GluN1 deletions in D1- and A2A-expressing cell types reveal distinct modes of behavioral regulation

Prefrontal cortex parvalbumin interneurons exhibit decreased excitability and potentiated synaptic strength after ethanol reward learning

The prefrontal cortex (PFC) is intimately associated with behavioral characteristics of alcohol use disorders, including high motivation to drink and difficulty with moderation. Thus, continued mechanistic research investigating PFC cells and targets altered by ethanol experiences should inform translational efforts to craft new, efficacious treatments. Inhibitory interneurons expressing parvalbumin (PV-INs) comprise only a minor fraction of cells within the PFC, yet these cells are indispensable for coordinating PFC ensemble function, oscillatory activity, and subcortical output. Based on this, PV-INs represent an exciting target for the rational design of breakthrough treatments for alcohol use disorders. Here, we assessed experience-dependent physiological adaptations via ethanol place conditioning. By manipulating the timing of administration relative to conditioning sessions, equivalent ethanol exposure can form either rewarding or aversive memories in different individuals. Here, we found that female mice and male mice on a C57BL/6J background display conditioned place preference (CPP) or aversion (CPA) to an intoxicating dose of ethanol (2 g/kg, intraperitoneal [i.p.]) without overt differences between sexes. Ethanol reward learning was associated with decreased PV-IN excitability in deep layer prelimbic PFC, whereas PV-INs from CPA mice were not different from controls. Furthermore, PV-INs from mice in the CPP group, but not the CPA group, displayed potentiated excitatory synaptic strength that emerged during 1 week of abstinence. Taken together, these findings illustrate that synaptic and intrinsic adaptations associated with ethanol can depend on an individual's experience. These studies provide further context and support for PFC PV-INs as intriguing targets for modulating alcohol associations.

DNA Epigenetics in Addiction Susceptibility

Addiction is a chronically relapsing neuropsychiatric disease that occurs in some, but not all, individuals who use substances of abuse. Relatively little is known about the mechanisms which contribute to individual differences in susceptibility to addiction. Neural gene expression regulation underlies the pathogenesis of addiction, which is mediated by epigenetic mechanisms, such as DNA modifications. A growing body of work has demonstrated distinct DNA epigenetic signatures in brain reward regions that may be associated with addiction susceptibility. Furthermore, factors that influence addiction susceptibility are also known to have a DNA epigenetic basis. In the present review, we discuss the notion that addiction susceptibility has an underlying DNA epigenetic basis. We focus on major phenotypes of addiction susceptibility and review evidence of cell type-specific, time dependent, and sex biased effects of drug use. We highlight the role of DNA epigenetics in these diverse processes and propose its contribution to addiction susceptibility differences. Given the prevalence and lack of effective treatments for addiction, elucidating the DNA epigenetic mechanism of addiction vulnerability may represent an expeditious approach to relieving the addiction disease burden.

Cell-Type-Specific Adaptions in Striatal Medium-Sized Spiny Neurons and Their Roles in Behavioral Responses to Drugs of Abuse

Drug addiction is defined as a compulsive pattern of drug-seeking- and taking- behavior, with recurrent episodes of abstinence and relapse, and a loss of control despite negative consequences. Addictive drugs promote reinforcement by increasing dopamine in the mesocorticolimbic system, which alters excitatory glutamate transmission within the reward circuitry, thereby hijacking reward processing. Within the reward circuitry, the striatum is a key target structure of drugs of abuse since it is at the crossroad of converging glutamate inputs from limbic, thalamic and cortical regions, encoding components of drug-associated stimuli and environment, and dopamine that mediates reward prediction error and incentive values. These signals are integrated by medium-sized spiny neurons (MSN), which receive glutamate and dopamine axons converging onto their dendritic spines. MSN primarily form two mostly distinct populations based on the expression of either DA-D1 (D1R) or DA-D2 (D2R) receptors. While a classical view is that the two MSN populations act in parallel, playing antagonistic functional roles, the picture seems much more complex. Herein, we review recent studies, based on the use of cell-type-specific manipulations, demonstrating that dopamine differentially modulates dendritic spine density and synapse formation, as well as glutamate transmission, at specific inputs projecting onto D1R-MSN and D2R-MSN to shape persistent pathological behavioral in response to drugs of abuse. We also discuss the identification of distinct molecular events underlying the detrimental interplay between dopamine and glutamate signaling in D1R-MSN and D2R-MSN and highlight the relevance of such cell-type-specific molecular studies for the development of innovative strategies with potential therapeutic value for addiction. Because drug addiction is highly prevalent in patients with other psychiatric disorders when compared to the general population, we last discuss the hypothesis that shared cellular and molecular adaptations within common circuits could explain the co-occurrence of addiction and depression. We will therefore conclude this review by examining how the nucleus accumbens (NAc) could constitute a key interface between addiction and depression.

