Characterization of GABA A Receptor Function in Human Temporal Cortical
Neurons.
Gibbs, John W., Yun-Fu Zhang, Chang-Qing Kao, Kathryn L. Holloway, Kwang-Soo
Oh and Douglas A. Coulter.
Departments of Neurology, Anatomy, and Neurosurgery Medical College of
Virginia, and the MCV Comprehensive Epilepsy Institute of Virginia
Commonwealth University, Richmond, VA 23298-0599.
APStracts 2:0302N, 1995.
SUMMARY AND CONCLUSIONS
1. Surgically resected tissue from the tip of the human temporal lobe of seven
patients undergoing temporal lobectomy was employed to study functional
properties of GABAergic inhibition mediated through activation of GABA A
receptors, using patch-clamp recording techniques in acutely isolated neurons
and in slices of human temporal cortex. 2. Human temporal cortical pyramidal
neurons from surgically resected tissue could be acutely isolated using
conventional methods. These neurons appeared normal in morphology, in their
intrinsic membrane properties, and in their response to application of
exogenous GABA. 3. Application of GABA to acutely isolated human temporal
cortical neurons elicited a large current with an average reversal potential
of -65 mV, presumably mediated through a GABA A -activated chloride
conductance. Application of varying concentrations of GABA generated a
concentration/response relationship which could be well-fitted by a
conventional sigmoidal curve, with an EC 50 of 25.5 æM and a Hill coefficient
of 1.0. 4. Coapplication of the benzodiazepine clonazepam and 10 æM GABA
augmented the amplitude of the GABA response. The concentration-dependence of
this benzodiazepine augmentation could be best-fitted by an equation assuming
that the benzodiazepine interacted with two distinct binding sites, with
differing potencies. The high potency site had an EC 50 of 0.06 nM, and
maximally contributed 38.5% augmentation to the total effect of clonazepam.
The lower potency site had an EC 50 of 16.4 nM, and contributed 66.1% maximal
augmentation to the overall effect of clonazepam. These data derived from
adult human temporal cortical neurons were very similar to our findings in
adult rat sensory cortical neurons (Oh et al., 1995). 5. The effects of
equimolar concentrations (100 nM) of clonazepam, a BZ 1 and BZ 2 agonist,
and zolpidem, a selective BZ 1 agonist, on acutely isolated human temporal
cortical neurons were also investigated. Zolpidem and clonazepam were equally
effective (71.5% vs. 65.0% respectively) in potentiating GABA responses
elicited by application of 10æM GABA. This suggests that many of the
functional benzodiazepine receptors in these neurons were of the BZ 1
variety. 6. GABAergic synaptic inhibition was also studied using patch clamp
recordings in slices of human temporal cortex. Extracellular stimulation at
the white matter/gray matter border elicited compound synaptic events in layer
II-V cortical neurons. These events usually consisted of an early EPSP, and a
late multiphasic IPSP. Application of either clonazepam or zolpidem (both at
100 nM) to the slice during extracellular stimulation reversibly augmented the
late compound IPSP. 7. Spontaneous IPSPs were also recorded in approximately
50% of human temporal cortical neurons. These events did not have a preceding
EPSP and were usually monopolar, with a single exponential rise and decay.
This supported the idea that these events were triggered by spontaneous
activity of GABAergic interneurons. Bath application of either clonazepam or
zolpidem (both at 100 nM) to the slice during ongoing spontaneous IPSP
activity increased the amplitude and lengthened the time constant of decay of
these events. 8. To our knowledge, this is one of the first detailed
characterizations of the functional properties of GABA A -mediated inhibition
in human cortical neurons using patch clamp recordings in both isolated cells
and slices of resected temporal cortex. Isolated pyramidal neurons exhibited
GABA A -mediated currents which were comparable in many aspects to GABA
currents recorded from adult rat cortical neurons, including similar GABA
concentration/response curves, and similar two differing potency site effects
for clonazepam augmentation of GABA currents. In addition, evoked and
spontaneous IPSPs recorded in human cortical neurons appeared similar to IPSPs
in rat cortical neurons, both in their properties and modulation by
benzodiazepines. This study demonstrates the feasibility of conducting
detailed functional characterizations of receptor properties of human cortical
neurons, a necessary step towards furthering our understanding of similarities
and differences between receptor properties of human neurons and neurons in
various experimental animal preparations.
Received 13 April 1995; accepted in final form 23 October 1995.
APS Manuscript Number J248-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 6 November 95