Origin of transmitters released by electrical stimulation from a small metabolically very active vesicular pool of cholinergic synapses in guinea-pig cerebral cortex

Function suggests nano-structure: towards a unified theory for secretion rate, a statistical mechanics approach

The inventory of secretory granules along the plasma membrane can be viewed as maintained in two restricted compartments. The release-ready pool represents docked granules available for an initial stage of fast, immediate secretion, followed by a second stage of granule set-aside secretion pool, with significantly slower rate. Transmission electron microscopy ultra-structural investigations correlated with electrophysiological techniques and mathematical modelling have allowed the categorization of these secretory vesicle compartments, in which vesicles can be in various states of secretory competence. Using the above-mentioned approaches, the kinetics of single vesicle exocytosis can be worked out. The ultra-fast kinetics, explored in this study, represents the immediately available release-ready pool, in which granules bound to the plasma membrane are exocytosed upon Ca(2+) influx at the SNARE rosette at the base of porosomes. Formalizing Dodge and Rahamimoff findings on the effect of calcium concentration and incorporating the effect of SNARE transient rosette size, we postulate that secretion rate (rate), the number (X) of intracellular calcium ions available for fusion, calcium capacity (0 ≤ M ≤ 5) and the fusion nano-machine size (as measured by the SNARE rosette size K) satisfy the parsimonious M-K relation rate ≈ C × [Ca(2+)](min(X,M))e(-K/2).

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.

Rapid reuse of readily releasable pool vesicles at hippocampal synapses

Functional presynaptic vesicles have been subdivided into readily releasable (RRP) and reserve (RP) pools. We studied recycling properties of RRP vesicles through differential retention of FM1-43 and FM2-10 and by varying the time window for FM dye uptake. Both approaches indicated that vesicles residing in the RRP underwent rapid endocytosis (tau approximately 1s), whereas newly recruited RP vesicles were recycled slowly (tau approximately 30 s). With repeated challenges (hypertonic or electrical stimuli), the ability to release neurotransmitter recovered 10-fold more rapidly than restoration of FM2-10 destaining. Finding neurotransmission in the absence of destaining implied that rapidly endocytosed RRP vesicles were capable of reuse, a process distinct from repopulation from the RP. Reuse would greatly expand the functional capabilities of a limited number of vesicles in CNS terminals, particularly during intermittent bursts of activity.

Genetics of synaptic vesicle function: toward the complete functional anatomy of an organelle

Synaptic transmission starts with the release of neurotransmitters by exocytosis of synaptic vesicles. As a relatively simple organelle with a limited number of components, synaptic vesicles are in principle accessible to complete structural and functional genetic analysis. At present, the majority of synaptic vesicle proteins has been characterized, and many have been genetically analyzed in mice, Drosophila, and Caenorhabditis elegans. These studies have shown that synaptic vesicles contain proteins with diverse structures and functions. Although the genetic studies are as yet unfinished, they promise to lead to a full description of synaptic vesicles as macromolecular machines involved in all aspects of presynaptic neurotransmitter release.

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.

The heterogeneity of vesicular acetylcholine storage in cholinergic nerve terminals

The vesicular hypothesis of quantal acetylcholine release describes the process by which discrete packages (or quanta) of the transmitter are released from nerve terminals through the exocytosis of the content of synaptic vesicles. However, cholinergic synaptic vesicles can no longer be vaguely regarded as simple membrane bound 'sacks' of the transmitter. Modern molecular, biochemical, morphological and electrophysiological research has revealed them to be complex cellular structures with a heterogeneous mixture of functions. Thus, not all synaptic vesicle populations are formed under the same circumstances and there are variations in the releasability of synaptic vesicle populations. This review briefly outlines some of the experimental research that has lead to our current thinking on the heterogeneity of vesicular acetylcholine storage in cholinergic nerve terminals. In addition, a model for vesicular acetylcholine storage and release is presented that attempts to accommodate many of the modern ideas concerning cholinergic synaptic vesicle function and interaction.

