Metabotropic glutamate receptor 3 as a potential therapeutic target for psychiatric and neurological Disorders

Abstract

Glutamate is a major excitatory neurotransmitter in the central nervous system (CNS) and abnormalities in the glutamatergic system underlie various CNS disorders. As metabotropic glutamate receptor 3 (mGlu₃ receptor) regulates glutamatergic transmission in various brain areas, emerging literature suggests that targeting mGlu₃ receptors can be a novel approach to the treatment of psychiatric and neurological disorders. For example, mGlu₃ receptor negative allosteric modulators (NAMs) induce rapid antidepressant-like effects in both acute and chronic stress models. Activation of mGlu₃ receptors can enhance cognition in the rodents modeling schizophrenia-like pathophysiology. The mGlu₃ receptors expressed in the astrocytes induce neuroprotective effects. Although polymorphisms in GRM3 have been shown to be associated with addiction, there is not significant evidence about the efficacy of mGlu₃ receptor ligands in rodent models of addiction. Collectively, drugs targeting mGlu₃ receptors may provide an alternative approach to fill the unmet clinical need for safer and more efficacious therapeutics for CNS disorders.

Glutamate is the primary excitatory neurotransmitter in the mammalian central nervous system (CNS). It is a key neuromodulator that regulates synaptic plasticity and several brain functions. A growing body of evidence suggests that abnormalities in the glutamatergic neurotransmission play important role in the development of several psychiatric disorders like major depressive disorders (MDD), bipolar disorders, and schizophrenia. Excessive glutamate release can cause neuronal toxicity and has been linked to neurodegenerative disorders, like Parkinson’s disease, amyotrophic lateral sclerosis (ALS), and Huntington’s disease (Olloquequi et al., 2018). Considering the role of glutamate in the CNS, pharmacological interventions targeting the components of the glutamatergic system may provide a therapeutic benefit to patients suffering from these neurological disorders.

In the CNS, glutamate acts through two classes of receptors: ionotropic glutamate (iGlu) receptors and metabotropic glutamate(mGlu) receptors that are found throughout the nervous system (Nakanishi, 1992) (Figure-1). Ionotropic GluRs (iGluRs) are ligand-gated ion channels that mediate fast synaptic transmission. Based upon the ligand binding profile and sequence homology, iGlu receptors are subdivided into three main families: the α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) receptor, kainate (KAR) receptor and the N-methyl-d-aspartate (NMDA) receptor (Hollmann and Heinemann, 1994) (Figure-1A). A functional iGlu receptor is a tetrameric complex of receptor subunit proteins that can be identical subunits (homomeric receptors) or heterogeneous (heterogeneous receptors) (Traynelis et al., 2010). Each subunit has one large extracellular (EC) amino-terminal domain, one EC ligand-binding domain, three transmembrane domains (TMD), and an intracellular carboxy-terminal domain (CTD) (Traynelis et al., 2010). AMPA receptors are tetrameric complexes composed of subunits GluA1–4, which can be assembled into functional homomeric or heteromeric complexes. All AMPA receptor subunits have a glutamate binding site (Hollmann and Heinemann, 1994). Kainate receptors are comprised of high-affinity subunits GluK1–2 and low-affinity subunits GluK5–7. Low-affinity subunits can form homo-oligomeric receptors, while high-affinity subunits GluK1 and GluK2 must combine with GluK5, GluK6, or GluK7 to form function heteromeric receptors (Herb et al., 1992). NMDA receptors are obligatory heterotetramers comprising two GluN1 with either two GluN2 (GluN2A-GluN2D) or a combination of GluN2 and GluN3 (GluN3A and GluN3B) subunits (Ulbrich and Isacoff, 2008). Glutamate binds to GluN2 subunits (Furukawa et al., 2005; Laube et al., 1997), while glycine is a ligand for GluN1 and GluN3 subunits (Kuryatov et al., 1994). Therefore, GluN1/GluN2 NMDA receptors have glutamate and glycine as obligate coagonists, whereas GluN1/GluN3 NMDA receptors’ pharmacological properties are different from conventional GluN2 containing NMDA receptors and require glycine alone for activation (Chatterton et al., 2002; Smothers and Woodward, 2007).

Figure 1:

(A) Representative structure of an ionotropic glutamate (iGlu) receptor subunit. The iGluRs consist of one extracellular (EC) amino-terminal domain and one ligand binding domain (LBD). The LBD is connected to three transmembrane domains (TMDs). The transmembrane domain is in turn connected to intracellular (IC) C-terminal tail. In iGluRs, ligands bind to the orthosteric site in the LBD and the allosteric modulators bind to the ATD or LBD and channel blockers bind within the ion-channel forming TMD. (B) Representative structure of a metabotropic glutamate (mGlu) receptor subunit. The mGlu receptors have a larger EC domain known as Venus-flytrap domain (VFD) which is connected to seven transmembrane-spanning domains via cysteine rich domain (CRD). The transmembrane domain is in turn connected to intracellular C-terminal tail. The VFD binds glutamate and other orthosteric agonists, whereas the allosteric modulators bind in the TMD. These figures were created with BioRender.com.

The mGlu receptors are class C G-protein coupled receptors (GPCRS) and exist as constitutive dimers (homodimer and heterodimer). Based on G-protein coupling, ligand binding, sequence homology, and function, they are classified into three groups. Group I includes mGlu₁ and mGlu₅, group II contains mGlu₂ and mGlu₃, and group III has mGlu₄, mGlu₆, mGlu₇ and mGlu₈ receptors (Niswender and Conn, 2010) (Figure-1B). Except for the mGlu₆ receptor, which is expressed only in the retina, all others mGlu receptors are widely expressed in the CNS. The mGlu receptors have a large EC N-terminal domain known as the Venus flytrap domain (VFD) linked to the heptahelical TMD via a cysteine-rich domain (CRD; nine cysteine residues). The TMD is connected to intracellular CTD (Pin et al., 2003). The VFD contains the binding site for glutamate, divalent cations and other orthosteric ligands (Kunishima et al., 2000). Ligand binding to the VFD induces conformational changes that are propagated via CRDs to TMD- C-terminal tail and intracellular signaling partners to modulate cellular processes (Niswender and Conn, 2010). Group I mGlu receptors couple to G_q/G₁₁ G proteins and ligand binding activates phospholipase Cβ leading to hydrolysis of phosphoinositide (PI) to diacylglycerol and inositol 1,4,5-trisphosphate (IP₃). In contrast, group II and group III receptors are coupled primarily to G_i/o proteins, and activation of these receptors leads to inhibition of adenylyl cyclase. Besides canonical GPCR signaling, mGlu receptors can couple to other signaling pathways like mitogen-activated protein kinase/extracellular receptor kinase, mammalian target of rapamycin /p70 S6 kinase, and phosphatidyl inositol 3-kinase pathways to regulate various aspects of synaptic plasticity (Hou and Klann, 2004; Iacovelli et al., 2002; Li et al., 2007; Page et al., 2006).

