Choline: high-affinity uptake by rat brain synaptosomes

Autoradiography of 14C-choline uptake in endplates and skeletal muscle of mice

The uptake of 14C-choline into the axonal part of the motor endplate and muscle of mouse diaphragms was investigated by autoradiography. With i.v. doses of 0.1 microgram/g choline chloride, the uptake into the nerve endings is fast (less than 2 min) and into muscle slower (greater than 2 min). With higher doses (1.0 microgram/g) the uptake into muscle tissue is accelerated. The radioactivity in the endplates decreases with a halflife time of 20 min and remains constant in the muscle fibres over 60 min. Denervation by cutting the phrenic nerve reduces the uptake into endplates by 40% within 14 h, but probably induces uptake into regenerating Schwann cells during 30 days. Som compounds with choline-like structure (hemicholinium-3, decamethylen-dicholine, triethyl-choline) reduce the high-affinity uptake of choline into the nerve endings with sublethal doses, whereas tetraethylammonium and N-hydroxyethyl-4-(1-naphthylvinyl)-pyridinium, an inhibitor of cholinacetyltransferase, are less active. Half lethal doses of cocaine, imipramine and reserpine have no significant action on uptake of choline into the endplates. Chlorpromazine slightly diminishes the uptake into endplates. Chlorpromazine and imipramine reduce uptake into the muscle fibres.

An evaluation of fitting methods for the sum of two hyperbolas: application to uptake studies

Analysis of data in terms of the sum of two rectangular hyperbolas is frequently required in solute uptake studies. Four methods for such analysis have been compared. Three are based on least-squares fitting whereas the fourth (partition method I) is an extension of a single hyperbola fitting procedure based on non-parametric statistics. The four methods were tested using data sets which had been generated with two primary types of random, normal error in the dependent variable: one of constant error variance and the other of constant coefficient of variation. The methods were tested on further data sets which were obtained by incorporating single 10% bias errors at different positions in the original two sets. Partition method I consistently gave good estimates for the four parameters defining the double hyperbola and was highly insensitive to the bias errors. The least-squares procedures performed well under conditions satisfying the least-squares assumptions regarding error distribution, but frequently gave poor estimates when these assumptions did not hold. Our conclusion is that in view of the errors inherent in many solute uptake experiments it would usually be preferable to analyse data by a method such as partition method I rather than to rely on a least-squares procedure.

Origin of the acetyl moiety of acetylcholine synthesized in rat striatal synaptosomes

The subcellular localization of the AcCoA compartment supplying the cytoplasmic choline acetyltransferase (ChAc, EC 2.3.1.6) was investigated using a purified preparation of rat striatal synaptosomes (B fraction). It was first demonstrated that the SRA of the [14C]ACh synthesized during a 10 min incubation period was equal to the SRA of the [2-14C] and the [3-14C]pyruvate added to the isolated nerve terminal suspension. The experimental results can be summarised as follows: (i) No modification in the amount of [14C]ACh synthesized from [2-14C]pyruvatetion in the amount of [14C]ACh synthesized from [2-13C]pyruvate could be detected after the addition of high concentrations of either carnitine, acetylcarnitine or acetyl phosphate to the synaptosomal suspension. (ii) Under experimental conditions in which the amount of [1,5-14C]citrate taken up by passive diffusion into the cholinergic nerve endings would allow detection of the possible formation of the labelled ester, no [14C]ACh could be recovered. (iii) The SRA's of the individual carbon atoms of the Krebs cycle intermediary compounds when the cycle is fed with [2-14C] and [3-14C]pyruvate were calculated as a function of the STA's of each of these two precursors (a and a' respectively), of the number of 14CO2 dpm produced in the Krebs cycle from each of these two labelled compounds (D2 and D3 respectively), and as the function of the rate y of exchanges of molecules between the tricarboxylic acid cycle and other metabolic compartments. The experimental value obtained from a 10 min incubation, after the nerve endings had reached a steady metabolic activity, indicate that if the acetyl moiety of ACh was derived from some Krebs cycle intermediary compounds, its SRA could never exceed 55 per cent that of the [2-14C]pyruvate from which it is produced, (iv) No correlation could be found between the rate of [14C]ACh formation and changes in the Krebs cycle activity induced by sodium cyanide, 2-4 dinitrophenol and Ca2+ free medium. (v) The lack of significant [14C]ACh synthesis from [1-14C]acetate in striatal synaptosomes is consistent with the failure of fluoroacetate to modify the amounts of 14CO2 as well as of [14C]ACh formed from [2-14C]pyruvate. These results were interpreted as a confirmation of the presence of a low AcCoA synthetase activity in the nerve terminals. To reconcile all these data, it is proposed that pyruvate is transformed into AcCoA outside the mitochondria by the action of some cytoplasmic pyruvate dehydrogenase-like enzyme.

