Effect of tetanus toxin on the accumulation of the permeant lipophilic cation tetraphenylphosphonium by guinea pig brain synaptosomes

Analysis of synaptic vesicle endocytosis in synaptosomes by high-content screening

Small molecules modulating synaptic vesicle endocytosis (SVE) may ultimately be useful for diseases where pathological neurotransmission is implicated. Only a small number of specific SVE modulators have been identified to date. Slow progress is due to the laborious nature of traditional approaches to study SVE, in which nerve terminals are identified and studied in cultured neurons, typically yielding data from 10-20 synapses per experiment. We provide a protocol for a quantitative, high-throughput method for studying SVE in thousands of nerve terminals. Rat forebrain synaptosomes are attached to 96-well microplates and depolarized; SVE is then quantified by uptake of the dye FM4-64, which is imaged by high-content screening. Synaptosomes that have been frozen and stored can be used in place of fresh synaptosomes, reducing the experimental time and animal numbers required. With a supply of frozen synaptosomes, the assay can be performed within a day, including data analysis.

Methamphetamine-induced rapid and reversible changes in dopamine transporter function: an in vitro model

This laboratory has demonstrated that a single methamphetamine (METH) injection rapidly and reversibly decreases the activity of the dopamine transporter (DAT), as assessed ex vivo in synaptosomes prepared from treated rats. This decrease does not occur because of residual drug introduced by the original injection or nor is it associated with a change in binding of the DAT ligand WIN35428. The purpose of this study was to elucidate the mechanism or mechanisms of this METH effect by determining whether direct application of this stimulant to synaptosomes causes changes in DAT similar to those observed ex vivo. Similar to the ex vivo effect, incubation of striatal synaptosomes with METH decreased DAT activity, but not WIN35428 binding: the effect on activity was not eliminated by repeated washing of synaptosomes. Also, as observed ex vivo, incubation with 3,4-methylenedioxymethamphetamine, but not cocaine or methylphenidate, caused a METH-like reduction in DAT function. The rapid and reversible METH-induced diminution in DAT activity did not occur because of a change in membrane potential, as assessed in vitro and ex vivo by [(3)H]tetraphenylphosphonium accumulation. However, the METH-related decline in DAT function may be attributed to phosphorylation because NPC15437, a protein kinase C inhibitor, attenuated the METH-induced decline in DAT function. Similarities between previously reported effects ex vivo of a single METH injection on serotonin and norepinephrine transporter function and effects of direct METH application in vitro were also found. Together, these data demonstrate that the in vitro incubation model mimics the rapid and reversible effects observed after a single METH injection.

Methamphetamine-induced rapid and reversible changes in dopamine transporter function: an in vitro model

This laboratory has demonstrated that a single methamphetamine (METH) injection rapidly and reversibly decreases the activity of the dopamine transporter (DAT), as assessed ex vivo in synaptosomes prepared from treated rats. This decrease does not occur because of residual drug introduced by the original injection or nor is it associated with a change in binding of the DAT ligand WIN35428. The purpose of this study was to elucidate the mechanism or mechanisms of this METH effect by determining whether direct application of this stimulant to synaptosomes causes changes in DAT similar to those observed ex vivo. Similar to the ex vivo effect, incubation of striatal synaptosomes with METH decreased DAT activity, but not WIN35428 binding: the effect on activity was not eliminated by repeated washing of synaptosomes. Also, as observed ex vivo, incubation with 3,4-methylenedioxymethamphetamine, but not cocaine or methylphenidate, caused a METH-like reduction in DAT function. The rapid and reversible METH-induced diminution in DAT activity did not occur because of a change in membrane potential, as assessed in vitro and ex vivo by [(3)H]tetraphenylphosphonium accumulation. However, the METH-related decline in DAT function may be attributed to phosphorylation because NPC15437, a protein kinase C inhibitor, attenuated the METH-induced decline in DAT function. Similarities between previously reported effects ex vivo of a single METH injection on serotonin and norepinephrine transporter function and effects of direct METH application in vitro were also found. Together, these data demonstrate that the in vitro incubation model mimics the rapid and reversible effects observed after a single METH injection.

