Subcellular origin of cholinergic transmitter release from mouse brain

Cytosolic choline acetyltransferase binds specifically to cholinergic plasma membrane of rat brain synaptosomes to generate membrane-bound enzyme

Uncovering the way membrane-bound choline acetyltransferase (ChAT) interacts with membranes and with which membrane in cholinergic neurons may help in understanding its role in acetylcholine metabolism. Subfractionation of rat hippocampal synaptosomes aiming to separate synaptic vesicles from plasma membranes shows that membrane-bound ChAT is bound to plasma membrane. Either detergents or urea and alkali can solubilize membrane-bound enzyme. Detergent-solubilized enzyme has a higher sedimentation rate than urea-alkali solubilized or cytosolic ChAT. Once dissociated, membrane-bound ChAT reassociates specifically with cholinergic plasma membranes, a process that was abolished by previous treatment of membranes with trypsin. Cytosolic ChAT behaves similarly. Thus, in cholinergic synaptosomes, ChAT exists as cytosolic and peripheral activity. Cytosolic ChAT generates peripheral enzyme most probably by interacting with a protein of plasma membrane of cholinergic nerve terminals. This "receptor" protein might regulate the amount of membrane-bound ChAT in cholinergic neurons.

Evidence to suggest that extracellular acetate is accumulated by rat hippocampal cholinergic nerve terminals for acetylcholine formation and release

It is well established that extracellular choline is transported into central cholinergic nerve terminals by 'high' and 'low' affinity processes to form the neurotransmitter acetylcholine (ACh). The intent of the present investigation was to ascertain whether extracellular acetate might also be transported into central cholinergic nerve terminals to form ACh. To test this possibility, rat hippocampal tissue was incubated with varying concentrations of extracellular [1-(14)C]acetate (0.1-100 microM) and the uptake of [1-(14)C]acetate and the amount of [14C]ACh formed by the tissue determined. The results indicated that the uptake of extracellular [1-(14)C]acetate was temperature-dependent and saturable having an apparent Michaelis constant (Km) of 22 microM. The formation of [14C]ACh in the tissue as a function of extracellular [1-(14)C]acetate appeared to occur by both 'high' and 'low' affinity processes with apparent Km values of 0.5 and 19.6 microM, respectively. In other experiments, three inhibitors (lithium, allicin and sodium) of acetyl CoA synthetase (EC 6.2.1.1 acetate: CoA ligase), the enzyme which converts acetate to acetyl CoA when ATP and CoA are present, inhibited [1-(14)C]acetate uptake and the amount of [14C]ACh formed from that [1-(14)C]acetate. Additionally, vesamicol, an inhibitor of ACh transport into synaptic vesicles, blocked the filling of a synaptic vesicle-enriched fraction of hippocampal tissue with newly synthesized [14C]ACh formed from extracellular [1-(14)C]acetate. High K+ depolarization of hippocampal tissue loaded with extracellular [1-(14)C]acetate not only increased the synthesis but also the release of [14C]ACh. These results suggest that extracellular acetate is recycled by rat hippocampal cholinergic nerve terminals for the formation and release of ACh. They also suggest that the enzyme acetyl CoA synthetase mediates extracellular acetate uptake into hippocampal cholinergic nerve terminals by metabolizing it to acetyl CoA and thereby creating a diffusion gradient for it to follow.

Reduced evoked release of acetylcholine in the rodent neocortex following traumatic brain injury

Neocortical acetylcholine (ACh) release was examined in awake, freely-moving rats at 14 days following lateral controlled cortical impact. Extracellular ACh was measured prior to and after an intraperitoneal administration of scopolamine, which evokes ACh release by blocking autoreceptors. At 14 days post-injury there was a significant reduction in scopolamine-evoked ACh release. The data suggest that neocortical cholinergic neurotransmission is chronically compromised, and may contribute to post-traumatic memory deficits.

