Functional properties and developmental regulation of nicotinic acetylcholine receptors on embryonic chicken sympathetic neurons

Nicotinic acetylcholine receptors: Conventional and unconventional ligands and signaling

Postsynaptic nAChRs in the peripheral nervous system are critical for neuromuscular and autonomic neurotransmission. Pre- and peri-synaptic nAChRs in the brain modulate neurotransmission and are responsible for the addictive effects of nicotine. Subtypes of nAChRs in lymphocytes and non-synaptic locations may modulate inflammation and other cellular functions. All AChRs that function as ligand-gated ion channels are formed from five homologous subunits organized to form a central cation channel whose opening is regulated by ACh bound at extracellular subunit interfaces. nAChR subtype subunit composition can range from α7 homomers to α4β2α6β2β3 heteromers. Subtypes differ in affinities for ACh and other agonists like nicotine and in efficiencies with which their channels are opened and desensitized. Subtypes also differ in affinities for antagonists and for positive and negative allosteric modulators. Some agonists are "silent" with respect to channel opening, and AChRs may be able to signal metabotropic pathways by releasing G-proteins independent of channel opening. Electrophysiological studies that can resolve single-channel openings and molecular genetic approaches have allowed characterization of the structures of ligand binding sites, the cation channel, and the linkages between them, as well as the organization of AChR subunits and their contributions to function. Crystallography and cryo-electron-microscopy are providing increasing insights into the structures and functions of AChRs. However, much remains to be learned about both AChR structure and function, the in vivo functional roles of some AChR subtypes, and the development of better pharmacological tools directed at AChRs to treat addiction, pain, inflammation, and other medically important issues. This article is part of the special issue on 'Contemporary Advances in Nicotine Neuropharmacology'.

Sympathetic tales: subdivisons of the autonomic nervous system and the impact of developmental studies

Remarkable progress in a range of biomedical disciplines has promoted the understanding of the cellular components of the autonomic nervous system and their differentiation during development to a critical level. Characterization of the gene expression fingerprints of individual neurons and identification of the key regulators of autonomic neuron differentiation enables us to comprehend the development of different sets of autonomic neurons. Their individual functional properties emerge as a consequence of differential gene expression initiated by the action of specific developmental regulators. In this review, we delineate the anatomical and physiological observations that led to the subdivision into sympathetic and parasympathetic domains and analyze how the recent molecular insights melt into and challenge the classical description of the autonomic nervous system.

Generating diversity: Mechanisms regulating the differentiation of autonomic neuron phenotypes

Sympathetic and parasympathetic postganglionic neurons innervate a wide range of target tissues. The subpopulation of neurons innervating each target tissue can express unique combinations of neurotransmitters, neuropeptides, ion channels and receptors, which together comprise the chemical phenotype of the neurons. The target-specific chemical phenotype shown by autonomic postganglionic neurons arises during development. In this review, we examine the different mechanisms that generate such a diversity of neuronal phenotypes from the pool of apparently homogenous neural crest progenitor cells that form the sympathetic ganglia. There is evidence that the final chemical phenotype of autonomic postganglionic neurons is generated by both signals at the level of the cell body that trigger cell-autonomous programs, as well as signals from the target tissues they innervate.

Mapping of presynaptic nicotinic acetylcholine receptors using fluorescence imaging of neuritic calcium

Neuronal nicotinic receptors (nAChRs) appear to function at both pre- and postsynaptic sites, to modulate the release of neurotransmitter, and to mediate synaptic transmission, respectively. Localization of functional nAChRs at presynaptic structures has only been possible under the best of circumstances where the presynaptic structure is very large allowing direct nAChR channel recording. We report here a novel and simple method that allows the visualization of stimulus-evoked changes in Fura-2 fluorescence in the presynaptic structures of essentially any neuron type in vitro. Following 'loading' of all neurons by incubation with the calcium-sensitive dye, Fura-2-AM, we selectively reduced the fluorescent signal in the postsynaptic neuron by injecting the Fura-2 quenching agent, Mn(2+), into the postsynaptic neuron. After quenching, nicotine treatment elicits calcium transients that can be observed in spatially distinct regions of neurite bundles contacting the Mn(2+)-infused neuron. Thus, the approach described allows one to readily map the distribution of activated nAChRs on presynaptic inputs in vitro.

The role of neuronal nicotinic acetylcholine receptor subunits in autonomic ganglia: lessons from knockout mice

Neuronal nicotinic acetylcholine receptors (nAChR), composed of 12 subunits (alpha2-alpha10, beta2-beta4), are expressed in autonomic ganglia, playing a central role in autonomic transmission. The repertoire of nicotinic subunits in autonomic ganglia includes alpha3, alpha5, alpha7, beta2 and beta4 subunits. In the last 10 years, heterologous expression studies have revealed much about the nature of neuronal nAChRs. However, there is only limited understanding of subunit actions in autonomic system. Functional deletions of subunit by gene knockout in animals could overcome these limitations. We review recent studies on nAChRs on autonomic ganglia for physiological and pharmacological properties and potential locations of the subunits.

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.

