Actions of dendrotoxin on K+ channels and neuromuscular transmission in Drosophila melanogaster, and its effects in synergy with K+ channel-specific drugs and mutations

The blockade of K+ channels and enhancement of neuromuscular transmission by dendrotoxin (DTX), a convulsant peptide from mamba snake venom, were examined in normal and mutant larval preparations of Drosophila. Two-microelectrode voltage-clamp experiments showed that DTX reduced the transient K+ current, IA, in muscle membrane. This effect was suppressed by raising the Mg2+ concentration or by lowering the temperature. The interaction of DTX with Mg2+ was further analyzed at a low cation concentration, at which DTX reduced both IA and the delayed rectifier IK. These results were correlated with the action of DTX on the neuromuscular junction. Its facilitatory effect on excitatory junctional potentials (EJPs) was relatively mild but the effect was drastically enhanced when combined with certain mutations and K+ channel blocking drugs, leading to repetitive or prolonged giant EJPs. Only the mutations or drugs that reduced IK or the Ca2(+)-dependent K+ current, ICF, could yield these synergistic effects with DTX. In contrast, the abnormal EJPs caused by the mutation or drug that blocked IA were not further enhanced by DTX, indicating that DTX also affects IA at the neuromuscular junction. Thus, the A-type K+ channels in muscle and nerve terminals appeared very similar in their sensitivity to the specific toxin, drugs and mutations examined here.

Effects of memantine on the twitch tension of mouse diaphragm

The effects of memantine (50-175 microM) on the post-tetanic potentiation of the twitch tension were studied on the isolated mouse nerve diaphragm preparation. Memantine completely abolished the twitch tension elicited indirectly while it had no effect on the directly elicited twitch tension. Memantine also decreased the post-tetanic potentiation of amplitude of endplate potential and twitch tension. The duration of tetanic stimulation that induced a maximal decrease of twitch tension was 10-20 s. It is suggested that the effect of memantine on post-tetanic potentiation may be due to its voltage- and time-dependent effect on the ion channel-acetylcholine receptor complex.

The effect of carbamazepine on the post-tetanic twitch tension in the diaphragm of the mouse

The effects of carbamazepine (0.084-0.25 mM) on the post-tetanic potentiation of the twitch tension, were studied on the isolated phrenic nerve diaphragm preparation of the mouse. Carbamazepine decreased the post-tetanic potentiation of the twitch tension. The maximal depressant effect was found after higher frequencies and longer durations of stimulation. After repetitive stimulation, the amplitude of the endplate potential and the frequency of miniature endplate potentials were potentiated. Carbamazepine decreased the post-tetanic potentiation of the amplitude of endplate potential and the frequency of miniature endplate potentials. The directly-elicited muscle action potential was not affected when post-tetanic potentiation of the twitch tension was decreased. It is concluded that carbamazepine suppressed the post-tetanic potentiation of the indirectly-elicited twitch tension, mainly due to its pre-synaptic inhibitory effect.

The effects of myasthenic immunoglobulin G on neuromuscular transmission in mouse diaphragm

The effect of immunoglobulin G (IgG) from a patient with myasthenia gravis (MG) on neuromuscular transmission of an isolated mouse diaphragm preparation was studied by using electrophysiological methods. The frequency and amplitude of miniature endplate potentials (MEPPs) of the diaphragms obtained from mice injected intraperitoneally with purified IgG from MG patient were reduced by about 13% and 20%, respectively in comparison with results from normal mouse diaphragms. The amplitude of endplate potential (EPP) and the quantal content of EPPs were also reduced by about 21% and 7%, respectively. No significant changes in EPP and MEPP amplitudes were seen in the diaphragms of mice injected with normal IgG serum. It is postulated IgG from myasthenia gravis acts to interfere with both the receptor and the nerve terminal functions.

Effect of phencyclidine on post-tetanic twitch tension of the mouse diaphragm preparation

The effects of phencyclidine(PCP) on the post-tetanic potentiation(PTP) of twitch tension were studied on the isolated mouse phrenic nerve diaphragm preparation. Phencyclidine increased directly elicited twitch tension while it decreased post-tetanic potentiation of the indirectly elicited twitch tension. The maximal depression effect of the PTP was found after higher frequencies and longer durations of stimulation. After repetitive stimulation, the amplitude of endplate potential was potentiated. Phencyclidine decreased the post-tetanic potentiation of the amplitude of endplate potential while the quantal content of the endplate potential was not affected. 4-Aminopyridine increased both directly and indirectly elicited twitch tension while it did not inhibit the post-tetanic potentiation of the twitch tension. It is concluded that phencyclidine suppressed the post-tetanic potentiation of the indirectly elicited twitch tension. The depressant effect may be mainly due to its effect on the acetylcholine receptor-ionic channel complex of the motor endplate.

