Cocaine-induced adaptations in metabotropic inhibitory signaling in the mesocorticolimbic system

Region-specific alterations in the expression and phosphorylation of NMDA receptor subunits in the rat prefrontal cortex and dorsal striatum accompanying behavioral sensitization induced by cocaine and ethanol

Behavioral sensitization is defined as the enhanced behavioral response to drugs of abuse after repeated exposure, which can serve as a behavioral model of addiction. Our previous study demonstrated that behavioral cross-sensitization occurs between cocaine and ethanol, suggesting commonalities between these drugs. N-methyl-d-aspartate (NMDA) receptors play important roles in synaptic plasticity, learning, memory, and addiction-associated behaviors. However, little is known about whether NMDA receptor-mediated signaling regulation is a common feature following behavioral sensitizations induced by cocaine and ethanol. Thus, the present study examined the expression of phospho-S896-NR1, NR2A, and NR2B subunits in the prefrontal cortex and dorsal striatum following reciprocal cross-sensitization between cocaine and ethanol. We also examined the mRNA expression of the NR2A and NR2B subunits. In the ethanol-sensitized state, phosphorylation of NR1 and expression of NR2A and NR2B subunits were increased in both the prefrontal cortex and dorsal striatum. In the cocaine-sensitized state, phosphorylation of NR1 and expression of the NR2A and NR2B subunits were increased in the prefrontal cortex but not in the dorsal striatum. Corresponding changes in mRNA expression were observed in the ethanol-sensitized state but not in the cocaine-sensitized state. Acute treatment with either cocaine or ethanol had no effect on the phosphorylation and expression of NMDA receptor subunits in either the prefrontal cortex or dorsal striatum, regardless of the sensitization state. These results indicate a partially overlapping neural mechanism for cocaine and ethanol that may induce the development of behavioral sensitization.

RGS6 negatively regulates inhibitory G protein signaling in dopamine neurons and positively regulates binge-like alcohol consumption in mice

BACKGROUND AND PURPOSE

Drugs of abuse, including alcohol, increase dopamine in the mesocorticolimbic system via actions on dopamine neurons in the ventral tegmental area (VTA). Increased dopamine transmission can activate inhibitory G protein signalling pathways in VTA dopamine neurons, including those controlled by GABAB and D2 receptors. Members of the R7 subfamily of regulator of G protein signalling (RGS) proteins can regulate inhibitory G protein signalling, but their influence on VTA dopamine neurons is unclear. Here, we investigated the influence of RGS6, an R7 RGS family memberthat has been implicated in the regulation of alcohol consumption in mice, on inhibitory G protein signalling in VTA dopamine neurons.

EXPERIMENTAL APPROACH

We used molecular, electrophysiological and genetic approaches to probe the impact of RGS6 on inhibitory G protein signalling in VTA dopamine neurons and on binge-like alcohol consumption in mice.

KEY RESULTS

RGS6 is expressed in adult mouse VTA dopamine neurons and it modulates inhibitory G protein signalling in a receptor-dependent manner, tempering D2 receptor-induced somatodendritic currents and accelerating deactivation of synaptically evoked GABAB receptor-dependent responses. RGS6-/- mice exhibit diminished binge-like alcohol consumption, a phenotype replicated in female (but not male) mice lacking RGS6 selectively in VTA dopamine neurons.

CONCLUSIONS AND IMPLICATIONS

RGS6 negatively regulates GABAB - and D2 receptor-dependent inhibitory G protein signalling pathways in mouse VTA dopamine neurons and exerts a sex-dependent positive influence on binge-like alcohol consumption in adult mice. As such, RGS6 may represent a new diagnostic and/or therapeutic target for alcohol use disorder.

Neuronal G protein-gated K+ channels

G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels exert a critical inhibitory influence on neurons. Neuronal GIRK channels mediate the G protein-dependent, direct/postsynaptic inhibitory effect of many neurotransmitters including γ-aminobutyric acid (GABA), serotonin, dopamine, adenosine, somatostatin, and enkephalin. In addition to their complex regulation by G proteins, neuronal GIRK channel activity is sensitive to PIP₂, phosphorylation, regulator of G protein signaling (RGS) proteins, intracellular Na⁺ and Ca²⁺, and cholesterol. The application of genetic and viral manipulations in rodent models, together with recent progress in the development of GIRK channel modulators, has increased our understanding of the physiological and behavioral impact of neuronal GIRK channels. Work in rodent models has also revealed that neuronal GIRK channel activity is modified, transiently or persistently, by various stimuli including exposure drugs of abuse, changes in neuronal activity patterns, and aversive experience. A growing body of preclinical and clinical evidence suggests that dysregulation of GIRK channel activity contributes to neurological diseases and disorders. The primary goals of this review are to highlight fundamental principles of neuronal GIRK channel biology, mechanisms of GIRK channel regulation and plasticity, the nascent landscape of GIRK channel pharmacology, and the potential relevance of GIRK channels to the pathophysiology and treatment of neurological diseases and disorders.

Infralimbic cortex pyramidal neuron GIRK signaling contributes to regulation of cognitive flexibility but not affect-related behavior in male mice

Dysfunction of the infralimbic cortical (ILC) region of the medial prefrontal cortex (mPFC) is thought to be an underlying factor in both affect- and cognition-related behavioral deficits that co-occur across neuropsychiatric disorders. Increasing evidence highlights pathological imbalances in prefrontal pyramidal neuron excitability and associated aberrant firing as an underlying factor in this dysfunction. G protein-gated inwardly rectifying K⁺ (GIRK/Kir3) channels mediate excitability of mPFC pyramidal neurons, however the functional role of these channels in ILC-dependent regulation of behavior and pyramidal neuron excitation is unknown. The present study used a viral-cre approach in male mice harboring a 'floxed' version of the kcnj3 (Girk1) gene, to disrupt GIRK1-containing channel expression in pyramidal neurons within the ILC. Loss of GIRK1-dependent signaling increased excitability and spike firing of pyramidal neurons but did not alter affective behavior measured in an elevated plus maze, forced swim test, or progressive ratio test of motivation. Alternatively, ablation of GIRK1 impaired performance in an operant-based attentional set-shifting task designed to assess cognitive flexibility. These data highlight a unique role for GIRK1 signaling in ILC pyramidal neurons in the regulation of strategy shifting but not affect and suggest that these channels may represent a therapeutic target for treatment of cognitive deficits in neuropsychiatric disease.

