Purification and characterization of a nonvesicular vesamicol-binding protein from electric organ and demonstration of a related protein in mammalian brain

In vitro and in vivo characterization of two C-11-labeled pet tracers for vesicular acetylcholine transporter

PURPOSE

The vesicular acetylcholine transporter (VAChT) is a specific biomarker for imaging presynaptic cholinergic neurons. Herein, two potent and selective (11)C-labeled VAChT inhibitors were evaluated in rodents and nonhuman primates for imaging VAChT in vivo.

PROCEDURES

For both (-)-[(11)C]2 and (-)-[(11)C]6, biodistribution, autoradiography, and metabolism studies were performed in male Sprague Dawley rats. Positron emission tomography (PET) brain studies with (-)-[(11)C]2 were performed in adult male cynomolgus macaques; 2 h dynamic data was acquired, and the regions of interest were drawn by co-registration of the PET images with the MRI.

RESULTS

The resolved enantiomers (-)-2 and (-)-6 were very potent and selective for VAChT in vitro (K i  < 5 nM for VAChT with >35-fold selectivity for VAChT vs. σ receptors); both radioligands, (-)-[(11)C]2 and (-)-[(11)C]6, demonstrated high accumulation in the VAChT-enriched striatum of rats. (-)-[(11)C]2 had a higher striatum to cerebellum ratio of 2.4-fold at 60 min; at 30 min, striatal uptake reached 0.550 ± 0.086 %ID/g. Uptake was also specific and selective; following pretreatment with (±)-2, striatal uptake of (-)-[(11)C]2 in rats at 30 min decreased by 50 %, while pretreatment with a potent sigma ligand had no significant effect on striatal uptake in rats. In addition, (-)-[(11)C]2 displayed favorable in vivo stability in rat blood and brain. PET studies of (-)-[(11)C]2 in nonhuman primates indicate that it readily crosses the blood-brain barrier (BBB) and provides clear visualization of the striatum; striatal uptake reaches the maximum at 60 min, at which time the target to nontarget ratio reached ~2-fold.

CONCLUSIONS

The radioligand (-)-[(11)C]2 has high potential to be a suitable PET radioligand for imaging VAChT in the brain of living subjects.

Development of (18)F-labeled radiotracers for neuroreceptor imaging with positron emission tomography

Positron emission tomography (PET) is an in vivo molecular imaging tool which is widely used in nuclear medicine for early diagnosis and treatment follow-up of many brain diseases. PET uses biomolecules as probes which are labeled with radionuclides of short half-lives, synthesized prior to the imaging studies. These probes are called radiotracers. Fluorine-18 is a radionuclide routinely used in the radiolabeling of neuroreceptor ligands for PET because of its favorable half-life of 109.8 min. The delivery of such radiotracers into the brain provides images of transport, metabolic, and neurotransmission processes on the molecular level. After a short introduction into the principles of PET, this review mainly focuses on the strategy of radiotracer development bridging from basic science to biomedical application. Successful radiotracer design as described here provides molecular probes which not only are useful for imaging of human brain diseases, but also allow molecular neuroreceptor imaging studies in various small-animal models of disease, including genetically-engineered animals. Furthermore, they provide a powerful tool for in vivo pharmacology during the process of pre-clinical drug development to identify new drug targets, to investigate pathophysiology, to discover potential drug candidates, and to evaluate the pharmacokinetics and pharmacodynamics of drugs in vivo.

Development of novel PET probe [¹¹C](R,R)HAPT and its stereoisomer [¹¹C](S,S)HAPT for vesicular acetylcholine transporter imaging: a PET study in conscious monkey

