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

Anticholinesterase induces nicotinic receptor modulation

The effects of carbamate anticholinesterases, pyridostigmine and physostigmine, on the function of the nicotinic receptor (nAChR) in TE671 cells was studied, precluding their inhibition of acetylcholine hydrolysis by carbachol usage. In radioassay, the simultaneous application of carbachol and carbamates dose-dependently decreased carbachol-induced 22Na+ influx, compared with carbachol activation alone. Increasing cell preincubation in the presence of carbamates, however, potentiated influx at low concentrations in a time-dependent manner. This facilitating effect of carbamates, even at high concentrations, was significantly increased by washing out these drugs and was blocked by pretreatment with diisopropylfluorophosphate. Similar results were also obtained in whole-cell patch-clamp study. There were insignificant changes in desensitization properties during facilitation. It is thus supposed that facilitation cannot be explained by the inhibition of acetylcholine hydrolysis. These results support a previous hypothesis that acetylcholinesterase might modulate nAChR by an unknown mechanism. In addition, the clinical effects of carbamates may be partly attributed to this facilitation.

Effects of organophosphates on rabbit pyramidal cells firing pattern and hippocampal theta rhythm

The effects of the irreversible acetylcholinesterase (AChE) antagonist paraoxon (Px) on hippocampal neurophysiology were investigated and compared to those of physostigmine in urethane-anaesthetized rabbits. Hippocampal CA1 EEG signals were analyzed by power spectra. Following intracarotid administration, the two drugs induced a similar fundamental low-frequency theta power peak while the appearance of a second theta harmonic was commonly found under Px. Again, inhibition of CA1 pyramidal cells firing was significantly more pronounced after Px injection than after physostigmine. A potent inhibitory action was also described following local Px iontophoretic application. However, a discrepancy appeared between the effects of Px and the classical cholinergic drugs (acetylcholine, physostigmine). The results indicate that Px and physostigmine have a rather similar influence on the septo-hippocampal pathway and support suggestions that Px could act within local hippocampal circuitry through other systems than the cholinergic system exclusively.

Xanthine derivatives IBMX and S-9977-2 potentiate transmission at an Aplysia central cholinergic synapse

In an attempt to investigate the role of cAMP-dependent phosphorylations on synaptic transmission at an Aplysia cholinergic buccal ganglion synapse, the effects of xanthine derivatives such as 3-isobutyl-1-methylxanthine (IBMX), which is well known to inhibit phosphodiesterase activity thereby promoting cAMP accumulation, and a novel xanthine derivative, S-9977-2 were evaluated. They were found to potentiate cholinergic transmission by significantly increasing the time constant of decay (Tc) of inhibitory postsynaptic currents (IPSCs). The postsynaptic origin of the phenomenon was supported by the observation that responses to the ionophoretic application of acetylcholine (ACh) were also potentiated in duration as well as in amplitude. No effects of S-9977-2 on the ACh-gated Cl- channel conductance or mean open time were observed. The finding that responses to the hydrolysis-resistant cholinergic analogue carbachol were unaffected by the two xanthines suggested that the observed effects were at least partly caused by an inhibition of acetylcholinesterase (AChE) activity. That these substances inhibit AChE activity was confirmed in vitro. Phosphorylation processes nonetheless appear to be partly involved in the synaptic effect of the xanthines as the kinase blocker H-8 blocked part of the IPSC Tc lengthening. Possible mechanisms are discussed.

Histamine and FLRFamide regulate acetylcholine release at an identified synapse in Aplysia in opposite ways

