Brain-derived neurotrophic factor in the brain of Xenopus laevis may act as a pituitary neurohormone together with mesotocin

The role of brain-derived neurotrophic factor in the regulation of cell growth and gene expression in melanotrope cells of Xenopus laevis

Brain-derived neurotrophic factor (BDNF) is, despite its name, also found outside the central nervous system (CNS), but the functional significance of this observation is largely unknown. This review concerns the expression of BDNF in the pituitary gland. While the presence of the neurotrophin in the mammalian pituitary gland is well documented its functional significance remains obscure. Studies on the pars intermedia of the pituitary of the amphibian Xenopus laevis have shown that BDNF is produced by the neuroendocrine melanotrope cells, its expression is physiologically regulated, and the melanotrope cells themselves express receptors for the neurotrophin. The neurotrophin has been shown to act as an autocrine factor on the melanotrope to promote cell growth and regulate gene expression. In doing so BDNF supports the physiological function of the cell to produce and release α-melanophore-stimulating hormone for the purpose of adjusting the animal's skin color to that of its background.

Plasticity of melanotrope cell regulations in Xenopus laevis

This review focuses on the plasticity of the regulation of a particular neuroendocrine transducer cell, the melanotrope cell in the pituitary pars intermedia of the amphibian Xenopus laevis. This cell type is a suitable model to study the relationship between various external regulatory inputs and the secretion of an adaptive endocrine message, in this case the release of α-melanophore-stimulating hormone, which activates skin melanophores to darken when the animal is placed on a dark background. Information about the environmental conditions is processed by various brain centres, in the hypothalamus and elsewhere, that eventually control the activity of the melanotrope cell regarding hormone production and secretion. The review discusses the roles of these hypothalamic and extrahypothalamic nuclei, their neurochemical messengers acting on the melanotrope, and the external stimuli they mediate to control melanotrope cell functioning.

Analysis of the melanotrope cell neuroendocrine interface in two amphibian species, Rana ridibunda and Xenopus laevis: a celebration of 35 years of collaborative research

This review gives an overview of the functioning of the hypothalamo-hypophyseal neuroendocrine interface in the pituitary neurointermediate lobe, as it relates to melanotrope cell function in two amphibian species, Rana ridibunda and Xenopus laevis. It primarily but not exclusively concerns the work of two collaborating laboratories, the Laboratory for Molecular and Cellular Neuroendocrinology (University of Rouen, France) and the Department of Cellular Animal Physiology (Radboud University Nijmegen, The Netherlands). In the course of this review it will become apparent that Rana and Xenopus have, for the most part, developed the same or similar strategies to regulate the release of α-melanophore-stimulating hormone (α-MSH). The review concludes by highlighting the molecular and cellular mechanisms utilized by thyrotropin-releasing hormone (TRH) to activate Rana melanotrope cells and the function of autocrine brain-derived neurotrophic factor (BDNF) in the regulation of Xenopus melanotrope cell function.

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.

Ultrastructural and neurochemical architecture of the pituitary neural lobe of Xenopus laevis

The melanotrope cell in the amphibian pituitary pars intermedia is a model to study fundamental aspects of neuroendocrine integration. They release alpha-melanophore-stimulating hormone (alphaMSH), under the control of a large number of neurochemical signals derived from various brain centers. In Xenopus laevis, most of these signals are produced in the hypothalamic magnocellular nucleus (Mg) and are probably released from neurohemal axon terminals in the pituitary neural lobe, to stimulate alphaMSH-release, causing skin darkening. The presence in the neural lobe of at least eight stimulatory factors implicated in melanotrope cell control has led us to investigate the ultrastructural architecture of this neurohemal organ, with particular attention to the diversity of neurohemal axon terminals and their neurochemical contents. Using regular electron microscopy, we here distinguish six types of neurohemal axon terminal, on the basis of the size, shape and electron-density of their secretory granule contents. Subsequently, we have identified the neurochemical contents of these terminal types by immuno-electron microscopy and antisera raised against not only the 'classical' neurohormones vasotocin and mesotocin but also brain-derived neurotrophic factor, cocaine- and amphetamine-regulated transcript peptide, corticotropin-releasing factor, metenkephalin, pituitary adenylyl cyclase-activating polypeptide, thyrotropin-releasing hormone and urocortin-1. This has revealed that each terminal type possesses a unique set of neurochemical messengers, containing at least four, but in some cases up to eight messengers. These results reveal the potential of the Mg/neural lobe system to release a wide variety of neurochemical messengers in a partly co-ordinated and partly differential way to control melanotrope cell activity as well as ion and water balance regulatory organs, in response to various, continuously changing, environmental stimuli.

