Nicotinic agonists antagonize quantal size increases and evoked release at frog neuromuscular junction

Activation of Neuronal Nicotinic Receptors Inhibits Acetylcholine Release in the Neuromuscular Junction by Increasing Ca2+ Flux through Cav1 Channels

Cholinergic neurotransmission is a key signal pathway in the peripheral nervous system and in several branches of the central nervous system. Despite the fact that it has been studied extensively for a long period of time, some aspects of its regulation still have not yet been established. One is the relationship between the nicotine-induced autoregulation of acetylcholine (ACh) release with changes in the concentration of presynaptic calcium levels. The mouse neuromuscular junction of m. Levator Auris Longus was chosen as the model of the cholinergic synapse. ACh release was assessed by electrophysiological methods. Changes in calcium transients were recorded using a calcium-sensitive dye. Nicotine hydrogen tartrate salt application (10 μM) decreased the amount of evoked ACh release, while the calcium transient increased in the motor nerve terminal. Both of these effects of nicotine were abolished by the neuronal ACh receptor antagonist dihydro-beta-erythroidine and Cav1 blockers, verapamil, and nitrendipine. These data allow us to suggest that neuronal nicotinic ACh receptor activation decreases the number of ACh quanta released by boosting calcium influx through Cav1 channels.

Localized infusions of the partial alpha 7 nicotinic receptor agonist SSR180711 evoke rapid and transient increases in prefrontal glutamate release

The ability of local infusions of the alpha 7 nicotinic acetycholine receptor (α7 nAChR) partial agonist SSR180711 to evoke glutamate release in prefrontal cortex was determined in awake rats using a microelectrode array. Infusions of SSR180711 produced dose-dependent increases in glutamate levels. The lower dose (1.0μg in 0.4μL) evoked a rapid rise (∼1.0s) in glutamate (1.41±0.30μM above baseline). The higher dose (5.0μg) produced a similarly rapid, yet larger increase (3.51±0.36μM above baseline). After each dose, the glutamate signal was cleared to basal levels within 7-18s. SSR180711-evoked glutamate was mediated by the α7 nAChR as co-infusion of the selective α7 nAChR antagonist α-bungarotoxin (10.0μM)+SSR1808711 (5.0μg) reduced the effect of 5.0μg alone by 87% (2.62 vs. 0.35μM). Finally, the clearance of the SSR180711 (5.0μg)-evoked glutamate was bidirectionally affected by drugs that inhibited (threo-beta-benzyl-oxy-aspartate (TβOA), 100.0μM) or facilitated (ceftriaxalone, 200mg/kg, i.p.) excitatory amino acid transporters. TβOA slowed both the clearance (s) and rate of clearance (μM/s) by 10-fold, particularly at the mid-late stages of the return to baseline. Ceftriaxone reduced the magnitude of the SSR180711-evoked increase by 65%. These results demonstrate that pharmacological stimulation of α7 nAChRs within the prefrontal cortex is sufficient to evoke rapid yet transient increases in glutamate levels. Such increases may underlie the cognition-enhancing effects of the drug in animals; further justifying studies on the use of α7 nAChR-positive modulators in treating cognition-impairing disorders in humans.

Neuromuscular activity of BaTX, a presynaptic basic PLA2 isolated from Bothrops alternatus snake venom

