The development of the noradrenergic transmitter phenotype in postganglionic sympathetic neurons

The diversity of neuronal phenotypes in rodent and human autonomic ganglia

Selective sympathetic and parasympathetic pathways that act on target organs represent the terminal actors in the neurobiology of homeostasis and often become compromised during a range of neurodegenerative and traumatic disorders. Here, we delineate several neurotransmitter and neuromodulator phenotypes found in diverse parasympathetic and sympathetic ganglia in humans and rodent species. The comparative approach reveals evolutionarily conserved and non-conserved phenotypic marker constellations. A developmental analysis examining the acquisition of selected neurotransmitter properties has provided a detailed, but still incomplete, understanding of the origins of a set of noradrenergic and cholinergic sympathetic neuron populations, found in the cervical and trunk region. A corresponding analysis examining cholinergic and nitrergic parasympathetic neurons in the head, and a range of pelvic neuron populations, with noradrenergic, cholinergic, nitrergic, and mixed transmitter phenotypes, remains open. Of particular interest are the molecular mechanisms and nuclear processes that are responsible for the correlated expression of the various genes required to achieve the noradrenergic phenotype, the segregation of cholinergic locus gene expression, and the regulation of genes that are necessary to generate a nitrergic phenotype. Unraveling the neuron population-specific expression of adhesion molecules, which are involved in axonal outgrowth, pathway selection, and synaptic organization, will advance the study of target-selective autonomic pathway generation.

Sympathetic tales: subdivisons of the autonomic nervous system and the impact of developmental studies

Remarkable progress in a range of biomedical disciplines has promoted the understanding of the cellular components of the autonomic nervous system and their differentiation during development to a critical level. Characterization of the gene expression fingerprints of individual neurons and identification of the key regulators of autonomic neuron differentiation enables us to comprehend the development of different sets of autonomic neurons. Their individual functional properties emerge as a consequence of differential gene expression initiated by the action of specific developmental regulators. In this review, we delineate the anatomical and physiological observations that led to the subdivision into sympathetic and parasympathetic domains and analyze how the recent molecular insights melt into and challenge the classical description of the autonomic nervous system.

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

Neurotrophins and target interactions in the development and regulation of sympathetic neuron electrical and synaptic properties

The electrical and synaptic properties of neurons are essential for determining the function of the nervous system. Thus, understanding the mechanisms that control the appropriate developmental acquisition and maintenance of these properties is a critical problem in neuroscience. A great deal of our understanding of these developmental mechanisms comes from studies of soluble growth factor signaling between cells in the peripheral nervous system. The sympathetic nervous system has provided a model for studying the role of these factors both in early development and in the establishment of mature properties. In particular, neurotrophins produced by the targets of sympathetic innervation regulate the synaptic and electrophysiological properties of postnatal sympathetic neurons. In this review we examine the role of neurotrophin signaling in the regulation of synaptic strength, neurotransmitter phenotype, voltage-gated currents and repetitive firing properties of sympathetic neurons. Together, these properties determine the level of sympathetic drive to target organs such as the heart. Changes in this sympathetic drive, which may be linked to dysfunctions in neurotrophin signaling, are associated with devastating diseases such as high blood pressure, arrhythmias and heart attack. Neurotrophins appear to play similar roles in modulating the synaptic and electrical properties of other peripheral and central neuronal systems, suggesting that information provided from studies in the sympathetic nervous system will be widely applicable for understanding the neurotrophic regulation of neuronal function in other systems.