Disrupting D1-NMDA or D2-NMDA receptor heteromerization prevents cocaine's rewarding effects but preserves natural reward processing

Addictive drugs increase dopamine in the nucleus accumbens (NAc), where it persistently shapes excitatory glutamate transmission and hijacks natural reward processing. Here, we provide evidence, from mice to humans, that an underlying mechanism relies on drug-evoked heteromerization of glutamate N-methyl-d-aspartate receptors (NMDAR) with dopamine receptor 1 (D1R) or 2 (D2R). Using temporally controlled inhibition of D1R-NMDAR heteromerization, we unraveled their selective implication in early phases of cocaine-mediated synaptic, morphological, and behavioral responses. In contrast, preventing D2R-NMDAR heteromerization blocked the persistence of these adaptations. Interfering with these heteromers spared natural reward processing. Notably, we established that D2R-NMDAR complexes exist in human samples and showed that, despite a decreased D2R protein expression in the NAc, individuals with psychostimulant use disorder display a higher proportion of D2R forming heteromers with NMDAR. These findings contribute to a better understanding of molecular mechanisms underlying addiction and uncover D2R-NMDAR heteromers as targets with potential therapeutic value.

Cannabinoid type 1 receptors in A2a neurons contribute to cocaine-environment association

RATIONALE

Cannabinoid type 1 receptors (CB1Rs) are widely expressed within the brain's reward circuits and are implicated in regulating drug induced behavioral adaptations. Understanding how CB1R signaling in discrete circuits and cell types contributes to drug-related behavior provides further insight into the pathology of substance use disorders.

OBJECTIVE AND METHODS

We sought to determine how cell type-specific expression of CB1Rs within striatal circuits contributes to cocaine-induced behavioral plasticity, hypothesizing that CB1R function in distinct striatal neuron populations would differentially impact behavioral outcomes. We crossed conditional Cnr1^fl/fl mice and striatal output pathway cre lines (Drd1a -cre; D1, Adora2a -cre; A2a) to generate cell type-specific CB1R knockout mice and assessed their performance in cocaine locomotor and associative behavioral assays.

RESULTS

Both knockout lines retained typical locomotor activity at baseline. D1-Cre x Cnr1^fl/fl mice did not display hyperlocomotion in response to acute cocaine dosing, and both knockout lines exhibited blunted locomotor activity across repeated cocaine doses. A2a-cre Cnr1^fl/fl, mice did not express a preference for cocaine paired environments in a two-choice place preference task.

CONCLUSIONS

This study aids in mapping CB1R-dependent cocaine-induced behavioral adaptations onto distinct striatal neuron subtypes. A reduction of cocaine-induced locomotor activation in the D1- and A2a-Cnr1 knockout mice supports a role for CB1R function in the motor circuit. Furthermore, a lack of preference for cocaine-associated context in A2a-Cnr1 mice suggests that CB1Rs on A2a-neuron inhibitory terminals are necessary for either reward perception, memory consolidation, or recall. These results direct future investigations into CB1R-dependent adaptations underlying the development and persistence of substance use disorders.

Dendritic spine density is increased on nucleus accumbens D2 neurons after chronic social defeat

Stress alters the structure and function of brain reward circuitry and is an important risk factor for developing depression. In the nucleus accumbens (NAc), structural and physiological plasticity of medium spiny neurons (MSNs) have been linked to increased stress-related and depression-like behaviors. NAc MSNs have opposing roles in driving stress-related behaviors that is dependent on their dopamine receptor expression. After chronic social defeat stress, NAc MSNs exhibit increased dendritic spine density. However, it remains unclear if the dendritic spine plasticity is MSN subtype specific. Here we use viral labeling to characterize dendritic spine morphology specifically in dopamine D2 receptor expressing MSNs (D2-MSNs). After chronic social defeat, D2-MSNs exhibit increased spine density that is correlated with enhanced social avoidance behavior. Together, our data indicate dendritic spine plasticity is MSN subtype specific, improving our understanding of structural plasticity after chronic stress.

From Signaling Molecules to Circuits and Behaviors: Cell-Type-Specific Adaptations to Psychostimulant Exposure in the Striatum

