REGULATION OF ACETYLCHOLINE SYNTHESIS: CONTROL OF CHOLINE TRANSPORT AND ACETYLATION IN SYNAPTOSOMES

Abstract— The high affinity transport of choline (Ch) and the synthesis of acetylcholine (ACh) were measured in synaptosomes by measuring the utilization of [²H₄]Ch. The synthesis of ACh was reduced under several conditions which reduce the availability of acetyl coenzyme A (AcCoA) including no glucose added, replacement of glucose with succinate or impairment of glucose utilization by bromopyr-uvate, NaCN, or pentobarbital. These conditions did not reduce the amount of unacetylated [²H₄]Ch in the synaptosomes indicating that the high affinity transport of Ch is not directly coupled to the synthesis of ACh.     

Linear feedback control of acetylcholine level in the presynaptic terminal

A dynamic analysis of acetylcholine (ACh) level during nervous signal transmission was performed by means of computer simulation. The rate equation expressed in a system of non-linear ordinary differential equations represents the dynamic aspects of the fundamental metabolic processes in the chemical transmission at the synapse in terms of the relevant metabolite fluxes and the reactions of choline acetyltransferase, ACh receptor and acetylcholinesterase functioning in two putative homogeneous and open compartments. After a transmitter release, the ACh level in the presynaptic terminal cannot be restored by supplying the substrates of acetyl-CoA (AcCoA) and choline (Ch) at constant influx rates for ACh synthesis. A simple regulatory mechanism of linear feedback of the ACh level for variation of the substrate influx rates can accomplish the desired replenishment of ACh, in which the influx rate of AcCoA characterizes the response speed and its ratio to that of Ch governs the maintenance of the ACh level. The CoA level is ineffective for this regulatory mechanism.     

Mechanisms controlling choline transport and acetylcholine synthesis in motor nerve terminals during electrical stimulation

Electrical stimulation of the chick ciliary nerve leads to a frequency-dependent increase in the Na+-dependent high affinity uptake of [3H]choline (SDHACU) and its conversion to acetylcholine (ACh) in the nerve terminals innervating the iris muscle. The forces that drive this choline (Ch) uptake across the presynaptic membrane were evaluated. Depolarization with increased [K+] out or veratridine decreases Ch accumulation. In addition to the electrical driving force, energy is provided by the Na+ gradient. Inhibition of the Na,K-ATPase decreased the Ch taken up. Thus, changes in the rate of Ch transport are dependent on the electrochemical gradients for both Ch and Na+. Ch uptake and ACh synthesis were increased after a conditioning preincubation with high [K+] out or veratridine. As is the case for electrical stimulation, this acceleration of Ch uptake and ACh synthesis was strongly dependent on the presence of Ca++ in the incubation medium. Na+ influx through a TTX-sensitive channel also contributed to this acceleration. Inasmuch as membrane depolarization reduces the initial velocity of Ch uptake and ACh synthesis, their increases during electrical stimulation therefore cannot be the direct effect of the depolarization phase of the action potential. Instead they are the result of the ionic fluxes accompanying the presynaptic spike. It is concluded that stimulation of Ch uptake and ACh synthesis by nerve activity depends first, on the ACh release elicited by Ca++ influx after depolarization and second, on the activation of the Na,K-ATPase due to Na+ entry. Furthermore, it is suggested that the release of ACh after stimulation drives translocation of cytoplasmic ACh into a protected compartment (probably vesicular). This recompartmentation of intraterminal ACh stimulates ACh synthesis by mass action, allowing further accumulation of Ch.     

