Localization of alpha-bungarotoxin binding sites in the ciliary ganglion of the embryonic chick: an autoradiographic study at the light and electron microscopic level

Evaluating Commercially Available Antibodies for Rat α7 Nicotinic Acetylcholine Receptors

Alpha7 nicotinic acetylcholine receptors (α7 nAChRs) are important drug targets in neurological disorders and inflammation, making their detection and localization by validated antibodies highly desirable. However, tests in knockout animals raised questions about specificity of antibodies to mouse α7 nAChRs. To date, methods for validating antibodies for rat or human α7 nAChR have not been reported. We developed a gel-shift assay for western blots using GH4C1 cells expressing either native rat receptors or α7 nAChR-green fluorescent protein (GFP) chimeras to evaluate seven commercially available α7 nAChR antibodies. Blots with anti-GFP antibody detected GFP or α7 nAChR-GFP expressed in GH4C1 cells, and ¹²⁵I-α-bungarotoxin binding and RNA analysis demonstrated α7 nAChR expression. Validated samples were used to evaluate α7 nAChR antibodies by western blot and immunofluorescence studies. These methods confirmed that two of seven α7 nAChR antibodies identify gel-shifts for α7 nAChR/nAChR-GFP but only one antibody demonstrated low background and significant immunofluorescence differences between wild-type and α7 nAChR expressing GH4C1 cells. However, that polyclonal antibody displayed lot-to-lot variability. Our findings suggest that careful validation methods are required for all α7 nAChR receptor species and antibody lots and that the gel-shift assay may allow for relatively rapid antibody screening.

Fast synaptic transmission in the goldfish CNS mediated by multiple nicotinic receptors

Usually nicotinic receptors in the central nervous system only influence the strength of a signal between neurons. At a few critical connections, for instance some of those involved in the flight response, nicotinic receptors not only modulate the signal, they actually determine whether a signal is conveyed or not. We show at one of the few such connections accessible for study, up to three different nicotinic receptor subtypes mediate the signal. The subtypes appear to be clustered in separate locations. Depending on the number and combination of the subtypes present the signal can range from short to long duration and from low to high amplitude. This provides a critical connection with a built-in plasticity and may enable it to adapt to a changing environment.

Nicotinic receptors concentrated in the subsynaptic membrane do not contribute significantly to synaptic currents at an embryonic synapse in the chicken ciliary ganglion

Rapid synaptic transmission at the calyciform synapse in the embryonic chicken ciliary ganglion is mediated by two classes of nicotinic receptors: those containing alpha3 subunits [alpha3-nicotinic ACh receptors (nAChRs)] and those containing alpha7 subunits (alpha7-nAChRs). alpha3-nAChRs and alpha7-nAChRs are differentially distributed on the cell surface; alpha3-nAChRs are concentrated at postsynaptic densities, whereas both alpha7-nAChRs and alpha3-nAChRs are found extrasynaptically on somatic spines. I explored the contribution of alpha3-nAChRs and alpha7-nAChRs to uniquantal responses, measured as mEPSCs, or as evoked responses under low release probability conditions. The contribution that each nAChR makes to uniquantal response shape was determined by blocking one nAChR type; pharmacologically isolated alpha7-nAChR responses were kinetically fast (rise time, 0.32 +/- 0.02 ms; decay time, 1.66 +/- 0.18 ms; mean +/- SD; n = 6 cells), whereas pharmacologically isolated alpha3-nAChR responses were slow (rise time, 1.28 +/- 0.35 ms; decay time, 6.71 +/- 1.46 ms; n = 8 cells). In the absence of antagonists, most cells (11 of 14) showed heterogeneity in the kinetics of uniquantal responses, with approximately 25% of events exhibiting fast, alpha7-nAChR-like kinetics and approximately 75% of events exhibiting the kinetics expected of coactivation of alpha7-nAChRs and alpha3-nAChRs. Cells rarely showed significant numbers of uniquantal responses with slow, alpha3-nAChR-like kinetics, which was unexpected given that alpha3-nAChRs alone are concentrated at postsynaptic densities. The only site where ACh quanta can activate both alpha3-nAChRs and alpha7-nAChRs readily is on the somatic spines, where alpha7-nAChRs and alpha3-nAChRs are present extrasynaptically. At the calyciform synapse, rapid synaptic transmission is mediated apparently without participation of ionotropic receptors concentrated at postsynaptic densities.

Exocytosis unbound

The accepted theory of vesicular release of neurotransmitter posits that only a single vesicle per synapse can fuse with the membrane following action potential invasion, and this exocytotic event is limited to the ultrastructurally defined presynaptic active zone. Neither of these dictums is universally true. At certain synapses, more than a single vesicle can be released per action potential, and there is growing evidence that neuronal exocytosis can occur from sites that are unremarkable in electron micrographs. The first discrepancy extends the dynamic range of synapses, whereas the second enables faster and more robust chemical transmission at sites distant from morphologically defined synapses. Taken together, these attributes expand the capabilities of cellular communication in the nervous system.

Evidence for ectopic neurotransmission at a neuronal synapse

Neurotransmitter release is well known to occur at specialized synaptic regions that include presynaptic active zones and postsynaptic densities. At cholinergic synapses in the chick ciliary ganglion, however, membrane formations and physiological measurements suggest that release distant from postsynaptic densities can activate the predominantly extrasynaptic alpha7 nicotinic receptor subtype. We explored such ectopic neurotransmission with a novel model synapse that combines Monte Carlo simulations with high-resolution serial electron microscopic tomography. Simulated synaptic activity is consistent with experimental recordings of miniature excitatory postsynaptic currents only when ectopic transmission is included in the model, broadening the possibilities for mechanisms of neuronal communication.

Hidden function of neuronal nicotinic acetylcholine receptor β2 subunits in ganglionic transmission: comparison to α5 and β4 subunits

Neuronal nicotinic acetylcholine receptors (nAChR), which modulate fast excitatory postsynaptic potentials (f-EPSP), are located on both pre- and postganglionic sites in the autonomic nervous system (ANS). The receptor subunits alpha3, alpha5, alpha7, beta2 and beta4 are present in autonomic ganglia in various combinations and modulate acetylcholine (ACh) transmission. In the present study, autonomic functions were systemically examined in mice lacking beta2 subunits (beta2-/-) to further understand the functional role of beta2 subunits in modulating ganglionic transmission. The results show normal autonomic functions, both under physiological conditions and in perturbed conditions, on thermoregulation, pupillary size, heart rate responses and ileal contractile reactions. This suggests that the function of beta2-containing receptors in ganglionic transmission is hidden by the predominant beta4 containing receptors and confirms previous studies which suggest that alpha3alpha5beta4 nAChRs are sufficient for autonomic transmission. On the other hand, beta2-containing receptors have only a minor function on postsynaptic responses to ACh, but may modulate ACh release presynaptically, although there is no evidence for this.

Nicotinic acetylcholine receptors: from structure to brain function

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels and can be divided into two groups: muscle receptors, which are found at the skeletal neuromuscular junction where they mediate neuromuscular transmission, and neuronal receptors, which are found throughout the peripheral and central nervous system where they are involved in fast synaptic transmission. nAChRs are pentameric structures that are made up of combinations of individual subunits. Twelve neuronal nAChR subunits have been described, alpha2-alpha10 and beta2-beta4; these are differentially expressed throughout the nervous system and combine to form nAChRs with a wide range of physiological and pharmacological profiles. The nAChR has been proposed as a model of an allosteric protein in which effects arising from the binding of a ligand to a site on the protein can lead to changes in another part of the molecule. A great deal is known about the structure of the pentameric receptor. The extracellular domain contains binding sites for numerous ligands, which alter receptor behavior through allosteric mechanisms. Functional studies have revealed that nAChRs contribute to the control of resting membrane potential, modulation of synaptic transmission and mediation of fast excitatory transmission. To date, ten genes have been identified in the human genome coding for the nAChRs. nAChRs have been demonstrated to be involved in cognitive processes such as learning and memory and control of movement in normal subjects. Recent data from knockout animals has extended the understanding of nAChR function. Dysfunction of nAChR has been linked to a number of human diseases such as schizophrenia, Alzheimer's and Parkinson's diseases. nAChRs also play a significant role in nicotine addiction, which is a major public health concern. A genetically transmissible epilepsy, ADNFLE, has been associated with specific mutations in the gene coding for the alpha4 or beta2 subunits, which leads to altered receptor properties.

