Channel properties of an insect neuronal acetylcholine receptor protein reconstituted in planar lipid bilayers

Natural and artificial ion channels for biosensing platforms

The single-molecule selectivity and specificity of the binding process together with the expected intrinsic gain factor obtained when utilizing flow through a channel have attracted the attention of analytical chemists for two decades. Sensitive and selective ion channel biosensors for high-throughput screening are having an increasing impact on modern medical care, drug screening, environmental monitoring, food safety, and biowarefare control. Even virus antigens can be detected by ion channel biosensors. The study of ion channels and other transmembrane proteins is expected to lead to the development of new medications and therapies for a wide range of illnesses. From the first attempts to use membrane proteins as the receptive part of a sensor, ion channels have been engineered as chemical sensors. Several other types of peptidic or nonpeptidic channels have been investigated. Various gating mechanisms have been implemented in their pores. Three technical problems had to be solved to achieve practical biosensors based on ion channels: the fabrication of stable lipid bilayer membranes, the incorporation of a receptor into such a structure, and the marriage of the modified membrane to a transducer. The current status of these three areas of research, together with typical applications of ion-channel biosensors, are discussed in this review.

A review of experimental techniques used for the heterologous expression of nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors (nAChRs) are members of the Cys-loop family of neurotransmitter-gated ion channels, a family that also includes receptors for gamma-aminobutyric acid, glycine and 5-hydroxytryptamine. In humans, nAChRs have been implicated in several neurological and psychiatric disorders and are major targets for pharmaceutical drug discovery. In addition, nAChRs are important targets for neuroactive pesticides in insects and in other invertebrates. Historically, nAChRs have been one of the most intensively studied families of neurotransmitter receptors. They were the first neurotransmitter receptors to be biochemically purified and the first to be characterized by molecular cloning and heterologous expression. Although much has been learnt from studies of native nAChRs, the expression of recombinant nAChRs has provided dramatic advances in the characterization of these important receptors. This review will provide a brief history of the characterization of nAChRs by heterologous expression. It will focus, in particular, upon studies of recombinant nAChRs, work that has been conducted by many hundreds of scientists during a period of almost 30 years since the molecular cloning of nAChR subunits in the early 1980s.

Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor-like subunit with homology to the Drosophila melanogaster Dbeta1 subunit

The cloning, sequencing and functional expression of Sgbeta1, a novel locust (Schistocerca gregaria) non-alpha nicotinic acetylcholine receptor (nAChR) subunit is described. This subunit shows 80% identity with the Drosophila melanogaster Dbeta1 and 92% identity with the Locusta migratoria beta1, non-alpha subunits but only 38% identity to Sgalpha1 (also referred to as alphaL1), a previously cloned S. gregaria nAChR alpha-subunit. When expressed in Xenopus laevis oocytes, Sgbeta1 does not respond to nicotine. Responses to nicotine are observed, however, in oocytes co-expressing Sgalpha1 and Sgbeta1, but the pharmacology is indistinguishable from that of currents produced by expressing Sgalpha1 alone. We conclude that either Sgbeta1 does not co-assemble with Sgalpha1, or that it is unable to contribute to the functional properties of the receptor, in the Xenopus oocyte expression system.

4,5-Dihydro-1H-pyrazole Derivatives with Inhibitory nNOS Activity in Rat Brain: Synthesis and Structure−Activity Relationships

In an attempt to find new compounds with neuroprotective activity, we have designed, synthesized and characterized 19 new nNOS inhibitors with a 4,5-dihydro-1H-pyrazole structure. Compounds 11r [1-cyclopropanecarbonyl-3-(2-amino-5-chlorophenyl)-4,5-dihydro-1H-pyrazole] and 11e [1-cyclopropanecarbonyl-3-(2-amino-5-methoxyphenyl)- 4,5-dihydro-1H-pyrazole] show the highest activities with inhibition percentages of 70% and 62%, respectively. A structure-activity relationship for the nNOS inhibition can be established from the structural comparison of these new pyrazole derivatives and the described synthetic kynurenines 10.

Inhibition of nNOS activity in rat brain by synthetic kynurenines: structure-activity dependence

The overstimulation of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors is involved in excitotoxicity, a process participating in neurodegeneration that characterizes some neurological disorders and acute cerebral insults. In looking for compounds with neuroprotective properties, a series of kynurenine derivatives were synthesized, and their effects on both the NMDA and nNOS activity in rat striatum were evaluated. Two compounds, 15a (2-acetamido-4-(2-amino-5-methoxyphenyl)-4-oxobutyric acid) and 15c (2-butyramido-4-(2-amino-5-methoxyphenyl)-4-oxobutyric acid), displayed more potent activities than the other synthetic compounds tested for the inhibition of NMDA excitability and nNOS activity. Two other compounds, 18a (2-acetamido-4-(3-methoxyphenyl)-4-oxobutyric acid) and 18c (2-butyramido-4-(3-methoxyphenyl)-4-oxobutyric acid), that have the same structure as 15a and 15c, except the amino group in R(1), showed different effects. Whereas compound 18a showed lower electrophysiological potency than compounds 15a and 15c in the inhibition of the NMDA-dependent excitability, compound 18c showed the opposite effect. Moreover, compounds 18a and 18c were unable to modify nNOS activity. The remaining kynurenines tested behave like compound 18a. These results suggest that a structure-related activity of these synthetic kynurenines and a N-H bond in a specific direction is necessary for some kynurenine analogues to inhibit nNOS activity.

