Cocaine Experience Enhances Thalamo-Accumbens N-Methyl-D-Aspartate Receptor Function

Activation of Metabotropic Glutamate Receptor 3 Modulates Thalamo-accumbal Transmission and Rescues Schizophrenia-Like Physiological and Behavioral Deficits

BACKGROUND

Polymorphisms in the gene encoding for metabotropic glutamate receptor 3 (mGlu3) are associated with an increased likelihood of schizophrenia diagnosis and can predict improvements in negative symptoms following treatment with antipsychotics. However, the mechanisms by which mGlu3 can regulate brain circuits involved in schizophrenia pathophysiology are not clear.

METHODS

We employed selective pharmacological tools and a variety of approaches including whole-cell patch-clamp electrophysiology, slice optogenetics, and fiber photometry to investigate the effects of mGlu3 activation on phencyclidine (PCP)-induced impairments in thalamo-accumbal transmission and sociability deficits. A chemogenetic approach was used to evaluate the role of thalamo-accumbal transmission in PCP-induced sociability deficits.

RESULTS

We first established that PCP treatment augmented excitatory transmission onto dopamine D1 receptor-expressing medium spiny neurons (D1-MSNs) in the nucleus accumbens (NAc) and induced sociability deficits. Our studies revealed a selective increase in glutamatergic synaptic transmission from thalamic afferents to D1-MSNs in the NAc shell. Chemogenetic silencing of thalamo-accumbal inputs rescued PCP-induced sociability deficits. Pharmacological activation of mGlu3 normalized PCP-induced impairments in thalamo-accumbal transmission and sociability deficits. Mechanistic studies revealed that mGlu3 activation induced robust long-term depression at synapses from the thalamic projections onto D1-MSNs in the NAc shell.

CONCLUSIONS

These data demonstrate that activation of mGlu3 decreases thalamo-accumbal transmission and thereby rescues sociability deficits in mouse modeling schizophrenia-like symptoms. These findings provide novel insights into the NAc-specific mechanisms and suggest that agents modulating glutamatergic signaling in the NAc may provide a promising approach for treating negative symptoms in schizophrenia.

Dissociable encoding of motivated behavior by parallel thalamo-striatal projections

The successful pursuit of goals requires the coordinated execution and termination of actions that lead to positive outcomes. This process relies on motivational states that are guided by internal drivers, such as hunger or fear. However, the mechanisms by which the brain tracks motivational states to shape instrumental actions are not fully understood. The paraventricular nucleus of the thalamus (PVT) is a midline thalamic nucleus that shapes motivated behaviors via its projections to the nucleus accumbens (NAc)1,2,3,4,5,6,7,8 and monitors internal state via interoceptive inputs from the hypothalamus and brainstem.3,9,10,11,12,13,14 Recent studies indicate that the PVT can be subdivided into two major neuronal subpopulations, namely PVTD2(+) and PVTD2(-), which differ in genetic identity, functionality, and anatomical connectivity to other brain regions, including the NAc.4,15,16 In this study, we used fiber photometry to investigate the in vivo dynamics of these two distinct PVT neuronal types in mice performing a foraging-like behavioral task. We discovered that PVTD2(+) and PVTD2(-) neurons encode the execution and termination of goal-oriented actions, respectively. Furthermore, activity in the PVTD2(+) neuronal population mirrored motivation parameters such as vigor and satiety. Similarly, PVTD2(-) neurons also mirrored some of these parameters, but to a much lesser extent. Importantly, these features were largely preserved when activity in PVT projections to the NAc was selectively assessed. Collectively, our results highlight the existence of two parallel thalamo-striatal projections that participate in the dynamic regulation of goal pursuits and provide insight into the mechanisms by which the brain tracks motivational states to shape instrumental actions.

D1 and D2 medium spiny neurons in the nucleus accumbens core have distinct and valence-independent roles in learning

At the core of value-based learning is the nucleus accumbens (NAc). D1- and D2-receptor-containing medium spiny neurons (MSNs) in the NAc core are hypothesized to have opposing valence-based roles in behavior. Using optical imaging and manipulation approaches in mice, we show that neither D1 nor D2 MSNs signal valence. D1 MSN responses were evoked by stimuli regardless of valence or contingency. D2 MSNs were evoked by both cues and outcomes, were dynamically changed with learning, and tracked valence-free prediction error at the population and individual neuron level. Finally, D2 MSN responses to cues were necessary for associative learning. Thus, D1 and D2 MSNs work in tandem, rather than in opposition, by signaling specific properties of stimuli to control learning.

Inhibition of the glutamatergic PVT-NAc projections attenuates local anesthetic-induced neurotoxic behaviors

INTRODUCTION

Local anesthetic-induced neurotoxicity contributes to perioperative nerve damage; however, the underlying mechanisms remain unclear. Here, we investigated the role of the paraventricular thalamus (PVT)-nucleus accumbens (NAc) projections in neurotoxicity induced by ropivacaine, a local anesthetic agent.

METHODS

Ropivacaine (58 mg/kg, intraperitoneal administration) was used to construct the local anesthetic systemic toxicity (LAST) mice model. We first identified neural projections from the PVT to the NAc through the expression of a retrograde tracer and virus. The inhibitory viruses (rAAV-EF1α-DIO-hm4D(Gi)-mCherry-WPREs: AAV2/retro and rAAV-CaMKII-CRE-WPRE-hGh: AAV2/9) were injected into the mice model to assess the effects of the specific inhibition of the PVT-NAc pathway on neurological behaviors in the presence of clozapine-N-oxide. The inhibition of the PVT-NAc pathway was evaluated by immunofluorescence staining of c-Fos-positive neurons and Ca2+ signals in CaMKIIa neurons.

RESULTS

We successfully identified a circuit connecting the PVT and NAc in C57BL/6 mice. Ropivacaine administration induced the activation of the PVT-NAc pathway and seizures. Specific inhibition of NAc-projecting CaMKII neurons in the PVT was sufficient to inhibit the neuronal activity in the NAc, which subsequently decreased ropivacaine-induced neurotoxicity.

CONCLUSION

These results reveal the presence of a dedicated PVT-NAc circuit that regulates local anesthetic-induced neurotoxicity and provide a potential mechanistic explanation for the treatment and prevention of LAST.

