The use of a tyrosine-hydroxylase cDNA probe to study the neurotransmitter plasticity of rat sympathetic neurons in culture

Infarction-induced cytokines cause local depletion of tyrosine hydroxylase in cardiac sympathetic nerves

Myocardial infarction causes a heterogeneity of noradrenergic transmission that contributes to the development of ventricular arrhythmias and sudden cardiac death. Ischaemia-induced alterations in sympathetic transmission include regional variations in cardiac noradrenaline (NA) and in tyrosine hydroxylase, the rate-limiting enzyme in NA synthesis. Inflammatory cytokines that act through gp130 are elevated in the heart after myocardial infarction. These cytokines decrease expression of tyrosine hydroxylase in sympathetic neurons, and indirect evidence suggests that they contribute to the local depletion of tyrosine hydroxylase in the damaged left ventricle. However, gp130 cytokines are also important for the survival of cardiac myocytes following damage to the heart. To examine the effect of cytokines on tyrosine hydroxylase and NA content in cardiac nerves we used gp130(DBH-Cre/lox) mice, which have a deletion of the gp130 receptor in neurons expressing dopamine beta-hydroxylase. The absence of neuronal gp130 prevented the loss of tyrosine hydroxylase in cardiac sympathetic nerves innervating the left ventricle 1 week after ischaemia-reperfusion compared with wild-type C57BL/6J mice. Surprisingly, restoration of tyrosine hydroxylase in the damaged ventricle did not return neuronal NA content to normal levels. Noradrenaline uptake into cardiac nerves was significantly lower in gp130 knockout mice, contributing to the lack of neuronal NA stores. There were no significant differences in left ventricular peak systolic pressure, dP/dt(max) or dP/dt(min) between the two genotypes after myocardial infarction, but ganglionic blockade revealed differences in autonomic tone between the genotypes. Stimulation of the heart with dobutamine or release of endogenous NA with tyramine generated similar responses in both genotypes. Thus, the removal of gp130 from sympathetic neurons prevents the post-infarct depletion of tyrosine hydroxylase in the left ventricle, but does not alter NA content or cardiac function.

Postinfarct sympathetic hyperactivity differentially stimulates expression of tyrosine hydroxylase and norepinephrine transporter

The balance between norepinephrine (NE) synthesis, release, and reuptake is disrupted after acute myocardial infarction, resulting in elevated extracellular NE. Stimulation of sympathetic neurons in vitro increases NE synthesis and the synthetic enzyme tyrosine hydroxylase (TH) to a greater extent than it increases NE reuptake and the NE transporter (NET), which removes NE from the extracellular space. We used TGR(ASrAOGEN) transgenic rats, which lack postinfarct sympathetic hyperactivity, to test the hypothesis that increased cardiac sympathetic nerve activity accounts for the imbalance in TH and NET expression in these neurons after myocardial infarction. TH and NET mRNA levels were identical in the stellate ganglia of unoperated TGR(ASrAOGEN) rats compared with Sprague Dawley (SD) controls, but the threefold increase in TH and twofold increase in NET mRNA seen in the stellate ganglia of SD rats 1 wk after ischemia-reperfusion was absent in TGR(ASrAOGEN) rats. Similarly, the increase in TH and NET protein observed in the base of the SD ventricle was absent in the base of the TGR (ASrAOGEN) ventricle. Neuronal TH content was depleted in the left ventricle of both genotypes, whereas NET was unchanged. Basal heart rate and cardiac function were similar in both genotypes, but TGR(ASrAOGEN) hearts were more sensitive to the β-agonist dobutamine. Tyramine-induced release of endogenous NE generated similar changes in ventricular pressure and contractility in both genotypes, but postinfarct relaxation was enhanced in TGR(ASrAOGEN) hearts. These data support the hypothesis that postinfarct sympathetic hyperactivity is the major stimulus increasing TH and NET expression in cardiac neurons.

Chronic depolarization stimulates norepinephrine transporter expression via catecholamines

Chronic depolarization increases norepinephrine (NE) uptake and expression of the norepinephrine transporter (NET) in sympathetic neurons, but the mechanisms are unknown. Depolarization of sympathetic neurons stimulates catecholamine synthesis, and several studies suggest that NET can be regulated by catecholamines. It is not clear if the depolarization-induced increase in NET is because of nerve activity per se, or is secondary to elevated catecholamines. To determine if induction of NET mRNA was a result of increased catecholamines, we used pharmacological manipulations to (i) inhibit tyrosine hydroxylase activity in neurons depolarized with 30 mm KCl, thereby preventing increased catecholamines, or (ii) stimulate tyrosine hydroxylase activity in the absence of depolarization. Inhibiting the depolarization-induced increase in catecholamines prevented the up-regulation of NET mRNA, but did not block the increase in tyrosine hydroxylase (TH) mRNA. Furthermore, stimulating catecholamine production in the absence of depolarization elevated NE uptake, NET protein, and NET mRNA in sympathetic neurons. Similarly, elevating endogenous catecholamines in SK-N-BE2M17 neuroblastoma cells increased NE uptake and NET expression. These data suggest that chronic depolarization of sympathetic neurons induces NET expression through increasing catecholamines, and that M17 neuroblastoma cells provide a model system in which to investigate catechol regulation of NET expression.