The available mGlu receptor-selective compounds are generally allosteric modulators (compounds that modulate the influence of an agonist at a receptor), which “fine-tune” receptor activity and may have lesser side-effects than those associated with orthosteric ligands (ligands that bind to the same binding site as endogenous ligands). Along these lines, a growing body of evidence shows that modulating metabotropic glutamate receptor 3 (mGlu₃ receptor) function can exhibit significant therapeutic efficacy in preclinical models of psychiatric disorders. Therefore, in this review, we have summarized clinical and preclinical evidence linking mGlu₃ receptors with various psychiatric and neurological disorders (MDD, schizophrenia, addiction disorders and neurodegenerative disorders) (Table-1, 2, 3). Additionally, we have focused on the recent developments of mGlu₃ receptor-selective pharmacological tools (negative allosteric modulators) and the possibility of utilizing these ligands for as novel therapeutics.

Table 1.

Summary of the antidepressant- and anxiolytic-like effects of Group II mGlu Receptor ligands

Compound	Experimental Organisms/strains	Behavioral Tests	Dose and route of Administration	Drug treatment	Behavioral effects	References
MGS0039	Rats	Forced swim test (FST)	1, 3 mg/kg, i.p.	24 and 1 h prior to the FST	Decreased immobility time	Chaki et al., 2004
	ICR mice	Tail suspension Test (TST)	1, 3 mg/kg, i.p.	60 min prior to the TST	Reduced immobility time	Chaki et al., 2004
	CORT treated ddY mice	FST	1.0 mg/kg, i.p.	60 min prior to the FST	Decreased CORT-induced increase in immobility time	Ago et al., 2013
	SDS exposed mice	TST and FST	1.0 mg/kg, i.p.	24 and 48 h prior to FST and TST, respectively	Reduced stress-induced increase in immobility time	Dong et al., 2017
	SDS treated mice	Sucrose Preference Test (SPT)	1.0 mg/kg, i.p.	3 and 7 days prior to the test	Restored SDS-induced deficits in sucrose preference	Dong et al., 2017
TP0473292	CSDS exposed C57BL/6 mice	TST and FST	3.0 mg/kg; i.p.	24 and 48 h prior to the TST and FST, respectively	Reduced immobility time	Dong et al., 2022
	CSDS exposed C57BL/6 mice	SPT	3.0 mg/kg; i.p.	3 and 7 days prior to SPT	Enhanced sucrose preference in stressed mouse	Dong et al., 2022
LY341495	ICR mice	TST	1, 3 mg/kg, i.p.	60 min prior to the TST	Decreased immobility time	Chaki et al., 2004
	Rats	FST	1, 3 mg/kg, i.p.	24 and 1 h prior to the FST	Reduced immobility time	Chaki et al., 2004
	C57Bl6/J mice	TST and FST	1, 3, and 10 mg/kg; i.p.	60 min prior to the TST	Reduced immobility time	Campo et al., 2011
	Sprague-Dawley rats	SPT	3 mg/kg, i.p.	48 h and 10 days prior to SPT	Reversed CUS-induced impairments in sucrose preference	Dwyer at al., 2013
	CORT treated ddY mice	FST	0.3 mg/kg, i.p.	60 min prior to the FST	Decreased CORT-induced increase in immobility time	Ago et al., 2013
	Rats	FST	1, 3 mg/kg; i.p.	24 h and 30 min prior to the FST	Attenuated increased immobility time	Koike et al., 2013
	Sprague-Dawley rats	FST	3 mg/kg; i.p.	24 h prior to the FST	Decreased immobility time	Koike and Chaki, 2014
	C57BL/6J mice	FST	1 mg/kg; i.p.	30 min prior to the FST	Reduced immobility time	Fukumoto et al., 2016
	CD-1 mice	TST	0.3, 1, 3, 10 mg/kg; i.p.	30 min prior to the TST	Decreased immobility time	Witkin et al., 2016
	Swiss mice	FST	1, 3, 10 mg/kg; i.p.	30 min prior to the FST	Reduced immobility time	Witkin et al., 2016
	Sprague-Dawley rats	FST	0.3, 1, 3, 10 mg/kg; i.p.	30 min prior to the FST	Decreased immobility time	Witkin et al., 2016
	CD-1 mice	Inescapable shock (IES)	3 mg/kg; i.p.	35 min or 24 h prior to IES	Attenuated the development of escape deficits	Highland et al., 2019
RO4491533	C57Bl6/J mice	TST	30, 100 mg/kg; i.p.	60 min prior to the TST	Decreased immobility time	Campo et al., 2011
	C57Bl6/J mice	FST	3, 10, 30, 100 mg/kg; i.p.	60 min prior to the FST	Decreased immobility time	Campo et al., 2011
LY3020371	Sprague-Dawley rats	FST	0.3, 1, 3, 10 mg/kg; i.v.	120 min prior to the FST	Reduced immobility time	Witkin et al., 2017a
	Swiss mice	FST	10 mg/kg; i.p.	30 min prior to the FST	Reduced immobility time	Witkin et al., 2017a
VU650786	CD-1 mice	TST	3 mg/kg; i.p.	Acute	Decreased immobility time	Engers et al., 2015
	C57BL/6J mice	Progressive ratio test	30 mg/kg; i.p.	Acute	Prevented motivational deficits induced by acute restraint stress	Joffe et al., 2019
	C57BL/6J mice	TST and FST	30 mg/kg; i.p.	Acute	Decreased total immobility time	Joffe et al., 2020
	CVS and CORT exposed C57BL/6J mice	SPT	30 mg/kg; i.p.	Acute	Restored CVS and CORT-induced decrease in sucrose preference.	Joffe et al., 2020
	CVS and CORT exposed C57BL/6J mice	TST and FST	30 mg/kg; i.p.	Acute	Increased CVS and CORT-induced decrease in latency to immobility	Joffe et al., 2020
VU6010572	CD-1 mice	TST	3 mg/kg; i.p.	Acute	Reduced immobility time	Engers et al., 2017

CORT- corticosterone; SDS- social defeat stress; CVS- chronic variable stress; i.p.- intraperitoneal; i.v. intravenous

Table 2.

Summary of the beneficial effects of Group II receptor ligands in behaviors relevant for schizophrenia and substance use disorders

