Choline metabolism in the cerebral cortex of guinea pigs. Stable-bound acetylcholine

Two distinct profiles of fMRI and neurophysiological activity elicited by acetylcholine in visual cortex

fMRI changes are typically assumed to be due to changes in neural activity, although whether this remains valid under the influence of neuromodulators is relatively unknown. Here, we found evidence that intracortical acetylcholine elicits distinct profiles of fMRI and electrophysiological activity in visual cortex. Two patterns of cholinergic activity were observed, depending on the distance to the injection site, although neurovascular coupling was preserved. Our results illustrate the effects of neuromodulators on fMRI and electrophysiological responses and show that these depend on neuromodulator concentration and kinetics.

Cholinergic neuromodulation is involved in all aspects of sensory processing and is crucial for processes such as attention, learning and memory, etc. However, despite the known roles of acetylcholine (ACh), we still do not how to disentangle ACh contributions from sensory or task-evoked changes in functional magnetic resonance imaging (fMRI). Here, we investigated the effects of local injection of ACh on fMRI and neural signals in the primary visual cortex (V1) of anesthetized macaques by combining pharmaco-based MRI (phMRI) with electrophysiological recordings, using single electrodes and electrode arrays. We found that local injection of ACh elicited two distinct profiles of fMRI and neurophysiological activity, depending on the distance from the injector. Near the injection site, we observed an increase in the baseline blood oxygen-level-dependent (BOLD) and cerebral blood flow (CBF) responses, while their visual modulation decreased. In contrast, further from the injection site, we observed an increase in the visually induced BOLD and CBF modulation without changes in baseline. Neurophysiological recordings suggest that the spatial correspondence between fMRI responses and neural activity does not change in the gamma, high-gamma, and multiunit activity (MUA) bands. The results near the injection site suggest increased inhibitory drive and decreased metabolism, contrasting to the far region. These changes are thought to reflect the kinetics of ACh and its metabolism to choline.

Vesicular integrity in Parkinson's disease

The defining motor characteristics of Parkinson's disease (PD) are mediated by the neurotransmitter dopamine (DA). Dopamine molecules spend most of their lifespan stored in intracellular vesicles awaiting release and very little time in the extracellular space or the cytosol. Without proper packaging of transmitter and trafficking of vesicles to the active zone, dopamine neurotransmission cannot occur. In the cytosol, dopamine is readily oxidized; excessive cytosolic dopamine oxidation may be pathogenic to nigral neurons in PD. Thus, factors that disrupt vesicular function may impair signaling and increase the vulnerability of dopamine neurons. This review outlines the many mechanisms by which disruption of vesicular function may contribute to the pathogenesis of PD. From direct inhibition of dopamine transport into vesicles by pharmacological or toxicological agents to alterations in vesicle trafficking by PD-related gene products, variations in the proper compartmentalization of dopamine can wreak havoc on a functional dopamine pathway. Findings from patient populations, imaging studies, transgenic models, and mechanistic studies will be presented to document the relationship between impaired vesicular function and vulnerability of the nigrostriatal dopamine system. Given the deleterious effects of impaired vesicular function, strategies aimed at enhancing vesicular function may be beneficial in the treatment of PD.

Synaptic vesicle recycling: genetic and cell biological studies

We review mainly the work from our research group here. Our focus has been on the use of genetic methods to delineate the mechanisms of synaptic vesicle recycling and cellular trafficking. Acute temperature-sensitive paralytic mutants have been of particular value in this approach. We have primarily used screens for suppressor and enhancer mutations to identify genetic loci coding for proteins that interact with Dynamin in Drosophila. In addition, we have used reverse genetic approaches to investigate few other candidate molecules that may play a role in synaptic vesicle endocytosis. We have in particular discussed at some length the role of endocytic accessory proteins Stoned and Eps15 in vesicle recycling.

Three-photon microscopy shows that somatic release can be a quantitatively significant component of serotonergic neurotransmission in the mammalian brain

Recent experiments on monoaminergic neurons have shown that neurotransmission can originate from somatic release. However, little is known about the quantity of monoamine available to be released through this extrasynaptic pathway or about the intracellular dynamics that mediate such release. Using three-photon microscopy, we directly imaged serotonin autofluorescence and investigated the total serotonin content, release competence, and release kinetics of somatic serotonergic vesicles in the dorsal raphe neurons of the rat. We found that the somata of primary cultured neurons contain a large number of serotonin-filled vesicles arranged in a perinuclear fashion. A similar distribution is also observed in fresh tissue slice preparations obtained from the rat dorsal raphe. We estimate that the soma of a cultured neuron on an average contains about 9 fmoles of serotonin in about 450 vesicles (or vesicle clusters) of < or =370 nm average diameter. A substantial fraction (>30%) of this serotonin is released with a time scale of several minutes by K(+)-induced depolarization or by para-chloroamphetamine treatment. The amount of releasable serotonin stored in the somatic vesicles is comparable to the total serotonin content of all the synaptic vesicles in a raphe neuron, indicating that somatic release can potentially play a major role in serotonergic neurotransmission in the mammalian brain.

