Alpha-bungarotoxin binding and central nervous system nicotinic acetylcholine receptors

Nicotinic agonists regulate alpha-bungarotoxin binding sites of TE671 human medulloblastoma cells

The TE671 human medulloblastoma cell line expresses a variety of characteristics of human neurons. Among these characteristics is the expression of membrane-bound high-affinity binding sites for alpha-bungarotoxin, which is a potent antagonist of functional nicotinic acetylcholine receptors on these cells. These toxin binding sites represent a class of nicotinic receptor isotypes present in mammalian brain. Treatment of TE671 cells during proliferative growth phase with nicotine or carbamylcholine, but not with muscarine or d-tubocurarine, induced up to a five-fold increase in the density of radiolabeled toxin binding sites in crude membrane fractions. This effect was blocked by co-incubation with the nicotinic antagonists d-tubocurarine and decamethonium, but not by mecamylamine or by muscarinic antagonists. Following a 10-13 h lag phase upon removal of agonist, recovery of the up-regulated sites to control values occurred within an additional 10-20 h. These studies indicate that the expression of functional nicotinic acetylcholine receptors on TE671 cells is subject to regulation by nicotinic agonists. Studies of the murine CNS have consistently indicated nicotine-induced up-regulation of nicotinic acetylcholine receptors, thereby supporting the identification of the toxin binding site on these cells as the functional nicotinic receptor. Although a mechanism for this effect is not apparent, nicotine-induced receptor blockade does not appear to be involved.

Nicotine enhances cerebral glucose utilization in central components of the rat visual system

The effect of nicotine in visual system components of the rat brain was examined using the 2-deoxy-D-[1-14C]-glucose ([14C]DG) technique. Nicotine was administered subcutaneously (SC) at doses of 0.1, 0.3, and 1.0 mg/kg 2 min before the infusion of the radiotracer. Local cerebral glucose utilization (LCGU) was stimulated by nicotine in a dose-dependent manner in many brain regions associated with the visual system. Increases of over 100% were seen in the superior colliculus, nucleus of the optic tract, and portions of the accessory optic system (medial and dorsal terminal nuclei, and the inferior fasciculus). Statistically significant increases were also observed in the lateral geniculate nucleus, the lateral terminal nucleus and the cerebellum. The effects were blocked by pretreatment with mecamylamine and by enucleation. The findings lend support to the involvement of the nicotinic cholinergic system in the processing of visual information or visual-motor function.

Sulfhydryl modification of two nicotinic binding sites in mouse brain

Two distinct binding sites with properties corresponding to those expected for nicotinic cholinergic receptors can be identified in brain by the specific binding of nicotine (or acetylcholine) and alpha-bungarotoxin. The effects of modification of these binding sites by treatment with the disulfide-reducing agent dithiothreitol were examined in tissue prepared from DBA mouse brains. Treatment with dithiothreitol reduced the binding measured with either ligand, and reoxidization of the disulfides fully restored binding. The effects of dithiothreitol treatment appeared to be due to a reduction in the maximal binding of nicotine and to a decrease in the binding affinity for alpha-bungarotoxin. Agonist affinity for the alpha-bungarotoxin binding site was reduced by treatment with low concentrations of dithiothreitol. The nicotine binding sites remaining after disulfide treatment displayed rates of ligand association and dissociation similar to those of unmodified tissue, but treatment of previously unmodified tissue with dithiothreitol accelerated the rate of nicotine dissociation. After reduction, both binding sites could be selectively alkylated with bromoacetylcholine. The results suggest that both putative nicotinic receptors in brain respond similarly to disulfide reduction and that their responses resemble those known for the nicotinic receptor of electric tissue.

Perturbations of locomotor activity rhythms following suprachiasmatic bungarotoxin infusion

The actions of intracranial injections of alpha bungarotoxin (BTX) on locomotor activity rhythms were examined in male rats. The hypothalamic suprachiasmatic region is known to be a locus of high affinity BTX binding although the potential roles of this receptor system are unknown. Rats were stereotaxically implanted with cannulae aimed just dorsal to the SCN (or cortex for control injections). Free-running locomotor activity rhythms in darkness were constantly monitored. Seventy-eight percent (78%) of the animals injected with BTX in the SCN region had phase shifts that were outside the 99% confidence limits of control animals. Infusion of either saline (into the SCN) or BTX into cortex were without effects. Doses of BTX varied from 125 fmol to 600 pmol. At the highest dose a substantial fraction of the animals showed both period changes and loss of rhythmicity as well as phase shifts. Although nearly all of the animals injected with BTX experienced phase shifts the direction of the shifts were not consistently correlated with the circadian time of injection. However, the sensitivity of the animals varied systematically with the smallest shifts resulting after injection at CT12 and CT16. These results argue that BTX does not influence the SCN pacemaker as an entraining signal but does potently perturb the circadian system.

Distribution of nicotinic cholinergic receptors in the rat tel- and diencephalon: a quantitative receptor autoradiographical study using [3H]-acetylcholine, [alpha-125I]bungarotoxin and [3H]nicotine

The topographical distribution of [alpha-125I]bungarotoxin [125I]BTX, [3H]nicotine ([3H]Nic), [3H]acetylcholine ([3H]ACh) (in the presence of atropine) binding in rat tel- and diencephalon was investigated using a quantitative receptor autoradiographical technique. With the [3H]ACh and [3H]Nic radioligands, a strong labelling was observed in various thalamic nuclei, including the medial habenula, a moderate labelling in different areas of the cortex cerebri, the nucleus caudatus putamen, the nucleus accumbens and tuberculum olfactorium and a uniform weak labelling in the hypothalamus. When the binding data for [3H]Nic were plotted against binding data for [3H]ACh in various brain nuclei, a significant correlation was obtained. Considering [125I]BTX, the strongest labelling was observed in the lateral mammillary nucleus and the hilus gyrus dentatus of the hippocampal formation. A weak labelling occurred in areas such as the nucleus causatus putamen, the thalamus and the cerebral cortex. No significant correlation was therefore obtained between the degree of [125I]BTX binding in various brain nuclei and the degree of binding observed with [3H]Nic or [3H]ACh. The present results underline the view that the high-affinity [3H]Nic and [3H]ACh binding sites label the same cholinergic nicotinic receptor binding site, while [125I]BTX labels another subpopulation of nicotinic cholinergic receptors, predominantly found in discrete areas of the hypothalamus and the limbic cortex.