4-(1-Naphthylvinyl)pyridine Decreases Brain Acetylcholine In Vivo, but Does Not Alter the Level of Acetyl-CoA

The effects of intraperitoneally administered 4-(1-naphthylvinyl)pyridine (NVP; 200 mg/kg) on the concentrations of acetylcholine (ACh), choline (Ch), and acetyl-CoA (AcCoA) in rat striatum, cortex, hippocampus, and cerebellum were investigated. Twenty minutes after treatment, the content of ACh was significantly diminished, whereas that of Ch was increased. In response to stress (swimming for 20 min), these changes were enhanced. However, the AcCoA content did not change in any of the brain regions. It is thus very likely that the decrease of brain ACh concentration induced by NVP is due to the drug's effect on choline acetyltransferase (ChAT) and/or the reduction of the high-affinity Ch uptake, and not on the availability of AcCoA. Presumably, the pharmacologically diminished activity of ChAT may become the rate-limiting factor in the maintenance of ACh levels in cholinergic neurons.     

EFFECT OF PRETREATMENT UNDER VARIOUS CATIONIC CONDITIONS ON ACETYLCHOLINE CONTENT AND CHOLINE TRANSPORT IN RAT WHOLE BRAIN SYNAPTOSOMES

The effects of different ionic environments were measured on the concentration of acetyl-choline (ACh) from synaptosomes and their effect on subsequent high affinity choline (Ch) transport and ACh synthesis after resuspension of the synaptosomes in the normal Krebs medium. KCl (40 mM) was used to induce ACh release and reduce synaptosomal ACh content. The effects of Na⁺ omission, Ca²⁺ omission, and high Mg²⁺ on spontaneous (KC1: 4.75 mM) and potassium induced (KC1: 40 mM) ACh release and other cholinergic parameters are presented. The high affinity transport of Ch was more highly correlated with the reciprocal of the ACh level (r= 0.934, P= 9.7 × 10^-4) than with the ACh release rate during preincubation (r= 0.792, P= 3.4 × 10^-2). The results are consistent with the view that the consequences of the various ionic conditions on Ch transport and ACh synthesis are dependent on their effects on intrasynaptosomal ACh levels and only secondarily on synaptosomal ACh release.     

Inhibition of choline efflux results in enhanced acetylcholine synthesis and release in the guinea-pig corticocerebral synaptosomes.

Synthesis and release of [3H]acetylcholine ([3H]ACh) were measured in synaptosomes from the guinea pig cerebral cortex after preloading with [3H]choline ([3H]Ch). We demonstrate here that inhibition of choline (Ch) efflux results in an increase in acetylcholine (ACh) synthesis and release. Our findings are as follows: (1) inhibition of [3H]Ch efflux by hemicholinium-3 (HC-3) (100 microM), increased the levels of both the released (116% of control) and the residing (115% of control) [3H]ACh. (2) The muscarinic agonist, McN-A-343 (100 microM), which was previously shown to inhibit Ch efflux, also increased the released (121% of control) and the residing (109% of control) [3H]ACh. (3) Omission of Na+ ions (which are required for Ch transport) from the incubation medium had similar effects to those observed with McN-A-343 and HC-3. These results suggest inverse relationships between Ch efflux on one hand, and ACh synthesis and release on the other hand. (4) Depolarization with 50 mM K+, or with the K+ channel blocker, 4-aminopyridine (100 microM), also increased the total level of [3H]ACh (113 and 107% of nondepolarized synaptosomes, respectively). However, whereas conditions that inhibit Ch transport such as HC-3, McN-A-343 and "no sodium" increased both the residing and the released [3H]ACh depolarization with high K+ or 4-aminopyridine reduced the residing (79 and 87% of control, respectively) and increased only the released [3H]ACh (182 and 148% of control, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)     

