Role of nerve growth factor in the development of rat sympathetic neurons in vitro. III. Effect on acetylcholine production

The effect of nerve growth factor (NGF) on the development of cholinergic sympathetic neurons was studied in cultures grown either on monolayers of dissociated rat heart cells or in medium conditioned by them. In the presence of rat heart cells the absolute requirement of neurons for exogenous NGF was partially spared. The ability of heart cells to support neuronal survival was due at least in part to production of a diffusable NGF-like substance into the medium. Although some neurons survived on the heart cell monolayer without added NGF, increased levels of exogenous NGF increased neuronal survival until saturation was achieved at 0.5 microgram/ml 7S NGF. The ability of neurons to produce acetylcholine (ACh) from choline was also dependent on the level of exogenous NGF. In mixed neuron-heart cell cultures, NGF increased both ACh and catecholamine (CA) production per neuron to the same extent; saturation occurred at 1 microgram/ml 7S NGF. As cholinergic neurons developed in culture, they became less dependent on NGF for survival and ACh production, but even in older cultures approximately 40% of the neurons died when NGF was withdrawn. Thus, NGF is as necessary for survival, growth, and differentiation of sympathetic neurons when the neurons express cholinergic functions as when the neurons express adrenergic functions (4, 5).     

Role of nerve growth factor in the development of rat sympathetic neurons in vitro. I. Survival, growth, and differentiation of catecholamine production

To study the effect of nerve growth factor (NGF) on neuronal survival, growth, and differentiation, cultures of dissociated neonatal rat sympathetic neurons virtually free of other cell types were maintained for 3-4 wk. In the absence of NGF, the neurons did not survive for more than a day. Increased levels of NGF increased neuronal survival and growth (total protein and total lipid phosphate); saturation occurred at 0.5 microgram/ml 7S NGF. Neuronal differentiation examined by measuring catecholamine (CA) production from tyrosine also depended on the level of NGF in the culture medium. As the NGF concentration was raised, CA production per neuron, per nanogram protein, or per picomole lipid phosphate increased until saturation was achieved between 1 and 5 microgram/ml 7S NGF. Thus, NGF induces neuronal survival, growth, and differentiation of CA production in a dose-dependent fashion. Neuronal growth and differentiation were quantitatively compared in the presence of the high and low molecular weight forms of NGF; no significant functional differences were found.     

The survival of early chick sympathetic neurons in vitro is dependent on a suitable substrate but independent of NGF

The neuronal cell population of lumbosacral sympathetic ganglia from 7-day-old chick embryos is characterized by a high proportion of cells with the ability to proliferate in culture (Rohrer and Thoenen, 1987). It is now demonstrated that neither proliferation nor survival of these neurons depend on the presence of nerve growth factor (NGF). However, neuronal survival did depend on the culture substrate used: on laminin, E7 neurons survived and their number increased due to proliferation, whereas on fibronectin (FN) or a substrate of molecules from heart cell-conditioned medium (HCM) a significant number of the cells died during early culture periods. Less than 70 and 50% of the number of neurons surviving on a laminin substrate were found on FN and HCM, respectively, after 3 days in culture. Although NGF did not affect neuronal survival, a small increase in neurite extension on these substrates was observed in the presence of NGF. Furthermore, although NGF did not prevent neuronal death after extended culture periods, this could be prevented by elevated extracellular potassium concentrations. Sympathetic neurons of E8 chick embryos however showed a strikingly different response to NGF compared with those of E7: whereas neuronal survival on laminin was not influenced by NGF, a significant effect of NGF on survival and on neurite extension was observed for E8 neurons on a HCM substrate. In contrast to cells from E7 and E8 embryos, the majority of neurons from E11 chick embryos required NGF for survival even on a laminin substrate as described previously (D. Edgar, R. Timpl, and H. Thoenen, 1984, EMBO J. 3, 1463-1468). These results demonstrate that while sympathetic neurons from E7 chick embryos do not depend on the soluble neurotrophic factor NGF for survival in vitro, they are dependent on molecules of the extracellular matrix. With increasing age, the survival requirements demonstrated in vitro change toward the classical pattern of NGF dependency. Low amounts of laminin-like immunoreactivity were shown to be present in sympathetic ganglia of E7 chick embryos which were then shown to increase as development proceeded. These data indicate that laminin may play a role in the survival and development of chick sympathetic neurons not only in vitro, but also in vivo.     