Development of satellite glia in mouse sympathetic ganglia: GDNF and GFR alpha 1 are not essential

The phenotypic development of satellite cells in mouse sympathetic ganglia was examined by localizing the transcription factors, Sox10 and Phox2b, the neuronal marker, tyrosine hydroxylase (TH), and brain-derived fatty acid binding protein (B-FABP), which identifies glial precursors and mature glia. In E10.5 mice, most cells in the sympathetic chain expressed both Sox10 and Phox2b, with a minority of cells expressing Sox10 only or Phox2b only. In E11.5 mice, the majority of cells expressed Sox10 only or Phox2b only. B-FABP was colocalized with Sox10 in satellite glial precursors, which were located on the periphery of the ganglion. There was no overlap between B-FABP and Phox2b or B-FABP and TH. During subsequent development, the number of B-FABP+ cells increased and they became more common deep within the ganglion. In E12.5 and E18.5 mice, there was no overlap between Sox10 and Phox2b, and 98% of Sox10 cells were also B-FABP+. Satellite glial precursors in E11.5-E15.5 mice also expressed the GDNF-binding molecule, GFRalpha1. B-FABP immunoreactive cells did not express Ret or NCAM, two potential signaling molecules for GDNF/GFRalpha1. In E12.5 and E18.5 mice lacking GFRalpha1 or GDNF, the development of B-FABP immunoreactive satellite cells was normal, and hence neither GDNF or GFRalpha1 are essential for the development of satellite glia in sympathetic ganglia.

The role of GDNF family ligand signalling in the differentiation of sympathetic and dorsal root ganglion neurons

The diversity of neurons in sympathetic ganglia and dorsal root ganglia (DRG) provides intriguing systems for the analysis of neuronal differentiation. Cell surface receptors for the GDNF family ligands (GFLs) glial cell-line-derived neurotrophic factor (GDNF), neurturin and artemin, are expressed in subpopulations of these neurons prompting the question regarding their involvement in neuronal subtype specification. Mutational analysis in mice has demonstrated the requirement for GFL signalling during embryonic development of cholinergic sympathetic neurons as shown by the loss of expression from the cholinergic gene locus in ganglia from mice deficient for ret, the signal transducing subunit of the GFL receptor complex. Analysis in mutant animals and transgenic mice overexpressing GFLs demonstrates an effect on sensitivity to thermal and mechanical stimuli in DRG neurons correlating at least partially with the altered expression of transient receptor potential ion channels and acid-sensitive cation channels. Persistence of targeted cells in mutant ganglia suggests that the alterations are caused by differentiation effects and not by cell loss. Because of the massive effect of GFLs on neurite outgrowth, it remains to be determined whether GFL signalling acts directly on neuronal specification or indirectly via altered target innervation and access to other growth factors. The data show that GFL signalling is required for the specification of subpopulations of sensory and autonomic neurons. In order to comprehend this process fully, the role of individual GFLs, the transduction of the GFL signals, and the interplay of GFL signalling with other regulatory pathways need to be deciphered.

Cholinergic differentiation occurs early in mouse sympathetic neurons and requires Phox2b

The generation of neurotransmitter identity in the autonomic nervous system is a classical model system to study the development of neuronal diversity. Analysis of the expression of genes coding for enzymes of noradrenaline biosynthesis in the sympathoadrenal system allowed the characterization of factors involved in the differentiation of the noradrenergic transmitter phenotype. The development of cholinergic properties in the autonomic system is less well understood. Here we show that expression of mRNAs for choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter (VAChT), both encoded by the cholinergic gene locus, is induced in mouse sympathetic ganglia at embryonic day 11 (E11). Positive cells amount to more than 50% of Phox2b-positive sympathetic cells at cervical levels. The proportion declines caudally, decreasing to approximately 20% of Phox2b-positive cells at lower thoracic levels. In the adrenal anlage, ChAT and VAChT mRNA are largely undetectable at E11 and E13. In mice homozygous for a mutational inactivation of the transcription factor Phox2b, ChAT and VAChT mRNA expression is absent from sympathetic ganglia. The data show that expression from the cholinergic gene locus is regulated differently in sympathetic neurons and adrenal chromaffin cells. Phox2b is required for development of cholinergic neurons but does not suffice to support cholinergic properties in chromaffin cells.