Selective antagonism of nicotine actions in the rat cerebellum with alpha-bungarotoxin

Nicotine, locally applied to identified neurons in the rat cerebellar cortex, excites inhibitory interneurons, but depresses the discharge of Purkinje cells. Alpha-bungarotoxin blocked the excitatory actions of nicotine on the inhibitory interneurons. The antagonism of nicotine excitatory actions is largely irreversible and also insurmountable with higher doses of nicotine. The antagonism by alpha-bungarotoxin is, in addition, selective since there is no blockade of the inhibitory actions of nicotine on Purkinje neurons. The present data suggest that the excitatory actions of nicotine on inhibitory interneurons are mediated by neuromuscular-type nicotinic receptors in the cerebellum. Moreover, the present data also supports the hypothesis of multiple nicotinic sites of action in mammalian brain.

Molecular studies of the neuronal nicotinic acetylcholine receptor family

Nicotinic acetylcholine receptors on neurons are part of a gene family that includes nicotinic acetylcholine receptors on skeletal muscles and neuronal alpha bungarotoxin-binding proteins that in many species, unlike receptors, do not have an acetylcholine-regulated cation channel. This gene superfamily of ligand-gated receptors also includes receptors for glycine and gamma-aminobutyric acid. Rapid progress on neuronal nicotinic receptors has recently been possible using monoclonal antibodies as probes for receptor proteins and cDNAs as probes for receptor genes. These studies are the primary focus of this review, although other aspects of these receptors are also considered. In birds and mammals, there are subtypes of neuronal nicotinic receptors. All of these receptors differ from nicotinic receptors of muscle pharmacologically (none bind alpha bungarotoxin, and some have very high affinity for nicotine), structurally (having only two types of subunits rather than four), and, in some cases, in functional role (some are located presynaptically). However, there are amino acid sequence homologies between the subunits of these receptors that suggest the location of important functional domains. Sequence homologies also suggest that the subunits of the proteins of this family all evolved from a common ancestral protein subunit. The ligand-gated ion channel characteristic of this superfamily is formed from multiple copies of homologous subunits. Conserved domains responsible for strong stereospecific association of the subunits are probably a fundamental organizing principle of the superfamily. Whereas the structure of muscle-type nicotinic receptors appears to have been established by the time of elasmobranchs and has evolved quite conservatively since then, the evolution of neuronal-type nicotinic receptors appears to be in more rapid flux. Certainly, the studies of these receptors are in rapid flux, with the availability of monoclonal antibody probes for localizing, purifying, and characterizing the proteins, and cDNA probes for determining sequences, localizing mRNAs, expressing functional receptors, and studying genetic regulation. The role of nicotinic receptors in neuromuscular transmission is well understood, but the role of nicotinic receptors in brain function is not. The current deluge of data using antibodies and cDNAs is beginning to come together nicely to describe the structure of these receptors. Soon, these techniques may combine with others to better reveal the functional roles of neuronal nicotinic receptors.

Internalization of (125I) alpha-bungarotoxin into rat suprachiasmatic nucleus neurons and dendrites

The distribution and rate of internalization of (125I) alpha-bungarotoxin (alpha-BuTX) was evaluated by quantitative electron microscopic autoradiography in rat suprachiasmatic nucleus. The ultrastructural distribution of silver grains was studied by both line source and 50% probability circle analyses. Line source analysis demonstrated that with increasing time following intracerebroventricular infusion, ligand is internalized from neuronal membranes. At all time points, real grain distribution was significantly different from randomly generated hypothetical grains. Quantification of silver grain localization by probability circle analysis indicated that membrane bound sources were mainly associated with axo-dendritic appositions, regardless of the length of time the tissue was exposed to radioligand. The compartment containing synaptic terminals was the most enriched when comparing real to hypothetical grains. By eight hours after intracerebroventricular infusion of specifically labelled alpha-BuTX binding sites were most likely to be within neurons and dendrites. These studies demonstrate that the majority of alpha-BuTX binding sites remain membrane bound with respect to time and may be associated with synaptic transmission; a significant proportion of silver grains are internalized within SCN neurons and dendrites.

Ultrastructural distribution of alpha-bungarotoxin binding sites in the suprachiasmatic nucleus of the rat hypothalamus

The distribution of (125I) alpha bungarotoxin (alpha-BTX) binding sites in the suprachiasmatic nucleus (SCN) of the adult female rat was examined by electron-microscopic autoradiography. The ultrastructural distribution of silver grains was analysed by line source, direct point count, and 50% probability circle methods. Real grain distribution was significantly different from that of randomly generated hypothetical grains. Line source analysis demonstrated two populations of sources: one associated with membranes, and one inside neuronal structures. Probability circle analysis of shared grains indicated that membrane-bound-radioactive sources were mainly associated with axo-dendritic appositions. Only a small proportion of labeled neuronal interfaces exhibited synaptic differentiations in the plane of section. However, the compartment containing synaptic terminals was the most enriched when comparing real to hypothetical grains. Probability circle analysis of exclusive grains demonstrated that sources that were not associated with neuronal plasma membranes were likely to be within nerve cell bodies and dendrites. It is concluded that the majority of specifically labelled alpha-BTX binding sites in the SCN is membrane bound, and may be associated with axodendritic synaptic transmission. The presence of a significant proportion of the label in the soma and dendrites of suprachiasmatic neurons 24 h after ventricular infusion suggests that some of the labeled binding sites (junctional or nonjunctional) may be internalized within these two compartments.

