The effect of acetylcholine upon mammalian motor nerve terminals

Effect of temperature on motor responses in organophosphate intoxication

We studied the effect of temperature on median motor responses in a 41-year-old man with organophosphate intoxication. At 32 degrees C, a normal amplitude compound motor action potential (CMAP) and a smaller spontaneous repetitive discharge (SRMAP) were present. With warming to 39 degrees C, the CMAP amplitude decreased 20%, while the SRMAP amplitude decreased 33%. With cooling to 14 degrees C, the CMAP amplitude decreased 9%, while the SRMAP became unobtainable. Possible mechanisms for these findings are discussed.

Physiological and regenerative acetylcholine release from motor nerve: differential inhibitions by vesamicol and omega-agatoxin IVA

Stimulation of mammalian motor neurons can elicit Ca(2+)-dependent regenerative release of acetylcholine and prolonged endplate depolarization when the enzymatic degradation of the neurotransmitter is inhibited. Unlike physiological phasic release of acetylcholine, the regenerative release is sensitive to L-type Ca2+ channel blockers. We studied the effects of vesamicol (an inhibitor of active transport of acetylcholine into synaptic vesicles) and omega-agatoxin IVA (a blocker of the motor nerve P-type Ca2+ channel) on these two types of acetylcholine release to compare the vesicle pools and Ca2+ channels responsible for the release. When coupled with repetitive stimulations, vesamicol decreased mean amplitude of miniature endplate potentials, resulting in a skewed distribution to lower amplitude, reduced quantal content of endplate potentials and decreased immediate available pool of acetylcholine. omega-Agatoxin IVA had no effect on miniature endplate potential but inhibited quantal content of endplate potential. The mean inhibitory concentration was around 5-10 nM. Vesamicol and omega-agatoxin IVA decreased the probability of triggering regenerative release. However, the magnitude and duration of regenerative release, once triggered, were not depressed by either agent. It appears that the majority of Ca2+ necessary for regenerative release is translocated via omega-agatoxin IVA-insensitive Ca2+ channels, which can be activated by prolonged depolarization of nerve terminals induced by accumulated acetylcholine. The results suggest that different Ca2+ channels are activated in the regenerative (L-type) and phasic (P-type) acetylcholine release, which utilize different pools of synaptic vesicles.

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.

Pharmacological studies on cutaneous inflammation induced by ultraviolet irradiation (1): quantification of erythema by reflectance colorimetry and correlation with cutaneous blood flow

This study was conducted to quantify the intensity of ultraviolet (UV) erythema in guinea pigs, a method for evaluating anti-inflammatory drugs, and to clarify any correlation of erythema with cutaneous blood flow. Skin color and cutaneous blood flow in non-administered and indomethacin-administered animals were measured by a colorimeter and a laser Doppler flowmeter over time after UV-irradiation treatment. Skin color was indicated by a XYZ colorimetric system and L*a*b* color space. In either colorimetric system, the values of two indices, x and y or a* and b*, increased along with the intensification of erythema. The increase in the chroma (C*) value calculated from a* and b* was UV-dose-dependent. This value was significantly suppressed by indomethacin 0.5-4 hr after irradiation, and it was found to be a clear and sensitive index for evaluating the suppressive effect of drugs. Cutaneous blood flow also increased with UV irradiation. Indomethacin significantly suppressed this increase 2-3 hr after UV irradiation. The changes of cutaneous blood flow correlated with those of C*. These results suggested C* was a suitable parameter to quantify UV erythema, and the change of skin color in UV erythema reflected the change of cutaneous blood flow.

Effects of tetrodotoxin, Ca2+ absence, d-tubocurarine and vesamicol on spontaneous acetylcholine release from rat muscle

1. Rat hemidiaphragms were incubated in a physiological low-K+ medium without stimulation and the amount of acetylcholine (ACh) released was measured radioenzymatically. Cholinesterases were inhibited by paraoxon. 2. In the presence of 1 microM tetrodotoxin (TTX), the amount of ACh released during a 2 h incubation was lowered by 40%. A similar decrease was observed in the absence of Ca2+ and in the presence of 10 microM-d-tubocurarine (dTC). The effects of TTX combined with Ca2+ removal, and of TTX combined with dTC were no greater than those of TTX, dTC or Ca2+ removal alone. TTX and dTC had no effect on the release of ACh from diaphragms 4 days after denervation. 3. The reduction of spontaneous ACh release observed in the presence of TTX or dTC or in the absence of Ca2+ is best interpreted on the assumption that about 40% of the ACh release was due to the impulse activity known to be generated in intramuscular motor nerve branches by the ACh which accumulates after the inhibition of cholinesterases. 4. In the presence of 1 and 10 microM vesamicol (AH5183, 2-(4-phenylpiperidino)-cyclohexanol), the release of ACh was also diminished by approximately 40%. Vesamicol did not augment the inhibition of release produced by TTX or by the omission of Ca2+.(ABSTRACT TRUNCATED AT 250 WORDS)

Temperature dependence of carbachol-induced modulation of miniature end-plate potential frequency in rats

In the rat soleus, the frequency of miniature end-plate potentials (MEPP) did not change after application of 10(-5) M of the cholinomimetic drug carbachol between 18 degrees C and 34 degrees C but decreased by 40% at physiological temperatures of 37-38 degrees C. The carbachol-induced decrease in MEPP frequency was not eliminated by 10(-7) to 10(-8) M atropine or 3 x 10(-7) (+)-tubocurarine similarly as had been previously found at frog neuromuscular junction.

