IONIC CURRENTS IN CRUSTACEAN NEUROSECRETORY-CELLS

The patterns of electrical activity and membrane characteristics of a population of neurosecretory-cell somata in the X-organ of the crayfish were investigated with microelectrodes and whole-cell, voltage-clamp techniques. Some neurons (56%) were silent but could be excited by intracellular current injection; other cells showed spontaneous tonic activity (35%), and some had spontaneous bursting activity (9%). The spiking activity was abolished by tetrodotoxin (TTX) exposure and by severing the axon near the cell body. After axotomy, only a small, slow, regenerative depolarization remained that could be blocked by Cd2+. Under voltage clamp the steady-state I-V curve in low [Ca2+](i) (9 X 10-9 M) showed a slope conductance of 16.7 ± 3.9 (SD) nS (n = 10) at -50 mV and zero current potential of -50.1 ± 7.7 mV. In current-clamp mode these neurons were either silent or fired tonically. With high [Ca2+](i) (1.7 X 10-6 M) both the slope conductance and inward and outward currents were reduced. In some neurons high [Ca2+](i) reveals a negative slope resistance in the range of -46 to -41 mV. It could be supressed by removing [Na+](o), but it was TTX insensitive. These are the neurons that under current clamp showed bursting activity. The main inward current in cell somata was a Ca2+ current of 2 ± 0.6 nA (n = 18), activated at -40 mV and peaking at 20 mV. It showed relaxation with prolonged pulses. No Na+-dependent, TTX-sensitive inward currents were recorded with short (100-ms) pulses in axotomized neurons. Two outward currents could be distinguished. One activated at -40 mV with a sigmoidal time course and slight relaxation for depolarizations >0 mV during 400-ms pulses. It was blocked by tetraethylammonium (TEA), and its characteristics are similar to the delayed rectifier or I(K) current. The other was a transient current activated from -90 mV holding potential. It was blocked by 4 aminopyridine (4-AP) and is similar to I(A) current. Two populations of cells could be identified on the basis of the voltage dependence of the steady-state inactivation of I(A).