Dorsal raphe stimulation relays a reward signal to the ventral tegmental area via GluN2C NMDA receptors

BACKGROUND

Glutamate relays a reward signal from the dorsal raphe (DR) to the ventral tegmental area (VTA). However, the role of the different subtypes of N-methyl-D-aspartate (NMDA) receptors is complex and not clearly understood. Therefore, we measured NMDA receptors subunits expression in limbic brain areas. In addition, we studied the effects of VTA down-regulation of GluN2C NMDA receptor on the reward signal that arises from DR electrical stimulation.

METHODS

Using qPCR, we identified the relative composition of the different Grin2a-d subunits of the NMDA receptors in several brain areas. Then, we used fluorescent in situ hybridization (FISH) to evaluate the colocalization of Grin2c and tyrosine hydroxylase (TH) mRNA in VTA neurons. To assess the role of GluN2C in brain stimulation reward, we downregulated this receptor using small interfering RNA (siRNA) in rats self-stimulating for electrical pulses delivered to the DR. To delineate further the specific role of GluN2C in relaying the reward signal, we pharmacologically altered the function of VTA NMDA receptors by bilaterally microinjecting the NMDA receptor antagonist PPPA.

RESULTS

We identified GluN2C as the most abundant subunit of the NMDA receptor expressed in the VTA. FISH revealed that about 50% of TH-positive neurons colocalize with Grin2c transcript. siRNA manipulation produced a selective down-regulation of the GluN2C protein subunit and a significant reduction in brain stimulation reward. Interestingly, PPPA enhanced brain stimulation reward, but only in rats that received the nonactive RNA sequence.

CONCLUSION

The present results suggest that VTA glutamate neurotransmission relays a reward signal initiated by DR stimulation by acting on GluN2C NMDA receptors.

Identification of miRNA-mediated gene regulatory networks in L-methionine exposure counteracts cocaine-conditioned place preference in mice

Background and Aims: Methionine has been proven to inhibit addictive behaviors of cocaine dependence. This study aimed to identify the potential mechanisms of MET relating to its inhibitory effects on cocaine induced cellular and behavioral changes. Methods: MRNA and miRNA high-throughput sequencing of the prefrontal cortex in a mouse model of cocaine conditioned place preference (CPP) combined with L-methionine was performed. Differentially expressed miRNAs (DE-miRNAs) and differentially expressed genes (DEGs) regulated by cocaine and inhibited by L-methionine were identified. DEGs were mapped to STRING database to construct a protein-protein interaction (PPI) network. Then, the identified DEGs were subjected to the DAVID webserver for functional annotation. Finally, miRNA-mRNA regulatory network and miRNA-mRNA-TF regulatory networks were established to screen key DE-miRNAs and coregulation network in Cytoscape. Results: Sequencing data analysis showed that L-methionine reversely regulated genes and miRNAs affected by cocaine. Pathways associated with drug addiction only enriched in CS-down with MC-up genes targeted by DE-miRNAs including GABAergic synapse, Glutamatergic synapse, Circadian entrainment, Axon guidance and Calcium signaling pathway. Drug addiction associated network was formed of 22 DEGs including calcium channel (Cacna1c, Cacna1e, Cacna1g and Cacng8), ephrin receptor genes (Ephb6 and Epha8) and ryanodine receptor genes (Ryr1 and Ryr2). Calcium channel gene network were identified as a core gene network modulated by L-methionine in response to cocaine dependence. Moreover, it was predicted that Grin1 and Fosb presented in TF-miRNA-mRNA coregulation network with a high degree of interaction as hub genes and interacted calcium channels. Conclusion: These identified key genes, miRNA and coregulation network demonstrated the efficacy of L-methionine in counteracting the effects of cocaine CPP. To a certain degree, it may provide some hints to better understand the underlying mechanism on L-methionine in response to cocaine abuse.

Cocaine preference and neuroadaptations are maintained by astrocytic NMDA receptors in the nucleus accumbens

Cocaine-associated memories induce cravings and interfere with the ability of users to cease cocaine use. Reducing the strength of cue-drug memories by facilitating extinction may have therapeutic value for the treatment of cocaine addiction. Here, we demonstrate the expression of GluN1/2A/2C NMDA receptor currents in astrocytes in the nucleus accumbens core. Selective ablation of GluN1 subunit from astrocytes in the nucleus accumbens enhanced extinction of cocaine preference memory but did not affect cocaine conditioning or reinstatement. Repeated cocaine exposure up-regulated GluN2C subunit expression and increased astrocytic NMDA receptor currents. Furthermore, intra-accumbal inhibition of GluN2C/2D-containing receptors and GluN2C subunit deletion facilitated extinction of cocaine memory. Cocaine-induced neuroadaptations including dendritic spine maturation and AMPA receptor recruitment were absent in GluN2C knockout mice. Impaired retention of cocaine preference memory in GluN2C knockout mice was restored by exogenous administration of recombinant glypican 4. Together, these results identify a previously unknown astrocytic GluN2C-containing NMDA receptor mechanism underlying maintenance of cocaine preference memory.

Cocaine restricts nucleus accumbens feedforward drive through a monoamine-independent mechanism

Parvalbumin-expressing fast-spiking interneurons (PV-INs) within feedforward microcircuits in the nucleus accumbens (NAc) coordinate goal-directed motivational behavior. Feedforward inhibition of medium spiny projection neurons (MSNs) is initiated by glutamatergic input from corticolimbic brain structures. While corticolimbic synapses onto MSNs are targeted by the psychostimulant, cocaine, it remains unknown whether cocaine also exerts acute neuromodulatory actions at collateralizing synapses onto PV-INs. Using whole-cell patch-clamp electrophysiology, optogenetics, and pharmacological tools in transgenic reporter mice, we found that cocaine decreases thalamocortical glutamatergic drive onto PV-INs by engaging a monoamine-independent mechanism. This mechanism relies on postsynaptic sigma-1 (σ₁) activity, leading to the mobilization of intracellular Ca²⁺ stores that trigger retrograde endocannabinoid signaling at presynaptic type-1 cannabinoid receptors (CB₁R). Cocaine-evoked CB₁R activity occludes the expression of CB₁R-dependent long-term depression (LTD) at this synaptic locus. These findings provide evidence that acute cocaine exposure targets feedforward microcircuits in the NAc and extend existing models of cocaine action on mesolimbic reward circuits.

