceral nucleus in the isthmus, and for distinguishing territories in the primary viscerosensory column and octavolateral region. Comparison of the data obtained within a segmental neuromeric context indicates that some CB-ir and CR-ir populations in polypteriform fishes are shared with other ray-finned fishes, but other positive structures appear to have evolved following the separation between polypterids and other ray-finned fishes.

Integrative neuro-endocrine pathways in the control of reproduction in lamprey: a brief review

The gonadotropin-releasing hormone (GnRH) system is well known as the main regulator of reproductive physiology in vertebrates. It is also part of a network of brain structures and pathways that integrate information from the internal and external milieu and coordinate the adaptive behavioral and physiological responses to social and reproductive survival needs. In this paper we review the state of knowledge of the GnRH system in relation to the behavior, external, and internal factors that control reproduction in one of the oldest lineage of vertebrates, the lampreys.

Neuronal mechanisms of respiratory pattern generation are evolutionary conserved

A brainstem region, the paratrigeminal respiratory group (pTRG), has been suggested to play a crucial role in the respiratory rhythm generation in lampreys. However, a detailed characterization of the pTRG region is lacking. The present study performed on isolated brainstem preparations of adult lampreys provides a more precise localization of the pTRG region with regard to both connectivity and neurochemical markers. pTRG neurons projecting to the vagal motoneuronal pool were identified in a restricted area of the rostral rhombencephalon at the level of the isthmic Müller cell I1 close to sulcus limitans of His. Unilateral microinjections of lidocaine, muscimol, or glutamate antagonists into the pTRG inhibited completely the bilateral respiratory activity. In contrast, microinjections of glutamate agonists enhanced the respiratory activity, suggesting that this region is critical for the respiratory pattern generation. The retrogradely labeled pTRG neurons are glutamatergic and surrounded by terminals with intense substance P immunoreactivity. Cholinergic neurons were seen close to, and intermingled with, pTRG neurons. In addition, α-bungarotoxin binding sites (indicating nicotinic receptors) were found throughout the pTRG area and particularly on the soma of these neurons. During apnea, induced by blockade of ionotropic glutamate receptors within the same region, microinjections of 1 μm substance P or 1 mm nicotine into the pTRG restored rhythmic respiratory activity. The results emphasize the close similarities between the pTRG and the mammalian pre-Bötzinger complex as a crucial site for respiratory rhythmogenesis. We conclude that some basic features of the excitatory neurons proposed to generate respiratory rhythms are conserved throughout evolution.

Regional distribution of calretinin and calbindin-D28k expression in the brain of the urodele amphibian Pleurodeles waltl during embryonic and larval development

The sequence of appearance of calretinin and calbindin-D28k immunoreactive (CRir and CBir, respectively) cells and fibers has been studied in the brain of the urodele amphibian Pleurodeles waltl. Embryonic, larval and juvenile stages were studied. The early expression and the dynamics of the distribution of CBir and CRir structures have been used as markers for developmental aspects of distinct neuronal populations, highlighting the accurate extent of many regions in the developing brain, not observed on the basis of cytoarchitecture alone. CR and, to a lesser extent, CB are expressed early in the central nervous system and show a progressively increasing expression from the embryonic stages throughout the larval life and, in general, the labeled structures in the developing brain retain their ability to express these proteins in the adult brain. The onset of CRir cells primarily served to follow the development of the olfactory bulbs, subpallium, thalamus, alar hypothalamus, mesencephalic tegmentum, and distinct cell populations in the rhombencephalic reticular formation. CBir cells highlighted the development of, among others, the pallidum, hypothalamus, dorsal habenula, midbrain tegmentum, cerebellum, and central gray of the rostral rhombencephalon. However, it was the relative and mostly segregated distribution of both proteins in distinct cell populations which evidenced the developing regionalization of the brain. The results have shown the usefulness in neuroanatomy of the analysis during development of the onset of CBir and CRir structures, but the comparison with previous data has shown extensive variability across vertebrate classes. Therefore, one should be cautious when comparing possible homologue structures across species only on the basis of the expression of these proteins, due to the variation of the content of calcium-binding proteins observed in well-established homologous regions in the brain of different vertebrates.