Evidence to suggest that cytosolic acetylcholine in rat hippocampal nerve terminals is not directly transferred into synaptic vesicles for release

Rat hippocampal minces were loaded with [acetyl 1-14C]acetylcholine ([14C]ACh) in the presence of the "poorly penetrating" acetylcholinesterase (EC 3.1.1.7; AChE) inhibitor echothiophate and the effect of high K+ depolarization determined on the subcellular storage and release of [14C]ACh and its metabolites. Results indicated that high K+ did not augment the release of [14C]ACh. Rather, it increased the release of [14C]acetate while simultaneously reducing the level of [14C]ACh in the cytosolic (S3) fraction. When the identical experiment was performed with paraoxon, a "penetrating" AChE inhibitor, high K+ still did not increase the release of [14C]ACh. However, paraoxon prevented the K(+)-induced loss of [14C]ACh from the cytosolic fraction as well as the K(+)-induced gain of [14C]acetate in the release medium. When minces were loaded with [14C]ACh in the presence of echothiophate and subsequently subjected to high K+ depolarization in the absence or presence of vesamicol (AH5183; (-)-trans-2-[4-phenylpiperidino] cyclohexanol), a drug which blocks the refilling of synaptic vesicles with ACh, the amount of endogenous ACh released was reduced approximately 50%. Conversely, the amount of [14C]ACh released was not reduced at all. These results suggest that cytosolic ACh is not directly transported into synaptic vesicles for release when hippocampal nerve terminals are depolarized. Rather, its hydrolysis is accelerated in response to depolarization. A working hypothesis explaining the importance of the depolarization-induced breakdown of cytosolic ACh to central ACh metabolism is presented.

Basal and scopolamine-evoked release of hippocampal acetylcholine following traumatic brain injury in rats

This study employed in vivo microdialysis in awake, freely-moving Sprague-Dawley rats to examine acetylcholine (ACh) release in the dorsal hippocampus at 14 days following lateral controlled cortical impact. Extracellular levels of ACh were measured prior to and after an intraperitoneal administration of scopolamine (1 mg/kg), which evokes ACh release by blocking autoreceptors. At 14 days post injury there were no differences in basal ACh levels. However, injury produced a significant reduction in scopolamine-evoked ACh release. The data suggest that cholinergic neurotransmission remains chronically compromised, and thus may contribute to previously documented post traumatic spatial memory deficits.

Characterization of choline O-acetyltransferase (ChAT) in the BALB/C mouse spleen

The synthesizing enzyme, Choline-O-acetyl transferase (ChAT) (EC 2.3.1.6) and the degradation enzyme, acetylcholinesterase (EC 3.1.1.7) for the neurotransmitter acetylcholine, have been anatomically and biochemically characterized in the thymus of the BALB/C mouse. In the present study we continue to analyze the possibility of cholinergic immunomodulation of immune tissues by determining if ChAT is present in the BALB/C mouse spleen. Our enzymatic evaluation of ChAT activity in splenic extracts revealed .05 nmoles/min/mg protein as compared to .1 nmoles/min/mg of protein activity in controls prepared from whole brain extracts. No detectable levels of ChAT activity were observed in the serum. Immunoblotting and immunoprecipitating using the anti ChAT monoclonal antibody, MB16, demonstrated two bands in the brain and one band in the spleen. Membrane bound ChAT in the brain was composed of two subunits with apparent molecular weights of 28 and 50 kDa. The spleen demonstrated only one form of ChAT with an apparent molecular weight of 28 kDa. Immunoprecipitation of the enzyme from both the brain and spleen resulted in a recovery of 59% and 60% of the activity respectively.