Nicotinic actions of oxotremorine on murine skeletal muscle. Evidence against muscarinic modulation of acetylcholine release

The effects of oxotremorine, arecoline and muscarine on neuromuscular transmission of mouse or rat phrenic nerve-diaphragm were investigated. For some studies of endplate potentials (e.p.p.s) the preparation was immobilized by cutting muscle fibers. Oxotremorine (0.3-10 microM) depolarized endplate membranes, reduced miniature e.p.p. amplitudes but increased frequency, induced spontaneous neural discharges and muscle fasciculations, and produced contracture of denervated mouse diaphragm. In mouse and young rat preparations pretreated with Mn2+, Co2+, Ni2+, Cd2+ or low Ca2+ Tyrode to depress evoked acetylcholine release, oxotremorine 0.3-1 microM increased indirect twitches as well as amplitudes and quantal contents of e.p.p.s. These increases were not observed when the synaptic transmission was not depressed, nor in adult rat preparations. The augmentation by oxotremorine of evoked acetylcholine release persisted in preparations pretreated with neostigmine (1 microM) and tetrodotoxin (20 nM), which inhibited acetylcholinesterase and oxotremorine-induced spontaneous neural discharges. These effects of oxotremorine were mimicked by arecoline but not by muscarine and were antagonized by tubocurarine (0.3 microM) but not by atropine (0.1-10 microM). Atropine alone did not affect indirect twitches, synaptic transmission, tetanic responses evoked by direct stimulation of diaphragms, nor the durations of muscle action potential. The direct twitch responses were only slightly increased by oxotremorine at 2-3 microM. Oxotremorine at high concentrations (greater than 2 microM), depressed indirect twitches and e.p.p. amplitude, and accelerated the run-down of trains of e.p.p.s. The IC50 on indirect twitches was reduced by pretreatment with diltiazem or proadifen, which are known to promote receptor desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Selective antagonism to succinylcholine-induced depolarization by alpha-bungarotoxin with respect to the mode of action of depolarizing agents

1. The interactions of alpha-bungarotoxin or tubocurarine with the neuromuscular block and endplate depolarization induced by succinylcholine (SCh) in the phrenic nerve-diaphragm preparation of mice were studied in order to elucidate the role of depolarization by SCh in the neuromuscular blockade. 2. The SCh concentrations required to depress the indirect twitch response by 20% and the evoked endplate potential in cut muscle preparations by 80% were 10 microm and 6 microM, respectively, while only 2 microM SCh was needed to induce maximal endplate depolarization from -80 mV to about -60 mV. 3. SCh blocked the neuromuscular transmission synergistically with either alpha-bungarotoxin or tubocurarine. There was an initial partial reversal of the neuromuscular inhibition caused by tubocurarine, but not that by alpha-bungarotoxin. 4. alpha-Bungarotoxin (0.025 microM) antagonized SCh (10 microM)-induced depolarization more effectively than it depressed miniature endplate potentials and the antagonism was insurmountable by increasing SCh concentration. By contrast, tubocurarine preferentially depressed miniature endplate potentials and antagonized SCh-depolarization competitively. 5. The above difference was attributed to the irreversible nature of alpha-bungarotoxin binding to acetylcholine receptors, to the slow diffusion of the toxin molecule into the synaptic cleft and thus to the more rapid binding with perijunctional receptors compared with junctional ones. 6. It is concluded that the sustained depolarization of the endplate by SCh results largely from an action on the perijunctional receptor in mice and, unlike cats, the neuromuscular block by SCh is not due to the depolarization per se but rather to a direct attenuation of endplate potential.

The nature of the presynaptic effects of (+)-tubocurarine at the mouse neuromuscular junction

1. The effects of (+)-tubocurarine (TC) on tetanic run-down and quantum content of end-plate potentials (EPPs) were investigated in cut-fibre preparations of mouse diaphragm. 2. (+)-Tubocurarine, 0.15 microM, halved the amplitude of spontaneous miniature EPPs (MEPPs) and steepened the tetanic run-down of EPPs evoked at 10 Hz by increasing the quantum content of the first EPP of the train while having no effect on quantum content of plateau EPPs. With stimulation at 1 Hz, there was little run-down and the quantum content of all EPPs was increased by TC. 3. The use of binomial statistics to analyse release indicated that after TC the increase in the quantum content of the first EPP in the train at 10 Hz was due to an increase in n and that during the run-down there was a decrease in p so that plateau EPP quantum content at 10 Hz was not different from control. 4. To elucidate a possible role of cholinoreceptors in the presynaptic effects of TC, studies were made on the effects of pancuronium or of alpha-bungarotoxin (BTX), with concentrations and exposure times where they had postsynaptic effects equal to 0.15 microM-TC. The run-down of EPPs was unaffected by BTX, while pancuronium steepened it to a lesser extent than TC. 5. The anticholinesterase, ecothiopate, decreased the quantum content of plateau EPPs only at high frequencies of stimulation (50 Hz) and did not affect the presynaptic effects of TC at 10 Hz. 6. At concentrations which reduced MEPP amplitude, atropine (10 microM) or hexamethonium (50 microM) had no effect on EPP run-down. 7. These results indicate that TC could have presynaptic effects via a presynaptic acetylcholine receptor, but that such a receptor may not have the same binding specificities as the postsynaptic receptor.