Nicotinic alpha 7 receptors: synaptic options and downstream signaling in neurons

Nicotinic receptors are cation-ion selective ligand-gated ion channels that are expressed throughout the nervous system. Most have significant calcium permeabilities, enabling them to regulate calcium-dependent events. One of the most abundant is a species composed of the alpha 7 gene product and having a relative calcium permeability equivalent to that of NMDA receptors. The alpha 7-containing receptors can be found presynaptically where they modulate transmitter release, and postsynaptically where they generate excitatory responses. They can also be found in perisynaptic locations where they modulate other inputs to the neuron and can activate a variety of downstream signaling pathways. The effects the receptors produce depend critically on the sites at which they are clustered. Instructive preparations for examining alpha 7-containing receptors are the rat hippocampus, where they are thought to play a modulatory role, and the chick ciliary ganglion, where they participate in throughput transmission as well as regulatory signaling. Relatively high levels of alpha 7-containing receptors are found in the two preparations, and the receptors display a variety of synaptic options and functions in the two cases. Progress is starting to be made in understanding the mechanisms responsible for localizing the receptors at specific sites and in identifying components tethered in the vicinity of the receptors that may facilitate signal transduction and downstream signaling.

Regulatory mechanisms that govern nicotinic synapse formation in neurons

Individual cholinoceptive neurons express high levels of different neuronal nicotinic acetylcholine receptor (nAChR) subtypes, and target them to the appropriate synaptic regions for proper function. This review focuses on the intercellular and intracellular processes that regulate nAChR expression in vertebrate peripheral nervous system (PNS) and central nervous system (CNS) neurons. Specifically, we discuss the cellular and molecular mechanisms that govern the induction and maintenance of nAChR expression-innervation, target tissue interactions, soluble factors, and activity. We define the regulatory principles of interneuronal nicotinic synapse differentiation that have emerged from these studies. We also discuss the molecular players that target nAChRs to the surface membrane and the interneuronal synapse.

Extrasynaptic alpha 7-nicotinic acetylcholine receptor expression in developing neurons is regulated by inputs, targets, and activity

Alpha7-nicotinic acetylcholine receptors (nAChRs) are widely expressed in the vertebrate nervous system. alpha7-nAChR functions include postsynaptic transmission, modulating neurotransmitter release, reinforcing nicotine addiction, and a role in neurological disorders, such as schizophrenia and Alzheimer's disease. In chick parasympathetic ciliary ganglion (CG) neurons, alpha7-nAChRs are excluded from the synapse and localize perisynaptically. Despite their extrasynaptic distribution, the highly Ca2+-permeable alpha7-nAChRs have important synapse-related Ca2+-dependent signaling functions in the CG. We show here that the synaptic partners regulate alpha7-nAChR expression during synapse formation in embryonic CG neurons in situ. The absence of inputs and target tissues cause reductions in alpha7-nAChR mRNA and protein levels that primarily resemble those seen for synaptic alpha3-nAChRs. However, there is a difference in their regulation. alpha7-nAChR levels are downregulated by reduced activity, whereas alpha3-nAChR levels are not. We propose that the activity-dependent regulation of extrasynaptic alpha7-nAChR levels may be an important mechanism for postsynaptic CG neurons to detect changes in presynaptic activity levels and respond with Ca2+-dependent plasticity changes in gene expression.

Synaptic vesicle recycling at two classes of release sites in giant nerve terminals of the embryonic chicken ciliary ganglion

Rapid synaptic transmission in the embryonic chicken ciliary ganglion occurs through the activation of two distinct classes of nicotinic acetylcholine receptors (AChRs): those containing alpha3 subunits (alpha 3*-AChRs) and those containing alpha7 subunits (alpha 7*-AChRs). alpha3*-AChRs are found on ciliary neurons in clusters at synaptic sites on the cell body, whereas alpha7* -AChRs are found on somatic spines, which historically were thought not to have release sites in the embryo. However, Shoop et al. (Shoop et al. [1999] J. Neurosci. 19:692-704) recently described release sites having pre- and postsynaptic densities on somatic spines. We used transmission electron microscopy to compare the structure of synaptic sites on spines with those on the smooth surfaced part of the cell. We find that the two populations of sites are similar in active zone length, number of vesicles, and distance between vesicles and active zone. To study the functional properties of these sites, we examined their stimulation-dependent uptake and release of the extracellular tracer horseradish peroxidase (HRP). We found that each class of release sites both took up and released HRP in a stimulation- and calcium-dependent manner. The mean fraction of synaptic vesicles labeled with tracer was similar for the two populations, both after loading ( approximately 45%) and after unloading ( approximately 7%). Thus we detect no differences between these two anatomically distinct classes of release sites, other than their incidence: sites on spines occurred only 12% as often as those on the cell body. The release sites on somatic spines presumably underlie synaptic responses attributable to alpha7*-AChRs.

Ultrastructure of a somatic spine mat for nicotinic signaling in neurons

Chick ciliary neurons have somatic spines grouped in discrete clumps or mats tightly folded against the soma and enriched in nicotinic receptors containing alpha7 subunits. An embryonic ciliary neuron has one to two dozen such spine mats, all overlaid by a large presynaptic calyx engulfing the cell. Three-dimensional tomographic reconstruction from serial thick sections revealed 13 somatic spines in one complete spine mat on a ciliary neuron late in embryogenesis. The spines varied in morphology and usually were branched but had numerous similarities to dendritic spines, including mean length, volume, surface area, presence of endoplasmic reticulum, and occasional multivesicular bodies. The spines invariably were connected to the soma via a narrow neck of approximately 0.2 micrometer in diameter as found for dendritic spines, suggesting restricted access from spine lumen to soma. A prominent difference between dendritic and somatic spines is the absence of postsynaptic densities from most somatic spines both on embryonic and adult ciliary neurons. Transmitter access to receptors on the spines may occur either by lateral diffusion from release sites over nearby postsynaptic densities or by release directly onto spines from the overlying calyx lined with vesicles. The latter is less likely in the adult, where some spines are adjacent to but not overlaid by vesicle-enriched presynaptic structures. The anatomical configuration of spine mats suggests coordinate spine activation by transmitter release into a confined volume while spine morphology is used to control the chemical consequences of synaptic signaling.

