Persistence of an amine uptake system in cultured rat sympathetic neurons which use acetylcholine as their transmitter

Stages of neurotransmitter development in autonomic neurons

The ontogeny of neurotransmitters in autonomic neurons proceeds through the successive stages of early expression, definitive expression, modulation, and regulation, extending from embryonic life to maturity. Although different extracellular signals influence development at different stages, a number of signals that influence development continue to govern transmitter function during maturity. The sequential ontogenetic stages parallel the progressive restriction of mutability of phenotypic expression; however, some degree of neuronal mutability appears to persist through maturity.

Growth and development of sympathetic neurons in tissue culture

Adrenergic neurons from the superior cervical ganglion of the neonatal rat, when studied under certain culture conditions, develop cholinergic properties including hexamethonium-sensitive synaptic interactions, choline acetyltransferase activity and synaptic endings containing clear vesicles. Evidence from correlative biochemical, physiological and morphological studies on populations of neurons indicates that cholinergic function is acquired by the majority of neurons and not by subpopulation. The factors that influence the development of cholinergic function in culture include the presence of non-neuronal cells, the addition of human placental serum and chick embryo extract to the culture medium as well as the stage of development at which the neurons are placed in culture. Neurons from mature rats, maintained as explants in culture, develop low choline acetyltransferase activity and the synaptic endings containing dense-cored vesicles. In contrast, if dissociated, these adult neurons develop several cholinergic characteristics. Studies to determine which adrenergic properties are retained in neurons expressing cholinergic characteristics have shown an increase in the activities of tyrosine hydroxylase and dopamine beta-hydroxylase in both explanted and dissociated perinatal neurons. In addition, tyrosine hydroxylase has been localized immunocytochemically in neurons identified as cholinergic by electrophysiological methods.

Norepinephrine transporter expression in cholinergic sympathetic neurons: differential regulation of membrane and vesicular transporters

Sympathetic neurons that undergo a noradrenergic to cholinergic change in phenotype provide a useful model system to examine the developmental regulation of proteins required to synthesize, store, or remove a particular neurotransmitter. This type of change occurs in the sympathetic sweat gland innervation during development and can be induced in cultured sympathetic neurons by extracts of sweat gland-containing footpads or by leukemia inhibitory factor. Sympathetic neurons initially produce norepinephrine (NE) and contain the vesicular monoamine transporter 2 (VMAT2), which packages NE into vesicles, and the norepinephrine transporter (NET), which removes NE from the synaptic cleft to terminate signaling. We have used a variety of biochemical and molecular techniques to test whether VMAT2 and NET levels decrease in sympathetic neurons which stop producing NE and make acetylcholine. In cultured sympathetic neurons, NET protein and mRNA decreased during the switch to a cholinergic phenotype but VMAT2 mRNA and protein did not decline. NET immunoreactivity disappeared from the developing sweat gland innervation in vivo as it acquired cholinergic properties. Surprisingly, NET simultaneously appeared in sweat gland myoepithelial cells. The presence of NET in myoepithelial cells did not require sympathetic innervation. VMAT2 levels did not decrease as the sweat gland innervation became cholinergic, indicating that NE synthesis and vesicular packaging are not coupled in this system. Thus, production of NE and the transporters required for noradrenergic transmission are not coordinately regulated during cholinergic development.

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.

Transiently catecholaminergic (TC) cells in the bowel of the fetal rat: precursors of noncatecholaminergic enteric neurons

