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

Abstract

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 Ca 2ϩ 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 Ni 2ϩ , 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 DF 2 and proctolin, which occur in many crustaceans, potentiated evoked transmitter release differentially. Proctolin increased release at SCE and FCE endings, and DF 2 increased release only at FCE endings. Selective blocking of Ca 2ϩ 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 DF 2 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.

Peptide-Induced Modulation of Synaptic Transmission and Escape Response inDrosophilaRequires Two G-Protein-Coupled Receptors

“…Although it has been previously suggested that FMRFamidelike peptide-induced modulation of arthropod neuromuscular synapses can result from modulation of N-type calcium channels (Rathmayer et al, 2002), we provide strong evidence for an alternate modulatory pathway. An approximate third-or fourthpower relationship between transmitter release and calcium level at release sites suggests that changes in calcium influx are not responsible for the increased evoked calcium response (Jan and Jan, 1976;Robinson et al, 2002;Okamoto et al, 2005).…”

Retrograde Gbb signaling through the Bmp type 2 receptor Wishful Thinking regulates systemic FMRFa expression inDrosophila

“…Early studies in the locust suggested that FMRF-like peptides enhance synaptic transmission at the neuromuscular junction (Robb and Evans, 1994), and more recent studies in crustaceans suggest that these peptides modulate presynaptic Ca 2+ -channel activity (Rathmayer et al, 2002). In Drosophila, peptides produced from the FMRFa gene can also enhance synaptic efficacy at the neuromuscular junction when perfused…”

Identification and characterization of a G protein-coupled receptor for the neuropeptide proctolin in Drosophila melanogaster

“…It was purified based on its myotropic actions on the insect hindgut, and was subsequently found to have wide distribution throughout the arthropods (2)(3)(4). In the crab Cancer, proctolin has potent effects on defined neural circuits within the stomatogastric system (5) and at the neuromuscular junction (6). In insects, proctolin is a major peptide cotransmitter and it modulates contractions of both somatic and visceral muscles (7,8).…”