Structure and diversity of insect nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAChR) is an agonist-regulated ion-channel complex responsible for rapid neurotransmission. The vertebrate nAChR, assembled from five homologous subunits, penetrates the synaptic membrane. Different subunit combinations lead to receptor subtypes with distinctive pharmacological profiles. In comparison with mammalian nAChRs, the insect receptor is poorly understood relative to functional architecture and diversity. Several genes for Drosophila, Locusta and Myzus encoding insect nAChR subunits have been identified, although the functional assembly and presence of different subtypes of these receptors are not defined. The insect nAChR is the primary target site for the neonicotinoid insecticides, thereby providing an incentive to explore its functional architecture with neonicotinoid radioligands, photoaffinity probes and affinity chromatography matrices. This review considers the current understanding of the structure and diversity of insect nAChRs based mainly on recent studies in molecular biology and protein biochemistry.

Neuronal nicotinic acetylcholine receptors from Drosophila: two different types of alpha subunits coassemble within the same receptor complex

Although neuronal nicotinic acetylcholine receptors from insects have been reconstituted in vitro more than a decade ago, our knowledge about the subunit composition of native receptors as well as their functional properties still remains limited. Immunohistochemical evidence has suggested that two alpha subunits, alpha-like subunit (ALS) and Drosophila alpha2 subunit (Dalpha2), are colocalized in the synaptic neuropil of the Drosophila CNS and therefore may be subunits of the same receptor complex. To gain further understanding of the composition of these nicotinic receptors, we have examined the possibility that a receptor may imbed more than one alpha subunit using immunoprecipitations and electrophysiological investigations. Immunoprecipitation experiments of fly head extracts revealed that ALS-specific antibodies coprecipitate Dalpha2, and vice versa, and thereby suggest that these two alpha subunits must be contained within the same receptor complex, a result that is supported by investigations of reconstituted receptors in Xenopus oocytes. Discrimination between binary (ALS/beta2 or Dalpha2/beta2) and ternary (ALS/Dalpha2/beta2) receptor complexes was made on the basis of their dose-response curve to acetylcholine as well as their sensitivity to alpha-bungarotoxin or dihydro-beta-erythroidine. These data demonstrate that the presence of the two alpha subunits within a single receptor complex confers new receptor properties that cannot be predicted from knowledge of the binary receptor's properties.

Nicotinic acetylcholine currents of cultured Kkenyon cells from the mushroom bodies of the honey bee Aapis mellifera

1. Acetylcholine-induced currents of mushroom body Kenyon cells from the honey bee Apis mellifera were studied using the whole-cell configuration of the patch clamp technique. Pressure application of 1 mM acetylcholine (ACh) induced inward currents with amplitudes between -5 and -500 pA. 2. The cholinergic agonists ACh and carbamylcholine had almost equal potencies of current activation at concentrations between 0.01 and 1 mM; nicotine was less potent. The muscarinic agonist oxotremorine did not elicit any currents. 3. Approximately 80 % of the ACh-induced current was irreversibly blocked by 1 microM alpha-bungarotoxin. Atropine (1 mM) did not block the ACh-induced current. 4. Upon prolonged ACh application the current desensitized with a time course that could be approximated by the sum of two exponentials (tau1 = 276 +/- 45 ms (mean +/- s.e.m. ) for the fast component and tau2 = 2.4 +/- 0.7 s for the slow component). 5. Noise analyses of whole-cell currents yielded elementary conductances of 19.5 +/- 2.4 pS for ACh and 23.7 +/- 5.0 pS for nicotine. The channel lifetimes, calculated from the frequency spectra, were tauo = 1.8 ms for ACh and tauo = 2.5 ms for nicotine. 6. Raising the external calcium concentration from 5 to 50 mM shifted the reversal potential of the ACh-induced current from +4. 6 +/- 0.9 to +37.3 +/- 1.3 mV. The calcium-to-sodium permeability ratio (PCa : PNa) was 6.4. 7. In high external calcium solution (50 mM) the ACh-induced current rectified in an outward direction at positive membrane potentials. 8. We conclude that Kenyon cells express nicotinic ACh receptors with functional profiles reminiscent of the vertebrate neuronal nicotinic ACh receptor subtype.

