Catecholamines in the yolk sac epithelium of the rat

Chicken embryos can maintain heart rate during hypoxia on day 4 of incubation

Acute exposure to hypoxic conditions is a frequent natural event during the development of bird eggs. However, little is known about the effect of such exposure on the ability of young embryos in which cardiovascular regulation is not yet developed to maintain a normal heart rate (HR). To address this question, we studied the effect of 10-20 min of exposure to moderate or severe acute hypoxia (10% or 5% O₂, respectively) on the HR of day 4 (D4) chicken embryos. In ovo, video recording of the beating embryo heart inside the egg revealed that severe, but not moderate, hypoxia resulted in significant HR changes. The HR response to severe hypoxia consisted of two phases: the first phase, consisting of an initial decrease in HR, was followed by a phase of partial HR recovery. Upon the restoration of normoxia, after an overshoot period of a few minutes, the HR completely recovered to its basal level. In vitro (isolated heart preparation), the first phase of the HR response to severe hypoxia was strengthened (nearly complete heart silencing) compared to that in ovo, and the HR recovery phase was greatly attenuated. Furthermore, neither an overshoot period nor complete HR recovery after hypoxia was observed. Thus, the D4 chicken embryo heart can partially maintain its rhythm during hypoxia in ovo, but not in vitro. Some factors from the egg, such as catecholamines, are likely to be critical for avian embryo responding to hypoxic condition and survival.

Evidence for a critical role of catecholamines for cardiomyocyte lineage commitment in murine embryonic stem cells

Catecholamine release is known to modulate cardiac output by increasing heart rate. Although much is known about catecholamine function and regulation in adults, little is known about the presence and role of catecholamines during heart development. The present study aimed therefore to evaluate the effects of different catecholamines on early heart development in an in vitro setting using embryonic stem (ES) cell-derived cardiomyocytes. Effects of catecholamine depletion induced by reserpine were examined in murine ES cells (line D3, αPIG44) during differentiation. Cardiac differentiation was assessed by immunocytochemistry, qRT-PCR, quantification of beating clusters, flow cytometry and pharmacological approaches. Proliferation was analyzed by EB cross-section measurements, while functionality of cardiomyocytes was studied by extracellular field potential (FP) measurements using microelectrode arrays (MEAs). To further differentiate between substance-specific effects of reserpine and catecholamine action via α- and β-receptors we proved the involvement of adrenergic receptors by application of unspecific α- and β-receptor antagonists. Reserpine treatment led to remarkable down-regulation of cardiac-specific genes, proteins and mesodermal marker genes. In more detail, the average ratio of ∼40% spontaneously beating control clusters was significantly reduced by 100%, 91.1% and 20.0% on days 10, 12, and 14, respectively. Flow cytometry revealed a significant reduction (by 71.6%, n = 11) of eGFP positive CMs after reserpine treatment. By contrast, reserpine did not reduce EB growth while number of neuronal cells in reserpine-treated EBs was significantly increased. MEA measurements of reserpine-treated EBs showed lower FP frequencies and weak responsiveness to adrenergic and muscarinic stimulation. Interestingly we found that developmental inhibition after α- and β-adrenergic blocker application mimicked developmental changes with reserpine. Using several methodological approaches our data suggest that reserpine inhibits cardiac differentiation. Thus catecholamines play a critical role during development.

Gene expression of catecholamine synthesizing enzymes and beta adrenoceptor subtypes during rat embryogenesis

Timed-pregnant Sprague-Dawley rats were killed between gestational day (GD) 8 and 10, and embryos were explanted and separated into developmental stages according to a modified Theiler's system. Total RNA from each stage was isolated and subjected to reverse transcription-polymerase chain reaction (RT-PCR) assays to examine gene expression of catecholamine synthesizing enzymes and three subtypes of beta adrenoceptors. Expression of these genes was detected at much earlier stages than previously reported, and each enzyme and receptor subtype showed a different pattern of gene expression. For example, mRNA for tyrosine hydroxylase, the rate-limiting enzyme for catecholamine synthesis, was detected as early as stage 10a, late GD 8, before the neural crest cells appear (stage 12, mid GD 10). This contradicts the common belief that catecholamines are produced only in the cells of sympathoadrenal lineage which originate from the neural crest cells and the cells of central nervous system. Results from the present study indicate that catecholamine synthesis is not limited to the cells of sympathoadrenal lineage.

