A BDNF infusion into the medial prefrontal cortex suppresses cocaine seeking in rats

Corticostriatal contributions to dysregulated motivated behaviors in stress, depression, and substance use disorders

Coordinated network activity, particularly in circuits arising from the prefrontal cortex innervating the ventral striatum, is crucial for normal processing of reward-related information which is perturbed in several psychiatric disorders characterized by dysregulated reward-related behaviors. Stress-induced depression and substance use disorders (SUDs) both share this common underlying pathology, manifested as deficits in perceived reward in depression, and increased attribution of positive valence to drug-predictive stimuli and dysfunctional cognition in SUDs. Here we review preclinical and clinical data that support dysregulation of motivated and reward-related behaviors as a core phenotype shared between these two disorders. We posit that altered processing of reward-related stimuli arises from dysregulated control of subcortical circuits by upstream regions implicated in executive control. Although multiple circuits are directly involved in reward processing, here we focus specifically on the role of corticostriatal circuit dysregulation. Moreover, we highlight the growing body of evidence indicating that such abnormalities may be due to heightened neuroimmune signaling by microglia, and that targeting the neuroimmune system may be a viable approach to treating this shared symptom.

Working Memory Performance Predicts, but Does Not Reduce, Cocaine and Cannabinoid Seeking in Adult Male Rats

BACKGROUND

Cognitive deficits reflecting impaired executive function are commonly associated with psychiatric disorders, including substance use. Cognitive training is proposed to improve treatment outcomes for these disorders by promoting neuroplasticity within the prefrontal cortex, enhancing executive control, and mitigating cognitive decline due to drug use. Additionally, brain derived neurotrophic factor (BDNF) can facilitate plasticity in the prefrontal cortex and reduce drug-seeking behaviors. We investigated whether working memory training could elevate BDNF levels in the prefrontal cortex and if this training would predict or protect against cocaine or cannabinoid seeking.

METHODS

Adult male rats were trained to perform a "simple" or "complex" version of a delayed-match-to-sample working memory task. Rats then self-administered cocaine or the synthetic cannabinoid WIN55,212-2 and were tested for cued drug seeking during abstinence. Tissue from the prefrontal cortex and dorsal hippocampus was analyzed for BDNF protein expression.

RESULTS

Training on the working memory task enhanced endogenous BDNF protein levels in the prelimbic prefrontal cortex but not the dorsal hippocampus. Working memory training did not impact self-administration of either drug but predicted the extent of WIN self-administration and cocaine seeking during abstinence.

CONCLUSIONS

These results suggest that working memory training promotes endogenous BDNF but does not alter drug-seeking or drug-taking behavior. However, individual differences in cognitive performance before drug exposure may predict vulnerability to future drug use.

How life events may confer vulnerability to addiction: the role of epigenetics

Substance use disorder (SUD) represents a large and growing global health problem. Despite the strong addictive potency of drugs of abuse, only a minority of those exposed develop SUDs. While certain life experiences (e.g., childhood trauma) may increase subsequent vulnerability to SUDs, mechanisms underlying these effects are not yet well understood. Given the chronic and relapsing nature of SUDs, and the length of time that can elapse between prior life events and subsequent drug exposure, changes in SUD vulnerability almost certainly involve long-term epigenetic dysregulation. To validate this idea, functional effects of specific epigenetic modifications in brain regions mediating reinforcement learning (e.g., nucleus accumbens, prefrontal cortex) have been investigated in a variety of animal models of SUDs. In addition, the effects of epigenetic modifications produced by prior life experiences on subsequent SUD vulnerability have been studied, but mostly in a correlational manner. Here, we review how epigenetic mechanisms impact SUD-related behavior in animal models and summarize our understanding of the relationships among life experiences, epigenetic regulation, and future vulnerability to SUDs. Despite variations in study design, epigenetic modifications that most consistently affect SUD-related behavior are those that produce predominantly unidirectional effects on gene regulation, such as DNA methylation and histone phosphorylation. Evidence explicitly linking environmentally induced epigenetic modifications to subsequent SUD-related behavior is surprisingly sparse. We conclude by offering several directions for future research to begin to address this critical research gap.

Vagus nerve stimulation (VNS) modulates synaptic plasticity in the rat infralimbic cortex via Trk-B receptor activation to reduce drug-seeking

Drugs of abuse cause changes in the prefrontal cortex (PFC) and associated regions that impair inhibitory control over drug-seeking. Breaking the contingencies between drug-associated cues and the delivery of the reward during extinction learning reduces relapse. Vagus nerve stimulation (VNS) has previously been shown to enhance extinction learning and reduce drug-seeking. Here we determined the effects of VNS-mediated release of brain-derived neurotrophic factor (BDNF) on extinction and cue-induced reinstatement in rats trained to self-administer cocaine. Pairing 10 days of extinction training with VNS facilitated extinction and reduced drug-seeking behavior during reinstatement. Rats that received a single extinction session with VNS showed elevated BDNF levels in the medial PFC as determined via an enzyme-linked immunosorbent assay (ELISA). Systemic blockade of Tropomyosin receptor kinase B (TrkB) receptors during extinction, via the TrkB antagonist ANA-12, decreased the effects of VNS on extinction and reinstatement. Whole-cell recordings in brain slices showed that cocaine self-administration induced alterations in the ratio of AMPA and NMDA receptor-mediated currents in layer 5 pyramidal neurons of the infralimbic cortex (IL). Pairing extinction with VNS reversed cocaine-induced changes in glutamatergic transmission by enhancing AMPAR currents, and this effect was blocked by ANA-12. Our study suggests that VNS consolidates extinction of drug-seeking behavior by reversing drug-induced changes in synaptic AMPA receptors in the IL, and this effect is abolished by blocking TrkB receptors during extinction, highlighting a potential mechanism for the therapeutic effects of VNS in addiction.

Peripheral neurotrophin levels during controlled crack/cocaine abstinence: a systematic review and meta-analysis

Cocaine/crack abstinence periods have higher risk of relapse. Abstinence as initial part of the recovery process is affected by learning and memory changes that could preserve the addictive cycle. To further understand how the interruption of cocaine/crack consumption affects neurotrophin level we performed the present systematic review and meta-analysis following the PRISMA statement (number CRD42019121643). The search formula was conducted in PubMed, Web of Science, Embase, ScienceDirect, and Google Scholar databases. The inclusion criterion was cocaine use disorder in 18 to 60-year-old people, measuring at least one neurotrophin in blood before and after a controlled abstinence period. Studies without pre-post design were excluded. Five investigations had nine different reports, four of them were subjected to a meta-analysis (n = 146). GRADE risk of bias method was followed. Individual studies reported increased peripheral brain derived neurotrophic factor (BDNF) after abstinence, evidence pooled by Hedge's g showed no significant change in BDNF after abstinence. Relevant heterogeneity in the length of the abstinence period (12-32 days), last cocaine/crack consumption monitoring and blood processing were detected that could help to explain non-significant results. Further improved methods are suggested, and a potential BDNF augmentation hypothesis is proposed that, if true, would help to understand initial abstinence as a re-adaptation period influenced by neurotrophins such as the BDNF.

BDNF as a therapeutic candidate for cocaine use disorders

Cocaine self-administration disturbs intracellular signaling in multiple reward circuitry neurons that underlie relapse to drug seeking. Cocaine-induced deficits in prelimbic (PL) prefrontal cortex change during abstinence, resulting in different neuroadaptations during early withdrawal from cocaine self-administration than after one or more weeks of abstinence. Infusion of brain-derived neurotrophic factor (BDNF) into the PL cortex immediately following a final session of cocaine self-administration attenuates relapse to cocaine seeking for an extended period. BDNF affects local (PL) and distal subcortical target areas that mediate cocaine-induced neuroadaptations that lead to cocaine seeking. Blocking synaptic activity selectively in the PL projection to the nucleus accumbens during early withdrawal prevents BDNF from decreasing subsequent relapse. In contrast, blocking synaptic activity selectively in the PL projection to the paraventricular thalamic nucleus by itself decreases subsequent relapse and prior intra-PL BDNF infusion prevents the decrease. Infusion of BDNF into other brain structures at different timepoints after cocaine self administration differentially alters cocaine seeking. Thus, the effects of BDNF on drug seeking are different depending on the brain region, the timepoint of intervention, and the specific pathway that is affected.