Carbon-11-labeled (R,R)trans-8-methyl-2-hydroxy-3-[4-[2-aminophenyl]piperizinyl]-tetralin ([(11)C](R,R)HAPT) and its stereoisomer [(11)C](S,S)HAPT were developed for imaging vesicular acetylcholine transporters (VAChTs), exclusively located in presynaptic cholinergic neurons. Both positron emission tomography (PET) probes were evaluated in the brain of conscious monkey (Macaca mulatta) using high-resolution PET. Time-activity curves (TACs) of [(11)C](R,R)HAPT peaked within 5 min after the injection in all regions except the caudate and putamen, both of which showed peaks around 20 min postinjection. The regional distribution patterns of [(11)C](R,R)HAPT determined as total distribution volume (V(t)) were highest in the putamen, high in the caudate, intermediate in the amygdala, hippocampus, and thalamus, lower in the cingulate gyrus and frontal, temporal, and occipital cortices, and lowest in the cerebellum. In contrast, the distribution and TACs of [(11)C](S,S)HAPT were homogeneous in all regions. The uptake of [(11)C](R,R)HAPT was reduced by 1 mg/kg (-)-vesamicol, a specific VAChT antagonist, in all regions except the cerebellum, but not by 0.1 mg/kg SA4503, a specific sigma-1 receptor agonist. These results well reflect the in vitro affinity assessments using rat cerebral membranes. They also demonstrate that [(11)C](R,R)HAPT is a potential PET probe for noninvasive and quantitative imaging of VAChT in the living brain.

Possible important pair of acidic residues in vesicular acetylcholine transporter

Invariant E309 is in contact with critical and invariant D398 in a three-dimensional homology model of vesicular acetylcholine transporter (VAChT, TC 2.A.1.2.13) [Vardy, E., et al. (2004) Protein Sci. 13, 1832-1840]. In the work reported here, E309 and D398 in human VAChT were mutated singly and together to test their functions, assign pK values to them, and determine whether the residues are close to each other in three-dimensional space. Mutants were stably expressed in the PC12(A123.7) cell line, and transport and binding properties were characterized at different pH values using radiolabeled ligands and filtration assays. Contrary to a prior conclusion, the results demonstrate that most D398 mutants do not bind the allosteric inhibitor vesamicol even weakly. Earlier work showed that most D398 mutants do not transport ACh. D398 therefore probably is the residue that must deprotonate with a pK of 6.5 for binding of vesamicol and with a pK of approximately 5.9 for transport of ACh. Because E309Q has no effect on VAChT functions at physiological pH, E309 has no apparent critical role. However, radical mutations in E309 cause decreases in ACh and vesamicol affinities and total loss of ACh transport. Unlike wild-type VAChT, which exhibits a peak of [(3)H]vesamicol binding centered at pH 7.4, mutants E309Q, E309D, E309A, and E309K all exhibit peaks of binding centered at pH >or=9. The combination of high pH and mutated E309 apparently produces a relaxed (in contrast to tense) conformation of VAChT that binds vesamicol exceptionally tightly. No compensatory interactions between E309 and D398 in double mutants were discovered. Proof of a close spatial relationship between E309 and D398 was not found. Nevertheless, the data are more consistent with the homology model than an alternative hydropathy model of VAChT that likely locates E309 far from D398 and the ACh binding site in three-dimensional space. Also, a probable network of interactions involving E309 and an unknown residue having a pK of 10 has been revealed.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Synthesis and evaluation of radiolabeled piperazine derivatives of vesamicol as SPECT agents for cholinergic neurons

To diagnose and investigate neurodegenerative diseases affecting cholinergic neuron density, piperazine derivatives of vesamicol were synthesized and evaluated. Previously, we reported that trans-5-iodo-2-hydroxy-3-[4-phenylpiperazinyl] tetralin (DRC140, 1) possessed high selectivity for vesicular acetylcholine transporter (VAChT). In present study of the effect of alkyl substituents, we observed that the introduction of a methyl group into the ortho or meta positions of the phenyl group of 1 increased affinity for VAChT. trans-5-Iodo-2-hydroxy-3-[4-[2-methylphenyl] piperazinyl]tetralin (2) displayed high affinity and specificity for VAChT. The regional distributions of radioactivity in the rat brain correlated well with known patterns of central cholinergic innervation. [(123)I]2 is a potentially useful compound for SPECT imaging.

