Ontogenesis of the asymmetric parapineal organ in the zebrafish epithalamus

The parapineal organ is a midline-derived epithalamic structure that in zebrafish adopts a left-sided position at embryonic stages to promote the development of left-right asymmetries in the habenular nuclei. Despite extensive knowledge about its embryonic and larval development, it is still unknown whether the parapineal organ and its profuse larval connectivity with the left habenula are present in the adult brain or whether, as assumed from historical conceptions, this organ degenerates during ontogeny. This paper addresses this question by performing an ontogenetic analysis using an integrative morphological, ultrastructural and neurochemical approach. We find that the parapineal organ is lost as a morphological entity during ontogeny, while parapineal cells are incorporated into the posterior wall of the adult left dorsal habenular nucleus as small clusters or as single cells. Despite this integration, parapineal cells retain their structural, neurochemical and connective features, establishing a reciprocal synaptic connection with the more dorsal habenular neuropil. Furthermore, we describe the ultrastructure of parapineal cells using transmission electron microscopy and report immunoreactivity in parapineal cells with antibodies against substance P, tachykinin, serotonin and the photoreceptor markers arrestin3a and rod opsin. Our findings suggest that parapineal cells form an integral part of a neural circuit associated with the left habenula, possibly acting as local modulators of the circuit. We argue that the incorporation of parapineal cells into the habenula may be part of an evolutionarily relevant developmental mechanism underlying the presence/absence of the parapineal organ in teleosts, and perhaps in a broader sense in vertebrates.

Diversification of habenular organization and asymmetries in teleosts: Insights from the Atlantic salmon and European eel

Habenulae asymmetries are widespread across vertebrates and analyses in zebrafish, the reference model organism for this process, have provided insight into their molecular nature, their mechanisms of formation and their important roles in the integration of environmental and internal cues with a variety of organismal adaptive responses. However, the generality of the characteristics identified in this species remains an open question, even on a relatively short evolutionary scale, in teleosts. To address this question, we have characterized the broad organization of habenulae in the Atlantic salmon and quantified the asymmetries in each of the identified subdomains. Our results show that a highly conserved partitioning into a dorsal and a ventral component is retained in the Atlantic salmon and that asymmetries are mainly observed in the former as in zebrafish. A remarkable difference is that a prominent left-restricted pax6 positive nucleus is observed in the Atlantic salmon, but undetectable in zebrafish. This nucleus is not observed outside teleosts, and harbors a complex presence/absence pattern in this group, retaining its location and cytoarchitectonic organization in an elopomorph, the European eel. These findings suggest an ancient origin and high evolvability of this trait in the taxon. Taken together, our data raise novel questions about the variability of asymmetries across teleosts and their biological significance depending on ecological contexts.

Anatomical and histological investigation of the pineal gland in the lake van fish (Alburnus tarichi (Güldenstädt, 1814))

The pineal gland and melatonin secreted from the gland regulate the biological clock and adaptation to seasonal changes, glucose balance, nutrition and locomotor activities. In this study, the pineal gland of the Lake Van fish was examined anatomically and histologically. The melatonin level secreted from the pineal gland was determined in fish plasma sampled from both lakes and streams during reproduction migration. The pineal gland in the Lake Van fish, as in other teleost fish, is located in the head, under the translucent pineal window, which does not contain many pigment cells. The gland consists of pineal vesicle and pineal stalk parts on the dorsal sac in the Lake Van fish. It was determined that the pineal gland showed good vascularity. The presence of pinealocytes and different types of cells in the pineal organ was determined histologically. Pinealocytes were intensely localized in the lumen of the pineal vesicle. The plasma melatonin level increased in fish passing from lake to stream for reproductive migration.

Input of sensory, limbic, basal ganglia and pallial/cortical information into the ventral/lateral habenula: Functional principles in anuran amphibians

The habenula - a phylogenetically old brain structure present in all vertebrates - is involved in pain processing, reproductive behaviors, sleep-wake cycles, stress responses, reward, and learning. We performed intra- and extracellular recordings of ventral habenula (VHb) neurons in the isolated brain of anurans and revealed similar cell and response properties to those reported for the lateral habenula of mammals. We identified tonic regular, tonic irregular, rhythmic firing, and silent VHb neurons. Transitions between these firing patterns were observed during spontaneous activity. Electrical stimulation of various brain areas demonstrated VHb input of auditory, optic, limbic, basal ganglia, and pallial information. This resulted in three different response behaviors in VHb neurons: excitation, inhibition, or alternating facilitation and suppression of neuronal activity. Spontaneously changing activity patterns were observed to modulate, reset, or suppress the response behavior of VHb neurons, indicating a gating mechanism. This could be a network status or context dependent selection mechanism for which information are transmitted to task relevant brain areas (i.e., sensory system, limbic system, basal ganglia). Furthermore, alternating facilitation and suppression sequences upon auditory nerve stimulation correlated positively fictive motor activities recorded via the compound potential of the vagal nerve. Stimulation of the auditory nerve or the habenula led to facilitation, suppression, or alternating facilitation and suppression of neuronal activity in putative dopaminergic neurons. Due to complex habenula feedback loops with basal ganglia, limbic, and sensory systems, the habenula involvement in a variety of functions might therefore be explained by a modulatory effect on a task-relevant input stream.