The origin of the anticholinesterase-induced repetitive activity of the phrenic nerve-diaphragm preparation of the rat in vitro

1. Action potentials have been recorded in contracting muscle cells of the phrenic nerve-diaphragm preparation from rats. After the organophosphorous anticholinesterase, ecothiopate, some cells fired repetitive action potentials. In 0.1 mM [Mg2+]o the repetitive activity was generated presynaptically or postsynaptically, and in 1 mM [Mg2+]o, probably only postsynaptically. 2. The repetitive action potentials in muscle were generated ectopically about 0.2 mm away from the usual site. 3. In 1 mM [Mg2+]o, spontaneous action potentials in muscle were generated presynaptically. These were often followed by repetitive action potentials generated either presynaptically or postsynaptically. 4. The initiation of centripetal action potentials in the phrenic nerve was coincident with the repetitive firing in the muscle. 5. The results are discussed in relation to the manner of generation of repetitive activity in nerve and muscle after anticholinesterases.

The resting release of acetylcholine by a retinal neuron

The cholinergic amacrine cells of the rabbit retina secrete acetylcholine by two mechanisms. One is activated by stimulation of the retina by light or depolarization of the amacrine cells by K+ ions. It requires the presence of extracellular Ca2+. The second is independent of extracellular Ca2+ and is unaffected by large depolarizations of the cells. It bears some similarity to the acetylcholine 'leakage' described at the neuromuscular junction. Although the Ca2+-independent mechanism accounts for about two thirds of the total acetylcholine release in the dark, the amount of acetylcholine released in this way is small compared with the release of acetylcholine triggered by stimulation of the retina with light. Its biological significance is unclear.

Presynaptic actions of curare and atropine on quantal acetylcholine release at a central synapse of Aplysia

1. In a cholinergic synaptic couple in the buccal ganglion of Aplysia california, where the synaptic areas are situated close to the somata (500 micron), we were able to control transmitter release by stimulating the cell body of the presynaptic neurone with long depolarizing pulses in the presence of tetrodotoxin (TTX). 2. Statistical analysis of noise occurring at the peak of the long-depolarization-induced post-synaptic current (p.s.c.) responses allowed us to calculate the amplitude and the decay time of the miniature post-synaptic currents (m.p.s.c.s). These data were used to calculate the quantal content of the responses. 3. Bath-applied tubocurarine reduced the amplitude of the long-depolarization-induced p.s.c. more than that of the m.p.s.c.s, indicating that tubocurarine exerts a depressive presynaptic action on the quantal content of the post-synaptic responses. 4. Tubocurarine injected into the presynaptic neurone blocked synaptic transmission without decreasing the size of the m.p.s.c.s probably by acting on the mechanism of transmitter release. 5. Bath-applied atropine (10(-6) and 10(-5) M) caused a slight decrease of the m.p.s.c.s but the long-depolarization-induced p.s.c.s increased, as did the quantal content. Higher concentrations of atropine depressed strongly both the m.p.s.c. and the quantal content. 6. Injection of atropine into the presynaptic neurone had the same effect as its bath application, probably due to the leakage of the drug into the synaptic cleft; the effect depended on the concentration reached in the cleft, i.e. on the quantity of injected drug. The synapses of the neighbouring cholinergic neurone were also affected by this leak of atropine. 7. The presence of nicotinic presynaptic receptors blocked by tubocurarine, and muscarinic presynaptic receptors blocked by atropine, which regulate synaptic transmission by facilitating and depressing the ACh release respectively, is discussed.

Presynaptic nicotine receptors mediating a positive feed-back on transmitter release from the rat phrenic nerve

The effects of 1,1-dimethyl-4-phenylpiperazinium (DMPP) and of nicotine receptor antagonists on [3H]acetylcholine release from the rat phrenic nerve preincubated with [3H]choline were investigated in the absence and presence of cholinesterase inhibitors (presynaptic effects). Additionally, the effects of hexamethonium and tubocurarine on the muscle contraction of the indirectly stimulated diaphragm were examined (postsynaptic effects). DMPP (1-30 microM) increased (76-92%), whereas hexamethonium (0.001-1 mM) and tubocurarine (1-10 microM) decreased (52-60%) the release of [3H]acetylcholine following a train of 100 pulses at 5 Hz. The release caused by a longer train (750 pulses at 5 Hz) was only slightly affected by DMPP and tubocurarine. In the presence of neostigmine (10 microM) neither tubocurarine nor DMPP significantly modulated the evoked [3H]acetylcholine release. High DMPP concentrations (10 and 30 microM) enhanced the evoked release only when the pretreatment interval was reduced from 15 min to 20 s. Tubocurarine and hexamethonium concentration-dependently inhibited the end-organ response. Hexamethonium was 250-fold more potent on presynaptic than on postsynaptic nicotine receptors. It is concluded that the motor nerve terminals are endowed with presynaptic nicotine receptors. These autoreceptors mediate a positive feed-back mechanism that can be triggered by previously released endogenous acetylcholine. Receptor desensitization can be produced by high agonist concentrations (endogenous or exogenous agonists) and is probably one mechanism to limit the autofacilitatory process. The presynaptic receptors appear to differ in their pharmacological properties from the postsynaptic receptors.