Effects of chronic nicotine treatment on the accumulation of [3H]tetraphenylphosphonium by cerebral cortical synaptosomes

Chronic exposure of rats to nicotine increases the number of [3H]nicotine binding sites in the brain; however, it is not clear whether nicotinic cholinergic receptor function is altered as well. In this study, we have used [3H]tetraphenylphosphonium as a probe of synaptosomal membrane potential to investigate whether exposure to nicotine in vivo alters the ability of cerebral cortical synaptosomes to maintain a potential difference and to depolarize in response to in vitro nicotine. Treatment of rats for 14 days with 0.475 mg of nicotine base/day via subcutaneously implanted minipumps resulted in a decrease in the synaptosomal accumulation of [3H]tetraphenylphosphonium in physiological buffer, corresponding to a decrease in estimated membrane potential from -55 mV to -50 mV. The onset of the decrease in membrane potential occurred after 7 days of in vivo nicotine treatment and was significantly correlated with an increase in [3H]nicotine binding to cerebral cortical synaptosomal (P2) membranes. Nicotine, at in vitro concentrations of 3-1,000 microM, decreased [3H]tetraphenylphosphonium accumulation in cerebral cortical synaptosomes from control animals. When compared to accumulation in buffer alone, in vitro nicotine and other nicotinic agonists did not significantly decrease [3H]tetraphenylphosphonium accumulation in cerebral cortical synaptosomes prepared from rats treated with nicotine in vivo. These studies provide evidence that chronic treatment with nicotine results in an average lower membrane potential in cerebral cortical synaptosomes and in functional down-regulation of the depolarization response to nicotinic cholinergic receptor stimulation.

Interactions of bilirubin with isolated presynaptic nerve terminals: functional effects on the uptake and release of neurotransmitters

1. The functional effects of bilirubin:albumin solutions (10:1, mol/mol) on several synaptosomal functions were investigated using rat cortical, striatal, and hippocampal synaptosomes prepared by iso-osmotic Percoll/sucrose gradient centrifugation. 2. Bilirubin (10-80 microM) depolarized synaptosomes in a tetrodotoxin-insensitive manner as assessed by the equilibrium distribution of tetra-[3H]phenylphosphonium. Depolarization induced by bilirubin was of a lesser magnitude than that caused by KCl or veratridine. Steady-state pH gradients across the synaptosomal membrane were determined using the transmembrane distribution of [14C]methylamine. Bilirubin (20-40 microM) did not modify the intracellular pH in physiological buffers. The pigment effected a 0.14 delta pH change when the synaptosomes were suspended in a Ca2+ and Na+ free choline medium containing ouabain. 3. Bilirubin (20-80 microM) had no effect of its own on [7,8-3H] dopamine release from striatal synaptosomes. In contrast, it inhibited the initial rate of synaptosomal uptake of the catecholamine and its intrasynaptosomal content at 10 min. The pigment (20 and 40 microM) reduced the 35 mM KCl-induced release of endogenous acetylcholine from hippocampal synaptosomes by 20 and 36%, respectively. 4. The association of bilirubin with synaptic plasma membrane vesicles was characterized by a chloroform:methanol 2:1 (v/v) extraction method. At total concentrations of 10 to 80 microM bilirubin, the molar percentage of the pigment in synaptic plasma membrane phospholipids was 1-4%. 5. It is proposed that the two main functional consequences of the bilirubin-nerve ending interaction are an impairment of specific membrane-bound neurotransmitter uptake mechanisms and a reduction of the response to depolarizing stimuli. This may be the basis for rapid alterations in synaptic transmission documented in early reversible bilirubin encephalopathy.

Analysis of the membrane potential of rat- and mouse-liver mitochondria by flow cytometry and possible applications

