Glycine receptor transmembrane regions and alcohol action: linking atoms to behavior.

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Harris RA1, Blednov YA1, Borghese CM1, Trudell JR2

1Waggoner Center for Alcohol and Addiction Research, University of Texas, Austin, USA.
2Department of Anesthesia, Stanford University School of Medicine, Stanford, USA.

Studies of recombinant receptors show that ethanol enhances the function of homomeric glycine receptors (GlyR) containing α1, α2 or α3 subunits (McCracken et al., 2013). However, most studies of effects of ethanol are focused on GlyR containing the α1 subunit (Borghese et al., 2012) and the role of GlyR containing α2 or α3 subunits are mostly unknown even though GLRA3 is a candidate gene for alcohol dependence (Han et al., 2013). To evaluate these GlyR, we tested a battery of ethanol-related behaviors in global null mutant mice lacking either GlyR α2 or GlyR α3 (Blednov et al., 2015). Mice lacking GlyR α2 or GlyR α3 did not show those changes in behavioral responses that were altered in mice with knock-in mutations in GlyR α1. Those changes included acoustic startle reflex and sensitivity to sedative effects of ethanol, flurazepam, or ketamine. Mice lacking GlyR α2 showed reduced ethanol intake and preference in the continuous two-bottle choice test. In contrast, mice lacking GlyR α3 showed increased ethanol intake and preference in the 24-hour intermittent access test. Both mutant lines showed increased ethanol conditioned taste aversion (CTA), but in different components. To construct knock-in (KI) mice with normal GlyR α2 function but lacking ethanol modulation, we probed the transmembrane (TM) regions critical for alcohol action on GlyRs (Horani et al., 2015) and tested 12 mutations. We selected L298A because it did not change sensitivity to glycine, taurine or Zn++ or the maximal glycine-induced currents, but eliminated ethanol-induced potentiation, both in the absence or presence of Zn++. KI mice with this mutation were behaviorally normal, but showed the same change in CTA that was found in the GlyR α2 null mutant mice. Thus, deletion of GlyR α2 or GlyR α3 subunits leads to the changes of distinct ethanol behaviors (ethanol intake and aversion to ethanol) and the changes in these behavioral effects of ethanol are subunit-specific. The KI mice suggest that changes in CTA are partially due to alcohol actions on GlyR α2. Overall, GlyR containing α2 or α3 subunits can be important for specific behavioral actions of ethanol, including consumption and aversion. Supported by NIH/NIAAA (AA U01 13520 – INIA Project; AA06399).