Neural nicotinic acetylcholine responses in solitary mammalian retinal ganglion cells

Using the patch-clamp technique, whole-cell recordings from solitary rat retinal ganglion cells in culture have established the nicotinic nature of the acetylcholine responses in these central neurons. Currents produced by acetylcholine (5-20 mumol/l) or nicotine (5-20 mumol/l) reversed in polarity near -5 mV and were unaffected by atropine (10 mumol/l). Agonist-induced currents were blocked by low doses (2-10 mumol/l) of the classical 'ganglionic' antagonists hexamethonium and mecamylamine, as well as by d-tubocurarine and dihydro-beta-erythroidine (the latter two do not discriminate clearly between ganglionic and neuromuscular junction receptors). Treatment with the potent neuromuscular blocking agent alpha-bungarotoxin (10 mumol/l) did not affect the cholinergic responses of these cells, while toxin F (0.2 mumol/l), a neural nicotinic receptor antagonist, readily abolished acetylcholine-induced currents. Thus, the experiments performed to date show that the nicotinic responses of retinal ganglion cells in the central nervous system share the pharmacology of autonomic ganglion cells in the peripheral nervous system. The ionic current carried by the nicotinic channels was selective for cations, similar to that described for nicotinic channels in other tissues. In addition, single-channel currents elicited by acetylcholine were observed in whole-cell recordings with seals greater than 5 G omega as well as in occasional outside-out patches of membrane. These acetylcholine-activated events, which had a unitary conductance of 48 pS and a reversal potential of 0 mV, represent the ion channels that mediate the neural nicotinic responses observed in these experiments on retinal ganglion cells.

Postnatal development of cholinergic enzymes and receptors in mouse brain

The developmental profiles for the cholinergic enzymes acetylcholinesterase and choline acetyltransferase, and the muscarinic and nicotinic receptors were determined in whole mouse brain. The enzyme activities (per milligram of protein) increased steadily from birth, reaching adult levels at 20 days of age. These increases were primarily due to increases in Vmax. Muscarinic receptor numbers, measured by [3H]quinuclidinyl benzilate binding, also increased from birth to 25 days of age. Brain nicotinic receptors were measured with the ligands L-[3H]nicotine and alpha-[125I]-bungarotoxin. Neonatal mouse brain had approximately twice the number of alpha-bungarotoxin binding sites found in adult mouse brain. Binding site numbers rose slightly until 10 days of age, after which they decreased to adult values, which were reached at 25 days of age. The nicotine binding site was found in neonatal brain at concentrations comparable to those at the alpha-bungarotoxin site followed by a steady decline in nicotine binding until adult values were reached. Thus, brain nicotinic and muscarinic systems develop in totally different fashions; the quantity of muscarinic receptors increases with age, while the quantity of nicotinic receptors decreases. It is conceivable that nicotinic receptors play an important role in directing the development of the cholinergic system.

[3H]N-methyl-carbamylcholine, a new radioligand specific for nicotinic acetylcholine receptors in brain

The present study characterized the binding of [3H]N-methyl-carbamylcholine ([3H]methyl-carbachol), a new radioligand, to rat cerebral cortex membranes and demonstrated the autoradiographic distribution of these sites in rat brain. With atropine used to block muscarinic acetylcholine sites and nicotine to define non-specific binding, [3H]methyl-carbachol bound specifically, saturably and with high affinity (Kd = 11.0 nM, Bmax = 118.4 fmol/mg protein and Hill coefficient = 0.92) to a population of presumably nicotinic sites in cerebral cortex membranes. When nicotine was used to block nicotinic acetylcholine sites and atropine to define non-specific binding there was no specific binding of [3H]methyl-carbachol (concentrations up to 45 nM) to possible muscarinic sites in cerebral cortex membranes. The binding parameters under non-selective conditions (without blockade of either muscarinic or nicotinic acetylcholine sites) had very similar values to those obtained under nicotinic conditions (Kd = 8.0 nM, Bmax = 125.0 fmol/mg protein and Hill coefficient = 0.98). [3H]Methyl-carbachol binding was potently inhibited by nicotinic agonists and antagonists but only poorly displaced by muscarinic agents. Autoradiographic studies evidenced high densities of [3H]methyl-carbachol binding sites in the interpeduncular nucleus, various thalamic nuclei, superior colliculus and layers III/IV of the cortex. Such a distribution was very similar to those previously reported for nicotinic acetylcholine sites and other radioligands. These results suggest that [3H]methyl-carbachol is a specific radioligand of the neuronal nicotinic receptor. Its stability and high selectivity constitute distinct advantages over previously used nicotinic radioligands such as acetylcholine and nicotine.

Acetylcholine as a neurotransmitter in the vertebrate retina

The evidence for the existence of acetylcholine as a neurotransmitter in the vertebrate retina is reviewed. There is evidence for the existence of a cholinergic system in every retina studied to date; therefore, it appears that acetylcholine is both essential and ubiquitous at this level of the visual system. Particular attention is directed to descriptions of the possible functions of acetylcholine in the retina, and formation of testable models which will serve to elucidate some of the details of cholinergic neurotransmission in the retina.

Nicotinic receptor abnormalities in Alzheimer's and Parkinson's diseases

The status of cholinergic receptors in dementia is related to the question of potential cholinergic therapy. Whilst muscarinic receptor binding is generally reported to be normal or near normal, findings are reported which indicate substantial reductions of hippocampal nicotinic (high affinity nicotine) binding (occurring in conjunction with decreased choline acetyltransferase) in both Alzheimer's and Parkinson's but not Huntington's disease. A further indication that nicotinic receptor function may be abnormal in Alzheimer's disease is the extensive loss of an endogenous compound, detected for the first time in human brain, which inhibits normal nicotinic binding. Both receptor binding and the inhibitor are also substantially decreased with increasing age in the normal hippocampus.