Decrease of quantal size and quantal content during tetanic stimulation detected by focal recording

End-plate potentials and miniature end-plate potentials were recorded focally (i.e., over a limited area of the end-plate with several or possibly only one active zone) in a cutaneous pectoris from neuromuscular junction during a prolonged (1-6 min) tetanic (20-100 Hz) nerve stimulation. End-plate potential amplitudes decreased and became more variable with prolonged stimulation. Synaptic depression thus occurs even when synaptic output is low, if release is evoked from only a few active zones, suggesting that there is little if any vesicular replenishment between the active zones. The probability density function of the end-plate potential amplitudes has been obtained using the Parzen estimate with a Gaussian weighting function, to reduce the number of end-plate potentials needed for the same accuracy. Quantal size of the end-plate potentials was estimated from the slope of the best fitted line to the prominent and apparently equidistant peaks of probability density functions or from the spectrogram of the probability density function of end-plate potentials. Quantal contents were initially (+/- S.D.):5.7 +/- 2.9, ranged from 2 to 12, and in all cases examined (n = 11) decreased with prolonged tetanic stimulation. The rates of the decrease of end-plate potentials amplitudes (and quantal contents) from different segments of the same nerve terminal were often different, even when they were initially comparable. This suggests that some active zones or some areas of the end-plates become depleted much faster than others. Quantal sizes of the nerve evoked and the spontaneously released quanta were generally similar at low frequencies of stimulation (0.5-2 Hz). Both decreased with high frequency stimulation, but the decrease of the quantal sizes of nerve evoked quanta was usually more pronounced. At different loci of the same end-plate the contribution of lower quantal size to the synaptic depression varied widely (from < 5% to > 80%). In conclusion lower quantal size can contribute significantly to synaptic depression. At uneven decrease of quantal sizes over the whole nerve terminal helps to explain both aspects of synaptic depression (lower synaptic efficacy and greater variability of quantal responses.

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.

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.

Effect of AH5183 (vesamicol) on cholinergic transmission in intact airway smooth muscle

The effect of the vesicular acetylcholine (ACh) transport blocker trans-2-(4- phenyl-piperidino)-cyclohexanol (AH5183) was studied in bronchial smooth muscle during activation of the vagus nerve. AH5183 inhibited in a dose-dependent manner the Ca(2+)-sensitive electrically induced smooth muscle contractions in vitro with a half-inhibitory concentration (IC50) of 1.6 +/- 0.4 microM. The inhibition was complete within 68 +/- 1 min (n = 8) at approximately 20 microM AH5183 and was partly reversible after washing of the preparations. AH5183 (20 microM) reduced the level of endogenous ACh by 47.4 +/- 7.6% (n = 4) during this time period. The effect of AH5183 is most likely prejunctional, since the contractions induced post-junctionally by carbachol were not altered by AH5183. The irreversible anticholinesterase, soman, increased the tonus of airway smooth muscle as a result of accumulation of spontaneously released ACh from prejunctional leakage. AH5183 had no effect on this increase of muscle contraction. The present results show that the nerve-evoked release of ACh comes from an AH5183-sensitive pool, probably a vesicular pool, whereas leakage of ACh presumably comes from the cytoplasmic pool in airway smooth muscle.

Evaluation by reverse phase HPLC of [3H]acetylcholine release evoked from the myenteric plexus of the rat

Myenteric plexus-longitudinal muscle strips isolated from the small intestine of rats were incubated with [3H]choline to measure the synthesis and the release of [3H]acetylcholine. To separate different radioactive compounds (acetylcholine, choline, phosphorylcholine) from both the tissue and the overflow a new method, the reverse phase HPLC, was used. The radiochromatogram following the injection of a [3H]choline-standard and a [14C]acetylcholine-standard onto the HPLC showed a clear separation of both isotopes with a recovery rate of roughly 100%. Incubation of the muscle strips with [3H]choline caused the synthesis of [3H]acetylcholine (30,000 dpm/preparation) that increased 2-fold, when the electrical field stimulation during labelling was increased from 0.2 Hz to 1 Hz. Electrical field stimulation (3 Hz, 2 min) caused an increase in tritium efflux that was abolished by the removal of extracellular calcium or by the addition of tetrodotoxin. Analysis by reverse phase HPLC of the overflow showed that the stimulated increase in tritium overflow was balanced by the enhanced release of [3H]acetylcholine. whereas the overflow of [3H]choline was not affected by the electrical field stimulation. Oxotremorine (1 mumol/l) suppressed the release of [3H]acetylcholine by 60%. Scopolamine (0.1 mumol/l) prevented this inhibition and, given alone, enhanced the release of [3H]acetylcholine by 43%. The release of [3H]acetylcholine evoked at 0.2, 2 or 20 Hz did not consistently decline at increasing frequencies. The present experiments show the synthesis and the calcium-dependent release of [3H]acetylcholine from the myenteric plexus-longitudinal muscle preparation of rats correspondingly to the same in-vitro preparation isolated from guinea-pigs. Muscarinic autoinhibition operates also in the small intestine of rats.(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.

