Layer- and cell-type-specific tonic GABAergic inhibition of pyramidal neurons in the rat visual cortex

Tonic inhibition mediated by persistent activation of gamma-aminobutyric acid(A) (GABA(A)) receptors by ambient GABA plays a crucial role in the regulation of network excitability and neuronal signal processing. Varying degrees in the strength of tonic inhibition were detected across different cell types throughout the brain. Since sensory information flows through cortical layers in a specific order, the characteristics of tonic inhibition in different cortical layers are of interest. Therefore, we examined the properties of tonic inhibition in pyramidal neurons (PyNs) throughout the rat visual cortex. Layer 2/3 PyNs and burst-spiking PyNs in layers 5 and 6 showed prominent tonic GABA(A) currents. Tonic GABA(A) currents in layer 4 star PyNs and regular-spiking PyNs in layers 5 and 6 were much weaker. The magnitude of tonic currents correlated well with the inhibition of spike generation. The amplitude of tonic GABA(A) currents measured with bicuculline and gabazine, the two different GABA(A) receptor blockers, did not differ. The differences in the expression levels of extrasynaptic GABA(A) receptors might be the major contributor to the differences in tonic GABA(A) currents among cell types. Furthermore, alpha 5 subunits might contribute significantly to tonic currents in infragranular burst-spiking PyNs, especially in layer 5. These results suggest that ambient GABA might exert differential effects on the neuronal integration in a layer- and cell-type-specific manner and thus contribute to the processing of sensory properties by selectively tuning the signals flowing through the visual cortex.