Botulinum toxin prevents stimulus-induced backfiring produced by neostigmine in the mouse phrenic nerve-diaphragm

The origin of motor nerve antidromic activity (backfiring) induced by anticholinesterase treatment was examined in the mouse phrenic nerve-hemidiaphragm preparation. Botulinum toxin was used to determine whether backfiring is due to (a) a direct effect of the cholinesterase inhibitor on the nerve terminal, or (b) an indirect effect via the prolongation of the action of acetylcholine. In previously untreated control preparations, neostigmine produced spontaneous and stimulus-induced antidromic activity in the phrenic nerve when rapidly introduced into the diaphragm via its vasculature. This activity could be reversibly blocked by d-tubocurarine and decamethonium, but not by atropine. Neostigmine-induced backfiring did not occur in preparations in which transmitter release was blocked with botulinum toxin. Infusion of a small bolus of a high concentration of acetylcholine following neostigmine treatment resulted in a short-term increase in the incidence of antidromic activity, followed by block, in both controls and botulinum toxin-treated preparations. It is concluded that transmitter release is necessary for the production of backfiring following cholinesterase inhibition since neostigmine alone does not elicit antidromic activity in botulinum toxin-treated preparations at concentrations which are effective in controls. Our results support the hypothesis that the effects of neostigmine on the motoneurone terminal are mediated by the prolonged action of acetylcholine that occurs with inhibition of acetylcholinesterase.

Axonal transport of alpha-bungarotoxin binding sites in rat sciatic nerve

[125I]alpha-Bungarotoxin (alpha-BuTX) binding sites accumulate both proximal and distal to a ligature positioned around the sciatic nerve of rats. [125I]alpha-BuTX binding sites, localized using quantitative receptor autoradiography, were found to accumulate at nerve ligatures at a relatively constant rate which suggests that they undergo both anterograde and retrograde axonal transport. [125I]alpha-BuTX binding to sections of ligated sciatic nerve was saturable with apparent dissociation constants of 0.97 nM proximal and 0.53 nM distal to the ligature. D-Tubocurarine, nicotine, decamethonium and atropine displaced [125]alpha-BuTX from sciatic nerve sections with affinities comparable to those previously reported for the toxin binding component of rat brain. These data indicate that [125I]alpha-BuTX binding sites pharmacologically similar to those of rat brain are transported in sciatic nerve. Axonally transported toxin binding sites may correspond to those previously localized to the plasma membrane of peripheral nerve axons and on the terminals of motor neurons.

Inhibition of acetylcholinesterase accelerates axon terminal withdrawal at the developing rat neuromuscular junction

In developing skeletal muscles, the rate at which superfluous innervation is lost from the endplates depends on the general level of neuromuscular activity. Whether it is activity of the presynaptic or postsynaptic structures (or both) that is critical is not well established. In this work, we transitorily inhibited the AChE of soleus muscle in postnatal rats, in order to increase postsynaptic activity, without directly altering activity of the nerve terminals. We then followed the time course of disappearance of axon terminals from the endplates of treated and normal muscles, using electron-microscope techniques. Three hours after inhibition of AChE, the muscle fibres exhibited local supercontracture and ultrastructural damage in the region of the endplate, consistent with local elevation of Ca2+ levels. At the same time, small electron-opaque vesicles, apparently of muscular origin, appeared in the synaptic cleft. The nerve terminals, however, were entirely normal in number and appearance. One day after treatment, endplates of esterase-inhibited muscles showed accelerated loss of nerve terminals, compared to endplates of normally developing muscles. No further loss of nerve terminals occurred, once AChE activity returned at the endplate. These results suggest that the rate at which superfluous nerve terminals retract from the developing neuromuscular junction is regulated by the level of activation of the muscle. It seems likely that activity of postsynaptic sites may similarly regulate changes in innervation patterns, in other developing or adapting neuro-neuronal or neuro-effector systems.

Presynaptic function modified by acetylcholine-receptor interaction in experimental autoimmune myasthenia gravis

Rats with experimental autoimmune myasthenia gravis (EAMG) were studied to test if the postsynaptic receptor disease would be modified by a compensating presynaptic effect of acetylcholine (ACh) being mediated through the presynaptic receptor. In normal rat phrenic-diaphragms, the ACh quantum content was estimated using the cut-muscle technique by which the evoked release of transmitter can be measured in the absence of blocking agents; values were significantly increased when the ACh receptor was blocked by d-tubocurarine. In rats with EAMG, this presynaptic autoregulation acted to compensate the postsynaptic failure, but did not reach the range of d-tubocurarine-treated controls. Results may be explained by either the difference between the receptor sites for myasthenic antibodies and d-tubocurarine or the immunological property of the presynaptic receptor which may differ from the postsynaptic one.

