Effect of 2-(4-phenylpiperidino)cyclohexanol on acetylcholine release and subcellular distribution in rat striatal slices

Long-Term Depression of Striatal DA Release Induced by mGluRs via Sustained Hyperactivity of Local Cholinergic Interneurons

The cellular mechanisms regulating dopamine (DA) release in the striatum have attracted much interest in recent years. By in vitro amperometric recordings in mouse striatal slices, we show that a brief (5 min) exposure to the metabotropic glutamate receptor agonist DHPG (50 μM) induces a profound depression of synaptic DA release, lasting over 1 h from DHPG washout. This long-term depression is sensitive to glycine, which preferentially inhibits local cholinergic interneurons, as well as to drugs acting on nicotinic acetylcholine receptors and to the pharmacological depletion of released acetylcholine. The same DHPG treatment induces a parallel long-lasting enhancement in the tonic firing of presumed striatal cholinergic interneurons, measured with multi-electrode array recordings. When DHPG is bilaterally infused in vivo in the mouse striatum, treated mice display an anxiety-like behavior. Our results demonstrate that metabotropic glutamate receptors stimulation gives rise to a prolonged depression of the striatal dopaminergic transmission, through a sustained enhancement of released acetylcholine, due to the parallel long-lasting potentiation of striatal cholinergic interneurons firing. This plastic interplay between dopamine, acetylcholine, and glutamate in the dorsal striatum may be involved in anxiety-like behavior typical of several neuropsychiatric disorders.

Role of calcium on the modulation of spontaneous acetylcholine efflux by the D2 dopamine receptor subtype in rat striatal synaptosomes

The role of calcium in the modulation of spontaneous [3H]acetylcholine ([3H]ACh) efflux through presynaptic D2 dopamine hetero-receptors was investigated in rat striatal synaptosomes. The kinetic studies of [3H]ACh efflux in the presence or absence of Ca2+ were carried out in nonstimulating conditions. When Ca2+ was omitted from the superfusion medium, a notable and significant (P<0.001) decrease of tritium efflux (39%) was obtained. While [3H]ACh efflux was insensitive to tetrodotoxin (TTX) 1 microM, cadmium (10 microM), a nonselective antagonist of calcium channels, significantly reduced the tritium efflux by 24% (P<0.001), while the L-type calcium antagonist, nifedipine, (30 microM) inhibited the tritium efflux by only 10% (P<0.02). 2-(4-Fenylpiperidine)cyclohexanol (vesamicol), an inhibitor of the vesicular [3H]ACh carrier, significantly depressed the spontaneous tritium efflux in the presence of Ca2+ (60%; P<0.001) and in a low-calcium medium (20%; P<0.001). Although 1 microM of 7-hydroxy-N,N-di-n-propyl-2-aminotetraline (7-OH-DPAT) inhibited spontaneous [3H]ACh efflux in the presence of calcium, this dopaminergic agonist did not modify the neurotransmitter release in either the low-Ca2+ medium or in the presence of vesamicol. These results suggest that the spontaneous [3H]ACh efflux is a process involving a Ca2+-dependent component (39%), sensitive to calcium channel-blockers and vesamicol, in rat striatal synaptosomes. In addition, activation of the D2 dopamine hetero-receptor only modulates the calcium-dependent component of spontaneous [3H]ACh efflux.

Halothane enhances exocytosis of [3H]-acetylcholine without increasing calcium influx in rat brain cortical slices

1. The effect of halothane on the release of [3H]-acetylcholine ([3H]-ACh) in rat brain cortical slices was investigated. 2. Halothane (0.018 mM) did not significantly affect the basal and the electrical field stimulation induced release of [3H]-ACh. However, halothane (0.063 mM) significantly increased the basal release of [3H]-ACh and this effect was additive with the electrical field stimulation induced release of [3H]-ACh. 3. The release of [3H]-ACh induced by 0.063 mM halothane was independent of the extracellular sodium and calcium ion concentration and was decreased by tetracaine, an inhibitor of Ca(2+)-release from intracellular stores or dantrolene, an inhibitor of Ca(2+)-release from ryanodine-sensitive stores 4. Using 2-(4-phenylpiperidino)-cyclohexanol (vesamicol), a drug that blocks the storage of ACh in synaptic vesicles, we investigated whether exocytosis of this neurotransmitter is involved in the effect of halothane. Vesamicol significantly decreased the release of [3H]-ACh evoked by halothane. 5. It is suggested that halothane may cause a Ca2+ release from intracellular stores that increases [3H]-ACh exocytosis in rat brain cortical slices.