Sex differences in risk behavior parameters in adolescent mice: Relationship with brain-derived neurotrophic factor in the medial prefrontal cortex

Adolescence is as a period of development characterized by impulsive and risk-seeking behaviors. Risk behaviors (RB) involves exposure to dangerous or negative consequences to achieve goal-directed behaviors, such as reward-seeking. On the other hand, risk aversion/assessment behaviors allow the individual to gather information or avoid potentially threatening situations. Evidence has suggested that both behavioral processes, RB and risk assessment (RA), may have sex-differences. However, sex-specific behavioral patterns implicated in RB and RA are not fully understood. To address that, we investigated sex differences in risk-behavioral parameters in a decision-making task developed for rodents. In addition, we investigated the potential role of sex-dependent differences in gene expression of brain-derived neurotrophic factor (BDNF) exon IV in the medial prefrontal cortex (mPFC), which has been implicated to mediate PFC-related behavioral dysfunctions. Male and female C57BL/6J adolescent mice were evaluated in the elevated plus-maze (EPM) to assess anxiety-like behaviors and in the predator-odor risk taking (PORT) task. The PORT task is a decision-making paradigm in which a conflict between the motivation towards reward pursuit and the threat elicited by predatory olfactory cues (coyote urine) is explored. After behavioral testing, animals were euthanized and BDNF exon IV gene expression was measured by RT-qPCR. Comparative and correlational analyses for behavioral and molecular parameters were performed for both sexes. We observed that female mice spent more time exploring the middle chamber of the PORT apparatus in the aversive condition, which is an indicative of avoidance behavior. Female mice also had a higher latency to collect the reward than male mice and presented less time exploring the open arms of the EPM. BDNF exon IV gene expression was higher among females, and there was a positive correlation between the BDNF and PORT behavioral parameters. Our findings suggest sex-dependent effects in the PORT task. Females presented higher RA and avoidance behavior profile and expressed higher levels of BDNF exon IV in the mPFC. Moreover, higher BDNF expression was correlated with RA behaviors, which suggests that adolescent females tend to evaluate the risks more than adolescent males and that BDNF gene expression may be mediating decision-making processes.

Neural correlates and determinants of approach-avoidance conflict in the prelimbic prefrontal cortex

The recollection of environmental cues associated with threat or reward allows animals to select the most appropriate behavioral responses. Neurons in the prelimbic (PL) cortex respond to both threat- and reward-associated cues. However, it remains unknown whether PL regulates threat-avoidance vs. reward-approaching responses when an animals’ decision depends on previously associated memories. Using a conflict model in which male Long–Evans rats retrieve memories of shock- and food-paired cues, we observed two distinct phenotypes during conflict: (1) rats that continued to press a lever for food (Pressers) and (2) rats that exhibited a complete suppression in food seeking (Non-pressers). Single-unit recordings revealed that increased risk-taking behavior in Pressers is associated with persistent food-cue responses in PL, and reduced spontaneous activity in PL glutamatergic (PL^GLUT) neurons during conflict. Activating PL^GLUT neurons in Pressers attenuated food-seeking responses in a neutral context, whereas inhibiting PL^GLUT neurons in Non-pressers reduced defensive responses and increased food approaching during conflict. Our results establish a causal role for PL^GLUT neurons in mediating individual variability in memory-based risky decision-making by regulating threat-avoidance vs. reward-approach behaviors.

Trophic factors as potential therapies for treatment of major mental disorders

Notwithstanding major advances in psychotherapeutics, their efficacy and specificity remain limited. The slow onset of beneficial outcomes and numerous adverse effects of widely used medications remain of chief concern, warranting in-depth studies. The majority of frontline therapies are thought to enhance the endogenous monoaminergic drive, to initiate a cascade of molecular events leading to lasting functional and structural plasticity. They also involve alterations in trophic factor signalling, including brain-derived neurotrophic factor (BDNF), VGF (non-acronymic), vascular endothelial growth factor (VEGF), fibroblast growth factor 2 (FGF2), glial cell-derived neurotrophic factor (GDNF), and others. In several major mental disorders, emerging data suggest protective and restorative effects of trophic factors in preclinical models, when applied on their own. Antidepressant outcomes of VGF and FGF2, for instance, were shown in experimental animals, while BDNF and GDNF prove useful in the treatment of addiction, schizophrenia, and autism spectrum disorders. The main challenge with the effective translation of these and other findings in the clinic is the knowledge gap in action mechanisms with potential risks, as well as the lack of effective platforms for validation under clinical settings. Herein, we review the state-of-the-art and advances in the therapeutic use of trophic factors in several major neuropsychiatric disorders.

Psychedelics and Other Psychoplastogens for Treating Mental Illness

Psychedelics have inspired new hope for treating brain disorders, as they seem to be unlike any treatments currently available. Not only do they produce sustained therapeutic effects following a single administration, they also appear to have broad therapeutic potential, demonstrating efficacy for treating depression, post-traumatic stress disorder (PTSD), anxiety disorders, substance abuse disorder, and alcohol use disorder, among others. Psychedelics belong to a more general class of compounds known as psychoplastogens, which robustly promote structural and functional neural plasticity in key circuits relevant to brain health. Here we discuss the importance of structural plasticity in the treatment of neuropsychiatric diseases, as well as the evidence demonstrating that psychedelics are among the most effective chemical modulators of neural plasticity studied to date. Furthermore, we provide a theoretical framework with the potential to explain why psychedelic compounds produce long-lasting therapeutic effects across a wide range of brain disorders. Despite their promise as broadly efficacious neurotherapeutics, there are several issues associated with psychedelic-based medicines that drastically limit their clinical scalability. We discuss these challenges and how they might be overcome through the development of non-hallucinogenic psychoplastogens. The clinical use of psychedelics and other psychoplastogenic compounds marks a paradigm shift in neuropsychiatry toward therapeutic approaches relying on the selective modulation of neural circuits with small molecule drugs. Psychoplastogen research brings us one step closer to actually curing mental illness by rectifying the underlying pathophysiology of disorders like depression, moving beyond simply treating disease symptoms. However, determining how to most effectively deploy psychoplastogenic medicines at scale will be an important consideration as the field moves forward.

The role of circTmeff-1 in incubation of context-induced morphine craving

A progressive increase in drug craving following drug exposure is an important trigger of relapse. CircularRNAs (CircRNAs), key regulators of gene expression, play an important role in neurological diseases. However, the role of circRNAs in drug craving is unclear. In the present study, we trained mice to morphine conditioned place preference (CPP) and collected the nucleus accumbens (NAc) sections on abstinence day 1 (AD1) and day 14 (AD14) for RNA-sequencing. CircTmeff-1, which was highly expressed in the NAc core, was associated with incubation of context-induced morphine craving. The gain- and loss- of function showed that circTmeff-1 was a positive regulator of incubation. Simultaneously, the expression of miR-541-5p and miR-6934-3p were down-regulated in the NAc core during the incubation period. The dual luciferase reporter, RNA pulldown, and fluorescence insitu hybridization assays confirmed that miR-541-5p and miR-6934-3p bind to circTmeff-1 selectively. Furthermore, bioinformatics and western blot analysis suggested that vesicle-associated membrane protein 1 (VAMP1) and neurofascin (NFASC), both overlapping targets of miR-541-5p and miR-6934-3p, were highly expressed during incubation. Lastly, AAV-induced down-regulation of circTmeff-1 decreased VAMP1 and NFASC expression and incubation of morphine craving. These findings suggested that circTmeff-1, a novel circRNA, promotes incubation of context-induced morphine craving by sponging miR-541/miR-6934 in the NAc core. Thus, circTmeff-1 represents a potential therapeutic target for context-induced opioid craving, following prolonged abstinence.

Rottlerin, BDNF, and the impairment of inhibitory avoidance memory

RATIONALE AND OBJECTIVE

As a eukaryotic elongation factor 2 kinase (eEF2K) inhibitor and a mitochondrial uncoupler, oncologists have extensively studied rottlerin. Neuroscientists, however, have accumulated scarce data on the role of rottlerin in affective and cognitive functions. Only two prior studies have, respectively, documented its antidepressant-like effect and how it impairs psychostimulant-supported memory. Whether or not rottlerin would affect aversive memory remains unknown. Hence, we sought to investigate the effects of rottlerin on aversive memory in the inhibitory avoidance (IA) task in mice.

MATERIALS AND METHODS

Male C57BL/6J mice were trained to acquire the IA task. Rottlerin (5 mg/kg, i.p. or 3 μg bilaterally in the hippocampus) or the vehicle was administered before footshock training (acquisition), after footshock training (consolidation), after the memory reactivation (reconsolidation), and before the test (retrieval) in the IA task.

RESULTS

Systemic and intrahippocampal rottlerin impaired the acquisition, consolidation, and retrieval of IA memory, without affecting the reconsolidation process. Rottlerin (5 mg/kg, i.p.) induced a fast-onset and long-lasting increase in the brain-derived neurotrophic factor (BDNF) protein levels in the mouse hippocampus. Systemic injection of 7,8-dihydroxyflavone (7,8-DHF, 30 mg/kg), a BDNF tropomyosin receptor kinase B (TrkB) agonist impaired IA memory consolidation, and treatment with K252a (5 μg/kg), a Trk receptor antagonist, reversed the suppressing effect of rottlerin on IA memory consolidation.

CONCLUSION

Rottlerin impairs IA memory consolidation through the enhancement of BDNF signaling in the mouse hippocampus. Excessive brain BDNF levels can be detrimental to cognitive function. Rottlerin is likely to affect the original memory-associated neuroplasticity. Thus, it can be combined with exposure therapy to facilitate the forgetting of maladaptive aversive memory, such as post-traumatic stress disorder (PTSD).