Subcellular distribution of mammalian tachykinins in rat basal ganglia

A combined differential and density gradient centrifugation procedure was used to study the subcellular localisation of the mammalian tachykinins in rat caudateputamen and substantia nigra. Substance P, neurokinin A, neuropeptide K, and neurokinin B were found to be concentrated in the synaptosomal fractions and in fractions containing heavy synaptic vesicles in both regions studied. In contrast, the catecholamines dopamine and noradrenaline had a more widespread distribution throughout the gradient. HPLC analysis of the immunoreactivity recovered showed that the tachykinin immunoreactivity coeluted with the relevant synthetic tachykinins, except in the soluble gradient fraction where neurokinin A immunoreactivity eluted in position consistent with neurokinin A3-10. These results suggest that, in the basal ganglia, the mammalian tachykinins are localised in fractions containing large dense cored synaptic vesicles. This vesicular localisation would be consistent with the proposed role of the tachykinins as neurotransmitters and neuromodulators.

Acetylcholine mobilization in a sympathetic ganglion in the presence and absence of 2-(4-phenylpiperidino)cyclohexanol (AH5183)

The present experiments measured the release of acetylcholine (ACh) by the cat superior cervical ganglia in the presence of, and after exposure to, 2-(4-phenylpiperidino)cyclohexanol (AH5183), a compound known to block the uptake of ACh by cholinergic synaptic vesicles. We confirmed that AH5183 blocks evoked ACh release during preganglionic nerve stimulation when approximately 13-14% of the initial ganglial ACh stores had been released; periods of rest in the presence of the drug did not promote recovery from the block, but ACh release recovered following the washout of AH5183. ACh was synthesized in AH5183-treated ganglia, as determined by the synthesis of [3H]ACh from [3H]choline, and this [3H]ACh could be released by stimulation following drug washout. The specific activity of the released ACh matched that of the tissue's ACh, and thus we conclude that ACh synthesized in the presence of AH5183 is a releasable as pre-existing ACh stores once the drug is removed. We tested the relative releasability of ACh synthesized during AH5183 exposure (perfusion with [3H]choline) and that synthesized during recovery from the drug's effects (perfusion with [14C]choline: the ratio of [3H]ACh to [14C]ACh released by stimulation was similar to the ratio in the tissue. These results suggest that the mobilization of ACh for release by ganglia during recovery from an AH5183-induced block is independent of the conditions under which the ACh was synthesized. Unlike nerve impulses, black widow spider venom (BWSV) induced the release of ACh from AH5183-blocked ganglia, even in the drug's continued presence. Venom-induced release of ACh from AH5183-treated ganglia was not less than the venom-induced release from tissues not exposed to AH5183. This effect of BWSV was attributed to the action of the protein, alpha-latrotoxin, because an anti-alpha-latrotoxin antiserum blocked the venom's action. ACh synthesized during AH5183 exposure was labelled from [3H]choline, and subsequent treatment with BWSV released [3H]ACh with the same temporal pattern as the release of total ACh. To exclude a nonexocytotic origin for the [3H]ACh released by BWSV, ganglia were preloaded with [3H]diethylhomocholine to form [3H]acetyldiethylhomocholine, an ACh analogue excluded from vesicles; the venom did not increase the rate of [3H]acetyldiethylhomocholine efflux. It is concluded that a vesicular ACh pool insensitive to the inhibitory action of AH5183 might exist and that this vesicular pool is not mobilized by electrical stimulation to exocytose in the presence of AH5183, but it is by BWSV.

Differential release of [3H]acetylcholine from the rat phrenic nerve-hemidiaphragm preparation by electrical nerve stimulation and by high potassium

