GABA(A) receptor-mediated miniature postsynaptic currents and alpha-subunit expression in developing cortical neurons

Previous studies have described maturational changes in GABAergic inhibitor y synaptic transmission in the rodent somatosensory cortex during the early postnatal period. To determine whether alterations in the functional prope rties of synaptically localized GABA(A) receptors (GABA(A)Rs) contribute to development of inhibitory transmission, we used the whole cell recording t echnique to examine GABAergic miniature postsynaptic currents (mPSCs) in de veloping cortical neurons. Neurons harvested from somatosensory cortices of newborn mice showed a progressive, eightfold increase in GABAergic mPSC fr equency during the first 4 wk of development in dissociated cell culture. A twofold decrease in the decay time of the GABAergic mPSCs, between 1 and 4 wk, demonstrates a functional change in the properties of GABA(A)Rs mediat ing synaptic transmission in cortical neurons during development in culture . A similar maturational profile observed in GABAergic mPSC frequency and d ecay time in cortical neurons developing in vivo (assessed in slices), sugg ests that these changes in synaptically localized GABA(A)Rs contribute to d evelopment of inhibition in the rodent neocortex. Pharmacological and rever se transcription-polymerase chain reaction (RT-PCR) studies were conducted to determine whether changes in subunit expression might contribute to the observed developmental alterations in synaptic GABA(A)Rs. Zolpidem (300 nM) , a subunit-selective benzodiazepine agonist with high affinity for alpha 1 -subunits, caused a reversible slowing of the mPSC decay kinetics in cultur ed cortical neurons. Development was characterized by an increase in the po tency of zolpidem in modulating the mPSC decay, suggesting a maturational i ncrease in percentage of functionally active GABA(A)Rs containing alpha 1 s ubunits. The relative expression of alpha 1 versus alpha 5 GABA(A)R subunit mRNA in cortical tissue, both in vivo and in vitro, also increased during this same period. Furthermore, single-eel RT-multiplex PCR analysis reveale d more rapidly decaying mPSCs in individual neurons in which alpha 1 versus alpha 5 mRNA was amplified. Together these data suggest that changes in al pha-subunit composition of GABA(A)Rs contribute to the maturation of GABAer gic mPSCs mediating inhibition in developing cortical neurons.