Electrically induced release of acetylcholine from denervated Schwann cells

Perisynaptic Schwann Cells: Guardians of Neuromuscular Junction Integrity and Function in Health and Disease

The neuromuscular junction (NMJ) is a highly reliable synapse to carry the control of the motor commands of the nervous system over the muscles. Its development, organization, and synaptic properties are highly structured and regulated to support such reliability and efficacy. Yet, the NMJ is also highly plastic, able to react to injury, and able to adapt to changes. This balance between structural stability and synaptic efficacy on one hand and structural plasticity and repair on another hand is made possible by perisynaptic Schwann cells (PSCs), glial cells at this synapse. They regulate synaptic efficacy and structural plasticity of the NMJ in a dynamic, bidirectional manner owing to their ability to decode synaptic transmission and by their interactions with trophic-related factors. Alteration of these fundamental roles of PSCs is also important in the maladapted response of NMJs in various diseases and in aging.

Functional adaptation of glial cells at neuromuscular junctions in response to injury

Synaptic elements from neuromuscular junctions (NMJs) undergo massive morphological and functional changes upon nerve injury. While morphological changes of NMJ‐associated glia in response to injury has been investigated, their functional properties remain elusive. Perisynaptic Schwann cells (PSCs), glial cells at the NMJ, are essential for NMJ maintenance and repair, and are involved in synaptic efficacy and plasticity. Importantly, these functions are regulated by PSCs ability to detect synaptic transmission through, notably, muscarinic (mAChRs) and purinergic receptors' activation. Using Ca²⁺ imaging and electrophysiological recordings of synaptic transmission at the mouse NMJ, we investigated PSC receptors activation following denervation and during reinnervation in adults and at denervated NMJs in an ALS mouse model (SOD1^G37R). We observed reduced PSCs mAChR‐mediated Ca²⁺ responses at denervated and reinnervating NMJs. Importantly, PSC phenotypes during denervation and reinnervation were distinct than the one observed during NMJ maturation. At denervated NMJs, exogenous activation of mAChRs greatly diminished galectin‐3 expression, a glial marker of phagocytosis. PSCs Ca²⁺ responses at reinnervating NMJs did not correlate with the number of innervating axons or process extensions. Interestingly, we observed an extended period of reduced PSC mAChRs activation after the injury (up to 60 days), suggesting a glial memory of injury. PSCs associated with denervated NMJs in an ALS model (SOD1^G37R mice) did not show any muscarinic adaptation, a phenotype incompatible with NMJ repair. Understanding functional mechanisms that underlie this glial response to injury may contribute to favor complete NMJ and motor recovery.

In Vitro Testing of Neurotoxicity

Many of the toxic compounds that are at large in the environment represent a risk to our neuronal functions. Chemicals may have a direct or indirect effect on the nervous system and they may interfere with general biochemical properties or specific neuronal structures and processes. In this review, a brief presentation of the major neurotoxicological targets is given, together with a discussion of some aspects of the use of different in vitro models for screening purposes and mechanistic studies. It is believed that in vitro methods offer special opportunities for the development of new neurotoxicological assays, and that this development will mainly involve cultured model systems. Therefore, a presentation of nerve and glia tissue culture methods is given, followed by an overview of how information on the action of mercury and mercurials, excitotoxins and acrylamide has been obtained through the use of cultured cell models. It is concluded that the developmental potential in cell neurotoxicology lies within the areas of separation and identification of cells representative for different structures in the nervous system, co-cultivation of different cell types, in vivo/in vitro (ex vivo) procedures, chemically defined media, metabolic competent cultures of human cells and improved physiological conditions for cultivation and exposure.

Schwann cells participate in synapse elimination at the developing neuromuscular junction

During the initial stages of innervation of developing skeletal muscles, the terminal branches of axons from multiple motor neurons form neuromuscular junctions (NMJs) on a small region of each muscle fiber, the motor endplate. Subsequently, the number of axonal inputs at the endplate region is reduced so that, at maturity, each muscle fiber is innervated by the terminals of a single motor neuron. The Schwann cells associated with the axon terminals are involved in the removal of these synapses but do not select the axon that is ultimately retained on each fiber. Schwann cells perform this function by disconnecting terminal branches from the myofiber surface and by attacking them phagocytically. Here we discuss how this behavior is regulated and argue that such regulation is not unique to development of neuromuscular innervation but is also expressed in the response of the mature NMJ to various manipulations and pathologies.

Perisynaptic Schwann Cells at the Neuromuscular Synapse: Adaptable, Multitasking Glial Cells

The neuromuscular junction (NMJ) is engineered to be a highly reliable synapse to carry the control of the motor commands of the nervous system over the muscles. Its development, organization, and synaptic properties are highly structured and regulated to support such reliability and efficacy. Yet, the NMJ is also highly plastic, able to react to injury and adapt to changes. This balance between structural stability and synaptic efficacy on one hand and structural plasticity and repair on another hand is made possible by the intricate regulation of perisynaptic Schwann cells, glial cells at this synapse. They regulate both the efficacy and structural plasticity of the NMJ in a dynamic, bidirectional manner owing to their ability to decode synaptic transmission and by their interactions via trophic-related factors.

The dynamics of recycled acetylcholine receptors at the neuromuscular junction in vivo

At the peripheral neuromuscular junction (NMJ), a significant number of nicotinic acetylcholine receptors (AChRs) recycle back into the postsynaptic membrane after internalization to intermingle with not-yet-internalized ;pre-existing' AChRs. However, the way in which these receptor pools are maintained and regulated at the NMJ in living animals remains unknown. Here, we demonstrate that recycled receptors in functional synapses are removed approximately four times faster than pre-existing receptors, and that most removed recycled receptors are replaced by new recycled ones. In denervated NMJs, the recycling of AChRs is significantly depressed and their removal rate increased, whereas direct muscle stimulation prevents their loss. Furthermore, we show that protein tyrosine phosphatase inhibitors cause the selective accumulation of recycled AChRs in the peri-synaptic membrane without affecting the pre-existing AChR pool. The inhibition of serine/threonine phosphatases, however, has no effect on AChR recycling. These data show that recycled receptors are remarkably dynamic, and suggest a potential role for tyrosine dephosphorylation in the insertion and maintenance of recycled AChRs at the postsynaptic membrane. These findings may provide insights into long-term recycling processes at less accessible synapses in the central nervous system in vivo.