Glutamatergic neuronal populations in the forebrain of the sea lamprey, Petromyzon marinus: an in situ hybridization and immunocytochemical study

Despite the importance of glutamate as a major excitatory neurotransmitter in the brain, the distribution of glutamatergic populations in the brain of most vertebrates is still unknown. Here, we studied for the first time the distribution of glutamatergic neurons in the forebrain of the sea lamprey (Petromyzon marinus), belonging to the most ancient group of vertebrates (agnathans). For this, we used in situ hybridization with probes for a lamprey vesicular glutamate transporter (VGLUT) in larvae and immunofluorescence with antiglutamate antibodies in both larvae and adults. We also compared glutamate and γ-aminobutyric acid (GABA) immunoreactivities in sections using double-immunofluorescence methods. VGLUT-expressing neurons were observed in the olfactory bulb, pallium, septum, subhippocampal lobe, preoptic region, thalamic eminence, prethalamus, thalamus, epithalamus, pretectum, hypothalamus, posterior tubercle, and nucleus of the medial longitudinal fascicle. Comparison of VGLUT signal and glutamate immunoreactivity in larval forebrain revealed a consistent distribution of positive cells, which were numerous in most regions. Glutamate-immunoreactive cell populations were also found in similar regions of the adult forebrain. These include mitral-like cells of the olfactory bulbs and abundant cells in the lateral pallium, septum, and various diencephalic regions, mainly in the prethalamus, thalamus, habenula, pineal complex, and pretectum. Only a small portion of the glutamate-immunoreactive cells showed colocalization with GABA, which was observed mainly in the olfactory bulb, telencephalon, hypothalamus, ventral thalamus, and pretectum. Comparison with glutamatergic cells observed in rodent forebrains suggests that the regional distribution of glutamatergic cells does not differ greatly in lampreys and mammals.

Evolutionary conservation of the habenular nuclei and their circuitry controlling the dopamine and 5-hydroxytryptophan (5-HT) systems

The medial (MHb) and lateral (LHb) habenulae are a small group of nuclei that regulate the activity of monoaminergic neurons. Disruptions to these nuclei lead to deficits in a range of cognitive and motor functions from sleep to decision making. Interestingly, the habenular nuclei are present in all vertebrates, suggesting that they provide a common neural mechanism to influence these diverse functions. To unravel conserved habenula circuitry and approach an understanding of their basic function, we investigated the organization of these nuclei in the lamprey, one of the phylogenetically oldest vertebrates. Based on connectivity and molecular expression, we show that the MHb and LHb circuitry is conserved in the lamprey. As in mammals, separate populations of neurons in the LHb homolog project directly or indirectly to dopamine and serotonin neurons through a nucleus homologous to the GABAergic rostromedial mesopontine tegmental nucleus and directly to histamine neurons. The pallidal and hypothalamic inputs to the LHb homolog are also conserved. In contrast to other species, the habenula projecting pallidal nucleus is topographically distinct from the dorsal pallidum, the homolog of the globus pallidus interna. The efferents of the MHb homolog selectively target the interpeduncular nucleus. The MHb afferents arise from sensory (medial olfactory bulb, parapineal, and pretectum) and not limbic areas, as they do in mammals; consequently, the "context" in which this circuitry is recruited may have changed during evolution. Our results indicate that the habenular nuclei provide a common vertebrate circuitry to adapt behavior in response to rewards, stress, and other motivating factors.

Role of neurokinin receptors and ionic mechanisms within the respiratory network of the lamprey