Nicotinic acetylcholine receptor subunit proteins alpha7 and beta4 decrease in the superior cervical ganglion after axotomy

Synaptic transmission in the superior cervical ganglion (SCG) is mediated by nicotinic acetylcholine receptors (nAChR). After transection of the postganglionic nerves of the SCG in the adult rat, the transcript levels of four of the five nAChR subunits present in the ganglion, alpha3, alpha5, alpha7, and beta4, decrease dramatically. In the present study, the effect of axotomy on nAChR subunit expression was examined at the protein level, focusing on the alpha7 and beta4 subunits. Immunohistochemistry with monoclonal antibody mAb306 (for the alpha7 subunit) and polyclonal antibody 4886 (for the beta4 subunit) showed that immunoreactivities for both alpha7 and beta4 subunits were concentrated in neurons in the intact ganglion. Results from double staining with antibodies to these subunits and to tyrosine hydroxylase, the enzyme that catalyzes the rate-limiting step in the biosynthesis of the sympathetic neurotransmitter norepinephrine, demonstrated that most neurons in the SCG express both the alpha7 and beta4 subunits. Three days after axotomy, the number of immunolabeled neurons and the intensity of the immunostaining per labeled neuron were decreased for both subunits. Decreases in subunit levels were also observed by Western blot analysis. Observing changes in these subunits over time after surgery revealed that, while the protein level of the alpha7 subunit recovered substantially within 2 weeks after the lesion, that of the beta4 subunit stayed low. These data demonstrate that decreases in nicotinic receptor subunits are among the changes in proteins that occur in axotomized sympathetic neurons, and suggest that these decreases may contribute to the depression in ganglionic synaptic transmission observed in axotomized ganglia.

Functional nicotinic acetylcholine receptors that mediate ganglionic transmission in cardiac parasympathetic neurons

Nicotinic acetylcholine receptors (nAChRs) mediate ganglionic transmission in the peripheral autonomic nervous system in mammals. Functional neuronal nAChRs have been shown to assemble from a combination of alpha and beta subunits, including alpha3, alpha5, alpha7, beta2, and beta4 in RNA-injected oocytes, but the subunit composition of functional neuronal nAChRs in vivo in mammals remains unknown. We examined the subunit composition of functional nAChRs in the intracardiac parasympathetic ganglion in a physiologically intact system in vivo. We report here that localized perfusion of the canine intracardiac ganglion in situ with an antagonist specific for nAChRs containing an alpha3/beta2 subunit interface (alpha-conotoxin MII 100-200 nm) resulted in reversible attenuation of the sinus cycle length (SCL) response by approximately 70% to electrical stimulation of the preganglionic vagus nerve. Perfusion with antagonist specific for receptors containing an alpha3/beta4 subunit interface (alpha-conotoxin AuIB 1 micrometer) resulted in attenuation in SCL responses (approximately 20%) compared with baseline when applied by itself, but not in animals pretreated with alpha-conotoxin MII. Perfusion of the ganglion with alpha-bungarotoxin (1 micrometer, which blocks alpha7 receptors) caused a reduction in SCL response by approximately 30% compared with baseline when perfused on its own and when added after blockade with MII and AuIB. Perfusion with hexamethonium bromide resulted in complete blockade of ganglionic transmission, confirming total perfusion of the ganglion and the nicotinic nature of ganglionic transmission at this synapse. Immunohistochemistry using monoclonal antibodies against specific nicotinic subunits confirmed the presence of alpha3, alpha7, beta2, and beta4 subunits. We conclude that functional ganglionic transmission in the canine intracardiac ganglion is mediated primarily by receptors containing an alpha3/beta2 subunit interface, with a smaller contribution by receptors containing alpha7 nAChRs. Despite the presence of beta4 subunits in functional channels, a contribution of a distinct alpha3/beta4 receptor population that does not include an alpha3/beta2 subunit interface was less clear.

Cytoskeletal links of neuronal acetylcholine receptors containing alpha 7 subunits

Nicotinic acetylcholine receptors serve a variety of signaling functions in the nervous system depending on cellular location, but little is known about mechanisms responsible for tethering them at specific sites. Among the most interesting are receptors containing the alpha7 gene product, because of their abundance and high relative permeability to calcium. On chick ciliary ganglion neurons alpha7-containing receptors are highly concentrated on somatic spines folded into discrete patches on the cell. We show that the spines contain filamentous actin and drebrin. After cell dissociation, the actin slowly redistributes, the spines retract, and the alpha7-containing receptors disperse and are subsequently lost from the surface. Latrunculin A, a drug that depolymerizes filamentous actin, accelerates receptor dispersal, whereas jasplikinolide, a drug that stabilizes the actin cytoskeleton, preserves large receptor clusters and prevents receptor loss from the surface. The receptors are resistant to extraction by nonionic detergent even after latrunculin A treatment. Other, less abundant, nicotinic receptors on the neurons are readily solubilized by the detergent even though these receptors are located in part on the spines. The results demonstrate that the actin cytoskeleton is important for retaining receptor-rich spines and indicate that additional cytoskeletal elements or molecular interactions specific for alpha7-containing receptors influence their fate in the membrane. The cytoskeletal elements involved are not dependent on the architecture of the postsynaptic density because alpha7-containing receptors are excluded from such sites on ciliary ganglion neurons.

Receptor targeting and heterogeneity at interneuronal nicotinic cholinergic synapses in vivo

Within a single neuron the correct targeting of the diverse neurotransmitter receptor types to discrete synaptic regions is crucial for proper function. However, the molecular mechanisms that underlie neuronal receptor clustering and targeting are still largely undefined. Here we report advances in defining the mechanisms that mediate nicotinic acetylcholine receptor (nAChR) targeting to interneuronal synapses. Recent in vivo studies have demonstrated that one subunit plays a critical role in the differentiation of nicotinic cholinergic synapses on vertebrate autonomic neurons. The major cytoplasmic loop of the alpha3 subunit targets specific nAChR subtypes to the synapse. In contrast, nAChR complexes that lack the alpha3 targeting domain are excluded and are perisynaptic. Additional studies have demonstrated a greater complexity to alpha3-nAChR targeting due to a unique postsynaptic receptor microheterogeneity - under one presynaptic terminal, alpha3-nAChR clusters are separate, but proximal to, glycine receptor (GlyR) clusters in discrete postsynaptic membrane microregions. The surprising coexistence under one nerve ending of separate clusters of receptors that respond to different fast-acting transmitters with opposing functions may represent a novel mechanism for modulating synaptic activity. Overall, the receptor targeting and clustering studies reviewed in this issue suggest that a common mechanism underlies the formation of the diverse types of interneuronal synapses but differs from that responsible for neuromuscular junction assembly in vertebrates.

Neuronal nicotinic receptors in non-neuronal cells: new mediators of tobacco toxicity?

The nicotinic acetylcholine receptors are prototypic ionotropic receptors that mediate fast synaptic transmission. However, also non-excitable cells, and particularly the tegumental cells that line external and internal body surfaces, express acetylcholine receptors of neuronal type sensitive to nicotine. Bronchial epithelial cells, endothelial cells of blood vessels and skin keratinocytes express neuronal nicotinic receptors composed of alpha(3), alpha(5), beta(2) and beta(4) subunits, similar to those expressed in sympathetic ganglia, and neuronal nicotinic receptors composed of alpha(7) subunits. Neuronal nicotinic receptors in tegumental cells are involved in modulating cell shape and motility, and therefore in maintaining the integrity of the surfaces lined by those cells. Neuronal nicotinic receptors in non-neuronal tissues may modulate other functions, including cell proliferation and differentiation. Acetylcholine is synthesized, secreted and degraded by a variety of cells, including the tegumental cells that express neuronal nicotinic receptors. Thus, acetylcholine may function as a local "hormone" that is able to modulate cell functions that require fast adaptation to new conditions. The presence of neuronal nicotinic receptors sensitive to nicotine in tissues known to be involved in tobacco toxicity, like bronchi and blood vessels, raises the possibility that they mediate some of the toxic effects of smoking.

