Distribution and abundance of metabotropic glutamate receptor subtype 2 in rat brain revealed by [³H]LY354740 binding in vitro and quantitative radioautography: Correlation with the sites of synthesis, expression, and agonist stimulation of [³⁵S]GTPγs binding

Abstract

Until recently, there was a lack of selective radioligands for the subtypes of metabotropic glutamate (mGlu) receptors. [(3)H]LY354740 ((+)-2-aminobicyclo[3,1,0]hexane-2,6-dicarboxylic acid), a selective agonist for group II receptors (mGlu2 and -3, which are negatively coupled to cAMP production), has now been used to map their brain distribution and abundance by in vitro binding and quantitative radioautography. The selective cation dependence of its binding allowed the discrimination between mGlu2 and mGlu3 receptor labeling. Thus, in the presence of Ca(2+) and Mg(2+) ions, the agonist bound selectively to mGlu2 receptors as evidenced by: 1) the correlative distribution and abundance of binding sites (highest in the lacunosum moleculare of the hippocampus and lowest in white matter) with mGlu2 receptor mRNA and protein revealed by in situ hybridization histochemistry and immunohistochemistry, respectively; 2) its selective pharmacology; and 3) the distribution of LY354740-stimulated [(35)S]GTPgammaS binding (25-97% above basal, according to the brain region), revealing G protein-coupled receptor coupling to G(i) proteins. Nonspecific binding (in the presence of 10 muM DCG-IV, a group II-selective, mGlu2-preferring, receptor agonist) was <10% of total. In adjacent sections, the distribution of binding sites for [(3)H]DCG-IV was very similar. This extensive study paves the way for investigations of the regional expression and regulation of mGlu2 receptors in human CNS diseases, such as Alzheimer's disease, which may reveal their functional roles and identify potential therapeutic drug targets. Indeed, it has recently been demonstrated (Higgins et al. [2004] Neuropharmacology 46:907-917) that pharmacological manipulation of mGlu2 receptors influences cognitive performance in the rodent.

Activation of Group II Metabotropic Glutamate Receptors in the Nucleus Accumbens Shell Attenuates Context-Induced Relapse to Heroin Seeking

“…mGluR 2 in these brain areas is selectively expressed on presynaptic sites (Testa et al, 1998;Richards et al, 2005). Results from anatomical studies do not indicate differential distribution of mGluR 2 or mGluR 3 receptors in the NAc core, shell, and striatum (Petralia et al, 1996;Ohishi et al, 1998;Richards et al, 2005). In the NAc, agonist activation of mGluR 2/3 receptors decreases excitatory synaptic transmission via presynaptic mechanisms (Manzoni et al, 1997) and decreases glutamate levels, as measured by microdialysis .…”

“…mGluR 3 is expressed on both pre-and postsynaptic sites in the NAc and the caudate-putamen (Ohishi et al, 1993b;Tamaru et al, 2001). mGluR 2 in these brain areas is selectively expressed on presynaptic sites (Testa et al, 1998;Richards et al, 2005). Results from anatomical studies do not indicate differential distribution of mGluR 2 or mGluR 3 receptors in the NAc core, shell, and striatum (Petralia et al, 1996;Ohishi et al, 1998;Richards et al, 2005).…”

“…mGluR 2 and mGluR 3 are expressed in the NAc (Ohishi et al, 1993a, b;Tamaru et al, 2001;Richards et al, 2005), and this brain area receives glutamatergic projections from the prefrontal cortex, thalamus, basolateral amygdala, and hippocampus (Brog et al, 1993;Heimer et al, 1997;Groenewegen et al, 1999b). Infusions of an mGluR 2/3 agonist into the NAc decrease glutamate release .…”

Metabotropic glutamate receptor mGluR2/3 and mGluR5 binding in the anterior cingulate cortex in psychotic and nonpsychotic depression, bipolar disorder and schizophrenia: implications for novel mGluR-based therapeutics

“…Receptor autoradiography studies were based on protocols described previously (mGluR2/3; 18,19 mGluR5 20,21 ). Autoradiography exposures were optimized for mGluR2/3 and mGluR5 to ensure that each cortical layer could be quantified on a linear scale and none of the cortical layers with high expression were overexposed.…”

Group II metabotropic glutamate receptors (mGlu2/3) in drug addiction

“…Group II metabotropic receptor (mGlu 2/3 receptors) family includes 2 subtypes both coupled to Gi proteins; mGlu2 receptors are expressed outside the active zone on presynaptic axon terminals to negatively regulate neurotransmitter release, while mGlu3 receptors are localized pre- and post-synaptically as well as on glia with a less clear overall function, but including negative regulation of transmitter release (Ohishi, Shigemoto et al 1993; Testa, Friberg et al 1998; Schoepp 2001; Tamaru, Nomura et al 2001; Richards, Messer et al 2005). mGlu 2/3 receptors can be homosynaptic, regulating glutamate release, or heterosynaptic regulating release of dopamine and γ-aminobutyric acid (GABA) (Hu, Duffy et al 1999; Schoepp 2001; Karasawa, Yoshimizu et al 2006; Xi, Kiyatkin et al 2010).…”