Membrane-Mediated Activity of Local Anesthetics

The activity of local anesthetics (LAs) has been attributed to the inhibition of ion channels, causing anesthesia. However, there is a growing body of research showing that LAs act on a wide range of receptors and channel proteins far beyond simple analgesia. The current concept of ligand recognition may no longer explain the multitude of protein targets influenced by LAs. We hypothesize that LAs can cause anesthesia without directly binding to the receptor proteins just by changing the physical properties of the lipid bilayer surrounding these proteins and ion channels based on LAs' amphiphilicity. It is possible that LAs act in one of the following ways: They 1) dissolve raft-like membrane microdomains, 2) impede nerve impulse propagation by lowering the lipid phase transition temperature, or 3) modulate the lateral pressure profile of the lipid bilayer. This could also explain the numerous additional effects of LAs besides anesthesia. Furthermore, the concepts of membrane-mediated activity and binding to ion channels do not have to exclude each other. If we were to consider LA as the middle part of a continuum between unspecific membrane-mediated activity on one end and highly specific ligand binding on the other end, we could describe LA as the link between the unspecific action of general anesthetics and toxins with their highly specific receptor binding. This comprehensive membrane-mediated model offers a fresh perspective to clinical and pharmaceutical research and therapeutic applications of local anesthetics. SIGNIFICANCE STATEMENT: Local anesthetics, according to the World Health Organization, belong to the most important drugs available to mankind. Their rediscovery as therapeutics and not only anesthetics marks a milestone in global pain therapy. The membrane-mediated mechanism of action proposed in this review can explain their puzzling variety of target proteins and their thus far inexplicable therapeutic effects. The new concept presented here places LAs on a continuum of structures and molecular mechanisms in between small general anesthetics and the more complex molecular toxins.

Drugs of Abuse Differentially Alter the Neuronal Excitability of Prefrontal Layer V Pyramidal Cell Subtypes

The medial prefrontal cortex (mPFC) plays an important role in regulating executive functions including reward seeking, task flexibility, and compulsivity. Studies in humans have demonstrated that drugs of abuse, including heroin, cocaine, methamphetamine, and alcohol, alter prefrontal function resulting in the consequential loss of inhibitory control and increased compulsive behaviors, including drug seeking. Within the mPFC, layer V pyramidal cells, which are delineated into two major subtypes (type I and type II, which project to subcortical or commissurally to other cortical regions, respectively), serve as the major output cells which integrate information from other cortical and subcortical regions and mediate executive control. Preclinical studies examining changes in cellular physiology in the mPFC in response to drugs of abuse, especially in regard to layer V pyramidal subtypes, are relatively sparse. In the present study, we aimed to explore how heroin, cocaine, methamphetamine, ethanol, and 3,4-methylenedioxypyrovalerone (MDPV) alter the baseline cellular physiology and excitability properties of layer V pyramidal cell subtypes. Specifically, animals were exposed to experimenter delivered [intraperitoneal (i.p.)] heroin, cocaine, the cocaine-like synthetic cathinone MDPV, methamphetamine, ethanol, or saline as a control once daily for five consecutive days. On the fifth day, whole-cell physiology recordings were conducted from type I and type II layer V pyramidal cells in the mPFC. Changes in cellular excitability, including rheobase (i.e., the amount of injected current required to elicit action potentials), changes in input/output curves, as well as spiking characteristics induced by each substance, were assessed. We found that heroin, cocaine, methamphetamine, and MDPV decreased the excitability of type II cells, whereas ethanol increased the excitability of type I pyramidal cells. Together, these results suggest that heroin, cocaine, MDPV, and methamphetamine reduce mPFC commissural output by reducing type II excitability, while ethanol increases the excitability of type I cells targeting subcortical structures. Thus, separate classes of abused drugs differentially affect layer V pyramidal subtypes in the mPFC, which may ultimately give rise to compulsivity and inappropriate synaptic plasticity underlying substance use disorders.

Maternal crack cocaine use in rats leads to depressive- and anxiety-like behavior, memory impairment, and increased seizure susceptibility in the offspring

Crack users suffer the effects of cocaine present in the drug and the action of other active compounds from its pyrolysis. An emergent fact is an increase in the number of pregnant crack cocaine users. Studies suggest that crack cocaine and its metabolites cross the placenta, promoting premature birth, fever, irritability, sweating, and seizures in the early months of life. In children, the effects of crack cocaine have been associated with cognitive deficits, difficulty in verbalization, aggressiveness, and depression, besides enhancing the susceptibility to epileptic seizures, including status epilepticus (SE) in adulthood. Therefore, we investigated the effect of maternal exposure to smoke crack cocaine on several behavioral parameters in the offspring during adulthood. A series of behavioral tests and intrahippocampal pilocarpine (H-PILO) microinjection at sub-convulsive and convulsive doses in a rat model demonstrated that exposure to crack cocaine during the embryonic period leads to anxiogenic-like behavior and long-term memory impairment in both genders and promotes depressive-like behavior in the female. Besides, crack cocaine offspring exposed to a sub-convulsive H-PILO dose showed higher susceptibility to SE, increased seizure frequency, and neurodegeneration, while animals that received a convulsive dose of H-PILO displayed no alteration in SE severity. Taken together, our data suggest that crack cocaine exposure during the gestational period leads to an increased predilection for anxiety and depression, long-term memory deficits, and reduction in the threshold for developing epileptic seizures associated with neuronal death, which predispose crack cocaine babies to develop neuropsychological disorders.