Microinjection of the BDNF receptor antagonist ANA-12 into the nucleus accumbens and medial-prefrontal cortex attenuates morphine-induced reward memory, and alterations of BDNF levels and apoptotic cells in rats

This study was designed to examine the effects of intra- nucleus accumbens (NAc) of BDNF receptor antagonist ANA-12 on the acquisition and expression and intra- medial-prefrontal cortex (mPFC) of ANA-12 on the extinction and reinstatement of morphine-induced conditioned place preference (CPP) and also BDNF levels and apoptotic neurons in the NAc and mPFC of rats. In this study, adult male Wistar rats (200-250 g) were used. Two separate cannulas were inserted bilaterally into the NAc and/or mPFC. ANA-12 (3 μg/0.5 μl/side) was injected into the NAc and/or mPFC to evaluate the rewarding effects of morphine using a CPP paradigm. Then, the levels of BDNF and apoptotic in the NAc and mPFC were assessed at the end of each treatment phase using ELISA and TUNEL methods, respectively. All of vehicle-treated rats following morphine CPP showed the increase of BDNF levels and apoptotic neurons in the NAc and mPFC. ANA-12 significantly attenuated the acquisition and expression of morphine-induced CPP, BDNF levels and apoptotic neurons in the NAc during the acquisition, but not the expression phase. Also, ANA-12 significantly facilitated the extinction, but no effect on reinstatement of morphine CPP, and decreased BDNF levels and apoptotic neurons in the mPFC during the extinction, but not the reinstatement. We conclude that blocking TrkB with ANA-12 showed therapeutic effects on morphine-associated reward memory and neuronal death in the NAc and mPFC induced by morphine CPP. Thus, the BDNF-TrkB signaling may be important in the acquisition, expression, extinction, but not the reinstatement of morphine CPP.

Experimentally altered male mating behaviour affects offspring exploratory behaviour via nongenetic paternal effects

Evidence is emerging that fathers can have nongenetic effects on the phenotypes of their offspring. Most studies have focused on the role that nongenetic modifications to sperm can have on offspring phenotype; however, fathers can also have nongenetic effects on offspring through their interactions with females, called female-mediated paternal effects. These effects can occur in situations where male phenotype, e.g. behaviour or morphology, affects female stress and/or provisioning of offspring. These effects are potentially widespread, but few studies have explicitly investigated the role of female-mediated paternal effects on offspring phenotype. Here, we asked if male mating interactions can affect offspring via female mediated paternal effects in the Trinidadian guppy, Poecilia reticulata. To do this, we manipulated mating behaviour by: (i) administering a drug known to affect the neurotransmitter dopamine, and (ii) varying the familiarity of potential mates, which affects attractiveness in this species. With these treatments, we successfully manipulated the mating behaviour of male guppies and female preference for those males. Further, we found significant effects of sire mating behaviour, sire drug treatment, and parental familiarity status on behavioural measures of offspring anxiety in response to a novel object. Because Control offspring of 'familiar' and 'unfamiliar' pairs differed in their behaviour, our results cannot be solely attributed to potential nongenetic modifications to sperm caused by the drug. These results emphasize the importance of female-mediated paternal effects, including those caused by altered male mating behaviour, in shaping offspring phenotype.

Growth Factors and Alcohol Use Disorder

Neurotrophic growth factors were originally characterized for their support in neuronal differentiation, outgrowth, and survival during development. However, it has been acknowledged that they also play a vital role in the adult brain. Abnormalities in growth factors have been implicated in a variety of neurological and psychiatric disorders, including alcohol use disorder (AUD). This work focuses on the interaction between alcohol and growth factors. We review literature suggesting that several growth factors play a unique role in the regulation of alcohol consumption, and that breakdown in these growth factor systems is linked to the development of AUD. Specifically, we focus on the brain-derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF), fibroblast growth factor 2 (FGF2), and insulin growth factor 1 (IGF-1). We also review the literature on the potential role of midkine (MDK) and pleiotrophin (PTN) and their receptor, anaplastic lymphoma kinase (ALK), in AUD. We show that alcohol alters the expression of these growth factors or their receptors in brain regions previously implicated in addiction, and that manipulations on these growth factors and their downstream signaling can affect alcohol-drinking behaviors in animal models. We conclude that there is a need for translational and clinical research to assess the therapeutic potential of new pharmacotherapies targeting these systems.

Increased relapse to cocaine-seeking in a genetic model for depression

The greatest difficulty in treating cocaine addiction is the enormous rates of relapse, which occur despite immense negative consequences. Relapse risks are even greater in addicts with comorbid depression, perhaps because they use drugs to alleviate depressive symptoms. Only a few preclinical studies have examined this comorbidity, mostly exploring depressive-like effects following drug exposure. We examined rats from two different depression-like models: (a) chronic-mild-stress (CMS), which respond to antidepressant medications and (b) depressed-rat-line (DRL), a genetic model of selective breeding, which is less responsive to antidepressant medications. We tested addictive behaviors in a cocaine self-administration procedure, including the "conflict model," where drug-seeking and relapse encounter adverse consequences: an electrified grid in front of the drug-delivering lever. Following behavioral testing, we explored a potential association between behavioral outcomes and protein expression of brain-derived neurotrophic factor (BDNF). We found that DRL rats self-administer more cocaine compared with both CMS and controls, while CMS and control groups did not differ significantly. Notably, DRL but not CMS rats, displayed higher rates of relapse than controls, and expressed higher levels of BDNF in the prelimbic cortex (PLC). Potential translation of these results suggest that medication-resistant depressed patients tend to consume more drugs and are more susceptible to relapse. The increase in PLC BDNF levels is consistent with previous rat models of depression, and concomitantly, with its suggested role in promoting cocaine-seeking.

Facilitative effects of environmental enrichment for cocaine relapse prevention are dependent on extinction training context and involve increased TrkB signaling in dorsal hippocampus and ventromedial prefrontal cortex

Cocaine-cue extinction training combined with brief interventions of environmental enrichment (EE) was shown previously to facilitate extinction and attenuate reacquisition of cocaine self-administration in rats. It is unknown whether or not the usefulness of this approach would be undermined if extinction training took place in a novel rather than familiar context. Drawing on previous studies involving pharmacological interventions, we hypothesized that the facilitative effects of EE for cocaine relapse prevention would be independent of the context used for extinction training. Rats trained to self-administer cocaine underwent cocaine-cue extinction training in either the familiar self-administration context or a novel context, with or without EE. Rats then were tested for reacquisition of cocaine self-administration in the familiar context. Target brain regions were lysed and probed for memory-related changes in receptors for glutamate and BDNF by western blotting. Contrary to our hypothesis, the facilitative effects of EE for cocaine relapse prevention were dependent on the context used for extinction training. While EE facilitated extinction regardless of context used, it inhibited cocaine relapse only after extinction training in the familiar context. EE was associated with increased GluA2 in nucleus accumbens, TrkB in dorsal hippocampus and activated TrkB in ventromedial prefrontal cortex. Of these, the changes in dorsal hippocampus and ventromedial prefrontal cortex mirrored outcomes of the cocaine relapse tests in that these changes were specific to rats receiving EE plus extinction training in the familiar context. These findings support a role for hippocampal-prefrontal BDNF-TrkB signaling in extinction-based relapse prevention strategies involving EE.

Human Plasma BDNF Is Associated With Amygdala-Prefrontal Cortex Functional Connectivity and Problem Drinking Behaviors

BACKGROUND

Preclinical studies suggest that decreased levels of brain-derived neurotrophic factor in the amygdala play a role in anxiety and alcohol use disorder. The association between brain-derived neurotrophic factor levels and amygdala function in humans with alcohol use disorder is still unclear, although neuroimaging studies have also implicated the amygdala in alcohol use disorder and suggest that alcohol use disorder is associated with disrupted functional connectivity between the amygdala and prefrontal cortex during aversive states.

METHODS

The current study investigated whether plasma brain-derived neurotrophic factor levels in individuals with and without alcohol use disorder (n = 57) were associated with individual differences in amygdala reactivity and amygdala-prefrontal cortex functional connectivity during 2 forms of aversive responding captured via functional magnetic resonance imaging: anxiety elicited by unpredictable threat of shock and fear elicited by predictable threat of shock. We also examined whether brain-derived neurotrophic factor and brain function were associated with binge drinking episodes and alcohol use disorder age of onset.

RESULTS

During anxiety, but not fear, lower levels of plasma brain-derived neurotrophic factor were associated with less connectivity between the left amygdala and the medial prefrontal cortex and the inferior frontal gyrus. In addition, within individuals with alcohol use disorder (only), lower levels of brain-derived neurotrophic factor and amygdala-medial prefrontal cortex functional connectivity during anxiety were associated with more binge episodes within the past 60 days and a lower age of alcohol use disorder onset. There were no associations between brain-derived neurotrophic factor levels and focal amygdala task reactivity.

CONCLUSIONS

Together, the results indicate that plasma brain-derived neurotrophic factor levels are related to amygdala circuit functioning in humans, particularly during anxiety, and these individual differences may relate to drinking behaviors.