Effects of calyculin A and okadaic acid on acetylcholine release and subcellular distribution in rat hippocampal formation

The mechanisms regulating the compartmentation of acetylcholine (ACh) and the relationship between transmitter release and ACh stores are not fully understood. In the present experiments, we investigated whether the inhibitors of serine/threonine phosphatases 1 and 2A, calyculin A and okadaic acid, alter subcellular distribution and the release of ACh in rat hippocampal slices. Calyculin A and okadaic acid significantly (p < 0.05) depleted the occluded ACh of the vesicular P3 fraction, but cytoplasmic ACh contained in the S3 fraction was not significantly affected. The P3 fraction is known to be heterogeneous; calyculin A and okadaic acid reduced significantly (p < 0.05) the amount of ACh recovered with a monodispersed fraction (D) of synaptic vesicles, but the other nerve terminal bound pools (E-F and G-H) were not so affected. K+-evoked ACh release decreased significantly (p < 0.01) in the presence of calyculin A and okadaic acid, suggesting that fraction D's vesicular store of ACh contributes to transmitter release. The loss of ACh from synaptic vesicle fractions prepared from tissue exposed to phosphatase inhibitors appeared not to result from a reduced ability to take up ACh. Thus, when tissue was allowed to synthesize [3H]ACh from [3H]choline, the ratio of [3H]ACh in the S3 to P3 fractions was not much changed by exposure of tissue to calyculin A or okadaic acid; furthermore, the specific activity of ACh recovered from the D fraction was not reduced disproportionately to that of cytosolic ACh. The changes are considered to reflect reduced synthesis of ACh by tissue treated with the phosphatase inhibitors, rather than an effect on vesicle uptake mechanisms. Thus, exposure of tissue to calyculin A or okadaic acid appears to produce selective depletion of tissue ACh content in a subpopulation of synaptic vesicles, suggesting that phosphatases play a role in ACh compartmentation.

D2 dopamine receptor activation inhibits basal and forskolin-evoked acetylcholine release from dissociated striatal cholinergic interneurons

We tested whether D2 ligands inhibit basal and forskolin-stimulated [3H]ACh release from dissociated striata, as opposed to striatal slices. Quinpirole inhibited both basal (40% maximal inhibition; IC50 approximately 50 nM) and 10 microM forskolin-stimulated release (80% inhibition; IC50 approximately 25 nM quinpirole) and both actions were blocked by a D2 antagonist. Vesamicol prevented the quinpirole and forskolin actions. The ability of D2 agonists to inhibit basal and cyclase-stimulated acetylcholine release emanating from vesamicol-sensitive vesicles appears to be tonically suppressed by inhibitory elements within striatal circuitry.

Modulation of hippocampal norepinephrine release by cholinergic agonists is altered by AF64A lesion

The effect of lesioning hippocampal cholinergic neurons with the neurotoxin AF64A on the ability of cholinergic agonists to modulate stimulation-induced release of 3H-norepinephrine (NE) from rat hippocampal slices was studied. Rats received intracerebroventricular injections of either AF64A (ethylcholine mustard aziridinium, 2 nmol) or vehicle (sham operated). Six weeks after treatment, release of 3H-NE evoked by electrical stimulation (2 Hz, 2 min) in the presence or absence of cholinergic agonists and/or antagonists was measured. Activation of M2 receptors with oxotremorine (in the presence of the M1 antagonist pirenzepine) caused a small inhibition of NE release, which was abolished in hippocampi from AF64A-treated rats. The Kd for high-affinity binding of the selective M2 ligand [3H] AF-DX 384 was increased 10-fold in lesioned tissues. The M1 selective agonist McN-A-343 produced a significant enhancement of NE release, which was unchanged by AF64A lesion. Binding studies with [3H] pirenzepine showed no change in the affinity or number of M1 receptors. Nicotine also caused a significant enhancement of evoked NE release, but this effect was markedly reduced in tissues from AF64A-treated rats. AF64A treatment caused a twofold decrease in the number of [3H] nicotine binding sites. This study suggests that long-term lesion of hippocampal cholinergic neurons with AF64A alters the function of postsynaptic muscarinic M2 and nicotinic cholinergic receptors that modulate the release of NE in the hippocampus.

Effects of systemic administration of 2-(4-phenyl-piperidino)-cyclohexanol (vesamicol) and an organophosphate DDVP on the cholinergic system in brain regions of rats

