Effect of alpha-bungarotoxin on retinotectal synaptic transmission in the goldfish and the toad

Inhibition of nicotinic receptor mediated ion fluxes in rat sympathetic ganglia by BGT II-S1 a potent phospholipase

The mechanism of action of the bungarotoxin fraction II-S1 (BGT II-S1), which copurifies with alpha-bungarotoxin (alpha-BGT) and inhibits nicotinic transmission, has been further characterized. BGT II-S1 (1 microM) inhibited the carbachol (100 microM) or nicotine (50 microM) stimulated uptake of [3H]agmatine into rat sympathetic ganglia by 73% and 52%, respectively. These responses were inhibited 90% by D-tubocurarine (100 microM), but unaffected by alpha-BGT (1 microM) or atropine (10 microM), suggesting that BGT II-S1 affects nicotinic function at a postsynaptic site. Binding of physiologically active [125I]BGT II-S1 could be demonstrated to intact sympathetic ganglia; however, the binding could not be displaced by nicotinic agents, suggesting that BGT II-S1 is not interacting at the receptor. Because some neurotoxins produce their effect at the synapse through a phospholytic action, the phospholipase activity of BGT II-S1 was determined. The results demonstrate that BGT II-S1 is a very potent calcium dependent phospholipase. In addition, conditions which abolished the toxin's phospholytic activity prevented its effects on nicotinic transmission and on nicotinic receptor mediated ion fluxes. These include irreversible inhibition of enzymic activity by treatment of BGT II-S1 with p-bromophenacylbromide, as well as reversible inhibition of the phospholipase by substitution of Ba2+ or Sr2+ ions for Ca2+ ions in the physiological medium. Thus, in rat sympathetic ganglia, BGT II-S1 blocks the nicotinic receptor mediated movement of ions across the membrane. This is probably not due to a direct interaction at the nicotinic acetylcholine recognition site; rather, it may be an ion channel associated effect which is mediated by alterations in the phospholipid environment of the receptor complex or of the membrane. Although BGT II-S1 also has presynaptic actions, in a cultured system of postsynaptic cells, it could prove a useful tool to study the role of phospholipids in neuronal nicotinic receptor regulation.

Real-time optical imaging of naturally evoked electrical activity in intact frog brain

A major obstacle to understanding the function and development of the vertebrate brain is the difficulty in monitoring dynamic patterns of electrical activity in the millesecond time domain; this has limited investigations of such phenomena as modular organization of functional units, seizure activities and spreading depression. The use of voltage-sensitive dyes and the recent development of the use of an array of photodiodes has provided a unique technique for monitoring the dynamic patterns of electrical activity in real time from a variety of invertebrate or vertebrate neuronal preparations including the rat cortex. In the present study, this technique has been used to investigate the intact optic tectum of the frog. We demonstrate that optical measurements can be used for real-time imaging of spatio-temporal patterns of neuronal responses and for identification of functional units evoked by natural visual stimuli. We report also the structure of the new voltage-sensitive probe that facilitates the in vivo applications of this technique.

Properties of monoclonal antibodies to nicotinic acetylcholine receptor from chick muscle

Four stable, hybrid-cell lines secreting monoclonal antibodies to distinct determinants on the nicotinic acetylcholine receptor from chick muscle have been established. These were characterised by the following criteria: immunoglobulin isotype, ability to produce experimental autoimmune myasthenia gravis in mice and reactivity towards homologous and heterologous acetylcholine receptor proteins. Two monoclonal antibodies were found to inhibit the reaction of alpha-bungarotoxin with homologous acetylcholine receptor; in addition one of these, on binding to receptor-toxin, induced a rapid dissociation of the complex (t1/2 = 0.5 h at 23 degrees C). Three of the antibody preparations recognised epitopes on this receptor from muscle of other species and two of these caused experimental autoimmune myasthenia gravis in BALB/c mice following passive transfer. The latter two recognised to significant extents the alpha-bungarotoxin binding component purified from chick optic lobe and brain cortex. Sedimentation analysis demonstrated that two of the monoclonal antibodies form a distinct size (s20, w = 12S) of complex with the receptor of chick muscle which most probably corresponds to a 1:1 attachment of antibody and receptor; this may involve cross-linking of two determinants within the same oligomer. A similar observation was made with the alpha-bungarotoxin binding component from optic lobe using one of the cross-reacting antibodies. Another monoclonal antibody was found to be capable of forming much heavier complexes with the receptor from chick muscle, these are thought to involve inter-molecular cross-linking of oligomers. The observed properties of these antibodies are discussed in relation to their myasthenogenicity and with reference to the extent of structural similarities between the peripheral nicotinic acetylcholine receptor and the alpha-bungarotoxin binding protein from brain.