Compound	Experimental Organisms/strains	Behavioral Tests	Dose and route of Administration	Drug treatment	Behavioral effects	References
ZJ43	Sprague-Dawley rats	Locomotor test and stereotypy	150 mg/kg, i.p.	20 min prior to PCP treatment	Reduced PCP-induced motor activation, stereotypic circling behavior, and head movements	Olszewski et al., 2004
	NIH-Swiss mice	Jumping Behavior	200 mg/kg; i.p.	5 min prior to MK801 treatment	Reduced MK-801-induced jumping behavior	Olszewski et al., 2008
	DBA/2 mice	Stereotypy	50, 100, 150 mg/kg, i.p.	10 min prior to PCP treatment	Reduced PCP-induced stereotypic movements	Olszewski et al., 2008
	DBA/2 mice	Resident-Intruder Assay	150 mg/kg, i.p.	10 min prior to PCP treatment	Reversed PCP-induced decrease in escape behavior	Olszewski et al., 2008
	C57BL/6NCr mice	Novel Object recognition (NOR)	100, 150 mg/kg, i.p.	20 min prior to MK801	Rescued MK-801-induced cognitive deficits in NOR	Olszewski et al. 2012b
	C57BL/6NCr mice	locomotor test	100, 150 mg/kg, i.p.	15 min prior to AMPH treatment	Lowered AMPH-induced hyperlocomotion	Olszewski et al. 2012b
	C57BL/6NCr, Grm2 KO and Grm3 KO mice	NOR	150 mg/kg; i.p.	30 min prior to the test	Reversed ethanol-induced deficits in NOR in Grm2 KO and WT mice, but not in Grm3 KO	Olszewski et al., 2017
	C57BL/6NCr mice	Locomotor test	100, 150 mg/kg, i.p.	15 min prior to ethanol administration	Reduced ethanol-induced motor activation	Olszewski et al., 2017
	C57BL/6NCr mice	Rotarod test	150 mg/kg, i.p.	15 min prior to ethanol administration	Reduced ethanol-induced impairments in motor coordination	Olszewski et al., 2017
2-PMPA	Long-Evans rats	Cocaine self-administration, intracranial electrical brain-stimulation reward	100 mg/kg, i.p.	30 min prior to cocaine injection	Inhibited intravenous cocaine self-administration under progressive-ratio reinforcement conditions, reduced cocaine-induced brain-stimulation reward	Xi et al., 2010a
	Long-Evans rats	Cocaine self-administration	30–100 mg/kg, i.p.	30 min prior to the test session	Inhibited intravenous cocaine self-administration	Xi et al., 2010b
	Grm3 KO, Grm2 KO and WT mice	Hyperlocomotion	100 mg/kg; i.p.	30 min prior to PCP treatment	Reduced PCP-induced motor activation in Grm2 KO and WT mice	Olszewski et al., 2012a
	C57BL/6NCr mice	Locomotor test	100, 150 mg/kg, i.p.	15 min prior to AMPH treatment	Reduced AMPH-induced hyperlocomotion	Olszewski et al. 2012b
	C57BL/6NCr mice	Hyperlocomotion	10, 100, 150 mg/kg, i.p.	15 min prior to PCP treatment	Reduced PCP-induced hyperlocomotion	Olszewski et al. 2012b
	C57BL/6NCr mice	Novel Object recognition (NOR)	10, 100 mg/kg, i.p.	20 min prior to MK801 treatment	Rescued MK-801-induced cognitive deficits in NOR	Olszewski et al. 2012b
	C57BL/6NCr mice	NOR	100 mg/kg; i.p.	30 min prior to the test	Reversed ethanol-induced deficits in NOR in Grm2 KO and WT mice, but not in Grm3 KO	Olszewski et al., 2017
	C57BL/6NCr mice	Rotarod test	50, 100 mg/kg, i.p.	15 min prior to ethanol administration	Reduced ethanol-induced impairments in motor coordination	Olszewski et al., 2017
	3xTg mice	NOR	100 mg/kg; i.p.	31 min prior to the test	Reversed short-term memory deficits in NOR	Olszewski et al., 2017
NAAG	Long-Evans rats	Cocaine self-administration, intracranial electrical brain-stimulation reward	300 μg/side, i.n.	30 min prior to cocaine injection	inhibited intravenous cocaine self-administration under progressive-ratio reinforcement conditions, reduced cocaine-induced brain-stimulation reward	Xi et al., 2010a
VU0477950	C57BL/6 mice	Fear Extinction	100 mg/kg; i.p.	30 minutes prior to the test	Impaired fear extinction learning	Walker et al. 2015
LY2794193	Sprague-Dawley Rats	Locomotor test	10, 30 mg/kg, i.p.	30 minutes prior to PCP treatment	Reduced PCP-induced hyperactivity	Monn et al. 2018
LY379268	C57BL/6 mice	Trace Fear Conditioning	3 mg/kg; i.p.	30 min prior to the test	Restored sub-chronic PCP-induced deficits in acquisition of TFC task	Dogra et al. 2021
BINA	Rhesus macaques	oculomotor delayed response	0.001 mg/kg; i.m.	60 min prior to the test	Improvement in delayed response performance	Jin et al. 2018
N-acetylcystine	Sprague Dawley-derived albino rats	Cocaine-induced reinstatement	60 mg/kg, s.c.	4 h prior to cocaine administration	Prevented cocaine-induced reinstatement in mGlu2/3 receptor dependent manner	Moran et al. 2005
	Sprague-Dawley rats	Electrophysiology	100 mg/kg, i.p.	2.5 h prior to electrophysiology	Restored cocaine induced impairments in long termpotentiation and long-term depression in the NAcc core.	Moussawi et al., 2009

AMPH- Amphetamine; i.p.- intraperitoneal; s.c. subcutaneous; i.n.- intranasal

Table 3.

Summary of the studies indicating neuroprotective effects of mGlu₃ receptor activation.

Compound	Experimental Organisms/system	Assays	Concentration	Drug treatment	Treatment effects	References
LY379268	SOD1G93A mice	Rotarod test, western blotting, IHC	0.5, 1, 5 mg/kg; s.c.	6 μl/day (chronic activation)	Improved neurologic disability and motor coordination and increased spinal GDNF levels	Battaglia et al., 2015
	C57BL/6J Mice	Western blotting	0.5, 1 mg/kg, i.p	24 h	Increased GDNF levels in the spinal cord	Battaglia et al., 2015
	C57BL/6J Mice	Western blotting/IHC	0.5, 1, 5 mg/kg/day; s.c.	6 μl/day/28 days	Increased GDNF levels in the cervical and thoraco-lumbar spinal cord	Battaglia et al., 2015
	Grm3 and Grm2 KO (CD-1 background)	Western blotting	1 mg/kg, i.p.	24 h	Increased GDNF levels in the spinal cord of Grm2 KO and WT mice (not in Grm3 KO)	Battaglia et al., 2015
	Cultured spinal cord astrocytes	Western blotting	1 μM	24 h	Increased GDNF levels in the cultured astrocytes from Grm2 KO and WT mice, but not from Grm3 KO	Battaglia et al., 2015
	Mixed cultures	ICC	1 μM	24 h	protected motor neurons against kainate toxicity in cultures containing spinal cord astrocytes from WT or Grm2 KO mice	Battaglia et al., 2015
	Cultured astrocytes	sAPPα ELISA, RT-PCR	0.5, 1, 1, 10 μM	3 h, 16h, 24h	Induced expression of α-secretase, PPAR-γ and enhances sAPPα release from cultured astrocytes	Durand et al., 2014
	Cultured astrocytes	RT-PCR	0.1 μM	3 h	Induced BDNF expression	Durand et al., 2017
	Neurons treated with CM derived from astrocytes treated with LY379268	TUNEL			medium from LY379268-treated astrocytes protected hippocampal neurons from Aβ-induced cell death	Durand et al., 2017
	Astrocyte-neuronal coculture	TUNEL	0.1 μM	24 h	Promoted Aβ uptake	Durand et al., 2017
	Primary culture Microglia	ICC	0.1 μM	24 h	induced phagocytic activity and Aβ uptake by microglia	Durand et al., 2017
	Cultured Astrocytes	ICC	CM derived from LY379268 (0.1 μM, 3h) astrocytes	24 h	increased Aβ uptake	Durand et al. 2019
	Cultured astrocytes	Flow cytometry, SOD activity, ROS measurement	0.1 μM	3 treatments (48 h apart)	Decreased ROS production induced by aging in astrocytes, protected astrocytes from apoptosis induced by aging, prevented reduction in SOD activity caused by aging	Turati et al. 2020
DCG-IV	Astrocytes and neuronal cultures	TGFβ measurement assay, Microscopy	1 μM	10 min	Delayed increase in both intracellular and extracellular levels of TGFβ, decreased neurotoxicity	Bruno et al., 1998