We have previously isolated a Lys49 phospholipase A(2) homolog (BaTX) from Bothrops alternatus snake venom using a combination of molecular exclusion chromatography and reverse phase HPLC and shown its ability to cause neuromuscular blockade. In this work, we describe a one-step procedure for the purification of this toxin and provide further details of its neuromuscular activity. The toxin was purified by reverse phase HPLC and its purity and molecular mass were confirmed by SDS-PAGE, MALDI-TOF mass spectrometry, amino acid analysis and N-terminal sequencing. BaTX (0.007-1.4 microM) produced time-dependent, irreversible neuromuscular blockade in isolated mouse phrenic nerve-diaphragm and chick biventer cervicis preparations (time to 50% blockade with 0.35 microM toxin: 58+/-4 and 24+/-1 min, respectively; n=3-8; mean+/-S.E.) without significantly affecting the response to direct muscle stimulation. In chick preparations, contractures to exogenous acetylcholine (55 and 110 microM) or KCl (13.4 mM) were unaltered after complete blockade by all toxin concentrations. These results, which strongly suggested a presynaptic mechanism of action for this toxin, were reinforced by (1) the inability of BaTX to interfere with the carbachol-induced depolarization of the resting membrane, (2) a significant decrease in the frequency and amplitude of miniature end-plate potentials, and (3) a significant reduction (59+/-4%, n=12) in the quantal content of the end-plate potentials after a 60 min incubation with the toxin (1.4 microM). In addition, a decrease in the organ bath temperature from 37 degrees C to 24 degrees C and/or the replacement of calcium with strontium prevented the neuromuscular blockade, indicating a temperature-dependent effect possibly mediated by enzymatic activity.

The Neurotransmitter Cycle and Quantal Size

Changes in the response to release of a single synaptic vesicle have generally been attributed to postsynaptic modification of receptor sensitivity, but considerable evidence now demonstrates that alterations in vesicle filling also contribute to changes in quantal size. Receptors are not saturated at many synapses, and changes in the amount of transmitter per vesicle contribute to the physiological regulation of release. On the other hand, the presynaptic factors that determine quantal size remain poorly understood. Aside from regulation of the fusion pore, these mechanisms fall into two general categories: those that affect the accumulation of transmitter inside a vesicle and those that affect vesicle size. This review will summarize current understanding of the neurotransmitter cycle and indicate basic, unanswered questions about the presynaptic regulation of quantal size.

Pharmacological and structural characterization of a novel phospholipase A2 from Micrurus dumerilii carinicauda venom

We have isolated a new phospholipase A2 (MiDCA1) from the venom of the coral snake Micrurus dumerilii carinicauda. This toxin, which had a molecular mass of 15,552Da, shared high sequence homology with the PLA2 toxins MICNI A and B from Micrurus nigrocinctus venom (77.7% and 73.1%, respectively). In chick biventer cervicis preparations, MiDCA1 produced concentration- and time-dependent neuromuscular blockade that reached 100% after 120 min (2.4 microM, n = 6); contractures to exogenously applied carbachol (8 microM) and KCl (13 mM) were still seen after complete blockade. In mouse phrenic-nerve diaphragm preparations, MiDCA1 (2.4 microM; n = 6) caused triphasic changes followed by partial neuromuscular blockade. Intracellular recordings of end-plate potentials (EPPs) and miniature end-plate potentials (MEPPs) from mouse diaphragm preparations showed that MiDCA1 increased the quantal content by 386+/-12% after 10 min (n = 14; p<0.05) and caused a triphasic change in the frequency of MEPPs. MiDCA1 also decreased the resting membrane potential, an effect that was prevented by tetrodotoxin and/or low extracellular calcium, but not by d-tubocurarine. The toxin increased the amplitude of mouse sciatic-nerve compound action potentials by 30+/-9% (0.6 microM; p<0.05). Potassium currents elicited in freshly dissociated dorsal root ganglia neurones were blocked by 31+/-1% (n = 4; p<0.05) in the presence of 2.4 microM MiDCA1. These results show that MiDCA1 is a new presynaptic phospholipase A2 that produces neuromuscular blockade in vertebrate nerve-muscle preparations. The triphasic effects seen in mammalian preparations and the facilitatory response were probably caused mainly by the activation of sodium channels, complemented by the blockade of nerve terminal potassium channels. The inability of d-turocurarine to prevent the depolarization by MiDCA1 indicated that cholinergic nicotinic receptors were not involved in this phenomenon.

