Anger management: Mechanisms of glutamate receptor-mediated synaptic plasticity underlying animal aggression

Abstract

Excessive and recurring violent aggression is a serious concern for society and a symptom of many psychiatric diseases. Substance abuse, attack experience, and social and traumatic stress increase vulnerability to developing this type of aggression. Glutamate receptors are an intriguing target for long-term treatment. This review will assess the importance of glutamate receptors and glutamatergic pathways in aggression, focusing on the role of glutamate receptor-mediated synaptic plasticity in experience-dependent long-lasting aggression. By synthesizing what is known about glutamatergic systems in aggression, it is hoped more effective treatments can be developed.

1. Introduction

Aggression is an innate, evolutionarily advantageous trait mammals use to acquire resources, establish hierarchies, or protect oneself from harm (Nelson and Trainor, 2007). However, excessive displays of aggression are typically maladaptive and are a hallmark of psychiatric diseases such as PTSD (Miczek et al., 2013; Nelson and Trainor, 2007). A major determinant for excessive and recurring violent aggression is prior attack, stressful experience or substance abuse (Miczek et al., 2013; Nordman et al., 2020a, 2020b; Stagkourakis et al., 2020). The impact of social and stressful experience or substance abuse in shaping neural circuits that drive future excessive aggression is only starting to be understood. A more thorough examination of the molecular, cellular, and circuit-level mechanisms will be essential in developing new and better treatments to combat the scourge of excessive aggression.

The primary mechanism by which experience alters neural pathways in the brain is synaptic plasticity (Squire and Kandel, 2009). Synaptic plasticity modifies the strength and efficacy of information transfer at the connection points between neurons known as synapses (Squire and Kandel, 2009). Here, synapses undergo molecular changes that increase or decrease their response to synaptic input or undergo structural changes that lead to the addition of new synapses or subtraction of existing ones. While it is typically presented as a mechanism for learning and memory, synaptic plasticity is increasingly appreciated as a means to alter innate and adaptive behavioral responses (Nordman et al., 2020a, 2020b; Stagkourakis et al., 2020).

Glutamate receptors are a key source of synaptic plasticity. Glutamate receptors are found at the surface of synapses where they mediate excitatory neurotransmission by binding the neurotransmitter glutamate, which is synthesized and released by glutamatergic neurons (Willard and Koochekpour, 2013). There are two classes of glutamate receptors: ionotropic receptors, typically found in the postsynaptic compartment of a neuronal synapse, and metabotropic receptors, which are widely distributed on both presynaptic and postsynaptic compartments. Both classes are important for social behaviors that include aggression (Araki et al., 2014; Borland et al., 2020; Shaltiel et al., 2008; Shimizu et al., 2016; Vekovischeva et al., 2007; Zoicas and Kornhuber, 2019a, 2019b) (Fig. 1).

Fig. 1. Glutamatergic circuitry of aggression.