GABAergic polygenic risk for cocaine use disorder is negatively correlated with precuneus activity during cognitive control in African American individuals

Impaired cognitive control has been implicated in cocaine use disorder (CUD). GABAergic treatments have been proposed for CUD. Here we examined relationships between GABAergic genes and neural correlates of cognitive control in CUD. We analyzed two independent African American cohorts: one of >3000 genomewide-genotyped subjects with substance dependence and another of 40 CUD and 22 healthy control (HC) subjects who were exome-array genotyped and completed an fMRI Stroop task. We used five association thresholds to select variants of GABAergic genes in the reference cohort, yielding five polygenic risk scores (i.e., CUD-GABA-PRSs) for the fMRI cohort. At p < 0.005, the CUD-GABA-PRSs, which aggregated relative risks of CUD from 89 variants harboring in 16 genes, differed between CUD and HC individuals in the fMRI sample (p = 0.013). This CUD-GABA-PRS correlated inversely with Stroop-related activity in the left precuneus in CUD (r = -80.58, p_FWE < 0.05) but not HC participants. Post-hoc seed-based connectivity analysis of the left precuneus identified reduced functional connectivity to the posterior cingulate cortex (PCC) in CUD compared to HC subjects (p = 0.0062) and the degree of connectivity correlated with CUD-GABA-PRSs in CUD individuals (r = 0.287, p = 0.036). Our findings suggest that the GABAergic genetic risk of CUD in African Americans relates to precuneus/PCC functional connectivity during cognitive control. Identification of these GABAergic processes may be relevant targets in CUD treatment. The novel identification of 16 GABAergic genes may be investigated further to inform treatment development efforts for this condition that currently has no medication with a formal indication for its treatment.

Molecular changes in the nucleus accumbens and prefrontal cortex associated with the locomotor sensitization induced by coca paste seized samples

RATIONALE

In previous studies, we have demonstrated that seized samples of a smokable form of cocaine, also known as coca paste (CP), induced behavioral sensitization in rats. Interestingly, this effect was accelerated and enhanced when the samples were adulterated with caffeine. While the cocaine phenomenon is associated with persistent functional and structural alterations in the prefrontal cortex (PFC) and nucleus accumbens (NAc), the molecular mechanisms underlying the CP sensitization and the influence of caffeine remains still unknown.

OBJECTIVE

We examined the gene expression in NAc and mPFC after the expression caffeine-adulterated and non-adulterated CP locomotor sensitization.

METHODS

The locomotor sensitization was established in C57BL/6 mice, repeatedly treated with a CP-seized sample adulterated with caffeine (CP-2) and a non-adulterated one (CP-1). We then assessed the mRNA expression of receptor subunits of the dopaminergic and glutamatergic systems in the medial PFC (mPFC) and NAc. Other molecular markers (e.g., adenosinergic, endocannabinoid receptor subunits, and synaptic plasticity-associated genes) were also analyzed.

RESULTS

Only CP-2-treated mice expressed locomotor sensitization. This phenomenon was associated with increased Drd1a, Gria1, Cnr1, and Syn mRNA expression levels in the NAc. Drd3 mRNA expression levels were only significantly increased in mPFC of CP-2-treated group.

CONCLUSIONS

Our results demonstrated that caffeine actively collaborates in the induction of the molecular changes underlying CP sensitization. The present study provides new knowledge on the impact of active adulterants to understand the early dependence induced by CP consumption.

Heroin exposure and withdrawal differentially influence expression of NMDA receptor NR2 subunits in the prelimbic region of rat medial prefrontal cortex

It is widely reported that drug addiction involves the strengthening of specific reward circuits through N-methyl-d-aspartic acid receptor (NMDAR)-dependent synaptic potentiation, and several lines of evidence strongly implicate NMDA receptor 2 (NR2) subunits in drug abuse. To explore the potential mechanism of heroin dependence, this study examined changes in the expression levels of NR2 subunits NR2A-D in the prelimbic (PL) region of the medial prefrontal cortex (mPFC) after repeated heroin administration and subsequent abstinence. The conditioned place preference (CPP) test confirmed successful induction of heroin dependence and withdrawal. Western blotting and qRT-PCR revealed no differences in NR2A subunit expression among heroin-exposure, heroin-withdrawal, and control group rats; in contrast, expression of NR2B was significantly higher in the heroin-exposure group, whereas expression levels of NR2C and NR2D were significantly higher in the heroin-withdrawal group relative to the controls. Further studies are needed to identify the functional significance based on alterations of NR2 subunits.

Cocaine-induced Changes in the Expression of NMDA Receptor Subunits

Cocaine use disorder is manifested by repeated cycles of drug seeking and drug taking. Cocaine exposure causes synaptic transmission in the brain to exhibit persistent changes, which are poorly understood, while the pharmacotherapy of this disease has not been determined. Multiple potential mechanisms have been indicated to be involved in the etiology of co-caine use disorder. The glutamatergic system, especially N-methyl-D-aspartate (NMDA) receptors, may play a role in sever-al physiological processes (synaptic plasticity, learning and memory) and in the pathogenesis of cocaine use disorder. The composition of the NMDA receptor subunits changes after contingent and noncontingent cocaine administration and after drug abstinence in a region-specific and time-dependent manner, as well as depending on the different protocols used for co-caine administration. Changes in the expression of NMDA receptor subunits may underlie the transition from cocaine abuse to dependence, as well as the transition from cocaine dependence to cocaine withdrawal. In this paper, we summarize the cur-rent knowledge regarding neuroadaptations within NMDA receptor subunits and scaffolding proteins observed following voluntary and passive cocaine intake, as well as the effects of NMDA receptor antagonists on cocaine-induced behavioral changes during cocaine seeking and relapse.

Mechanisms of cannabinoid CB2 receptor-mediated reduction of dopamine neuronal excitability in mouse ventral tegmental area

BACKGROUND

We have recently reported that activation of cannabinoid type 2 receptors (CB₂Rs) reduces dopamine (DA) neuron excitability in mouse ventral tegmental area (VTA). Here, we elucidate the underlying mechanisms.