Washed and purified rat- or mouse-liver mitochondria exhibiting high membrane integrity and metabolic activity were studied by flow cytometry. The electrophoretic accumulation/redistribution of cationic lipophilic probes, rhodamine 123, safranine O and a cyanine derivative, 3,3'-dihexyloxadicarbocyanine iodide, during the energization process was studied and was consistent with the generation of a negative internal membrane potential. An exception to this was nonylacridine orange which spontaneously bound to the mitochondrial membrane by hydrophobic interactions via its hydrocarbon chain. Energized purified mitochondria stained with potentiometric dyes exhibited both higher fluorescence and population homogeneity than the non-energized or deenergized (nigericin plus valinomycin) mitochondria. By contrast, under non-energized or deenergized conditions, the mitochondrial population exhibited fluorescence intensity heterogeneity related to the residual membrane potential; two subpopulations were evident, one of low fluorescence which may be related to the autofluorescence of the mitochondria (plus non-specific dye binding) and a second population which exhibited high fluorescence. Flow cytometry of the unpurified, simply washed, rat-liver mitochondria stained with rhodamine 123, a classically used dye, provided evidence of their heterogeneity in terms of light-scattering properties and membrane-potential-related fluorescence. One third of the washed mitochondria were found to be non-functional by such assays. The fluorescence of purified rat-liver mitochondria due to the membrane potential built up by endogenous substrates indicates heterogeneity of the mitochondrial population with respect to levels of endogenous substrates. The low-angle light scattering increases upon energization and provides some original information about the shape and modification of the inner mitochondrial conformation accompanying the energization. The heterogeneity of the rat liver mitochondrial population, from a structural, metabolic (existence of endogenous substrates) and functional (active and non-active mitochondrial population dispersion) point of view could thus be demonstrated by flow-cytometry analysis. Two animal models were examined with regard to the alteration of the mitochondrial membrane potential under the effects of drugs (rat-liver mitochondria), and the effects of ammonium toxicity (mouse-liver mitochondria). These results are promising and open new perspectives in the study of mitochondriopathies.

Prevention of chemically induced synaptosomal changes

The ability of altered environmental conditions to modulate some properties of synaptosomes has been studied. Incubation conditions used included the presence of methyl mercury or an organochlorine insecticide: chlordecone. Other adverse chemical conditions during incubation were the absence of calcium salts from the incubation medium or the addition of agents bringing about enhanced oxidative conditions. Synaptosomal parameters studied were the cytosolic level of free, ionic calcium, [Ca2+]i, the extent of depolarization-induced uptake of radioactive calcium, and the permeability of the limiting membrane. In addition, peroxidative activity was estimated by quantitation of thiobarbituric acid-reactive material. All these facets of synaptosomal function were responsive to the presence of these potentially deleterious changes in the incubation medium. While the response of [Ca2+]i was potentially in either direction, all adverse conditions increased synaptosomal permeability as evaluated by leakage of fura-2 into the extracellular compartment. Pretreatment with ganglioside GM1 in some situations or alpha-tocopherol in others could either wholly or partially prevent the onset of such altered synaptosomal characteristics.

Affinity-purified tetanus neurotoxin interaction with synaptic membranes: properties of a protease-sensitive receptor component

The pharmacokinetic interaction of an affinity-purified 125I-labeled tetanotoxin fraction with guinea pig brain synaptosomal preparations was investigated. Binding of tetanotoxin was time- and temperature-dependent, was proportional to protein concentration, and was saturable at about 8 X 10(-9) M as estimated by a solid-surface binding assay. Binding was optimal at pH 6.5 under low ionic strength buffer and was almost entirely blocked by gangliosides or antitoxin. In analogy to intact nerve cells, binding of toxin to membranes resulted in a tight association operationally defined as sequestration. Binding and sequestration were abolished after membrane pretreatment with sialidase. The enzyme could not dissociate the membrane-bound toxin formed at 4 or 37 degrees C under low ionic strength conditions, which is in part compatible with internalization as defined in nerve cell cultures. In the latter system the toxin could be removed at 4 degrees C but not at 37 degrees C. Binding was significantly reduced upon pretreatment of guinea pig brain membranes by a variety of hydrolytic enzymes. Trypsin and chymotrypsin inhibited binding between 55% and 68% while bacterial protease abolished it by 91-95%. The effect was species-specific as it was not seen in rat or bovine synaptosomes. Collagenase and hyaluronidase had little or no inhibitory effect when applied to synaptosomes (27% and 9%) but inhibited binding to synaptic vesicles by 56% and 49%, respectively. Phospholipases A2 and C caused 42-43% inhibition of binding in vesicles and less than 22% in synaptosomes.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of bilirubin on the membrane potential of rat brain synaptosomes

