Factors affecting choline transport by the cat superior cervical ganglion during and following stimulation, and the relationship between choline uptake and acetylcholine synthesis

Regulated recycling and plasma membrane recruitment of the high-affinity choline transporter

The high-affinity choline transporter (CHT1) is responsible for uptake of choline from the synaptic cleft and supplying choline for acetylcholine synthesis. CHT1 internalization by clathrin-coated vesicles is proposed to represent a mechanism by which high-affinity choline uptake can be modulated. We show here that internalized CHT1 is rapidly recycled back to the cell surface in both human embryonic kidney cells (HEK 293 cells) and SH-SY5Y neuroblastoma cells. This rapidly recycling pool of CHT1 comprises about 10% of total CHT1 protein. In the SH-SY5Y neuroblastoma cell line K(+)-depolarization promotes Ca(2+)-dependent increase in the rate of CHT1 recycling to the plasma membrane without affecting the rate of CHT1 internalization. K(+)-depolarization also increases the size of the pool of CHT1 protein that can be mobilized to the plasma membrane. Thus, the activity-dependent increase in plasma membrane CHT1 localization appears to be regulated by two mechanisms: (i) an increase in the rate of externalization of the intracellular CHT1 pool; and (ii) the recruitment of additional intracellular transporters to the recycling pool.

Choline transport for phospholipid synthesis

Choline is an essential nutrient for all cells because it plays a role in the synthesis of the membrane phospholipid components of the cell membranes, as a methyl-group donor in methionine metabolism as well as in the synthesis of the neurotransmitter acetylcholine. Choline deficiency affects the expression of genes involved in cell proliferation, differentiation, and apoptosis, and it has been associated with liver dysfunction and cancer. Abnormal choline transport and metabolism have been implicated in a number of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Therefore, the study of choline transport and the characteristics of choline transporters are of central importance to understanding the mechanisms that underlie membrane integrity and cell signaling in such disorders. Kinetic studies with radiolabeled choline and inhibitors distinguish three systems for choline transport: (i) low-affinity facilitated diffusion, (ii) high-affinity, Na+-dependent transport, and (iii) intermediate-affinity, Na+-independent transport. It is only recently, however, that the proteins having transport characteristics of at least one of these systems have been identified. They include (i) polyspecific organic cation transporters (OCTs) with low affinity for choline, (ii) high-affinity choline transporters (CHT1s), and (iii) intermediate-affinity choline transporter-like (CTL1) proteins. CHT1 and CTL1 but not OCT transporters are selectively inhibited with hemicholinium-3 and essentially display characteristics of specialized transporters for targeted choline metabolism. CHT1 is abundant in neurons and almost exclusively supplies choline for acetyl-choline synthesis. The focus here is more on newly-discovered CTL1 choline transporters. They are expressed in different organisms and cell types, apparently not for the biosynthesis of acetylcholine but for the production of the most abundant metabolite of choline, the membrane lipid phosphatidylcholine.

Regulation of choline transporter surface expression and phosphorylation by protein kinase C and protein phosphatase 1/2A

The Na(+)/Cl(-)-dependent, hemicholinium-3-sensitive choline transporter (CHT) provides choline for acetylcholine biosynthesis. Recent studies show that CHT contains canonical protein kinase C (PKC) serine and threonine residues. We examined the ability of PKC and serine/threonine protein phosphatase 1/2A (PP1/PP2A) to regulate CHT function, surface expression, and phosphorylation. In mouse crude striatal and hippocampal synaptosomes, PKC activators beta-phorbol 12-myristate 13-acetate (beta-PMA) and beta-phorbol 12,13-dibutyrate produced time- and concentration-dependent reductions in CHT function. PP1/PP2A inhibitors okadaic acid (OKA) and calyculin A (CL-A) produced a time- and concentration-dependent decrease in CHT function. However, tautomycin (PP1 inhibitor) and cyclosporin A (PP2B inhibitor) failed to alter CHT-mediated choline uptake. Choline transport kinetic studies following beta-PMA, OKA, and CL-A treatment revealed a reduction in the maximal choline transport velocity (V(max)) with no change in K(m) for choline. These modulators also produced no change in the total levels of CHT protein in the crude hippocampal and striatal synaptosomes; however, surface biotinylation studies using the membrane-impermeant N-hydroxysuccinimide-biotin in crude synaptosomes following treatment with beta-PMA, OKA, and CL-A indicate significant reductions of CHT levels in biotinylated fractions. Pretreatment with OKA alone, but not beta-PMA, significantly augmented the phosphorylation level of CHT proteins. Our findings suggest that neuronal PKC and PP1/PP2A activity may establish the level of function and surface expression of CHT. These studies also provide the first evidence that CHT is a phosphoprotein and that the basal PP1/PP2A activity may have a dominant role in controlling the levels of CHT phosphorylation.