Neuronal transmitter stores of the phrenic nerve were labelled under different conditions. Subsequently, transmitter release evoked by electrical nerve stimulation and by a high potassium-low sodium solution was studied. Incubation of the end-plate preparation with [3H]choline at rest led to the synthesis of [3H]acetylcholine which could not be released by electrical nerve stimulation but it was released by high potassium-low sodium solution, independent of the presence of extracellular calcium. When the end-plate preparation was labelled during stimulation at 1 Hz, prolonged periods of electrical nerve stimulation released 83% of the total releasable [3H]transmitter pool in a completely calcium-dependent manner. After exhaustion of the electrically releasable pool, high potassium-low sodium solution still caused a significant outflow. Without a preceding exhaustion of the [3H]acetylcholine pool, high potassium-low sodium solution released a similar amount in the absence of extracellular calcium or after pretreatment with the intracellular calcium chelating substance, Quin-2. When evoked transmitter release was studied at different temperatures (36, 26 and 16 degrees C) Q 10 values of 1.6 and 1.0 were found for the release caused by electrical nerve stimulation and high potassium-low sodium solution (calcium-independent effect), respectively. After labelling during a short interval (2 min) but at a high stimulation rate (50 Hz), only 72% of the releasable [3H]transmitter could be released by electrical nerve stimulation, whereas the outflow due to the calcium-independent effect of high potassium-low sodium solution increased from 17 (labelling during stimulation at 1 Hz) to 28%. It is suggested that the calcium-independent effect of high potassium-low sodium solution reflects the release of acetylcholine from the cytoplasmic compartment, as this outflow occurred after labelling at rest and increased when cytoplasmic synthesis was enhanced by a high loading stimulation. In contrast to high potassium-low sodium solution, propagated nerve activity cannot release acetylcholine synthesized at rest (presumed to be cytoplasmic), but only [3H]acetylcholine synthesized during quantal release (presumed to be vesicular). The absolute requirement of extracellular calcium for electrically stimulated release suggests an exocytotic release mechanism. The low Q 10 value of 1.6 does not fit into the concept of a carrier- or channel-operated release mechanism.(ABSTRACT TRUNCATED AT 400 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.

Effects of chronic dietary administration of the cholinergic false precursor N-amino-N,N-dimethylaminoethanol on behavior and cholinergic parameters in rats

The choline analog, N-amino-N,N-dimethylaminoethanol (NADe), was fed ad libitum (chloride salt; 0.5%) to weanling rats in a low choline, low methionine synthetic diet. Control rats were fed choline chloride (0.5%) in place of NADe. Initial observation and behavioral screen tests of grasp strength, startle reflex, righting reflex, analgesia (hot plate test) and body temperature did not reveal any toxic effects caused by NADe, although both experimental and control groups gained weight more slowly than rats fed standard lab chow. After 25 days on the diet, the performance of rats fed NADe in a one-trial passive avoidance test was significantly impaired compared to control rats. There was no difference between experimental and control rats in sensitivity to foot shock or in activity monitored in a closed field. A subjective, 6-component behavioral rating scale indicated rats fed NADe were resistant to handling but not aggressive. These behavioral results were similar in two separate feeding experiments using deuterium-labeled and unlabeled NADe. The twitch response of isolated rat phrenic nerve-diaphragms during stimulation did not show any impairment of neuromuscular function in rats fed NADe. Receptor binding experiments indicated there were no differences between experimental and control rats in tritiated quinuclidinyl benzilate [( 3H]QNB) binding capacity in cortex, heart and ileum. Competitive [3H]QNB binding with carbachol indicated there was no difference in the IC50's measured in cortex homogenates. Acetylcholinesterase (AChE) and choline acetyltransferase (ChAT) activities in cortex were similar in experimental and control groups.(ABSTRACT TRUNCATED AT 250 WORDS)

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)

Subcellular distribution of neuropeptide Y-like immunoreactivity in guinea pig neocortex

Neuropeptide Y-like immunoreactivity (NPY-LI) was enriched in synaptosomal fractions of neocortex, which on lysis yielded vesicle-rich fractions. The distribution of NPY-LI on a sucrose density gradient was similar to that of somatostatin, with a concentration in heavy vesicles. The peptides were not found in light vesicles in contrast to the distribution of noradrenaline. Both homogenate and vesicular NPY-LI coeluted with synthetic NPY on reverse-phase HPLC.

In vivo acetylation of homocholine and beta-methylcholine in rat brain

A nitrogen phosphorus-gas chromatographic procedure was modified to determine the extent of in vivo acetylation of the choline analogs homocholine and beta-methylcholine. Infusion of homocholine (18 mumoles) for 2 hours into the lateral ventricle of the rat produced 2.3 nmoles/gram of acetylhomocholine which represented 0.035% of the detected homocholine. Infusion of the same quantity of beta-methylcholine produced 1.0 nmole/gram of acetyl-beta-methylcholine representing 0.025% of the detected beta-methylcholine. Although pretreatment with hemicholinium-3 reduced the amount of acetylated product formed from either analog, the reduction was significant only for acetyl-beta-methylcholine (p less than 0.01).

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.

