Brain regional distribution of GABAA receptors exhibiting atypical GABA agonism: Roles of receptor subunits

Abstract

The major inhibitory neurotransmitter in the brain, γ-aminobutyric acid (GABA), has only partial efficacy at certain subtypes of GABAA receptors. To characterize these minor receptor populations in rat and mouse brains, we used autoradiographic imaging of t-butylbicyclophosphoro[35S]thionate ([35S]TBPS) binding to GABAA receptors in brain sections and compared the displacing capacities of 10 mM GABA and 1 mM 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP), a competitive GABA-site agonist. Brains from GABAA receptor α1, α4, δ, and α4 + δ subunit knockout (KO) mouse lines were used to understand the contribution of these particular receptor subunits to “GABA-insensitive” (GIS) [35S]TBPS binding. THIP displaced more [35S]TBPS binding than GABA in several brain regions, indicating that THIP also inhibited GIS-binding. In these regions, GABA prevented the effect of THIP on GIS-binding. GIS-binding was increased in the cerebellar granule cell layer of δ KO and α4 + δ KO mice, being only slightly diminished in that of α1 KO mice. In the thalamus and some other forebrain regions of wild-type mice, a significant amount of GIS-binding was detected. This GIS-binding was higher in α4 KO mice. However, it was fully abolished in α1 KO mice, indicating that the α1 subunit was obligatory for the GIS-binding in the forebrain. Our results suggest that native GABAA receptors in brain sections showing reduced displacing capacity of [35S]TBPS binding by GABA (partial agonism) minimally require the assembly of α1 and β subunits in the forebrain and of α6 and β subunits in the cerebellar granule cell layer. These receptors may function as extrasynaptic GABAA receptors.

MODULATION OF THE GABA_A RECEPTOR BY SRIF IN RAT THALAMUS

“…These two compounds inhibit the [ 35 S]-TBPS binding in similar manner to somatostatin (Chigr et al, 2002). These findings are different from previous data showing that a proportion of GABA A receptors in the thalamus displaying atypical allosteric coupling between the agonist and channel sites: the so-called GABAinsensitive [ 35 S]-TBPS binding (Sinkkonen et al, 2001a;2001b;Halonen et al, 2009).…”

“…35 S]-TBPS binding by many positive-GABA agonists such as neurosteroids has been used as of a highly of allosteric interactions of neurosteroids with the GABA A receptor (Vincens et al, 1992;Atack et al, 2007;Halonen et al, 2009).…”

“…This is surprising since this structure plays important functional roles (Saalmann and Kastner, 2011;Wang et al, 2011). Furthermore, this brain region is enriched in somatostatin containing neuronal elements (Wang et al, 2000) GABA (Geis and Borst, 2012) and GABA receptors (Edgar and Schwartz, 1990;Halonen et al, 2009). Somatostatin and GABA A receptors in this region play important roles in a variety of physiological functions (Leresche et al, 2000;Wang, 2011;Wang et al, 2011).…”

Altered Localization of the δ Subunit of the GABAA Receptor in the Thalamus of α4 Subunit Knockout Mice

“…Thus, combined loss or decrease of the two subunits in the thalamus and dentate gyrus could contribute to the increased seizure susceptibility in both α4 and δ subunit KO mice [8, 47, 48]. These findings are also consistent with marked decreases in high affinity muscimol binding in both the α4 and δ KO mice, and essentially all such binding is lost in the double α4/δ KO mouse [49]. Together these results emphasize the normally strong partnership of the δ and α4 subunits of the GABA A R.…”

Molecular Annotation of Integrative Feeding Neural Circuits

“…Gabrb2 (Fig. 4D), is a Subunit of the GABA A receptor (Halonen et al, 2009) and Lrig2 (Fig. 4E-F), is a member of the leucine-rich repeats and immunoglobulin-like domains (LRIG) family (Guo et al, 2004).…”