Experiments were done to study the fate of transient catecholaminergic (TC) cells that develop in the rodent gut during ontogeny. When they are first detected, at Day E11 in rats, TC cells are distributed along the vagal pathway, in advance of the descending fibers of the vagus nerves, and in the foregut. The early TC cells coexpress the immunoreactivities of several neural markers, including 150-kDa neurofilament protein, peripherin, microtubule associated protein (MAP) 5, and growth-associated protein (GAP)-43, with those of the catecholamine biosynthetic enzymes tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH). All cells in the fetal rat bowel at Day E11 that express neural markers also express TH immunoreactivity. The primitive TC cells also express the immunoreactivities of neural cell adhesion molecule (N-CAM), neuropeptide Y (NPY), and nerve growth factor (NGF) receptor (and NGF receptor mRNA). By Day E12 TC cells are found along the vagal pathway and throughout the entire preumbilical bowel. At this age TC cells acquire additional characteristics, including MAP 2 and synaptophysin immunoreactivities and acetylcholinesterase activity, which indicate that they continue to mature as neurons. In addition, TC cells of the rat are immunostained at Day E12 by the NC-1 monoclonal antibody, which in rats labels multiple cell types including migrating cells of neural crest origin. Despite their neural properties, at least some TC cells divide and therefore are neural precursors and not terminally differentiated neurons. At Day E10 TH mRNA-containing cells were not detected by in situ hybridization; however, by Day E11 TH mRNA was detected in sympathetic ganglia and in scattered cells in the mesenchyme of the foregut and vagal pathway. At this age, the number of enteric and vagal cells containing TH mRNA is about 30% less than the number of cells containing TH immunoreactivity in adjacent sections. The ratio of TH mRNA-containing cells to TH-immunoreactive vagal and enteric cells is even less at Day E12, especially in more caudal regions of the preumbilical bowel. A similar decline in the ratio of TH mRNA-containing to TH-immunoreactive cells was not observed in sympathetic ganglia. After Day E12 TH mRNA cannot be detected in enteric or vagal cells by in situ hybridization; nevertheless, TH immunoreactivity continues to be present through Day E14. DBH, NPY, and NGF receptor immunoreactivities are expressed by TH-immunoreactive transitional cells in the fetal rat gut after TH mRNA is no longer detectable.(ABSTRACT TRUNCATED AT 400 WORDS)

Facilitation of noradrenergic character of sympathetic neurons by co-culturing with heart cells

Noradrenergic properties of peripheral sympathetic neurons obtained from 10-12-day-old chick embryos were examined under various culture conditions. Sympathetic neurons supported by nerve growth factor in serum-free or serum-containing medium took up significant and almost equivalent amounts of [3H]norepinephrine. The uptake was markedly enhanced when neurons were co-cultured with heart cells, either in the absence or presence of nerve growth factor, for 3 days. The facilitatory effect of heart cells on the uptake was persistent only if the nerve growth factor was present. In its absence there was a gradual decrease in the uptake. Endogenous norepinephrine content was increased by several fold when sympathetic neurons were grown with either heart cells or in a medium conditioned by the heart cells. Sympathetic neurons initially selected in culture by nerve growth factor in regular medium and then exposed to a conditioned medium for 3 days exhibited a marked facilitation of [3H]norepinephrine uptake. The number of surviving neurons was almost constant when culture media were changed. Choline acetyltransferase activity of neurons grown in heart-conditioned medium plus nerve growth factor was not significantly higher than that of neurons grown in regular medium plus nerve growth factor. The overall conclusion of the study is that the noradrenergic character of sympathetic neurons can be further enhanced by heart cells or a medium conditioned by these cells.

MHC-specific cytotoxic T lymphocyte killing of dissociated sympathetic neuronal cultures

Experiments were conducted to determine whether neurons in culture can serve as targets for immunologic attack mediated by major histocompatibility complex (MHC)-specific cytotoxic T lymphocytes (CTLs) which recognize Class I antigens. Allogeneic C3H/He primary neuronal cultures were quickly destroyed after CTL addition, while syngeneic C57BL/6J neurons were not lysed. Alterations in the distribution of chromatin were the first ultrastructural changes that occurred, followed by loss of nuclear morphology, cytosolic changes, and eventually fragmentation of both the nucleus and cytosol. With Campenot chambers, it was possible to separate the membrane and nuclear lesions. CTLs exposed to neurites, but separated from the cell body by the chamber barrier, caused degeneration of neurites but did not cause lysis and cell death. Neuronal lysis mediated by antibody and complement appeared to be distinct from CTL-mediated lysis. These experiments demonstrate that neurons in culture are targets for MHC-specific CTLs, and therefore probably express functional levels of Class I antigens. The signal for killing by CTLs is not retrogradely transported from the neurite to the cell body, and morphologic events following CTL-neuron interaction resemble those that occur in dividing tumor target cell populations.

