Development of a neuronal pressure sensor

Neuronal sensory systems are capable of performing very complex signal processing functions. Reconstruction of such sensory systems in vitro should enable whole-cell biological sensors to be generated that possess inherent signal processing capabilities. In this paper, the results of preliminary investigations to produce a mechanosensory neuronal network are presented. An in vitro network of rat dorsal root ganglion neurons has been produced on a microelectrode plate revealing an interesting rhythmical pattern of spontaneous discharges. This periodic activity has been shown to be disrupted following the application of a static pressure to the cell culture. These results indicate that neuronal networks represent a practical system that may be used for the development of intelligent, whole-cell, biological sensors.

Immunocytochemical localization of a putative inhibitory amino acid receptor subunit in the parasitic nematodes Haemonchus contortus and Ascaris suum

A rabbit antiserum was raised against a synthetic peptide corresponding to a region near the N-terminus of the Haemonchus contortus inhibitory amino acid receptor subunit, HG1. The antiserum recognized a recombinant form of the N-terminal domain of the subunit on Western blots and reacted with the ventral nerve cord of H. contortus in immunofluorescence experiments. Immunofluorescence was also detected in specific head neurons of H. contortus: these were tentatively identified as ring motor- and inter-neurons, plus a possible sensory neuron equivalent to the AQR cell of Caenorhabditis elegans. In the roundworm Ascaris suum, immunoreactivity was limited to the muscle arms, the post-synaptic component of the neuromuscular junction. The possible ligand of receptors containing the HG1 subunit is discussed in the light of this expression pattern.

Alternative splicing of a Caenorhabditis elegans gene produces two novel inhibitory amino acid receptor subunits with identical ligand binding domains but different ion channels

Two full-length cDNAs, gbr-2A and gbr-2B, encoding inhibitory amino acid receptor subunits have been amplified and cloned from Caenorhabditis elegans mRNA. The 5' 732 bp of the two cDNAs, encoding 237 amino acids, are identical. The 3' 758 bp of the gbr-2B cDNA are present within the 3' untranslated region of the gbr-2A clone. As a result, the two cDNAs are predicted to encode subunits which share a common extracellular N-terminal sequence of 237 amino acids, but different, though closely related, C-terminal sequences which include four predicted membrane-spanning regions. A search of the EMBL database revealed that the sequences of the two subunits are most closely related to the alpha-subunit of the C. elegans avermectin receptor. Northern blot analysis showed the presence of two related mRNAs of approximately 2.2 and 1.5 kb in a developmentally mixed population of C. elegans. The genomic DNA sequence confirms that both mRNAs were transcribed from the same gene, gbr-2, suggesting that the closely related 3' sequences have arisen as a result of a partial gene duplication event. We propose that C. elegans is utilising alternative splicing to generate receptor subunits with identical extracellular, ligand-binding domains but different transmembrane, channel forming domains.

Molecular modelling of the nicotinic acetylcholine receptor transmembrane region in the open state

A model of the nicotinic acetylcholine receptor transmembrane region has been constructed which may represent the channel in its open-state. The positions of helices flanking the ion channel match those observed by electron microscopy and previously reported by others. Residues labelled, mutated or by other means known to have a strong influence on ion flux are each accessible from the lumen of the modelled channel. The model provides new insights into our current understanding of the ion channel structure, and suggests some novel explanations for the results of labelling and mutation studies such as those involving ion channel blockers and residue-dependent changes in ion selectivity.

Reporter gene constructs suggest that the Caenorhabditis elegans avermectin receptor beta-subunit is expressed solely in the pharynx

Gene promoter/LacZ reporter constructs were made in order to analyse the expression of the beta-subunit of the Caenorhabditis elegans glutamate-gated Cl- channel (Glu-Cl) receptor. Southern blot analysis of the C. elegans cosmid C35E8 identified a 4kbp EcoRI fragment which contained the 5' portion of the Glu-Cl beta coding sequence together with 5' flanking sequences. This was subcloned and used as the template for polymerase chain reaction (PCR) amplification of a DNA fragment encoding the first 24 amino acid residues of Glu-Cl beta together with 1.4 kbp of 5' genomic sequence. The fragment was subcloned into the LacZ expression vector pPD22.11 to form a translational reporter fusion. After injection of the construct into worms, six stably transformed lines were established and assayed for beta-galactosidase activity. Stained nuclei were observed in the pharyngeal metacorpus in adults and in all larval stages, and stained nuclei were seen in many embryos undergoing morphogenesis. Additional stained nuclei towards the terminal bulb of the pharynx were observed in larval stages. These results provide further evidence that the Glu-Cl receptor mediates the glutamatergic inhibition of pharyngeal muscle via the M3 motor neurone and point to inhibition of pharyngeal pumping as a major mode of action for avermectins.