Monoclonal antibodies with defined specificities for Torpedo nicotinic acetylcholine receptor cross-react with Drosophila neural tissue

A panel of monoclonal antibodies with known specificity for the well-characterized nicotinic acetylcholine receptor from the electroplax of Torpedo californica, many of which cross-react with the mammalian muscle acetylcholine receptor, were examined for cross-reactivity in the fly, Drosophila melanogaster. Monoclonal antibodies with specificities for different epitopes on the transmembrane receptor complex from Torpedo cross-react with different regional subsets of neural tissue in Drosophila. Axonal tracts, neuropil, mechano-sensory bristle elements and photoreceptors, each are detected by separate monoclonal antibody classes corresponding to different epitope domains. A preliminary characterization of an antigenic determinant in Drosophila heads recognized by one of the cross-reacting monoclonal antibodies is presented. Monoclonal antibodies such as these may be useful in identifying molecules of homologous structure or function, possibly including a neuronal acetylcholine receptor.

Members of a nicotinic acetylcholine receptor gene family are expressed in different regions of the mammalian central nervous system

Nicotinic acetylcholine receptors found in the peripheral and central nervous system differ from those found at the neuromuscular junction. Recently we isolated a cDNA clone encoding the alpha subunit of a neuronal acetylcholine receptor expressed in both the peripheral and central nervous system. In this paper we report the isolation of a cDNA encoding the alpha subunit of a second acetylcholine receptor expressed in the central nervous system. Thus it is clear that there is a family of genes coding for proteins with sequence and structural homology to the alpha subunit of the muscle nicotinic acetylcholine receptor. Members of this gene family are expressed in different regions of the central nervous system and, presumably, code for subtypes of the nicotinic acetylcholine receptor.

Synthetic peptides corresponding to sequences of snake venom neurotoxins and rabies virus glycoprotein bind to the nicotinic acetylcholine receptor

Peptides corresponding to portions of loop 2 of snake venom curare-mimetic neurotoxins and to a structurally similar region of rabies virus glycoprotein were synthesized. Interaction of these peptides with purified Torpedo electric organ acetylcholine receptor was tested by measuring their ability to block the binding of 125I-labeled alpha-bungarotoxin to the receptor. In addition, inhibition of alpha-bungarotoxin binding to a 32-residue synthetic peptide corresponding to positions 173-204 of the alpha-subunit was determined. Neurotoxin and glycoprotein peptides corresponding to toxin loop 2 inhibited labeled toxin binding to the receptor with IC50 values comparable to those of nicotine and the competitive antagonist d-tubocurarine and to the alpha-subunit peptides with apparent affinities between those of d-tubocurarine and alpha-cobratoxin. Substitution of neurotoxin residue Arg37, the proposed counterpart of the quaternary ammonium of acetylcholine, with a negatively charged Glu residue reduced the apparent affinity about 10-fold. Peptides containing the neurotoxin invariant residue Trp29 and 10- to 100-fold higher affinities than peptides lacking this residue. These results demonstrate that relatively short synthetic peptides retain some of the binding ability of the native protein from which they are derived, indicating that such peptides are useful in the study of protein-protein interactions. The ability of the peptides to compete alpha-bungarotoxin binding to the receptor with apparent affinities comparable to those of other cholinergic ligands indicates that loop 2 of the neurotoxins and the structurally similar segment of the rabies virus glycoprotein act as recognition sites for the acetylcholine receptor. Invariant toxin residues Arg37 and Trp29 and their viral homologs play important, although not essential, roles in binding, possibly by interaction with complementary anionic and hydrophobic subsites on the acetylcholine receptor. The alpha-subunit peptide most likely contains all of the determinants for binding of the toxin and glycoprotein peptides present on the alpha-subunit, because these peptides bind to the 32-residue alpha-subunit peptide with the same or greater affinity as to the intact subunit.

Cat carotid body chemoreceptor responses before and after nicotine receptor blockade with alpha-bungarotoxin

The nature of nicotine receptors in the carotid body was studied in anesthetized, paralyzed and artificially ventilated cats. Chemoreceptor discharge in single or few-fiber preparations of the carotid sinus nerve was measured during isocapnic hypoxia, hyperoxic hypercapnia and in response to nicotine injections before and after administration of alpha-bungarotoxin (10 cats) and after alpha-bungarotoxin plus mecamylamine (7 cats) which binds to neuromuscular-type nicotine cholinergic receptors. alpha-Bungarotoxin caused a slight enhancement of the chemoreceptor response to hypoxia without affecting the chemoreceptor stimulation by nicotine. Mecamylamine (1-5 mg, i.v.), a ganglionic-type nicotinic receptor blocker, had no further effect on the response to hypoxia while it completely abolished the chemoreceptor stimulation by nicotine. Thus the nicotinic receptors in the cat carotid body which elicit excitation of chemosensory fibers appear to be of the ganglionic-type. Blockade of neuromuscular and ganglionic types of nicotinic receptors in the carotid body by alpha-bungarotoxin and mecamylamine does not attenuate the chemosensory responses to either hypoxia or hypercapnia. These nicotinic receptors therefore, do not appear to play an essential role in hypoxic or hypercapnic chemoreception in the cat carotid body.

Purification and characterization of a nicotinic acetylcholine receptor from rat brain

We previously reported the immunoaffinity purification of an acetylcholine receptor from chicken brain that did not bind alpha-bungarotoxin but did bind nicotine and other cholinergic agonists. Antisera and monoclonal antibodies raised against this receptor crossreacted with a receptor from rat brain that had similar pharmacological properties, and also bound to functional acetylcholine receptors in chicken ciliary ganglion cells and rat PC12 cells. Here we report purification of the receptor from rat brain using monoclonal antibody (mAb) 270 raised against receptor from chicken brain. This receptor, similar in size to monomers of receptor from Torpedo electric organ, contained two subunits--apparent Mr, 51,000 and 79,000. The Mr 51,000 subunit was bound by antisera to alpha subunits of receptor from Torpedo electric organ and by mAb 270, which is specific for the Mr 49,000 subunit analogue of receptor from chicken brain. Both subunits were bound by mAb 286, which also binds both subunits of receptors from chicken brain. The alpha-bungarotoxin binding component was purified from the same extracts. It consisted of four subunits of apparent Mr 44,700, 52,300, 56,600, and 65,200. The basic structure of receptors from muscle had evolved to an (alpha)2 beta gamma delta subunit stoichiometry by the time of primitive elasmobranches and is now little changed in mammals. The apparent (alpha)2(beta)2 or (alpha)3(beta)2 structure of the neuronal acetylcholine receptors that we have purified may derive from an early gene duplication event in the evolution of the extended gene family, which now also includes receptors from ganglia and muscle as well as neuronal alpha-bungarotoxin binding sites.

