Effects of high K+ concentrations on the growth and development of ciliary ganglion neurons in cell culture

Membrane Depolarization Inhibits BIMEL Upregulation but Prevents Neuronal Apoptosis Primarily by Increasing Cellular GSH Levels

Sympathetic neurons deprived of nerve growth factor (NGF) die by apoptosis. Chronic depolarization with elevated concentrations of extracellular potassium ([K⁺]_E) supports long-term survival of these and other types of neurons in culture. While depolarization has long been used to support neuronal cultures, little is known about the mechanism. We explored how chronic depolarization of NGF-deprived mouse sympathetic neurons in culture blocks apoptotic death. First, we determined the effects of elevated [K⁺]_E on proapoptotic BH3-only proteins reported to be upregulated in sympathetic neurons after NGF withdrawal. Upregulation of BIM_EL was blocked by depolarization while upregulation of PUMA was not. BMF levels did not increase after NGF withdrawal, and elevated [K⁺]_E had no effect on its expression. dp5/HRK was not detectable. A large increase in production of mitochondria-derived reactive species (RS), including reactive oxygen species (ROS), occurs in NGF-deprived sympathetic neurons. Suppressing these RS prevents cytochrome c release from mitochondria and apoptosis. The addition of high [K⁺]_E to cultures rapidly blocked increased RS and cytochrome c release. Elevated [K⁺]_E caused an increase of the cellular antioxidant glutathione (GSH). Preventing this increase prevented the elevated [K⁺]_E from blocking cytochrome c release and death. While suppression of BIM_EL upregulation by elevated [K⁺]_E may contribute to high [K⁺]_E pro-survival effects, we conclude that elevated [K⁺]_E prevents apoptotic death of NGF-deprived sympathetic neurons primarily via an antioxidant mechanism.

Role of electrical activity of neurons for neuroprotection

Neurons of the central nervous system (CNS) of adult mammals can be damaged in a variety of ways. Most neurons rapidly die after injury. Even if the injured CNS neurons do not die in a short time, the neurons eventually die because they are not able to regenerate their axons to reconnect with their normal targets. In addition, neurons are normally not replaced. Therefore, much work has been directed toward understanding of the molecular regulation of the CNS degeneration following injury, and different experimental strategies are being used to try to protect the damaged neurons. Following axonal lesion, the neurons not only need to survive but also to reconnect to be functionally relevant, and efforts are directed toward not only survival but also axonal regeneration and proper rewiring of injured neurons. Recent experimental data suggest that electrical activity, endogenous or exogenous, can enhance neuronal survival and regeneration in vitro and in vivo. This chapter reviews the evidence that have been obtained on the role of neuronal electrical activity on neuroprotection. We will develop perspectives toward neuroprotection and regeneration of adult lesioned CNS neurons based on electrical activity-dependent cell survival that may be applicable to various diseases of the CNS.

Specific vulnerability of mouse spinal cord motoneurons to membrane depolarization

Intracellular calcium (Ca(2+)) concentration determines neuronal dependence on neurotrophic factors (NTFs) and susceptibility to cell death. Ca(2+) overload induces neuronal death and the consequences are thought to be a probable cause of motoneuron (MN) degeneration in neurodegenerative diseases. In the present study, we show that membrane depolarization with elevated extracellular potassium (K(+)) was toxic to cultured embryonic mouse spinal cord MNs even in the presence of NTFs. Membrane depolarization induced an intracellular Ca(2+) increase. Depolarization-induced toxicity and increased intracellular Ca(2+) were blocked by treatment with antagonists to some of the voltage-gated Ca(2+) channels (VGCCs), indicating that Ca(2+) influx through these channels contributed to the toxic effect of depolarization. Ca(2+) activates the calpains, cysteine proteases that degrade a variety of substrates, causing cell death. We investigated the functional involvement of calpain using a calpain inhibitor and calpain gene silencing. Pre-treatment of MNs with calpeptin (a cell-permeable calpain inhibitor) rescued MNs survival; calpain RNA interference had the same protective effect, indicating that endogenous calpain contributes to the cell death caused by membrane depolarization. These findings suggest that MNs are especially vulnerable to extracellular K(+) concentration, which induces cell death by causing both intracellular Ca(2+) increase and calpain activation.

Dynamics of intracellular oxygen in PC12 Cells upon stimulation of neurotransmission

Neurotransmission, synaptic plasticity, and maintenance of membrane excitability require high mitochondrial activity in neurosecretory cells. Using a fluorescence-based intracellular O2 sensing technique, we investigated the respiration of differentiated PC12 cells upon depolarization with 100 mm K+. Single cell confocal analysis identified a significant depolarization of the plasma membrane potential and a relatively minor depolarization of the mitochondrial membrane potential following K+ exposure. We observed a two-phase respiratory response: a first intense spike lasting approximately 10 min, during which average intracellular O2 was reduced from 85-90% of air saturation to 55-65%, followed by a second wave of smaller amplitude and longer duration. The fast rise in O2 consumption coincided with a transient increase in cellular ATP by approximately 60%, which was provided largely by oxidative phosphorylation and by glycolysis. The increase of respiration was orchestrated mainly by Ca2+ release from the endoplasmic reticulum, whereas the influx of extracellular Ca2+ contributed approximately 20%. Depletion of Ca2+ stores by ryanodine, thapsigargin, and 4-chloro-m-cresol reduced the amplitude of respiratory spike by 45, 63, and 71%, respectively, whereas chelation of intracellular Ca2+ abolished the response. Uncoupling of the mitochondria with the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone amplified the responses to K+; elevated respiration induced a profound deoxygenation without increasing the cellular ATP levels reduced by carbonyl cyanide p-trifluoromethoxyphenylhydrazone. Cleavage of synaptobrevin 2 by tetanus toxin, known to reduce neurotransmission, did not affect the respiratory response to K+, whereas the general excitability of d PC12 cells increased.

PACAP support of neuronal survival requires MAPK- and activity-generated signals

Pituitary adenylate cyclase-activating polypeptide (PACAP) is expressed in the parasympathetic ciliary ganglion (CG) and modulates nicotinic acetylcholine receptor function. PACAP also provides trophic support, promoting partial survival of CG neurons in culture and full survival when accompanied by membrane depolarization. We probed the adenylate cyclase (AC) and phospholipase-C (PLC) transduction cascades stimulated by PACAP to determine their respective roles in supporting neuronal survival and examined their interaction with signals generated by membrane activity. While PLC-dependent signaling was dispensable, AC-generated signals proved critical for PACAP to support neuronal survival. Specifically, PACAP-supported survival was mimicked by 8Br-cAMP and blocked by inhibiting either PKA or the phosphorylation of mitogen-activated protein kinase (MAPK). The ability of PACAP to promote survival was additionally dependent on spontaneous activity as blocking Na+ or Ca2+ channel currents completely abrogated trophic effects. Our results underscore the importance of coordinated MAPK- and activity-generated signals in transducing neuropeptide-mediated parasympathetic neuronal survival.

Norman K. Wessells: a life in science

"In its triple role as locomotory organelle, as a site of deposition of new surface material for the elongating axon, and a source of microspikes (sensory probes), the growth cone becomes the key to axon elongation" Yamada et al. (1971).