Vesamicol is known to inhibit the transport of acetylcholine (ACh) into synaptic vesicles in vitro, but much less is known about its effects in the brain in vivo. To assess the effect of vesamicol in vivo, we examined cholinergic parameters, such as the subcellular distribution of ACh, activities of enzymes, uptake of choline, and muscarinic receptor binding in the striatum, hippocampus, and cerebral cortex of rats 30 and 60 min after intraperitoneal injection of vesamicol (3 mg/kg) or of vesamicol in combination with DDVP (5 mg/kg), which was administered 10 min before vasamicol. The levels of cytosolic ACh increased in all regions of the brain after injection of vesamicol, while those of vesicular ACh decreased in all regions except for the striatum. The increase in the levels of extracellular ACh and cytosolic ACh in the striatum induced by DDVP was generally enhanced after injection of vesamicol, Vesamicol did not reduce the level of vesicular ACh when DDVP had been injected previously. Vesamicol did not induce any significant changes in the activities of enzymes, choline uptake, or binding of [6H]quinuclidinyl benzilate to the muscarinic ACh receptors in the three regions. Changes in the cholinergic parameters caused by DDVP were not reversed by the combined administration of DDVP with vesamicol. The present results indicate that vesamicol can inhibit the transport of ACh into synaptic vesicles in the brain tissue in vivo, although it cannot reverse the effects of DDVP that has been injected prior to vesamicol.

Evidence to suggest that cytosolic acetylcholine in rat hippocampal nerve terminals is not directly transferred into synaptic vesicles for release

Rat hippocampal minces were loaded with [acetyl 1-14C]acetylcholine ([14C]ACh) in the presence of the "poorly penetrating" acetylcholinesterase (EC 3.1.1.7; AChE) inhibitor echothiophate and the effect of high K+ depolarization determined on the subcellular storage and release of [14C]ACh and its metabolites. Results indicated that high K+ did not augment the release of [14C]ACh. Rather, it increased the release of [14C]acetate while simultaneously reducing the level of [14C]ACh in the cytosolic (S3) fraction. When the identical experiment was performed with paraoxon, a "penetrating" AChE inhibitor, high K+ still did not increase the release of [14C]ACh. However, paraoxon prevented the K(+)-induced loss of [14C]ACh from the cytosolic fraction as well as the K(+)-induced gain of [14C]acetate in the release medium. When minces were loaded with [14C]ACh in the presence of echothiophate and subsequently subjected to high K+ depolarization in the absence or presence of vesamicol (AH5183; (-)-trans-2-[4-phenylpiperidino] cyclohexanol), a drug which blocks the refilling of synaptic vesicles with ACh, the amount of endogenous ACh released was reduced approximately 50%. Conversely, the amount of [14C]ACh released was not reduced at all. These results suggest that cytosolic ACh is not directly transported into synaptic vesicles for release when hippocampal nerve terminals are depolarized. Rather, its hydrolysis is accelerated in response to depolarization. A working hypothesis explaining the importance of the depolarization-induced breakdown of cytosolic ACh to central ACh metabolism is presented.

Inhibition of potassium-stimulated acetylcholine release from rat brain cortical slices by two high-affinity analogs of vesamicol

In this work, we investigated the effects of two structural analogs of the drug vesamicol, which inhibits the vesicular acetylcholine (ACh) transport, on the potassium-stimulated release of ACh from rat brain cortical slices. These vesamicol analogs, 4-aminobenzovesamicol (ABV) and (trans)-cyclohexovesamicol (transDec), were almost as potent as vesamicol in inhibiting the evoked release of ACh from cortex slices. Similar to vesamicol, the presence of these analogues inhibited the ability of ACh newly-synthesized from [3H]choline to become releasable. However, vesamicol's action was reversible, while ABV and transDec caused a persistent block of this [3H]ACh release. In addition, vesamicol did not affect the release of pre-stored [3H]ACh, but ABV and transDec partially inhibited the release of [3H]ACh in this condition, suggesting that the two latter drugs may alter some of the steps posterior to the entry of [3H]ACh into synaptic vesicles. The rank order of potency for these drugs to reduce ACh release (vesamicol = transDec > ABV) is close to the rank order for inhibition of ACh vesicular transport (transDec > vesamicol > ABV), but is completely different from the order of affinities of these drugs for the vesamicol receptor (ABV > transDec > > vesamicol). These results suggest that although these two vesamicol analogs are able to block ACh release due to their effects on the vesicular transport system, they may have other unexpected actions not shared by vesamicol.

Vesamicol blocks the recovery, by recycling cholinergic electromotor synaptic vesicles, of the biophysical characteristics of the reserve population

The effect of vesamicol on the ability of recycling cholinergic synaptic vesicles to recover, during a period of post-stimulation rest, the biophysical properties of the reserve pool was studied in prestimulated perfused blocks of the electric organ of the electric ray, Torpedo marmorata, a tissue rich in cholinergic synapses. The effect of the drug was analysed by high-resolution centrifugal density-gradient fractionation in a zonal rotor of the extracted vesicles. The two vesicle fractions were identified by their ATP and acetylcholine content and the recycled vesicles by their acquisition of [3H]acetylcholine derived from [3H]acetate in the perfusate. Vesamicol (10 microM) blocked the uptake of tritiated acetylcholine by recycled vesicles and also prevented them from rejoining the reserve pool. This is consistent with a previously formulated model of the recovery process, whereby the increase in the acetylcholine and ATP content of the recycled vesicles which takes place during a post-stimulus period of rest increases their osmotic load and thus their content of free water. Vesamicol, by blocking acetylcholine uptake, also blocks rehydration of the recycled vesicles and thus the accompanying decrease in their density to the value characteristic of fully charged vesicles.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Suppression by cholinesterase inhibition of a Ca(2+)-independent efflux of [3H]acetylcholine from the neuromuscular junction of the isolated rat diaphragm

