GABAergic dynamics in thalamocortical circuits: Insights into absence seizures and hyperinhibition

Over the last decade, experimental and clinical evidence has shaped the excitation/inhibition imbalance doctrine, which serves as our contemporary understanding of the mechanisms underlying various epilepsies. In addition, emerging findings have highlighted the peculiar role of γ-aminobutyric acid (GABA), the brain's main inhibitory neurotransmitter, as an underappreciated factor in hyperexcitation disorders such as epilepsy. Intriguingly, the unique neuroanatomy and synaptic physiology of thalamocortical (TC) circuits have made this axis a distinctive field of study. In this review, we focus on the current literature that explores the detailed structure of the GABAergic system in TC circuits under both normal conditions and absence seizures. Augmented excitability of TC neurons as well as cortical somatosensory neurons due to differential activity of various GABAergic neurons and synapses contributes to hyperoscillatory circuits in absence seizures. Furthermore, subtle yet significant differential expression of GABA receptor subunits in the thalamus and somatosensory cortex plays crucial roles in the pathophysiology of absence seizures. Convincing evidence also underscores the pivotal importance of the lower expression of GABA transporters, particularly in astrocytes, which increases synaptic concentration of GABA and promotes persistent tonic inhibition in the thalamus. This, in turn, creates a priming membrane potential that facilitates activation of transient (T)-type calcium channels, thereby increasing the excitability of TC neurons. In summary, this review accentuates hyperinhibition as a leading pathophysiology of absence seizures. Thus, there is a pressing need to develop targeted pharmacologic interventions or precision medicine approaches to treat absence seizures and potentially other TC related disorders.