The principal inhibitory neurotransmitter in the mammalian brain, ?-aminobutyric acid (GABA), is thought to regulate memory processes by activating transient inhibitory postsynaptic currents. Here we describe a nonsynaptic, tonic form of inhibition in mouse CA1 pyramidal neurons that is generated by a distinct subpopulation of GABA type A receptors (GABAARs). This tonic inhibitory conductance is predominantly mediated by a5 subunit-containing GABAARs (a5GABAARs) that have different pharmacological and kinetic properties compared to postsynaptic receptors. GABAARs that mediate the tonic conductance are well suited to detect low, persistent, ambient concentrations of GABA in the extracellular space because they are highly sensitive to GABA and desensitize slowly. Moreover, the tonic current is highly sensitive to enhancement by amnestic drugs. Given the restricted expression of a5GABAARs to the hippocampus and the association between reduced a5GABAAR function and improved memory performance in behavioral studies, our results suggest that tonic inhibition mediated by a5GABAARs in hippocampal pyramidal neurons plays a key role in cognitive processes. The ?-aminobutyric acid (GABA) subtype A receptor (GABAAR) is a pentameric anion-selective ion channel that assembles from different classes of subunits (a1-6, ß1-3, ?1-3, d, p, ?, and e) (1). The combination of various GABAAR subunits confers different biophysical and pharmacological properties and regulates regional and subcellular patterns of distribution (2, 3). The subunit composition critically determines agonist affinity, receptor kinetics, and sensitivity to a variety of clinically important drugs, including benzodiazepines and general anesthetics. Studies of gene-targeted mice have implicated specific GABAAR subunit isoforms in critical aspects of information processing in the brain. Notably, a5-null mutant mice (a5-/-) exhibit improved performance in the water maze model of spatial learning, a hippocampus-dependent learning task (4). Further, mice carrying a point mutation at position 105 of the a5 subunit (H105R) experience an unexpected selective reduction of a5GABAARs in hippocampal pyramidal neurons and improved performance for learning tasks (5). Pharmacological studies further support the involvement of a5GABAARs in learning processes; for example, a5 subunit-selective inverse agonists such as L-655,708 enhance learning performance in rats in the Morris water maze test (6, 7). Moreover, a5 subunit-selective inverse agonists exhibit desirable nootropic effects without causing adverse convulsant activities associated with nonselective GABAAR inverse agonists. Thus, inhibition of a5GABAARs presents an attractive strategy for developing memory-enhancing drugs. The neuronal substrates underlying improved cognitive performance associated with reduced a5GABAAR function remain unknown. The a5 subunit has a unique and limited pattern of distribution in the mammalian brain. Although a5-containing receptors constitute <5% of the total GABAAR population, they are abundantly expressed in the hippocampus, where they account for >20% of all GABAARs (8-12). The a5 subunit is present at considerably lower levels in the cerebral cortex and is virtually absent from most brain regions (3, 10, 13). The relatively restricted expression of the a5 subunit isoform in the hippocampus provides an unprecedented opportunity to establish links between specific populations of GABAARs and the mechanisms of learning and memory. Thus, it is of great interest to determine the neuronal correlate(s) underlying the improved behavioral performance in a5-/- mice (4, 5). Immunocytochemistry and in situ hybridization studies indicate that a5GABAARs are localized primarily to extrasynaptic regions of pyramidal neurons in the CA1 and CA3 regions of the hippocampus (5, 11, 14, 15). In cerebellar granule neurons, a6dGABAARs that are localized primarily to extrasynaptic regions (2, 16, 17) generate a persistent tonic inhibitory conductance that is distinct from inhibitory synaptic currents (18). This tonic conductance regulates neuronal excitability and possibly information processing in the cerebellar cortex (18-20). In dentate gyrus granule cells, GABAARs containing the d subunit, and likely a4 subunits, also generate a tonic current (21). The subunit composition of GABAARs that underlie a tonic current in the CA1 region of the hippocampus has not been determined. Whole-cell recordings from rat hippocampal CA1 pyramidal cells and cultured hippocampal neurons show two distinct forms of GABAergic inhibition: a persistent low-amplitude tonic current and a transient synaptic current (22, 23). Low, ambient concentrations of ?-aminobutyric acid (GABA) activate a tonic current rather than spontaneous GABAAR openings (22). We previously showed that the benzodiazepine midazolam enhances the tonic current in cultured hippocampal neurons, indicating that the underlying GABAAR complexes lack a4 and d subunits but contain ? subunits in combination with a1, a2, a3, or a5 subunits (22). The GABAARs that generate tonic and synaptic currents in hippocampal neurons have different pharmacological and biophysical properties (22, 24, 25). This differential sensitivity to GABAergic compounds could result from different molecular compositions of the underlying receptors, conditions of receptor activation, or a combination of these factors. Here, we test the hypothesis that GABAARs composed of a unique complement of subunits generate a tonic current in cultured hippocampal neurons and CA1 pyramidal cells; specifically, a5GABAARs underlie tonic, but not synaptic, inhibitory currents.