(A) Naturally occurring attack circuitry. Ai. The dorsal raphe (DR) regulates attack escalation via an AMPAR and NMDAR-dependent mechanism (Takahashi et al., 2015). The dotted line indicates hypothesized input from the prefrontal cortex (PFC). Aii. Attack termination is regulated by a glutamatergic DR - medial amygdala (MeA) pathway (Nordman and Li, 2020). Aiii. Attack duration is mediated by a glutamatergic DR - medial orbitofrontal cortex (MeOC) pathway (Nordman and Li, 2020). Aiv. Attack is initiated by an NMDAR-dependent glutamatergic ventromedial hypothalamus (VmHvl) – periaqueductal gray (PAG) pathway (Falkner et al., 2020). Av. Defensive rage is regulated by an NMDAR-dependent basolateral amygdala (BLA) - PAG pathway (Shaikh et al., 1994; Shaikh and Siegel, 1994). Avi. The lateral amygdala (LA) regulates attack via an NMDAR-dependent mechanism (Bacq et al., 2018). Avii. The bed nucleus of the stria terminalis (BNST) regulates attack via an mGluR7-dependent mechanism (Masugi-Tokita et al., 2016). Aviii. Attack is regulated by an AMPAR-dependent glutamatergic posterior amygdala (PA) - VmHvl pathway (Zha et al., 2020). (B) Substance-induced attack circuitry. Bi. Glutamatergic lateral hypothalamic (LH) neurons project onto the BNST to mediate anabolic steroid-induced aggression via an AMPAR-dependent mechanism (Carrillo et al., 2011). Bii. Alcohol heightens aggression by activating GluN2D NMDARs in the medial PFC (mPFC) (Newman et al., 2018). Biii-iv. GluN2A/B is increased in the mPFC and striatum of morphine-induced aggressive mice (Peregud et al., 2012). (C) Instigated and primed aggression. Ci. Attack experience primes aggression by potentiating NMDAR-dependent glutamatergic MeApv-VmHvl and MeApv-BNST pathways (Nordman, J.C. et al., 2020). Cii. Repeated attack experience potentiates and induces structural plasticity in a glutamatergic PA-VmHvl pathway (Stagkourakis et al., 2020). Ciii. Repeated attack experience increases mGluR1, mGluRS, and AMPAR expression at the nucleus accumbens (NAc) (Borland et al., 2020). (D) Social isolation and traumatic stress-induced aggression. Di-ii. Social isolation-induced aggression increases AMPAR expression in the central amygdala (CeA) and PFC (Araki et al., 2014; Shimizu et al., 2016). Diii. An AMPA-dependent glutamatergic PA-VmHvl pathway regulates attack induced by social isolation (Zha et al., 2020). Div. GluN2B expression in the LA is decreased in social isolation-induced aggression (Bacq et al., 2018). Dv. GluN2A/B expression is increased in the hippocampus (Hip) of social isolation-induced aggressive mice (Chang et al., 2015; Kishi et al., 2017; Meyer et al., 2004; Zhao et al., 2009). Dvi. Glutamatergic ventral hippocampus (vHip) neurons mediate social isolation-dependent foot shock-induced attack through the VmH (Chang and Gean, 2019). Dvii. Social isolation and foot shock induce aggression by potentiating glutamatergic MeApv-VmHvl and MeApv-BNST pathways (Nordman et al., 2020b).

2. Ionotropic glutamate receptors in aggression

The ionotropic glutamate receptors are heteromeric tetramers that depolarize the neuronal membrane by conducting the cations Na⁺, K⁺, and Ca⁺⁺ (Willard and Koochekpour, 2013). The three major types of ionotropic glutamate receptors are the α-Amino-3--hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMDAR), the N-methyl-d-aspartate receptor (NMPAR), and the kainate receptor. Each is named after the specific pharmacological agent that activates it. AMPARs and kainate receptors passively conduct ions when bound by glutamate. NMDARs are only active when bound by glutamate and the neurotransmitter glycine at certain voltage thresholds, allowing them to serve as coincidence detectors.

Due to their fast kinetics, high calcium permeability, and location in the synapse, AMPARs exert a large amount of control over NMDARs, which only open when the postsynaptic compartment where they reside is sufficiently depolarized. NMDARs, in turn, govern the insertion or removal of AMPARs at the postsynaptic surface. This is believed to be the primary mechanism for synaptic plasticity in the brain. Kainate receptors, though less understood, appear to play more of a modulatory role in excitatory signaling and synaptic plasticity (See (Nair et al., 2021) for an excellent recent review). Notably, all three ionotropic glutamate receptor subtypes are involved in regulating naturally occurring attacks (Fig. 1A), with AMPARs and NMDARs also controlling experience-dependent aggression (Fig. 1B–D).

