Metabotropic glutamate receptor 5 binding and protein expression in schizophrenia and following antipsychotic drug treatment
Introduction
Whilst the precise cause of schizophrenia is unknown, evidence suggests dysfunction of glutamatergic signalling contributes to all aspects of schizophrenia symptomatology including negative and cognitive deficits. Although the major theories relate to a hypofunction of the NMDA receptor (NMDAR) (Marek et al., 2010), metabotropic glutamate receptors (mGluR) have also been implicated (Niswender and Conn, 2010). Metabotropic glutamate receptor 5 (mGluR5) is one of eight known glutamate-specific G-protein coupled receptors which has been shown to uniquely modulate NMDAR activity through a physical and functional link (Tu et al., 1999, Alagarsamy et al., 2002, Luccini et al., 2007). Consequently, mGluR5 has become of interest as a potential target for antipsychotic treatment (see Matosin and Newell, 2013). However, evidence indicates that mGluR5 itself may also play a role in the pathophysiology of schizophrenia (Krivoy et al., 2008).
mGluR5 knock-out mice exhibit deficits in prepulse inhibition, memory deficits, and hyperlocomotion (Kinney et al., 2003, Brody et al., 2004). Similarly, mGluR5 negative allosteric modulators (NAMs) induce social interaction deficits, impaired working memory and reduced instrumental learning (Homayoun et al., 2004, Koros et al., 2006), implying a direct relationship between reduced mGluR5 function and the manifestation of schizophrenia-like symptoms. Moreover, mGluR5 positive allosteric modulators (PAM) are effective in the attenuation of cognitive as well as negative and positive schizophrenia-like symptoms in NMDAR antagonist and dopaminergic animal models (Matosin and Newell, 2013). These studies highlight a possible role for mGluR5 in schizophrenia pathology as well as the therapeutic potential of targeting mGluR5 to regulate glutamatergic NMDAR signalling, particularly for the treatment of negative and cognitive aspects of schizophrenia symptomatology.
Although animal studies suggest that mGluR5 is involved in the pathology of schizophrenia, studies characterising mGluR5 in post-mortem human brain tissue have found minimal changes in mRNA (Ohnuma et al., 1998, Ohnuma et al., 2000, Richardson-Burns et al., 2000, Volk et al., 2010) and protein (Gupta et al., 2005, Corti et al., 2011). This suggests that mGluR5 is unaltered in the pathological state, which is beneficial for novel therapeutics that modulate NMDAR signalling through mGluR5. However, alteration of the specific binding site(s) could render these therapeutics unsuitable. Despite this, examination of binding to mGluR5, specifically at the 6-methyl-2-(phenylethynyl)pyridine (MPEP) allosteric binding site (the specific target of mGluR5 modulators), has never before been investigated in schizophrenia.
It is crucial to characterise mGluR5, both in response to the pathological state and to current antipsychotics to decipher not only if it is a viable target for future therapeutics, but also to determine if mGluR5 is already affected by current antipsychotic drugs (APDs). For the first time, we measured the binding density as well as the protein density of mGluR5 in the dorsolateral prefrontal cortex (DLPFC), a brain region specifically involved in cognitive function (Eisenberg and Berman, 2010), in a large cohort of schizophrenia and control subjects. Furthermore, we analysed mGluR5 binding following typical and atypical APD treatment to determine if these agents have short- or long-term adaptive effects on mGluR5.
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Human post-mortem brain samples
Tissue was acquired from the New South Wales Tissue Resource Centre (TRC) and its use in this investigation was approved by, and conducted under the guidelines of the Human Research Ethics Committees at the University of Wollongong (HE99/222) and the University of New South Wales (HREC07261). 37 schizophrenia subjects (including 7 schizoaffective) diagnosed according to DSM-IV and 37 controls were matched by the TRC and the Schizophrenia Research Laboratory (SRL) (Table 1). APD treatment
Diagnosis related effects
[3H]MPEP binding density was highly expressed, with a homogenous distribution across the cortical layers (Fig. 1a). One case (control) was excluded from analysis for [3H]MPEP binding due to technical issues. Despite this, mean pH, age at death, PMI, freezer storage time, brain weight and RNA integrity number (RIN) did not differ between schizophrenia and control groups (− 1.525 < t > 1.264, 0.132 < p > 0.955). Independent t-tests revealed no significant change in [3H]MPEP binding density in
Discussion
mGluR5 is a potential therapeutic target for several neuropsychiatric disorders, including schizophrenia (Matosin and Newell, 2013, Vinson and Conn, 2012). For the first time we have examined binding to mGluR5 in schizophrenia patients and in the brains of rats following APD treatment. We report no change in mGluR5 binding or protein density in schizophrenia subjects compared to controls. Furthermore, we found that olanzapine and haloperidol, APDs commonly used in a clinical setting to treat
Role of funding source
The funding sources had no role in this study, including study design, data collection and publication decisions.
Contributors
All authors contributed to the study design and have contributed to and approved the final manuscript.
Conflict of interest
All authors declare that they have no conflicts of interest.
Acknowledgements
This work was supported by the Schizophrenia Research Institute, utilising infrastructure funding from the NSW Ministry of Health. Post-mortem brain tissues were received from the NSW Tissue Resource Centre which is supported by the National Health and Medical Research Council of Australia, Schizophrenia Research Institute and the National Institute of Alcohol Abuse and Alcoholism (NIH (NIAA) R24AA012725). The authors wish to thank the Schizophrenia Research Laboratory for preparation of the
References (44)
- et al.
NMDA-induced phosphorylation and regulation of mGluR5
Pharmacol. Biochem. Behav.
(2002) - et al.
NMDA-induced potentiation of mGluR5 is mediated by activation of protein phosphatase 2B/calcineurin
Neuropharmacology
(2005) - et al.
Altered levels of glutamatergic receptors and Na +/K + ATPase-α1 in the prefrontal cortex of subjects with schizophrenia
Schizophr. Res.
(2011) Psychopharmacological treatment of schizophrenia: what do we have, and what could we get?
Neuropharmacology
(2012)- et al.
Density of metabotropic glutamate receptors 2 and 3 (mGluR2/3) in the dorsolateral prefrontal cortex does not differ with schizophrenia diagnosis but decreases with age
Schizophr. Res.
(2011) - et al.
Haloperidol induces higher Homer1a expression than risperidone, olanzapine and sulpiride in striatal sub-regions
Psychiat. Res.
(2010) - et al.
N-methyl-d-aspartate (NMDA) receptor dysfunction or dysregulation: the final common pathway on the road to schizophrenia?
Brain Res. Bull.
(2010) - et al.
The possible involvement of metabotropic glutamate receptors in schizophrenia. European Neuropsychopharmacol
Eur. Neuropsychopharmacol.
(2008) - et al.
Metabotropic glutamate receptor 5 in the pathology and treatment of schizophrenia
Neurosci. Biobehav. Rev.
(2013) - et al.
Expression of the human excitatory amino acid transporter 2 and metabotropic glutamate receptors 3 and 5 in the prefrontal cortex from normal individuals and patients with schizophrenia
Mol. Brain Res.
(1998)