Elsevier

Brain Research

Volume 1197, 4 March 2008, Pages 47-62
Brain Research

Research Report
Distribution of metabotropic glutamate 2 and 3 receptors in the rat forebrain: Implication in emotional responses and central disinhibition

https://doi.org/10.1016/j.brainres.2007.12.057Get rights and content

Abstract

The receptor localization of metabotropic glutamate receptors (mGlu) 2 and 3 was examined by using in situ hybridization and a well-characterized mGlu2-selective antibody in the rat forebrain. mGlu2 was highly and discretely expressed in cell bodies in almost all of the key regions of the limbic system in the forebrain, including the midline and intralaminar structures of the thalamus, the association cortices, the dentate gyrus of the hippocampus, the medial mammillary nucleus, and the lateral and basolateral nuclei of the amygdala. Moreover, presynaptic mGlu2 terminals were found in most of the forebrain structures, especially in the lateral part of the central nucleus of the amygdala, and the CA1 region of the hippocampus. Although some overlaps exist, such as in the hippocampus and the amygdala, the expression of mGlu3 mRNA, however, appeared to be more disperse, compared with that of mGlu2 mRNA. These distribution results support previous behavioral studies that the mGlu2 and 3 receptors may play important roles in emotional responses. In addition to its expression in glia, mGlu3 was distinctively expressed in cells in the GABAergic reticular nucleus of the thalamus. Local infusion of a non-selective mGlu2/3 agonist, LY379268, in the reticular nucleus of the thalamus, significantly reduced GABA release, suggesting that mGlu3 may also play a role in central disinhibition.

Introduction

Excessive activation of limbic brain regions, such as the basolateral nucleus of the amygdala and the hippocampus, is thought to be responsible for many symptoms of chronic stress and anxiety disorders (McEwen, 2005). Glutamate is believed to play an important role in this activation. Thus, the Gi/o-coupled group II metabotropic glutamate receptors (mGlus) have been targeted as a novel approach for the treatment of anxiety disorders. Since this approach bears a different mechanism of action than the classical benzodiazepines (Tizzano et al., 2002), it is expected that the new therapy will avoid the undesirable side effects associated with benzodiazepine treatment, which include sedation and drug dependence. In the rat and mouse, non-selective mGlu2/3 agonists are found to be efficacious in a number of anxiety models, such as blocking the expression of fear-potentiated startle, increasing the open-arm time in elevated plus maze, and reducing stress-induced hyperthermia (Monn et al., 1997, Helton et al., 1998, Ferris et al., 2001, Spooren et al., 2002, Swanson et al., 2005). The mGlu2/3 agonist LY354740 can also block the expression of fear-potentiated startle and CO2-induced anxiety in humans (Grillon et al., 2003, Schoepp et al., 2003), and successfully alleviate symptoms in patients with general anxiety disorder (Schoepp, 2004, Dunayevich et al., 2007).

The exact site of action of the mGlu2/3 agonists is not clear. Based on c-Fos activation studies, it has been hypothesized that these agonists elicit their therapeutic effect by decreasing excitation and enhancing inhibition in pathways associated with anxiety (Linden et al., 2004, Swanson et al., 2005). However, due to high homology of the mGlu2 and the mGlu3 receptors, the existing agonists bind to both receptor subtypes. Thus, it is unclear whether the effects of mGlu2/3 agonists on anxiety are mediated through mGlu2 or mGlu3, or if activation of both receptors is necessary to achieve efficacy. While development of mGlu2 and mGlu3 selective compounds is needed to clarify roles of these receptors in anxiety, detailed mapping of their expression in the brain will be helpful in exploring which receptor is involved in the therapeutic effects of non-selective agonists.

The expression of mGlu2 and mGlu3 mRNA and/or proteins in the CNS has been previously reported (Testa et al., 1994, Ohishi et al., 1993a, Ohishi et al., 1993b, Ohishi et al., 1998, Petralia et al., 1996, Tamaru et al., 2001, Crook et al., 2002). In these studies, the receptor subtype distribution is largely based on in situ hybridization experiments due to the limited availability of convincing subtype-selective antibodies. In the present study, we have re-examined and compared the detailed mGlu2 and mGlu3 mRNA distribution in the rat forebrain, with an emphasis on regions that are associated with anxiety responses. Moreover, we have further characterized and confirmed the selectivity of one antibody against mGlu2 that was previously reported (Neki et al., 1996, Ohishi et al., 1998). Using this mGlu2-selective antibody, we have mapped the mGlu2 protein expression in the pre- and postsynaptic components in the rat forebrain. To explore the possible function of mGlu3 in the thalamus, we have also investigated the effects of the mixed mGlu2/3 agonist LY379268 on GABA release from the GABAergic reticular nucleus of the thalamus, a region that expresses a high level of mGlu3 but not mGlu2.

