Search

Effects of lophotoxin (LTX), a neurotoxin isolated from Pacific sea whips Lophogorgia rigida and Lophogorgia chelensis, on neuromuscular transmission were assessed in the rat phrenic nerve-hemidiaphragm preparation using conventional microelectrode recording techniques, and in the frog cutaneous pectoris preparation using two microelectrode voltage clamp techniques. LTX (2-25 microM) produced a progressive, irreversible block of miniature endplate potential (m.e.p.p.) and endplate potential (e.p.p.) amplitude. M.e.p.p. amplitude histograms were shifted markedly in the direction of lower amplitude by LTX. These effects occurred following a latency of 25-40 min. The latency to onset of block was decreased with increasing LTX concentrations. In some preparations, LTX produced a transient increase in m.e.p.p. frequency during the first 5 min of application; however, m.e.p.p. frequency then declined to complete block. The depressant effect of LTX on m.e.p.p. and e.p.p. amplitude progressed to complete block irrespective of the LTX concentration. LTX also blocked the endplate depolarization produced by iontophoretic application of acetylcholine (ACh). The resting membrane potential of skeletal muscle fibres was unaffected by LTX. In voltage clamp experiments, LTX (15 microM) depressed the peak amplitude of the endplate current (e.p.c.) nearly uniformly at potentials between -120 and +60 mV. LTX did not affect the e.p.c. reversal potential or the kinetics of e.p.c. decay suggesting that LTX does not block open ACh channels. E.p.c. block by LTX was also progressive and irreversible. The results indicate that LTX blocks neuromuscular transmission by a postjunctional action. The binding site of LTX may be different from that of ACh.

The presynaptic and postsynaptic effects of MDL 12,330A, an adenylate cyclase inhibitor in several biological tissues, were studied at motor endplates in frog cutaneous pectoris nerve-muscle preparations. This agent increased both spontaneous quantal acetylcholine (ACh) release and neurally-evoked ACh release approximately twofold during the first 20-40 min of application. The increased ACh release was accompanied by a profound irreversible depression in the amplitudes of the miniature endplate potentials (m.e.p.ps) and endplate potentials (e.p.ps). The response to iontophoretically-applied ACh was reduced in parallel with the amplitude of the spontaneous m.e.p.ps, indicating that the depression of synaptic transmission was postsynaptic in origin. Endplates were voltage-clamped to study the postsynaptic depression in more detail. It was observed that the peak endplate current (e.p.c.) was depressed without concomitant changes in: the kinetics of e.p.c. decay, the relationship between peak e.p.c. and membrane potential, the ACh equilibrium potential or the voltage sensitivity of the e.p.c. decay. This suggests that MDL 12,330A reduces the postsynaptic sensitivity to ACh by a voltage-dependent block of the cholinoceptor. The presynaptic enhancement and the postsynaptic depression of junctional transmission produced by MDL 12,330A are discussed in conjunction with current theories of the role of adenylate cyclase and cyclic nucleotides at nicotinic cholinergic synapses.

Actions of two structurally related toxins, T-17 and brevetoxin-B, isolated from the red-tide dinoflagellate, Ptychodiscus brevis, were studied on the giant axon of the squid and the neuromuscular junctions of the frog and rat. T-17 toxin caused a large increase in the frequency of miniature endplate potentials at nanomolar concentrations. In one typical case with a frog endplate, the frequency increased from 1.9 s-1 before application of 3.5 nM T-17 to 69.3 s-1 within 5 min after application. In the rat muscle, the mean frequency increased from 1.39 s-1 in control to 11.93 s-1 after application of 23.2 nM T-17. The increase in miniature endplate potential frequency was reversed by the addition of 1 microM tetrodotoxin, and was not observed in a solution containing elevated Mg2+ and reduced Ca2+ concentrations. External or internal application of T-17 toxin (2-5 microM) or brevetoxin-B (10-30 microM) to intact or internally perfused squid axons caused a depolarization of the membrane. This depolarization was abolished by the removal of external Na+ or by addition of tetrodotoxin to the external solution. In voltage clamped squid giant axons, exposure to T-17 toxin or brevetoxin-B increased the non-inactivating component of the tetrodotoxin-sensitive sodium current. The sodium current was activated at potentials 15 to 40 mV more negative than control. It is proposed that these toxins modify a fraction of the sodium channels to a form which opens at potentials more negative than normal and which inactivates to a lesser extent. This mechanism would predict a depolarization of the nerve membrane at the neuromuscular junction, thus explaining the increased discharge of transmitter.