Analysis of conditional paralytic mutants in Drosophila sarco-endoplasmic reticulum calcium ATPase reveals novel mechanisms for regulating membrane excitability

Individual contributions made by different calcium release and sequestration mechanisms to various aspects of excitable cell physiology are incompletely understood. SERCA, a sarco-endoplasmic reticulum calcium ATPase, being the main agent for calcium uptake into the ER, plays a central role in this process. By isolation and extensive characterization of conditional mutations in the Drosophila SERCA gene, we describe novel roles of this key protein in neuromuscular physiology and enable a genetic analysis of SERCA function. At motor nerve terminals, SERCA inhibition retards calcium sequestration and reduces the amplitude of evoked excitatory junctional currents. This suggests a direct contribution of store-derived calcium in determining the quantal content of evoked release. Conditional paralysis of SERCA mutants is also marked by prolonged neural activity-driven muscle contraction, thus reflecting the phylogenetically conserved role of SERCA in terminating contraction. Further analysis of ionic currents from mutants uncovers SERCA-dependent mechanisms regulating voltage-gated calcium channels and calcium-activated potassium channels that together control muscle excitability. Finally, our identification of dominant loss-of-function mutations in SERCA indicates novel intra- and intermolecular interactions for SERCA in vivo, overlooked by current structural models.

Extension of glial processes by activation of Ca2+-permeable AMPA receptor channels

AMPA type-glutamate receptor channels (AMPARs) assembled without the GluR2 (GluR-B) subunit are characterized by high Ca2+ permeability, and are expressed abundantly in cerebellar Bergmann glial cells. Here we show that the morphology of cultured Bergmann glia-like fusiform cells derived from the rat cerebellum was changed by manipulating expression of Ca2+-permeable AMPARs using adenoviral vector-mediated gene transfer. Converting endogenous Ca2+-permeable AMPARs into Ca2+-impermeable channels by viral-mediated transfer of GluR2 gene induced retraction of glial processes. In contrast, overexpression of Ca2+-permeable AMPARs markedly elongated glial processes. The process extension was blocked by 2,3-Dihydroxy-6-nitro-7-sulfamoylbenzo(F)quinoxaline (NBQX), a specific antagonist of AMPAR. These results indicate that glutamate regulates the morphology of glial processes by activating Ca2+-permeable AMPARs.

Synaptic development is controlled in the periactive zones of Drosophila synapses

A cell-adhesion molecule fasciclin 2 (FAS2), which is required for synaptic growth and still life (SIF), an activator of RAC, were found to localize in the surrounding region of the active zone, defining the periactive zone in Drosophila neuromuscular synapses. BetaPS integrin and discs large (DLG), both involved in synaptic development, also decorated the zone. However, shibire (SHI), the Drosophila dynamin that regulates endocytosis, was found in the distinct region. Mutant analyses showed that sif genetically interacted with Fas2 in synaptic growth and that the proper localization of SIF required FAS2, suggesting that they are components in related signaling pathways that locally function in the periactive zones. We propose that neurotransmission and synaptic growth are primarily regulated in segregated subcellular spaces, active zones and periactive zones, respectively.

Tetanic stimulation recruits vesicles from reserve pool via a cAMP-mediated process in Drosophila synapses

At Drosophila neuromuscular junctions, there are two synaptic vesicle pools, namely the exo/endo cycling pool (ECP) and the reserve pool (RP). We studied the recruitment process from RP using a fluorescent dye, FMI-43. During high-frequency nerve stimulation, vesicles in RP were recruited for release, and endocytosed vesicles were incorporated into both pools, whereas with low-frequency stimulation, vesicles were incorporated into and released from ECP. Release of vesicles from RP was detected electrophysiologically after emptying vesicles in the ECP of transmitter by a H+ pump inhibitor. Recruitment from RP was depressed by inhibitors of steps in the cAMP/PKA cascade and enhanced by their activators. In rutabaga (rut) with low cAMP levels, mobilization of vesicles from RP during tetanic stimulation was depressed, while it was enhanced in dunce (dnc) with high cAMP levels.

Activation of metabotropic glutamate receptors enhances synaptic transmission at the Drosophila neuromuscular junction

We examined the effects of activation of metabotropic glutamate receptors (mGluRs) on glutamatergic synaptic transmission at the neuromuscular junction of newly hatched Drosophila larvae. In nominally Ca(2+)-free solutions puff-application of low concentrations of glutamate evoked a transient frequency increase of miniature synaptic currents (mSCs). The mean amplitude of mSCs was unaffected, suggesting that this effect was presynaptic. Similar alterations of the mSC frequency were obtained using the mGluR agonists, (S)-4C3HPG, DCG-IV, or (1S,3S)-ACPD, but not when using agonists for ionotropic glutamate receptors, NMDA, AMPA or kainate. An mGluR antagonist, MCCG-I, blocked the effect of agonists on the mSC frequency. An adenylate cyclase activator, forskolin, and a cAMP analog, CPT-cAMP, mimicked the effects of mGluR activation. Meanwhile, an adenylate cyclase inhibitor, SQ22,536, blocked the mGluR agonist-induced effects, and in rutabaga, an adenylate-cyclase-defective mutant, the effect of the agonist was greatly reduced. In the presence of external Ca2+, (S)-4C3HPG decreased the failure rate and increased the mean amplitude of stimulus-evoked SCs, while MCCG-I decreased the amplitudes. We suggest that at the larval Drosophila neuromuscular junction endogenous glutamate released at the terminal potentiates synaptic transmission via a process involving cAMP.