We have suggested that in the lamprey, a medullary region called the paratrigeminal respiratory group (pTRG), is essential for respiratory rhythm generation and could correspond to the pre-Bötzinger complex (pre-BötC), the hypothesized kernel of the inspiratory rhythm-generating network in mammals. The present study was performed on in vitro brainstem preparations of adult lampreys to investigate whether some functional characteristics of the respiratory network are retained throughout evolution and to get further insights into the recent debated hypotheses on respiratory rhythmogenesis in mammals, such as for instance the "group-pacemaker" hypothesis. Thus, we tried to ascertain the presence and role of neurokinins (NKs) and burst-generating ion currents, such as the persistent Na(+) current (I(NaP)) and the Ca(2+)-activated non-specific cation current (I(CAN)), described in the pre-Bötzinger complex. Respiratory activity was monitored as vagal motor output. Substance P (SP) as well as NK1, NK2 and NK3 receptor agonists (400-800 nM) applied to the bath induced marked increases in respiratory frequency. Microinjections (0.5-1 nl) of SP as well as the other NK receptor agonists (1 microM) into the pTRG increased the frequency and amplitude of vagal bursts. Riluzole (RIL) and flufenamic acid (FFA) were used to block I(NaP) and I(CAN), respectively. Bath application of either RIL or FFA (20-50 microM) depressed, but did not suppress respiratory activity. Coapplication of RIL and FFA at 50 microM abolished the respiratory rhythm that, however, was restarted by SP microinjected into the pTRG. The results show that NKs may have a modulatory role in the lamprey respiratory network through an action on the pTRG and that I(NaP) and I(CAN) may contribute to vagal burst generation. We suggest that the "group-pacemaker" hypothesis is tenable for the lamprey respiratory rhythm generation since respiratory activity is abolished by blocking both I(NaP) and I(CAN), but is restored by enhancing network excitability.

Building an asymmetric brain: development of the zebrafish epithalamus

The human brain exhibits notable asymmetries. Little is known about these symmetry deviations; however scientists are beginning to understand them by employing the lateralized zebrafish epithalamus as a model. The zebrafish epithalamus consists of the pineal and parapineal organs and paired habenular nuclei located bilateral to the pineal complex. While zebrafish pineal and parapineal organs arise from a common population of cells, parapineal cells undergo a separate program that allows them to migrate left of the pineal anlage. Studying the processes that lead to brain laterality in zebrafish will allow a better understanding of how human brain laterality is established.

Glucocorticoids play a key role in circadian cell cycle rhythms

Clock output pathways play a pivotal role by relaying timing information from the circadian clock to a diversity of physiological systems. Both cell-autonomous and systemic mechanisms have been implicated as clock outputs; however, the relative importance and interplay between these mechanisms are poorly understood. The cell cycle represents a highly conserved regulatory target of the circadian timing system. Previously, we have demonstrated that in zebrafish, the circadian clock has the capacity to generate daily rhythms of S phase by a cell-autonomous mechanism in vitro. Here, by studying a panel of zebrafish mutants, we reveal that the pituitary–adrenal axis also plays an essential role in establishing these rhythms in the whole animal. Mutants with a reduction or a complete absence of corticotrope pituitary cells show attenuated cell-proliferation rhythms, whereas expression of circadian clock genes is not affected. We show that the corticotrope deficiency is associated with reduced cortisol levels, implicating glucocorticoids as a component of a systemic signaling pathway required for circadian cell cycle rhythmicity. Strikingly, high-amplitude rhythms can be rescued by exposing mutant larvae to a tonic concentration of a glucocorticoid agonist. Our work suggests that cell-autonomous clock mechanisms are not sufficient to establish circadian cell cycle rhythms at the whole-animal level. Instead, they act in concert with a systemic signaling environment of which glucocorticoids are an essential part.

To guarantee normal growth and to avoid tumor formation, the timing of cell division must be under strict control. Remarkably, cells, from bacteria to man, often divide only at certain times of day, suggesting the influence of internal biological clocks. A central pacemaker structure in the brain controls diurnal rhythms of behavior and hormone release. However, biological clocks are also encountered in almost every cell type (so-called “peripheral” clocks), in which they regulate daily changes in cell biology, including cell division. Very little is known to date about how the two clock systems interact. Here, by examining zebrafish strains with defects in hormone production, we find that peripheral clocks require the steroid hormone cortisol to generate daily rhythms of cell proliferation. Interestingly, the daily changes in cortisol levels observed in normal zebrafish are not required to achieve this control; treating the cortisol-deficient strains with constant levels of a drug that mimics the effects of cortisol restores normal cell-division rhythms. Thus, it appears that internal cell timers cooperate with hormonal signals to regulate the timing of cell division.