Differential modulation of nicotinic acetylcholine receptor subtypes and synaptic transmission in chick sympathetic ganglia by PGE(2)

The diversity of neuronal nicotinic acetylcholine receptors (nAChRs) is likely an important factor in the modulation of synaptic transmission by acetylcholine and nicotine. We have tested whether postsynaptic nAChRs are modulated in a subtype-specific manner by prostaglandin E(2) (PGE(2)), a regulator of neuronal excitability in both the central and peripheral nervous systems, and examined the effects of PGE(2) on nicotinic transmission. Somatodendritic nAChRs in chick lumbar sympathetic ganglia include four nAChR subtypes distinguished on the basis of conductance and kinetic profile. Nanomolar PGE(2) applied to the extrapatch membrane differentially regulates opening probability (Po), frequency and the opening duration of each nAChR channel subtype in cell-attached patches. PGE(2) decreases the Po of the predominant nAChR subtype (36 pS) and significantly increases Po and open duration of the 23 pS subtype. The 23 pS subtype is gated by the alpha 7-selective agonist choline, and choline-gated currents are inhibited by alpha-bungarotoxin. To examine whether PGE(2) modulates nAChRs at synaptic sites, we studied the effects of PGE(2) on amplitude and decay of synaptic currents in visceral motoneuron-sympathetic neuron co-cultures. PGE(2) significantly decreases the amplitude of miniature excitatory postsynaptic currents (mEPSCs), consistent with the predominant inhibition by PGE(2) of all but the 23 pS subtype. The time constant of mEPSCs at PGE(2)-treated synapses is prolonged, which is also consistent with an increased contribution of the longer open duration of the 23 pS nAChR subtype with PGE(2) treatment. To examine the presynaptic effect of PGE(2), nanomolar nicotine was used. Nicotine induces facilitation of synaptic transmission by increasing mEPSC frequency, an action thought to involve presynaptic, alpha 7-containing nAChRs. In the presence of PGE(2), nicotine-induced synaptic facilitation persists. Thus the net effect of PGE(2) is to alter the profile of nAChRs contributing to synaptic transmission from larger conductance, briefer opening channels to smaller conductance, longer opening events. This subtype-specific modulation of nAChRs by PGE(2) may provide a mechanism for selective activation and suppression of synaptic pathways mediated by different nAChR subtype(s) at both pre- and postsynaptic sites.

Nicotinic acetylcholine receptor gene expression in developing chick autonomic ganglia

The developmental expression patterns of ten genes encoding nicotinic acetylcholine receptor subunits were analyzed using Northern blots and in situ hybridization in chick peripheral ganglia of neural crest, placodal and dual embryonic origin. The superior cervical and ciliary ganglia were investigated in detail because they accumulated relatively abundant transcripts of the alpha3, beta4, alpha5 and alpha7 genes. In the superior cervical ganglion, these four mRNA species had similar developmental time-courses. They appeared at embryonic day 8 (E8), increased steadily until E16 and maintained a rather high plateau level until E18. In the ciliary ganglion, alpha7 transcripts were already abundant at E6, increased until E10, and considerably decreased thereafter. High-resolution in situ hybridization showed that alpha7 transcripts were present in all cell types of the E6 ciliary ganglion, whereas they were restricted to large neuronal somas at E16. Transfections with a reporter gene under the control of the alpha7 promoter demonstrated that a sharp developmental divide occurred at E11-12, after which stage the promoter was activatable in neurons exclusively.

Target-specific control of nicotinic receptor expression at developing interneuronal synapses in chick

Neuronal differentiation and development of synaptic specializations are strongly influenced by cellular interactions. We compared the effects of interaction with distinct autonomic targets on the molecular and biophysical differentiation of 'upstream' neuron-neuron synapses. Contact with cardiac tissue induced expression of nicotinic receptor channels (nAChRs) distinct from those induced by renal tissue in presynaptic autonomic neurons. The kinetics of cholinergic currents at interneuronal synapses are dictated by the peripheral target contacted. Analysis of the nAChR channel subtypes and subunits in individual neurons demonstrated that the profile of transmitter receptors expressed at mature neuron-neuron synapses develops from the convergent influences of input-derived (anterograde) and target-specific (retrograde) signals.

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.

Functional contribution of the alpha7 subunit to multiple subtypes of nicotinic receptors in embryonic chick sympathetic neurones

1. Many studies of the alpha7 subunit of the neuronal nicotinic acetylcholine receptor (nAChR) family have demonstrated that this alpha-bungarotoxin (alpha-BgTx)-binding neuronal receptor can participate in ACh-gated channels. Heterologous expression studies reveal that alpha7 subunits form homomeric channels of unusually high Ca2+ permeability. However, the physiological role of the alpha7 subunit in native neuronal nAChR channels is less clear. 2. We present evidence that the alpha7 subunit contributes to the function of at least three subtypes of native nAChR expressed by embryonic chick sympathetic neurones. These subtypes are functionally distinct from heterologously expressed homomeric alpha7 nAChRs as well as homomeric-like currents described in studies of hippocampal and parasympathetic neurones. 3. The proposed nAChRs differ from one another and from homomeric alpha7 nAChRs in their sensitivity to block by alpha7 subunit-specific antagonists: alpha-BgTx and methyllycaconitine (MLA). While MLA blocks 60 % of the macroscopic ACh response, alpha-BgTx inhibits a small component of the macroscopic current described by slow-on and slow-off kinetics. 4. Functional deletion of the alpha7 subunit by antisense oligonucleotide treatment eliminates the susceptibility of the nAChRs to block by both MLA and alpha-BgTx. 5. Single channel recordings combined with pharmacological and antisense-mediated 'deletion' techniques reveal that alpha-BgTx-sensitive alpha7-containing nAChRs have a small unitary conductance (18 pS), brief open time kinetics and relatively low open probability (Po). MLA-sensitive alpha7 nAChRs are characterized by a conductance of approximately 35 pS, intermediate burst duration, and a relatively high Po. 6. The third nAChR subtype deleted by alpha7 antisense treatment is characterized by a unitary conductance of 50 pS and prolonged opening duration. 7. We propose that these three populations of native alpha7-containing nAChRs are distinct heteromeric complexes that include other alpha and/or beta nAChR subunits.