The effect of preganglionic nerve stimulation on the accumulation of certain analogues of choline by a sympathetic ganglion

1. Cat superior cervical ganglia were perfused with a Krebs solution containing 10(-6) M [3H]homocholine (2-hydroxypropyl-trimethylammonium) or 10(-5) M [14C]triethylcholine (2-hydroxyethyl-triethylammonium). Preganglionic nerve stimulation (20 Hz) increased the accumulation of homocholine (3-2-fold) and of triethylcholine (2-1-fold). This increased accumulation during stimulation was not the result of increased metabolism. 2. The increased accumulation of homocholine or triethylcholine induced by pregnaglionic nerve stimulation was not reduced by tubocurarine or by atropine, but it was blocked by choline and by hemicholinium. These results suggested that preganglionic nerve stimulation increased choline analogue accumulation into cholinergic nerve terminals. 3. The increased accumulation of homocholine or of triethylcholine induced by preganglionic nerve stimulation was reduced when the Ca2+ concentration was reduced and was abolished in the absence of Ca2+. However, changes in the Mg2+ concentration which depressed acetylcholine (ACh) release by amounts comparable to those induced by altered Ca2+ concentrations did not alter the uptake of homocholine or triethylcholine. It is concluded that the uptake of choline analogues is not regulated by transmitter release but that stimulation increases the uptake of the choline analogues by a Ca2+-dependent mechanism. 4. The accumulation of ACh by ganglia perfused with a Krebs solution containing choline and high MgSO4 (18 mM) was measured. The ACh content of these ganglia did not increase, although choline transport presumably exceeded that necessary for ACh synthesis to replace released ACh. It is concluded that choline transport does not limit ACh synthesis in ganglia.

Cholinesterase activity and choline uptake in intact nerve cell cultures

Choline uptake and ecto-cholinesterase activities have been measured in intact astroblast and neuroblast cultures. The data show that choline uptake is dependent upon the ionic composition of the culture medium and is sensitive to metabolic inhibitors. However, the high concentrations of the inhibitors necessary for the inhibition of the uptake and some thermodynamic properties qould suggest a facilitated transport rather than an active uphill process. Preincubation of the cultures with various inhibitors of cholinesterases shows no direct parallelism between inhibition of choline high affinity uptake (apparent Km approximately equal to 10-6 M) and inhibition of ecto-acetylcholinesterase (EC 3.1.1.7).

Mechanism of action of beta-bungarotoxin on synaptosomal preparations

The neurochemical activity of beta-bungarotoxin was investigated using a synaptosomal preparation of rat cerebral cortices. In preparations preincubated with [3H]choline in order to label acetylcholine the toxin caused a rapid release of the transmitter, which was calcium dependent but only a little affected by a depolarizing concentration of potassium. beta-Bungarotoxin was also shown to be a potent inhibitor of the high affinity transport system for choline, producing 50% inhibition at a concentration of 50 nM. These findings explain the observed electrophysiological effects of the toxin. Electron microscopy revealed no discernible effect of 0.1 muM beta-bungarotoxin on either synaptic vesicles or mitochondria. Neither the release of transmitter nor the inhibition of choline uptake by the toxin was affected by the presence of an inhibitor of phospholipase activity.

Inhibition by botulinum toxin of depolarization-evoked release of (14C)acetylcholine from synaptosomes in vitro

1. Cerebral-cortex synaptosomes were shown to synthesize (14C)acetylcholine after incubation with (14C)choline, and 25mM-KCl released (14C)acetylcholine (but not (14C)choline) into the medium by a Ca2+-dependent and Mg2+-sensitive process. 2. The K+-stimulated release of (14C)acetylcholine was inhibited by more than 80% after preincubation of the synaptosomes with 10(5) mouse lethal doses of botulinum toxin/ml. (14C)choline uptake, (14C)acetylcholine synthesis, intrasynaptosomal K+ and occluded lactate dehydrogenase were unaffected by the toxin. It also failed to prevent the K+-stimulated release of (3H)noradrenaline and (14C)glycine from synaptosomes. 3. Fractionation of hypo-osmotically shocked synaptosomes revealed that more than 75% of the radioactive acetylcholine was in the cytoplasmic compartment, although the vesicle pellet contained more total acetylcholine than the cytoplasmic pool. Consequently the specific radioactivity of acetylcholine in the cytoplasmic pool was almost 5 times that of the vesicles. This distribution was unaffected by preincubation with botulinum toxin. It is concluded that the toxin acts directly on the release of acetylcholine, rather than influencing its storage. 4. After K+-stimulation, toxin-inhibited synaptosomes contained increased amounts of total acetylcholine, which suggests that its rate of synthesis is controlled by depolarization rather than release.