To establish circadian cell cycle rhythms, cell-autonomous clock mechanisms act in concert with a systemic signaling environment of which glucocorticoids are an essential part.

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.

Afferent Connections of the Optic Tectum in Lampreys: An Experimental Study

Tectal afferents were studied in adult lampreys of three species (Ichthyomyzon unicuspis, Lampetra fluviatilis, and Petromyzon marinus) following unilateral BDA injections into the optic tectum (OT). In the secondary prosencephalon, neurons projecting to the OT were observed in the pallium, the subhipoccampal lobe, the striatum, the preoptic area and the hypothalamus. Following tectal injections, backfilled diencephalic cells were found bilaterally in: prethalamic eminence, ventral geniculate nucleus, periventricular prethalamic nucleus, periventricular pretectal nucleus, precommissural nucleus, magnocellular and parvocellular nuclei of the posterior commissure and pretectal nucleus; and ipsilaterally in: nucleus of Bellonci, periventricular thalamic nucleus, nucleus of the tuberculum posterior, and the subpretectal tegmentum, as well as in the pineal organ. At midbrain levels, retrogradely labeled cells were seen in the ipsilateral torus semicircularis, the contralateral OT, and bilaterally in the mesencephalic reticular formation and inside the limits of the retinopetal nuclei. In the hindbrain, tectal projecting cells were also bilaterally labeled in the dorsal and lateral isthmic nuclei, the octavolateral area, the sensory nucleus of the descending trigeminal tract, the dorsal column nucleus and the reticular formation. The rostral spinal cord also exhibited a few labeled cells. These results demonstrate a complex pattern of connections in the lamprey OT, most of which have been reported in other vertebrates. Hence, the lamprey OT receives a large number of nonvisual afferents from all major brain areas, and so is involved in information processing from different somatic sensory modalities.

Distribution of neuropeptide FF-like immunoreactive structures in the lamprey central nervous system and its relation to catecholaminergic neuronal structures

The neuropeptide FF (NPFF) is an octapeptide of the RFamide-related peptides (FaRPs) that was primarily isolated from the bovine brain. Its distribution in the CNS has been reported in several mammalian species, as well as in some amphibians. Therefore, in order to gain insight in the evolution on the expression pattern of this neuropeptide in vertebrates, we carried out an immunohistochemical study in the sea lamprey, Petromyzon marinus. The distribution of NPFF-like-immunoreactive (NPFF-ir) structures in the lamprey brain is, in general, comparable to that previously described in other vertebrate species. In lamprey, most of the NPFF-ir cells were found in the hypothalamus, particularly in two large populations, the bed nucleus of the tract of the postoptic commissure and the tuberomammillary area. Numerous NPFF-ir cells were also observed in the rostral rhombencephalon, including a population in the dorsal isthmic gray and the reticular formation. Additional labeled neurons were found inside the preoptic region, the parapineal vesicle, the periventricular mesencephalic tegmentum, the descending trigeminal tract, the nucleus of the solitary tract, as well as in the gray matter of the spinal cord. The NPFF-ir fibers were widely distributed in the brain and the spinal cord, being, in general, more concentrated throughout the basal plate. The presence of NPFF-ir fibers in the lamprey neurohypophysis suggests that the involvement of NPFF-like substances in the hypothalamo-hypophyseal system had emerged early during evolution.

The interplay between the pineal complex and the habenular nuclei in lower vertebrates in the context of the evolution of cerebral asymmetry

This paper presents an overview on the epithalamus of vertebrates, with particular reference to the pineal and to the asymmetrical organization of the habenular nuclei in lower vertebrates. The relationship between the pineal and the habenulae in the course of phylogenesis is here emphasized, taking data in the frog as example. Altogether the data support the hypothesis, put forward also in earlier studies, of a correlation of habenular asymmetry in lower vertebrates with phylogenetic modification of the pineal complex. The present re-visitation was also stimulated by recent data on the asymmetrical expression of Nodal genes, which involves the pineal and habenular structures in zebrafish. The comparative analysis of data, from cyclostomes to mammals, suggests that transformation of epithalamic structures may play an important role in brain evolution. In addition, in mammals, including rodents, a remarkable complexity has evolved in the organization of the habenulae and their functional interactions with the pineal gland. The evolution of these two epithalamic structures seems to open also new perspectives of knowledge on their implication in the regulation of biological rhythms.