AMPARs are intriguing because of their immediate and persistent effects on aggression. For example, systemic injections of AMPAR antagonists suppress consummatory aggression in Turku Aggressive mice and social isolation-induced aggression in ddY mice (Araki et al., 2014; Vekovischeva et al., 2007). Mice with functionally reduced AMPARs and AMPAR knock-out (KO) mice show significantly less inter-male aggression than their wild-type (WT) littermates (Vekovischeva et al., 2004). Direct infusion of AMPAR antagonists into the BNST of male Syrian hamsters suppresses offensive attack behavior potentiated by anabolic steroids (Carrillo et al., 2011) (Fig. 1Bi). By contrast, the AMPAR subunit GluA1 is enriched at synapses in the nucleus accumbens (NAc) of female Syrian hamsters with prior attack experience (Borland et al., 2020) (Fig. 1Ciii). AMPAR levels are also higher in the amygdala and prefrontal cortex (PFC) of social isolation-induced aggressive mice compared to non-aggressive controls (Araki et al., 2014; Shimizu et al., 2016) (Fig. 1Di and Dii). A recent study demonstrated that glutamatergic projections from the posterior amygdala (PA) to the ventrolateral aspect of the ventromedial hypothalamus (VmHvl) regulate social isolation-induced aggression through an AMPAR-dependent mechanism (Zha et al., 2020) (Fig. 1Diii). These results demonstrate a clear role for AMPARs in attack behavior and suggest that AMPARs in several brain regions are involved in experience-dependent aggression.

The NMDARs have a more nuanced role in aggression. For example, high doses of the non-competitive NMDAR antagonist ketamine decrease aggression in socially isolated mice, but low doses increase it (Shin et al., 2019; Takahashi et al., 1984). Conversely, low doses of the NMDAR antagonist PCP decrease intermale aggression in socially isolated and female-paired mice, while high doses (6-10 mg/kg) increase aggression in socially isolated mice but have no effect in female-paired mice (Tyler and Miczek, 1982; Wilmot et al., 1987). The low-affinity NMDAR antagonist memantine has been shown to reduce aggression in Alzheimer’s disease and dementia patients (Cummings et al., 2008; Wilcock et al., 2008). However, other studies have shown the opposite effect (Da Re et al., 2015; Huey et al., 2005). Memantine and another NMDAR antagonist, 1-amino-1,3,3,5,5-pentamethyl-cyclohexane hydrochloride (MRZ 2/579), can reduce morphine-induced aggression in mice, though neither drug alters aggression in drug-naïve mice (Sukhotina and Bespalov, 2000). Moreover, systemic and PFC-infused memantine can heighten aggression when coupled with ethanol (Newman et al., 2012; Newman et al., 2018) (Fig. 1Bii). These results indicate that NMDAR drugs have distinct effects on aggression, likely owing to their unique effects on receptor kinetics and synaptic plasticity.

NMDAR effects on aggression are also subunit-specific. The best-studied NMDAR subunits in aggression are GluN1 and GluN2A-D. These receptors are highly expressed in regions that regulate attack behavior, such as the amygdala, (PFC), hippocampus, hypothalamus, NAc, brain stem, and striatum (Chen and Hong, 2018; Newman et al., 2018; Zoicas and Kornhuber, 2019b). GluN1 KO mice display overall reduced intermale aggression (Duncan et al., 2004; Halene et al., 2009). Decreases in GluN2B expression in the lateral amygdala (LA) are associated with naturally occurring attack behavior and social isolation-induced aggression (Bacq et al., 2018) (Fig. 1Avi and Fig. 1Div). However, GluN2A and GluN2B expression are increased in the hippocampus of social isolation-induced aggressive mice (Chang et al., 2015; Meyer et al., 2004; Zhao et al., 2009) (Fig. 1Dv) and in the frontal cortex and striatum of morphine-induced aggressive mice (Peregud et al., 2012) (Fig. 1Biii–iv). In addition, GluN2B-dependent NMDAR currents are elevated in the ventral hippocampus of social isolation-dependent, foot shock-induced aggressive mice, which mediates attack through the VmH (Chang and Gean, 2019; Chang et al., 2015) (Fig. 1Dv–vi). Interestingly, alcohol-heightened aggression leads to an increase in the expression of GluN2D, but not GluN2A or GluN2B, in the mPFC, and may mediate the aggression-suppressing effects of memantine and ketamine after alcohol consumption (Newman et al., 2018) (Fig. 1Bii). These findings demonstrate that the role of NMDARs on aggression is highly dependent on context, experience, and brain region.

