Dynamics and plasticity of peptidergic control centres in the retino-brain-pituitary system of Xenopus laevis

Bidirectional crosstalk between Hypoxia-Inducible Factor and glucocorticoid signalling in zebrafish larvae

In the last decades in vitro studies highlighted the potential for crosstalk between Hypoxia-Inducible Factor-(HIF) and glucocorticoid-(GC) signalling pathways. However, how this interplay precisely occurs in vivo is still debated. Here, we use zebrafish larvae (Danio rerio) to elucidate how and to what degree hypoxic signalling affects the endogenous glucocorticoid pathway and vice versa, in vivo. Firstly, our results demonstrate that in the presence of upregulated HIF signalling, both glucocorticoid receptor (Gr) responsiveness and endogenous cortisol levels are repressed in 5 days post fertilisation larvae. In addition, despite HIF activity being low at normoxia, our data show that it already impedes both glucocorticoid activity and levels. Secondly, we further analysed the in vivo contribution of glucocorticoids to HIF activity. Interestingly, our results show that both glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) play a key role in enhancing it. Finally, we found indications that glucocorticoids promote HIF signalling via multiple routes. Cumulatively, our findings allowed us to suggest a model for how this crosstalk occurs in vivo.

A zebrafish model of glucocorticoid resistance shows serotonergic modulation of the stress response

One function of glucocorticoids is to restore homeostasis after an acute stress response by providing negative feedback to stress circuits in the brain. Loss of this negative feedback leads to elevated physiological stress and may contribute to depression, anxiety, and post-traumatic stress disorder. We investigated the early, developmental effects of glucocorticoid signaling deficits on stress physiology and related behaviors using a mutant zebrafish, gr(s357), with non-functional glucocorticoid receptors (GRs). These mutants are morphologically inconspicuous and adult-viable. A previous study of adult gr(s357) mutants showed loss of glucocorticoid-mediated negative feedback and elevated physiological and behavioral stress markers. Already at 5 days post-fertilization, mutant larvae had elevated whole body cortisol, increased expression of pro-opiomelanocortin (POMC), the precursor of adrenocorticotropic hormone (ACTH), and failed to show normal suppression of stress markers after dexamethasone treatment. Mutant larvae had larger auditory-evoked startle responses compared to wildtype sibling controls (gr(wt)), despite having lower spontaneous activity levels. Fluoxetine (Prozac) treatment in mutants decreased startle responding and increased spontaneous activity, making them behaviorally similar to wildtype. This result mirrors known effects of selective serotonin reuptake inhibitors (SSRIs) in modifying glucocorticoid signaling and alleviating stress disorders in human patients. Our results suggest that larval gr(s357) zebrafish can be used to study behavioral, physiological, and molecular aspects of stress disorders. Most importantly, interactions between glucocorticoid and serotonin signaling appear to be highly conserved among vertebrates, suggesting deep homologies at the neural circuit level and opening up new avenues for research into psychiatric conditions.

About a snail, a toad, and rodents: animal models for adaptation research

Neural adaptation mechanisms have many similarities throughout the animal kingdom, enabling to study fundamentals of human adaptation in selected animal models with experimental approaches that are impossible to apply in man. This will be illustrated by reviewing research on three of such animal models, viz. (1) the egg-laying behavior of a snail, Lymnaea stagnalis: how one neuron type controls behavior, (2) adaptation to the ambient light condition by a toad, Xenopus laevis: how a neuroendocrine cell integrates complex external and neural inputs, and (3) stress, feeding, and depression in rodents: how a neuronal network co-ordinates different but related complex behaviors. Special attention is being paid to the actions of neurochemical messengers, such as neuropeptide Y, urocortin 1, and brain-derived neurotrophic factor. While awaiting new technological developments to study the living human brain at the cellular and molecular levels, continuing progress in the insight in the functioning of human adaptation mechanisms may be expected from neuroendocrine research using invertebrate and vertebrate animal models.