The synaptic vesicle cycle

Neurotransmitter release is mediated by exocytosis of synaptic vesicles at the presynaptic active zone of nerve terminals. To support rapid and repeated rounds of release, synaptic vesicles undergo a trafficking cycle. The focal point of the vesicle cycle is Ca2+-triggered exocytosis that is followed by different routes of endocytosis and recycling. Recycling then leads to the docking and priming of the vesicles for another round of exo- and endocytosis. Recent studies have led to a better definition than previously available of how Ca2+ triggers exocytosis and how vesicles recycle. In particular, insight into how Munc18-1 collaborates with SNARE proteins in fusion, how the vesicular Ca2+ sensor synaptotagmin 1 triggers fast release, and how the vesicular Rab3 protein regulates release by binding to the active zone proteins RIM1 alpha and RIM2 alpha has advanced our understanding of neurotransmitter release. The present review attempts to relate these molecular data with physiological results in an emerging view of nerve terminals as macromolecular machines.

Synaptotagmin 7 splice variants differentially regulate synaptic vesicle recycling

The speed of synaptic vesicle recycling determines the efficacy of neurotransmission during repetitive stimulation. Synaptotagmins are synaptic C(2)-domain proteins that are involved in exocytosis, but have also been linked to endocytosis. We now demonstrate that upon expression in transfected neurons, a short splice variant of synaptotagmin 7 that lacks C(2)-domains accelerates endocytic recycling of synaptic vesicles, whereas a longer splice variant that contains C(2)-domains decelerates recycling. These results suggest that alternative splicing of synaptotagmin 7 acts as a molecular switch, which targets vesicles to fast and slow recycling pathways.

Limited numbers of recycling vesicles in small CNS nerve terminals: implications for neural signaling and vesicular cycling

The tiny nerve terminals of central synapses contain far fewer vesicles than preparations commonly used for analysis of neurosecretion. Photoconversion of vesicles rendered fluorescent with the dye FM1-43 directly identified vesicles capable of engaging in exo-endocytotic recycling following stimulated Ca(2+) entry. This recycling pool typically contained 30-45 vesicles, only a minority fraction (15-20% on average) of the total vesicle population. The smallness of the recycling pool would severely constrain rates of quantal neurotransmission if classical pathways were solely responsible for vesicle recycling. Fortunately, vesicles can undergo rapid retrieval and reuse in addition to conventional slow recycling, to the benefit of synaptic information flow and neuronal signaling.

Effects of calyculin A and okadaic acid on acetylcholine release and subcellular distribution in rat hippocampal formation

The mechanisms regulating the compartmentation of acetylcholine (ACh) and the relationship between transmitter release and ACh stores are not fully understood. In the present experiments, we investigated whether the inhibitors of serine/threonine phosphatases 1 and 2A, calyculin A and okadaic acid, alter subcellular distribution and the release of ACh in rat hippocampal slices. Calyculin A and okadaic acid significantly (p < 0.05) depleted the occluded ACh of the vesicular P3 fraction, but cytoplasmic ACh contained in the S3 fraction was not significantly affected. The P3 fraction is known to be heterogeneous; calyculin A and okadaic acid reduced significantly (p < 0.05) the amount of ACh recovered with a monodispersed fraction (D) of synaptic vesicles, but the other nerve terminal bound pools (E-F and G-H) were not so affected. K+-evoked ACh release decreased significantly (p < 0.01) in the presence of calyculin A and okadaic acid, suggesting that fraction D's vesicular store of ACh contributes to transmitter release. The loss of ACh from synaptic vesicle fractions prepared from tissue exposed to phosphatase inhibitors appeared not to result from a reduced ability to take up ACh. Thus, when tissue was allowed to synthesize [3H]ACh from [3H]choline, the ratio of [3H]ACh in the S3 to P3 fractions was not much changed by exposure of tissue to calyculin A or okadaic acid; furthermore, the specific activity of ACh recovered from the D fraction was not reduced disproportionately to that of cytosolic ACh. The changes are considered to reflect reduced synthesis of ACh by tissue treated with the phosphatase inhibitors, rather than an effect on vesicle uptake mechanisms. Thus, exposure of tissue to calyculin A or okadaic acid appears to produce selective depletion of tissue ACh content in a subpopulation of synaptic vesicles, suggesting that phosphatases play a role in ACh compartmentation.

Factors governing the appearance of small-mode miniature endplate currents at the snake neuromuscular junction

At the snake neuromuscular junction, following a short period of high frequency motor nerve stimulation, a population of 'small-mode' miniature endplate currents (MEPCs) is seen. These small-mode MEPCs are believed to be due to the release of acetylcholine from incompletely refilled recycling synaptic vesicles [Searl et al., Neuroscience, 35 (1991) 145-156]. This study determines the role of the trans-vesicular membrane proton gradient in the generation of small-mode MEPCs. Preserving the trans-vesicular membrane proton gradient during synaptic vesicle exocytosis by exposure of snake nerve/muscle preparations to an extracellular pH approximately equal to the intravesicular pH, leads to an augmentation of the amplitude of the stimulation-induced small-mode MEPCs. Conversely, pretreatment of snake neuromuscular preparations with 10 mM ammonium ions, to buffer intravesicular protons and hence dissipate the trans-vesicular membrane proton gradient, leads to an inhibition of the stimulation-induced appearance of small-mode MEPCs. The data are consistent with the theory that the reduced acetylcholine content of recycled synaptic vesicles in the snake is a consequence of an incomplete restoration of the trans-vesicular membrane proton gradient following synaptic vesicle exocytosis. Thus, in recycling synaptic vesicles the limiting step in the refilling process appears to be generation of the trans-membrane proton gradient and not the transport of acetylcholine into the vesicle.