The hemicholinium-3 sensitive high affinity choline transporter is internalized by clathrin-mediated endocytosis and is present in endosomes and synaptic vesicles

Synthesis of acetylcholine depends on the plasma membrane uptake of choline by a high affinity choline transporter (CHT1). Choline uptake is regulated by nerve impulses and trafficking of an intracellular pool of CHT1 to the plasma membrane may be important for this regulation. We have generated a hemagglutinin (HA) epitope tagged CHT1 to investigate the organelles involved with intracellular trafficking of this protein. Expression of CHT1-HA in HEK 293 cells establishes Na+-dependent, hemicholinium-3 sensitive high-affinity choline transport activity. Confocal microscopy reveals that CHT1-HA is found predominantly in intracellular organelles in three different cell lines. Importantly, CHT1-HA seems to be continuously cycling between the plasma membrane and endocytic organelles via a constitutive clathrin-mediated endocytic pathway. In a neuronal cell line, CHT1-HA colocalizes with the early endocytic marker green fluorescent protein (GFP)-Rab 5 and with two markers of synaptic-like vesicles, VAMP-myc and GFP-VAChT, suggesting that in cultured cells CHT1 is present mainly in organelles of endocytic origin. Subcellular fractionation and immunoisolation of organelles from rat brain indicate that CHT1 is present in synaptic vesicles. We propose that intracellular CHT1 can be recruited during stimulation to increase choline uptake in nerve terminals.

Regulation of hemicholinium-3 sensitive choline uptake in Xenopus laevis oocytes by the second C2 domain of synaptotagmin

A size-fractionated torpedo electric lobe cDNA library was screened for the neuronal choline transporter by functional expression in oocytes. A clone, TLC2B, was isolated that induced a component of choline uptake that was hemicholinium-3 sensitive and inhibited by the substitution of lithium for sodium at low choline concentrations. However, [3H]choline uptake by both injected and non-injected oocytes were characterized by high affinity constants, suggesting that TLC2B could be affecting a native choline transporter. Indeed, hemicholinium-3 sensitive choline uptake could also be induced by preincubation of non-injected oocytes with a protein kinase C inhibitor, H-7. By sequence analysis and immuno-precipitation, the peptide produced by injection of TLC2B cRNA was identified as a soluble 24 kDa C-terminal fragment of the neuronal protein, synaptotagmin. Full length synaptotagmin was, however, ineffective in the functional test. The peptide encoded by TLC2B corresponds to the second protein kinase C-homologous domain of torpedo synaptotagmin, and like other soluble C2 domain peptides, was capable of calcium-dependent translocation to membranes. Its action on choline uptake in oocytes was, however, abolished by the addition of calcium in the presence of a calcium ionophore. These results suggest that the interaction of certain C2 domains, such as the C-terminal domain of synaptotagmin, with more specific targets may be anulled in the presence of calcium due to its absorption to membrane phospholipids.

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.

Acetylcholine synthesis by a sympathetic ganglion in the presence of 2-(4-phenylpiperidino)cyclohexanol (AH5183) and picrylsulfonic acid

The present experiments measured the release and the synthesis of acetylcholine (ACh) by cat sympathetic ganglia in the presence of 2-(4-phenylpiperidino)cyclohexanol (AH5183 or vesamicol) and/or picrylsulfonic acid (TNBS), two compounds known to have the ability to block the uptake of ACh by cholinergic synaptic vesicles in vitro. We confirmed that, in stimulated (5 Hz) perfused (30 min) ganglia, AH5183 depressed ACh release and ACh tissue content increased by 86 +/- 6% compared to contralateral ganglia used as controls. Preganglionic activity increased ACh release by a similar amount in the presence (19.9 +/- 1.0 pmol/min) or absence (20.5 +/- 2.4 pmol/min) of TNBS. The final tissue ACh content was also similar in the presence (1,668 +/- 166 pmol) or absence (1,680 +/- 56 pmol) of TNBS. However, the AH5183-induced increase of tissue ACh content (86 +/- 6%) was abolished completely when AH5183 was perfused with 1.5 mM TNBS (-3.0 +/- 1.0%). This inhibition of ACh synthesis, observed in TNBS-AH5183-perfused ganglia, was not dependent upon further inhibition of ACh release beyond that caused by AH5183 alone, because 14.0 +/- 1.9% of the transmitter store was released by preganglionic nerve stimulation in the presence of TNBS plus AH5183 and this was similar in the presence of AH5183 without TNBS (14.0 +/- 0.6%). Moreover, when ganglia were first treated with TNBS and then stimulated in the presence of AH5183, an increase of 64 +/- 6% of the ganglionic ACh content occurred, and this increase was not statistically different from the increase measured with AH5183 alone (86 +/- 6%).(ABSTRACT TRUNCATED AT 250 WORDS)