The potential of radioiodinated (-)-m-iodovesamicol for diagnosing cholinergic deficit dementia

We investigated changes in the brain distribution of (-)-[(125)I]-m-iodovesamicol [(-)-[(125)I]mIV] in cholinergic denervation rats produced by a unilateral lesion of the nucleus basalis magnocellularis (NBM). Dual-tracer ex vivo autoradiographic analysis using (-)-[(125)I]mIV and [(99m)Tc]HMPAO was conducted to the effect of regional cerebral perfusion on the brain distribution of (-)-[(125)I]mIV in a unilateral NBM-lesioned rat. (-)-[(125)I]mIV binding in the ipsilateral cortex to the lesion significantly reduced by 10.4 %, compared with that in the contralateral cortex, while (-)-[(125)I]mIV binding in the ipsilateral caudate putamen, hippocampus and thalamus did not change. The rate of reduction in the (-)-[(125)I]mIV binding (10.4 %) was significantly higher than that of [(99m)Tc]HMPAO accumulation (4.0%) in the ipsilateral cortex to the lesion (P < 0.01). These results suggested that radioiodinated (-)-mIV may be useful in the study of dementia characterized by degeneration of the cholinergic neurotransmitter system, such as Alzheimer's disease.

Piperazine analog of vesamicol: in vitro and in vivo characterization for vesicular acetylcholine transporter

The probes to detect vesicular acetylcholine transporter (VAChT) in vivo are important to evaluate the mapping and function in cholinergic system. To develop high-specific and high-affinity radiotracer for single photon emission computed tomography, we investigated piperazine analogs which replaced the piperidine ring of (-)-vesamicol with a piperazine ring. We found that the piperazine analog of iodobenzovesamicol, trans-5-iodo-2-hydroxy-3-[4-phenylpiperazinyl] tetralin (DRC140), had high affinity for VAChT in rat brain. We carried out binding assay in subcellular fraction of the rat brain. The highest B(max) for [(125)I]-DRC140 binding was observed in the synaptic vesicle fraction (1,751 fmol/mg protein), followed by the crude vesicle (821 fmol/mg protein) and the P2 fraction (187 fmol/mg protein). These K(d) values were similar to the affinity of highly purified synaptic vesicular fraction (K(d) = 0.3 nM) with a one-site model. The possibility that [(125)I]-DRC140 recognizes sigma receptor was excluded by our finding large inhibition constants (K(i) = 849 nM for haloperidol, K(i) = 3,052 nM for 1,3-di(2-tolyl)guanidine). In vivo distribution studies with the [(123)I]-DRC140 in rats showed a rapid brain uptake. The highest brain area was in striatum, followed by frontal cortex, occipital cortex, and hippocampus. The lowest brain area was cerebellum. The radioactivity of high-accumulated areas in ex vivo autoradiography was reduced by a preinjection of (-)-vesamicol and these levels were reduced to the radioactivity in cerebellum. These results show that [(125)I]-DRC140 can provide extremely high specific tracer with excellent brain permeability as a ligand for single photon emission computed tomography.

Autoradiographic comparison of [3H](-)nicotine, [3H]cytisine and [3H]epibatidine binding in relation to vesicular acetylcholine transport sites in the temporal cortex in Alzheimer's disease

The laminar binding distribution of three nicotinic receptor agonists, [3H](-)nicotine, [3H]cytisine, and [3H]epibatidine, and their relation to the [3H]vesamicol binding, which is known to represent the vesicular acetylcholine transport sites, was performed employing in vitro autoradiography on the medial temporal cortex (Brodmann area 21). Autopsied brain tissue from nine Alzheimer patients and seven age-matched controls were used. The binding pattern of the three nicotinic ligands in the normal cortex was in general similar, showing binding maxima in the cortical layers I, III and V. The binding of [3H](-)nicotine, [3H]cytisine, and [3H]epibatidine was lower in the older controls and more uniform throughout the layers as compared with younger controls. There was a significant age-related decrease in the binding of the three nicotinic ligands within the controls (age range: 58 to 89 years; P[3H](-)nicotine = 0.002, P[3H]epibatidine = 0.010, P[3H]cytisine = 0.037). In the older controls, the [3H]epibatidine binding was much decreased as compared with that of [3H](-)nicotine and [3H]cytisine. This may indicate a higher selectivity of [3H]epibatidine for a nicotinic receptor subtype that is particularly affected by aging. The laminar binding pattern of [3H]vesamicol showed one maximum in the outer cortical layers II/III. The [3H]vesamicol binding did not change with aging. The binding of all ligands was significantly decreased in all layers of the temporal cortex in Alzheimer's disease, but the [3H]vesamicol binding decreased only half as much as the nicotinic receptors. Also, choline acetyltransferase activity was percentually more reduced than [3H]vesamicol binding in Alzheimer's disease. The cortical laminar binding pattern of all 3H-ligands was largely absent in the Alzheimer's disease cases. The less severe loss of vesicular acetylcholine transport sites as compared with the loss of the nicotinic receptors and choline acetyltransferase activity may suggest that vesamicol binding sites might be more preserved in presynaptic terminals still existing and thereby expressing compensatory capacity to maintain cholinergic activity.

