Contributions of the GABAA receptor alpha6 subunit to phasic and tonic inhibition revealed by a naturally occurring polymorphism in the alpha6 gene.
GABAA receptors (GABARs) are heteromultimeric proteins composed of five subunits. The specific subunit composition determines critical properties of a GABAR such as pharmacological sensitivities and whether the receptor contributes to synaptic or extrasynaptic forms of inhibition. Classically, synaptic but not extrasynaptic GABARs are thought to respond to benzodiazepines, whereas the reverse has been suggested for ethanol. To examine the effects of subunit composition on GABAR function in situ, we took advantage of two naturally occurring alleles of the rat gene for GABAR subunit alpha6 (Gabra6(100R) and Gabra6(100Q)). Depending on their subunit partners, these two variants of alpha6 can lead to differential sensitivities to benzodiazepines and ethanol. An examination of synaptic and extrasynaptic GABA-mediated currents in cerebellar granule cells from Gabra6(100R/100R) and Gabra6(100Q/100Q) rats uncovered marked allele-dependent differences in benzodiazepine sensitivity. Unexpectedly, we found that the benzodiazepines flunitrazepam and diazepam enhanced extrasynaptic inhibition mediated by delta subunit-containing GABARs in Gabra6(100Q/100Q) rats. Complementary experiments on recombinant GABARs confirmed that, at subsaturating [GABA], flunitrazepam potentiates alpha6/delta subunit-containing GABARs. Based on data and a simple theoretical analysis, we estimate that the average extrasynaptic [GABA] is approximately 160 nm in perfused slices. These results (1) demonstrate contributions of alpha6 subunits to both synaptic and extrasynaptic GABA responses, (2) establish that delta subunit-containing GABARs are benzodiazepine sensitive at subsaturating [GABA] and, (3) provide an empirical estimate of extrasynaptic [GABA] in slices.
Santhakumar
Vijayalakshmi
V
Department of Neurobiology, David Geffen School of Medicine, University of California, Los Angeles, California 90095, USA.
Hanchar
H Jacob
HJ
Wallner
Martin
M
Olsen
Richard W
RW
Otis
Thomas S
TS
D000818:Animals; D001569:Benzodiazepines / Q000494:pharmacology; D002525:Cerebellar Cortex / Q000502:physiology; D004351:Drug Resistance / Q000235:genetics; D000431:Ethanol / Q000494:pharmacology; D005110:Extracellular Space / Q000187:drug effects / Q000235:genetics; D005260:Female; D009433:Neural Inhibition / Q000187:drug effects / Q000235:genetics*; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D009865:Oocytes; D009924:Organ Culture Techniques; D018408:Patch-Clamp Techniques; D011110:Polymorphism, Genetic / Q000235:genetics*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011963:Receptors, GABA-A / Q000235:genetics*; D013569:Synapses / Q000187:drug effects / Q000235:genetics / Q000378:metabolism*; D013570:Synaptic Membranes / Q000187:drug effects / Q000235:genetics; D009435:Synaptic Transmission / Q000187:drug effects / Q000235:genetics; D014982:Xenopus laevis; D005680:gamma-Aminobutyric Acid / Q000378:metabolism*
C546173:Gabra6 protein, rat; D011963:Receptors, GABA-A; D001569:Benzodiazepines; D000431:Ethanol; D005680:gamma-Aminobutyric Acid