Immunocytochemical properties of stellate ganglion neurons during early postnatal development

Neurotransmitter features in sympathetic neurons are subject to change during development. To better understand the neuroplasticity of sympathetic neurons during early postnatal ontogenesis, this study was set up to immunocytochemically investigate the development of the catecholaminergic, cholinergic, and peptidergic phenotypes in the stellate ganglion of mice and rats. The present study was performed on Wistar rats and Swiss mice of different ages (newborn, 10-day-old, 20-day-old, 30-day-old, and 60-day-old). To this end, double labeling for tyrosine hydroxylase (TH), choline acetyltransferase (ChAT), vasoactive intestinal (poly)peptide (VIP), neuropeptide Y (NPY), galanin (GAL), and somatostatin (SOM) was applied. The results obtained indicate that the majority of the neurons in the stellate ganglion of both species were TH-positive from birth onward and that a large part of these neurons also contained NPY. The percentage of neurons containing TH and NPY invariably increased with age up to 60 days postnatally. A smaller portion of the stellate ganglion neurons contained other types of neuropeptides and showed a distinct chronological pattern. The proportion of VIP- and ChAT-positive neurons was maximal in 10-day-old animals and then decreased up to 60 days of age, whereas the number of SOM-positive cells in rats significantly decreased from birth onward. In newborn rats, VIP-, ChAT- and SOM-positive neurons were largely TH-positive, while their proportions decreased in 10-day-old and older rats. Accordingly, the largest part of VIP-positive neurons also expressed SOM immunoreactivity at birth, after which the number of neurons containing both peptides diminished. The VIP- and SOM-positive cells did not contain NPY in any of the age groups studied. In rats up to 10 days of life, GAL-immunoreactive (-IR) neurons were scarce, after which their number increased to reach a maximal value in 30-day-old animals and then declined again. The SOM-reactive cells had the smallest size in all rats, while the largest neurons were those containing ChAT. In the mouse stellate ganglion, VIP- and ChAT-IR neurons were larger in comparison to NPY- and TH-IR cells. Our study further revealed some species differences: compared to mice the proportion of neurons containing TH and NPY was higher in rats at all ages under study. Furthermore, no GAL-immunostained neurons were found in mice and the number of SOM-positive cells in mice was limited compared to that observed in rats. In conclusion, the development of neurotransmitter composition is complete in rats and mice by their second month of life. At this age, the percentages of immunopositive cells have become similar to those reported in adult animals.

Histochemical features of neurons in the cat stellate ganglion during postnatal ontogenesis

Changes in the distribution of NADPH-diaphorase (NADPH-d) and acetylcholinesterase (AChE) were studied in neurons of the stellate ganglion in newborn, 10-, 20-day-old, 1-, 2-, 4- and 6-month-old kittens. AChE-positive neurons were revealed in the stellate ganglion (SG) from birth onwards. The number of these neurons increased until 20 days of postnatal life and then declined in 1- and 2-month-old kittens. A small number of weakly stained, NADPH-d-positive cells were found in newborn kittens, while intensely stained neurons first appeared in 10-day-old animals and increased in number up to the second month of life. The size of AChE-positive neurons was larger in comparison with NADPH-d-positive cells in the stellate ganglion of all animals under study. We suggest that putative vasodilator neurons or cells innervating sweat glands exhibit different development patterns from the moment of birth.

Control of postganglionic neurone phenotype by the rat pineal gland

As neurones develop they are faced with choices as to which genes to express, to match their final phenotype to their role in the nervous system. A number of processes can guide these decisions. Within the autonomic and sensory nervous systems, there are a handful of examples that suggest that one mechanism that may match phenotype to function is the presence of target-derived differentiation factors. We tested whether the rat pineal gland controls the expression of a neuropeptide (neuropeptide Y) and a calcium-binding protein (calbindin) in sympathetic postganglionic neurones that innervate it. We first showed that the chemical phenotype of sympathetic neurones innervating the rat pineal includes the expression of both neuropeptide Y and the calcium-binding protein, calbindin. After transplanting the pineal gland of neonatal rats into the submandibular salivary gland of neonatal hosts, it was innervated by sympathetic axons from the surrounding salivary gland tissue, which do not normally express neuropeptide Y and calbindin. The presence of the pineal gland led to the appearance of neuropeptide Y and calbindin in many of the postganglionic neurones that innervated the graft. From these findings we suggest that, like the rodent sweat gland, the pineal gland generates a signal that can direct the neurochemical phenotype of innervating sympathetic neurones.