Muscarinic receptor loss and preservation of presynaptic cholinergic terminals in hippocampal sclerosis

PURPOSE

Prior single-photon emission tomography studies showed losses of muscarinic acetylcholine receptor (MAChR) binding in patients with refractory mesial temporal lobe epilepsy. Experimental animal studies demonstrated transient losses of MAChR due to electrically induced seizures originating in the amygdala. However, the relations between cholinergic synaptic markers, seizures, and underlying neuropathology in human temporal lobe epilepsy are unknown. We tested the hypotheses that human brain MAChR changes are attributable to hippocampal sclerosis (HS), and that HS resembles axon-sparing lesions in experimental animal models.

METHODS

We measured MAChR binding-site density, an intrinsic neuronal marker, within the hippocampal formation (HF) in anterior temporal lobectomy specimens from 10 patients with HS and in 10 autopsy controls. Binding-site density of the presynaptic vesicular acetylcholine transporter (VAChT) was measured as a marker of extrinsic cholinergic afferent integrity. MAChR and VAChT results were compared with neuronal cell counts to assess their relations to local neuronal losses.

RESULTS

Reduced MAChR binding-site density was demonstrated throughout the HF in the epilepsy specimens compared with autopsy controls and correlated in severity with reductions in cell counts in several HF regions. In contrast to MAChR, VAChT binding-site density was unchanged in the epilepsy specimens compared with autopsy controls.

CONCLUSIONS

Reduction in MAChR binding in HS is attributable to intrinsic neuronal losses. Sparing of afferent septal cholinergic terminals is consistent with the hypothesis that an excitotoxic mechanism may contribute to the development of HS and refractory partial epilepsy in humans.

[18F]fluoroethoxy-benzovesamicol, a PET radiotracer for the vesicular acetylcholine transporter and cholinergic synapses

Loss of cholinergic transmission in the cortex and hippocampus is a characteristic feature of Alzheimer's disease, and visualization of functional cholinergic synapses in the brain with PET could be a useful method for studying this degenerative condition in living humans. We investigated [18F]fluoroethoxybenzovesamicol, (-)-[18F] FEOBV,(-)-(2R,3R)-trans-2-hydroxy-3-(4-phenylpiperidino)-5-(2-[18F ]fluoroethoxy)-1,2,3,4-tetralin, a high affinity positron emitting ligand for the vesicular acetylcholine transporter, as a potential in vivo cholinergic synapse mapping agent. Rodent biodistribution, dosimetry, stereospecificity of biological effects, pharmacologic blocking studies, in vivo rodent brain autoradiography and metabolites were examined. (-)-[18F]FEOBV brain uptake following intravenous injection was robust, with 2.65% dose/brain in mice at 5 min, and the regional localization matched the known distributions of presynaptic cholinergic markers at later times. Both the cholinergic localization and curare-like effects of FEOBV were associated with the "(-)"-enantiomer exclusively. (-)-[18F]FEOBV regional brain distribution in rodents was changed little by pretreatment with haloperidol, (+)-3-PPP, or E-2020, indicating FEOBV, unlike other vesamicol analogs, did not interact in vivo with dopamine or sigma receptor systems. Autoradiography of rat brain 3 h following i.v. injection of (-)-[18F]FEOBV showed high localization in brain areas rich in presynaptic cholinergic elements. Metabolic defluorination in rodents was modest, and analysis of brain tissue following tracer administration found FEOBV as the only extractable radioactive species. (-)-[18F]FEOBV dosimetry calculated from rat data estimate 10 mCi doses can be given to humans. These studies show FEOBV maps cholinergic areas with high specificity in vivo, and may provide a noninvasive means to safely and accurately gauge the functional integrity of cholinergic synapses in man using PET.