The effect of the neurotoxic pigment bilirubin on the membrane potential of rat brain synaptosomes was studied by using the tetraphenylphosphonium ion (TTP+) technique. Bilirubin induces a rapid depolarization of synaptosomes, as reflected by an efflux of previously accumulated [3H]TTP+. This phenomenon persisted when the membrane potential across either the plasma membrane of the synaptosome or the inner membrane of the entrapped mitochondria was selectively depressed, thus indicating that both components of the synaptosomal membrane potential were affected by bilirubin. Bovine serum albumin, used at a albumin/bilirubin molar ratio of 1:1, had the capacity to completely prevent and reverse the effect of bilirubin. This fact demonstrates that the bilirubin-induced TPP+ release from synaptosomes is a reversible process that requires the presence of bilirubin interacting with the synaptosomal membranes. These results, together with the inhibition by bilirubin of [3H]TPP+ and [2-14C]acetate uptake by synaptosomal plasma membrane vesicles isolated from rat brain, suggest that bilirubin depresses the membrane potential across the synaptosomal plasma membrane by a mechanism involving alterations in ion permeability. This effect could be of relevance in the pathogenesis of bilirubin encephalopathy.

Electrophysiological and neurobiochemical evidence for the blockade of a potassium channel by dendrotoxin

The effects of dendrotoxin (DTX), a toxic peptide from Dendroaspis angusticeps venom, were studied electrophysiologically on peripheral frog nerve fibres, and biochemically on large synaptosomes from rat brain. On nerve fibres, DTX reduced the amplitude and prolonged the duration of the action potential; even at 0.1 nmol/l DTX produced significant effects. Maximum block of potassium currents occurred at about 30 nmol/l. Turning on of the remaining current was slowed. Reversibility was incomplete. The reduction of potassium currents was between 31% and 85% at 85 nmol/l DTX (n = 8). The remainder appeared to be resistant to DTX. Sodium channels were not affected. On large synaptosomes DTX (above 1 nmol/l) produced a slight depolarization, indicated by an outward shift of the lipophilic cation tetraphenylphosphonium, and promoted the release of radioactivity after preloading with [3H] GABA. DTX had similar potency but lower efficacy in this respect than sea anemone toxin II (ATX II). In contrast to the effects of ATX II, those due to DTX were only partially inhibited by tetrodotoxin. The actions of 4-aminopyridine resembled those of DTX, but the latter was about 500 times more potent. The electrophysiological data provide direct evidence for blockade of a potassium channel by DTX. This action is sufficient to explain the biochemical observations, although additional effects on synaptosomes cannot be excluded.

Potassium channels in isolated presynaptic nerve terminals from rat brain

86Rb efflux from pinched-off rat brain presynaptic nerve terminals (synaptosomes) was used to measure the K permeability of the terminals. Synaptosomes were pre-loaded with 86Rb and the suspensions were then filtered on glass fibre filters. The terminals trapped on the filters were superfused with 'efflux solutions', and the effluent and filters were then counted. 86Rb efflux into physiological saline (PSS) containing 5 mM-K and 145 mM-Na was about 0.4% of the 86Rb load per second (component 'R'). Increasing extracellular K concentration [( K]o), or adding veratridine and sea anemone toxin, stimulated efflux; presumably by depolarizing the nerve terminals. The K-stimulated 86Rb efflux was a graded function of [K]o. High [K]o evoked at least three components of efflux: a 'fast phase' (T) that apparently inactivated in less than 1 s, a 'slower phase' (S) that was linear for 3-5 s, and a Ca-dependent phase (C). Some, but not all, of the slow phase 86Rb efflux (component S) may be attributable to increased efflux mediated by the 'resting' K permeability mechanism when the driving force is increased by depolarization. K efflux was also studied and was found to be qualitatively similar to 86Rb efflux. 86Rb: 42K permeability ratios were 0.6-0.8 for most components of the efflux. Raising the Mg concentration in the efflux solution shifted the 86Rb efflux versus [K]o curve in the direction of increased [K]o. This shift may be the result of screening of surface charges by Mg. Several agents that block various K channels in other preparations inhibited K-stimulated 86Rb efflux in synaptosomes: tetraethylammonium (TEA), tetrabutylammonium (TBA), and 4-aminopyridine (4-AP). The fast component (T) of high [K]o-stimulated 86Rb efflux was selectively blocked by low concentrations of 4-AP (apparent half-maximal inhibition, KI = 0.1-0.2 mM); it was also blocked by TEA (KI = 0.6 mM) and TBA (KI = 0.8-1.0 mM). Dose-response curves for inhibition of component T by all three agents were monophasic. the slow component (S) of the K-stimulated 86Rb efflux was much less sensitive to all three agents, than was component T; the broad dose-response curves were consistent with the view that two (or more) different K conductances may contribute to component S.(ABSTRACT TRUNCATED AT 400 WORDS)