Choline and acetylcholine metabolism in rat neostriatal slices

Choline (Ch) uptake and release and acetylcholine (ACh) synthesis and release have been studied by gas chromatography mass spectrometry (GCMS) in slices of rat neostriatum in vitro to assess the effects of depolarization by 25 mM K+ and the influence of elevated concentrations of Ch in the incubation medium. During the first 60 min after preparation, 25 mM K+ increased ACh release by 182% and reduced ACh levels by 40%. The rate of ACh synthesis was unchanged. After a 1-h equilibration period, the rate of ACh synthesis was considerably less (2.41 nmol mg-1 h-1, compared to 9.78 nmol mg-1 h-1). Exposure to 25 mM K+ during the second hour increased the rate to 6.47 nmol mg-1 h-1. During the first 10 min of exposure to 25 mM K+, ACh synthesis was reduced, regardless of incubation. Increasing concentrations of external [2H4]Ch apparently favored initial rates of net ACh synthesis, since the rank order of initial net ACh synthesis rates is the same as the rank order of external [2H4] Ch concentration under both normal and depolarized conditions. However, the only significant effect of external [2H4]Ch on ACh metabolism was that it increased ACh release during the initial 10 min, when the preparation was depolarized with K+. The efflux of endogenous [2H0]Ch was increased initially (10 min) and slowed over a 60-min period by 25 mM K+, and increased when [2H4]Ch in the medium was increased. Changes in ACh synthesis and release were dependent upon the time exposure of slices to high K+, and the results suggest that Ch favors initial rates of ACh synthesis, but that Ch influences ACh release primarily under conditions of stress (i.e., depolarization).     

Choline is transported by vesicular acetylcholine transporter

Previously published results appeared to show that vesicular acetylcholine transporter (VAChT) does not transport choline (Ch). Because it is uniquely suited to detect transport of weakly bound substrates, a recently developed assay that detects transmembrane reorientation of the substrate binding site was used to re-examine transport selectivity. Rat VAChT was expressed in PC12(A1237) cells, postnuclear supernatant-containing microvesicles was prepared, and the reorientation assay was conducted with unlabeled Ch and tetramethylammonium (TMA). Also, [(14)C]Ch and [(3)H]acetylcholine (ACh) were used in an optimized accumulation assay. The results demonstrate that Ch is transported at least as well as ACh is, but with sevenfold lower affinity. Even TMA is transported, but with 26-fold lower affinity. Ch transport by VAChT is of interest in view of the possibilities that Ch (i) occurs at higher concentration than ACh does in terminal cytoplasm under some conditions, and (ii) is an agonist for alpha 7 nicotinic receptors.     

Effect of oxotremorine,physostigmine, and scopolamine on brain acetylcholine synthesis: A study using HPLC

The synthesis rate of brain acetylcholine (ACh) was estimated in mice following i.v. administration of [³H]choline (Ch). The measurements were performed 1 min after the tracer injection, using the [³H]ACh/[³H]Ch specific radioactivity ratio as an index of ACh synthesis rate. Endogenous and labeled Ch and ACh were quantified using HPLC methodology. Oxotremorine and physostigmine (0.5 mg/kg, i.p.) increased the steady state concentration of brain ACh by +130% and 84%, respectively and of Ch by +60% (oxotremorine); they decreased ACh synthesis by 62 and 55%, respectively. By contrast, scopolamine (0.7 mg/kg, i.p.) decreased the cerebral content of Ch by –26% and of ACh by –23% without enhancing the synthesis of ACh. The results show the utility of HPLC methodology in the investigation of ACh turnover.     

The source of choline for acetylcholine synthesis in brain

More is known about the synthesis and metabolism of acetylcholine (ACh) than other choline (Ch) containing compounds in the brain in spite of the fact that ACh represents only a small fraction of the total Ch esters. This review will attempt to summarize the evidence for the source of Ch in the brain and its relation to the turnover of ACh. Ch is a precursor not only for ACh but also for phosphoryl Ch and phospholipids. It appears that in the rat a bound form of Ch in the brain can produce free Ch which can leave the brain, be converted to ACh or be reutilized for phospholipid synthesis. There is evidence that one of the sources of free Ch that is utilized for ACh synthesis is outside the cholinergic nerve terminal.     