The development of postganglionic sympathetic neurons: coordinating neuronal differentiation and diversification

The fine-tuned operation of the nervous system is accomplished by a diverse set of neurons which differ in their morphology, biochemistry and, consequently, their functional properties. The accurate interconnection between different neuron populations and their target tissues is the prerequisite for physiologically appropriate information processing. This is exemplified by the regulatory action of the autonomic nervous system in vertebrates to sustain homeostasis under changing physiological demands. For this purpose, the coordination of divergent regulatory responses is required in a multitude of tissues spread over the entire body. To meet this task, diverse neuronal populations interact at different levels. In the sympathetic system. chemical relations between preganglionic and postganglionic neurons appear to differ along the rostrocaudal axis. In addition, postganglionic neurons innervating different target tissues at a segmental level have distinct properties. Differences in their preganglionic innervation and their integrative membrane properties result in diverse activation patterns upon reflex stimulation. Moreover, postganglionic neurons differ in the transmitter molecules they employ to convey information to the target tissues. The segregation of noradrenaline and acetylcholine to different populations of postganglionic sympathetic neurons is well established. A combination of cellular and molecular approaches has begun to uncover how such a complex system may be generated during development. Growth and transcription factors involved in noradrenergic and cholinergic differentiation are characterised. Interestingly, they can also promote the expression of proteins involved in transmitter secretion. As the proteins participating in the vesicle cycle are expressed in many neuron populations, whereas the enzymes of transmitter biosynthesis are restricted to subpopulations of neurons, the findings suggest that early in neuronal development subpopulation-specific and more widely expressed neuronal properties can be commonly induced. Still, many details concerning the signals involved in the induction of the neurotransmitter synthesis and release machinery remain to be worked out. Likewise, the regulatory processes resulting in differences of electrophysiological membrane properties and the specific recognition between pre- and postganglionic neurons have to be determined. Ultimately, this will lead to an understanding at the molecular level of the development of a nervous system with diverse neuronal populations that are specifically interconnected to distinct input neurons and target tissues as required for the performance of a complex regulatory function.

Induction of cholinergic function in cultured sympathetic neurons by periosteal cells: cellular mechanisms

Periosteum, the connective tissue surrounding bone, alters the transmitter properties of its sympathetic innervation during development in vivo and after transplantation. Initial noradrenergic properties are downregulated and the innervation acquires cholinergic and peptidergic properties. To elucidate the cellular mechanisms responsible, sympathetic neurons were cultured with primary periosteal cells or osteoblast cell lines. Both primary cells and an immature osteoblast cell line, MC3T3-E1, induced choline acetyltransferase (ChAT) activity. In contrast, lines representing marrow stromal cells or mature osteoblasts did not increase ChAT. Growth of periosteal cells with sympathetic neurons in transwell cultures that prevent direct contact between the neurons and periosteal cells or addition of periosteal cell-conditioned medium to neuron cultures induced ChAT, indicating that periosteal cells release a soluble cholinergic inducing factor. Antibodies against LIFRbeta, a receptor subunit shared by neuropoietic cytokines, prevented ChAT induction in periosteal cell/neuron cocultures, suggesting that a member of this family is responsible. ChAT activity was increased in neurons grown with periosteal cells or conditioned medium from mice lacking either leukemia inhibitory factor (LIF) or LIF and ciliary neurotrophic factor (CNTF). These results provide evidence that periosteal cells influence sympathetic neuron phenotype by releasing a soluble cholinergic factor that is neither LIF nor CNTF but signals via LIFRbeta.

