TY - JOUR T1 - GABA<sub>A</sub> Receptor-Mediated Tonic Inhibition in Thalamic Neurons JF - The Journal of Neuroscience JO - J. Neurosci. SP - 11553 LP - 11563 DO - 10.1523/JNEUROSCI.3362-05.2005 VL - 25 IS - 50 AU - David W. Cope AU - Stuart W. Hughes AU - Vincenzo Crunelli Y1 - 2005/12/14 UR - http://www.jneurosci.org/content/25/50/11553.abstract N2 - Tonic GABAA receptor-mediated inhibition is typically generated byδ subunit-containing extrasynaptic receptors. Because the δ subunit is highly expressed in the thalamus, we tested whether thalamocortical (TC) neurons of the dorsal lateral geniculate nucleus (dLGN) and ventrobasal complex exhibit tonic inhibition. Focal application of gabazine (GBZ) (50 μm) revealed the presence of a 20 pA tonic current in 75 and 63% of TC neurons from both nuclei, respectively. No tonic current was observed in GABAergic neurons of the nucleus reticularis thalami (NRT). Bath application of 1μm GABA increased tonic current amplitude to ∼70 pA in 100% of TC neurons, but it was still not observed in NRT neurons. In dLGN TC neurons, the tonic current was sensitive to low concentrations of the δ subunit-specific receptor agonists allotetrahydrodeoxycorticosterone (100 nm) and 4,5,6,7-tetrahydroisoxazolo[5,4-c]-pyridin-3-ol (THIP) (100 nm) but insensitive to the benzodiazepine flurazepam (5 μm). Bath application of low concentrations of GBZ (25–200 nm) preferentially blocked the tonic current, whereas phasic synaptic inhibition was primarily maintained. Under intracellular current-clamp conditions, the preferential block of the tonic current with GBZ led to a small depolarization and increase in input resistance. Using extracellular single-unit recordings, block of the tonic current caused the cessation of low-threshold burst firing and promoted tonic firing. Enhancement of the tonic current by THIP hyperpolarized TC neurons and promoted burst firing. Thus, tonic current in TC neurons generates an inhibitory tone. Its modulation contributes to the shift between different firing modes, promotes the transition between different behavioral states, and predisposes to absence seizures. ER -