GABAergic system of the pineal organ of an elasmobranch (Scyliorhinus canicula): a developmental immunocytochemical study

The present immunocytochemical study provides evidence of a previously unrecognized, rich, gamma-aminobutyric acid (GABA)-ergic innervation of the pineal organ in the dogfish (Scyliorhinus canicula). In this elasmobranch, the pineal primordium is initially detected at embryonic stage 24 and grows to form a long pineal tube by stage 28. Glutamic acid decarboxylase (GAD)-immunoreactive (-ir) fibers were first observed at stage 26, and by stage 28, thin GAD-ir fibers were detectable at the base of the pineal neuroepithelium. In pre-hatchling embryos, most fibers gave rise to GAD-ir boutons that were localized in the basal region of the neuroepithelium, although a smaller number of labeled terminals ascended to the pineal lumen. A few pale GAD-ir perikarya were observed within the pineal organ of stage 29 embryos, but GAD-ir perikarya were not observed at other developing stages or in adults. In contrast, GABA immunocytochemistry revealed the presence of GABAergic perikarya and fibers in the pineal organ of late stage embryos and adults. Although high densities of GABAergic cells were observed in the paracommissural pretectum, posterior tubercle, and tegmentum of dogfish embryos (regions previously demonstrated to contain pinealopetal cells), the presence of GABA-ir perikarya in the pineal organ strongly suggests that the rich GABAergic innervation of the elasmobranch pineal organ is intrinsic. This contrasts with the central origin of GABAergic fibers in the pineal gland of some mammals.

Immunohistochemical distribution of tachykinins in the CNS of the lamprey Petromyzon marinus

The presence of tachykinins in the CNS of vertebrates has been known for many decades, and numerous studies have described their distribution in mammals. Tachykinins were also reported in the CNS of lampreys using immunohistochemistry, chromatography, and radioimmunoassay methods, but the use of substance P (SP)-specific antibodies to reveal those tachykinins could have led to an underestimation of their number in this genus. Therefore, we carried out a new immunohistochemical study on Petromyzon marinus using a commercial polyclonal antibody that binds not only to mammalian SP, but also to other neurokinins. This antibody labeled all previously described lamprey tachykinin-containing neuronal populations, but more important, labeled new populations in several parts of the brain. These include the dorsal gray of the rostral spinal cord, the dorsal column nuclei, the octavolateral area, the nucleus of the solitary tract, the medial rhombencephalic reticular formation, the lateral tegmentum of the rostral rhombencephalon, the torus semicircularis, the optic tectum, the habenula, the mammillary area, the dorsal thalamic area, the lateral hypothalamus, and the septum area. Preabsorption experiments confirmed the binding of the antibody to neurokinins and allowed us to propose that the CNS of P. marinus contains at least two different tachykinins.

The dorsal diencephalic conduction system of zebrafish as a model of vertebrate brain lateralisation

Lateralisation is an attractive and intriguing feature of the vertebrate CNS studied for decades in the different disciplines of the neurosciences. Due to the complexity of the phenomena and intrinsic limitations of the approaches used to date, it has been difficult to establish useful links between the different, and usually distant, levels of lateralisation e.g. between genetics, morphology, physiology and behaviour. Recently, the dorsal diencephalon of the teleost zebrafish has emerged as a valuable model to begin addressing this issue and as a result unravel the role of vertebrate CNS lateralisation. Zebrafish is a well-established genetic system that allows a 'bottom up' ('gene to behaviour') approach to the study of lateralisation. In fact, it is the single vertebrate system to date in which asymmetric gene expression in the brain has been directly linked to asymmetric morphology. Zebrafish offers several experimental advantages that allow the study of brain lateralisation using a wide range of experimental tools, from study of gene function through in vivo analysis of morphology and physiology to behavioural assessments. Altogether, these features will allow the establishment of operational links between lower (genetics and morphology) and upper (physiology and behaviour) levels of brain lateralisation.

