Regulator of G Protein Signaling Protein Suppression of Gαo Protein-Mediated α2A Adrenergic Receptor Inhibition of Mouse Hippocampal CA3 Epileptiform Activity

Activation of G protein-coupled α 2 adrenergic receptors (ARs) inhibits epileptiform activity in the hippocampal CA3 region. The specific mechanism underlying this action is unclear. This study investigated which subtype(s) of α 2 ARs and G proteins (Gα o or Gα i ) are involved in this response using recordings of mouse hippocampal CA3 epileptiform bursts. Application of epinephrine (EPI) or norepinephrine (NE) reduced the frequency of bursts in a concentration-dependent manner: (-)EPI > (-)NE >>> (+)NE. To identify the α 2 AR subtype involved, equilibrium dissociation constants (p K b ) were determined for the selective αAR antagonists atipamezole (8.79), rauwolscine (7.75), 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride (WB-4101; 6.87), and prazosin (5.71). Calculated p K b values correlated best with affinities determined previously for the mouse α 2A AR subtype ( r = 0.98, slope = 1.07). Furthermore, the inhibitory effects of EPI were lost in hippocampal slices from α 2A AR-but not α 2C AR-knockout mice. Pretreatment with pertussis toxin also reduced the EPI-mediated inhibition of epileptiform bursts. Finally, using knock-in mice with point mutations that disrupt regulator of G protein signaling (RGS) binding to Gα subunits to enhance signaling by that G protein, the EPI-mediated inhibition of bursts was significantly more potent in slices from RGS-insensitive Gα o G184S heterozygous (Gα o +/GS) mice compared with either Gα i2 G184S heterozygous (Gα i2 +/GS) or control mice (EC 50 = 2.5 versus 19 and 23 nM, respectively). Together, these findings indicate that the inhibitory effect of EPI on hippocampal CA3 epileptiform activity uses an α 2A AR/Gα o protein-mediated pathway under strong inhibitory control by RGS proteins. This suggests a possible role for RGS inhibitors or selective α 2A AR agonists as a novel antiepileptic drug therapy.