Specific binding of alpha-bungarotoxin to synaptic membranes in rat sympathetic ganglion: computer best-fit analysis of electron microscope radioautographs

In the rat superior cervical sympathetic ganglion (SCG), alpha-bungarotoxin (alpha BT) demonstrates binding that is saturable and inhibited by nicotinic ligands. However, alpha BT does not inhibit the physiological response of ganglionic neurons to preganglionic stimulation or to exogenously applied acetylcholine. Thus the specificity of alpha BT for ganglionic nicotinic cholinergic receptors has been questioned. The present study provides a morphological localization of the binding sites of 125I-labelled alpha BT in the rat SCG using the method of Blackett and Parry on electron microscopic radioautographs. The distribution of grains resulting from specific binding was calculated by subtracting the nonspecific distribution (alpha BT in the presence of D-tubocurarine, a known nicotinic ligand) from the total grain distribution (alpha BT alone). A hypothetical grain distribution was obtained based on the geometrical properties of the tissue sections. A computer minimizing routine was employed to adjust the relative weights of each of the potential sources of hypothetical grains until a 'best-fit' with the real grain distributions occurred. The nonspecific binding of alpha BT was uniform across all tissue components, with the exception of a significant concentration on the membrane of the ganglion cell body. By contrast, the specific binding of alpha BT was highly localized to synaptic membranes, and to a lesser extent, to dendritic membranes.

Functional anatomy of the tectum mesencephali of the goldfish. An explorative analysis of the functional implications of the laminar structural organization of the tectum

The present paper is aimed at an exploration of the possible functional significance of the laminar organization of the goldfish tectum at both the cellular and the synaptic level. For this purpose (1) the data concerning the structure of the teleostean tectum are surveyed, (2) a conceptual framework of the intratectal connectivity in the goldfish is proposed, (3) the electrophysiological data concerning the teleosteam tectum are surveyed and (4) the degree of correlation between the structural and physiological data available is discussed. Apart from the retina, tectal afferents originate from at least 10 other brain centers. At least 5 of these projections appear to be topographically organized. Tectal afferents, neurons as well as synapses reveal a characteristic intratectal lamination pattern. Tectal efferents project to at least 10 brain centers, and have until now been shown to arise from 6 cell types. The structural data surveyed allow the construction of a conceptual framework of tectal circuitry on the basis of 3 starting points. (1) The existence of at least 8 presynaptic zones or laminae, each containing a characteristic set of presynaptic structures (afferents and axons of interneurons). (2) The fact that the tectal postsynaptic structures (somata and dendrites of tectal neurons) each have a characteristic location, extension and synaptic density, which determines the relative importance of the different presynaptic zones for each cell type. (3) The laminar specificity hypothesis, which implies that presynaptic structures that coexist in a particular presynaptic zone terminate without preference on all types of postsynaptic structures within that zone. The conceptual framework of tectal circuitry is quantified in terms of connectivity index and connective importance. Analysis of the framework constructed leads to a detailed description of the intratectal pathways involved in the processing of the 4 main streams of tectal input (i.e. visual, toral, telencephalic and 'deep' input). It was concluded that the laminar organization of the tectum is primarily relevant for multimodal integration and that the tectal cell types each receive a characteristic sample out of the multimodal information available in the different tectal layers.(ABSTRACT TRUNCATED AT 400 WORDS)

Kappa-bungarotoxin: a probe for the neuronal nicotinic receptor in the avian ciliary ganglion

