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The Journal of Neuroscience, February 1, 2002, 22(3):708-717

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

Werner Rathmayer, Stjefan Djokaj, Aleksandr Gaydukov, and Sabine Kreissl

Faculty of Biology, University of Konstanz, D-78457 Konstanz, Germany

Most crustacean muscle fibers receive double excitatory innervation by functionally different motor neurons termed slow and fast. By using specific -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. -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 -conotoxin-sensitive channels also participate in transmission at crustacean neuromuscular terminals and are colocalized with -agatoxin-sensitive channels in an axon-type-specific distribution. -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 -toxins in the presence of peptides revealed that the target of proctolin-mediated modulation is the -agatoxin-sensitive channel (P/Q-like), that of DF₂ the -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.

Key words: P/Q-type Ca²⁺ channels; N-type Ca²⁺ channels; R-type Ca²⁺ channels; crustacea; DF₂; proctolin; RFamide; axon-type specific peptidergic modulation; -agatoxin; -conotoxin

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Copyright © 2002 Society for Neuroscience  0270-6474/02/223708-10$05.00/0

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