Effect of neomycin on post-tetanic twitch tension of the mouse diaphragm preparation

The effect of calcium (0.5-6 mM) and neomycin (0.1-0.2 mM) on the maximum post-tetanic twitch tension (MTT) and post-tetanic depression (PTD) of the indirectly elicited twitch tension was studied on the mouse isolated phrenic nerve-diaphragm preparation. The effect of neomycin on MTT of directly stimulated twitch tension was also tested in (+)-tubocurarine pretreated preparations. Three-dimensional plots between MTT and frequency and duration of indirect tetanic stimulation revealed that the frequencies and durations inducing maximal MTT were 500 Hz for 20 s in 0.5 mM CaCl2, 100 Hz for 5 s in 2 mM CaCl2 and 100 Hz for 10 s in 6 mM CaCl2. The frequency and duration inducing maximal PTD was 100 Hz for 20 s in 0.5 mM CaCl2, but there was no PTD in 2 mM or 6 mM CaCl2. Neomycin was associated with significantly greater MTT than in control if the duration of tetanic stimulation was 1 or 2 s, while it was associated with less MTT if the duration of tetanic stimulation was 10 or 20 s. Neomycin caused PTD in 2 mM CaCl2; sometimes the depressive effect was so severe that twitch tension was abolished. The maximal depression effect was found after 100 Hz tetanic stimulation for 20 s. Increasing the extracellular calcium concentration to 6 mM antagonized the effects of neomycin on MTT and PTD, whereas neostigmine (1.6 microM) antagonized the effect partially. Neomycin had no effect on the MTT or PTD of the directly stimulated twitch tension. It is concluded that neomycin alters the conditions of tetanic stimulation inducing MTT.

Effects of alpha-bungarotoxin and d-tubocurarine on the post-tetanic potentiation of the mouse diaphragm twitch tension

Effects of alpha-bungarotoxin (alpha-BuTX, 0.037-0.062 microM) and d-tubocurarine (d-Tc, 0.7-2.2 microM) on the post-tetanic potentiation (PTP) of the twitch tension were studied on the isolated mouse diaphragm preparation. In normal physiological solution, maximum PTP was obtained after 100 Hz tetanic stimulation for 5 sec. In preparations partially blocked by alpha-BuTX, PTP was less than in control preparations if tetanically stimulated at 100-200 Hz for 5-10 sec. In contrast, in preparations partially blocked by d-Tc (1-2.2 microM), PTP was increased if a higher frequency (500 Hz) was applied. This increase of the effect was independent of the duration of tetanic stimulation. Thus, d-Tc and alpha-BuTX have a different effect on PTP. The irreversibility of alpha-BuTX may contribute to the differences in mode of action of alpha-BuTX and d-Tc on the PTP of the twitch tension of the isolated mouse diaphragm.

The ionic requirements for the production of action potential in Achatina fulica Ferussac neuron

The ionic requirement for the production of directly elicited action potentials of a tonically auto-active neuron (TAN) in the subesophageal ganglia of the giant African snail, Achatina fulica Ferussac, was studied electrophysiologically. Calcium free Ringer solution containing 1 mM EDTA reversibly abolished the directly elicited action potential. Verapamil (10 micrograms/ml) or cocaine (4 mg/ml) decreased both amplitude and Vmax of the action potentials. The amplitude of the action potential was also slightly decreased in sodium free choline Ringer. However, tetrodotoxin did not significantly affect either the amplitude or Vmax of the directly elicited action potentials. The results suggest that the ionic requirement for generating action potential in snail neuron is not an ordinary sodium spike. Both calcium and sodium ions may participate in carrying charges across the membrane of the action potential.

Effect of chlordecone exposure on thermoregulation in the rat

Intraperitoneal administration of chlordecone (20-80 mg/kg) caused a dose-related fall in the rectal temperatures of rats at ambient temperatures of 8 and 22 degrees C. The chlordecone-induced hypothermia was brought about by a decrease in metabolism. The chlordecone hypothermia was antagonized by pretreatment of animals with haloperidol (a dopaminergic receptor antagonist), but not by phentolamine (an alpha-adrenergic receptor antagonist), propranolol (a beta-adrenergic antagonist), atropine (a cholinergic receptor antagonist), or p-chlorophenylalanine (a serotonin depletor). In addition, the dopamine levels in the hypothalamus were elevated significantly when evaluated 1 h after an intraperitoneal dose of chlordecone. The data suggest that systemic administration of chlordecone may act through the dopaminergic mechanisms in the hypothalamus to induce hypothermia (or decreased metabolic heat production) in rats.