The optically determined size of exo/endo cycling vesicle pool correlates with the quantal content at the neuromuscular junction of Drosophila larvae

According to the current theory of synaptic transmission, the amplitude of evoked synaptic potentials correlates with the number of synaptic vesicles released at the presynaptic terminals. Synaptic vesicles in presynaptic boutons constitute two distinct pools, namely, exo/endo cycling and reserve pools (). We defined the vesicles that were endocytosed and exocytosed during high K+ stimulation as the exo/endo cycling vesicle pool. To determine the role of exo/endo cycling vesicle pool in synaptic transmission, we estimated the quantal content electrophysiologically, whereas the pool size was determined optically using fluorescent dye FM1-43. We then manipulated the size of the pool with following treatments. First, to change the state of boutons of nerve terminals, motoneuronal axons were severed. With this treatment, the size of exo/endo cycling vesicle pool decreased together with the quantal content. Second, we promoted the FM1-43 uptake using cyclosporin A, which inhibits calcineurin activities and enhances endocytosis. Cyclosporin A increased the total uptake of FM1-43, but neither the size of exo/endo cycling vesicle pool nor the quantal content changed. Third, we increased the size of exo/endo cycling vesicle pool by forskolin, which enhances synaptic transmission. The forskolin treatment increased both the size of exo/endo cycling vesicle pool and the quantal content. Thus, we found that the quantal content was closely correlated with the size of exo/endo cycling vesicle pool but not necessarily with the total uptake of FM1-43 fluorescence by boutons. The results suggest that vesicles in the exo/endo cycling pool primarily participate in evoked exocytosis of vesicles.

Two distinct pools of synaptic vesicles in single presynaptic boutons in a temperature-sensitive Drosophila mutant, shibire

In a temperature-sensitive Drosophila mutant, shibire, synaptic vesicles are completely depleted in nerve terminals after stimulation at 34 degrees C, but upon returning to 22 degrees C, endocytosis resumes. In this study, synaptic vesicles in the boutons of nerve terminals at the mutant neuromuscular junction were loaded with a fluorescent dye, FM1-43, during vesicle reformation at 22 degrees C after complete depletion at 34 degrees C. We found two distinct pools of synaptic vesicles, namely an exo/endo cycling pool, located in the periphery of the bouton, and a reserve pool, located in its center. Cytochalasin D treatment eliminated the reserve pool and reduced synaptic transmission evoked by high frequency stimulation. Thus, the reserve pool may play a crucial role for sustaining high frequency synaptic transmission.

Altered bcl-2 and bax expression and intracellular Ca2+ signaling in apoptosis of pancreatic cells and the impairment of glucose-induced insulin secretion

Apoptosis is the process of cellular self-destruction, and genes such as bcl-2 and bax are known to inhibit and promote apoptosis, respectively. In this study, we show that apoptosis can be induced in pancreatic beta-cell lines, and we investigate the apoptotic pathways through the bcl-2 and bax genes and intracellular Ca2+. Serum deprivation induces apoptosis in the MIN6 and RINm5F pancreatic beta-cell lines, and alters the bcl-2 messenger RNA (mRNA) and protein. KCl, BayK, A23187, and ionomycin elicit an elevation of cytosolic/nuclear Ca2+, which, however, is insufficient to evoke apoptosis or to alter bcl-2 or bax mRNA expression in MIN6 cells. The extracellular Ca2+ chelators, EGTA and 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, tetrapotassium salt, hydrate, evoke apoptosis and also alter the ratio of bcl-2 to bax mRNA and protein concomitantly with the depletion of cytosolic/nuclear Ca2+. This indicates that there are at least two apoptotic pathways in pancreatic beta-cells: through serum deprivation and through a decrease in cytosolic/nuclear Ca2+. MIN6 cells exhibit reduced insulin secretion induced by glucose regardless of the molecular pathway of apoptosis. Apoptosis in pancreatic beta-cells, therefore, may be closely related to the impairment of insulin secretion in certain pathological conditions such as diabetes mellitus.

Suramin inhibits glucose-induced Ca2+ response in single rat pancreatic beta cells

It is reported that glucose releases ATP with insulin from pancreatic beta cells. Suramin (50-200 microM), a purinoceptor blocker, reversibly and dose-dependently inhibited the rise in cytoplasmic calcium concentration ([Ca2+]i) induced by glucose and by ATP in single beta cells. Suramin did not inhibit the glucose-induced increase in NAD(P)H fluorescence. The rise in [Ca2+]i induced by tolbutamide (an inhibitor of ATP-sensitive K+ channels), arginine (a cationic amino acid) and acetylcholine was unaffected by suramin (50-200 microM). This suggests that suramin inhibits the glucose-induced Ca2+ response without an inhibitory effect on glucose metabolism, K+ channels, voltage-dependent Ca2+ influx or Ca2+ release from internal Ca2+ stores and that a purinergic mechanism is involved in the glucose response in beta cells.