METHODS

Patch-clamp recordings were performed in mouse VTA slices and dissociated single VTA DA neurons.

FINDINGS

Using cell-attached recording in VTA slices, bath-application of CB₂R agonists (JWH133 or five other CB₂R agonists) significantly reduced VTA DA neuron action potential (AP) firing rate. Under the patch-clamp whole-cell recording model, JWH133 (10 μM) mildly reduced the frequency of miniature excitatory postsynaptic currents (mEPSCs) but not miniature inhibitory postsynaptic currents (mIPSCs). JWH133 also did not alter evoked EPSCs or IPSCs. In freshly dissociated VTA DA neurons, JWH133 reduced AP firing rate, delayed AP initiation and enhanced AP after-hyperpolarization. In voltage-clamp recordings, JWH133 (1 μM) enhanced M-type K⁺ currents and this effect was absent in CB₂^-/- mice and abolished by co-administration of a selective CB₂R antagonist (10 μM, AM630). CB₂R-mediated inhibition in VTA DA neuron firing can be mimicked by M-current opener (10 μM retigabine) and blocked by M-current blocker (30 μM XE991). In addition, enhancement of neuronal cAMP by forskolin (10 μM) reduced M-current and increased DA neuron firing rate. Finally, pharmacological block of synaptic transmission by NBQX (10 μM), D-APV (50 μM) and picrotoxin (100 μM) in VTA slices failed to prevent CB₂R-mediated inhibition, while intracellular infusion of guanosine 5'-O-2-thiodiphosphate (600 μM, GDP-β-S) through recording electrode to block postsynaptic G-protein function prevented JWH133-induced reduction in AP firing.

INTERPRETATION

Our results suggest that CB₂Rs modulate VTA DA neuron excitability mainly through an intrinsic mechanism, including a CB₂R-mediated reduction of intracellular cAMP, and in turn enhancement of M-type K⁺ currents. FUND: This research was supported by the Barrow Neuroscience Foundation, the BNI-BMS Seed Fund, and CNSF (81771437).

Chronic unpredictable stress promotes cell-specific plasticity in prefrontal cortex D1 and D2 pyramidal neurons

Exposure to unpredictable environmental stress is widely recognized as a major determinant for risk and severity in neuropsychiatric disorders such as major depressive disorder, anxiety, schizophrenia, and PTSD. The ability of ostensibly unrelated disorders to give rise to seemingly similar psychiatric phenotypes highlights a need to identify circuit-level concepts that could unify diverse factors under a common pathophysiology. Although difficult to disentangle a causative effect of stress from other factors on medial prefrontal cortex (PFC) dysfunction, a wealth of data from humans and rodents demonstrates that the PFC is a key target of stress. The present study sought to identify a model of chronic unpredictable stress (CUS) which induces affective behaviors in C57BL6J mice and once established, measure stress-related alterations in intrinsic excitability and synaptic regulation of mPFC layer 5/6 pyramidal neurons. Adult male mice received 2 weeks of 'less intense' stress or 2 or 4 weeks of 'more intense' CUS followed by sucrose preference for assessment of anhedonia, elevated plus maze for assessment of anxiety and forced swim test for assessment of depressive-like behaviors. Our findings indicate that more intense CUS exposure results in increased anhedonia, anxiety, and depressive behaviors, while the less intense stress results in no measured behavioral phenotypes. Once a behavioral model was established, mice were euthanized approximately 21 days post-stress for whole-cell patch clamp recordings from layer 5/6 pyramidal neurons in the prelimbic (PrL) and infralimbic (IL) cortices. No significant differences were initially observed in intrinsic cell excitability in either region. However, post-hoc analysis and subsequent confirmation using transgenic mice expressing tdtomato or eGFP under control of dopamine D1-or D2-type receptor showed that D1-expressing pyramidal neurons (D1-PYR) in the PrL exhibit reduced thresholds to fire an action potential (increased excitability) but impaired firing capacity at more depolarized potentials, whereas D2-expressing pyramidal neurons (D2-PYR) showed an overall reduction in excitability and spike firing frequency. Examination of synaptic transmission showed that D1-and D2-PYR exhibit differences in basal excitatory and inhibitory signaling under naïve conditions. In CUS mice, D1-PYR showed increased frequency of both miniature excitatory and inhibitory postsynaptic currents, whereas D2-PYR only showed a reduction in excitatory currents. These findings demonstrate that D1-and D2-PYR subpopulations differentially undergo stress-induced intrinsic and synaptic plasticity that may have functional implications for stress-related pathology, and that these adaptations may reflect unique differences in basal properties regulating output of these cells.

GIRK Channel Activity in Dopamine Neurons of the Ventral Tegmental Area Bidirectionally Regulates Behavioral Sensitivity to Cocaine