Addiction is characterized by a compulsive pattern of drug seeking and consumption and a high risk of relapse after withdrawal that are thought to result from persistent adaptations within brain reward circuits. Drugs of abuse increase dopamine (DA) concentration in these brain areas, including the striatum, which shapes an abnormal memory trace of drug consumption that virtually highjacks reward processing. Long-term neuronal adaptations of gamma-aminobutyric acidergic striatal projection neurons (SPNs) evoked by drugs of abuse are critical for the development of addiction. These neurons form two mostly segregated populations, depending on the DA receptor they express and their output projections, constituting the so-called direct (D₁ receptor) and indirect (D₂ receptor) SPN pathways. Both SPN subtypes receive converging glutamate inputs from limbic and cortical regions, encoding contextual and emotional information, together with DA, which mediates reward prediction and incentive values. DA differentially modulates the efficacy of glutamate synapses onto direct and indirect SPN pathways by recruiting distinct striatal signaling pathways, epigenetic and genetic responses likely involved in the transition from casual drug use to addiction. Herein we focus on recent studies that have assessed psychostimulant-induced alterations in a cell-type-specific manner, from remodeling of input projections to the characterization of specific molecular events in each SPN subtype and their impact on long-lasting behavioral adaptations. We discuss recent evidence revealing the complex and concerted action of both SPN populations on drug-induced behavioral responses, as these studies can contribute to the design of future strategies to alleviate specific behavioral components of addiction.

Ferulic acid through mitigation of NMDA receptor pathway exerts anxiolytic-like effect in mouse model of maternal separation stress

Background Experiencing early-life stress plays an important role in the pathophysiology of anxiety disorders. Ferulic acid is a phenolic compound found in some plants which has several pharmacological properties. N-methyl-D-aspartate (NMDA) receptors are involved in the pathophysiology of mood disorders. In this study we aimed to assess the anxiolytic-like effect of ferulic acid in a mouse model of maternal separation (MS) stress by focusing on the possible involvement of NMDA receptors. Methods Mice were treated with ferulic acid (5 and 40 mg/kg) alone and in combination with NMDA receptor agonist/antagonist. Valid behavioral tests were performed, including open field test (OFT) and elevated plus maze test (EPM), while quantitative real time polymerase chain reaction (qRT-PCR) was used to evaluate gene expression of NMDA subunits (GluN2A and GluN2B) in the hippocampus. Results Findings showed that treatment of MS mice with ferulic acid increased the time spent in the central zone of the OFT and increased both open arm time and the percent of open arm entries in the EPM. Ferulic acid reduced the expression of NMDA receptor subunit genes. We showed that administration of NMDA receptor agonist (NMDA) and antagonist (ketamine) exerted anxiogenic and anxiolytic-like effects, correspondingly. Results showed that co-administration of a sub-effective dose of ferulic acid plus ketamine potentiated the anxiolytic-like effect of ferulic acid. Furthermore, co-administration of an effective dose of ferulic acid plus NMDA receptor agonist (NMDA) attenuated the anxiolytic-like effect of ferulic acid. Conclusions In deduction, our findings showed that NMDA, partially at least, is involved in the anxiolytic-like effect of ferulic acid in the OFT and EPM tests.

The molecular and cellular mechanisms of depression: a focus on reward circuitry

Depression is a complex disorder that takes an enormous toll on individual health. As affected individuals display a wide variation in their clinical symptoms, the precise neural mechanisms underlying the development of depression remain elusive. Although it is impossible to phenocopy every symptom of human depression in rodents, the preclinical field has had great success in modeling some of the core affective and neurovegetative depressive symptoms, including social withdrawal, anhedonia, and weight loss. Adaptations in select cell populations may underlie these individual depressive symptoms and new tools have expanded our ability to monitor and manipulate specific cell types. This review outlines some of the most recent preclinical discoveries on the molecular and neurophysiological mechanisms in reward circuitry that underlie the expression of behavioral constructs relevant to depressive symptoms.

mGlu2 and mGlu3 Negative Allosteric Modulators Divergently Enhance Thalamocortical Transmission and Exert Rapid Antidepressant-like Effects

Non-selective antagonists of metabotropic glutamate receptor subtypes 2 (mGlu₂) and 3 (mGlu₃) exert rapid antidepressant-like effects by enhancing prefrontal cortex (PFC) glutamate transmission; however, the receptor subtype contributions and underlying mechanisms remain unclear. Here, we leveraged newly developed negative allosteric modulators (NAMs), transgenic mice, and viral-assisted optogenetics to test the hypothesis that selective inhibition of mGlu₂ or mGlu₃ potentiates PFC excitatory transmission and confers antidepressant efficacy in preclinical models. We found that systemic treatment with an mGlu₂ or mGlu₃ NAM rapidly activated biophysically unique PFC pyramidal cell ensembles. Mechanistic studies revealed that mGlu₂ and mGlu₃ NAMs enhance thalamocortical transmission and inhibit long-term depression by mechanistically distinct presynaptic and postsynaptic actions. Consistent with these actions, systemic treatment with either NAM decreased passive coping and reversed anhedonia in two independent chronic stress models, suggesting that both mGlu₂ and mGlu₃ NAMs induce antidepressant-like effects through related but divergent mechanisms of action.