Rapsyn variants in ciliary ganglia and their possible effects on clustering of nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) containing the alpha7 gene product can influence a range of cellular events in neurons, depending on receptor location. On chick ciliary neurons, the receptors are concentrated on somatic spines, but little is known about mechanisms responsible for sequestering them there. Rapsyn is a 43-kDa protein essential for clustering nicotinic receptors at the vertebrate neuromuscular junction. RT-PCR confirmed previous studies showing that the chick ciliary ganglion expresses rapsyn transcripts, including several splice variants lacking part or all of exon 2. Heterologous expression of rapsyn constructs, together with nicotinic receptor constructs, shows that chicken full-length rapsyn can induce clustering of both muscle and neuronal nicotinic receptors. Splice variants lacking one or both leucine zipper motifs of exon 2 are unable to cluster the receptors, though, like full-length rapsyn, they cluster themselves. Immunological analysis demonstrates the presence of full-length rapsyn in chick muscle extracts but fails to detect either full-length or splice-variant versions of rapsyn at significant levels in ganglion extracts. The results suggest that rapsyn does not cluster alpha7-nAChRs on ciliary neurons in any way similar to that of receptors at the neuromuscular junction where rapsyn and the receptors are present in approximately equimolar amounts.

Extracellular calcium regulates responses of both alpha3- and alpha7-containing nicotinic receptors on chick ciliary ganglion neurons

Neuronal nicotinic receptors are generally both permeable to calcium and potentiated by it. We have examined acute calcium regulation of both native alpha7-containing and the less abundant alpha3-containing nicotinic receptors on chick ciliary ganglion neurons. Most of the receptors are concentrated on somatic spines tightly overlaid in situ by a large presynaptic calyx. Whole cell patch-clamp recording from dissociated neurons using perforated patch-clamp techniques indicates that the rapidly desensitizing nicotinic response of alpha7-containing receptors achieves maximum amplitude in 2 mM calcium; both lower and higher concentrations of calcium are less effective. Barium and strontium but not magnesium can substitute for calcium in potentiating the response. Neither calcium current through the receptors nor calcium action at intracellular sites is necessary. These latter conclusions are supported by current-voltage analysis of the nicotine-induced response, ion substitution experiments, and internal perfusion of the cells with 1,2-bis-(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) via a conventional patch pipette. Varying the agonist concentration indicates that some of the calcium-dependent enhancement may involve a shift in the dose-response curve for agonist binding, but much of the effect is also likely to involve increased receptor responsiveness. Blockade of alpha7-containing receptors with alpha-bungarotoxin showed that the heteromeric alpha3-containing nicotinic receptors also undergo calcium-dependent potentiation. Calcium did not have a major effect on the desensitization rate of either receptor class but did have a selective effect on the rise time of alpha7-containing receptors. Analysis of stably transfected cells expressing an alpha7 gene construct showed that the calcium potentiation observed for native receptors did not require neuron-specific modifications or components and that it could be seen with the natural agonist acetylcholine. Receptor dependence on extracellular calcium may provide a regulatory mechanism for constraining synaptic signaling, avoiding local depletion of external calcium, and limiting calcium buildup in postsynaptic compartments.

Neuronal acetylcholine receptors with alpha7 subunits are concentrated on somatic spines for synaptic signaling in embryonic chick ciliary ganglia

Nicotinic acetylcholine receptors containing alpha7 subunits are widely distributed in the vertebrate nervous system. In the chick ciliary ganglion such receptors generate large synaptic currents but appear to be excluded from postsynaptic densities on the cells. We show here that alpha7-containing receptors are concentrated on somatic spines in close proximity to putative sites of presynaptic transmitter release. Intermediate voltage electron microscopy on thick sections, together with tomographic reconstruction, permitted three-dimensional analysis of finger-like projections emanating from cell bodies. The projections were identified as spines based on their morphology, cytoskeletal content, and proximity to presynaptic elements. Both in situ and after ganglionic dissociation, the spines were grouped on the cell surface and tightly folded into mats. Immunogold labeling of receptors containing alpha7 subunits showed them to be preferentially concentrated on the somatic spines. Postsynaptic densities were present in vivo both on the soma near spines and occasionally on the spines themselves. Synaptic vesicle-filled projections from the presynaptic calyx were interdigitated among the spines. Moreover, the synaptic vesicles often abutted the membrane and sometimes included Omega profiles as if caught in an exocytotic event, even when no postsynaptic densities were juxtaposed on the spine. The results suggest several mechanisms for delivering transmitter to alpha7-containing receptors, and they support new ideas about synaptic signaling via spines. They also indicate that neurons must have specific mechanisms for targeting alpha7-containing receptors to desired locations.

Developmental changes in the nicotinic responses of ciliary ganglion neurons

The accumulation of functional neurotransmitter receptors by neurons during development is an essential part of synapse formation. Chick ciliary ganglion neurons express two kinds of nicotinic receptors. One is abundant, contains the alpha7 gene product, rapidly desensitizes, and binds alpha-bungarotoxin. The other is less abundant, contains multiple gene products (alpha3, beta4, alpha5, and beta2 subunits), slowly desensitizes, and binds the monoclonal antibody mAb 35. Rapid application of agonist to freshly dissociated neurons elicits responses from both classes of receptors. Between embryonic days 8 and 15, the whole cell response of alpha3-containing receptors increases fivefold in peak amplitude and, normalized for cell growth, 1.7-fold in current density. In addition, the response decays more slowly in older neurons, suggesting a developmental decrease in the rate of desensitization. The whole cell response of alpha7-containing receptors increases 10-fold in peak amplitude over the same period and 3-fold in current density. No change in the rate of desensitization was apparent for alpha7-containing receptors with developmental age, but analysis was limited by overlap in responses from the two kinds of receptors. Indirect immunofluorescence measurements on dissociated neurons showed that the relative levels of alpha7-containing receptors on the soma increased during development to the same extent as the whole cell response attributed to them. In contrast, the relative levels of alpha3-containing receptors increased more during the same time period than did the whole cell response they generated. The immunofluorescence analysis also showed that both classes of receptors become distributed in prominent clusters on the cell surface as a function of developmental age. The results indicate that during this period of synaptic consolidation on the neurons, the two major classes of functional nicotinic receptors undergo substantial upregulation; alpha3-containing receptors as a class may undergo changes in receptor properties as well.

Presence of nicotinic acetylcholine receptors in cat carotid body afferent system

With immunocytochemical techniques using a monoclonal antibody for alpha7 subunits of neuronal nicotinic acetylcholine receptors, we have found these subunits to be exclusively expressed in nerve fibers in the carotid body. Double-immunostaining showed that alpha7 subunit-positive nerve endings enveloped tyrosine hydroxylase-positive glomus cells. Some carotid sinus nerve fibers and tyrosine hydroxylase-positive petrosal ganglion neurons also expressed alpha7 subunits. These data support a role for acetylcholine in carotid body neurotransmission.

The long internal loop of the alpha 3 subunit targets nAChRs to subdomains within individual synapses on neurons in vivo

Different types of neurotransmitter receptors coexist within single neurons and must be targeted to discrete synaptic regions for proper function. In chick ciliary ganglion neurons, nicotinic acetylcholine receptors (nAChRs) containing alpha 3 and alpha 5 subunits are concentrated in the postsynaptic membrane, whereas alpha-bungarotoxin receptors composed of alpha 7 subunits are localized perisynaptically and excluded from the synapse. Using retroviral vector-mediated gene transfer in vivo, we show that the long cytoplasmic loop of alpha 3 targets chimeric alpha 7 subunits to the synapse and reduces endogenous nAChR surface levels, whereas the alpha 5 loop does neither. These results show that a particular domain of one subunit targets specific receptor subtypes to the interneuronal synapse in vivo. Moreover, our findings suggest a difference in the mechanisms that govern assembly of interneuronal synapses as compared to the neuromuscular junction in vertebrates.