Endplate preparations of the left rat hemidiaphragm were incubated with [3H]choline to label neuronal acetylcholine stores. Elevation of the concentration (13.5-135 mmol/l) of extracellular potassium chloride (KCl) stimulated the release of [3H]acetylcholine in a concentration-dependent manner. KCl (27 mmol/l) still caused a significant efflux of [3H]acetylcholine in a Ca(2+)-free medium. Inhibitors of cholinesterase (physostigmine, diisopropylfluorophosphate) suppressed by 80% this Ca(2+)-independent efflux of [3H]acetylcholine. Vesamicol (10 mumol/l), the blocker of the vesicular acetylcholine carrier, also suppressed the stimulated, Ca(2+)-independent efflux of [3H]acetylcholine. The inhibitory effect of physostigmine was not prevented by muscarine or nicotine receptor antagonists, but the inhibitory effect was lost when the stimulus strength was increased (81 mmol/l KCl). The present experiments showed cholinesterase inhibition to suppress a Ca(2+)-independent efflux of [3H]acetylcholine, probably by interference with a membrane-bound acetylcholine carrier.

Acetylcholine synthesis is modulated by acetylcholine content of cytosolic fraction but not by that of releasable fraction

Synthesis and release of acetylcholine (ACh) in the rat hippocampal slices were examined to clarify the mechanism of modulation of ACh synthesis. Treatment with 2-(4-phenylpiperidino)cyclohexanol (AH5183, 50 microM), an inhibitor of ACh transport from cytosol to synaptic vesicles, inhibited the increase in ACh content of the membrane-bound fraction which is readily releasable, but did not affect the cytosolic ACh content. Under these conditions, the total ACh content reached a plateau value. These results indicate that ACh synthesis is modulated by cytosolic ACh content but not by the vesicular fraction.

Phosphoethanolamine enhances high-affinity choline uptake and acetylcholine synthesis in dissociated cell cultures of the rat septal nucleus

Dissociated rat septal nucleus cells cultured in defined medium exhibited twofold increases in the maximal rates of sodium-dependent, high-affinity choline uptake and acetylcholine formation when grown in the presence of phosphoethanolamine. The effect was concentration-dependent (EC50 = 15 microM) and appeared to be associated with in vitro maturation of cholinergic neurons rather than with enhanced survival. Choline acetyltransferase, acetylcholinesterase, and choline kinase activities were unaffected by this treatment. The effect of phosphoethanolamine was specific for cholinergic neurons, because treatment with this compound did not alter the kinetic constants for high-affinity neuronal uptake of gamma-aminobutyric acid or dopamine. The action appeared to be mediated primarily through activation of the sodium-dependent, high-affinity transport mechanism for choline as opposed to alterations in the storage and release of acetylcholine.

Routes of acetylcholine leakage from cytosolic and vesicular compartments of rat motor nerve terminals

Acetylcholine efflux at the rat neuromuscular junction was assayed following blockage of ACh transport into synaptic vesicles by 2-(4-phenylpiperidino) cyclohexanol (AH5183). [2H4]Choline was used as a labeled precursor. AH5183 completely blocked ACh efflux from the cytosolic compartment but had comparatively less effect on release from the unlabeled vesicular pool. Tissue [2H4]ACh levels increased after AH5183 addition due to cytosolic ACh retention. Thus, ACh in the non-vesicular pool (calculated to be 34% of the total ACh) may efflux solely via the AH5183-sensitive ACh transporter inserted into the terminal membrane. ACh released from the vesicular fraction was about 100-fold more than could be accounted for by miniature end-plate potentials; possible causes of this overestimate are discussed.

Suppression of in vivo neostriatal acetylcholine release by vesamicol: evidence for a functional role of vesamicol receptors in brain

Experiments examined the effects of peripheral and central administration of the vesicular acetylcholine transport blocker vesamicol (AH5183) on the content, synthesis, and release of acetylcholine in the rat brain in vivo. In time course studies, a single intraperitoneal dose of DL-vesamicol (5 mg/kg) rapidly and reversibly (within 2 h) doubled the content of acetylcholine in the striatum and hippocampus, without affecting choline levels or the rate of transmitter synthesis. In microdialysis experiments, the same peripheral dose of drug produced a reversible 55% reduction in endogenous striatal acetylcholine release. A similar inhibitory effect was produced by direct intrastriatal perfusion with vesamicol. Moreover, this effect of vesamicol was (a) concentration-dependent and saturable (EC50 = 68 nM), (b) rapidly reversible, (c) stereospecific for the L-isomer, and (d) poorly mimicked by a vesamicol analog with lower plasma membrane permeability. This profile of effects is consistent with an interaction with a specific vesamicol receptor as defined by previous in vitro binding studies. These results support a functional role for vesamicol receptors in modulating central cholinergic transmission in vivo.