Binding of [3H]hemicholinium-3 to the high-affinity choline transporter in electric organ synaptosomal membranes

Sodium-dependent binding of [3H]hemicholinium-3 was observed to be 10-fold higher with presynaptic membranes from the electric organ than with electroplaque membranes and this binding site copurified with synaptosomal membranes. The KD for specific [3H]hemicholinium-3 binding was found to be 31 +/- 4 nM and the Bmax, 5.0 +/- 0.2 pmol/mg protein; a Ki of 16 nM was estimated for hemicholinium-3 as a competitive inhibitor of high-affinity choline transport in electric organ synaptosomes. Choline and choline analogues were equally potent as inhibitors of [3H]choline uptake and [3H]hemicholinium-3 binding. Tubocurarine and oxotremorine also inhibited uptake and binding, but carbachol was without effect in both tests. These findings suggest that [3H]hemicholinium binds to the high-affinity choline transporter present at the cholinergic nerve terminal membrane. A comparison of maximal velocities for choline transport and the maximal number of hemicholinium-3 binding sites indicated that the high-affinity choline transporter has an apparent turnover number of about 3s-1 at 20 degrees C under resting conditions. The high transport rates observed in electric organ synaptosomes are likely due to the high density of high-affinity choline transporters in this tissue, estimated on the basis of [3H]hemicholinium-3 binding to be of the order of 100/micron2 of synaptosomal membrane.

Cholinergic properties of embryonic chick sensory neurons

Experiments were carried out to determine the cholinergic properties of sensory neurons of the chick embryo by measuring the choline acetyltransferase activity (ChAT) and [3H]choline uptake. The choline acetyltransferase activity in the dorsal root ganglia of an 8-day-old chick embryo was 24.2 +/- 2.52, which increased to 45.4 +/- 9.69 pmol ACh/mg protein/min in the ganglia of 12-day-old embryos. Sensory neurons derived from dorsal root ganglia of 10-day-old embryos and maintained in a serum-free culture medium supplemented with insulin, transferrin and nerve growth factor (NGF) also contained significant amounts of ChAT (21.9 pmol ACh/mg protein/min). Omission of NGF resulted in neuronal death, and the enzyme activity could not be measured in these cultures. A specific inhibitor of ChAT, hydroxyethyl naphthylvinyl pyridine (NVP), when added to the assay mix produced a dose-dependent inhibition of ChAT from cultured neurons. Cultured sensory neurons incubated with [3H]choline followed by repeated washouts took up and retained [3H]choline. The uptake of [3H]choline was reduced by about 45% when NaCl, in the incubation medium, was replaced by LiCl. A specific inhibitor of choline uptake, hemicholinium-3, caused about 75% inhibition of [3H]choline uptake. It is implied that sensory neurons of the chick dorsal root ganglia express cholinergic properties during development.

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

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

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

These experiments measured the effect of 2-(4-phenylpiperidino)cyclohexanol (AH5183) on the release of acetylcholine (ACh) and its subcellular distribution in slices of rat striatum incubated in vitro. The AH5183, a drug that blocks the uptake of ACh by isolated synaptic vesicles, reduced the release of ACh from slices stimulated to release transmitter in response to K+ depolarization. Tissue stimulated in the presence of AH5183 contained more ACh in a nerve terminal cytoplasmic fraction than did tissue stimulated in the drug's absence, but stimulation in AH5183's presence reduced the amount of ACh measured in fractions containing synaptic vesicles. The depletion of ACh caused by stimulating tissue in the presence of AH5183 was more evident in the fraction of nerve terminal ACh occluded within synaptic vesicles as isolated by gradient centrifugation (fraction D) than it was in other nerve terminal occluded stores. It is concluded that the synaptic vesicles isolated as fraction D under the present experimental conditions likely contain releasable transmitter. The AH5183 also depressed the spontaneous release of ACh from incubated slices of striatum and this effect was evident in the presence or the absence of medium Ca2+. It is suggested that this effect might indicate that the process of spontaneous ACh release measured neurochemically results, in part, from an AH5183-sensitive carrier-mediated process.