An inhibitory role of calcineurin in endocytosis of synaptic vesicles at nerve terminals of Drosophila larvae

In this study, we tested a hypothesis that activation of calcineurin, Ca2+/calmodulin-dependent protein phosphatase 2B, is an initiating signal for synaptic vesicle endocytosis. We examined effects of calcineurin inhibitors, cyclosporin A or FK506 and calmodulin inhibitors on stimulus-induced FM1-43 uptake into nerve terminals of Drosophila larvae. Fluorescent FM1-43 labels recycling synaptic vesicles in nerve terminals. Pretreatment with cyclosporin A (5-40 microM) or with FK506 (5-10 microM) enhanced FM1-43 uptake induced by high (60 mM) K+ in a dose-dependent manner. The effect required some preincubation time of about 10 min. The nerve terminals loaded with FM1-43 were destained by electrical nerve stimulation in the cyclosporin A-pretreated preparations, confirming that FM1-43 was taken up into synaptic vesicles. Pretreatment with rapamycin (2 or 20 microM), a structural analog of FK506 which has no effect on calcineurin, or calyculin A (0.3-50 nM), an inhibitor of protein phosphatase 1 and 2A, had no detectable effect on FM1-43 uptake. On the other hand, pretreatment with trifluoperazine (1-50 microM) or with phenoxybenzamine (100 microM), inhibitors of calmodulin, enhanced FM1-43 uptake. Since endocytosis is coupled with exocytosis, it is possible that the enhancement of FM1-43 uptake results from facilitation of exocytosis. However, the frequency of spontaneous junctional potentials and the mean amplitude of evoked potentials did not change after the cyclosporin A treatment, suggesting that the exocytosis process was not significantly affected by the drug. Furthermore, we can temporally separate synaptic vesicle exocytosis and endocytosis in a Drosophila mutant, shibire (shi(ts1)). By taking advantage of this mutation, we showed that cyclosporin A and trifluoperazine enhanced synaptic vesicle recycling by directly acting on the endocytotic process. Present results are not compatible with the hypothesis, but suggest that calcineurin inhibits synaptic vesicle recycling.

PACAP/VIP receptors in pancreatic beta-cells: their roles in insulin secretion

Pituitary adenylate cyclase-activating polypeptide (PACAP) is a neuropeptide belonging to the vasoactive intestinal polypeptide (VIP)/glucagon/secretin family. We have isolated a third PACAP receptor subtype, designated PACAPR-3, by molecular cloning. The cDNA encoding PACAPR-3 has been isolated from a mouse insulin-secreting beta-cell line MIN6 cDNA library. Mouse PACAPR-3 is a protein of 437 amino acids that has 50% and 51% identity with rat PACAP type I and type II receptors, respectively. We have expressed PACAPR-3 in mammalian cells and Xenopus oocytes. PACAPR-3 binds to VIP as well as PACAP-38 and -27, with a slightly higher affinity for PACAP-38, and is positively coupled to adenylate cyclase. PACAP-38, -27, and VIP evoked Ca2+ activated-Cl- currents in Xenopus oocytes. RNA blotting studies reveal that PACAPR-3 mRNA is expressed widely in tissues and cell lines, including pancreatic islets, insulin-secreting cell lines (MIN6, HIT-T15, and RINm5F), lung, brain, stomach, colon, and heart. Furthermore, insulin secretion from the MIN6 cells is stimulated significantly by PACAP-38 and VIP. The possible mechanisms of insulin secretion by PACAP and VIP are also discussed.

A monitor of secretion from single pancreatic beta-cells

ATP is known to be coreleased with insulin from pancreatic beta-cells. To monitor insulin secretion from single beta-cells, a single beta-cell was surrounded in culture by Fura 2-loaded calf pulmonary artery endothelium (CPAE) cells, which can detect the ATP. CPAE cells did not respond with an elevation in cytoplasmic calcium concentration ([Ca2+]i) to either tolbutamide (100 mumol/l) or kainate (1 mmol/l) but did respond with an elevation in [Ca2+]i to ATP (0.1-10 mumol/l) without desensitization and in a dose-dependent manner. A brief application of tolbutamide (10 mumol/l) increased [Ca2+]i in both the beta-cell and the adjacent CPAE cells in co-culture. Suramin (100 mumol/l), an ATP-receptor blocker, inhibited the tolbutamide-induced elevation in [Ca2+]i in the CPAE cells but did not inhibit the elevation in [Ca2+]i in the beta-cell, confirming that the insulin secretagogue-induced Ca2+ response in CPAE cells in co-culture is mediated by ATP released from the beta-cell. When co-culture of the beta-cell and CPAE cells was stimulated by kainate (1 mmol/l) and then tolbutamide (10 mumol/l), the CPAE cells showed elevations in [Ca2+]i in response to kainate and tolbutamide during elevation in [Ca2+]i in the beta-cell. This strongly suggests that insulin secretion as well as an increase in [Ca2+]i in response to different agents, i.e., kainate and tolbutamide, occurs in a single beta-cell. A long exposure of tolbutamide (100 mumol/l, 4 min) resulted in a long-lasting elevation in [Ca2+]i in the beta-cell.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression and role of ionotropic glutamate receptors in pancreatic islet cells

