Effects of several 'membrane stabilizing' agents on frog neuromuscular junction

Acute and chronic effects of clozapine on cholinergic transmission in cultured mouse superior cervical ganglion neurons

Cholinergic dysfunction contributes to cognitive deficits in schizophrenia. The atypical antipsychotic clozapine improves cognition in patients with schizophrenia, possibly through modulation of the cholinergic system. However, little is known about specific underlying mechanisms. We investigated the acute and chronic effects of clozapine on cholinergic synaptic transmission in cultured superior cervical ganglion (SCG) neurons. Spontaneous excitatory postsynaptic currents (sEPSCs) were detected and were reversibly inhibited by the nicotinic receptor antagonist d-tubocurarine, confirming that the synaptic responses were primarily mediated by nicotinic receptors. Bath application of clozapine at therapeutic concentrations rapidly and reversely inhibited both the amplitude and frequency of sEPSCs in a concentration-dependent manner, without changing either rise or decay time, suggesting that clozapine effects have both presynaptic and postsynaptic origins. The acute effects of clozapine on sEPSCs were recapitulated by chronic treatment of SCG cultures with similar concentrations of clozapine, as clozapine treatment for 4 d reduced the frequency and amplitude of sEPSCs without affecting their kinetics. Cell survival analysis indicated that SCG neuron cell counts after chronic clozapine treatment were comparable to the control group. These results demonstrate that therapeutic concentrations of clozapine suppress nicotinic synaptic transmission in SCG cholinergic synapses, a simple in vitro preparation of cholinergic transmission.

Inhibition of skeletal muscle nicotinic receptors by the atypical antipsychotic clozapine

We have previously observed that certain atypical antipsychotic drugs reduce the amplitude and duration of miniature end-plate currents (EPCs) at the frog neuromuscular junction (Effects of atypical antipsychotics on vertebrate neuromuscular transmission, Nguyen, Q.-T., Yang, J., Miledi, R. Neuropharmacology 42, 2002, 670-676), therefore suggesting that these drugs act on nicotinic acetylcholine receptors. In this study we examined the effects of the atypical antipsychotic clozapine on nicotinic receptors of frog neuromuscular end-plates or in Xenopus oocytes expressing the alpha(1)beta(1)gamma delta mouse skeletal muscle nicotinic receptor. At neuromuscular junctions, postsynaptic currents were reduced by micromolar concentrations of clozapine. This compound also acted presynaptically by increasing the quantal content of EPCs of muscles without noticeably affecting paired-pulse facilitation. In oocytes, clozapine inhibited alpha(1)beta(1)gamma delta receptors with an IC(50) of 10 microM and a Hill coefficient of 1. Blockage of alpha(1)beta(1)gamma delta receptors by clozapine bears several hallmarks of open-channel blockers, including faster response decays, strong voltage dependence of the block, large rebound currents upon wash, and reduction of peak responses even at saturating concentrations of acetylcholine. However, clozapine increased the EC(50) for acetylcholine and its blocking effect was enhanced by preincubation. These results suggest that clozapine antagonizes muscle nicotinic receptors by blocking open channels, and possibly also by another mechanism which still remains to be investigated.

Chlorpromazine, but not chlorpromazine sulphoxide, stimulates transmitter release from motor nerve terminals

Treatment of frog neuromuscular preparations with chlorpromazine (5 mumol/l) resulted in a marked rise in miniature endplate potential (MEPP) frequency of greater than 100% within 30 min, and an increase in evoked transmitter release (quantal content 5-15) of about 35%. Treatment with chlorpromazine sulphoxide (5 microM), a derivative of chlorpromazine with a much lower affinity for calmodulin, had very little effect on either form of transmitter release. It is concluded that stimulatory effects of calmodulin-binding drugs at the nerve terminal may well be exerted through calmodulin inhibition. The stimulatory effect of chlorpromazine on MEPP frequency was markedly reduced in preparations bathed in EGTA-containing Ca2+-free saline, but the response was largely restored by raising the temperature by 3-4 degrees C. It is argued that despite this partial dependence on [Ca2+]o, stimulation of transmitter secretion by chlorpromazine is likely to be mediated by inhibition of calmodulin-activated Ca2+-ATPases, and consequent elevation of [Ca2+]i.

Postsynaptic inhibitory effects of phenothiazines at cholinergic synapses may not involve calmodulin

Treatment of frog cutaneous-pectoris nerve-muscle preparations with calmidazolium (R 24571), a calmodulin-inhibitor, at concentrations of 2 X 10(-7) mol/l and 5 X 10(-7) mol/l had no discernable effect on MEPP amplitude. 10(-6) mol/l calmidazolium caused a small (10-35%) increase in MEPP amplitude in most preparations. The phenothiazine calmodulin-inhibitor chlorpromazine (5 X 10(-6) mol/l) caused a clear reduction in MEPP amplitude (20%) after 30 min treatment. Similar experiments carried out with chlorpromazine sulphoxide (a derivative of chlorpromazine that is 60 X less potent in inhibition of calmodulin-activated enzymes) produced data that were very similar to those obtained with chlorpromazine. It is concluded that the postsynaptic inhibitory effect of phenothiazines at cholinergic synapses is unlikely to involve calmodulin.