Patching the glia reveals the functional organisation of the brain

The neuroglia was initially conceived by Rudolf Virchow as a non-cellular connective tissue holding neurones together. In 1894, Carl Ludwig Schleich proposed a hypothesis of fully integrated and interconnected neuronal-glial circuits as a substrate for brain function. This hypothesis received direct experimental support only hundred years later, after several physiological techniques, and most notably the patch-clamp method, were applied to glial cells. These experiments have demonstrated the existence of active and bi-directional neuronal-glial communications, integrating neuronal networks and glial syncytium into one functional circuit. The data accumulated during last 15 years prompt rethinking of the neuronal doctrine towards more inclusive concept, which regards both neurones and glia as equally responsible for information processing in the brain.

Axon branch removal at developing synapses by axosome shedding

In many parts of the developing nervous system, the number of axonal inputs to each postsynaptic cell is dramatically reduced. This synapse elimination has been extensively studied at the neuromuscular junction, but how axons are lost is unknown. Here, we combine time-lapse imaging of fluorescently labeled axons and serial electron microscopy to show that axons at neuromuscular junctions are removed by an unusual cellular mechanism. As axons disappear, they shed numerous membrane bound remnants. These "axosomes" contain a high density of synaptic organelles and are formed by engulfment of axon tips by Schwann cells. After this engulfment, the axosome's contents mix with the cytoplasm of the glial cell. Axosome shedding might underlie other forms of axon loss and may provide a pathway for interactions between axons and glia.

Characterization of neuronal nicotinic acetylcholine receptors in the membrane of unmyelinated human C-fiber axons by in vitro studies

Application of acetylcholine to peripheral nerve terminals in the skin is a widely used test in studies of human small-fiber functions. However, a detailed pharmacological profile and the subunit composition of nicotinic acetylcholine receptors in human C-fiber axons are not known. In the present study, we recorded acetylcholine-induced changes of the excitability and of the intracellular Ca2+ concentration in C-fiber axons of isolated human nerve segments. In addition, using immunohistochemistry, an antibody of a subtype of nicotinic acetylcholine receptor was tested. Acetylcholine and agonists reduced the current necessary for the generation of action potentials in C fibers by <or=30%. This increase in axonal excitability was accompanied by a rise in the free intracellular Ca2+ concentration. The following rank order of potency for agonists was found: epibatidine >> 5-Iodo-A-85380 > 1,1-dimethyl-4-phenylpiperazinium iodide > nicotine > cytisine > acetylcholine; choline had no effect. The epibatidine-induced increase in axonal excitability was blocked by mecamylamine and, less efficiently, by methyllycacontine and dihydro-beta-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the alpha5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of alpha3, alpha5, and beta4 but not of alpha4/beta2 or of alpha7 subunits in the axonal membrane of unmyelinated human C fibers. In addition, the observations suggest that the axonal membrane of C fibers in isolated segments of human sural nerve can be used as a model for presumed cholinergic chemosensitivity of axonal terminals.

Perisynaptic Schwann cells at the neuromuscular junction: nerve- and activity-dependent contributions to synaptic efficacy, plasticity, and reinnervation

Glial cells are increasingly recognized for their important contributions to CNS and PNS synaptic function. Perisynaptic Schwann cells, which are glial cells at the neuromuscular junction, have proven to be an exceptionally useful model for studying these roles. Recent studies have shown that they detect and reciprocally modulate synaptic efficacy in an activity-dependent manner in the short term. In addition, perisynaptic Schwann cells guide reinnervating nerve sprouts after deinnervation, and many important parameters of this are dependent on synapse activity. Thus, it is hypothesized that perisynaptic Schwann cells are key integrators in a continuum of synaptic efficacy, stability, and plasticity at the neuromuscular junction, which is important for maintaining and restoring synaptic efficacy.

A population of oligodendrocytes derived from multipotent neural precursor cells expresses a cholinergic phenotype in culture and responds to ciliary neurotrophic factor

Because oligodendrocytes and their precursors possess receptors for classical transmitters, and neurotransmitters such as glutamate and noradrenaline can mediate oligodendroglial proliferation and differentiation, it is possible that other neurotransmitters can also exert regulatory roles in oligodendrocyte function. We used mitogen-proliferated multipotent neuroepithelial precursors (neurospheres) and identified oligodendroglia that expressed markers traditionally found in cholinergic neurons. Regardless of culture conditions, there existed a large population of cells that resembled oligodendrocytes morphologically and coexpressed the oligodendrocyte-specific marker galactocerebroside (GalC) and the acetylcholine (ACh)-synthesizing enzyme choline acetyltransferase (ChAT). These cells did not express neuronal markers, and whole-cell recordings from cells with similar morphology displayed only outward currents in response to depolarizing voltage steps, further supporting their oligodendroglial identity. Another cholinergic marker, the vesicular ACh transporter, was also detected in GalC(+) oligodendrocytes. Furthermore, neurospheres cultured in the presence of the cholinergic receptor antagonist atropine showed a decrease in the number of GalC(+) spheres, implicating the muscarinic ACh receptor in oligodendrocyte development. The actions of neurotrophins and ciliary neurotrophic factor (CNTF) on these ChAT(+) oligodendrocytes were examined. Among these, CNTF treatment significantly increased oligodendrocytic process outgrowth. These results demonstrate classical cholinergic neuronal markers in oligodendrocytes as well as an effect of muscarinic receptor blockade on oligodendrocyte differentiation.