Carbachol phase shifts circadian activity rhythms in ovariectomized rats

Ovariectomy in the rat abolishes alpha-bungarotoxin (alpha-BTX) binding within the suprachiasmatic nucleus (SCN), a putative circadian pacemaker, and markedly reduces cholinergic activation of SCN cells. Circadian rhythms of running wheel activity were monitored continuously in intact male and female and chronically ovariectomized rats before and after intraventricular carbachol injections. Carbachol produced similar phase shifts of the free-running rhythms in all groups. The results suggest that carbachol can influence the circadian system of rats by activation of cholinergic receptor sites independent of those binding alpha-BTX within the SCN.

Characterization of L-[3H]nicotine binding in human cerebral cortex: comparison between Alzheimer's disease and the normal

Putative nicotine receptors in the human cerebral cortex were characterized with L-[3H]nicotine, L-[3H]Nicotine binding was enhanced by the addition of Ca2+ and abolished in the presence of Na3EDTA. Association and dissociation of the ligand were rapid at 25 degrees C with t1/2 values of 2 and 3 min, respectively. Saturation binding analysis revealed an apparent single class of sites with a dissociation constant of 5.6 nM and a Hill coefficient of 1.05. There was no effect of postmortem interval on the density of binding sites assayed up to 24 h in rat frontoparietal cortex. Nicotine binding in human cortical samples was also unaltered by increasing sampling delay. In human cortical membranes, binding site density decreased with normal aging. Receptor affinity and concentration in samples of frontal cortex (Brodmann area 10) from patients with Alzheimer's disease were comparable to age-matched control values. Samples of infratemporal cortex (Brodmann area 38) from patients with Alzheimer's disease had a 50% reduction in the number of L-[3H]nicotine sites. Choline acetyltransferase activity was significantly decreased in both cortical areas. Enzyme activities in the temporal pole were reduced to 20% of control values. These data indicate that postsynaptic nicotine receptors are spared in the frontal cortex in Alzheimer's disease. In the infratemporal cortex, significant numbers of receptors remain despite the severe reduction in choline acetyltransferase activity. Replacement therapy directed at these sites may be warranted in Alzheimer's disease.

Flight induced by microinjection of D-tubocurarine or alpha-bungarotoxin into medial hypothalamus or periaqueductal gray matter: cholinergic or GABAergic mediation?

Microinjections of various doses (50-300 ng) of the nicotinic antagonist D-tubocurarine (TUBO) into the rat's medial hypothalamus (MH) or dorsal periaqueductal gray (PAG) produced flight reactions characterized by jumps. Two different types of flight reactions were produced depending on whether the drug was injected into the MH or into the PAG. MH injections provoked an increase in both locomotor activity and rearing together with well-oriented jumps. PAG injections provoked either freezing reactions or running with explosive jumps, but no increase in rearing. In addition, the rat exhibited an asymmetry in responsiveness to tactile stimulation. These reactions also differed depending on whether the drug was injected into the dorsal or ventral PAG. Behavioral reactions similar to those produced by TUBO were also produced by microinjection of the GABA receptor antagonist bicuculline into the same brain sites. Among the 4 putative cholinergic antagonists tested under the same conditions only alpha-bungarotoxin produced effects that were qualitatively similar to those induced by D-tubocurarine or bicuculline. Gallamine and hemicholinium produced tremor when injected into sites located near the ventricular system at either the MH or the PAG level, while vocalizations were only produced by PAG injections. Hexamethonium produced no marked effect. The hypothesis that flight reactions induced by D-tubocurarine or alpha-bungarotoxin do not result from their antinicotinic action but rather from a direct effect on GABAergic transmission is discussed.

Cholinergic receptors in cognitive disorders

Cholinergic receptors (muscarinic subtypes M1 and M2, and putative nicotinic binding) have been examined in the hippocampus obtained at autopsy from a variety of patients with cognitive disorders (Alzheimer's, Parkinson's, and Huntington's diseases, Down's Syndrome and alcoholic dementia) and compared with neurologically normal controls and cases of Motor Neuron disease. In all of the disorders associated with a pre-synaptic cortical cholinergic deficit reflected by an extensive loss of choline acetyltransferase (Alzheimer's disease, Parkinson's disease and Down's Syndrome) there was a substantial reduction in the binding of (3H) nicotine to the nicotinic receptor. By contrast reductions in both muscarinic subtypes (M1 and M2) were apparent to only a moderate extent in Alzheimer's disease, whereas in Parkinson's disease binding was significantly increased (apparently not in relation to anti-cholinergic drug treatment) in the non-demented but not demented cases. A further abnormality detected in Alzheimer's disease but not the other disorders investigated was a decrease in an endogenous inhibitor of nicotinic binding, the identity of which is as yet unknown but which may be a candidate for a possible endogenous modulator of the nicotinic receptor. These observations suggest that in Alzheimer's disease not only muscarinic but also nicotinic receptor function should be considered in relation both to future therapeutic strategies and, in the search for a clinical marker which might be of diagnostic value, to potential probes of the cortical cholinergic system.

Amino acid sequence of toxin F, a snake venom toxin that blocks neuronal nicotinic receptors

The amino acid sequence was determined for toxin F, a component of Bungarus multicinctus venom that blocks nicotinic transmission in the chick ciliary ganglion and the rat superior cervical ganglion. Toxin F was purified by a procedure that includes preparative isoelectric focusing and ion exchange chromatography. Seventy nanomolar toxin F blocks nicotinic transmission in the chick ciliary ganglion; however, the toxin only weakly blocks the binding of 125I-alpha-bungarotoxin to membranes derived from Torpedo californica electroplax (IC50 = 1 microM). These data raise the possibility that toxin F may preferentially recognize neuronal nicotinic receptors. Toxin F focused identically on an isoelectric focusing gel with samples of two similar toxins, bungarotoxin 3.1 and kappa-bungarotoxin. The sequence of toxin F is identical with that recently reported for kappa-bungarotoxin. When the N-terminal portion of bungarotoxin 3.1 was sequenced, it was found to be identical to the other two toxins. These and other data suggest that the 3 toxins are, in fact, the same.