Ferrocenoyl derivatives of alamethicin: redox-sensitive ion channels

The synthesis and single-channel characterization of two redox-active C-terminal derivatives of alamethicin are herein described. The reduced [Fe(II)] forms of ferrocenoyl-alamethicin (Fc-ALM) and 1'-carboxyferrocenoyl-alamethicin (cFc-ALM) are shown to form voltage-dependent ion channels at cis positive potentials in planar lipid bilayers (PLB) with conductance properties similar to those of alamethicin. In situ oxidation of Fc-ALM [to Fe(III)] in the PLB apparatus causes a time-dependent elimination of channel openings, which can be restored by an increase in the transbilayer potential. In contrast, oxidation of cFc-ALM leads to the formation of shorter-lived channels. Pretreatment of the ferrocenoyl peptides with oxidizing agent alters their single-channel properties in a qualitatively similar manner, establishing that the changes in channel properties in the presence of oxidizing agents are due specifically to ferrocenoyl oxidation. We suggest that the redox sensitivity of these ferrocene-containing ion channels may be governed by a combination of the following factors: (1) changes in hydrophobicity; (2) alteration of peptide molecular dipole; and (3) alterations in tendencies toward self-association. However, oxidation induced changes in peptide conformation cannot be ruled out. Our results provide evidence that it is possible to engineer channel-forming peptides that respond to specific changes in the chemical environment.

A mixed helix-beta-sheet model of the transmembrane region of the nicotinic acetylcholine receptor

We have modelled the transmembrane region of the alpha 7 nicotinic acetylcholine receptor as a mixed alpha-helical/beta-sheet structure. The model was mainly based on the crystal structure of a pore-forming toxin, heat-labile enterotoxin. This is a pentameric protein having a central pore or channel composed of five alpha-helices, one from each of the 5 B subunits that form this pentamer. The remainder of this structure is beta-sheet, loops and a short alpha-helix, not included in the model. The model uses this channel as a template to build the transmembrane region, from M1 to the middle of M3. The remainder of M3 and M4 were built de novo as alpha-helices. Great consideration was given to labelling data available for the transmembrane region. In general terms, the shape of the model agrees very well with that obtained independently by electron microscopic analysis and the secondary structure predicted by the model is in accord with that estimated independently by Fourier transform infrared spectroscopy. The M2 helical region of the model is only slightly kinked, contrary to what is inferred from electron microscopic analysis, but has the same overall shape and form. On the membrane face of the model, the presence of deep pockets may provide the structural basis for the distinction between annular and non-annular lipid binding sites. Also, the transmembrane region is clearly asymmetric in the direction perpendicular to the membrane, and this may have strong influence on the surrounding lipid composition of each leaflet of the cytoplasmic membrane.

A nicotinic acetylcholine receptor subunit from insect brain forms a non-desensitising homo-oligomeric nicotinic acetylcholine receptor when expressed in Xenopus oocytes

The locust alpha-like nicotinic receptor subunit alpha L1 was expressed in Xenopus oocytes. Small but reproducible currents were elicited by application of high concentrations of nicotine, demonstrating that alpha L1 is capable of forming homo-oligomeric channels. Nicotine-evoked currents were blocked by alpha-bungarotoxin and methyllycaconitine. Comparison with chick alpha 7 receptors showed that the two receptors differ with respect to nicotine sensitivity and time course of evoked currents. Nicotine dose-response curves gave EC50 values of 24 and 830 microM for alpha 7 and alpha L1 respectively. Whereas alpha 7 responses showed characteristic fast onset and rapid desensitization within 3 s, alpha L1 currents displayed a slow onset and showed no tendency to desensitize during 45 s of agonist application. Thus alpha L1 is a novel nicotine subunit for the further exploration of structure-function relationships of ligand-gated ion channels. The question of the subunit composition of native insect receptors remains open.

The beta-subunit of Caenorhabditis elegans avermectin receptor responds to glycine and is encoded by chromosome 1

A full-length cDNA encoding the beta subunit of the recently described avermectin receptor was amplified from Caenorhabditis elegans mRNA. When this cDNA was injected into Xenopus oocytes a dose-dependent response to glycine was observed, together with a smaller response to 1 mM GABA. The EC50 of the glycine response was similar to that described previously for glutamate (0.38 mM). Hybridisation of the cDNA to polytene filters identified three yeast artificial chromosome clones that gave a positive signal, Y37B3, Y38E5, and Y24C9, all of which are mapped to chromosome 1. Hybridisation to a series of cosmid clones covering this area further mapped the gene encoding this subunit to the region -2,818 to -2,824.

Evolutionary history of the ligand-gated ion-channel superfamily of receptors

The fast-acting ligand-gated ion channels (LGICs) constitute a group that encompasses nicotinic ACh, 5-HT3, GABAA and glycine receptors. Undoubtedly, they all share a common evolutionary ancestor, and the group can therefore be considered to be a gene superfamily. Because the members of the superfamily are all receptors, it is reasonable to suppose that their common ancestor must also have been some type of receptor, and because the receptors are made of similar subunits, the ancestor was probably homo-oligomeric. Although we failed to find a group of proteins that are related evolutionarily to this superfamily, the analysis of the evolutionary relationships within the superfamily is possible and can give rise to information about the evolution of the structure and function of present-day receptors and indeed of the nervous system itself.