Organizing Effects of Rapsyn on Neuronal Nicotinic Acetylcholine Receptors

Targeting receptors to appropriate locations on the cell surface is a critical task for neurons. We have examined the possibility that rapsyn controls the distribution of nicotinic receptors on neurons as it does nicotinic receptors on muscle fibers. Cotransfection of QT6 cells with rapsyn and neuronal nicotinic receptor cDNA constructs produced receptor aggregates or clusters that codistributed in part with rapsyn protein. Though all nicotinic receptor subtypes tested were affected by rapsyn, receptors containing the alpha7 gene product were among the most responsive. In addition, rapsyn caused a portion of the nicotinic receptors containing alpha7 subunits to become resistant to solubilization with nonionic detergent and to display a marked increase in metabolic stability. The results are consistent with rapsyn linking the receptors to cytoskeletal elements and suggest that it may play an organizing role determining the fate and location of nicotinic receptors on neurons. Copyright 1998 Academic Press.

Differential regulation of levels of nicotinic receptor subunit transcripts in adult sympathetic neurons after axotomy

Axotomy of adult peripheral neurons produces decreases in the levels of transcripts for a number of proteins involved in synaptic transmission. For example, tyrosine hydroxylase and neuropeptide Y mRNA decrease in axotomized sympathetic neurons in the superior cervical ganglion (SCG). In the present study, the effects of axotomy on the expression of nicotinic receptor subunit transcripts were examined in the SCG and the results were compared to those produced by deafferentation and explantation. Normally, neurons in the SCG express five different nicotinic subunits: alpha3, alpha5, alpha7, beta2, and beta4. Forty-eight hours after axotomy in vivo or explantation, dramatic decreases in these transcripts were seen, except for beta2, which increased. In contrast, deafferentation of the SCG had negligible effects on any of these transcripts. Both leukemia inhibitory factor (LIF) and nerve growth factor (NGF) have been shown to play a role in the decrease in neuropeptide Y mRNA expression after axotomy. In the cases of these nicotinic receptor transcripts, however, similar decreases were seen in wild-type and LIF knockout animals. Furthermore, administration of an antiserum to NGF in intact animals produced no changes in transcript levels. On the other hand, providing exogenous NGF to axotomized SCG in vivo or in explant cultures partially prevented the decreases in the transcripts for alpha3, alpha5, alpha7, and beta4. These data indicate that axotomy produces dramatic decreases in the expression of several nicotinic receptor subunit transcripts, and that the molecular signals underlying these changes differ from those previously shown to mediate the decrease in neuropeptide Y expression.

Inhibitory effects of arachidonic acid on nicotinic transmission in bullfrog sympathetic neurons

Inhibitory effects of arachidonic acid on nicotinic transmission in bullfrog sympathetic neurons. J. Neurophysiol. 78: 2396-2401, 1997. Arachidonic acid (AA, 0.2-40 mu M) reversibly reduced the amplitude of the fast excitatory postsynaptic potentials and the underlying currents (fast EPSCs) of bullfrog sympathetic neurons evoked by preganglionic nerve stimulation in a Ca2+-deficient solution. AA reduced the acetylcholine (ACh)-induced nicotinic currents (nIACh) evoked by brief applications of ACh to the ganglion cells in a dose-related manner. AA reduced the maximum amplitude of nIACh estimated from the dose-response relationship without causing an appreciable change in the apparent dissociation constant. Indomethacin (2 mu M) and nordihydroguaiaretic acid (20 mu M), blockers of cyclooxygenase and lipoxygenase pathways, respectively, had no effect on the inhibition of fast EPSC by AA. AA did not obviously affect the preganglionic nerve terminal spike configuration, synaptic delay, facilitation, quantal content of transmitter release, or the presynaptic long-term potentiation elicited by the repetitive stimulation applied to the preganglionic nerve fibers. These results suggest that AA acts on an allosteric site of the nicotinic receptor-channel complex either directly or indirectly and in turn inhibits ion permeation through these channels without affecting the release of ACh from preganglionic nerve terminals.

Rapid synaptic transmission in the avian ciliary ganglion is mediated by two distinct classes of nicotinic receptors

We analyzed the kinetics and pharmacology of EPSCs in two kinds of neurons in the embryonic avian ciliary ganglion. Whole-cell voltage-clamp recordings revealed that the singly innervated ciliary neurons had large-amplitude (1.5-8.0 nA) EPSCs that could be classified according to the kinetics of their falling phases. Most of the neurons responded with an EPSC the falling phase of which followed a double exponential time course with time constants of approximately 1 and 10 msec. The EPSCs of the remaining ciliary neurons followed a single time constant ( approximately 8 msec). Multiple innervated choroid neurons had smaller-amplitude responses (0.2-1.5 nA when all inputs were activated) that appeared to contain only a slowly decaying component (tau = 12 msec). The fast and slow components of EPSC decay seen in most ciliary neurons could be pharmacologically isolated with two toxins against nicotinic acetylcholine receptors (AChRs). The fast component was blocked by 50 nM alpha-bungarotoxin (alpha-BuTx), which binds alpha7-subunit-containing AChRs. The slow component was selectively blocked by 50 nM alpha-conotoxin MII (alpha-CTx-MII), which blocks mammalian AChRs containing an alpha3/beta2 subunit interface. A combination of both alpha-BuTx and alpha-CTx-MII abolished nearly all evoked current. Similar pharmacological results were found for ciliary neurons with monoexponentially decaying EPSCs and for choroid neurons. These results suggest that nerve-evoked transmitter acts on at least two different populations of AChRs on autonomic motor neurons in the ciliary ganglion.

Pharmacology of neuronal nicotinic acetylcholine receptor subtypes

The search for the physiological function of nicotinic receptors on neurons in the brain began with their discovery. It was initially assumed that, as in ganglia and at the neuromuscular junction, nicotinic receptors would gate fast synaptic transmission in the brain. The best functional evidence now, however, points to a role in modifying the release of other transmitters. This does not preclude a postsynaptic role in transmission for nicotinic receptors in the brain, but attempts to locate such a synapse have not been successful. If fast nicotinic synapses are present in the brain, they are probably low in number and may be masked by other more prevalent synapses (such as glutamatergic) so identification will not be easy. The extent of diversity of nicotinic receptors is substantial. At the molecular level this is reflected in the number of different genes that encode receptor subunits and the multiple possible combinations of subunits that function in expression systems. From the cellular level there is a broad diversity of properties of native receptors in neurons. Some useful pharmacological tools allow the limited identification of subunits in native receptors. For example, block by alpha-bungarotoxin identifies alpha 7, alpha 8, or alpha 9 subunits; activation of a receptor by cytisine indicates an alpha 7 or beta 4 subunit; and neuronal bungarotoxin block identifies a beta 2 subunit. Despite the clues to identity gained by careful use of these agents, we have not been able to identify all the components of any native receptor based on pharmacological properties assessed from expression studies. When both pharmacological and biophysical properties of a receptor are taken into consideration, none of the combinations tested in oocytes mimics native receptors exactly. The reason for this discrepancy has been debated at length; it is possible that oocytes do not faithfully manufacture neuronal nicotinic receptors. For example, they may not correctly modify the protein after translation or they may allow a combination of subunits that do not occur in vivo. Another possibility is that correct combinations of subunits have not yet been tested in oocytes. Data from immunoprecipitation experiments suggest that many receptors contain three or more different subunits. Results from further experiments injecting combinations of three or more subunits into oocytes may be enlightening. The diversity of receptors may allow targeting of subtypes to specific locations. Nicotinic receptors are located presynaptically, preterminally, and on the cell soma. The function of the nicotinic receptors located on innervating axons is presumably to modify the release of other neurotransmitters. It is an attractive hypothesis that nicotinic receptors might be involved in modifying the weight of central synapses; however, in none of the regions where this phenomenon has been described is there any evidence for axoaxonal contacts. The presynaptic receptors described so far are pharmacologically unique; therefore, if there are different subtypes of nicotinic receptors modifying the release of different transmitters, they may provide a means of exogenously modifying the release of a particular transmitter with drugs. There are still many basic unanswered questions about nicotinic receptors in the brain. What are the compositions of native nicotinic receptors? What is their purpose on neurons? Although there is clearly a role presynaptically, what is the function of those located on the soma? Neuronal nicotinic receptors are highly permeable to calcium, unlike muscle nicotinic receptors, and this may have important implications for roles in synaptic plasticity and development. Finally, why is there such diversity? (ABSTRACT TRANCATED)