Local anaesthetic activity of vesamicol in the electric organ of Torpedo

Synaptic transmission in intact pieces of the Torpedo electric organ treated with vesamicol (2-(4-phenylpiperidino)cyclohexanol, formerly AH5183) was elicited by trains of repetitive electrical stimulation at different frequencies. When the frequency of stimulation was increased from 10 to 50 or 100 Hz, micromolar concentrations of vesamicol enhanced the tetanic rundown of the successive tissue responses. This effect was already detectable with 10 microM vesamicol. It was dramatically potentiated with concentrations of 50 or 100 microM vesamicol, which caused complete failure of transmission after usually less than 10 responses. The drug was unequivocally demonstrated to act by depressing the evoked release of acetylcholine as a consequence of a highly frequency- and concentration-dependent impairment of Na+ channel function in afferent axons. It is concluded that, in the electric organ, vesamicol blocks transmission by acting as a local anaesthetic. This action of micromolar concentrations of vesamicol must be taken into account especially during high-rate nerve activity.

Ca2+o-independent veratridine-evoked acetylcholine release from striatal slices is not inhibited by vesamicol (AH5183): mobilization of distinct transmitter pools

The effect of 2-(4-phenylpiperidino)cyclohexanol (AH5183 or vesamicol), a compound known to block the uptake of acetylcholine (ACh) into cholinergic synaptic vesicles, on the release of endogenous and [14C]ACh from slices of rat striatum was investigated. ACh release was evoked either by electrical stimulation or by veratridine. The effect of electrical stimulation was entirely dependent on external Ca2+. By contrast, veratridine (40 microM) also enhanced ACh release in the absence of Ca2+. Indeed, with veratridine two components were clearly distinguished: one dependent on external Ca2+ and the other not. Vesamicol inhibited [14C]ACh release evoked by both veratridine and electrical stimulation in the presence of external Ca2+, provided it was added to the tissue prior to loading with [14C]choline. With the same treatment vesamicol only slightly affected the release of endogenous ACh. Under the same conditions the Ca2(+)-independent [14C]ACh release evoked by veratridine was not prevented by vesamicol. The differential responsiveness to vesamicol suggests that ACh pools involved in Ca2+o-dependent ACh release are different from those mobilized during Ca2+o-independent ACh release.

The effect of 2-(4-phenylpiperidino)cyclohexanol (AH-5183), tityustoxin and ouabain on the release of acetylcholine and its mobilization from cytoplasmic and vesicular pools of rat brain cortical slices

The effect of vesicular acetylcholine (ACh) transport blocker 2-(4-phenylpiperidino)cyclohexanol (AH-5183) on the subcellular storage and release of ACh was studied in rat brain cortical slices. AH-5183 reduced the release of ACh from cortical slices stimulated by tityustoxin and ouabain. Tissue stimulated in the presence of AH-5183 contained more ACh in both the nerve terminal synaptic vesicles and cytoplasmic fraction than did tissue stimulated in drug's absence. Thus, AH-5183 blocked the tityustoxin and ouabain induced release of ACh from both cytoplasmic and vesicular pools. AH-5183 also depressed the spontaneous release of ACh from incubated slices and, in this condition, the drug had no effect in the subcellular distribution of ACh. It is suggested that AH-5183 interferes with the process of ACh release independent of its blocking action on ACh transport into the synaptic vesicles.

In favour of the vesicular hypothesis: neurochemical evidence that vesamicol (AH5183) inhibits stimulation-evoked release of acetylcholine from neuromuscular junction

1. The effects of optical isomers of vesamicol (2-(4-phenylpiperidino) cyclohexanol), an inhibitor of acetylcholine (ACh) storage, on stimulation-evoked release of [3H]-acetylcholine [( 3H]-ACh) from the neuromuscular junction have been studied in the region of the mouse hemidiaphragm which contains the motor endplates, and which can easily be loaded with [3H]-choline. This method made it possible to detect exclusively the Cao-dependent release of [3H]-ACh in response to stimulation, and therefore to test the vesicular hypothesis. 2. (-)-Vesamicol was approximately 20 times more potent than (+)-vesamicol in reducing stimulation-evoked release of [3H]-ACh. 3. 4-Aminopyridine, a potassium channel blocker, enhanced the release of ACh in response to stimulation, but failed to increase release from hemidiaphragm which had been pretreated with (-)-vesamicol. 4. The fact that (-)-vesamicol inhibited the release of [3H]-ACh in response to electrical stimulation only when it was administered prior to the loading of the tissue with [3H]-choline, and had no effect when the stores had already been filled with labelled [3H]-ACh indicates that the stimulation-evoked release of [3H]-ACh is of vesicular origin and (-)-vesamicol has no effect on the release process. This is the first neurochemical evidence for the vesicular origin of stimulation-evoked release of ACh from the neuromuscular junction.