CEDIT: a C interface and macro facility for protein sequence alignment editing in colour with Microsoft Word 5.0 for PCs

CEDIT, a C interface and macro facility that provides for the colour editing of protein sequence alignments (up to 2000 sequences, 5000 residues each) using Microsoft Word 5.0 for PCs is presented. CEDIT uses the ability of MS-Word 5.0 to display letters with the desired colour to easily identify conservative homologies across the sequences. A glossary file with useful macros for the sequence editing is provided, along with several utilities programs for error checking, estimating sequence similarities and homology significance. CEDIT has a menu interface and a context sensitive help.

Homologies and disparities of glutamate receptors: a critical analysis

Based on analysis of aligned amino acid sequences the following statements are made: (i) There is evolutionary homology between the N-terminal extracellular region of ionotropic Glutamate receptors/Kainate Binding Proteins and a family of procaryote amino acid binding proteins. (ii) Homology of the N-terminal extracellular domain of the metabotropic glutamate receptors with a family of receptors with a guanylate cyclase intracellular domain appears to be valid. (iii) There is no evidence for homology between the N-terminal extracellular domain of the nicotinic Acetylcholine, GABA, Glycine and 5HT3 receptors and that of the ionotropic Glutamate receptors/Kainate Binding proteins. (iv) The proposal of homology for the N-terminal extracellular domain of metabotropic Glutamate receptors and that of ionotropic Glutamate receptors does not appear to hold.

Agonist pharmacology of the neuronal alpha 7 nicotinic receptor expressed in Xenopus oocytes

The potencies and efficacies of seven agonists at chick alpha 7 nicotinic receptors expressed in Xenopus oocytes were determined by whole cell recording. (+)-Anatoxin-a was the most potent agonist (EC50 = 0.58 microM) and acetylcholine was the least potent (EC50 = 320 microM). The rank order of agonist potencies was: (+)-anatoxin-a >> cytisine > (-)-nicotine > (+)-nicotine > DMPP > 1-acetyl-4-methylpiperazine methiodide > acetylcholine. DMPP evoked only very small currents: comparison of maximally effective agonist concentrations showed that DMPP was only one-fifth as efficacious as other agonists. Previously published IC50 values for rat brain [125I]alpha-bungarotoxin sites show a similar agonist profile, and the identity of homo-oligomeric alpha 7 receptors with native alpha-bungarotoxin-sensitive neuronal nicotinic receptors is discussed.

(+)-Anatoxin-a is a potent agonist at neuronal nicotinic acetylcholine receptors

The effects of the nicotinic agonist (+)-anatoxin-a have been examined in four different preparations, representing at least two classes of neuronal nicotinic receptors. (+)-Anatoxin-a was most potent (EC50 = 48 nM) in stimulating 86Rb+ influx into M10 cells, which express the nicotinic receptor subtype comprising alpha 4 and beta 2 subunits. A presynaptic nicotinic receptor mediating acetylcholine release from hippocampal synaptosomes was similarly sensitive to (+)-anatoxin-a (EC50 = 140 nM). alpha-Bungarotoxin-sensitive neuronal nicotinic receptors, studied using patch-clamp recording techniques, required slightly higher concentrations of this alkaloid for activation: Nicotinic currents in hippocampal neurons were activated by (+)-anatoxin-a with an EC50 of 3.9 microM, whereas alpha 7 homooligomers reconstituted in Xenopus oocytes yielded an EC50 value of 0.58 microM for (+)-anatoxin-a. In these diverse preparations, (+)-anatoxin-a was between three and 50 times more potent than (-)-nicotine and approximately 20 times more potent than acetylcholine, making it the most efficacious nicotinic agonist thus far described.

BIOSITE: a program for the interactive comparison of aligned homologous protein sequences

A program, BIOSITE, providing for the interactive visual comparison of aligned homologous amino-acid sequences is presented, including an example of its application. The program allows for two types of comparison sequence to be generated: an 'identity' sequence and a 'difference' sequence. These may be used on subsets of sequences and in further comparisons to identify candidate sites involved in a distinct functional property. The program should prove a useful tool for biologists engaged in understanding sequence--function relationships.

Inhibition of muscarinic cholinergic receptors by disulfide reducing agents and arsenicals. Differential effect on locust and rat

Muscarinic receptors are altered by sulfhydryl reagents. Arsenic compounds, which have been used as insecticides, exert their toxic effects by combining with sulfhydryl groups. We compared the action of arsenicals and other sulfhydryl reagents on the muscarinic receptor from invertebrate and vertebrate species (locust and rat). Disulfide-reducing reagents dithiothreitol (DTT) and British Anti-Lewisite (BAL), but not arsenicals, inhibited [3H]quinuclidinyl benzilate ([3H]QNB) binding. However, after disulfide reduction, arsenicals caused a further inhibition of muscarinic binding. The effect of DTT + arsenicals was largely irreversible. The locust receptors were more sensitive to the action of both disulfide reagents either in the absence or presence of arsenicals than the rat receptors. The sulfhydryl reagent p-chloromercuric benzoate (PCMB) was more effective at inhibiting the locust receptors than the rat receptors, but addition of arsenicals did not cause further inhibition in either the locust or rat receptors. In locust, DTT + cacodylate and DTT + arsenite caused a reduction in the number of sites without modifying the affinity of [3H]QNB binding. In rat, DTT + arsenite caused a decrease in the affinity, while DTT + cacodylate caused a decrease in the affinity of [3H]QNB binding and its number of sites. Competition experiments after DTT + cacodylate showed that the IC50 and the Hill coefficient (nH) remained unchanged in the locust. In the rat, the IC50 for atropine was increased without alteration in the nH, and both parameters were increased for carbachol. These results are explained assuming that the binding site of the locust receptor has a disulfide group similar to that of the mammalian receptor, but that the hydrophobic interactions within the binding site are weaker in the locust receptor. The higher sensitivity of the insect receptor to sulfhydryl reagents could be of interest for developing methods of pest control.