Calmodulin, synchronous and asynchronous release of neurotransmitter

Evidence collected from studies on a wide range of secretory cells suggests that calmodulin may play an important role in stimulus-secretion coupling. Work on synaptosomes, central synaptic preparations and chromaffin cell preparations indicates that calmodulin probably also acts as the intracellular Ca2+-receptor for secretion in neuronal cells, Ca2+-binding resulting in activation of protein kinases and phosphorylation of certain secretory vesicle proteins. Studies on the effects of calmodulin-binding drugs at peripheral synapses have given surprising results, particularly the finding that evoked (synchronous) transmitter release is not suppressed by calmodulin inhibition, though asynchronous release can be markedly inhibited. It is suggested that the insensitivity of synchronous release to drug treatment is due to the fact that only vesicle-bound calmodulin is involved in this form of transmitter secretion. Asynchronous release, however, involves recruitment of cytosolic calmodulin and can therefore be inhibited by calmodulin-binding drugs.

Effects of vinblastine in the frog neuromuscular junction during repetitive stimulation

Vinblastine (10(-5) to 10(-4) M) applied in a Mg-blocked sciatic nerve-sartorius muscle preparation of the frog at the beginning of 20 min of repetitive stimulation of the nerve at 10 Hz, markedly inhibited the facilitation period. There was a smaller increase of end-plate potential amplitude, of quantal content, and of miniature end-plate potential (MEPP) frequency in comparison with that found in experiments without the drug. This depressant effect was more evident when the preparation was preincubated in vinblastine: synaptic responses often disappeared after a few minutes, whereas MEPP frequency attained a maximum at the 5th min. The results suggest a multiple site of action for vinblastine. First, an alteration of both vesicle and plasmalemma membrane, as revealed by the increase of MEPP frequency and by the change of the binomial release features. Second, the drug could damage the intracellular transport system, interfering with the mobilization of the vesicles toward the nerve endings, as shown by the very low or absent facilitation.

Role of calcium in triggering the release of transmitters at the neuromuscular junction

Calcium ions have a key role in triggering the release of packaged transmitter at the amphibian neuromuscular junction and of the chromaffin granules at the adrenal medulla. It is suggested that (i) proteins on the vesicle and plasma membranes are of particular importance in promoting membrane fusion and exocytosis (ii) they may be divalent cation-stimulated ATPases, which form the calcium-binding sites or have a specific calcium-binding protein in close molecular apposition (iii) these ATPases in synaptic vesicles and chromaffin granules also generate a protonmotive force which is associated with the uptake of transmitter (iv) the osmotic properties of the vesicle may be important during fission, but it is not suggested that chemiosmotic effects are involved in Ca2+-triggered fusion (v) the action of calcium is markedly co-operative (vi) the adrenal medullary cell and the n.m.j. may differ in the Ca2+-binding site; there is evidence for the involvement of calmodulin in granule-plasmalemma fusion in the chromaffin cells, but not at present (surprisingly) for a role of this Ca2+-binding protein at the n.m.j. (vii) exocytosis requires MgATP (viii) phosphorylation of the ATPase may well be involved; phosphorylation via cAMP does not seem to be involved in fusion in either system (ix) the ATPase may undergo configurational changes during exocytosis and is markedly sensitive to the physical state of its phospholipid environment and to the oxidation of its -SH groups.

Diamide, temperature and spontaneous transmitter release at the neuromuscular junction: stimulation of exocytosis by a direct effect on membrane fusion?

The thiol-oxidizing agent diamide markedly increases m.e.p.p. frequency at the frog neuromuscular junction, even at low [Ca2+]0 and also when the mitochondria are uncoupled with DNP. The effect is reversed by dithioerythritol and is very temperature-sensitive, with a marked transition at 16 degrees C; m.e.p.p. frequency is raised 2- to 5-fold at 13-15 degrees C and 55- to 60-fold at 17-20 degrees C. Diamide increases the frequency of large amplitude m.e.p.p.s, the effect being explicable as the fusion of two or more vesicles. It is concluded that (a) diamide does not act at the Ca2+ channels of the plasma membrane, nor at the mitochondria. It affects the release system directly via an alteration of membrane protein --SH groups; (b) the eventual decline in m.e.p.p. frequency after DNP treatment is because of the exhaustion of mitochondrial Ca2+ rather than a depletion of quanta; (c) the major effect of temperature is on the release mechanism, perhaps via a phase-change in the phospholipoproteins of the plasmalemma or vesicles, rather than an elevation of [Ca2+]i; (d) either diamide or temperatures above 16 degrees C make Ca2+ more effective in promoting vesicle-plasmalemma fusion.

The effect of prednisolone on neuromuscular transmission in the rat diaphragm

Prednisolone, in concentrations of 0.004--0.032 mmol/l, increased the amplitude of the miniature end-plate potentials (MEPPs). A maximum increase to 134% of the control values was seen at 0.016 mmol/l. At higher prednisolone concentrations the MEPP amplitude gradually decreased to reach 77% of the control value at 0.62 mmol/l. The MEPP frequency was increased to twice the control value at 0.62 mmol/l. The quantal content of the end-plate potential (EPP), however, was not influenced by prednisolone. The response to iontophoretically applied acetylcholine was diminished, especially at 0.62 mmol/l prednisolone. Prednisolone, therefore, caused presynaptic effects as was shown by an increase in unitary MEPP amplitude and by a considerable number of giant MEPPs, which at increasing prednisolone concentrations was antagonized increasingly by a postsynaptic depressant effect. These results also provide an explanation for the adverse effects of prednisolone on the end-plate potential and on neuromuscular transmission.