Carrier-mediated inhibition of choline acetyltransferase

The biochemical and ultrastructural examinations in central cholinergic damage of the rat induced by the intraperitoneal administration of AF64A

Ethylcholine mustard aziridinium ion (AF64A), a synthesized cholinergic neurotoxin, was administered via intraperitoneal injection to the rat to study its effect on the central cholinergic nervous system. A single or consecutive daily injection of AF64A for 10 days resulted in a persistent reduction of acetylcholine (ACh) content in the several tested regions of the brain in the following order: hippocampus greater than cerebral cortex = striatum, the degree was the greatest in the hippocampus. Both resting and K(+)-stimulated release of ACh from the hippocampus were also significantly reduced 24 hr after a single injection of AF64A. Furthermore, daily injection of AF64A for 10 days induced a significant reduction of choline acetyltransferase (ChAT) activity in the homogenate obtained from the hippocampus but not from the cerebral cortex and striatum. ChAT activity in the crude synaptosomal fraction of the cerebral cortex was also significantly decreased. These results suggest that intraperitoneal administration of AF64A could induce cholinergic hypofunction more selectively in the nerve terminals. The high affinity choline uptake, which is located mainly on cholinergic nerve terminals, was not affected by the administration of AF64A. Any notable changes of ultrastructure in the cholinergic nerve terminals after the administration were not observed in all three regions examined. The present findings suggested that intraperitoneal administration of AF64A induces a specific damage of cholinergic nerve terminals by inhibiting ChAT activity. The cholinergic damage was most prominent in the hippocampus.

Cholinergic but not GABAergic neuronal markers are decreased in primary neuronal cultures treated with choline mustard

Nitrogen mustard analogues of choline are potent irreversible inhibitors of high-affinity choline transport at the cholinergic presynaptic nerve terminal in vitro. Ethylcholine mustard aziridinium ion, and to a lesser extent choline mustard aziridinium ion, have been used as tools to chemically lesion cholinergic neurons in the central nervous system. The selectivity of these compounds as neurotoxins for cholinergic neurons in vivo has been questioned and the mechanism by which they mediate neuronal death has not been elucidated. The objective of the present study was to investigate the selectivity of choline mustard aziridinium ion on embryonic rat brain neurons maintained in primary culture. The effect of choline mustard aziridinium ion on levels of cholinergic neuronal markers was compared with markers for GABAergic neurons as a measure of neuronal specificity. Choline mustard aziridinium ion at 10 and 30 microM irreversibly inhibited hemicholinium-sensitive, high-affinity choline transport into the cultured neurons without altering sodium-dependent, high-affinity GABA transport. Similarly, incubation of the neurons for 30 min in the presence of 10 microM choline mustard aziridinium ion led to a decrease in choline acetyltransferase activity of the cultures which was maintained for 72 h; glutamic acid decarboxylase activity was not altered under the same experimental conditions. Protein and DNA content and DNA-to-protein ratios of the choline mustard aziridinium ion-treated cultures were monitored as indicators of generalized cellular damage.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake and metabolism of [3H]choline mustard by cholinergic nerve terminals from rat brain