Sequence and functional expression of a single alpha subunit of an insect nicotinic acetylcholine receptor

We report the isolation and sequence of a cDNA clone that encodes a locust (Schistocerca gregaria) nervous system nicotinic acetylcholine receptor (AChR) subunit (alpha L1). The calculated molecular weight of the unglycosylated polypeptide, which contains in the proposed extracellular domain two adjacent cysteine residues which are characteristic of alpha (ligand binding) subunits, is 60,641 daltons. Injection into Xenopus oocytes, of RNA synthesized from this clone in vitro, results in expression of functional nicotinic receptors in the oocyte membrane. In these, nicotine opens a cation channel; the receptors are blocked by both alpha-bungarotoxin (alpha-Bgt) and kappa-bungarotoxin (kappa-Bgt). Reversible block of the expressed insect AChR by mecamylamine, d-tubocurarine, tetraethylammonium, bicuculline and strychnine has also been observed. These data are entirely consistent with previously reported electrophysiological studies on in vivo insect nicotinic receptors and also with biochemical studies on an alpha-Bgt affinity purified locust AChR. Thus, a functional receptor exhibiting the characteristic pharmacology of an in vivo insect nicotinic AChR can be expressed in Xenopus oocytes by injection with a single subunit RNA.

Methyllycaconitine: a selective probe for neuronal alpha-bungarotoxin binding sites

The ability of methyllycaconitine (MLA) to inhibit the binding of [125I]alpha-bungarotoxin to rat brain membranes, frog and human muscle extracts and the human muscle cell line TE671 has been measured. MLA showed a markedly higher affinity for the rat brain site (Ki 1.4 x 10(-9) M) than for the muscle receptors (Ki 10(-5)-10(-6) M). Structure modelling techniques were used to fit the structure of MLA to a nicotinic pharmacophore model. MLA is the first low molecular weight ligand to be shown to discriminate between muscle nicotinic receptors and their alpha-bungarotoxin-binding counterpart in the brain, and as such may be a useful structural probe for pursuing the structural and functional properties of the neuronal protein.

Neurotrophic activities of normal and motor neurone disease patient skeletal muscle extract on cultured rat spinal neurones

Cultured foetal rat spinal neurones, grown in serum-free medium from day 3, respond to neuronotrophic factors present in adult human skeletal muscle extracts. Levels of total protein, neurofilament protein and choline acetyltransferase (ChAT) activity are all increased approx. 2 to 3-fold at 200 micrograms extract/ml medium. Culture survival is also extended from an average of 21 days to more than 30 days. Extracts prepared from skeletal muscle biopsies of motor neurone disease (MND) patients do not differ significantly from non-MND patient muscle extracts in their ability to boost the parameter levels or survival of the cultures. The results show no evidence that neuronotrophic factors are lacking in the skeletal muscle of MND patients.

The anticonvulsant MK-801 interacts with peripheral and central nicotinic acetylcholine receptor ion channels

The effects of MK-801 [( +]-5-methyl-10,11-dihydro-5H-di-benzo[a, d]cyclohepten-5,10-imine) on peripheral and central nicotinic receptors were studied using electrophysiological and biochemical techniques. MK-801 depressed the peak amplitude and accelerated the decay of end-plate currents. The drug (1-10 microM) decreased the frequency of activation of acetylcholine (ACh)-induced single-channel currents in addition to shortening the mean open and burst times of channels activated by either ACh or (+)anatoxin-a (AnTX). MK-801 (10-40 microM) depressed the single potentials and trains of ACh and AnTX-induced potentials in chronically denervated rat soleus muscles. MK-801 blocked the twitch responses (20-100 microM) of both frog sartorius and rat diaphragm muscles evoked by stimulation of their respective nerves. Also this drug (less than 1 microM) decreased the frequency of channels activated by AnTX or ACh in outside-out patch membranes of rat retinal ganglion cells with minimal changes in the channel open time. MK-801 (10-25 microM) depressed (-)nicotine-evoked gamma-amino[2,3-3H]butyric acid release from rat hippocampal synaptosomes; however, it failed to affect the binding of [3H](-)nicotine to brain membranes and also failed to interfere with the binding of [125I]alpha-bungarotoxin to either frog muscle or Torpedo membranes. On the other hand, MK-801 inhibited the binding of [3H]perhydrohistrionicotoxin to Torpedo membranes and such an effect was more pronounced in the presence of carbamylcholine. Neither AnTX nor any other nicotinic agonist increased the binding of [3H]MK-801 to the N-methyl-D-aspartate receptor ion channel complex. The actions of MK-801 were evident at concentrations comparable with those needed to block N-methyl-D-aspartate receptors. These results demonstrate the existence of at least three different types of nicotinic AChR, all of which were blocked noncompetitively by MK-801.