Dopamine synthesis in synaptosomes: relation of autoreceptor functioning to pH, membrane depolarization, and intrasynaptosomal dopamine content

Factors affecting dopamine (DA) synthesis in rat striatal synaptosomes were examined by measuring the conversion of [3H]tyrosine (Tyr) to [3H]DA. Any [3H]DA that was synthesized was extracted into a toluene-based scintillation cocktail and quantitated by liquid scintillation spectrometry. The extraction was facilitated using di-(2-ethylhexyl) phosphoric acid (DEHP), a liquid cation exchanger. DA, apomorphine, and other DA agonists were much less potent inhibitors of DA synthesis in striatal synaptosomes at pH 6.2 than at pH 7.2. 3-(3-Hydroxyphenyl)-N-n-propylpiperidine (3-PPP), a putative DA autoreceptor agonist, was inactive at pH 6.2. However, at pH 7.2, 3-PPP did inhibit DA synthesis. This inhibition was reversed by sulpiride, a DA receptor antagonist, but not by benztropine, a DA uptake blocker, suggesting that 3-PPP inhibits DA synthesis by stimulating the DA autoreceptor. DA release from synaptosomes was much greater at pH 6.2 than at pH 7.2, most probably because the synaptosomal membrane appears to be depolarized at pH 6.2, as measured by the accumulation of [3H]tetraphenylphosphonium ions. Since tyrosine hydroxylase is inhibited by DA, this finding suggested that low assay buffer pH (i.e., pH 6.2) might interfere with the ability of 3-PPP and other DA agonists to inhibit DA synthesis, by promoting DA release. Likewise, reserpine and tetrabenazine, compounds which disrupt vesicular DA storage, were much less effective inhibitors of DA synthesis at pH 6.2 (high basal DA release). Moreover, D-amphetamine and high buffer potassium concentrations, treatments which promote DA release, also interfered with the ability of 3-PPP to inhibit DA synthesis. Thus, modulation of the release of DA in equilibrium with tyrosine hydroxylase may be a mechanism by which the DA autoreceptor regulates DA synthesis.

Factors influencing the accumulation of tetraphenylphosphonium cation in HeLa cells

Exposure of HeLa cells to tetraphenylphosphonium cation (TPP+) results in a rapid accumulation intracellularly, and a steady-state level is reached within 10 min. Accumulation of [3H]TPP+ in HeLa cells is reduced under the following conditions: (i) after preincubation of cells in buffered saline or in medium containing two- to fourfold higher concentrations of amino acids, (ii) exposure to the alkylating agent L-1-tosylamido-2-phenyl-ethylchloromethyl ketone, (iii) ouabain-mediated inhibition of the Na+, K+ ATPase, and (iv) high external K+ concentrations. In contrast, addition of serum increases the uptake of TPP+. In synchronized cells, intracellular levels of TPP+ differ at various stages of cell cycle and are lowest in mitosis.

Factors influencing the accumulation of tetraphenylphosphonium cation in HeLa cells

Exposure of HeLa cells to tetraphenylphosphonium cation (TPP+) results in a rapid accumulation intracellularly, and a steady-state level is reached within 10 min. Accumulation of [3H]TPP+ in HeLa cells is reduced under the following conditions: (i) after preincubation of cells in buffered saline or in medium containing two- to fourfold higher concentrations of amino acids, (ii) exposure to the alkylating agent L-1-tosylamido-2-phenyl-ethylchloromethyl ketone, (iii) ouabain-mediated inhibition of the Na+, K+ ATPase, and (iv) high external K+ concentrations. In contrast, addition of serum increases the uptake of TPP+. In synchronized cells, intracellular levels of TPP+ differ at various stages of cell cycle and are lowest in mitosis.

