Excitability and depolarization-release characteristics of excitatory nerve terminals in a tail muscle of spiny lobster

In the deep abdominal L₁-extensor muscle of the spiny lobster (Panulirus penicillatus) quantal excitatory postsynaptic currents (EPSCs) were recorded through macropatch-clamp electrodes. Release of transmitter quanta from terminals was also elicited by depolarizing current pulses given through the recording electrode. The majoritiy of terminals were excitable: on increasing the depolarization pulses, release was triggered at a threshold in an all-ornothing manner. If excitation was blocked by tetrodotoxin (TTX), release was graded with depolarization reaching the amplitude of the all-or-nothing response at pulse amplitudes several times higher than the former threshold level. Some inexcitable terminals were also found: in these, release was graded for increasing depolarization pulses, and TTX did not alter the depolarization-release relation. Among the other types of terminals studied with the same technique, the proportion of excitable terminals in this lobster tail muscle is higher than in the crayfish opener and lower than in the frog's cutaneous pectoris muscle. The contribution of the increase in intraterminal Ca concentration to the control of release was estimated using facilitation of a test EPSC as an indicator of Ca inflow during a preceding depolarization pulse. This facilitation was found to have a maximum at a certain pulse amplitude, {Mathematical expression}, and to decline for larger depolarizations. Release, however, rose considerably for depolarizations larger than those effected at {Mathematical expression}.It is concluded that, like in crayfish and frog motor terminals, release is controlled directly by depolarization in addition to the control by Ca-inflow.