Regulation of noradrenergic function by inflammatory cytokines and depolarization

Although the sympathetic neurons innervating the heart are exposed to the inflammatory cytokines cardiotrophin-1 (CT-1), interleukin-6 (IL-6) and tumor necrosis factor alpha (TNFalpha) after myocardial infarction, the effects of these cytokines on noradrenergic function are not well understood. We used cultured sympathetic neurons to investigate the effects of these cytokines on catecholamine content, the tyrosine hydroxylase co-factor, tetrahydrobiopterin (BH4), and norepinephrine (NE) uptake. CT-1, but not IL-6 or TNFalpha, suppressed NE uptake and catecholamines in these neurons, whereas CT-1 and, to a lesser extent, IL-6 decreased BH4 content. CT-1 exerted these effects by decreasing tyrosine hydroxylase, GTP cyclohydrolase (GCH) and NE transporter mRNAs, while IL-6 lowered only GCH mRNA. The neurons innervating the heart are also activated by the central nervous system after myocardial infarction. We examined the combined effect of depolarization and cytokines on noradrenergic function. In CT-1-treated cells, depolarization caused a small increase in BH4 and NE uptake, and a large increase in catecholamines. These changes were accompanied by increased TH, GCH and NE transporter mRNAs. CT-1 and depolarization regulate expression of noradrenergic properties in an opposing manner, and the combined treatment results in elevated cellular catecholamines and decreased NE uptake relative to control cells.

Quantification of transcriptional activities of reporter gene constructs in primary cultures of sympathetic neurons

Primary cultures of sympathetic neurons provide an attractive cellular model for investigating the mechanisms of neurotransmitter phenotypic plasticity. However, it has not been possible to transfect these neurons by conventional techniques, and this has been a major impediment to molecular investigations of neuronal gene expression in this system. Here, reporter plasmids were transferred into the nuclei of cultured sympathetic neurons by microinjection. We developed and improved this procedure and were able to measure the transcriptional activities of two coinjected promoters in small groups of neurons, and even from a single neuron. Promoter activities can thus be quantified and normalized relative to that of a constitutively expressed promoter, allowing correction for variability in the injection and assay procedures. High and low promoter activities can be reliably quantified. Importantly, this method can be used not only for reporter plasmids but also for DNA fragments containing only a promoter and reporter gene without any vector sequence that might interfere with promoter. Using this approach, we measured neuronal promoter activities and found that one promoter region of the gene encoding choline acetyltransferase was up-regulated by more than sevenfold by leukemia inhibitory factor. This method thus provides the means to investigate the function of neuronal genes and the mechanisms that regulate their transcription in cultured sympathetic neurons.

Calcium-dependent regulation of cholinergic cell phenotype in the hypothalamus in vitro

Glutamate is a major fast excitatory neurotransmitter in the CNS including the hypothalamus. Our previous experiments in hypothalamic neuronal cultures showed that a long-term decrease in glutamate excitation upregulates ACh excitatory transmission. Data suggested that in the absence of glutamate activity in the hypothalamus in vitro, ACh becomes the major excitatory neurotransmitter and supports the excitation/inhibition balance. Here, using neuronal cultures, fura-2 Ca(2+) digital imaging, and immunocytochemistry, we studied the mechanisms of regulation of cholinergic properties in hypothalamic neurons. No ACh-dependent activity and a low number (0.5%) of cholinergic neurons were detected in control hypothalamic cultures. A chronic (2 wk) inactivation of N-methyl-D-aspartate (NMDA) ionotropic glutamate receptors, L-type voltage-gated Ca(2+) channels, calmodulin, Ca(2+)/calmodulin-dependent protein kinases II/IV (CaMK II/IV), or protein kinase C (PKC) increased the number of cholinergic neurons (to 15-24%) and induced ACh activity (in 40-60% of cells). Additionally, ACh activity and an increased number of cholinergic neurons were detected in hypothalamic cultures 2 wk after a short-term (30 min) pretreatment with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid tetrakis(acetoxy-methyl) ester (BAPTA AM; 2.5 microM), a membrane permeable Ca(2+)-chelating agent that blocks cytoplasmic Ca(2+) fluctuations. An increase in the number of cholinergic neurons following a chronic NMDA receptor blockade was likely due to the induction of cholinergic phenotypic properties in postmitotic noncholinergic neurons, as determined using 5-bromo-2'-deoxyuridine (BrdU) labeling. In contrast, a chronic inactivation of non-NMDA glutamate receptors or cGMP-dependent protein kinase had little effect on the expression of ACh properties. The data suggest that Ca(2+), at normal intracellular concentrations, tonically suppresses the development of cholinergic properties in hypothalamic neurons. However, a decrease in Ca(2+) influx into cells (through NMDA receptors or L-type Ca(2+) channels), inactivation of intracellular Ca(2+) fluctuations, or downregulation of Ca(2+)-dependent signal transduction pathways (CaMK II/IV and PKC) remove the tonic inhibition and trigger the development of cholinergic phenotype in some hypothalamic neurons. An increase in excitatory ACh transmission may represent a novel form of neuronal plasticity that regulates the activity and excitability of neurons during a decrease in glutamate excitation. This type of plasticity has apparent region-specific character and is not expressed in the cortex in vitro; neither increase in ACh activity nor change in the number of cholinergic neurons were detected in cortical cultures under all experimental conditions.

Specific frequencies of spontaneous Ca2+ transients upregulate GAD 67 transcripts in embryonic spinal neurons

Spontaneous Ca2+ transients expressed prior to synaptogenesis regulate the developmental appearance of GABA in cultured Xenopus spinal neurons. We find that glutamic acid decarboxylase (GAD) immunoreactivity is also Ca(2+)-dependent and parallels the appearance of GABA. We show that xGAD 67 transcripts first appear in the embryonic spinal cord during the period in which these Ca2+ spikes are generated, in a pattern that is temporally and spatially appropriate to account for differentiation of GABAergic interneurons. RNase protection and competitive quantitative RT-PCR demonstrate that transcript levels are approximately threefold greater when neurons are cultured in the presence of extracellular Ca2+ that permits generation of transients than when cultured in its absence. The frequency of spontaneous Ca2+ spikes plays a crucial role in the regulation of transcripts, since reimposition of Ca2+ transients at the frequency generated in cultured neurons rescues normal expression. We conclude that naturally occurring low frequencies of these Ca2+ transients regulate levels of xGAD 67 mRNA in differentiating neurons.