Phosphorylation of rat brain choline acetyltransferase and its relationship to enzyme activity

Choline acetyltransferase catalyzes the formation of acetylcholine from choline and acetyl-CoA in cholinergic neurons. The present study examined conditions for modulation of kinase-mediated phosphorylation of this enzyme. By using a monospecific polyclonal rabbit anti-human choline acetyltransferase antibody to immunoprecipitate cytosolic and membrane-associated subcellular pools of enzyme from rat hippocampal synaptosomes, we determined that only the cytosolic fraction of the enzyme (67,000 +/- 730 daltons) was phosphorylated under basal, unstimulated conditions. The quantity of this endogenous phosphoprotein was dependent, in part, upon the level of intracellular calcium, with 32Pi incorporation into the enzyme in nerve terminals incubated in nominally calcium-free medium only 43 +/- 7% of control. The corresponding enzymatic activity of cytosolic choline acetyltransferase did not appear to be altered by lowered cytosolic calcium, whereas membrane-associated choline acetyltransferase activity was decreased to 58 +/- 11% of control. Depolarization of synaptosomes with 50 microM veratridine neither altered the extent of phosphorylation or specific activity of cytosolic choline acetyltransferase, nor induced detectable phosphorylation of membrane-associated choline acetyltransferase, although the specific activity of the membrane-associated enzyme was increased to 132 +/- 5% of control. In summary, phosphorylation of choline acetyltransferase does not appear to regulate cholinergic neurotransmission by a direct action on catalytic activity of the enzyme.

Evidence to suggest that the spontaneous release of acetylcholine from rat hippocampal tissue is carrier-mediated

The effect of L- and D-stereoisomers of 2-(4-phenylpiperidino) cyclohexanol (AH 5183) on the spontaneous release of acetylcholine (ACh) from rat hippocampal tissue was studied. L-AH 5183 was approximately 100 times more potent than was D-AH 5183 in reducing spontaneous ACh release. Spontaneous ACh release was also temperature dependent. These results may suggest that the spontaneous release of ACh from brain tissue is carrier-mediated.

Effect of external high potassium and pH on the uptake of choline in glial and neuronal cells in culture

The Vmax of the uptake of choline was increased in nerve cell cultures by lowering (from 7.4 to 6.5) or increasing (from 7.4 to 8.1) the pH. In neurons no effect was observed on the value of the Km's of the uptake of either the apparent high or low affinity components. In glial cells only a low affinity component was measured at pH 6.5 and diffusion was observed at pH 8.1. An excess of K+ ions in the incubation medium reproduced the increase in Vmax observed with changes in pH suggesting a possible dependence of the uptake of choline upon the H+ and OH- gradients. Taking into account the characteristics already known of the transport of choline into nerve cells, such a dependence adds new insight in the mechanisms underlying the transport and indicates another possible regulation of choline entry, eventually directed towards the synthesis of acetylcholine.

Veratridine-induced breakdown of cytosolic acetylcholine in rat hippocampal minces: an intraterminal form of acetylcholinesterase or choline O-acetyltransferase?

Rat hippocampal minces were loaded with N-methyl-[3H]acetylcholine ([3H]ACh) in the presence of the 'poorly penetrating' acetylcholinesterase (EC 3.1.1.7, AChE) inhibitor echothiophate and the effect of the depolarizing agent veratridine determined on the subcellular storage and release of [3H]ACh and [3H]choline. Results indicated that veratridine stimulated the release of [3H]ACh from a crude vesicular fraction (P3) by a Ca2+-dependent process, while simultaneously accelerating the breakdown of cytosolic (S3) [3H]ACh. A portion of the [3H]choline derived from the hydrolyzed S3 [3H]ACh was donated to the P3 fraction for [3H]ACh formation and release. When the identical experiment was done using hippocampal minces from septal lesioned rats, veratridine did not stimulate either the Ca2+-dependent release of [3H]ACh or the hydrolysis of cytosolic [3H]ACh. Incubation of control hippocampal minces with paraoxon, an AChE inhibitor which can penetrate cholinergic nerve terminals more rapidly than echothiophate, prevented veratridine from stimulating the Ca2+-dependent release of [3H]ACh from the P3 fraction. Instead, it then stimulated the Ca2+-independent release of [3H]ACh from the S3 fraction. When minces were incubated with the choline O-acetyltransferase (EC 2.3.1.6, ChAT) inhibitor 4-(1-naphthyl)vinyl pyridine (NVP), veratridine was no longer able to stimulate the Ca2+-dependent release of labelled ACh either. Instead, veratridine stimulated the Ca2+-independent release of labelled ACh from the S3 fraction. NVP also abolished the veratridine-induced, Ca2+-dependent release of total ACh. Both paraoxon and NVP inhibited the reversible reaction of ionically bound ChAT prepared from rat brain when tested in vitro, yet paraoxon was much less potent than NVP, and was unable to inhibit this reaction at the low concentration which prevented the veratridine induced breakdown of S3 [3H]ACh during mince incubation. Veratridine depolarization of hippocampal minces stimulated the activity of a membrane-bound fraction of ChAT associated with the P3 fraction, but this fraction of ChAT did not become more sensitive to inhibition by paraoxon during tissue incubation. Veratridine depolarization of minces also increased the activity of membrane-bound AChE, but this enzyme was not inhibited by the low NVP concentration which prevented the veratridine-induced breakdown of S3 [3H]ACh. The veratridine-induced increase in membrane-bound ChAT activity was dependent on the presence of extracellular Ca2+ in the incubation medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Membrane-bound choline acetyltransferase from human brain: purification and properties

