Selective modulation of cholinergic properties in cultures of avian embryonic sympathetic ganglia

Sweat gland innervation is pioneered by sympathetic neurons expressing a cholinergic/noradrenergic co-phenotype in the mouse

Classic neurotransmitter phenotypes are generally predetermined and develop as a consequence of target-independent lineage decisions. A unique mode of target-dependent phenotype instruction is the acquisition of the cholinergic phenotype in the peripheral sympathetic nervous system. A body of work suggests that the sweat gland plays an important role to determine the cholinergic phenotype at this target site. A key issue is whether neurons destined to innervate the sweat glands express cholinergic markers before or only after their terminals make target contact. We employed cholinergic-specific over-expression of the vesicular acetylcholine transporter (VAChT) in transgenic mice to overcome sensitivity limits in the detection of initial cholinergic sweat gland innervation. We found that VAChT immunoreactive nerve terminals were present around the sweat gland anlage already from the earliest postnatal stages on, coincident selectively at this sympathetic target with tyrosine hydroxylase-positive fibers. Our results provide a new mechanistic model for sympathetic neuron-target interaction during development, with initial selection by the target of pioneering nerve terminals expressing a cholinergic phenotype, and subsequent stabilization of this phenotype during development.

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.

Opposing functions of GDNF and NGF in the development of cholinergic and noradrenergic sympathetic neurons

We identified a population of mature sympathetic neurons in which Ret, the receptor for glial cell line-derived neurotrophic factor (GDNF), is coexpressed with the neurotrophin-3 (NT3) receptor TrkC and choline acetyltransferase. In a complementary population the nerve growth factor receptor TrkA is coexpressed with the norepinephrine transporter. In accordance with these in vivo results, GDNF and neurturin promote the expression of cholinergic marker genes in sympathetic chain explants, similar to NT3 and ciliary neuronotrophic factor (CNTF). To define intracellular signaling mechanisms commonly activated by NT3, GDNF, or CNTF to promote cholinergic differentiation, we have analyzed the activation of intracellular signaling cascades. Signal transducer and activator of transcription-3 (STAT3) was strongly activated by CNTF but not by GDNF or NT3 and hence is not essential for cholinergic differentiation. We conclude that cholinergic properties can be regulated by neurotrophic factors from three different protein families, whereas noradrenergic properties are promoted by NGF.

Neurotrophin-3 promotes the cholinergic differentiation of sympathetic neurons

Neurotrophins influence the epigenetic shaping of the vertebrate nervous system by regulating neuronal numbers during development and synaptic plasticity. Here we attempt to determine whether these growth factors can also regulate neurotransmitter plasticity. As a model system we used the selection between noradrenergic and cholinergic neurotransmission by paravertebral sympathetic neurons. Developing sympathetic neurons express the neurotrophin receptors TrkA and TrkC, two highly related receptor tyrosine kinases. Whereas the TrkA ligand nerve growth factor (NGF) has long been known to regulate both the survival and the expression of noradrenergic traits in sympathetic neurons, the role of TrkC and of its ligand neurotrophin-3 (NT3) has remained unclear. We found that TrkC expression in the avian sympathetic chain overlaps substantially with that of choline acetyltransferase. In sympathetic chain explants, transcripts of the cholinergic marker genes choline acetyltransferase and vasoactive intestinal polypeptide were strongly enriched in the presence of NT3 compared with NGF, whereas the noradrenergic markers tyrosine hydroxylase and norepinephrine transporter were reduced. The transcription factor chicken achaete scute homolog 1 was coexpressed with cholinergic markers. The effects of NT3 are reversed and antagonized by NGF. They are independent of neuronal survival and developmentally regulated. These results suggest a role for NT3 as a differentiation factor for cholinergic neurons and establish a link between neurotrophins and neurotransmitter plasticity.

Phenotype plasticity and immunocytochemical evidence for ChAT and D beta H co-localization in fetal pig superior cervical ganglion cells

The early expression of the cholinergic phenotype in sympathetic neurons was already studied in superior cervical ganglion cells derived from rat, quail and chicken embryo. In the present work, we set up a neuron culture derived from the superior cervical ganglia of fetal pigs. The yield is 1000 times of that of a neonatal rat [17], 100 times of a 10- to 13-day-old chick embryo [26] and 20 times of a 10-day-old quail embryo [3]. This high yield will greatly facilitate further biochemical studies concerning neuronal differentiation. Using these cells as a model, the phenotype plasticity was studied by both biochemical and immunocytochemical methods in normal physiological medium, in a high KCl (30 mM) medium and in a splenocyte co-culture. The phenotype shift occurs in the normal physiological medium and in the splenocyte co-culture, but not in the high KCl medium. Taking into account the species difference, the fetal pig superior cervical ganglion neurons behave in a comparable manner as reported in earlier studies for other animal models. Moreover, for the first time, using immunocytochemical methods, direct evidence for a co-localization of choline-acetyl-transferase and dopamine-beta-hydroxylase in mammalian fetal sympathetic neurons, at least during a certain period, is given.

