Max E. Joffe

Biological substrates of addiction

This review is an introduction to addiction, the reward circuitry, and laboratory addiction models. Addiction is a chronic disease hallmarked by a state of compulsive drug seeking that persists despite negative consequences. Most of the advances in addiction research have centered on the canonical and contemporary drugs of abuse; however, addictions to other activities and stimuli also exist. Substances of abuse have the potential to induce long-lasting changes in the brain at the behavioral, circuit, and synaptic levels. Addiction-related behavioral changes involve initiation, escalation, and obsession to drug seeking and much of the current research is focused on mapping these manifestations to specific neural pathways. Drug abuse is well known to recruit components of the mesolimbic dopamine system, including the nucleus accumbens and ventral tegmental area. In addition, altered function of a wide variety of brain regions is tightly associated with specific manifestations of drug abuse. These regions peripheral to the mesolimbic pathway likely play a role in specific observed comorbidities and endophenotypes that can facilitate, or be caused by, substance abuse. Alterations in synaptic structure, function, and connectivity, as well as epigenetic and genetic mechanisms are thought to underlie the pathologies of addiction. In preclinical models, these persistent changes are studied at the levels of molecular pharmacology and biochemistry, ex vivo and in vivo electrophysiology, radiography, and behavior. Coordinating research efforts across these disciplines and examining cell type- and circuit-specific phenomena are crucial components for translating preclinical findings to viable medical interventions that effectively treat addiction and related disorders. WIREs Cogn Sci 2014, 5:151-171. doi: 10.1002/wcs.1273 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

Partial mGlu 5 Negative Allosteric Modulators Attenuate Cocaine-Mediated Behaviors and Lack Psychotomimetic-Like Effects

Cocaine abuse remains a public health concern for which pharmacotherapies are largely ineffective. Comorbidities between cocaine abuse, depression, and anxiety support the development of novel treatments targeting multiple symptom clusters. Selective negative allosteric modulators (NAMs) targeting the metabotropic glutamate receptor 5 (mGlu5) subtype are currently in clinical trials for the treatment of multiple neuropsychiatric disorders and have shown promise in preclinical models of substance abuse. However, complete blockade or inverse agonist activity by some full mGlu5 NAM chemotypes demonstrated adverse effects, including psychosis in humans and psychotomimetic-like effects in animals, suggesting a narrow therapeutic window. Development of partial mGlu5 NAMs, characterized by their submaximal but saturable levels of blockade, may represent a novel approach to broaden the therapeutic window. To understand potential therapeutic vs adverse effects in preclinical behavioral assays, we examined the partial mGlu5 NAMs, M-5MPEP and Br-5MPEPy, in comparison with the full mGlu5 NAM MTEP across models of addiction and psychotomimetic-like activity. M-5MPEP, Br-5MPEPy, and MTEP dose-dependently decreased cocaine self-administration and attenuated the discriminative stimulus effects of cocaine. M-5MPEP and Br-5MPEPy also demonstrated antidepressant- and anxiolytic-like activity. Dose-dependent effects of partial and full mGlu5 NAMs in these assays corresponded with increasing in vivo mGlu5 occupancy, demonstrating an orderly occupancy-to-efficacy relationship. PCP-induced hyperlocomotion was potentiated by MTEP, but not by M-5MPEP and Br-5MPEPy. Further, MTEP, but not M-5MPEP, potentiated the discriminative-stimulus effects of PCP. The present data suggest that partial mGlu5 NAM activity is sufficient to produce therapeutic effects similar to full mGlu5 NAMs, but with a broader therapeutic index.

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

BACKGROUND Excitatory synaptic transmission in the nucleus accumbens (NAc) is a key biological substrate underlying behavioral responses to psychostimulants and susceptibility to relapse. Studies have demonstrated that cocaine induces changes in glutamatergic signaling at distinct inputs to the NAc. However, consequences of cocaine experience on synaptic transmission from the midline nuclei of the thalamus (mThal) to the NAc have yet to be reported. METHODS To examine synapses from specific NAc core inputs, we recorded light-evoked excitatory postsynaptic currents following viral-mediated expression of channelrhodopsin-2 in the mThal, prefrontal cortex (PFC), or basolateral amygdala from acute brain slices. To identify NAc medium spiny neuron subtypes, we used mice expressing tdTomato driven by the promoter for dopamine receptor subtype 1 (D1). We recorded N-methyl-D-aspartate receptor (NMDAR) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) properties to evaluate synaptic adaptations induced by cocaine experience, a 5-day cocaine exposure followed by 2 weeks of abstinence. RESULTS Excitatory inputs to the NAc core displayed differential NMDAR properties, and cocaine experience uniquely altered AMPAR and NMDAR properties at mThal-D1(+), mThal-D1(-), and PFC-D1(+) synapses, but not at PFC-D1(-) synapses. Finally, at mThal-D1(+) synapses, cocaine enhanced GluN2C/D function and NMDAR-dependent synaptic plasticity. CONCLUSIONS Our results identify contrasting cocaine-induced AMPAR and NMDAR modifications at mThal-NAc and PFC-NAc core synapses. These changes include an enhancement of NMDAR function and plasticity at mThal-D1(+) synapses. Incorporation of GluN2C/D-containing NMDARs most likely underlies these phenomena and represents a potential therapeutic target for psychostimulant use disorders.