Nicotinic modulation of [3H]dopamine release from striatal synaptosomes: pharmacological characterisation

Presynaptic nicotinic acetylcholine receptors on striatal nerve terminals modulate the release of dopamine. We have compared the effects of a number of nicotinic agonists and antagonists on a perfused synaptosome preparation preloaded with [3H]dopamine. (-)-Nicotine, acetylcholine, and the nicotinic agonists cytisine and 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP), at micromolar concentrations, stimulated the release of [3H]dopamine from striatal nerve terminals. Carbamylcholine was a much weaker agonist. The actions of (-)-nicotine, cytisine, and DMPP were inhibited by low concentrations of the nicotinic antagonists dihydro-beta-erythroidine, mecamylamine, pempidine, and neosurugatoxin; alpha-bungarotoxin was without effect, and extending the time of exposure to this toxin resulted in only very modest inhibition. This pharmacology points to a specific nicotinic receptor mechanism that is clearly distinct from that at the neuromuscular junction. Atropine failed to antagonise the effects of acetylcholine and carbamylcholine, suggesting that no muscarinic component is involved. The nicotinic receptor ligands (-)-[3H]nicotine and 125I-alpha-bungarotoxin bound to specific sites enriched in the synaptosome preparation. Drugs tested on the perfused synaptosomes were examined for their ability to interact with these two ligand binding sites in brain membranes. The differential sensitivity to the neurotoxins alpha-bungarotoxin and neosurugatoxin of the 125I-alpha-bungarotoxin and (-)-[3H]nicotine binding sites, respectively, leads to a tentative correlation of the (-)-[3H]nicotine site with the presynaptic nicotinic receptor on striatal nerve terminals.

Modeling of agonist binding to the ligand-gated ion channel superfamily of receptors

A generalized model is presented of agonist binding to ligand-gated ion channels (LGICs). Broad similarity in the structure of agonists suggests that the binding sites of LGICs may have evolved from a protobinding site. Aligned sequence data identified as a candidate for such a site a highly conserved 15 residue stretch of primary structure in the N-terminal extracellular region of all known LGIC subunits. We modeled this subregion, termed the cys-loop, as a rigid, amphiphilic beta-hairpin and propose that it may form a major determinant of a conserved structural binding cleft. In the model of the binding complex (1) an invariant aspartate residue at position 11 of the cys-loop is the anionic site interacting with the positively charged amine group of agonists, (2) a local dipole within the pi-electron system of agonists is favorably oriented in the electrostatic field of the invariant aspartate, (3) the epsilon ring-proton of a conserved aromatic residue at the turn of the cys-loop interacts orthogonally with the agonist pi-electron density at its electronegative center, and (4) selective recognition is partly a result of the type of amino acid residue at position 6 of the cys-loop. Additionally, formation of a hydrogen bond between the electronegative atom of the pi-electron system of agonist and a complementary group in the receptor may be important in the high-affinity binding of agonists.

Differential effect of ethanol on muscarinic cholinergic binding to rat and locust neural membranes

We have compared the effect of ethanol, a membrane perturbant, on the muscarinic binding sites in neural membranes from a vertebrate (rat) and an insect (locust). The binding of the muscarinic antagonist [3H]quinuclidinyl benzilate ([3H]QNB) to both rat and locust neural membranes was inhibited by ethanol at 10-500 mM concentrations; but this inhibition was greater in the locust. Ethanol (500 mM) increased the apparent dissociation constant (KD') of [3H]QNB binding to rat membranes from 0.13 +/- 0.01 nM in control to 0.20 +/- 0.02 nM; there was also an small but significant reduction in the number of binding sites Bmax. In locust, 500 mM ethanol reduced the Bmax of [3H]QNB binding from 590 +/- 30 in control to 320 +/- 40 pmol/g protein; no significant alteration in the KD was detected. The dissociation rate constant (koff) of [3H]QNB increased from 0.020 +/- 0.003 in controls to 0.031 +/- 0.004 (min-1) in the presence of 500 mM ethanol, the association rate constant (Kon) did not change significantly. In locust, 500 mM ethanol did not affect either Kon or Koff. Competition experiments revealed that the binding affinities of both the agonist carbamylcholine and the antagonist atropine to the rat membranes were reduced in the presence of ethanol. In contrast, ethanol caused no alteration in the binding affinities of these ligands to the locust membranes. This differential effect of ethanol on rat and locust muscarinic binding suggests a difference in the hydrophobic domains and/or the membrane interactions of the muscarinic receptors in the two species.