Effect of chronic methamphetamine injection on levels of BDNF mRNA and its CpG island methylation in prefrontal cortex of rats

Methamphetamine (METH) is a highly addictive psychostimulant. Its abuse causes problems in cognition, attention, or psychiatric conditions such as psychosis. Prefrontal cortex is involved in many aspects of drug addiction and in mental disorders similar to those triggered by METH. Brain-derived neurotrophic factor (BDNF), plays important roles in modulating different aspects of addiction, and is implicated in psychiatric conditions reminiscent of those suffered by METH-abusers. Male Wistar rats were intra-peritoneally injected with METH (8 mg/kg/day) for 14 days while control group received normal saline. After extraction of prefrontal cortices, expression of BDNF IV splice variant and methylation level of its CpG island were evaluated. The relative expression of BDNF IV in METH-treated group was 2.15 fold higher than the control group. Seven out of 29 CpG sites were significantly hypomethylated in the METH group, although none survived Bonferroni adjustment. However, the overall methylation level of the 29 CpGs was significantly lower in METH cases than in controls. We discuss the importance of our results and its implications in detail.

Action-Outcome Expectancies Require Orbitofrontal Neurotrophin Systems in Naïve and Cocaine-Exposed Mice

Cocaine use during adolescence decreases the likelihood that individuals will seek treatment for recurrent drug use. In rodents, developmental cocaine exposure weakens action-consequence decision-making, causing a deferral to familiar, habit-like behavioral response strategies. Here, we aimed to improve action-outcome decision-making. We found that acute pharmacological stimulation of the tyrosine/tropomyosin receptor kinase B (trkB) via 7,8-dihydroxyflavone (7,8-DHF) or 3,4-methylenedioxymethamphetamine (MDMA) blocked cocaine-induced habit biases by strengthening memory for action-outcome associations. We believe that MDMA acts by stimulating neurotrophin/trkB systems in the orbitofrontal cortex (OFC), a region involved in prospectively evaluating the consequences of one's action, because 1) MDMA also increased brain-derived neurotrophic factor (BDNF) in the OFC, 2) MDMA corrected habit biases due to Bdnf loss in the OFC, and 3) overexpression of a truncated isoform of trkB occluded the memory-enhancing effects of MDMA. Thus, selecting actions based on their consequences requires BDNF-trkB in the OFC, the stimulation of which may improve goal attainment in both drug-naïve and cocaine-exposed individuals. SIGNIFICANCE STATEMENT: Cocaine use during adolescence decreases the likelihood that individuals will seek treatment for recurrent drug use, even as adults. Understanding how early-life cocaine exposure impacts goal-oriented action and prospective decision-making in adulthood is thus important. One key aspect of goal-directed decision-making is anticipating the consequences of one's actions, a process that likely involves the orbitofrontal cortex (OFC). In rodents, developmental cocaine exposure weakens action-consequence decision-making, causing a deferral to familiar, habit-like behavioral response strategies. Here, we report that we can improve memory for action-consequence relationships by stimulating neurotrophic factors, which support cell survival, development, and plasticity in the brain. With strengthened action-consequence associations, cocaine-exposed mice regain the ability to optimally select actions based on their likely outcomes. Brain region-selective manipulations reveal that neurotrophin systems in the OFC are necessary for stable memory of action-consequence relationships.

A Single Ketamine Infusion Combined With Mindfulness-Based Behavioral Modification to Treat Cocaine Dependence: A Randomized Clinical Trial

OBJECTIVE

Research has suggested that subanesthetic doses of ketamine may work to improve cocaine-related vulnerabilities and facilitate efforts at behavioral modification. The purpose of this trial was to test whether a single ketamine infusion improved treatment outcomes in cocaine-dependent adults engaged in mindfulness-based relapse prevention.

METHODS

Fifty-five cocaine-dependent individuals were randomly assigned to receive a 40-minute intravenous infusion of ketamine (0.5 mg/kg) or midazolam (the control condition) during a 5-day inpatient stay, during which they also initiated a 5-week course of mindfulness-based relapse prevention. Cocaine use was assessed through self-report and urine toxicology. The primary outcomes were end-of-study abstinence and time to relapse (defined as first use or dropout).

RESULTS

Overall, 48.2% of individuals in the ketamine group maintained abstinence over the last 2 weeks of the trial, compared with 10.7% in the midazolam group (intent-to-treat analysis). The ketamine group was 53% less likely (hazard ratio=0.47; 95% CI=0.24, 0.92) to relapse (dropout or use cocaine) compared with the midazolam group, and craving scores were 58.1% lower in the ketamine group throughout the trial (95% CI=18.6, 78.6); both differences were statistically significant. Infusions were well tolerated, and no participants were removed from the study as a result of adverse events.

CONCLUSIONS

A single ketamine infusion improved a range of important treatment outcomes in cocaine-dependent adults engaged in mindfulness-based behavioral modification, including promoting abstinence, diminishing craving, and reducing risk of relapse. Further research is needed to replicate these promising results in a larger sample.

Activation of Trace Amine-Associated Receptor 1 Stimulates an Antiapoptotic Signal Cascade via Extracellular Signal-Regulated Kinase 1/2

Methamphetamine (MA) is highly addictive and neurotoxic, causing cell death in humans and in rodent models. MA, along with many of its analogs, is an agonist at the G protein-coupled trace amine-associated receptor 1 (TAAR1). TAAR1 activation protects against MA-induced degeneration of dopaminergic neurons, suggesting that TAAR1 plays a role in regulating MA-induced neurotoxicity. However, the mechanisms involved in TAAR1's role in neurotoxicity and cell death have not been described in detail. In this study, we investigated the apoptosis pathway in Taar1 wild-type (WT) and knockout (KO) mice and in cells expressing the recombinant receptor. Bcl-2, an antiapoptotic protein, was upregulated ∼3-fold in the midbrain area (substantial nigra and ventral tegmental area) in Taar1 KO compared with WT mice, and MA significantly increased Bcl-2 expression in WT mice but decreased Bcl-2 expression in KO mice. The proapoptotic protein Bax did not differ across genotype or in response to MA. Bcl-2 expression was significantly upregulated by the TAAR1 agonist RO5166017 ((S)-4-[(ethyl-phenyl-amino)-methyl]-4,5-dihydro-oxazol-2-ylamine) in cells expressing the recombinant mouse TAAR1. Additionally, activation of TAAR1 by RO5166017 increased phosphorylation of extracellular signal-regulated kinase (ERK) 1/2, and protein kinase B (AKT), but only inhibition of ERK1/2 phosphorylation prevented TAAR1-induced increases in Bcl-2 levels, indicating that TAAR1 activation increases Bcl-2 through an ERK1/2-dependent pathway. All changes to ERK1/2 pathway intermediates were blocked by the TAAR1 antagonist, N-(3-ethoxyphenyl)-4-(1-pyrrolidinyl)-3-(trifluoromethyl) benzamide. These findings suggest that TAAR1 activation protects against MA-induced cell apoptosis and TAAR1 may play a role in cell death in neurodegenerative diseases. SIGNIFICANCE STATEMENT: Methamphetamine stimulates TAAR1, a G protein-coupled receptor. The role and mechanisms for TAAR1 in methamphetamine-induced neurotoxicity are not known. Here, we report that, in genetic mouse models and cells expressing the recombinant receptor, TAAR1 activates the ERK1/2 pathway but not the AKT pathway to upregulate the antiapoptotic protein Bcl-2, which protects cells from drug-induced toxicity.

Repeated cocaine exposure dysregulates BDNF expression and signaling in the mesocorticolimbic pathway of the adolescent rat

Objectives: Long-term abstinence following cocaine exposure up-regulates brain-derived neurotrophic factor (BDNF) expression in the mesocorticolimbic pathway. Given the increased vulnerability to drug abuse typical of adolescence, we hypothesized that changes in BDNF expression may become manifest early after the end of cocaine treatment in the adolescent brain.Methods: Rats received cocaine injections from postnatal day 28 (PND28) to PND42 and the mesocorticolimbic expression of BDNF was measured by real-time PCR and Western blotting at PND43.Results: In the ventral tegmental area, BDNF-tropomyosin receptor kinase B (TrΚB) expression and phosphorylation are enhanced while the intracellular signaling is unaltered. In the nucleus accumbens (NAc) shell and core, BDNF and its signaling were down-regulated. In the prelimbic (PL) cortex, we found reduced BDNF expression and increased phosphoprylation of TrΚB, ERK and AKT. In the infralimbic (IL) cortex, increased BDNF expression was coupled with reduced activity and expression of its downstream targets. To evaluate the role of glutamate on BDNF-independent changes, we investigated the expression of the transporter GLT-1 and the activation of the NMDA receptor subunit GluN2B, which were both increased in the PL cortex while reduced in the IL cortex.Conclusions: Our results show that adolescent cocaine exposure modulates BDNF system early after treatment in the mesocorticolimbic pathway, identifying a complex but specific set of changes that could provide clues for treatment.

