Stress augments the rewarding memory of cocaine via the activation of brainstem-reward circuitry

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

Social defeat stress enhances the rewarding effects of cocaine through α1A adrenoceptors in the medial prefrontal cortex of mice

Various stressors potentiate the rewarding effects of cocaine and contribute to cocaine cravings. However, it remains unclear whether psychosocial stress enhances the rewarding effects of cocaine. Accordingly, this study employed a cocaine-conditioned place preference (CPP) paradigm combined with social defeat (SD) exposure to investigate the effects of acute SD stress on cocaine reward in male mice. We found that SD stress immediately before the posttest significantly increased cocaine CPP, and systemic blockade of α1 adrenoceptors, but not β adrenoceptors, suppressed this increase. Fiber photometry recordings with GRABNE1m sensors revealed increased noradrenaline (NA) levels in the medial prefrontal cortex (mPFC) in test mice in response to attacks by aggressor mice during SD. Moreover, the SD stress-induced enhancement of CPP was effectively suppressed by intra-mPFC infusion of an α1 adrenoceptor antagonist. In vitro whole-cell recordings demonstrated that silodosin, an α1A, but not α1B or α1D, adrenoceptor antagonist, inhibited NA-induced depolarizing currents and facilitation of excitatory synaptic transmissions. Consistently, intra-mPFC silodosin infusion significantly suppressed the SD stress-induced CPP enhancement. Conversely, intra-mPFC infusion of α1A adrenoceptor agonist augmented cocaine CPP in the absence of stress exposure. Additionally, intranasal silodosin administration attenuated the SD stress-induced enhancement of CPP, and chemogenetic inhibition of mPFC excitatory neurons also suppressed the SD stress-induced CPP enhancement. Together, these findings suggest that NA stimulation of α1A adrenoceptors and the subsequent activation of mPFC pyramidal cells may contribute to SD stress-induced amplification of the rewarding effects of cocaine, and intranasal silodosin administration may hold therapeutic potential for mitigating stress-associated cocaine craving.

Viral-mediated expression of Erk2 in the nucleus accumbens regulates responses to rewarding and aversive stimuli

Second-messenger signaling within the mesolimbic reward circuit is involved in both the long-lived effects of stress and in the underlying mechanisms that promote drug abuse liability. To determine the direct role of kinase signaling within the nucleus accumbens, specifically mitogen-activated protein kinase 1 (ERK2), in mood- and drug-related behavior, we used a herpes-simplex virus to up- or down-regulate ERK2 in adult male rats. We then exposed rats to a battery of behavioral tasks including the elevated plus-maze, open field test, forced-swim test, conditioned place preference, and finally cocaine self-administration. Herein, we show that viral overexpression or knockdown of ERK2 in the nucleus accumbens induces distinct behavioral phenotypes. Specifically, over expression of ERK2 facilitated depression- and anxiety-like behavior while also increasing sensitivity to cocaine. Conversely, down-regulation of ERK2 attenuated behavioral deficits, while blunting sensitivity to cocaine. Taken together, these data implicate ERK2 signaling, within the nucleus accumbens, in the regulation of affective behaviors and modulating sensitivity to the rewarding properties of cocaine.

Involvement of the ghrelin system in the maintenance and reinstatement of cocaine-motivated behaviors: a role of adrenergic action at peripheral β1 receptors

