Single Channel Properties of a G Protein-Coupled Inward Rectifier Potassium Channel in Brain Neurons. GRIGG, J. J., T. KOZASA, Y. NAKAJIMA AND S. NAKAJIMA. Department of Anatomy and Cell Biology and the Department of Pharmacology, University of Illinois at Chicago, College of Medicine, Chicago, Illinois, 60612.
APStracts 2:0278N, 1995.
SUMMARY AND CONCLUSIONS
1. In cultured rat locus coeruleus neurons somatostatin or met-enkephalin induces an inwardly rectifying K + conductance. This inward rectifier was analyzed at the single channel level. 2. Using the inside-out patch clamp, GTP application to the cytoplasmic side in the presence of somatostatin or met- enkephalin in the pipette produced a large increase in channel activity, which disappeared upon switching from GTP to GDP. 3. The unitary conductance was nearly equal to 30 pS at -95 mV with [K + ] o = 156 mM and [K + ] i = 124 mM at 23 o C. The channel showed burst behavior, and the closed time histogram was fit by 2 exponentials with the fast time constant being 0.4 msec. The burst time histogram was also fit by 2 exponentials with time constants of 0.24 and 2.0 msec (at 10 nM somatostatin). When the somatostatin concentration was changed from 500 nM to 1 nM, the kinetic behavior of the channel did not change, except that the open probability of the patch ( Np o ) was decreased. 4. The current-voltage relation of the unitary channel current showed inward rectification. The reversal potential coincided with the K + equilibrium potential, and it shifted according to a change in the K + equilibrium potential. 5. In the presence of external somatostatin, the application of GTP_S to the cytoplasmic side induced an irreversible activation of this channel. 6. These results indicate that this K + channel is the microscopic counterpart of the somatostatin- or met-enkephalin-induced inwardly rectifying K + current in whole-cell recording, and that the channel is activated by a G protein without a diffusible second messenger. Thus, this channel is identified as a neuronal G protein-coupled inward rectifier K + channel. 7. Analysis of the burst behavior, based on a close-close-open kinetic model, revealed that there are at least 4 states in the K + channel, a short gap, a longer closing, a short opening, and a long opening, and that the neuronal inward rectifier is activated at faster rates than the atrial inward rectifier. 

Received 7 June 1995; accepted in final form 31 August 1995.
APS Manuscript Number J364-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.