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At least 19 recordsLinked to original sources
PubMed1977

Postsynaptic membranes in the electric tissue of Narcine: IV. Isolation and characterization of the nicotinic receptor protein.

The nicotinic receptor protein of the electric tissue of Narcine was purified in several different media by partial isolation of postsynaptic membranes and affinity chromatography. Protease inhibitors were found to be necessary to prevent degradation of the protein, and both EDTA and Tris buffer were used in addition to prevent intramolecular crosslinking of 44,000 and 58,000 dalton subunits by tissue factors. The intact protein was found to have a molecular weight close to 400,000, and appears to be composed of four subunits of 44,000 daltons, two to three of 48,000, one of 58,000 and one of 65,000. All the subunits are glycoproteins and their amino acid compositions show similar hydrophobicity and acidity, suggesting similar positioning in postsynaptic membranes. Crosslinking experiments showed that acetylcholine and alpha-bungarotoxin bind to the smallest subunit, and suggest the juxtaposition of at least two of these subunits, and of all four toxin molecules bound to a receptor molecule. Morphological studies of the protein in membranes and after purification indicated cylindrial molecules with central cores.

Acetylcholine↗
PubMed1978

Distribution of an alpha-bungarotoxin-binding cholinergic nicotinic receptor in rat brain.

Cholinergic nicotinic receptors in rat brain were demonstrated by the use of the potent nicotinic antagonist [125I]alpha-bungarotoxin [125I]alpha-Btx). Biochemical studies on binding of [125I]alpha-Btx to rat hippocampal homogenates revealed saturable binding sites which are protected by nicotine, D-tuborcurarine and acetylcholine but not by atropine or oxotremorine. The hippocampus and hypothalamus displayed relatively high [125I]alpha-Btx specific binding whereas the cerebellum was devoid of specific binding. Other regions displayed intermediate binding levels. Analysis of the regional distribution of [125I]alpha-Btx binding by autoradiography of frontal brain sections revealed high labeling in the hippocampus, hypothalamic supraoptic, suprachiasmatic and periventricular nuclei, ventral lateral geniculate and the mesencephalic dorsal tegmental nucleus. It is suggested that the limbic forebrain and midbrain structures as well as sensory nuclei are the main nicotinic cholinoceptive structures in the brain.

Animals↗
PubMed1978

Blockade of desensitization of nicotinic receptors of the cat adrenal medulla by concanavalin A.

1 The possibility of concanavalin A (Con A) blocking the development of desensitization of nicotinic receptors of the cat adrenal gland has been investigated. 2 During perfusion of the adrenal gland with Krebs-bicarbonate solution containing acetylcholine (ACh), the rate of catecholamine (CA) secretion was very high in the first 2 min; thereafter, as perfusion with ACh was continued the output fell, to reach about 20% of the initial value in 10 minutes. When the adrenal gland was pretreated with Con A, the subsequent desensitization of release during continued infusion of ACh was prevented. 3 When the adrenal gland was perfused with high K+ solution, there was always a large initial secretion of CA, and as perfusion with high K+ continued the output fell, to reach about 15% of the initial rate in 10 minutes. Con A did not affect the rate of CA secretion induced by high K+. 4 It is tentatively suggested that Con A blocks the desensitization of CA secretion evoked by ACh by interaction with the glycoprotein moiety of the nicotinic receptor of adrenal chromaffin cells.

Acetylcholine↗
PubMed1978

Interaction of the acetylcholine (nicotinic) receptor protein from Torpedo marmorata electric organ with monolayers of pure lipids.

Membrane fragments rich in cholinergic (nicotinic) receptor protein were purified from the electric organ of Torpedo marmorata. Their lipid composition is essentially characterized by the prominence of cholesterol, phosphatidylethanolamine and phosphatidylcholine, long-chain fatty acyl constituents, and the absence of sphingomyelin. Solubilised receptor was purified from these fragments and the concentration of sodium cholate lowered by dialysis to 0.01% (w/v). When this preparation was injected under a lipid monolayer, an increase of surface pressure developed, which was not observed with the detergent alone nor in the absence of lipid film. When covalently radiolabelled receptor preparations were injected at a constant surface pressure the radioactivity recovered with the film was proportional to the increase in area. It is concluded that the pressure or area increases are due to the penetration of the cholinergic receptor protein into the lipid film. Incorporation experiments into films formed from various pure lipids showed that the protein interacts more readily with cholesterol than with ergosterol, phosphatidylcholine, or other phospholipids. Its affinity is also higher for long-chain phosphatidylcholines than for short-chain ones. The degree of unsaturation and fluidity of the 3-sn-phosphatidylcholine (lecithin) films are of secondary importance. Parallel experiments with covalently and non-covalently labelled receptor preparations showed that part of the protein recovered with the film lost its alpha-toxin binding ability during the penetration. Similar data were obtained with the receptor purified from Electrophorus electricus electric organ.