Biphasic striatal acetylcholine release during and after transient cerebral ischemia in gerbils

Acetylcholine (ACh) release into the extracellular space was measured by HPLC with electrochemical detection after in vivo intracerebral microdialysis in the striatum of gerbils subjected to 15 min of bilateral carotid artery occlusion followed by 5 h of recirculation. Tissue ACh and choline (Ch) contents were also determined during ischemia and after 5, 30, 60, and 120 min of reflow. Fifteen minutes of ischemia led to a significant transient increase in extracellular ACh concentration (threefold after 7.5 min of ischemia) concomitant with a reduced endogenous ACh level (-62%) and increased tissue Ch content (ninefold). Recirculation significantly reduced the ACh release during the early period of reflow (-50% vs. basal level), followed by a significant increase in ACh release between 1 and 3 h of reflow (45-55% vs. basal level) and subsequent normalization. Simultaneously, a "rebound" of tissue ACh level occurred in the early period of reflow (fourfold vs. ischemic value), followed by gradual normalization after 2 h of reperfusion, whereas a rapid decrease in tissue Ch levels was found after 30 min of reflow. These findings represent the first demonstration of a biphasic release of ACh during ischemia and reperfusion, as assessed by intracerebral microdialysis in gerbils.

Acetylcholine transport, storage, and release

ACh is released from cholinergic nerve terminals under both resting and stimulated conditions. Stimulated release is mediated by exocytosis of synaptic vesicle contents. The structure and function of cholinergic vesicles are becoming known. The concentration of ACh in vesicles is about 100-fold greater than the concentration in the cytoplasm. The AChT exhibits the lowest binding specificity among known ACh-binding proteins. It is driven by efflux of protons pumped into the vesicle by the V-type ATPase. A potent pharmacology of the AChT based on the allosteric VR has been developed. It has promise for clinical applications that include in vivo evaluation of the density of cholinergic innervation in organs based on PET and SPECT. The microscopic kinetics model that has been developed and the very low transport specificity of the vesicular AChT-VR suggest that the transporter has a channel-like or multidrug resistance protein-like structure. The AChT-VR has been shown to be tightly associated with proteoglycan, which is an unexpected macromolecular relationship. Vesamicol and its analogs block evoked release of ACh from cholinergic nerve terminals after a lag period that depends on the rate of release. Recycling quanta of ACh that are sensitive to vesamicol have been identified electrophysiologically, and they constitute a functional correlate of the biochemically identified VP2 synaptic vesicles. The concept of transmitter mobilization, including the observation that the most recently synthesized ACh is the first to be released, has been greatly clarified because of the availability of vesamicol. Differences among different cholinergic nerve terminal types in the sensitivity to vesamicol, the relative amounts of readily and less releasable ACh, and other aspects of the intracellular metabolism of ACh probably are more apparent than real. They easily could arise from differences in the relative rates of competing or sequential steps in the complicated intraterminal metabolism of ACh rather than from fundamental differences among the terminals. Nonquantal release of ACh from motor nerve terminals arises at least in part from the movement of cytoplasmic ACh through the AChT located in the cytoplasmic membrane, and it is blocked by vesamicol. Possibly, the proteoglycan component of the AChT-VR produces long-term residence of the macromolecular complex in the cytoplasmic membrane through interaction with the synaptic matrix. The preponderance of evidence suggests that a significant fraction of what previously, heretofore, had been considered to be nonquantal release from the motor neuron actually is quantal release from the neuron at sites not detected electrophysiologically.(ABSTRACT TRUNCATED AT 400 WORDS)

Mobilization of the readily releasable pool of acetylcholine from a sympathetic ganglion by tityustoxin in the presence of vesamicol

The present experiments tested whether preganglionic stimulation and direct depolarization of nerve terminals by tityustoxin could mobilize similar or different pools of acetylcholine (ACh) from the cat superior cervical ganglia in the presence of 2-(4-phenylpiperidino)cyclohexanol (vesamicol, AH5183), an inhibitor of ACh uptake into synaptic vesicles. In the absence of vesamicol, both nerve stimulation and tityustoxin increased ACh release. In the presence of vesamicol, the release of ACh induced by tityustoxin was inhibited, and just 16% of the initial tissue content could be released, a result similar to that obtained with electrical stimulation under the same condition. When the impulse-releasable pool of ACh had been depleted, tityustoxin still could release transmitter, amounting to some 10% of the ganglion's initial content. This pool of transmitter seemed to be preformed in the synaptic vesicles, rather than synthesized in response to stimuli, as tityustoxin could not release newly synthesized [3H]ACh formed in the presence of vesamicol, and hemicholinium-3 did not prevent the toxin-induced release. In contrast to the results with tityustoxin, preganglionic stimulation could not release transmitter when impulse-releasable or toxin-releasable compartments had been depleted. Our results confirm that vesamicol inhibits the mobilization of transmitter from a reserve to a more readily releasable pool, and they also suggest that, under these experimental conditions, there might be some futile transmitter mobilization, apparently to sites other than nerve terminal active zones.