Neuronal nicotinic receptors in the locust Locusta migratoria. Cloning and expression

We have identified five cDNA clones that encode nicotinic acetylcholine receptor (nAChR) subunits expressed in the nervous system of the locust Locusta migratoria. Four of the subunits are ligand-binding alpha subunits, and the other is a structural beta subunit. The existence of at least one more nAChR gene, probably encoding a beta subunit, is indicated. Based on Northern analysis and in situ hybridization, the five subunit genes are expressed. localpha1, localpha3, and locbeta1 are the most abundant subunits and are expressed in similar areas of the head ganglia and retina of the adult locust. Because Loc<alpha3 binds alpha-bungarotoxin with high affinity, it may form a homomeric nAChR subtype such as the mammalian alpha7 nAChR. Localpha1 and Locbeta1 may then form the predominant heteromeric nAChR in the locust brain. localpha4 is mainly expressed in optic lobe ganglionic cells and localpha2 in peripherally located somata of mushroom body neurons. localpha3 mRNA was additionally detected in cells interspersed in the somatogastric epithelium of the locust embryo, suggesting that this isoform may also be involved in functions other than neuronal excitability. Transcription of all nAChR subunit genes begins approximately 3 days before hatching and continues throughout adult life. Electrophysiological recordings from head ganglionic neurons also indicate the existence of more than one functionally distinct nAChR subtype. Our results suggest the existence of several nAChR subtypes, at least some of them heteromeric, in this insect species.

Effects of nicotinic and muscarinic ligands on embryonic neurones of Periplaneta americana in primary culture: a whole cell clamp study

The pharmacological properties of acetylcholine (ACh) receptors of cultured neurones from embryonic cockroach brains were studied using the whole-cell configuration of the patch-clamp technique. More than 90% of the studied neurones responded to ACh by a monophasic inward current, the intensity of which varied from cell to cell. The sequence of potency of the five tested agonists was ACh > nicotine=carbamylcholine > suberyldicholine=oxotremorine. The dose-response relationship was complex, suggesting the existence of two populations of receptors: high-affinity receptors (extrapolated K(d) around 10(-7) M) and low-affinity receptors (extrapolated K(d) around 5x10(-5) M). The current-voltage relationship of the induced current was linear between -80 and -40 mV and the extrapolated reversal potential was not significantly different from 0 mV. The sequence of decreasing potency of the antagonists of the ACh response was: methyllycaconitine > alpha-bungarotoxin > mecamylamine > curare > strychnine > bicuculline > atropine > picrotoxin. These results show: (1) that, in embryonic brain neurones, the response to ACh corresponds to the opening of non-selective cationic channels; and (2) that the pharmacology of the ACh receptors is mainly but not solely nicotinic. The nature of the single events which underlie this response, as well as the structure of the channels (homo or hetero-oligomeric) remain to be investigated.

Insect neurotransmission: neurotransmitters and their receptors

The roles of acetylcholine, dopamine, octopamine, tyramine, 5-hydroxytryptamine, histamine, glutamate, 4-aminobutanoic acid (gamma-aminobutyric acid) and a range of peptides as insect neurotransmitters are evaluated in terms of the criteria used to identify transmitters. Of the biogenic amines considered, there is good evidence that acetylcholine, dopamine, octopamine, 5-hydroxytryptamine, and histamine should be considered to be neurotransmitters, but the case for tyramine is less convincing at the moment. The evidence supporting neurotransmitter roles for glutamate and gamma-aminobutyric acid at specific insect synapses is overwhelming, but much work remains to be undertaken before the full significance of these molecules in the insect nervous system is appreciated. Attempts to characterise biogenic amine and amino acid receptors using pharmacological and molecular biological techniques have revealed considerable differences between mammalian and insect receptors. The number of insect neuropeptides isolated and identified has increased spectacularly in recent years, but genuine physiological or biochemical functions can be assigned to very few of these molecules. Of these, only proctolin fulfills the criteria expected of a neurotransmitter, and the recent discovery of proctolin receptor antagonists should enable the biology of this pentapeptide to be explored fully.

Molecular evolution of the nicotinic acetylcholine receptor: an example of multigene family in excitable cells

An extensive phylogenetic analysis of the nicotinic-acetylcholine-receptor subunit gene family has been performed by cladistic and phenetic methods. The conserved parts of amino acid sequences have been analyzed by CLUSTAL V and PHYLIP software. The structure of the genes was also taken in consideration. The results show that a first gene duplication may have occurred before the appearance of Bilateria. Three subfamilies then appeared: I--the neuronal alpha-bungarotoxin binding-site subunits (alpha 7, alpha 8); III--the neuronal nicotinic subunits (alpha 2-alpha 6, beta 2-beta 4), which also contain the muscle acetylcholine-binding subunit (alpha 1); and IV--the muscle non-alpha subunits (beta 1, gamma, delta, epsilon). The Insecta subunits (subfamily II) could be orthologous to family III and IV. Several tissular switches of expression from neuron to muscle and the converse can be inferred from the extant expression of subunits and the reconstructed trees. The diversification of the neuronal nicotinic subfamily begins in the stem lineage of chordates, the last duplications occurring shortly before the onset of the mammalian lineage. Such evolution parallels the increase in complexity of the cholinergic systems.