Crosslinking of the receptors for immunoglobulin E depolarizes the plasma membrane of rat basophilic leukemia cells

Aggregation of the receptor for IgE on mast cells, basophils, and a tumor analog, rat basophilic leukemia (RBL) cells, induces a calcium-dependent degranulation of the cells. We have measured the membrane potential (delta psi) of RBL cells during this reaction by using the tetraphenylphosphonium ion (Ph4P+) equilibration technique. We observed a 20-45% reduction in ionophore-sensitive Ph4P+ accumulation. The phenomenon persisted under conditions expected to collapse the mitochondrial membrane potential, consistent with the effect being due to a change in delta psi of the plasma membrane. We estimated that the change reflects a depolarization of 20 mV (from -90 to -70 mV, interior negative). Whereas degranulation fails to occur in the absence of external Ca2+, this was not true of the depolarization, indicating that the latter was not a consequence of secretion. When aggregation of the receptor is induced by reaction of the cell-bound IgE with a multivalent antigen, the secretory reaction can be halted by adding a univalent hapten. In this case, complete repolarization occurs. Equivalent depolarization was observed in the absence of Na+ but was diminished when both Ca2+ and Na+ were absent. Together, the data suggest that aggregation of the receptor opens ion channels and that the latter disappear promptly when the receptors are disaggregated. It is plausible that formation of these channels leads to the entry of Ca2+ and is an early and critical consequence of the aggregation of the receptors, thereby leading to degranulation.

Action and binding of palytoxin, as studied with brain membranes

Palytoxin in concentrations as low as 10(-11) to 10(-12) M promotes the outflow of the lipophilic [3H]tetraphenylphosphonium ion from particulate brain cortex of guinea-pigs and rats, and from preloaded crude synaptosomes of rats, which indicates depolarization. The outflow is not influenced by tetrodotoxin or the calcium channel blocker nimodipin, or by substitution of choline for Na+ ions. It is increased by Ca2+ and by borate, the latter interacting with the toxin itself. To assess the fixation of palytoxin to biological membranes, a binding step was installed before the depolarization step. Palytoxin binds to membranes from rat brain, liver, kidney, human and dog erythrocytes, and to a lesser degree to liposomes made from rat brain or erythrocyte lipids. Binding is reversible. It is decreased by mild physical pretreatments of crude synaptosomes. Palytoxin binding is increased in the presence of micromolar concentrations of Ca2+ or borate. It is concluded that the potentiation of palytoxin actions by Ca2+ or borate is at least partially due to the promotion of its binding.

alpha-latrotoxin of black widow spider venom depolarizes the plasma membrane, induces massive calcium influx, and stimulates transmitter release in guinea pig brain synaptosomes

The effect of alpha-latrotoxin from black widow spider venom upon guinea pig cerebral cortical synaptosomes is described. Plasma membrane potential (delta psi p), in situ mitochondrial membrane potential (delta psi m), Ca2+ transport, gamma-amino[3H]butyrate release, [3H]noradrenaline release, and synaptosomal ATP were monitored under parallel conditions. Potentials were determined both isotopically and with a tetraphenylphosphonium-selective electrode. alpha-Latrotoxin depolarizes delta psi p selectively, both in the presence and absence of Ca2+. A slight toxin-induced depolarization of delta psi m is a consequence of a massive Ca2+ uptake across the plasma membrane. Depolarization of delta psi p is insensitive to tetrodotoxin, and Ca2+ entry is only partially inhibited by verapamil. Release of [3H]noradrenaline and gamma-amino[3H]butyrate is markedly stimulated by the toxin in the presence of Ca2+, and this effect is only slightly reduced in Ca2+-free conditions.

Influx of calcium, strontium, and barium in presynaptic nerve endings

Depolarization-induced (potassium-stimulated) influx of 45Ca, 85Sr, and 133Ba was measured in synaptosomes prepared from rat brain. There are two phases of divalent cation entry, "fast" and "slow;" each phase is mediated by channels with distinctive characteristics. The fast channels inactivate (within 1 s) and are blocked by low concentrations (less than 1 micro M) of La. The slow channels do not inactivate (within 10 s), and are blocked by high concentrations (greater than 50 micro M) of La. Divalent cation influx through both channels saturates with increasing concentrations of permeant divalent cation; in addition, each permeant divalent cation species competitively blocks the influx of other permeant species. These results are consistent with the presence of "binding sites" for divalent cations in the fast and slow channels. The Ca:Sr:Ba permeability ratio, determined by measuring the influx of all three species in triple-label experiments, was 6:3:2 for the fast channel and 6:3:1 for the slow channel. A simple model for ion selectivity, based on the presence of a binding site in the channel, could account well for slow and, to some extent, for fast, channel selectivity data.