Accumulation, acetylation, and releasability of diethylhomocholine from a sympathetic ganglion

Superior cervical ganglia of the cat perfused with [14C]diethylhomocholine [( 14C]DEHCh) synthesized acetyldiethylhomocholine (ADEHCh), but rather little of this ester was released by subsequent preganglionic nerve stimulation. Stimulation evoked the release of an appreciable amount of unchanged DEHCh when ganglia had been exposed to the analogue in the absence of choline (Ch), but did not do so when exposed to both Ch and DEHCh. The release of DEHCh was Ca2+ dependent, and was not the result of the release and subsequent hydrolysis of ADEHCh. This is the first clear demonstration of the release of an unacetylated compound from mammalian tissue; therefore, the characteristics of the transmitter release mechanism are further defined. The effect of preganglionic nerve stimulation on the uptake and acetylation of DEHCh was also measured. Stimulated ganglia accumulated approximately 4 times more labeled analogue and synthesized 7.5 times more ADEHCh than did rested ganglia. Stimulated ganglia perfused with 2-(4-phenylpiperidino)cyclohexanol, a compound considered to inhibit acetylcholine (ACh) release by inhibiting its transport into synaptic vesicles, accumulated 3.4 times as much and acetylated 6 times as much DEHCh as did rested ganglia. When the concentration of Mg2+ in the perfusion medium was increased to block ACh release, accumulation of the labelled analogue was enhanced by stimulation, but its acetylation was increased much less than during perfusion with normal medium. It is concluded that the synthesis of ADEHCh is subject to the same regulation as is ACh synthesis and that the activation of ester synthesis during activity can be dissociated from ester release.(ABSTRACT TRUNCATED AT 250 WORDS)

Reconstitution of carrier-mediated choline transport in proteoliposomes prepared from presynaptic membranes of Torpedo electric organ, and its internal and external ionic requirements

Proteoliposomes made by a butanol-sonication technique from electric organ presynaptic membranes showed choline transport activity. In contrast to intact nerve terminals, the uptake of choline was dissociated from its conversion to acetylcholine in this preparation. The kinetics of choline uptake by proteoliposomes was best described by two Michaelis-Menten components. At a low concentration of choline, uptake was inhibited by hemicholinium-3 and required external Na+ and, thus, closely resembled high-affinity choline uptake by intact cholinergic nerve terminals. Choline transport could be driven by the Na+ gradient and by the transmembrane potential (inside negative) but did not directly require ATP. External Cl-, but not a Cl- gradient, was needed for choline transport activity. It is suggested that internal K+ plays a role in the retention of choline inside the proteoliposome. Proteoliposomes should prove a useful tool for both biochemical and functional studies of the high-affinity choline carrier.

Uptake of acetate and propionate by isolated nerve endings from the electric organ of Torpedo marmorata and their incorporation into choline esters

The uptake and incorporation into choline esters of acetate and propionate by electric organ synaptosomes were compared, with the aim of better understanding the basis for the selectivity of choline ester synthesis shown by this tissue for acetate. It was found that propionate uptake, like acetate uptake, was a temperature-dependent, saturable process. Both uptake mechanisms had similar affinities for their substrates, but the maximal velocity of propionate uptake was considerably lower than that of acetate uptake; and less of the accumulated propionate was used for choline ester synthesis than of the accumulated acetate. While acetate was a good inhibitor of propionate uptake, propionate was a very poor inhibitor of acetate uptake. This finding, in addition to the observation that the two uptakes were not affected in the same way by changes in pH, led to the suggestion that acetate uptake and propionate uptake reflect different processes. In both cases, however, the pH dependence of uptake indicated that these substrates cross the membrane as the charged species. Acetate uptake and acetylcholine synthesis remained closely associated under various experimental conditions, while propionate uptake could be dissociated from the synthesis of propionylcholine. Hence, it appears that acetate is taken up by a specific, high-velocity mechanism linked to acetylcholine synthesis, whereas propionate uptake may represent a less specific mechanism.