The objective of this study was to measure the uptake and metabolism of [3H]choline mustard aziridinium ion in rat brain synaptosomes. In previous investigations, we showed that this compound binds irreversibly to the choline carrier thereby inhibiting choline transport into nerve terminals; it also acts as both a substrate and inhibitor of the acetylcholine biosynthetic enzyme choline acetyltransferase. We now report that [3H]choline mustard aziridinium ion was transported into purified rat brain synaptosomes by a hemicholinium-sensitive mechanism, but at only a fraction of the rate of uptake of [3H]choline. Following 5 min incubation with the nerve terminal preparation, uptake of [3H]choline mustard aziridinium ion was 20% of that of [3H]choline transport, but this fell to 10% of [3H]choline accumulation at 30 min incubation. Apparent Michaelis constants derived from double reciprocal plots of velocity of transport versus substrate concentration revealed that the apparent affinity constants (Km) of the high-affinity choline carrier for [3H]choline mustard aziridinium ion and [3H]choline were not different (1.44 +/- 0.15 and 2.14 +/- 0.80 microM for choline and choline mustard aziridinium ion, respectively). Increasing the incubation time from 5 to 30 min, during which time a proportion of the high-affinity choline carriers were irreversibly inactivated by choline mustard aziridinium ion, did not alter the binding affinity for this compound. The maximum velocity of transport (Vmax) for the two compounds were significantly different with the maximum uptake of [3H]choline mustard aziridinium ion being 19.5% of that for choline at 5 min incubation, and falling to only 10.6% of the maximum rate of choline transport by 30 min incubation. [3H]Choline mustard aziridinium ion transported into synaptosomes on the high-affinity choline carrier was metabolized, with 27% being recovered as [3H]acetylcholine mustard aziridinium ion, 27% as [3H]phosphorylcholine mustard aziridinium ion, 7% as unmetabolized [3H]choline mustard aziridinium ion and 16% recovered as an unidentified metabolite. In parallel samples, [3H]choline taken up into synaptosomes was recovered as [3H]acetylcholine (71%) and unmetabolized [3H]choline (18%) with no net production of [3H]phosphorylcholine. Acetylation of [3H]choline mustard aziridinium ion amounted to only 7.6% of [3H]acetylcholine synthesized under the same conditions. These results show clearly that choline mustard aziridinium ion was accumulated into the cholinergic nerve terminals by the high-affinity choline carrier, but the amount was small relative to the uptake of choline and probably restricted by progressive inactivation of the transporters through covalent bond formation.

Synaptosomal "membrane-bound" choline acetyltransferase is most sensitive to inhibition by choline mustard

The objectives of the present study were to validate the presence of cytoplasmic and membrane-associated pools of choline acetyltransferase (ChAT) in rat brain synaptosomes, and to evaluate inhibition of these different forms of the enzyme by the nitrogen mustard analogue of choline, choline mustard aziridinium ion (ChM Az). The relative distribution of ChAT and lactate dehydrogenase (LDH) was followed in subfractions of synaptosomes to establish whether ChAT activity associated with salt-washed presynaptic membranes represents membrane-bound protein rather than cytosolic enzyme trapped within undisrupted synaptosomes or revesiculated membrane fragments. The percentage of total synaptosomal ChAT activity (14%) recovered in the final membrane pellet always exceeded that of LDH (6%), lending support to the hypothesis that much of the ChAT associated with the membranes was a membrane bound form of the enzyme. Incubation of purified synaptosomes with ChM Az led to irreversible inhibition of ChAT activity; this loss of enzyme activity could not be accounted for by lysis of nerve terminals during incubation in the presence of the mustard analogue. Subfractionation of the ChM Az-treated nerve terminals revealed that the membrane-bound form of ChAT was inhibited to the greatest extent, followed by the ionically membrane-associated enzyme, with the activity of the water-solubilized enzyme not differing significantly from control. Preparation of the synaptosomal ChAT subfractions from untreated nerve terminals prior to incubation with varying concentrations of ChM Az or naphthylvinylpyridine revealed that under these conditions water-solubilized, ionically membrane-associated, and detergent-solubilized membrane-bound pools of ChAT were not differentially inhibited by either compound.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversible and irreversible inhibition of high-affinity choline transport caused by ethylcholine aziridinium ion

The effect of ethylcholine aziridinium ion (AF64A) on choline transport in hippocampal, striatal, and cerebrocortical synaptosomes was studied. Synaptosomes prepared from these three brain regions were equally sensitive to AF64A. Low concentrations of AF64A produced a reversible inhibition (IC50 values = 1.35-2.25 microM), whereas higher concentrations produced an irreversible inhibition (IC50 values = 25-30 microM), which started as competitive. The irreversible component of the inhibition was independent of extracellular Na+ concentration, a finding suggesting that the choline transporter is alkylated at its outward position. The kinetics of the inhibition were rapid and similar in the three brain regions examined. The high-affinity choline transport was more sensitive to the toxin than the low-affinity choline transport. Based on these results, we propose a kinetic model that explains the reversible and the irreversible inhibitions induced by AF64A. The possible relationships between the concentrations that in vitro produce reversible and irreversible inhibition and those that in vivo produce selective and nonselective cholinergic hypofunction are discussed.