Conditions increasing the adrenergic properties of dissociated chick superior cervical ganglion neurons grown in long-term culture

Neurons dissociated from the embryonic chick superior cervical ganglion (SCG) were separated from ganglionic nonneuronal cells using a density gradient formed with Percoll. The sympathetic neurons were then grown for 3-4 weeks in serum containing medium on a polyornithine substrate precoated with heart-conditioned medium. Both catecholamine (CA) and acetylcholine (ACh) are synthesized and accumulated by these neurons, but the amount of CA is higher and increases much more over time in culture than the amount of ACh. The cultures become therefore more adrenergic with time. We report here that the adrenergic properties of these cells can be enhanced. A 3-fold increase in CA synthesis, as expressed on a per neuron basis, is obtained by increasing neuron cell density 3- to 4-fold. ACh synthesis, however, is decreased at high neuronal density. Optimal CA production is obtained at densities of 120-150,000 neurons/cm2. This effect is due to direct cell contact since it cannot be transferred to low density cultures by medium conditioned by high density cultures. Nerve growth factor concentrations 5-10-fold higher than the amount necessary for optimal neuronal survival (1 microgram/ml 7S NGF) increases CA production but do not affect ACh synthesis. This effect is highest at low plating densities (20-30,000 neurons/cm2, 10-fold increase) and progressively decreases with increasing neuronal density. No increase is obtained in high density cultures where CA production is maximal. In addition, we made the novel observation that medium conditioned by chick liver cells in culture (LCM) increases CA production approximately 4-fold, whereas it does not increase ACh production by the SCG neurons. Work is in progress to biochemically characterize the active component(s) present in the LCM and to determine whether they favor the survival of a subpopulation of adrenergic neurons possible present in these ganglia. Alternatively, the adrenergic differentiation of neurons initially capable of synthesizing both CA and ACh could be selectively increased by LCM.     

The novel neurotrophin-regulated neuronal development-associated protein, NDAP, mediates apoptosis

We identified a novel gene and named it, "neuronal development-associated protein (NDAP)". We detected NDAP mRNA presence in most tissues including the brain where it was present in the area from the external granular layer to the multiform layer in the cerebral cortex, and in CA1, CA2, CA3 and the dentate gyrus in the hippocampus. Its expression increased transiently in primary cultures of 2-4 day neurons and 1-2 week astrocytes and was significantly reduced in older cultures. Treatment by the neurotrophin, NT-3, significantly attenuated the decline of NDAP in neurons from days 2 to 10, whereas growth factors such as GDNF and insulin, and high potassium levels did not. To elucidate the effects of neurotrophins, we treated day 5 neurons with NT-3, BDNF or NGF for 48 h. NT-3 and BDNF both inhibited downregulation of NDAP mRNA levels but NGF slightly enhanced the already present downregulation; this effect of NGF was significant when examined in day 3 neurons. To investigate the potential function of NDAP, we over-expressed an NDAP-EGFP fusion protein in 4-week-old astrocytes. The newly expressed NDAP gradually aggregated into membrane-bound structures and eventually led to cell death through apoptosis by 24 h. Significant levels of cell death were also observed in NDAP-EGFP transfected HEK293 cells. Thus maintenance of high NDAP levels may cause apoptosis. The different regulations of NDAP expression by neurotrophins indicate that the expression of NDAP might be a checkpoint for apoptosis during neuronal development.     

NGF utilizes c-Ret via a novel GFL-independent, inter-RTK signaling mechanism to maintain the trophic status of mature sympathetic neurons.

During postnatal development, sympathetic neurons lose their dependence upon NGF for survival but continue to require NGF for soma and process growth and for development of a mature neurotransmitter phenotype. Although c-Ret is expressed in sympathetic neurons during this period, its function in these transitional processes is unclear. The level of Ret phosphorylation markedly increased with postnatal age in SCG neurons in vitro and in vivo. Postnatal Ret phosphorylation did not require either GFLs or GFR(alpha) coreceptors. Instead, NGF promoted age-dependent Ret phosphorylation with delayed kinetics both in vitro and in vivo. Functionally, maximal NGF-dependent trophism of mature sympathetic neurons required Ret, but not GFR(alpha) coreceptors. Therefore, NGF promotes phosphorylation of the heterologous RTK Ret resulting in augmented growth, metabolism, and gene expression.     