Modulatory effect of substance P to the brain stem locomotor command in lampreys

Substance P initiates locomotion when injected in the brain stem of mammals. This study examined the possible role of this peptide on the supraspinal locomotor command system in lampreys. Substance P was bath applied or locally injected into an in vitro isolated brain stem, and the effects of the drug were examined on reticulospinal cells and on the occurrence of swimming in a semi-intact preparation. Bath applications of substance P induced sustained depolarizations occurring rhythmically in intracellularly recorded reticulospinal cells. Spiking activity was superimposed on the depolarizations and swimming was induced. The sustained depolarizations were abolished by tetrodotoxin, and substance P did not affect the membrane resistance of reticulospinal cells nor their firing properties, suggesting that it did not directly effect reticulospinal cells. To establish where the effects were exerted, successive lesions of the brain stem were made as well as local applications of the drug in the brain stem. Removing the mesencephalon abolished the sustained depolarizations, whereas large ejections of the drug in the mesencephalon excited reticulospinal cells and elicited bouts of swimming. More local injections into the mesencephalic locomotor region (MLR) also elicited swimming. After an injection of substance P, the current threshold needed to induce locomotion by MLR stimulation was decreased, and the size of the postsynaptic responses of reticulospinal cells to MLR stimulation was increased. Substance P also reduced the frequency of miniature spontaneous postsynaptic currents in reticulospinal cells. Taken together, these results suggest that substance P plays a neuromodulatory role on the brain stem locomotor networks of lampreys.

Cholinergic elements in the zebrafish central nervous system: Histochemical and immunohistochemical analysis

Recently, the zebrafish has been extensively used for studying the development of the central nervous system (CNS). However, the zebrafish CNS has been poorly analyzed in the adult. The cholinergic/cholinoceptive system of the zebrafish CNS was analyzed by using choline acetyltransferase (ChAT) immunohistochemistry and acetylcholinesterase (AChE) histochemistry in the brain, retina, and spinal cord. AChE labeling was more abundant and more widely distributed than ChAT immunoreactivity. In the telencephalon, ChAT-immunoreactive (ChAT-ir) cells were absent, whereas AChE-positive neurons were observed in both the olfactory bulb and the telencephalic hemispheres. The diencephalon was the region with the lowest density of AChE-positive cells, mainly located in the pretectum, whereas ChAT-ir cells were exclusively located in the preoptic region. ChAT-ir cells were restricted to the periventricular stratum of the optic tectum, but AChE-positive neurons were observed throughout the whole extension of the lamination except in the marginal stratum. Although ChAT immunoreactivity was restricted to the rostral tegmental, oculomotor, and trochlear nuclei within the mesencephalic tegmentum, a widespread distribution of AChE reactivity was observed in this region. The isthmic region showed abundant AChE-positive and ChAT-ir cells in the isthmic, secondary gustatory and superior reticular nucleus and in the nucleus lateralis valvulae. ChAT immunoreactivity was absent in the cerebellum, although AChE staining was observed in Purkinje and granule cells. The medulla oblongata showed a widespread distribution of AChE-positive cells in all main subdivisions, including the octavolateral area, reticular formation, and motor nuclei of the cranial nerves. ChAT-ir elements in this area were restricted to the descending octaval nucleus, the octaval efferent nucleus and the motor nuclei of the cranial nerves. Additionally, spinal cord motoneurons appeared positive to both markers. Substantial differences in the ChAT and AChE distribution between zebrafish and other fish species were observed, which could be important because zebrafish is widely used as a genetic or developmental animal model.