Cocaine addiction is a significant medical and public concern. Despite decades of research effort, development of pharmacotherapy for cocaine use disorder remains largely unsuccessful. This may be partially due to insufficient understanding of the complex biological mechanisms involved in the pathophysiology of this disorder. In the present study, we show that: (1) elevation of ghrelin by cocaine plays a critical role in maintenance of cocaine self-administration and cocaine-seeking motivated by cocaine-conditioned stimuli; (2) acquisition of cocaine-taking behavior is associated with the acquisition of stimulatory effects of cocaine by cocaine-conditioned stimuli on ghrelin secretion, and with an upregulation of ghrelin receptor mRNA levels in the ventral tegmental area (VTA); (3) blockade of ghrelin signaling by pretreatment with JMV2959, a selective ghrelin receptor antagonist, dose-dependently inhibits reinstatement of cocaine-seeking triggered by either cocaine or yohimbine in behaviorally extinguished animals with a history of cocaine self-administration; (4) JMV2959 pretreatment also inhibits brain stimulation reward (BSR) and cocaine-potentiated BSR maintained by optogenetic stimulation of VTA dopamine neurons in DAT-Cre mice; (5) blockade of peripheral adrenergic β1 receptors by atenolol potently attenuates the elevation in circulating ghrelin induced by cocaine and inhibits cocaine self-administration and cocaine reinstatement triggered by cocaine. These findings demonstrate that the endogenous ghrelin system plays an important role in cocaine-related addictive behaviors and suggest that manipulating and targeting this system may be viable for mitigating cocaine use disorder.

Noradrenergic circuits and signaling in substance use disorders

The central noradrenergic system innervates almost all regions of the brain and, as such, is well positioned to modulate many neural circuits implicated in behaviors and physiology underlying substance use disorders. Ample pharmacological evidence demonstrates that α1, α2, and β adrenergic receptors may serve as therapeutic targets to reduce drug -seeking behavior and drug withdrawal symptoms. Further, norepinephrine is a key modulator of the stress response, and stress has been heavily implicated in reinstatement of drug taking. In this review, we discuss recent advances in our understanding of noradrenergic circuitry and noradrenergic receptor signaling in the context of opioid, alcohol, and psychostimulant use disorders.

Reduced Volume of a Brainstem Substructure in Adolescents with Problematic Smartphone Use

Objectives

Despite the growing concern regarding the adverse effects related to problematic smartphone use (PSU), little is known about underlying morphologic changes in the brain. The brainstem is a deep brain structure that consists of several important nuclei associated with emotions, sensations, and motor functions. In this study, we sought to examine the difference in the volume of brainstem substructures among adolescents with and without PSU.

Methods

A total of 87 Korean adolescents participated in this study. The PSU group (n=20, age=16.2±1.1, female:male=12:8) was designated if participants reported a total Smartphone Addiction Proneness Scale (SAPS) score of ≥42, whereas the remaining participants were assigned to the control group (n=67, age=15.3±1.7, female:male=19:48). High-resolution T1 magnetic resonance imaging was performed, and the volume of each of the four brainstem substructures [midbrain, pons, medulla, and superior cerebellar peduncle (SCP)] was measured. Analysis of covariance was conducted to reveal group differences after adjusting for effects of age, gender, whole brain-stem volume, depressive symptoms, and impulsivity.

Results

The PSU group showed a significantly smaller volume of the SCP than the control group (F=8.273, p=0.005). The volume of the SCP and the SAPS score were negatively correlated (Pearson’s r=-0.218, p=0.047).

Conclusion

The present study is the first to reveal an altered volume of the brainstem substructure among adolescents with PSU. This finding suggests that the altered white matter structure in the brainstem could be one of the neurobiological mechanisms underlying behavioral changes in PSU.

Stress Adaptation and the Brainstem with Focus on Corticotropin-Releasing Hormone