The interaction of snake alpha-neurotoxins with neuronal membranes has been examined in the chick ciliary ganglion. Some, but not all, alpha-neurotoxins block nicotinic transmission in this ganglion. alpha-Bungarotoxin (ABgT), the major alpha-neurotoxin in the venom of Bungarus multicinctus, does not block transmission at high concentrations (1.2 microM) although it binds (Kd = 1 nM) to a pharmacologically nicotinic site in the ganglion. A toxin (kappa-bungarotoxin, KBgT) has been purified from the venom of Bungarus multicinctus. KBgT has a molecular weight of 6500 daltons and a pI of 9.1. KBgT is a potent inhibitor of nicotinic transmission in the ciliary ganglion, producing a reversible (overal several hours) blockade at 75 nM. Pre-exposure of ganglia to 1.2 microM ABgT does not prevent the effects of KBgT, indicating that the blockade occurs at a site distinct from that recognized by ABgT. Binding of [125I]KBgT to ciliary ganglia reveals two binding sites: one which has previously been characterized by [125I]ABgT and one which is not identified by [125I]ABgT. Both of these [125I]KBgT binding sites are blocked following pre-treatment of ganglia with the irreversible nicotinic affinity agent bromoacetylcholine. A two-site model is proposed to account for these observations. One site (the ABgT binding site) is seen by both ABgT and KBgT, and has as yet no physiological function associated with it. The second site is recognized only by the physiologically active KBgT, and may represent binding of the toxin to the physiologically detected nicotinic receptor.

Activity sharpens the map during the regeneration of the retinotectal projection in goldfish

In the regenerating retinotectal projection of goldfish, we have used intraocular injections of tetrodotoxin (TTX) to determine whether activity plays a role in organizing or refining the retinotopic map. Repeated injections produced a continuous 27-day block without producing extraocular effects or causing deleterious effects in the retinal ganglion cells. The retinotectal maps regenerated in the TTX fish were normally organized but the multiunit receptive fields were grossly enlarged. In control regenerates, 1-3 units (arbors of retinal ganglion cell axons) were simultaneously recorded at each penetration and their combined receptive field averaged 11-12 degrees, nearly the same as for single units. In TTX fish each penetration yielded at least 5-10 units whose receptive fields were clustered over a wider area averaging 27 degrees across. Individual ganglion cell receptive fields were assessed both by tectal and by intraretinal recording and were not enlarged. Many fish were recorded up to 4 months after the release from TTX block, but no further refinement of the maps occurred. If the nerve was recrushed and regenerated a second time without TTX, a normal map was formed, ruling out any permanent changes in the retinal ganglion cells or in the tectum. Blocks during various portions of the regeneration process showed that lack of activity during the process of axonal elongation (first 2 weeks) does not cause enlargement of the multiunit receptive fields, but lack of activity during the period of synapse formation and maturation (14-34 days) does. The results are discussed in terms of an activity-dependent stabilization of synapses. Neighboring retinal ganglion cells are known to fire in a statistically correlated fashion and this could help in their elimination of incorrect branches following an early period of diffuse connections.

Differences between embryos and adults in the plasticity of somatosensory afferents to the axolotl tectum

The plasticity of somatosensory afferents in the adult axolotl tectum was studied in two ways. First, normal adult axolotls were monocularly enucleated, and second, adult animals which had been monocular since embryogenesis had the tectum contralateral to the remaining eye excised. After a survival time of about one year the brains of these animals were studied electrophysiologically and histochemically. In the enucleated adults, the deprived tectum lacked visual activity, and the acetylcholinesterase staining, dense in the superficial neuropil of the normally innervated tectum, was much reduced. Somatosensory units remained deep in the tectal neuropil, as in normally innervated tecta. Serotonergic terminals associated with somatosensory responses were correspondingly largely restricted to the deeper tectal neuropil in these animals. This result is in contrast to that of embryonic enucleation where somatosensory activity and serotonergic terminals are found in the superficial tectal neuropil of the adult. Autoradiographic results from the tectal excision experiments showed that retinal axons in adults regrew to innervate the ipsilateral tectum when deprived of their normal targets. Associated with the ipsilateral innervation both visual units and acetylcholinesterase staining were found in the superficial tectum. In spite of this change in the visual innervation of the tectum, there was little difference in the distribution of somatosensory responses or serotonergic terminals; both remained relatively superficial and appeared to be unaffected by the late ingrowing retinal axons. This result is in contrast to the findings from embryonic transplants to genetically eyeless embryos which, as adults, show somatosensory responses and serotonergic endings restricted to the deeper tectal neuropil.