Localization of a nicotinic acetylcholine receptor-like antigen in the thoracic nervous system of embryonic locusts, Schistocerca gregaria

Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subsequent immunoblotting of neuronal membrane proteins derived from thoracic ganglia of adult Locusta and Schistocerca reveal that a polyclonal antiserum raised against the Locusta nicotinic acetylcholine receptor (nAChR), binds strongly to an identical polypeptide band corresponding to 65 kDa in both locust species. This polyclonal antiserum was used to analyze the distribution of antigenic sites within the developing thoracic central nervous system of Schistocerca embryos. Axonal outgrowths from the earliest differentiated neurons are first labeled between 30% and 35% development. By 40% to 45% development, labeled granules appear in the cytoplasm of neuronal cell bodies. When the developing neuropil is first enclosed at approximately 45% to 50% development, it appears uniformly labeled, but by 55% development, unlabeled areas appear that represent the sites of future tracts and commissures. By 75%, an adult pattern of neuropil immunogenicity is established in which synaptic regions are stained but tracts and commissures are not. This suggests that during the early development of the thoracic nervous system nAChR-like antigenic sites are evenly distributed, but later become concentrated in the developing synaptic areas.

Physiological properties of neuronal nicotinic receptors reconstituted from the vertebrate beta 2 subunit and Drosophila alpha subunits

Three cDNAs (ALS, D alpha 2 and ARD) isolated from the nervous system of Drosophila and encoding putative nicotinic acetylcholine receptor subunits were expressed in Xenopus oocytes in order to study their functional properties. Functional receptors could not be reconstituted from any of these subunits taken singly or in twos and threes. In contrast, large evoked currents (in the microA range) were consistently observed upon agonist application on oocytes co-injected with ALS or D alpha 2 in combination with the chick beta 2 structural subunit. The ALS/beta 2 and D alpha 2/beta 2 receptors are highly sensitive to acetylcholine and nicotine, and their physiological properties resemble those of native or reconstituted receptors from vertebrates. Although the physiological properties of ALS/beta 2 and D alpha 2/beta 2 receptors are quite similar, clear differences appear in their pharmacological profiles. The ALS/beta 2 receptor is highly sensitive to alpha-bungarotoxin while the D alpha 2/beta 2 receptor is totally insensitive to this agent. These results demonstrate that the Drosophila ALS and D alpha 2 cDNAs encode neuronal nicotinic subunits responding to physiological concentrations of the agonists acetylcholine and nicotine.

Immunohistochemical localization of a ligand-binding and a structural subunit of nicotinic acetylcholine receptors in the central nervous system of Drosophila melanogaster

The distribution of two subunits of nicotinic acetylcholine receptors in the developing and the differentiated central nervous system of Drosophila melanogaster was studied. With subunit-specific antibodies raised against the ligand-binding alpha-like subunit ALS and the putative non-ligand-binding subunit ARD, we find both ALS-like and ARD-like immunoreactivity widely distributed in most neuropiles of the optic lobes, the protocerebrum, the deutocerebrum and the thoracic ganglion of the adult fly. With a single exception, namely in the lamina of the visual system, the antigens recognized by the two types of antibodies are colocalized. This observation is consistent with previous immunoprecipitation data indicating that the ALS and ARD proteins are integral components of the same hetero-oligomeric receptor that binds the nicotinic antagonist alpha-bungarotoxin with high affinity. During embryonic development ARD-like immunoreactivity is first detectable in approximately 10 hour old embryos. Both subunits are consistently detected in the central nervous system of the late embryo, the three larval stages, and all prepupal and pupal stages. During metamorphosis the optic stalk is transiently immunoreactive with anti-ARD, but not with anti-ALS antiserum. Although in larvae and adults, immunoreactivity with both types of antibodies is most abundant in synaptic regions, in embryos and pupae strong staining of cortical cell body layers is observed, in particular with anti-ARD antisera. As these developmental periods coincide with strong accumulation of ARD transcripts, the cell body staining may reflect newly synthesized and assembled receptors, while the functional ARD- and ALS-containing receptor may be destined for synapses.

Distribution of acetylcholine receptors in the central nervous system of adult locusts

A polyclonal antibody raised against nicotinic acetylcholine receptor protein from purified locust neuronal membrane was used to analyse the distribution of antigenic sites within the central nervous system of adult Schistocerca gregaria. Light microscopic examination showed that all principal neuropiles in the thoracic ganglia label with the antibody but that the major tracts and commissures do not. Analysis of this pattern of staining in the electron microscope reveals that the receptor is present on specific synaptic and extrajunctional neuronal membranes in the neuropile. Antigenic sites are also evident on the plasma membranes and within the cytoplasm adjacent to Golgi complexes of some neuronal somata, suggesting that these neurones synthesise nicotinic acetylcholine receptors. In addition to neuronal labelling, there is evidence that the receptor is also present on the membranes of three types of glial cells. The implications of this pattern of receptor distribution are discussed.