Using transgenic animal models in neuroendocrine research: lessons from Xenopus laevis

Transgenic animals are commonly employed to explore the function of individual proteins. Transgenic animal models include the mouse, the zebrafish, and the South African clawed toad Xenopus laevis. In contrast to mice and zebrafish, with Xenopus transgenesis DNA integration is mostly achieved in the one-cell stage. Moreover, Xenopus (as well as zebrafish) eggs are relatively large, the embryos are transparent, a large offspring is generated, and maintenance of the offspring is easy. In our transgenic studies in Xenopus, we focus on the well-characterized neuroendocrine melanotrope cells of the pituitary pars intermedia that are regulated during the process of adaptation of Xenopus to a changing environment. When the animal is placed on a black background, the melanotrope cells produce and process large amounts of the prohormone proopiomelanocortin (POMC). We apply stable melanotrope-specific transgenesis that is achieved by mixing a Xenopus POMC-promoter/transgene construct with sperm nuclei and injecting this mixture into unfertilized eggs. Since in the melanotrope cells the POMC promoter is much more active in black-adapted animals, the level of transgene expression can be manipulated by placing the animal on either a black or a white background. In this paper we review the possibilities of the Xenopus melanotrope-specific transgenic approach. Following a brief overview of the functioning of Xenopus melanotrope cells, stable melanotrope-specific transgenesis is discussed and our transgenic studies on brain-derived neurotrophic factor and secretory pathway components are described as examples of the transgenic approach in a physiological context and close to the in vivo situation.

Neurochemistry and plasticity of the median eminence and neural pituitary lobe in relation to background adaptation of Xenopus laevis

Using immunocytochemistry and morphometry we have supported our hypothesis that magnocellular neurons in the preoptic area of the brain of Xenopus laevis release identical sets of neuropeptides containing not only the previously identified vasotocin, mesotocin, corticotropin-releasing factor, thyrotropin-releasing hormone, brain-derived neurotrophic factor, urocortin 1, and pituitary adenylate cyclase-activating peptide but also mesotocin and met-enkephalin from both neurohemal areas in the pituitary neural lobe and in the median eminence. We also show that the external zone of the median eminence is plastic, depending in size on the state of background illumination.

Brain-derived neurotrophic factor in infants <32 weeks gestational age: correlation with antenatal factors and postnatal outcomes