Choline Transport and Metabolism in Soman-or Sarin-Intoxicated Brain

The metabolism and blood-brain transport of choline (Ch) were investigated in perfused canine brain under control conditions and for 60 min after inhibition of brain cholinesterases by the organophosphorus (OP) compounds soman (pinacolylmethylphosphonofluoridate). Ch and acetylcholine (ACh) in blood and brain samples were analyzed using gas chromatography-mass spectrometry methods. Net transport of Ch was determined by Ch analysis in arterial and venous samples. Unidirectional transport of [3H]Ch was determined using the indicator dilution method. During control perfusion periods of 90 min, net efflux of brain Ch occurred at a rate of 1.6 +/- 0.4 nmol/g/min, and the Ch content of the recirculated perfusate increased 10-fold to approximately 8 microM. Brain Ch content increased in proportion to the increase in perfusate Ch level, but brain ACh was unaltered. Rapid administration of soman (100 micrograms) or sarin (400 micrograms) into the arterial perfusate after a 40-min control period resulted in a greater than 10-fold increase in ACh content in cerebral cortex, brainstem, and hippocampus. The ACh content of cerebellum increased only slightly. The Ch level in all four brain regions studied also increased two- to fourfold above control levels. Ch efflux from brain, however, decreased to 0.2 +/- 0.1 nmol/g/min during the 60 min after OP exposure. Unidirectional influx of [3H]Ch was 0.49 +/- 0.07 nmol/g/min before and did not change significantly 10 or 40 min after OP exposure, thus indicating that the Ch transporter of the brain endothelial cell is not directly inhibited.2+ Based on these results, it is proposed that (a) efflux of brain Ch occurs from the extracellular compartment, which becomes depleted when ACh breakdown is inhibited;(ABSTRACT TRUNCATED AT 250 WORDS)     

Certain aspects of acetylcholine metabolism in teleost retina

Acetylcholine (ACh) synthesis and release from isolated superfused retina of the teleostEugerres plumieri has been studied under different physiological conditions. The retinas were superfused with Krebs-Ringer solutions containing [¹⁴C]choline and the extracellular space of 32% was determined by [³H]inulin. The retina accumulates choline (Ch) from the superfusion medium and this process is mediated by a high affinity transport system with aK _m of 1.82 M. The incorporated Ch is mainly utilized for the synthesis of ACh. The ACh content of the light-adapted retina is not significantly different from that of a dark-adapted one. However, the release of [¹⁴C]ACh from the light-adapted retina was 52% higher as compared to the release from the dark-adapted retina. Flicker stimulation induced a larger increase in ACh release, than from either light or dark adapted retina, proportional to flicker frequency. The results suggest that changes in ACh utilization were related to the function of cellular units responsible for light changes transduction rather than light detection.In partial fulfillment of a MSc degree.     

Post-ischemic regional changes in acetylcholine synthesis following transient forebrain ischemia in gerbils

The synthesis rate of brain acetylcholine (ACh) was estimated 30 min and 5 days following transient forebrain ischemia performed by 10 min bilateral carotid occlusion in gerbils. ACh synthesis was evaluated from the conversion of radiolabeled choline (Ch) into ACh after an i.v. administration of [methyl-³H]Ch. Endogenous and labeled Ch and ACh were quantified by HPLC. The synthesis rate of ACh was significantly decreased following 30 min of recirculation. The reductions reached 55.4% in the hippocampus, 51.2% in the cerebral cortex and 44.4% in the striatum. Five days after ischemia, the values returned to normal in the cerebral cortex and in the striatum, while ACh synthesis remained selectively lowered (–30.4%, p<0.01) in the hippocampus. These cholinergic alterations may account for both early and delayed post-ischemic behavioral and mnesic deficits.     