The relationship between stimulus-induced antidromic firing and twitch potentiation produced by paraoxon in rat phrenic nerve-diaphragm preparations

The relationship between stimulus-induced repetitive antidromic firing (ADF) in the motor nerve and twitch potentiation produced by the organophosphate anticholinesterase paraoxon, has been investigated in rat diaphragm preparations. Little or no ADF was produced by paraoxon in preparations bathed in a Tyrode solution containing 1 mM calcium and 1 mM magnesium ions although the preparations showed marked twitch potentiation. Increases in the calcium:magnesium ion ratio produced a ratio-dependent increase in the ADF but had no consistent effect on peak twitch potentiation. Dithiothreitol, a disulphide bond reducing agent which decreases the affinity of acetylcholine for nicotinic cholinoceptors, abolished ADF but only modified the time course of twitch potentiation. Dithiothreitol, a disulphide bond reducing agent which decreases the affinity of acetylcholine for nicotinic cholinoceptors, abolished ADF but only modified the time course of twitch potentiation. It is concluded that the initiation of ADF is a consequence of the prolonged action of acetylcholine within the synaptic cleft following inhibition of acetylcholinesterase, and that ADF is not the only mechanism by which twitch potentiation can be produced.

A comparison of the effects of acute and chronic cholinesterase inactivation on spontaneous transmitter release

The irreversible inhibitor of acetylcholinesterase (AChE), paraoxon, when given in vivo to rats in a single injection (0.23 mg/kg s.c.) raised the miniature endplate potential (MEPP) frequency to values greater than 3 times control levels in 34% of the fibers in the rat phrenic nerve-hemidiaphragm preparation. The elevated MEPP frequencies were observed in areas of extensive muscle twitching and were associated with high frequencies of giant MEPPs. Following 3 daily injections of paraoxon; the overall MEPP frequency was reduced below control levels, the frequency of giant MEPPs returned to normal, and a greater percentage of fibers showed no spontaneous activity. This depressant effect of chronic AChE inhibition on the overall MEPP frequency diminished during 1-2 weeks of daily paraoxon treatment (0.12 mg/kg s.c. paraoxon/day). After one week of recovery from 14 daily paraoxon injections (0.12 mg/kg, 1 injection/day), the original response to a single injection (0.23 mg/kg) was restored. In an attempt to determine whether paraoxon exerts its effects on spontaneous release by depolarizing the presynaptic terminal, the effect of increases in the potassium concentration on the MEPP and giant potential frequency were examined in control (saline injection) preparations, and preparations treated with 1 or 3 daily injections of paraoxon. The results suggest that paraoxon does not act by reducing the presynaptic membrane potential, but may interact more directly with the mechanism(s) responsible for regulating the release of MEPPs and giant MEPPs.

Direct and indirect effects of an organophosphorus acetylcholinesterase inhibitor and of an oxime on a neuro-neuronal synapse

The action of an irreversible inhibitor of acetylcholinesterase (AChE), the organophosphorus compound, ecothiopate iodide, and of a reactivator of phosphorylated AChE, contrathion, were analysed on acetylcholine (ACh) receptors and cholinergic synaptic transmission in the buccal ganglion of Aplysia. At high concentration (above 10(-4)mol X 1(-1), both compounds exerted a curare-like depression on ACh receptors which was reversible with washing. Both compounds reversibly facilitated the current response to ionophoretic application of ACh and increased the evoked postsynaptic current (PSC) as well as the miniature postsynaptic currents (MPSCs). All responses also showed an increase in decay time. These modifications, when induced by ecothiopate iodide were irreversible by washing; however they could be reversed if first washed with contrathion. Neither the organophosphate compound or the oxime did change the number of quanta released per impulse. The current response to ionophoretic application of carbachol also increased after ecothiopate iodide was added. In the limits of the method used, the conductance and opening time of postsynaptic ionic channels opened by ACh were not found to be modified by the two compounds. It was concluded that the facilitatory action of the organophosphorus inhibitors cannot be solely explained by the inhibition of ACh hydrolysis.

Influence of presynaptic receptors on neuromuscular transmission in rat

The presence and physiological significance of acetylcholine (ACh) receptors on motor nerve terminals was examined at the rat diaphragm neuromuscular junction. Intracellular recording techniques were used to monitor end-plate potentials (EPP), miniature end-plate potentials (MEPP), and resting potentials of the muscle fibers. Muscle action potentials were blocked by the cut-muscle technique. Quantal release was determined by the ratio EPP/MEPP, after correcting for nonlinear summation. Blockade of acetylcholinesterase with eserine and neostigmine was tested to determine the influence of residual ACh on transmitter release. Partial blockade of ACh receptors with curare was examined to further clarify the role of these presynaptic receptors. The experiments demonstrate that residual ACh inhibits transmitter release and that blockade of ACh receptors enhances transmitter release. It is concluded that presynaptic ACh receptors exist and that they serve an important physiological function. It is suggested that the presynaptic ACh receptors normally serve to limit transmitter release in a negative feedback pathway.