Illumination controls differentiation of dopamine neurons regulating behaviour

Specification of the appropriate neurotransmitter is a crucial step in neuronal differentiation because it enables signalling among populations of neurons. Experimental manipulations demonstrate that both autonomous and activity-dependent genetic programs contribute to this process during development, but whether natural environmental stimuli specify transmitter expression in a neuronal population is unknown. We investigated neurons of the ventral suprachiasmatic nucleus that regulate neuroendocrine pituitary function in response to light in teleosts, amphibia and primates. Here we show that altering light exposure, which changes the sensory input to the circuit controlling adaptation of skin pigmentation to background, changes the number of neurons expressing dopamine in larvae of the amphibian Xenopus laevis in a circuit-specific and activity-dependent manner. Neurons newly expressing dopamine then regulate changes in camouflage colouration in response to illumination. Thus, physiological activity alters the numbers of behaviourally relevant amine-transmitter-expressing neurons in the brain at postembryonic stages of development. The results may be pertinent to changes in cognitive states that are regulated by biogenic amines.

Calcium channel kinetics of melanotrope cells in Xenopus laevis depend on environmental stimulation

We have tested the hypothesis that the type and kinetics of voltage-activated Ca(2+) channels in a neuroendocrine cell depend on the cell's long-term external input. For this purpose, the presence and kinetics of both low (LVA) and high-voltage-activated (HVA) L-type Ca(2+) channels have been assessed in melanotrope pituitary cells of the amphibian Xenopus laevis. The secretory activity of this cell type can readily be manipulated in vivo by changing the animal's environmental light condition, from a black to a white background. We here show that, compared to white background-adapted Xenopus, melanotropes from black background-adapted frogs have (1) a much larger size, as revealed by their 2.5 times larger membrane capacitance (P<0.001), (2) a 2 times higher HVA current density (P<0.05), (3) a clearly smaller Ca(2+)-dependent inactivation (10%; P<0.05), (4) L-type channels with 5 times slower activation and inactivation kinetics (P<0.05), and (5) slower kinetics of L-type channels that become faster and more similar to those in white-background adapted cells when the intracellular Ca(2+)-buffering capacity is reduced. Furthermore, white-adapted melanotropes possess LVA-type Ca(2+) channels, which are lacking from cells from black-adapted animals. The melanotrope calmodulin mRNA level does not differ between the two adaptation states. These results indicate that HVA L-type channel kinetics differ in relation to environmentally induced changes in cellular secretory state, probably mediated via intracellular Ca(2+)-buffering, whereas the occurrence of LVA Ca(2+) channels may depend on environmentally controlled channel gene expression.

Spatiotemporal sequence of appearance of NPFF-immunoreactive structures in the developing central nervous system of Xenopus laevis

Neuropeptide FF-like immunoreactive (NPFFir) cells and fibers were analyzed through development of Xenopus laevis. The first NPFFir cells appeared in the embryonic hypothalamus, which projected to the intermediate lobe of the hypophysis, the brainstem and spinal cord. Slightly later, scattered NPFFir cells were present in the olfactory bulbs and ventral telencephalon. In the caudal medulla, NPFFir cells were observed in the nucleus of the solitary tract only at embryonic and early larval stages. Abundant NPFFir cells and fibers were demonstrated in the spinal cord. The sequence of appearance observed in Xenopus shares many developmental features with mammals although notable differences were observed in the telencephalon and hypothalamus. In general, NPFF immunoreactivity developed earlier in amphibians than in mammals.

