MedicalResearch.com Interview with:
Dr. Paul Davies PhD
Tufts University School of Medicine
Department of Neuroscience
Boston, MA 02111
MedicalResearch.com: What is the background for this study? What are the main findings?
Response: Inhibition in the brain regulates neuronal action potential generation, too little inhibition can directly cause conditions such as epileptic seizures, neurodevelopment disorders, and neurodegenerative disorders. The main type of inhibition in the mammalian brain occurs when the neurotransmitter γ-aminobutyric acid (GABA) binds to the GABA type A receptors (GABAARs), a ligand-gated ion channel. Once bound with GABA, the receptor changes shape to open the ion channel allowing negative charged chloride ions to flow through into the cell and inhibiting excitation steaming from positive charged ions flowing through opposing excitatory ion channels. GABAARs mediate both synaptic (phasic) and extrasynaptic (tonic) inhibitory neurotransmission in the CNS and are the sites of action of benzodiazepines, barbiturates, general anesthetics and neuro-active steroids. We have been focused on the extrasynaptic GABAARs that mediate tonic inhibition. In the dentate gyrus of the hippocampus, neocortex, striatum and the thalamus tonic inhibition is largely dependent on GABAARs composed of α4, β2/3, and δ subunits. Neuro-active steroids play a central role in regulating behavior via their ability to allosterically enhance GABAARs, particularly extrasynaptic α4-containing GABAARs. Allosteric enhancement means that neuro-active steroids bind to GABAARs and cause a further change in the structure of the ion channel allowing it to remain open for longer.
For the last few decades, allosteric enhancement of GABAARs by neuro-active steroids has been the prevailing explanation for how the steroids increase inhibition in the brain. However, recently we described a new mechanism where the neuro-active steroid, THDOC, increased the association of protein kinase C (PKC) with extrasynaptic α4β3 subunit-containing GABAARs. The increase in PKC-mediated phosphorylation of α4 and β3 subunits leads to an increase in membrane insertion from intracellular stores, an increase in GABAAR stability in the membrane, and a prolonged increase of tonic inhibition, even after when the neuro-active steroids have been removed.
For this present study we asked whether other neuro-active steroids demonstrated the same metabotropic activity. We tested another endogenous neuro-active steroid, allopregnanolone (ALLO), and the synthetic neuro-active steroid, ganaxolone. In collaboration with SAGE Therapeutics, we also tested another synthetic neuro-active steroid, SGE-516.
We found that all the neuro-active steroids tested were able to allosterically potentiate both synaptic and, to a lesser degree, extrasynaptic GABAARs. Short 15-minute exposures to neuro-active steroids resulted in significantly increase in phosphorylation of β3 subunits, and long lasting enhancement of tonic current. These increases were metabotropic in nature, being dependent upon PKC mediated phosphorylation. Following this short 15-minute exposure we saw a change in synaptic currents only with SGE-516 suggesting a selectivity of this metabotropic pathway to extrasynaptic GABAARs. Although ganaxolone was an effective allosteric modulator, it did not produce a metabotropic enhancement of tonic current suggesting that not all neuroactive steroids work through this pathway.
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