ICC- immunocytochemistry; IHC-immunohistochemistry; TUNEL-terminal deoxynucleotidyl transferase dUTP nick end labeling; SOD-Superoxide dismutase; ROS- reactive oxygen species; TGF-β- Transforming Growth Factor-β; sAPP- Soluble amyloid precursor protein; ELISA- enzyme-linked immunosorbent assay; RT-PCR- reverse transcription polymerase chain reaction; i.p.- intraperitoneal; s.c. subcutaneous; CM- conditioned medium

Metabotropic glutamate receptor 3 (mGlu₃)

Metabotropic glutamate receptor 3 (mGlu₃) is a member of group II mGlu receptors. Group II metabotropic receptors are primarily presynaptic receptors, while mGlu₃ receptors are also present at postsynaptic sites and glia (Jin et al., 2018; Woo et al., 2022). Because of its coupling to G_i/o G-proteins, activation of the mGlu₃ receptor inhibits adenylyl cyclase activity leading to a decrease in the production of cyclic adenosine monophosphate (cAMP) and activity of cAMP-dependent protein kinases (Figure-2). Besides canonical G-protein mediated signaling, the mGlu₃ receptors can signal through other non-canonical pathways and can potentiate signaling mediated by G_q- coupled receptors. For example, activation of neuronal mGlu₃ receptors supports mGlu₅ receptor (a G_q-coupled receptor) induced signaling and contributes to the stimulation of mGlu₅ receptor-mediated PI hydrolysis in early postnatal life (Di Menna et al., 2018; Schoepp et al., 1996). This neuronal mGlu_3–5 receptor interaction has potential implications for enhancing cognition and sculpturing the influence of the mGlu₅ receptor on neurotoxicity (Di Menna et al., 2018; Dogra et al., 2021). Additionally, mGlu₃ receptor activation in the cortical pyramidal neurons can potentiate somatic Ca²⁺ mobilization induced by mGlu₅ receptor activation (Di Menna et al., 2018). Interestingly, mGlu₃ receptor-induced long-term depression (LTD) in the prefrontal cortex (PFC) requires activation of endogenous mGlu₅ receptors, but activation of mGlu₃ receptors does not potentiate mGlu₅-induced LTD in the hippocampus (Dogra et al., 2021) indicating selective effects of mGlu₃ receptors on mGlu₅ receptor-mediated signaling.

Figure 2:

Schematic depicting mGlu₃ receptor-dependent functions in pyramidal neurons and astrocytes. Activation of mGlu₃ receptors in the postsynaptic neurons mobilizes endocannabinoids (2-arachindonylglycerol; 2-AG) via a mGlu₅ receptor-dependent signaling. Mobilization of 2-AG decreases the release of GABA from local interneurons leading to decreased inhibitory tone on pyramidal neurons and altered threshold for synaptic plasticity via N-methyl-D-aspartate (NMDA) receptors. Facilitating synaptic pathway through this mechanism enhances hippocampal-dependent cognition and can rescue cognitive deficits in rodents modeling schizophrenia-like pathophysiology. Activation of astrocytic mGlu₃ receptors can promote the release of brain derived neurotrophic factor (BDNF) that has neuroprotective effects. Activation of mGlu₃ receptor in the astrocytes also enhances the synthesis and release of anti-inflammatory cytokine transforming growth factor-β (TGF β) from astrocyte that protects neurons from excitotoxic insults. Astrocytic mGlu₃ receptor activation also enhances glial derived neurotropic factor (GDNF) levels indicating potential role of enhancing mGlu₃ receptor function in slowing neurodegeneration. AMPA- α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid; GABA- γ-Aminobutyric acid; LTP- long-term potentiation; EAAT1- Excitatory amino acid transporter 1; EAAT2- Excitatory amino acid transporter 1. This figure was created with BioRender.com.

While mGlu_3–5 receptor interaction has been shown to increase the extent of long-term potentiation (LTP) at SC-CA1 synapse (Figure-2), coincident activation of mGlu₃ receptors with β-adrenergic receptors (βAR) in the astrocytes in the CA1 of hippocampus can counteract the effects of βAR activation on the synaptic plasticity (Walker et al., 2017). Studies employing Grm3 (the gene coding for mGlu₃ receptor) knockout (KO) mice reveal that coactivation of mGlu₃ with βARs induces an increase in cAMP accumulation and increased adenosine release. The adenosine in turn acts on A1 adenosine receptors present in excitatory SC terminals that can restrain the effects of βAR on synaptic plasticity at the SC-CA1 synapse (Walker et al., 2017). These findings suggest that activation of the glial mGlu₃ receptor can modulate the effect of excessive βAR activation (for example, during intense stress) and may present a protective mechanism to inhibit neuronal injury. Moreover, activation of mGlu₃ receptors expressed on the astrocytes increases the expression of glutamate aspartate transporters (GLAST) and glutamate transporter-1 (GLT-1), and promotes glutamate uptake from the synapse, and thereby, limiting glutamate spread across the synapse (Aronica et al., 2003). These findings point towards an exciting possibility of targeting mGlu₃ receptors to regulate EC glutamate levels during pathological conditions and restricting excitotoxic brain damage. Further, it will be exciting to determine whether these mechanisms exist at different synapses in other brain areas.

The expression of mGlu₃ receptors is found in several brain areas implicated in the regulation of cognition and stress-related behaviors (Jin et al., 2018; McOmish et al., 2016; Woo et al., 2022). In the rodent brain, strong mGlu₃ receptor mRNA expression is observed in the cortex, basolateral amygdala (BLA), reticular thalamic nucleus, striatal matrix and nucleus accumbens (NAcc) (McOmish et al., 2016; Woo et al., 2022). The mGlu₃ receptors are also expressed in the CA1 of the hippocampus, the dentate gyrus (granular layer), claustrum and superficial gray superior colliculus (Dogra et al., 2021; McOmish et al., 2016). To note, The levels of mGlu₃ receptors have been reported to decrease with age, which might be related to cognitive decline seen with advanced ages (Hernandez et al., 2018), and the impact of age-related downregulation of mGlu₃ receptor is worth further investigation.