Synaptic currents generated by neuronal acetylcholine receptors sensitive to alpha-bungarotoxin

Nicotinic acetylcholine receptors are widely distributed throughout the nervous system, but their functions remain largely unknown. One of the most abundant is a class of receptors that contains the alpha 7 gene product, has a high relative permeability to calcium, and binds alpha-bungarotoxin. Here, we report that receptors sensitive to alpha-bungarotoxin, though concentrated in perisynaptic clusters on neurons, can generate a large amount of the synaptic current. Residual currents through other nicotinic receptors are sufficient to elicit action potentials, but with slower rise times. This demonstrates a postsynaptic response for alpha-bungarotoxin-sensitive receptors on neurons and suggests that the functional domain of the postsynaptic membrane is broader than previously recognized.

Tyrosine kinase inhibitors alter composition of nicotinic receptors on neurons

Protein tyrosine kinase (PTK) inhibitors were used to examine the roles of tyrosine phosphorylation in synaptic function. We show here that two different PTK inhibitors, herbimycin A and lavendustin A, both selectively downregulate a subpopulation of nicotinic acetylcholine receptors (AChRs) on chick ciliary ganglion neurons in culture. The downregulation requires a number of hours to occur and involves only those receptors containing the alpha 3, alpha 5, and beta 4 gene products. Not affected are AchRs that additionally contain the beta 2 gene product or AchRs that are made up of the alpha 7 gene product. The downregulation preferentially targets receptors destined for the cell surface and has little effect on the large pool of intracellular receptors. The receptor loss is not additive with that seen in the presence of either cycloheximide or tunicamycin, two compounds that the block appearance of new receptors. The downregulation induced by herbimycin A in surface receptors is accompanied by a specific decrement in the amount of alpha 3 protein in the cells. The results indicate that PTKs, either by phosphorylating AChR gene products directly or by acting through intermediary proteins, regulate the size and composition of the AChR pool maintained on the cell surface. Receptor regulation by PTKs may provide a mechanism for long-term control of synaptic signaling between neurons.

Elevation of intracellular calcium levels in neurons by nicotinic acetylcholine receptors

The recognition that intracellular free calcium serves as a ubiquitous intracellular signal has motivated efforts to elucidate mechanisms by which cells regulate calcium influx. One route of entry that may offer both spatial and temporal fine resolution for altering calcium levels is that provided by cation-permeable, ligand-gated ion channels. Biophysical measurements as well as calcium imaging techniques demonstrate that neuronal nicotinic acetylcholine receptors as a class have a high relative permeability to calcium; some subtypes equal or exceed all other known receptors in this respect. Activation of nicotinic receptors on neurons can produce substantial increases in intracellular calcium levels by direct passage of calcium through the receptor channel. When multiple classes of nicotinic receptors are expressed by the same neuron, each appears capable of increasing calcium in the cell but may differ with respect to location, temporal response, agonist sensitivity, or regulation in achieving it. As a result, nicotinic receptors must be considered strong candidates for signaling molecules through which neurons regulate a diverse array of cellular events.

Synaptosomal binding of 125I-labelled daboiatoxin, a new PLA2 neurotoxin from the venom of Daboia russelli siamensis

Daboiatoxin (DbTx), the PLA2 neurotoxin from Daboia russelli siamensis venom, was shown to bind specifically and saturably to rat cerebrocortical synaptosomes and synaptic membrane fragments. Two families of binding sites were detected by equilibrium binding analysis in the presence and absence of Ca2+. Scatchard analysis of biphasic plateaus revealed Kdl 5 nM and Bmax1, 6 pmoles/mg protein, and Kd2 80 nM and Bmax2 20 pmoles/mg protein, respectively, for the high- and low-affinity binding sites. The binding of 125I-DbTx to synaptosomes did not show marked dependence on Ca2+, Mg2+, Co2+ and Sr2+. Native DbTx was the only strong competitor to 125I-DbTx synaptosomal binding (IC50 12.5 nM, KI 5.5 nM). Two other crotalid PLA2 neurotoxins, crotoxin CB and mojave toxin basic subunit, and nontoxic C. Atrox PLA2 enzyme, were relatively weaker inhibitors, while two viperid PLA2 neurotoxins, ammodytoxin A and VRV PL V, were very weak inhibitors. Crotoxin CA was a poor inhibitor even at microM concentrations, whereas no inhibitory effect at all was observed with crotoxin CACB, ammodytoxin C, VRV PL VIIIa, taipoxin, beta-bungarotoxin, or with PLA2 enzymes from N. naja venom, E. schistosa venom, bee venom and porcine pancreas. All other pharmacologically active ligands examined (epinephrine, norepinephrine, histamine, choline, dopamine, serotonin, GABA, naloxone, WB-4101, atropine, hexamethonium and alpha-bun-garotoxin) also failed to interfere with 125I-DbTx binding. As those competitors that showed partial inhibition were effective only at microM concentration range compared to the Kd (5 nM) of 125I-DbTx synaptosomal binding, DbTx could well recognize a different neuronal binding site. Rabbit anti-DbTx polyclonal antisera completely blocked the specific binding. When a range of Ca2+ and K+ channels modulators were examined, Ca2+ channel blockers (omega-conotoxins GVIA and MVIIC, taicatoxin, calciseptine and nitrendiprene) did not affect the binding even at high concentrations, while charybdotoxin was the only K+ channel effector that could partially displace 125I-DbTx synaptosomal binding amongst the K+ channel blockers tested (apamin, dendrotoxin-I, iberiotoxin, MCD-peptide, 4-aminopyridine and tetraethylammonium), suggesting that neither K+ nor Ca2+ channels are associated with DbTx binding sites.

Synaptic and extrasynaptic distribution of two distinct populations of nicotinic acetylcholine receptor clusters in the frog cardiac ganglion

We examined the distribution of neuronal nicotinic acetylcholine receptor clusters in relation to synaptic sites on autonomic neurons in the frog heart using immunofluorescence techniques and laser scanning confocal microscopy. Acetylcholine receptor clusters were visualized using the rat anti-Electrophorus acetylcholine receptor monoclonal antibody no. 22 and cyanine 3.18-labelled goat anti-rat secondary antibody. Synaptic boutons were labelled with the mouse anti-synaptic vesicle protein SV2, monoclonal antibody no. 10h and cyanine 5.18-labelled goat anti-mouse secondary antibody. Acetylcholine receptor clusters on the neuronal surface exist in two populations that vary in size, staining intensity, and surface distribution. The more prominent population consists of large, brightly stained clusters numbering 30 +/- 15 per cell, while the second class is smaller and less brightly stained and numbers over 100 per cell. The large clusters tend to be organized into groups of 2-6 members. This arrangement results from the fact that 80% of the large clusters colocalize at synaptic boutons and that single boutons can have several associated clusters. The remaining 20% of large/bright acetylcholine receptor clusters are extrasynaptic, but they, too, are clustered and are found in close proximity to synaptic boutons. The small/dim acetylcholine receptor clusters are randomly distributed over the cell surface. The large/bright synaptic acetylcholine receptor clusters presumably underlie fast excitatory synaptic transmission. The small/dim clusters and the large/bright extrasynaptic clusters may represent intermediates in the metabolism of large/bright synaptic clusters.