1. The effects of histamine and FLRFamide (Phe-Leu-Arg-Phe-NH2) on acetylcholine (ACh) release were studied in the buccal ganglion of Aplysia californica on an identified synapse (buccal ganglion inhibitory synapse, BGIS) involved in a small neuronal circuit controlling the feeding behaviour. The inhibitory postsynaptic current (IPSC) evoked by a presynaptic spike in the voltage-clamped postsynaptic neurone was decreased by histamine and increased by FLRFamide. 2. Histamine and FLRFamide modified the amplitude of the presynaptic spike. To test if these drugs acted directly on presynaptic calcium influx, we evoked transmitter release by 3 s depolarizations of the presynaptic neurone (to +10 mV) under voltage clamp to avoid modifications of presynaptic membrane polarization induced by changes in presynaptic voltage-dependent K+ and/or Na+ conductances. 3. Statistical analysis of this evoked long-duration (3 s) induced postsynaptic current (LDIPSC) allowed us to calculate the amplitude and the decay time of the miniature postsynaptic current and consequently the number of quanta released by the presynaptic terminal. 4. The amplitude of the LDIPSC decreased during the 3 s presynaptic depolarization. This was not due to a lack of available transmitter, since LDIPSC amplitude could be maintained constant by a 'clamp of the release of ACh' which adequately depolarized the presynaptic neurone, but rather to changes in the calcium influx into the presynaptic neurone. 5. FLRFamide increased more the initial portions of the LDIPSC than the final portions. This effect of FLRFamide was only reduced and delayed by atropine or curare, antagonists of muscarinic-like and nicotinic-like autoreceptors previously demonstrated to be present at the same terminal. Activation of the nicotinic-like receptors, which also increased transmitter release, induced a modification of the shape of the LDIPSC which was completely different from that due to FLRFamide. 6. Histamine decreased the amplitude of the LDIPSC. This effect was more pronounced at the beginning of the response. The effects of histamine were insensitive to curare and atropine, but were completely blocked by cimetidine, a specific histamine receptor antagonist. 7. The modifications of the shape and of the amplitude of the LDIPSC by FLRFamide and histamine suggested that these molecules alter presynaptic influx of calcium. This was confirmed by the analysis of calcium current recorded from the presynaptic neurone: the calcium inward current in the presynaptic neurone was increased by FLRFamide and reduced by histamine, whereas the activation of autoreceptors had no measurable effect on calcium current.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of protein kinase C by presynaptic FLRFamide receptors facilitates transmitter release at an aplysia cholinergic synapse

Modulation of evoked quantal transmitter release by protein kinase C (PKC) was investigated at an identified cholinergic neuro-neuronal synapse of the Aplysia buccal ganglion. Evoked acetylcholine release was increased by a diacylglycerol analog that activates PKC and was decreased by H-7, a blocker of PKC. FLRFamide facilitated evoked quantal release by increasing presynaptic Ca2+ influx. The inhibition of PKC by H-7 prevented both the increase of presynaptic Ca2+ influx and the facilitation of evoked acetylcholine release induced by the activation of presynaptic FLRFamide receptors. These results provide evidence that the activation of PKC could be a step in the intracellular pathway by which FLRFamide receptors increase evoked quantal acetylcholine release.

Receptor-mediated presynaptic facilitation of quantal release of acetylcholine induced by pralidoxime in Aplysia

1. Possible interactions of contrathion (pralidoxime sulfomethylate), a reactivator of phosphorylated acetylcholinesterase (AChE), with the regulation of cholinergic transmission were investigated on an identified synapse in the buccal ganglion of Aplysia californica. 2. Transmitter release was evoked either by a presynaptic action potential or, under voltage clamp, by a long depolarization of the presynaptic cell. At concentrations higher than 10(-5) M, bath-applied contrathion decreased the amplitude of miniature postsynaptic currents and increased their decay time. At the same time, the quantal release of ACh was transiently facilitated. The facilitatory effect of contrathion was prevented by tubocurarine but not by atropine. Because in this preparation, these drugs block, respectively, the presynaptic nicotinic-like and muscarinic-like receptors involved in positive and negative feedback of ACh release, we proposed that contrathion activates presynaptic nicotinic-like receptors. 3. Differential desensitization of the presynaptic receptors is proposed to explain the transience of the facilitatory action of contrathion on ACh release. 4. The complexity of the synaptic action of contrathion raises the possibility that its therapeutic effects in AChE poisonings are not limited to AChE reactivation.

Both presynaptic nicotinic-like and muscarinic-like autoreceptors regulate acetylcholine release at an identified neuro-neuronal synapse of Aplysia