Dopamine (DA) neurons of the VTA have been widely implicated in the cellular and behavioral responses to drugs of abuse. Inhibitory G protein signaling mediated by GABA_B receptors (GABA_BRs) and D₂ DA receptors (D₂Rs) regulates the excitability of VTA DA neurons, DA neurotransmission, and behaviors modulated by DA. Most of the somatodendritic inhibitory effect of GABA_BR and D₂R activation on DA neurons reflects the activation of G protein-gated inwardly rectifying K⁺ (GIRK) channels. Furthermore, GIRK-dependent signaling in VTA DA neurons can be weakened by exposure to psychostimulants and strengthened by phasic DA neuron firing. The objective of this study was to determine how the strength of GIRK channel activity in VTA DA neurons influences sensitivity to cocaine. We used a Cre-dependent viral strategy to overexpress the individual GIRK channel subunits in VTA DA neurons of male and female adult mice, leading to enhancement (GIRK2) or suppression (GIRK3) of GIRK channel activity. Overexpression of GIRK3 decreased somatodendritic GABA_BR- and D₂R-dependent signaling and increased cocaine-induced locomotor activity, whereas overexpression of GIRK2 increased GABA_BR-dependent signaling and decreased cocaine-induced locomotion. Neither manipulation impacted anxiety- or depression-related behavior, despite the link between such behaviors and DA signaling. Together, these data show that behavioral sensitivity to cocaine in mice is inversely proportional to the strength of GIRK channel activity in VTA DA neurons and suggest that direct activators of the unique VTA DA neuron GIRK channel subtype (GIRK2/GIRK3 heteromer) could represent a promising therapeutic target for treatment of addiction.SIGNIFICANCE STATEMENT Inhibitory G protein signaling in dopamine (DA) neurons, including that mediated by G protein-gated inwardly rectifying K⁺ (GIRK) channels, has been implicated in behavioral sensitivity to cocaine. Here, we used a viral approach to bidirectionally manipulate GIRK channel activity in DA neurons of the VTA. We found that decreasing GIRK channel activity in VTA DA neurons increased behavioral sensitivity to cocaine, whereas increasing GIRK channel activity decreased behavioral sensitivity to cocaine. These manipulations did not alter anxiety- or depression-related behaviors. These data highlight the unique GIRK channel subtype in VTA DA neurons as a possible therapeutic target for addiction.

Prefrontal-accumbens opioid plasticity: Implications for relapse and dependence

In addiction, an individual's ability to inhibit drug seeking and drug taking is thought to reflect a pathological strengthening of drug-seeking behaviors or impairments in the capacity to control maladaptive behavior. These processes are not mutually exclusive and reflect drug-induced modifications within prefrontal cortical and nucleus accumbens circuits, however unlike psychostimulants such as cocaine, far less is known about the temporal, anatomical, and cellular dynamics of these changes. We discuss what is known regarding opioid-induced adaptations in intrinsic membrane physiology and pre-/postsynaptic neurotransmission in principle pyramidal and medium spiny neurons in the medial prefrontal cortex and nucleus accumbens from electrophysiological studies and explore how circuit specific adaptations may contribute to unique facets of opioid addiction.

Chemogenetic Manipulations of Ventral Tegmental Area Dopamine Neurons Reveal Multifaceted Roles in Cocaine Abuse

Ventral tegmental area (VTA) dopamine (DA) neurons perform diverse functions in motivation and cognition, but their precise roles in addiction-related behaviors are still debated. Here, we targeted VTA DA neurons for bidirectional chemogenetic modulation during specific tests of cocaine reinforcement, demand, and relapse-related behaviors in male rats, querying the roles of DA neuron inhibitory and excitatory G-protein signaling in these processes. Designer receptor stimulation of G_q signaling, but not G_s signaling, in DA neurons enhanced cocaine seeking via functionally distinct projections to forebrain limbic regions. In contrast, engaging inhibitory G_i/o signaling in DA neurons blunted the reinforcing and priming effects of cocaine, reduced stress-potentiated reinstatement, and altered behavioral strategies for cocaine seeking and taking. Results demonstrate that DA neurons play several distinct roles in cocaine seeking, depending on behavioral context, G-protein-signaling cascades, and DA neuron efferent targets, highlighting their multifaceted roles in addiction.SIGNIFICANCE STATEMENT G-protein-coupled receptors are crucial modulators of ventral tegmental area (VTA) dopamine neuron activity, but how this metabotropic signaling impacts the complex roles of dopamine in reward and addiction is poorly understood. Here, we bidirectionally modulate dopamine neuron G-protein signaling with DREADDs (designer receptors exclusively activated by designer drugs) during a variety of cocaine-seeking behaviors, revealing nuanced, pathway-specific roles in cocaine reward, effortful seeking, and relapse-like behaviors. G_q and G_s stimulation activated dopamine neurons, but only G_q stimulation robustly enhanced cocaine seeking. G_i/o inhibitory signaling reduced some, but not all, types of cocaine seeking. Results show that VTA dopamine neurons modulate numerous distinct aspects of cocaine addiction- and relapse-related behaviors, and point to potential new approaches for intervening in these processes to treat addiction.

A high-refined carbohydrate diet facilitates compulsive-like behavior in mice through the nitric oxide pathway

Obesity is characterized by abnormal adipose tissue expansion and is associated with chronic inflammation. Obesity itself may induce several comorbidities, including psychiatric disorders. It has been previously demonstrated that proinflammatory cytokines are able to up-regulate inducible nitric oxide synthase (iNOS) and nitric oxide (NO) release, which both have a role in compulsive related behaviors.

OBJECTIVE

To evaluate whether acute or chronic consumption of a high-refined carbohydrate-containing (HC) diet will modify burying-behavior in the Marble Burying Test (MBT) through augmentation of NO signaling in the striatum, a brain region related to the reward system. Further, we also verified the effects of chronic consumption of a HC diet on the reinforcing effects induced by cocaine in the Conditioned Place Preference (CPP) test.

METHODS

Male BALB/c mice received a standard diet (control diet) or a HC diet for 3 days or 12 weeks.

RESULTS

An increase in burying behavior occurred in the MBT after chronic consumption of a HC diet that was associated with an increase of nitrite levels in the striatum. The pre-treatment with Aminoguanidine (50 mg/kg), a preferential inhibitor of iNOS, prevented such alterations. Additionally, a chronic HC diet also induced a higher expression of iNOS in this region and higher glutamate release from striatal synaptosomes. Neither statistical differences were observed in the expression levels of the neuronal isoform of NOS nor in microglia number and activation. Finally, the reinforcing effects induced by cocaine (15 mg/kg, i.p.) during the expression of the conditioned response in the CPP test were not different between the chronically HC diet fed mice and the control group. However, HC diet-feeding mice presented impairment of cocaine-preference extinction.

CONCLUSION

Altogether, our results suggest that the chronic consumption of a HC diet induces compulsive-like behavior through a mechanism possibly associated with NO activation in the striatum.