Comparison of the Schwann cell surface and Schwann cell extracellular matrix as promoters of neurite growth

The ability of Schwann cells to influence the direction and rate of neurite growth was investigated in a tissue culture model of the bands of Büngner of injured peripheral nerve. The arrangement of this culture system allowed testing of the growth-promoting properties of the Schwann cell surface and extracellular matrix (ECM) assembled by Schwann cells rather than soluble substances secreted into conditioned medium. Various components of peripheral nerve were examined separately as substrata for regenerating neurites: (i) Schwann cells and their ECM; (ii) Schwann cells alone; (iii) Schwann cell ECM alone; (iv) Schwann cells, fibroblasts, and their assembled ECM; (v) Schwann cells, their ECM and neurites; and (vi) purified laminin. Regenerating peripheral neurites were from explants of foetal rat dorsal root ganglia, which had been cultured for several weeks to rid them of accompanying non-neuronal cells, or from explants of foetal rat superior cervical ganglia, which contained non-neuronal cells. CNS neurites from the somatosensory cortex of embryonic rats were also studied; these neurites may be either first growing or regenerating. Neurites from all types of explants studied grew longer and were guided on a substratum of Schwann cells or Schwann cell ECM compared with a collagen substratum. The presence of fibroblasts during ECM assembly did not enhance the neurite growth-promoting activity. The design of the experiments suggested that the factors by which the Schwann cells or their ECM promoted and guided neurite outgrowth were surface-bound rather than medium-borne. Electron microscopic examination showed that neurites grew on either Schwann cell surfaces or basal lamina material. Attempts to define the chemical nature of the neurite growth-promoting effect of ECM by partial enzymatic digestion did not identify any single component as essential. Purified laminin was a more effective promoter of outgrowth of peripheral neurites than were Schwann cells or Schwann cell ECM. Cortical explants also grew on laminin, but neurites were accompanied on this substratum by a massive migration of non-neuronal cells; the neurites appeared to extend primarily on the non-neuronal cells rather than by direct attachment to the laminin substratum. This characteristic outgrowth of cortical non-neuronal cells on laminin was not consistently seen on Schwann cell ECM. In conclusion, either the Schwann cell surface or the ECM produced and assembled by Schwann cells promotes neurite outgrowth and guides that outgrowth from the several types of peripheral and CNS neurons studied in this report.

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.

Characterization of the vesicular monoamine transporter in cultured rat sympathetic neurons: persistence upon induction of cholinergic phenotypic traits

The binding of [3H]dihydrotetrabenazine ([3H]TBZOH), a specific ligand of the reserpine-sensitive monoamine transporter in brain and adrenal medulla storage vesicles, has been measured in cultured sympathetic neurons from newborn rat in relation to their neurotransmitter phenotype. As shown previously, neurons cultured in the absence of muscle-conditioned medium displayed high activities in catecholamine synthesizing enzymes and low levels of choline acetyltransferase, and neurons cultured in conditioned medium displayed the reverse pattern (J. P. Swerts, A. Le Van Thai, A. Vigny, and M. J. Weber, Dev. Biol. 100, 1-11, 1983). The density of [3H]TBZOH binding sites as well as their subcellular distribution were identical in both types of cultures. Two other structures rich in choline acetyltransferase, the electric organ of Torpedo and the ciliary ganglion of the chick embryo did not contain measurable amounts of [3H]TBZOH binding sites, suggesting that the monoamine transporter is not an ubiquitous component of cholinergic synaptic vesicles. These data suggest that the synthesis of the monoamine transporter in sympathetic neurons is not coregulated with the syntheses of the three norpinephrine synthesizing enzymes. It is proposed that the same population of synaptic vesicles can accumulate acetylcholine or catecholamine, depending only upon which neurotransmitter synthesizing enzymes are expressed by sympathetic neurons.