Functional contribution of the alpha5 subunit to neuronal nicotinic channels expressed by chick sympathetic ganglion neurones

1. Heterologous expression studies of the alpha5 subunit of the neuronal acetylcholine receptor (nAChR) gene family have demonstrated that it can participate in the function of ACh-gated channels if co-expressed with another alpha- and a beta-subunit. Previous studies also indicate prominent expression of alpha5 in both central and peripheral nervous systems. The participation of alpha5 in native nAChRs and its functional role in these channels is, however, unknown. 2. In this study, we present evidence that alpha5 has a role in at least two distinct subtypes of nAChR complexes expressed by embryonic chick sympathetic neurones. 3. alpha5 contributes not only to agonist but also to antagonist sensitivity of natively expressed nAChR channels. Functional deletion of the alpha5 subunit by antisense oligonucleotide treatment removes the nAChRs with relatively low affinity to ACh and cytisine. Deletion of alpha5 also eliminates channels that are blocked by the alpha7-specific antagonist methyllycaconitine (MLA) while increasing the percentage of current carried by nAChRs that are sensitive to alpha-bungarotoxin (alpha-BgTx). 4. Single channel analyses indicate that functional deletion of alpha5 results in the deletion of both the 'brief' and 'long' open duration, 50 pS subtypes of nAChR channels while increasing the expression of the 18 pS, alpha-BgTx-sensitive native nAChRs normally detected in sympathetic neurones at later developmental stages. 5. The biophysical and pharmacological profiles of native nAChRs revealed by this study and previous work are discussed in the context of a proposed model of the nAChR channels expressed by chick sympathetic neurones throughout development.

Changes in the properties of developing glycine receptors in cultured mouse spinal neurons

We studied several neurophysiological properties of in vitro maturing glycine receptors in mouse spinal cord neurons cultured for various times: 3-7 days (early), 10-12 days (intermediate), and 17-24 days (mature), using whole-cell and gramicidin-perforated techniques. The glycine-activated Cl- conductance increased about 6-fold during in vitro development, and the current density increased from 177+/-42 pA/pF in early to 504+/-74 pA/pF in mature neurons. The sensitivity to glycine increased transiently from 39+/-2.8 microM in early neurons to 29+/-1 microM in intermediate neurons. Using whole-cell recordings, we found that ECl did not change during development. With the gramicidin-perforated technique, on the other hand, ECl shifted from -27 to -52 mV with development. Thus, immature neurons were depolarized by the activation of glycine receptors, whereas mature neurons were hyperpolarized. The current decayed (desensitized) during the application of 500 microM glycine. The decay was single exponential and the time constant increased from 2,212+/-139 msec in early neurons to 4,580+/-1,071 msec in mature neurons. Picrotoxin (10 microM) inhibited the current to a larger extent in early neurons (46+/-6% of control), and the sensitivity of these receptors to strychnine (IC50) increased from 23+/-3 nM to 9+/-1 nM in mature neurons. In conclusion, several properties of spinal glycine receptors changed during in vitro neuronal maturation. This indicates that, similar to GABA(A) receptors, the functions of these receptors are developmentally regulated. These changes should affect the excitability of spinal neurons as well as other maturation processes.

Modulation of nicotinic AChR channels by prostaglandin E2 in chick sympathetic ganglion neurons

The effects of prostaglandin E2 (PGE2), an important metabolite of arachidonic acid, were studied on the activity of nicotinic AChR channels in cultured chick sympathetic ganglion neurons. In whole cell recordings, PGE2 (25 nM) inhibited significantly the ACh-evoked macroscopic current. In cell-attached patch recordings, PGE2 significantly inhibited single AChR channel currents as a result of a decrease in the frequency of channel opening, with no change in open time and conductance. PGE2 did not alter the extent or rate of agonist-induced desensitization of the AChR channels. These effects are specific since the related compound PGD2 had no effect on AChR channel function. Because there is an abundant endogenous production of PGE2 within sympathetic ganglia in response to certain stimuli, the inhibition of AChR channel function by PGE2 could serve an important role to modulate synaptic transmission in the sympathetic nervous system.

A cysteine-rich isoform of neuregulin controls the level of expression of neuronal nicotinic receptor channels during synaptogenesis

We report here that neuregulin (NRG) isoforms with a conserved cysteine-rich domain (CRD) in their N terminus regulate expression of nicotinic acetylcholine receptors (nAChRs) at developing interneuronal synapses and report the isolation of transmembrane NRG isoforms with this CRD within the N-terminal portion. CRD-NRG mRNA and immunoreactive protein are detected early in developing presynaptic (visceral motor) neurons. The levels of expression of CRD-NRG peak prior to the formation of synapses with their postsynaptic partners, the ganglionic sympathetic neurons. Recombinant CRD-NRG mimics the effects of presynaptic input on target neurons. Functional deletion of CRD-NRG from presynaptic neurons abolishes the upregulation of nAChR expression induced by input-derived soluble material. Thus, CRD-NRG appears to be both a necessary and a sufficient signal for the control of neuronal nAChR expression during synaptogenesis.