Loading and recycling of synaptic vesicles in the Torpedo electric organ and the vertebrate neuromuscular junction

In vertebrate motor nerve terminals and in the electromotor nerve terminals of Torpedo there are two major pools of synaptic vesicles: readily releasable and reserve. The electromotor terminals differ in that the reserve vesicles are twice the diameter of the readily releasable vesicles. The vesicles contain high concentrations of ACh and ATP. Part of the ACh is brought into the vesicle by the vesicular ACh transporter, VAChT, which exchanges two protons for each ACh, but a fraction of the ACh seems to be accumulated by different, unexplored mechanisms. Most of the vesicles in the terminals do not exchange ACh or ATP with the axoplasm, although ACh and ATP are free in the vesicle interior. The VAChT is controlled by a multifaceted regulatory complex, which includes the proteoglycans that characterize the cholinergic vesicles. The drug (-)-vesamicol binds to a site on the complex and blocks ACh exchange. Only 10-20% of the vesicles are in the readily releasable pool, which therefore is turned over fairly rapidly by spontaneous quantal release. The turnover can be followed by the incorporation of false transmitters into the recycling vesicles, and by the rate of uptake of FM dyes, which have some selectivity for the two recycling pathways. The amount of ACh loaded into recycling vesicles in the readily releasable pool decreases during stimulation. The ACh content of the vesicles can be varied over eight-fold range without changing vesicle size.

Vesicle size and transmitter release at the frog neuromuscular junction when quantal acetylcholine content is increased or decreased

We investigated whether the synaptic vesicles at the neuromuscular junction change size when their acetylcholine (ACh) content is altered. The size of the miniature endplate potential (MEPP) increased 3- or 4-fold in preparations pre-treated in a hypertonic solution in which the anion was gluconate. We measured the dimensions of synaptic vesicles in such preparations and in controls. The size of the vesicles and size distribution were indistinguishable. Quanta contained about half of the usual amount of ACh in preparations stimulated in the presence of hemicholinium-3, an inhibitor of choline uptake, or in NH(4)(+), which diminishes the proton gradient for ACh uptake into the vesicles. Neither treatment changed the size of the synaptic vesicles. ACh content and vesicle size were both decreased in preparations stimulated in (-)-vesamicol, an inhibitor of ACh uptake in vesicles. Since the other inhibitors decreased ACh content by a similar amount without altering vesicle size, (-)-vesamicol may decrease vesicle size by acting on another target. We also found that a hypertonic solution in which the anion was aspartate increased quantal size similar to gluconate. Both anions have high hydration energy and a large volume. When these treatments increased quantal size the mean 20-80 % rise time of MEPPs recorded with an extracellular electrode was 170 micros. In the controls it was 97 micros. Perhaps some of the added ACh is bound within the vesicles, which slows the rise. Our major conclusion is that ACh content can change notably without any change in the size of the synaptic vesicles.

Repetitive nerve stimulation decreases the acetylcholine content of quanta at the frog neuromuscular junction

We investigated how elevated quantal release produced by motor nerve stimulation affects the size of the quanta. The motor nerve was stimulated at 10 Hz in preparations in which excitation-contraction coupling was disrupted. Two hundred stimuli reduced the size of the time integrals of the miniature endplate currents ([integral]MEPCs), measured at the same junction immediately after stimulation, by 16 %. Three thousand stimuli reduced size by 23 %. When the solution contained 10 microM neostigmine (NEO) 3000 stimuli reduced [integral]MEPCs by 60 %, because with acetylcholinesterase (AChE) inhibited, [integral]MEPC size is more sensitive to changes in acetylcholine (ACh) content. Similar decreases in miniature endplate potential size ([integral]MEPP) followed repetitive stimulation of contracting preparations. The depolarization produced by iontophoretic pulses of ACh was scarcely changed by 3000 nerve stimuli at 10 Hz, suggesting that the decreases in miniature sizes are largely due to less ACh released per quantum. Following 3000 stimuli at 10 Hz the sizes of the [integral]MEPCs increased back to pre-stimulus values with a half-time of 8-10 min. Recovery was blocked by (-)-vesamicol (VES), by hemicholinium-3 (HC3) and by nicotinic cholinergic agonists - all of which inhibit ACh loading into synaptic vesicles. The number of quanta in the total store was estimated by releasing them with carbonyl cyanide m-chlorophenylhydrazone (CCCP). CCCP releases fewer quanta after stimulation than from unstimulated controls. After resting for hours following stimulation, the releasable number increased, even when ACh loading inhibitors were present. We conclude that the inhibitors do not block a significant fraction of the ACh loading into reformed reserve vesicles and propose that ACh can be loaded in a series of steps.