The epithalamus of the developing and adult frog: calretinin expression and habenular asymmetry in Rana esculenta

Expression of the calcium binding protein (CaBP) calretinin (CR) was studied with immunohistochemistry in the pineal complex and habenular nuclei (HN) of the developing and adult frog Rana esculenta. The frog pineal complex is a medial structure formed by two interconnected components, the frontal organ and the pineal organ or epiphysis; the habenular nuclei are bilateral and are asymmetric due to subdivision of the left dorsal nucleus into medial and lateral components. In the pineal complex, calretinin immunostaining of cells and fibers was consistently observed in developing and adult frogs. In the habenulae, calretinin immunoreactivity exhibited instead marked variations during development, and was expressed only in cells of the medial subnucleus of the left dorsal habenula. In particular, calretinin was detected at larval stages, peaked during metamorphosis, was markedly downregulated at the end of metamorphosis, and was evident again in adulthood. This sequence of calretinin expression was confirmed by quantitative analysis of immunoreactive cells in the left habenula. In tadpoles, calretinin-positive cells exhibited a dorsoventral gradient of density, while in adulthood, they were distributed throughout the dorsoventral extent of the medial subnucleus. The study demonstrates a peculiar developmental pattern, with transient downregulation, of asymmetric calretinin expression in the frog epithalamus. The findings indicate that calcium and calcium buffering systems may play critical roles in neurogenetic and neuronal migration processes implicated in the formation of the asymmetric habenular portion in amphibians. In addition, the reappearance of calretinin expression in the adult frog supports a distinct functional role of the asymmetric habenular component in amphibians.

Development of GABA-immunoreactive cells in the spinal cord of the sea lamprey,P. marinus

The lamprey spinal cord increases in length and size during all its life cycle; thus, it is expected that new cells will be generated. This expectation suggests that the locomotor circuits must be continuously remodeled. Key elements in the cellular network controlling locomotor behavior are inhibitory cells. Here, we studied the gamma-aminobutyric acid-immunoreactive (GABA-ir) cells in the lamprey spinal cord during postembryonic development. Three major populations of GABA-ir cells were identified according to their distribution: those located in the gray matter, those contacting the cerebrospinal liquid (LC cells), and those located in the white matter. The results show (1). the number of GABA-ir cells per segment increase from prolarvae (<10 mm) to adulthood; (2). the lower number of GABA-ir cells in 100 microm of spinal cord is 66 +/- 7, found in premetamorphic larvae, and the highest is 107 +/- 6, found in postmetamorphic animals; (3). the gray matter and LC GABA-ir cells show different variations in number depending on the developmental period. Thus, in the 10-mm larvae, the gray matter GABA-ir cells are more abundant than LC cells, whereas in the young postmetamorphic specimens, the contrary occurs. Most of the GABA-ir cells located in the white matter were classified as edge cells. They increase in number from the beginning of the prolarval period, where there are not white matter-positive cells, to the middle larval period, where there are 9 +/- 4 GABA-ir edge cells per segment. This value was unaltered in later periods, where GABA-ir edge cells represent 20-30% of the total number of edge cells per segment. The increase in number of GABA-ir cells in these populations during a specific point of the lamprey life cycle may indicate different inhibitory requirements of the locomotor circuit at different developmental periods.

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'.

Ontogeny of gamma-aminobutyric acid-immunoreactive neurons in the rhombencephalon and spinal cord of the sea lamprey

The development of neurons expressing gamma-aminobutyric acid (GABA) in the rhombencephalon and spinal cord of the sea lamprey (Petromyzon marinus) was studied for the first time with an anti-GABA antibody. The earliest GABA-immunoreactive (GABAir) neurons appear in late embryos in the basal plate of the isthmus, caudal rhombencephalon, and rostral spinal cord. In prolarvae, the GABAir neurons of the rhombencephalon appear to be distributed in spatially restricted cellular domains that, at the end of the prolarval period, form four longitudinal GABAir bands (alar dorsal, alar ventral, dorsal basal, and ventral basal). In the spinal cord, we observed only three GABAir longitudinal bands (dorsal, intermediate, and ventral). The larval pattern of GABAir neuronal populations was established by the 30-mm stage, and the same populations were observed in premetamorphic and adult lampreys. The ontogeny of GABAergic populations in the lamprey rhombencephalon and spinal cord is, in general, similar to that previously described in mouse and Xenopus.

