Neuromuscular transmission without sodium activation of the presynaptic nerve terminal in the lobster.

Abstract

Na-independent synaptic transmission was studied in the inhibitory synapse of the walking leg of the spiny lobster (P. japonicus). After loading the preparation with tetrodotoxin (TTX), brief depolarizing current injected in the inhibitory axon produced a small action potential, which propagated to the nerve terminal and gave rise to inhibitory postsynaptic potentials (i.p.s.p.). The presynaptic action potential, in the presence of TTX, failed to propagate after removing Na+ in the solution. The TTX-resistant action potential was decreased but not blocked by 30 mM-CoCl2. When 4-aminopyridine (4-AP) was added to low Na+ or Na-free solution containing TTX, synaptic transmission was restored. When the duration of the current pulse was increased, graded i.p.s.p. were evoked. In high Ca2+ solutions containing K blockers, action potentials with prolonged duration were evoked. The action potential of the presynaptic axon of the lobster neuromuscular junction depends on both Na+ and Ca2+.