Axon-glia interactions modulate axonal excitability in mammalian unmyelinated nerves

A period of electrical activity in unmyelinated nerve fibers is followed by a post-tetanic hyperpolarization (PTH), generated by the hyperactivity of the electrogenic Na(+)-K(+) pump. In order to protect the membrane potential against these strong hyperpolarizations, different types of axonal inward currents are activated during the PTH. We investigated in the rabbit vagus nerve one of these currents, which was activated by carbamylcholine (CCh). We observed that the effect of CCh on the PTH amplitude could be blocked or reversed with scopolamine. Moreover, the PTH amplitude increased when scopolamine alone was added to the perfusate, indicating that an endogenous muscarinic agonist was liberated in the preparation during the period of electrical activity. This CCh-activated current was TEA but not Ba(2+) or Cs(+) sensitive. It has been shown previously that muscarinic acetylcholine receptors (mAChRs) in the rabbit vagus nerve are located on the axonal but not glial membrane and that Schwann cells express several types of purinergic receptors, which activation evoke Ca(2+) transients in Schwann cells. We hypothesise that during electrical activity axons release a transmitter, presumably ATP. This transmitter evoke in the neighbouring Schwann cells a Ca(2+)-dependent liberation of a endogenous muscarinic agonist, which in turn activates a TEA-sensitive inward current in axons. We suggest that the major purpose of this mechanism is the control of the membrane potential during and after a period of intense electrical activity when the Na(+)-K(+) pump generates a robust PTH.

ATP stimulates calcium-dependent glutamate release from cultured astrocytes

ATP caused a dose-dependent, receptor-mediated increase in the release of glutamate and aspartate from cultured astrocytes. Using calcium imaging in combination HPLC we found that the increase in intracellular calcium coincided with an increase in glutamate and aspartate release. Competitive antagonists of P(2) receptors blocked the response to ATP. The increase in intracellular calcium and release of glutamate evoked by ATP were not abolished in low Ca(2+)-EGTA saline, suggesting the involvement of intracellular calcium stores. Pre-treatment of glial cultures with an intracellular Ca(2+) chelator abolished the stimulatory effects of ATP. Thapsigargin (1 microM), an inhibitor of Ca(2+)-ATPase from the Ca(2+) pump of internal stores, significantly reduced the calcium transients and the release of aspartate and glutamate evoked by ATP. U73122 (10 microM, a phospholipase C inhibitor, attenuated the ATP-stimulatory effect on calcium transients and blocked ATP-evoked glutamate release in astrocytes. Replacement of extracellular sodium with choline failed to influence ATP-induced glutamate release. Furthermore, inhibition of the glutamate transporters p-chloromercuri-phenylsulfonic acid and Ltrans-pyrolidine-2,4-dicarboxylate failed to impair the ability of ATP to stimulate glutamate release from astrocytes. However, an anion transport inhibitor, furosemide, and a potent Cl(-) channel blocker, 5-nitro-2(3-phenylpropylamino)-benzoate, reduced ATP-induced glutamate release. These results suggest that ATP stimulates excitatory amino acid release from astrocytes via a calcium-dependent anion-transport sensitive mechanism.

Rapid and reversible effects of activity on acetylcholine receptor density at the neuromuscular junction in vivo

Quantitative fluorescence imaging was used to study the regulation of acetylcholine receptor (AChR) number and density at neuromuscular junctions in living adult mice. At fully functional synapses, AChRs have a half-life of about 14 days. However, 2 hours after neurotransmission was blocked, the half-life of the AChRs was now less than a day; the rate was 25 times faster than before. Most of the lost receptors were not quickly replaced. Direct muscle stimulation or restoration of synaptic transmission inhibited this process. AChRs that were removed from nonfunctional synapses resided for hours in the perijunctional membrane before being locally internalized. Dispersed AChRs could also reaggregate at the junction once neurotransmission was restored. The rapid and reversible alterations in AChR density at the neuromuscular junction in vivo parallel changes thought to occur in the central nervous system at synapses undergoing potentiation and depression.

Tripartite synapses: glia, the unacknowledged partner

According to the classical view of the nervous system, the numerically superior glial cells have inferior roles in that they provide an ideal environment for neuronal-cell function. However, there is a wave of new information suggesting that glia are intimately involved in the active control of neuronal activity and synaptic neurotransmission. Recent evidence shows that glia respond to neuronal activity with an elevation of their internal Ca2+ concentration, which triggers the release of chemical transmitters from glia themselves and, in turn, causes feedback regulation of neuronal activity and synaptic strength. In view of these new insights, this article suggests that perisynaptic Schwann cells and synaptically associated astrocytes should be viewed as integral modulatory elements of tripartite synapses.

Modulation of synaptic efficacy and synaptic depression by glial cells at the frog neuromuscular junction

The ability of perisynaptic glial cells to modulate transmitter release and synaptic depression was studied at the frog neuromuscular junction (nmj). Injection of GTPgammaS in perisynaptic Schwann cells (PSCs), glial cells at this synapse, induced a reduction in the amplitude of nerve-evoked synaptic responses but had no effect on the frequency, the amplitude, or the duration of the miniature endplate currents (MEPCs). Also, paired pulse facilitation was not affected. The reduction in transmitter release was mediated by pertussis toxin-(PTX) sensitive and insensitive G proteins. Blockade of G proteins in PSCs with GDPbetaS reduced synaptic depression induced by high frequency trains of stimuli, whereas activation of G proteins occluded it. Hence, the activation by endogenous neurotransmitters of G proteins in PSCs induced a profound depression in neurotransmitter release.

Recovery of acetylcholine release and choline acetyltransferase activity after freezing-induced degeneration of rat soleus muscle

In order to study the effect of synaptic contact on the amounts of choline acetyltransferase (ChAT) and acetylcholine (ACh) in the nerve terminals and on their ability to release ACh, a freeze-thaw procedure was developed as a means to induce long lasting degeneration of rat soleus muscle. It was found that 4 days after the freeze thaw procedure the preparation did not contract upon direct electric stimulation and the level of creatine kinase (CK) was below detection. The preparation contained about 15% of the ChAT activity and 15% of the ACh content of the controls. The ACh release evoked by 50 mM KCl was 25% of controls, but it was, when expressed as a fraction of the ACh content, about twice as high as that in control muscles. At day 12, the preparation still did not contract and the level of CK was less than 5% of controls. The ChAT activity and the ACh content were 40% and 20% of controls, respectively. However, no release of ACh could be evoked by 50 mM KCl. At days 28 and 58 the preparation contracted upon stimulation of the nerve; the CK activity had recovered to about 20% and the ACh content to 40%, while the ChAT activity did not increase above 40%. The KCl-evoked ACh release had recovered to 20-30% of controls. The results indicate that freezing destroyed muscle cells and most intramuscular nerve branches. Subsequent regeneration of muscle fibres was slow, probably because freezing had killed many satellite cells in the muscle. Because the ChAT activity at day 12 had recovered when CK was almost absent and the preparation failed to contract, we conclude that there was expression of ChAT activity in 'nerve terminals' which do not make contact with regenerated muscle cells, although little if any ACh was released from these sites.