Neurons assemble acetylcholine receptors with as many as three kinds of subunits while maintaining subunit segregation among receptor subtypes

A family of genes encoding neuronal acetylcholine receptor (AChR) subunits has been identified and cloned from vertebrates. Expression studies have implied that as few as one or two kinds of subunits may be sufficient to construct neuronal AChRs and that multiple pair-wise combinations of the gene products are capable of generating functional receptors. We show here that a class of AChRs with a predominantly synaptic location on neurons contains receptors having at least three types of subunits and that the subunits are encoded by the alpha 3, beta 4, and alpha 5 AChR genes. In addition, we show that a class of extrasynaptic AChRs on the same neurons contains the alpha 7 subunits but lacks the alpha 3, beta 4, and alpha 5 subunits. The results demonstrate that native AChRs on neurons are more complex in composition than previously appreciated and suggest that constraints on subunit interactions limit the kinds of receptor species produced.

Acetylcholine receptor/channel molecules of insects

Acetylcholine-gated ion channels of the nicotinic type are abundant in the nervous system of insects. The channels are permeable to Na+, K+ and probably Ca(2+), and unlike most vertebrate neuronal nicotinic acetylcholine receptors the receptor/channel molecule is blocked by alpha-bungarotoxin (alpha-Bgt). Such alpha-Bgt-sensitive receptors are present at synapses and on cell bodies of insect neurones. Single channel recordings have shown the existence of multiple conductances of nAChRs. Studies on several different insect preparations have provided evidence for more than one open state and several closed states of insect nAChRs. Functional insect nAChR channels have now been investigated in situ, following reconstitution of a purified protein in bilayers, and as a result of expressing in Xenopus oocytes messenger RNA encoding receptor subunits.

The alpha 5 gene product assembles with multiple acetylcholine receptor subunits to form distinctive receptor subtypes in brain

The acetylcholine receptor (AChR) alpha 5 gene has been classified as a member of the AChR gene family based on sequence homology. Expression studies, however, have yet to identify a function for the alpha 5 gene product or even to demonstrate an interaction with known AChR subunits. We report here that the alpha 5 gene product is identical to the 49 kd protein previously found on immunoblots of AChRs purified from brain and ciliary ganglia. In brain the alpha 5 gene product is present both in alpha 3- and in alpha 4-based receptor subtypes, while in the ganglion it is found in an alpha 3-based receptor subtype concentrated in postsynaptic membrane. Immunoprecipitation experiments with subunit-specific monoclonal antibodies indicate that some native AChRs are likely to have at least three kinds of subunits, with two being of the alpha type. These findings support new views about the construction of AChRs in neurons.

How complex is the nicotinic receptor system of insects?

In insects, nicotinic acetylcholine receptors (nAChRs) are confined to the nervous system. It is a long-standing open question whether the insect nicotinic cholinergic receptor system is less complex than that of the vertebrate nervous system. Simplicity can be conceived in two ways. (1) Fewer receptor subtypes may exist. (2) Single receptors may have a more primitive (homo-oligomeric) quaternary structure. Recent approaches to the molecular cloning of insect nAChRs may contribute valuable new information to this issue. Thus, the identification of multiple genes encoding proteins similar to vertebrate nAChR subunits implicates a remarkable heterogeneity for these receptors. The discovery of putatively non-ligand-binding subunits hints to the existence of vertebrate-like hetero-oligomeric nAChRs. However, the simultaneous occurrence of homo-oligomeric receptors must still be considered.

Multiple conductances of neuronal nicotinic acetylcholine receptors

In the presence of acetylcholine, cationic channels with three different conductances were recorded from neurones of the dissociated housefly (Musca domestica). Large conductance (80 pS) channels, resembling those that are abundant in reconstitution studies with a 65 kDa alpha-bungarotoxin affinity purified polypeptide, were detected in situ. The two larger conductance channels (80 pS; 32 pS) exhibited open and closed times that were best fitted by multiple exponential functions, indicating the presence of at least two open states. A third conductance (20 pS) showed brief, sparse openings and was least frequently observed. The 32 pS channel was the most common.

Neuronal nicotinic acetylcholine receptors in Drosophila: antibodies against an alpha-like and a non-alpha-subunit recognize the same high-affinity alpha-bungarotoxin binding complex

ALS and ARD proteins are thought to represent a ligand binding and a structural subunit, respectively, of Drosophila nicotinic acetylcholine receptors (nAChRs). Here, antibodies raised against fusion constructs encompassing specific regions of the ALS and ARD proteins were used to investigate a potential association of these two polypeptides. Both ALS and ARD antisera removed 20-30% of the high-affinity binding sites for the nicotinic antagonist 125I-alpha-bungarotoxin (125I-alpha-Btx) from detergent extracts of fly head membranes. Combinations of both types of antisera also precipitated the same fraction of alpha-Btx binding sites, a result suggesting that both polypeptides are components of the previously defined class I 125I-alpha-Btx binding sites in the Drosophila CNS. 125I-alpha-Btx binding to a MS2 polymerase-ALS fusion protein containing the predicted antagonist binding region showed that the ALS protein indeed constitutes the ligand binding subunit of a nicotinic receptor complex. These data are consistent with neuronal nAChRs in Drosophila containing at least two types of subunits, ligand binding and structural ones.