Optical analysis of synaptic vesicle recycling at the frog neuromuscular junction

The fluorescent dyes FM1-43 and RH414 label motor nerve terminals in an activity-dependent fashion that involves dye uptake by synaptic vesicles that are recycling. This allows optical monitoring of vesicle recycling in living nerve terminals to determine how recycled vesicles reenter the vesicle pool. The results suggest that recycled vesicles mix with the pool morphologically and functionally. One complete cycle of release of transmitter, recycling of a vesicle, and rerelease of transmitter appears to take about 1 minute.

Effects of potassium depolarization on intracellular compartmentalization of ATP in cholinergic synaptosomes isolated from Torpedo electric organ

It is well known that acetylcholine (ACh) and ATP are co-stored and co-released in nerve terminals of the electric organ of Torpedo. Cholinergic synaptosomes were subjected to a cycle of freezing and thawing showing that ATP is distributed in two operational pools like those described for ACh. The bound pool is resistant to freezing and thawing, and it is presumably protected by membranes. When metabolically active ATP was prelabelled with [3H]adenosine, 76% of the radioactivity was associated with the free pool of ATP. When the preparation was depolarized in a calcium containing medium, there was a decrease in the specific radioactivity of ATP in the free pool and an increase in the bound pool. These results reflect that the patterns of distribution of ACh and ATP, in this synaptosomal preparation, are similar in resting conditions and during K+ depolarization.

Characterization of prejunctional muscarinic autoreceptors in the guinea-pig trachea

1. The effects of ten muscarinic antagonists on electrically evoked [3H]-acetylcholine release and muscle contraction were compared in an epithelium-free preparation of the guinea-pig trachea that had been preincubated with [3H]-choline. 2. The M3-selective antagonists UH-AH 37, 4-diphenyl-acetoxy-N-piperidine methobromide and para-fluorohexahydrosiladiphenidol were more potent in reducing the contractile response than in facilitating the evoked [3H]-acetylcholine release. Hexahydrosiladiphenidol did not discriminate between pre- and postjunctional effects. The rank order of the postjunctional potencies of the ten antagonists as well as the postjunctional pA2 values obtained for hexahydrosiladiphenidol (7.95) and AQ-RA (7.08) identified the muscular receptor as an M3 subtype. 3. The M2-selective antagonists methoctramine, AF-DX 116 and AQ-RA 741 were more potent in facilitating the evoked [3H]-acetylcholine release than in inhibiting the contractile response. The increase in release by low concentrations of methoctramine, AF-DX 116 and AQ-RA 741 was paralleled by an enhancement of the stimulation-evoked contractions. 4. Comparison of the pre- and postjunctional potencies of the M1-, M2- and M3-selective antagonists suggests that autoinhibition of acetylcholine release is mediated via an 'M2-like' receptor which differs from the cardiac type M2 receptor in its relatively high affinity for hexahydrosiladiphenidol.

Cholinergic synaptic vesicles are metabolically and biophysically heterogeneous even in resting terminals

The metabolic heterogeneity of synaptic vesicles in the cholinergic nerve terminals of the electromotor neurons of Torpedo marmorata has been studied in resting tissue by evaluating the molecular acetylcholine content (MAC) of synaptic vesicles after extraction from frozen and crushed tissue and high-resolution centrifugal density gradient separation in a zonal rotor. Although vesicular acetylcholine was distributed in the gradient as a single, more or less symmetrical peak, 3 subpopulations of synaptic vesicles could be identified: a small, relatively light subpopulation of low MAC on the ascending limb of the acetylcholine peak, designated V0, a main population of fully charged vesicles designated V1, and a small, denser subpopulation also of low MAC on the descending limb of the acetylcholine peak, designated V2. The mean proportions and MACs of the 3 pools were: V0, 13%, 58,000; V1, 53%, 246,000; V2, 34%, 79,000. When tritiated acetate was perfused through excised blocks of electric organ for 1-2 h before vesicle isolation, the specific radioactivity of the acetylcholine in the V0 and V2 pools was 10-30 times higher than in the V1 pool. This suggests that both the V0 and V2 pools are not generated by the isolation procedure but are present in the intact endings and are functionally active. On the basis of their density and uptake of newly synthesized acetylcholine, the V0 and V2 pools were identified with the previously described VP0 pool of axonal vesicles and the VP2 pool of recycling vesicles in stimulated nerve terminals respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of morphine on Na+,K(+)-ATPase from homogenate of synaptosomes and of synaptic membrane of rat cerebral cortex