Stereoselective nicotine-induced release of dopamine from striatal synaptosomes: concentration dependence and repetitive stimulation

Using a sensitive perfusion system we have studied the nicotine-induced release of [3H]dopamine ([( 3H]DA) from striatal synaptosomes. Nicotine-evoked release was concentration dependent with an EC50 of 3.8 microM. The response to 1 microM nicotine was comparable to that to 16 mM K+; 10 microM veratridine evoked a larger response. All three stimuli were Ca2+ dependent but only the response to veratridine was blocked by tetrodotoxin. Repetitive stimulations by 1 microM (-)-nicotine (100 microliters) at 30-min intervals resulted in similar levels of [3H]DA release; higher concentrations of (-)-nicotine resulted in an attenuation of the response particularly following the third stimulation. This may reflect desensitisation or tachyphylaxis of the presynaptic nicotinic receptor. The action of nicotine was markedly stereoselective: a 100-fold higher concentration of (+)-nicotine was necessary to evoke the same level of response as 1 microM (-)-nicotine. It is proposed that these presynaptic nicotinic receptors on striatal terminals are equivalent to high-affinity nicotine binding sites described in mammalian brain.

An ELISA for the detection of anti-acetylcholine receptor antibodies using biotinylated alpha-bungarotoxin

An antibody-capture enzyme immunoassay has been developed for the detection of anti-acetylcholine receptor (AChR) antibodies in tissue culture supernatants using biotinylated alpha-bungarotoxin (B alpha BGT). Immunoglobulins in culture supernatants were bound indirectly to microtitre plates via an anti-globulin antibody already coupled to polyvinyl plates. Anti-AChR antibodies were then detected by incubation with AChR crude extract. Bound AChR was revealed by incubation with B alpha BGT followed by horseradish peroxidase-conjugated avidin. This assay is specific, more sensitive than the commonly used double antibody radioimmunoassay, avoids the use of radioactive material, is practical for large numbers of samples and is particularly suitable for detecting anti-AChR antibodies in tissue culture supernatants.

Purification and partial characterization of 4-aminobutyrate:2-oxoglutarate aminotransferase from sheep brain and locust ganglia

We report here the first purification to homogeneity of 4-aminobutyrate: 2-oxoglutarate aminotransferase (EC 2.6.1.19) (GABA-T) from an invertebrate source (locust) and its initial comparison with that of GABA-T from mammalian brain (sheep). The enzyme from both organisms was found to be a dimer of similar-sized subunits, with a native Mr of approx. 97,000. The pI of GABA-T from the locust was 6.7 and that of the sheep enzyme was 5.5. Michaelis constants for 4-aminobutyric acid (GABA) and 2-oxoglutarate were respectively 0.79 +/- 0.16 mM and 0.27 +/- 0.08 mM for the locust enzyme and 2.2 +/- 0.24 mM and 0.22 +/- 0.11 mM for the sheep enzyme. 5-(Aminomethyl)-3-isoxazolol (muscimol) was a competitive inhibitor of both enzymes, whereas 5-amino-1,3-cyclohexadienylcarboxylic acid (gabaculine) acted as a potent suicide substrate. However, 3-aminopropane-1-sulphonic acid, diaminobutyric acid, 1,2,3,4-tetrahydro-1-methyl-3-pyridinecarboxylic acid (isoguvacine), beta-(aminomethyl)-4-chlorobenzenepropanoic acid (baclofen), bicuculline and picrotoxin did not inhibit either enzyme at concentrations below 100 mM. Polyclonal antisera raised against GABA-T from the sheep failed to cross-react with the enzyme from locust in either an Ouchterlony immunodiffusion plate or a competitive enzyme-linked immunosorbent assay. The purification procedures differed considerably. Ion-exchange chromatography, which was found suitable for the purification of GABA-T from the sheep, was ineffective with locust enzyme, which was finally purified by hydrophobic-interaction chromatography and chromatofocusing.

Methyllycaconitine and (+)-anatoxin-a differentiate between nicotinic receptors in vertebrate and invertebrate nervous systems

Specific high-affinity binding sites for 125I-alpha-bungarotoxin and (-)-[3H]nicotine have been measured in rat brain and locust (Schistocerca gregaria) ganglia. The binding sites for 125I-alpha-bungarotoxin had similar Kd values of 1.5 x 10(-9) and 0.8 x 10(-9) M for rat and locust preparations, respectively; the corresponding values for the (-)-[3H]nicotine-binding site were 9.3 x 10(-9) and 1.7 x 10(-7) M. Methyllycaconitine (MLA) potently inhibited 125I-alpha-bungarotoxin binding in both rat and locust. MLA was a less effective inhibitor of (-)-[3H]nicotine binding whereas (+)-anatoxin-a was a very potent inhibitor at this site in the rat but not in the locust. These data suggest that (+)-anatoxin-a is a useful probe for the high-affinity nicotine-binding receptor in vertebrate brain, whereas MLA is a preferential probe for the subclass of receptor that binds alpha-bungarotoxin.