Plasticity in fast synaptic inhibition of adult oxytocin neurons caused by switch in GABA(A) receptor subunit expression

We found that magnocellular oxytocin neurons in adult female rats exhibit an endogenous GABA(A) receptor subunit switch around parturition: a decrease in alpha1:alpha2 subunit mRNA ratio correlated with a decrease in allopregnanolone potentiation and increase in decay time constant of the GABA(A) receptor-mediated IPSCs in these cells. The causal relationship between changes in alpha1:alpha2 mRNA ratio and the ion channel kinetics was confirmed using in vitro antisense deletion. Further, GABA(A) receptors exhibited a tonic inhibitory influence upon oxytocin release in vivo, and allopregnanolone helped to restrain oxytocin neuron in vitro firing only before parturition, when the alpha1:alpha2 subunit mRNA ratio was still high. Such observations provide evidence for the physiological significance of GABA(A) receptor subunit heterogeneity and plasticity in the adult brain.

The molecular biology of neuronal nicotinic acetylcholine receptors

The molecular cloning of genes encoding neuronal nicotinic acetylcholine receptors (nAChRs) has made possible a better understanding of the pharmacology and toxicology of cholinergic compounds. Neuronal nAChRs are related in structure to the nAChRs present at the neuromuscular junction. They are composed of multiple subunits designated either alpha and beta. Eight alpha and three beta subunit genes have been cloned. The alpha subunits contain the ligand binding sites, whereas beta subunits are structural subunits that contribute to the function of the receptor. A large number of nAChRs can be formed from different combinations of alpha and beta subunits. Different combinations of alpha and beta subunits can produce receptors in vitro with distinct ion conducting properties. Each subunit gene is expressed in a distinct pattern in the nervous system. The expression of at least some of the nAChR subunit genes is regulated during development and by cell-cell interactions. Each neuronal nAChR subtype has a distinct pharmacology. Both alpha and beta subunits contribute to the pharmacological properties of each subtype. The expression of multiple nAChR subtypes may allow for precise control of neurotransmission mediated by acetylcholine in diverse populations of neurons.

Functional properties of neuronal nicotinic acetylcholine receptor channels expressed in transfected human cells

To study how subunit composition affects the functional properties of neuronal nicotinic acetylcholine receptors (nAChRs), we examined the behaviour of acetylcholine (ACh)-induced single-channel currents in human BOSC 23 cells transiently transfected with various subunit cDNA combinations. For all nAChRs examined (chick and rat alpha 3 beta 4, chick alpha 3 beta 2, alpha 4 beta 2, alpha 7 and alpha 8), expression levels were high enough to allow measurements of acetylcholine-evoked whole-cell currents and nicotine-elicited Ca2+ transients as well as the functional characterization of nAChR channels. Unitary acetylcholine-evoked events of alpha 8 nAChR had a slope conductance of 23 pS, whereas two conductance classes (19-23 and 32-45 pS) were identified for all other nAChR channels. The mean channel open times were significantly longer for homomeric alpha 7 and alpha 8 nAChRs (6-7 ms) than for heteromeric nAChRs (1-3 ms), with the exception of alpha 3 beta 4 nAChRs (8.4 ms for rat, 7 ms for chick). At least two species of heterologously expressed nAChRs (alpha 3 beta 4 and alpha 3 beta 2) exhibited single-channel characteristics similar to those reported for native receptors. The variety of nAChR channel conductance and kinetic properties encountered in human cells transfected with nAChR subunits contributes to the functional diversity of nAChRs in nerve cells.

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)

Single-channel recording in brain slices reveals heterogeneity of nicotinic receptors on individual neurons within the chick lateral spiriform nucleus

We examined the functional properties of central nicotinic acetylcholine receptors at the single-channel level using tight-seal, voltage-clamp techniques. Single-channel currents were recorded from cell-attached patches on lateral spiriform neurons in chick brain slices. These neurons are known to express functional nicotinic receptors that are insensitive to the antagonists alpha-bungarotoxin and kappa-bungarotoxin, and which exhibit a high affinity for nicotine and other nicotinic agonists. Single-channel openings were observed in 84% of patches (n = 118) when the nicotinic agonists acetylcholine (1-100 microM), carbamylcholine (3-100 microM), or nicotine (3-10 microM) were present in the patch pipette. In contrast, single-channels were markedly reduced in number or entirely absent when the nicotinic antagonist dihydro-beta-erythroidine was present along with acetylcholine (n = 7) or when no agonist was present in the pipette (n = 22). Single-channel openings displayed inward rectification at depolarized potentials, and were dependent on extracellular sodium. Between 1 and 30 microM acetylcholine, a dose-response relationship was observed between agonist concentration and single-channel open probability during the first minute following seal formation. Multiple classes of single nicotinic channels, with calculated mean slope conductances of 15, 31, 40, and approximately 70 pS, were observed in membrane patches on different neurons within the lateral spiriform nucleus, and even within single patches on individual neurons. We conclude that neurons within the lateral spiriform nucleus express functionally heterogeneous nicotinic receptors and that in some neurons different nicotinic receptor subtypes are present in close proximity to each other on the same cell surface.

Innervation and target tissue interactions differentially regulate acetylcholine receptor subunit mRNA levels in developing neurons in situ

Neurons engage in two distinct types of cell-cell interactions: they receive innervation and establish synapses on target tissues. Regulatory events that influence synapse formation and function on developing neurons are largely undefined. We show here that nicotinic acetylcholine receptor (AChR) subunit transcript levels are differentially regulated by innervation and target tissue interactions in developing chick ciliary ganglion neurons in situ. Using ganglia that have developed in the absence of pre- or postganglionic tissues and quantitative RT-PCR, we demonstrate that alpha 3 and beta 4 transcript levels are increased by innervation and target tissue interactions. In contrast, alpha 5 transcript levels are increased by innervation, but target tissues have little effect. Whole-cell ACh-induced currents, used to estimate the number of functional AChRs, change in correlation with alpha 3 and beta 4, but not alpha 5, transcript levels. A model is proposed in which the changes in AChR subunit expression regulate levels of synaptic activity, which is a critical determinant of synapse stabilization and elimination, and neuronal cell death.