Cocaine- and amphetamine-regulated transcript peptide in the nucleus accumbens shell inhibits cocaine-induced locomotor sensitization to transient over-expression of α-Ca2+ /calmodulin-dependent protein kinase II

Cocaine- and amphetamine-regulated transcript (CART) peptide is a widely distributed neurotransmitter that attenuates cocaine-induced locomotor activity when injected into the nucleus accumbens (NAc). Our previous work first confirmed that the inhibitory mechanism of the CART peptide on cocaine-induced locomotor activity is related to a reduction in cocaine-enhanced phosphorylated Ca²⁺ /calmodulin-dependent protein kinaseIIα (pCaMKIIα) and the enhancement of cocaine-induced D3R function. This study investigated whether CART peptide inhibited cocaine-induced locomotor activity via inhibition of interactions between pCaMKIIα and the D3 dopamine receptor (D3R). We demonstrated that lentivirus-mediated gene transfer transiently increased pCaMKIIα expression, which peaked at 10 days after microinjection into the rat NAc shell, and induced a significant increase in Ca²⁺ influx along with greater behavioral sensitivity in the open field test after intraperitoneal injections of cocaine (15 mg/kg). However, western blot analysis and coimmunoprecipitation demonstrated that CART peptide treatment in lentivirus-transfected CaMKIIα-over-expressing NAc rat tissues or cells prior to cocaine administration inhibited the cocaine-induced Ca²⁺ influx and attenuated the cocaine-increased pCaMKIIα expression in lentivirus-transfected CaMKIIα-over-expressing cells. CART peptide decreased the cocaine-enhanced phosphorylated cAMP response element binding protein (pCREB) expression via inhibition of the pCaMKIIα-D3R interaction, which may account for the prolonged locomotor sensitization induced by repeated cocaine treatment in lentivirus-transfected CaMKIIα-over-expressing cells. These results provide strong evidence for the inhibitory modulation of CART peptide in cocaine-induced locomotor sensitization. Cover Image for this issue: doi: 10.1111/jnc.14187.

The involvement of brain-derived neurotrophic factor in 3,4-methylenedioxymethamphetamine-induced place preference and behavioral sensitization

3,4-Methylenedioxymethamphetamine (MDMA) is known to induce dependence and psychosis in humans. Brain-derived neurotrophic factor (BDNF) is involved in the synaptic plasticity and neurotrophy in midbrain dopaminergic neurons. This study aimed to investigate the role of BDNF in MDMA-induced dependence and psychosis. A single dose of MDMA (10mg/kg) induced BDNF mRNA expression in the prefrontal cortex, nucleus accumbens, and amygdala, but not in the striatum or the hippocampus. However, repeated MDMA administration for 7 days induced BDNF mRNA expression in the striatum and hippocampus. Both precursor and mature BDNF protein expression increased in the nucleus accumbens, mainly in the neurons. Additionally, rapidly increased extracellular serotonin levels and gradually and modestly increased extracellular dopamine levels were noted within the nucleus accumbens of mice after repeated MDMA administration. Dopamine receptor antagonists attenuated the effect of repeated MDMA administration on BDNF mRNA expression in the nucleus accumbens. To examine the role of endogenous BDNF in the behavioral and neurochemical effects of MDMA, we used mice with heterozygous deletions of the BDNF gene. MDMA-induced place preference, behavioral sensitization, and an increase in the levels of extracellular serotonin and dopamine within the nucleus accumbens, were attenuated in BDNF heterozygous knockout mice. These results suggest that BDNF is implicated in MDMA-induced dependence and psychosis by activating the midbrain serotonergic and dopaminergic neurons.

Stable immediate early gene expression patterns in medial prefrontal cortex and striatum after long-term cocaine self-administration

The transition from casual to compulsive drug use is thought to occur as a consequence of repeated drug taking leading to neuroadaptive changes in brain circuitry involved in emotion and cognition. At the basis of such neuroadaptations lie changes in the expression of immediate early genes (IEGs) implicated in transcriptional regulation, synaptic plasticity and intracellular signalling. However, little is known about how IEG expression patterns change during long-term drug self-administration. The present study, therefore, compares the effects of 10 and 60-day self-administration of cocaine and sucrose on the expression of 17 IEGs in brain regions implicated in addictive behaviour, i.e. dorsal striatum, ventral striatum and medial prefrontal cortex (mPFC). Increased expression after cocaine self-administration was found for 6 IEGs in dorsal and ventral striatum (c-fos, Mkp1, Fosb/ΔFosb, Egr2, Egr4, and Arc) and 10 IEGs in mPFC (same 6 IEGs as in striatum, plus Bdnf, Homer1, Sgk1 and Rgs2). Five of these 10 IEGs (Egr2, Fosb/ΔFosb, Bdnf, Homer1 and Jun) and Trkb in mPFC were responsive to long-term sucrose self-administration. Importantly, no major differences were found between IEG expression patterns after 10 or 60 days of cocaine self-administration, except Fosb/ΔFosb in dorsal striatum and Egr2 in mPFC, whereas the amount of cocaine obtained per session was comparable for short-term and long-term self-administration. These steady changes in IEG expression are, therefore, associated with stable self-administration behaviour rather than the total amount of cocaine consumed. Thus, sustained impulses to IEG regulation during prolonged cocaine self-administration may evoke neuroplastic changes underlying compulsive drug use.

Correlating behaviour and gene expression endpoints in the dopaminergic system after modafinil administration in mouse

The mechanisms of action of modafinil continue to be poorly characterised and its potential for abuse in preclinical models remains controverted. The aim of this study was to further elucidate the mechanism of action of modafinil, through a potential behavioural and molecular association in the mouse. A conditioned place preference (CPP) paradigm was implemented to investigate the rewarding properties of modafinil. Whole genome expression and qRT-PCR analysis were performed on the ventral tegmental area (VTA), nucleus accumbens (NAC) and prefrontal cortex (PFC) of modafinil-treated and control animals. Modafinil administration (65 mg/kg) induced an increase in locomotor activity, an increase in the change of preference for the drug paired side after a conditioning period as well as changes to gene expression profiles in the VTA (120 genes), NAC (23 genes) and PFC (19 genes). A molecular signature consisting of twelve up-regulated genes was identified as common to the three brain regions. Multiple linear correlation analysis showed a strong correlation (R(2)>0.70) between the behavioural and molecular endpoints in the three brain regions. We show that modafinil had a concomitant effect on CPP, locomotor activity, and up-regulation of interferon-γ (IFN-γ) regulated genes (Gbp2, Gbp3, Gbp10, Cd274, Igtp), while correlating the latter set of genes with behaviour changes evaluated through the CPP. A potential association can be proposed based on the dysregulation of p47 family genes and Gbp family of IFN-γ induced GTPases. In conclusion, these findings suggest a link between the behavioural and molecular events in the context of modafinil administration.