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

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

Rapid changes in synaptic vesicle cytochemistry after depolarization of cultured cholinergic sympathetic neurons

Sympathetic neurons taken from rat superior cervical ganglia and grown in culture acquire cholinergic function under certain conditions. These cholinergic sympathetic neurons, however, retain a number of adrenergic properties, including the enzymes involved in the synthesis of norepinephrine (NE) and the storage of measurable amounts of NE. These neurons also retain a high affinity uptake system for NE; despite this, the majority of the synaptic vesicles remain clear even after incubation in catecholamines. The present study shows, however, that if these neurons are depolarized before incubation in catecholamine, the synaptic vesicles acquire dense cores indicative of amine storage. These manipulations are successful when cholinergic function is induced with either a medium that contains human placental serum and embryo extract or with heart-conditioned medium, and when the catecholamine is either NE or 5-hydroxydopamine. In some experiments, neurons are grown at low densities and shown to have cholinergic function by electrophysiological criteria. After incubation in NE, only 6% of the synaptic vesicles have dense cores. In contrast, similar neurons depolarized (80 mM K+) before incubation in catecholamine contain 82% dense-cored vesicles. These results are confirmed in network cultures where the percentage of dense-cored vesicles is increased 2.5 to 6.5 times by depolarizing the neurons before incubation with catecholamine. In both single neurons and in network cultures, the vesicle reloading is inhibited by reducing vesicle release during depolarization with an increased Mg++/Ca++ ratio or by blocking NE uptake either at the plasma membrane (desipramine) or at the vesicle membrane (reserpine). In addition, choline appears to play a competitive role because its presence during incubation in NE or after reloading results in decreased numbers of dense-cored vesicles. We conclude that the depolarization step preceding catecholamine incubation acts to empty the vesicles of acetylcholine, thus allowing them to reload with catecholamine. These data also suggest that the same vesicles may contain both neurotransmitters simultaneously.

Fetal rat sympathetic neurons maintained in a serum-free medium retain induced cholinergic characteristics

When maintained in some serum-containing media, fetal rat sympathetic neurons acquire substantial choline acetyltransferase activity and form cholinergic synapses in vitro. However, when they are maintained in a serum-free, defined culture medium, choline acetyltransferase activity is not detected and cholinergic synapses are not observed. In this study, we have examined the effects of various times of exposure to a medium inducing cholinergic function on the properties of neurons subsequently maintained in defined medium. We report that 2-day, but not 2-h, exposure to this inducing medium causes a long-lasting (greater than 6 weeks) increase (7-10-fold) in the activity of choline acetyltransferase and that, under these conditions, sympathetic neurons in vitro form cholinergic, electrical and mixed function cholinergic and electrical synapses. We conclude that a relatively brief exposure to media inducing cholinergic function can cause long-lasting changes in the functional properties of sympathetic neurons in vitro.

Regulation of enzymes responsible for neurotransmitter synthesis and degradation in cultured rat sympathetic neurons. I. Effects of muscle-conditioned medium

The enzymatic machinery for neurotransmitter synthesis and breakdown have been compared in sister cultures of newborn rat sympathetic neurons grown for 12-28 days either in the presence (CM+ cultures) or in the absence (CM- cultures) of a culture medium conditioned by rat skeletal muscle cells. Neuron numbers, total protein, and lactate dehydrogenase activities were identical in CM+ and CM- cultures. Choline acetyltransferase activity was 27- to 100-fold higher in homogenates of CM+ than CM- cultures, whereas acetylcholinesterase activity was 2.5-fold lower. The activities of tyrosine hydroxylase (TOH), DOPA decarboxylase, and dopamine beta-hydroxylase were all about twofold lower in homogenates from CM+ cultures. All these effects were also observed in homogenates of sympathetic neuron cultures grown with and without a macromolecular factor partially purified from CM (Weber, J. (1981). Biol. Chem. 256, 3447-3453.). Experiments of mixing homogenates from CM+ and CM- cultures suggested that the differences in each of the enzyme activities did not result from differences in the concentrations of hypothetical reversible enzyme activators and/or inhibitors. In addition, the deficit in TOH activity in CM+ cultures resulted from a decrease in the enzymatic Vmax with no significant variation in the apparent Km's for the substrate and the cofactor. An identical decrease in the Vmax was observed if TOH was assayed under phosphorylating or nonphosphorylating conditions, suggesting that this decrease did not result from differences in the state of enzyme phosphorylation. Immunoprecipitation curves of TOH activity by an anti-TOH antiserum were parallel when performed on homogenates from CM+ and CM- cultures, suggesting a difference in the number of enzyme molecules without detectable alteration of their kinetic properties.

