Contribution of potassium conductances to a time-dependent transition in electrical properties of a cockroach motoneuron soma

The cell body of the cockroach (Periplaneta americana) fast coral depressor motoneuron (D-f) displays a time-dependent change in excitability. Immedia tely after dissection, depolarization evokes plateau potentials, but after several hours all-or-none action potentials are evoked. Because K channel b lockers have been shown to produce a similar transition in electrical prope rties, we have used current-clamp, voltage-clamp and action-potential-clamp recording to elucidate the contribution of different classes of K channel to the transition in electrical activity of the neuron. Apamin had no detec table effect on the neuron, but charybdotoxin (ChTX) caused a rapid transit ion from plateau potentials to spikes in the somatic response of D-f, to de polarization. In neurons that already produced spikes when depolarized, ChT X increased spike amplitude but did not increase their duration nor decreas e the amplitude of their afterhyperpolarization. 4-Aminopyridine (4-AP) (wh ich selectively blocks transient K currents) did not cause a transition fro m plateau potentials to spikes but did enhance oscillations superimposed on plateau potentials. When applied to neurons that already generated spikes when depolarized, 4-AP could augment spike amplitude, decrease the latency to the first spike, and prolong the afterhyperpolarization. Evidence sugges ts that the time-dependent transition in electrical properties of this moto neuron soma may result, at least in part, from a fall in calcium-dependent potassium current (I-K,I-Ca), consequent on a gradual reduction in Ca2+ ii. Voltage-clamp experiments demonstrated directly that outward K currents in this neuron do fall with a time course that could be significant in the tr ansition of electrical properties. Voltage-clamp experiments also confirmed the ineffectiveness of apamin and showed that ChTX blocked most of I-K,I-C a. Application of Cd2+ (0.5 mM), however, caused a small additional suppres sion in outward current. Calcium-insensitive outward currents could be divi ded into transient (4-AP-sensitive) and sustained components. The action-po tential-clamp technique revealed that the ChTX-sensitive current undenwent sufficient activation during the depolarizing phase of plateau potentials t o enable it to shunt inward conductances. Although the ChTX-sensitive condu ctance apparently makes little contribution to spike repolarization, the Ch TX-resistant I-K,I-Ca does make a significant contribution to this phase of the action potential. The 4-AP-sensitive current began to develop during t he rising phase of both action potentials and plateau potentials but had li ttle effect on the electrical activity of the neuron, probably because of i ts relatively small amplitude.