Choline acetyltransferase (ChAT; EC 2.3.1.6) was separated from human caudate/putamen into three fractions by successive extractions into a potassium phosphate buffer, a high salt (NaCl) buffer and a buffer containing 0.6% Triton X-100. The Triton-X-solubilized fraction is the membrane-bound ChAT (mChAT) and represents about 40% of the total ChAT. After centrifugation, mChAT was precipitated by ammonium sulfate at 35-65% saturation. The crude enzyme preparation was fractionated in turn on a DEAE-Sepharose, a hydroxylapatite and a phosphocellulose columns. Finally, mChAT was applied to a CoA-Sepharose column equilibrated with buffer containing 100 mM choline chloride and was specifically eluted with buffer containing acetyl-CoA. The presence of both substrates greatly stabilized the enzyme and ChAT was recovered almost quantitatively. The final preparation of mChAT has a specific activity of 37.2 mumol of acetylcholine synthesized per min-mg protein. The purified mChAT has a pH optimum of 8.3. It migrated as two bands on SDS-PAGE with molecular weights of 67,000 and 62,000 daltons, respectively. Immunoblot autoradiography showed that an antiserum prepared previously against soluble ChAT also cross-reacted with both bands of mChAT, indicating that both forms of this enzyme are related. Furthermore, as previously reported for soluble ChAT, Fab-Sepharose chromatography could be used for the purification of mChAT and this preparation also resolved into two bands on 10% SDS gel.

The effect of the acetylcholine transport blocker 2-(4-phenylpiperidino) cyclohexanol (AH5183) on the subcellular storage and release of acetylcholine in mouse brain

The effect of the acetylcholine (ACh) transport blocker 2-(4-phenylpiperidino) cyclohexanol (AH5183) on the subcellular storage and release of acetylcholine was studied in mouse forebrain. Results indicated that AH5183 reduced the amount of ACh released from mouse forebrain minces by high K+ and veratridine over the identical concentration range as it inhibits the active transport of ACh into synaptic vesicles isolated from the electric organ of Torpedo. However, AH5183 did not block the K+- or veratridine-induced reduction of cytoplasmic (S3) ACh. Also, it did not block the loss of vesicular (P3) ACh caused by these depolarizing agents. It did, however, cause a disappearance of nerve ending ACh which was partially matched by a selective gain in the choline content of the P3 fraction. When minces of mouse forebrain were pretreated in high K+ to deplete the S3 and P3 fractions of their ACh content and then subsequently incubated in normal Krebs with [14C]choline, AH5183, at a concentration which reduces ACh release by 50%, did not affect the repletion of P3 stores with newly synthesized [14C]ACh. At somewhat higher concentrations, however, AH5183 reduced the amount of [14C]ACh in the P3 fraction without affecting the amount of [14C]ACh in the S3 fraction. At these concentrations it did not inhibit extracellular choline transport or ChAT activity. These results suggest that AH5183 may reduce the amount of ACh released from central cholinergic nerve terminals in response to depolarization through a combination of effects: (1) it may facilitate the breakdown or loss of ACh stored in the vesicular fraction; (2) it may also block the transport of newly synthesized ACh into the vesicular fraction.