Neural tube defects: a review of human and animal studies on the etiology of neural tube defects

Although neural tube defects are a common congenital anomaly, their etiology is not known. Human studies have emphasized the pathology and epidemiology of the defects and suggest that in the majority of cases the etiology is multifactorial. Factors which appear possibly to be important are genetic predisposition, maternal illness, and fetal drug exposure. Animal studies have utilized naturally occurring neural tube defects and teratologically induced lesions. No animal model has been convincingly established as the equivalent of human neural tube defects. However, animal models have allowed investigation of the mechanisms of suggested human teratogens and determination of the pathogenesis of naturally occurring animal defects. Their most important contribution has been in furthering the understanding of the normal mechanisms of neural tube closure. It may be through this understanding that the etiology of human neural tube defects will be determined.

Specific cellular expression of monoamine oxidase B during early stages of quail embryogenesis

Monoamine oxidase-B (MAO-B) is one of two distinct molecular forms of MAO that in part regulate the cellular levels of biogenic amines. In order to determine whether discrete cell populations known to express aminergic properties in the vertebrate embryo also express MAO-B, the distribution of MAO-B-immunoreactive cells was examined in early developing quail embryos. Two major patterns of staining emerge. First, tissues known to express several aminergic characteristics are initially MAO-B positive at early stages of development and continue to express immunoreactivity through Zacchei stage 20, the oldest stage examined. These include cells of the sympathetic and some cranial and trunk sensory ganglia (beginning at stage 13), the pancreas (stage 14), and the brain stem raphe (stage 14). Second, other structures that contain or accumulate biogenic amines are transiently MAO-B immunopositive during early stages of development. These tissues include extraembryonic yolk sac and presumptive gut endoderm (with most intense staining between stages 8 and 13), the ventral trunk neural tube (stages 14 and 16), and the notochord (stages 8-10). MAO-B immunoreactivity disappears from these regions at different stages, and none are MAO-B positive by stage 19-20. Other structures without known aminergic properties during early development also stain; these include liver (stage 20), mesenchymal cells that surround the Wolffian duct and lung buds (stages 14 and 18), and endothelial cells surrounding the dorsal aorta (stages 14 and 20). In general, however, MAO-B appears to be distributed in embryonic tissues that can use this enzyme to regulate biogenic amine levels either transiently or during initial phenotypic expression of aminergic traits.

Identification of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian palate

A direct radioligand binding technique utilizing a beta-adrenergic antagonist [3H]Dihydroalprenolol [( 3H]DHA) was employed in the identification and characterization of fetal palatal beta-adrenergic receptors. [3H]DHA binding was saturable (Bmax 16 fmol/mg protein) with high affinity and an apparent equilibrium dissociation constant (KD) of 1.5 nM. Binding of [3H]DHA was displaced by the competitive beta-adrenergic antagonist propranolol in a concentration-dependent manner. Dissociation kinetic studies demonstrated almost complete reversibility of radioligand binding within 60 min. The functionality of these beta-adrenergic receptors was demonstrated by showing that fetal palatal mesenchymal cells responded to catecholamine agonists with dose-dependent accumulations of intracellular cAMP. This effect could be entirely blocked by the beta-antagonist, propranolol. The relative potency order of catecholamines in eliciting an elevation of cellular cAMP was characteristic of a beta 2-adrenergic receptor-mediated response: (-) isoproterenol greater than (-) epinephrine greater than (-) norepinephrine. In addition, this response was found to be stereospecific with (-) isoproterenol being significantly more potent than (+) isoproterenol. Both the [3H]DHA binding characteristics and the catecholamine sensitivity of fetal palatal tissue support the presence of adenylate cyclase-coupled beta-adrenergic receptors in the developing mammalian secondary palate.