Evidence for neurotransmitter plasticity in vivo: developmental changes in properties of cholinergic sympathetic neurons

We have examined the cholinergic sympathetic innervation of sweat glands in footpads of adult and developing rats. Acetylcholinesterase staining reveals a plexus of heavily stained fibers in the sweat glands of adult rats. Reaction product appears among and around bundles of axons that lie at a considerable distance from the cells of the secretory tubule. Each bundle contains 8-12 axons that possess numerous varicosities and contain small clear and large dense core vesicles. The glands of the hindpaws and their innervation develop during the first three weeks after birth. Catecholamine-containing axons were associated with the forming glands. At 7 and 10 days, intensely fluorescent fibers surrounded the tubules, and all of the axon profiles associated with the glands contained small granular vesicles (SGV) after permanganate fixation to reveal vesicular stores of norepinephrine. At 14 days the sweat gland plexus was less intensely fluorescent than at earlier ages and relatively few SGV were present. By 21 days, no endogenous catecholamine fluorescence and no SGV were detectable. However, following exposure to exogenous catecholamine, fluorescent fibers were present in the sweat glands of mature rats and they corresponded in position and density to the plexus localized with acetylcholinesterase staining. Catecholamine uptake was blocked by incubation in the cold and by desmethylimipramine and was not observed in cholinergic parasympathetic fibers in the iris or salivary glands. After intraperitoneal administration of 5-hydroxydopamine and permanganate fixation, all the axons in the sweat glands contained a few SGV. Thus, the developing sweat glands appear to be innervated by noradrenergic axons that lose their stores of endogenous catecholamines but not their capacity for uptake and storage as they elaborate an axonal plexus in the maturing glands. These observations support the hypothesis that cholinergic sympathetic neurons appear to undergo a transition from noradrenergic to cholinergic function during development in vivo similar to that previously described in cell culture.

Mechanism of uptake and retrograde axonal transport of noradrenaline in sympathetic neurons in culture: reserpine-resistant large dense-core vesicles as transport vehicles

The uptake and retrograde transport of noradrenaline (NA) within the axons of sympathetic neurons was investigated in an in vitro system. Dissociated neurons from the sympathetic ganglia of newborn rats were cultured for 3-6 wk in the absence of non-neuronal cells in a culture dish divided into three chambers. These allowed separate access to the axonal networks and to their cell bodies of origin. [3H]NA (0.5 X 10(-6) M), added to the axon chambers, was taken up by the desmethylimipramine- and cocaine-sensitive neuronal amine uptake mechanisms, and a substantial part was rapidly transported retrogradely along the axons to the nerve cell bodies. This transport was blocked by vinblastine or colchicine. In contrast with the storage of [3H]NA in the axonal varicosities, which was totally prevented by reserpine (a drug that selectively inactivates the uptake of NA into adrenergic storage vesicles), the retrograde transport of [3H]NA was only slightly diminished by reserpine pretreatment. Electron microscopic localization of the NA analogue 5-hydroxydopamine (5-OHDA) indicated that mainly large dense-core vesicles (700-1,200-A diam) are the transport compartment involved. Whereas the majority of small and large vesicles lost their amine dense-core and were resistant to this drug. It, therefore, seems that these vesicles maintained the amine uptake and storage mechanisms characteristic for adrenergic vesicles, but have lost the sensitivity of their amine carrier for reserpine. The retrograde transport of NA and 5-OHDA probably reflects the return of used synaptic vesicle membrane to the cell body in a form that is distinct from the membranous cisternae and prelysosomal structures involved in the retrograde axonal transport of extracellular tracers.

Simultaneous ultrastructural visualization of acetylcholinesterase activity and tritiated norepinephrine uptake in renal nerves

In this investigation we have combined the methods of ultrastructural demonstration of acetylcholinesterase activity with electron microscopic autoradiography for the demonstration of norepinephrine uptake. The results show electron-dense deposits indicative of acetylcholinesterase activity associated with perivascular axons overlaid by concentrations of silver grains representing exogenous tritiated norepinephrine. Forty-five percent of the intervaricose regions and 19% of the varicosities overlaid by autoradiographic grains showed "moderate" amounts of cholinesterase staining. A greater proportion of autoradiographic grains was observed on the varicosities than in the intervaricose regions; however, the amount of acetylcholinesterase activity was greater in the intervaricose regions than in the varicosities. This investigation provides evidence for the presence of periaxonal acetylcholinesterase staining in adrenergic axons in the rat kidney.