Animals↗
PubMed1977

Postsynaptic membranes in the electric tissue of Narcine: II. A freeze-fracture study of nicotinic receptor molecules.

The ventral, postsynaptic membranes of the electroplaques of Narcine were found to containe intramembranous particles similar in location, packing density (about 5700/micron 2), transmembrane position and end appearance to nicotinic acetylcholine receptor-channel molecules. In fixed tissue the particles were limited to the cytoplasmic lamina, while in unfixed tissue an equivalent number were found symmetrically in both laminae. Four populations of particle diameters were observed in each unfixed lamina, even though other morphological evidence indicates the presence of large number of molecules of uniform structure, and biochemical studies of isolated postsynaptic membranes indicate that at least 70% of the membrane protein is receptor-channel protein. Intramembranous particles in dorsal, non-innervated electroplaque membranes, presumably representing Na+, K+-associated ATPase and other channel proteins, were found to have similar characteristics to particles in ventral membranes. Receptor-channel molecules cannot, therefore, be distinguished from other intrinsic membrane proteins by freeze-replication alone.

Animals↗
PubMed1978

Properties of an alpha-bungarotoxin-binding cholinergic nicotinic receptor from Drosophila melanogaster.

alpha-[125I]Bungarotoxin specifically binds to homogenates of Drosophila melanogaster head at levels of 0.3-0.8 pmol/mg protein. The dissociation constant calculated from rates of association and dissociation of toxin.receptor complex, is 0.6.10(-9) M. Ca2+, and to a lesser extent Na+, inhibit the reaction. alpha-[125I]Bungarotoxin binding is inhibited by low concentrations of unlabelled toxin, nicotinic ligands and eserine, but not by low concentrations of muscarinic ligands, decamethonium or an organophosphate. The receptor is membrane bound and can be partially released into 100 000 X g supernatant by combination of 1 M NaCl and 1% Triton X-100. Most of the activity in the supernatant sediments after further centrifugation at 200 000 X g for 2 h. Toxin binding sites are distinct from acetylcholinesterase molecules as revealed by pharmacological, biochemical and genetic techniques. The gene for the toxin-binding nicotinic receptor in Drosophila is apparently not located adjacent to the gene for acetylcholinesterase.

Acetylcholinesterase↗
PubMed1979

Immunologic similarities between the hypothalamic alpha-bungarotoxin receptor and the Torpedo californica nicotinic cholinergic receptor.

The solubilized rat central nervous system (hypothalamic) nicotinic cholinergic receptor and the Torpedo nicotinic cholinergic receptors are immunologically similar and show greater than 60% immunologic cross-reactivity using a double-antibody technique. Antibodies to the Electrophorus and Torpedo receptors also decrease the rate of alpha-bungarotoxin binding to these membraneous receptors. It is concluded that the Torpedo and hypothalamic nicotinic receptors are immunologically similar and that receptor binding sites for alpha-bungarotoxin and antibodies are physically close. These studies indicate that alpha-bungarotoxin can be used to study the nicotinic cholinergic receptor of the rat hypothalamus.

Animals↗
PubMed1979

Nicotinic cholinergic receptors in brain synaptosomes.

In order to elucidate pharmacological characteristics of nicotinic receptors in the brain, competitive binding of nicotine analogues or cholinergic agents to brain particles was studied utilizing [3H]nicotine. The binding of [3H]nicotine to brain crude nitochondrial or synaptosomal fraction was progressively inhibited by the addition of increasing amounts of native nicotine or nornicotine, but cotinine had little effect. Of the myelin, synaptosomal and mitochondrial subfractions of the crude mitochondrial fraction, [3H]nicotine binding was almost exclusively confined to synaptosomes. In this binding, the affinity constant was 1.97 X 10(9)/M and the maximum binding capacity was 7.30 X 40(-8) M/g of protein. This binding was reduced by 50% in the presence of 4 X 10(-7) M D-tubocurarine and 10(-6) M carbamylcholine, while atropine was the least effective of the drugs tested. These findings suggest the possible existence of nicotinic cholinergic receptors in the synaptic region. A study of regional differences in nicotinic cholinergic receptors showed that higher specific bindings of nicotine to synaptosomes occurred in the hypothalamus, hippocampus and thalamus. These findings suggest that the nicotinic cholinergic mechanism plays an important role in these regions of the central nervous system.

Animals↗