Although the excitatory amino acid glutamate and its receptors play crucial roles in many functions of the central nervous system (CNS), their presence in the peripheral tissues has remained unclear. In the present study, we have identified kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and N-methyl-D-aspartate (NMDA) receptor subtype mRNAs in pancreatic islets, using reverse transcriptase polymerase chain reaction (RT-PCR). Intracellular calcium ([Ca2+]i) measurements and electrophysiological recordings indicate that kainate, AMPA, and NMDA all elicit increases of [Ca2+]i in single pancreatic beta-cells and depolarize them. In addition, kainate and AMPA stimulate insulin secretion from isolated pancreatic islets, whereas NMDA does not. Also, immunocytochemical study shows the presence of intense glutaminase immunoreactivity in pancreatic alpha-cells and intrapancreatic ganglia, a finding compatible with the possibility that glutamate is released from alpha-cells as well as from neurons. Because the inhibitory amino acid gamma-amino butyric acid (GABA) is present in beta-cells as well as in neurons and inhibits glucagon secretion from alpha-cells, the present study suggests that glutamate and GABA are coordinated in the regulation of hormone secretion in pancreatic islets.

Functional neuronal ionotropic glutamate receptors are expressed in the non-neuronal cell line MIN6

We report that a non-neuronal cell line, MIN6, derived from insulin-secreting pancreatic beta-cells, naturally expresses functional ionotropic glutamate receptors. Electrophysiological recordings show that kainate, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and N-methyl-D-aspartate (NMDA) depolarize single MIN6 cells and evoke inward ionic currents. These agents also increase the intracellular calcium concentration in MIN6 cells. Furthermore, insulin secretion from MIN6 cells is stimulated by kainate, AMPA, and NMDA. The presence of AMPA/kainate and NMDA receptor subtypes is confirmed by reverse transcriptase-polymerase chain reaction. These results demonstrate that ionotropic glutamate receptors with properties similar to those in neuronal cells are expressed in a non-neuronal cell line, MIN6. Thus, MIN6 provides a useful and valuable model system for biochemical, pharmacological, and physiological studies of ionotropic glutamate receptors.

Isolation of sympathonectin; a substrate-bound protein which induces preferential growth of sympathetic fibers in vitro

A protein named 'sympathonectin', was purified from chick heart cell-conditioned medium (HCM), on the basis of its ability to direct the neurite outgrowth of cultured sympathetic neurons. The most purified fraction showed a doublet band on SDS-PAGE with an apparent molecular weight of 370 kDa. The biological activity of sympathonectin was over 100 times higher than that of laminin. Immunoblotting with anti-sympathonectin of the 100,000 x g pellet (particulate) fraction of HCM identified a distinct 370 kDa band; this molecule did not react to the anti-laminin serum. The immunohistochemical analysis showed that the antibody against sympathonectin stained heart tissue but not skeletal muscle tissue, whereas anti-laminin serum stained both tissues. These results suggest that sympathonectin may play a role during sympathetic innervation of target organs.

A variant form of laminin is responsible for the neurite outgrowth-promoting activity in conditioned medium from a squamous carcinoma cell line

We examined the effects of conditioned medium (CM) obtained from a squamous carcinoma cell line, termed SAS, on chick sympathetic neuritic outgrowth. Neurons grown on a substratum coated with CM extended their neurites. Antisera raised against human laminin and mouse EHS laminin immunoprecipitated the neurite outgrowth-promoting factor in CM. The most purified fraction of CM contained a 740-kDa protein which reacted with the anti-laminin sera and was composed of three polypeptides of 330 kDa, 215 kDa and 195 kDa. The 215-kDa and 195-kDa polypeptides, but not the 330-kDa polypeptide were shown to be antigenically related to mouse laminin and human laminin by immunoblotting. The presence of merosin M chain in SAS cells was ruled out by using the method of reverse transcription polymerase chain reaction (RT-PCR). These results suggest that the 740-kDa protein is a laminin variant having a novel heavy chain of 330 kDa and is responsible for neurite outgrowth-promoting activity in CM.

Characterization of substances which promote or repel sympathetic fiber growth in vitro

To determine whether a recognition mechanism is involved in determination of sympathetic innervation patterns of various tissues, tissue-derived substances were applied to a restricted test surface region of dishes and the responses of cultured sympathetic neurites were examined. Sympathetic fibers exhibited a turning or ramifying response, resulting in a dense fiber growth on test regions coated with particulate (adheron) fractions of a conditioned-medium (CM) from expansor secundariorum, heart, peripheral blood vessel or abdominal aorta, whereas on test regions coated with those from lung, skeletal muscle or dorsal aorta the neurite growth was repelled and sparse fiber growth was observed. Particulate fractions of brain- or gizzard-CM had no effect. These patterns in vitro were in parallel with the dense sympathetic innervation in expansor secundariorum, heart, peripheral blood vessel and abdominal aorta, but little or no sympathetic innervation in lung, skeletal muscle and dorsal aorta in vivo. These results suggest that adheron particles may participate in determination of sympathetic innervation patterns. Activity which repels or promotes the sympathetic fiber growth was inactivated by pronase E or trypsin but not by DNase or neuraminidase. Repelling activity was lost after treatment with heparinase or heparitinase but not with chondroitinase ABC or hyaluronidase. Promoting activity was retained after treatment with these glycosidases. These results suggest that the factor(s) possessing a repellent effect is a heparan sulfate proteoglycan and one(s) possessing a promoting effect is a protein.