ATP stimulates release of excitatory amino acids from cultured Schwann cells

The release of excitatory amino acids from Schwann cell cultures in the rat was monitored using high-performance liquid chromatography. The basal concentration of glutamate and aspartate was 33 +/- 4 nM (mean +/- S.E.M., n = 12) and 8 +/- 1 nM (mean +/- S.E.M., n = 12), respectively. ATP (100 microM) caused a receptor-mediated increase in release of glutamate and aspartate from Schwann cell cultures. Bath application of adenosine (100 microM) was without effect on release of excitatory amino acids suggesting involvement of P2 receptors. Suramin, a competitive antagonist at P2 receptors, prevented the response to ATP. The release of excitatory amino acids evoked by ATP was not abolished in calcium-depleted saline. Pretreatment of the Schwann cultures with 50 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetracetic acid-acetoxymethyl ester (BAPTA-AM) abolished the effect of ATP. ATP-evoked release of glutamate from cultured Schwann cells was significantly reduced by thapsigargin (1 microM), an inhibitor of Ca(2+)-ATPase of the Ca2+ pump of internal stores. U73122, a selective inhibitor of receptor-coupled phospholipase C-dependent processes, abolished stimulatory effect of ATP suggesting that ATP's action is mediated through an inositol 1,4,5-triphosphate-sensitive calcium store. The action of ATP was not blocked by L-trans-pyrrolidine-2,4-dicarboxylate, an inhibitor of the electrogenic glutamate transporter, nor was it blocked in Na(+)-free medium, and glutamate release was not stimulated by a depolarizing stimulus, suggesting that ATP-evoked release of glutamate from Schwann cells is not due to the reversal of the glutamate uptake. An anion transport blocker, furosemide, reduced ATP-induced glutamate release. These results suggest that ATP-stimulated glutamate and aspartate release from Schwann cells may be through a calcium-dependent furosemide-sensitive mechanism.

Muscarinic Ca2+ responses resistant to muscarinic antagonists at perisynaptic Schwann cells of the frog neuromuscular junction

1. Acetylcholine causes a rise of intracellular Ca2+ in perisynaptic Schwann cells (PSCs) of the frog neuromuscular junction. The signalling pathway was characterized using the fluorescent Ca2+ indicator fluo-3 and fluorescence microscopy. 2. Nicotinic antagonists had no effect on Ca2+ responses evoked by ACh and no Ca2+ responses were evoked with the nicotinic agonist nicotine. The muscarinic agonists muscarine and oxotremorine-M induced Ca2+ signals in PSCs. 3. Ca2+ responses remained unchanged when extracellular Ca2+ was removed, indicating that they are due to the release of Ca2+ from internal stores. Incubation with pertussis toxin did not alter the Ca2+ signals induced by muscarine, but did block depression of transmitter release induced by adenosine and prevented Ca2+ responses in PSCs induced by adenosine. 4. The general muscarinic antagonists atropine, quinuclidinyl benzilate and N-methyl-scopolamine failed to block Ca2+ responses to muscarinic agonists. Atropine (at 20,000-fold excess concentration) also failed to reduce the proportion of cells responding to a threshold muscarine concentration sufficient to cause responses in less than 50% of cells. Only the allosteric, non-specific blocker, gallamine (1-10 microM) was effective in blocking muscarine-induced Ca2+ responses. 5. In preparations denervated 7 days prior to experiments, low concentrations of atropine reversibly and completely blocked Ca2+ responses to muscarine. 6. The lack of blockade by general muscarinic antagonists in innervated, in situ preparations suggests that muscarinic Ca2+ responses at PSCs are not mediated by any of the five known muscarinic receptors or that post-translational modification prevented antagonist binding.

Activation of nicotinic acetylcholine receptors increases the rate of fusion of cultured human myoblasts

1. Fusion of myogenic cells is important for muscle growth and repair. The aim of this study was to examine the possible involvement of nicotinic acetylcholine receptors (nAChR) in the fusion process of myoblasts derived from postnatal human satellite cells. 2. Acetylcholine-activated currents (ACh currents) were characterized in pure preparations of freshly isolated satellite cells, proliferating myoblasts, myoblasts triggered to fuse and myotubes, using whole-cell and single-channel voltage clamp recordings. Also, the effect of cholinergic agonists on myoblast fusion was tested. 3. No nAChR were observed in freshly isolated satellite cells. nAChR were first observed in proliferating myoblasts, but ACh current densities increased markedly only just before fusion. At that time most mononucleated myoblasts had ACh current densities similar to those of myotubes. ACh channels had similar properties at all stages of myoblast maturation. 4. The fraction of myoblasts that did not fuse under fusion-promoting conditions had no ACh current and thus resembled freshly isolated satellite cells. 5. The rate of myoblast fusion was increased by carbachol, an effect antagonized by alpha-bungarotoxin, curare and decamethonium, but not by atropine, indicating that nAChR were involved. Even though a prolonged exposure to carbachol led to desensitization, a residual ACh current persisted after several days of exposure to the nicotinic agonist. 6. Our observations suggest that nAChR play a role in myoblast fusion and that part of this role is mediated by the flow of ions through open ACh channels.

Synthesis and release of an acetylcholine-like compound by human myoblasts and myotubes

1. Exogenously applied acetylcholine (ACh) is a modulator of human myoblast fusion. Using a chemiluminescent method, we examined whether an endogenous ACh-like compound (ACh-lc) was present in, and released by, pure human myogenic cells. 2. Single, freshly isolated satellite cells and proliferating myoblasts contained 15 and 0.5 fmol ACh-lc, respectively. Cultured myotubes contained ACh-lc as well. Also, ACh-like immunoreactivity was detected in all myogenic cells. 3. Part of the ACh-lc was synthesized by choline acetyltransferase (ChAT), as indicated by the reduction of ACh-lc content when bromoACh was present in the culture medium, and by direct measurements of ChAT activity. Also, ChAT-like immunoreactivity was observed in all myogenic cells. 4. Myoblasts and myotubes released ACh-lc spontaneously by a partially Ca(2+)-dependent mechanism. 5. The application by microperfusion of medium conditioned beforehand by myoblasts (thus presumably containing ACh-lc) onto a voltage-clamped myotube induced inward currents resembling ACh-induced currents in their kinetics, reversal potential, and sensitivity to nicotinic antagonists. 6. In vitro, the spontaneously released ACh-lc promoted myoblast fusion but only in the presence of an anticholinesterase. 7. Our observations indicate that human myogenic cells synthesize and release an ACh-lc and thereby promote the fusion process that occurs in muscle during growth or regeneration.

