Abstract
Activation of G protein-coupled α2 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 α2ARs and G proteins (Gαo or Gαi) 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 α2AR subtype involved, equilibrium dissociation constants (pKb) 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 pKb values correlated best with affinities determined previously for the mouse α2AAR subtype (r = 0.98, slope = 1.07). Furthermore, the inhibitory effects of EPI were lost in hippocampal slices from α2AAR-but not α2CAR-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 GαoG184S heterozygous (Gαo+/GS) mice compared with either Gαi2G184S heterozygous (Gαi2+/GS) or control mice (EC50 = 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 α2AAR/Gαo protein-mediated pathway under strong inhibitory control by RGS proteins. This suggests a possible role for RGS inhibitors or selective α2AAR agonists as a novel antiepileptic drug therapy.
Footnotes
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This work was supported by the North Dakota Experimental Program to Stimulate Competitive Research through the National Science Foundation (NSF) [Grant EPS-0447679]; NSF Faculty Early Career Development Award [Grant 0347259]; NSF Research Experience for Undergraduates Site [Grant 0639227]; NSF Research Experience for Teachers [Grant 0639227]; National Institutes of Health National Institute on Drug Abuse [Grant 5-R01-DA17963]; National Institutes of Health National Institute of General Medical Sciences [Grant 5-R01-GM039561]; and National Institutes of Health National Center for Research Resources INBRE Program [Grant P20-RR016741].
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Preliminary reports of these findings were presented at the 2007 annual meeting of the American Society for Biochemistry and Molecular Biology (ASBMB) Northwest Regional Undergraduate Affiliate Network; 2007 October 26-27; Moorhead, MN; and the 2008 annual meetings of the ASBMB and the American Society for Pharmacology and Experimental Therapeutics, 2008 April 5-9, San Diego, CA.
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B.L.G. and B.W.N. contributed equally to this work.
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ABBREVIATIONS: CNS, central nervous system; ACSF, artificial cerebral spinal fluid; AR, adrenergic receptor; CA3, cornu ammonis 3; EPI, epinephrine; GPCR, G-protein coupled receptor; KO, knockout; NE, norepinephrine; RGS, regulator of G-protein signaling; WB-4101, 2-(2,6-dimethoxyphenoxyethyl)aminomethyl-1,4-benzodioxane hydrochloride; WT, wild type; PTX, pertussis toxin; JP-1302, N-[4-(4-methyl-1-piperazinyl)phenyl]-9-acridinamine dihydrochloride.
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↵1 Current affiliation: Schroeder Middle School, Grand Forks, North Dakota.
- Received December 18, 2008.
- Accepted February 18, 2009.
- The American Society for Pharmacology and Experimental Therapeutics
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