Vesicular acetylcholine transporter density and Alzheimer's disease

We have evaluated the vesamicol analogue meta-[125I]iodobenzyltrozamicol {(+)-[125I]MIBT} as a probe to assess cholinergic terminal integrity in the human temporal cortex. Saturation binding analysis, using 5-aminobenzovesamicol (ABV) to define nonspecific binding, revealed a high-affinity binding site with a Kd value of 4.3 +/- 1.2 nM in the temporal cortex of the young control subjects. Similar affinity values were observed for (+)-[125I]MIBT binding in aged control subjects (Kd = 3.4 +/- 0.5 nM) and AD patients (Kd = 3.0 +/- 0.8 nM). In contrast, Bmax values for young subjects, aged controls and AD patients were 31.2 +/- 6.3, 17.0 +/- 2.0 and 9.4 +/- 1.6 pmol/g, respectively, clearly reflecting significant reductions in (+)-[125I]MIBT binding site density with aging and age-related neuropathology. Moreover, the decrease in (+)-[125I]MIBT binding was correlated with choline acetyltransferase activities (r = 0.72) in the AD temporal cortex. These results suggest that when selective ligands are used, the vesicular acetylcholine transporter can be a useful marker protein for assessing the loss of cholinergic projections in AD and related disorders.

The role of vesicular transport proteins in synaptic transmission and neural degeneration

Classical neurotransmitters are synthesized in the cytoplasm, so they require transport into secretory vesicles for regulated exocytotic release. Previous work has identified distinct vesicular transport activities for the different classical transmitters, and all depend on the H+-electrochemical gradient across the vesicle membrane but differ in the extent to which they rely on the chemical and electrical components of this gradient. Drugs that interfere with vesicular amine transport have implicated this activity in psychiatric disease. Selection for a cDNA encoding vesicular amine transport in the neurotoxin MPP+ also implicates the activity in Parkinson's disease. Molecular cloning of vesicular monoamine transporters shows sequence similarity to bacterial antibiotic resistance proteins, supporting a role for transport in detoxification and defining a novel mammalian gene family that now also includes a transporter for acetylcholine. Current work focuses on the mechanism of transport and the role that regulation of activity and its subcellular localization have in transmitter release, behavior, and neural degeneration.

In vivo mapping of cholinergic terminals in normal aging, Alzheimer's disease, and Parkinson's disease

To map presynaptic cholinergic terminal densities in normal aging (n = 36), Alzheimer's disease (AD) (n = 22), and Parkinson's disease (PD) (n = 15), we performed single-photon emission computed tomography using [123I]iodobenzovesamicol (IBVM), an in vivo marker of the vesicular acetylcholine transporter. We used coregistered positron emission tomography with [18F]fluorodeoxyglucose for metabolic assessment and coregistered magnetic resonance imaging for atrophy assessment. In controls (age, 22-91 years), cortical IBVM binding declined only 3.7% per decade. In AD, cortical binding correlated inversely with dementia severity. In mild dementia, binding differed according to age of onset, but metabolism did not. With an onset age of less than 65 years, binding was reduced severely throughout the entire cerebral cortex and hippocampus (about 30%), but with an onset age of 65 years or more, binding reductions were restricted to temporal cortex and hippocampus. In PD without dementia, binding was reduced only in parietal and occipital cortex, but demented PD subjects had extensive cortical binding decreases similar to early-onset AD. We conclude that cholinergic neuron integrity can be monitored in living AD and PD patients, and that it is not so devastated in vivo as suggested by postmortem choline acetyltransferase activity (50-80%).

Vesamicol receptor mapping of brain cholinergic neurons with radioiodine-labeled positional isomers of benzovesamicol