Compared effects of two vesicular acetylcholine uptake blockers, AH5183 and cetiedil, on cholinergic functions in Torpedo synaptosomes: acetylcholine synthesis, choline transport, vesicular uptake, and evoked acetylcholine release

We examined the effects of two drugs, AH5183 and cetiedil, demonstrated to be potent inhibitors of acetylcholine (ACh) transport by isolated synaptic vesicles on cholinergic functions in Torpedo synaptosomes. AH5183 exhibited a high specificity toward vesicular ACh transport, whereas cetiedil was shown to inhibit both high-affinity choline uptake and vesicular ACh transport. Choline acetyltransferase was not affected by either drug. High external choline concentrations permitted us to overcome cetiedil inhibition of high-affinity choline transport, and thus synthesis of [14C]ACh in treated preparations was similar to that in controls. We then tested evoked ACh release in drug-treated synaptosomes under conditions where ACh translocation into the vesicles was blocked. We observed that ACh release was impaired only in cetiedil-treated preparations; synaptosomes treated with AH5183 behaved like the controls. Thus, this comparative study on isolated nerve endings allowed us to dissociate two steps in drug action: upstream, where both AH5183 and cetiedil are efficient blockers of the vesicular ACh translocation, and downstream, where only cetiedil is able to block the ACh release process.

Effect of 2-(4-phenylpiperidino)cyclohexanol (AH 5183) on the veratridine-induced increase in acetylcholine synthesis by rat hippocampal tissue

The intent of this study was to determine whether the drug 2-(4-phenylpiperidino)cyclohexanol (AH 5183 or vesamicol) might inhibit the veratridine-induced increase in acetylcholine (ACh) synthesis by reducing the veratridine-induced activation of a detergent-soluble choline-O-acetyltransferase (EC 2.3.1.6; ChAT) fraction associated with a vesicle-bound store of ACh. When minces of rat hippocampal tissue were loaded with [14C]choline and subsequently depolarized with veratridine, an increase in the synthesis of [14C]ACh occurred that could be abolished by L-AH 5183 (75 nM). When minces were depolarized with veratridine in the presence of L-AH 5183 (75 nM), the depolarization-induced activation of a detergent-soluble ChAT fraction associated with a vesicle-bound store of ACh was blocked. Conversely, the veratridine-induced activation of a water-soluble ChAT fraction believed to be cytosolic was not. AH 5183 also blocked the repletion of the vesicle-bound store with newly synthesized ACh following veratridine-induced depletion of ACh, a result that appeared to be mediated by an effect on the synthesis of ACh at the vesicular surface. These results suggest that veratridine depolarization of rat hippocampal nerve terminals stimulates the synthesis of ACh by activating a detergent-soluble fraction of ChAT closely associated with synaptic vesicle release sites. ACh synthesis and transport at the vesicular surface may be influenced by a common AH 5183-sensitive regulatory protein.

Effects of 2-(4-phenylpiperidino)cyclohexanol (AH5183) and barium ions on frog neuromuscular transmission

1. By applying electrophysiological techniques such as frequency facilitation, tetanic run-down and depression, recovery from depression and post-tetanic potentiation (PTP) of the end-plate potential (EPP), the effects on frog neuromuscular transmission of 2-(4-phenylpiperidino)cyclohexanol (AH5183), a compound known to inhibit specifically the loading of newly synthesized acetylcholine (ACh) molecules into synaptic vesicles, and Ba2+, a selective activator of the augmentation phase of PTP, were investigated to elucidate whether these were related to ACh turnover. 2. Effects of AH5183 and Ba2+ ions were frequency dependent. At low frequency of stimulation, both agents showed essentially no effects on the EPPs recorded from Mg2+-blocked preparations. 3. AH5183 pivoted the log-linear frequency facilitation relation clockwise without altering the intercept on the ordinate, whereas Ba2+ ions did so counter-clockwise. As is the case with Ca2+ ions, Sr2+ ions shifted the relation upwards leaving its slope unaffected. 4. AH5183 selectively depressed the component of potentiation in PTP while the effect of Ba2+ ions was a specific increase in the augmentation phase of PTP. 5. Ba2+ ions increased the amplitude of EPPs in the late depressed phase during the tetanic run-down and depression experiment, but 4-aminopyridine and Ca2+ ions failed to do so. 6. AH5183 increased, Ba2+ ions reduced but Ca2+ ions did not change the constant of recovery from depression of the EPP measured on curarized preparations. 7. The present results suggested that mobilization of the ACh quanta readily available for release might be a common mechanism underlying both frequency facilitation and two components of PTP (augmentation and potentiation). The term 'frequency facilitation' would be more comprehensive if it were re-termed 'frequency augmentation' or 'frequency potentiation'.