Functional properties of the nicotinic and glutamatergic receptors

Several important physiological processes such as plasticity, memory, cell death, and rhythmic firing involve the N-methyl-D-aspartate (NMDA)-type of glutamatergic receptor. Nicotinic acetylcholine receptors (AChR), recently demonstrated in the central nervous system (CNS), are also of great interest. We have used several ligands to study the physiology and pharmacology of the agonist recognition sites of these receptors and kinetic properties of associated ion channels using whole-cell, cell-attached or outside-out variants of the patch-clamp technique. Enzymatically dissociated frog interosseal muscles were used to study peripheral AChRs, and tissue cultured or acutely dissociated hippocampal neurons and retinal ganglion cells (RGCs) for CNS receptors. For reproducible and fast solution changes when recording in the whole-cell configuration, we modified the "U"-shaped tube system to obtain different outputs from the same outflow port. We used fluorescent rhodamine-labeled latex microspheres to identify RGCs. Our studies provide important information regarding the molecular mechanisms of several clinically used agents. Additionally, similar actions of noncompetitive agents on the ion channels of the nicotinic ACh and NMDA receptors support the concept of a receptor ion channel superfamily.

A neuronal nicotinic acetylcholine receptor in crayfish neurons

In warm-blooded vertebrates, neuronal nicotinic acetylcholine receptors (nAChRs) are distinguished from muscle endplate receptors by their ligand affinities and sensitivity to several toxins. In the crayfish optic lobe, synaptic and acetylcholine (ACh)-elicited responses are blocked by toxins (F-toxin and neosurugatoxin) selective for neuronal nAChRs and are insensitive to the alpha-neurotoxins selective for endplate nAChRs.

Neuronal acetylcholine receptor channels from insects: a comparative electrophysiological study

The channel properties of nicotinic acetylcholine receptor subtypes in the nervous system of insects (Locusta migratoria) have been characterized. Single channel measurements were performed using patch-clamp techniques as well as planar lipid bilayer reconstitution approaches. In reconstitution experiments using receptor-preparations isolated from neuronal membranes by alpha-toxin affinity chromatography, a ligand-gated channel type was found, which showed a high conductance and a short mean lifetime. Patch-clamp experiments on synapse-free somata of isolated nerve cells revealed an acetylcholine-gated channel type with a lower conductance but a longer lifetime. The two different agonist-activated channel types are supposed to represent synaptic and extrasynaptic acetylcholine receptors.

Embryonic and adult neurones of the housefly (Musca domestica) in culture

This paper provides a detailed description of the dissociation and maintenance in culture of cells derived from embryonic tissue and from the central nervous system of adult housefly, Musca domestica. Dissociation of embryonic tissue produced several cell types of which muscle and neurone-like cells predominated. These cells were morphologically distinct once development had started in vitro with muscle cells often producing rhythmic contractions. Neurones had small cell bodies (less than 10 mum) and showed extensive outgrowth of neurites. Neuronal cell bodies readily sealed onto patch pipettes and with pipette solutions containing nicotinic agonists, single channel currents could be recorded. With cell-attached patches, these currents were inward at the cell resting potential and reversed with depolarization. Channels with two different conductances could be seen in the majority of patches.

Immunocytochemical localization of nicotinic acetylcholine receptors in the terminal abdominal ganglion of the cockroach (Periplaneta americana)

A polyclonal, monospecific antiserum raised against a nicotinic acetylcholine receptor protein affinity-purified from insect nervous tissue, was employed to demonstrate the localization of antigenic sites in the neuropile of the terminal (sixth) abdominal ganglion of the cockroach Periplaneta americana. In agreement with previously published autoradiographic mapping of specific [125I]alpha-bungarotoxin binding sites, specific areas of the central neuropile of this ganglion were densely stained, but not the cercal afferent axons. No staining was detected corresponding to the dense, peripheral, partly non-specific binding of alpha-bungarotoxin seen in autoradiographs of the same tissue. Certain peripherally located neuronal cell bodies, including the cell body of giant interneuron 2, contained intracellularly located antigenic sites.

alpha-Bungarotoxin interacts with the rat brain tachykinin receptors

alpha-Bungarotoxin (alpha Bgt) was shown to inhibit the binding of the 125I-labeled substance P (SP) and eledoisin (EL) to the rat brain membranes with Kl values of 8.0 +/- 5.0 x 10(-8) and 1.1 +/- 0.5 x 10(-6) M, respectively. Lower inhibitory activity was manifested by several other postsynaptically acting snake venom neurotoxins. The alpha Bgt inhibition of SP binding with a Kl value of 8.5 +/- 5.5 x 10(-8) M to solubilized preparations of the rat brain membranes was demonstrated. The capacity to displace SP was found for d-tubocurarine and phencyclidine, although at concentrations considerably higher than those affecting the nicotinic acetylcholine receptors (AChRs). The results obtained suggest that some of the alpha Bgt-binding polypeptides, distinct from neuronal AChRs, may be functionally associated with the tachykinin receptors (TchR).