Effects of morphine on noradrenaline release from rat cerebrocortical synaptosomes and on the Na+,K(+)-ATPase activity in homogenates of synaptosomes and of synaptic membranes were examined. Both morphine (10(-3)-10(-5) M) and methionine-enkephalin (M-Enk; 10(-5) M) inhibited the enhanced [3H]noradrenaline [( 3H]NA) release evoked by high concentrations of K+ from synaptosomes and these inhibitory actions were antagonized by naloxone (10(-4), 10(-5) M). Morphine (10(-3)-10(-5) M) and M-Enk (10(-5) M) stimulated the Na+,K(+)-ATPase activity in homogenates of synaptosomes but not of synaptic membranes in the incubation medium containing 2.2 X 10(-6)-4.7 X 10(-7) M free Ca2+ and these stimulatory effects were antagonized by naloxone. In homogenates of synaptic membranes, the same concentrations of morphine and M-Enk stimulated the Na+,K(+)-ATPase activity suppressed by FeCl2 (5 X 10(-7) M) but not by CuCl2 nor ZnCl2, and these stimulatory effects were antagonized by naloxone. Significant levels of Fe2+ were liberated from synaptosomes during the preparation of synaptic membrane using distilled water. These results suggest that both morphine and M-Enk stimulate the suppressed Na+,K(+)-ATPase activity by interacting with Fe2+ at opioid receptor sites, and they may play a role in the suppression of membrane depolarization and/or the release of NA through their stimulatory action on the Na+,K(+)-ATPase activity probably suppressed by Fe2+ in the rat cerebral cortex.

Inhibition of noradrenaline release from cerebrocortical synaptosomes and stimulation of synaptosomal Na+,K(+)-ATPase activity by morphine in rats

The effects of morphine on noradrenaline (NA) release from rat cerebrocortical synaptosomes and on the synaptosomal Na+,K(+)-ATPase activity were determined. Morphine (10(-3)-10(-5) M) caused a dose-related inhibition of enhanced prelabelled [3H]NA release evoked by a high concentration of K+ from synaptosomes and this inhibitory action of morphine was antagonized by the specific antagonist naloxone (10(-4), 10(-5) M). Morphine dose-dependently stimulated the synaptosomal Na+,K(+)-ATPase activity but not Ca2(+)-ATPase activity in the incubation medium containing 2.2 x 10(-6)-4.7 x 10(-7) M free Ca2+, and this stimulatory effect was antagonized by naloxone. These results suggest that morphine may have some role in the suppression of membrane depolarization and/or the release of NA through its stimulatory action on the Na+,K(+)-ATPase activity in rat cerebral cortex.

Inhibition of Na+,K+-ATPase activity by phospholipase A2 and several lysophospholipids: possible role of phospholipase A2 in noradrenaline release from cerebral cortical synaptosomes

p-Bromophenacyl bromide (PBPB), quinacrine and indomethacin, which inhibit phospholipase A2 (PLA2; EC 3.1.1.4) activity in several tissues, caused a dose-dependent inhibition of prelabelled [3H]noradrenaline ([3H]NA) release evoked by high concentrations of K+ from rat cerebral cortical synaptosomes. Release of prelabelled [3H]NA was caused by natural lysophosphatidic acid (LPA; 10(-6)-10(-5) g mL-1) and lysophosphatidylcholine (LPC; 10(-6)-10(-5) g mL-1) and synthetic LPA (6 x 10(-6), 2 x 10(-5) M) and LPC (6 x 10(-6), 2 x 10(-5) M), but not by natural lysophosphatidylserine (LPS; 10(-5) g mL-1), lysophosphatidylethanolamine (LPE; 10(-5) g mL-1) and lysophosphatidylinositol (LPI; 10(-5) g mL-1). The release evoked by natural LPA and LPC could be inhibited only marginally by PBPB and quinacrine. Phosphatidic acid (PA)-specific and phosphatidylcholine (PC)-specific PLA2 activities from rat cerebral cortical synaptosomes were stimulated in incubation medium containing high concentrations of K+ or calcium ionophore A23187. Low concentrations of PLA2 (10(-6)-10(-8) g mL-1, from bee venom) inhibited the synaptic membrane Na+,K+-ATPase activity in incubation media with intracellular levels of free Ca2+. Several lysophospholipids (LPLs), metabolites of the PLA2 type, also inhibited the synaptic membrane Na+,K+-ATPase activity in a dose-dependent manner. The minimum effective concentrations of natural LPA, LPC, LPS, LPI and LPE were 10(-6), 4.7 x 10(-6), 10(-5), 4.7 x 10(-5) and 4.7 x 10(-5) g mL-1, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of blockade of choline uptake by hemicholinium-3 on synaptic output

The decrease in amplitude of endplate potentials that occurs with high frequency nerve stimulation at the rat phrenic-diaphragm preparation is greater after blockade of choline uptake by hemicholinium-3. The effect is mainly presynaptic and occurs after presumably only a small fraction of the total number of releasable quanta has been discharged. Moreover, when the phrenic nerve is stimulated with a rapid sequence of short tetanic trains, the decrease of the amplitude of both the "first" and the "last" endplate potential of each train which is usually monoexponential becomes not only greater but also biphasic. The effect on the "first" endplate potentials is particularly large. This can be interpreted as further evidence that newly synthesized acetylcholine preferentially replenishes the immediately available store of quanta released by nerve stimulation.