Programming of Dopaminergic Neurons by Neonatal Sex Hormone Exposure: Effects on Dopamine Content and Tyrosine Hydroxylase Expression in Adult Male Rats

We sought to determine the long-term changes produced by neonatal sex hormone administration on the functioning of midbrain dopaminergic neurons in adult male rats. Sprague-Dawley rats were injected subcutaneously at postnatal day 1 and were assigned to the following experimental groups: TP (testosterone propionate of 1.0 mg/50 μL); DHT (dihydrotestosterone of 1.0 mg/50 μL); EV (estradiol valerate of 0.1 mg/50 μL); and control (sesame oil of 50 μL). At postnatal day 60, neurochemical studies were performed to determine dopamine content in substantia nigra-ventral tegmental area and dopamine release in nucleus accumbens. Molecular (mRNA expression of tyrosine hydroxylase) and cellular (tyrosine hydroxylase immunoreactivity) studies were also performed. We found increased dopamine content in substantia nigra-ventral tegmental area of TP and EV rats, in addition to increased dopamine release in nucleus accumbens. However, neonatal exposure to DHT, a nonaromatizable androgen, did not affect midbrain dopaminergic neurons. Correspondingly, compared to control rats, levels of tyrosine hydroxylase mRNA and protein were significantly increased in TP and EV rats but not in DHT rats, as determined by qPCR and immunohistochemistry, respectively. Our results suggest an estrogenic mechanism involving increased tyrosine hydroxylase expression, either by direct estrogenic action or by aromatization of testosterone to estradiol in substantia nigra-ventral tegmental area.

GIRK Channels Modulate Opioid-Induced Motor Activity in a Cell Type- and Subunit-Dependent Manner

G-protein-gated inwardly rectifying K(+) (GIRK/Kir3) channel activation underlies key physiological effects of opioids, including analgesia and dependence. GIRK channel activation has also been implicated in the opioid-induced inhibition of midbrain GABA neurons and consequent disinhibition of dopamine (DA) neurons in the ventral tegmental area (VTA). Drug-induced disinhibition of VTA DA neurons has been linked to reward-related behaviors and underlies opioid-induced motor activation. Here, we demonstrate that mouse VTA GABA neurons express a GIRK channel formed by GIRK1 and GIRK2 subunits. Nevertheless, neither constitutive genetic ablation of Girk1 or Girk2, nor the selective ablation of GIRK channels in GABA neurons, diminished morphine-induced motor activity in mice. Moreover, direct activation of GIRK channels in midbrain GABA neurons did not enhance motor activity. In contrast, genetic manipulations that selectively enhanced or suppressed GIRK channel function in midbrain DA neurons correlated with decreased and increased sensitivity, respectively, to the motor-stimulatory effect of systemic morphine. Collectively, these data support the contention that the unique GIRK channel subtype in VTA DA neurons, the GIRK2/GIRK3 heteromer, regulates the sensitivity of the mouse mesolimbic DA system to drugs with addictive potential.

Intrinsic plasticity: an emerging player in addiction

Exposure to drugs of abuse, such as cocaine, leads to plastic changes in the activity of brain circuits, and a prevailing view is that these changes play a part in drug addiction. Notably, there has been intense focus on drug-induced changes in synaptic excitability and much less attention on intrinsic excitability factors (that is, excitability factors that are remote from the synapse). Accumulating evidence now suggests that intrinsic factors such as K+ channels are not only altered by cocaine but may also contribute to the shaping of the addiction phenotype.

Sex differences in GABA(B)R-GIRK signaling in layer 5/6 pyramidal neurons of the mouse prelimbic cortex

The medial prefrontal cortex (mPFC) has been implicated in multiple disorders characterized by clear sex differences, including schizophrenia, attention deficit hyperactivity disorder, post-traumatic stress disorder, depression, and drug addiction. These sex differences likely represent underlying differences in connectivity and/or the balance of neuronal excitability within the mPFC. Recently, we demonstrated that signaling via the metabotropic γ-aminobutyric acid receptor (GABABR) and G protein-gated inwardly-rectifying K(+) (GIRK/Kir3) channels modulates the excitability of the key output neurons of the mPFC, the layer 5/6 pyramidal neurons. Here, we report a sex difference in the GABABR-GIRK signaling pathway in these neurons. Specifically, GABABR-dependent GIRK currents recorded in the prelimbic region of the mPFC were larger in adolescent male mice than in female counterparts. Interestingly, this sex difference was not observed in layer 5/6 pyramidal neurons of the adjacent infralimbic cortex, nor was it seen in young adult mice. The sex difference in GABABR-GIRK signaling is not attributable to different expression levels of signaling pathway components, but rather to a phosphorylation-dependent trafficking mechanism. Thus, sex differences related to some diseases associated with altered mPFC function may be explained in part by sex differences in GIRK-dependent signaling in mPFC pyramidal neurons.