GABA binding to receptor sites in locust supraoesophageal ganglia

The binding of 4-amino-n-[2,3-3H]butyric acid (GABA) to receptor sites in the supraoesophageal ganglia of the locust Schistocerca gregaria is reported. Binding is saturable with a Kd of 30 nM and a Bmax of 150 fmol/mg protein. Binding is sodium-independent with a pH optimum of 6.8 and the pharmacological properties of the site suggest a receptor rather than an uptake or transport protein. The assay is being utilised in a comparative study of the binding sites of the GABA receptor and the enzyme 4-aminobutyrate: 2-oxoglutarate amino-transferase (EC 2.6.1.19, GABA-T). GABA binds to at least 4 proteins in the nervous system of vertebrates: the GABAA and GABAB receptors, GABA-T the enzyme involved in the GABA shunt, and the GABA transport system. In the invertebrates the status of these GABA-binding proteins is less well established. There are reports of a GABA receptor complex resembling the GABAA receptor; GABA-T activity has been reported and we have recently purified the enzyme from locust ganglia; it is assumed that GABA uptake systems are present in invertebrates. Proteins with different functions which specifically bind the same ligand are interesting from an evolutionary point of view. Are they distinct gene products or is the sub-unit of the receptor which binds GABA an enzyme which has lost the ability to bind pyroxidal phosphate? Do either receptor or enzyme differ significantly from their mammalian counterparts?

The neurotoxin histrionicotoxin interacts with the putative ion channel of the nicotinic acetylcholine receptors in the central nervous system

Perhydrohistrionicotoxin at micromolar concentrations blocked the nicotine-evoked transmitter release from perfused striatal (dopaminergic) and hippocampal (cholinergic) nerve terminals. Perhydrohistrionicotoxin failed to compete with [3H]nicotine for its high-affinity binding site in rat brain, suggesting that the action of this toxin on central nicotinic receptors is noncompetitive. From the dose-response curve, 50% inhibition of nicotine-evoked striatal dopamine release occurred at 5 microM perhydrohistrionicotoxin, a value similar to that obtained in frog sartorius muscle and Electrophorus electroplax. This close agreement may suggest that the ionic channel of the presynaptic nicotinic acetylcholine receptor of brain neurons has similar properties to those of the peripheral receptor.

Immunological factors in neuronal degeneration with particular reference to motor neurone disease

Cellular and humoral immunoreactivity to neuronal antigens was investigated in patients with motor neurone disease (MND). Lymphocytes from patients with MND and normal healthy controls were cultured with a membrane fraction prepared from cultured spinal cord neurones. 4 out of 14 patients with MND and 0 out of 9 normal controls showed a significantly increased stimulation index. An enzyme-linked immunoabsorbent assay (ELISA) was established to detect antibodies to synaptic membrane fraction prepared from human motor cortex. Sera from MND patients showed a significantly increased immunoglobulin binding with respect to normal control sera. Antineuronal antibody production by MND lymphocytes was studied by using Epstein-Barr virus transformation followed by fusion with a mouse myeloma cell line. Antibody-producing clones were isolated. This procedure would allow a more detailed analysis of the antineuronal antibody production in MND.

Subcellular fractionation and distribution of cholinergic binding sites in fetal human brain

Conventional subcellular fractionation techniques have been applied to human fetal brain (13-15 weeks gestation) and the fractions have been characterized by assaying for marker enzymes, cholinergic binding sites and electron microscopy. Fractionation of the homogenate resulted in a nuclear pellet (P1), a crude mitochondrial pellet (P2) and a supernatant (S2). Further resolution of the P2 fraction by density gradient centrifugation resulted in two bands at the gradient interfaces and a pellet. The P2 and subsequently the P2B fraction contained intact plasma membrane profiles as judged by the predominance of adenylate cyclase activity and the presence of occluded lactate dehydrogenase which constituted over 70% of the total activity in these fractions. Morphological examination of the gradient fractions revealed that the P2B fraction contains membrane bound structures which resemble synaptosomes prepared from neonatal rat brain. These structures have a granular matrix in which mitochondria and frequently, neurofilaments were observed. Very few synaptic vesicles were present and there was no evidence for post synaptic attachments. The cholinergic markers choline acetyltransferase, acetylcholinesterase and receptor sites defined by quinuclidinyl benzilate and alpha-bungarotoxin binding were enriched in fractions P2 and P2B which contained the bulk of nerve ending particles. This enriched preparation of fetal synaptosomes may be valuable for functional studies on pre-synaptic terminals in developing brain.

Decreased autologous mixed lymphocyte reaction in myasthenia gravis

Myasthenia gravis is an autoimmune disease in which the autoantigen, the acetylcholine receptor at the neuromuscular junction, is well characterized. As with many other autoimmune diseases, however, the basic cause of immune malfunction is unknown. The autologous mixed lymphocyte reaction (AMLR) involves the proliferation of T lymphocytes when co-cultured with autologous non-T cells and may reflect in vivo mechanisms of immune control. We have measured the AMLR in 22 patients with myasthenia gravis and found the magnitudes of the peak responses to be significantly depressed compared to those of 41 normal healthy controls. Proliferative responses of T cells from myasthenic patients to the mitogens, Concanavalin A and Phytohaemagglutinin-P, were also found to be significantly depressed relative to controls. These abnormal immune cell responses can be, in part, interpreted in terms of defective suppressor cell functions in myasthenia gravis.