Relationship of Intracellular Calcium to Dependence on Nerve Growth Factor in Dorsal Root Ganglion Neurons in Cell Culture

During development, neural crest-derived sensory neurons require nerve growth factor (NGF) for survival, but lose this dependency postnatally. Similarly, dissociated embryonic sensory neurons lose their NGF dependence during the first 3 weeks in cell culture. It has been hypothesized that, in sympathetic neurons, intracellular levels of calcium are related to trophic factor dependence. In vitro during the period in which embryonic-day-15 sensory neurons become independent of NGF, intracellular calcium concentrations progressively increased in parallel to the decline in NGF dependence. This elevation of intracellular calcium was directly related to the absolute age of the neurons, not to the length of time in culture. Without NGF, immature sensory, i.e., dependent, neurons survived in the presence of high extracellular potassium, a condition that produces elevated intracellular calcium. In another paradigm, measurements of intracellular calcium were determined in NGF-dependent neurons "committed to die" after NGF withdrawal. These measurements were determined prior to the time that extensive morphological changes, consistent with cell death, were noted by phase-contrast microscopy. No elevation in intracellular calcium was found in these dying neurons, but rather, a small decrease was observed prior to the disintegration of the neurons. These findings support the hypothesis that trophic factor dependence of neurons may be inversely related to levels of intracellular calcium.     

Skin-derived nerve growth factor blocks programmed cell death in the trigeminal ganglia but does not enhance neuron proliferation.

Development of the cutaneous sensory nervous system is dependent on the production of neurotrophic factors, such as nerve growth factor (NGF), by the skin. Limited synthesis of NGF in developing skin is thought to underlie programmed cell death and cause a 50% neuronal loss. This loss does not occur in transgenic mice that overexpress NGF in the skin, which have double the number of neurons (J. Neurosci. 14 (1994) 1422). To determine whether increased NGF blocks neuronal death and/or increases neuronal precursor replication, we analyzed the trigeminal ganglia at embryonic days E12.5, E14.5 and E16.5 using transferase-mediated dUTP nick-end labeling (TUNEL) and bromodeoxyuridine labeling. Results show that excess target-derived NGF causes a major decrease in the percent of TUNEL-labeled neurons without affecting the percent of replicating neurons. Analysis of RNA and protein expression suggests this block in cell death is mediated via the anti-apoptotic protein bcl-2.     

Characterization of apoptosis in cultured rat sympathetic neurons after nerve growth factor withdrawal

Sympathetic neurons depend on nerve growth factor (NGF) for their survival both in vivo and in vitro. In culture, the neurons die after NGF withdrawal by an autonomous cell death program but whether these neurons die by apoptosis is under debate. Using vital DNA stains and in situ nick translation, we show here that extensive chromatin condensation and DNA fragmentation occur before plasma membrane breakdown during the death of NGF-deprived rat sympathetic neurons in culture. Furthermore, kinetic analysis of chromatin condensation events within the cell population is consistent with a model which postulates that after NGF deprivation nearly all of the neurons die in this manner. Although the dying neurons display membrane blebbing, cell fragmentation into apoptotic bodies does not occur. Apoptotic events proceed rapidly at around the time neurons become committed to die, regardless of neuronal culture age. However the duration of NGF deprivation required to commit neurons to die, and the rate at which apoptosis occurs, increase with culture age. Thus, within the first week of culture, apoptosis is the predominant form of cell death in sympathetic neurons.     

SHP-1 negatively regulates neuronal survival by functioning as a TrkA phosphatase

Nerve growth factor (NGF) mediates the survival and differentiation of neurons by stimulating the tyrosine kinase activity of the TrkA/NGF receptor. Here, we identify SHP-1 as a phosphotyrosine phosphatase that negatively regulates TrkA. SHP-1 formed complexes with TrkA at Y490, and dephosphorylated it at Y674/675. Expression of SHP-1 in sympathetic neurons induced apoptosis and TrkA dephosphorylation. Conversely, inhibition of endogenous SHP-1 with a dominant-inhibitory mutant stimulated basal tyrosine phosphorylation of TrkA, thereby promoting NGF-independent survival and causing sustained and elevated TrkA activation in the presence of NGF. Mice lacking SHP-1 had increased numbers of sympathetic neurons during the period of naturally occurring neuronal cell death, and when cultured, these neurons survived better than wild-type neurons in the absence of NGF. These data indicate that SHP-1 can function as a TrkA phosphatase, controlling both the basal and NGF-regulated level of TrkA activity in neurons, and suggest that SHP-1 regulates neuron number during the developmental cell death period by directly regulating TrkA activity.     

