TWO TEMPORALLY OVERLAPPING "DELAYED-RECTIFIERS" DETERMINE THE VOLTAGE- DEPENDENT POTASSIUM CURRENT PHENOTYPE IN CULTURED HIPPOCAMPAL INTERNEURONS. Chikwendu, Adaora & Chris J. McBain. Unit on Cellular and Synaptic Physiology, Lab. of Cellular and Molecular Neurophysiology, Rm. 5A72, Bldg. 49, NICHD-LCMN, 49 Convent Drive, Bethesda MD 20892-4495.
APStracts 3:0072N, 1996.
SUMMARY AND CONCLUSIONS
1. Whole-cell voltage clamp recordings were used to characterize the calcium- independent "delayed-rectifier" potassium currents of GABA-positive st. radiatum - lacunosum - moleculare interneurons in primary culture derived from neonate rats (P5- P7). 2. Two distinct current phenotypes were observed, which we termed "sustained" and "slowly-inactivating". Despite possessing similar voltage-dependent activation properties, currents differed in their time- dependent inactivation properties and their kinetics of activation and deactivation. The phenotypes of the observed currents did not change during the time in vitro. The total current phenotype observed in any cell likely resulted from the temporal overlap of the two current components expressed in different relative proportions. 3. Externally applied 4AP selectively blocked the slowly-inactivating current component, by a use-dependent, but voltage- independent mechanism, suggesting that channel activation is required for 4AP to interact with its binding site. In contrast, the sustained current component was unaffected by 4AP. 4. Both the slowly-inactivating and sustained current phenotypes were sensitive to externally applied TEA. The IC 50 of block by TEA was lower in cells expressing predominantly the sustained current component. 5. Currents recorded in the presence of internally applied TEA were of a slowly-inactivating phenotype, suggesting that [TEA] i preferentially blocked the sustained current component. 6 . When test pulses were preceded by a prepulse to -100mV, a transient A-type current component was observed but in contrast to pyramidal neurons and other interneuron types, this transient current contributed only a minor component to the total initial peak current. 7. In conclusion, two distinct, temporally overlapping potassium current phenotypes were observed on st. L-M interneurons. The overall phenotype was determined by the relative proportion of each current component. The absence of a prominent transient current suggests that the two delayed-rectifier currents play a critical role in determining the firing characteristics of these interneurons. 

Received 2 February 1995; accepted in final form 10 April 1996.
APS Manuscript Number J88-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 1 May 96