Accumbens brain-derived neurotrophic factor (BDNF) transmission inhibits cocaine seeking

Brain-derived neurotrophic factor (BDNF) regulates a variety of physiological processes, and several studies have explored the role of BDNF in addiction-related brain regions like the nucleus accumbens core (NAcore). We sought to understand the rapid effects of endogenous BDNF on cocaine seeking. Rats were trained to self-administer cocaine and extinguished. We then microinjected two inhibitors of BDNF stimulation of tropomyosin receptor kinase B (TrkB), the non-competitive receptor antagonist ANA-12 and TrkB/Fc, a fusion protein that binds BDNF and prevents TrkB stimulation. Blocking TrkB or inactivating BDNF in NAcore potentiated active lever pressing, showing that endogenous BDNF tone was present and supplying inhibitory tone on cue-induced reinstatement. To determine if exogenous BDNF also negatively regulated reinstatement, BDNF was microinjected into NAcore 15 minutes before cue-induced reinstatement. BDNF decreased cocaine seeking through TrkB receptor binding, but had no effect on inactive lever pressing, spontaneous or cocaine-induced locomotion, or on reinstated sucrose seeking. BDNF-infusion potentiated within trial extinction when microinjected in the NAcore during cue- and context + cue induced reinstatement, and the inhibition of lever pressing lasted at least 3 days post injection. Although decreased reinstatement endured for 3 days when BDNF was administered prior to a reinstatement session, when microinjected before an extinction session or in the home cage, BDNF did not alter subsequent cued-reinstatement. Together, these data show that endogenous BDNF acts on TrKB to provide inhibitory tone on reinstated cocaine seeking, and this effect was recapitulated by exogenous BDNF.

Multigenerational epigenetic inheritance: One step forward, two generations back

Modifications to DNA and histone proteins serve a critical regulatory role in the developing and adult brain, and over a decade of research has established the importance of these "epigenetic" modifications in a wide variety of brain functions across the lifespan. Epigenetic patterns orchestrate gene expression programs that establish the phenotypic diversity of various cellular classes in the central nervous system, play a key role in experience-dependent gene regulation in the adult brain, and are commonly implicated in neurodevelopmental, psychiatric, and neurodegenerative disease states. In addition to these established roles, emerging evidence indicates that epigenetic information can potentially be transmitted to offspring, giving rise to inter- and trans-generational epigenetic inheritance phenotypes. However, our understanding of the cellular events that participate in this information transfer is incomplete, and the ability of this transfer to overcome complete epigenetic reprogramming during embryonic development is highly controversial. This review explores the existing literature on multigenerational epigenetic mechanisms in the central nervous system. First, we focus on the cellular mechanisms that may perpetuate or counteract this type of information transfer, and consider how epigenetic modification in germline and somatic cells regulate important aspects of cellular and organismal development. Next, we review the potential phenotypes resulting from ancestral experiences that impact gene regulatory modifications, including how these changes may give rise to unique metabolic phenotypes. Finally, we discuss several caveats and technical limitations that influence multigenerational epigenetic effects. We argue that studies reporting multigenerational epigenetic changes impacting the central nervous system must be interpreted with caution, and provide suggestions for how epigenetic information transfer can be mechanistically disentangled from genetic and environmental influences on brain function.

Epigenetic effects of paternal cocaine on reward stimulus behavior and accumbens gene expression in mice

Paternal cocaine use causes phenotypic alterations in offspring behavior and associated neural processing. In rodents, changes in first generation (F1) offspring include drug reward behavior, circadian timing, and anxiety responses. This study, utilizing a murine (C57BL/6J) oral cocaine model, examines the effects of paternal cocaine exposure on fundamental characteristics of offspring reward responses, including: 1) the extent of cocaine-induced effects after different durations of sire drug withdrawal; 2) sex- and drug-dependent differences in F1 reward preference; 3) effects on second generation (F2) cocaine preference; and 4) corresponding changes in reward area (nucleus accumbens) mRNA expression. We demonstrate that paternal cocaine intake over a single ˜40-day spermatogenic cycle significantly decreased cocaine (but not ethanol or sucrose) preference in a sex-specific manner in F1 mice from sires mated 24 h after drug withdrawal. However, F1 offspring of sires bred 4 months after withdrawal did not exhibit altered cocaine preference. Altered cocaine preference also was not observed in F2's. RNASeq analyses of F1 accumbens tissue revealed changes in gene expression in male offspring of cocaine-exposed sires, including many genes not previously linked to cocaine addiction. Enrichment analyses highlight genes linked to CNS development, synaptic signaling, extracellular matrix, and immune function. Expression correlation analyses identified a novel target, Fam19a4, that may negatively regulate many genes in the accumbens, including genes already identified in addiction. Collectively, these results reveal that paternal cocaine effects in F1 offspring may involve temporally limited epigenetic germline effects and identify new genetic targets for addiction research.

Ibogaine Administration Modifies GDNF and BDNF Expression in Brain Regions Involved in Mesocorticolimbic and Nigral Dopaminergic Circuits

Ibogaine is an atypical psychedelic alkaloid, which has been subject of research due to its reported ability to attenuate drug-seeking behavior. Recent work has suggested that ibogaine effects on alcohol self-administration in rats are related to the release of Glial cell Derived Neurotrophic Factor (GDNF) in the Ventral Tegmental Area (VTA), a mesencephalic region which hosts the soma of dopaminergic neurons. Although previous reports have shown ibogaine’s ability to induce GDNF expression in rat midbrain, there are no studies addressing its effect on the expression of GDNF and other neurotrophic factors (NFs) such as Brain Derived Neurotrophic Factor (BDNF) or Nerve Growth Factor (NGF) in distinct brain regions containing dopaminergic neurons. In this work, we examined the effect of ibogaine acute administration on the expression of these NFs in the VTA, Prefrontal Cortex (PFC), Nucleus Accumbens (NAcc) and the Substantia Nigra (SN). Rats were i.p. treated with ibogaine 20 mg/kg (I₂₀), 40 mg/kg (I₄₀) or vehicle, and NFs expression was analyzed after 3 and 24 h. At 24 h an increase of the expression of the NFs transcripts was observed in a site and dose dependent manner. Only for I₄₀, GDNF was selectively upregulated in the VTA and SN. Both doses elicited a large increase in the expression of BDNF transcripts in the NAcc, SN and PFC, while in the VTA a significant effect was found only for I₄₀. Finally, NGF mRNA was upregulated in all regions after I₄₀, while I₂₀ showed a selective upregulation in PFC and VTA. Regarding protein levels, an increase of GDNF was observed in the VTA only for I₄₀ but no significant increase for BDNF was found in all the studied areas. Interestingly, an increase of proBDNF was detected in the NAcc for both doses. These results show for the first time a selective increase of GDNF specifically in the VTA for I₄₀ but not for I₂₀ after 24 h of administration, which agrees with the effective dose found in previous self-administration studies in rodents. Further research is needed to understand the contribution of these changes to ibogaine’s ability to attenuate drug-seeking behavior.

Different doses of methamphetamine alter long-term potentiation, level of BDNF and neuronal apoptosis in the hippocampus of reinstated rats

Methamphetamine (METH) is a psychostimulant. The precise mechanisms of its effects remain unknown and current relapse treatments have low efficacy. However, brain-derived neurotrophic factor (BDNF) and neuronal plasticity are essential contributors, despite paradoxical reports and a lack of comprehensive studies. Therefore, we investigated the effects of different doses of METH on long-term potentiation (LTP), BDNF expression and neuronal apoptosis in the hippocampus of reinstated rats. Rats were injected intraperitoneally with METH (1, 5, or 10 mg/kg) or saline, and trained in a conditioned place preference paradigm. Following implementation of the reinstatement model, electrophysiology, western blotting and TUNEL assay were performed to assess behavior, LTP components, BDNF expression, and neuronal apoptosis, respectively. The results demonstrated that the preference scores, population spike amplitude and BDNF expression markedly decreased in the METH (10 mg/kg) group compared with the other groups. In contrast, METH (5 mg/kg) significantly increased these factors more than the control group. There was no change in variables between METH (1 mg/kg) and the control group. Also, apoptosis of the hippocampus was increased in the METH (10 mg/kg) group compared with the METH (5 mg/kg) group. These results suggest that alterations in synaptic plasticity, expression of BDNF and neuronal apoptosis in the hippocampus has a vital role in the context-induced reinstatement of METH seeking.

Effects of a histone deacetylase 3 inhibitor on extinction and reinstatement of cocaine self-administration in rats

RATIONALE

A challenge in treating substance use disorder is that successful treatment often does not persist, resulting in relapse and continued drug seeking. One approach to persistently weaken drug-seeking behaviors is to pair exposure to drug-associated cues or behaviors with delivery of a compound that may strengthen the inhibition of the association between drug cues and behavior.

OBJECTIVES

We evaluated whether a selective histone deacetylase 3 (HDAC3) inhibitor could promote extinction and weaken contextual control of operant drug seeking after intravenous cocaine self-administration.

METHODS

Male Long-Evans rats received a systemic injection of the HDAC3 inhibitor RGFP966 either before or immediately after the first extinction session. Persistence of extinction was tested over subsequent extinction sessions, as well as tests of reinstatement that included cue-induced reinstatement, contextual renewal, and cocaine-primed reinstatement. Additional extinction sessions occurred between each reinstatement test. We also evaluated effects of RGFP966 on performance and motivation during stable fixed ratio operant responding for cocaine and during a progressive ratio of reinforcement.