Ontogeny of galanin-immunoreactive elements in chicken embryo autonomic nervous system

To elucidate the main ontogenetic steps of galanin immunoreactivity within the extrinsic nerve supply of the alimentary tract, we undertook an immunohistochemical study of chicken embryo specimens. Fluorescence and streptavidin-biotin-peroxidase protocols were combined, using a galanin polyclonal antiserum, on transverse serial sections obtained from chicken embryos from embryonic Day 3 (E3) to hatching, and from 9-day-old newborn chicks. Galanin-immunoreactive cells were first detected at E3.5 within the pharyngeal pouch region, the nodose ganglion, the primary sympathetic chain, primitive splanchnic branches and the caudal portion of the Remak ganglion. At E5.5 galanin-immunoreactive cells and fibers appeared in the secondary (paravertebral) sympathetic chain, splanchnic nerves, peri- and preaortic plexuses, adrenal gland anlage and visceral nerves. Galanin-immunoreactive cells also lay scattered along the vagus nerve, and in the intermediate zone of the thoracolumbar spinal cord. At E18, galanin-immunoreactive cells and fibers were found along the entire Remak ganglion and around the gastrointestinal blood vessels. In post-hatching-9-day old chicks, the para- and prevertebral ganglia, but not the intermediate zone of the spinal cord, contained galanin-immunoreactive cells. Data indicate the presence of a consistent "galaninergic" nerve system supplying the chick embryonal gut wall. Whether this system has growth or differentiating role remains to be demonstrated. Its presence and distribution pattern in the later stages clearly support its well known role as a visceral neuromodulator of gut function.

Neurochemical differentiation of functionally distinct populations of autonomic neurons

The coeliac ganglion of guinea pigs displays a unique topographical arrangement of neurochemically and functionally distinct populations of sympathetic neurons. The authors used multiple-labeling immunohistochemistry to investigate the neurochemical differentiation of these neurons during embryonic and fetal development. Sympathoadrenal precursors, located on either side of the abdominal aorta, were intensely immunoreactive for tyrosine hydroxylase (TH-IR), neurofilament, and the human natural killer 1 antibody at midembryonic stages (Carnegie stages 16-19). During late embryonic stages (stages 20-23), a single bilobed ganglion had formed. At this time, neuropeptide Y immunoreactivity (NPY-IR) was widely expressed in sympathetic neurons (with moderate TH-IR) and chromaffin cells (with intense TH-IR). The onset of somatostatin (Som-IR) expression followed that of NPY-IR and was restricted to sympathetic neurons. However, at late embryonic stages, most TH-IR neurons with Som-IR also expressed NPY-IR (a combination of peptides not found in the mature coeliac ganglion). Between late embryonic stages and the end of the early fetal period, there was a significant increase in the proportion of neurons in lateral regions that had both NPY-IR and TH-IR. At the same time, there was an increase in the proportion of neurons in medial regions that had both Som-IR and TH-IR. Neurons expressing both Som-IR and TH-IR were rarely observed in lateral regions of the coeliac ganglion. Thus, a clear topography within the coeliac ganglion is established during late embryonic and early fetal stages of development and reflects that found in the mature animal by the end of the early fetal period.

Evidence for an evolutionary conserved role of bone morphogenetic protein growth factors and phox2 transcription factors during noradrenergic differentiation of sympathetic neurons. Induction of a putative synexpression group of neurotransmitter-synthesizing enzymes

The noradrenergic transmitter phenotype in postganglionic sympathetic neurons is induced early during embryonic development in avian and mammalian primary sympathetic ganglia. The simultaneous expression of tyrosine hydroxylase and dopamine beta-hydroxylase, enzymes of the noradrenaline biosynthesis pathway, indicates that different genes contributing to the noradrenergic transmitter phenotype are regulated as a synexpression group. This conclusion is supported by the demonstration of bone morphogenetic protein (BMP) growth factors and Phox2 transcription factors being necessary for the expression of both tyrosine hydroxylase and dopamine beta-hydroxylase in differentiating sympathetic neurons. The close similarity in the expression patterns of the relevant genes as well as in the function of BMPs and Phox2s between avian and mammalian embryos strongly suggests that noradrenergic induction occurs along a conserved signalling pathway in these vertebrate classes.