Human Retinas Synthesize and Release Acetylcholine

Human retinas have the capacity to synthesize and release [3H]acetylcholine ([3H]ACh) after an incubation in [3H]choline ([3H]Ch). Synthesis of [3H]Ch by retinal homogenates was determined using either high-voltage paper electrophoresis (HVPE) or a two-step enzymatic/extraction assay for separating [3H]ACh from [3H]Ch. The enzymatic/extraction assay is shown to be accurate over a wide range of concentrations (10(-6)-10(-12) M). Homogenates of human retina synthesize [3H]ACh from [3H]Ch. We find an approximate Km of 50 microM and a Vmax of about 20 nmol/mg protein/h (at 37 degrees C) for the synthesis of labeled ACh by retinal homogenates. Human retinas also release [3H]ACh after a pulse of [3H]Ch. Release of labeled transmitter is stimulated by potassium depolarization. The potassium-stimulated release is partially blocked by magnesium or cobalt ions. Release data were analyzed by both the enzymatic/extraction assay and HVPE; the results are qualitatively identical in both cases. The data reported here provide additional evidence for cholinergic neurotransmission in the human retina.     

Measurement of Acetylcholine Turnover Rate in Brain: An Adjunct to a Simple HPLC Method for Choline and Acetylcholine

Abstract: An existing method for measuring acetylcholine (ACh) and choline (Ch) is shown to be useful formeasuring the turnover rate of ACh in mouse brain. Methl-[3H]Ch is injected into mice. They are killed atdifferent times by microwave irradiation and Ch and AChextracted and separated by reverse-phase HPLC. Ch andACh are converted to hydrogen peroxide by a post-column enzyme reaction. Hydrogen peroxide, which isdirectly related to the tissue content of Ch or ACh, isdetermined electrochemically. The fractions that corre-spond to the detector response for Ch and ACh are col-lected for the measurement of radioactivity. In this wayspecific radioactivities of endogenous Ch and ACh areestimated in the same sample. We used the specific ra-dioactivity values determined by this procedure to esti-mate the turnover of ACh for striatum, cerebral cortex, and hippocampus of the mouse.     

ACETYLCHOLINE TRANSLOCATION IN SYNAPTIC VESICLE GHOSTS IN VITRO

Abstract— Translocation of acetylcholine (ACh) into cholinergic synaptic vesicles depleted of ACh and ATP was studied by Sephadex gel filtration. The hypo-osmotically shocked vesicles become transiently leaky, but retain ACh under iso-osmotic conditions. Intravesicular accumulation of [³H]ACh is due to a simple diffusional equilibration. Addition of 2mM-ATP and Mg²⁺ to the incubation medium is without effect. When acetylcoenzyme A (AcCoA) and choline (Ch ⁺) are used in place of preformed ACh, the intravesicular concentration of ACh does not exceed that of the newly synthesized, extravesicular ACh. However, in the absence of Na ⁺ the quantity of [³H]ACh associated with the vesicles increased, presumably due to ACh binding to ion-exchanger sites in the vesicles.     

Regulation of high affinity choline uptake

High affinity uptake of choline, the rate-limiting, regulatory step for the synthesis of acetylcholine (ACh), was found to be regulated via presynaptic auto- and heteroreceptors. The transport rate was reduced by a muscarinic agonist and neuropeptides, but was significantly enhanced by octopamine. Intracellular messengers, including cyclic nucleotides, appear to modulate the transport activity, apparently by activating specific protein kinases.     

Acetylcholine release and choline uptake by cuttlefish (Sepia officinalis) optic lobe synaptosomes

Acetylcholine (ACh), which is synthesized from choline (Ch), is believed to hold a central place in signaling mechanisms within the central nervous system (CNS) of cuttlefish (Sepia officinalis) and other coleoid cephalopods. Although the main elements required for cholinergic function have been identified in cephalopods, the transmembrane translocation events promoting the release of ACh and the uptake of Ch remain largely unsolved. The ACh release and Ch uptake were quantitatively studied through the use of in vitro chemiluminescence and isotopic methods on a subcellular fraction enriched in synaptic nerve endings (synaptosomes) isolated from cuttlefish optic lobe. The ACh release evoked by K+ depolarization was found to be very high (0.04 pmol ACh.s(-1).mg(-1) protein). In response to stimulation by veratridine, a secretagogue (a substance that induces secretion) that targets voltage-gated Na+ channels, the release rate and the total amount of ACh released were significantly lower, by 10-fold, than the response induced by KCl. The high-affinity uptake of choline was also very high (31 pmol Ch.min(-1).mg(-1) protein). The observed ACh release and Ch uptake patterns are in good agreement with published data on preparations characterized by high levels of ACh metabolism, adding further evidence that ACh acts as a neurotransmitter in cuttlefish optic lobe.     

