Effects of Ca2+ channel blockers on transmitter release and presynaptic currents at the frog neuromuscular junction.

Abstract

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.

Direct Measurements of Presynaptic Calcium and Calcium-Activated Potassium Currents Regulating Neurotransmitter Release at CulturedXenopusNerve–Muscle Synapses

“…This is consistent with findings at mature frog neuromuscular junctions, at which it has been shown that -CgTX blocks all evoked transmitter release (Kerr and Yoshikami, 1984;Koyano et al, 1987;Katz et al, 1995) and some spontaneous release (Grinnell and Pawson, 1989). In the present study, nimodipine blocked ϳ17% of I Ca , suggesting the presence of L-type Ca channels as well.…”

The Neuromuscular Junctions of the Slow and the Fast Excitatory Axon in the Closer of the CrabEriphia spinifronsAre Endowed with Different Ca2+Channel Types and Allow Neuron-Specific Modulation of Transmitter Release by Two Neuropeptides

“…In frog and lizard neuromuscular synapses, N-or L-type channels mediate transmission (Lindgren and Moore, 1989;Katz et al, 1995;Arenson and Gill, 1996).…”

Presynaptic Ca2+/Calmodulin-Dependent Protein Kinase II Modulates Neurotransmitter Release by Activating BK Channels atCaenorhabditis elegansNeuromuscular Junction

“…The BK channel is an attractive candidate. In virtually all synapses examined, the BK channel is a prominent K ϩ channel at presynaptic nerve endings (Anderson et al, 1988;Farley and Rudy, 1988;Lindgren and Moore, 1989;Tabti et al, 1989;Morita and Barrett, 1990;Sivaramakrishnan et al, 1991;Robitaille et al, 1993;Wangemann and Takeuchi, 1993;Katz et al, 1995;Vatanpour and Harvey, 1995;Knaus et al, 1996;Sun et al, 1999;Zhou et al, 1999;Yazejian et al, 2000;Misonou et al, 2006), where it colocalizes with voltage-gated Ca 2ϩ channels (Robitaille et al, 1993;Issa and Hudspeth, 1994;Yazejian et al, 1997;Yazejian et al, 2000), and may serve as an important negative regulator of neurotransmitter release (Robitaille et al, 1993;Wang et al, 2001;Raffaelli et al, 2004). Several lines of evidence suggest that BK channel activity may be modulated by CaMKII: (1) BK channels reconstituted into artificial lipid bilayers are activated by ATP, which is blocked by a CaMKII inhibitor (Muller et al, 1996); (2) interactions between 14-3-3 and Slob, which are proteins that control Drosophila BK channel activity, may be modulated by CaMKII (Zhou et al, 1999); and (3) BK channels contribute to afterhyperpolarization of mouse vestibular nucleus neurons in a CaMKII-dependent manner (Nelson et al, 2005).…”