Peptidergic, catecholaminergic and morphological properties of avian chromaffin cells are modulated distinctively by growth factors

Most neurons and endocrine cells are known to co-express a 'classical neurotransmitter' with one or more neuropeptides. Although their expression has been shown to be modulated by differentiation factors, it is not known if particular combinations of neurotransmitter/neuropeptide(s) are co-regulated. We have analyzed the effect of nerve growth factor (NGF), neurotrophin-3 (NT-3), brain derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF), basic fibroblast growth factor (bFGF) and transforming growth factor beta 1 (TGF-beta 1) on the modulation of neuroactive substances co-expressed by avian chromaffin cells. The content of the neuropeptides neuropeptide Y (NPY), enkephalin (ENK) and somatostatin (SS) was measured by radioimmunoanalysis, and the content of the catecholamines norepinephrine (NE) and epinephrine (E) by high pressure liquid chromatography-electrochemistry (HPLC-EC). In addition, the morphological differentiation of chromaffin cells in response to the growth factors was assessed. All of the studied factors had distinct effects on the chromaffin content of neuropeptides and catecholamines. Our results show that the modulation of CAs and neuropeptides, and among the neuropeptides themselves is completely dissociated. Moreover, the cellular responses to the different growth factors show that neurochemical properties are modulated independently of morphological ones.

Peripheral expression and biological activities of GDNF, a new neurotrophic factor for avian and mammalian peripheral neurons

Glial cell line-derived neurotrophic factor (GDNF) is a neurotrophic polypeptide, distantly related to transforming growth factor-beta (TGF-beta), originally isolated by virtue of its ability to induce dopamine uptake and cell survival in cultures of embryonic ventral midbrain dopaminergic neurons, and more recently shown to be a potent neurotrophic factor for motorneurons. The biological activities and distribution of this molecule outside the central nervous system are presently unknown. We report here on the mRNA expression, biological activities and initial receptor binding characterization of GDNF and a shorter spliced variant termed GDNF beta in different organs and peripheral neurons of the developing rat. Both GDNF mRNA forms were found to be most highly expressed in developing skin, whisker pad, kidney, stomach and testis. Lower expression was also detected in developing skeletal muscle, ovary, lung, and adrenal gland. Developing spinal cord, superior cervical ganglion (SCG) and dorsal root ganglion (DRG) also expressed low levels of GDNF mRNA. Two days after nerve transection, GDNF mRNA levels increased dramatically in the sciatic nerve. Overall, GDNF mRNA expression was significantly higher in peripheral organs than in neuronal tissues. Expression of either GDNF mRNA isoform in insect cells resulted in the production of indistinguishable mature GDNF polypeptides. Purified recombinant GDNF promoted neurite outgrowth and survival of embryonic chick sympathetic neurons. GDNF produced robust bundle-like, fasciculated outgrowth from chick sympathetic ganglion explants. Although GDNF displayed only low activity on survival of newborn rat SCG neurons, this protein was found to increase the expression of vasoactive intestinal peptide and preprotachykinin-A mRNAs in cultured SCG neurons. GDNF also promoted survival of about half of the neurons in embryonic chick nodose ganglion and a small subpopulation of embryonic sensory neurons in chick dorsal root and rat trigeminal ganglia. Embryonic chick sympathetic neurons expressed receptors for GDNF with Kd 1-5 x 10(-9) M, as measured by saturation and displacement binding assays. Our findings indicate GDNF is a new neurotrophic factor for developing peripheral neurons and suggest possible non-neuronal roles for GDNF in the developing reproductive system.

Depolarization effects on the peptidergic phenotypes of chick sympathetic and adrenal cells

Depolarizing stimuli are among the factors known to influence the phenotypic plasticity of nerve cells. In order to determine the prevalence of the depolarization effects in terms of cell and neuropeptide phenotypes, we have analyzed the effect of potassium (K+)-induced depolarization on the avian sympathoadrenal system. The expression of three peptidergic phenotypes, somatostatin (SS), neuropeptide Y (NPY) and enkephalin (Enk) by two cell types, adrenal and sympathetic, was studied under different depolarizing regimens. Cells from the sympathetic paravertebral ganglion and adrenal gland of 10-11-day chick embryos were cultured and the peptide levels were measured by radioimmunoassays. Chronic depolarization causes differential effects on the peptidergic phenotypes increasing NPY and Enk but decreasing SS in both adrenal and sympathetic cultures. However, shorter exposures to depolarizing stimuli revealed diverse effects on NPY and Enk phenotypes and even between adrenal and sympathetic cells. Moreover, the maintenance of the effects after removal of the depolarizing stimuli showed additional differences among the phenotypes. Our results are not compatible with a previously established hypothesis stating that depolarization increases the synthesis of whichever neurotransmitters a neuron is already producing. They provide evidence indicating that the depolarization effect is much more complex than originally thought, and serve to initiate an in depth probe into the effect of depolarization of cellular plasticity.