Developmental changes in the responses of rat chromaffin cells to neuronotrophic and neurite-promoting factors

This study describes the survival and neurite outgrowth behaviors of cultured adrenal medullary (chromaffin) cells obtained from postnatal rats 1 day (D1) to 100 days (D100) old in response to nerve growth factor (NGF), chick eye ciliary neuronotrophic factor (CNTF), and laminin. In the absence of trophic factors the 4-day survival of cultured chromaffin cells (relative to the number of cells attached at 2 hr) increased from one-third of the cells at D1 to 40% at D8 and 90-100% at D16 and older stages. At saturating concentrations NGF increased cell survival at D8 by 90%, but failed to support all chromaffin cells present at 2 hr. In contrast, CNTF supported the survival of all cells at D8. At D1 NGF and CNTF had only a very small effect on survival during the 4-day culture period, although both factors clearly enhanced the numbers of surviving cells after 8 days. Either NGF or CNTF also elicited neurite outgrowth from rat chromaffin cells, which amounted to approximately 15-20% at D1 and D8 and subsequently decreased to about 5-8% at D30 and virtually zero at D100. At this last age both factors applied together clearly elicited neurites. Such a potentiating effect of NGF and CNTF was also seen at earlier postnatal ages. Laminin did not affect neurite growth at D30 in the absence of trophic factors, as already described for D8 rat chromaffin cells. In the presence of NGF, however, it increased neurite length and branching during a 4-day culture period and even enhanced neurite recruitment at later culture times. These data suggest that rat chromaffin cells undergo age-related changes in their responses to NGF and CNTF and that laminin modulates their neurite outgrowth behaviors in the presence of trophic factors.     

Differential Effects of Insulin on Choline Acetyltransferase and Glutamic Acid Decarboxylase Activities in Neuron-Rich Striatal Cultures

We studied the effects of insulin, nerve growth factor (NGF), and tetrodotoxin (TTX) on cellular metabolism and the activity of glutamic acid decarboxylase (GAD) and choline acetyltransferase (ChAT) in neuron-rich cultures prepared from embryonic day 15 rat striatum. Insulin (5 micrograms/ml) increased glucose utilization, protein synthesis, and GAD activity in cultures plated over a range of cell densities (2,800-8,400 cells/mm2). TTX reduced GAD activity; NGF had no effect on GAD activity. Insulin treatment reversibly reduced ChAT activity in cultures plated at densities of greater than 4,000 cells/mm2, and the extent of this reduction increased with increasing cell density. The number of acetylcholinesterase-positive neurons was not reduced by insulin, suggesting that insulin acts by down-regulating ChAT rather than by killing cholinergic neurons. Insulin-like growth factor-1 (IGF-1) reduced ChAT activity at concentrations 10-fold lower than insulin, suggesting that insulin's effect on ChAT may involve the IGF-1 receptor. NGF increased ChAT activity; TTX had no effect on ChAT activity. These results suggest that striatal cholinergic and GABAergic neurons are subject to differential trophic control.     

Geometry of isolated sensory neurons in culture. Effects of embryonic age and culture substratum

Sensory neurons were dissociated from lumbar dorsal root ganglia of embryonic chick and put into culture, either directly or after removing non-neuronal cells by density gradient centrifugation. The cells were grown on culture substrata of various kinds in medium containing nerve growth factor (NGF). After 24 h the cultures were fixed, mounted and analysed. Lengths of neurites were measured, and the numbers of primary processes formed at the cell body and of growth cones were counted. From these values, the rates of growth cone advance and frequency of growth cone branching were calculated. Neuronal outgrowths increased strikingly in length and complexity with embryonic age; there was a 3.5-fold increase in total neurite length and a 3-fold increase in the number of growth cones when neurons from 15-day embryos (E15) were compared with those from 8-day embryos (E8) grown on the same substratum (glass). Growth was markedly greater on surfaces prepared with laminin or conditioned medium compared with plain glass or air-dried collagen. When E15 neurons grown on glass were compared with those grown on laminin, for example, a 2.5-fold increase in total neurite length and a 3-fold increase in the number of growth cones was observed. Calculations showed that a major factor in these changes was an increase in the frequency of growth cone branching. The number of initial processes emanating from the cell body changed with age, but not with the different substrata tested. Non-neuronal cells when present in low numbers and in contact with neurons did not appear to influence neuronal geometry in a systematic way. Our results document the fact that both external factors (in this case, the nature of the culture substratum) and intrinsic factors (stage of development of the neuron) can influence the geometry of neurite outgrowth.     