Coding of neuronal differentiation by calcium transients

Excitability has long been recognized as the basis for rapid signaling in the mature nervous system, but roles of channels and receptors in controlling slower processes of differentiation have been identified only more recently. Voltage-dependent and transmitter-activated channels are often expressed at early stages of development prior to synaptogenesis, and allow influx of Ca(2+). Here we examine the functions of spontaneous transient elevations of intracellular Ca(2+) in embryonic neurons. These Ca(2+) transients abruptly raise levels of Ca(2+) as much as tenfold, for brief periods, repeatedly, and can be highly localized. Like cloudbursts on the developing landscape, Ca(2+) transients modulate growth and stimulate differentiation, in a frequency-dependent manner, probably by changes in phosphorylation or proteolysis of regulatory and structural proteins in local regions. We review the mechanisms by which Ca(2+) transients are generated and their effects in regulating motility via the cytoskeleton and differentiation via transcription.

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.

Histone hyperacetylating agents stimulate promoter activity of human choline acetyltransferase gene in transfection experiment

Butyrate (5 mM), Trichostatin A (1 microM) or Trapoxin A (30 nM) increased choline acetyltransferase (ChAT) activity in cultured rat sympathetic neurons 3- to 8-fold in 2 days. On the contrary, the three drugs decreased ChAT activity in human CHP126 cells. Butyrate had little effect on ChAT mRNA level in these cells, suggesting post-transcriptional mechanisms for the decrease in ChAT activity. However, transient transfection experiments using CHP126 cells revealed that the M promoter, but not the R promoter, of human ChAT gene was activated 20- to 130-fold by the three hyperacetylating agents. A butyrate-responsive element was localized in the 1 kbp region upstream of exon M. Constructs containing in addition the genomic segment between exons M and 1 displayed maximal basal activity and inducibility by butyrate, suggesting the presence of butyrate-activated promoter/enhancer elements in this region. The stimulatory effects of butyrate and Trichostatin A were also observed in stably transfected CHP126 clones, suggesting that the chromatin environment was not preventing the induction of the endogenous ChAT gene by butyrate. Rather, the data suggest different chromatin organizations for the stable transgene and the endogenous ChAT gene.

Target determination of neurotransmitter phenotype in sympathetic neurons

While the majority of sympathetic neurons are noradrenergic, a minority population are cholinergic. At least one population of cholinergic sympathetic neurons arises during development by a target-dependent conversion from an initial noradrenergic phenotype. Evidence for retrograde specification has been obtained from transplantation studies in which sympathetic neurons that normally express a noradrenergic phenotype throughout life were induced to innervate sweat glands, a target normally innervated by cholinergic sympathetic neurons. This was accomplished by transplanting footpad skin containing sweat gland primordia from early postnatal donor rats to the hairy skin region of host rats. The sympathetic neurons innervating the novel target decreased their expression of noradrenergic traits and developed choline acetyltransferase (ChAT) activity. In addition, many sweat gland-associated fibers acquired acetylcholinesterase (AChE) staining and VIP immunoreactivity. These studies indicate that sympathetic neurons in vivo alter their neurotransmitter phenotype in response to novel environmental signals and that sweat glands play a critical role in the cholinergic and peptidergic differentiation of the sympathetic neurons that innervate them. The sweat gland-derived cholinergic differentiation factor is distinct from leukemia inhibitory factor and ciliary neurotrophic factor, two well-characterized cytokines that alter the neurotransmitter properties of cultured sympathetic neurons in a similar fashion. Recent studies indicate that anterograde signalling is also important for the establishment of functional synapses in this system. We have found that the production of cholinergic differentiation activity by sweat glands requires sympathetic innervation, and the acquisition and maintenance of secretory competence by sweat glands depends upon functional cholinergic innervation.

Galanin expression increases in adult rat sympathetic neurons after axotomy

Changes in neuropeptide expression occur in sensory, motor, and sympathetic neurons following axotomy. The particular pattern of peptide changes that occurs varies among the three cell types. We have studied the regulation in the rat superior cervical ganglion of the expression of galanin, a peptide previously shown to increase in axotomized sensory and motor neurons. While normally only an occasional neuron exhibiting galanin-like immunoreactivity is found in this ganglion, at two days after transection of the postganglionic internal and external carotid nerves, immunostaining can be observed in many neurons throughout the ganglion. Similar changes are found when ganglia are placed in organ culture for two days. The distribution of immunostained neurons after section of only one of the postganglionic trunks suggests that changes in galanin-like immunoreactivity occur only within neurons whose axons are transected. None the less, even when both nerve trunks are transected, only about half of the neurons in the ganglion exhibit galanin-like immunoreactivity, indicating that only a proportion of the axotomized neurons exhibit a detectable response. The few immunostained neurons seen after section of the cervical sympathetic trunk may also represent axotomized neurons. Galanin-like immunoreactivity extracted from the ganglion co-chromatographs with authentic galanin, and the level of this immunoreactivity increases dramatically after axotomy and explantation, and modestly after decentralization. These same manipulations produce parallel increases in the level of galanin messenger RNA. Together, the findings indicate that the expression of galanin increases in sympathetic neurons after axotomy. Galanin is thus the first neuropeptide whose expression has been shown to increase after transection of all three types of peripheral axons that have been studied.