In the CNS, mGlu₃ receptors are activated by glutamate as well as N-acetylaspartylglutamate (NAAG) (Ghose et al., 1997; Neale, 2011; Wroblewska et al., 1997). NAAG is a peptide neurotransmitter and is co-released with glutamate. It is catabolized by glutamate carboxypeptidase II (NAAG peptidases). Therefore, compounds that block the catabolism of synaptic NAAG (NAAG peptidase inhibitors) can regulate mGlu₃ receptor-dependent functions in the CNS (Neale and Olszewski, 2019; Zhong et al., 2006). These classes of compounds are increasingly in use to investigate the significance of NAAG and the role of mGlu₃ receptors in various disease conditions. Besides NAAG modulators, the discovery of mGlu₃ receptor-selective negative allosteric modulators (NAMs) has further advanced the field (Engers et al., 2017; Engers et al., 2015). Using these selective pharmacological tools along with the receptor-selective conditional KOs, several exciting discoveries have facilitated the mGlu₃ receptor research and have advanced our understanding of how mGlu₃ receptors regulate glutamatergic transmission in different brain areas during psychiatric disorders like depression and schizophrenia (Dogra et al., 2021; Joffe et al., 2019; Joffe et al., 2020; Walker et al., 2015).

Negative allosteric modulators of mGlu₃ receptor have potential utility as antidepressants

Major Depressive Disorders (MDD) are one of the most widespread health concerns and a leading cause of disability worldwide. A growing body of evidence indicates that altered glutamatergic neurotransmission may contribute to the pathophysiology of MDD, and establish the glutamatergic system as an interesting target for designing novel therapeutics for MDD (Dogra and Conn, 2021; Duman et al., 2019). Specifically, rapid and sustained antidepressant effects of ketamine in patients suffering from treatment-refractory depression (TRD) (Duman, 2018) have prompted researchers to increase focus on agents regulating glutamatergic signaling in the CNS as potential novel antidepressants.

Along these lines, mGlu_2/3 receptor antagonists and NAMs have been shown to induce rapid and sustained antidepressant-like responses in both acute and chronic models of depression (Table-1) (Ago et al., 2013; Campo et al., 2011; Chaki et al., 2004; Dong et al., 2022; Dong et al., 2017; Dwyer et al., 2013; Fukumoto et al., 2016; Joffe et al., 2020; Koike et al., 2013; Podkowa et al., 2015; Witkin et al., 2017a). Further, mGlu_2/3 antagonists reduced the learned helplessness induced by chronic corticosterone (CORT) treatment (Ago et al., 2013; Koike et al., 2013), and reduced immobility time in the tail suspension test (TST) in CD-1 mice (Witkin et al., 2016). Moreover, pharmacological blockade of mGlu_2/3 receptors using LY341495 during stress prevented the development of inescapable shock and CORT-induced escape deficits (Highland et al., 2019). It is important to note that effective dosages of conventional antidepressants like imipramine, desipramine, and fluoxetine do not show efficacy in the chronic CORT model (Ago et al., 2013; Iijima et al., 2010), while a single injection of ketamine can induce antidepressant-like effects in this model (Koike et al., 2011, 2013). Similarly, the CD-1 strain of mice shows a reduced sensitivity to acute administration of AD therapeutics like fluvoxamine, amitriptyline and imipramine in TST (van der Heyden et al., 1987). Therefore, both chronic CORT treatment and CD-1 mice can be considered preclinical models of TRD and the efficacy of mGlu_2/3 receptor antagonists in these models advise the potential utility of mGlu_2/3 antagonists for TRD.

Interestingly, mGlu_2/3 receptor antagonists exert antidepressant-like effects comparable to ketamine in several preclinical behavioral tests and may share some neural mechanisms with ketamine to induce antidepressant effects (Dong et al., 2017; Fukumoto et al., 2016; Koike and Chaki, 2014; Witkin et al., 2017a; Witkin et al., 2016). Additionally, administration of mGlu_2/3 receptor antagonist does not recapitulate ketamine-associated adverse effects like abuse liability, cognitive deficits, motor abnormalities and neurotoxicity issues (Witkin et al., 2017b) indicating that mGlu_2/3 receptor antagonists possess desired safety profile that could support development of drug candidates for further studies in patients. Based on this exciting preclinical literature, TP0473292 (TS-161), a prodrug of mGlu_2/3 antagonist TP0178894, has been launched into clinical trials for TRD (Watanabe et al., 2022). The results from phase-1, double-blind, placebo-controlled clinical trials in healthy objects revealed that TP0473292 is safe and well-tolerated and is a promising clinical candidate for the evaluation of efficacy in patients with depression.

Similar to the mGlu_2/3 receptor antagonists, the mGlu₃ receptor-selective NAMs can induce antidepressant- and anxiolytic-like effects in various preclinical models. For instance, mGlu₃ receptor-selective NAM, VU650786, dose-dependently inhibited the number of marbles buried by CD-1 mice in the marble-burying test (Engers et al., 2015) indicating potential anxiolytic-like effects of mGlu₃ receptor inhibition. Additionally, VU650786 reduced the immobility time in the acute model of behavioral despair (forced swim test (FST) and tail suspension test (TST)) (Engers et al., 2015; Joffe et al., 2020). Excitingly, acute treatment with VU650786 rescued anhedonia (as accessed by an increase in sucrose preference) as well as behavioral despair measured as the increase in latency to become immobile in FST and TST in chronic CORT and chronic variable stress models of depression (Joffe et al., 2020). These findings indicate that single administration of mGlu₃ receptor NAMs can rapidly reverse anhedonia and behavioral despair in preclinical chronic stress models and suggest that mGlu₃ receptor NAM display an efficacy profile comparable to mGlu_2/3 receptor antagonists and other rapid-acting antidepressants.

The mGlu₃ receptor NAM, VU650786, was further modified using a reductionist optimization approach and the resulting novel mGlu₃ receptor NAM, VU6010576, also showed efficacy for reducing immobility time comparable to ketamine (30 mg/kg; i.p.) in TST in CD-1 mice (Engers et al., 2017). Recently published mechanistic studies revealed that systemic treatment with a mGlu₃ receptor NAM, VU650786, activates unique pyramidal cell ensembles in the PFC (Joffe et al., 2020). It also attenuates the mGlu_2/3 receptor agonist, LY379268, induced long-term depression at thalamocortical synapses. Interestingly, inhibition of thalamocortical synapses using a chemogenetic approach blocked the effects of mGlu₃ receptor NAM in TST and FST indicating that mGlu₃ receptor NAM may act through these synapses to induce antidepressant-like effects. Moreover, the mGlu₃ receptor NAM can prevent acute restraint stress-induced impairments in plasticity at amygdalo-cortico synapses and motivational deficits (Joffe et al., 2019), which further strengthens the utility of mGlu₃ receptor NAM for treating stress-related psychiatric disorders where motivational deficits are often observed. Taken together, the above-mentioned literature indicates that selective modulators of mGlu₃ receptors may provide novel approaches to address mood disorders and indicate their potential utility for disorders like schizophrenia where depression a is comorbid condition.

