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.

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Presynaptic inhibition exerted by the common inhibitor on the closer and opener muscles and by the specific inhibitor on the opener muscle was investigated in the crab Eriphia spinifrons. In the closer muscle, activation of GABA(B) receptors by baclofen reduced the mean quantal content of excitatory junctional currents by about 25%. Blocking GABA(B) receptors with CGP 55845 diminished presynaptic inhibition at a similar percentage. GABA(B) receptor-mediated presynaptic inhibition is linked to G proteins. Application of pertussis toxin eliminated about 25% of the inhibition exerted by the common inhibitory neuron. GABA(B) receptors participate in presynaptic inhibition at release boutons of the slow and the fast closer excitor at a similar percentage. In the opener muscle, presynaptic inhibition of transmitter release from the same endings of the opener excitor was about 15% stronger with the specific inhibitor than with the common inhibitor. About 10% of the presynaptic inhibition produced by either one of the two inhibitors could be abolished by blocking GABA(B) receptors. The amplitudes of the excitatory junctional currents in the opener were reduced in the presence of baclofen by about 25%, suggesting that synaptic terminals of the opener excitor are endowed with a similar percentage of GABA(B) receptors as terminals of the slow and the fast closer excitors. Baclofen had no effect on postsynaptic inhibition, indicating that GABA(B) receptors are not involved in postsynaptic neuromuscular inhibition.