Evidence for the role of non-quantal acetylcholine in the maintenance of the membrane potential of rat skeletal muscle

1. Resting membrane potentials of rat diaphragm muscles cultured in Trowell T8 medium were measured in vitro. After 3 hr in culture the resting membrane potential of muscle fibres within 2.5 mm of nerve section (;near') was -68.3 +/- 0.4 mV (nineteen preparations). This was significantly lower (P < 0.001) than the resting potential (-74.0 +/- 0.4 mV) measured in muscle fibres 8-10 mm from the site of nerve section (;far') in the same preparations. A difference between the ;near' and the ;far' fibres was maintained in muscles cultured for 6 and 12 hr. Miniature end-plate potentials were present in both ;near' and ;far' fibres cultured for 3 and 6 hr and ceased after 12-15 hr.2. The presence of carbamylcholine (10(-7) or 10(-8) M) maintained the resting membrane potential of ;near' fibres close to that of ;far' fibres at 3, 6 and 12 hr. For example, at 3 hr in the presence of 10(-8) M-carbamylcholine the mean resting potential was 75.6 +/- 0.5 mV in ;near' fibres and 76.1 +/- 0.4 mV in ;far' fibres (four preparations). A similar effect was produced in preparations exposed to anticholinesterases: diisopropylphosphorofluoridate (DFP) (10(-7) M), neostigmine (10(-7) M) or physostigmine (10(-5) M).3. Agents that blocked acetylcholine receptors had the reverse effect. In the presence of alpha-bungarotoxin (1 mug/ml.) or d-tubocurarine (10(-5) M) the resting membrane potential of ;far' fibres was reduced to the level of ;near' fibres over the 24 hr period of observation. For example, at 3 hr in the presence of alpha-bungarotoxin the mean resting potential was 67.2 +/- 0.5 mV in ;near' fibres and 68.5 +/- 0.6 mV in ;far' fibres (six preparations). The effect of d-tubocurarine was reversible.4. When muscles were cultured in Ca(2+)-free medium containing 1 mM-EGTA and 10 mM-Mg(2+), there was no difference in membrane potential between ;near' and ;far' fibres and physostigmine (10(-5) M) was ineffective in raising the membrane potential of ;near' fibres.5. It is suggested that non-quantal acetylcholine released from nerve terminals maintains the membrane potential of muscle fibres through a Ca(2+)-dependent mechanism.

Binding of horseradish peroxidase-alpha-bungarotoxin to axonal membranes at the node of Ranvier

The binding of horseradish peroxidase (HRP)-labeled alpha-bungarotoxin (alpha-BuTx) was investigated in rat sciatic nerve. Activity was found to be localized to the axolemma of myelinated nerve fibers at the nodes of Ranvier. Activity was also seen in other regions of the axolemma where the myelin sheath was separated from the axon by enzymatic treatment. Pretreatment of nerves with native alpha-BuTx or curare blocked the binding of HRP-alpha-BuTx to the axonal membranes. This study demonstrates binding of alpha-BuTx to axonal membranes although the nature and significance of the toxin receptor is uncertain.

Do motor-nerve terminals have gamma-aminobutyric acid receptors?

1 gamma-Aminobutyric acid (GABA, 0.1 to 1 mM) had no significant effect on the amplitude, rise time, half decay time or frequency of miniature endplate potentials (m.e.p.ps) at the frog or mouse neuromuscular junctions in vitro. 2 Addition of GABA (1 mM) to preparations previously treated with 11 mM K+-Ringer did not cause any further increase in m.e.pp. frequency. GABA also failed to increase the m.e.p.p. frequency in a low Cl--Ringer. 3 GABA (0.1 to 1 mM) did not reduce the high m.e.p.p. frequency induced by veratrine (20 to 40 mg/l). 4 GABA (0.5 to 1 mM) did not affect the amplitude of the extracellularly-recorded nerve terminal spike, whereas 15 mM [K+] reduced the spike. 5 The quantal content (m) of the evoked endplate potential was not significantly altered by GABA; 9 mM [K+] significantly increased m. 6 When external d.c. potential differences were recorded in a three-chambered bath, GABA (0.1 to 1 mM) produced a very small depolarization if applied to the phrenic nerve trunk, but not if applied to the pre-terminal axon/motor nerve terminal region. Carbachol (0.3 to 1 mM) evoked a small depolarization when applied to the nerve terminal chamber. 7 These results fail to provide evidence for the existence of GABA receptors on motor nerve terminals.