A quantitative study of the reinnervation of the goldfish optic tectum following optic nerve crush

Stereological and quantitative morphometric methods were used to study changes in the stratum fibrosum et griseum superficialis (SFGS), the major retinal target, in optic tectum of goldfish, during regeneration of the optic nerve. Orthograde transport of HRP by optic axons was used to characterize the retinal projection in SFGS. Profiles of HRP-labeled optic terminals contained rounded vesicles, contacted small dendrites, and were distributed uniformly throughout the area of SFGS sampled; labeling density estimates indicate that at least 37% of the total terminal population in SFGS is retinal in origin. Partial denervation of the tectum by optic nerve crush is accompanied by a loss of 40% of the total terminal population in SFGS and by a marked decrease of SFGS thickness. Entry of massive numbers of regenerating optic axons into SFGS begins about 3 weeks postoperatively, about the time that some visual function recovers and produces a marked increase in SFGS thickness which persists for several months postoperatively. The area occupied by regenerating axons and the number of terminals in the tectum only approach preoperative levels 3 months postoperatively. The recovery of normal synaptic number is therefore delayed several months beyond the time of entry of regenerative axonal sprouts. The results indicate that return of synaptic number to normal is temporally associated with the reduction of the excess number of regenerating optic axons and that both these processes are prolonged.

Presence of an endogenous factor which inhibits binding of alpha-bungarotoxin 2.2 to its receptor

Cerebral cortical membranes and supernatant from rat were prepared by centrifugation of tissue homogenates at 45,000 g for 10 min. The supernatant fraction thus obtained was found to significantly inhibit alpha-bungarotoxin binding to the membrane preparation. After a 3 min incubation period, the supernatant inhibited toxin binding by approximately 65%, while the inhibition declined to about 40% after 30 min of incubation, presumably due to the slow reversibility of alpha-bungarotoxin binding. The choice of buffer was found to be an important determinant of the degree of inhibition observed, with 10 mM Tris pH 7.4 providing the most effective condition. This inhibition of toxin binding to cortical membranes by the 45,000 g supernatant was shown not to be due to adsorption of the radiolabeled compound to soluble or residual particulate material in the supernatant fraction. Specificity of the supernatant for the alpha-bungarotoxin site was demonstrated; a supernatant fraction could be prepared which inhibited alpha-bungarotoxin binding by 50% but had no effect on [3H]spiroperidol (DA2 and 5-HT2), [3H]prazosin (alpha 1-adrenergic, [3H]5-hydroxytryptamine (5-HT1) and [3H]quinuclidinylbenzilate (muscarinic cholinergic) binding. The inhibition of toxin binding also occurred in several other CNS regions including hippocampus, brainstem, spinal cord and cerebellum with an 80 to 90% inhibition of binding occurring in the latter two regions. In addition, the 45,000 g cortical supernatant completely prevented the binding of alpha-bungarotoxin to extrajunctional neuromuscular receptors and inhibited the binding to junctional receptors by 50%. Supernatants prepared from heart, liver and kidney or bovine serum albumin, at a concentration similar to the supernatant fraction, did not alter radiolabeled toxin binding to cortical membranes, while supernatant prepared from striated muscle tissue was effective. These results suggest there may be an endogenous ligand for the alpha-bungarotoxin 2.2 binding site in tissues which receive nicotinic cholinergic innervation.

Blockade of transmission in rat sympathetic ganglia by a toxin which co-purifies with alpha-bungarotoxin

Bungarus multicinctus venom was fractionated into its toxin components using ion-exchange chromatography on CM-Sephadex. According to previous reports, rechromatography of fraction II on a CM cellulose column yields chemically homogenous alpha-bungarotoxin (II2) of molecular weight 9000. However, in our hands, using the identical purification procedure, two discrete proteins of molecular weight 9000 and 15,000 were obtained as demonstrated by SDS gel electrophoresis. Subsequent fractionation of this alpha-bungarotoxin fraction (II2) was achieved on Sephadex G-50. The 9000 weight component (labelled II-S2) was identical to alpha-bungarotoxin; at a concentration of 1 microgram/ml it blocked transmission at the neuromuscular junction but did not block nicotinic responses in rat sympathetic ganglia. Very different properties were exhibited by II-SI, the 15,000 molecular weight component; it inhibited ganglionic transmission but was ineffective at the neuromuscular junction at the same concentration (1 microgram/ml). BGT II-S1 was equipotent in blocking the ganglionic action potential in the presence or absence of eserine; thus, it is not acting as an acetylcholinesterase by increasing acetylcholine breakdown. In the presence of toxin, [3H]choline incorporation into ganglionic acetylcholine during preganglionic stimulation was not altered, suggesting that the toxin did not block transmission by a presynaptic mechanism. Thus, the site of action of the toxin appears to be postsynaptic although it did not affect depolarization of the ganglia induced by carbachol.

