Cation modulation of GABA_A receptors in brain sections of AT and ANT rats

Abstract 

Changes in magnesium ion (Mg²⁺) concentration may be implicated in alcohol-related behaviors through modulation of neuronal excitability by actions on ligand-gated ion channels. To study whether putative Mg²⁺-binding sites differ between two rat lines, alcohol-insensitive (AT) and alcohol-sensitive (ANT) rats, selectively outbred for differential sensitivity to the motor-impairing effect of ethanol, we compared the effect of Mg²⁺ on [³⁵S]tert-butylbicyclophosphorothionate ([³⁵S]TBPS) binding to GABA_A receptors with the use of ligand autoradiographic analyses of brain sections from these rats. There were some slight differences between the rat lines in modulation of the binding in the forebrain. A low concentration of Mg²⁺ (0.1 mM) inhibited basal [³⁵S]TBPS binding more efficiently in the central gray matter and hippocampus in the ANT rats than in the AT rats. In the presence of γ-aminobutyric acid, the effect of a low concentration of Mg²⁺ was higher in the caudate-putamen and inner layer of the cerebral cortex in the AT rats than in the ANT rats. No difference between the rat lines was found at a higher (3 mM) Mg²⁺ concentration. Furosemide, a GABA_A antagonist selective for cerebellar granule cell–specific α6β2/3 subunit–containing receptors, was less efficient in antagonizing the Mg²⁺-induced inhibition of [³⁵S]TBPS binding in the ANT rats than in the AT rats. Another divalent cation, zinc ion, was less efficient in displacing [³⁵S]TBPS binding from the cerebellar granule cell layer in the ANT rats than in the AT rats, whereas a trivalent cation, lanthanum ion, produced identical modulation of the binding in the two rat lines. The results indicate that the alcohol-sensitive ANT rats have altered cerebellar granule cell–specific α6 subunit–containing GABA_A receptors and seem to indicate that these receptors might be implicated in the sensitivity difference of the rat lines to ethanol and sedative drugs.

Keywords: GABA_A receptors, Alcohol sensitivity, Cerebellum, Animal model, Cage convulsant, TBPS, Furosemide, Magnesium, Zinc, Lanthanum