Regulation and critical role of potassium homeostasis in apoptosis

Programmed cell death or apoptosis is broadly responsible for the normal homeostatic removal of cells and has been increasingly implicated in mediating pathological cell loss in many disease states. As the molecular mechanisms of apoptosis have been extensively investigated a critical role for ionic homeostasis in apoptosis has been recently endorsed. In contrast to the ionic mechanism of necrosis that involves Ca(2+) influx and intracellular Ca(2+) accumulation, compelling evidence now indicates that excessive K(+) efflux and intracellular K(+) depletion are key early steps in apoptosis. Physiological concentration of intracellular K(+) acts as a repressor of apoptotic effectors. A huge loss of cellular K(+), likely a common event in apoptosis of many cell types, may serve as a disaster signal allowing the execution of the suicide program by activating key events in the apoptotic cascade including caspase cleavage, cytochrome c release, and endonuclease activation. The pro-apoptotic disruption of K(+) homeostasis can be mediated by over-activated K(+) channels or ionotropic glutamate receptor channels, and most likely, accompanied by reduced K(+) uptake due to dysfunction of Na(+), K(+)-ATPase. Recent studies indicate that, in addition to the K(+) channels in the plasma membrane, mitochondrial K(+) channels and K(+) homeostasis also play important roles in apoptosis. Investigations on the K(+) regulation of apoptosis have provided a more comprehensive understanding of the apoptotic mechanism and may afford novel therapeutic strategies for apoptosis-related diseases.

Ca2+ clearance at growth cones produced by crayfish motor axons in an explant culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 microM) and CCCP (10 microM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

Ca2+ Clearance at Growth Cones Produced by Crayfish Motor Axons in an Explant Culture

Intracellular free Ca2+ concentration ([Ca2+]i) plays an important role in the regulation of growth cone (GC) motility; however, the mechanisms responsible for clearing Ca2+ from GCs have not been examined. We studied the Ca2+-clearance mechanisms in GCs produced by crayfish tonic and phasic motor axons by measuring the decay of [Ca2+]i after a high [K+] depolarizing pulse using fura-2AM. Tonic motor axons regenerating in explant cultures develop GCs with more rapid Ca2+ clearance than GCs from phasic axons. When Na/Ca exchange was blocked by replacing external Na+ with N-methyl-d-glucamine (NMG), [Ca2+]i decay was delayed in both tonic and phasic GCs. Tonic GCs appear to have higher Na/Ca exchange activity than phasic ones since reversal of Na/Ca exchange by lowering external Na+ caused a greater increase in [Ca2+]i for tonic than phasic GCs. Application of the mitochondrial inhibitors, Antimycin A1 (1 μM) and CCCP (10 μM), demonstrated that mitochondrial Ca2+ uptake/release was more prominent in phasic than tonic GCs. When both Na/Ca exchange and mitochondria were inhibited, the plasma membrane Ca2+ ATPase was effective in extruding Ca2+ from tonic, but not phasic GCs. We conclude that Na/Ca exchange plays a prominent role in extruding large Ca2+ loads from both tonic and phasic GCs. High Na/Ca exchange activity in tonic GCs contributes to the rapid decay of [Ca2+]i in these GCs; low rates of Ca2+ extrusion plus the release of Ca2+ from mitochondria prolongs the decay of [Ca2+]i in the phasic GCs.

Developmental cell death in vivo: rescue of neurons independently of changes at target tissues

Programmed cell death is a prominent feature of neural development that is regulated by a variety of cell-cell interactions. We used the avian ciliary ganglion to dissect the relative contributions of target tissues vs. ganglionic inputs in regulating cell death. The two populations of the ciliary ganglion innervate different targets: choroid neurons innervate vasculature, whereas ciliary neurons innervate the iris and ciliary body. By counting after labeling all neurons with Islet-1 and choroid neurons with anti-somatostatin, we determined that alpha-bungarotoxin (alpha-btx) at 12.5 microg/day rescued only ciliary neurons, whereas 75 microg/day rescued both ciliary and choroid neurons. It is unlikely that alpha-btx acted by blocking nerve transmission at both targets because the choroid vasculature lacked transcripts for alpha-btx binding molecules. In addition, no inherent trophic activity could be ascribed to alpha-btx, and survival could not be attributed to differences in total trophic activity of eyes from saline vs. alpha-btx-treated embryos. In contrast, the alpha7 antagonist alpha-methyllycaconitine (MLA) rescued ciliary neurons at 2.6 microg/day, whereas 26 microg/day rescued choroid neurons. Nerve terminals of ciliary neurons rescued with alpha-btx were significantly larger; however, differences in nerve terminal size or branching of axons were not observed in ciliary neurons rescued with MLA or choroid neurons rescued by either MLA or alpha-btx. Our results suggest that neuronal survival can be promoted independently of changes at the target tissues when orthograde signals acting by means of neuronal alpha7 nicotinic receptors are blocked.

A cyclic AMP-regulated negative feedforward system for neuritogenesis revealed in a neuroblastomaxglioma hybrid cell line

We examined the role of second messengers during the neuritogenesis that accompanies neuronal differentiation in a neuroblastomaxglioma hybrid cell line (NG108-15). NG108-15 cells extended neurites after treatment with dibutyryl cyclic AMP. This dibutyryl cyclic AMP treatment evoked the synthesis of voltage-dependent Ca(2+) channel proteins in the cells. The number of neurites was decreased by Ca(2+) influx under condition of high K(+). Interestingly, the increase of neurites stimulated by dibutyryl cyclic AMP and the decrease of neurites caused by high K(+) were both reversible. This is the first study to demonstrate that cyclic AMP regulates a negative feedforward system for neuritogenesis, which links with Ca(2+) signaling. Such a dual role of cyclic AMP may play an important part in precise neurite targeting.

In vitro inhibition of antirecoverin immunoglobulin-mediated death of mammalian photoreceptor cells

Cancer-associated retinopathy (CAR) is a blinding disease, which can be mediated by autoimmune reactions with a specific calcium-binding retinal protein, recoverin. A number of recent studies demonstrate that agents that mobilize intracellular calcium can protect neurons from apoptotic death induced by a variety of insults. In this study, we investigated the effect of one such agent, potassium, on the survival of mammalian rod photoreceptors exposed to antirecoverin IgG. Primary cell cultures of rat retinal neurons were grown in a chemically defined medium, and cells were exposed to antirecoverin IgG for 72 hr in various concentrations of potassium and the surviving cells counted. Rod photoreceptors were quantitated using antirhodopsin immunofluorescence microscopy, and total cell numbers were determined by 4',6-diamidino-2-phenylindole (DAPI) staining of nuclei. Apoptosis was evaluated by TdT-mediated biotin-dUTP nick-end labeling (TUNEL), cell death-detection ELISA, and DNA laddering. The present study shows that elevated extracellular K+ ([K+](o)) protects retinal neurons from antirecoverin antibody-mediated cell death. The protective effects of ([K+](o)) were shown to be time- and dose-dependent. The inhibition of antirecoverin IgG-mediated death of photoreceptors by elevated ([K+](o)) suggests that the mobilization of internal calcium stores rescues the cells by interfering with apoptotic signal transduction pathways. These data also suggest that the death of photoreceptor cells occurring in CAR possibly can be prevented by reagents and/or environmental changes that mobilize intracellular calcium.

