Subnuclear development of the zebrafish habenular nuclei requires ER translocon function

The dorsal habenular nuclei (Dh) of the zebrafish are characterized by significant left–right differences in gene expression, anatomy, and connectivity. Notably, the lateral subnucleus of the Dh (LsDh) is larger on the left side of the brain than on the right, while the medial subnucleus (MsDh) is larger on the right compared to the left. A screen for mutations that affect habenular laterality led to the identification of the sec61a-like 1(sec61al1) gene. In sec61al1^c163 mutants, more neurons in the LsDh and fewer in the MsDh develop on both sides of the brain. Generation of neurons in the LsDh occurs more rapidly and continues for a longer time period in mutants than in WT. Expression of Nodal pathway genes on the left side of the embryos is unaffected in mutants, as is the left sided placement of the parapineal organ, which promotes neurogenesis in the LsDh of WT embryos. Ultrastructural analysis of the epithalamus indicates that ventricular precursor cells, which form an epithelium in WT embryos, lose apical-basal polarity in sec61al1^c163 mutants. Our results show that in the absence of sec61al1, an excess of precursor cells for the LsDh exit the ventricular region and differentiate, resulting in formation of bilaterally symmetric habenular nuclei.     

Ontogeny of melatonin, Per2 and E4bp4 light responsiveness in the chicken embryonic pineal gland.

The chicken pineal gland possesses the capacity to generate circadian oscillations, is able to synchronize to external light:dark cycles and can generate an hormonal output--melatonin. We examined the light responses of the chicken pineal gland and its effects on melatonin and Per2, Bmal1 and E4bp4 expression in 19-day old embryos and hatchlings during the dark phase, subjective light phase and in constant darkness. Expression of Per2 and E4bp4 were rhythmic under light:dark conditions, but the rhythms of E4bp4 and Bmal1 mRNA did not persist in constant darkness in 19-day old embryos. Per2 mRNA expression persisted in constant darkness, but with a reduced amplitude. Per2 expression was inducible by light only during the subjective day. Melatonin release was inhibited by light only at end of the dark phase and during the subjective light phase in embryos. Our data demonstrate that the embryonic avian pineal pacemaker is light sensitive and can generate rhythmic output, however the effects of light were diminished in chick embryos in compared to hatchlings.     

Ontogeny of melatonin, Per2 and E4bp4 light responsiveness in the chicken embryonic pineal gland

The chicken pineal gland possesses the capacity to generate circadian oscillations, is able to synchronize to external light:dark cycles and can generate an hormonal output--melatonin. We examined the light responses of the chicken pineal gland and its effects on melatonin and Per2, Bmal1 and E4bp4 expression in 19-day old embryos and hatchlings during the dark phase, subjective light phase and in constant darkness. Expression of Per2 and E4bp4 were rhythmic under light:dark conditions, but the rhythms of E4bp4 and Bmal1 mRNA did not persist in constant darkness in 19-day old embryos. Per2 mRNA expression persisted in constant darkness, but with a reduced amplitude. Per2 expression was inducible by light only during the subjective day. Melatonin release was inhibited by light only at end of the dark phase and during the subjective light phase in embryos. Our data demonstrate that the embryonic avian pineal pacemaker is light sensitive and can generate rhythmic output, however the effects of light were diminished in chick embryos in compared to hatchlings.     

The state of peroxidation processes in the basal nuclei of the brain under conditions of an altered photoperiod

The state ofperoxidation processes in the basal nuclei (the nucleus caudatus, globus pallidus, nucleus accumbens, amigdaloid complex) of the rat's brain under conditions of an altered photoperiod has been studied. A disturbance of regular photoperiodicity was shown to result in metabolic changes in the mentioned structures of the brain. Enhancement of the processes of lipid and protein peroxidation was observed in the basal nuclei under constant light, the intensity of fibrinolysis and proteolysis increases, the activity of the enzymes of antioxidant defense decreases. Heterodirectional changes of fibrinolysis and proteolysis in individual structures, a decrease of free radical processes against a background of accumulation of modified proteins were observed under conditions of constant darkness.     