Enhanced function of NR2C/2D-containing NMDA receptor in the nucleus accumbens contributes to peripheral nerve injury-induced neuropathic pain and depression in mice

N-methyl-d-aspartate receptors (NMDARs) dysfunction in the nucleus accumbens (NAc) participates in regulating many neurological and psychiatric disorders such as drug addiction, chronic pain, and depression. NMDARs are heterotetrameric complexes generally composed of two NR1 and two NR2 subunits (NR2A, NR2B, NR2C and NR2D). Much attention has been focused on the role of NR2A and NR2B-containing NMDARs in a variety of neurological disorders; however, the function of NR2C/2D subunits at NAc in chronic pain remains unknown. In this study, spinal nerve ligation (SNL) induced a persistent sensory abnormity and depressive-like behavior. The whole-cell patch clamp recording on medium spiny neurons (MSNs) in the NAc showed that the amplitude of NMDAR-mediated excitatory postsynaptic currents (EPSCs) was significantly increased when membrane potential held at −40 to 0 mV in mice after 14 days of SNL operation. In addition, selective inhibition of NR2C/2D-containing NMDARs with PPDA caused a larger decrease on peak amplitude of NMDAR-EPSCs in SNL than that in sham-operated mice. Appling of selective potentiator of NR2C/2D, CIQ, markedly enhanced the evoked NMDAR-EPSCs in SNL-operated mice, but no change in sham-operated mice. Finally, intra-NAc injection of PPDA significantly attenuated SNL-induced mechanical allodynia and depressive-like behavior. These results for the first time showed that the functional change of NR2C/2D subunits-containing NMDARs in the NAc might contribute to the sensory and affective components in neuropathic pain.

Kappa opioid receptor modulation of excitatory drive onto nucleus accumbens fast-spiking interneurons

The dynorphin/kappa opioid receptor (KOR) system within the nucleus accumbens (NAc) contributes to affective states. Parvalbumin fast-spiking interneurons (PV-FSIs), a key component of feedforward inhibition, participate in integration of excitatory inputs to the NAc by robustly inhibiting select populations of medium spiny output neurons, therefore greatly influencing NAc dependent behavior. How the dynorphin/KOR system regulates feedforward inhibition in the NAc remains unknown. Here, we elucidate the molecular mechanisms of KOR inhibition of excitatory transmission onto NAc PV-FSIs using a combination of whole-cell patch-clamp electrophysiology, optogenetics, pharmacology, and a parvalbumin reporter mouse. We find that postsynaptic KOR stimulation induces long-term depression (LTD) of excitatory synapses onto PV-FSI by stimulating the endocytosis of AMPARs via a PKA and calcineurin-dependent mechanism. Furthermore, KOR regulation of PV-FSI synapses are input specific, inhibiting thalamic but not cortical inputs. Finally, following acute stress, a protocol known to elevate dynorphin/KOR signaling in the NAc, KOR agonists no longer inhibit excitatory transmission onto PV-FSI. In conclusion, we delineate pathway-specific mechanisms mediating KOR control of feedforward inhibitory circuits in the NAc and provide evidence for the recruitment of this system in response to stress.

GluN3-Containing NMDA Receptors in the Rat Nucleus Accumbens Core Contribute to Incubation of Cocaine Craving

Cue-induced cocaine craving progressively intensifies (incubates) after withdrawal from cocaine self-administration in rats and humans. In rats, the expression of incubation ultimately depends on Ca²⁺-permeable AMPARs that accumulate in synapses onto medium spiny neurons (MSNs) in the NAc core. However, the delay in their accumulation (∼1 month after drug self-administration ceases) suggests earlier waves of plasticity. This prompted us to conduct the first study of NMDAR transmission in NAc core during incubation, focusing on the GluN3 subunit, which confers atypical properties when incorporated into NMDARs, including insensitivity to Mg²⁺ block and Ca²⁺ impermeability. Whole-cell patch-clamp recordings were conducted in MSNs of adult male rats 1-68 d after discontinuing extended-access saline or cocaine self-administration. NMDAR transmission was enhanced after 5 d of cocaine withdrawal, and this persisted for at least 68 d of withdrawal. The earliest functional alterations were mediated through increased contributions of GluN2B-containing NMDARs, followed by increased contributions of GluN3-containing NMDARs. As predicted by GluN3-NMDAR incorporation, fewer MSN spines exhibited NMDAR-mediated Ca²⁺ entry. GluN3A knockdown in NAc core was sufficient to prevent incubation of craving, consistent with biotinylation studies showing increased GluN3A surface expression, although array tomography studies suggested that adaptations involving GluN3B also occur. Collectively, our data show that a complex cascade of NMDAR and AMPAR plasticity occurs in NAc core, potentially through a homeostatic mechanism, leading to persistent increases in cocaine cue reactivity and relapse vulnerability. This is a remarkable example of experience-dependent glutamatergic plasticity evolving over a protracted window in the adult brain.SIGNIFICANCE STATEMENT "Incubation of craving" is an animal model for the persistence of vulnerability to cue-induced relapse after prolonged drug abstinence. Incubation also occurs in human drug users. AMPAR plasticity in medium spiny neurons (MSNs) of the NAc core is critical for incubation of cocaine craving but occurs only after a delay. Here we found that AMPAR plasticity is preceded by NMDAR plasticity that is essential for incubation and involves GluN3, an atypical NMDAR subunit that markedly alters NMDAR transmission. Together with AMPAR plasticity, this represents profound remodeling of excitatory synaptic transmission onto MSNs. Given the importance of MSNs for translating motivation into action, this plasticity may explain, at least in part, the profound shifts in motivated behavior that characterize addiction.

Thalamo-Nucleus Accumbens Projections in Motivated Behaviors and Addiction

The ventral striatum, also called nucleus accumbens (NAc), has long been known to integrate information from cortical, thalamic, midbrain and limbic nuclei to mediate goal-directed behaviors. Until recently thalamic afferents have been overlooked when studying the functions and connectivity of the NAc. However, findings from recent studies have shed light on the importance and roles of precise Thalamus to NAc connections in motivated behaviors and in addiction. In this review, we summarize studies using techniques such as chemo- and optogenetics, electrophysiology and in vivo calcium imaging to elucidate the complex functioning of the thalamo-NAc afferents, with a particular highlight on the projections from the Paraventricular Thalamus (PVT) to the NAc. We will focus on the recent advances in the understanding of the roles of these neuronal connections in motivated behaviors, with a special emphasis on their implications in addiction, from cue-reward association to the mechanisms driving relapse.

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.