Kainate receptors were also found to regulate aggression in mice. Socially isolated mice lacking the kainate receptor subunit GluK2 (GluR6/GRIK2) display heightened aggression during the resident intruder test and standard opponent test (Shaltiel et al., 2008). These mice are also naturally aggressive, robustly attacking their litter and cage mates while in their home cages. Lithium treatment suppresses the attack behavior, suggesting that mania may be responsible for the enhanced aggression. It remains unclear, however, which molecular mechanisms and brain areas are responsible for this behavioral phenotype.

One recent study demonstrated a role for glutamatergic neurons in attack experience-dependent aggression (Fig. 1Ci). Here, attack experience briefly heightened aggression by potentiating glutamatergic synapses between the posterior ventral segment of the medial amygdala (MeApv) and the VmHvl (MeApv-VmHvl) and medial aspect of the bed nucleus of the stria terminalis (MeApv-BNSTm) (Nordman et al., 2020a). The MeApv is a glutamatergic subnucleus of the amygdala responsible for social behaviors such as mating and aggression (Choi et al., 2005) and the VmH and BNST are brain regions widely shown to regulate attack behavior (Lin et al., 2011; Masugi-Tokita et al., 2016). Optogenetically potentiating these pathways using high-frequency photostimulation (HFPS) produced pathway-specific effects on aggression, with the MeApv-VmHvl increasing the duration of an attack and the MeApv-BNSTm increasing the frequency of attacks during a test session. Intriguingly, optogenetically weakening these pathways using low-frequency photostimulation (LFPS) suppressed the feature-specific aggression increase brought on by attack experience or HFPS. These alterations are driven by an NMDAR-dependent mechanism, as demonstrated by systemic injections of MK-801. A companion study showed that traumatic-stress induced aggression depended on a similar mechanism (Nordman et al., 2020b) (Fig. 1Dvii). Similar results were found in another recent study showing that repeated attack-experience over days can produce long-lasting increases in aggression by inducing structural and synaptic plasticity changes between the PA and the VmHvl, as determined by in vivo electrophysiology and ex vivo measurements of the AMPA/NMDA ratio (Stagkourakis et al., 2020) (Fig. 1Cii).

3. Metabotropic glutamate receptors in aggression

Metabotropic glutamate receptors (mGluRs) are members of the G-protein coupled receptor (GPCR) superfamily (Kim et al., 2008; Willard and Koochekpour, 2013). Instead of conducting ions through a central pore, when bound by glutamate, mGluRs activate an intracellular protein cascade of second messengers. These second messengers can activate, alter the conductance, or influence the cell surface expression of ion channels that include the NMDAR, AMPAR, and Kainate receptor. mGluR second messenger pathways can also regulate transcription factors that control gene expression, and therefore are integral players in long-term and structural plasticity (Kim et al., 2008).

mGluRs fall into three functional groups based on sequence homology and pharmacological properties: the group I mGluRs, which include mGluR1 and mGluR5; the group II mGluRs, which include mGluR2 and mGluR3; the group III mGluRs, which include mGluR4, mGluR6, mGluR7, and mGluR8. The group I mGluRs are predominantly postsynaptic and mediate excitatory signaling. The group II and III mGluRs are predominantly presynaptic and regulate neurotransmitter release. Group I mGluRs activate the excitatory Gq/G11 pathway, which in turn activates protein kinase C (PKC) and promotes intracellular calcium release. Group II and III mGluRs activate the inhibitory Gi/o pathway, inhibiting cyclic adenosine monophosphate (cAMP) production and protein kinase A activation. Both the Gq/11 and Gi/o pathways are responsible for inducing persistent modifications to the synapse and thus to memory, characteristic of the late phase of LTP. Notably, all three mGluR groups have been shown to regulate aggression.