Biochemical and morphological characterization of sympathetic neurons grown in a chemically-defined medium

Previous studies have demonstrated that individual neurons from neonatal rat superior cervical ganglion express a mixed adrenergic-cholinergic phenotype when grown under certain tissue culture conditions. The expression of this phenotype is critically influenced by a number of undefined components present in the culture medium. In the present study, we have examined whether superior cervical ganglion neurons grown on a chemically defined serum-free medium similarly develop dual transmitter expression, or if under these conditions, neurons express only those properties characteristic of their adrenergic heritage. To address this issue, we established that superior cervical ganglion neurons could be maintained in culture for extended periods on the defined medium described by Bottenstein & Sato in the absence of supporting cells. We then studied the biochemical, immunocytochemical and ultrastructural characteristics of these neurons. We found that in defined medium, superior cervical ganglion neurons continued to express, in a modified form, certain of their expected adrenergic properties, including the development of tyrosine hydroxylase and dopamine-beta-hydroxylase activities, stores of endogenous norepinephrine, synaptic vesicles with dense cores and tyrosine hydroxylase-immunoreactive staining properties. Superior cervical ganglion neurons grown on a defined medium did not, however, acquire cholinergic traits in culture. In this paper we show that choline acetyltransferase activity did not reach detectable levels; the comparison paper documents that cholinergic synapses were not formed. We concluded that superior cervical ganglion neurons, grown under serum-free culture conditions, develop certain properties characteristic of adrenergic neurons and do not express a mixed adrenergic-cholinergic phenotype. A comparison paper describes the electrophysiological properties of these neurons and demonstrates the frequent occurrence of electrotonic synapses in these cultures.

Electrotonic synapses are formed by fetal rat sympathetic neurons maintained in a chemically-defined culture medium

Principal neurons from the superior cervical ganglia of rat fetuses were maintained for up to 101 days in dissociated cell cultures in a serum-free, chemically-defined medium; non-neuronal cells were killed by the periodic addition of fluorodeoxyuridine to the medium. Intracellular recordings, obtained at various times between 16th and 98th day in vitro, showed that these neurons could generate substantial (up to 90 mV) action potentials in response to depolarizing current injections; these responses were dependent on tetrodotoxin-sensitive Na+ channels, cobalt-sensitive Ca++ channels, and tetraethylammonium-sensitive K+ channels. Action potentials were often followed by prominent, long hyperpolarizing after-potentials (10-15 mV, greater than 150 ms); the duration of these after-potentials was reduced by the addition of Co++ (2-5 mM) to the perfusate. Acetylcholine depolarized these neurons by a hexamethonium-sensitive mechanism. To determine whether sympathetic neurons formed synapses in a defined medium, intracellular recordings were obtained from pairs of neighboring neurons. Synaptic interactions were frequently observed at all times in vitro (up to 60% of all pairs tested). At many synapses, both hyperpolarizing and depolarizing DC potential changes spread from one neuron to another. At other synapses, the spread of DC potential changes could not be directly demonstrated; however, interactions at such synapses were not inhibited by antagonists of several neurotransmitters, by elevation of the Mg++/Ca++ ratio, or by the addition of Co++. Thus most, if not all, of the synaptic interactions among sympathetic neuron were electronic; such electrical synapses were not observed among dorsal root ganglion neurons maintained in the same medium. These data indicate that, when maintained in a chemically-defined culture medium, sympathetic neurons of rat fetuses express many of the basic membrane properties observed in neurons of superior cervical ganglia recently removed from adult rats. However, fetal sympathetic neurons maintained in this defined medium also differ from their counterparts in vivo; they adopt a mode of synaptic transmission (electrical) that has not been observed in the sympathetic ganglia of the adult rat. Thus, as late as the 21st embryonic day, not only the choice of neurotransmitter, but also the mode of transmission has not been irrevocably determined in sympathetic neurons.

