Analysis of the Interactions between GABA(A) Receptors and T3 using Electrophysiology and Molecular Dynamics Simulations

Abstract

Disorders of the thyroid cause a multitude of neurological dysfunctions including depression, anxiety, and psychosis. Thyroid hormones have been primarily thought to act via genomic mechanisms throughout the organism; however, recently another mechanism has been proposed for the adult brain. We hypothesize that the thyroid hormone triiodothyronine (T3) acts directly on GABA(A) receptors via a mechanism similar to that of neurosteroids. Previous electrophysiology experiments on expressed recombinant GABA(A) receptors demonstrated an inhibition of GABA responses in the presence of T3, and, at much higher concentrations of T3 alone, a direct stimulation of receptor activity. Current experiments are being conducted to investigate stereoselective effects and interactions with other compounds known to bind directly to the GABA(A) receptor. using a GABA(A) receptor homology model based on the crystal structure of the related Glutamate-gated chloride channel, atomic resolution molecular dynamics simulations were conducted to analyze the possible interaction of T3 and the GABA(A) receptor. In simulations, T3 is stable in binding sites in the transmembrane domain of the GABA(A)receptor in a region that is associated with the activation by neurosteroids. Alchemical free energy perturbation calculations are underway to test the affinity of T3 for the binding site in the GABA(A) receptor. Our results provide strong evidence supporting earlier experimental findings indicating a role of T3 in regulating the activity of GABA(A) receptors in brain, while bringing additional insight into both the molecular binding mode and the mechanism of modulation.