Functional partnership between mGlu3 and mGlu5 metabotropic glutamate receptors in the central nervous system

ABSTRACT mGlu5 receptors are involved in mechanisms of activity‐dependent synaptic plasticity, and are targeted by drugs developed for the treatment of CNS disorders. We report that mGlu3 receptors, which are traditionally linked to the control of neurotransmitter release, support mGlu5 receptor signaling in neurons and largely contribute to the robust mGlu5 receptor‐mediated polyphosphoinositide hydrolysis in the early postnatal life. In cortical pyramidal neurons, mGlu3 receptor activation potentiated mGlu5 receptor‐mediated somatic Ca2+ mobilization, and mGlu3 receptor‐mediated long‐term depression in the prefrontal cortex required the endogenous activation of mGlu5 receptors. The interaction between mGlu3 and mGlu5 receptors was also relevant to mechanisms of neuronal toxicity, with mGlu3 receptors shaping the influence of mGlu5 receptors on excitotoxic neuronal death. These findings shed new light into the complex role played by mGlu receptors in physiology and pathology, and suggest reconsideration of some of the current dogmas in the mGlu receptor field. HIGHLIGHTSmGlu5 and mGlu3 receptors functionally interact in the central nervous system.mGlu3 receptors support mGlu5 receptor signaling in the early postnatal life.mGlu3 receptor activation potentiate mGlu5 receptor‐mediated somatic Ca2+ mobilization.mGlu3 and mGlu5 receptors interact to induce LTD in the prefrontal cortex.This interaction is relevant to mechanisms of neuronal toxicity.

Nuclear Magnetic Resonance Methods for Metabolic Fluxomics

Fluxomics, through its core methodology of metabolic flux analysis (MFA), enables quantification of carbon traffic through cellular biochemical pathways. Isotope labeling experiments aid MFA by providing information on intracellular fluxes, especially through parallel and cyclic pathways. Nuclear magnetic resonance (NMR) and mass spectrometry (MS) are two complementary methods to measure abundances of isotopomers generated in these experiments. 2-D [(13)C, (1)H] heteronuclear correlation NMR spectra can detect (13)C isotopes coupled to protons and thus noninvasively separate molecules and atoms with a specific isotopic content from a mixture of molecular species. Furthermore, the fine structures of the peaks in these spectra can reveal scalar couplings between chemically bonded carbon atoms in the sample, from which isotopomer abundances can be quantified. This chapter introduces methods for NMR sample preparation and spectral acquisition of 2-D [(13)C, (1)H] correlation maps, followed by a detailed presentation of methods to process the spectra and quantify isotopomer abundances. We explain the use of the software NMRViewJ for spectral visualization and processing, as well as MATLAB scripts developed by us for peak extraction, deconvolution of overlapping peaklets, and isotopomer abundance quantification. Finally, we discuss the applications of NMR-derived isotopomer data toward quantitatively understanding metabolic pathways.

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

ABSTRACT N‐methyl‐d‐aspartate receptors (NMDARs) are profound regulators of glutamate neurotransmission and behavior. To coordinate components of the limbic system, the dorsal and ventral striatum integrate cognitive and emotional information towards the execution of complex behaviors. Striatal outflow is conveyed by medium spiny neurons (MSNs), which can be dichotomized by expression of dopamine receptor subtype 1 (D1) or adenosine receptor subtype 2A (A2A). To examine how striatal NMDAR function modulates reward‐related behaviors, we generated D1‐ and A2A‐specific genetic deletions of the obligatory GluN1 subunit. Interestingly, we observed no differences in any GluN1−/− genotype in reward learning as assessed by acquisition or extinction of cocaine conditioned place preference (CPP). Control and A2A‐GluN−/− mice exhibited robust cocaine‐primed reinstatement, however this behavior was markedly absent in D1‐GluN−/− mice. Interestingly, dual D1‐/A2A‐GluN−/− mice displayed an intermediate reinstatement phenotype. Next, we examined models of exploration, anxiety, and despair, states often associated with relapse to addiction‐related behavior, to determine NMDAR contribution in D1 and A2A cell types to these behaviors. D1‐GluN1−/− mice displayed aberrant exploratory locomotion in a novel environment, but the phenotype was absent in dual D1/A2A‐GluN1−/− mice. In contrast A2A‐GluN1−/− mice displayed a despair‐resistant phenotype, and this phenotype persisted in dual D1/A2A‐GluN−/− mice. These data support the hypothesis that cell type‐specific NMDAR signaling regulates separable behavioral outcomes related to locomotion, despair, and relapse. This article is part of the Special Issue entitled ‘Ionotropic glutamate receptors’. HIGHLIGHTSAcquisition of cocaine CPP remains intact in D1‐GluN1−/− and A2A‐GluN1−/− mice.D1‐GluN1−/− mice do not reinstate CPP following cocaine challenge.D1‐GluN1−/− mice display abnormal exploration; rescued by A2A‐GluN1−/− co‐deletion.A2A‐GluN1−/− mice and D1/A2A‐GluN1−/− mice resist behavioral despair.