Histamine H3 Receptor Function Biases Excitatory Gain in the Nucleus Accumbens

BACKGROUND

Histamine (HA), a wake-promoting monoamine implicated in stress-related arousal states, is synthesized in histidine decarboxylase-expressing hypothalamic neurons of the tuberomammillary nucleus. Histidine decarboxylase-containing varicosities diffusely innervate striatal and mesolimbic networks, including the nucleus accumbens (NAc). The NAc integrates diverse monoaminergic inputs to coordinate motivated behavior. While the NAc expresses various HA receptor subtypes, mechanisms by which HA modulates NAc circuit dynamics are undefined.

METHODS

Using male D1tdTomato transgenic reporter mice, whole-cell patch-clamp electrophysiology, and input-specific optogenetics, we employed a targeted pharmacological approach to interrogate synaptic mechanisms recruited by HA signaling at glutamatergic synapses in the NAc. We incorporated an immobilization stress protocol to assess whether acute stress engages these mechanisms at glutamatergic synapses onto D₁ receptor-expressing [D1(+)] medium spiny neurons (MSNs) in the NAc core.

RESULTS

HA negatively regulates excitatory gain onto D1(+)-MSNs via presynaptic H₃ receptor-dependent long-term depression that requires G_βγ-directed Akt-GSK3β signaling. Furthermore, HA asymmetrically regulates glutamatergic transmission from the prefrontal cortex and mediodorsal thalamus, with inputs from the prefrontal cortex undergoing robust HA-induced long-term depression. Finally, we report that acute immobilization stress attenuates this long-term depression by recruiting endogenous H₃ receptor signaling in the NAc at glutamatergic synapses onto D1(+)-MSNs.

CONCLUSIONS

Stress-evoked HA signaling in the NAc recruits H₃ heteroreceptor signaling to shift thalamocortical input onto D1(+)-MSNs in the NAc. Our findings provide novel insight into an understudied neuromodulatory system within the NAc and implicate HA in stress-associated physiological states.

Psychostimulant-Induced Adaptations in Nucleus Accumbens Glutamatergic Transmission

Carrying different aspects of emotional and motivational signals, glutamatergic synaptic projections from multiple limbic and paralimbic brain regions converge to the nucleus accumbens (NAc), in which these arousing signals are processed and prioritized for behavioral output. In animal models of drug addiction, some key drug-induced alterations at NAc glutamatergic synapses underlie important cellular and circuit mechanisms that promote subsequent drug taking, seeking, and relapse. With the focus of cocaine, we review changes at NAc glutamatergic synapses that occur after different drug procedures and abstinence durations, and the behavioral impact of these changes.

Spermidine, a positive modulator of the NMDA receptor, facilitates extinction and prevents the reinstatement of morphine-induced conditioned place preference in mice

RATIONALE

Individuals with opioid use disorders often relapse into drug-seeking behavior after recalling memories linked to the drug use experience. Improving extinction efficacy has been used as a strategy to treat substance use disorders and suppress relapse. Although N-methyl-D-aspartate receptor (NMDAr) agonists facilitate acquisition, consolidation, and extinction, no study has addressed whether spermidine (SPD), a natural polyamine ligand of the NMDA receptor, facilitates the extinction and reinstatement of morphine-induced conditioned place preference (CPP).

OBJECTIVES AND METHODS

The aim of the present study was to investigate the effect of SPD, an NMDAr agonist, on the extinction and reinstatement of morphine-induced CPP in mice. Adult male albino Swiss mice received saline (0.9% NaCl) or morphine (5 mg/kg) intraperitoneally (i.p.) and were respectively confined to a black or a white compartment for 30 min for four consecutive days for CPP induction. SPD (10-30 mg/kg, i.p.) or ifenprodil (NMDAr antagonist, 0.1-1 mg/kg, i.p.) were injected 15 min before extinction training.

RESULTS

SPD and ifenprodil facilitated the extinction of morphine-induced CPP. SPD treatment during the extinction period impaired reinstatement induced by a priming dose of morphine (1.25 mg/kg). Ifenprodil (0.1 mg/kg) prevented the facilitatory effect of spermidine on the extinction of morphine-induced CPP but did not prevent reinstatement induced by morphine.

CONCLUSIONS

These results suggest that SPD facilitated the extinction of morphine-induced CPP by modulating the polyamine binding site of the NMDA receptor. Our findings reveal important effects of SPD and ifenprodil on the re-exposure-induced decrease in morphine-induced CPP, which may be promising for developing novel pharmacological strategies to treat opioid use disorder.

Hypothalamic proteome changes in response to nicotine and its withdrawal are potentially associated with alteration in body weight

Nicotine, a major addictive component in tobacco, plays an important role in the changes of body weight upon smoking and its cessation. Here we showed that nicotine-treated mice exhibited weight loss and nicotine withdrawal led to weight gain. Using TMT-based proteomic analysis, we obtained the different hypothalamic protein profiles in response to nicotine and its withdrawal. A total of ~5000 proteins were identified from the hypothalamus with 50 altered proteins upon 28-day nicotine treatment and 28 altered proteins upon 15-day nicotine withdrawal. Of the altered proteins, CASP3, LCMT2, GRIN2D, CCNT2, FADS3 and MRPS18B were inversely changed in response to nicotine and withdrawal, coincidence with the change of body weight. Of them, CASP3, LCMT2, GRIN2D and CCNT2 were found to be associated with several GO terms and KEGG pathways linking with cell apoptosis, neurotransmission and metabolism. Further Western blot and RT-qPCR analyses confirmed that the levels of the 4 proteins CASP3, LCMT2, GRIN2D and CCNT2, instead of their mRNA transcripts, altered in response to nicotine and withdrawal. Thus this study provides nicotine- and withdrawal-induced hypothalamic protein profiles and suggests potential roles of these altered proteins in the change of body weight. SIGNIFICANCE: Cigarette smoking is one of important factors harming human health. Most smokers tend to have lower body weights and smoking cessation often lead to overweight or obesity, which is an important reason for smokers to insist on smoking. It is known that nicotine, a critical component in tobacco, is associated with the alteration in body weight by affecting hypothalamic function. Through TMT-based proteomic analysis, this study identified differential hypothalamic protein profiles in response to nicotine treatment and its withdrawal, and 4 nicotine- and withdrawal-induced contrary proteins CASP3, LCMT2, GRIN2D and CCNT2 are involved in several enriched GO terms and KEGG pathways, which are associated with cell apoptosis, neurotransmission and metabolism. Our study may provide novel targets for further investigation of the molecular mechanisms of nicotine- and withdrawal-induced alteration in body weight.