RESULTS

RGFP966 administered before the first extinction session led to significantly less responding during subsequent extinction and reinstatement tests compared to vehicle-injected rats. Follow-up studies found that these effects were not likely due to a performance deficit or a change in motivation to self-administer cocaine, as injections of RGFP966 had no effect on stable responding during a fixed or progressive ratio schedule. In addition, RGFP966 administered just after the first extinction session had no effect during early extinction and reinstatement tests, but weakened long-term responding during later extinction sessions.

CONCLUSIONS

These results suggest that a systemic injection of a selective HDAC3 inhibitor can enhance extinction and suppress reinstatement after cocaine self-administration. The finding that behavioral and pharmacological manipulations can be combined to decrease drug seeking provides further potential for treatment by epigenetic modulation.

Role of prefrontal cortex in the extinction of drug memories

It has been recognized that drug addiction engages aberrant process of learning and memory, and substantial studies have focused on developing effective treatment to erase the enduring drug memories to reduce the propensity to relapse. Extinction, a behavioral intervention exposing the individuals to the drug-associated cues repeatedly, can weaken the craving and relapse induced by drug-associated cues, but its clinic efficacy is limited. A clear understanding of the neuronal circuitry and molecular mechanism underlying extinction of drug memory will facilitate the successful use of extinction therapy in clinic. As a key component of mesolimbic system, medial prefrontal cortex (mPFC) has received particular attention largely in that PFC stands at the core of neural circuits for memory extinction and manipulating mPFC influences extinction of drug memories and subsequent relapse. Here, we review the recent advances in both animal models of drug abuse and human addicted patients toward the understanding of the mechanistic link between mPFC and drug memory, with particular emphasis on how mPFC contributes to the extinction of drug memory at levels ranging from neuronal architecture, synaptic plasticity to molecular signaling and epigenetic regulation, and discuss the clinic relevance of manipulating the extinction process of drug memory to prevent craving and relapse through enhancing mPFC function.

Distinct regulation pattern of Egr-1, BDNF and Arc during morphine-withdrawal conditioned place aversion paradigm: Role of glucocorticoids

Negative affective aspects of opiate abstinence contribute to the persistence of substance abuse. Importantly, interconnected brain areas involved in aversive motivational processes, such as the ventral tegmental area (VTA) and medial prefrontal cortex (mPFC), become activated when animals are confined to withdrawal-paired environments. In the present study, place aversion was elicited in sham and adrenalectomized (ADX) animals by conditioned naloxone-precipitated drug withdrawal following exposure to chronic morphine. qPCR was employed to detect the expression of brain derived neurotrophic factor (Bdnf) and the immediate early genes (IEG) early growth response 1 (Egr-1) and activity-regulated cytoskeletal-associated protein (Arc) mRNAs in the VTA and mPFC at different time points of the conditioned place aversion (CPA) paradigm: after the conditioning phase and after the test phase. Sham + morphine rats exhibited robust CPA, which was impaired in ADX + morphine animals. Egr-1 and Arc were induced in the VTA and mPFC after morphine-withdrawal conditioning phase. Furthermore, Bdnf expression was enhanced in the VTA during the test phase. Bdnf induction seemed to be glucocorticoid-dependent, given that was correlated with HPA axis function and was not observed in morphine-dependent ADX animals. In addition, BDNF regulation and function was opposite in the VTA and mPFC during aversive-withdrawal memory retrieval. Our results suggest that IEGs and BDNF in these brain regions may play key roles in mediating the negative motivational component of opiate withdrawal.

Biphasic effect of abstinence duration following cocaine self-administration on spine morphology and plasticity-related proteins in prelimbic cortical neurons projecting to the nucleus accumbens core

Cocaine self-administration (SA) in rats dysregulates glutamatergic signaling in the prelimbic (PrL) cortex and glutamate release in the nucleus accumbens (NA) core, promoting cocaine seeking. PrL adaptations that affect relapse to drug seeking emerge during the first week of abstinence, switching from an early (2 h) hypoglutamatergic state to a later (7 days) hyperglutamatergic state. Different interventions that normalize glutamatergic signaling in PrL cortex at each timepoint are necessary to suppress relapse. We hypothesized that plasticity-related proteins that regulate glutamatergic neurotransmission as well as dendritic spine morphology would be biphasically regulated during these two phases of abstinence in PrL cortical neurons projecting to the NA core (PrL-NA core). A combinatorial viral approach was used to selectively label PrL-NA core neurons with an mCherry fluorescent reporter. Male rats underwent 2 weeks of cocaine SA or received yoked-saline infusions and were perfused either 2 h or 7 days after the final SA session. Confocal microscopy and 3D reconstruction analyses were performed for Fos and pCREB immunoreactivity (IR) in the nucleus of layer V PrL-NA core neurons and GluA1-IR and GluA2-IR in apical dendritic spines of the same neurons. Here, we show that cocaine SA decreased PrL-NA core spine head diameter, nuclear Fos-IR and pCREB-IR, and GluA1-IR and GluA2-IR in putative mushroom-type spines 2 h after the end of cocaine SA, whereas the opposite occurred following 1 week of abstinence. Our findings reveal biphasic, abstinence duration-dependent alterations in structural plasticity and relapse-related proteins in the PrL-NA core pathway after cocaine SA.

ARC and BDNF expression after cocaine self-administration or cue-induced reinstatement of cocaine seeking in adolescent and adult male rats

Recreational drug use peaks during adolescence. Our research with adolescent vs adult male rats, however, shows that rats taking cocaine as adolescents have lower levels of cue-induced reinstatement of drug-seeking than adults, despite similar levels of intravenous (i.v.) cocaine self-administration. Lower rates of reinstatement in younger rats could be explained by higher levels of brain plasticity. Two neuroplasticity-related genes, activity-regulated cytoskeletal-associated gene (Arc) and brain-derived neurotrophic factor (Bdnf), influence cocaine self-administration and cue-induced reinstatement. We tested whether reinstatement of cocaine seeking correlates with expression of these genes in reinforcement-related brain regions. Adolescent and adult male rats (postnatal day 35 or 83-95 at start) were allowed to acquire lever-pressing maintained by i.v. infusions of cocaine in daily 2-h sessions over 13 days. At one of three experimental time points, rats were sacrificed and tissue collected to analyze Arc and Bdnf mRNA by in situ hybridization in the entire medial prefrontal cortex and entire nucleus accumbens, as well as relevant subregions: prelimbic cortex, infralimbic cortex, and nucleus accumbens core and shell. Despite taking similar amounts of cocaine, adolescents reinstated less than adults. Gene expression was most notable in the prelimbic cortex, was generally higher in adolescent-onset groups, and was higher with longer abstinence. These data partially support the hypothesis that higher levels of Arc and/or Bdnf gene expression in reinforcement-related brain regions of younger animals contribute to lower rates of extinction responding and/or reinstatement. Future studies should include mechanistic analysis of Arc, Bdnf, and their signaling pathways in age-dependent effects of cocaine.

Minding the brain: the role of pharmacotherapy in substance-use disorder treatment

With its medicalization as a brain-based disease, addiction has come to be regarded as amenable to biomedical treatment approaches, most commonly pharmacotherapy. Various vulnerabilities are recognized to contribute to maladaptive substance use, and have been linked to diverse neurobiological alterations that may be targeted with pharmacotherapy: withdrawal, craving and cue reactivity, and aberrant reward processing are the most significant. Here, we summarize current thinking regarding pharmacotherapy for substance-use disorders, grouping medications by the type of vulnerability they propose to address and providing insight into their neurobiological mechanisms. We also examine the limitations of the brain-based disease model in addiction treatment, especially as these shortcomings pertain to the place of pharmacotherapy in recovery. We conclude by sketching a framework whereby medications might be integrated fruitfully with other interventions, such as behavioral, existential, or peer-based treatments, targeting aspects of addiction beyond neurobiological deficits.

Divergent Prelimbic Cortical Pathways Interact with BDNF to Regulate Cocaine-seeking

A single BDNF microinfusion into prelimbic (PrL) cortex immediately after the last cocaine self-administration session decreases relapse to cocaine-seeking. The BDNF effect is blocked by NMDAR antagonists. To determine whether synaptic activity in putative excitatory projection neurons in PrL cortex is sufficient for BDNF's effect on relapse, the PrL cortex of male rats was infused with an inhibitory Designer Receptor Exclusively Activated by Designer Drugs (DREADD) viral vector driven by an αCaMKII promoter. Immediately after the last cocaine self-administration session, rats were injected with clozapine-N-oxide 30 min before an intra-PrL BDNF microinfusion. DREADD-mediated inhibition of the PrL cortex blocked the BDNF-induced decrease in cocaine-seeking after abstinence and cue-induced reinstatement after extinction. Unexpectedly, DREADD inhibition of PrL neurons in PBS-infused rats also reduced cocaine-seeking, suggesting that divergent PrL pathways affect relapse. Next, using a cre-dependent retroviral approach, we tested the ability of DREADD inhibition of PrL projections to the NAc core or the paraventricular thalamic nucleus (PVT) to alter cocaine-seeking in BDNF- and PBS-infused rats. Selective inhibition of the PrL-NAc pathway at the end of cocaine self-administration blocked the BDNF-induced decrease in cocaine-seeking but had no effect in PBS-infused rats. In contrast, selective inhibition of the PrL-PVT pathway in PBS-infused rats decreased cocaine-seeking, and this effect was prevented in BDNF-infused rats. Thus, activity in the PrL-NAc pathway is responsible for the therapeutic effect of BDNF on cocaine-seeking whereas inhibition of activity in the PrL-pPVT pathway elicits a similar therapeutic effect in the absence of BDNF.SIGNIFICANCE STATEMENT The major issue in cocaine addiction is the high rate of relapse. However, the neuronal pathways governing relapse remain unclear. Using a pathway-specific chemogenetic approach, we found that BDNF differentially regulates two key prelimbic pathways to guide long-term relapse. Infusion of BDNF in the prelimbic cortex during early withdrawal from cocaine self-administration decreases relapse that is prevented when neurons projecting from the prelimbic cortex to the nucleus accumbens core are inhibited. In contrast, BDNF restores relapse when neurons projecting from the prelimbic cortex to the posterior paraventricular thalamic nucleus are inhibited. This study demonstrates that two divergent cortical outputs mediate relapse that is regulated in opposite directions by infusing BDNF in the prelimbic cortex during early withdrawal from cocaine.

