1. The involvement of the different types of voltage-dependent calcium channels (VDCCs) Ca2+ is a key link between the arrival of an action potential Llinas, Sugimori, Hillman & Cherksey, 1992). More recently, at the synaptic terminal and the release of neurotransmitter the Q-type, a channel closely related to the P-type, and the (Katz, 1969). The rapid increase in [Ca2+]i that occurs upon R-type, a channel resistant to any of the drugs and toxins depolarization of the terminal membrane is accomplished by commonly used to block and discriminate the other types of the opening of voltage-dependent calcium channels (VDCCs) VDCC, have been described (Zhang, et al. 1993). Many of that allow the entry of Ca2P from the extracellular space these VDCCs, particularly the N-, L-and P/Q-types, have (Llinas, Steinberg & Walton, 1976). Several types of VDCC been shown to play a role in Ca2P entry during synaptic have been described based on their biophysical and transmission at different synapses

1 The e ects of the voltage-dependent calcium channel (VDCC) blockers o-agatoxin IVA (o-AgaIVA), o-conotoxin GVIA (o-CgTx), o-conotoxin MVIIC (o-MVIIC) and o-conotoxin MVIID (o-MVIID) were evaluated on transmitter release in the mouse diaphragm preparation. The e ects of o-AgaIVA and o-MVIIC were also evaluated on the perineurial calcium and calcium-dependent potassium currents, I Ca and I K(Ca) , respectively, in the mouse levator auris preparation. 2 The P-and Q-type VDCC blocker o-AgaIVA (100 nM) and P-Q-and N-type channel blockers o-MVIIC (1 mM) and o-MVIID (3 mM) strongly reduced transmitter release (480 ± 90% blockade) whereas the selective N-type channel blocker o-CgTx (5 mM) was ine ective. 3 The process of release was much more sensitive to o-MVIIC (IC 50 =39 nM) than to o-MVIID (IC 50 =1.4 mM). After almost completely blocking transmitter release (quantal content *0.3% of its control value) with 3 mM o-MVIIC, elevating the external [Ca 2+ ] from 2 to 10 mM induced an increase of *20 fold on the quantal content of the endplate potential (e.p.p.) (from 0.2+0.04 to 4.8+1.4). 4 Nerve-evoked transmitter release in a low Ca 2+ -high Mg 2+ medium (low release probability, quantal content = 2+0.1) had the same sensitivity to o-AgaIVA (IC 50 =16.8 nM) as that in normal saline solutions. In addition, K + -evoked transmitter release was also highly sensitive to the action of this toxin (IC 50 =11.5 nM; 100 nM 495% blockade). The action of o-AgaIVA on transmitter release could be reversed by toxin washout if the experiments were carried out at 31 ± 338C. Conversely, the e ect of oAgaIVA persisted even after two hours of toxin washout at room temperature. 5 Both the calcium and calcium-dependent potassium presynaptic currents, I Ca and I K(Ca) , respectively, were highly sensitive to low concentrations (10 ± 30 nM) of o-AgaIVA. The I Ca and the I K(Ca) were also strongly reduced by 1 mM o-MVIIC. The most marked di erence between the action of these two toxins was the long incubation times required to achieve maximal e ects with o-MVIIC. 6 In summary these results provide more evidence that synaptic transmission at the mammalian neuromuscular junction is mediated by Ca 2+ entry through P-and/or Q-type calcium channels.

1. The effects of the calcium channel blockers, funnel‐web spider toxin (FTX), omega‐agatoxin IVA (omega‐Aga IVA) and omega‐conotoxin GVIA (omega‐CgTX), were tested on transmitter release and presynaptic currents in frog motor nerve endings. 2. Evoked transmitter release was blocked by FTX (IC50 = 0.02 microliter ml‐1) and omega‐CgTX (1 microM) but was not affected by omega‐Aga IVA (0.5 microM). When FTX (0.1 microliter ml‐1) was assayed on spontaneous release either in normal Ringer solution or in low Ca(2+)‐high Mg2+ solution, it was found not to affect miniature endplate potential (MEPP) amplitude but to increase MEPP frequency by approximately 2‐fold in both conditions. 3. Presynaptic calcium currents (ICa), measured by the perineurial technique in the presence of 10 mM tetraethylammonium chloride (TEA) and 200 microM BaCl2 to block K+ currents, were blocked by omega‐CgTX (5 microM), partially blocked by FTX (1 microliter ml‐1) and not affected by omega‐Aga IVA (0.5 microM). 4. The presynaptic calcium‐activated potassium current (IK(Ca)) measured by the perineurial technique in the presence of 0.5 microM 3,4‐aminopyridine (DAP) to block voltage‐dependent K+ currents, was strongly affected by charybdotoxin (ChTX) (300 nM) and completely abolished by BaCl2 (200 microM). This current was also blocked by omega‐CgTX (5 microM) and by CdCl2 (200 microM) but was not affected by FTX (1 microliter ml‐1). The blockade by omega‐CgTX could not be reversed by elevating [Ca]o to 10 mM. 5. The results suggest that in frog synaptic terminals two omega‐CgTX‐sensitive populations might coexist. The transmitter release process seems to be mediated by calcium influx through a omega‐CgTX‐ and FTX‐sensitive population.