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.

Heterosynaptic GABAB Receptor Function within Feedforward Microcircuits Gates Glutamatergic Transmission in the Nucleus Accumbens Core

Complex circuit interactions within the nucleus accumbens (NAc) facilitate goal-directed behavior. Medium spiny neurons (MSNs) mediate NAc output by projecting to functionally divergent brain regions, a property conferred, in part, by the differential projection patterns of D1- and D2 dopamine receptor-expressing MSNs. Glutamatergic afferents to the NAc direct MSN output by recruiting feedforward inhibitory microcircuits comprised of parvalbumin (PV)-expressing interneurons (INs). Furthermore, the GABA_B heteroreceptor (GABA_BR), a G_i/o-coupled G-protein-coupled receptor, is expressed at glutamatergic synapses throughout the mesolimbic network, yet its physiological context and synaptic mechanism within the NAc remains unknown. Here, we explored GABA_BR function at glutamatergic synapses within PV-IN-embedded microcircuits in the NAc core of male mice. We found that GABA_BR is expressed presynaptically and recruits a noncanonical signaling mechanism to reduce glutamatergic synaptic efficacy at D1(+) and D1(-) (putative D2) MSN subtypes. Furthermore, PV-INs, a robust source of neuronal GABA in the NAc, heterosynaptically target GABA_BR to selectively modulate glutamatergic transmission onto D1(+) MSNs. These findings elucidate a new mechanism of feedforward inhibition and refine mechanisms by which GABA_B heteroreceptors modulate mesolimbic circuit function.SIGNIFICANCE STATEMENT Glutamatergic transmission in the nucleus accumbens (NAc) critically contributes to goal-directed behaviors. However, intrinsic microcircuit mechanisms governing the integration of these synapses remain largely unknown. Here, we show that parvalbumin-expressing interneurons within feedforward microcircuits heterosynaptically target GABA_B heteroreceptors (GABA_BR) on glutamate terminals. Activation of presynaptically-expressed GABA_BR decreases glutamatergic synaptic strength by engaging a non-canonical signaling pathway that interferes with vesicular exocytotic release machinery. These findings offer mechanistic insight into the role of GABA_B heteroreceptors within reward circuitry, elucidate a novel arm to feedforward inhibitory networks, and inform the growing use of GABA_BR-selective pharmacotherapy for various motivational disorders, including addiction, major depressive disorder, and autism (Cousins et al., 2002; Kahn et al., 2009; Jacobson et al., 2018; Stoppel et al., 2018; Pisansky et al., 2019).

Cell-type and projection-specific dopaminergic encoding of aversive stimuli in addiction

Drug addiction is a major public health concern across the world for which there are limited treatment options. In order to develop new therapies to correct the behavioral deficits that result from repeated drug use, we need to understand the neural circuit dysfunction that underlies the pathophysiology of the disorder. Because the initial reinforcing effects of drugs are dependent on increases in dopamine in reward-related brain regions such as the mesolimbic dopamine pathway, a large focus of addiction research has centered on the dysregulation of this system and its control of positive reinforcement and motivation. However, in addition to the processing of positive, rewarding stimuli, there are clear deficits in the encoding and valuation of information about potential negative outcomes and how they control decision making and motivation. Further, aversive stimuli can motivate or suppress behavior depending on the context in which they are encountered. We propose a model where rewarding and aversive information guides the execution of specific motivated actions through mesocortical and mesolimbic dopamine acting on D1- and D2- receptor containing neuronal populations. Volitional drug exposure alters the processing of rewarding and aversive stimuli through remodeling of these dopaminergic circuits, causing maladaptive drug seeking, self-administration in the face of negative consequences, and drug craving. Together, this review discusses the dysfunction of the circuits controlling different types of aversive learning as well as how these guide specific discrete behaviors, and provides a conceptual framework for how they should be considered in preclinical addiction models.

Metabotropic glutamate receptor subtype 3 gates acute stress-induced dysregulation of amygdalo-cortical function

Stress can precipitate or worsen symptoms of many psychiatric disorders by dysregulating glutamatergic function within the prefrontal cortex (PFC). Previous studies suggest that antagonists of group II metabotropic glutamate (mGlu) receptors (mGlu₂ and mGlu₃) reduce stress-induced anhedonia through actions in the PFC, but the mechanisms by which these receptors act are not known. We now report that activation of mGlu₃ induces long-term depression (LTD) of excitatory transmission in the PFC at inputs from the basolateral amygdala. Our data suggest mGlu₃-LTD is mediated by postsynaptic AMPAR internalization in PFC pyramidal cells, and we observed a profound impairment in mGlu₃-LTD following a single, 20-min restraint stress exposure. Finally, blocking mGlu₃ activation in vivo prevented the stress-induced maladaptive changes to amydalo-cortical physiology and motivated behavior. These data demonstrate that mGlu₃ mediates stress-induced physiological and behavioral impairments and further support the potential for mGlu₃ modulation as a treatment for stress-related psychiatric disorders.

Cocaine Blocks Effects of Hunger Hormone, Ghrelin, Via Interaction with Neuronal Sigma-1 Receptors

Despite ancient knowledge on cocaine appetite-suppressant action, the molecular basis of such fact remains unknown. Addiction/eating disorders (e.g., binge eating, anorexia, bulimia) share a central control involving reward circuits. However, we here show that the sigma-1 receptor (σ1R) mediates cocaine anorectic effects by interacting in neurons with growth/hormone/secretagogue (ghrelin) receptors. Cocaine increases colocalization of σ1R and GHS-R1a at the cell surface. Moreover, in transfected HEK-293T and neuroblastoma SH-SY5Y cells, and in primary neuronal cultures, pretreatment with cocaine or a σ1R agonist inhibited ghrelin-mediated signaling, in a similar manner as the GHS-R1a antagonist YIL-781. Results were similar in G protein-dependent (cAMP accumulation and calcium release) and in partly dependent or independent (ERK1/2 phosphorylation and label-free) assays. We provide solid evidence for direct interaction between receptors and the functional consequences, as well as a reliable structural model of the macromolecular σ1R-GHS-R1a complex, which arises as a key piece in the puzzle of the events linking cocaine consumption and appetitive/consummatory behaviors.