Pharmacological study on angusticeps-type toxins from mamba snake venoms

Five angusticeps-type toxins, F7, F8 and C10S2C2 from Dendroaspis angusticeps and C and FS2 from D. polylepis polylepis, were tested for action on the chick biventer cervicis nerve-muscle, the frog rectus abdominis muscle and the mouse phrenic nerve-diaphragm preparations. In the chick muscle, none of these toxins exhibited any stimulatory effect up to 100 micrograms/ml. In the frog muscle, the response to acetylcholine, but not to carbachol, was enhanced dose dependently by F7 and C. No appreciable effect was observed with the other three toxins. In the mouse diaphragm, also only F7 and C augmented responses to indirect stimulation and produced spontaneous fasciculations. On tetanic stimulation, a marked Wedensky inhibition was observed. Their stimulatory effect was abolished by d-tubocurarine. In the presence of d-tubocurarine as well as in the denervated mouse diaphragm, neither toxin increased responses to direct stimulation. In low-calcium (0.6 mM) or high magnesium (4.2 mM) medium, the stimulatory effect of both toxins was markedly attenuated. The resting membrane potential of the mouse diaphragm was not changed. The amplitude and frequency of MEPPs and the quantal content and the half-decay time of EPPs was increased. Both toxins also produced a stimulatory effect on the isolated guinea-pig ileum, which was abolished by atropine. In the rat atrial preparation, both toxins caused negative inotropic and chronotropic effects, which were reversed by atropine. If pretreated with atropine, these effects were completely prevented. Both F7 and C markedly inhibited the cholinesterase activity of the homogenized mouse diaphragm and frog rectus abdominis muscle but not that of the chick biventer cervicis muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

A pharmacological study of the effect of carbamazepine on neuromuscular transmission in the rat diaphragm

The effects of carbamazepine (0.042-0.42 mM) on neuromuscular transmission were studied on the isolated rat phrenic nerve diaphragm preparation using standard pharmacological and electrophysiological methods. Carbamazepine decreased (1) the antidromic activity of the phrenic nerve, (2) the amplitude of the endplate potential (EPP) and miniature endplate potential (MEPP), (3) the quantal content of the endplate potential, (4) the indirectly-elicited twitch tension, (5) the muscle contracture in chronically denervated muscle induced by acetylcholine (ACh) and (6) the amplitude of the compound phrenic nerve action potential, in a concentration-dependent manner. The antidromic activity of the phrenic nerve was the most affected, while the phrenic nerve compound action potential was least affected. However, the IC50 for carbamazepine (the concentration of carbamazepine that inhibited 50% of the response) was in the same order of concentration, i.e. 0.11-0.3 mM. Compared with the effect of carbamazepine on the indirectly-elicited twitch tension with its actions described above, it is concluded that carbamazepine interfered with the neuromuscular activity by inhibiting pre- and postsynaptic process and conduction in the phrenic nerve.

Mode of neuromuscular blocking action of toxic phospholipases A2 from Vipera ammodytes venom

The effects of toxic phospholipases A2 ( fraxtions "j", "k1" and "k2") isolated from the venom of Vipera ammodytes were studied on the chick biventer cervicis muscle and the mouse phrenic nerve-diaphragm preparations. In the chick muscle, all of these PLA2s caused neuromuscular (N-M) blockade without producing contracture or affecting the response of the muscle to acetylcholine. In the mouse diaphragm, these PLA2s inhibited completely the indirectly elicited contraction without affecting that evoked directly. The order of their N-M blocking potency is "k2" greater than "k1" greater than or equal to "j". In a low Ca2+ (0.5 mM) medium, they produced a triphasic change in the indirectly elicited contractions: an initial inhibition followed by an enhancement and then a progressive depression leading to complete N-M blockade. The frequency of miniature endplate potentials (m.e.p.p.s) in the mouse diaphragm first increased 2-3 fold and then gradually decreased after "k2" treatment, while the amplitude of m.e.p.p.s did not decrease even after the evoked release of transmitter failed. Giant potentials and bursts of m.e.p.p.s were frequently observed. The quantal content of e.p.p.s was first increased and then decreased gradually. The resting membrane potential was only slightly reduced at 30 micrograms per ml. The ultrastructure of motor nerve terminals in the "k2"-intoxicated mouse diaphragm showed an increase in omega-shaped indentation in the axolemma. The mitochondria in the nerve terminal were swollen and vacuolized. No structural changes were found in the muscle fibers, fibrocytes and myelinated axons in the diaphragm. It is concluded that the toxic PLA2s from Vipera ammodytes venom produce a N-M blockade by acting selectively on the presynaptic site.

