The role of GABA_A receptors in hippocampus-dependent cognitive functions

The hippocampus is a central structure for learning and memory, integrating excitatory and inhibitory activity to support cognitive processing. Among inhibitory mechanisms, phasic and tonic inhibition via γ-aminobutyric acid type A receptors (GABA_ARs) play pivotal and subtype-specific roles in shaping hippocampal excitability, oscillations, and plasticity. This review collects and presents evidence from molecular, electrophysiological, pharmacological, and behavioral studies to delineate how distinct GABA_AR subtypes, specifically α1, α2, α4, and α5GABA_ARs modulate hippocampus-dependent cognitive functions. α5GABA_ARs are enriched in the hippocampus, where they are localized primarily on dendritic shafts of pyramidal neurons. They mediate tonic and slow phasic inhibition that regulate signal sparsity and memory interference. Their expression and synaptic localization are dynamically regulated by neuronal activity, linking inhibitory plasticity to learning and memory. Genetic and pharmacologic manipulations reveal age-dependent bidirectional effects: α5-negative allosteric modulators enhance learning and memory in young adult animals, whereas α5-positive allosteric modulators alleviate cognitive deficits in aging animals, and in stress or neuropsychiatric models, underscoring context-dependent roles. Other GABA_AR subtypes also contribute, e.g., α1GABA_ARs mediate benzodiazepine-induced anterograde amnesia, α2GABA_ARs modulate contextual and spatial learning and hippocampal theta oscillations, and α4GABA_ARs mediate tonic inhibition critical for pubertal synaptic pruning and hormone-dependent plasticity. Collectively, hippocampal GABA_AR diversity enables the control of inhibitory activity and strength essential for mnemonic function. Understanding subtype-specific contributions illuminates mechanisms of cognitive impairment in disorders such as Down syndrome and major depressive disorder, and enhances the understanding and the development of α-subunit-targeted allosteric modulators as interventions to restore hippocampal inhibitory balance and cognitive performance.