Prejunctional effects of the nicotinic ACh receptor agonist dimethylphenylpiperazinium at the rat neuromuscular junction

1. We have studied the effects of the nicotinic acetylcholine (ACh) receptor agonist dimethylphenylpiperazinium (DMPP) on the evoked release of ACh from motor terminals in the rat isolated hemidiaphragm using an electrophysiological approach. 2. DMPP (1-4 microM) had no effect on the rate of spontaneous quantal ACh release but increased the number of quanta of ACh released per impulse during 50 Hz stimulation. The DMPP-induced increase in evoked ACh release was dependent on the frequency of stimulation, being absent when it was reduced to 0.5 Hz, but was not Ca2+ dependent, being unaffected at 50 Hz by a 4-fold decrease in the extracellular Ca2+ concentration. 3. The facilitation of evoked ACh release at 50 Hz by 2 microM DMPP was abolished by 10 microM of the calmodulin antagonist W7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulphonamide hydrochloride) and, in the presence of W7, 2 microM DMPP depressed evoked ACh release at 0.5 Hz. The ability of the nicotinic ACh receptor antagonist vecuronium (1 microM) to depress evoked ACh release at 50 Hz was also abolished by 10 microM W7. 4. The present findings demonstrate, using an electrophysiological technique, that DMPP can produce changes in the evoked ACh release from rat motor nerve terminals that are consistent with the existence of facilitatory nicotinic ACh receptors on the motor nerve endings. Further, they indicate a role for calmodulin-dependent systems in this facilitatory effect of the compound.

Localizing quantal currents along frog neuromuscular junctions

1. We spatially localized the origins of quantal currents by recording simultaneously with two intracellular electrodes and employing the prediction of the one-dimensional cable equations that the time integrals of the resulting voltage changes fall off exponentially with distance. 2. Miniature endplate potentials (MEPPs) were more frequent near the centre of the endplate. In contrast to some work using other methods, we did not find MEPPs originating at the margins of the endplate to be strikingly smaller. 3. Spontaneous MEPPs and uniquantal endplate potentials (EPPs) were released over the same length of endplate and with the same relative probabilities at different regions. 4. Nicotinic agonists decreased evoked quantal output, but did not change the length over which uniquantal EPPs were generated. We conclude they do not block nerve conduction in the terminals. 5. Data sets were obtained with an extracellular electrode and two intracellular electrodes. The extracellular electrode was invariably near the centre of the region in which congruous MEPPs appeared to be generated. However, the range in the calculated positions of the synchronous MEPPs was as long as 0.8 mm. Therefore, it may be possible that extracellular electrodes have a longer recording range than commonly assumed.

Multiphoton-excited fluorescence of fluorogen-labeled neurotransmitters

Fluorescence detection of fluorogen-labeled neurotransmitters is demonstrated using 100 fs pulses from a titanium-sapphire mode-locked laser to achieve molecular excitation by simultaneous absorption of two and three photons of near-IR radiation. Two-photon excitation spectra are determined for the naphthalene-2,3 dicarboxaldehyde derivative of glycine and the fluorescamine derivative of leucine enkephalin, with the peak excitation cross section (o2) approximately equal to 1 x 10(-50) cm4 s/photon for both species. Three-photon-excitation fluorescence is demonstrated for o-phthaldialdehyde-labeled glutamate using excitation wavelengths between 965 and 1012 nm. The three-photon excitation cross section (o3) remains nearly constant in this wavelength range, with an absolute value of approximately 10(-84)-10(-85) cm6 s2/photon 2. Rapid cycling of analytes through the fluorescent excited state and detection that is free from background caused by Rayleigh and Raman scatter combine to make multiphoton-excited fluorescence a highly sensitive approach for detecting trace amounts of neurotransmitters. Measurements of two-photon-excited fluorescence of fluorescamine-labeled bradykinin and analysis of multiphoton-excited background reveal the potential of this method to detect fewer than 1000 neurotransmitter molecules.