The development of catecholaminergic nerves in the spinal cord of rat. II. Regional development

The development of noradrenergic and dopaminergic nerves in 5 regions of the developing spinal cord of rat, from fetal day (FD) 16, to the young adult stage was studied. The normal synthetic capacity of adrenergic nerves in the ventral horn of the cervical and lumbar regions developed at the same time, and at the same rate, despite their spatial separation, and before similar development of the noradrenergic nerves in the dorsal horn and zona intermedia. In the ventral horn, the synthesis of NE from injected L-DOPA, as well as the release and metabolism of NE are well-established at 12 h (ND 0.5) after birth. In the dorsal horn these developments occur later at ND 4. Except in the dorsal horn of the cervical region, there was no easily observable, consistent pattern in the development of regional spinal dopaminergic innervation. The capacity of the developing cord to synthesize dopamine (DA) from injected DOPA is significantly developed at FD 16 (the earliest time studied), and peaked in all regions as early as ND 4. Control experiments indicate that 100%, and only 10% respectively of NE and DA synthetized from injected DOPA, occurred in descending monoaminergic fibers. Norepinephrine is synthesized exclusively in noradrenergic nerves. Cells appear transiently in the developing cord at FD 18, that are capable of synthesizing catecholamines (probably mainly DA) from injected DOPA. During postnatal development of the cord, and to a less extent in the adult, the network of catecholaminergic nerves actually present, is more extensive than that normally revealed during routine fluorescence microscopy. The results are discussed in the context of current attempts to understand the functional importance of catecholaminergic nerves in the mammalian spinal cord, and elsewhere in the CNS.

Development of catecholaminergic nerves in the spinal cord of the rat

The development of the noradrenergic and dopaminergic innervations in the spinal cord of rat was studied using fluorescence histochemical and neurochemical methods. From fetal day (FD) 16 to neonatal day (ND) 26, the cord increased in weight by 4-6 mg/day, except for the period between ND 14 and 20, when the increase was 13 mg/day. Norepinephrine was first detectable in the whole cord at ND 18, and then increased rapidly thereafter, peaking at ND 14, then declining at the end of neonatal life to the values found in the young adult spinal cord. The innervation in the intermediolateral cell column of the thoracic cord appeared to be more extensive at ND 14 than in the adult, raising the possibility of the selective destruction of a part of this noradrenergic innervation during later development. The nerve terminals in the ventral horn were first visualized clearly at birth, with a pattern similar to that of the adult. When the fetal locus coeruleus is transplanted into the transected spinal cord of the adult rat, it induces an extensive proliferation of the cut rostral axons in the ventral horn specifically. It is proposed that the transplanted fetal locus coeruleus produces a neurotrophic substance which stimulates the proliferation of the cut rostral axons derived from the locus coeruleus. Dopamine was first detectable in the cord at ND 20. Unlike noradrenergic nerves, dopaminergic nerves developed slowly throughout neonatal life. The adult innervation presumably develops slowly between ND 26 and young adulthood. In the fetus and very young neonate, DA was most concentrated in the thoracic region. Dopamine metabolism in the cord during neonatal life was a fraction of that found in the adult. It is concluded that the spinal dopaminergic and noradrenergic innervations develop with quite different time sequences. The rapid peaking of the noradrenergic innervation of ND 14 may play a significant role in the overall development and functional maturation of the cord.

Hormonal and humoral influences on brain development

This review discusses evidence for neurotransmitters as developmental signals in such ontogenic processes as neural tube formation (neurulation), germinal cell proliferation, and neuronal and glial differentiation during brain organogenesis, as well as evidence for other roles of these neurotransmitters in non-neuronal tissues of vertebrates and invertebrates. Evidence also is presented for hormonal regulation of brain development during postnatal neurogenesis and for interrelationships which may link neurotransmitters and hormones in a humoral milieu, providing a variety of control mechanisms for the central and peripheral nervous system during key phases of their development. Given the evidence for neurotransmitters and hormones as coordinating influence on neural ontogeny, it is possible that drugs, stress, and environmental influences may have the ability to perturb particular aspects of these developmental systems if present during those "critical periods" when such humoral influences are important for normal ontogeny.

Effect of central nervous system acting drugs on brain cell replication in vitro

The role in the regulation of cell replication of the neurotransmitter compounds and the drugs which affect their balance was studied in vitro, using morphologically preserved brain slices. Compounds affecting noradrenergic, dopaminergic and serotoninergic neurotransmitter systems reduced the brain cell replication, measured in terms of the rate of [3H]thymidine incorporation into DNA. The reduction was dose dependent and half-maximal effects were obtained at about 1-5 x 10(-4) M concentrations. Although agonists and antagonists both showed similar inhibitory effect, the action of agonists was reversed by the appropriate antagonists. Also, the pharmacologically active isomers were several-fold more effective than the inactive isomers in forebrain slices, although with cerebellar slices the selectivity was less marked. Cyclic nucleotides and drugs affecting cholinergic neurotransmitter systems were apparently ineffective. These results indicate that monoamines may be involved in the regulation of cell replication in the developing brain. Furthermore, as some of the CNS acting drugs tested are suspected behavioural teratogens the present results suggest that the reported behavioural abnormalities in the offspring may be related, in part, to a chronologically determined interference with the formation of certain cell types.