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)

Membrane-bound choline acetyltransferase in Torpedo electric organ: a marker for synaptosomal plasma membranes?

The enzyme choline-O-acetyltransferase catalyses the biosynthesis of acetylcholine from acetyl coenzyme A and choline and is considered as one of the best markers for cholinergic nerve endings. The distribution of this enzymatic activity was analysed during the purification of plasma membranes of purely cholinergic nerve endings isolated from the electric organ of the fish Torpedo marmorata. This tissue, which receives a profuse and purely cholinergic innervation, can be considered as being a "giant" neuromuscular synapse. The isolated nerve endings (synaptosomes) were first osmotically disrupted and their plasma membranes isolated by equilibrium density centrifugation (discontinuous followed by continuous sucrose gradients). Choline acetyltransferase activity was found to exist in three forms: (1) a soluble form (the major one) present in the cytoplasm of the nerve endings, (2) a form which is ionically associated with membranes and which can be solubilized by washing exhaustively the membrane fraction with solutions of high ionic strength (0.5 M NaCl) and (iii) a form which is non-ionically bound to membranes and cannot be solubilized with high salt solution. The soluble and the non-ionically bound activities exhibited very similar affinities for choline (1.34 and 1.64 mM, respectively). The non-ionically membrane-associated form of choline acetyltransferase was found to "copurify" with the cholinergic synaptosomal plasma membranes of Torpedo, its specific activity being increased from 122 (crude fraction) to 475 (purified membrane fraction) nmol/h/mg protein. An enrichment was also observed for another cholinergic marker, the enzyme acetylcholinesterase, but not for the nicotinic receptor to acetylcholine, a marker for postsynaptic membranes. No choline acetyltransferase activity could be detected in preparations of synaptic vesicles that were highly purified from the electric organ. Also, the non-ionically associated form of choline acetyltransferase activity was hardly detectable (2.4 nmol/h/mg protein) in fractions enriched in axonal membranes prepared from the cholinergic electric nerves innervating the electric organ. The partition into soluble and membrane-bound activity was also analysed for choline acetyltransferase present in human placenta, a rich source for the enzyme but a non-innervated tissue. In this case the great majority of the enzyme appeared as soluble activity. Very low levels of non-ionically membrane-bound activity were found to be present in a crude membrane fraction from human placenta (2.8 nmol/h/mg protein).(ABSTRACT TRUNCATED AT 400 WORDS)

Veratridine-induced release of acetylcholine from mouse forebrain minces: dependence on the hydrolysis of cytoplasmic acetylcholine for a source of choline

The importance of depolarization induced hydrolysis of cytoplasmic acetylcholine (ACh) in providing choline for the veratridine-and high K+-induced release of acetylcholine was studied in mouse forebrain minces. Results indicated that a loss of hydrolyzable cytoplasmic ACh prior to depolarization reduced the amount of ACh released by veratridine but not the amount released by high K+. The reduction in the veratridine-induced release of ACh did not occur during the first 5 min of incubation. Loss of vesicular ACh prior to depolarization reduced both the veratridine- and K+-induced release of ACh during the first 5 min of incubation. Blockade of extra-cellular choline transport by hemicholinium (HC-3) did not affect the veratridine-induced release of ACh during a 10 min incubation period unless the cytoplasmic pool of ACh had first been depleted and was unavailable as a source of choline. In contrast, HC-3 reduced the K+-induced release of ACh from brain tissue with normal stores of cytoplasmic ACh. These results indicate that both depolarizing agents primarily stimulate the release of preformed ACh from a vesicular fraction during the first 5 min of mince incubation. Thereafter, they both stimulate the release of newly synthesized ACh, however, they differ in one important respect. The principal source of choline for the veratridine-induced release of newly synthesized ACh appears to be the cytoplasmic pool of ACh, whereas the major source of choline for the K+-induced release of newly synthesized ACh appears to be extracellular choline.