Ethanol Experience Enhances Glutamatergic Ventral Hippocampal Inputs to D1 Receptor-Expressing Medium Spiny Neurons in the Nucleus Accumbens Shell

A growing number of studies implicate alterations in glutamatergic signaling within the reward circuitry of the brain during alcohol abuse and dependence. A key integrator of glutamatergic signaling in the reward circuit is the nucleus accumbens, more specifically, the dopamine D1 receptor-expressing medium spiny neurons (D1-MSNs) within this region, which have been implicated in the formation of dependence to many drugs of abuse including alcohol. D1-MSNs receive glutamatergic input from several brain regions; however, it is not currently known how individual inputs onto D1-MSNs are altered by alcohol experience. Here, we investigate input-specific adaptations in glutamatergic transmission in response to varying levels of alcohol experience. Virally mediated expression of Channelrhodopsin in ventral hippocampal (vHipp) glutamate neurons of male mice allowed for selective activation of vHipp to D1-MSN synapses. Therefore, we were able to compare synaptic adaptations in response to low and high alcohol experience in vitro and in vivo Alcohol experience enhanced glutamatergic activity and abolished LTD at vHipp to D1-MSN synapses. Following chronic alcohol experience, GluA2-lacking AMPARs, which are Ca permeable, were inserted into vHipp to D1-MSN synapses. These findings support the reversal of alcohol-induced insertion of Ca-permeable AMPARs and the enhancement of glutamatergic activity at vHipp to D1-MSNs as potential targets for intervention during early exposure to alcohol.SIGNIFICANCE STATEMENT Given the roles of the nucleus accumbens (NAc) in integrating cortical and allocortical information and in reward learning, it is vital to understand how inputs to this region are altered by drugs of abuse such as alcohol. The strength of excitatory inputs from the ventral hippocampus (vHipp) to the NAc has been positively associated with reward-related behaviors, but it is unclear whether or how ethanol affects these inputs. Here we show that vHipp-NAc synapses indeed are altered by ethanol exposure, with vHipp glutamatergic input to the NAc being enhanced following chronic ethanol experience. This work provides insight into ethanol-induced alterations of vHipp-NAc synapses and suggests that, similarly to drugs such as cocaine, the strengthening of these synapses promotes reward behavior.

Cocaine-Induced Structural Plasticity in Input Regions to Distinct Cell Types in Nucleus Accumbens

BACKGROUND

The nucleus accumbens (NAc) is a brain region implicated in pathological motivated behaviors such as drug addiction and is composed predominantly of two discrete populations of neurons, dopamine receptor-1- and dopamine receptor-2-expressing medium spiny neurons (D1-MSNs and D2-MSNs, respectively). It is unclear whether these populations receive inputs from different brain areas and whether input regions to these cell types undergo distinct structural adaptations in response to the administration of addictive drugs such as cocaine.

METHODS

Using a modified rabies virus-mediated tracing method, we created a comprehensive brain-wide monosynaptic input map to NAc D1- and D2-MSNs. Next, we analyzed nearly 2000 dendrites and 125,000 spines of neurons across four input regions (the prelimbic cortex, medial orbitofrontal cortex, basolateral amygdala, and ventral hippocampus) at four separate time points during cocaine administration and withdrawal to examine changes in spine density in response to repeated intraperitoneal cocaine injection in mice.

RESULTS

D1- and D2-MSNs display overall similar input profiles, with the exception that D1-MSNs receive significantly more input from the medial orbitofrontal cortex. We found that neurons in distinct brain areas projecting to D1- and D2-MSNs display different adaptations in dendritic spine density at different stages of cocaine administration and withdrawal.

CONCLUSIONS

While NAc D1- and D2-MSNs receive input from similar brain structures, cocaine-induced spine density changes in input regions are quite distinct and dynamic. While previous studies have focused on input-specific postsynaptic changes within NAc MSNs in response to cocaine, these findings emphasize the dramatic changes that occur in the afferent input regions as well.

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.

Mechanisms underlying prelimbic prefrontal cortex mGlu3/mGlu5-dependent plasticity and reversal learning deficits following acute stress

Stress can precipitate or worsen symptoms of many psychiatric illnesses. Dysregulation of the prefrontal cortex (PFC) glutamate system may underlie these disruptions and restoring PFC glutamate signaling has emerged as a promising avenue for the treatment of stress disorders. Recently, we demonstrated that activation of metabotropic glutamate receptor subtype 3 (mGlu₃) induces a postsynaptic form of long-term depression (LTD) that is dependent on the activity of another subtype, mGlu₅. Stress exposure disrupted this plasticity, but the underlying signaling mechanisms and involvement in higher-order cognition have not yet been investigated. Acute stress was applied by 20-min restraint and early reversal learning was evaluated in an operant-based food-seeking task. We employed whole-cell patch-clamp recordings of layer 5 prelimbic (PL)-PFC pyramidal cells to examine mGlu₃-LTD and several mechanistically distinct mGlu₅-dependent functions. Acute stress impaired both mGlu₃-LTD and early reversal learning. Interestingly, potentiating mGlu₅ signaling with the mGlu₅ positive allosteric modulator (PAM) VU0409551 rescued stress-induced deficits in both mGlu₃-LTD and reversal learning. Other aspects of PL-PFC mGlu₅ function were not disrupted following stress; however, signaling downstream of mGlu₅-Homer interactions, phosphoinositide-3-kinase (PI3K), Akt, and glycogen synthase kinase 3β was implicated in these phenomena. These findings demonstrate that acute stress disrupts early reversal learning and PL-PFC-dependent synaptic plasticity and that potentiating mGlu₅ function can restore these impairments. These findings provide a framework through which modulating coordinated mGlu₃/mGlu₅ signaling may confer benefits for the treatment of stress-related psychiatric disorders.

Synaptic Plasticity in the Nucleus Accumbens: Lessons Learned from Experience

Synaptic plasticity contributes to behavioral adaptations. As a key node in the reward pathway, the nucleus accumbens (NAc) is important for determining motivation-to-action outcomes. Across animal models of motivation including addiction, depression, anxiety, and hedonic feeding, selective recruitment of neuromodulatory signals and plasticity mechanisms have been a focus of physiologists and behaviorists alike. Experience-dependent plasticity mechanisms within the NAc vary depending on the distinct afferents and cell-types over time. A greater understanding of molecular mechanisms determining how these changes in synaptic strength track with behavioral adaptations will provide insight into the process of learning and memory along with identifying maladaptations underlying pathological behavior. Here, we summarize recent findings detailing how changes in NAc synaptic strength and mechanisms of plasticity manifest in various models of motivational disorders.