In vivo and in vitro development of serotonergic neurons

The monoamines are one of the earliest developing neurotransmitter systems in the mammalian brain. The first part of this paper describes the normal ontogeny of the serotonergic (5-HT) system in the rat brain as studied using long survival 3H-thymidine autoradiography (time of neuronal genesis, time of origin) and the Falck-Hillarp histofluorescence method, electron microscopy, and immunocytochemistry (anti-5-HT). Due to their early ontogeny relative to other brain regions, 5-HT neurons (as well as monoamine neurons in general) have been suggested to exert some type of "trophic" influence on brain development. Results of pharmacological experiments designed to inhibit 5-HT synthesis in the embryonic rat brain by maternal treatment with p-chlorophenylalanine (pCPA) at a time when this monoamine might exert such an influence are discussed with regard to effects on the time course of neuronal genesis (time of origin) of 5-HT neurons and their target cells. These results, which prompted us to propose that 5-HT might act as a "differentiation signal" for certain of its target cells, are now discussed in light of our more recent immunocytochemical-autoradiographic studies (anti-5-HT, 3H-thymidine) which morphologically demonstrate close associations between developing 5-HT neurons and proliferating neuroepithelial cells in the embryonic brain. Postnatal studies using this immunocytochemical-autoradiographic method also provide evidence for interactions of 5-HT axons with proliferating glioblasts in the developing cerebellum and with immature granule cells and their precursors in the hippocampus. These findings, in conjunction with the results of our pCPA experiments, further enhance the possibility that 5-HT neurons could exert an epigenetic influence on the development of less differentiated cells with which they come into contact. Finally, preliminary studies using dissociated cell cultures containing 5-HT neurons suggest that interactions between 5-HT neurons and glial elements may be important for the differentiation of these neurons in vitro. Whether 5-HT neurons in turn influence the development of glial or neuronal cells in these cultures remains to be determined. These studies are evaluated with regard to a possible pre-transmission role for 5-HT during key phases of neuronal and glial genesis.

Roles for serotonin in neuroembryogenesis

Possible non-transmitter roles for 5-HT in different phases of early neuroembryogenesis have been discussed based upon experimental evidence from the rat and chick. Fluorescence histochemical studies have demonstrated sites of uptake and synthesis of 5-HT in the chick embryo during the first few days of incubation. These sites are located in discrete regions of the notochord and floor plate of the neural tube as well as in extra-neural regions such as the somites and primitive gut. The 5-HT patterns are distinctly different from those observed for the uptake and synthesis of norepinephrine in embryos of the same age. Spatio-temporal changes in the distribution of these sites during closure of the neural tube suggest a role for 5-HT in various aspects of neural tube development. Moreover, the non-overlapping localization of 5-HT and norepinephrine raises the possibility that these two amines may exert different and perhaps cooperative influences on early neurogenic processes in the chick. In the rat, autoradiographic and biochemical studies concerning the consequences of 5-HT depletion in the embryo for development of different brain regions have provided evidence that 5-HT acts as a "differentiation signal" regulating the time of neuronal genesis in those cell populations which will eventually receive 5-HT innervation. Although the details of this system are as yet unknown, these studies suggest that 5-HT (and possibly the other monoamine transmitters) may actually "mold" the construction of their own circuitry during neurogenesis. Further, the ability of drugs and stress to interact with this process during that period of gestation when the monoamines are required as humoral signals suggests that maternal influences can interfere with ontogeny of this circuitry during pre- and possibly postnatal development. It is not yet clear whether the data in chicks and rats can be directly analogized from the one species to the other. Nevertheless, the evidence that sites of 5-HT uptake and/or synthesis are present during the earliest phases of neurogenesis in the chick and the observation that 5-HT depletion can alter the time of genesis of 5-HT target cells in the rat provide a new context for the consideration of possible actions of 5-HT prior to its role as a neurotransmitter substance.