Uptake, Metabolism, and Releasability of Ethyl Analogues of Homocholine by Rat Brain

Ethyl analogues of homocholine were synthesized and used to describe further the specificities of the processes involved in choline uptake and acetylation and acetylcholine storage and release. Monoethylhomocholine, diethylhomocholine, and triethylhomocholine decreased the transport of choline into rat brain synaptosomes. The mono- and diethyl compounds were taken up into synaptosomes with similar affinity for the transport system as choline (5.8, 8.5, and 5.5 microM, respectively) but at a somewhat slower rate (11.3, 8.5, and 37.3 nmol/g original tissue/h, respectively); the triethyl analogue was not transported at the concentrations tested, which further defines the structural specificity of the transport system. L-Carnitine did not affect the transport of the analogues. The in situ acetylation of mono- and diethylhomocholine by slices of rat cerebral cortex was measurable, but the in vitro acetylation by choline acetyltransferase solubilized from rat forebrain was not. Acetylation of the diethyl analogue by slices of cerebellar cortex was less than 20% of that by slices of cerebral cortex. Subcellular fractionation of cerebral slices showed that acetyldiethylhomocholine localized preferentially to the cytosolic rather than vesicular stores, indicating specificity of the mechanism responsible for the incorporation of acetylated product into the vesicles. The release of acetyldiethylhomocholine and of acetylcholine was tested from sliced brain that had been incubated with the precursors. Both esters were released spontaneously but stimulation with increased K+ concentration enhanced the release of acetylcholine without changing the release of acetyldiethylhomocholine, suggesting that evoked transmitter release occurred from a vesicular store.

Depolarization of mouse forebrain minces with veratridine and high K+: failure to stimulate the Ca2+ independent, spontaneous release of acetylcholine from the cytoplasm due to hydrolysis of the acetylcholine stored there

Both high K+ and veratridine, depolarizing agents with different mechanisms of action, lowered the ACh content of the cytoplasmic (S3) fraction of mouse forebrain minces incubated in a Ca2+-free Krebs solution, without stimulating ACh release or altering the level of ACh in the vesicle-bound (P3) fraction. Veratridine increased the level of choline in the P3 fraction by the same amount as it reduced the level of ACh in the S3 fraction, and these changes did not occur in the presence of tetrodotoxin (TTX). Pretreatment of minces in normal Krebs increased the ACh but not the choline content of the S3 fraction. Following this expansion of the S3 ACh content, veratridine caused an even greater loss of S3 ACh, and increased the Ca2+-independent release of ACh slightly. Under these conditions, veratridine also stimulated the Ca2+ independent release of choline, and this increase exceeded that obtained for the Ca2+-independent release of ACh. Preincubation in normal Krebs with paraoxon did not alter the S3 ACh content after 5 min, but raised it by 78% after 30 min. Under the latter conditions of pretreatment, veratridine then stimulated the Ca2+-independent release of ACh even more, but did not stimulate the release of choline. These results suggest that depolarization of brain tissue does not facilitate the Ca2+-independent release of ACh from the cytoplasm because a portion of ACh stored there is hydrolyzed. When the cytoplasmic level of ACh is sufficiently elevated prior to depolarization, then some ACh escapes hydrolysis and is released independently of Ca2+. It is suggested that the depolarization-induced hydrolysis of cytoplasmic ACh may be mediated by an intraterminal form of AChE and may, in addition to the hydrolysis of extracellular ACh, provide substrate for the formation and release of ACh by the vesicle-bound fraction.