A nicotinic acetylcholine receptor regulating cell adhesion and motility is expressed in human keratinocytes

Acetylcholine is synthesized and released by human epidermal keratinocytes and modulates the adhesion and motility of these cells. To understand the molecular basis of the effects of acetylcholine on keratinocytes, we investigated the presence, pharmacology, structure, and function of nicotinic acetylcholine receptors in human epidermal keratinocytes. Patch-clamp studies indicated that keratinocytes express acetylcholine receptors with ion gating and pharmacologic properties similar to those observed so far only in neurons, and containing the alpha 3 subunit. Specific binding of the receptor-specific ligand 125I-kappa-bungarotoxin revealed approximately 5500 binding sites per cell on undifferentiated keratinocytes in cell cultures and approximately 35,400 binding sites per cell on mature keratinocytes freshly isolated from human neonatal foreskins. Antibody binding and polymerase chain reaction experiments demonstrated the presence of alpha 3, beta 2, and beta 4 nicotinic receptor subunits. Binding of subunit-specific antibodies indicated that nicotinic receptors were associated with the suprabasal keratinocytes in epidermis and localized to the cell membranes of differentiated keratinocytes in cell cultures. Acetylcholine and the nicotinic agonist nicotine increased cell-substrate and cell-cell adherence of cultured keratinocytes and stimulated their lateral migration. The specific antagonists kappa-bungarotoxin and mecamylamine caused cell detachment and abolished migration. Thus, a nicotinic receptor expressed in keratinocytes may mediate acetylcholine control of keratinocyte adhesion and motility.

The characterization of alpha-bungarotoxin receptors on the surface of parasympathetic neurons in the frog heart

Receptors for alpha-bungarotoxin are found on the surface of parasympathetic neurons in the frog cardiac ganglion by light microscopic autoradiography. Competition studies suggest that these receptors are cholinergic and indicate that they are also recognized by neuronal bungarotoxin (kappa-bungarotoxin). These receptors are outnumbered by those recognized exclusively by neuronal bungarotoxin. Unlike neuronal bungarotoxin receptors, alpha-bungarotoxin receptors are not concentrated at synaptic sites. Fluorescence techniques fail to find evidence for clusters of alpha-bungarotoxin receptors anywhere on the neuronal surface. The possible function of these receptors, which apparently do not play a role in fast synaptic transmission, is discussed.

Effects of retinal lesions upon the distribution of nicotinic acetylcholine receptor subunits in the chick visual system

Immunohistochemistry was used in this study to evaluate the effects of retinal lesions upon the distribution of neuronal nicotinic acetylcholine receptor subunits in the chick visual system. Following unilateral retinal lesions, the neuropil staining with an antibody against the beta 2 receptor subunit, a major component of alpha-bungarotoxin-insensitive nicotinic receptors, was dramatically reduced or completely eliminated in all of the contralateral retinorecipient structures. On the other hand, neuropil staining with antibodies against two alpha-bungarotoxin-sensitive receptor subunits, alpha 7 and alpha 8, was only slightly affected after retinal lesions. Decreased neuropil staining for alpha 7-like immunoreactivity was only observed in the nucleus of the basal optic root and layers 2-4 and 7 of the optic tectum. For alpha 8-like immunoreactivity, slight reduction of neuropil staining could be observed in the ventral geniculate complex, griseum tecti, nucleus lateralis anterior, nucleus lentiformis mesencephali, layers 4 and 7 of the tectum, and nucleus suprachiasmaticus. Taken together with previous data on the localization of nicotinic receptors in the retina, the present results indicate that the beta 2 subunit is transported from retinal ganglion cells to their central targets, whereas the alpha 7 and alpha 8 subunit immunoreactivity appears to have a central origin. The source of these immunoreactivities could be, at least in part, the stained perikarya that were observed to contain alpha 7 and alpha 8 subunits in all retinorecipient areas. In agreement with this hypothesis, the beta 2 subunit of the nicotinic acetylcholine receptors was not frequently found in perikarya of the same areas.

The sensitivity of nicotinic synapses in bullfrog sympathetic ganglia to alpha-bungarotoxin and neuronal-bungarotoxin

1. The sensitivity of nicotinic synapses to alpha-bungarotoxin (alpha-Bgt) and neuronal-bungarotoxin (n-Bgt) was measured in the B and C cell systems of bullfrog paravertebral sympathetic ganglia 9 and 10 by recording extracellular compound postganglionic action potentials from the rami communicantes. 2. High concentrations (10 microM) of alpha-Bgt applied for up to 8 h had no effect upon synaptic transmission in either the B or C cell system. Ganglia pretreated with collagenase were also insensitive to alpha-Bgt. In control experiments on isolated sartorius muscle preparations, nerve-evoked twitches were fully blocked by 30-100 nM alpha-Bgt. 3. Nicotinic transmission in the B and C cell systems was reversibly blocked by 30-300 nM n-Bgt. Block appeared within 25-45 min of exposure to toxin and reversed fully with a half-time of 40-80 min. This was indistinguishable from washout times after block by 100 microM (+)-tubocurarine. 4. The results demonstrate close parallels between the bungarotoxin sensitivity of neuronal nicotinic receptors mediating ganglionic transmission in functional subclasses of bullfrog sympathetic neurones and the bungarotoxin sensitivity which has been reported for autonomic in avian and mammalian preparations.

Neuronal acetylcholine receptors that bind alpha-bungarotoxin with high affinity function as ligand-gated ion channels

Neuronal membrane components that bind alpha-bungarotoxin with high affinity can increase intracellular levels of free calcium, demonstrating the components function as nicotinic receptors. Though such receptors often contain the alpha 7 gene product, which by itself can produce ionotropic receptors in Xenopus oocytes, numerous attempts have failed to demonstrate an ion channel function for the native receptors on neurons. Using rapid application of agonist, we show here that the native receptors are ligand-gated ion channels which are cation selective, prefer nicotine over acetylcholine, and rapidly desensitize. Much of the calcium increase caused in neurons by the receptors under physiological conditions appears to result from their depolarizing the membrane sufficiently to trigger calcium influx through voltage-gated channels.

Thymopoietin, a polypeptide ligand for the alpha-bungarotoxin binding site in brain: an autoradiographic study

Thymopoietin, a 48-49-amino acid polypeptide present in the thymus gland, was investigated as a potential ligand for the neuronal nicotinic alpha-bungarotoxin binding site in rat brain. Binding of [125I]alpha-bungarotoxin to whole rat brain sections was inhibited by thymopoietin in a concentration-dependent manner with an IC50 of 30.0 +/- 8.2 nM as compared to 1.1 +/- 0.3 nM for alpha-bungarotoxin. However, at concentrations of thymopoietin of up to 1 microM, [3H]nicotine binding to high affinity sites was not inhibited. Thysplenin, a polypeptide with considerable homology to thymopoietin did not affect [125I]alpha-bungarotoxin binding. These results suggest that thymopoietin selectively interacts with the nicotinic alpha-bungarotoxin binding site labelled by [125I]alpha-bungarotoxin rather than the neuronal nicotinic receptor(s) labelled by [3H]nicotine. Autoradiographic studies revealed that 1 microM thymopoietin almost completely inhibited [125I]alpha-bungarotoxin binding in all brain regions. Computer-assisted image analysis of displacement curves was performed on various brain areas rich in alpha-bungarotoxin binding, such as the dorsal endopiriform nucleus, fields 1 and 2 of Ammon's horn, the polymorph cell layer of the dentate gyrus and cortical layers 4 and 5. Thymopoietin inhibited [125I]alpha-bungarotoxin binding with similar potency in all these regions, suggesting that it interacted at the same site in the different brain areas. The IC50 values averaged over the six regions were 24.6 +/- 2.8 nM for thymopoietin and 1.2 +/- 0.2 nM for alpha-bungarotoxin. These results show that thymopoietin specifically interacted with the alpha-bungarotoxin site with a similar potency in different brain regions. It is suggested that thymopoietin represents a selective ligand for alpha-bungarotoxin binding sites in brain.