Cultured rat spinal cord neurons: interaction with motor neuron disease immunoglobulins

Conditions have been developed for the culture of rat spinal cord neurons in serum-free media supplemented with hormones and growth factors. Neurons were identified by immunofluorescence-labeled anti-neurofilament antibody, and their growth was monitored by assay of choline acetyltransferase and cholinesterase activities. Activities of these enzymes were considerably higher than those of comparable cultures in serum supplemented media in which there were visibly many more nonneuronal cells. Serum immunoglobulins from patients with motor neuron disease showed enhanced binding to rat spinal cord cells maintained in both serum-supplemented and serum-free media, as compared with those from normal healthy individuals. Enhanced binding was more marked with the latter cells, presumably because of the higher proportion of neuronal cells in these cultures. Serum immunoglobulins from patients with other neurologic disorders showed a similar binding to that of the normal controls. The results demonstrate the presence of an immune response to spinal cord cell membrane components in patients with motor neuron disease, although whether the response is primary or secondary in the disease process remains unclear.

Complement-dependent toxicity of serum from myasthenic patients to muscle cells in culture

The toxicity of myasthenic sera to rat myotubes in monolayer culture was examined by measuring the release of [Me-3H]carnitine from pre-loaded cells. In the presence of guinea pig complement, heat-inactivated serum samples from 9 out of 13 myasthenic patients showed clear myotoxicity, in contrast to 0 out of 11 normal controls and 0 out of 6 polymyositis patients. Neither heat-inactivated sera alone nor guinea pig complement sera alone showed myotoxicity. Removal of anti-acetylcholine receptor (anti-AChR) antibodies from a myasthenic serum sample by affinity absorption led to loss of myotoxicity. Myotoxicity of myasthenic sera could, in most cases, be confirmed by light microscopy. These results support the idea that complement-mediated cell damage, initiated by anti-AChR antibodies, contributes to post-synaptic membrane degeneration in myasthenia gravis.

Partial purification and characterisation of an acetylcholine receptor with nicotinic properties from the supraoesophageal ganglion of the locust (Schistocerca gregaria)

An alpha-bungarotoxin-binding component has been partially purified from the supraoesophageal ganglion of the locust, (Schistocerca gregaria). The component binds alpha-bungarotoxin with a Kd of about 1.7 nM and this value changes little throughout the purification procedure. The specific binding activity ranges from 1.18 pmol alpha-bungarotoxin bound/mg protein for the membrane-bound site up to a maximum of 230 pmol bound/mg protein for the partially purified component. The pharmacological properties of the membrane-bound site are predominantly nicotinic. Affinity labelling of the binding species with 4-(N-maleimido)-[3H]benzyltrimethylammonium suggests that the binding is associated with a peptide of Mr 58000. Polyacrylamide gel electrophoresis of the partially purified of binding component shows three major bands corresponding to Mr of 60000, 41000 and 25000. We suggest that the binding component can be tentatively identified as a nicotinic acetylcholine receptor.

Glutamate receptor binding in insects and mammals

High affinity stereospecific binding sites for L-glutamate have been reported in several regions of mammalian brain. The binding sites in the hippocampus and cerebellum have been studied more extensively than binding in other brain regions. The hippocampal and cerebellar binding sites show similar properties with respect to their pharmacology and their independence of Na+. There is evidence, particularly good in the case of hippocampus, of mechanisms that may regulate the availability of the binding sites in both brain areas. Some progress has been made with the isolation of the hippocampal binding site but the protein has not been extensively characterised. In the case of insect muscle, high-affinity stereospecific binding of L-glutamate to whole membrane preparations, to detergent-solubilised membranes and to isolated proteolipids has been reported. Much greater variability in the binding characteristics is seen than is the case with the mammalian brain preparations. Preliminary experiments suggest that at least four distinct binding sites may be present on insect muscle. The complete characterisation of glutamate binding sites is at present precluded by a lack of potent agonists and antagonists. However, recent advances in the pharmacological classification of receptor sites for the excitatory amino acids in mammalian brain could provide sufficient information to permit the identification of the binding sites as synaptic receptors. Invertebrate toxins whose site of action is the insect neuromuscular junction may well prove to be useful tools with which to isolate and characterise the synaptic receptor proteins.

An assessment of radioimmunoassay procedures for determination of anti-acetylcholine receptor antibodies in the sera of patients with myasthenia gravis

A reproducible radioimmunoassay procedure for the determination of anti-acetylcholine receptor antibodies in the sera of patients with myasthenia gravis is described and examined in detail. The assay combines features of a number of methods previously outlined and allows repeat determinations of antibody titre in a given myasthenic serum sample with coefficient of variation 6%. The mean +/- standard deviation for normal human serum anti-acetylcholine receptor antibodies was found by this procedure to be 0.024 +/- 0.033 nmol/l alpha-bungarotoxin binding sites whereas the range for myasthenic patients was 0-139.14 nmol/l with a mean value of 7.55 nmol/l alpha-bungarotoxin binding sites.