Multiple promoters of human choline acetyltransferase and aromatic L-amino acid decarboxylase genes

The promoter regions of human choline acetyltransferase (ChAT) and aromatic L-amino acid decarboxylase (AADC) genes have been analyzed by transient transfection assays. AADC gene is transcribed from two alternative noncoding first exons, 1N and 1NN, expressed in pheochomocytoma and hepatoma cells, respectively. 5' flanking sequences of exon 1 N (from 9000 to 147 bp) display promoter activity in SK-N-BE neuroblastoma cells, but not in MC-I-XC cholinergic neuroepithelioma cells, and in AADC-rich non-neuronal cells. On the contrary, 5' flanking sequences of exon 1 NN (from 1117 to 119 bp) display high promoter activity in human hepatoma cells HepG2, but not in SK-N-BE cells, suggesting high degrees of specificity of promoters N and NN for AADC-expressing neuronal and non-neuronal cells, respectively. Preliminary evidence suggests that leukemia inhibitory factor suppresses the activity of the neuronal promoter in cultured sympathetic neurons. Two alternative first exons, R and M, have been localized in human ChAT gene, and the corresponding promoters characterized in cholinergic PC12 and NG-108-15 cells, and in non-cholinergic neuro2A cells. Several positively or negatively acting cis elements have been localized in the two promoters, as well as a cAMP-inducible, enhancer-like element in the second intron. Among the various cell lines studied, there was no correlation between promoter activities and the expression of the endogenous ChAT gene, suggesting that the fine-tuning of ChAT gene expression is controlled by silencer elements which remain to be localized.

The neuropeptide VIP regulates the expression of the tyrosine hydroxylase gene in cultured avian sympathetic neurons

The neuropeptide vasoactive intestinal polypeptide (VIP) increases the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in the biosynthesis of catecholamines, in cultured chicken sympathetic neurons. We report here that VIP acts by increasing TH mRNA levels in these cells. Induction of TH mRNA is transient and reaches maximal values 6-8 h after the addition of the peptide to the cultures. TH mRNA levels return to control values after 1-2 days. The quail cDNA probe detects a single mRNA species of approximately 9 kb in RNA extracted both from embryonic chicken sympathetic neurons and adult quail adrenal medulla.

Retinoic acid induces cholinergic differentiation of cultured newborn rat sympathetic neurons

Many studies provide evidence that retinoic acid (RA), an endogenous derivative of vitamin A, plays a role in the development of the nervous system. We now report that RA controls the neurotransmitter phenotype of post-mitotic rat sympathetic neurons in cell culture. RA added to the culture medium increased the specific activity of choline acetyltransferase (ChAT) and the level of acetylcholine (ACh). Concomitantly, RA reduced the specific activities of two catecholamine synthetic enzymes, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) and the level of norepinephrine (NE). After a 2 week treatment with 5 microM RA, ChAT was increased by 5-10 fold, whereas TH and DBH were decreased by 10-15 fold and 2-3 fold, respectively, as compared to sympathetic neurons grown in the absence of RA. The modulation of the activity of the three enzymes was dose-dependent and followed a similar time course. The decrease of TH expression was demonstrated to be due to a decreased number of TH molecules.

Bradykinin elevates tyrosine hydroxylase and dopamine beta-hydroxylase mRNA levels in PC12 cells

Bradykinin is known to rapidly elevate intracellular calcium leading to secretion of neurotransmitters and short term activation of tyrosine hydroxylase (TH). In this study we examined the effect of bradykinin on mRNA levels of two catecholamine biosynthetic enzymes. Treatment of PC12 cells with 1 microM bradykinin for 3 h markedly elevated both TH and dopamine beta-hydroxylase (DBH) mRNA levels.

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

We have studied the expression of catecholaminergic and cholinergic phenotypes in sympathetic ganglia removed from 7- to 10-day-old quail embryos and grown in vitro under different conditions. Quantitative data were obtained by measuring the conversion of (3H) tyrosine and (3H) choline to catecholamines (CA) and acetylcholine (ACh), respectively. In explant cultures, large amounts of both neurotransmitters were synthesized from the onset, but CA generally predominated, the molar ratios of CA:ACh being, on average, of the order of 2:1. If the ganglia were dissociated before plating, there was a selective increase in ACh synthesis (three- to fivefold) such that the CA:ACh ratio fell strikingly. The early expression of the cholinergic phenotype appears to be species-specific in that, under identical conditions, dissociated cell cultures of newborn mouse superior cervical ganglia were overwhelmingly catecholaminergic (CA:ACh ratio of approximately 40:1) and ACh synthesis was only just detectable. Addition of veratridine (1.5 microM) either to explant or to dissociated cell cultures of embryonic quail sympathetic ganglia barely altered CA-synthesizing ability; in contrast, ACh synthesis and accumulation were stimulated about threefold. This effect, which we found to correspond to a quantitatively similar increase in the activity of choline acetyltransferase (ChAT), was completely blocked by tetrodotoxin, indicating that it was due to Na(+)-dependent depolarization. A preferential stimulation of ACh production was also observed when the concentration of K+ was raised to 20 mM. Veratridine treatment of cultures of presumptive sympathoblasts, in the form of sclerotome-associated neural crest cells, had identical effects. Our results reveal the quantitative importance of ACh-related properties in avian sympathetic ganglia from the earliest stages of their development and suggest that depolarization may be one of the factors selectively enhancing expression of the cholinergic phenotype during ontogeny. In these respects, the neurochemical differentiation of sympathetic neurons unfolds according to dissimilar scenarios in birds and mammals.

Cell interactions that determine sympathetic neuron transmitter phenotype and the neurokines that mediate them

The transmitter properties of both developing and mature sympathetic neurons are plastic and can be modulated by a number of environmental cues. Cell culture studies demonstrate that noradrenergic neurons can be induced to become cholinergic and that the expression of neuropeptides can be altered. Similar changes in transmitter phenotype occur in vivo. During development, noradrenergic neurons that innervate eccrine sweat glands acquire cholinergic and peptidergic function. This change is dependent upon interactions with the target tissue. Following injury of sympathetic neurons in developing and adult animals, striking alterations take place in peptide expression. Ciliary neurotrophic factor and cholinergic differentiation factor/leukemia inhibitory factor, members of a family that includes several hematopoietic cytokines, induce cholinergic function and modulate neuropeptide expression in cultured sympathetic neurons. Studies in progress provide evidence that members of this new cytokine family influence the transmitter phenotype of sympathetic neurons not only in vitro but also in vivo.