Activation of mGlu₃ receptors can rescue deficits associated with schizophrenia

Schizophrenia is a debilitating mental disorder that involves the dysfunction of multiple brain areas. A growing body of evidence reveals that changes in glutamatergic neurotransmission may contribute to the pathophysiology of schizophrenia and the agents reversing these changes could provide symptomatic relief to schizophrenia patients. Along these lines, mGlu₃ receptors have emerged as a potential therapeutic target for treating schizophrenia-associated behavioral and physiological deficits (Dogra and Conn, 2022). Genome-wide association studies (GWAS) have identified genetic variants in GRM3 as potential risk factors for developing schizophrenia (Fujii et al., 2003; Jablensky et al., 2011; Mounce et al., 2014; Saini et al., 2017). Further, the risk alleles in GRM3 have been found to be associated with schizophrenia-related endophenotypes (Chang et al., 2015; Egan et al., 2004; Kinoshita et al., 2015). Such as, single nucleotide polymorphisms (SNPs) in GRM3 were associated with cognitive impairments (Chang et al., 2015; Egan et al., 2004; Jablensky et al., 2011) and lesser activation of the PFC during cognitive task performance in schizophrenia (Kinoshita et al., 2015). Also, schizophrenia risk SNP in GRM3 was associated with lower levels of N-acetylaspartate (NAA) and mRNA of glial glutamate transporter excitatory amino acid transporter 2 (EAAT2) in the PFC (Egan et al., 2004; Marenco et al., 2006). As reduced levels of NAA and EAAT2 protein have been related to impaired executive function and working memory in schizophrenia patients (Galinska et al., 2007; Spangaro et al., 2012), it is possible that the GRM3 genotype might alter glutamatergic neurotransmission in the PFC and performance in the PFC-dependent cognitive tasks, and thereby increase the risk for schizophrenia.

Interestingly, patients having GRM3 variant rs1468412 (which is associated with altered dysfunction of the glutamatergic system (Xia et al., 2012)) have been shown to have more adverse changes in the performance of working memory tasks after antipsychotic treatment (Bishop et al., 2015). Also, GRM3 polymorphisms (rs274622, rs6465084 and rs724226) can predict the improvement of negative symptoms following antipsychotic treatment (Bishop et al., 2005; Bishop et al., 2015; Fijal et al., 2009). These association studies suggest that genetic variants in GRM3 can influence the responsiveness to antipsychotic treatment and indicate the possibility of utilizing this knowledge to identify a subgroup of patients susceptible to experiencing adverse effects on cognition following antipsychotic medication. Further, polymorphisms in GRM3 have also been linked to the white matter integrity in schizophrenia patients (Mounce et al., 2014), which is considered one of the predictors of response to antipsychotics (Kraguljac et al., 2021; Reis Marques et al., 2014). These studies provide a piece of evidence that GRM3 is a susceptibility locus for schizophrenia and have persuaded researchers to probe if activating or modulating mGlu₃ receptor function can be used to reverse schizophrenia-like deficits in preclinical setups.

Based on various theories proposed to explain the underlying pathology of schizophrenia, several pharmacological disease models are currently in use preclinically. For example, as the hyperdopaminergic state is the core pathophysiology of schizophrenia, initial animal models were developed using pharmacological modifications like amphetamine treatment to induce psychotic-like changes (Jones et al., 2011). Similarly, based on the clinical evidence that administration of NMDA antagonists like phencyclidine (PCP) and ketamine in health individuals induce symptoms like those seen in schizophrenia (Hu et al., 2015), NMDA antagonists are extensively used to mimic schizophrenia-like deficits. Interestingly, initial studies have shown beneficial effects of NAAG peptidase inhibitors in reducing motor abnormalities induced by NMDA antagonists dizocilpine (MK801) and PCP (Olszewski et al., 2004; Olszewski et al., 2008). PCP-induced stereotypic movements (cycling behavior, head movements) and negative symptoms were also reduced by the NAAG peptidase inhibitor, ZJ43 (Olszewski et al., 2004; Olszewski et al., 2008). Moreover, ZL43 could inhibit amphetamine-induced hyperactivity when administered before or after amphetamine treatment (Olszewski et al., 2012b). Similarly, another inhibitor, 2-PMPA, blocked PCP- and amphetamine-induced motor activation (Olszewski et al., 2012b). Also, both ZL43 and 2-PMPA blocked PCP-induced increase in the glutamate levels in the medial PFC (mPFC) and NAcc (Zuo et al., 2012), and significantly blocked MK801-induced impairments in the novel object recognition when administered before the acquisition trials (Olszewski et al., 2012b). All these studies indicate the possibility that elevation of synaptic NAAG levels may represent a novel therapeutic approach for treating all symptom domains in schizophrenia.

Interestingly, the above-mentioned effects of NAAG inhibitors were blocked by the mGlu_2/3 receptor antagonist, LY341495 revealing an essential role of the mGlu_2/3 receptors in mediating beneficial effects of NAAG peptidase inhibitors in rodents mimicking schizophrenia-like symptoms. Further, studies employing receptor-specific KO mice have shown that the effects of NAAG peptidase inhibitor, 2-PMPA, on PCP-induced motor activation were absent in Grm3 KO mice (Olszewski et al., 2012a) suggesting an essential role of mGlu₃ receptors in rescuing schizophrenia-like symptoms induced by NMDA antagonist. These findings are supported by studies in the Grm3 KO mice revealing abnormalities in the expression of interneuron-related genes and neuronal synchrony in the PFC (Imbriglio et al., 2019) similar to those seen in schizophrenia patients (Anderson et al., 2020; Kaar et al., 2019; Kang et al., 2018; Spencer et al., 2003; Toker et al., 2018). These findings raise the possibility that insults to the mGlu₃ receptors may induce cortical neuronal abnormalities and can predispose individuals to schizophrenia. Mechanistic studies conducted by Jin et al. 2017 revealed that activation of mGlu₃ using endogenous agonist NAAG enhances the firing of dorsolateral PFC (DLPFC) Delay cells during a spatial working memory task performance in Rhesus macaques (Jin et al., 2018). This study stipulates that mGlu₃ receptors are positioned to strengthen the high-order cognitive circuits in DLPFC and support a large body of the above-mentioned literature linking GRM3 polymorphisms with impairments in the PFC function.

Recently, the development of pharmacological probes has helped researchers to evaluate the role of mGlu₃ receptors in the regulation of CNS physiology and related behaviors (Table-2). For example, subcutaneous injections of mGlu3 receptor agonist, LY2794193 has been shown to decrease PCP-induced hyperlocomotion in a dose-dependent manner (Monn et al., 2018). Using receptor-selective KOs and NAMs, it has been shown that activation of mGlu₃ receptors induces LTD in the PFC (Joffe et al., 2019; Walker et al., 2015). To note, this LTD is expressed post-synaptically and is dependent upon intracellular calcium (Ca²⁺) mobilization induced by activation of mGlu₃ receptors in layer V pyramidal neurons (Walker et al., 2015). Further mechanistic studies revealed that co-activation of another class of mGlu receptor, mGlu₅, is essential for mGlu₃ receptor-induced LTD in the PFC (Di Menna et al., 2018). Recently, a similar partnership between mGlu₃ and mGlu₅ receptors has also been observed in the CA1 region of the hippocampus. In the CA1 pyramidal neurons of the hippocampus, activation of mGlu₃ receptors has been shown to induce metaplastic changes to enhance synaptic plasticity at the SC-CA1 synapse (Figure-2) (Dogra et al., 2021). This metaplasticity is dependent upon the mGlu₅ and endocannabinoid receptor-mediated disinhibitory mechanisms (Dogra et al., 2021). Excitingly, activation of mGlu₃ receptors has been shown to improve fear extinction (Walker et al., 2015), and promotes associative learning in trace fear conditioning tasks (Dogra et al., 2021) indicating that activation of mGlu₃ receptors may rescue cognitive deficits in psychiatric disorders. Since these functions of mGlu₃ receptors are dependent upon the activation of mGlu₅ receptors, it will be exciting to evaluate the efficacy of mGlu₃ receptor activation on other testes of cognition modulated by mGlu₅ receptors (Dogra and Conn, 2022). Moreover, activation of the mGlu₃ receptors has been shown to be neuroprotective (Caraci et al., 2011). Therefore, the pharmacological tools that selectively activate the mGlu₃ receptor may have the potential to reduce the risk of causing neurotoxic insults while treating cognitive deficits associated with schizophrenia.