Mode of neuromuscular blocking action of a toxic phospholipase A2 from Pseudocerastes fieldi (Field's horned viper) snake venom

The effects of a toxic phospholipase A2 (Fr.Cb) isolated from the venom of Pseudocerastes fieldi were studied on the chick biventer cervicis muscle and the mouse phrenic nerve--diaphragm preparations. In the chick muscle, Fr.Cb (10 micrograms/ml) caused complete neuromuscular blockade without producing contracture or affecting the response of the muscle to acetylcholine. In the mouse diaphragm, Fr.Cb blocked the indirectly elicited contraction without affecting that evoked directly. In a low calcium medium (0.5 mM), Fr.Cb produced a triphasic change of the indirectly elicited contractions. The frequency of miniature endplate potentials (m.e.p.p.s) in the mouse diaphragm was first increased 3--4 fold 40 min after toxin (10 micrograms/ml) application, then gradually decreased, while the amplitude of m.e.p.p.s. was not decreased, even after the evoked release of transmitter had failed. Giant m.e.p.p.s were frequently observed. The quantal content first increased and then decreased gradually. The resting membrane potential and the compound phrenic nerve action potential were not significantly affected by the toxin at 10 micrograms/ml after 2 hr of incubation. The motor nerve terminals in the Fr.Cb intoxicated mouse diaphragm showed swelling and vacuolization of both synaptic vesicles and mitochondria. It is concluded that the toxin produces a neuromuscular blockade by acting selectively on the presynaptic site.

Phencyclidine interactions with the ionic channel of the acetylcholine receptor and electrogenic membrane

The effects of phencyclidine (PCP) were studied on the electrogenic and chemosensitive properties of the neuromuscular junction of skeletal muscle as well as on the binding sites on the acetylcholine (AcCho) receptor and its ionic channel in the electric organ membranes of the electric ray. The directly elicited muscle twitch was markedly potentiated by prolonging the falling phase of the muscle action potential and blocking delayed rectification. The indirectly elicited muscle twitch was transiently potentiated and then blocked by PCP at concentrations below 60 muM. PCP blocked miniature endplate potentials and AcCho sensitivities at the junctional region of innervated muscle, blocked the extrajunctional sensitivity of the chronically denervated muscle, and significantly depressed the peak amplitude of the endplate current (EPC) in a voltage- and time-dependent manner. PCP also caused acceleration of the time course of EPC decay and shortening of the mean life-time of the open ionic channel. The effects of PCP were not due to inhibition of AcCho receptor sites because PCP did not protect against the quasi-irreversible inhibition of receptor sites by alpha-bungarotoxin, nor did it inhibit binding of [(3)H]AcCho or [(125)I-labeled alpha-bungarotoxin to the receptor sites. On the other hand, PCP blocked the binding of [(3)H]perhydrohistrionicotoxin to the sites of the ionic channel of the AcCho receptor. The data suggest that PCP reacts with the electrogenic K(+) channel and the ionic channel associated with the AcCho receptor in the open as well as the closed conformation.

Amantadine: neuromuscular blockade by suppression of ionic conductance of the acetylcholine receptor

Amantadine hydrochloride decreases the sensitivity of denervated mammalian muscle to iontophoretically applied acetylcholine. The drug depresses the amplitude of the end-plate current and reverses the slope of the relation between half-decay time and membrane potential suggesting that it alters the ionic conductance that is mediated by the acetylcholine receptor. Binding studies confirm that amantadine acts on the ion conductance modulator rather than the acetylcholine receptor.

Is the contractile response to exogenous acetylcholine due to a presynaptic effect?

Whether the contractile response induced by exogenous acetylcholine (ACh) chiefly involved the pre- or post-synaptic junctional site of the motor endplate was studied by using the cat gastrocnemius nerve muscle preparation poisoned with beta-bungarotoxin (beta-BuTX), a toxin isolated from the venom of Bungarus multicinctus which acts presynaptically. 2 After neuromuscular transmission was completely blocked by beta-BuTX, the dose-response curve of the contractile response induced by close intra-arterial injection of ACh, was compared with that of the control. No appreciable difference was observed. 3 In contrast, the response to ACh was completely abolished when neuromuscular transmission was blocked by alpha-bungarotoxin, a toxin isolated from the same venom which acts postsynaptically. 4 It is concluded that postjunctional site of the motor end-plate is chiefly involved in the contractile response produced by exogenous ACh.