Neocortical cholinergic enzyme and receptor activities in the human fetal brain

In the human fetus, obtained postmortem at estimated gestational ages of 8-22 weeks, biochemical activities of cortical choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) were comparable to those of adult brain tissue. In contrast cholinergic receptor binding, including muscarinic M1 and M2 subtypes (measured by displacement of [3H]N-methylscopolamine with, respectively, pirenzepine and carbachol) and [3H]nicotine (putative nicotinic) binding were undetectable before 13-14 weeks and even at 22 weeks were substantially (three- to fourfold) below the respective adult values. Cortical ChAT activity decreased significantly with gestational age whereas binding to the three receptors, including the proportion M1/M2, increased significantly. AChE was present at all ages investigated as the two molecular monomeric (G1) and tetrameric (G4) forms. The proportion of G4, which was much more soluble in fetal compared with adult cortex, increased approximately threefold. Histochemically AChE, although intense in the nucleus of Meynert, was generally confined to subcortical white matter at early fetal developmental periods, appearing later in the cortex localized to nerve fibres and occasional cell bodies. These observations suggest that during the second trimester of human fetal development, cortical cholinergic function may be preceded by relatively high ChAT activity and paralleled not only by increasing receptor binding but also by a proportional increase in the tetrameric form and histochemical reactivity of AChE.

3H-nicotine- and 125I-alpha-bungarotoxin-labeled nicotinic receptors in the interpeduncular nucleus of rats. II. Effects of habenular deafferentation

The cholinergic innervation of the interpeduncular nucleus (IPN) is wholly extrinsic and is greatly attenuated by bilateral habenular destruction. We describe changes in the labeling of putative nicotinic receptors within this nucleus at 3, 5, or 11 days after bilateral habenular lesions. Adjacent tissue sections of the rat IPN were utilized for 3H-nicotine and 125I-alpha-bungarotoxin (125I-BTX) receptor autoradiography. Compared to sham-operated controls, habenular destruction significantly reduced autoradiographic 3H-nicotine labeling in rostral (-25%), intermediate (-13%), and lateral subnuclei (-36%). Labeling in the central subnucleus was unchanged. Loss of labeling was maximal at the shortest survival time (3 days) and did not change thereafter. In order to establish whether this loss was due to a reduction in the number or the affinity of 3H-nicotine-binding sites, a membrane assay was performed on microdissected IPN tissue from rats that had received surgery 3 days previously. Bilateral habenular lesions produced a 35% reduction of high-affinity 3H-nicotine-binding sites, with no change in binding affinity. Bilateral habenular lesions reduced 125I-BTX labeling in the intermediate subnuclei, and a slight increase occurred in the rostral subnucleus. In the lateral subnuclei, 125I-BTX labeling was significantly reduced (27%) at 3 days but not at later survival times. In view of the known synaptic morphology of the habenulointerpeduncular tract, it is concluded that a subpopulation of 3H-nicotine binding sites within the IPN is located on afferent axons and/or terminals. This subpopulation, located within rostral, intermediate, and lateral subnuclei, may correspond to presynaptic nicotinic cholinergic receptors. Sites that bind 125I-BTX may include a presynaptic subpopulation located in the lateral and possibly the intermediate subnuclei.

3H-nicotine and 125I-alpha-bungarotoxin-labeled nicotinic receptors in the interpeduncular nucleus of rats. I. Subnuclear distribution

The distribution of nicotinic receptors within the interpeduncular nucleus (IPN) was determined in male rats following in vitro labeling with the cholinergic ligands 3H-nicotine and 125I-alpha-bungarotoxin (BTX). Autoradiographic images of two rostrocaudal levels of IPN were analyzed by computer-assisted densitometry and the optical density contributed by displaceable labeling was determined in the rostral, central, intermediate, and lateral subnuclei. 3H-nicotine labeling density within the four subnuclei differs significantly at both levels of IPN. The greatest density of labeling is localized in the rostral subnucleus, followed in order of diminishing density by the central, intermediate, and lateral subnuclei. Labeling within the rostral subnucleus is prominently localized within its central zone. In the central subnucleus, a dense concentration of binding sites is apparent in the middle region, adjacent to less dense vertically oriented columns; 3H-nicotine binding sites in the lateral subnuclei appear to be most concentrated medially, adjacent to the intermediate subnuclei. 125I-BTX labeling density within the four subnuclei also differs significantly at both levels of IPN. The greatest density of labeling is found in the rostral subnucleus, followed in order of decreasing density by the lateral, central, and intermediate subnuclei. The ovoid regions of the rostral subnucleus contain dense 125I-BTX labeling. In the lateral subnuclei, 125I-BTX binding appears to be predominantly along the lateral margins of the subnucleus. The present data indicate that the IPN contains two distinct populations of putative cholinergic nicotinic receptors identified, respectively, by 3H-nicotine and 125I-BTX labeling. Each population of labeled receptors is uniquely localized in patterns that suggest differences in density within and across subnuclei.

