, 2007) and neurosteroids (which are brain-synthesized metabolites of ovarian and adrenal cortical steroid hormones) act as anesthetics through an action on δ-GABAARs

(Stell et al., 2003). Indeed, the loss of δ-GABAARs is associated with an attenuated response to neurosteroid-induced anesthesia (Mihalek et al., 1999). Other important general anesthetics such as propofol and isoflurane enhance tonic BAY 73-4506 manufacturer inhibition in hippocampal neurons (Bai et al., 2001), thalamic relay neurons (Jia et al., 2008b), and neocortical neurons (Drasbek et al., 2007). However, the amnesia-inducing effect, but not the anesthetic potency of isoflurane, is altered in α4 knockout mice, which also lack δ-GABAARs on the cell surface (Rau et al., 2009), demonstrating that extrasynaptic GABAARs are not a primary site of action for all anesthetics. Neurosteroids are among the most powerful regulators of GABAAR function in the CNS (Belelli and Lambert, 2005, Chisari Small molecule library et al., 2010, Mitchell et al., 2008 and Reddy, 2010). The first example of this robust modulatory effect was discovered nearly 30 years ago (Harrison and Simmonds, 1984) for the synthetic steroid alphaxalone (5α-pregnan- 3α-ol-11,20 dione). Shortly after, it was demonstrated that a metabolite of the ovarian steroid hormone progesterone (allopregnanolone, also called 3α-hydroxy-5α-pregnan-20-one, or 3α,5α-tetrahydroprogesterone, or 5α-pregnan-3α-ol-20-one,

or 5α3α-THPROG) and a metabolite of the stress steroid deoxycorticosterone (aka 5α3α-THDOC) are potent barbiturate-like MTMR9 ligands of GABAARs (Majewska et al., 1986). Our first collaborative research (Stell et al., 2003) demonstrated that δ-GABAARs are a preferred site of action for neurosteroids at low (nanomolar) concentrations. This preferred action probably reflects a simple property of these receptors: GABA is not an efficacious agonist at δ-GABAARs (Chisari et al., 2010), which means that the coupling of GABA binding to channel opening is not efficient. Because neurosteroids increase the likelihood that GABA will open the channel

(Chisari et al., 2010), they can enhance the efficacy of GABA at δ-GABAARs and thus modulate receptor activity, while this is less likely at other GABAARs where GABA is already an efficacious agonist. Perhaps δ-GABAARs are the preferred site of action for paracrine neurosteroid signaling where the neurosteroids synthesized in another cell (e.g., astrocyte) must travel through the extracellular space to act on extrasynaptic δ-GABAARs. Neurosteroid synthesis in astrocytes is regulated by the mitochondrial 18 kD translocator protein TSPO (the peripheral benzodiazepine receptor by its former name) for which the drug XBD173 is an excellent nonsedative anxiolytic and antipanic agent (Rupprecht et al., 2009). The mitochondrial TSPO is also in CNS neurons where it may mediate autologous effects of neurosteroids on neuronal excitability in brain slices following benzodiazepine (Tokuda et al.