Thymopoietin, a thymic polypeptide, potently interacts at muscle and neuronal nicotinic alpha-bungarotoxin receptors

Current studies suggest that several distinct populations of nicotinic acetylcholine (ACh) receptors exist. One of these is the muscle-type nicotinic receptors with which neuromuscular nicotinic receptor ligands and the snake toxin alpha-bungarotoxin interact. alpha-Bungarotoxin potently binds to these nicotinic receptors and blocks their function, two characteristics that have made the alpha-toxin a very useful probe for the characterization of these sites. In neuronal tissues, several populations of nicotinic receptors have been identified which, although they share a nicotinic pharmacology, have unique characteristics. The alpha-bungarotoxin-insensitive neuronal nicotinic receptors, which may be involved in mediating neuronal excitability, bind nicotinic agonists with high affinity but do not interact with alpha-bungarotoxin. Subtypes of these alpha-toxin-insensitive receptors appear to exist, as evidenced by findings that some are inhibited by neuronal bungarotoxin whereas others are not. In addition to the alpha-bungarotoxin-insensitive sites, alpha-bungarotoxin-sensitive neuronal nicotinic receptors are also present in neuronal tissues. These latter receptors bind alpha-bungarotoxin with high affinity and nicotinic agonists with an affinity in the microM range. The function of the nicotinic alpha-bungarotoxin receptors are as yet uncertain. Thymopoietin, a polypeptide linked to immune function, appears to interact specifically with nicotinic receptor populations that bind alpha-bungarotoxin. Thus, in muscle tissue where alpha-bungarotoxin both binds to the receptor and blocks activity, thymopoietin also potently binds to the receptor and inhibits nicotinic receptors-mediated function. In neuronal tissues, thymopoietin interacts only with the nicotinic alpha-bungarotoxin site and not the alpha-bungarotoxin-insensitive neuronal nicotinic receptor population. These observations that thymopoietin potently and specifically interacts with nicotinic alpha-bungarotoxin-sensitive receptors in neuronal and muscle tissue, together with findings that thymopoietin is an endogenously occurring agent, could suggest that this immune-related polypeptide represents a ligand for the alpha-bungarotoxin receptors. The function of thymopoietin at the alpha-bungarotoxin receptor is as yet uncertain; however, a potential trophic, as well as other roles are suggested.

Nicotinic receptors that bind alpha-bungarotoxin on neurons raise intracellular free Ca2+

Many populations of vertebrate neurons have a membrane component that binds alpha-bungarotoxin and cholinergic ligands. Despite the abundance of this component and its similarities to nicotinic receptors, its function has remained controversial. Using a fluorescence assay, we show here that activation of the component elevates the intracellular concentration of free Ca2+, demonstrating a receptor function for the toxin-binding component. Whole-cell voltage-clamp and intracellular recordings did not detect a significant current resulting from receptor activation, possibly because the currents were small or the receptors rapidly desensitized. The rise in intracellular free Ca2+ caused by the receptor was prevented by Ca2+ channel blockers. This suggests a signaling cascade likely to have important regulatory consequences for the neuron.

Developmental changes in transmitter sensitivity and synaptic transmission in embryonic chicken sympathetic neurons innervated in vitro

Dispersed neurons from embryonic chicken sympathetic ganglia were innervated in vitro by explants of spinal cord containing the autonomic preganglionic nucleus or somatic motor nucleus. The maturation of postsynaptic acetylcholine (ACh) sensitivity and synaptic activity was evaluated from ACh and synaptically evoked currents in voltage-clamped neurons at several stages of innervation. All innervated cells are more sensitive to ACh than uninnervated neurons regardless of the source of cholinergic input. Similarly, medium conditioned by either dorsal or ventral explants mimics innervation by enhancing neuronal ACh sensitivity. This increase is due to changes in the rate of appearance of ACh receptors on the cell surface. There are also several changes in the nature of synaptic transmission with development in vitro, including an increased frequency of synaptic events and the appearance of larger amplitude synaptic currents. In addition, the mean amplitude of the unit synaptic current mode increases, as predicted from the observed changes in postsynaptic sensitivity. Although spontaneous synaptic current amplitude histograms with multimodal distributions are seen at all stages of development, histograms from early synapses are typically unimodal. Changes in the synaptic currents and ACh sensitivity between 1 and 4 days of innervation were paralleled by an increase in the number of synaptic events that evoked suprathreshold activity in the postsynaptic neurons. The early pre- and postsynaptic differentiation described here for interneuronal synapses formed in vitro may be responsible for increased efficacy of synaptic transmission during development in vivo.

Evidence for thymopoietin and thymopoietin/alpha-bungarotoxin/nicotinic receptors within the brain

Thymopoietin, a polypeptide hormone of the thymus that has pleiotropic actions on the immune, endocrine, and nervous systems, potently interacts with the neuromuscular nicotinic acetylcholine receptor. Thymopoietin binds to the nicotinic alpha-bungarotoxin (alpha-BGT) receptor in muscle and, like alpha-BGT, inhibits cholinergic transmission at this site. Evidence is given that radiolabeled thymopoietin similarly binds to a nicotinic alpha-BGT-binding site within the brain and does so with the characteristics of a specific receptor ligand. Thus specific binding to neuronal membranes was saturable, of high affinity (Kd = 8 nM), linear with increased tissue concentration, and readily reversible; half-time was approximately 5 min for association and 10 min for dissociation. Binding of 125I-labeled thymopoietin was displaced not only by unlabeled thymopoietin but also by alpha-BGT and the nicotinic receptor ligands d-tubocurarine and nicotine; various other receptor ligands (muscarinic, adrenergic, and dopaminergic) did not affect binding of 125I-labeled thymopoietin. Thymopoietin was shown by ELISA to be present in brain extracts, displacement curves of thymus and brain extracts being parallel to the standard thymopoietin curve, and Western (immuno) blot identified in brain and thymus extracts a thymopoietin-immunoreactive polypeptide of the same molecular mass as purified thymopoietin polypeptide. We conclude that thymopoietin and thymopoietin-binding sites are present within the brain and that the receptor for thymopoietin is the previously identified nicotinic alpha-BGT-binding site of neuronal tissue.

Brain alpha-bungarotoxin binding protein cDNAs and MAbs reveal subtypes of this branch of the ligand-gated ion channel gene superfamily

alpha-Bungarotoxin (alpha Bgt) is a potent, high-affinity antagonist for nicotinic acetylcholine receptors (AChRs) from muscle, but not for AChRs from neurons. Both muscle and neuronal AChRs are thought to be formed from multiple homologous subunits aligned around a central cation channel whose opening is regulated by ACh binding. In contrast, the exact structure and function of high-affinity alpha Bgt binding proteins (alpha BgtBPs) found in avian and mammalian neurons remain unknown. Here we show that cDNA clones encoding alpha BgtBP alpha 1 and alpha 2 subunits define alpha BgtBPs as members of a gene family within the ligand-gated ion channel gene superfamily, but distinct from the gene families of AChRs from muscles and nerves. Subunit-specific monoclonal antibodies raised against bacterially expressed alpha BgtBP alpha 1 and alpha 2 subunit fragments reveal the existence of at least two different alpha BgtBP subtypes in embryonic day 18 chicken brains. More than 75% of all alpha BgtBPs have the alpha 1 subunit, but no alpha 2 subunit, and a minor alpha BgtBP subtype (approximately 15%) has both the alpha 1 and alpha 2 subunits.