Molecules secreted from target and non-target tissues promote and repel sympathetic fiber distribution in vitro

By using an in vitro assay system detecting the ability of nerve terminals to recognize the test molecules, I found that sympathetic fibers distributed densely on substrates coated with the particulate (adheron) fractions of growth conditioned medium from expansor secundariorum (target tissue), but did not on substrates coated with those from skeletal muscle (non-target). This result suggests that adheron particles are involved in the haptotactic process of specific sympathetic innervation on the target tissue.

An in vitro method assaying recognition ability of nerve terminals for special substances

The present study provides a simple in vitro method for detecting the ability of nerve terminals to recognize special molecules. Test molecules were applied to a restricted narrow region on the surface of culture dishes, and dissociated sympathetic neurons which were embedded in a collagen gel were placed on the surface next to the region. Sympathetic fibers distributed densely on the laminin-applied region, whereas they were missing on the heparin- or hyaluronic acid-applied regions. The neurites showed no response to the fibronectin- or heparan sulfate-applied region. These results indicate that sympathetic fibers have an affinity for laminin, whereas they recognize a repellent property of heparin or hyaluronic acid.

Selective promotion of neuronal adhesion by target tissue-derived materials

Chick sympathetic nerves densely innervate expansor secundariorum muscle, but not skeletal muscle. By contrast, parasympathetic ciliary nerves innervate striated muscle, but no parasympathetic innervation occurs in expansor secundariorum muscle. The present study revealed accordingly that sympathetic ganglionic neurons from chick embryos adhered firmly to a dish precoated with expansor secundariorum muscle-conditioned medium (ECM), but not to one precoated with skeletal muscle-conditioned medium (SCM), while parasympathetic ciliary ganglionic neurons adhered to a dish precoated with SCM, but not to one precoated with ECM. These results indicate that there are certain materials which mediate specific adhesion between neurons and their target cells.

Sympathetic innervation-mediating protein

The present study was performed to elucidate the mechanisms and molecular basis of specific innervation of the peripheral tissues. Chick sympathetic nerve fibers densely innervate expansor secundariorum muscle, but not skeletal muscle. When a sympathetic ganglion was cultured in collagen gel with muscle explants, the ganglion extended neurites towards two types of muscles to the same extent. Dissociated sympathetic ganglion neurons adhered firmly to the dish precoated with materials from expansor secundariorum but not to the dish precoated with those from skeletal muscle. Sympathetic nerve fibers were found densely distributed on the substrate from expansor secundariorum but not on the substrate from skeletal muscle. These results suggest that neuronal recognition mechanisms are involved in the process of selective sympathetic innervation of the expansor secundariorum muscle of the chick. The protein which caused the dense distribution of sympathetic fibers as the substrate has been purified from heart-cell-conditioned medium. The most purified fraction showed a single band with an apparent molecular weight of 370,000 daltons on SDS-PAGE under non-reducing and reducing conditions. The biological activity of the protein was over 100 times higher than that of laminin. Antiserum was raised against the factor in heart-cell-conditioned medium that induced the dense distribution of sympathetic fibers in culture. Addition of the antiserum to the culture medium inhibited the dense distribution of sympathetic fibers on the purified protein-substrate without affecting the general growth of the fibers. Subcutaneous injection of the antiserum into the chicks inhibited the regeneration of adrenergic fibers following 6-hydroxydopamine-induced axotomy in peripheral tissues (heart, spleen, kidney and blood vessel).(ABSTRACT TRUNCATED AT 250 WORDS)

[Mechanism for acetylcholine receptor localization at nerve-muscle synapse]

This paper summarizes the hypothesis proposed to explain the mechanism for AChR localization at the neuromuscular synapse. Two theories have been proposed to explain the neuronal control of extrajunctional AChR. One theory claimed that motoneurons decreased the ACh sensitivity of the extrajunctional membrane through neurotrophic influence. However, direct electrical stimulation of denervated muscles resulted in a decrease of extrajunctional ACh sensitivity, supporting the other hypothesis that loss of extrajunctional AChR of the innervated muscle is directly related to muscle activity per se. AChR clusters (high density of AChR) at the neuro-muscular junction were supposed to result from the association of nerves with preexisting AChR clusters. However, Xenopus nerve-muscle cocultures clearly demonstrated that AChR clusters at the neuromuscular junction were formed after the nerve came in contact with the muscle membrane. Two hypothesis are proposed for nerve-induced formation of AChR clusters. Preferential insertion of AChR into the end-plate was suggested by the finding that AChR messenger RNA was more abundant near to than far from the end-plate in adult muscle fibers. On the other hand, in cultured and embryonic muscles, AChR clusters were formed at nerve-muscle junctions through receptor redistribution which was mediated by the passive diffusion-trap mechanism.