Developmental regulation of multiple nicotinic AChR channel subtypes in embryonic chick habenula neurons: contributions of both the alpha 2 and alpha 4 subunit genes

Habenula neurons from both early and late stage embryonic chickens express multiple subtypes of nicotinic acetylcholine receptor channels (nAChRs). The channel subtypes expressed by habenula neurons are similar in functional properties, but apparently distinct in subunit composition, from their peripheral counterparts in autonomic ganglia. Early in development, nicotine activates four classes of neuronal bungarotoxin (nBGT)-sensitive channels (approx. conductance = 15, 30, 50, 60pS) that are intermingled on the surface of habenula neuronal somata. In neurons removed from older animals, nAChR channel activity has increased 4- to 40-fold and channel subtypes have become spatially segregated from one another. Analysis of the profile of nAChR subunit gene expression by polymerase chain reaction indicates that several of the alpha-type subunit genes, including alpha 2,3,4,5,7, and alpha 8, as well as both beta 2 and beta 4, are expressed. Treatment of the neurons with subunit specific antisense oligonucleotides reveals that the alpha 2 and alpha 4 (but not alpha 3) subunits contribute to the functional profile of native nAChRs expressed by habenula neurons. Consideration of the functional properties and apparent subunit composition of autonomic ganglion nAChRs in the chick suggests that habenula neurons may utilize a very distinct set of subunit combinations to produce an array of nAChR channel subtypes similar in both conductance and pharmacological profile to those expressed by sympathetic neurons.

Residues 1 to 80 of the N-terminal domain of the beta subunit confer neuronal bungarotoxin sensitivity and agonist selectivity on neuronal nicotinic receptors

Standard two electrode voltage clamp techniques were used to investigate the response of neuronal nicotinic acetylcholine receptors, expressed in Xenopus oocytes, to various agonists and neuronal bungarotoxin (NBT). The beta subunit is an important determinant of the receptor's pharmacological profile. Co-expression of alpha 4 and beta 2 subunits produced a receptor that was relatively insensitive to cytisine and nicotine and inhibited by NBT, whilst the alpha 4 beta 4 combination produced a receptor that was highly sensitive to cytisine and nicotine but resistant to toxin. The first 80 amino acids of the N-terminal domain of the beta subunit are implicated in these characteristics, since the combination of alpha 4 with a hybrid beta subunit comprising amino acids 1-->80 of beta 2 and 81-->416 of beta 4 became relatively insensitive to nicotine and cytisine and resistant to inhibition by neuronal bungarotoxin.

Muscle membrane preparation restores sensitivity to acetylcholine in cultured chick ciliary ganglion neurons

Ciliary ganglion (CG) neurons grown in culture in the absence of muscle cells rapidly lose sensitivity to acetylcholine (ACh), while neurons grown in the presence of muscle or muscle cell membranes maintain sensitivity to ACh for extended periods of time. The present study examined whether exposure to muscle membrane preparation or stimulation of cAMP-dependent processes could restore sensitivity to ACh in cultured neurons which had lost responsiveness to ACh. CG neurons from 11- to 14-day-old chick embryos were grown on collagen substrate in the absence of muscle cells. Sensitivity to ACh was assessed by measuring peak current responses following application of ACh (IACh) to neurons under whole-cell voltage clamp. In control cultures IACh decreased from an average of 837 pA the day of plating to 145 pA following 4 days in culture. Stimulation of cAMP-dependent processes with forskolin and 3-isobutyl-1-methylxanthine (IBMX) or 8'Br-cAMP and IBMX had variable effects on IACh. These treatments increased peak IACh in some neurons maintained in culture for less than 48 h. Treatment with these agents decreased peak IACh in cultures which were more than 48 h old. Exposure of neurons, which had lost sensitivity to ACh in culture, to muscle membranes increased IACh 2- to 3-fold over 24 to 48 h. This membrane-induced restoration of sensitivity to ACh was blocked by exposure to the protein synthesis inhibitor cycloheximide. Stimulation of cAMP-dependent processes in neurons exposed to muscle membrane decreased IACh. In conclusion, these results indicate that some element associated with the membranes of muscle cells has the ability to restore ACh responsiveness to CG neurons which have become insensitive to ACh in culture.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular cloning of a human neuronal nicotinic acetylcholine receptor beta 3-like subunit

Three cDNA clones homologous to rat neuronal nicotinic receptors were identified in a human brain stem library by screening at low stringency with a mixture of rat alpha 5, beta 2 and beta 4 subunit probes. Positive clones were further analysed by hybridisation at higher stringency with individual rat nicotinic receptor probes. One positive clone was found to encode a protein that exhibited characteristic features of a member of the ligand-gated ion channel class of protein subunits, and showed 89% homology with a rat beta 3 neuronal nicotinic receptor subunit. Northern blots demonstrated the presence of a 1.7 kb transcript in RNA extracted from adult human Pons. This clone has therefore been designated the human beta 3 neuronal nicotinic receptor subunit.