Neuronal acetylcholine receptors in Drosophila: mature and immature transcripts of the ard gene in the developing central nervous system

The ARD protein is a Drosophila homolog of vertebrate nicotinic acetylcholine receptor (AChR) polypeptides. Here, an analysis of transcripts of the corresponding ard gene is presented. In situ hybridization experiments revealed ard gene expression in nervous tissue only. During development, ard transcripts are prevalent in late embryos, pupae, and newly eclosed flies. Both the spatial and the temporal pattern of ard gene expression is consistent with the ARD protein being part of a neuronal AChR that is produced in large amounts during major periods of neuronal differentiation. In situ hybridization with an intron-specific probe indicated codistribution of immature and mature ard RNAs in pupae and adult flies. In addition to the mature 3.2 kb RNA species, two large immature transcripts are found in newly eclosed flies but not in embryos, suggesting a developmentally regulated processing of ard RNA.

Protein phosphorylation of nicotinic acetylcholine receptors

The nicotinic acetylcholine receptor (nAcChR) is a ligand-gated ion channel found in the postsynaptic membranes of electric organs, at the neuromuscular junction, and at nicotinic cholinergic synapses of the mammalian central and peripheral nervous system. The nAcChR from Torpedo electric organ and mammalian muscle is the most well-characterized neurotransmitter receptor in biology. It has been shown to be comprised of five homologous (two identicle) protein subunits (alpha 2 beta gamma delta) that form both the ion channel and the neurotransmitter receptor. The nAcChR has been purified and reconstituted into lipid vesicles with retention of ion channel function and the primary structure of all four protein subunits has been determined. Protein phosphorylation is a major posttranslational modification known to regulate protein function. The Torpedo nAcChR was first shown to be regulated by phosphorylation by the discovery that postsynaptic membranes contain protein kinases that phosphorylate the nAcChR. Phosphorylation of the nAcChR has since been shown to be regulated by the cAMP-dependent protein kinase, protein kinase C, and a tyrosine-specific protein kinase. Phosphorylation of the nAcChR by cAMP-dependent protein kinase has been shown to increase the rate of nAcChR desensitization, the process by which the nAcChR becomes inactivated in the continued presence of agonist. In cultured muscle cells, phosphorylation of the nAcChR has been shown to be regulated by cAMP-dependent protein kinase, a Ca2+-sensitive protein kinase, and a tyrosine-specific protein kinase. Stimulation of the cAMP-dependent protein kinase in muscle also increases the rate of nAcChR desensitization and correlates well with the increase in nAcChR phosphorylation. The AcChR represents a model system for how receptors and ion channels are regulated by second messengers and protein phosphorylation.

The lipid environment of the nicotinic acetylcholine receptor in native and reconstituted membranes

Detailed knowledge of the membrane framework surrounding the nicotinic acetylcholine receptor (AChR) is key to an understanding of its structure, dynamics, and function. Recent theoretical models discuss the structural relationship between the AChR and the lipid bilayer. Independent experimental data on the composition, metabolism, and dynamics of the AChR lipid environment are analyzed in the first part of the review. The composition of the lipids in which the transmembrane AChR chains are inserted bears considerable resemblance among species, perhaps providing this evolutionarily conserved protein with an adequate milieu for its optimal functioning. The effects of lipids on the latter are discussed in the second part of the review. The third part focuses on the information gained on the dynamics of AChR and lipids in the membrane, a section that also covers the physical properties and interactions between the protein, its immediate annulus, and the bulk lipid bilayer.

Reconstitution of acetylcholine receptors into planar lipid bilayers

Obviously, bilayer reconstitution experiments have largely contributed to the understanding of the AChR-channel function. Nevertheless, at present there are many unanswered questions concerning the minimum structural requirements for AChR-channel function, agonist cooperativity, and different types of AChR. Another complex of parameters important for receptor function which must be explored in much more detail, is the dependence of AChR-channel function on membrane composition and its physical state. This important but rather neglected field is predestined to be explored by reconstitution techniques. All the results on AChRs reconstituted in planar lipid bilayers cannot adequately be discussed without the data obtained by other techniques, thus coming back to the statements already mentioned in the introductory section about strategies for investigating ion channels in general. Only such a strategy can lead to a molecular understanding of channel function.