Corticosteroids and neuromuscular transmission: electrophysiological investigation of the effects of prednisolone on normal and anticholinesterase-treated neuromuscular junction

The effect of prednisolone on indirectly stimulated rat muscle twitch was investigated at normal and prostigmine-treated neuromuscular junctions. In vivo, predenisolone up to 150 mg/kg body weight did not affect twitch contraction in normal animals. In neostigmine-pretreated animals, however, doses between 12.5 and 90 mg/kg could entirely abolish the anticholinesterase-induced twitch augmentation. In vitro, prednisolone produced a depressant effect on the twitch of a normal phrenic nerve diaphragm preparation which could amount to 20%. When the preparation was pretreated with neostigmine the augmented twitch could be depressed by 10(-3) to 10(-6) mol/l prednisolone to levels below the untreated control. Part of this effect is owing to a suppression of the neostigmine-induced, stimulus-bound repetitive firing of the motor nerve terminals, but to explain the full effect a further inhibitory action on neuromuscular transmission must be assumed. The latter could be accounted for by a depolarizing interaction of prednisolone and neostigmine on the nerve terminals resulting in conduction block. An action of prednisolone on postsynaptic receptors could also be considered. Such effects of the glucocorticoid might contribute to the exacerbation of muscular weakness occasionally observed in patients with myasthenia gravis at the beginning of steroid therapy.

Presynaptic action of curare

1. As a result of a conditioning phrenic nerve stimulus, end-plate currents (e.p.c.s) in a voltage clamped uncurarized cut diaphragm show a facilitation which reaches its maximum at 30-40 msec and subsequently decays with a time constant from 150 to 200 msec. In curarized (cut or uncut) diaphragms, however, the conditioning stimulus causes a depression which reaches its maximal value at 10 msec and then decays slowly with a time constant of about 3 sec. This indicates that curare strongly interferes with the process of transmitter release. 2. The presynaptic action of curare is also evident if short tetanic trains are given. In uncurarized preparations e.p.c.s decay in size much more slowly than in curarized preparations, and usually show a transient facilitation. 3. These results can be explained in terms of a model where curare blocks presynaptic depolarizing action of ACh. As a result of this presumed curare action a small increase in Ca permeability and subsequent entry of Ca associated with depolarization are also blocked, and the facilitation resulting from that entry of Ca is abolished.

Possible role of thiamine in neuromuscular transmission

Pyrithiamine (50 mg/kg), a thiamine antagonist, decreased the muscle twitches of the rat masseter muscle at stimulation frequencies above 1 Hz 40--80 min after an i.v. injection. The post-tetanic potentiation (PTP) induced by nerve stimulation of the masseter muscle was abolished by pyrithiamine. Administration of thiamine restored the muscle twitches at stimulation frequencies above 1 Hz and the PTP. The muscle twitches elicited by direct muscle stimulation were not affected by pyrithiamine treatment. The abolishment of the PTP was accompanied by a decrease in thiamine and thiamine-diphosphate. The pyruvate level in the blood was unchanged after pyrithiamine treatment. Oxythiamine, on the other hand, had no effect on the PTP but increased the pyruvate level in the blood. Fern extract which contains thiaminase I also abolished the PTP--an effect reversible by the addition of thiamine. The frequency-induced depression of the muscle twitches induced by pyrithiamine was similar to the effect of low doses of d-tubocurarine (8 microgram/kg). The results support the hypothesis that thiamine may play a role in neuromuscular transmission.

Presynaptic origin of penicillin after discharges at mammalian nerve terminals

The site of origin and mechanism underlying the generation of repetitive after-discharges produced by penicillin was studied in the isolated rat phrenic nerve-hemidiaphragm preparation. Application of low concentrations of sodium penicillin to the bathing solution initiated bursts of antidromic action potentials originating at or near the motor nerve terminals following single orthodromic stimuli to the nerve. Afterdischarges could not be elicited by direct stimulation of the muscle fibers alone, or when the nerve trunk was isolated from the neuromuscular junction and exposed to penicillin. D-Tubocurarine applied in doses sufficient to abolish postsynaptic responses did not diminish penicillin-induced after discharges. At concentrations which most reliably produced repetitive firing (5000 IU/ml; 8.5 mM), penicillin did not accelerate the frequency of spontaneous transmitter release (MEPPs), yet significantly increased the relative excitability of nerve endings to extracellular stimulation. It is concluded that penicillin acts directly and preferentially on presynaptic nerve terminals to induce repetitive afterdischarges which arise independently of postsynaptic depolarization, transmitter-mediated potassium efflux, or muscle fiber contraction. The results suggest that the convulsant effects of penicillin at a mammalian neuromuscular junction are due to non-depolarizing alterations in the intrinsic excitability of the terminal membrane which increase the probability of suprathreshold depolarizations during the recovery period of spike electrogenesis. Several models of the mechanisms which might produce hyperexcitability at presynaptic nerve terminals are discussed.

The role of cyclic nucleotides in the CNS

On the basis of the information presented in this review, it is difficult to reach any firm decision regarding the role of cyclic AMP (or cyclic GMP) in synaptic transmission in the brain. While it is clear that cyclic nucleotide levels can be altered by the exposure of neural tissues to various neurotransmitters, it would be premature to claim that these nucleotides are, or are not, essential to the transmission process in the pre-or post-synaptic components of the synapse. In future experiments with cyclic AMP it will be necessary to consider more critically whether the extracellularly applied nucleotide merely provides a source of adenosine and is thus activating an extracellularly located adenosine receptor, or whether it is actually reaching the hypothetical sites at which it might act as a second messenger. The application of cyclic AMP by intrcellular injection techniques should minimize this particular problem, although possibly at the expense of new diffulties. Prio blockade of the adenosine receptor with agents such as theophylline or adenine xylofuranoside may also assist in the categorization of responses to extracellularly applied cyclic AMP as being a result either of activation of the adenosine receptor or of some other mechanism. Utimately, the developement of highly specific inhibitor for adenylate cyclase should provide a firm basis from which to draw conclusions about the role of cyclic AMP in synaptic transmission. Similar considerations apply to the action of cyclic GMP and the role of its synthesizing enzyme, guanylate cyclase. The use of phosphodiesterase inhibitors in studies on cyclic nucleotides must also be approached with caution. The diverse actions of many of these compounds, which include calcium mobilization and block of adenosine uptake, could account for many of the results that have been reported in the literature.