Formation and release of [3H]acetylcholine in the rat urinary bladder strip

The relationship between different frequencies of loading stimulation and [3H]acetylcholine (ACh) formation and release from nerve terminals has been investigated in extratrigonal strips of the urinary bladder of the rat. An increase in frequency (0.2, 0.4 and 0.8 Hz) for the 30 min incubations with [3H]choline produced an enhancement of storage of [3H]ACh from 19.5 to 34% of total tritium content in the tissue. Higher frequencies (1.6 and 3.2 Hz) failed to increase storage further on. The [3H]choline content did not vary significantly. Electrical field stimulation at 2 Hz (360 shocks) produced a release of tritium. The evoked outflow was higher when the strip was loaded at 0.8 Hz than at the other frequencies tested. Both [3H]ACh and [3H]choline were measured in the perfusate of strips preloaded at 0.8 Hz. Most of the induced outflow was found to be [3H]ACh, as in previous experiments carried out using 0.2 Hz as a loading frequency. The findings suggest that in the rat urinary bladder strip loading at 0.8 Hz is suitable for increasing the formation and the resulting release of [3H]ACh during electrical stimulation.

Cerebellar Na+,K+-ATPase activity is increased during early postnatal development of the estrogenized female rat

The activities of Na+,K+-ATPase and Mg2+-ATPase were measured in the crude P2 synaptosomal fraction of the cerebellum through age 35 days in female rats injected s.c. with 500 micrograms estradiol benzoate 24 h after birth. Estrogenization did not affect Mg2+-ATPase. However, the activity of Na+,K+-ATPase was significantly increased above control values between ages 5 and 20 days. These data demonstrate an age-dependent estrogen-induced effect on cerebellar Na+,K+-ATPase during early postnatal development.

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.

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

These experiments measured the release and the synthesis of acetylcholine (ACh) by cat sympathetic ganglia in the presence of 2-(4-phenylpiperidino) cyclohexanol (AH5183), an agent that blocks the uptake of ACh into synaptic vesicles. Evoked transmitter release during short periods of preganglionic nerve stimulation was not affected by AH5183, but release during prolonged stimulation was not maintained in the drug's presence, whereas it was in the drug's absence. The amount of ACh releasable by nerve impulses in the presence of AH5183 was 194 +/- 10 pmol, which represented 14 +/- 1% of the tissue ACh store. The effect of AH5183 on ACh release was not well antagonized by 4-aminopyridine (4-AP), and not associated with inhibition of stimulation-induced calcium accumulation by nerve terminals. It is concluded that AH5183 blocks ACh release indirectly, and that the proportion of stored ACh releasable in the compound's presence represents transmitter in synaptic vesicles available to the release mechanism. The synthesis of ACh during 30 min preganglionic stimulation in the presence of AH5183 was 2,448 +/- 51 pmol and in its absence it was 2,547 +/- 273 pmol. Thus, as the drug decreased ACh release it increased tissue content. The increase in tissue content of ACh in the presence of AH5183 was not evident in resting ganglia; it was evident in stimulated ganglia whether or not tissue cholinesterase was inhibited; it was increased by 4-AP and reduced by divalent cation changes expected to decrease calcium influx during nerve terminal depolarization.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory action of phosphatidylinositol on synaptosomal (Na+ + K+)-ATPase activity

Phosphatidylinositol and several other phospholipids were tested for their ability to influence the (Na+ + K+)-ATPase activity of the cortical synaptic membrane from rats at various levels of free Ca2+. Phosphatidylinositol, but not phosphatidylethanolamine, phosphatidylcholine nor phosphatidylserine, markedly inhibited this enzyme activity, when the free Ca2+ concentration in the incubation media was less than 2.5 X 10(-6) M. This result suggests that phosphatidylinositol may play a role in the depolarization and/or the release of neurotransmitters or intracellular substances in the brain.

Separation of recycling and reserve synaptic vesicles from cholinergic nerve terminals of the myenteric plexus of guinea pig ileum