Multigenerational and transgenerational effects of paternal exposure to drugs of abuse on behavioral and neural function

Addictions are highly heritable disorders, with heritability estimates ranging from 39% to 72%. Multiple studies suggest a link between paternal drug abuse and addiction in their children. However, patterns of inheritance cannot be explained purely by Mendelian genetic mechanisms. Exposure to drugs of abuse results in epigenetic changes that may be passed on through the germline. This mechanism of epigenetic transgenerational inheritance may provide a link between paternal drug exposure and addiction susceptibility in the offspring. Recent studies have begun to investigate the effect of paternal drug exposure on behavioral and neurobiological phenotypes in offspring of drug-exposed fathers in rodent models. This review aims to discuss behavioral and neural effects of paternal exposure to alcohol, cocaine, opioids, and nicotine. Although a special focus will be on addiction-relevant behaviors, additional behavioral effects including cognition, anxiety, and depressive-like behaviors will be discussed.

Overexpression of neuronal RNA-binding protein HuD increases reward induced reinstatement of an instrumental response

The neuronal RNA-binding protein HuD is involved in synaptic plasticity and the molecular mechanisms of learning and memory. Previously, we have shown that HuD is upregulated after both spatial and addiction-associated forms of learning, such as conditioned place preference. However, what role HuD plays in non-drug dependent learning and memory is not fully understood. In order to elucidate the role that HuD plays in non-drug appetitive behavior, we assessed mice over-expressing HuD (HuD_OE) throughout the forebrain on the acquisition of an instrumental response for a non-sucrose food reward utilizing a touch-screen paradigm. Next, we examined whether HuD level would alter the extinction or reward-induced reinstatement of responding. We found that HuD_OE acquired and extinguished the instrumental response at rates similar to control littermates with no significant alterations in secondary measures of motor behavior or motivation. However, HuD_OE reinstated their responding for food reward at rates significantly higher than control animals after a brief presentation of reward. These results suggest that HuD positively regulates the reinstatement of natural reward seeking and supports the role of HuD in forms of learning and memory associated with seeking of appetitive rewards.

BDNF infusion into the MPN mag is sufficient to restore copulatory behavior in the castrated Syrian hamster

Testosterone plays a key role in the expression of male sex behavior by influencing cellular activity and synapses within the magnocellular medial preoptic nucleus (MPN mag), a sub-nucleus of the medial preoptic area (MPOA) in the Syrian hamster. Although the mechanisms underlying hormonally-induced synaptic plasticity in this region remain elusive, the data suggests that an increase in synaptic density may mediate testosterone's effects on copulation. As brain derived neurotrophic factor (BDNF) plays an integral role in regulating synaptic plasticity and gonadal steroids regulate the levels of BDNF, we hypothesize that BDNF may mediate the effects of gonadal hormones on copulatory behavior. To test this hypothesis, we infused BDNF or controls into the MPN mag of long-term castrates. Our results indicate that BDNF, but not the controls, restored copulatory behavior in castrated male Syrian hamsters. Furthermore, the rise of BDNF expression in the MPOA preceded the rise of synaptophysin following testosterone replacement in castrated males. These data are consistent with our hypothesis, implicating a role for BDNF in mediating testosterone's action on copulation and suggest that the delay in testosterone's restoration of copulation is, in part, due to the delay in the increase of BDNF and synaptophysin.

Adolescent THC exposure in female rats leads to cognitive deficits through a mechanism involving chromatin modifications in the prefrontal cortex

BACKGROUND

Increasing cannabis consumption among adolescents, studies that link its early use with mental illnesses, and the political debate on cannabis legalization together call for an urgent need to study molecular underpinnings of adolescent brain vulnerability. The emerging role of epigenetic mechanisms in psychiatric diseases led us to hypothesize that epigenetic alterations could play a role in causes and subsequent development of the depressive/psychotic-like phenotype induced by adolescent, but not adult, Δ9-tetrahydrocannabinol (THC) exposure in female rats.

METHODS

We performed a time-course analysis of histone modifications, chromatin remodelling enzymes and gene expression in the prefrontal cortex of female rats after adolescent and adult THC exposure. We also administered a specific epigenetic drug (chaetocin) with THC to investigate its impact on THC-induced behavioural alterations.

RESULTS

Adolescent THC exposure induced alterations of selective histone modifications (mainly H3K9me3), impacting the expression of genes closely associated with synaptic plasticity. Changes in both histone modifications and gene expression were more widespread and intense after adolescent treatment, suggesting specific adolescent susceptibility. Adolescent THC exposure significantly increased Suv39H1 levels, which could account for the enhanced H3K9me3. Pharmacological blockade of H3K9me3 during adolescent THC treatment prevented THC-induced cognitive deficits, suggesting the relevant role played by H3K9me3 in THC-induced effects.

LIMITATIONS

Only female rats were investigated, and the expression studies were limited to a specific subset of genes.

CONCLUSION

Through a mechanism involving SUV39H1, THC modifies histone modifications and, thereby, expression of plasticity genes. This pathway appears to be relevant for the development of cognitive deficits.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

Intra-prelimbic cortical inhibition of striatal-enriched tyrosine phosphatase suppresses cocaine seeking in rats

Cocaine self-administration in rats results in dysfunctional neuroadaptations in the prelimbic (PrL) cortex during early abstinence. Central to these adaptations is decreased phospho-extracellular signal-regulated kinase (p-ERK), which plays a key role in cocaine seeking. Normalizing ERK phosphorylation in the PrL cortex immediately after cocaine self-administration decreases subsequent cocaine seeking. The disturbance in ERK phosphorylation is accompanied by decreased phosphorylation of striatal-enriched protein tyrosine phosphatase (STEP), indicating increased STEP activity. STEP is a well-recognized ERK phosphatase but whether STEP activation during early abstinence mediates the decrease in p-ERK and is involved in relapse is unknown. Here, we show that a single intra-PrL cortical microinfusion of the selective STEP inhibitor, TC-2153, immediately after self-administration suppressed post-abstinence context-induced relapse under extinction conditions and cue-induced reinstatement, but not cocaine prime-induced drug seeking or sucrose seeking. Moreover, an intra-PrL cortical TC-2153 microinfusion immediately after self-administration prevented the cocaine-induced decrease in p-ERK within the PrL cortex during early abstinence. Interestingly, a systemic TC-2153 injection at the same timepoint failed to suppress post-abstinence context-induced relapse or cue-induced reinstatement, but did suppress cocaine prime-induced reinstatement. These data indicate that the STEP-induced ERK dephosphorylation in the PrL cortex during early abstinence is a critical neuroadaptation that promotes relapse to cocaine seeking and that systemic versus intra-PrL cortical inhibition of STEP during early abstinence differentially suppresses cocaine seeking.

PPARγ agonism attenuates cocaine cue reactivity

Cocaine use disorder is a chronic relapsing condition characterized by compulsive drug seeking and taking even after prolonged abstinence periods. Subsequent exposure to drug-associated cues can promote intense craving and lead to relapse in abstinent humans and rodent models. The responsiveness to these cocaine-related cues, or 'cue reactivity', can trigger relapse and cocaine-seeking behaviors; cue reactivity is measurable in cocaine-dependent humans as well as rodent models. Cue reactivity is thought to be predictive of cocaine craving and relapse. Here we report that PPARγ agonism during abstinence from cocaine self-administration reduced previously active lever pressing in Sprague Dawley rats during cue-reactivity tests, while administration of the PPARγ antagonist, GW9662, reversed this effect. PPARγ agonism also normalized nuclear ERK activity in the medial prefrontal cortex and hippocampus which was reversed with GW9662. Our results support the utility of PPARγ agonism as a relapse prevention strategy to maintain abstinence in the presence of cocaine-associated cues.