Structure, diversity, and ionic permeability of neuronal and muscle acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) form a family of ligand-gated, cation-selective channels that are concentrated at cholinergic synapses on vertebrate neurons and muscle cells. At the neuromuscular endplate, muscle nAChRs bind acetylcholine released by the presynaptic motor neuron. The receptors then undergo a conformational change that opens their ion channels. Cations move passively through the water-filled pores down their electrochemical gradients, completing synaptic transmission by depolarizing the postsynaptic muscle. The channel only weakly discriminates among permeant cations, which include all monovalent and divalent cations that are small enough to fit through the narrowest cross section. The membrane-spanning region of the pore is lined by uncharged domains that are bracketed by residues with net negative charge. The pore has large entrance vestibules, especially facing extracellularly. The narrowest cross-section is located near the cytoplasmic end of the membrane-spanning region, and this short narrow region probably provides the main cation binding site that is directly in the permeation pathway. Neuronal nAChRs share many of the properties of muscle nAChRs, but the neuronal receptor subtypes are more heterogenous genetically, pharmacologically, and functionally. There are especially important functional differences between muscle and neuronal nAChRs. For example, neuronal nAChRs are more highly permeable to Ca2+ and physiological levels of Ca2+ very potently modulate neuronal nicotinic currents. This variety of nAChRs suggests that these receptor/channels serve many roles in the excitable tissues of vertebrates.

Uptake of antisense oligonucleotides and functional block of acetylcholine receptor subunit gene expression in primary embryonic neurons

Several recent studies have used antisense oligonucleotides in the nervous system to probe the functional role of particular gene products. Since antisense oligonucleotide-mediated block of gene expression typically involves uptake of the oligonucleotides, we have characterized the mechanism of this uptake into developing neurons from embryonic chickens. Antisense oligonucleotides (15 mers) added to the bathing media are taken up into the embryonic chicken sympathetic neurons maintained in vitro. A portion of the oligonucleotide uptake is temperature dependent and saturates at extracellular oligonucleotide concentrations > or = 20 microM. This temperature sensitive, saturable component is effectively completed by single nucleotides of ATP and AMP and is reminiscent of receptor-mediated endocytosis of oligonucleotides described in non-neuronal cells. The efficiency of the oligonucleotide uptake system is dependent on the developmental stage of the animal but independent of the number of days that the neurons are maintained in vitro. Following the uptake of antisense oligonucleotides directed against ion channel subunit genes expressed by these neurons (nicotinic acetylcholine receptor subunit alpha 3; nAChR alpha 3), biophysical assays reveal that the functional expression of the target gene is largely blocked. Thus the number of wild type nAChR channels expressed is decreased by approximately 80%-90%. Furthermore, following antisense deletion of alpha 3, "mutant" nAChRs with distinct functional characteristics are expressed. In sum, these studies characterize the uptake of antisense oligonucleotide and demonstrate the functional block of specific gene expression in primary developing neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of GABAergic connections in vitro: increasing efficacy of synaptic transmission is not accompanied by changes in miniature currents

Development of inhibitory synaptic transmission was studied using a dissociated cell culture from the superior colliculus of neonatal rat. Patch-clamp recordings in the whole-cell configuration were performed to measure evoked (single-cell-activated) inhibitory postsynaptic currents (IPSCs), miniature IPSCs and current responses to maximal concentrations of exogenous gamma-aminobutyric acid (GABA). Over a period of 3 weeks in vitro (DIV3-24), the fraction of synaptically coupled neurons raised from 0% to 76%. Evoked IPSCs were first observed at DIV5. They had an average amplitude of 33.9 pA during the first week (n = 13) and 129.7 pA during the fourth week (n = 48). This increase by a factor of 3.8 represents a significant rise in the efficacy of GABAergic transmission during in vitro development. However, no developmental change has been observed in the average amplitudes of miniature somatic IPSCs. The latter remained at an average level of about 9 pA (symmetrical chloride concentration and a driving force of 68 mV). No increase was found also in whole-cell current densities induced by saturating concentrations of exogenous GABA. Our results suggest that under the given conditions, synapse maturation was primarily the result of presynaptic sprouting. This conclusion is further supported by bouton counts in immunostained collicular cultures, where the number of axosomatic and axodendritic GABAergic contacts per neuron increased from 0.54 and 0.37, respectively, at DIV3, to 13.84 and greater than 23.1, at DIV24. The overall density of GABAergic neurons decreased during this period from about 41,000/cm2 to 15,600 cm2, indicating that a growing number of contacts is formed by a declining number of presynaptic neurons.

Diversity in primary structure and function of neuronal nicotinic acetylcholine receptor channels

Neuronal nicotinic acetylcholine receptors are oligomeric protein complexes whose component subunits are each encoded by a family of homologous genes. The current challenge is to determine the functional contributions of the related subunits to the receptor-linked ion channels they compose and to uncover the physiological impact of the distinct channel classes expressed in vivo. In the past year, new approaches to the analysis of these receptors have yielded important insights into their stoichiometry, pharmacology and functional properties.