Acetylcholine and choline in rat adrenals and brain cortex prisms incubated at elevated concentrations of choline in the medium

Experiments were performed with rat adrenals and brain cortex prisms incubated in vitro in order to clarify whether it is possible to increase their acetylcholine (ACh) content by adding a high concentration of choline to the medium and whether the additional ACh formed can be released by subsequent depolarization. After 60 min incubation with 0.5 mmol/l choline, the concentration of ACh in the adrenals was increased by 116% (compared to the incubation without added choline), while in cortical prisms the observed increase (by 37%) was statistically non-significant. The content of ACh in both tissues was raised by paraoxon during incubations without added choline, but paraoxon did not augment the increased concentration of ACh in tissues incubated with added 0.5 mmol/l choline. The ACh that accumulated in the adrenals during 60 min preincubations with added choline could be released during subsequent depolarizing incubations; the release was Ca2+ independent. In contrast to brain cortex prisms and to the adrenals preincubated without choline, no resynthesis of ACh occurred during the period of depolarization in the adrenals preincubated with 0.5 mmol/l choline. Large amounts of choline accumulated in both tissues during incubations with 0.5 mmol/l choline and the accumulated choline could be released by depolarization; the release of choline from the adrenals was Ca2+ independent. Free choline was produced in the adrenals (presumably from choline esters) during the periods of depolarization. The reason for differences between the effects of increased concentrations of choline on ACh in the adrenals and in brain cortex is not known.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence to suggest that the spontaneous release of acetylcholine from rat hippocampal tissue is carrier-mediated

The effect of L- and D-stereoisomers of 2-(4-phenylpiperidino) cyclohexanol (AH 5183) on the spontaneous release of acetylcholine (ACh) from rat hippocampal tissue was studied. L-AH 5183 was approximately 100 times more potent than was D-AH 5183 in reducing spontaneous ACh release. Spontaneous ACh release was also temperature dependent. These results may suggest that the spontaneous release of ACh from brain tissue is carrier-mediated.

Post- and presynaptic effects of vesamicol (AH5183) on the frog neuromuscular junction

The effects of vesamicol (AH5183), a blocker of acetylcholine transport, on the voltage-clamped neuromuscular junction of the frog were studied. Vesamicol (15-30 microM) reduced the peak height of the ionophoretically applied acetylcholine-induced current. The amplitude of the evoked endplate current was also decreased in the presence of vesamicol (30 microM). The endplate current was reduced further when the nerve was tetanically stimulated. The reduction of the endplate current after tetanic stimulation in the presence of vesamicol was due to a decrease in the mean quantal content. The decay time constants of the evoked endplate current and the miniature endplate current were also decreased by vesamicol. It was concluded that vesamicol acts as a postsynaptic blocker at the endplate. This neurotoxin could also decrease the immediately available quanta in the presynaptic nerve terminal, but the mechanism of action of vesamicol on transmitter release remains obscure.

Calcium-independent release of acetylcholine from electric organ synaptosomes and its changes by depolarization and cholinergic drugs

Chemiluminescent detection was applied to measure the continuous spontaneous Ca2+-independent liberation of acetylcholine (ACh) from Torpedo electric organ synaptosomes. Differentiation between the release of ACh and choline was achieved by inhibiting cholinesterases with phospholine, and a way to quantify the continuous release was devised. The method permitted measurements during short time intervals from minute amounts of tissue and without an accumulation of ACh in the medium. Synaptosomes continuously liberated small amounts of ACh during incubations in the presence of 3 mM K+ and in the absence of Ca2+. The spontaneous liberation of ACh was similar both quantitatively and qualitatively at pH values of 8.6 and 7.8. It was unaltered by MgCl2 (10.4 mM), 2-(4-phenylpiperidino)cyclohexanol (10 microM), ouabain (104 microM), atropine (10 microM), and valinomycin (102 nM). Carbamoylcholine brought about a decrease, which could be partially reversed by atropine. The Ca2+-independent output of ACh was increased considerably when the concentration of K+ ions was raised (eightfold at 103 and 35-fold at 203 mM K+). Carbamoylcholine (104 microM) blocked the increase in ACh release produced by high K+; this effect of carbamoylcholine was not reversed by atropine (10 microM). When Ca2+ was added to synaptosomes depolarized by a high concentration of K+, the amount of ACh released during the first 1-3 min after the addition of Ca2+ was at least 20 times higher than in the absence of Ca2+, but the release returned rapidly to predepolarization values. Similarly high values of ACh release could be achieved by adding Ca2+ plus the ionophore A23187 and even higher values by adding Ca2+ plus gramicidin.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine mobilization in a sympathetic ganglion in the presence and absence of 2-(4-phenylpiperidino)cyclohexanol (AH5183)