Role of Src Family Kinases in BDNF-Mediated Suppression of Cocaine-Seeking and Prevention of Cocaine-Induced ERK, GluN2A, and GluN2B Dephosphorylation in the Prelimbic Cortex

Models of relapse have demonstrated that neuroadaptations in reward circuits following cocaine self-administration (SA) underlie reinstatement of drug-seeking. Dysregulation of the pathway from the prelimbic (PrL) cortex to the nucleus accumbens is implicated in reinstatement. A single BDNF infusion into the PrL cortex following a final cocaine SA session results in attenuation of reinstatement of cocaine-seeking. Inhibiting BDNF's receptor, TrkB, ERK/MAP kinase activation, or NMDA receptors blocks this attenuating effect, indicating that the interaction between glutamate-mediated synaptic activity and TrkB signaling is imperative to BDNF's suppressive effect on drug-seeking. Src family kinases (SFKs) are involved in both NMDA-mediated activation of TrkB- and TrkB-mediated tyrosine phosphorylation of NMDA receptors. We hypothesized that infusion of the SFK inhibitor, PP2, into the PrL cortex prior to a BDNF infusion, immediately after the end of the last cocaine SA session, would block BDNF's ability to suppress reinstatement of cocaine-seeking in rats with a cocaine SA history. PP2, but not the negative control, PP3, blocked BDNF's suppressive effect on context-induced relapse after 1 week of abstinence and cue-induced reinstatement after extinction. As previously reported, infusion of BDNF into the PrL cortex blocked cocaine SA-induced dephosphorylation of ERK, GluN2A, and GluN2B-containing receptors. Inhibition of SFKs using PP2 blocked BDNF-mediated phosphorylation of GluN2A, GluN2B, and ERK. These data indicate that SFK activity is necessary for BDNF-mediated suppression of cocaine-seeking and reversal of cocaine-induced dephosphorylation of key phosphoproteins in the prefrontal cortex related to synaptic plasticity.

Systemic Delivery of a Brain-Penetrant TrkB Antagonist Reduces Cocaine Self-Administration and Normalizes TrkB Signaling in the Nucleus Accumbens and Prefrontal Cortex

Cocaine exposure alters brain-derived neurotrophic factor (BDNF) expression in the brain. BDNF signaling through TrkB receptors differentially modulates cocaine self-administration, depending on the brain regions involved. In the present study, we determined how brain-wide inhibition of TrkB signaling affects cocaine intake, the motivation for the drug, and reinstatement of drug taking after extinction. To overcome the inability of TrkB ligands to cross the blood-brain barrier, the TrkB antagonist cyclotraxin-B was fused to the nontoxic transduction domain of the tat protein from human immunodeficiency virus type 1 (tat-cyclotraxin-B). Intravenous injection of tat-cyclotraxin-B dose-dependently reduced cocaine intake, motivation for cocaine (as measured under a progressive ratio schedule of reinforcement), and reinstatement of cocaine taking in rats allowed either short or long access to cocaine self-administration. In contrast, the treatment did not affect operant responding for a highly palatable sweet solution, demonstrating that the effects of tat-cyclotraxin-B are specific for cocaine reinforcement. Cocaine self-administration increased TrkB signaling and activated the downstream Akt pathway in the nucleus accumbens, and had opposite effects in the prefrontal cortex. Pretreatment with tat-cyclotraxin-B normalized protein levels in these two dopamine-innervated brain regions. Cocaine self-administration also increased TrkB signaling in the ventral tegmental area, where the dopaminergic projections originate, but pretreatment with tat-cyclotraxin-B did not alter this effect. Altogether, our data show that systemic administration of a brain-penetrant TrkB antagonist leads to brain region-specific effects and may be a potential pharmacological strategy for the treatment of cocaine addiction.

SIGNIFICANCE STATEMENT

Brain-derived neurotrophic factor (BDNF) signaling through TrkB receptors plays a well established role in cocaine reinforcement. However, local manipulation of BDNF signaling yields divergent effects, depending on the brain region, thereby questioning the viability of systemic TrkB targeting for the treatment of cocaine use disorders. Our study provides first-time evidence that systemic administration of a brain-penetrant TrkB antagonist (tat-cyclotraxin-B) reduces several behavioral measures of cocaine dependence, without altering motor performance or reinforcement by a sweet palatable solution. In addition, although cocaine self-administration produced opposite effects on TrkB signaling in the nucleus accumbens and prefrontal cortex, tat-cyclotraxin-B administration normalized these cocaine-induced changes in both brain regions.

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.

Cocaine self-administration disrupted by the N-methyl-D-aspartate receptor antagonist ketamine: a randomized, crossover trial

Repeated drug consumption may progress to problematic use by triggering neuroplastic adaptations that attenuate sensitivity to natural rewards while increasing reactivity to craving and drug cues. Converging evidence suggests a single sub-anesthetic dose of the N-methyl-D-aspartate receptor antagonist ketamine may work to correct these neuroadaptations and restore motivation for non-drug rewards. Using an established laboratory model aimed at evaluating behavioral shifts in the salience of cocaine now vs money later, we found that ketamine, as compared to the control, significantly decreased cocaine self-administration by 67% relative to baseline at greater than 24 h post-infusion, the most robust reduction observed to date in human cocaine users and the first to involve mechanisms other than stimulant or dopamine agonist effects. These findings signal new directions in medication development for substance use disorders.

Glutamatergic neurotransmission in the prefrontal cortex mediates the suppressive effect of intra-prelimbic cortical infusion of BDNF on cocaine-seeking

Cocaine self-administration induces dysfunctional neuroadaptations in the prefrontal cortex that underlie relapse to cocaine-seeking. Cocaine self-administration disturbs glutamatergic transmission in the nucleus accumbens that is prevented by infusion of brain-derived neurotrophic factor (BDNF) into the prelimbic area of the prefrontal cortex. Intra-prelimbic infusion of BDNF decreases cocaine-seeking in a TrkB-ERK MAP kinase-dependent manner. Neuronal activity triggers an interaction between TrkB receptors and NMDA receptors, leading to ERK activation. In the present study, infusion of the GluN2A-containing NMDA receptor antagonist, TCN-201, or the GluN2B-containing NMDA receptor antagonist, Ro-25-6981, into the prelimbic cortex of rats blocked the suppressive effect of BDNF on cocaine-seeking. During early withdrawal from cocaine self-administration, tyrosine phosphorylation of ERK, GluN2A, and GluN2B in the prelimbic cortex was reduced and this reduction of phospho-proteins was prevented by intra-prelimbic BDNF infusion. TCN-201 infusion into the prelimbic cortex inhibited the BDNF-mediated increase in pERK and pGluN2A whereas Ro-25-6981 infusion into the prelimbic cortex blocked BDNF-induced elevation of pERK and pGluN2B, indicating that both GluN2A- and GluN2B-containing NMDA receptors underlie BDNF-induced ERK activation. These data demonstrate that BDNF-mediated activation of GluN2A- and GluN2B-containing NMDA receptors underlies ERK activation in the prelimbic cortex during early withdrawal, preventing subsequent relapse to cocaine-seeking.

The Medial Orbitofrontal Cortex Regulates Sensitivity to Outcome Value

An essential component of goal-directed decision-making is the ability to maintain flexible responding based on the value of a given reward, or "reinforcer." The medial orbitofrontal cortex (mOFC), a subregion of the ventromedial prefrontal cortex, is uniquely positioned to regulate this process. We trained mice to nose poke for food reinforcers and then stimulated this region using CaMKII-driven Gs-coupled designer receptors exclusively activated by designer drugs (DREADDs). In other mice, we silenced the neuroplasticity-associated neurotrophin brain-derived neurotrophic factor (BDNF). Activation of Gs-DREADDs increased behavioral sensitivity to reinforcer devaluation, whereas Bdnf knockdown blocked sensitivity. These changes were accompanied by modifications in breakpoint ratios in a progressive ratio task, and they were recapitulated in Bdnf(+/-)mice. Replacement of BDNF selectively in the mOFC in Bdnf(+/-)mice rescued behavioral deficiencies, as well as phosphorylation of extracellular-signal regulated kinase 1/2 (ERK1/2). Thus, BDNF expression in the mOFC is both necessary and sufficient for the expression of typical effort allocation relative to an anticipated reinforcer. Additional experiments indicated that expression of the immediate-early gene c-fos was aberrantly elevated in the Bdnf(+/-)dorsal striatum, and BDNF replacement in the mOFC normalized expression. Also, systemic administration of an MAP kinase kinase inhibitor increased breakpoint ratios, whereas the addition of discrete cues bridging the response-outcome contingency rescued breakpoints in Bdnf(+/-)mice. We argue that BDNF-ERK1/2 in the mOFC is a key regulator of "online" goal-directed action selection.

SIGNIFICANCE STATEMENT

Goal-directed response selection often involves predicting the consequences of one's actions and the value of potential payoffs. Lesions or chemogenetic inactivation of the medial orbitofrontal cortex (mOFC) in rats induces failures in retrieving outcome identity memories (Bradfield et al., 2015), suggesting that the healthy mOFC serves to access outcome value information when it is not immediately observable and thereby guide goal-directed decision-making. Our findings suggest that the mOFC also bidirectionally regulates effort allocation for a given reward and that expression of the neurotrophin BDNF in the mOFC is both necessary and sufficient for mice to sustain stable representations of reinforcer value.