Spinal cholinergic neurons activated during locomotion: localization and electrophysiological characterization

The objective of the present study was to determine the location of the cholinergic neurons activated in the spinal cord of decerebrate cats during fictive locomotion. Locomotion was induced by stimulation of the mesencephalic locomotor region (MLR). After bouts of locomotion during a 7-9 h period, the animals were perfused and the L(3)-S(1) spinal cord segments removed. Cats in the control group were subjected to the same surgical procedures but no locomotor task. The tissues were sectioned and then stained by immunohistochemical methods for detection of the c-fos protein and choline acetyltransferase (ChAT) enzyme. The resultant c-fos labeling in the lumbar spinal cord was similar to that induced by fictive locomotion in the cat. ChAT-positive cells also clearly exhibited fictive locomotion induced c-fos labeling. Double labeling with c-fos and ChAT was observed in cells within ventral lamina VII, VIII, and possibly IX. Most of them were concentrated in the medial portion of lamina VII close to lamina X, similar in location to the partition and central canal cells found by Barber and collaborators. The number of ChAT and c-fos-labeled neurons was increased following fictive locomotion and was greatest in the intermediate gray, compared with dorsal and ventral regions. The results are consistent with the suggestion that cholinergic interneurons in the lumbar spinal cord are involved in the production of fictive locomotion. Cells in the regions positive for double-labeled cells were targeted for electrophysiological study during locomotion, intracellular filling, and subsequent processing for ChAT immunohistochemistry. Three cells identified in this way were vigorously active during locomotion in phase with ipsilateral extension, and they projected to the contralateral side of the spinal cord. Thus a new population of spinal cord cells can be defined: cholinergic partition cells with commissural projections that are active during the extension phase of locomotion.

Long-term depolarization changes morphological parameters of PC12 cells

It is well known that neuronal differentiation is strongly dependent on the intracellular level of free calcium ions ([Ca2+]i). In the present study the morphological and intracellular free calcium concentration changes were compared on PC12 pheochromocytoma cells cultured in control conditions and in a medium with high KCl level. Culturing PC12 cells in a medium with 20-30 mM KCl deprived of nerve growth factor supported cell proliferation and rapid growth of small neurite-like processes. However, their lengths did not increase with prolongation of the time of culturing. During culturing with 40 mM KCl the growth of these processes became blocked; the cells stopped proliferating and showed signs of degeneration. Measurements of [Ca2+]i level during the first days of PC12 cells culturing in a hyperpotassium medium indicate that such changes in this level could be an important factor in the induction of the observed morphological alterations; however, other effects induced by membrane depolarization may also be responsible for them.

Opposing effects of excitatory amino acids on chick embryo spinal cord motoneurons: excitotoxic degeneration or prevention of programmed cell death

Acute administration of a single dose of NMDA on embryonic day (E) 7 or later induces a marked excitotoxic injury in the chick spinal cord, including massive necrotic motoneuron (MN) death. When the same treatment was performed before E7, little, if any, excitotoxic response was observed. Chronic treatment with NMDA starting on E5 prevents the excitotoxic response produced by a later "acute" administration of NMDA. Additionally, chronic NMDA treatment also prevents the later excitotoxic injury induced by non-NMDA glutamate receptor agonists, such as kainate or AMPA. Chronic NMDA treatment also reduces normal MN death when treatment is maintained during the period of naturally occurring programmed cell death (PCD) of MNs and rescues MNs from PCD induced by early peripheral target deprivation. The trophic action of chronic NMDA treatment appears to involve a downregulation of glutamate receptors as shown by both a reduction in the obligatory NR1 subunit protein of the NMDA receptor and a decrease in the kainate-induced Co(2+) uptake in MNs. Both tolerance to excitotoxicity and trophic effects of chronic NMDA treatment are prevented by the NMDA receptor antagonist MK-801. Additionally, administration of MK-801 alone results in an increase in MN PCD. These data indicate for the first time that early activation of NMDA receptors in developing avian MNs in vivo has a trophic, survival-promoting effect, inhibiting PCD by a target-independent mechanism that involves NMDA receptor downregulation.

NMDA and non-NMDA receptor-mediated excitotoxicity are potentiated in cultured striatal neurons by prior chronic depolarization

The excitatory input from cortex and/or thalamus to striatum appears to promote the maturation of glutamate receptors on striatal neurons, but the mechanisms by which it does so have been uncertain. To explore the possibility that the excitatory input to striatum might influence glutamate receptor maturation on striatal neurons, at least in part, by its depolarizing effect on striatal neurons, we examined the influence of chronic KCl depolarization on the development of glutamate receptor-mediated excitotoxic vulnerability and glutamate receptors in cultured striatal neurons. Dissociated striatal neurons from E17 rat embryos were cultured for 2 weeks in Barrett's medium containing either low (3 mM) or high (25 mM) KCl. The vulnerability of these neurons to NMDA receptor agonists (NMDA and quinolinic acid), non-NMDA receptor agonists (AMPA and KA), and a metabotropic glutamate receptor agonist (trans-ACPD) was examined by monitoring cell loss 24 h after a 1-h agonist exposure. We found that high-KCl rearing potentiated the cell loss observed with 500 microM NMDA or 250 microM KA and yielded cell loss with 250 microM AMPA that was not evident under low KCl rearing. In contrast, neither QA up to 5 mM nor trans-ACPD had a significant toxic effect in either KCl group. ELISA revealed that chronic high KCl doubled the abundance of NMDA NR2A/B, AMPA GluR2/3, and KA GluR5-7 receptor subunits on cultured striatal neurons and more than doubled AMPA GluR1 and GluR4 subunits, but had no effect on NMDA NR1 subunit levels. These receptor changes may contribute to the potentiation of NMDA and non-NMDA receptor-mediated excitotoxicity shown by these neurons following chronic high-KCl rearing. Our studies suggest that membrane depolarization produced by corticostriatal and/or thalamostriatal innervation may be required for maturation of glutamate receptors on striatal neurons, and such maturation may be important for expression of NMDA and non-NMDA receptor-mediated excitotoxicity by striatal neurons. Striatal cultures raised under chronically depolarized conditions may, thus, provide a more appropriate culture model to study the role of NMDA or non-NMDA receptor subtypes in excitotoxicity in striatum.

Upregulation of calcium homeostatic mechanisms in chronically depolarized rat myenteric neurons

Perturbations of intracellular Ca2+ ion concentration ([Ca2+]i) have important effects on numerous neuronal processes and influence development and survival. Neuronal [Ca2+]i is, in large part, dependent on activity, and changes in activity levels can alter how neurons handle calcium (Ca). To investigate the ability of neuronal Ca homeostatic mechanisms to adapt to the persistent elevation of [Ca2+]i, we used optical and electrophysiological recording techniques to measure [Ca2+]i transients in neurons from the rat myenteric plexus that had been chronically depolarized by growth in culture medium containing elevated (25 mM) KCl. When studied in normal saline, neurons that had previously been chronically depolarized for 3-5 days had briefer action potentials than control neurons, their action potentials produced smaller, more rapidly decaying increases in [Ca2+]i, and voltage-clamp pulses with action potential waveforms evoked smaller Ca currents than in control neurons. Simultaneous voltage-clamp measurements and calcium imaging revealed that increases in the Ca handling capacities of the chronically depolarized neurons permitted them to limit the amplitudes of action potential-evoked [Ca2+]i transients and to restore [Ca2+]i to basal levels more rapidly than control neurons. Release of Ca from endoplasmic reticulum-based Ca stores made smaller contributions to action potential-evoked [Ca2+]i transients in chronically depolarized neurons even though those neurons had larger caffeine-releasable Ca stores. Endoplasmic reticulum-based Ca sequestration mechanisms appeared to contribute to the faster decay of [Ca2+]i transients in chronically depolarized neurons. These results demonstrate that when neurons experience prolonged perturbations of [Ca2+]i, they can adjust multiple components of their Ca homeostatic machinery. Appropriate utilization of this adaptive capability should help neurons resist potentially lethal metabolic and environmental insults.