Synaptic regulation of glial protein expression in vivo

We investigated signaling between individual nerve terminals and perisynaptic Schwann cells, the teloglial cells that cover neuromuscular junctions. When deprived of neuronal activity in vivo, either by motor nerve transection or tetrodotoxin injection, perisynaptic Schwann cells rapidly up-regulated glial fibrillary acidic protein. Addition of transcription or translation inhibitors to excised muscles prevented this increase. Stimulation of cut nerves prevented glial fibrillary acidic protein increases even when postsynaptic nicotinic receptors were blocked, but not when neurotransmitter release was blocked with omega-conotoxin GVIA. We conclude that there is a nerve terminal to glial signal, requiring presynaptic neurotransmitter release, which regulates perisynaptic Schwann cell genes. This may be a general principle since many types of glial are sensitive to transmitters applied in vitro or released in situ.

Functional colocalization of calcium and calcium-gated potassium channels in control of transmitter release

We examined, using physiological and morphological techniques, the distribution of Ca(2+)-gated K+ (gKca) channels relative to the location of Ca2+ channels and transmitter release sites at the frog neuromuscular junction (NM). Charybdotoxin (ChTx) and iberiotoxin, blockers of gKca channels with large conductances, increase transmitter release at the frog NMJ. Intracellular Ca2+ buffers with rapid binding kinetics, dimethyl BAPTA and BAPTA, prevented the effect of ChTx, but EGTA, a Ca2+ buffer with similar affinity for Ca2+ but slower binding kinetics, did not. Dimethyl BAPTA and BAPTA, but not EGTA, caused a temporary increase in transmitter release. Labeling of gKca channels with ChTx-biotin revealed a series of bands located at the sites of Ca2+ channels, but this labeling did not occur in denervated preparations. Cross sections of NMJs revealed that gKca channels are clustered in the presynaptic membrane facing the postsynaptic membrane. We conclude that gKca channels are strategically clustered at the neurotransmitter release sites, where they can be quickly activated by Ca2+ entering the terminal.

Effects of muscarinic agonists and depolarizing agents on inositol monophosphate accumulation in the rabbit vagus nerve

The effects of muscarinic agonists and depolarizing agents on inositol phospholipid hydrolysis in the rabbit vagus nerve were assessed by the measurement of [3H]inositol monophosphate production in nerves that had been preincubated with [3H]inositol. After 1 h of drug action, carbachol, oxotremorine, and arecoline increased the inositol monophosphate accumulation, though the maximal increase induced by these agonists differed. Addition of the muscarinic antagonists atropine or pirenzepine shifted the carbachol dose-response curves to the right, without decreasing the carbachol maximal stimulatory effects. The KB for pirenzepine was 35 nM, which is characteristic of muscarinic high-affinity binding sites coupled to phosphoinositide turnover and often associated with the M1 receptor subtype. On the other hand, agents known to depolarize or to increase the intracellular Ca2+ concentration, e.g., elevated extracellular K+, ouabain, Ca2+, and the Ca2+ ionophore A23187, also increased inositol monophosphate accumulation. These effects were not mediated by the release of acetylcholine, as suggested by the fact that they could not be potentiated by the addition of physostigmine nor inhibited by the addition of atropine. The Ca(2+)-channel antagonist Cd2+, also known to inhibit the Na+/Ca2+ exchanger, was able to block the effects of K+ and ouabain, but did not alter those of carbachol. These results suggest that depolarizing agents increase inositol monophosphate accumulation in part through elevation of the intracellular Ca2+ concentration and that muscarinic receptors coupled to phosphoinositide turnover are present along the trunk of the rabbit vagus nerve.

Transmitter release increases intracellular calcium in perisynaptic Schwann cells in situ

Glial cells isolated from the nervous system are sensitive to neurotransmitters and may therefore be involved in synaptic transmission. The sensitivity of individual perisynaptic Schwann cells to activity of a single synapse was investigated, in situ, at the frog neuromuscular junction by monitoring changes in intracellular Ca2+ in the Schwann cells. Motor nerve stimulation induced an increase in intracellular Ca2+ in these Schwann cells; this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of the cotransmitters acetylcholine and ATP evoked Ca2+ responses even in the absence of extracellular Ca2+. Successive trains of nerve stimuli or applications of transmitters resulted in progressively smaller Ca2+ responses. We conclude that transmitter released during synaptic activity can evoke release of intracellular Ca2+ in perisynaptic Schwann cells. This Ca2+ signal may play a role in the maintenance or modulation of a synapse. These data show that synaptic transmission involves three cellular components with both postsynaptic and glial components responding to transmitter secretion.