Neurotrophins (NTs) play important roles in brain growth and development. Cord blood (CB) brain-derived neurotrophic factor (BDNF) concentrations increase with gestational age but data regarding postnatal changes are limited. We measured BDNF concentrations after birth in 33 preterm infants <32-wk gestation. Serum was collected at birth (CB), at day 2, between day 6 and 10 (D6), at day 30 (D30), and at day 60 (D60). BDNF concentrations fell on D2 (p = 0.03), recovered by D6 (p = 0.10), and continued to rise thereafter at D30 (p = 0.06) and D60 (p = 0.01) compared with CB. CB BDNF concentrations positively correlated with duration of rupture of membranes (r = 0.43, p = 0.04). Antenatal steroids (ANS, p = 0.02), postnatal steroids (PNS, p = 0.04), and retinopathy of prematurity (ROP, p = 0.02) were identified as significant factors in multivariate analyses. The median (25-75th interquartile range) CB BDNF concentrations were higher in infants who received a complete course ANS compared with those who received a partial course [1461 (553-2064) versus 281 (171-536) pg/mL, p = 0.04]. BDNF concentrations negatively correlated with the use of PNS at D30 (r = -0.53, p = 0.002) and at D60 (r = -0.55, p = 0.009). PNS use was associated with reduced concentrations of BDNF at D30 [733 (101-1983) versus 2224 (1677- 4400) pg/mL, p = 0.004] and at D60 [1149 (288-2270) versus 2560 (1337-5166) pg/mL, p = 0.01]. BDNF concentrations on D60 in infants who developed ROP (n = 16) were lower than those who did not develop ROP (n = 7) [1417 (553-2540) versus 3593 (2620-7433) pg/mL, respectively, p = 0.005]. Our data suggests that BDNF concentrations rise beyond the first week of age. BDNF concentrations correlate with factors that influence neurodevelopment outcomes.

Brain distribution and evidence for both central and neurohormonal actions of cocaine- and amphetamine-regulated transcript peptide in Xenopus laevis

We tested the hypothesis that, in the amphibian Xenopus laevis, cocaine- and amphetamine-regulated transcript peptide (CARTp) not only has widespread actions in the brain but also acts as a local factor in endocrine pituitary cells and/or is neurohemally secreted into the circulation to control peripheral targets. CARTp-immunoreactive cells occur in the olfactory bulb, nucleus accumbens, amygdala, septum, striatum, nucleus of Bellonci, ventrolateral nucleus, central thalamic nucleus, preoptic nuclei, and suprachiasmatic nucleus, and particularly in the medial pallium, ventromedial nucleus, hypothalamus, Edinger-Westphal nucleus, optic tectum, raphe nuclei, central gray, nucleus of the solitary tract, and spinal cord. From the hypothalamic magnocellular nucleus, CARTp-containing axons run to the neurohemal median eminence, and to the neural pituitary lobe to form neurohemal terminals, as shown by immunoelectron microscopy. Starvation increases the number of CARTp-cells in the optic tectum by 46% but has no effect on such cells in the torus semicircularis. CARTp does not affect in vitro release of alpha-melanophore-stimulating hormone from pituitary melanotrope cells. Our results support the hypothesis that in X. laevis, CARTp not only has multiple and not exclusively feeding-related actions in the brain but is also secreted as a neurohormone 1) into the portal system to control endocrine targets in the pituitary distal lobe and 2) from neurohemal axon terminals in the neural pituitary lobe to act peripherally. The differences in CARTp distribution between X. laevis and Rana esculenta may be related to different environmental and physiological conditions such as feeding, sensory information processing, and locomotion.

Actions of PACAP and VIP on melanotrope cells of Xenopus laevis

The neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal polypeptide (VIP) are implicated in the regulation of gene expression and hormone secretion in mammalian melanotrope cells and a mammalian pro-opiomelanocortin (POMC)-producing tumor cell line, but the physiological relevance of this regulation is elusive. The purpose of the present study was to establish if these peptides affect biosynthetic and secretory processes in a well-established physiological model for endocrine cell functioning, the pituitary melanotrope cells of the amphibian Xenopus laevis, which hormonally control the process of skin color adaptation to background illumination. We show that both PACAP and VIP are capable of stimulating the secretory process of the Xenopus melanotrope cell. As the peptides are equipotent, they may exert their actions via a VPAC receptor. Moreover, PACAP stimulated POMC biosynthesis and POMC gene expression. Strong anti-PACAP immunoreactivity was found in the pituitary pars nervosa (PN), suggesting that this neurohemal organ is a source of neurohormonal PACAP action on the melanotropes in the intermediate pituitary. We propose that the PACAP/VIP family of peptides has a physiological function in regulating Xenopus melanotrope cell activity during the process of skin color adaptation.