Choline Uptake and Acetylcholine Synthesis in Synaptosomes: Investigations Using Two Different Labeled Variants of Choline

Abstract: Using sequential incubations in media of different K⁺ composition, we investigated the dynamics of choline (Ch) uptake and acetylcholine (ACh) synthesis in rat brain synaptosomal preparations, using two different deuterated variants of choline and a gas chromatographic-mass spectrometric (GC-MS) assay for ACh and Ch. Synaptosomes were preincubated for 10 min in a Krebs medium with or without high K⁺ and with 2 μM-[²H₉]Ch. At the end of the preincubation all variants of ACh and Ch were measured in samples of the pellet and medium. In the second incubation (4 min) samples of synaptosomes were resuspended in normal or high K⁺ solutions containing [²H₄]Ch (2 μM) and all variants of ACh and Ch were measured in the pellet and medium at the end of this period. This protocol allowed us to compare the effects of preincubation in normal or high K⁺ solution on the metabolism during a second low or high K⁺ incubation of a [²H₉]Ch pool accumulated during the preincubation period. Moreover, we were able to compare and contrast the effects of this protocol on [²H₉]Ch metabolism versus [²H₄]Ch metabolism. The most striking result we obtained was that [²H₉]Ch that had been retained by the synaptosomes after the preincubation was not acetylated during a subsequent incubation in normal or high K⁺ media. This result suggests that if an intraterminal pool of Ch is involved in ACh synthesis, the size of this pool is below the limits of detection of our assay. We have confirmed the observation that a prior depolarizing incubation results in an enhanced uptake of Ch during a second incubation in normal K⁺ Krebs. Moreover, Ch uptake is stimulated by prior incubation under depolarizing conditions relative to normal preincubation when the second incubation is in a high K⁺ solution. These results are discussed in terms of current models of the regulation of ACh synthesis in brain.     

Choline transporter‐like protein 4 (CTL4) links to non‐neuronal acetylcholine synthesis

Synthesis of acetylcholine (ACh) by non‐neuronal cells is now well established and plays diverse physiologic roles. In neurons, the Na⁺‐dependent, high affinity choline transporter (CHT1) is absolutely required for ACh synthesis. In contrast, some non‐neuronal cells synthesize ACh in the absence of CHT1 indicating a fundamental difference in ACh synthesis compared to neurons. The aim of this study was to identify choline transporters, other than CHT1, that play a role in non‐neuronal ACh synthesis. ACh synthesis was studied in lung and colon cancer cell lines focusing on the choline transporter‐like proteins, a five gene family choline‐transporter like protein (CTL)1–5. Supporting a role for CTLs in choline transport in lung cancer cells, choline transport was Na⁺‐independent and CTL1–5 were expressed in all cells examined. CTL1, 2, and 5 were expressed at highest levels and knockdown of CTL1, 2, and 5 decreased choline transport in H82 lung cancer cells. Knockdowns of CTL1, 2, 3, and 5 had no effect on ACh synthesis in H82 cells. In contrast, knockdown of CTL4 significantly decreased ACh secretion by both lung and colon cancer cells. Conversely, increasing expression of CTL4 increased ACh secretion. These results indicate that CTL4 mediates ACh synthesis in non‐neuronal cell lines and presents a mechanism to target non‐neuronal ACh synthesis without affecting neuronal ACh synthesis.