Neurotransmitters as Neurotrophic Factors: a New Set of Functions

At the start of this review, factors were deemed trophic if they stimulated mitosis, permitted neural cell survival, promoted neurite sprouting and growth cone motility, or turned on a specific neuronal phenotype. The in vitro evidence from cell cultures is overwhelming that both neurotransmitters and neuropeptides can have such actions. Furthermore, the same chemical can exert several of these effects, either on the same or on different cell populations. Perhaps the most striking example is that of VIP, which can stimulate not only mitosis, but also survival and neurite sprouting of sympathetic ganglion neuroblasts (Pincus et al., 1990a,b). The in vivo data to support the in vitro experiments are starting to appear. A role for VIP in neurodevelopment is supported by in vivo studies that show behavioral deficits produced in neonatal rats by treatment with a VIP antagonist (Hill et al., 1991). The work of Shatz' laboratory (Chun et al., 1987; Ghosh et al., 1990) suggests that neuropeptide-containing neurons, transiently present, serve as guideposts for thalamocortical axons coming in to innervate specific cortical areas. Along similar lines, Wolff et al. (1979) demonstrated gamma-aminobutyric acid-accumulating glia in embryonic cortex that appeared to form axoglial synapses and suggested the possibility that gamma-aminobutyric acid released from the glia might play a role in synaptogenesis by increasing the number of postsynaptic thickenings. Meshul et al. (1987) have provided evidence that astrocytes can regulate synaptic density in the developing cerebellum. The work of Zagon and McLaughlin (1986a,b, 1987) has shown that naltrexone, an antagonist of the endogenous opioid peptides, affects both cell number and neuronal sprouting. Lauder's laboratory (Lauder et al., 1982) has shown a role for 5-HT in regulation of the proliferation of numerous cell types. These studies illustrate another important point, that neurotransmitters and neuropeptides function in communication not only between neurons, but also between neurons and glial cells, and between glial cells. Given that astrocytes can express virtually all of the neural receptors and can produce at least some of the neurotransmitters and neuropeptides, they must now be considered equal partners in the processes of intercellular communication in the nervous system, including the trophic responses. The actions of neurotransmitters and neuropeptides have to be considered in terms of a broad spectrum of actions that range from the trophic actions described in this review, to the classic transmitter actions, to potential roles in neurotoxicity.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca2+ waves in astrocytes

The glial cell is the most numerous cell type in the central nervous system and is believed to play an important role in guiding brain development and in supporting adult brain function. One type of glial cell, the astrocyte also may be an integral computational element in the brain since it undergoes neurotransmitter-triggered signalling. Here we review the role of the astrocyte in the central nervous system, emphasizing receptor-mediated Ca2+ physiology. One focus is the recent discovery that the neurotransmitter glutamate induces a variety of intracellular Ca2+ changes in astrocytes. Simple Ca2+ spikes or intracellular Ca2+ oscillations often appear spatially uniform. However, in many instances, the Ca2+ rise has a significant spatial dimension, beginning in one part of the cell it spreads through the rest of the cell in the form of a wave. With high enough agonist concentration an astrocyte syncitium supports intercellular waves which propagate from cell to cell over relatively long distances. We present results of experiments using more specific pharmacological glutamate receptor agonists. In addition to describing the intercellular Ca2+ wave we present evidence for another form of intercellular signalling. Some possible functions of a long-range glial signalling system are also discussed.

Family of human neuronal external surface epitopes defined by Torpedo monoclonal antibodies

We are employing a library of monoclonal antibodies (MAbs) that were made to Torpedo cholinergic synaptosomes to identify conserved, physiologically vital epitopes of the neuronal surface. Our particular interest is in those epitopes that are present on some but not all neurons. In the present study we screened this library on different cell lines, the neuronal cell lines PC12, NG108, MC-IXC, and SY5Y, and the endocrine cell lines GH-3 and HIT. Of these cell lines, only SY5Y cells bind MAbs that define neuronal surface subsets. Utilizing its parent cell line, SK-N-SH, we verified that six MAbs, Tor 25, Tor 103, Tor 190, Tor 201, Tor 219, and Tor 233, bind the external neuronal surface. The cytolocalization of all six MAbs is very similar: the membrane of the cell body and its processes are finely outlined in a punctate distribution. Western blot analyses of Torpedo electric organ homogenates, a highly enriched source of antigenic material, revealed that each MAb identifies multiple polypeptides, two of which have the relative mobilities of 180 kD and 67 kD. In a screen of peripheral nerves from cases of amyotrophic lateral sclerosis (ALS), we found that all these MAbs revealed surface alterations; some displayed a decrease in binding, while others displayed an increase. The combined data provide evidence that these epitopes belong to an important, complex family of polypeptides of the external neuronal surface.

Direct measurement of ACh release from exposed frog nerve terminals: constraints on interpretation of non-quantal release

1. Acetylcholine (ACh) release from enzymatically exposed frog motor nerve terminals has been measured directly with closely apposed outside-out clamped patches of Xenopus myocyte membrane, rich in ACh receptor channels. When placed close to the synaptic surface of the terminal, such a membrane patch detects both nerve-evoked patch currents (EPCs) and spontaneous quantal 'miniature' patch currents (MPCs), from a few micrometres length of the terminal, in response to ACh release from the nearest three to five active zones. 2. Chemical measurements of ACh efflux from whole preparations revealed a spontaneous release rate of 4.1 pmol (2 h)-1, and no significant difference in resting efflux between enzyme-treated and control preparations. The ratio of enzyme-treated to contralateral control muscle efflux averaged 1.17, indicating that enzyme treatment did not affect spontaneous ACh release. Vesamicol (1.7 microM), which blocks the ACh transporter in synaptic vesicles, decreased the spontaneous release of ACh to 67% of control. 3. In the absence of nerve stimulation, the frequency of single-channel openings recorded by outside-out patch probes adjacent to nerve terminals was very low (1-2 min-1), and little different at a distance of hundreds of micrometres, suggesting that if ACh was continually leaking from the terminal in a non-quantal fashion, the amount being released near active zone regions on the terminal was below the limit of detection with the patches. 4. Direct measurements of the sensitivity of the patches, coupled with calculated ACh flux rates, lead to the conclusion that the amount of ACh released non-quantally from the synaptic surface of the frog nerve terminal is less than one-tenth the amount expected if all non-quantal release is from this region of the terminal membrane. 5. Following a series of single nerve shocks or a 50 Hz train of nerve stimuli, the frequency of asynchronous single-channel openings increased for several seconds. This transient increase in channel openings was not sensitive to movement of the patch electrode a significant distance (4 microns) away from the active sites, or to manipulations previously reported to block non-quantal transmitter leakage, including addition of 10 mM-Ca2+ or 1.7 microM-vesamicol to the bath. These channel openings appear to be due to an accumulation of ACh which originated from many evoked quanta, and not the effect of locally increased non-quantal ACh release due to nerve stimulation. 6. We conclude that transmitter leakage at adult frog terminals is either localized to a source other than the synaptic surface of the nerve terminal, or released in a widespread and diffuse fashion from many sources, which may include the nerve terminal.