The present experiments measured the release of acetylcholine (ACh) by the cat superior cervical ganglia in the presence of, and after exposure to, 2-(4-phenylpiperidino)cyclohexanol (AH5183), a compound known to block the uptake of ACh by cholinergic synaptic vesicles. We confirmed that AH5183 blocks evoked ACh release during preganglionic nerve stimulation when approximately 13-14% of the initial ganglial ACh stores had been released; periods of rest in the presence of the drug did not promote recovery from the block, but ACh release recovered following the washout of AH5183. ACh was synthesized in AH5183-treated ganglia, as determined by the synthesis of [3H]ACh from [3H]choline, and this [3H]ACh could be released by stimulation following drug washout. The specific activity of the released ACh matched that of the tissue's ACh, and thus we conclude that ACh synthesized in the presence of AH5183 is a releasable as pre-existing ACh stores once the drug is removed. We tested the relative releasability of ACh synthesized during AH5183 exposure (perfusion with [3H]choline) and that synthesized during recovery from the drug's effects (perfusion with [14C]choline: the ratio of [3H]ACh to [14C]ACh released by stimulation was similar to the ratio in the tissue. These results suggest that the mobilization of ACh for release by ganglia during recovery from an AH5183-induced block is independent of the conditions under which the ACh was synthesized. Unlike nerve impulses, black widow spider venom (BWSV) induced the release of ACh from AH5183-blocked ganglia, even in the drug's continued presence. Venom-induced release of ACh from AH5183-treated ganglia was not less than the venom-induced release from tissues not exposed to AH5183. This effect of BWSV was attributed to the action of the protein, alpha-latrotoxin, because an anti-alpha-latrotoxin antiserum blocked the venom's action. ACh synthesized during AH5183 exposure was labelled from [3H]choline, and subsequent treatment with BWSV released [3H]ACh with the same temporal pattern as the release of total ACh. To exclude a nonexocytotic origin for the [3H]ACh released by BWSV, ganglia were preloaded with [3H]diethylhomocholine to form [3H]acetyldiethylhomocholine, an ACh analogue excluded from vesicles; the venom did not increase the rate of [3H]acetyldiethylhomocholine efflux. It is concluded that a vesicular ACh pool insensitive to the inhibitory action of AH5183 might exist and that this vesicular pool is not mobilized by electrical stimulation to exocytose in the presence of AH5183, but it is by BWSV.

[3H]vesamicol binding in brain: autoradiographic distribution, pharmacology, and effects of cholinergic lesions

An autoradiographic analysis of high-affinity binding sites for the vesicular acetylcholine transport blocker [3H]vesamicol (2-(4-phenylpiperidino) cyclohexanol; AH 5183) was conducted in rat brain. [3H]Vesamicol binding was displaced 52-99% by DPPN [( 2,3,4,8]-decahydro-3-(4-phenyl-1-piperidinyl)-2-napthalenol) (IC50 = 14 nM) and by ketanserin (500 nM), haloperidol (43 nM), and vesamicol analogs, but not by drugs selective for adenosine, adrenergic, amino acid, calcium channel, monoaminergic, opioid, PCP, sigma, or several other receptor classes. [3H]Vesamicol binding was most concentrated in the interpeduncular nucleus and fifth and seventh cranial nerve nuclei. Moderate binding was found in the lateral caudate-putamen, medial nucleus accumbens, olfactory tubercle, vertical and horizontal diagonal bands of Broca, and basolateral amygdala. The distribution of [3H]vesamicol binding was similar to distributions of acetylcholine (r = 0.88), acetylcholine esterase (r = 0.97), choline acetyltransferase (ChAT) (r = 0.97), and [3H]hemicholinium-3 binding sites (r = 0.95-0.99). Lower correlations were obtained between [3H]vesamicol and muscarinic receptor densities (r = 0.50-0.70). Few exceptions to the match between binding and cholinergic neuronal markers were found, e.g., the molecular layer of the cerebellum and the thalamus. Lesions of cholinergic neuronal projections to the neocortex or hippocampus reduced [3H]vesamicol binding in each of these regions, but to a lesser extent than reductions in ChAT. [3H]Vesamicol binding sites appear to be anatomically associated with brain cholinergic neurons, a locus that is consistent with the control by this site of vesicular acetylcholine uptake.