Nicotinic acetylcholine receptor from chick optic lobe

An alpha-bungarotoxin-sensitive nicotinic cholinergic receptor from chick optic lobe has been completely purified. Its standard sedimentation coefficient is 9.1 S. The value near 12 S reported for the related component from other brain regions can be reproduced when the initial extraction is by Triton X-100 (rather than Lubrol PX), but other protein is then complexed with it. A single subunit of apparent molecular weight 54,000 is detected, and this subunit is specifically labeled by bromo-[3H]acetylcholine, but only after disulfide reduction. The same size subunit likewise is labeled in the protein (purified similarly) from the rest of the chick brain which can also bind alpha-bungarotoxin and nicotinic ligands. Immunological crossreactivity is demonstrated between both of these proteins with an antiserum to pure acetylcholine receptor from skeletal muscle. The acetylcholine receptor from chick optic lobe and the alpha-bungarotoxin-binding protein from the rest of the brain appear similar or identical by a series of criteria and are related to (but with differences from) peripheral acetylcholine receptors.

Differential accessibility to two insect neurones does not account for differences in sensitivity to alpha-bungarotoxin

The nicotinic acetylcholine receptor probe alpha-bungarotoxin (1.0 x 10(-7) M) blocks the depolarising response to ionophoretic application of acetylcholine onto the cell body membrane of the fast coxal depressor motoneurone (Df) of desheathed cockroach (Periplaneta americana) metathoracic ganglia, but at the same concentration is completely ineffective in blocking the depolarising action af acetylcholine on dorsal unpaired median (DUM) neurones in the same ganglion. The possibility that this is due to differences in accessibility of the toxin to the neurones has been tested by a combination of ionophoretic injection of horseradish peroxidase into single neurones with a study of the distribution of the exogenous tracer lanthanum, which is of similar effective size to alpha-bungarotoxin. The peripherally located cell body membranes and the fine axonal processes of Df and DUM neurones of desheathed metathoracic ganglia are equally accessible to lanthanum. Differential accessibility to the two cell types does not account therefore for the differences in sensitivity to alpha-bungarotoxin.

Analysis of alpha-bungarotoxin binding in the goldfish central nervous system

We report here the equilibrium, kinetic, and pharmacological analysis of alpha-125I-bungarotoxin (alpha-125I-Bgt) binding to a Triton X-100-solubilized goldfish brain synaptosomal fraction. In addition, a refined analysis of equilibrium binding to a particulate synaptosomal fraction is presented. Equilibrium binding from both particulate and soluble fractions revealed an apparent heterogeneity of binding sites. Kinetic analysis of the soluble receptor revealed linear association kinetics and nonlinear dissociation kinetics. The dissociation curve suggested the presence of at least two rate constants. Potential sources of the binding heterogeneity found in both the equilibrium binding and dissociation kinetics experiments are (1) multiple receptor species, (2) multiple ligand species, and (3) different, possibly interconvertible, states of a single receptor type. No evidence for the first two alternatives was found. Support for the third alternative was obtained by observing the effect of cholinergic ligands on alpha-125I-Bgt dissociation. Carbamylcholine and d-tubocurarine increased the apparent proportion of rapidly dissociating sites, suggesting that the two binding affinities can be interconverted and may arise from a single receptor type. Evidence concerning the identity of the alpha-Bgt binding protein as a nicotinic acetylcholine receptor is discussed.

Sexual dimorphism in alpha-bungarotoxin binding capacity in the mouse amygdala

A sex difference in alpha-bungarotoxin binding capacity in the mouse amygdala has been demonstrated by quantitative light microscopic autoradiography. The difference persisted even under widely different steroid-hormonal environment. In addition, it was observed that the binding capacities in both sexes were reversibly activated by administration of either testosterone or estradiol. Neonatal castration, on the other hand, permanently altered the toxin binding capacity in the adult male mouse. These data suggest the possibility that neonatal sex steroids irreversibly modify the cholinergic nicotinic mechanism in the developing mouse amygdala, while the hormones reversibly modulate the mechanism when applied in adulthood.