Structure-function relationships in nicotinic acetylcholine receptors

1. A combination of molecular, biochemical, electrophysiological and immunological approaches has begun to resolve some of the questions about structure-function relationships of nicotinic acetylcholine receptors (AchRs). 2. Current structural studies suggest that models of the subunits which propose four transmembrane domains are correct. 3. It is also probable that the carboxy termini of the subunits are extracellular, while the putative amphpathic helix is intracellular. 4. Electrophysiological and ligand-binding experiments suggest that the M2 region forms the wall of the ion channel. 5. We have isolated clones from PC12 and rat brain cDNA libraries which we have shown, by functional expression, code for members of a gene family of nicotinic acetylcholine receptor subunits. 6. In situ hybridization studies have shown that the neuronal receptor subunit mRNAs are expressed in the mammalian central nervous system. 7. The muscle and neuronal nicotinic AchR subtypes we have expressed show differences in their pharmacological properties. 8. The isolation and identification of clones which code for receptors and voltage-activated ion channels will help in the understanding of a variety of disease states and assist in the design of drugs which are specific for unique molecular targets.

Central nicotinic acetylcholine receptors in the chicken and Drosophila CNS: biochemical and molecular biology approaches

Putative neuronal nicotinic acetylcholine receptors (nAChRs) were investigated using biochemical and molecular biology approaches. In the chick visual system, a nicotinic cholinergic binding site was localized on a polypeptide of Mr 57,000 using the potent antagonist, alpha-bungarotoxin (alpha-Btx). Ion flux experiments indicate that this membrane protein is different from the toxin-insensitive nAChR present in vertebrate neurons. Crosshybridization with a Torpedo nAChR cDNA probe allowed isolation of a cholinergic receptor cDNA (ARD) from the Drosophila CNS. Analysis of the corresponding gene, its transcripts and the ARD protein are presented here.

Comparison of acetylcholine and alpha-bungarotoxin binding sites in insects and vertebrates

1. The nervous tissue of locusts contains high affinity as well as low affinity binding sites for acetylcholine which display a similar nicotinic pharmacology. 2. Hill plot analysis indicated a non-cooperative binding of acetylcholine. 3. In membrane preparations from locust ganglia and mouse brain the number of binding sites for ACh was about ten fold lower than for BGTX, whereas in membranes from electric tissue both sites occurred in similar concentrations. 4. Drug binding studies suggest that the high affinity binding sites for ACh and BGTX in preparations from insect and mouse are different; whereas in electric tissue both sites are very similar. 5. Precipitation experiments using immobilized BGTX and specific antibodies indicated that in insect nervous tissue as in electric tissue the ACh and BGTX binding sites are located on the same receptor molecule and occupy distinct partially overlapping binding sites, whereas in the vertebrate brain both sites are located on distinct binding proteins.

Regulation of nicotinic acetylcholine receptors by protein phosphorylation

Neurotransmitter receptors and ion channels play a critical role in the transduction of signals at chemical synapses. The modulation of neurotransmitter receptor and ion channel function by protein phosphorylation is one of the major regulatory mechanisms in the control of synaptic transmission. The nicotinic acetylcholine receptor (nAcChR) has provided an excellent model system in which to study the modulation of neurotransmitter receptors and ion channels by protein phosphorylation since the structure and function of this receptor have been so extensively characterized. In this article, the structure of the nAcChR from the electric organ of electric fish, skeletal muscle, and the central and peripheral nervous system will be briefly reviewed. Emphasis will be placed on the regulation of the phosphorylation of nAcChR by second messengers and by neurotransmitters and hormones. In addition, recent studies on the functional modulation of nicotinic receptors by protein phosphorylation will be reviewed.

The cross-linking reagent dimethyl suberimate modifies the target size of an insect nervous system nicotinic acetylcholine receptor

Radiation inactivation and simple target theory were employed to determine the molecular weight of an insect CNS alpha-bungarotoxin binding component in the presence and absence of a cross-linking reagent, dimethyl suberimate. In the presence of the cross-linker, the number of binding sites decreased, and the apparent molecular weight (236,000) was approximately double the control value (112,000). This, together with sedimentation data, suggests that the lower value represents only a portion of the insect nicotinic receptor molecule. A model is presented to account for the increase in target size and reduction in the number of alpha-[3H]bungarotoxin binding sites in the presence of dimethyl suberimate.

Binding sites for [3H]-acetylcholine and 125I-alpha-bungarotoxin in the optic ganglion of Loligo pealii

1. In the optic ganglion of Loligo pealii, binding sites for [3H]-acetylcholine (KD: 5.2 x 10(-7) M; Bmax: 1.7 x 10(-11) mol/g tissue) and 125I-alpha-bungarotoxin (KD: 3.3 x 10(-9) M; Bmax: 9.7 x 10(-11) mol/g tissue) were observed. 2. Both sites are blocked by nicotinic compounds, but differ significantly in their affinity for individual ligands, with the acetylcholine site preferentially binding agonists, and the toxin site, antagonists. 3. The acetylcholine site is substantially more thermolabile than the toxin site. 4. A partial separation of the two binding activities is accomplished by sucrose density centrifugation. 5. These observations and a comparison with other tissues (Torpedo californica electroplaque; chick optic lobe; rat brain) suggest the presence, in the squid, of more than one kind of neuronal nicotinic receptor.