Activation of astrocytic mGlu₃ receptors is neuroprotective:

Alterations in the components of glutamatergic signaling have also been reported in neurodegenerative diseases. For example, in Alzheimer’s disease (AD), decreased expression and function of astrocytic glutamate transporters have been observed (Kirvell et al., 2006; Masliah et al., 1996; Scott et al., 2011). Additionally, amyloid-β (Aβ), which is accumulated in the AD brain and plays a key role in the disease progression (Nicoll and Weller, 2003), has been shown to induce inhibitory effects on glutamate uptake by astrocytes (Matos et al., 2008; Parpura-Gill et al., 1997) that can change the availability of extrasynaptic glutamate. Considering the role of astrocytes in regulating glutamate homeostasis, significant attention has been shifted to evaluating the effects of mGlu receptor ligands on the astrocytes (Table-3). Astrocytes primarily express mGlu₅ and mGlu₃ receptors (Balazs et al., 1997; Condorelli et al., 1997; Nakahara et al., 1997; Schools and Kimelberg, 1999; Woo et al., 2022), whereas no expression of mGlu₂ receptors and other mGlu receptors have been found on these cells.

Interestingly, activation of the mGlu₃ receptors increases the release of neuroprotective soluble amyloid precursor protein from the cultured astrocytes by inducing α-secretase expression which stimulates the removal of Aβ plaques (Durand et al., 2014; Durand et al., 2019). However, this is not the only neuroprotective mechanism modulated by mGlu₃ receptors in AD (Figure-2). Activation of astrocytic mGlu₃ receptors can also increase brain-derived neurotropic factor (BDNF) release and induce clearance of amyloids from the EC space (Durand et al., 2017). Moreover, mGlu₃ receptor stimulation in the astrocytes induces anti-oxidant effects and prevents Aβ- induced hippocampal neuronal death (Durand et al., 2017; Turati et al., 2020). Excitingly, activation of astrocytic mGlu₃ receptors enhances the formation and release of anti-inflammatory cytokine, transforming growth factor-β (TGF β), which can protect neurons from excitotoxic insults (Bruno et al., 1998). These studies raise an exciting possibility of activating/potentiating mGlu₃ receptor function as a therapeutic strategy to slow down Aβ-related neurodegeneration in AD. Along these lines, decreased expression of mGlu₃ receptors has been reported in the hippocampal astrocytes of the PDAPP-J20 mouse model of AD (Durand et al., 2014) which might contribute to the development of AD-like pathology in these mice. Similar changes in the mGlu₃ receptor expression have been reported in the aged astrocytes (Turati et al., 2020), which can be increased after treatment with group II agonist, LY379268.

Besides astrocytes, decreased Grm3 expression has also been reported in the developing microglia following inflammatory challenges (Zinni et al., 2021). Further, genetic deletion of Grm3 mimicked the pro-inflammatory phenotype observed in the inflammatory challenged microglial. Similarly, genetic loss of mGlu₃ receptors has been shown to amplify ischemic brain damage in mice (Mastroiacovo et al., 2021). To note, increased infarct size as well as increased expression of proinflammatory genes (Il-1β, TNF-1α, COX-2, IL-6, and CD86) in the peri-infarct area have been reported (Mastroiacovo et al., 2021). These studies suggest that selective activation of mGlu₃ receptors may be used as a therapeutic intervention in brain injuries marked by a high level of inflammation. Additionally, acute as well as chronic treatment with mGlu_2/3 receptor agonist, LY379268, enhanced glial derived neurotropic factor (GDNF) and GLT-1 levels in the spinal cord of WT as well as Grm2 (gene coding for mGlu₂ receptor) KO mice (Battaglia et al., 2015). The effects of LY379268 were absent in Grm3 KO mice, indicating the role of mGlu₃ receptor activation in LY379268-induced enhancement of GDNF in the spinal cord. Excitingly, chronic treatment with LY379268 rescued the spinal cord motor neurons in SOD1G93A mice (a mouse model for amyotrophic lateral sclerosis, ALS) (Battaglia et al., 2015). To note, the activation of mGlu₃ receptors protects cultured spinal cord motor neurons via increased GDNF production (Battaglia et al., 2015), indicating the potential role of enhancing mGlu₃ receptor function in slowing neuronal degeneration in ALS patients.

mGlu₃ receptor may be involved in the pathology of substance use disorders

A tremendous amount of literature suggests a crucial role of glutamatergic signaling in the development and maintenance of behavioral and plasticity changes associated with addiction. Because of the ability to regulate glutamate release in the brain areas implicated in addiction, the mGlu₃ receptors are one of the molecular targets of drug-induced plasticity changes in the brain. An association between minor alleles of two GRM3 SNPs (rs274618 and rs274622) and an increased risk of heroin dependence has been revealed in a Chinese population (Jia et al., 2014). Of note, the carrier addicts of these alleles had a relatively shorter duration for the transition from the first use to dependence as compared to the homozygous for the major alleles. These studies indicate that genetic variants in GRM3 may confer an increased risk of developing heroin dependence. Interestingly, GRM3 is one of the top candidate genes that is shared between alcoholism and other psychiatric disorders (bipolar disorders, schizophrenia and stress behaviors) (Levey et al., 2014). Sequence variants in GRM3 (rs6465084 and rs148754219) are associated with alcohol dependence in male subjects (O’Brien et al., 2014; Xia et al., 2014). A mouse genome wide-microarray analysis from the PFC has revealed Grm3 as one of the hub genes in the acute ethanol responsive networks in the mice (Wolen et al., 2012). Further, a transcriptome analysis in the ethanol naive rat strains that are genetically selected to prefer or avoid ethanol has shown a differential baseline regulation of Grm3 in the PFC (Worst et al., 2005). This could be a mechanism responsible for altered neurotransmitter release following exposure to substance of abuse.