Development of substance P receptors on rat motoneurons in vitro

Experiments were performed to examine the influence of interneuronal interactions on the expression of neurotransmitter receptors by developing mammalian CNS neurons. Receptors for the neuropeptide, substance P (SP), were assayed on embryonic rat motoneurons and other spinal cord neurons developing in vitro by the binding of 125I-SP to live neurons. Scatchard analysis showed the presence of high-affinity binding sites, and binding competition assays using SP, neurokinin A, or neurokinin B indicated that the high-affinity 125I-SP binding sites on these neurons were type NK1 tachykinin receptors, or SP receptors (SPRs). Neurons in the spinal cords of rats at Embryonic Day 14 displayed no SPRs. Cell-surface SPRs were detected on spinal cord neurons within 24 hr after they were placed in culture, however, and the level of 125I-SP binding increased for several days. SPRs were assayed on spinal motoneurons that had been identified by retrograde labeling with a fluorescent tracer, isolated in high purity by fluorescence-activated cell sorting (FACS), and maintained in culture. Motoneurons grown in isolation from other neurons developed SPRs in vitro along the same time course as neurons in heterogeneous spinal cord cultures. These results show that rat spinal motoneurons can express SPRs early in their development, and they suggest that the initial expression of SPRs by developing motoneurons does not require interaction with other neurons.

Calcium permeability and modulation of nicotinic acetylcholine receptor-channels in rat parasympathetic neurons

Neuronal nicotinic acetylcholine (ACh)-activated currents in rat parasympathetic ganglion cells were examined using whole-cell and single-channel patch clamp recording techniques. The whole-cell current-voltage (I-V) relationship exhibited strong inward rectification and a reversal (zero current) potential of -3.9 mV in nearly symmetrical Na+ solutions (external 140 mM Na+/internal 160 mM Na+). Isosmotic replacement of extracellular Na+ with either Ca2+ or Mg2+ yielded the permeability (Px/PNa) sequence Mg2+ (1.1) > Na+ (1.0) > Ca2+ (0.65). Whole-cell ACh-induced current amplitude decreased as [Ca2+]0 was raised from 2.5 mM to 20 mM, and remained constant at higher [Ca2+]0. Unitary ACh-activated currents recorded in excised outside-out patches had conductances ranging from 15-35 pS with at least three distinct conductance levels (33 pS, 26 pS, 19 pS) observed in most patches. The neuronal nicotinic ACh receptor-channel had a slope conductance of 30 pS in Na+ external solution, which decreased to 20 pS in isotonic Ca2+ and was unchanged by isosmotic replacement of Na+ with Mg2+. ACh-activated single channel currents had an apparent mean open time (tau 0) of 1.15 +/- 0.16 ms and a mean burst length (tau b) of 6.83 +/- 1.76 ms at -60 mV in Na+ external solution. Ca(2+)-free external solutions, or raising [Ca2+]0 to 50-100 mM decreased both the tau 0 and tau b of the nAChR channel. Varying [Ca2+]0 produced a marked decrease in NP0, while substitution of Mg2+ for Na+ increased NP0. These data suggest that activation of the neuronal nAChR channel permits a substantial Ca2+ influx which may modulate Ca(2+)-dependent ion channels and second messenger pathways to affect neuronal excitability in parasympathetic ganglia.

Functional contribution of neuronal AChR subunits revealed by antisense oligonucleotides

Although multiple related genes encoding nicotinic acetylcholine receptor (AChR) subunits have been identified, how each of these subunits contributes to AChRs in neurons is not known. Sympathetic neurons express four classes of AChR channels and six AChR subunit genes (alpha 3, alpha 4, alpha 5, alpha 7, beta 2, and beta 4). The contribution of individual subunits to AChR channel subtypes in these neurons was examined by selective deletion with antisense oligonucleotides. An alpha 3 antisense oligonucleotide decreased the number and altered the properties of the normally expressed ACh-activated channels. The remaining AChR channels have distinct biophysical and pharmacological properties that indicate an important functional contribution of the alpha 7 subunit.

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.

Conductance and kinetic properties of single nicotinic acetylcholine receptor channels in rat sympathetic neurones

1. The unitary conductance of nicotinic acetylcholine (ACh) receptor channels in rat sympathetic neurones has been studied. Conductance estimates varied from 26-48 pS with a mean of 36.8 pS in 1 mM-Ca2+. The main conductance level varied from patch to patch and the presence (or absence) of additional conductance levels also varied. 2. The channels showed large open channel noise and experiments with 300 mM-NaCl in the patch pipette substantially increased the open channel noise. The appearance of detectable step-like transitions within this noise strongly suggested the existence of closely spaced discrete levels. 3. Removal of divalent cations from the external solution increased the unitary channel conductance. Altering the main permeant ion in divalent-free solutions gave the following conductance sequence: K+ (93 pS) greater than Cs+ (61 pS) greater than Na+ (51 pS) greater than Li+(23 pS). 4. Replacement of Na+ by Cs+ in the external solution considerably reduced the current evoked by ACh in whole-cell recordings and the channel-opening frequency in outside-out patches. 5. The kinetic properties of channels activated by ACh and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP) were also studied. At low concentrations of ACh and DMPP the gap distributions were complex and best fitted by the sum of four exponential components. Individual activations (bursts) were interrupted by the two shortest closed periods the briefer of which had time constants of 36 microseconds for ACh and 67 microseconds for DMPP. 6. The distribution of burst lengths had two components for each agonist, each component making up about 50% of the total area under the distribution. For ACh, the time constant of the longer component (12.2 ms) was similar to the decay time constant of excitatory postsynaptic potentials (EPSCs) at similar temperature and potential. For DMPP the time constant of the longer component was 17.6 ms. 7. The relative number of brief gaps per long burst was much larger for ACh than for DMPP. Therefore the corrected mean open time for ACh (0.86 ms) was much shorter than that for DMPP (2.3 ms). 8. In terms of receptor mechanism, the values of the channel opening equilibrium constant (beta/alpha) estimated from these numbers (ACh, 23; DMPP, 25) suggest that both agonists are efficaceous. 9. DMPP is a potent blocker of the channel with an equilibrium dissociation constant (KB) of around 50 microM and blockage gaps of around 1 ms duration. ACh also blocks the channel but with a higher KB of around 470 microM.(ABSTRACT TRUNCATED AT 400 WORDS)