Effects of ciliary neurotrophic factor (CNTF) and depolarization on neuropeptide expression in cultured sympathetic neurons

We examined the effects of ciliary neurotrophic factor (CNTF) and depolarization, two environmental signals that influence noradrenergic and cholinergic function, on neuropeptide expression by cultured sympathetic neurons. Sciatic nerve extract, a rich source of CNTF, increased levels of vasoactive intestinal peptide (VIP), substance P, and somatostatin severalfold while significantly reducing levels of neuropeptide Y (NPY). No change was observed in the levels of leu-enkephalin (L-Enk). These effects were abolished by immunoprecipitation of CNTF-like molecules from the extract with an antiserum raised against recombinant CNTF, and recombinant CNTF caused changes in neuropeptide levels similar to those of sciatic nerve extract. Alterations in neuropeptide levels by CNTF were dose-dependent, with maximal induction at concentrations of 5-25 ng/ml. Peptide levels were altered after only 3 days of CNTF exposure and continued to change for 14 days. Depolarization of sympathetic neuron cultures with elevated potassium elicited a different spectrum of effects; it increased VIP and NPY content but did not alter substance P, somatostatin, or L-Enk. Depolarization is known to block cholinergic induction in response to heart cell conditioned medium and we found that it blocked the induction of choline acetyltransferase (ChAT) and peptides by recombinant cholinergic differentiation factor/leukemia inhibitory factor (CDF/LIF). In contrast, it did not antagonize the effects of CNTF on either ChAT activity or neuropeptide expression. Thus, while CNTF has effects on neurotransmitter properties similar to those previously reported for CDF/LIF, the actions of these two factors are differentially modulated by depolarization, suggesting that the mechanisms of cholinergic and neuropeptide induction for the two factors differ. In addition, in contrast to CDF/LIF, CNTF did not alter levels of ChAT, VIP, substance P, or somatostatin in cultured dorsal root ganglion neurons. These observations indicate that CNTF and depolarization affect the expression of neuropeptides by sympathetic neurons and provide evidence for an overlapping yet distinct spectrum of actions of the two neuronal differentiation factors, CNTF and CDF/LIF.

Cholinergic differentiation factor (CDF/LIF) promotes survival of isolated rat embryonic motoneurons in vitro

We present evidence that the cholinergic differentiation factor (CDF), originally purified from cardiac and skeletal muscle cell-conditioned medium and found to be identical to leukemia inhibitory factor (LIF), promotes survival of embryonic day 14 rat motoneurons in vitro. These neurons were retrogradely labeled with the fluorescent tracer Dil and enriched on a density gradient or purified to homogeneity by fluorescence-activated cell sorting. Subnanomolar concentrations of CDF/LIF supported the survival of 85% of the motoneurons that would have died between days 1 and 4 of culture. The enhanced survival was accompanied by a 4-fold increase in choline acetyltransferase (ChAT) activity per culture. CDF/LIF also increased ChAT activity in dorsal spinal cord cultures, but had no detectable effect on ChAT levels in septal or striatal neuronal cultures. For comparison, other neurotrophic molecules were tested on motoneuron cultures. Ciliary neurotrophic factor had effects on motoneuron survival similar to those of CDF/LIF, whereas basic fibroblast growth factor was somewhat less effective. Nerve growth factor had no effect on the survival of rat motoneurons.

Membrane depolarization by isotonic or hypertonic KCl: differential effects on mRNA levels of tyrosine hydroxylase and dopamine beta-hydroxylase mRNA in PC12 cells

Membrane depolarization is an important and common manipulation used to study the result of enhanced neuronal activity on adaptive changes, including alterations in gene expression. In this study, the effect of elevated KCl, under isotonic and hypertonic conditions, on the changes in mRNA levels of the catecholamine biosynthetic enzymes, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) was compared. Treatment of PC12 pheochromocytoma cells for several hours with 50 mM KCl, under conditions where osmolarity was maintained, induced TH mRNA levels several fold, without changing DBH mRNA levels (Kilbourne and Sabban, 1990). In contrast, 50 mM KCl added to culture media without adjusting the osmolarity did not alter TH mRNA levels for up to 24 h. Longer continuous exposure to this hypertonic depolarization condition reduced TH mRNA levels to about 10% of control levels. DBH mRNA levels also declined when PC12 cells were treated from 12 h to 5 days with hypertonic 50 mM KCl. The effect appeared to be specific, since actin mRNA levels were elevated about 2-fold with these same hypertonic treatments. As a control for osmotic changes, 50 mM NaCl was used and did not alter TH or DBH mRNA levels. Viability of the cells was maintained and total protein synthesis was reduced somewhat after 12 h of exposure to hypertonic 50 mM KCl, and remained relatively constant for as long as 4 days. Thus, there appears to be an interaction between osmolarity and elevated KCl since very different results of the effects of membrane depolarization on the mRNA levels for the catecholamine biosynthetic enzymes were obtained depending on the osmolarity of the cultures. The extent of elevation of TH mRNA with isotonic KCl was also dependent on cell density. At high cell densities, membrane depolarization no longer induced TH mRNA levels. The results of this study indicate the experimental parameters which can be crucial in studies of membrane depolarization.

Immunoaffinity purification and dose-response of cholinergic neuronal differentiation factor

A glycoprotein from heart cell-conditioned medium, cholinergic neuronal differentiation factor (CDF), causes a transition from noradrenergic to cholinergic phenotype in cultured rat sympathetic neurons. Although the transition has been known to occur in a dose-dependent manner and CDF has been purified, the examination of a complete dose-response of neurons to CDF has not been possible because sufficient quantities of pure CDF have not been available. A complete dose-response curve is essential for evaluating the biological response of the neurons, for assessing the physiological role of CDF and for understanding the mechanism of action of CDF. We report here an immunoaffinity-purification procedure for CDF with a 73.1% recovery using antibodies raised against a synthetic peptide homologous with the N-terminal region of CDF. This method produced pure CDF in quantities sufficient for examination of the full dose-response range of the neurons. Our main findings are the following. The dose-responses of acetylcholine and catecholamine metabolisms to CDF are different, although the same molecule affects both transmitters. While the half-maximal concentrations for acetylcholine induction (0.20 nM) and for catecholamine suppression (0.28 nM) are similar, the response of catecholamine metabolism begins slowly and saturates at a CDF concentration (5-20 nM) considerably higher than that of acetylcholine (0.6 nM). This may indicate that CDF affects multiple processes in catecholamine metabolism.