Ion channel expression by white matter glia: I. Type 2 astrocytes and oligodendrocytes

White matter is a compact structure consisting primarily of neuronal axons and glial cells. As in other parts of the nervous system, the function of glial cells in white matter is poorly understood. We have explored the electrophysiological properties of two types of glial cells found predominantly in white matter: type 2 astrocytes and oligodendrocytes. Whole-cells and single-channel patch-clamp techniques were used to study these cell types in postnatal rat optic nerve cultures prepared according to the procedures of Raff et al. (Nature, 303:390-396, 1983b). Type 2 astrocytes in culture exhibit a "neuronal" channel phenotype, expressing at least six distinct ion channel types. With whole-cell recording we observed three inward currents: a voltage-sensitive sodium current qualitatively similar to that found in neurons and both transient and sustained calcium currents. In addition, type 2 astrocytes had two components of outward current: a delayed potassium current which activated at 0 mV and an inactivating calcium-dependent potassium current which activated at -30 mV. Type 2 astrocytes in culture could be induced to fire single regenerative potentials in response to injections of depolarizing current. Single-channel recording demonstrated the presence of an outwardly rectifying chloride channel in both type 2 astrocytes and oligodendrocytes, but this channel could only be observed in excised patches. Oligodendrocytes expressed only one other current: an inwardly rectifying potassium current that is mediated by 30- and 120-pS channels. Because these channels preferentially conduct potassium from outside to inside the cell, and because they are open at the resting potential of the cell, they would be appropriate for removing potassium from the extracellular space; thus it is proposed that oligodendrocytes, besides myelinating axons, play an important role in potassium regulation in white matter. The conductances present in oligodendrocytes suggest a "modulated Boyle and Conway mechanism" of potassium accumulation.

Astrocytes play a role in regulation of synaptic density

Exposure of neonatal cerebellar explants to cytosine arabinoside destroys granule cells and arrests surviving glia in an early stage of maturation. Purkinje cells lack astroglial ensheathment and are hyperinnervated by sprouted Purkinje cell recurrent axon collateral terminals. Such granuloprival cultures were transplanted with optic nerve in order to supply mature glial cells. It was observed that not only were Purkinje cells almost completely ensheathed by astroglia, but there was a greater than 60% reduction in the number of somatic synapses compared to the non-transplanted granuloprival cultures. This astroglial ensheathment, which may be neuronally directed, could be the physical element provoking the reduction in the number of synapses.

Effects of denervation and direct electrical stimulation upon the post-hatching differentiation of posterior latissimus dorsi muscle in chicken

The influences of denervation and of direct electrical stimulation of denervated muscle upon the post-hatching differentiation of fibre types in the fast avian muscle posterior latissimus dorsi have been investigated. Denervation inhibits the normal decrease in number of muscle fibres exhibiting acid-stable myofibrillar ATPase activity and leads to weak oxidative activity in all the fibres. Direct stimulation at a low rhythm of denervated muscle induces the normal decrease of fibres exhibiting acid-stable myofibrillar ATPase but does not allow the occurrence of normal oxidative activity pattern. The results emphasize the role of muscular activity upon the differentiation of fibre types in a developing muscle.

Acetylcholine synthesis by adult bovine adrenal chromaffin cell cultures

Adrenal chromaffin cells normally synthesize and release catecholamines. In the present study, [3H]acetylcholine synthesis and another characteristic of cholinergic neurons, [3H]choline uptake, were studied in cultures of adult bovine adrenal chromaffin cells. Chromaffin cell cultures took up [3H]choline from the medium and acetylated the [3H]choline to form [3H]acetylcholine. The rate of [3H]acetylcholine synthesis increased after 19 days in culture and continued to increase up to 28 days in culture. [3H]Acetylcholine synthesis could be increased by stimulating the cells with a depolarizing concentration of K+. The ability for K+ to stimulate synthesis of [3H]acetylcholine developed only after 28 days in culture. [3H]Choline was taken up by the cultures through a single mechanism with a high (to intermediate) affinity for choline. [3H]Choline uptake was enhanced by Na+ omission in day-14 cultures, but was at least partially Na+-dependent in day-29 cultures. Hemicholinium-3 (IC50 less than 10 muM) inhibited [3H]choline uptake into chromaffin cell cultures. It is concluded that bovine adrenal chromaffin cells, maintained in culture, are able to exhibit cholinergic properties and this capacity is retained even by the mature adult cell.

Assays for cholinergic properties in cultured rat Schwann cells

Cultured rat Schwann cells did not contain detectable levels of choline-acetyltransferase (less than 0.5 pmol ACh min-1 mg-1 of protein) or of acetylcholinesterase and nicotinic acetylcholine receptors. After adding Schwann cells to primary rat myotube cultures, the level of cholineacetyl-transferase in the co-cultures increased after three weeks to as high as 5 pmol ACh min-1 mg-1. The activity appearing in co-cultures sedimented at approximately 4S, and was inhibited 50% by 4(1-napthylvinyl)pyridine in the concentration range of 10-50 microM. After treatment of co-cultures with anti rat neural antigen-1 (RAN-1) and complement, 70-80% of the activity was lost, suggesting that it is induced in the Schwann cells. Attempts to obtain the effect by exposure of Schwann cells to medium conditioned by the myotube cultures, or by co-culture with fibroblasts gave levels of activity at or below the limit of detection.

The influence of basal lamina on the accumulation of acetylcholine receptors at synaptic sites in regenerating muscle

If skeletal muscles are damaged in ways that spare the basal lamina sheaths of the muscle fibers, new myofibers develop within the sheaths and neuromuscular junctions form at the original synaptic sites on them. At the regenerated neuromuscular junctions, as at the original ones, the muscle fiber plasma membrane is characterized by infoldings and a high concentration of acetylcholine receptors (AChRs). The aim of this study was to determine whether or not the synaptic portion of the myofiber basal lamina sheath plays a direct role in the formation of the subsynaptic apparatus on regenerating myofibers, a question raised by the results of earlier experiments. The junctional region of the frog cutaneous pectoris muscle was crushed or frozen, which resulted in disintegration and phagocytosis of all cells at the synapse but left intact much of the myofiber basal lamina. Reinnervation was prevented. When new myofibers developed within the basal lamina sheaths, patches of AChRs and infoldings formed preferentially at sites where the myofiber membrane was apposed to the synaptic region of the sheaths. Processes from unidentified cells gradually came to lie on the presynaptic side of the basal lamina at a small fraction of the synaptic sites, but there was no discernible correlation between their presence and the effectiveness of synaptic sites in accumulating AChRs. We therefore conclude that molecules stably attached to the myofiber basal lamina at synaptic sites direct the formation of subsynaptic apparatus in regenerating myofibers. An analysis of the distribution of AChR clusters at synaptic sites indicated that they formed as a result of myofiber-basal lamina interactions that occurred at numerous places along the synaptic basal lamina, that their presence was not dependent on the formation of plasma membrane infoldings, and that the concentration of receptors within clusters could be as great as the AChR concentration at normal neuromuscular junctions.