Section snippets

Distribution of mGlu2 and mGlu3 mRNA in the forebrain

Although mGlu2 and mGlu3 belong to the same molecular family and have the highest homology within the family, the distribution of mGlu2 mRNA differed remarkably from that observed with the mGlu3 probes. These results, together with the observation that both mGlu2 and mGlu3 sense probes did not produce any specific detectable signals, suggest that the mGlu2 signals seen with the antisense probes are specific.

mGlu2 mRNA was distinctively expressed in the rat forebrain, especially in the limbic

Discussion

It has been demonstrated that among all metabotropic glutamate receptors, only mGlu2, mGlu3, mGlu5 and mGlu7 receptors are highly expressed in the rodent forebrain (Shigemoto and Mizuno, 2000). Our results confirm previous reports on the cellular localization of mGlu2 and mGlu3 receptors in the rat forebrain (Ohishi et al., 1993a, Ohishi et al., 1993b, Ohishi et al., 1998). Moreover, using confocal microcopy, our data provide direct evidence that mGlu2 receptors are extensively expressed in

Experimental procedures

All protocols regarding animal use were approved by the Merck Research Laboratory San Diego Institutional Committee for the Care and Use of Animals in Research and Education, in accordance with the guidelines of the National Institute of Health (NIH Publications No. 80-23) revised in 1996 and United States Department of Agriculture. All efforts were made to minimize the number of animals used and their suffering.

Acknowledgments

We thank Dr. Shigemoto for generously providing the monoclonal mGlu2 antibody.

References (67)

  • NekiA. et al.

    Pre- and postsynaptic localization of a metabotropic glutamate receptor, mGlu2, in the rat brain: an immunohistochemical study with a monoclonal antibody

    Neurosci. Lett.

    (1996)
  • OhishiH. et al.

    Distribution of the messenger RNA for a metabotropic glutamate receptor, mGlu2, in the central nervous system of the rat

    Neuroscience

    (1993)
  • OhishiH. et al.

    Distribution of a metabotropic glutamate receptor, mGlu2, in the central nervous system of the rat and mouse: an immunohistochemical study with a monoclonal antibody

    Neurosci. Res.

    (1998)
  • OzawaS. et al.

    Glutamate receptors in the mammalian central nervous system

    Prog. Neurobiol.

    (1998)
  • PetraliaR.S. et al.

    The metabotropic glutamate receptors, mGlu2 and mGlu3, show unique postsynaptic, presynaptic and glial localizations

    Neuroscience

    (1996)
  • PoisikO. et al.

    Metabotropic glutamate receptor 2 modulates excitatory synaptic transmission in the rat globus pallidus

    Neuropharmacology

    (2005)
  • PrattJ.A.

    The neuroanatomical basis of anxiety

    Pharmacol. Ther.

    (1992)
  • ShibataK. et al.

    An important role of the central amygdaloid nucleus and mammillary body in the mediation of conflict behavior in rats

    Brain Res.

    (1986)
  • ShigemotoR. et al.
  • SpoorenW.P. et al.

    Pharmacological and endocrinological characterisation of stress-induced hyperthermia in singly housed mice using classical and candidate anxiolytics (LY314582, MPEP and NKP608)

    Eur. J. Pharmacol.

    (2002)
  • TamaruY. et al.

    Distribution of metabotropic glutamate receptor mGlu3 in the mouse CNS: differential location relative to pre- and postsynaptic sites

    Neuroscience

    (2001)
  • TizzanoJ.P. et al.

    The anxiolytic action of mGlu2/3 receptor agonist, LY354740, in the fear-potentiated startle model in rats is mechanistically distinct from diazepam

    Pharmacol. Biochem. Behav.

    (2002)
  • TurnerJ.P. et al.

    Group II and III metabotropic glutamate receptors and the control of the nucleus reticularis thalami input to rat thalamocortical neurones in vitro

    Neuroscience

    (2003)
  • AlexanderG.M. et al.

    Presynaptic inhibition of corticothalamic feedback by metabotropic glutamate receptors

    J. Neurophysiol.

    (2005)
  • BeracocheaD.J. et al.

    Effects of mammillary body and mediodorsal thalamic lesions on elevated plus maze exploration

    Neuroreport

    (1991)
  • CapognaM.

    Distinct properties of presynaptic group II and III metabotropic glutamate receptor-mediated inhibition of perforant pathway-CA1 EPSCs

    Eur. J. Neurosci.

    (2004)
  • CharneyD.S. et al.

    A functional neuroanatomy of anxiety and fear: implications for the pathophysiology and treatment of anxiety disorders

    Crit. Rev. Neurobiol.

    (1996)
  • ConnP.J. et al.

    Pharmacology and functions of metabotropic glutamate receptors

    Annu. Rev. Pharmacol. Toxicol.

    (1997)
  • CoxC.L. et al.

    Glutamate inhibits thalamic reticular neurons

    J. Neurosci.

    (1999)
  • CrookJ.M. et al.

    Comparative analysis of group II metabotropic glutamate receptor immunoreactivity in Brodmann's area 46 of the dorsolateral prefrontal cortex from patients with schizophrenia and normal subjects

    Mol. Psychiatry

    (2002)
  • Dunayevich, E., Erickson, J., Levine, L., Landbloom, R., Schoepp, D.D., Tollefson, G.D., Efficacy and tolerability of...
  • Feeley KearneyJ.A. et al.

    mGlus: a target for pharmacotherapy in Parkinson disease

    Exp. Neurol.

    (2003)
  • FerrisP. et al.

    Interactions between LY354740, a group II metabotropic agonist and the GABA(A)-benzodiazepine receptor complex in the rat elevated plus-maze

    J. Psychopharmacol.

    (2001)
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