Expression of neuroserpin is linked to neuroendocrine cell activation

Inhibitors of serine proteases (serpins) are important regulators of intracellular and extracellular proteolytic pathways, and they function by forming an irreversible complex with their substrate. Neuroserpin represents a neuroendocrine-specific serpin family member that is expressed in brain regions displaying synaptic plasticity. In this study, we explored the biosynthesis of endogenous neuroserpin in a neuroendocrine model system, namely the melanotrope cells of Xenopus intermediate pituitary. The biosynthetic activity of these cells can be physiologically manipulated (high and low production of the prohormone proopiomelanocortin in black and white animals, respectively), resulting from a synaptic plasticity in innervating hypothalamic neurons. We found that neuroserpin was also differentially expressed in the Xenopus intermediate, but not anterior, pituitary with a 3-fold higher mRNA and more than 30-fold higher protein expression in the active vs. the inactive melanotrope cells. Two newly synthesized glycosylated forms of the neuroserpin protein (47 and 50 kDa) were produced and secreted by the active cells. Intriguingly, neuroserpin was found in an approximately 130-kDa sodium dodecyl sulfate-stable complex in the active, but not in the inactive, melanotrope cells, which correlated with the high and low proopiomelanocortin expression levels, respectively. In conclusion, we report on the biosynthesis of neuroserpin in a physiological context, and we find that the induction of neuroserpin expression and the formation of the 130-kDa neuroserpin-containing complex are linked to neuroendocrine cell activation.

Of lasers, mutants, and see-through brains: functional neuroanatomy in zebrafish

Behavioral functions are carried out by localized circuits in the brain. Although this modular principle is clearly established, the boundaries of modules, and sometimes even their existence, are still debated. Zebrafish might offer distinct advantages in localizing behaviors to discrete brain regions because of the ability to visualize, record from, and lesion precisely identified populations of neurons in the brain. In addition, genetic screens in zebrafish enable the isolation of mutations that disrupt neural pathways and/or behaviors, as an alternative lesioning technique with complementary strengths to laser ablations. For example, the Mauthner cell, a large identified neuron in the hindbrain, has been postulated to be both necessary and sufficient for the execution of escapes. We discuss in this review how experiments, using laser ablations, calcium imaging, and mutants have eroded this notion. Even in a simple behavior, such as escape, many parallel pathways appear to be involved with no single one being absolutely necessary. Lesion studies and the analysis of behavioral mutants are now also beginning to elucidate the functional architecture of the zebrafish visual system. Although still in an embryonic stage, the neuroanatomy of behaviors in zebrafish has a bright future.

Activity-dependent dynamics of coexisting brain-derived neurotrophic factor, pro-opiomelanocortin and alpha-melanophore-stimulating hormone in melanotrope cells of Xenopus laevis

Brain-derived neurotrophic factor (BDNF) is involved as an autocrine factor in the regulation of the secretory activity of the neuroendocrine pituitary melanotrope cells of Xenopus laevis. We studied the subcellular distribution of BDNF in Xenopus melanotropes using a combination of high-pressure freezing, cryosubstitution and immunoelectron microscopy. Presence of BDNF, pro-opiomelanocortin (POMC) and alpha-melanophore-stimulating hormone (alphaMSH) within melanotrope secretory granules was studied by triple-labelling immunoelectron microscopy. In addition, intracellular processing of BDNF was investigated by quantifying the number of immunogold particles in different stages of secretory granule maturation, in animals adapted to black or white background light conditions. The high-pressure freezing technique provides excellent preservation of both cellular ultrastructure and antigenicity. BDNF coexists with POMC and alphaMSH within secretory granules. BDNF-immunoreactivity increases along the secretory granule maturation axis (i.e. from electron-dense, via moderately electron-dense, to electron-lucent secretory granules). Immature, low immunoreactive, electron-dense secretory granules are assumed to contain mainly or even exclusively proBDNF. Strongly immunoreactive electron-lucent secretory granules represent the mature granule stage in which proBDNF has been processed to mature BDNF. Furthermore, in moderately electron-dense secretory granules, immunoreactivity is markedly (+79%) higher in black-adapted than in white-adapted animals, indicating that stimulation of melanotrope cell activity by the black background condition speeds up processing of BDNF from its precursor in this granule stage. It is concluded that, in the Xenopus melanotrope, BDNF biosynthesis and processing occur along the secretory granule maturation axis, together with that of POMC-derived alphaMSH, and that the environmental light condition not only controls the biosynthesis and secretion of BDNF and of POMC end-products, but also regulates the rate of their intragranular processing.

