Early postnatal switch in GABA_A receptor α-subunits in the reticular thalamic nucleus

The GABAergic neurons of the thalamic reticular nucleus (nRt) provide the primary source of inhibition within the thalamus. Using physiology, pharmacology, and immunohistochemistry in mice, we characterized postsynaptic developmental changes in these inhibitory projection neurons. First, at postnatal days 3-5 (P3-5), inhibitory postsynaptic currents (IPSCs) decayed very slowly, followed by a biphasic developmental progression, becoming faster at P6-8 and then slower again at P9-11 before stabilizing in a mature form around P12. Second, the pharmacological profile of GABA_A receptor (GABA_AR)- mediated IPSCs differed between neonatal and mature nRt neurons, and this was accompanied by reciprocal changes in α₃ (late) and α₅ (early) subunit expression in nRt. Zolpidem, selective for α₁- and α₃-containing GABAA_Rs, augmented only mature IPSCs, whereas clonazepam enhanced IPSCs at all stages. This effect was blocked by the α₅-specific inverse agonist L-655,708, but only in immature neurons. In α₃(H126R) mice, in which α₃-subunits were mutated to become benzodiazepine insensitive, IPSCs were enhanced compared with those in wild-type animals in early development. Third, tonic GABA_AR activation in nRt is age dependent and more prominent in immature neurons, which correlates with early expression of α₅- containing GABA_ARs. Thus neonatal nRt neurons show relatively high expression of α₅-subunits, which contributes to both slow synaptic and tonic extrasynaptic inhibition. The postnatal switch in GABA_AR subunits from α₅ to α₃ could facilitate spontaneous network activity in nRt that occurs at this developmental time point and which is proposed to play a role in early circuit development.