Neurotransmitters, KCl and antioxidants rescue striatal neurons from apoptotic cell death in culture

Striatal neurons grown in low density culture on serum-free media and in the absence of glia die within 3 days of plating. In this study, we sought to determine the mechanism of cell death (e.g., apoptosis) and whether trophic influences, such as, growth factors, neurotransmitters, antioxidants or KCl-mediated depolarization could improve their survival. We found that striatal neurons grown in this manner die via apoptosis unless treated with one of several different rescuing agents. One way to prevent the death of most striatal neurons was continual treatment with 5-20 microM dopamine (DA) or other monoamines. Although the survival effect of DA was mimicked by the specific D1 receptor agonist, SKF38393, no D1 or D2 receptor antagonists blocked the effect. As with DA, chronic depolarization with KCl (12-39 mM) or treatment with antioxidants, such as the vitamin E analog, Trolox (10-10-500 microM), or the hormone, melatonin (10-10-500 microM) also rescued striatal neurons from impending cell death. Surprisingly, growth factors, such as BDNF, bFGF, GDNF, NGF, NT3 and EGF, demonstrated no ability to rescue striatal neurons in this model, suggesting that death was not solely caused by the absence of essential trophic factors. We conclude that a variety of agents, but not growth factors, can prevent the demise of striatal neurons, presumably by neutralizing damage at one or more steps in the death cascade.

Role of cell-cell interactions in the developmental regulation of Ca2+-activated K+ currents in vertebrate neurons

The functional expression of the Ca2+-activated K+ current (IK[Ca]) is dependent on cell-cell interactions in developing chick autonomic neurons. In chick ciliary ganglion (CG) neurons, expression of macroscopic IK[Ca] coincides with the formation of synapses with target tissues. CG neurons that develop in vivo in the absence of normal target tissues fail to express functional IK[Ca], although voltage-activated Ca2+ currents and most other ionic currents are expressed at normal amplitudes and densities. CG neurons placed in cell culture prior to formation of synapses with target tissues also fail to express macroscopic IK[Ca]. However, CG neurons cultured in the presence of a heat- and trypsin-sensitive extract of target tissues express IK[Ca] at normal levels. Similarly, interactions with target tissue appear to regulate the expression of whole-cell IK[Ca] in developing chick sympathetic ganglion neurons, although the relevant trophic factors appear to be different from those required by CG neurons. In addition to target tissue interactions, an intact preganglionic innervation is required for the normal in vivo development of IK[Ca] in chick CG neurons. The trophic effects of the afferent innervation do not require synaptic activation of the CG neurons, indicating secretion of a trophic factor, possibly an isoform of beta-neuregulin. The results are consistent with the hypothesis that target- and nerve terminal-derived trophic factors interact at a posttranslational level in the regulation of a functional IK[Ca]. Together, this body of data demonstrates an essential role for cell-cell interactions in the differentiation of neuronal excitability.

Activity-dependent development of calcium regulation in growing motor axons

In cultured nerve cord explants from the crayfish (Procambarus clarkii), the normal impulse activity levels of growing motor axons determine their response to Ca2+ influx. During depolarization or Ca2+ ionophore application, normally active tonic motor axons continue to grow, whereas inactive phasic motor axons retract and often degenerate. To determine the role of Ca2+ regulation in this difference, we measured the intracellular free Ca2+ concentration ([Ca2+]i) with fura-2. Growth cones from tonic axons normally had a higher [Ca2+]i than those from phasic axons. When depolarized with 60 mM K+, growth cones and neurites from phasic axons had a [Ca2+]i three to four times higher than did those from tonic axons. This difference in Ca2+ regulation includes greater Ca2+-handling capacity for growing tonic axons; the increase in [Ca2+]i produced by the Ca2+ ionophore 4-bromo-A23187 (0.25 microM) is four to five times greater in phasic than in tonic axons, and the decline in [Ca2+]i at the end of a depolarizing pulse is three to four times faster in tonic axons than phasic ones. Blocking impulses in growing tonic axons for 2-3 d with tetrodotoxin reduces their capacity to regulate [Ca2+]i. Thus, growing tonic and phasic axons have differences in Ca2+ regulation that develop as a result of their different activity levels. These activity-dependent differences in Ca2+ regulation influence axon growth and degeneration and probably influence other neuronal processes that are mediated by changes in [Ca2+]i.

Influence of muscle cells on the development of calcium currents in Xenopus spinal neurons

The influence of muscle cells on the development of voltage-dependent Ca2+ currents was investigated in Xenopus spinal neurons grown in neuron muscle co-cultures or in muscle-free cultures. Whole-cell currents were separated into low- and high-voltage-activated currents. Developmental changes were assessed by comparing the results obtained at two different periods after plating: 5-10 h (young neurons) and 20-30 h (mature neurons). Our results show a drop in the incidence of low-voltage-activated Ca2+ current with time in both environments: the fraction of young versus mature neurons expressing this current was 67% and 36% in neuron-muscle co-cultures, and 69% and 23% in muscle-free cultures. In both neuron muscle and muscle-free cultures, the density of low-voltage-activated Ca2+ current (when expressed) did not change during the development. In contrast, the density of high-voltage-activated Ca2+ currents increased more than two-fold during the first 30 h in neuron muscle co-cultures, but remained unchanged in muscle-free cultures. This difference was not related to neuronal growth since the increase in neuronal membrane capacitance with time was similar in the two environments. In addition, direct cell-cell interaction through the establishment of functional neuron-muscle synaptic contacts did not further modify the overall expression of high-voltage-activated Ca2+ currents. In conclusion, these results suggest the presence of diffusible factors in neuron muscle co-cultures which up-regulate the expression of high-voltage-activated Ca2+ currents during neuronal development, but do not have any effect on low-voltage-activated Ca2+ currents.

Retinal axon growth cone responses to different environmental cues are mediated by different second-messenger systems

Numerous studies have shown that the developing tip of a neurite, the growth cone, can respond to environmental cues with behaviors such as guidance or collapse. To assess whether a given cell type can use more than one second-messenger pathway for a single behavior, we compared the influence of two well-characterized guidance cues on growth cones of chick temporal retinal ganglion cells. The first cue was the repulsive activity derived from the posterior optic tectum (p-membranes), and the second was the collapse-inducing activity derived from oligodendrocytes known as NI35/NI250. p-Membranes caused permanent growth cone collapse with no recovery after several hours, while NI35 caused transient collapse followed by recovery after about 10 min. The p-membrane-induced collapse was found to be Ca2+ independent, as shown using the Ca2+-sensitive dye Fura-2 and by the persistence of collapse in Ca2+-free medium. Dantrolene, a blocker of the ryanodine receptor, had only a minor effect on the collapse frequency caused by p-membranes. In contrast, the NI35-induced collapse was clearly Ca2+ dependent. [Ca2+]i increased sevenfold preceding collapse, and both dantrolene and antibodies against NI35 significantly reduced both the Ca2+ increase and the collapse frequency. Thus, even in a single cell type, growth cone collapse induced by two different signals can be mediated by two different second-messenger systems.