NGF in CNS: Experimental data and clinical implications

The presence of β-nerve growth factor (NGF) and its cell surface receptor (NGF-R) in the brain has been well established by a variety of experimental techniques in recent years. In particular, the molecular cloning of NGF and NGF-R as well as the development of sensitive two-site ELISA techniques for determining the levels of NGF and antibodies to NGF-R suitable for immunohistochemistry have led to rapid accumulation of data in this field from many laboratories. A main finding is the function of NGF in the cholinergic neurons of the basal forebrain, expressing NGF receptors and responding to the factor by increased activity of choline acetyltransferase, and the production of NGF in cortical areas and hippocampus comprising terminal areas for the cholinergic projections from the basal forebrain. In addition, findings suggest that additional neurons in the brain and spinal cord may utilize NGF, notably during development and possibly also after lesion of the adult CNS. Moreover, observations indicate that endogenous levels of NGF are lowered in the aged rat brain concomitant with losses of NGF-dependent neurons in the basal forebrain. The involvement of NGF in human neurodegenerative diseases is not established but the application of NGF to degenerating cholinergic neurons in Alzheimer patients may prove useful. A promising approach to achieve this goal is the production of biologically active, recombinant NGF.     

Retinoic acid induces NGF-dependent survival response and high-affinity NGF receptors in immature chick sympathetic neurons.

An important step in the development of peripheral sensory and sympathetic neurons is the onset of the survival response and dependence on the presence of nerve growth factor (NGF) or other neurotrophic factors. We have recently observed that immature sympathetic neurons from 7-day-old chick embryos are unable to become NGF-responsive in vitro and we have now used these cells to identify molecules that induce NGF-dependent neuronal survival. We found that retinoic acid (RA) induces the ability of these cells to survive in the presence of NGF. At RA concentrations of 10(-9)-10(-8)M virtually all neurons survived in the presence of NGF. RA was found to also induce the biologically active, high-affinity NGF receptor: high-affinity receptors were undetectable on dissociated E7 sympathetic neurons and were observed in vitro only in RA-treated neurons. These findings suggest that the induction of high-affinity NGF receptors may be sufficient to activate the survival response in sympathetic neurons and imply an important role for RA during neuron differentiation in the peripheral nervous system.     

Differential regulation of survival and growth in adult sympathetic neurons: An invitro study of neurotrophin responsiveness

The survival and growth of embryonic and postnatal sympathetic neurons is dependent on both NGF and NT3. While it has been established that adult sensory neurons survive independently of neurotrophins, the case is less clear for adult sympathetic neurons, where the studies of survival responses to neurotrophins have relied upon using long‐term cultures of embryonic neurons. We have previously established a method to culture purified young (7 day) and adult (12 week) sympathetic neurons isolated from adult rat superior cervical ganglia (SCG) in order to examine their survival and growth responses to neurotrophins. We now show that by 12 weeks after birth virtually all neurons (90%) survive for 24 h in the absence of neurotrophins. Neuron survival is unaffected by treatment with anti‐NGF antibodies (anti‐NGF) or with the tyrosine kinase inhibitor, K252a, confirming the lack of dependence on extrinsic neurotrophins. Duration of neuron survival in culture increases significantly between E19 and day 7 and week 12 posnatally, and is similarly unaffected by the presence of anti‐NGF or K252a. Saturating concentrations of NGF and NT3 are equipotent in promoting neurite extension and branching. However, we find that NGF is more potent than NT3 in promoting neurite growth, irrespective of postnatal age. The growth‐promoting effects of NGF and NT3 are almost entirely blocked by K252a, demonstrating that these effects are mediated via activation of Trk receptors, which therefore appear to remain crucial to plasticity of adult neurons. Our results indicate that maturing neurons acquire protection against cell death, induced in the absence of neurotrophin, while retaining their growth responsiveness to these factors. © 2001 John Wiley & Sons, Inc. J Neurobiol 47: 295–305, 2001     

Transforming growth factor beta has neurotrophic actions on sensory neurons in vitro and is synergistic with nerve growth factor.