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.

Metabotropic Glutamate Receptors in Alcohol Use Disorder: Physiology, Plasticity, and Promising Pharmacotherapies

Developing efficacious treatments for alcohol use disorder (AUD) has proven difficult. The insidious nature of the disease necessitates a deep understanding of its underlying biology as well as innovative approaches to ameliorate ethanol-related pathophysiology. Excessive ethanol seeking and relapse are generated by long-term changes to membrane properties, synaptic physiology, and plasticity throughout the limbic system and associated brain structures. Each of these factors can be modulated by metabotropic glutamate (mGlu) receptors, a diverse set of G protein-coupled receptors highly expressed throughout the central nervous system. Here, we discuss how different components of the mGlu receptor family modulate neurotransmission in the limbic system and other brain regions involved in AUD etiology. We then describe how these processes are dysregulated following ethanol exposure and speculate about how mGlu receptor modulation might restore such pathophysiological changes. To that end, we detail the current understanding of the behavioral pharmacology of mGlu receptor-directed drug-like molecules in animal models of AUD. Together, this review highlights the prominent position of the mGlu receptor system in the pathophysiology of AUD and provides encouragement that several classes of mGlu receptor modulators may be translated as viable treatment options.

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

&NA; 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 3&bgr; 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. HIGHLIGHTSPL‐PFC mGlu3/mGlu5‐LTD requires PI3K‐Akt signaling, and not Ca2+ mobilization.Acute stress disrupts mGlu3/mGlu5‐LTD without a gross impairment to mGlu5 function.Potentiating mGlu5 rescues LTD deficit following acute restraint stress.Potentiating mGlu5 ameliorates reversal learning deficit following acute stress.

T39. NEURAL MECHANISMS OF METABOTROPIC GLUTAMATE RECEPTOR 3 MEDIATED ENHANCEMENT OF SYNAPTIC PLASTICITY AND COGNITION

Abstract Background The group II metabotropic glutamate receptor 3 (mGlu3) is an emerging therapeutic target for schizophrenia, as research has demonstrated a link between mutations in the human gene encoding for mGlu3, GRM3, and clinical diagnosis of schizophrenia. Schizophrenia is known to be accompanied by debilitating cognitive impairments that negatively impact the overall quality of life of the patient. While current pharmacological therapeutics mainly target the positive symptoms, cognitive symptoms are often not effectively treated. Our recent discovery that mGlu3 and mGlu5 can act as signaling partners to modulate synaptic plasticity in the prefrontal cortex led us to hypothesize that mGlu3 may subserve similar functions to those of mGlu5 during hippocampal synaptic plasticity and hippocampal-dependent behaviors. Methods We directly tested this hypothesis using acute slice electrophysiology to investigate basal synaptic transmission as well as long-term plasticity in hippocampal slices. To test cognition, the associative fear learning behavioral assay, termed trace-fear conditioning, was used. C57bl/6 mice or CaMKII-cre;mGlu5-/- mice were used in all studies. Results We report that mGlu2/3 activation enhances hippocampal theta-burst (TBS)-induced LTP but was without effect on group I mGlu agonist-induced LTD The group II mGlu agonist enhancement of TBS-LTP was blocked by antagonists of mGlu3 or mGlu5. We next tested downstream mechanisms of group II mGlu induced LTP by chemically activating LTP with the group II agonist LY379268 in combination with selective antagonists. We verified the LTP was induced by mGlu3 activation but not mGlu2 using selective negative allosteric modulators of each subtype. Furthermore, mGlu5 negative allosteric modulation with MTEP blocked mGlu3-LTP, and conversely the mGlu5 positive allosteric modulator, VU0092273, enhanced mGlu3-LTP. The cannabinoid receptor type 1 antagonist AM251 was also capable of blocking mGlu3-LTP, suggesting cannabinoid signaling mechanistically drives this LTP. Having confirmed a role for mGlu5 in the mGlu3-LTP, we next verified that postsynaptic mGlu5 located on pyramidal neurons was necessary for mGlu3-LTP by utilizing CaMKII-cre;mGlu5-/- mice. It was found that hippocampal slices from these mice showed no enhancement of LTP when LY379268 was bath applied alone or in combination with TBS-stimulation. Behaviorally, we discovered that selective activation of mGlu3 by systemically injecting the group II mGlu agonist in combination with a selective mGlu2 negative allosteric modulator, VU6001966, causes an enhancement in the acquisition of trace-fear conditioning learning. This was also confirmed to be dependent on mGlu5 as both systemic pharmacological inhibition or genetic deletion of mGlu5 abolished this learning enhancement. Further testing of the ability of mGlu3 activation to augment other cognitive tasks is currently underway. Discussion These results taken together demonstrate mGlu3 enhances hippocampal LTP and hippocampal-dependent learning through mechanisms that involve both mGlu5 and CB1 receptor activation. This work provides a basic biological mechanism and preclinical therapeutic validation for mGlu3 as a target for neurological disorders in which cognition is disrupted such as schizophrenia.