Duration of melatonin regulates seasonal changes in song control nuclei of the house sparrow, Passer domesticus: independence from gonads and circadian entrainment

Avian behavior and physiology are temporally regulated by a complex circadian clock on both a daily and an annual basis. The circadian secretion of the hormone melatonin is a critical component of the regulation of circadian/daily processes in passerine birds, but there is little evidence that the gland regulates annual changes in primary reproductive function. Here it is shown that locomotor rhythms of house sparrows, Passer domesticus, which are made arrhythmic by either pinealectomy or maintenance in constant light, can be synchronized by daily administration of melatonin of different durations to simulate the melatonin profiles indicative of long and short photoperiods. Pinealectomized male sparrows maintained in constant darkness were entrained by both melatonin regimens. In both cases, testes were regressed and the song control nuclei were small. Intact male house sparrows maintained in constant light were also entrained to both melatonin regimens. However, sparrows that received a long duration melatonin cycle exhibited small song control nuclei, while sparrows that received short duration melatonin or no melatonin at all exhibited large song control nuclei. The data indicate that seasonal changes in melatonin duration contribute to the regulation of song control nuclei.     

Phase response relationships between light pulses and the melatonin rhythm in rats.

There is some controversy whether phase response curves constructed from studies conducted after acute release into constant darkness (Type II) or after prolonged constant darkness are comparable. This study investigated the effects of brief low-intensity light pulses on the onset of 6-sulphatoxymelatonin excretion in rats 48 to 60 h after lights-off and after 14 days of continuous darkness. In the former condition, maximum phase delays occurred between 4 and 6 h after expected lights-off, but no phase advances were observed within 2 days of the presentation of the stimulus. When the times of the pulses were plotted in relation to the individual onsets, peak light-induced phase delays occurred 0 to 2 h after melatonin onset. After 14 days in continuous darkness, the peak phase delays also occurred 0 to 2 h after melatonin onset and were slightly but significantly smaller. No significant phase advances were observed. In a separate small series of experiments, the temperature rhythm of rats was shown to be delayed by a comparable degree to that of melatonin by light pulses 2 and 4 h after expected lights-off under the Type II conditions. It is concluded that phase response curves conducted under Type I and Type II conditions are comparable.     

Directional asymmetry of the zebrafish epithalamus guides dorsoventral innervation of the midbrain target

The zebrafish epithalamus, consisting of the pineal complex and flanking dorsal habenular nuclei, provides a valuable model for exploring how left-right differences could arise in the vertebrate brain. The parapineal lies to the left of the pineal and the left habenula is larger, has expanded dense neuropil, and distinct patterns of gene expression from the right habenula. Under the influence of Nodal signaling, positioning of the parapineal sets the direction of habenular asymmetry and thereby determines the left-right origin of habenular projections onto the midbrain target, the interpeduncular nucleus (IPN). In zebrafish with parapineal reversal, neurons from the left habenula project to a more limited ventral IPN region where right habenular axons would normally project. Conversely, efferents from the right habenula adopt a more extensive dorsoventral IPN projection pattern typical of left habenular neurons. Three members of the leftover-related KCTD (potassium channel tetramerization domain containing) gene family are expressed differently by the left and right habenula, in patterns that define asymmetric subnuclei. Molecular asymmetry extends to protein levels in habenular efferents, providing additional evidence that left and right axons terminate within different dorsoventral regions of the midbrain target. Laser-mediated ablation of the parapineal disrupts habenular asymmetry and consequently alters the dorsoventral distribution of innervating axons. The results demonstrate that laterality of the dorsal forebrain influences the formation of midbrain connections and their molecular properties.     

The role of seed coat in early stages of soybean germination

The presence of the seed coat delays the early stages of germination of soybean embryos. This retardation seems to be due to the seed coat acting as a mechanical barrier to seedling. Although entire soybean seeds are insensitive to light, removal of the seed coat induces a light requiring condition. The early germination of naked seeds or isolated embryonic axes is enhanced by darkness. Red light produced a similar effect as white, providing evidence for the effective wavelength of the photo-inhibition. Far-red light acted similarly to darkness in promoting the early germination of the embryos. It is suggested that possibly light may act through phytochrome by the high irradiance system.     

The parapineal mediates left-right asymmetry in the zebrafish diencephalon

The dorsal diencephalon (or epithalamus) of larval zebrafish displays distinct left-right asymmetries. The pineal complex consists of the pineal organ anlage and an unpaired, left-sided accessory organ - the parapineal. The neighboring brain nuclei, the left and right dorsal habenulae, show consistent differences in their size, density of neuropil and gene expression. Mutational analyses demonstrate a correlation between the left-right position of the parapineal and the laterality of the habenular nuclei. We show that selective ablation of the parapineal organ results in the loss of habenular asymmetry. The left-sided parapineal therefore influences the left-right identity of adjacent brain nuclei, indicating that laterality of the dorsal diencephalon arises in a step-wise fashion.     

