Developmental maturation of synaptic and extrasynaptic GABA_A receptors in mouse thalamic ventrobasal neurones

Abstract

Thalamic ventrobasal (VB) relay neurones express multiple GABA A receptor subtypes mediating phasic and tonic inhibition. During postnatal development, marked changes in subunit expression occur, presumably reflecting changes in functional properties of neuronal networks. The aims of this study were to characterize the properties of synaptic and extrasynaptic GABA A receptors of developing VB neurones and investigate the role of the α 1 subunit during maturation of GABA-ergic transmission, using electrophysiology and immunohistochemistry in wild type (WT) and α 1 0/0 mice and mice engineered to express diazepam-insensitive receptors (α 1H101R , α 2H101R ). In immature brain, rapid (P8/9-P10/11) developmental change to mIPSC kinetics and increased expression of extrasynaptic receptors (P8-27) formed by the α 4 and δ subunit occurred independently of the α 1 subunit. Subsequently (≥ P15), synaptic α 2 subunit/gephyrin clusters of WT VB neurones were replaced by those containing the α 1 subunit. Surprisingly, in α 1 0/0 VB neurones the frequency of mIPSCs decreased between P12 and P27, because the α 2 subunit also disappeared from these cells. The loss of synaptic GABA A receptors led to a delayed disruption of gephyrin clusters. Despite these alterations, GABA-ergic terminals were preserved, perhaps maintaining tonic inhibition. These results demonstrate that maturation of synaptic and extrasynaptic GABA A receptors in VB follows a developmental programme independent of the α 1 subunit. Changes to synaptic GABA A receptor function and the increased expression of extrasynaptic GABA A receptors represent two distinct mechanisms for fine-tuning GABA-ergic control of thalamic relay neurone activity during development.

Molecular and functional heterogeneity of GABAergic synapses

“…However, ablation of GABA A Rs by targeted deletion of an α subunit variant often results in the disruption of postsynaptic gephyrin clusters and formation of large intra-cellular gephyrin aggregates [56,[72][73][74][75][76], confirming the dependence of gephyrin on GABA A Rs for postsynaptic localization. Importantly, in thalamic neurons expressing both α1-GABA A R postsynaptically and α4-GABA A R extrasynaptically, gephyrin clustering was not rescued following α1 subunit deletion, indicating that α4-GABA A R cannot interact with gephyrin [75,56].…”

“…Importantly, in thalamic neurons expressing both α1-GABA A R postsynaptically and α4-GABA A R extrasynaptically, gephyrin clustering was not rescued following α1 subunit deletion, indicating that α4-GABA A R cannot interact with gephyrin [75,56].…”

Postnatal development of synaptic properties of the GABAergic projection from the inferior colliculus to the auditory thalamus

“…Rise and decay times of P16 neurons showed deviations from the fitted lines to , and the correlation between rise/decay time and became even stronger when only P8 and P27 data were used (R ϭ 0.65, P Ͻ 0.001 for rise time vs. ; R ϭ 0.75, P Ͻ 0.05 for decay time vs. ) This deviation at P16 may reflect the presence of multiple GABA R subunits (␣ 1 , ␣ 2 , and ␣ 3 ) at this intermediate age that promote high kinetic variability (Eyre et al 2012). Thalamic nuclei undergo a sharp development shift from ␣ 2 [and to a lesser extent from ␣ 3 (Laurie et al 1992)] to ␣ 1 between P10 and P20 (Fritschy et al 1994;Peden et al 2008). Overall, evoked GABA A IPSC temporal properties did not show much change with age [similar to TRN-VB IPSCs between ages P8-P12 and P21-P30 (Huntsman and Huguenard 2000)], but further study is needed to characterize developmental changes in receptor subunit composition.…”

Tonic GABAA receptor conductance in medial subnucleus of the tractus solitarius neurons is inhibited by activation of μ-opioid receptors

“…However, this study was performed in neonatal rats (postnatal days 1-14). Since expression of the ␦-subunit is developmentally regulated, it is not likely to be fully expressed at this early age (Peden et al 2008). Another study did report the presence of GBZ-insensitive tonic currents (I tonic ) in second-order NTS neurons.…”