Genetic loss of GluN2B in D1-expressing cell types enhances long-term cocaine reward and potentiation of thalamo-accumbens synapses

Transient upregulation of GluN2B-containing NMDA receptors (R) in the nucleus accumbens (NAc) is proposed as an intermediate to long-term AMPAR plasticity associated with persistent cocaine-related behaviors. However, cell type- and input-specific contributions of GluN2B underlying lasting actions of cocaine remain to be elucidated. We utilized GluN2B cell type-specific knockouts and optogenetics to deconstruct the role of GluN2B in cocaine-induced NAc synaptic and behavioral plasticity. While reward learning was unaffected, loss of GluN2B in D1 dopamine receptor-expressing cells (D1) led to prolonged retention of reward memory. In control mice, prefrontal cortex (PFC)-D1(+) NAc AMPAR function was unaffected by cocaine exposure, while midline thalamus (mThal)-D1(+) NAc AMPAR function was potentiated but diminished after withdrawal. In D1-GluN2B^-/- mice, the potentiation of mThal-D1(+) NAc AMPAR function persisted following withdrawal, corresponding with continued expression of cocaine reward behavior. These data suggest NAc GluN2B-containing NMDARs serve a feedback role and may weaken reward-related memories.

Toll-like receptor 4 deficiency alters nucleus accumbens synaptic physiology and drug reward behavior

Behavioral manifestations of drug-seeking behavior are causally linked to alterations of synaptic strength onto nucleus accumbens (NAc) medium spiny neurons (MSN). Although neuron-driven changes in physiology and behavior are well characterized, there is a lack of knowledge of the role of the immune system in mediating such effects. Toll-like receptor 4 (TLR4) is a pattern recognition molecule of the innate immune system, and evidence suggests that it modulates drug-related behavior. Using TLR4 knockout (TLR4.KO) mice, we show that TLR4 plays a role in NAc synaptic physiology and behavior. In addition to differences in the pharmacological profile of N-methyl-d-aspartate receptors (NMDAR) in the NAc core, TLR4.KO animals exhibit a deficit in low-frequency stimulation-induced NMDAR-dependent long-term depression (LTD). Interestingly, the synaptic difference is region specific as no differences were found in excitatory synaptic properties in the NAc shell. Consistent with altered NAc LTD, TLR4.KO animals exhibit an attenuation in drug reward learning. Finally, we show that TLR4 in the NAc core is primarily expressed on microglia. These results suggest that TLR4 influences NAc MSN synaptic physiology and drug reward learning and behavior.

Emerging Role for Nucleus Accumbens Medium Spiny Neuron Subtypes in Depression

The ventral striatum (nucleus accumbens) and its role in mood, reward, and motivation has been the focus of significant research. Despite this interest, little work has addressed cell type-specific distinctions in medium spiny neurons (MSNs), the main projection neurons in the nucleus accumbens and dorsal striatum, and their function in relation to stress and depression. Previous work has shown opposing roles for D₁ and D₂ receptor MSN subtypes in depression-like outcomes to stress, particularly in regard to repeated neuronal stimulation and excitatory transmission. Yet the mechanisms of action are still unknown. We discuss potential mechanisms by which MSN subtype function promotes dichotomous behavioral outcomes caused by differences in cellular plasticity, subcellular signaling pathways, and genetic expression. This review aims to address our current understanding about the role of nucleus accumbens MSN subtypes in stress-related depression behavior and speculates on how currently understood mechanisms contribute to factors that control the activity of MSNs.

Do specific NMDA receptor subunits act as gateways for addictive behaviors?

Addiction to alcohol and drugs is a major social and economic problem, and there is considerable interest in understanding the molecular mechanisms that promote addictive drives. A number of proteins have been identified that contribute to expression of addictive behaviors. NMDA receptors (NMDARs), a subclass of ionotropic glutamate receptors, have been of particular interest because their physiological properties make them an attractive candidate for gating induction of synaptic plasticity, a molecular change thought to mediate learning and memory. NMDARs are generally inactive at the hyperpolarized resting potentials of many neurons. However, given sufficient depolarization, NMDARs are activated and exhibit long-lasting currents with significant calcium permeability. Also, in addition to stimulating neurons by direct depolarization, NMDARs and their calcium signaling can allow strong and/or synchronized inputs to produce long-term changes in other molecules (such as AMPA-type glutamate receptors) which can last from days to years, binding internal and external stimuli in a long-term memory trace. Such memories could allow salient drug-related stimuli to exert strong control over future behaviors and thus promote addictive drives. Finally, NMDARs may themselves undergo plasticity, which can alter subsequent neuronal stimulation and/or the ability to induce plasticity. This review will address recent and past findings suggesting that NMDAR activity promotes drug- and alcohol-related behaviors, with a particular focus on GluN2B subunits as possible central regulators of many addictive behaviors, as well as newer studies examining the importance of non-canonical NMDAR subunits and endogenous NMDAR cofactors.