Asymmetric pitx2 expression in medaka epithalamus is regulated by nodal signaling through an intronic enhancer

The epithalamic region of fishes shows prominent left-right asymmetries that are executed by nodal signaling upstream of the asymmetry-determining transcription factor pitx2. Previous reports have identified that nodal controls the left-sided pitx2 expression in the lateral plate mesoderm through an enhancer present in the last intron of this gene. However, whether similar regulation occurs also in the case of epithalamic asymmetry is currently unresolved. Here, we address some of the cis-regulatory information that control asymmetric pitx2 expression in epithalamus by presenting a Tg(pitx2:EGFP) 116-17 transgenic medaka model, which expresses enhanced green fluorescent protein (EGFP) under control of an intronic enhancer. We show that this transgene recapitulates epithalamic expression of the endogenous pitx2 and that it responds to nodal signaling inhibition. Further, we identify that three foxh1-binding sites present in this enhancer modulate expression of the transgene and that the second site is absolutely necessary for the left-sided epithalamic expression while the other two sites may have subtler regulative roles. We provide evidence that left-sided epithalamic pitx2 expression is controlled through an enhancer present in the last intron of this gene and that the regulatory logic underlying asymmetric pitx2 expression is shared between epithalamic and lateral plate mesoderm regions.

The evolution of asymmetric photosensitive structures in metazoans and the Nodal connection

Asymmetries are observed in a great number of taxa in metazoans. More particularly, functional lateralization and neuroanatomical asymmetries within the central nervous system have been a matter of intense research for at least two hundred years. While asymmetries of some paired structures/organs (e.g. eyes, ears, kidneys, legs, arms) constitute random deviations from a pure bilateral symmetry, brain asymmetries such as those observed in the cortex and epithalamus are directional. This means that molecular and anatomical features located on one side of a given structure are observed in most individuals. For instance, in humans, the neuronal tract connecting the language areas is enlarged in the left hemisphere. When asymmetries are fixed, their molecular mechanisms can be studied using mutants displaying different phenotypes: left or right isomerism of the structure, reversed asymmetry or random asymmetry. Our understanding of asymmetry in the nervous system has been widely enriched thanks to the characterization of mutants affecting epithalamus asymmetry. Furthermore, two decades ago, pioneering studies revealed that a specific morphogen, Nodal, active only on one side of the embryo during development is an important molecule in asymmetry patterning. In this review, I have gathered important data bringing insight into the origin and evolution of epithalamus asymmetry and the role of Nodal in metazoans. After a short introduction on brain asymmetries (chapter I), I secondly focus on the molecular and anatomical characteristics of the epithalamus in vertebrates and explore some functional aspects such as its photosensitive ability related to the pineal complex (chapter II). Third, I discuss homology relationship of the parapineal organ among vertebrates (chapter III). Fourth, I discuss the possible origin of the epithalamus, presenting cells displaying photosensitive properties and/or asymmetry in the anterior part of the body in non-vertebrates (chapter IV). Finally, I report Nodal signaling expression data and functional experiments performed in different metazoan groups (chapter V).

New perspective on the regionalization of the anterior forebrain in Osteichthyes

In the current model, the most anterior part of the forebrain (secondary prosencephalon) is subdivided into the telencephalon dorsally and the hypothalamus ventrally. Our recent study identified a new morphogenetic unit named the optic recess region (ORR) between the telencephalon and the hypothalamus. This modification of the forebrain regionalization based on the ventricular organization resolved some previously unexplained inconsistency about regional identification in different vertebrate groups. The ventricular-based comparison also revealed a large diversity within the subregions (notably in the hypothalamus and telencephalon) among different vertebrate groups. In tetrapods there is only one hypothalamic recess, while in teleosts there are two recesses. Most notably, the mammalian and teleost hypothalami are two extreme cases: the former has lost the cerebrospinal fluid-contacting (CSF-c) neurons, while the latter has increased them. Thus, one to one homology of hypothalamic subregions in mammals and teleosts requires careful verification. In the telencephalon, different developmental processes between Sarcopterygii (lobe-finned fish) and Actinopterygii (ray-finned fish) have already been described: the evagination and the eversion. Although pallial homology has been long discussed based on the assumption that the medial-lateral organization of the pallium in Actinopterygii is inverted from that in Sarcopterygii, recent developmental data contradict this assumption. Current models of the brain organization are largely based on a mammalian-centric point of view, but our comparative analyses shed new light on the brain organization of Osteichthyes.

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.