Cholinergic agonists decrease quantal output at the frog neuromuscular junction by targeting a calcium channel blocked by ω-conotoxin

Abstract

Nicotinic cholinergic agonists are known to decrease synchronous evoked quantal output at the frog neuromuscular junction [Van der Kloot 1993, J Physiol (Lond) 468:567-589]. Here we also show that carbachol decreases the frequency of miniature endplate potentials (FMEPP) in solutions containing elevated levels of K+ and Ca2+. Carbachol did not decrease FMEPP in hypertonic solutions or in solutions containing the Ca2+ ionophore ionomycin and Ca2+. We conclude that the nicotinic agonists decrease Ca2+ influx through voltage-gated Ca2+ channels. Carbachol did not alter two-pulse facilitation. A blocker of N-type Ca2+ channels, omega-conotoxin GVIA, antagonized the nicotinic agonist-induced decrease in evoked quantal output. The effect of carbachol was not altered by omega-conotoxin MVIIC, a blocker of P-type and certain other Ca2+channels. The Ca2+ channel targeted by the nicotinic agonists appears to be of the N-type.

Acetylcholine-Induced Inhibition of Presynaptic Calcium Signals and Transmitter Release in the Frog Neuromuscular Junction

“…Specific N-type Ca 2+ -channel antagonist conotoxin GVIA removes inhibition of [Ca 2+ ] i transients by carbachol, pointing to a primary role of N-type channels in providing Ca 2+ entry into the nerve terminal. Furthermore, our data about the role for N-type of Ca 2+ channels are fully compatible with earlier results of Van der Kloot et al (1997) who demonstrated that ω-conotoxin eliminated effects of cholinomimetics on quantal content in frog synapses. We conclude that in the frog neuromuscular synapse activation of presynaptic M 2 muscarinic and d-tubocurarine-sensitive nicotinic acetylcholine receptors by exogenous cholinomimetics and/or endogenous acetylcholine decreases quantal content of mediator secretion by reducing the entry of Ca 2+ ions into the nerve endings due to inhibition of voltage-gated N-type Ca 2+ channels.…”

“…Although this phenomenon has received a significant research attention in the past (Nikol'skiĭ and Giniatullin, 1979; Wessler, 1989; Macleod et al, 1994; Van der Kloot et al, 1997; Nikolsky et al, 2004), fine details of ACh-dependent presynaptic regulation remain obscure. Both direct action of ACh on exocytotic machinery (Linial et al, 1997) and inhibition of presynaptic Ca 2+ entry (Wu and Saggau, 1997; Parnas et al, 2000; Khaziev et al, 2012) have been suggested.…”

Negative cross‐talk between presynaptic adenosine and acetylcholine receptors

“…High K + , ionomycin or hyperosmotic solution of sucrose trigger transmitter release via activation of voltage‐dependent Ca 2+ channels, ionophore‐mediated Ca 2+ influx or Ca 2+ release from internal stores (Hunt & Silinsky, 1993; Van der Kloot et al ., 1997; Kashani et al ., 2001; Grishin et al ., 2005).…”

Cholinergic regulation of the evoked quantal release at frog neuromuscular junction

“…The ability of cholinergic drugs to change the quantal content of the evoked endplate current (EPC) has been shown by many investigators (Ciani & Edwards, 1963; Nikolsky & Giniatullin, 1979; Dunant & Walker, 1982; Vizi & Somogyi, 1989; Doležal & Tuèek, 1993; Re et al . 1993; Van der Kloot et al . 1997; Slutsky et al .…”