Triphenylmethylphosphonium is an ion channel ligand of the nicotinic acetylcholine receptor

The lipophilic cation triphenylmethylphosphonium (Ph3MeP+), which is widely used as a sensor for membrane potential with cells, organelles, and membrane vesicles, is shown also to accumulate in membranes rich in nicotinic acetylcholine receptor in a voltage-independent way. Evidence is presented that Ph3MeP+ in this system is bound to a cation-binding site of the ion channel that is part of the acetylcholine receptor complex. Binding is stimulated by cholinergic effectors (Kd = 13 microM in the absence of carbamoylcholine; Kd = 1.5 microM in the presence of 10 microM carbamoylcholine), and this stimulation is blocked by alpha-bungarotoxin. Ph3MeP+ blocks efflux of 22Na from receptor-rich microsacs and appears to compete with the channel ligand phencyclidine for a common binding site. In contrast to the binding of other proven channel ligands, Ph3MeP+-binding is not affected by desensitization.

Tetanus toxin and botulinum A neurotoxin inhibit and at higher concentrations enhance noradrenaline outflow from particulate brain cortex in batch

Tetanus toxin and, to a lesser degree, botulinum A toxin partially depress the basal and the potassium evoked outflow of [3H]noradrenaline from preloaded particulate rat forebrain cortex. The effect is due to the toxins and not to any contaminant, as shown by dialysis, heating and antitoxin treatment, and also by replacement of crystalline botulinum A toxin with purified neurotoxin. Tetanus toxin also depresses the outflow due to sea anemone toxin II, 4-aminopyridine and d-amphetamine. The effect of the toxins proceeds with time and strongly depends on temperature. Once manifest the tetanus toxin effect is not reversed by antitoxin. Pretreatment with V. cholerae neuraminidase degrades the long-chain gangliosides quantitatively to GM1. Tetanus toxin, applied subsequently remains fully active. High concentrations of tetanus toxin and botulinum A neurotoxin promote the outflow of small amounts of tritium within short incubation times. It is concluded: a) Tetanus toxin is a broad range neurotoxin which acts on processes subsequent to the depolarization step. b) Long-chain gangliosides are only binding sites, but not receptors of tetanus toxin. c) Botulinum A toxin is less potent but resembles tetanus toxin in both promoting and depressing the outflow of noradrenaline.

Synaptosomes from rat brain: morphology, compartmentation, and transmembrane pH and electrical gradients

Morphological studies of synaptosomes isolated from rat brains show that approximately 68% of the synaptosomes in these preparations contain synaptic vesicles (range, 62--72.5%). Approximately 30% of the synaptosomes contain mitochondria, and only less than 20% of the total mitochondria in good preparations are free and not enclosed in synaptic structures. The mitochondrial volume percent calculated on the basis of the measured cytochrome c content is 5% for synaptosomes isolated from anesthetized animals and 11% for synaptosomes isolated from unanesthetized animals. These numbers bracket the value of 8.7% obtained from electron micrographs. The volume percent of intrasynaptic vesicles is 1.5% as calculated from electron micrographs. The pH gradient between the extracellular pH and the mean intracellular pH is --0.45, as measured by equilibrium distributions of methylamine and dimethylamine, and --0.05, as determined by equilibrium distributions of 5,5-dimethyloxazolidine-2,4-dione and trimethylacetic acid. Analysis of these data shows that there cannot be a large pH gradient (alkaline inside) across the mitochondria, nor can the synaptic vesicle compartment be very large (less than 1.85%). Equilibrium distribution of [3H]triphenylmethylphosphonium ion in synaptosomal preparations gives a calculated apparent potential of --85 mV, in agreement with our previous value. Analysis of these data using the measured volumes of mitochondrial and intrasynaptic vesicular compartments (8.7 and 1.5%, respectively) gives a maximum possible transmitochondrial membrane potential of --59 mV.

Measurement of membrane potentials (psi) of erythrocytes and white adipocytes by the accumulation of triphenylmethylphosphonium cation

The accumulation of the lipophilic cation, triphenylmethylphosphonium, has been employed to determine the resting membrane potential in human erythrocytes, turkey erythrocytes, and rat white adipocytes. The triphenylmethylphosphonium cation equilibrates rapidly in human erythrocytes in the presence of low concentrations of the hydrophobic anion, tetraphenylborate. Tetraphenylborate does not accelerate the uptake of triphenylmethylphosphonium ion by adipocytes. The cell associated vs. extracellular distribution of the triphenylmethylphosphonium ion is proportional to changes in membrane potential. The distribution of this ion reflects the membrane potential determining concentration of the ion with dominant permeability in a "Nernst" fashion. The resting membrane potentials for the human erythrocyte, turkey erythrocyte, and rat white adipocyte were found to be -8.4 +/- 1.3, -16.8 +/- 1.1, and -58.3 +/- 5.0 mV, respectively, values which compare favorably with values obtained by other methods. In addition, changes in membrane potential can be assessed by following triphenylmethylphosphonium uptake without determining the intracellular water space. The method has been successfully applied to a study of hormonally induced changes in membrane potential of rat white adipocytes.