A new twist in an old story: the role for crosstalk of neuronal and trophic activity

A number of recent findings suggest a reciprocal interaction between neurotransmitters and neurotrophins functioning at the level of the synapse, which may be relevant not only for plasticity changes in the mature nervous system, but also for the development of synaptic connectivity and for survival or maturation of neurons prior to target contact. Thus, neurotrophin-induced attenuation of frequency-dependent depletion of releasable synaptic vesicle pools of neurotransmitter at synapses may participate in Hebbian and non-Hebbian forms of LTP, as a characteristic of mature synaptic contacts. Subsequent to nerve/target contact, neurotrophins also appear to mediate contact-induced enhancement of neurotransmitter release; this may participate in a developmental improvement of synapse efficacy, stabilization of synaptic contacts, and maturation of "conductive" functional synapses. Coincident with a transmitter-induced elevation of cytosolic Ca2+ levels within growth cones, a local neurotrophin-mediated increase in released neurotransmitter occurring subsequent to stabilization of a distinct synaptic contact may then participate in the refinement of synapses with retention of those neurites affected by neurotrophins and withdrawal of those neurites not affected by neurotrophins. Finally, prior to nerve/target contact, Ca2+ channel-generated spontaneous neuronal activity as well as co-expression of neurotrophins and their receptors may play a role in maturational changes.

Synchronization of neuronal activity promotes survival of individual rat neocortical neurons in early development

Neural activity is thought to play a significant role during the development of the cerebral cortex. In this study, we examined the effects of global activity block or enhancement and the effects of patterned firing on the ability of cultured rat neocortical neurons to survive during the second week in vitro, beyond the beginning of synaptogenesis. Blockade of neuronal activity by adding tetrodotoxin (TTX) and increasing magnesium concentration in the medium strongly reduced the survival of cortical cells. Increasing neuronal activity by raising the external potassium concentration significantly improved the survival of cortical neurons. We postulated that in a developing neuronal network the survival of nerve cells is regulated by synaptically mediated events that involve changes in the intracellular calcium concentration. To examine this question further, we monitored the activity of the developing network by optically recording the intracellular calcium signals of many neurons simultaneously. These recordings show that in low magnesium neocortical neurons express synchronized oscillation of their intracellular calcium concentration. The ability of a network to synchronize the changes in intracellular calcium of multiple cells appeared gradually during the second week in culture, paralleled by both an increase in the synaptic density and a decline in the number of surviving neurons. By examining the fate of identified cells several days after a recording session, we found that those nerve cells that were co-activated with other neurons had a significantly higher chance to survive than cells that did not participate in synchronized events. These experiments demonstrate that during early cortical network development cortical neurons show synchronized firing activity and that the survival of neurons is at least partially dependent on this pattern of neuronal activity.

Inhibition of phosphatidylinositol 3-kinase activity blocks depolarization- and insulin-like growth factor I-mediated survival of cerebellar granule cells

Depolarizing concentrations of potassium promote the survival of many neuronal cell types including cerebellar granule cells. To begin to understand the intracellular mediators of neuronal survival, we have tested whether the survival-promoting effect of potassium depolarization on cerebellar granule cells is dependent on either mitogen-activated protein (MAP) kinase or phosphatidylinositol 3-kinase (PI-3-K) activity. In 7-day cerebellar granule cell cultures, potassium depolarization activated both MAP kinase and PI-3-K. Preventing the activation of MAP kinase with the MEK1 inhibitor PD98059 did not affect potassium saving. In contrast, the survival-promoting effect of 25 mM potassium was negated by the addition of 30 microM LY 294002 or 1 microM wortmannin, two distinct inhibitors of PI-3-K. The cell death induced by PI-3-K inhibition was indistinguishable from the cell death caused by potassium deprivation; LY 294002-induced death included nuclear condensation, was blocked by cycloheximide, and had the same time course as potassium deprivation-induced cell death. Cerebellar granule cells can also be maintained in serum-free medium containing either 100 ng/ml insulin-like growth factor I (IGF-I) or 800 microM cAMP. PI-3-K inhibition completely blocked the survival-promoting activity of IGF-I, but had no effect on cAMP-mediated survival. These data indicate that the survival-promoting effects of depolarization and IGF-I, but not cAMP, require PI-3-K activity.

Posttranslational regulation of Ca(2+)-activated K+ currents by a target-derived factor in developing parasympathetic neurons

Macroscopic IK[Ca is not expressed in normal levels in chick ciliary ganglion (CG) neurons prior to synapse formation with target tissues, or in neurons developing in vitro or in situ in the absence of target tissues. Here, two chick CG slo partial cDNAs encoding IK[Ca channels were isolated, cloned, and sequenced. Both slo transcripts were readily detected in developing CG neurons prior to or in the absence of target tissue interactions. When CG neurons developed in vitro in the presence of target tissue (iris) extracts, a normal whole-cell IK[Ca was expressed. These effects did not require protein synthesis, and the activity was detectable throughout the stages of synapse formation in the iris. The active component has an apparent molecular weight of 40-60 kDa.

Identification of functional receptors for ciliary neurotrophic factor on chick ciliary ganglion neurons

Ciliary neurotrophic factor and an avian homolog, growth promoting activity, are members of the cytokine/neurokine family of trophic factors and have been proposed to function as survival and developmental factors for ciliary ganglion neurons in vivo. Here we identify for the first time functional receptors for ciliary neurotrophic factor and growth promoting activity on cultured ciliary ganglion neurons. [(125)I]Rat ciliary neurotrophic factor binding studies indicate that rat ciliary neurotrophic factor and growth promoting activity bind to these receptors with a single affinity, while human ciliary neurotrophic factor recognizes both a high- and low-affinity site. Comparison of the relative potency of human ciliary neurotrophic factor and avian growth promoting activity in biological assays indicates that growth promoting activity is three to five times more active in promoting survival and in regulating acetylcholine receptors. The binding of ciliary neurotrophic factor is specific, sensitive to phosphatidylinositol-specific phospholipase C and partially inhibited by leukemia inhibitory factor, but not inhibited by other members of the human neurokine family, including interleukin-6, interleukin-22 and oncostatin M. Cross-linking of [(125)I]rat ciliary neurotrophic factor to ciliary neurons results in the specific labeling of three proteins with estimated molecular masses of 153,000, 81,000 and 72,000. Only the 81,000 molecular weight component is released from the cells after treatment with phosphatidylinositol-specific phospholipase C, suggesting a membrane attachment via a glycosylphosphatidylinositol linkage. Stimulation with ciliary neurotrophic factor or growth promoting activity, but not by other neurokines, results in the rapid tyrosine phosphorylation of a 90,000 molecular weight protein that is inhibited by pretreatment with phosphatidylinositol-specific phospholipase C. In conclusion, we report here the pharmacological and functional properties of ciliary neurotrophic factor receptors on embryonic ciliary ganglion neurons. These results provide the means for elaborating the molecular mechanisms of ciliary neurotrophic factor action and understanding its physiological role in a defined neuronal population.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Suppression of programmed neuronal death by a thapsigargin-induced Ca2+ influx