Effect of serum-free medium on growth and differentiation of sympathetic neurons in culture

Primary cultures of dissociated superior cervical ganglia (SCG) were grown either in serum-containing medium (+FBS) or in serum-free, defined medium (N1). The cultures were then compared for several differentiated properties. Neuronal survival was similar in the two media. Background cell, especially fibroblast, proliferation was less in N1. Small, intensely fluorescent cells were occasionally seen only in +FBS. Catecholamine fluorescence in neuronal processes and cholinergic synaptic activity persisted beyond 1 month in culture in both media. Quantitatively, however, neuritic outgrowth, nerve terminal fluorescence, and synaptic interaction were less in N1. Muscarinic depolarization and electrical membrane properties, including the presence of at least 4 voltage-sensitive outward currents, were similar in the two media. Thus, SCG neurons differentiate in N1 essentially to the same degree as they do in +FBS.

Morphological and biochemical studies on the development of cholinergic properties in cultured sympathetic neurons. I. Correlative changes in choline acetyltransferase and synaptic vesicle cytochemistry

Under certain culture conditions, neonatal rat superior cervical ganglion neurons display not only a number of expected adrenergic characteristics but, paradoxically, also certain cholinergic functions such as the development of hexamethonium-sensitive synaptic contacts and accumulation of choline acetyltransferase (ChAc). The purpose of this study was to determine whether the entire population of cultured neurons was aquiring cholinergic capabilities, or whether this phenomenon was restricted to a subpopulation. After 1--6 and 8 wk in culture, neurons were fixed in KMnO4 after incubation in norepinephrine and prepared for electron microscopy analysis of synaptic vesicle content to determine whether vesicles were dense cored or clear. ChAc, acetylcholinesterase (AChE), and DOPA-decarboxylase (DDC) activities were assayed in sister cultures. In the period from 1 to 8 wk in culture, the average ChAc activity per neuron increased 1,100-fold, and the DDC and AChE activities increased 20- and 30-fold, respectively. After 1 wk in culture, 48 of 50 synaptic boutons contained predominantly dense-cored vesicles, but by 8 wk the synaptic vesicle population was predominantly of the clear type. At intermediate times, the vesicle population in many boutons was mixed. The morphology of the synaptic contacts on neuronal surfaces was that characteristic of autonomic systems, with no definite clustering of the vesicles adjacent to the area of contact. Increased vesicle size correlated with increasing age in culture and the presence of a dense core. Considering these data along with available physiological studies, we conclude that these cultures contain one population of neurons that is initially adrenergic. Over time, under conditions of this culture system, this population develops cholinergic mechanisms. That a neuron may, at a given time, express both cholinergic and adrenergic mechanisms is suggested by the approximately equal numbers of clear and dense-cored vesicles in the boutons found at the intermediate times.

[Role of cell-milieu interactions in the differentiation of nerve cells]

In this review some aspects of nerve cell development are studied from the point of view of the role of the environment on differentiation processes. In the first part, attention is focused on the early stages, with special emphasis on the commitment of the cells coming from the neural crest to acquire and keep a specialized phenotype. In the second part, attempts were made to understand the mechanisms of action of specific growth factors, the Nerve Growth Factor (NGF) being taken as a model molecule. Information presently available is presented on factors that may be implicated in the development of cell types other than those NGF-sensitive, with considerations as to whether the notion of specific growth factors can be generalized to all nervous cell types or not.

Selective loss of noradrenergic phenotypic characters in neuroblasts of the rat embryo

To define the fate of embryonic neuroblasts in rat gut, which transiently express several noradrenergic traits, we investigated the high-affinity uptake of norepinephrine. At 12.5 days of gestation, these cells exhibited immunoreactivity to tyrosine hydroxylase [tyrosine 3-monoxygenase; L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] and endogenous catecholamine fluorescence. However, by 13.5 days these noradrenergic neurotransmitter phenotypic characters essentially disappeared. In contrast, norepinephrine uptake, which was also apparent at 12.5 days, persisted at least through 17.5 days. These observations indicate that norepinephrine uptake develops as an additional noradrenergic characteristic in these cells and persists after the disappearance of other noradrenergic traits. Consequently, neurotransmitter phenotypic characters may be transiently displayed during normal development in vivo.