Membrane potentials in cell-free preparations from guinea pig cerebral cortex: effect of depolarizing agents and cyclic nucleotides

The distribution of [3H]triphenylmethylphosphonium ion between the medium and vesicular entities was examined in a cell-free, particulate preparation from guinea pig cerebral cortex. This distribution followed the Nernst relationship with regard to the external potassium ion concentration and, in physiological media, indicated the maintenance of a mean trans-membrane potential ranging from -58 to -78 mV. The neurotoxins batrachotoxin, veratridine, and grayanotoxin I, partially depolarized the preparation. Tetrodotoxin blocked the depolarization by batrachotoxin, veratridine, and gray-anotoxin I. The depolarization by these neurotoxins was potentiated by the presence of anemone toxin II and presumably reflected the response of vesicular components of neuronal origin. An additional potassium-sensitive depolarization probably represented the response of vesicular components of glial origin with an apparent transmembrane potential of -8 to -35 mV. No correlation could be demonstrated between changes in transmembrane potential and stimulation of cyclic AMP generation by a variety of agents in this preparation.

Electrochemical proton gradient in Micrococcus lysodeikticus cells and membrane vesicles

Using the distribution of weak acids to measure the pH gradient (delta pH; interior alkaline) and the distribution of the lipophilic cation [3H]tetraphenylphosphonium+ to monitor the membrane potential (delta psi; interior negative), we studied the electrochemical gradient or protons (delta mu- H+) across the membrane of Micrococcus lysodeikticus cells and plasma membrane vesicles. With reduced phenazine methosulfate as electron donor, intact cells exhibited a relatively constant delta mu- H+ (interior negative and alkaline) of -193 mV to -223 mV from pH 5.5 to pH 8.5. On the other hand, in membrane vesicles under the same conditions, delta mu- H+ decreased from a maximum value of -166 mV at pH 5.5 to -107 mV at pH 8.0 and above. This difference is related to a differential effect of external pH on the components of delta mu- H+. In intact cells, delta pH decreased from about -86 mV (i.e., 1.4 units) at pH 5.5 to zero at pH 7.8 and above, and the decreases in delta pH was accompanied by a reciprocal increase in delta psi from -110 mV at pH 5.5 to -211 mV at pH 8.0 and above. In membrane vesicles, the decrease in delta pH with increasing external pH was similar to that described for intact cells; however, delta psi increased from -82 mV at pH 5.5 to only -107 mV at pH 8.0 and above.

Membrane potential changes during mitogenic stimulation of mouse spleen lymphocytes

By monitoring differences in accumulation of the lipophilic cation [(3)H]tetraphenylphosphonium in media containing low or high potassium concentrations [Lichtshtein, D., Kaback, H. R. & Blume, A. J. (1979) Proc. Natl. Acad. Sci. USA 76, 650-654], the membrane potential of lymphocytes from various sources has been estimated. On the basis of this method, the potential of normal mouse spleen lymphocytes (T and B cells) is -65 +/- 2 mV (mean +/- SEM, interior negative). During the course of mitogenic stimulation by concanavalin A, lipopolysaccharide, or fetal calf serum, the membrane potential of murine spleen lymphocytes changes systematically according to the following pattern: (i) early depolarization lasting 2-3 hr, (ii) repolarization over the next 7 hr, and (iii) a final hyperpolarization phase during the last 24-48 hr. During repolarization and hyperpolarization, moreover, there is a direct correlation between the membrane potential and DNA synthesis, as judged by [(3)H]thymidine incorporation. By using isolated T and B cells, it is observed that concanavalin A depolarizes T cells only, whereas lipopolysaccharide depolarizes B cells only. Thus, both mitogens exhibit the same specificity for depolarization as for mitogenic stimulation. On the basis of these observations, it is suggested that the transition of lymphocytes from a resting state to mitotic activity is initiated by depolarization of the plasma membrane.