Choline and acetylcholine metabolism in slice cultures of the newborn rat septum

The incorporation of [3H]choline into acetylcholine and other choline-containing compounds was investigated in slice cultures of the septal area of newborn rats. At choline concentrations in the range of the high affinity transport mechanism (0.1-1 microM) most of the labeled choline was incorporated into phosphorylcholine, followed by lipids, acetylcholine and the free choline pool. Hemicholinium-3 (1-10 microM) lead to a marked decrease of acetylcholine synthesis, whereas choline accumulation or phosphorylcholine synthesis were not decreased. Both basal and K+-induced release of acetylcholine were Ca2+ dependent. The efflux of choline was not stimulated by high K+. When choline was absent from the incubation medium, the slices were able to liberate significant amounts of the [3H]choline previously incorporated into phospholipids, and were also able to synthesize some acetylcholine. In choline-free medium, acetylcholine synthesis was greatly enhanced by depolarization. During the period in culture, there was a decrease of the incorporation rate of [3H]choline into phosphorylcholine and an increase of the incorporation rate into acetylcholine. The tissue structure was well preserved after several weeks in culture. After staining for acetylcholinesterase, the cholinergic neurons in the cultures showed a similar morphology to that seen in situ. The main conclusions of the present study are: cholinergic neurons in slice cultures develop and behave in a manner which is very similar to their in situ counterparts; the main divergence from previous studies of choline metabolism in tissue culture is the substantial incorporation rate of choline into acetylcholine at choline concentrations in the range of the high affinity uptake mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the alkylation of choline acetyltransferase by choline mustard aziridinium ion

Although a potent irreversible inhibitor of high-affinity choline transport in rat brain synaptosomes, choline mustard aziridinium ion (ChM Az) appeared to be a relatively weak inhibitor of choline acetyltransferase (ChAT) in rat brain homogenates, and evidence for irreversible binding of this compound to the enzyme had not been established. Accordingly, the irreversible inactivation of partially purified rat brain ChAT by ChM Az was studied. This compound is a rather weak inhibitor of the enzyme, with 50% inhibition of ChAT activity achieved following 30 min incubation at 37 degrees C with 0.6 mM ChM Az. This result indicates that although ChM Az has affinity for many nucleophiles there was little diluting effect of the inhibitor in the crude brain homogenate which could be attributed to such reactions (50% inhibition caused by 1.8 mM ChM Az following 10 min incubation). Although the initial binding of ChM Az to ChAT may be of a competitive nature, irreversible bond formation resulted. The time-dependent alkylation reaction conformed to pseudo-first-order kinetics with an observed forward rate constant (kobs) of 0.173 min-1; the half-time (t 1/2) for irreversible binding was about 4 min. The irreversible inactivation of ChAT by ChM Az would appear to be slower than the alkylation of high-affinity choline carriers in synaptosomes by this compound, and the relatively weak inhibitory action of ChM Az against either partially purified ChAT or ChAT activity in crude rat brain homogenates is in striking contrast to previous evidence that ChAT in intact synaptosomes was inhibited irreversibly by lower concentrations of the inhibitor.

The effect of choline mustard on the rat superior cervical ganglia

The effect of choline mustard aziridinium ion (ChM) on the isolated superior cervical ganglion of the rat was investigated. In the presence of ChM (22.5 microM), stimulation at 1 Hz resulted in a slowly developing blockade of transmission, which did not occur at a stimulus frequency of 0.1 Hz. Choline (0.1 mM) slowed the onset of the blockade produced by stimulation at 1 Hz in the presence of ChM. The presence of excess thiosulphate ions prevented the action of ChM on the transmission in the superior cervical ganglion. Treatment of the ganglion with ChM (22.5 microM) only slightly inhibited the depolarization produced by carbachol (dose ratio 1.3), suggesting the drug produced a small degree of receptor blockade. [3H]-choline accumulation in the rat superior cervical ganglia displays several components: (a) sodium-dependent high affinity uptake (SDHAU) that can be activated further by preincubation in a high concentration of K+ ions; (b) sodium-dependent low affinity uptake (SILAU); (c) linear diffusional accumulation which does not saturate. Hemicholinium-3 selectively inhibits the activated sodium-dependent high affinity uptake, but is a weak inhibitor of resting sodium-dependent high affinity uptake and sodium-independent low affinity uptake. ChM inhibits both activated and resting sodium-dependent high affinity uptake, but is a very weak inhibitor of sodium-independent low affinity uptake. Homocholine shows similar selectivity. ChM inhibition of activated sodium-dependent high affinity uptake is very much more persistent than that of hemicholinium-3. Hemicholinium-3 and ChM both inhibit [3H]-acetylcholine synthesis.