Acetylcholine-rich synaptic vesicles were isolated from myenteric plexus-longitudinal muscle strips derived from the guinea pig ileum by the method of Dowe, Kilbinger, and Whittaker [J. Neurochem. 35, 993-1003 (1980)] using either unstimulated preparations or preparations field-stimulated at 1 Hz for 10 min using pulses of 1 ms duration and 10 V . cm-1 intensity. The organ bath contained either tetradeuterated (d4) choline (50 microM) or [3H]acetate (2 muCi . ml-1); d4 acetylcholine was measured by gas chromatography-mass spectrometry. As with Torpedo electromotor cholinergic vesicle preparations made under similar conditions the distribution of newly synthesized (d4 or [3H]) acetylcholine in the zonal gradient from stimulated preparations was not identical with that of endogenous (d0, [1H]) acetylcholine, but corresponded to a subpopulation of denser vesicles (equivalent to the VP2 fraction from Torpedo) that had preferentially taken up newly synthesized transmitter. The density difference between the reserve (VP1) and recycling (VP2) vesicles was less than that observed in Torpedo but this smaller difference can be accounted for theoretically by the difference in size between the vesicles of the two tissues. At rest, a lesser incorporation of labelled acetylcholine into the vesicle fraction was observed, and the peaks of endogenous and newly synthesized acetylcholine coincided. Stimulation in the absence of label followed by addition of label did not lead to incorporation of labelled acetylcholine, suggesting that the synthesis and storage of acetylcholine in this preparation and its recovery from stimulation is much more rapid than in Torpedo.

One-carbon metabolism in lectin-activated human lymphocytes

Serine is an essential amino acid for the lectin-mediated transformation of human peripheral blood lymphocytes due to the inability of this cell to synthesize sufficient quantities via either the phosphorylated pathway or by reversal of the serine hydroxymethyltransferase reaction to meet the metabolic demands. The level of intracellular serine is tightly regulated, and the culture medium concentration for optimum cellular transformation falls within a relatively narrow range. The three-carbon atom of serine is the major source of one-carbon units required for purine and pyrimidine nucleotide biosynthesis, but the key effect of both serine deprivation and of high medium serine levels would appear to be on protein synthesis. Although an alternative source of one-carbon units, as provided by high levels of formate in the culture medium, can partially reverse the effects of serine deprivation, the only other demonstrable source of one-carbon units, tryptophan, requires serine for its incorporation and subsequent metabolism. Methionine is also essential for lymphocyte transformation and is involved in the synthesis of a small amount of phosphatidylcholine, although most of this phospholipid is provided by choline and lysophosphatidylcholine from the serum-supplemented culture medium.

Differences in the osmotic fragility of recycling and reserve synaptic vesicles from the cholinergic electromotor nerve terminals of Torpedo and their possible significance for vesicle recycling

In this study we demonstrate differences in the osmotic fragility of two metabolically and physically heterogeneous synaptic vesicle populations from stimulated electromotor nerve terminals. When synaptic vesicles isolated on sucrose density gradients are submitted to solutions of decreasing osmolarity 50% of VP2-type vesicles lysed at (mean + S.E. (number of experiments] 332 +/- 14 (4) mosM and 50% of VP1-type vesicles lysed at 573 +/- 8 (3) mosM. These results indicate that recycling vesicles are more resistant to hypo-osmotic lysis and they are consistent with our earlier conclusion that changes in water content on recycling are secondary to changes in the content of the osmotically active small-molecular-mass constituents acetylcholine and ATP.

The occurrence of gamma-aminobutyrylcholine in mammalian brain-fact or artefact?

gamma-Aminobutyrylcholine (GABAch) has been reported to exist in mammalian brain tissue, but not, as yet, given a specific physiological role in the CNS. In order to investigate further its occurrence and function in the CNS, two new methods have been developed for its isolation and determination at the picomole level. Its isolation has been achieved by ammonium Reineckate precipitation or by cation-exchange followed by HPLC determination of the dansyl and o-phthaldialdehyde derivatives. Using these methods, no free endogenous GABAch (less than 80 pmol/g) was found in rat, guinea pig, cat, pig marmoset, or human brain tissue. No evidence was obtained, either in vitro or in vivo, for the incorporation of [14C]gamma-aminobutyric acid into GABACh. GABACh was hydrolysed at a low rate (maximum of 45 nmol/h/g of brain tissue) after incubation with rat, guinea pig, or cat brain minces and homogenates. These results fail to confirm the data of other investigators, and the possible reasons for this are discussed.

Differential labeling of depot and active acetylcholine pools in nondepolarized and potassium-depolarized rat brain synaptosomes

To test the hypothesis that a pool of newly synthesized acetylcholine (ACh) turns over independently of performed ACh, compartmentation and K+-evoked release of ACh were examined in perfused synaptosomal beds intermittently stimulated by 50 mM K+. In resting synaptosomes, endogenous and labeled ACh was distributed between synaptic vesicles and the cytoplasm in a dynamic equilibrium ratio of 4:6. In the absence of new ACh synthesis, five sequential K+-depolarizations caused a decremental release of performed labeled ACh totaling 30% of the initial transmitter store. Further depolarization evoked little additional release, despite the fact that 60% of the labeled ACh remained in these preparations. Release of the performed [14C]ACh was unaltered while new ACh was being synthesized from exogenous [3H]choline. Since the evoke release of [3H]ACh was maintained while that of [14C]ACh was decreasing, the [3H]ACh/[14C]ACh ratio in perfusate increased with each successive depolarization. This ratio was six to ten times higher than the corresponding ratio in vesicles or cytoplasm. These results indicate that the newly synthesized ACh did not equilibrate with either the depot vesicular or cytoplasmic ACh pools prior to release.