Feeding rhythms in constant light and constant darkness: the role of the eyes and the effect of melatonin infusion

Exposure to constant light abolishes circadian behavioral rhythms of locomotion and feeding as well as circulating melatonin rhythms in pigeons (Columba livia). To determine if feeding rhythmicity could be maintained in pigeons exposed to constant light, periodic infusions (10h/day) of melatonin were administered to pinealectomized and bilaterally retinectomized/pinealectomized pigeons under conditions of both constant darkness and constant light. The infusions were sufficient to entrain rhythmicity in pinealectomized pigeons in constant darkness and to restore and maintain rhythmicity in bilaterally retinectomized/pinealectomized pigeons in constant darkness. On subsequent exposure to constant light, rhythmicity remained phase locked to the melatonin infusions in bilaterally retinectomized/pinealectomized pigeons but was abolished in sighted pinealectomized birds. These results suggest that while endogenous melatonin rhythms are both necessary and sufficient to maintain behavioral rhythms in DD, their effect can be overridden by constant light but only if perceived by the eyes. Thus, constant light may abolish behavioral rhythmicity in intact pigeons (and perhaps in other species) by a mechanism other than suppression of endogenous melatonin rhythmicity. Such a mechanism might involve direct stimulation of locomotor or feeding activity by retinally perceived (but not by extra-retinally perceived) light, or alternatively by suppression of a hypothalamic oscillator that receives its major light input from the retinae.Abbreviations PX pinealectomized - EX bilaterally enucleated - LD light:dark cycle - LL constant light - DD constant darkness - DD_b constant darkness before exposure to constant light - DD_a constant darkness after exposure to constant light     

Interplay between Hsp90 and TPR domain-containing proteins in steroidogenic signaling

EGFL7 drives the formation of neurons from neural stem cells. In the embryonic and adult brain this process is essential for neurogenesis and homeostasis of the nervous system. The function of adult neurogenesis is not fully understood but maybe it supports life-long learning and brain repair after injuries such as stroke. The transition of neural stem cells into mature neurons is tightly regulated. One of the essential signaling pathways governing this process is the Notch pathway, which controls metazoan development. In a recent publication, we identified a novel non-canonical Notch ligand, EGFL7, and described its impact on neural stem cells.¹ We explored the molecular mechanisms, which this molecule affects to regulate the self-renewal capacity of neural stem cells and to promote their differentiation into neurons. In this review, we discuss the implications of our findings for adult neurogenesis and illustrate the potential of EGFL7 to serve as an agent to increase neurogenesis and the self-renewal potential of the brain.     

Born this way: Hippocampal neurogenesis across the lifespan

The capability of the mammalian brain to generate new neurons through the lifespan has gained much attention for the promise of new therapeutic possibilities especially for the aging brain. One of the brain regions that maintains a neurogenesis‐permissive environment is the dentate gyrus of the hippocampus. Here, new neurons are generated from a pool of multipotent neural progenitor cells to become fully functional neurons that are integrated into the brain circuitry. A growing body of evidence points to the fact that neurogenesis in the adult hippocampus is necessary for certain memory processes, and in mood regulation, while alterations in hippocampal neurogenesis have been associated with a myriad of neurological and psychiatric disorders. More recently, evidence has come to light that new neurons may differ in their vulnerability to environmental and disease‐related influences depending on the time during the life course at which they are exposed. Thus, it has been the topic of intense research in recent years. In this review, we will discuss the complex process and associated functional relevance of hippocampal neurogenesis during the embryonic/postnatal period and in adulthood. We consider the implications of hippocampal neurogenesis during the developmentally critical periods of adolescence and older age. We will further consider the literature surrounding hippocampal neurogenesis and its functional role during these critical periods with a view to providing insight into the potential of harnessing neurogenesis for health and therapeutic benefit.     