Ca2+ oscillations in melanotropes of Xenopus laevis: their generation, propagation, and function

The melanotrope cell of the amphibian Xenopus laevis is a neuroendocrine transducer that converts neuronal input concerning the color of background into an endocrine output, the release of alpha-melanophore-stimulating hormone (alpha-MSH). The cell displays intracellular Ca(2+) oscillations that are thought to be the driving force for secretion as well as for the expression of genes important to the process of background adaptation. Here we review the functioning of the Xenopus melanotrope cell, with emphasis on the role of Ca(2+) oscillations in signal transduction in this cell. We start by giving a general overview of the evolution of Ca(2+) as an intracellular messenger molecule. This is followed by an examination of the melanotrope as a neuroendocrine integrator cell. Then, the evidence that Ca(2+) oscillations drive the secretion of alpha-MSH is reviewed, followed by a similar analysis of the evidence that the same oscillations regulate the expression of proopiomelanocortin (POMC), the precursor protein for alpha-MSH. Finally, the possible importance of the pattern of Ca(2+) signaling to melanotrope cell function is considered.

Alpha-melanophore-stimulating hormone in the brain, cranial placode derivatives, and retina of Xenopus laevis during development in relation to background adaptation

The amphibian Xenopus laevis can adapt the color of its skin to the light intensity of the background. A key peptide in this adaptation process is alpha-melanophore-stimulating hormone (alpha-MSH), which is derived from proopiomelanocortin (POMC) and released by the endocrine melanotrope cells in the pituitary pars intermedia. In this study, the presence of alpha-MSH in the brain, cranial placode derivatives, and retina of developing Xenopus laevis was investigated using immunocytochemistry, to test the hypothesis that POMC peptide-producing neurons and endocrine cells have a common embryonic origin and a common function, i.e., controlling each other's activities and/or being involved in the process of physiological adaptation. The presence of alpha-MSH-positive cells in the suprachiasmatic nucleus, ventral hypothalamic nucleus, epiphysis, and endocrine melanotrope and corticotrope cells, which are all involved in regulation of adaptation processes, has been detected from stage 37/38 onward. This is consistent with the presumed common origin of these cells, the anterior neural ridge (ANR) of the neural-plate-stage embryo. The olfactory epithelium and the otic and epibranchial ganglia also contain alpha-MSH, indicating that these placodal derivatives originate from a common placodal domain continuous with the ANR. Furthermore, we demonstrate the presence of alpha-MSH in a subpopulation of retinal ganglion cells (RGCs), which is possibly also derived from the ANR. Immunoreactivity for alpha-MSH in RGCs that are located in the dorsal part of the retina is dependent on the background light intensity, suggesting that these cells are involved in the regulation of background adaptation. Taken together, the results support the hypothesis that POMC peptide-producing cells have a common embryonic origin and are involved in adaptation processes.

Comparative analysis of neuropeptide FF-like immunoreactivity in the brain of anuran (Rana perezi, Xenopus laevis) and urodele (Pleurodeles waltl) amphibians