Synaptic Plasticity in the Bed Nucleus of the Stria Terminalis: Underlying Mechanisms and Potential Ramifications for Reinstatement of Drug- and Alcohol-Seeking Behaviors

The bed nucleus of the stria terminalis (BNST) is a component of the extended amygdala that shows significant changes in activity and plasticity through chronic exposure to drugs and stress. The region is critical for stress- and cue-induced reinstatement of drug-seeking behaviors and is thus a candidate region for the plastic changes that occur in abstinence that prime addicted patients for reinstatement behaviors. Here, we discuss the various forms of long-term potentiation (LTP) and long-term depression (LTD) in the rodent BNST and highlight the way that these changes in excitatory transmission interact with exposure to alcohol and other drugs of abuse, as well as other stressors. In addition, we highlight potential areas for future research in this area, including investigating input- and cell-specific bidirectional changes in activity. As we continue to accrue foundational knowledge in the mechanisms and effects of plasticity in the BNST, molecular targets and treatment strategies that are relevant to reinstatement behaviors will also begin to emerge. Here, we briefly discuss the effects of catecholamine receptor modulators on synaptic plasticity in the BNST due to the role of norepinephrine in LTD and dopamine on the short-term component of LTP as well as the role that signaling at these receptors plays in reinstatement of drug- and alcohol-seeking behaviors. We hope that insights gained on the specific changes in plasticity that occur within the BNST during abstinence from alcohol and other drugs of abuse will provide insight into the biological underpinnings of relapse behavior in human addicts and inform future treatment modalities for addiction that tackle this complex biological problem.

mGlu1 and mGlu5 modulate distinct excitatory inputs to the nucleus accumbens shell

Glutamatergic transmission in the nucleus accumbens shell (NAcSh) is a substrate for reward learning and motivation. Metabotropic glutamate (mGlu) receptors regulate NAcSh synaptic strength by inducing long-term depression (LTD). Inputs from prefrontal cortex (PFC) and medio-dorsal thalamus (MDT) drive opposing motivated behaviors yet mGlu receptor regulation of these synapses is unexplored. We examined Group I mGlu receptor regulation of PFC and MDT glutamatergic synapses onto specific populations of NAc medium spiny neurons (MSNs) using D1tdTom BAC transgenic mice and optogenetics. Synaptically evoked long-term depression (LTD) at MDT-NAcSh synapses required mGlu₅ but not mGlu₁ and was specific for D1(+) MSNs, whereas PFC LTD was expressed at both D1(+) and D1(-) MSNs and required mGlu₁ but not mGlu₅. Two weeks after five daily non-contingent cocaine exposures (15 mg/kg), LTD was attenuated at MDT-D1(+) synapses but was rescued by the mGlu5-positive allosteric modulator (PAM) VU0409551. These results highlight unique plasticity mechanisms regulating specific NAcSh synapses.

Do Alcohol-Related AMPA-Type Glutamate Receptor Adaptations Promote Intake?

Ionotropic glutamate receptors (AMPA, NMDA, and kainate receptors) play a central role in excitatory glutamatergic signaling throughout the brain. As a result, functional changes, especially long-lasting forms of plasticity, have the potential to profoundly alter neuronal function and the expression of adaptive and pathological behaviors. Thus, alcohol-related adaptations in ionotropic glutamate receptors are of great interest, since they could promote excessive alcohol consumption, even after long-term abstinence. Alcohol- and drug-related adaptations in NMDARs have been recently reviewed, while less is known about kainate receptor adaptations. Thus, we focus here on functional changes in AMPARs, tetramers composed of GluA1-4 subunits. Long-lasting increases or decreases in AMPAR function, the so-called long-term potentiation or depression, have widely been considered to contribute to normal and pathological memory states. In addition, a great deal has been learned about the acute regulation of AMPARs by signaling pathways, scaffolding and auxiliary proteins, intracellular trafficking, and other mechanisms. One important common adaptation is a shift in AMPAR subunit composition from GluA2-containing, calcium-impermeable AMPARs (CIARs) to GluA2-lacking, calcium-permeable AMPARs (CPARs), which is observed under a broad range of conditions including intoxicant exposure or intake, stress, novelty, food deprivation, and ischemia. This shift has the potential to facilitate AMPAR currents, since CPARs have much greater single-channel currents than CIARs, as well as faster AMPAR activation kinetics (although with faster inactivation) and calcium-related activity. Many tools have been developed to interrogate particular aspects of AMPAR signaling, including compounds that selectively inhibit CPARs, raising exciting translational possibilities. In addition, recent studies have used transgenic animals and/or optogenetics to identify AMPAR adaptations in particular cell types and glutamatergic projections, which will provide critical information about the specific circuits that CPARs act within. Also, less is known about the specific nature of alcohol-related AMPAR adaptations, and thus we use other examples that illustrate more fully how particular AMPAR changes might influence intoxicant-related behavior. Thus, by identifying alcohol-related AMPAR adaptations, the specific molecular events that underlie them, and the cells and projections in which they occur, we hope to better inform the development of new therapeutic interventions for addiction.

Rapid Synaptogenesis in the Nucleus Accumbens Is Induced by a Single Cocaine Administration and Stabilized by Mitogen-Activated Protein Kinase Interacting Kinase-1 Activity

BACKGROUND

Repeated cocaine exposure produces new spine formation in striatal projection neurons (SPNs) of the nucleus accumbens. However, an acute exposure to cocaine can trigger long-lasting synaptic plasticity in SPNs leading to behavioral alterations. This raises the intriguing question as to whether a single administration of cocaine could enduringly modify striatal connectivity.

METHODS

A three-dimensional morphometric analysis of presynaptic glutamatergic boutons and dendritic spines was performed on SPNs 1 hour and 1 week after a single cocaine administration. Time-lapse two-photon microscopy in adult slices was used to determine the precise molecular-events sequence responsible for the rapid spine formation.

RESULTS

A single injection triggered a rapid synaptogenesis and persistent increase in glutamatergic connectivity in SPNs from the shell part of the nucleus accumbens, specifically. Synapse formation occurred through clustered growth of active spines contacting pre-existing axonal boutons. Spine growth required extracellular signal-regulated kinase activation, while spine stabilization involved transcription-independent protein synthesis driven by mitogen-activated protein kinase interacting kinase-1, downstream from extracellular signal-regulated kinase. The maintenance of new spines driven by mitogen-activated protein kinase interacting kinase-1 was essential for long-term connectivity changes induced by cocaine in vivo.