Group I mGluRs play a largely facilitatory role in experienced-dependent aggression. For example, inhibiting mGluR1s using the selective antagonist JNJ162959685 (Navarro et al., 2008) or mGluR5s using the selective antagonist 2-Methyl-6-(phenylethynyl)pyridine (MPEP) reduces social isolation-induced aggression (Navarro et al., 2006). MPEP has also been shown to decrease attack experience-induced aggression in female hamsters (Been et al., 2016), while attack experience increases group I mGluR expression and spine density in the nucleus accumbens (NAc) (Borland et al., 2020) (Fig 1Ciii). Paradoxically, another mGluR5 selective antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine hydrochloride (MTEP) was shown to reduce instigated aggressive behavior while heightening excessive aggression when coupled with alcohol self-administration (Newman et al., 2012). These results suggest group I mGluRs mediate attack behavior and play a role in experience-enhanced aggression and synaptic plasticity in a brain region-specific and context-dependent manner.

mGluR2, mGluR3, and mGluR7 agonists attenuate attack behavior. The group II mGluR agonist LY379268 reduces attack bite frequency in normally aggressive and alcohol-induced excessively aggressive mice (Newman et al., 2012). The mGluR7 agonist N,N’-Bis(diphenylmethyl)-1,2-ethanediamine dihydrochloride (AMN082) also inhibits attack behavior in normally aggressive mice (Navarro et al., 2009). Paradoxically, mGluR7 KO mice and intra-BNST infusions of the mGluR7 antagonist 6-(4-methoxyphenyl)-5-methyl-3-pyridin-4-ylisoxazolo[4,5-c]pyridin-4(5H)-one (MMPIP) reduces intermale aggression (Masugi-Tokita et al., 2016) (Fig. 1Avii). These results suggest that group II and group III mGluRs play an inhibitory role in attack behavior. However, like the type I mGluRs, these effects appear to be context-dependent and specific to a particular brain region and genotype.

4. In conclusion

Circuit, genetic, and pharmacological studies in mice and postmortem tissue analysis in humans have demonstrated a clear role for glutamate receptors in aggressive behavior. Glutamate receptors, therefore, represent a promising pharmacological target to treat excessive and recurring aggression. Furthermore, because of their importance in synaptic plasticity, glutamate receptor drugs may help prevent long-lasting aggression brought on by substance abuse and traumatic stress.

Clinically, the most successful glutamate receptor drugs to treat aggression have been those that target the NMDAR. Ketamine and memantine have shown the greatest promise, with ketamine reducing aggression associated with PTSD (Liriano et al., 2019) and memantine reducing aggression associated with Alzheimer’s disease and dementia (Cummings et al., 2008; Wilcock et al., 2008). However, both drugs have also been shown to increase aggression in humans and mice (Da Re et al., 2015; Dillon et al., 2003; Huey et al., 2005). Other clinically available NMDAR drugs may prove beneficial.

Drugs that target the glycine binding site, such as D-Serine and Sarcosine, and glycine transporter, have been effective at treating the positive and negative symptoms of schizophrenia (Lakhan et al., 2013), possibly by augmenting the expression of NMDARs at the surface of the synapse (de Bartolomeis et al., 2020). In clinical trials, the glutamate receptor antagonist amantadine reduces irritability and hyperactivity in children with autism and attention deficit hyperactivity disorder (ADHD) (Hosenbocus and Chahal, 2013). Future studies should examine the efficacy of these drugs in treating excessive aggression associated with psychiatric disease.

While this review primarily focuses on the role of synaptic plasticity in aggression, other non-Hebbian forms of glutamate receptor-mediated plasticity might be involved. One intriguing possibility is intrinsic plasticity, which consists of a change in the intrinsic excitability properties of a neuron that alters its firing probabilities (Abraham, 2008; Sehgal et al., 2013). mGluRs tightly regulate this process. Given the importance of mGluRs in aggression, it is interesting to consider whether mGluR antagonists that reverse excessive aggression after stress or substance abuse do so by altering intrinsic plasticity within aggression circuits.

Studies examining the role of glutamate receptors in aggression are still in their infancy. It is clear that the diversity of receptor types, binding sites, and expression patterns in the brain pose a significant challenge to their use in a clinical setting. While the future is bright, a more thorough mechanistic understanding will be essential to develop new and better therapies.