Developmental regulation of GDNF response and receptor expression in the enteric nervous system

The development of the enteric nervous system is dependent upon the actions of glial cell line-derived neurotrophic factor (GDNF) on neural crest-derived precursor cells in the embryonic gut. GDNF treatment of cultured enteric precursor cells leads to an increase in the number of neurons that develop and/or survive. Here we demonstrate that, although GDNF promoted an increase in neuron number at all embryonic ages examined, there was a developmental shift from a mitogenic to a trophic response by the developing enteric neurons. The timing of this shift corresponded to developmental changes in gut expression of GFR alpha-1, a co-receptor in the GDNF-Ret signaling complex. GFR alpha-1 was broadly expressed in the gut at early developmental stages, at which times soluble GFR alpha-1 was released into the medium by cultured gut cells. At later times, GFR alpha-1 became restricted to neural crest-derived cells. GFR alpha-1 could participate in GDNF signaling when expressed in cis on the surface of enteric precursor cells, or as a soluble protein. The GDNF-mediated response was greater when cell surface, compared with soluble, GFR alpha-1 was present, with the maximal response seen the presence of both cis and trans forms of GFR alpha-1. In addition to contributing to GDNF signaling, cell-surface GFR alpha-1 modulated the specificity of interactions between GDNF and soluble GFR alphas. These experiments demonstrate that complex, developmentally regulated, signaling interactions contribute to the GDNF-dependent development of enteric neurons.

Expression of neurexin Ialpha splice variants in sympathetic neurons: selective changes during differentiation and in response to neurotrophins

Neurexins are a surprisingly diverse group of alternatively spliced proteins possibly involved in neural cell recognition processes. We find neurexin Ialpha and its splice variants highly conserved between mammals and birds. In vivo, neurexin Ialpha is expressed in sympathetic neurons during target innervation and relative expression levels of splice variants change with development. In vitro, no such changes are observed in the absence of growth factors, indicating that interactions with the environment are required to modify the splicing pattern. Specific alterations in splice variant expression are induced in vitro by neurotrophins. Expression patterns of splice variants in vivo and neurotrophin-induced regulation without changes in cell composition in vitro demonstrate that neurexin splice variant expression varies during differentiation of individual neurons. Our data suggest that changes in neurexin splice variants contribute to alterations of neuronal cell surface properties during target innervation.

The expression of dopamine beta-hydroxylase, tyrosine hydroxylase, and Phox2 transcription factors in sympathetic neurons: evidence for common regulation during noradrenergic induction and diverging regulation later in development

During differentiation of sympathetic neurons in chick embryos, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) mRNAs become detectable during the same developmental period and are both induced by BMP 4. Later during sympathetic ganglion development, DBH is detectable in TH-positive and -negative cells. Moreover, BMPs reduce DBH mRNA in cultures of sympathetic neurons while leaving TH unaffected. The data provide evidence for a common regulation of TH and DBH early during sympathetic neuron differentiation and indicate that BMPs promote their initial expression but not the maintenance during later development. The time course of Phox2a and 2b expression suggests an evolutionary conserved role in noradrenergic induction. In addition, Phox2a, Phox2b, and c-ret may be involved in the differentiation of cholinergic sympathetic neurons.

Restriction of developmental potential during divergence of the enteric and sympathetic neuronal lineages