The possible involvement of cholinergic presynaptic receptors regulating evoked quantal acetylcholine (ACh) release was investigated at an identified cholinergic neuro-neuronal synapse in the buccal ganglion of Aplysia, using cholinergic agonists (carbachol, pilocarpine, oxotremorine) and/or antagonists (curare, atropine, hexamethonium). Bath applied carbachol or pilocarpine (10(-8) M to 10(-4) M) induced a decrease in the evoked quantal release of ACh. As the effects of carbachol were prevented by atropine (5.10(-6) M) and not by curare (10(-5) M), it was concluded that carbachol activated presynaptic muscarinic-like receptors implicated in a negative feed-back on ACh release. On the contrary, oxotremorine (up to 10(-4) M) induced a potentiation of ACh release which was suppressed by curare (4.10(-6) M) or hexamethonium (10(-5) M) but not by atropine (5.10(-6) M) pointing to the activation of presynaptic nicotinic-like receptors implicated in a positive feed-back on ACh release. Moreover, in the presence of curare, oxotremorine decreased ACh release: this suggested that oxotremorine also activated the presynaptic muscarinic-like receptors. These results revealed the conjoint presence, on the same terminal, of both muscarinic-like and nicotinic-like autoreceptors.

Modulation of an acetylcholine receptor responsiveness by filipin and chlorpromazine studied in neurons of Aplysia californica

The responsiveness of Aplysia acetylcholine receptors (AChR) was studied using a polyene antibiotic, filipin, which specifically complexes cholesterol, and another compound, chlorpromazine (CPZ), which inserts at the proteolipidic interface. Both substances enhanced the evoked postsynaptic responses or responses to iontophoretic application of carbachol only on the H-type receptor (opening a Cl-permeability), whereas at the same concentrations filipin was without effect on the D-type receptor (opening a cationic permeability) while CPZ depressed the D-type response. The facilitation observed specifically for the H-type receptor was similar to that previously described after acetylcholinesterase (AChE) inhibition or when low concentrations of detergents were applied to this preparation. No additive effect was obtained after the addition of chlorpromazine following a maximal potentiation obtained with an anticholinesterase agent. Since at Aplysia central neurons, AChE is a membranal protein, we propose that the facilitation of H-type responses is attributable to the removal of a modulatory action of AChE on AChR. Filipin or chlorpromazine might disrupt the interaction between AChR and AChE.

Fasciculin II, a protein inhibitor of acetylcholinesterase, tested on central synapses of Aplysia

Fasciculin II, a potential inhibitor of acetylcholinesterase (AChE), was tested on two types of Aplysia cholinergic receptors: H type, opening Cl- channels; and D type, opening cationic channels. Evoked postsynaptic inhibitory responses and responses to ionophoretic application of acetylcholine (ACh) or carbachol onto H-type receptors were potentiated in the presence of fasciculin II at 10(-9) M, whereas the same concentration of this drug was without effect on the evoked postsynaptic excitatory responses or on the application of ACh or carbachol on D-type receptors. The observed effects of fasciculin II were identical to those obtained with other inhibitors of AChE on the same preparation. The facilitatory effect on the carbachol response in H-type cells indicates that fasciculin II, as other AChE inhibitors, does not act on H-type synapses solely by blocking the hydrolysis of ACh. We concluded that fasciculin II was a good inhibitor of acetylcholinesterase on neuronal preparations in vivo. The results are further discussed as a new element in favor of a previously proposed hypothesis of a molecular interaction between AChE and ACh H-type receptors.

Noncholinesterase actions of an irreversible acetylcholinesterase inhibitor on synaptic transmission and membrane properties in autonomic ganglia

Superfusion of the organophosphorous acetylcholinesterase inhibitor soman (pinacolyl methylphosphonofluoridate; 0.01-25 microM) produced a dose-dependent reduction of extracellularly and intracellularly recorded synaptic responses in the isolated rat superior cervical ganglia at frequencies of orthodromic stimulation that do not normally produce synaptic depression. The magnitude of depression was dependent upon the frequency of stimulation (0.02-1 Hz), was maintained after the removal of soman from the superfusion solution, and recovered by over 65% during periods of inactivity. The depression of synaptic transmission produced by soman was not dependent upon the inhibition of acetylcholinesterase (AChE) activity by this agent. Transmission was increasingly depressed by doses of soman greater than those needed to inactivate all measurable ganglionic AChE activity. Dose-dependent depression of synaptic transmission in soman also occurred after pretreatment with the irreversible AChE inhibitor diisopropylphosphofluoridate (DFP; 100 microM), which inhibited greater than 98% of the AChE activity in the ganglia. Soman produced a decline in the input resistance, resting potential, spike amplitude, and spike threshold and a reduction in the hyperpolarizing afterpotential. Soman-induced depression of synaptic transmission was not due primarily to a blockade of postsynaptic nicotinic receptors. At concentrations of soman which produced significant depression in transmission, ganglionic depolarization produced by bath-applied carbamylcholine (carbachol) was either slightly depressed or facilitated. In the presence of soman, repetitive focal application of acetylcholine or carbachol did not reveal use-dependent desensitization. Muscarinic antagonists, atropine and pirenzepine, protected against the use-dependent depression of synaptic transmission induced by soman. These results suggest that a principal site of action for soman is at the presynaptic terminal and that this site is sensitive to muscarinic receptor blockade.