Dopaminergic neurons: an in vivo system for measuring drug interactions with presynaptic receptors

An in vivo system has been used to investigate the ability of dopamine agonists and antagonists to alter dopamine synthesis by acting at what appear to be presynaptic dopamine receptors. In order to eliminate postsynaptically induced changes in dopamine synthesis caused by the effects of these drugs on the firing rate of dopamine neurons, gammabutyrolactone was administered to block impulse flow in the nigro-neostriatal pathway. The accumulation of Dopa in the rat striatum after administration of Dopa decarboxylase inhibitor was used as an index of striatal tyrosine hydroxylase activity. It was found that administration of the dopamine agonists, apomorphine or ET-495 [1-(2-pyrimidyl)-piperonyl-piperazine], modified the apparent activity of striatal tyrosine hydroxylase when impulse flow was blocked in dopamine neurons. This presynaptic effect of apomorphine could be prevented by low doses of loxapine haloperidol and spiroperidol. Chlorpromazine, fluphenazine, and thioridizine were much less effective than the butyrophenones in blocking the effects of apomorphine. Molindone and (+) butaclamol, but not (-) butaclamol, reversed the presynaptic agonist effects, pimozide was a weak blocker and clozapine had no effect at all. All these neuroleptics except (-) butaclamol caused a significant increase in Dopa accumulation when impulse flow was intact. Compared with haloperidol the phenothiazines and pimozide appeared less potent in reversing the presynaptic effects of apomorphine than in blocking the behavioral effects of this agonist. Possible functional significance of the presynaptic dopamine receptors are considered.

Diamide acts intracellularly to enhance transmitter release: the differential permeation of diamide, DIP, DIP+1 and DIP+2 across the nerve terminal membrane

The actions of the new potent thiol oxidizing agents, diazene dicarboxylic acid bis (N'-methyl piperazide) (DIP) and the N'-methyl iodide (DIP + 1) and the bis-N'-methyl iodide (DIP + 2) salts of DIP, were tested at the frog neuromuscular junction. At 20 degrees C, DIP was as fast as the thiol oxidizing agent, diamide, in evoking transmitter release but was appreciably less effective at 6 degrees C. DIP + 1 and DIP + 2 did not increase transmitter release. Since the three agents are potent oxidizers of glutathione and since the effectiveness of the compounds appears to depend on their ability to exist, at least in part, in a neutral form at physiological pH, it is concluded that their action as promoters of transmitter release depends on their ability to permeate nerve terminal membranes. Thus, both diamide and DIP act to increase transmitter release by the intracellular oxidation of glutathione. The two charged agents, DIP + 1 and DIP + 2, are potent muscular depolarizing agents. It is probable that the quaternary nitrogen groups of these compounds render them cholinomimetics.

Tetanic fade during partial transmission failure produced by non-depolarizing neuromuscular blocking drugs in the cat

1. A comparison has been made of the effects of three acetylcholine antagonists--hexamethonium, tubocurarine and pancuronium--on maximal tetani of limb muscles of cats under chloralose anaesthesia. In most experiments, the indirectly stimulated soleus muscle was studied, but observations were also made on the tibialis anterior and flexor digitorum longus muscles. 2. When neuromuscular block was produced by intra-arterial injections of the acetylcholine antagonists, tetanic tension, though depressed in amplitude did not wane and there was little or no post-tetanic relief of the block as judged by the amplitude of subsequent twitches. On the other hand, during similar degrees of block produced by intravenous injections, tetanic tension rapidly waned, and, after the tetanus, transmission was temporarily facilitated, as evidenced by an increase in the amplitude of post-tetanic twitches. 3. Intravenously injected hexamethonium caused complete waning of tetanic tension in doses too small to depress twitch amplitude and which caused only a small depression of peak tetanic tension. In contrast, pancuronium caused only partial tetanic fade even in doses that produced pronounced depressions of twitch and tetanic tensions. The effects of tubocurarine fell between these extremes. 4. The results suggest that depression of peak tension and tetanic fade are independent effects of acetylcholine antagonists. It is postulated that the former is a consequence of block of post-junctional cholinoceptors, whereas the latter arises from an action of pre-junctional cholinoceptors. 5. The results obtained, together with those of other workers, led to the suggestion that transmitter acetylcholine, in addition to evoking the endplate potential, acts on the nonmyelinated nerve terminals in a positive feed-back mechanism that mobilizes transmitter to keep pace with release during high frequencies of stimulation.