Transforming growth factor beta (TGF beta) influences the growth and differentiation of a wide variety of nonneuronal cells (nnc) during embryogenesis and in response to wounding. In the present study TGF beta 1 and TGF beta 2 were examined for their neurotrophic actions on neonatal rat dorsal root ganglion (DRG) neurons with ganglionic nnc in dissociated cultures. TGF beta 1 and TGF beta 2 each increased both neuronal survival and levels of the peptide neurotransmitter substance P (SP) expressed per neuron as well as per culture. TGF beta 1 was maximally effective at a concentration of 40 pM, whereas TGF beta 2 was about 10-fold less potent. Survival effects promoted by simultaneous treatment with both factors were not additive. TGF beta 1 also changed the morphology and distribution of DRG nnc which resulted in clustering of DRG neurons on top of the nnc. Cotreatment of the cultures with two different anti-nerve growth factor (NGF) antibodies eliminated the neurotrophic effects of TGF beta 1. However, treatment with TGF beta 1 did not alter NGF mRNA expression in the cultures nor did it change the amount of NGF in the medium. Further, TGF beta 1 greatly enhanced survival effects and SP stimulation promoted by exogenous NGF at concentrations up to 100 ng/ml. The neurotrophic effects of TGF beta 1 were significantly attenuated by decreasing the proportion of the ganglionic nnc, suggesting a role for these cells in mediating TGF beta 1 action on the neurons. It is hypothesized that the neurotrophic activity of TGF beta depended upon the presence of molecules immunologically related to NGF and that the effects of TGF beta were synergistic with NGF. These observations suggest that TGF beta may play a role in the differentiation and regeneration of DRG neurons in vivo.     

Spatially and functionally distinct roles of the PI3-K effector pathway during NGF signaling in sympathetic neurons

NGF is a target-derived growth factor for developing sympathetic neurons. Here, we show that application of NGF exclusively to distal axons of sympathetic neurons leads to an increase in PI3-K signaling in both distal axons and cell bodies. In addition, there is a more critical dependence on PI3-K for survival of neurons supported by NGF acting exclusively on distal axons as compared to neurons supported by NGF acting directly on cell bodies. Interestingly, PI3-K signaling within both cell bodies and distal axons contributes to survival of neurons. The requirement for PI3-K signaling in distal axons for survival may be explained by the finding that inhibition of PI3-K in the distal axons attenuates retrograde signaling. Therefore, a single TrkA effector, PI3-K, has multiple roles within spatially distinct cellular locales during retrograde NGF signaling.     

Neuronal precursor cells in chick dorsal root ganglia: differentiation and survival in vitro

Neuronal precursor cells present in dorsal root ganglia (DRG) during early development have been previously shown to differentiate in vitro to neurons, as characterized by morphology, cell surface antigens, and electrophysiological properties (H. Rohrer, S. Henke-Fahle, T. El-Sharkawy, H. D. Lux, and H. Thoenen, 1985, Embo J. 4, 1709-1714). In the present study the conditions necessary for the initial differentiation and long-term survival of these cells were established, and the neurotransmitter phenotype of the newly differentiated neurons was analyzed. Neuronal precursor cells isolated from chick DRG at Embryonic Day 6 (E6) were found to require the presence of a polyornithine substrate coated with either laminin or fibronectin for initial neurite production and long-term survival. Neurons were unable to develop on polyornithine alone or on polyornithine coated with BSA. The survival and neurite outgrowth from neuronal precursor cells was not affected by the presence of nerve growth factor (NGF) during the first 9 hr in culture. NGF also had no effect on the proportion of cells expressing the neuron-specific Q211 antigen. However, after this initial differentiation period the neurons did require the presence of a survival factor. The neurons could be maintained for at least 6 days in culture both in the presence of NGF and in the presence of brain-derived neurotrophic factor (BDNF). At saturating concentrations of both survival factors no additive effects could be observed, indicating a complete overlap of NGF- and BDNF-responsiveness. Although the same proportion of cells survived with either NGF or BDNF during the first 3 days in culture, survival decreased in the presence of BDNF but not in the presence of NGF during the following 3 days in culture. The loss of BDNF responsiveness in vitro was also observed in vivo. After 6 days in culture about 70% of the neurons expressed substance P immunoreactivity, and approximately the same proportion was positive for myelin-associated glycoprotein immunoreactivity. The neurons did not express properties of adrenergic neurons such as tyrosine hydroxylase immunoreactivity or norepinephrine uptake. These findings indicate that the neuronal precursor cells from E6 DRG acquire the same characteristics in vitro as in their normal in vivo environment.