Spontaneous release of acetylcholine and acetylhomocholine from mouse forebrain minces: cytoplasmic or vesicular origin

The objective of this study was to determine the subcellular origin of cholinergic transmitter released spontaneously from mouse forebrain minces. To accomplish this objective, minces were pretreated in ionic media and then loaded with [14C]homocholine, an analog of choline, to form the false transmitter [14C]acetylhomocholine [( 14C]AHCh). The ratio of the false transmitter [14C]AHCh to the true transmitter ACh was then used as an index of cholinergic transmitter contents for both the cytoplasmic (S3) and vesicle-bound (P3) fractions. Three different pretreatment procedures were used to cause the following changes in S3 and P3 false to true transmitter ratios prior to spontaneous release: 1) a small increase in the S3 ratio of [14C]AHCh to acetylcholine (ACh) and a large increase in the P3 ratio of [14C] AHCh to ACh; 2) a decrease in the S3 ratio of [14C]AHCh to ACh and an increase in the P3 ratio of [14C]AHCh to ACh; 3) an increase in the P3 ratio of [14C]AHCh to ACh without affecting the S3 ratio of [14C]AHCh to ACh. The influence of each pretreatment on these subcellular ratios was then compared with its influence on the spontaneous release ratio of [14C]AHCh to ACh. In all 3 instances, the influence of pretreatment on the ratio of spontaneously released false and true cholinergic transmitters from minces coincided with the effect of pretreatment on the pre-release ratio of false to true transmitter in the S3 fraction. These results suggest that much of the cholinergic transmitter which is spontaneously released from mouse forebrain occurs from the cytroplasmic fraction.

Multiple forms of choline-O-acetyltransferase in mouse and rat brain: solubilization and characterization

Three forms of acetyl coenzyme A: choline-O-acetyltransferase (EC 2.3.1.6, ChAT) have been isolated from mouse and rat forebrain synaptosomes with a 100 mM sodium phosphate (NaP) buffer of pH 7.4, a high-salt solution (500 mM NaCl), and a 2% Triton DN-65 solution, respectively. The Triton-solubilized form of ChAT differed from the other two forms in its capacity to acetylate homocholine, its pH profile, and its sensitivity to denaturation. NaCl-solubilized ChAT could be distinguished from the other two forms with respect to pH profile, sensitivity to inhibition by 4-(1-naphthylvinyl) pyridine (in the presence of Triton), and apparent Km value for choline acetylation. The caudate and putamen of rat brain contained the highest amount of ChAT activity, based on tissue wet weight, and the cerebellum contained the least of the brain regions examined; only the cerebellum had more membrane-bound than soluble ChAT. Septal lesion reduced ChAT activity in the NaP- and Triton-solubilized fractions prepared from hippocampus by 68% and 64%, respectively, whereas it reduced the activity of the NaCl-solubilized fraction by only 21%. These results suggest that three different forms of ChAT may exist in both mouse and rat brain.

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.

Acetylation of choline and homocholine by membrane-bound choline-O-acetyltransferase in mouse forebrain nerve endings

The choline analog homocholine is not acetylated in vitro by choline-O-acetyltransferase (ChAT, EC 2.3.1.6), which is solubilized by 100 mM-sodium phosphate buffer washes of a crude vesicular fraction of mouse forebrain. However, both homocholine and choline are acetylated by a form of ChAT which is nonionically associated with a subcellular fraction of mouse forebrain containing membrane-associated organelles and occluded acetylcholine (P4). Acetylation of homocholine by membrane-associated ChAT is saturable. 4-(1-Naphthylvinyl)pyridine (NVP) inhibits the acetylation of both choline (60%) and homocholine (40%) by membrane-associated ChAT but reduces the acetylation of choline alone by soluble ChAT (76%). Choline and homocholine serve as competitive alternative substrates for the same membrane-associated ChAT, whereas homocholine acts only as a competitive inhibitor of choline acetylation by soluble ChAT. Acetylhomocholine competitively inhibits the acetylation of choline by both soluble and membrane-associated ChAT more dramatically than does the natural end product, acetylcholine.