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

Most crustacean muscle fibers receive double excitatory innervation by functionally different motor neurons termed slow and fast. By using specific ω-toxins we show that the terminals of the slow closer excitor (SCE) and the fast closer excitor (FCE) at a crab muscle are endowed with different sets of presynaptic Ca2+ channel types. ω-Agatoxin, a blocker of vertebrate P/Q-type channels, reduced the amplitude of EPSCs by decreasing the mean quantal content of transmitter release in both neurons by 70–85%, depending on the concentration. We provide the first evidence that ω-conotoxin-sensitive channels also participate in transmission at crustacean neuromuscular terminals and are colocalized with ω-agatoxin-sensitive channels in an axon-type-specific distribution. ω-Conotoxin, a blocker of vertebrate N-type channels, inhibited release by 20–25% only at FCE, not at SCE endings. Low concentrations of Ni2+, which block vertebrate R-type channels, inhibited release in endings of the SCE by up to 35%, but had little effects in FCE endings. We found that two neuropeptides, the FMRFamide-like DF2 and proctolin, which occur in many crustaceans, potentiated evoked transmitter release differentially. Proctolin increased release at SCE and FCE endings, and DF2 increased release only at FCE endings. Selective blocking of Ca2+ channels by different ω-toxins in the presence of peptides revealed that the target of proctolin-mediated modulation is the ω-agatoxin-sensitive channel (P/Q-like), that of DF2 the ω-conotoxin-sensitive channel (N-like). The differential effects of these two peptides allows fine tuning of transmitter release at two functionally different motor neurons innervating the same muscle.