Rat sympathetic neurons undergo programmed cell death (PCD) in vitro and in vivo when they are deprived of nerve growth factor (NGF). Chronic depolarization of these neurons in cell culture with elevated concentrations of extracellular potassium ([K+]o) prevents this death. The effect of prolonged depolarization on neuronal survival is thought to be mediated by a rise of intracellular calcium concentration ([Ca2+]i) caused by Ca2+ influx through voltage-gated channels. In this report we investigate the effects of chronic treatment of rat sympathetic neurons with thapsigargin, an inhibitor of intracellular Ca2+ sequestration. In medium containing a normal concentration of extracellular Ca2+ ([Ca2+]o), thapsigargin caused a sustained rise of intracellular Ca2+ concentration and partially blocked death of NGF-deprived cells. Elevating [Ca2+]o in the presence of thapsigargin further increased [Ca2+]i, suggesting that the sustained rise of [Ca2+]i was caused by a thapsigargin-induced Ca2+ influx. This treatment potentiated the effect of thapsigargin on survival. The dihydropyridine Ca2+ channel antagonist, nifedipine, blocked both a sustained elevation of [Ca2+]i and enhanced survival caused by depolarization with elevated [K+]o, suggesting that these effects are mediated by Ca2+ influx through L-type channels. Nifedipine did not block the sustained rise of [Ca2+]i or enhanced survival caused by thapsigargin treatment, indicating that these effects were not mediated by influx of Ca2+ through L-type channels. These results provide additional evidence that increased [Ca2+]i can suppress neuronal PCD and identify a novel method for chronically raising neuronal [Ca2+]i for investigation of this and other Ca(2+)-dependent phenomena.

An improved method detects differential NGF and BDNF gene expression in response to depolarization in cultured hippocampal neurons

Differential regulation of individual neurotrophins by impulse activity potentially allows transformation of instantaneous signalling into diverse, long-lasting neural alterations. To define the temporal profiles of trophin gene expression we examined nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) mRNAs in dissociated cell cultures of rat hippocampus using an improved solution hybridization technique. Traditional methods lack the precision and sensitivity to detect small changes during brief intervals and the facility to process large sample numbers simultaneously. This improved method has now allowed us to better define the dynamics of depolarization-induced changes in expression of individual trophin genes. Using elevated K+ as a depolarizing stimulus, NGF mRNA increased 40% after 48 h. In contrast, BDNF message rose almost 4-fold within 3 h and attained a maximal 6-fold increase within 6 h. Similar increases in BDNF mRNA levels were exhibited following treatment of cultures with glutamate, an excitatory neurotransmitter. To document the sensitivity of BDNF mRNA to depolarizing conditions, we examined expression after K+ withdrawal. BDNF message began decreasing within one hour post-depolarization, and returned to basal levels after 6 h. Observations indicate that BDNF and NGF mRNAs are induced differentially in response to impulse activity; BDNF message is acutely responsive to ongoing changes, whereas NGF mRNA responds more slowly and sluggishly. The physiological implications of this differential regulation are discussed.

Block of neuronal apoptosis by a sustained increase of steady-state free Ca2+ concentration

Programmed death is a ubiquitous feature of the development of the vertebrate nervous system. This death is prevented in vivo by trophic factors and by afferent input. Death of neurons can also be prevented in culture models of programmed death by trophic factors and by chronic depolarization with elevated concentrations of K+ in the culture medium. The latter effect is mediated by Ca2+ influx through voltage-gated channels and may prevent death by mimicking survival-promoting effects of naturally occurring electrical activity. Little is currently known about the mechanism by which either trophic factors or increased cytoplasmic Ca2+ promote survival.

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

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

Target-derived molecules that influence the development of neurons in the avian ciliary ganglion

The developing avian ciliary ganglion has been a particularly amenable system for the identification, isolation, and characterization of putative target-derived molecules that mediate retrograde interactions. To date a number of biochemically distinct activities that regulate neuronal survival, transmitter phenotype, and chemosensitivity of ciliary ganglion neurons have been identified. Of these, only two survival-promoting molecules have been purified to homogeneity: ciliary neurotrophic factor and a related molecule, growth-promoting activity. A somatostatin-inducing activity found in cultured choroid cells is very likely to be chick activin A. Other molecules that regulate acetylcholine and acetylcholine receptor expression comigrate on a gel filtration column at a molecular weight of 50-60 kD, but they have yet to be isolated. Once molecules that mimic retrograde influences are identified, a number of criteria must be met before their physiological significance can be established. These criteria are (1) availability of the molecule from the target at the appropriate time in development; (2) ability of the neurons to respond to the molecule at the appropriate time in development; (3) demonstration that blocking the activity or availability of the molecule is able to block the target-derived developmental change expressed in the neurons. Of the molecules that are thought to retrogradely influence ciliary neuron development, only growth-promoting activity is known to meet criteria 1 and 2, and experiments are currently underway to test whether inhibition of growth-promoting activity in vivo will exacerbate normal cell death.

Requirement for BDNF in activity-dependent survival of cortical neurons

Cultured embryonic cortical neurons from rats were used to explore mechanisms of activity-dependent neuronal survival. Cell survival was increased by the activation of voltage-sensitive calcium channels (VSCCs) but not by activation of N-methyl-D-aspartate receptors. These effects correlated with the expression of brain-derived neurotrophic factor (BDNF) induced by these two classes of calcium channels. Antibodies to BDNF (which block intracellular signaling by BDNF, but not by nerve growth factor, NT3, or NT4/5) reduced the survival of cortical neurons and reversed the VSCC-mediated increase in survival. Thus, endogenous BDNF is a trophic factor for cortical neurons whose expression is VSCC-regulated and that functions in the VSCC-dependent survival of these neurons.