Increased uptake of [3H]choline by rat superior cervical ganglion: an effect of dexamethasone

The high affinity uptake of [3H]choline by the superior cervical ganglion, isolated from the rat, was found to be increased by dexamethasone. Maximal increase (60-65% above control values) occurred at the steroid concentration of 5 X 10(-5) M. Other glucocorticoids (triamcinolone, corticosterone and hydrocortisone) were without an effect on the [3H]choline uptake. Following administration of dexamethasone (25 mg/kg, i.p.), there was a marked increase in the level of choline in the ganglion. The increase was 3-fold at 1 hr and 10-fold at 6 hr, and by 24 hr the choline levels still remained higher in the steroid-treated animals than in the controls. Levels of acetylcholine in the ganglion were also increased, beginning at 1 hr after the injection of steroid. The increase was 85% by 3 hr and 60% by 6 hr. Triamcinolone, a glucocorticoid that was without an effect on [3H]choline uptake in vitro, was also ineffective in altering the levels of choline and acetylcholine in vivo. It seems probable that the increase of choline uptake in the ganglion induced by dexamethasone may, at least in part, occur in the preganglionic cholinergic terminals, leading to increased synthesis of acetylcholine. Such an effect of dexamethasone provides another case of a selective steroid acting directly on nerve terminals by altering a transport mechanism.

Increased acetylcholine synthesis and release following presynaptic activity in a sympathetic ganglion

The acetylcholine (ACh) content of sympathetic ganglia increases above its normal level following a period of preganglionic nerve stimulation. In the present experiments, this extra ACh that accumulates following activity was labeled radioactively from [3H]choline and its specific activity was compared with that of ACh subsequently released during preganglionic nerve stimulation. The specific activity released ACh was similar to that of the total tissue ACh, suggesting that the extra ACh mixes fully with endogenous stores. The present experiments also show that transmitter release during neuronal stimulation is necessary for the poststimulation increase in transmitter store, However, the increase was not evident when transmitter release was induced by K+. It is concluded that both transmitter release and impulse invasion of the nerve terminals are necessary for the adaptive phenomenon to manifest itself. The role of choline delivery and choline acetyltransferase activity in generating the poststimulation increase in transmitter store was tested. When choline transport activity measured as choline analogue (homocholine) accumulation increased. ACh synthesis was increased and when transport activity was not increased, neither was ACh synthesis. There was no poststimulation increase in measured choline acetyltransferase activity.

The synthesis, storage, and release of propionylcholine by the electric organ of Torpedo marmorata

Little is known about the specificity of the mechanisms involved in the synthesis and release of acetylcholine for the acetyl moiety. To test this, blocks of tissue from the electric organ of Torpedo were incubated with either [1-14C]acetate or [1-14C]propionate, and the synthesis, storage, and release of [14C]acetylcholine and [14C]propionylcholine were compared. To obtain equivalent amounts of the two labeled choline esters, a 50-fold higher concentration of propionate than of acetate was needed. Following subcellular fractionation, similar proportions of [14C]acetylcholine and [14C]propionylcholine were recovered with synaptosomes and with synaptic vesicles. Furthermore, both labeled choline esters were protected to a similar extent from degradation during homogenization of tissue in physiological medium, indicating that the two choline esters were equally well incorporated into synaptic vesicles. Yet depolarization of tissue blocks by 50 mM KCl released much less [14C]propionylcholine than [14C]acetylcholine. During field stimulation of the tissue blocks, the difference between the releasibility of the two choline esters was less marked, but acetylcholine was still released in preference to propionylcholine. Evidence for specificity of the release mechanism was also obtained when the release of the two choline esters in response to field stimulation was compared in tissue blocks preincubated with both [3H]choline and [14C]propionate.

Synthesis of acetylcholine from acetate in a sympathetic ganglion

The present experiments tested whether acetate plays a role in the provision of acetyl-CoA for acetylcholine synthesis in the cat's superior cervical ganglion. Labeled acetylcholine was identified in extracts of ganglia that had been perfused for 20 min with Krebs solution containing choline (10(-5) M) and [3H], [1-14C], or [2-14C]acetate (10(-3) M); perfusion for 60 min or with [3H]acetate (10(-2) M) increased the labeling. The acetylcholine synthesized from acetate was available for release by a Ca2+-dependent mechanism during subsequent periods of preganglionic nerve stimulation. When ganglia were stimulated via their preganglionic nerves or by exposure to 46 mM K+, the labeling of acetylcholine from [3H]acetate was reduced when compared with resting ganglia. The reduced synthesis of acetylcholine from acetate during stimulation was not due to acetate recapture, shunting of acetate into lipid synthesis, or the transmitter release process itself. In ganglia perfused with [2-14C]glucose, the amount of labeled acetylcholine formed was clearly enhanced during stimulation. An increase in acetylcholine labeling from [3H]acetate was shown during a 15-min resting period following a 60-min period of preganglionic nerve stimulation (20 Hz). It is concluded that acetate is not the main physiological acetyl precursor for acetylcholine synthesis in this sympathetic ganglion, and that during preganglionic nerve stimulation there is enhanced delivery of acetyl-CoA to choline acetyltransferase from a source other than acetate.