In the peripheral nervous system, enteric and sympathetic neurons develop from multipotent neural crest cells. While local environmental signals in the gut and in the region of the sympathetic ganglia play a role in the choice of cell fate, little is known about the mechanisms that underlie restriction to specific neuronal phenotypes. We investigated the divergence and restriction of the enteric and sympathetic neuronal lineages using immuno-isolated neural crest-derived cells from the gut and sympathetic ganglia. Analysis of neuronal and lineage-specific mRNAs and proteins indicated that neural crest-derived cells from the gut and sympathetic ganglia had initiated neuronal differentiation and phenotypic divergence by E14.5 in the rat. We investigated the developmental potential of these cells using expression of tyrosine hydroxylase as a marker for a sympathetic phenotype. Tyrosine hydroxylase expression was examined in neurons that developed from sympathetic and enteric neuroblasts under the following culture conditions: culture alone; coculture with gut monolayers to promote enteric differentiation; or coculture with dorsal aorta monolayers to promote noradrenergic differentiation. Both enteric and sympathetic neuroblasts displayed developmental plasticity at E14.5. Sympathetic neuroblasts downregulated tyrosine hydroxylase in response to signals from the gut environment and enteric neuroblasts increased expression of tyrosine hydroxylase when grown on dorsal aorta or in the absence of other cell types. Tracking of individual sympathetic cells displaying a neuronal morphology at the time of plating indicated that neuroblasts retained phenotypic plasticity even after initial neuronal differentiation had occurred. By E19.5 both enteric and sympathetic neuroblasts had undergone a significant loss of their developmental potential, with most neuroblasts retaining their lineage-specific phenotype in all environments tested. Together our data indicate that the developmental potential of enteric and sympathetic neuroblasts becomes restricted over time and that this restriction takes place not as a consequence of initial neuronal differentiation but during the period of neuronal maturation. Further, we have characterized a default pathway of adrenergic differentiation in the enteric nervous system and have defined a transient requirement for gut-derived factors in the maintenance of the enteric neuronal phenotype.

Cellular and molecular determinants of sympathetic neuron development

The development of the sympathetic nervous system can be divided into three overlapping stages. First, the precursors of sympathetic neurons arise from undifferentiated neural crest cells that migrate ventrally, aggregate adjacent to the dorsal aorta, and ultimately differentiate into catecholaminergic neurons. Second, cell number is refined during a period of cell death when neurotrophic factors determine the number of neuronal precursors and neurons that survive. The final stage of sympathetic development is the establishment and maturation of synaptic connections, which for sympathetic neurons can include alterations in neurotransmitter phenotype. Considerable progress has been made recently in elucidating the cellular and molecular mechanisms that direct each of these developmental decisions. We review the current understanding of each of these, focusing primarily on events in the peripheral nervous system of rodents.

Changes in the physiological roles of neurotransmitters during individual development

The classical neurotransmitters (acetylcholine and biogenic monoamines) are multifunctional substances involved in intra- and intercellular signaling at all stages of ontogenesis in multicellular animals. A cyclical scheme is proposed to describe age-related changes in neurotransmitter functions at different stages of development from oocyte maturation to neuron formation. This may reflect not only the temporospatial organization of neurotransmitter processes, but also the origin of the functions of acetylcholine and biogenic monoamines from the protosynapses of the cleaved embryo to neuronal synapses.

Age-dependent neurotransmitter plasticity of ciliary ganglion neurons

We have examined neurotransmitter plasticity in postmitotic cholinergic neurons isolated from 6.5- to 11-day-old embryonic quail ciliary ganglia. Purified neurons were labeled with DiI, transplanted into the trunk of young chick embryos, and assayed for catecholamine content and [3H]thymidine uptake 4 to 5 days later. For ciliary neurons derived from 6.5- to 8-day-old embryos, as many as 25% (average of 9% overall) expressed catecholamines in the host sympathetic ganglia, migratory stream, aortic plexuses, and adrenal medulla. In contrast, neurons from >8-day-old ganglia did not acquire or produce detectable catecholamines, indicating a limited time period over which phenotypic conversion can occur in vivo. As a control, ciliary neurons were also injected into the head mesenchyme of young embryos; no catecholamine expression was observed. Interestingly, after transplantation some DiI-labeled postmitotic ciliary neurons took up [3H]thymidine with or without phenotypic change. These results suggest that phenotypic plasticity in ciliary neurons is age-dependent, is location-dependent, and may involve resumption of DNA replication, a characteristic feature of some differentiating adrenergic sympathetic neurons. Apoptosis of a few proliferating transplanted cells may be induced independently or in association with transmitter change.