Cellular composition of a cerebral hemisphere primary culture

In this overview attention is given to available markers and methods for characterizing cell elements in a culture system. Primary cultures from newborn rat cerebral hemispheres were grown for 14 days. The population of cells was dominated by astrocytic glial cells (60-70%), but cells with properties of macrophages, endothelial-like cells, mesenchymal-like cells, ependymal-like cells, and oligoblasts were also found. Neither mature neurons nor oligodendroglial cells were observed. The enrichment in astroglial-like cells makes the cultures a satisfactory astroglial-cell model, at least for some purposes.

Evidence that transmitter can be released from regions of the nerve cell other than presynaptic axon terminal: axonal release of acetylcholine without modulation

Release of acetylcholine from isolated preganglionic axons of sympathetic nerve trunk (cervical preganglionic sympathetic branch) of the cat was studied. In response to depolarization (KCl, 48.4 mM) acetylcholine was released into the eserinized Krebs solution. This release was shown to be dependent on extracellular Ca2+. Electrical stimulation (1 Hz) enhanced the release of acetylcholine from the isolated axonal preparation. The release by stimulation proved to be tetrodotoxin-sensitive and Ca2+-dependent. Evidence has been obtained that the acetylcholine released from sympathetic nerve trunks originates from the axon and not from Schwann cells: 5 days after section of the nerve, there was no release in response to stimulation. The release of acetylcholine from the axon is unlike that from axon terminals in that the rate of release cannot be enhanced by the inhibition of Na, K-adenosine 5'-triphosphatase (ouabain 2 X 10(-5) M) and cannot be modulated by noradrenaline (10(-6) M) or by morphine. Furthermore, although isolated nerve trunks took up [3H]choline by a hemicholinium-sensitive process, no radioactivity could be released upon electrical stimulation. It is suggested that the release of acetylcholine is not confined to axon terminals, but that it can be non-synaptically released by depolarization from axons provided Ca2+ is present.

Effect of parasympathetic denervation on acetylcholine levels in the rat parotid gland. Is there an extraneuronal pool of acetylcholine?

Parasympathetic denervation of the rat parotid gland by avulsion of the auriculotemporal nerve caused a marked and lasting decrease in gland weight. Parasympathectomy did not change the levels of choline in the gland but decreased by 60% the levels of acetylcholine (ACh) ten days after surgery and 65% at 28 days. It is puzzling that relatively high levels of ACh remained after parasympathetic denervation. The presence of additional cholinergic nerves that innervate the gland, or pass through it en route to other structures may account for some of the remaining ACh. Also, Schwann cells from denervated nerves might have contributed to some of the ACh. The existence of an extraneuronal source of ACh is considered.

Development of sympathetic innervation to proximal and distal arteries of the rat mesentery

The changes which occur during the post-natal development of sympathetic innervation to proximal and distal arteries of the rat mesentery have been examined using intracellular recording and histochemical techniques. In the youngest animals examined, single perivascular stimuli initiated slow depolarizing potentials which were not calcium-dependent. After day 4, single stimuli failed to initiate membrane potential changes in a proportion of preparations. This stage coincided with the period of extensive ramification of sympathetic nerve fibres over the surface of the arterioles. From day 9 onwards, membrane potential changes were again initiated by single stimuli in the distal arteries. These responses were distinct from those recorded from the younger animals and in many ways resembled excitatory junction potentials recorded from the arteries of mature animals. There was a gradient in the development of the innervation to the arteries of the rat mesentery, with that to the distal vessels maturing earlier than that to the more proximal vessels.

Muscle reinnervation--I. Restoration of transmitter release mechanisms

Following sciatic nerve crush the restoration of neuromuscular transmission in the extensor digitorum longus muscle of rat proceeds in a well defined manner: (a) as soon as the nerve-muscle contact is reformed, a subthreshold end-plate potential is recorded; no 'non-transmitting stage' is observed; (b) 24 hours later muscle action potentials are induced by nerve stimulation; (c) miniature end-plate potentials are absent or very rare at the newly reinnervated end-plates; their frequency returns to normal in about 4 weeks; (d) the frequency is also very much reduced in 30 mM K+ and hypertonic solutions and recovers slowly, in 4 and 5 weeks, respectively, while black widow spider venom is from the beginning as powerful as in normal neuromuscular junctions; (e) at the early stages of reinnervation the Ca2+-dependent release mechanisms are much stronger than control cases, while the Ca2+-independent mechanisms are weaker and recover in 5 weeks. The gradual reassembly and restoration of neurotransmitter release mechanisms of the extensor digitorum longus nerve terminal indicate the complexity of pre-synaptic ending organization.

Independent release of supranormal acetylcholine quanta at the rat neuromuscular junction

This electrophysiological study deals with the occurrence and with the mode of release of unusually large miniature end-plate potentials at the rat neuromuscular junction during physiological conditions. A specific limit for the normal miniature end-plate potential amplitude at each cell studied was determined after fitting the observed frequency-amplitude histogram to a Gaussian distribution. The relative abundance of giant miniature end-plate potentials was 4.15% at room temperature. The occurrence of giant miniature end-plate potentials was temperature dependent. The percentage of giant miniature end-plate potentials was 5.8% and 0.61% at 35 degrees C and at 16 degrees C, respectively. The amplitude-independence of the intervals between miniature end-plate potentials was demonstrated at room temperature as well as at 35 degrees C and at 16 degrees C. The results of this study show that giant miniature end-plate potentials are produced by acetylcholine packets which are released independently and that they are not a consequence of the synchronous release of several normal-sized quanta. Moreover, the results indicate that during physiological conditions a minor but regular proportion of the spontaneous release of acetylcholine is made up of larger packets, which produce miniature end-plate potentials of supranormal amplitude.