Recombinant cholinergic differentiation factor (leukemia inhibitory factor) regulates sympathetic neuron phenotype by alterations in the size and amounts of neuropeptide mRNAs

The cholinergic differentiation factor (CDF) in heart cells is identical to leukemia inhibitory factor (LIF). Recombinant CDF/LIF was shown to alter dramatically neurotransmitter production as well as the levels of several neuropeptides in cultured rat sympathetic neurons. Here it is shown that these changes are likely to be caused by alterations in the mRNA for these proteins and peptides. Growth in 1 nM recombinant CDF/LIF induces mRNA for acetyl CoA: choline-O-acetyltransferase [EC 2.3.1.6; choline acetyltransferase (ChAT)], somatostatin (SOM), substance P, and vasoactive intestinal polypeptide while lowering mRNA levels of tyrosine hydroxylase (EC 1.14.16.2) and neuropeptide Y (NPY). In addition, the sizes of the mRNAs for ChAT, SOM, and NPY are larger after recombinant CDF/LIF treatment.

Spontaneous calcium influx and its roles in differentiation of spinal neurons in culture

Stimulation of embryonic amphibian spinal neurons has been shown to produce calcium-dependent action potentials of long duration at early stages of development. These impulses become brief and sodium-dependent upon further differentiation. The neurons are now shown to exhibit spontaneous, transient elevations of intracellular calcium in culture during the early developmental period when activity produces greatest calcium influx. Removal of extracellular calcium during this period alone is sufficient to perturb differentiation, and influx through voltage-dependent calcium channels is shown to be required for standard development of neuronal phenotypes. No large changes in steady-state calcium levels occur in the cytoplasm during the maturation of cultured neurons despite a reduction of the calcium-dependent component of the impulse. Transient elevation of intracellular calcium is necessary for standard cytodifferentiation and may provide a link between electrical activity and gene expression.

Differential effect of membrane depolarization on levels of tyrosine hydroxylase and dopamine beta-hydroxylase mRNAs in PC12 pheochromocytoma cells

Membrane depolarization has been widely used to elucidate the response of the nervous system to prolonged neuronal activity or stress. We studied the effect of treating PC12 cells with membrane depolarizing stimuli, 50 mM KCl, or 150 microM veratridine, and the subsequent changes in the mRNA levels of the catecholamine biosynthetic enzymes, tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH). TH mRNA levels were found to increase 2- to 5-fold after continuous treatment for 1-12 h with 50 mM KCl. Depolarization with 150 microM veratridine had a similar effect on TH mRNA. In contrast, DBH mRNA levels were unchanged by either KCl or veratridine treatment. The role of calcium in the increase of TH mRNA levels elicited by depolarization was examined. The increase in TH mRNA was inhibited by the chelation of calcium with 3 mM EGTA. However, in contrast to their effect on phosphorylation of TH elicited by acute depolarization, the calcium channel blockers, nitrendipine and verapamil, and the calmodulin antagonists, W7 and trifluoperazine, did not prevent the increase in TH mRNA levels subsequent to several hours exposure to depolarizing stimuli. The calcium ionophore, A23187, alone was unable to induce TH mRNA levels. Thus, the increase in TH mRNA elicited by depolarization is mediated differently than the acute phosphorylation of the enzyme.

Induction of the vesicular monoamine transporter by elevated potassium concentration in cultures of rat sympathetic neurons

The expression of the vesicular monoamine transporter was studied in newborn rat sympathetic neurons and compared to that of the catecholamine biosynthesis enzymes tyrosine hydroxylase and dopamine-beta-hydroxylase. The vesicular monoamine transporter was assayed using the specific ligand [3H]dihydrotetrabenazine. In cultures grown for 10 days in the presence of 35 mM K+, tyrosine hydroxylase activity and the density of [3H]dihydrotetrabenazine binding sites were increased by a similar 2-3-fold factor, while dopamine-beta-hydroxylase activity and protein level were unchanged. Under these conditions, choline acetyltransferase activity was depressed by 90%. The induction of the vesicular monoamine transporter by high K+ was dependent upon Ca2+ entry through slow calcium channels since it was inhibited by the diphenylbutylpiperidine antagonist fluspirilene and by 20 mM Mg2+, and was enhanced by the dihydropyridine agonist, Bay K8644. The induction of the vesicular monoamine transporter by neuronal depolarization indicates the existence of a Ca2(+)-dependent mechanism of coregulation for this intrinsic component of monoaminergic synaptic vesicles and tyrosine hydroxylase. On the other hand, the apparent absence of dopamine-beta-hydroxylase induction is probably due to the continuous secretion of this intravesicular enzyme by the depolarized sympathetic neurons, an effect already observed in trans-synaptically stimulated adult sympathetic ganglion and adrenal medulla.

Transforming growth factor beta 1 is a potent survival factor for rat embryo motoneurons in culture

We have studied the effects of transforming growth factor beta 1 (TGF beta 1) on the survival of embryonic motoneurons in culture. For this purpose, E14 rat embryo motoneurons were purified to more than 90% homogeneity by cell sorting and cultured at low density on monolayers of cortex astrocytes. Subnanomolar concentrations of TGF beta 1 (40-500 pM) increased the survival of motoneurons 2-fold after 9-11 days in culture. The increase in choline acetyltransferase (ChAT) activity per culture caused by TGF beta 1 was attributable to its effects on survival. Comparable results were found with motoneurons cultured on lysed astrocytes, suggesting that the effects of the factor are not mediated by non-neuronal cells, but that motoneurons are a target for TGF beta 1.