Similarly, decreased expression of mGlu₃ receptor, but not mGlu₂ receptor, mRNA has also been observed in the dentate gyrus of rats administered with ethanol for 2 months (Simonyi et al., 2004). Changes in the expression of Grm3 were also observed in the postmortem hippocampus of subjects exposed to chronic alcohol (Enoch et al., 2014). Given the role of the hippocampus in the processing of contextual cues that are considered important in drug reinforcement behaviors and craving, and cognitive processes (Luo et al., 2011; Volkow et al., 2004), the above mentioned findings point toward the potential role/ association of mGlu₃ receptors in cognitive deficits and behavioral changes associated with chronic substance use. Along these lines, NAAG peptidase inhibitors have been shown to reverse short-term novel object recognition memory deficits induced by acute ethanol administration in WT but not Grm3 KO mice (Olszewski et al., 2017). Additional studies revealed the absence of ethanol-induced conditioned place preference in Grm3 KO mice (Lainiola et al., 2019) suggesting a role of mGlu₃ receptors in the neuroplastic changes required for ethanol conditioned place preference (CPP). On the contrary, Grm3 KO mice displayed increased place preference for methamphetamine in a conditioned place preference (CPP) task and increased methamphetamine toxicity in the mPFC and the NAcc (Busceti et al., 2021). Based on that it is conceivable that potentiating mGlu₃ receptors might confer a therapeutic benefit for treating methamphetamine addiction and the associated brain damage (Table-2).

Decreased levels of GRM3 have been observed in the postmortem hippocampus of cocaine addicts (Enoch et al., 2014). In addition, repeated cocaine exposure impairs mGlu_2/3 receptor function in the PFC and the NAcc (Huang et al., 2007; Xi et al., 2002). And withdrawal from cocaine self-administration attenuated the expression of LTP in the NAcc core subregion following stimulation of the PFC (Moussawi et al., 2009). These deficits in plasticity were restored by treatment with N-acetylcysteine (NAC) and the effects of NAC on LTP were blocked by administration of mGlu_2/3 antagonist, LY341495. These results along with the previous findings that NAC inhibits reinstatement of drug seeking via activation of mGlu_2/3 receptors (Moran et al., 2005) propose an essential role of mGlu_2/3 receptors in mediating cocaine-induced drug-seeking. Further, systemic administration of NAAG peptidase inhibitor, 2-PMPA, inhibited intravenous cocaine self-administration and cocaine-induced reinstatement of drug-seeking behavior (Xi et al., 2010a; Xi et al., 2010b). Similarly, microinjections of 2-PMPA or NAAG into the NAcc also inhibited cocaine-induced reinstatement and indicate that activation of mGlu₃ receptors may mediate these affects. Nevertheless, further studies utilizing receptor-selective ligands, and receptor and cell-type KOs are warranted to evaluate the role of mGlu₃ receptors in cocaine-induced plasticity and behavioral changes.

Concluding remarks and future directions

Emerging evidence suggests that alterations in the components of glutamatergic signaling play an important role in the pathophysiology of multiple CNS disorders (Dong et al., 2009; Javitt, 2004; Li et al., 2018; Ribeiro et al., 2017). Pharmacologically targeting of iGlu receptors is beneficial in several CNS disorders, but these manipulations produce several adverse effects including motor deficits, psychosis-like symptoms and neurotoxicity. A tremendous number of preclinical studies have shown that manipulating mGlu receptors using selective allosteric modulators has a potential for the treatment of CNS disorders (Dogra and Conn, 2021, 2022; Maksymetz et al., 2017). Among these, the mGlu₃ receptors have emerged as promising agents for the treatment of several psychiatric (Dogra and Conn, 2021, 2022) and neurodegenerative disorders (Caraci et al., 2012; Durand et al., 2013) (Table-1, 2, 3). For example, mGlu₃ receptor NAM induces rapid and sustained antidepressant-like effects in both acute and chronic models of depression (Joffe et al., 2020). The mGlu₃ receptor selective NAMs are also efficacious in the rodent models of treatment-resistant depression. Further, mGlu₃ receptor NAM can prevent motivational deficits induced by acute restraint stress (Joffe et al., 2019). In the future, it will be exciting to evaluate the effects of mGlu₃ receptor NAMs on motivation in the chronic models of depression. As mGlu_2/3 receptor antagonists lack side-effects associated with the NMDA receptor antagonists, the mGlu₃ receptor NAMs hold great potential as next-generation safer and more efficacious antidepressants.

The progress in the mGlu₃ receptor research is impeded by the lack of mGlu₃ receptor-selective pharmacological ligands (agonists and positive allosteric modulators). In that scenario, human gene association studies have played an important role in defining the role of mGlu₃ receptors in CNS disorders. For example, polymorphisms in GRM3 are associated with an increased risk of developing schizophrenia and cognitive impairments associated with schizophrenia (Egan et al., 2004; Fujii et al., 2003; Kinoshita et al., 2015; Saini et al., 2017). Similarly, sequence variants in the GRM3 are linked with an increased risk of developing heroin and alcohol dependence (Jia et al., 2014; O’Brien et al., 2014; Xia et al., 2014). These gene association studies have bolstered research efforts to test if modulation of mGlu₃ receptor function in selective brain areas can rescue schizophrenia-like and addiction-related pathophysiological changes. Along these lines, recently, it has been reported that selective activation of neuronal mGlu₃ receptors can rescue schizophrenia-like cognitive deficits in rodents (Dogra et al., 2021). Moreover, exciting evidence suggests that activation of astrocytic mGlu₃ receptors can induce neuroprotective effects by promoting the release of BDNF, TGF-β and GDNF, and by modulating glutamate transporters (Figure-2) (Battaglia et al., 2015; Durand et al., 2017). Activation of mGlu₃ receptors promotes cognition (Dogra et al., 2021), therefore, it will be exciting to evaluate the cognition enhancing effects of mGlu₃ receptors in various models of neurodegenerative diseases. Earlier work in these models has been done using mGlu_2/3 receptor agonist and/or mGlu₂- and mGlu₃-selective KOs. Considering the previously reported compensatory changes in these whole-body KOs (Lyon et al., 2008), testing the effects of mGlu₃ receptor activation using cell-specific conditional KOs can provide important insights into the beneficial effects of mGlu₃ receptors. Besides, considering the sex differences in the CNS disorders, understanding the molecular basis of the sex-differences as a biological variable can also provide useful insights to full harness the therapeutic potential of these receptors. Additionally, the findings that mGlu₃ receptors can shape the influence of mGlu₅ receptor activation on neurotoxic insults (Di Menna et al., 2018) might open new avenues of research in the field of neurodegenerative disorders.

In conclusion, extensive preclinical and clinical studies illuminate the potential of targeting mGlu₃ receptors to develop safer and more efficacious drugs for the treatment of psychiatric and neurodegenerative disorders. Further, continued research efforts utilizing mGlu₃ receptor-selective compounds will likely provide unparalleled insights into the mechanism of CNS disorders.

Highlights.

• Negative allosteric modulators of mGlu₃ receptors induce antidepressant-like effects in preclinical models of depression.

• Agonists of mGlu_2/3 receptors induce antipsychotic-like and procognitive effects in rodents.

• Activation of mGlu₃ receptors enhances cognition and can provide a novel approach for treating cognitive deficits in schizophrenia.

• Activation of astrocytic mGlu₃ receptors induces neuroprotective effects.