Physiological properties of newly formed synapses between sympathetic preganglionic neurons and sympathetic ganglion neurons

We have examined the physiological properties of transmission at newly formed synapses between sympathetic preganglionic neurons and sympathetic ganglion neurons in vitro. Chick neurons were labeled with fluorescent carbocyanine dyes before they were placed into culture (Honig and Hume, 1986), and were studied by making intracellular recordings during the first 2 weeks of coculture. Evoked monosynaptic excitatory postsynaptic potentials (EPSPs) were not observed until 48 h of coculture. Beyond this time, the frequency with which connected pairs could be found did not vary greatly with time. With repetitive stimulation, the evoked monosynaptic EPSPs fluctuated in amplitude from trial to trial and showed depression at frequencies as low as 1 Hz. To gain further information about the quantitative properties of transmission at newly formed synapses, we analyzed the pattern of fluctuations of delayed release EPSPs. In mature systems, delayed release EPSPs are known to represent responses to single quanta, or to the synchronous release of a small number of quanta. For more than half of the connections we studied, the histograms of delayed release EPSPs were extremely broad. This result suggested that either quantal responses are drawn from a continuous distribution that has a large coefficient of variation or that there are several distinct size classes of quantal responses. The pattern of fluctuations of monosynaptic EPSPs was consistent with both of these possibilities, and was inconsistent with the possibility that monosynaptic EPSPs are composed of quantal subunits with very little intrinsic variation. Although variation in the size of responses to single quanta might arise in a number of ways, one attractive explanation for our results is that the density and type of acetylcholine receptors varies among the different synaptic sites on the surface of developing sympathetic ganglion neurons.

Development of fast synaptic transmission in bullfrog sympathetic ganglia

Extracellular recordings were made of postsynaptic responses originating in sympathetic ganglia 9 and 10 of bullfrog tadpoles and adults. At stage III, when the length and diameter of the developing hindlimb bud are equal, preganglionic stimulation elicits postganglionic action potentials in spinal nerves 9 and 10 near the sciatic plexus. Although they fluctuate in amplitude, these responses follow short trains of repetitive stimuli at 20 Hz. Their mediation by nicotinic synapses was demonstrated by reversible blockade in low Ca2+, high Mg2+ Ringer and in nicotine. Parallel sympathetic B and C systems are clearly defined by stage III. They can be selectively activated by appropriate segmental stimulation of the sympathetic chain and are characterized by distinct conduction velocities which both lie in the C fiber range (less than 1 m/s). Throughout subsequent tadpole stages, the conduction velocities of the developing B and C systems gradually double while the magnitudes of their compound action potentials grow exponentially by 100-fold. Conduction velocities reach adult values after completion of metamorphosis. These results provide physiological evidence that synapse formation in sympathetic ganglia supplying the hindlimbs begins by the earliest stages of limb bud development, is selective, and progresses over a protracted period of months, prior to myelination.

The role of the beta 4-subunit in determining the kinetic properties of rat neuronal nicotinic acetylcholine alpha 3-receptors

1. Single-channel currents were recorded from Xenopus oocytes which had been injected with complementary RNAs (cRNAs) for the neuronal nicotinic acetylcholine receptor subunits alpha 3 and beta 4. The co-expression of alpha 3 and beta 4 gave rise to three different channel types with conductances of 22, 18 and 13 pS. 2. The activity arising from the expression of these two subunits was compared with that observed when the alpha 3 subunit was co-expressed with an alternative beta-subunit, beta 2. The alpha 3 beta 4-receptors differed from alpha 3 beta 2-receptors in conductance, open times and most notably, in burst kinetics. The association of the beta 4-subunit with the alpha 3-subunit results in receptors which have a high probability of re-opening after closing, yielding protracted bursts of activity not observed when the alpha 3-subunit is associated with the beta 2-subunit. 3. All three alpha 3 beta 4-channel types had similar burst kinetics. However, the 13 pS conductance channels showed an additional long-lived open state to varying degrees when clusters of activity within an individual record were examined.

Endogenous neurotransmitter activates N-methyl-D-aspartate receptors on differentiating neurons in embryonic cortex

Before synapses form in embryonic turtle cerebral cortex, an endogenous neurotransmitter activates N-methyl-D-aspartate (NMDA) channels on neurons in the cortical plate. Throughout cortical development, these channels exhibit voltage-dependent Mg2+ blockade and are antagonized by D-2-amino-5-phosphonovaleric acid, a selective NMDA receptor antagonist. The activation in situ of these nonsynaptic NMDA channels demonstrates a potential physiological substrate for control of early neuronal differentiation.

Substance P modulates single-channel properties of neuronal nicotinic acetylcholine receptors

Substance P (SP) is present in avian sympathetic ganglia and accelerates the decay rate of acetylcholine (ACh)-evoked macroscopic currents in sympathetic neurons. We demonstrate here that SP modulates ACh-elicited single channels in a manner consistent with an enhancement of ACh receptor (AChR) desensitization. Furthermore, since AChR channel function was monitored in cell-attached patches with SP applied to the extra-patch membrane, the peptide must act via a second messenger mechanism. SP specifically decreases the net ACh-activated single-channel current across the patch membrane by decreasing both channel opening frequency and mean open time kinetics. These experiments demonstrate that a peptide can modulate neuronal AChR function by a second messenger mechanism.