The neuropeptide FF (NPFF) is a member of the RFamide related peptides (FaRPs) that share the dipeptide Arg-Phe-NH2 at their C-terminal. It was originally isolated from bovine brain and its wide distribution has been demonstrated in the brain of several mammalian species. By means of an NPFF antiserum we have investigated the distribution pattern of NPFF-like immunoreactive cells and fibers in the brain of anuran and urodele amphibians. In both amphibian orders, the most conspicuous labeled cell population was found in the preoptic area and hypothalamus, primarily in the suprachiasmatic region. Numerous fibers reached the median eminence and the intermediate lobe of the hypophysis. Only in the anuran brain cells were observed in the pallium and septum. In the urodele, cells and fibers of the terminal nerve were distinctly labeled. Cell bodies were widely distributed in the reticular formation of anurans and, in both orders, a large cell population was found in the nucleus of the solitary tract and the spinal cord. Comparable fiber distribution between both orders exists in which the basal telencephalon (mainly the amygdaloid complex), the hypothalamus and the spinal cord are the regions most richly innervated. The distribution pattern of NPFF-like immunorective elements in the brain of amphibians, which only partly overlaps with those of other FaRPs, supports the notion that a NPFF-like peptide exists in amphibians. On the basis of its localization, this peptide may act as a hypophysiotropic neurohormone and be involved in background adaptation. Its wide distribution in similar zones of the brain in amphibians and mammals suggests that functional roles of this peptide have been conserved in vertebrate evolution.

Regulation of neurons in the suprachiasmatic nucleus of Xenopus laevis

In the amphibian Xenopus laevis, suprachiasmatic melanotrope-inhibiting neurons (SMINs) play an important role in the regulation of the background adaptation process. In this study, we investigated the innervation of the SMINs at the light- and electron- microscopical level. Immunocytochemistry in combination with confocal laser scanning microscopy revealed co-existence of neuropeptide Y (NPY) and synaptobrevin in spots in the direct vicinity of the SMINs, suggesting the existence of NPY-containing synapses on these cells. At the ultrastructural level, the SMINs showed a high degree of plasticity, containing more electron-dense vesicles and a larger extent of RER in white- than in black-adapted animals. In black-adapted animals, symmetric synapses (Gray type II) were observed on the soma of the SMINs, suggesting an inhibitory input to these cells. The synaptic profiles contained electron-lucent and electron-dense vesicles, indicating the involvement of both a classical neurotransmitter and a neuropeptide (possibly NPY) in this input. In white-adapted animals, synapses were only found at some distance from the SMIN somata. Our findings indicate a striking plasticity of the innervation of the SMINs in relation to background adaptation and support the hypothesis that the SMINs are innervated by NPY-containing interneurons that inhibit SMIN activity in black-adapted animals.

Evidence that brain-derived neurotrophic factor acts as an autocrine factor on pituitary melanotrope cells of Xenopus laevis

We have investigated the physiological regulation and functional significance of brain-derived neurotrophic factor (BDNF) in the endocrine melanotrope cells of the pituitary pars intermedia of the amphibian Xenopus laevis, which can adapt its skin color to the light intensity of its environment. In black-adapted animals, melanotrope cells produce and release alpha-melanophore-stimulating hormone (alpha-MSH). In white-adapted animals, the activity of melanotrope cells is inhibited by neuronal input. Using Western blotting and immunocytochemistry at the light and electron microscopical level, we have detected both the BDNF precursor and the mature BDNF protein in Xenopus melanotrope cells. In situ hybridization and RT-PCR revealed the presence of BDNF mRNA in the pituitary pars intermedia, indicating that BDNF is synthesized in the melanotropes. Real-time quantitative RT-PCR showed that levels of BDNF mRNA in melanotrope cells are about 25 times higher in black- than in white-adapted animals. Although there is no difference in the amount of stored mature BDNF, the amount of BDNF precursor protein is 3.5 times higher in melanotropes of black-adapted animals than in those of white-adapted animals. These data indicate that BDNF mRNA expression and BDNF biosynthesis are up-regulated in active melanotrope cells. Because immunoelectron microscopy showed that BDNF is located in melanotrope secretory granules, BDNF is probably coreleased with alpha-MSH via the regulated secretory pathway. Superfusion and (3)H-amino acid incorporation studies demonstrated that BDNF stimulates the release of alpha-MSH and the biosynthesis of its precursor protein, POMC. Our results provide evidence that BDNF regulates the activity of Xenopus melanotrope cells in an autocrine fashion.