CONCLUSIONS

Our study originally demonstrates that a single administration of cocaine is able to induce stable synaptic rewiring in the nucleus accumbens, which will likely influence responses to subsequent drug exposure. It also unravels a new functional role for cocaine-induced extracellular signal-regulated kinase pathway independently of nuclear targets. Finally, it reveals that mitogen-activated protein kinase interacting kinase-1 has a pivotal role in cocaine-induced connectivity.

Functional roles of orexin/hypocretin receptors in reward circuit

Since its first discovery in 1998, it has become clear that the orexinergic system plays an important role in regulating a number of functions including food, sex, social connections, and most prominently reward-related behaviors. Orexinergic neurons in the lateral hypothalamus project extensively to other brain areas, two most important of which are the ventral tegmental area and the nucleus accumbens that are involved in reward processing. In this review, we have presented the work in our laboratory along with the work of others and have discussed the possible functions we can infer from the research. We discuss the anatomy of the orexinergic system and its components followed by a presentation of other connected brain areas. The second part of this review discusses observed results from the morphine conditioned place preference test that sheds light on the possible role of the involved areas in reward processing. The complex circuits involved in reward processing are only beginning to be understood and we need to deepen our understanding regarding the nature of the interactions between all brain areas involved.

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.

Chemogenetic inhibition reveals midline thalamic nuclei and thalamo-accumbens projections mediate cocaine-seeking in rats

Drug addiction is a chronic disease that is shaped by alterations in neuronal function within the cortical-basal ganglia-thalamic circuit. However, our understanding of how this circuit regulates drug-seeking remains incomplete, and relapse rates remain high. The midline thalamic nuclei are an integral component of the cortical-basal ganglia-thalamic circuit and are poised to mediate addiction behaviors, including relapse. It is surprising that little research has examined the contribution of midline thalamic nuclei and their efferent projections in relapse. To address this, we expressed inhibitory, G_i/o -coupled DREADDs (Designer Receptors Exclusively Activated by Designer Drugs) in a subset of the midline thalamic nuclei or in midline thalamic nuclei neurons projecting to either the nucleus accumbens or the amygdala. We examined the effect of transiently decreasing activity of these neuronal populations on cue-induced and cocaine-primed reinstatement of cocaine-seeking. Reducing activity of midline thalamic nuclei neurons attenuated both cue-induced and cocaine-primed reinstatement, but had no effect on cue-induced reinstatement of sucrose-seeking or locomotor activity. Interestingly, attenuating activity of efferent projections from the anterior portion of midline thalamic nuclei to the nucleus accumbens blocked cocaine-primed reinstatement but enhanced cue-induced reinstatement. Decreasing activity of efferent projections from either the posterior midline thalamic nuclei to the nucleus accumbens or the midline thalamic nuclei to amygdala had no effect. These results reveal a novel contribution of subsets of midline thalamic nuclei neurons in drug-seeking behaviors and suggest that modulation of midline thalamic nuclei activity may be a promising therapeutic target for preventing relapse.

Reward Circuitry in Addiction

Understanding the brain circuitry that underlies reward is critical to improve treatment for many common health issues, including obesity, depression, and addiction. Here we focus on insights into the organization and function of reward circuitry and its synaptic and structural adaptations in response to cocaine exposure. While the importance of certain circuits, such as the mesocorticolimbic dopamine pathway, are well established in drug reward, recent studies using genetics-based tools have revealed functional changes throughout the reward circuitry that contribute to different facets of addiction, such as relapse and craving. The ability to observe and manipulate neuronal activity within specific cell types and circuits has led to new insight into not only the basic connections between brain regions, but also the molecular changes within these specific microcircuits, such as neurotrophic factor and GTPase signaling or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor function, that underlie synaptic and structural plasticity evoked by drugs of abuse. Excitingly, these insights from preclinical rodent work are now being translated into the clinic, where transcranial magnetic simulation and deep brain stimulation therapies are being piloted in human cocaine dependence. Thus, this review seeks to summarize current understanding of the major brain regions implicated in drug-related behaviors and the molecular mechanisms that contribute to altered connectivity between these regions, with the postulation that increased knowledge of the plasticity within the drug reward circuit will lead to new and improved treatments for addiction.

Viral strategies for targeting cortical circuits that control cocaine-taking and cocaine-seeking in rodents

Addiction to cocaine is a chronic disease characterized by persistent drug-taking and drug-seeking behaviors, and a high likelihood of relapse. The prefrontal cortex (PFC) has long been implicated in the development of cocaine addiction, and relapse. However, the PFC is a heterogeneous structure, and understanding the role of PFC subdivisions, cell types and afferent/efferent connections is critical for gaining a comprehensive picture of the contribution of the PFC in addiction-related behaviors. Here we provide an update on the role of the PFC in cocaine addiction from recent work that used viral-mediated optogenetic and chemogenetic tools to study the role of the PFC in drug-taking and drug-seeking behavior in rodents. Following overviews of rodent PFC neuroanatomy and of viral-mediated optogenetic and chemogenetic techniques, we review studies of manipulations within the PFC, followed by a review of work that utilized targeted manipulations to PFC inputs and outputs.

Alcohol induces input-specific aberrant synaptic plasticity in the rat dorsomedial striatum

Accumulated evidence suggests that the dorsomedial striatum (DMS) of the basal ganglia plays an essential role in pathological excessive alcohol consumption. The DMS receives multiple glutamatergic inputs. However, whether and how alcohol consumption distinctly affects these excitatory afferents to the DMS remains unknown. Here, we used optogenetics to selectively activate the rat medial prefrontal cortex (mPFC) and basolateral amygdala (BLA) inputs in DMS slices, and measured the effects of alcohol consumption on glutamatergic transmission in these corticostriatal and amygdalostriatal circuits. We found that excessive alcohol consumption increased AMPA receptor- and NMDA receptor (NMDAR)-mediated neurotransmission, as well as the GluN2B/NMDAR ratio, at the corticostriatal input to the DMS. The probability of glutamate release was increased selectively at the amygdalostriatal input. Interestingly, we discovered that paired activation of the mPFC and BLA inputs using dual-channel optogenetics induced robust long-term potentiation (LTP) of the corticostriatal input to the DMS. Taken together, these results indicate that excessive alcohol consumption potentiates glutamatergic transmission via a postsynaptic mechanism for the corticostriatal input and via a presynaptic mechanism for the amygdalostriatal input. These changes may in turn contribute to pathological alcohol consumption.