Segregation of Different GABA_AReceptors to Synaptic and Extrasynaptic Membranes of Cerebellar Granule Cells

TLDR

Cerebellar granule cells exhibit both phasic and tonic GABA_A‑mediated inhibition, with six subunits (α1, α6, β2, β3, γ2, δ) assembling into multiple receptor subtypes. The study aimed to determine whether distinct surface distributions of GABA_A receptors underlie phasic versus tonic inhibition in granule cells. Immunogold labeling of granule cell membranes revealed the spatial localization of individual subunits, distinguishing synaptic from extrasynaptic sites. Synaptic α1, α6, β2/3, and γ2 subunits were enriched at Golgi synapses, while δ subunits were exclusively extrasynaptic, supporting tonic inhibition via α6β2/3δ receptors and phasic inhibition via synaptic α1β2/3γ2, α6β2/3γ2, and α1α6β2/3γ2 complexes.

Abstract

Two types of GABA A receptor-mediated inhibition (phasic and tonic) have been described in cerebellar granule cells, although these cells receive GABAergic input only from a single cell type, the Golgi cell. In adult rats, granule cells express six GABA A receptor subunits abundantly (α1, α6, β2, β3, γ2, and δ), which are coassembled into at least four to six distinct GABA A receptor subtypes. We tested whether a differential distribution of GABA A receptors on the surface of granule cells could play a role in the different forms of inhibition, assuming that phasic inhibition originates from the activation of synaptic receptors, whereas tonic inhibition is provided mainly by extrasynaptic receptors. The α1, α6, β2/3, and γ2 subunits have been found by immunogold localizations to be concentrated in GABAergic Golgi synapses and also are present in the extrasynaptic membrane at a lower concentration. In contrast, immunoparticles for the δ subunit could not be detected in synaptic junctions, although they were abundantly present in the extrasynaptic dendritic and somatic membranes. Gold particles for the α6, γ2, and β2/3, but not the α1 and δ, subunits also were concentrated in some glutamatergic mossy fiber synapses, where their colocalization with AMPA-type glutamate receptors was demonstrated. The exclusive extrasynaptic presence of the δ subunit-containing receptors, together with their kinetic properties, suggests that tonic inhibition could be mediated mainly by extrasynaptic α 6 β 2/3 δ receptors, whereas phasic inhibition is attributable to the activation of synaptic α 1 β 2/3 γ 2 , α 6 β 2/3 γ 2 , and α 1 α 6 β 2/3 γ 2 receptors.