Abstract
Kainate receptors are a subtype of ionotropic glutamate receptors, permeable to cations and thus expected to have an excitatory depolarizing action on neurons. However, kainate receptor activation inhibits γ-aminobutyric acid release in the hippocampus through activation of protein kinase C in a pertussis toxin-dependent manner, suggesting a coupling of kainate receptors to G proteins. Thus, we directly investigated the G protein coupling of kainate receptors in the rat hippocampus by using a selective kainate receptor agonist, [3H](2S,4R)-4-methylglutamate ([3H]MGA). [3H]MGA bound to a single site to hippocampal membranes with a K D value of 32 nM and a B max value of 1024 fmol/mg protein. This binding likely represents kainate receptors because it was displaced by domoate (K i = 4 nM), kainate (K i = 11 nM), and 6-cyano-7-nitroquinoxaline-2,3-dione (K i = 1.4 μM), but not by α-amino-3-hydroxy-5-methyl-4-isoxazole-propionate (K i > 10 μM), (RS)-α-methyl-4-phosphonophenylglycine (K i > 10 μM), or (±)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (K i > 10 μM). Guanylylimidodiphosphate (30 μM), which uncouples all G protein-coupled receptors, shifted to the right the saturation curve of [3H]MGA (K D = 133 nM). This effect was mimicked by pretreatment of hippocampal membranes with modifiers of Gi/Go proteins [30 μMN-ethylmaleimide (K D = 98 nM) or 25 μg/ml pertussis toxin (K D = 95 nM)] but not by a modifier of Gs proteins [50 μg/ml cholera toxin (K D = 32 nM)]. Treatment of solubilized hippocampal membranes with pertussis toxin (25 μg/ml) decreased [3H]MGA affinity (K D = 105–113 nM), which was recovered by reconstitution of these pretreated solubilized hippocampal membranes with Gi/Go proteins (K D = 41–76 nM). These results indicate that hippocampal kainate receptors are coupled to Gi/Go proteins.
Footnotes
- Received February 2, 1999.
- Accepted April 21, 1999.
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Send reprint requests to: Dr. R. A.Cunha, Laboratory of Neurosciences, Faculty of Medicine, University of Lisbon, Avenida Prof. Egas Moniz, 1649-035 Lisbon, Portugal. E-mail:racunha{at}neurociencias.pt
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This work was supported by Fundação para a Ciência e Tecnologia (Praxis/2/2.1/SAU/1348/95).
- The American Society for Pharmacology and Experimental Therapeutics
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