Alzheimer's disease is characterized by progressive cerebral cholinergic neuronal degeneration. Radiotracer analogs of benzovesamicol, which bind with high affinity to the vesamicol receptor located on the uptake transporter of acetylcholine storage vesicles, may provide an in vivo marker of cholinergic neuronal integrity. Five positional isomers of racemic iodobenzovesamicol (4'-, 5-, 6-, 7-, and 8-IBVM) were synthesized, exchange-labeled with iodine-125, and evaluated as possible in vivo markers for central cholinergic neurons. Only two isomers, 5-IBVM (5) and 6-IBVM (10), gave distribution patterns in mouse brain consistent with cholinergic innervation: striatum >> hippocampus > or = cortex > hypothalamus >> cerebellum. The 24-h tissue-to-cerebellum concentration ratios for 5-IBVM (5) were 3-4-fold higher for striatum, cortex, and hippocampus than the respective ratios for 6-IBVM (10). Neither 8-IBVM (16) nor 4'-IBVM (17) exhibited selective retention in any of the brain regions examined. In the heart, only 5-IBVM (5) exhibited an atria-to-ventricles concentration ratio consistent with high peripheral cholinergic neuronal selectivity. The 7-IBVM (14) isomer exhibited an anomalous brain distribution pattern, marked by high and prolonged retention in the five brain regions, most notably the cerebellum. This isomer was screened for binding in a series of 26 different biological assays; 7-IBVM (14) exhibited affinity only for the delta-receptor with an IC50 of approximately 30 nM. Drug-blocking studies suggested that brain retention of 7-IBVM (14) reflects high-affinity binding to both vesamicol and delta-receptors. Competitive binding studies using rat cortical homogenates gave IC50 values for binding to the vesamicol receptor of 2.5 nM for 5-IBVM (5), 4.8 nM for 6-IBVM (10), and 3.5 nM for 7-IBVM (14). Ex vivo autoradiography of rat brain after injection of (-)-5-[125I]IBVM ((-)-[125I]5) clearly delineated small cholinergic-rich areas such as basolateral amygdala, interpeduncular nucleus, and facial nuclei. Except for cortex, regional brain levels of (-)-5-[123I]IBVM ((-)-[123I]5) at 4 h exhibited a linear correlation (r2 = 0.99) with endogenous levels of choline acetyltransferase.

CONCLUSION

Vesamicol receptor mapping of cholinergic nerve terminals in murine brain can be achieved with 5-IBVM (5) and less robustly with 6-IBVM (10), whereas the brain localization of 7-IBVM (14) reflects high-affinity binding to both vesamicol and delta-receptors.

Autoradiographic quantification of muscarinic cholinergic synaptic markers in bat, shrew, and rat brain

We employed radioligand binding autoradiography to determine the distributions of pre- and post-synaptic cholinergic radioligand binding sites in the brains of two species of bat, one species of shrew, and the rat. High affinity choline uptake sites were measured with [3H]hemicholinium, and presynaptic cholinergic vesicles were identified with [3H]vesamicol. Muscarinic cholinergic receptors were determined with [3H]scopolamine. The distribution patterns of the three cholinergic markers were similar in all species examined, and identified known major cholinergic pathways on the basis of enrichments in both pre- and postsynaptic markers. In addition, there was excellent agreement, both within and across species, in the regional distributions of the two presynaptic cholinergic markers. Our results indicate that pharmacological identifiers of cholinergic pathways and synapses, including the cholinergic vesicle transport site, and the organizations of central nervous system cholinergic pathways are phylogenetically conserved among eutherian mammals.

Effects of vesicular acetylcholine uptake blockers on frequency augmentation-potentiation in frog neuromuscular transmission

Vesamicol inhibits the vesicular loading of acetylcholine molecules. The effects of vesamicol and similarly acting compounds on neuromuscular transmission in frogs were investigated to determine whether these inhibitors-inhibit the frequency augmentation-potentiation of transmitter release. Various vesicular acetylcholine transport blockers suppressed the stimulation frequency-related release parameter, k, in a dose-dependent manner. Artane, cetiedil, chloroquine, ethodin, quinacrine, vesamicol and its benzyl-analogue, 2-(4-benzylpiperidino)cyclohexanol, had strong effects, while those of aminacrine, chlorpromazine, fluphenazine, imipramine, pyrilamine and thioridazine were weak. A significant correlation was observed between the biochemically reported values of IC50 and the electrophysiological inhibitory potencies on k at 20 microM. Contrary to expectations from the biochemical data, however, vesamicol and its benzyl-analogue showed equipotent inhibitory actions on the electrophysiological frequency augmentation-potentiation relation. Low sensitivity and low selectivity of the frequency augmentation-potentiation for vesamicol and its benzyl-analogue lead us to conclude that the vesicular acetylcholine transporter is not the site of the electrophysiological action of vesamicol and similarly acting chemicals.