Regulation of A-currents by cell-cell interactions and neurotrophic factors in developing chick parasympathetic neurones

1. The developmental regulation of ion channel expression was studied in parasympathetic neurones isolated from the chick ciliary ganglion. Whole-cell patch clamp recordings were made from ciliary ganglion neurones that were removed from the embryo on the ninth embryonic day (E9) and maintained in dissociated cell culture for an additional 4 days. Previous studies have shown that the expression of a transient voltage-activated K+ current (IA) is regulated by unidentified environmental stimuli during these developmental stages. 2. The effect of interactions between neurones and target tissue on the expression of IA was tested by co-culturing ciliary ganglion neurones with chick striated muscle cells. Neurones from the nerve-muscle co-cultures expressed normal amplitudes of IA, but the neurones did not express normal levels of IA when they were plated onto lysed muscle fibres. 3. The effect of interactions between ganglionic neurones and non-neuronal ganglionic cells was tested by culturing ganglia as explants rather than as dissociated cells. Neurones isolated from the explant cultures did not express normal levels of IA. Similarly, when dissociated ganglionic neurones were co-cultured with fibroblasts isolated from embryonic chick skin, they did not express normal amplitudes of IA. 4. Chronic depolarization caused by growing ciliary ganglion neurones in the presence of elevated K+ concentrations did not allow for the normal expression of IA, although it did promote the survival of these neurones in vitro. 5. Addition of 40 ng ml-1 of recombinant human ciliary neurotrophic factor (CNTF) or basic fibroblast growth factor (bFGF) to the cell culture medium had no effect on IA expression in developing chick ciliary ganglion neurones. However, 40 ng ml-1 of acidic fibroblast growth factor (aFGF) stimulated the expression of IA. All trophic factors promoted the growth and survival of ciliary ganglion neurones in vitro. 6. Dissociated ciliary ganglion neurones were maintained in a culture medium containing an aqueous extract of the whole brain. Neurones developing under these conditions expressed normal levels of IA. The stimulatory activity of the brain extract was destroyed by heating. 7. The expression of IA in chick ciliary ganglion neurones developing in vitro can be regulated by soluble growth factors and by interactions with certain other cell types. Similar interactions may regulate the expression of IA in ciliary ganglion neurones developing in situ.

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionate and kainate differently affect neuronal cytoarchitecture of rat cerebellar granule cells

Rat cerebellar granule cells cultured in media containing 12 mM KCl showed short life-span, did not branch, and died after 10 days in vitro. The cell exposure to N-methyl-D-aspartate (NMDA) or to kainate promoted both neuron survival and branching, reproducing the viability and the neurite extension routinely observed in cultures maintained in media containing 25 mM KCl. Exposure of neurons to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) resulted in an increased survival not associated with neuritic arborization. These results suggest that the glutamate ionotropic receptor subtypes differently contribute in elaborating neuronal morphogenesis.

Expression of horizontal cell phenotypes in monolayer cultures from immature rabbit retina

Using the sandwich culture technique introduced by Brewer and Cotman we have studied the in vitro differentiation of A- and B-type horizontal cells which represent two well characterized cell types of the rabbit retina. Neurons from immature (postnatal day 3) rabbit retinae were dissociated and grown on inverted coverslips for up to 5 weeks in a chemically defined medium. On the basis of morphological criteria and the staining pattern for several immunocytochemical and autoradiographic horizontal cell markers we have examined to what extent expression of a distinct mature neuronal phenotype can take place under the artificial conditions of monolayer cultures. After 14 days in vitro neurons could be identified which had acquired elaborate morphological features closely resembling those of A- and B-type horizontal cells, respectively. Axonless A-like cells had 2-4 stout primary dendrites. In agreement with in situ observations these cells showed immunoreactivity for neurofilament proteins (68 kDa, 200 kDa), calbindin-28 kDa and less strongly for vimentin. B-like neurons reached varying states of development. Ideally, they had dendritic trees with 6-8 primary processes extending radially from the soma and a single axon-like process which branched extensively to form a profuse neuritic arbor strikingly similar to axon terminal systems of B-type cells in the intact retina. B-like cells also stained for vimentin, calbindin-28 kDa and unexpectedly also for neurofilament proteins. Interestingly, however, neurofilaments became redistributed during in vitro development eventually resulting in their restricted localization in the 'axon terminal system'. This apparently reflects a developmental process which has escaped detection in situ so far. Both cell types were intensely labelled with antibodies to gamma-aminobutyric acid (GABA), the presumed horizontal cell transmitter, but high affinity uptake of this transmitter was practically undetectable by [3H]-GABA autoradiography. This was in agreement with observations in intact retinae. These results support the notion that once a neuron has reached a certain developmental state further differentiation and maintenance of its particular morphological and functional properties are primarily governed by intrinsic factors, but do not exclude that extrinsic signals have important modulatory functions.

Promotion of granule cell survival by high K+ or excitatory amino acid treatment and Ca2+/calmodulin-dependent protein kinase activity

Cerebellar granule cells in culture develop survival requirements which can be met either by chronic membrane depolarization (25 mM K+) or by stimulation of ionotropic excitatory amino acid receptors. We observed previously that this trophic effect is mediated via Ca2+ influx, either through dihydropyridine-sensitive, voltage-dependent calcium channels (activated directly by high K+ or indirectly by kainate) or through N-methyl-D-aspartate receptor-linked ion channels. Steps after Ca2+ entry in the transduction cascade mediating the survival-supporting effect of high K+ and excitatory amino acids have now been examined. Using protein kinase inhibitors (H-7, polymixin B and gangliosides), and modulating protein kinase C activity by treatment with the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, we obtained evidence against the involvement of protein kinase C and cyclic nucleotide-dependent protein kinases in the transduction cascade. On the other hand, calmidazolium (employed as a calmodulin inhibitor) counteracted the trophic effect of elevated K+ with high potency (IC50 0.3 microM), which exceeded by approximately 10-fold the potency for the blockade by the drug of voltage-sensitive calcium channels. The potency of calmidazolium in interfering with the N-methyl-D-aspartate rescue of cells was also much higher in comparison with the inhibition of 45Ca2+ influx through N-methyl-D-aspartate receptor-linked channels. Our results indicated that after calmodulin the next step in the trophic effects involves Ca2+/calmodulin-dependent protein kinase II activity. KN-62, a fairly specific antagonist of this enzyme, compromised elevated K+ or excitatory amino acid-supported cell survival with high potency (IC50 2.5 microM). In the relevant concentration range, KN-62 had little or no effect on Ca2+ entry through either voltage- or N-methyl-D-aspartate receptor-gated channels. Combining information on the toxic action of glutamate in "mature" granule cells with the trophic effect of either excitatory amino acids or high K+ treatment on "young" cells, we conclude that after the initial steps involving calcium in both cases the respective transduction pathways diverge. The toxic action of glutamate seems to be mediated through protein kinase C [Favaron et al. (1990) Proc. natn. Acad. Sci. U.S.A. 87, 1983-1987 whereas a Ca2+/calmodulin-dependent protein kinase, which can be inhibited by KN-62 (but is resistant to gangliosides and to inhibitors whose potency is higher for protein kinase C than for Ca2+ calmodulin-dependent protein kinases, such as H-7 and polymixin B), is involved critically in the trophic effect.

Suppression of programmed neuronal death by sustained elevation of cytoplasmic calcium

Chronic depolarization greatly increases the survival of many types of neurons in culture. In at least some cases this enhancement of survival consists of the suppression of programmed cell death, a type of death occurring in developing neurons deprived of sufficient neurotrophic factor support. Available evidence suggests that the effect of depolarization on survival is mediated by a sustained rise of cytoplasmic free Ca2+, apparently caused by influx of Ca2+ through voltage-gated channels. This review discusses what is known about the mechanism by which prolonged depolarization and increased intracellular Ca2+ promote survival.