High affinity choline uptake: an early index of cholinergic innervation in rat brain

The uptake of [3H]choline was investigated in nuclei-free homogenates or crude synaptosomal fractions (P2) from rat brain under various stages of development. A comparable sensitivity of uptake to treatment by hyposmotic shock suggested the involvement of synaptosomal populations in choline uptake in immature as well as in adult brains. However, significant changes in the "apparent" Km for the high affinity transport system and quantitative differences in the Na ion requirement for maximal uptake at 0.43 muM choline concentration were found during development; facts which suggested a greater contribution of the low affinity system in the more immature brains. Assuming that the uptake with high and low sensitivity to Na+ reduction reflected that via the high and low affinity system reslectively, we have attempted to obtain real Km values for the high affinity system. These Km values changed less than those measured directly, suggesting that the affinity constant for the high affinity system does not change during development. On these assumptions, the developmental changes of cholinergic synaptogenesis were examined in 5 distinct regions of the brain. It was found that the synaptogenesis begins several days earlier than the increase of choline acetyltransferase (ChAc) level in the frontal cortex, the hippocampus, the superior colliculus and the cerebellum. These regions may be included among the terminal-rich regions according to available evidence related to cholinergic systems. On the other hand, synaptogenesis accompanied the concomitant ChAc increase in the striatum, where the cholinergic interneurons are present. It is concluded that the increase of ChAc in the terminal-rich regions is delayed by the axoplasmic flow; therefore, the earlier index of cholinergic synaptogenesis in these regions is the high affinity uptake activity rather than the enzyme activity.

The effect of atropine on the turnover of acetylcholine in the mouse brain

The effect of atropine on the acetylcholine (ACh) turnover in the mouse brain has been studied and related to the central effect (motor activity) of the drug. At the threshold dose for maximal increase in motor activity, atropine had no measurable effect in the brain on the initial rate of formation of labelled ACh from labelled choline (Ch) i.v. injected. However, if atropine was injected 3 min after the injection of labelled Ch, when the labelled ACh had reached its peak value in the brain, there was a more rapid exponential decline of labelled ACh. This was assumed to be an indication that atropine increases the turnover of ACh in the brain. The specific radioactivity of ACh was not changed 2-17 min after the atropine injection, which indicates that atropine does not preferentially increase the release of newly synthetized ACh.

Renal tubular excretion of triethylcholine (TEC) in the chicken: enhancement and inhibition of renal excretion of choline and acetylcholine by TEC

1. [3H]-triethylcholine (TEC) was actively transported by the renal tubule of the chicken at a rate 85% that of simultaneously administered p-aminohippuric acid (PAH). 2. TEC was demonstrated to be transported by the organic cation transport system in the kidney through inhibition with quinine and the bio-cation choline. 3. When the infusion of TEC was increased to 2 times 10(-6) mol kg(-1) min(-1) reaching the infused kidney, the transport of [3H]-TEC was inhibited, suggesting that an excretory transport maximum for TEC in the renal tubules had been reached. 4. The excretion of both choline and acetylcholine was enhanced by TEC loads as low as 1 times 10(-18) mol kg(-1) min(-1). Enhancement continued as TEC infusion was increased up to approximately 1 times 10(-7) mol kg(-1) min(-1) at which point this enhancement was converted to inhibition. 5. Possible mechanisms for the biphasic effect of TEC on organic cation transport are discussed.