Neurotransmitters in the cerebral cortex

This article surveys the conventional neurotransmitters and modulatory neuropeptides that are found in the cerebral cortex and attempts to place them into the perspective of both intracortical circuitry and cortical disease. The distribution of these substances is related, where possible, to particular types of cortical neuron or to afferent or efferent fibers. Their physiological actions, where known, on cortical neurons are surveyed, and their potential roles in disease states such as the dementias, epilepsy, and stroke are assessed. Conventional transmitters that occur in afferent fibers to the cortex from brain-stem and basal forebrain sites are: serotonin, noradrenaline, dopamine, and acetylcholine. All of these except dopamine are distributed to all cortical areas: dopamine is distributed to frontal and cingulate areas only. The transmitter in thalamic afferent systems is unknown. Gamma aminobutyric acid (GABA) is the transmitter used by the majority of cortical interneurons and has a profound effect upon the shaping of receptive field properties. The vast majority of the known cortical peptides are found in GABAergic neurons, and the possibility exists that they may act as trophic substances for other neurons. Levels of certain neuropeptides decline in cases of dementia of cortical origin. Acetylcholine is the only other known transmitter of cortical neurons. It, too, is contained in neurons that also contain a neuropeptide. The transmitter(s) used by excitatory cortical interneurons and by the efferent pyramidal cells is unknown, but it may be glutamate or aspartate. It is possible that excitotoxins released in anoxic disease of the cortex may produce damage by acting on receptors for these or related transmitter agents.

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.

Acetylcholine operated ion channel and alpha-bungarotoxin binding site in a human neuroblastoma cell line reside on different molecules

In neurons, alpha-bungarotoxin is often associated with nicotinic receptor but does not always block the acetylcholine operated channel. In a human neuroblastoma cell line, IMR 32, we have demonstrated a large number of alpha-Bungarotoxin binding sites (2640 per cell in non differentiated cells and 4660 per cell in differentiated cells) in presence of 0 to 4 Acetylcholine activated-channels per cell. This neuronal cell line promises to be an useful model for the study of structure and function of the alpha-Bungarotoxin binding site not related to the nicotinic receptor.

Mapping of brain areas expressing RNA homologous to two different acetylcholine receptor alpha-subunit cDNAs

We have used an in situ RNA X RNA hybridization technique to determine, in the central nervous systems of the mouse and rat, the distribution of RNA homologous to cDNA clones encoding the alpha subunit of a putative neural nicotinic acetylcholine receptor and the alpha subunit of the muscle nicotinic acetylcholine receptor. Hybridization of the neural alpha-subunit probe was strongest in the medial habenula but was also detected consistently in the compact part of the substantia nigra and ventral tegmental area, in the neocortex, and in certain parts of the thalamus and hypothalamus. The in situ hybridization technique makes it possible to compile a map of brain regions containing cell bodies expressing RNA coding for a specific receptor type and subsequently to apply the techniques of molecular biology to study these brain receptors.

Mecamylamine blockade of nicotine responses: evidence for two brain nicotinic receptors

Mice of two inbred strains, DBA and C3H, were pretreated with mecamylamine before challenge with nicotine. Mecamylamine blocked nicotine-induced seizures, enhanced startle, and alterations in respiratory rate, Y-maze activity, heart rate and body temperature. Mecamylamine blocked nicotine-induced seizures and enhanced startle with IC50 values of less than 0.1 mg/kg. The other nicotine effects were blocked by mecamylamine with IC50 values between 0.8 and 2.3 mg/kg. Strain differences in sensitivity to mecamylamine blockade were also detected. These results suggest that nicotine elicits its effects at two receptors, which may be those labeled with [125I]-alpha-bungarotoxin and with [3H]-nicotine.

Contrasting effect of a brain supernatant extract on neuronal vs neuromuscular alpha-bungarotoxin receptors

The effects of a rat brain supernatant extract and a partially purified supernatant preparation from bovine brain were determined on the binding of [125I]alpha-bungarotoxin (alpha-BGT) to muscle membranes, as well as to membranes prepared from brain. In agreement with previous work, the supernatant preparations inhibited alpha-BGT binding to brain membranes in a dose-dependent fashion, (Brain Research, 245 (1982) 57-67); however, no significant effect of either of the preparations was observed on the binding of the toxin to muscle membranes. As well, the supernatant preparations did not affect binding of radiolabelled alpha-BGT to muscle cells in culture in competition binding experiments. The effect of long-term incubation of cells in culture with the supernatant preparations was subsequently determined. These studies showed that the binding of [125I]alpha-BGT increased markedly (300%) in the presence of a crude rat brain supernatant preparation, while incubation of the muscle cells in the presence of the partially purified bovine supernatant extract had no significant effect on radiolabelled toxin binding. In contrast, both the rat and bovine brain supernatant preparations significantly decreased (up to 65%) radiolabelled toxin binding to a cultured neuronal cell population, adrenal medullary chromaffin cells. These results suggest that an endogenous factor(s), present in brain extracts, differentially regulates the neuronal as compared to the neuromuscular nicotinic alpha-bungarotoxin binding sites.

Alpha-bungarotoxin binding to a high molecular weight component from lower vertebrate brain identified on dodecyl sulfate protein-blots

The binding of [125I]iodo-alpha-bungarotoxin [( 125]alpha-BuTX) to the dissociated alpha-subunit of Torpedo acetylcholine receptor (AChR) can be readily demonstrated in a modified 'protein-blot' analysis utilizing electrophoretically transferred, dissociated subunits immobilized onto positively charged nylon membranes which are then incubated directly with [125I]alpha-BuTX. We report here the use of the protein-blotting technique to detect the alpha-BuTX binding site present in the central nervous system of lower vertebrates and to characterize some of the physicochemical properties of the toxin binding site. High molecular weight (Mr greater than or equal to 200,000 and greater than or equal to 120,000) alpha-BuTX-binding components can be readily demonstrated in avian and fish brain extracts upon protein-blotting with [125I]alpha-BuTX following lithium dodecyl sulfate PAGE. Neither extensive reduction with dithiothreitol nor prior reduction followed by alkylation with iodoacetamide alter the mobility of the CNS-derived BuTX-binding sites. In contrast to our findings with Torpedo AChR or muscle AChR derived from a number of different species, no binding is observed in the molecular weight range of the alpha-subunit (Mr = 40,000) nor is any binding at any molecular weight observed in similar fractions prepared from adult, mammalian (rat, guinea pig) brain using this technique. These results demonstrate the existence in lower vertebrate brain of a BuTX binding site comparable in size to the AChR oligomeric complex of electric organ and muscle. They also suggest, however, striking structural differences between muscle AChR and the central neuronal BuTX-binding complex as well as a considerable difference between the neuronal BuTX-binding sites derived from lower and higher vertebrate brain.