Concanavalin A prevents acetylcholine receptor redistribution in Xenopus nerve-muscle cultures

During neuromuscular junction formation ACh receptors accumulate at the nerve-contact region. It has been shown that this is at least partly due to lateral migration of existing receptors in the membrane (Anderson et al., 1977). Randomly diffusing ACh receptor molecules in the membrane may be trapped at the nerve-contact region to form a high receptor density area. If this were the major mechanism, cross-linking ACh receptors by tetravalent concanavalin A (Con A) should immobilize receptors and prevent nerve-induced receptor accumulation. We examined the effect of Con A on nerve-induced receptor accumulation and on the mobility of ACh receptors in cultured Xenopus muscle cells. ACh receptors were stained with tetramethyl rhodamine conjugated alpha-bungarotoxin. The cells were then treated briefly with Con A, and neural tube cells were added to these cultures. The mobility of ACh receptors was measured by the fluorescence photobleaching recovery method. The Con A treatment prevented rapid diffusion of ACh receptors as well as nerve-induced receptor accumulation. Functional synapse formation was not inhibited by this treatment. In contrast, divalent succinyl Con A did not affect the mobility of ACh receptors nor prevent nerve-induced ACh receptor accumulation. When the Con A concentration was varied, the blocking effect on the nerve-induced receptor accumulation changed in parallel with the mobile fraction of receptors. Newly inserted ACh receptors after the Con A treatment were found to be mobile and to accumulate at the nerve-contact region. In these cultures, new receptors accumulated around old, immobilized receptors in some areas along the nerve contact. This observation suggests that new receptors were inserted elsewhere and migrated to the nerve-contact region surrounding immobilized old ones. In addition to the accumulation of receptors, the nerve disperses preexisting receptor clusters prior to induction of high-density regions along the contact area, and, at this early stage, denervation disperses nerve-induced receptor clusters in Xenopus cultures (Kuromi and Kidokoro, 1984a, b). When cultures were treated with Con A, neither of these events occurred, suggesting that these are also diffusion-mediated.

Formation of acetylcholine receptor clusters at neuromuscular junction in Xenopus cultures

The formation of acetylcholine receptor (AChR) clusters at the neuromuscular junction was investigated by observing the sequential changes in AChR cluster distribution on cultured Xenopus muscle cells. AChRs were labeled with tetramethylrhodamine-conjugated alpha-bungarotoxin (TMR-alpha BT). Before innervation AChRs were distributed over the entire surface of muscle cells with occasional spots of high density (hot spots). When the nerve contacted the muscle cell, the large existing hot spots disappeared and small AChR clusters (less than 1 micron in diameter) initially emerged from the background along the area of nerve contact. They grew in size, increased in number, and fused to form larger clusters over a period of 1 or 2 days. Receptor clusters did not migrate as a whole as observed during "cap" formation in B lymphocytes. The rate of recruitment of AChRs at the nerve-muscle junction varied from less than 50 binding sites to 1000 sites/hr for alpha BT. In this study the diffusion-trap mechanism was tested for the nerve-induced receptor accumulation. The diffusion coefficient of diffusely distributed AChRs was measured using the fluorescence photobleaching recovery method and found to be 2.45 X 10(-10) cm2/sec at 22 degrees C. There was no significant difference in these values among the muscle cells cultured without nerve, the non-nerve-contacted muscle cells in nerve-muscle cultures, and the nerve-contacted muscle cells. It was found that the diffusion of receptors in the membrane is not rate-limiting for AChR accumulation.

Denervation disperses acetylcholine receptor clusters at the neuromuscular junction in Xenopus cultures

The effect of denervation on acetylcholine receptor (AChR) cluster distribution on cultured Xenopus muscle cells has been examined in order to study the role of intact nerve in the maintenance of clusters at the nerve-muscle junction during development. AChRs on the muscle cell were labeled with tetramethyl rhodamine-conjugated alpha-bungarotoxin and sequential changes in AChR cluster distribution were examined with a fluorescence microscope using an image intensifier. Denervation was carried out by exposing the nerve cell body to a focused laser light of a high intensity. After this procedure the neurites originating from the cell quickly disintegrated and large AChR clusters associated with nerve divided into smaller clusters. Individual clusters subsequently decreased in size and finally disappeared. In about 30% of the cases new AChR clusters appeared at the extrajunctional region after denervation. These observations indicate that intact nerves are necessary for the maintenance of receptor localization at the nerve-muscle junction and that nerve-induced accumulation is seemingly reversible during the early period of synapse formation. We tested the idea that receptor clusters were lost due to diffusion of receptors in the muscle membrane after denervation. However, the rate of receptor cluster dispersal after denervation was much slower than that predicted by the diffusion model, suggesting that diffusion of receptors is not a rate-limiting step. Furthermore, we found that receptor clusters at the junction stabilize during days in culture. Thus, 80-90% of receptor clusters at the nerve-muscle junction disappeared at 7 hr after denervation in 1-day cocultures, while about 50% of receptor clusters remained after denervation in 3-day cocultures.