Peptidergic neurons of the crab, Cardisoma carnifex, in defined culture maintain characteristic morphologies under a variety of conditions

Peptidergic neurons dissociated from the neurosecretory cell group, the X-organ, of adult crabs (Cardisoma carnifex) show immediate outgrowth on unconditioned plastic dishes in defined medium. Most of the neurons can be categorized as small cells, branchers or veilers. A fourth type, "superlarge," found occasionally, has a soma diameter greater than 40 microns and multipolar outgrowth. We report here the effects on morphology that follow alterations of the standard defined culturing conditions. The three common types of neurons are present when cells are grown in crab saline or saline with L-glutamine and glucose (saline medium). Changes of pH between 7.0 to 7.9 have no effect. Osmolarity changes cause transient varicosities in small cells. In some veilers, pits rapidly appear in the veil and then disappear within 35 min. In cultures at 26 degrees C instead of 22 degrees C, veilers extend processes from the initial veil in a pattern similar to branchers, and the processes of adjacent veilers sometimes form appositions. Culturing in higher [K+]o medium ([K+]o = 15-110 mM; standard = 11 mM) has no long-term effect, but growth is arrested by [K+]o greater than 30 mM. Cultures were also grown in media in which [Ca2+]o ranged from 0.1 microM to 26 mM (standard = 13 mM). Outgrowth occurred from all neuronal types in all [Ca2+]o tested. Thus, the expression of different outgrowth morphologies occurs under a wide variety of culturing conditions.

Divergent effects of astroglial and microglial secretions on neuron growth and survival

Brain glia have a secretory capacity which can modulate neuronal function. Astrocytes release proteins which enhance neuronal survival and induce neuronal growth and differentiation. These effects can be blocked by antagonists of voltage-dependent calcium channels and may be partly mimicked by Bay K 8644, a calcium channel agonist. Two of these neurotrophic proteins appear, on the basis of their physical properties and effects on ciliary ganglion neurons, to be ciliary neurotrophic factor and basic fibroblast growth factor. Activated microglia release a heat- and protease-stable neurotoxin of low molecular weight. This neurotoxicity is blocked by NMDA receptor antagonists. Ciliary neurons exposed to the microglial neurotoxin exhibit an abnormal distribution of neurofilament immunoreactivity, which becomes concentrated in a perinuclear region, while the astroglial growth factors induce neurofilament organization into an extensive neuritic network. The astrocyte-released growth factors can counteract the effect of the microglial neurotoxin and lead to unimpaired neural differentiation in the presence of the neurotoxin.

Intracellular calcium regulates the survival of early sensory neurons before they become dependent on neurotrophic factors

To investigate the role of intracellular Ca2+ in the survival of developing neurons before they become neurotrophic factor dependent, we have studied chick embryo nodose neurons, which have a particularly protracted period of neuorophic factor independence. Pharmacological reduction of intracellular free Ca2+ or depletion of either Ca(2+)-regulated or inositol trisphosphate-regulated intracellular Ca2+ stores kills early neurotrophic factor-independent neurons, but has a negligible effect on older neurons growing in the presence of brain-derived neutrotrophic factor. Shortly before they become dependent on brain-derived neurotrophic factor, nodose neurons express L-type Ca2+ channels and their survival can be enhanced by depolarization-induced Ca2+ influx. We conclude that intracellular Ca2+ plays a role in regulating neuronal survival both prior to and after the onset of neurotrophic factor dependence, but does not mediate the survival-promoting effects of neurotrophic factors.

Development and maintenance of the neuronal cytoskeleton in aggregated cell cultures of fetal rat telencephalon and influence of elevated K+ concentrations

Serum-free aggregating cell cultures of fetal rat telencephalon were examined by biochemical and immunocytochemical methods for their development-dependent expression of several cytoskeletal proteins, including the heavy- and medium-sized neurofilament subunits (H-NF and M-NF, respectively); brain spectrin; synapsin I; beta-tubulin; and the microtubule-associated proteins (MAPs) 1, 2, and 5 and tau protein. It was found that with time in culture the levels of most of these cytoskeletal proteins increased greatly, with the exceptions of the particular beta-tubulin form studied, which remained unchanged, and MAP 5, which greatly decreased. Among the neurofilament proteins, expression of M-NF preceded that of H-NF, with the latter being detectable only after approximately 3 weeks in culture. Furthermore, MAP 2 and tau protein showed a development-dependent change in expression from the juvenile toward the adult form. The comparison of these developmental changes in cytoskeletal protein levels with those observed in rat brain tissue revealed that protein expression in aggregate cultures is nearly identical to that in vivo during maturation of the neuronal cytoskeleton. Aggregate cultures deprived of glial cells, i.e., neuron-enriched cultures prepared by treating early cultures with the antimitotic drug cytosine arabinoside, exhibited pronounced deficits in M-NF, H-NF, MAP 2, MAP 1, synapsin I, and brain spectrin, with increased levels of a 145-kDa brain spectrin breakdown product. These adverse effects of glial cell deprivation could be reversed by the maintenance of neuron-enriched cultures at elevated concentrations of KCl (30 mM). This chronic treatment had to be started at an early developmental stage to be effective, a finding suggesting that sustained depolarization by KCl is able to enhance the developmental expression and maturation of the neuronal cytoskeleton.

A "calcium set-point hypothesis" of neuronal dependence on neurotrophic factor

In this commentary, we discuss evidence suggesting that cytoplasmic free calcium concentration determines neurotrophic factor dependence. Developing sympathetic and neural crest-derived sensory neurons require nerve growth factor (NGF) for survival both in vivo and in vitro. Chronic depolarization of these cells in culture causes a modest sustained elevation of cytoplasmic calcium concentration and promotes their survival in the absence of NGF. The amount of calcium increase caused by depolarization is closely correlated with the ability of the cells to survive in NGF-free medium. At an optimal calcium concentration, that we refer to as a "set point," survival is equivalent to that of cells grown in the presence of NGF.

Depolarizing stimuli regulate nerve growth factor gene expression in cultured hippocampal neurons

Although trophic factors and neuronal activity have been implicated in regulating functional synaptic circuits, the relationship of trophic interaction to impulse activity in synaptogenesis remains unclear. Using cultured hippocampus as a model system, we provide direct evidence that depolarization and impulse activity specifically increase nerve growth factor gene expression in neurons. Depolarizing stimuli, such as a high K+ concentration or the Na+ channel agonist veratridine, elicited a 3-fold increase of nerve growth factor mRNA levels in both explant and dissociated cultures. Blockade of depolarization by tetrodotoxin prevented the increase of neuronal nerve growth factor mRNA. Further, nerve growth factor gene expression was stimulated by picrotoxin, a gamma-aminobutyric acid antagonist frequently used to enhance hippocampal neuronal activity. Impulse regulation of trophic gene function may be relevant to developmental synaptogenesis and synaptic strengthening in learning and memory.

Role of voltage-sensitive calcium channels in mitogenic stimulation of neuroblasts

The present study examines the role of Ca2+ in the regulation of sympathetic neuroblast mitosis. Employing a fully defined neuroblast culture system, we previously found that insulin growth factors (IGFs), depolarization and vasoactive intestinal peptide (VIP) regulated precursor mitosis. We now report that Ca2+ entry via voltage-sensitive channels was required for depolarization-stimulated mitogenesis. Ca2+ channel blockade with nitrendipine completely inhibited the increase in [3H]thymidine incorporation elicited by depolarizing stimuli including 30 mM KCl and the Na+ channel agonist veratridine. However, Ca2+ channel activity was not involved in the stimulation of DNA synthesis by IGFs or VIP. Thus, neuroblast mitosis may be regulated by multiple intracellular as well as extracellular signals.