Characterization of putative nicotinic acetylcholine receptors solubilized from rat brains

A membrane component of rat brains which binds [3H]acetylcholine in the presence of eserine and atropine was solubilized by various detergents and characterized. The affinity of this component for [3H]acetylcholine was increased 2-3-fold upon dissolution with non-ionic detergents. Pharmacological specificity of this component suggested its nicotinic cholinergic nature. The component cross-reacted with antisera raised against nicotinic acetylcholine receptors of Torpedo marmorata or Narke japonica. The component interacted with wheat germ agglutinin-conjugated Sepharose 4B, suggesting that it may be a glycoprotein, but did not bind appreciably to alpha-bungarotoxin-conjugated Sepharose 4B. Conversely, alpha-bungarotoxin binding component in the same preparation, which was bound to the toxin-conjugated gels, did not bind [3H]acetylcholine. These results demonstrate that nicotinic acetylcholine binding component and alpha-bungarotoxin binding component in brain membranes are distinct molecules and can be separated from each other.

A quantitative and morphometric evaluation of 125I-alpha-bungarotoxin binding in the rat hypothalamus

The distribution of 125I-alpha-bungarotoxin (alpha-BTX), a putative nicotinic cholinergic receptor ligand was studied both in vitro and in vivo in the suprachiasmatic nucleus (SCN), anterior hypothalamic area (AHA), and supraoptic nucleus (SON) of the hypothalamus. For in vitro studies 20 micron frozen frontal sections containing SCN were incubated with either radioligand or, unlabeled alpha-BTX plus 125I alpha-BTX and tissues were processed for light microscopic autoradiography. Areas of cresyl violet stained SCN sections were measured using a Bioquant Analysis System and grain counts and distributions were determined. For in vivo investigations third ventricular infusion of either 125I alpha-BTX, or unlabeled alpha-BTX with 125I alpha-BTX was performed, and 24 hours later animals were perfused pericardially and 1 micron serial plastic sections of the SCN were processed for light microscopic autoradiography. Localization of silver grains in 1 micron serial sections was evaluated in a double blind study. In vitro and in vivo labeling patterns in the hypothalamus were the same and compared well with previously examined paraffin-processed tissues from animals which had received third ventricular infusions of the neurotoxin. We observed a distinctive and specific labeling pattern of the SCN. Grains tended to localize diffusely and uniformly in more rostral regions, but clustered densely in the dorsal and lateral mid-SCN, and dorsally in the mid-caudal SCN. Grains were localized in the SCN where larger neurons were found. In the most caudal regions of the SCN no labeling was observed. Tissues from unlabeled alpha-BTX plus 125I alpha-BTX in vitro or in vivo studies did not demonstrate grain counts above background levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of the nicotinic alpha-bungarotoxin site in chromaffin cells in culture by a factor(s) endogenous to neuronal tissue

An endogenous factor(s) which affects the in vitro binding of (alpha-BGT) to rat brain membranes has previously been found in brain supernatant. This fraction, as well as a partially purified preparation of this material from bovine brain, is here shown to affect the binding of alpha-BGT to chromaffin cell membranes. To study possible long term effects, the supernatant extract was added to adrenal medullary chromaffin cells in culture. The cells were incubated for several days and at the end of this time, the medium bathing the cells, which contained the endogenous factor(s), was removed and alpha-BGT binding to the cells measured. Binding to control cultures had shown that alpha-BGT bound to the chromaffin cells in a saturable manner, with high affinity (Kd = 1.5 nM) and the specificity of a nicotinic receptor ligand. After incubation of the cells with supernatant factor, a marked decline in the number of alpha-BGT binding sites was observed with no change in affinity. This does not appear to be due to a detrimental effect on the cells as cell number did not appear to be decreased in the cultures preincubated with the supernatant extract and the DNA and protein content were similar in the control and treated cultures. The possibility that there was some non-specific detrimental effect to the chromaffin cell membrane was considered; however, the stimulated release of noradrenaline from the cells was not affected by treatment of the cultures in the presence of the supernatant fractions. In addition, tyrosine hydroxylase activity was significantly increased in the treated cultures. D-Tubo-curarine, an antagonist at the acetylcholine receptor, caused an increase in alpha-BGT binding after 7 days of treatment, while the agonist nicotine and choline had no effect. These results suggest that in brain supernatant there may exist an endogenous factor(s), which may function in the regulation of the nicotinic-like alpha-BGT receptors in neuronal cell.

Nicotinic acetylcholine binding sites in Alzheimer's disease

In Alzheimer's disease (AD), there is a loss of presynaptic cholinergic markers in the cerebral cortex, but the nature of cholinergic receptor changes is unclear. In this study, [3H]acetylcholine and [3H]nicotine were used to label nicotinic cholinergic binding sites in cerebral cortical tissues obtained at autopsy from patients with AD and from matched controls. A consistent and severe loss of nicotinic receptors was found in AD.

Interaction of monoclonal antibodies to electroplaque acetylcholine receptors with the alpha-bungarotoxin binding site of goldfish brain

Polyclonal and monoclonal antibodies raised against acetylcholine receptors from Torpedo californica and Electrophorus electricus electroplaque were tested for interaction with the [125I]alpha-bungarotoxin binding protein of goldfish brain. A subset of monoclonal antibodies which recognize the main immunogenic region of the alpha subunit of the Electrophorus acetylcholine receptor interacted at high affinity with the [125I]alpha-bungarotoxin binding protein. Using immunofluorescence, these antibodies were shown to label the same layers of the optic tectum as [125I]alpha-bungarotoxin.

Neutralization of alpha-bungarotoxin by monoclonal antibodies

We have produced monoclonal antibodies against alpha-bungarotoxin from Bungarus multicinctus, and controls were performed for their specificity and monoclonal character. The antibodies protected mice against the toxic effect of alpha-bungarotoxin for a few hours to six days, depending upon the chosen antibody (in comparison to a survival time of 79 min for the standard toxin dose). Antibodies can be grouped in several sets according to their duration of protection.