Nerve disperses preexisting acetylcholine receptor clusters prior to induction of receptor accumulation in Xenopus muscle cultures

We have investigated the sequential changes of acetylcholine receptor (AChR) distribution on identified Xenopus laevis muscle cells in culture before and after innervation. AChRs on muscle cells were stained with tetramethylrhodamine-conjugated alpha-bungarotoxin and the distribution of AChR clusters was examined on a fluorescence microscope using an image intensifier. Large receptor clusters were identified on muscle cells and their fate was followed afterward. In muscle cells cultured without neural tube cells, about one-half of the identified AChR clusters survived for 2 days. In nerve-muscle cocultures, preexisting AChR clusters survived longer on non-nerve-contacted muscle cells than on muscle cells cultured without nerve. However, in nerve-contacted muscle cells the great majority of preexisting AChR clusters dispersed within 2 days. The dispersal of preexisting AChR clusters preceded receptor accumulation along the path of nerve contact by about 12-16 hr. Therefore, an accelerated dispersal of receptor clusters in innervated muscle cells is not a consequence of receptor accumulation along the nerve. The preexisting AChR clusters located near and far from the nerve contact sites dispersed along a similar time course. Protease inhibitors, trasylol and leupeptin, reduced the nerve-induced dispersal of the preexisting AChR clusters in the period before AChR accumulation at the nerve contact sites but did not do so during the period when AChRs began to accumulate at nerve-muscle contact. The significance of the dispersal of preexisting receptor clusters is discussed with regard to neuromuscular junction formation.

Papaverine-insensitive and papaverine-sensitive drug-induced contractions of chick smooth muscle

The mechanisms by which acetylcholine (ACh) and noradrenaline (NA) induce muscle contraction were examined in the chick expansor secundariorum muscle in early life, because responsiveness to ACh disappeared and responsiveness to NA persisted during development. ACh-induced contractions decreased with lowered temperature, while NA-induced contractions increased between 30 and 20 degrees C. Papaverine depressed and antagonized NA-induced contractions, but did not depress ACh-induced contractions. The second application of ACh in Ca2+-free solution produced a negligible contraction. In contrast, even the third application of NA in Ca2+-free solution still evoked a contraction of more than 50% of the response in normal solution. These results suggest that the pathway from cholinergic receptor activation to contraction is mainly mediated through a papaverine-insensitive and external Ca2+-dependent process, whereas the pathway from adrenergic receptor activation to contraction is mainly mediated through a papaverine-sensitive and external Ca2+-independent process.

Neurotrophic substance develops tetrodotoxin-sensitive action potential and increases curare-sensitivity of acetylcholine response in cultured rat myotubes

We examined the trophic effects of a partially purified trophic substance from mouse spinal cord extract on the tetrodotoxin (TTX)-sensitivity of action potentials and on acetylcholine-sensitivity of rat skeletal myotubes in 7- and 8-day-old cultures. Many myotubes grown in control medium generate action potentials in the presence of TTX (10(-6) M). The addition of fraction E (Fr.E) from a Biogel P2 column, which exhibited trophic activity on adult denervated muscle in organ culture, decreased TTX-resistivity of action potentials of myotubes in cell culture. The trophic substance was also effective when further purified by paper chromatography and electrophoresis. The response to iontophoretically applied acetylcholine of Fr.E-treated myotubes was much more reduced by D-tubocurarine (10(-7) g/ml) than those of control cultured myotubes. No difference in morphological differentiation, protein synthesis, creatine phosphokinase activity or specific binding of [125I]alpha-bungarotoxin was observed between control and Fr.E-treated cultures. These results suggest that the trophic substance in Fr.E may be involved in the normal development of TTX-sensitive sodium channels and of acetylcholine receptor properties.

Partial purification and characterization of neutrophic substance affecting tetrodotoxin sensitivity of organ-cultured mouse muscle

From mouse spinal cord homogenate, we isolated a trophic substance which reverses the post-denervation decrease in tetrodotoxin sensitivity of action potential in organ-cultured extensor digitorum longus muscle of mouse and characterized its physicochemical properties. The trophic substance was separated from macromolecules in homogenate by gel filtration on Biogel P2 column. The partially purified trophic substance was heat-stable, acid-stable and alkaline-labile. The trophic activity was destroyed by lyophilization at neutral pH but not at acidic pH. The trophic activity was abolished by incubation with pronase or leucine aminopeptidase, but not by trypsin, chymotrypsin, thermolysin or carboxypeptidase A. The trophic substance passed through an ultrafiltration membrane UM10 freely. A small part of the trophic activity passed through a UM2 or UM05, and the rest was retained on the membranes. The trophic substance adsorbed on CM-Sephadex at pH 7.2 but passed through DEAE-Sephadex at pH 8.4. These results suggest that the trophic substance regulating tetrodotoxin sensitivity of action potential in mouse skeletal muscle is a peptide with a rather low molecular weight of less than 10,000 and that while the N-terminus of the peptide is free, the C-terminus is probably blocked. This peptide differs from other trophic substances reported previously by other investigators.