Translational regulation of somatostatin in cultured sympathetic neurons

Coculture of sympathetic neurons with ganglion nonneuronal cells elevated levels of preprosomatostatin mRNA but did not alter neuronal synthesis, content, or release of somatostatin. Treatment of sympathetic neurons with culture medium conditioned by exposure to ganglion nonneuronal cells similarly elevated preprosomatostatin mRNA. Treatment with conditioned medium elevated somatostatin levels in pure neuronal cultures, but not in neurons cocultured with nonneuronal cells. Conditioned medium also failed to increase peptide levels in neurons cultured on a substratum of killed nonneuronal cells, despite a large increase in preprosomatostatin mRNA. These observations suggest that contact of sympathetic neurons with nonneuronal cell membranes inhibits the increase in peptide synthesis, but not the increase in preprosomatostatin mRNA after treatment with conditioned medium. Thus neuronal interactions with nonneuronal cells regulate somatostatin metabolism at both the mRNA and peptide levels. Regulatory effects on the mRNA and the peptide are separable and do not necessarily occur in parallel, and translational controls may be the rate-limiting factors.

Different biological activities in conditioned media control the expression of a variety of neuropeptides in cultured sympathetic neurons

An intriguing question regarding neuronal development is how neurons choose which neurotransmitter and/or peptide to express among over 40 candidates. We find that heart cell conditioned medium (CM) induces a number of neuropeptides and/or their precursor mRNAs, as well as acetylcholine, in cultured rat sympathetic neurons: substance P, somatostatin, vasoactive intestinal polypeptide, enkephalin derivatives, and cholecystokinin, but not neuropeptide Y. Different patterns of peptide induction were observed for CMs from primary cultures of heart, gut, and skin. Acetylcholine and substance P were induced most effectively by serum-free heart cell CM; enkephalin derivatives were induced most effectively by skin cell CM; and somatostatin and vasoactive intestinal polypeptide were induced equally well by all of the CMs. These observations suggest the possibility that many distinct, diffusible factors can influence the choice of transmitter and/or peptide phenotype in developing neurons.

The role of Ca2+ channels of the L-type in neurotransmitter plasticity of cultured sympathetic neurons

We have studied the effects of Ca2+ antagonists and agonists on the development of choline acetyltransferase (ChAT), tyrosine hydroxylase (TOH) and acetylcholinesterase (AChE) in cultures of rat sympathetic neurons maintained for 6-9 days in low K+ (5 mM) or high K+ (35 mM) medium. Previous experiments have shown that high K+ medium increases TOH activity and TOH-mRNA level up to 3.5-fold and depresses the development of AChE, in particular of its asymmetric A12 form. Moreover, high K+ medium inhibits ChAT induction by 90% in muscle-conditioned medium (Raynaud et al., Dev. Biol., 119 (1987) 305-312; 121 (1987) 548-558). None of the Ca2+ antagonists tested affected the development of ChAT, TOH or AChE in low K+ medium. In high K+ medium, nitrendipine (3 microM) or fluspirilene (1 microM) fully restored ChAT induction by conditioned medium to the level observed in low K+ medium. Other drugs (1 microM) gave partial reversion: flunarizine greater than (+)-PN 200-110 greater than (-)-D-888 greater than cinnarizine = lidoflazine. On the other hand, ChAT induction was not restored by a calmodulin inhibitor, calmidazolium (1 microM). Fluspirilene, PN 200-110, and nitrendipine also totally abolished TOH induction by high K+ medium; fluspirilene (1 microM) suppressed the inhibitory effect of high K+ medium on AChE development and restored the development of A12 AChE. Conditioned medium also depresses AChE and blocks the development of A12 AChE (Swerts et al., Dev. Biol., 103 (1984) 230-234), but these effects were insensitive to fluspirilene. The Ca2+ agonist Bay K 8644 (1 microM) potentiated the effects of elevated K+ on both ChAT and TOH. The data suggest that the effects of long-term depolarization on ChAT, TOH and AChE are mediated by Ca2+ entry specifically through voltage-sensitive channels of the L-type. Our results on cultured sympathetic neurons raise the possibility that Ca2+ antagonists, which are widely used clinically, may affect the expression of neurotransmitter phenotypic traits in vivo and interfere with trans-synaptic induction of enzymes.

Complete sequence of a cDNA encoding an active rat choline acetyltransferase: a tool to investigate the plasticity of cholinergic phenotype expression

A cDNA clone encoding the complete sequence of an active rat choline acetyltransferase (ChoAcTase; acetyl-CoA:choline O-acetyltransferase, EC 2.3.1.6) has been isolated. Analysis of the deduced amino acid sequence reveals 85% and 31% identity with the porcine and Drosophila melanogaster enzymes, respectively. To further elucidate the molecular basis of neurotransmitter-related phenotypic plasticity, the expression of ChoAcTase mRNA was compared with that of tyrosine hydroxylase [TH; tyrosine 3-monooxygenase, L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2], in neurons from superior cervical ganglia grown in the following conditions: 1) normal medium, 2) high K+ medium, and 3) normal medium supplemented with 50% muscle-conditioned medium (CM). TH mRNA was expressed in all three media; its level rose in high K+ and decreased strikingly in the presence of CM. ChoAcTase mRNA could be visualized in CM, but fell to undetectable levels in normal and high K+ media. These results suggest that translational or post-translational mechanisms do not play a major role for the modulation of neurotransmitter-associated phenotype.

