Neurosteroid interactions with synaptic and extrasynaptic GABA(A) receptors: regulation of subunit plasticity, phasic and tonic inhibition, and neuronal network excitability.
BACKGROUND
Neurosteroids are steroids synthesized within the brain with rapid effects on neuronal excitability. Allopregnanolone, allotetrahydrodeoxycorticosterone, and androstanediol are three widely explored prototype endogenous neurosteroids. They have very different targets and functions compared to conventional steroid hormones. Neuronal γ-aminobutyric acid (GABA) type A (GABA(A)) receptors are one of the prime molecular targets of neurosteroids.


OBJECTIVE
This review provides a critical appraisal of recent advances in the pharmacology of endogenous neurosteroids that interact with GABA(A) receptors in the brain. Neurosteroids possess distinct, characteristic effects on the membrane potential and current conductance of the neuron, mainly via potentiation of GABA(A) receptors at low concentrations and direct activation of receptor chloride channel at higher concentrations. The GABA(A) receptor mediates two types of inhibition, now characterized as synaptic (phasic) and extrasynaptic (tonic) inhibition. Synaptic release of GABA results in the activation of low-affinity γ2-containing synaptic receptors, while high-affinity δ-containing extrasynaptic receptors are persistently activated by the ambient GABA present in the extracellular fluid. Neurosteroids are potent positive allosteric modulators of synaptic and extrasynaptic GABA(A) receptors and therefore enhance both phasic and tonic inhibition. Tonic inhibition is specifically more sensitive to neurosteroids. The resulting tonic conductance generates a form of shunting inhibition that controls neuronal network excitability, seizure susceptibility, and behavior.


CONCLUSIONS
The growing understanding of the mechanisms of neurosteroid regulation of the structure and function of the synaptic and extrasynaptic GABA(A) receptors provides many opportunities to create improved therapies for sleep, anxiety, stress, epilepsy, and other neuropsychiatric conditions.
Department of Neuroscience and Experimental Therapeutics, College of Medicine, Texas A&M University Health Science Center, 2008 Medical Research and Education Building, 8447 State Highway 47, Bryan, TX, 77807-3260, USA.
Chase Matthew
CM
Carver
Doodipala Samba
DS
Reddy
D000494:Allosteric Regulation / Q000502:physiology; D015113:Androstane-3,17-diol / Q000378:metabolism; D000818:Animals; D001921:Brain / Q000378:metabolism*; D003900:Desoxycorticosterone / Q000031:analogs & derivatives / Q000378:metabolism; D006801:Humans; D009415:Nerve Net / Q000502:physiology; D009433:Neural Inhibition; D009473:Neuronal Plasticity / Q000502:physiology; D018377:Neurotransmitter Agents / Q000378:metabolism*; D011280:Pregnanolone / Q000378:metabolism; D011963:Receptors, GABA-A / Q000378:metabolism*
D018377:Neurotransmitter Agents; D011963:Receptors, GABA-A; D015113:Androstane-3,17-diol; D003900:Desoxycorticosterone; C009413:tetrahydrodeoxycorticosterone; D011280:Pregnanolone