Retinoic acid functions as a key GABAergic differentiation signal in the basal ganglia

Although retinoic acid (RA) has been implicated as an extrinsic signal regulating forebrain neurogenesis, the processes regulated by RA signaling remain unclear. Here, analysis of retinaldehyde dehydrogenase mutant mouse embryos lacking RA synthesis demonstrates that RA generated by Raldh3 in the subventricular zone of the basal ganglia is required for GABAergic differentiation, whereas RA generated by Raldh2 in the meninges is unnecessary for development of the adjacent cortex. Neurospheres generated from the lateral ganglionic eminence (LGE), where Raldh3 is highly expressed, produce endogenous RA, which is required for differentiation to GABAergic neurons. In Raldh3?/? embryos, LGE progenitors fail to differentiate into either GABAergic striatal projection neurons or GABAergic interneurons migrating to the olfactory bulb and cortex. We describe conditions for RA treatment of human embryonic stem cells that result in efficient differentiation to a heterogeneous population of GABAergic interneurons without the appearance of GABAergic striatal projection neurons, thus providing an in vitro method for generation of GABAergic interneurons for further study. Our observation that endogenous RA is required for generation of LGE-derived GABAergic neurons in the basal ganglia establishes a key role for RA signaling in development of the forebrain.     

The effect of different wavelengths of light during incubation on the development of rhythmic pineal melatonin biosynthesis in chick embryos

Rhythmic pineal melatonin biosynthesis develops in chick embryos incubated under a light (L)-dark (D) cycle of polychromatic white light. The spectral sensitivity of the embryonic pineal gland is not known and was investigated in this study. Broiler breeder eggs (Ross 308, n=450) were incubated under white, red, green or blue light under the 12L : 12D cycle. Melatonin was measured in extracts of pineal glands by radioimmunoassay. The daily rhythm of pineal melatonin levels in 20-day-old chick embryos was confirmed during the final stages of embryonic life under all four wavelengths of light with expected higher concentrations during dark- than light-times. The highest pineal melatonin levels were determined in chick embryos incubated under red and white light and lower levels under green light. The incubation under blue light resulted in the lowest melatonin biosynthesis. Pineal melatonin concentrations increased substantially on post-hatching day two compared with pre-hatching levels and we did not find differences between birds incubated and kept in either white or green light. Our results demonstrate a selective sensitivity of the chick embryo pineal gland to different wavelengths of light. Rhythmic melatonin production is suggested as a possible mechanism, which transfers information about the quality of ambient light to the developing avian embryo.     

Effect of acute hypoxia on the intensity of free radical processes in the basal nuclei of the brain, and the rat behaviour in the open field test under conditions of altered photoperiod

The effect of acute hypoxia on the intensity of free radical processes in the basal nuclei (the nucleus caudatus, globus pallidus. nucleus accumbens. amygdaloid complex) of the brain, and the rat behaviour in the open field test has been studied under conditions of altered photoperiod. It has been shown that constant darkness levels the effect of acute hypoxia on the intensity of lipid peroxidation, preserves the activity of superoxide dismutase and catalase at a higher level, lowers the activity of glutathione peroxidase. Under light, the sensitivity of basal nuclei neurons to acute hypoxia is enhanced, the latter being reflected in intensification of lipid peroxidation at the expense of increased formation of dien conjugates. The activity of catalase at that considerably exceeds the level of even intact rats in all the structures. It has been established that an altered photoperiod modulates the effect of acute hypoxia on the parameters of rat's activity in the open field, the character of their change depending on the nature of a photophase change.     

Osteocalcin- and Osteopontin-Containing Neurons in the Rat Hind Brain

Immunohistochemistry for osteocalcin (OC) and osteopontin (OPN) was performed to know their distributions in the hind brain of adult rats. OC- and OPN-immunoreactivity (-ir) were detected in neuronal cell bodies, including perikarya and proximal dendrites and the neuropil. In the cranial nerve motor nuclei, numerous OC- and OPN-immunoreactive (-ir) neurons were detected. The neuropil in the cranial motor nuclei mostly showed strong OC- and OPN-staining intensity. The cranial nerve sensory nuclei and other relay and modulating structures in the lower brain stem also contained various numbers of OC- and OPN-ir neurons. The staining intensities in the neuropil were varied among these regions. In the cerebellar cortex, Purkinje cells and granule cells showed OPN-ir but not OC-ir. However, OC- and OPN-ir neurons were abundantly distributed throughout the cerebellar nuclei. The neuropil in the cerebellar nuclei showed moderate OC-ir and strong OPN-ir staining intensities. These findings indicate that the distribution patterns of OC- and OPN-ir neurons were similar in many structures within the hind brain. OC may play a role in modulating neuroprotective function of OPN.