Molecular and cellular targets of GABAergic anesthetics in the mesopontine tegmentum that enable pain-free surgery

The mesopontine tegmental anesthesia area (MPTA) is a focal brainstem locus which, when exposed to GABAergic agents, induces brain-state transitioning from wakefulness to unconsciousness. Correspondingly, MPTA lesions render animals relatively insensitive to GABAergic anesthetics delivered systemically. Using chemogenetics, we recently identified a neuronal subpopulation within the MPTA whose excitation induces this same pro-anesthetic effect. However, very few of these "effector-neurons" express synaptic γ2 -containing GABA A receptor isoforms and none express extrasynaptic δ -subunit containing receptors, suggesting that they are not the direct cellular target of GABAergic agents. Here we used pharmacological tools in rats to define the molecular target(s) of GABAergics in the MPTA. GABA microinjected into the MPTA at nanomolar concentrations, selective for GABA Aδ -Rs, proved to be pro-anesthetic as was blocking GABA reuptake. Likewise, low-concentration gaboxadol/THIP, also selective for GABA Aδ -Rs, was effective, whereas benzodiazepines and zolpidem, which selectively target GABA Aγ2 -Rs, were not. The GABAergic anesthetics pentobarbital and propofol proved pro-anesthetic when applied to the MPTA at the low concentrations present in the brain after systemic dosing. Glycinergic agonists which are inhibitory, but infective on GABA Aδ -Rs, and other non-GABAergic agonists tested, were at most only marginally effective. We conclude that GABA Aδ -Rs are the primary molecular target of GABAergic anesthetics in the MPTA. Immunolabeling revealed that this GABA A -R isoform is expressed exclusively by a distinct subpopulation of MPTA "δ-cells" that reside in close apposition to effector neurons. This suggests that during wakefulness, δ-cells serve as inhibitory interneurons which, when silenced by GABAergic agents, disinhibit (excite) the effector-neurons, triggering transition to unconsciousness.