Two types of neuronal muscarine receptors modulating acetylcholine release from guinea-pig myenteric plexus

Longitudinal muscle strips of the guinea-pig ileum were incubated with [3H]choline and the effects of muscarinic agonists on smooth muscle contraction and on spontaneous and electrically-evoked outflow of tritium were studied. Muscarine and pilocarpine concentration-dependently increased both muscle contraction and spontaneous outflow of [3H]ACh, and inhibited the electrically-evoked outflow of [3H]ACh. The increase in spontaneous outflow was prevented by tetrodotoxin and scopolamine, but not by hexamethonium. Oxotremorine (1-100 microM) did not increase the spontaneous outflow of tritium. Pirenzepine in concentrations of 10 and 100 nM hardly affected the muscle contractions induced by pilocarpine, but significantly antagonized the pilocarpine-evoked increases in [3H]ACh outflow. Likewise, pirenzepine (100 nM) antagonized more effectively the enhancement by muscarine of spontaneous outflow than the inhibitory effect of muscarine on the electrically-evoked release of [3H]ACh. Scopolamine (1 and 10 nM) antagonized to a similar extent the effects of pilocarpine on spontaneous outflow of [3H]ACh and on muscle contraction. The results suggest that the cholinergic nerves of the myenteric plexus are endowed with excitatory (ganglionic) and inhibitory (prejunctional) muscarine receptors which modulate the release of ACh and which differ in their affinities to pirenzepine.

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.

Incorrect interpretations of radioactive labelling experiments with false transmitters

In a number of recent studies attempts have been made to determine the subcellular origin of the released transmitter by comparison of the ratio of the radioactivity (14C/3H) of true and false transmitters between those released by stimulation and the subcellular fractions. It is shown that use of the isotopic or pseudo-molar ratios will lead to incorrect conclusions because varying degrees of isotopic dilution render the ratio meaningless. The only correct basis for comparison is the true molar ratio.

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.

The effect of physostigmine on the vagally induced muscarinic inhibition of noradrenaline release from the isolated perfused rabbit atria

1. Presynaptic cholinergic-adrenergic interactions were studied on isolated perfused rabbit atria with the extrinsic right vagus and sympathetic innervation intact. The transmitter stores were labelled with 14C-choline and 3H-noradrenaline. The radioactive compounds were separated on columns and determined by scintillation spectrometry. The stimulation-evoked overflow of both transmitters was calcium-dependent and abolished by tetrodotoxin. 2. Methacholine caused a concentration-dependent decrease of atrial tension development and 3H-noradrenaline overflow evoked by 3 Hz sympathetic stimulation. Vagus nerve stimulation (1-20 Hz), although nearly abolishing tension development at 20 Hz, decreased evoked 3H-noradrenaline overflow by not more than 18%. 3. Physostigmine decreased atrial cholinesterase activity by 80% and increased the fraction of stimulation-evoked unhydrolyzed 14C-acetylcholine in the persufates from 58 to 86%. However, the inhibition by vagus stimulation (1-10 Hz) of evoked 3H-noradrenaline overflow was smaller than in the absence of the drug. This was closely related to a decrease in acetylcholine overflow. Yet for a give fractional rate of acetylcholine release the muscarinic inhibition of noradrenaline overflow still did not exceed that observed in the absence of physostigmine. 4. It is concluded that the vagally induced control of noradrenaline release occurs at discrete sites rather than in a diffuse pattern at multiple terminal axon sites as is the case after exogenous muscarinic agonists.

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.

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.

N-Ethyl analogues of choline as precursors to cholinergic false transmitters

Cat superior cervical ganglia perfused with the choline analogue, diethylcholine, acetylated the analogue and acetyldiethylcholine was subsequently released from the ganglia in response to preganglionic nerve stimulation by a Ca2+-dependent mechanism. Rat cerebral cortical slices incubated with monoethylcholine or diethylcholine acetylated the choline analogues, and both acetylmonoethylcholine and acetyldiethylcholine were released from the slices in response to stimulation by a high K+ concentration. In brain slices pre-incubated with monoethylcholine, diethylcholine, or triethylcholine, acetylated derivatives of the choline analogues were found in both free and bound nerve-ending stores, similar to the subcellular localization of [3H]acetylcholine (ACh) in brain slices pre-incubated with [3H]choline; the relative distribution of each of the acetylated choline analogues between the two nerve-ending stores differed only slightly from that of [3H]ACh. Nerve-ending free and bound stores of acetyldiethylcholine were equally depleted when brain slices were stimulated under conditions that cause depletion of releasable acetyldiethylcholine stores; similar results were obtained with acetyltriethylcholine. It is concluded that the three acetylated ethyl analogues of choline fulfill the necessary criteria for identification as cholinergic false transmitters, and that, under the conditions of the present experiments with brain, the false transmitters are able to distribute similarly to ACh between nerve-ending stores.