Chronic cocaine disrupts mesocortical learning mechanisms

The addictive power of drugs of abuse such as cocaine comes from their ability to hijack natural reward and plasticity mechanisms mediated by dopamine signaling in the brain. Reward learning involves burst firing of midbrain dopamine neurons in response to rewards and cues predictive of reward. The resulting release of dopamine in terminal regions is thought to act as a teaching signaling to areas such as the prefrontal cortex and striatum. In this review, we posit that a pool of extrasynaptic dopaminergic D1-like receptors activated in response to dopamine neuron burst firing serve to enable synaptic plasticity in the prefrontal cortex in response to rewards and their cues. We propose that disruptions in these mechanisms following chronic cocaine use contribute to addiction pathology, in part due to the unique architecture of the mesocortical pathway. By blocking dopamine reuptake in the cortex, cocaine elevates dopamine signaling at these extrasynaptic receptors, prolonging D1-receptor activation and the subsequent activation of intracellular signaling cascades, and thus inducing long-lasting maladaptive plasticity. These cellular adaptations may account for many of the changes in cortical function observed in drug addicts, including an enduring vulnerability to relapse. Therefore, understanding and targeting these neuroadaptations may provide cognitive benefits and help prevent relapse in human drug addicts.

Cocaine-induced behavioral sensitization is associated with changes in the expression of endocannabinoid and glutamatergic signaling systems in the mouse prefrontal cortex

BACKGROUND

Endocannabinoids modulate the glutamatergic excitatory transmission by acting as retrograde messengers. A growing body of studies has reported that both signaling systems in the mesocorticolimbic neural circuitry are involved in the neurobiological mechanisms underlying drug addiction.

METHODS

We investigated whether the expression of both endocannabinoid and glutamatergic systems in the prefrontal cortex (PFC) were altered by an acute and/or repeated cocaine administration schedule that resulted in behavioral sensitization. We measured the protein and mRNA expression of the main endocannabinoid metabolic enzymes and the cannabinoid receptor type 1 (CB1). We also analyzed the mRNA expression of relevant components of the glutamate-signaling system, including glutamate-synthesizing enzymes, metabotropic receptors, and ionotropic receptors.

RESULTS

Although acute cocaine (10 mg/kg) produced no significant changes in the endocannabinoid-related proteins, repeated cocaine administration (20 mg/kg daily) induced a pronounced increase in the CB1 receptor expression. In addition, acute cocaine administration (10 mg/kg) in cocaine-sensitized mice (referred to as cocaine priming) induced a selective increase in the endocannabinoid-degrading enzymes fatty acid amide hydrolase (FAAH) and monoacylglycerol lipase (MAGL). These protein changes were accompanied by an overall decrease in the ratios of endocannabinoid synthesis/degradation, especially the N-acyl phosphatidylethanolamine phospholipase D/FAAH and diacylglycerol lipase alpha/MAGL ratios. Regarding mRNA expression, while acute cocaine administration produced a decrease in CB1 receptors and N-acyl phosphatidylethanolamine phospholipase D, repeated cocaine treatment enhanced CB1 receptor expression. Cocaine-sensitized mice that were administered priming injections of cocaine mainly displayed an increased FAAH expression. These endocannabinoid changes were associated with modifications in glutamatergic transmission-related genes. An overall decrease was observed in the mRNA expression of the glutamate-synthesizing gene kidney-type glutaminase (KGA), the metabotropic glutamate receptors (mGluR3 and GluR), and subunits of NMDA ionotropic receptors (NR1, NR2A, NR2B and NR2C) after acute cocaine administration, while mice repeatedly exposed to cocaine only displayed an increase in NR2C. However, in cocaine-sensitized mice primed with cocaine, this inhibition was reversed and a strong increase was detected in the mGluR5, NR2 subunits, and both GluR1 and GluR3.

CONCLUSIONS

These findings indicate that cocaine sensitization is associated with an endocannabinoid downregulation and a hyperglutamatergic state in the PFC that, overall, contribute to an enhanced glutamatergic input into PFC-projecting areas.

Optogenetic dissection of medial prefrontal cortex circuitry

The medial prefrontal cortex (mPFC) is critically involved in numerous cognitive functions, including attention, inhibitory control, habit formation, working memory and long-term memory. Moreover, through its dense interconnectivity with subcortical regions (e.g., thalamus, striatum, amygdala and hippocampus), the mPFC is thought to exert top-down executive control over the processing of aversive and appetitive stimuli. Because the mPFC has been implicated in the processing of a wide range of cognitive and emotional stimuli, it is thought to function as a central hub in the brain circuitry mediating symptoms of psychiatric disorders. New optogenetics technology enables anatomical and functional dissection of mPFC circuitry with unprecedented spatial and temporal resolution. This provides important novel insights in the contribution of specific neuronal subpopulations and their connectivity to mPFC function in health and disease states. In this review, we present the current knowledge obtained with optogenetic methods concerning mPFC function and dysfunction and integrate this with findings from traditional intervention approaches used to investigate the mPFC circuitry in animal models of cognitive processing and psychiatric disorders.

Repeated cocaine weakens GABA(B)-Girk signaling in layer 5/6 pyramidal neurons in the prelimbic cortex

Repeated cocaine exposure triggers adaptations in layer 5/6 glutamatergic neurons in the medial prefrontal cortex (mPFC) that promote behavioral sensitization and drug-seeking behavior. While suppression of metabotropic inhibitory signaling has been implicated in these behaviors, underlying mechanisms are unknown. Here, we show that Girk/K(IR)3 channels mediate most of the GABA(B) receptor (GABA(B)R)-dependent inhibition of layer 5/6 pyramidal neurons in the mPFC and that repeated cocaine suppresses this pathway. This adaptation was selective for GABA(B)R-dependent Girk signaling in layer 5/6 pyramidal neurons of the prelimbic cortex (PrLC) and involved a D₁/₅ dopamine receptor- and phosphorylation-dependent internalization of GABA(B)R and Girk channels. Persistent suppression of Girk signaling in layer 5/6 of the dorsal mPFC enhanced cocaine-induced locomotor activity and occluded behavioral sensitization. Thus, the cocaine-induced suppression of GABA(B)R-Girk signaling in layer 5/6 pyramidal neurons of the prelimbic cortex appears to represent an early adaptation critical for promoting addiction-related behavior.