Glycine receptors in cultured chick sympathetic neurons are excitatory and trigger neurotransmitter release

1. Total RNA isolated from embryonic chick paravertebral sympathetic ganglia was used in a reverse transcription-polymerase chain reaction (RT-PCR) assay with a pair of degenerate oligonucleotide primers deduced from conserved regions of mammalian glycine receptor alpha-subunits. Three classes of cDNA were identified which encode portions of the chicken homologues of the mammalian glycine receptor alpha 1, alpha 2 and alpha 3 subunits. 2. The presence of functional glycine receptors was investigated in the whole-cell configuration of the patch-clamp technique in neurons dissociated from the ganglia and kept in culture for 7-8 days. In cells voltage clamped to -70 mV, glycine consistently induced inward currents in a concentration-dependent manner and elicited half-maximal peak current amplitudes at 43 microM. 3. The steady-state current-voltage relation for glycine-induced currents was linear between +80 and -60 mV, but showed outward rectification at more hyperpolarized potentials. Reversal potentials of these currents shifted with changes in intracellular chloride concentrations and matched the calculated Nernst potentials for chloride. 4. beta-Alanine and taurine were significantly less potent than glycine in triggering inward currents, with half-maximal responses at 79 and 86 microM, respectively. At maximally active concentrations, beta-alanine-evoked currents were identical in amplitude to those induced by glycine. Taurine-evoked currents, in contrast, never reached the same amplitude as glycine-induced currents. 5. The classical glycine receptor antagonist strychnine reversibly reduced glycine-induced currents, with half-maximal inhibition occurring at 62 nM. Two more recently characterized glycine receptor antagonists, isonipecotic acid (half-maximal inhibition at 2 mM) and 7-trifluoromethyl-4-hydroxyquinoline-3-carboxylic acid (half-maximal inhibition at 67 microM), also blocked glycine-evoked currents in a reversible manner. The chloride channel blocker picrotoxin reduced glycine-evoked currents, with half-maximal effects at 348 microM. Inhibition by the glycine receptor channel blocker cyanotriphenylborate was half-maximal at 4 microM. 6. Apart from evoking inward currents, glycine occasionally triggered short (< 100 ms) spike-like currents which were abolished by hexamethonium and thus reflected synaptic release of endogenous acetylcholine. In addition, glycine caused Ca(2+)-dependent and tetrodotoxin-sensitive tritium overflow from neurons previously labelled with [3H]noradrenaline. This stimulatory action of glycine was reduced in the presence of strychnine and after treatment with the chloride uptake inhibitor furosemide (frusemide). 7. In 65% of neurons loaded with the Ca2+ indicator fura-2 acetoxymethyl ester, glycine increased the ratio of the fluorescence signal obtained with excitation wavelengths of 340 and 380 nm, respectively, which indicates a rise in intracellular Ca2+ concentration. 8. The results show that sympathetic neurons contain transcripts for different glycine receptor alpha-subunits and carry functional heteromeric glycine receptors which depolarize the majority of neurons to trigger transmitter release.

The developmental expression of choline acetyltransferase (ChAT) and the neuropeptide VIP in chick sympathetic neurons: evidence for different regulatory events in cholinergic differentiation

Cholinergic properties in chick sympathetic neurons are detectable early during development of paravertebral ganglia and mature after target contact. The cholinergic marker choline acetyltransferase (ChAT) is first detectable at embryonic day 6 and its expression partly overlaps with that of the noradrenergic marker tyrosine hydroxylase (TH). At late embryonic stages, when sympathetic neurons have established target contact, ganglia consist of two major neuronal populations, TH-positive noradrenergic neurons and cholinergic neurons that at this stage express vasoactive intestinal peptide (VIP) in addition to ChAT. The maturation of sympathetic neurons is paralleled by changes in their response to the neurokine ciliary neurotrophic factor (CNTF). These findings suggest that expression of neurotransmitter properties is controlled differentially before and during target innervation.