Ciliary neurotrophic factor induces cholinergic differentiation of rat sympathetic neurons in culture

Ciliary neurotrophic factor (CNTF) influences the levels of choline acetyltransferase (ChAT) and tyrosine hydroxylase (TH) in cultures of dissociated sympathetic neurons from newborn rats. In the presence of CNTF both the total and specific activity of ChAT was increased 7 d after culture by 15- and 18-fold, respectively, as compared to cultures kept in the absence of CNTF. Between 3 and 21 d in culture in the presence of CNTF the total ChAT activity increased by a factor of greater than 100. Immunotitration demonstrated that the elevated ChAT levels were due to an increased number of enzyme molecules. In contrast to the increase in ChAT levels, the total and specific activity levels of TH were decreased by 42 and 36%, respectively, after 7 d in culture. Half-maximal effects for both ChAT increase and TH decrease were obtained at CNTF concentrations of approximately 0.6 ng and maximal levels were reached at 1 ng of CNTF per milliliter of medium. The effect of CNTF on TH and ChAT levels were seen in serum-containing medium as well as in serum-free medium. CNTF was shown to have only a small effect on the long-term survival of rat sympathetic neurons. We therefore concluded that the effects of CNTF on ChAT and TH are not due to selective survival of cells that acquire cholinergic traits in vitro, but are rather due to the induction of cholinergic differentiation of noradrenergic sympathetic neurons.

Activity-dependent regulation of gene expression in muscle and neuronal cells

In both the central and the peripheral nervous systems, impulse activity regulates the expression of a vast number of genes that code for synaptic proteins, including neuropeptides, enzymes involved in neurotransmitter biosynthesis and degradation, and membrane receptors. In recent years, the mechanisms involved in these regulations became amenable to investigation by the methods of recombinant DNA technology. The first part of this review focuses on the activity-dependent control of nicotinic acetylcholine receptor biosynthesis in vertebrate muscle, a model case for the regulation of synaptic protein biosynthesis at the postsynaptic level. The second part summarizes some examples of neuronal proteins whose biosynthesis is under the control of transsynaptic impulse activity. The first, second, and third intracellular messengers involved in membrane-to-gene signaling are discussed, as are possible posttranscriptional control mechanisms. Finally, models are proposed for a role of neuronal activity in the genesis and stabilization of the synapse.

Evidence for neurotransmitter plasticity in vivo. II. Immunocytochemical studies of rat sweat gland innervation during development

Previous studies of the cholinergic sympathetic innervation of rat sweat glands provide evidence for a change in neurotransmitter phenotype from noradrenergic to cholinergic during development. To define further the developmental history of cholinergic sympathetic neurons, we have used immunocytochemical techniques to examine developing and mature sweat gland innervation for the presence of the catecholamine synthetic enzymes tyrosine hydroxylase (TH) and dopamine beta-hydroxylase (DBH) and for two neuropeptides present in the mature cholinergic innervation, vasoactive intestinal peptide (VIP) and calcitonin gene-related peptide (CGRP). In 7-day old animals, intensely TH- and DBH-immunoreactive axons were closely associated with the forming glands. The intensity of both the TH and DBH immunofluorescence decreased as the glands and their innervation developed. Neither TH-IR nor DBH-IR disappeared entirely; faint immunoreactivity for both enzymes was reproducibly detected in mature animals. In contrast to noradrenergic properties, the expression of peptide immunoreactivities appeared relatively late. No VIP-IR or CGRP-IR was detectable in the sweat gland innervation at 4 or 7 days. In some glands VIP-IR first appeared in axons at 10 days, and was evident in all glands by 14 days. CGRP-IR was detectable only after 14 days. In addition to VIP-IR and CGRP-IR, we examined the sweat gland innervation for several neuropeptides which have been described in noradrenergic sympathetic neurons including neuropeptide Y, somatostatin, substance P, and leu- and met-enkephalin; these peptides were not evident in either developing or mature sweat gland axons. Our observations provide further evidence for the early expression and subsequent modulation of noradrenergic properties in a population of cholinergic sympathetic neurons in vivo. In addition, the asynchronous appearance during development of the two neuropeptide immunoreactivities raises the possibility that the expression of peptide phenotypes may be controlled independently.

Comparison of the effects of elevated K+ ions and muscle-conditioned medium on the neurotransmitter phenotype of cultured sympathetic neurons

Neuronal depolarization and culture media conditioned by certain nonneuronal cells (CM) are known to exert opposite effects on the expression of cholinergic and noradrenergic traits in cultured rat sympathetic neurons. We have compared their effects on the developments of choline acetyltransferase (CAT), tyrosine hydroxylase (TOH), dopa decarboxylase (AADC) and acetylcholinesterase (AcChE) in these cultures. A macromolecular factor which was partially purified from CM increased CAT development in a dose-dependent manner and depressed the development of TOH and AADC by 5- to 10-fold. In the presence of intermediate concentrations of this partially purified factor, both CAT and catecholamine synthesizing enzymes developed to high levels, whereas high concentrations caused a long-lasting, but not total, impairment of TOH development. The effects of CM on both CAT and AADC activities resulted from variations in the number of immunotitratable enzyme molecules. Conversely, K+ ions (30-40 mM) depressed the development of CAT by 90% and stimulated TOH development 2.5-fold. Cultures grown with CM in high K+ medium had similar CAT and TOH activities as compared to those cultures grown without CM in low K+ medium suggesting that CM and K+ ions had antagonistic effects on the expression of these enzymes. However, K+ ions did not affect the development of AADC in these cultures. CM suppressed in a reversible manner the development of the 16 S form of AcChE. In the presence of 40 mM K+, the rate of development of AcChE was reduced. In particular, the development of 16 S AcChE was strikingly impaired, although not totally suppressed. The effect of elevated K+ ions on the percentage of 16 S AcChE was rapidly reversible. It is concluded that CM and elevated K+ ions have antagonistic effects on CAT and TOH, but not on AADC development; AcChE, in particular its asymmetric 16 S form, is regulated independently of the cholinergic/noradrenergic status of sympathetic neurons.