18F-labelled vesamicol derivatives: syntheses and preliminary in vivo small animal positron emission tomography evaluation

As possible presynaptic tracers for cholinergic function in humans, three 18F-labelled vesamicol analogs were synthesized for use in positron emission tomography (PET): cis-[18F]-4-fluoromethylvesamicol (FMV), [18F]-N-fluoroacetamidobenzovesamicol (FAA) and [18F]-N-ethyl-N-fluoroacetamidobenzovesamicol (NEFA). Radiolabelling was accomplished using [18F]fluoride and the corresponding tosylates, the syntheses of which are also described. Yields were on the order of 40-60, 5 and 40-60%, respectively. Dynamic studies of the biodistribution in rats of [18F]FAA and [18F]NEFA using PET were compared with those previously reported for [18F]FMV. Due to probable rapid metabolism, [18F]FAA was considered unsuitable as a ligand for in vivo imaging. [18F]NEFA, similar to [18F]FAA, displayed a more moderate cerebral uptake than that of [18F]FMV (2 vs 20-30%). Pretreatment with vesamicol blocked the cerebral uptake, indicating a specific interaction with the vesamicol binding site. The biodistribution of high specific activity [18F]NEFA with time could be described with a three-compartmental model. The evaluation of [18F]NEFA as a tracer for cholinergic function is currently being pursued in monkeys and humans.

Characterization of [3H]vesamicol binding in rat brain preparations

The binding of (1)-[3H]vesamicol was characterized in several subcellular fractions and brain regions of the rat. Binding to a lysed P2 fraction from the rat cerebral cortex reached equilibrium within 4 min at 37 degrees C and was reversible (dissociation half-time 4.9 min). At least two binding affinities were found in P2 fractions from the cerebral cortex (Kd: 21 nM and 980 nM), striatum (Kd: 28 nM and 690 nM), and cerebellum (Kd: 22 nM and 833 nM). High affinity Bmax values were highest in striatum (1.17 pmol/mg protein), followed by cerebellum (0.67 pmol/mg protein), and cerebral cortex (0.38 pmol/mg protein). Low affinity Bmax values were highest in cerebellum (5.2 pmol/mg protein), with similar values for cerebral cortex (3.7 pmol/mg protein) and striatum (3.8 pmol/mg protein). High affinity but not low affinity binding in each brain region was stereospecific. Another inhibitor of vesicular ACh-transport also displaced 1-vesamicol binding potently (IC50: 17 nM) and efficaciously (over 90%). Both high affinity and low affinity Bmax values for [3H]vesamicol-binding were highest in a partially purified synaptic vesicle fraction, followed by purified synaptosomes, crude membranes and P2 fractions. Specific binding was not observed in a mitochondria-enriched fraction. Crude membrane preparations of primary, neuron-enriched whole brain cultures also exhibited high (64 nM) and low affinity (1062 nM) [3H]vesamicol binding. Isoosmotic replacement of 0.18 M KCl in the binding-buffer with NaCl had no effect on binding. These results suggest that at least some high affinity [3H]vesamicol binding in rat brain preparations may be associated with synaptic vesicles, some of which may not be cholinergic in origin.

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)

The transport of neurotransmitters into synaptic vesicles

As investigations identify additional plasma membrane neurotransmitter transporters, attention has focused on the molecular basis of neurotransmitter transport into synaptic vesicles. The transport of biogenic amines into chromaffin granules has served as the paradigm for understanding vesicular transport. Recent work now describes the vesicular transport of other classical neurotransmitters, which occur by distinct but related mechanisms. To determine their biochemical basis, several of the transporters have been functionally reconstituted in liposomes. The ability of vesicular amine transport to protect against the neurotoxin MPP+ has permitted the isolation of the first cDNA clone for a member of this family, and the sequence establishes a relationship with drug-resistance transporters in bacteria.

Positron emission tomographic studies of central cholinergic nerve terminals

The aim of this study was to develop a quantitative method for the study of cholinergic nerve terminals in vivo. An 18F-labeled analogue of vesamicol ([18F]FMV) that binds with high affinity to synaptic vesicles from Torpedo electric organ was synthesized and evaluated in vivo in rats and monkeys by positron emission tomography (PET). In rats, the tracer was rapidly cleared from the blood and highly extracted into the brain, where it was specifically and irreversibly bound. In monkeys, a specific binding of the tracer was observed in brain regions known to contain cholinergic nerve terminals. Preinjection of non-labeled vesamicol prevented the cerebral binding of [18F]FMV to a high affinity site in both species. Our results are a major step towards quantitative human in vivo studies of presynaptic cholinergic functions.