Isolation and culture of motoneurons from embryonic chicken spinal cords

A method is described for isolating cholinergic alpha motoneurons from the spinal cord of chicken embryos at stage 17-18 (Hamburger and Hamilton numbering), at the time when it has been shown that motoneurons withdraw from the mitotic cycle but neurons of other types and glia are still mitotic. Fragments of the ventral half of the spinal cord are incubated for 24 hr in the presence of 10 microM 1-beta-D-arabinofuranosylcytosine in order to eliminate dividing cells and are subsequently dissociated into a suspension of single cells. The following evidence has been obtained and suggests that these cells are neuronal and appear to be alpha motoneurons: (i) they are resistant to the lethal effect of arabinofuranosylcytosine, and thus are postmitotic at stage 17-18; (ii) when grown in vitro, they exhibit morphological characteristics similar to those of ventral spinal neurons, which include the ability to be stained with silver, Nissl, methylene blue vital stain at pH 6.5-7.0, and choline acetyltransferase histochemistry; (iii) they have high choline acetyltransferase activity; (iv) they are capable of forming functional synapses with muscle.

Development of sympathetic nerve--muscle transmission in chick

The development of sympathetic nerve--muscle transmission was investigated pharmacologically by using the nerve-expansor secundariorum muscle preparation of 2- to 35-day-old chicks. The nerve-induced muscle contraction was not affected by atropine and neostigmine after hatch but was slightly depressed by phentolamine and completely depressed by guanethidine. Since the responsiveness of the muscle to some adrenomimetic drugs remained constant after hatch, adrenergic receptor function does not change during development. The nerve-induced muscle response was scarcely influenced by cocaine or reserpine in neonatal chick while it was potentiated by cocaine and changed by reserpine in mature chicks. These results suggest that cocaine- and reserpine-sensitive mechanisms gradually develop in sympathetic nerve function after hatch. The presence of a potentiating effect of cocaine on the nerve-induced muscle response in dimethylphenylpiperazinium-treated neonatal chick suggests that the development of sympathetic nerve function is regulated by nerve activity itself.

Inhibition of thalamic and hypothalamic somatosensory evoked potentials by stimulation of substantia nigra and its modification by morphine and methotrimeprazine (levomepromazine)

A brief electrical stimulation of the substantia nigra induced a marked and long lasting inhibition of the somatosensory evoked potentials recorded from the centrum medianum of the thalamus (CM) and posterior hypothalamic area (PHA) following sciatic stimulation in unanesthetized rabbits. The nigral inhibitory effect on CM was prolonged by the administration of morphine (4 mg/kg i.v.) but not influenced by that of methotrimeprazine (2-4 mg/kg i.v.). In contrast, the nigral inhibitory effect on PHA was enhanced by the injection of methotrimeprazine (2 mg/kg i.v.), but not changed by that of morphine (4 mg/kg i.v.). These results indicate that the inhibitory system originating from the substantia nigra operates on the somatosensory transmissions from the peripheral nerve to the thalamus and hypothalamus, and that morphine or methotrimeprazine in small doses induced a selective potentiation of the nigral inhibitory influence on the thalamus or hypothalamus, respectively.

Influence of sympathetic nerves on development of responsiveness of the chick smooth muscle to drugs

The role of sympathetic nerves in the development of responsiveness of smooth muscle to drugs was investigated using the expansor secondariorum muscle of 2- to 40-day-old chicks. The normal developmental decrease in the responsiveness of the muscle to acetylcholine was prevented by surgical transection of the nerve or chronic treatment with guanethidine, while it was facilitated by chronic treatment with dimethylphenylpiperazinium. Sensitivity of the muscle to noradrenaline, remaining constant during normal development, was increased by nerve section or guanethidine treatment, while it was slightly decreased by dimethylphenylpipe-azinium treatment. These results suggest that sympathetic nerves regulate the development of the responsiveness of the chick expansor secundariorum muscle to drugs, at least the developmental decrease in responsiveness to acetylcholine.

Changes in acetylcholine and noradrenaline sensitivity of chick smooth muscle wholly innervated by sympathetic nerve during development

Developmental changes in sensitivity of the isolated expansor sedumdariorum muscle of posthatching chicks to noradrenaline (NA), Acetylcholine (ACh) and some other drugs were investigated. This muscle responded to both NA and ACh in early life. The sensitivity to ACh decreased progressively with increasing age and disappeared on day 40 after hatching, however, a corresponding elevation of cholinesterase activity was not observed. The sensitivity to NA remained at the same level during the period of 2-60 days after hatching. The Contractile action of ACh on this muscle was not affected by d-tubocurarine, hexamethonium or phentolamine, but was completely abolished by atropine. These results suggest that there are at least two kinds of responsive sites on the expansor secumdariorum muscle in the new-born chick and that the sites sensitive to ACh degenerate progressively during the developmental processes. The cholinergic sensitive sites of this muscle in the new-born chick may be muscarinic.