Journal of Neurophysiology, Vol 73, Issue 1 160-171, Copyright © 1995 by APS

ARTICLES

Separation and identification of multiple potassium currents regulating the pacemaker activity of insect neurosecretory cells (DUM neurons)

F. Grolleau and B. Lapied
Centre National de la Recherche Scientifique URA 611, Universite d'Angers, France.

1. Whole cell voltage-clamp studies performed in isolated adult neurosecretory cells identified as dorsal unpaired median (DUM) neurons of the terminal abdominal ganglion of the cockroach Periplaneta americana have allowed us to reveal a complex voltage-dependent outward current regulating the pacemaker activity. 2. The global outward current remaining after tetrodotoxin treatment was activated by depolarization above -50 mV, showing steep voltage dependence and outward rectification. 3. We used tail current analysis to determine the ionic selectivity of this outward current. The reversal potentials for two extracellular potassium concentrations (-92.7 and -65.4 mV for 3.1 and 10 mM, respectively) is consistent with the expected equilibrium potential for potassium ions. 4. Both peak and sustained components of the global outward K+ current were reduced by external application of 20 mM tetraethylammonium chloride, 10 nM iberiotoxin, 1 nM charybdotoxin (CTX) and 1 mM cadmium chloride. Subtraction of current recorded in CTX solution from that in control solution revealed an unusual biphasic Ca(2+)-dependent K+ current. The fast transient current resistant to 5 mM 4-aminopyridine (4-AP) is distinguished by its dependence on holding potential and time course from the late sustained current. 5. In addition, two other components of CTX-resistant outward K+ current could be separated by sensitivity to 4-AP, time course, and voltage dependence. Beside a calcium-independent delayed outwardly rectifying current, a 4-AP-sensitive fast transient current resembling the A-current has been also identified. It activates at negative potential (about -65 mV) and unlike the A-current of other neurons, it inactivates rapidly with complex inactivation kinetics. A-like current is half-inactivated at -63.5 mV and half-activated at -35.6 mV. 6. Our findings demonstrate for the first time in DUM neuron cell bodies the existence of multiple potassium currents underlying the spontaneous electrical activity. Their identification and characterization represent a fundamental step in further understanding the pacemaker properties of these insect neurosecretory cells.

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