Local tissue interactions across the dorsal midline of the forebrain establish CNS laterality

The mechanisms that establish behavioral, cognitive, and neuroanatomical asymmetries are poorly understood. In this study, we analyze the events that regulate development of asymmetric nuclei in the dorsal forebrain. The unilateral parapineal organ has a bilateral origin, and some parapineal precursors migrate across the midline to form this left-sided nucleus. The parapineal subsequently innervates the left habenula, which derives from ventral epithalamic cells adjacent to the parapineal precursors. Ablation of cells in the left ventral epithalamus can reverse laterality in wild-type embryos and impose the direction of CNS asymmetry in embryos in which laterality is usually randomized. Unilateral modulation of Nodal activity by Lefty1 can also impose the direction of CNS laterality in embryos with bilateral expression of Nodal pathway genes. From these data, we propose that laterality is determined by a competitive interaction between the left and right epithalamus and that Nodal signaling biases the outcome of this competition.

Ontogeny of gamma-aminobutyric acid-immunoreactive neuronal populations in the forebrain and midbrain of the sea lamprey

Although brain organization in lampreys is of great interest for understanding evolution in vertebrates, knowledge of early development is very scarce. Here, the development of the forebrain and midbrain gamma-aminobutyric acid (GABA)-ergic systems was studied in embryos, prolarvae, and small larvae of the sea lamprey using an anti-GABA antibody. Ancillary immunochemical markers, such as proliferating cell nuclear antigen (PCNA), calretinin, and serotonin, as well as general staining methods and semithin sections were used to characterize the territories containing GABA-immunoreactive (GABAir) neurons. Differentiation of GABAir neurons in the diencephalon begins in late embryos, whereas differentiation in the telencephalon and midbrain was delayed to posthatching stages. In lamprey prolarvae, the GABAir populations appear either as compact GABAir cell groups or as neurons interspersed among GABA-negative cells. In the telencephalon of prolarvae, a band of cerebrospinal fluid-contacting (CSF-c) GABAir neurons (septum) was separated from the major GABAir telencephalic band, the striatum (ganglionic eminence) primordium. The striatal primordium appears to give rise to most GABAir neurons observed in the olfactory bulb and striatum of early larval stages. GABAir populations in the dorsal telencephalon appear later, in 15-30-mm-long larvae. In the diencephalon, GABAir neurons appear in embryos, and the larval pattern of GABAir populations is recognizable in prolarvae. A small GABAir cluster consisting mainly of CSF-c neurons was observed in the caudal preoptic area, and a wide band of scattered CSF-c GABAir neurons extended from the preoptic region to the caudal infundibular recess. A mammillary GABAir population was also distinguished. Two compact GABAir clusters, one consisting of CSF-c neurons, were observed in the rostral (ventral) thalamus. In the caudal (dorsal) thalamus, a long band extended throughout the ventral tier. The nucleus of the medial longitudinal fascicle contained an early-appearing GABAir population. The paracommissural pretectum of prolarvae and larvae contained a large group of non-CSF-c GABAir neurons, although it was less compact than those of the thalamus, and a further group was found in the dorsal pretectum. In the midbrain of larvae, several groups of GABAir neurons were observed in the dorsal and ventral tegmentum and in the torus semicircularis. The development of GABAergic populations in the lamprey forebrain was similar to that observed in teleosts and in mouse, suggesting that GABA is a very useful marker for understanding evolution of forebrain regions. The possible relation between early GABAergic cell groups and the regions of the prosomeric map of the lamprey forebrain (Pombal and Puelles [ 1999] J. Comp. Neurol. 414:391-422) is discussed in view of these results and information obtained with ancillary markers.

Proliferating cell nuclear antigen (PCNA) immunoreactivity and development of the pineal complex and habenula of the sea lamprey

The development of the pineal complex and the habenula of the sea lamprey was studied with proliferating cell nuclear antigen (PCNA) immunocytochemistry. The pineal organ and the habenula primordia appeared in late embryos, and neuron differentiation began in prolarvae, as indicated by the presence of PCNA-negative cells. The parapineal primordium could not be distinguished until early prolarval stages, and cell differentiation was delayed to the larval period. Although the number of cycling (PCNA-immunoreactive) cells gradually decreased during the larval period in the three organs studied, their patterns of differentiation were different. We conclude that the unusual developmental pattern observed is related with the complex life cycle of lampreys.

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.