γ-Aminobutyric acid (GABA) type A receptors mediate fast inhibitory synaptic transmission and have been implicated in responses to sedative/hypnotic agents (including neuroactive steroids), anxiety, and learning and memory. Using gene targeting technology, we generated a strain of mice deficient in the δ subunit of the GABA type A receptors. In vivo testing of various behavioral responses revealed a strikingly selective attenuation of responses to neuroactive steroids, but not to other modulatory drugs. Electrophysiological recordings from hippocampal slices revealed a significantly faster miniature inhibitory postsynaptic current decay time in null mice, with no change in miniature inhibitory postsynaptic current amplitude or frequency. Learning and memory assessed with fear conditioning were normal. These results begin to illuminate the novel contributions of the δ subunit to GABA pharmacology and sedative/hypnotic responses and behavior and provide insights into the physiology of neurosteroids.

γ-Aminobutyric acid type A receptors (GABA _A -Rs) mediate the bulk of rapid inhibitory synaptic transmission in the central nervous system. The β3 subunit is an essential component of the GABA _A -R in many brain regions, especially during development, and is implicated in several pathophysiologic processes. We examined mice harboring a β3 gene inactivated by gene targeting. GABA _A -R density is approximately halved in brain of β3-deficient mice, and GABA _A -R function is severely impaired. Most β3-deficient mice die as neonates; some neonatal mortality, but not all, is accompanied by cleft palate. β3-deficient mice that survive are runted until weaning but achieve normal body size by adulthood, although with reduced life span. These mice are fertile but mothers fail to nurture offspring. Brain morphology is grossly normal, but a number of behaviors are abnormal, consistent with the widespread location of the β3 subunit. The mice are very hyperactive and hyperresponsive to human contact and other sensory stimuli, and often run continuously in tight circles. When held by the tail, they hold all paws in like a ball, which is frequently a sign of neurological impairment. They have difficulty swimming, walking on grids, and fall off platforms and rotarods, although they do not have a jerky gait. β3-deficient mice display frequent myoclonus and occasional epileptic seizures, documented by electroencephalographic recording. Hyperactivity, lack of coordination, and seizures are consistent with reduced presynaptic inhibition in spinal cord and impaired inhibition in higher cortical centers and/or pleiotropic developmental defects.

Neuronal rhythmic activities within thalamocortical circuits range from partially synchronous oscillations during normal sleep to hypersynchrony associated with absence epilepsy. It has been proposed that recurrent inhibition within the thalamic reticular nucleus serves to reduce synchrony and thus prevents seizures. Inhibition and synchrony in slices from mice devoid of the γ-aminobutyric acid type-A (GABA _A ) receptor β ₃ subunit were examined, because in rodent thalamus, β ₃ is largely restricted to reticular nucleus. In β ₃ knockout mice, GABA _A -mediated inhibition was nearly abolished in reticular nucleus, but was unaffected in relay cells. In addition, oscillatory synchrony was dramatically intensified. Thus, recurrent inhibitory connections within reticular nucleus act as “desynchronizers.”

The neurotransmitter GABA mediates the majority of rapid inhibition in the CNS. Inhibition can occur via the conventional mechanism, the transient activation of subsynaptic GABA_Areceptors (GABA_A-Rs), or via continuous activation of high-affinity receptors by low concentrations of ambient GABA, leading to “tonic” inhibition that can control levels of excitability and network activity. The GABA_A-R α4 subunit is expressed at high levels in the dentate gyrus and thalamus and is suspected to contribute to extrasynaptic GABA_A-R-mediated tonic inhibition. Mice were engineered to lack the α4 subunit by targeted disruption of theGabra4gene. α4 Subunit knockout mice are viable, breed normally, and are superficially indistinguishable from WT mice. In electrophysiological recordings, these mice show a lack of tonic inhibition in dentate granule cells and thalamic relay neurons. Behaviorally, knockout mice are insensitive to the ataxic, sedative, and analgesic effects of the novel hypnotic drug, gaboxadol. These data demonstrate that tonic inhibition in dentate granule cells and thalamic relay neurons is mediated by extrasynaptic GABA_A-Rs containing the α4 subunit and that gaboxadol achieves its effects via the activation of this GABA_A-R subtype.

Studies using molecular modeling, genetic engineering, neurophysiology/pharmacology, and whole animals have advanced our understanding of where and how inhaled anesthetics act to produce immobility (minimum alveolar anesthetic concentration; MAC) by actions on the spinal cord. Numerous ligand- and voltage-gated channels might plausibly mediate MAC, and specific animo acid sites in certain receptors present likely candidates for mediation. However, in vivo studies to date suggest that several channels or receptors may not be mediators (e.g., γ-aminobutyric acid A, acetylcholine, potassium, 5-hydroxytryptamine-3, opioids, and α2-adrenergic), whereas other receptors/channels (e.g., glycine, N-methyl-d-aspartate, and sodium) remain credible candidates.