Metabolism of acetylcholine in the nervous system of Aplysia californica. III. Studies of an indentified cholinergic neuron

[3H] choline and [3H] acetyl CoA were injected into the cell body of an identified cholinergic neuron, the giant R2 of the Aplysia abdominal ganglion, and the fate and distribution of the radioactivity studied. Direct eveidence was obtained that the availabliity of choline to the enzymatic machinery limits synthesis. [3H] choline injected intrasomatically was converted to acetylcholine far more efficiently than choline taken up into the cell body from the bath. Synthesis from injected [3H] acety CoA was increased more than an order of magnitude when the cosubstrate was injected together with a saturating amount of unlabeled choline. In order to study the kinetics of acetylcholine synthesis in the living neuron, we injected [3H] choline in amounts resulting in a range of intracellular concentrations of about four orders of magnitude. The maximal velocity was 300 pmol of acetylcholine/cell/h and the Michaelis constant was 5.9 mM [3H] choline; these values agreed well with those previously reported for choline acetyltransferase assayed in extracts of Aplysia nervous tissue. [3H] acetylcholine turned over within the injected neuron with a half-life of about 9 h. The ultimate product formed was betaine. Subcellular distribution of [3H] acetylcholine was studied using differential and gradient centrifuagtion, gel filtration, and passage through cellulose acetate filters. A small portion of acetylcholine was contained in particulates the size and density expected of cholinergic vesicles.

Metabolism of acetylcholine in the nervous system of Aplysia californica. I. Source of choline and its uptake by intact nervous tissue

Although acetylcholine is a major neurotransmitter in Aplysia, labeling studies with methionine and serine showed that little choline was synthesized by nervous tissue and indicated that the choline required for the synthesis of acetylcholine must be derived exogenously. Aanglia in the central nervous system (abdominal, cerebral, and pleuropedals) all took up about 0.5 nmol of choline per hour at 9 muM, the concentration of choline we found in hemolymph. This rate was more than two orders of magnitude greater than that of synthesis from the labeled precursors. Ganglia accumulated choline by a process which has two kinetic components, one with a Michaelis constant between 2-8 muM. The other component was not saturated at 420 muM. Presumably the process with the high affinity functions to supply choline for synthesis of transmitter, since the efficiency of conversion to acetylcholine was maximal in the range of external concentrations found in hemolymph.

Metabolism of acetylcholine in the nervous system of Aplysia californica. II. Reginal localization and characterization of choline uptake

The choline required for synthesis of acetylcholine is derived exogenously by Aplysia ganglia. Under physiological conditions choline was taken up primarlily by neuropile and nerves and not by cholinergic cell bodies. In addition, compared with their contents of choline acetyltransferase, those components of nervous tissue which contain nerve terminals and axons synthesized acetylcholine far more efficiently. Choline was accumulated by high and low affinity uptake processes; the high affinity process appeared to be characteristic of cholinergic nuerons (Swartz, J. H., M. L. Eisenstadt, and H. Cedar.1975. J. Gen. Physiol. 65:255). The two uptake processes were similarly affected by temperature with a Q10 of 2.8. Both were dependent on a variety of ions in a complicated manner. High affinity uptake seemed to be more dependent on Na+, showed greater inhibition by ouabain, and was selectively inhibited by oxotremorine. We found that the functional state of neurons did not alter uptake of radioactive choline by either process, nor did it change the conversion to radioactive acetylcholine.

Effects of dopaminergic receptor agonists and antagonists on the activity of the neo-striatal cholinergic system

The effects of various neuroleptics and of apomorphine on the metabolism of ACh were examined in the neostriatum of the rat. For this purpose, a specific radio-enzymatic assay for brain ACh was used. This method is based on the preliminary purification of the choline esters by liquid cation exchange, separation of choline and ACh on thin layer chromatography plates, hydrolysis of ACh then reactylation of the choline moiety with a purified and stabilized rat brain choline acetyltransferase. The rat neostriatal ACh levels were decreased by neuroleptics of the phenothiazine and butyrophenone type and increased by apomorphine. An "in vivo" estimation of the rate of utilization of ACh was obtained by measuring the decline in neostriatal ACh content following the local microinjection of hemicholinium-3. This compound blocked almost totally the synthesis of ACh in these conditions. Chlorpromazine (15 mg/kg) enhanced neo-striatal ACh utilization and apomorphine (10 mg/kg) antagonized this effect. Neuroleptics did not effect ACh levels in the parietal cerebral cortex and the hippocampal formation. The modifications of the activity of neostriatal cholinergic neurons by chlorpromazine and apomorphine were still observed following the degeneration of the nigro-neostriatal dopaminergic fibers induced by the injection of 6-hydroxydopamine into the substantia nigra. The results strongly suggest that dopaminergic receptors as defined by their pharmacological interaction with neuroleptics and apomorphine are localized on neostriatal ACh neurons.