Accounting for the shapes and size distributions of miniature endplate currents

The current model does not account adequately for the characteristics of miniature endplate currents (MEPCs). We do not understand their relatively slow rise, the shape of their rise, their variable and sometimes prolonged decay, and the correlation between amplitude and decay time. If we assume that ACh is released from the vesicle through a pore and that the vesicle enlarges as it takes on additional transmitter, the predictions are more like MEPCs. However, previous measurements showed that after quantal size was increased the vesicles in the terminal were not enlarged. This need not be a problem, because some of the ACh is added to vesicles positioned at the active zones, a process known as second-stage loading. By using the false transmitter precursor monoethylcholine we provide additional evidence for second-stage loading. The distribution of quantal sizes at the junction usually does not follow a normal probability distribution; it is skewed to the right. The skew can be accounted for by a model incorporating second-stage loading in which the vesicles are released randomly, without regard to their ACh content. If the vesicles increase in size when they contain more transmitter, only vesicles at the active zone need swell.

Statistics for studying quanta at synapses: resampling and confidence limits on histograms

This paper describes some statistical methods for working with data on quantal sizes. Since quantal sizes often do not fit to normal probability distribution functions, statistics based on the normal distribution are inappropriate. Resampling methods can be used to determine confidence limits and to test whether two sets of data differ by chance. Some hypotheses about the nature of quanta are based on apparent peaks and valleys in histograms. Confidence limits can be placed on the bins in the histogram by using the Kolomorogov-Smirnov statistic or by resampling. The confidence limits should assist in the evaluation of the significance of the valleys.

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.

Impact of alpha-bungarotoxin on transmitter release at the neuromuscular junction of the rat

The drug, alpha-bungarotoxin (BTX) is believed to be a 'pure' nicotinic antagonist. Hence, use of this drug should avoid the secondary actions associated with other nicotinic antagonists. The hypothesis that the motor nerve terminal responds to the presence of acetylcholine (ACh) by releasing less transmitter was tested by examining the effects of BTX on end-plate potentials (EPPs), miniature end-plate potentials (MEPPs), and quantal release at the rat diaphragm neuromuscular junction. Analysis of EPP and MEPP amplitudes and quantal release demonstrate that BTX significantly increases transmitter release at the onset of tetanic stimulation (50 Hz). Like other nicotinic antagonists, BTX was not able to sustain enhanced quantal release during a brief train of 40 stimuli and resulted in greater decline in EPP amplitude during tetanic stimulation. The data suggests that negative feedback regulation by presynaptic autoreceptors only serves a functional role at the onset of stimulation and that other factors such as transmitter supply or adenosine regulation may serve to dominate transmitter release during maintained tetanic stimulation.

Inhibitory action of nicotinic antagonists on transmitter release at the neuromuscular junction of the rat

The effects of two nicotinic antagonists, d-tubocurarine (TC) and hexamethonium (HEX) were tested on the rat diaphragm neuromuscular junction during train-of-six stimuli to determine if a second action of these antagonists on evoked release could be demonstrated, in addition to its known impact of blocking the autoreceptor pathway. To minimize the autoreceptor pathway, the preparations were examined under low transmitter release conditions. It was observed that both compounds significantly depressed the end-plate potential amplitudes more than the miniature end-plate potential amplitudes, while also significantly depressing quantal release output. This inhibitory action is contrary to what is observed when transmitter release is high, where feedback regulation via the autoreceptors serves a prominent role. It is concluded that this depressive action on transmitter output contributes to onset of tetanic fade and that when higher concentrations of these antagonists are used this inhibitory action of TC and HEX may override autoreceptor feedback regulation.