Functional expression of A-currents in embryonic chick sympathetic neurones during development in situ and in vitro

1. The functional expression of transient voltage-activated K+ currents (IA) was examined using whole-cell recording techniques in embryonic chick sympathetic ganglion neurones that developed in situ and under various growth conditions in vitro. 2. The density of IA increased dramatically during development in sympathetic neurones isolated acutely between embryonic days 7 and 20 (E7-E20). The time course of IA inactivation became significantly faster between E7 and E13. With these protocols, neuronal differentiation and development occurred entirely in situ. 3. Sympathetic neurones isolated at E9 and maintained in vitro for 4 days did not express a normal IA compared to neurones isolated acutely at E13. Those neurones that were in physical contact with other neurones expressed normal densities of IA, but the resulting inactivation kinetics were abnormally slow. Sympathetic neurones that were cultured on the membrane fragments of lysed neurones expressed normal densities of IA even when they failed to make visible connections with other viable neurones, but the resulting inactivation kinetics were abnormally slow. Those cultured neurones that were not in physical contact with other cells or their membranes had markedly reduced densities of IA with abnormally slow inactivation kinetics. 4. Application of 5-100 ng ml-12.5 S nerve growth factor by itself did not promote normal A density of kinetics in E9 sympathetic neurones cultured for 4 days. 5. Sympathetic neurones that developed in vitro in physical contact with ventral spinal cord explants, cardiac myocytes or aortic smooth muscle cells expressed normal densities of IA, but the inactivation kinetics were abnormally slow. Cell culture media conditioned by these tissues failed to promote normal IA expression. Sympathetic neurones cultured as explants or maintained under depolarizing conditions did not express a normal IA. 6. Embryonic chick sympathetic neurones exhibit developmental changes in the density and kinetics of IA that can be regulated independently by extrinsic environmental factors including interactions with insoluble components of the plasma membranes of some cells.

Functional dependence of Ca(2+)-activated K+ current on L- and N-type Ca2+ channels: differences between chicken sympathetic and parasympathetic neurons suggest different regulatory mechanisms

The influx of Ca2+ ions controls many important processes in excitable cells, including the regulation of the gating of Ca(2+)-activated K+ channels (the current IK[Ca]). Various IK[Ca] channels contribute to the regulation of the action-potential waveform, the repetitive discharge of spikes, and the secretion of neurotransmitters. It is thought that large-conductance IK[Ca] channels must be closely colocalized with Ca2+ channels (ICa) to be gated by Ca2+ influx. We now report that IK[Ca] channels can be preferentially colocalized with pharmacologically distinct subtypes of voltage-activated Ca2+ channel and that this occurs differently in embryonic chicken sympathetic and parasympathetic neurons. The effects of various dihydropyridines and omega-conotoxin on voltage-activated Ca2+ currents (ICa) and Ca(2+)-activated K+ currents (IK[Ca]) were examined by using perforated-patch whole-cell recordings from embryonic chicken ciliary and sympathetic ganglion neurons. Application of nifedipine or omega-conotoxin each caused a 40-60% reduction in ICa, whereas application of S-(-)-BAY K 8644 potentiated ICa in ciliary ganglion neurons. But application of omega-conotoxin had little or no effect on IK[Ca], whereas nifedipine and S-(-)-BAY K 8644 inhibited and potentiated IK[Ca], respectively. These results indicate that IK[Ca] channels are preferentially coupled to L-type, but not to N-type, Ca2+ channels on chicken ciliary ganglion neurons. Chicken sympathetic neurons also express dihydropyridine-sensitive and omega-conotoxin-sensitive components of ICa. However, in those cells, application of omega-conotoxin caused a 40-60% reduction in IK[Ca], whereas nifedipine reduced IK[Ca] but only in a subpopulation of cells. Therefore, IK[Ca] in sympathetic neurons is either coupled to N-type Ca2+ channels or is not selectively coupled to a single Ca(2+)-channel subtype. The preferential coupling of IK[Ca] channels with distinct ICa subtypes may be part of a mechanism to allow for selective modulation of neurotransmitter release. Preferential coupling may also be important for the differentiation and development of vertebrate neurons.

Neuropoietic cytokines and activin A differentially regulate the phenotype of cultured sympathetic neurons

A number of cytokines sharing limited sequence homology have been grouped as a family because of partially overlapping biological activities, receptor subunit promiscuity, and the prediction of a shared secondary structure. Since several of these cytokines regulate gene expression and cell number in the nervous and hematopoietic systems, this specific group is termed the neuropoietic cytokine family. Using a reverse transcription-polymerase chain reaction-based assay system for monitoring the expression of multiple phenotypic markers in cultured sympathetic neurons, we present further evidence that, in addition to cholinergic differentiation factor/leukemia inhibitory factor and ciliary neurotrophic factor, oncostatin M, growth promoting activity, interleukin 6, and interleukin 11 belong in this family. In addition, one member of the transforming growth factor beta superfamily, activin A, shares a selective overlap with the neuropoietic family in the spectrum of neuropeptides that it induces in sympathetic neurons. The particular neuropeptides induced by activin A, however, demonstrate that the activity of this cytokine is distinct from that of the neuropoietic family. Twenty-six other cytokines and growth factors were without detectable activity in this assay.