Subcellular distribution of prolyl endopeptidase and cation-sensitive neutral endopeptidase in rabbit brain

The subcellular distribution of prolyl endopeptidase, and of cation-sensitive neutral endopeptidase, two enzymes actively metabolizing many neuropeptides, was determined in homogenates of rabbit brain. The subcellular distribution of both enzymes was more similar to lactate dehydrogenase, a cytoplasmic enzyme marker, than to choline acetyltransferase, a synaptosomal marker. Only 35% of the activity of these two neutral endopeptidases was found in the crude mitochondrial fraction (P2), the bulk of the remaining activity being associated with the high-speed supernatant. Prolyl endopeptidase and cation-sensitive neutral endopeptidase thus can be regarded as mainly cytoplasmic enzymes in the rabbit brain.

The subsynaptosomal distribution and release of [3H]acetylcholine synthesized by rat cerebral cortical synaptosomes

Synaptosomes were prepared from rat cerebral cortex and incubated in [3H]choline for periods ranging from 1 to 90 min. The [3H]ACh synthesized during this period was found only in the cytoplasm and in a membrane-associated fraction. A negligible amount of the newly formed [3H]ACh was recovered in the vesicular fraction despite concerted efforts to protect a hypothetical population of labile vesicles. The specific activity of the membrane-associated component, accounting for 21% of the total [3H]ACh, was by far the highest. This membrane-associated fraction was not released by hypotonic shock or homogenization and apparently was not in association with the monodisperse synaptic vesicles. The [3H]ACh was released in a calcium dependent manner. This investigation has determined that the ACh synthesized by synaptosomes is localized in only two fractions, cytoplasmic and membrane-associated; that this newly synthesized ACh can be released from synaptosomes by a process consistent with physiological release; and that at least part of the ACh released was originally present in the cytoplasm.

Acetylcholine turnover and compartmentation in rat brain synaptosomes

The turnover of acetylcholine (ACh) in rat brain synaptosomes and its compartmentation in the labile bound and stable bound pools were investigated. The P(2) fraction from rat brain was subjected to three sequential incubations, each terminated by centrifugation followed by determination of ACh concentrations by gas chromatography-mass spectrometry (GCMS): (1) Depletion phase: Incubation of synaptosomes at 37 percent C for 10 min in Na+ -free buffer containing 35 mM-KCl reduced the content of both labile bound and stable bound ACh by 40%. (2) Synthesis phase: incubation at 37 percent C with 2 micrometer-[(2)H(4)]choline resulted in accumulation of labeled and unlabeled ACh in both compartments. Addition of an anticholinesterase had little effect on stable bound ACh but greatly increased the content of labile bound ACh. This excess accumulated ACh was probably due to inhibition of intracellular acetylcholinesterase (AChE), because negligible uptake of ACh from the medium was observed. The effects on ACh synthesis of altered cation concentrations and metabolic inhibitors were examined. (3) Release phase: The tissue was incubated in the presence of 35 mM-KCl, 40 micrometer-paraoxon, and 20 micrometer-hemicholinium-3 (HC-3) (to inhibit further synthesis of ACh). Measurements of the compartmental localization of ACh at several time points indicated that ACh was being released from the labile bound fraction. In support of this conclusion, 20 mM-Mg2+ reduced ACh release and increased the labile bound ACh concentration.

Choline uptake and permanent memory storage

The uptake of 3H-choline and its incorporation into 3H-acetylcholine was studied in vitro on hippocampus slices obtained from animals showing a good or poor long-term memory. The animals were selected on the basis of their retention performance when tested by a brightness discrimination model. The 3H-choline uptake and the incorporation of 3H-choline into 3H-acetylcholine was higher in hippocampus slices from animals showing good retention compared to those from animals with poor retention. The level of high affinity uptake of choline into hippocampus slices may serve as an indicator of the cholinergic activity in this structure under in vitro conditions. The present findings suggest that individual differences in the activity level of the hippocampal cholinergic system do exist and are capable of influencing the retention of the individual animals to a variable degree.

High affinity choline transport and acetylCoA production in brain and their roles in the regulation of acetylcholine synthesis

This review describes recent advances made in the understanding of the regulation of acetylcholine synthesis in brain with regard to the availability of its two precursors, choline and acetylCoA. Choline availability appears to be regulated by the high affinity choline transport system. Investigations of the localization and inhibition of this system are reviewed. Procedures for measuring high affinity choline transport and their shortcomings are described. The kinetics and effects of previous in vivo and in vitro treatments on high affinity choline transport are reviewed. Kinetic and direct coupling of the transport and acetylation of choline are discussed. Recent investigations of the source of acetylCoA used for the synthesis of acetylcholine are reviewed. Three sources of acetylCoA have recently received support: citrate conversion catalyzed by citrate lyase, direct release of acetylCoA from mitochondria following its synthesis from pyruvate catalyzed by pyruvate dehydrogenase, and production of acetylCoA by cytoplasmic pyruvate dehydrogenase. Investigations indicating that acetylCoA availability may limit acetylcholine synthesis are reviewed. A model for the regulation of acetylcholine synthesis which incorporates most of the reviewed material is presented.