Mechanism of hydrogen ion transport in the diluting segment of frog kidney

Transepithelial H+ transport was studied in diluting segments of the isolated-perfused kidney of rana esculenta. The experiments were performed in controls as well as in K+-adapted and Na+-adapted animals (exposed to 50 mmol/l KCl or NaCl, resp. for at least 3 days). Conventional and single-barreled, liquid ion-exchanger H+-sensitive microelectrodes were applied in the tubule lumen to evaluate transepithelial H+ net flux (JHte) as well as limiting transepithelial electrical and H+ electrochemical potential differences (PDte, EHte) and luminal pH at zero net flux conditions. The measurements were made in absence (control) and presence of furosemide (5 X 10(-5) mol/l) or amiloride (10(-3) mol/l). EHte (lumen positive vs ground) was 19 +/- 3 mV in controls, 43 +/- 3 mV in K+ adapted but about zero in Na+ adapted animals. Using the corresponding PDte-values, steady state luminal pH of 7.63 +/- 0.05, 7.13 +/- 0.05 and 8.02 +/- 0.02 was calculated for the respective groups of animals (peritubular pH 7.80). In parallel, significant secretory JHte (from blood to lumen) was found in controls (14 +/- 2 pmol X cm-2 X s-1) which was stimulated by K+ adaptation (61 +/- 8 pmol X cm-2 X s-1) but reversed in direction by Na+-adaptation (-8 +/- 1 pmol X cm-2 X s-1). Amiloride inhibited secretory JHte.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of obidoxime chloride on native and sarin-poisoned frog neuromuscular junctions

The effect of the oxime reactivator obidoxime chloride (obidoxime) on single frog neuromuscular junctions has been studied in order to clarify its action on the acetylcholine receptor (AChR) and on the acetylcholine esterase (AChE), both before and after blocking its enzymatic activity with the organophosphorus compound sarin. Experiments iontophoretic application of obidoxime to end-plates demonstrated that it has a weak direct depolarizing effect. Furtheron, the drug is shown to possess a potentiating effect on the ACh-induced depolarization. After the AChE activity had been inhibited with sarin, obidoxime on the contrary decreases the depolarization induced by ACh. Both effects are fully reversible. It is concluded that obidoxime acts as an inhibitor of the AChE and as a partial antagonist of the AChR. The antagonistic effect on the receptor is usually masked by the predominating anticholinesterase effect. The effect of obidoxime on miniature end-plate potentials in long-time experiments on sarin-poisoned muscles, showed only weak signs of recovery from the action of the AChE inhibitor. Only focally higher concentration of the drug produced a more marked but short term recovery of the mepps, which is, however, supposed to be dependent on the AChR antagonism. It is still unclear how much of the varying therapeutic usefulness of obidoxime in clinical cases is due to its AChE reactivation and how much to the antagonistic effect on the AChR.

Possible role of acetylcholinesterase in regulation of postsynaptic receptor efficacy at a central inhibitory synapse of Aplysia

Most of the effects of acetylcholinesterase (AChE) on synaptic transmission are considered to be related to its acetylcholine (ACh) hydrolysing properties. This is clearly apparent from changes which occur in the characteristics of the miniature endplate potential and of the endplate potential at neuromuscular junctions when AChE is inhibited1-4 and during the development of enzymatic AChE activity at maturing synapses5. However, we report here that after inhibiting AChE in a cholinergic synapse in Aplysia, we found an increase not only in postsynaptic responses to presynaptic stimulation and to ionophoretic application of ACh on postsynaptic receptors, but also to ionophoretic application of carbachol. This could not be explained by the inhibition of the ACh hydrolysing function of the enzyme, as carbachol is not hydrolysed by AChE. A possible explanation of these observations is that inhibition of the enzyme affects a property of the ACh receptor (AChR) itself.