Stress adaptation is of utmost importance for the maintenance of homeostasis and, therefore, of life itself. The prevalence of stress-related disorders is increasing, emphasizing the importance of exploratory research on stress adaptation. Two major regulatory pathways exist: the hypothalamic–pituitary–adrenocortical axis and the sympathetic adrenomedullary axis. They act in unison, ensured by the enormous bidirectional connection between their centers, the paraventricular nucleus of the hypothalamus (PVN), and the brainstem monoaminergic cell groups, respectively. PVN and especially their corticotropin-releasing hormone (CRH) producing neurons are considered to be the centrum of stress regulation. However, the brainstem seems to be equally important. Therefore, we aimed to summarize the present knowledge on the role of classical neurotransmitters of the brainstem (GABA, glutamate as well as serotonin, noradrenaline, adrenaline, and dopamine) in stress adaptation. Neuropeptides, including CRH, might be co-localized in the brainstem nuclei. Here we focused on CRH as its role in stress regulation is well-known and widely accepted and other CRH neurons scattered along the brain may also complement the function of the PVN. Although CRH-positive cells are present on some parts of the brainstem, sometimes even in comparable amounts as in the PVN, not much is known about their contribution to stress adaptation. Based on the role of the Barrington’s nucleus in micturition and the inferior olivary complex in the regulation of fine motoric—as the main CRH-containing brainstem areas—we might assume that these areas regulate stress-induced urination and locomotion, respectively. Further studies are necessary for the field.

Pretreatment with High Mobility Group Box-1 Monoclonal Antibody Prevents the Onset of Trigeminal Neuropathy in Mice with a Distal Infraorbital Nerve Chronic Constriction Injury

Persistent pain following orofacial surgery is not uncommon. High mobility group box 1 (HMGB1), an alarmin, is released by peripheral immune cells following nerve injury and could be related to pain associated with trigeminal nerve injury. Distal infraorbital nerve chronic constriction injury (dIoN-CCI) evokes pain-related behaviors including increased facial grooming and hyper-responsiveness to acetone (cutaneous cooling) after dIoN-CCI surgery in mice. In addition, dIoN-CCI mice developed conditioned place preference to mirogabalin, suggesting increased neuropathic pain-related aversion. Treatment of the infraorbital nerve with neutralizing antibody HMGB1 (anti-HMGB1 nAb) before dIoN-CCI prevented both facial grooming and hyper-responsiveness to cooling. Pretreatment with anti-HMGB1 nAb also blocked immune cell activation associated with trigeminal nerve injury including the accumulation of macrophage around the injured IoN and increased microglia activation in the ipsilateral spinal trigeminal nucleus caudalis. The current findings demonstrated that blocking of HMGB1 prior to nerve injury prevents the onset of pain-related behaviors, possibly through blocking the activation of immune cells associated with the nerve injury, both within the CNS and on peripheral nerves. The current findings further suggest that blocking HMGB1 before tissue injury could be a novel strategy to prevent the induction of chronic pain following orofacial surgeries.

Neurochemical mechanisms and neurocircuitry underlying the contribution of stress to cocaine seeking

In individuals with substance use disorders, stress is a critical determinant of relapse susceptibility. In some cases, stressors directly trigger cocaine use. In others, stressors interact with other stimuli to promote drug seeking, thereby setting the stage for relapse. Here, we review the mechanisms and neurocircuitry that mediate stress-triggered and stress-potentiated cocaine seeking. Stressors trigger cocaine seeking by activating noradrenergic projections originating in the lateral tegmentum that innervate the bed nucleus of the stria terminalis to produce beta adrenergic receptor-dependent regulation of neurons that release corticotropin releasing factor (CRF) into the ventral tegmental area (VTA). CRF promotes the activation of VTA dopamine neurons that innervate the prelimbic prefrontal cortex resulting in D1 receptor-dependent excitation of a pathway to the nucleus accumbens core that mediates cocaine seeking. The stage-setting effects of stress require glucocorticoids, which exert rapid non-canonical effects at several sites within the mesocorticolimbic system. In the nucleus accumbens, corticosterone attenuates dopamine clearance via the organic cation transporter 3 to promote dopamine signaling. In the prelimbic cortex, corticosterone mobilizes the endocannabinoid, 2-arachidonoylglycerol (2-AG), which produces CB1 receptor-dependent reductions in inhibitory transmission, thereby increasing excitability of neurons which comprise output pathways responsible for cocaine seeking. Factors that influence the role of stress in cocaine seeking, including prior history of drug use, biological sex, chronic stress/co-morbid stress-related disorders, adolescence, social variables, and genetics are discussed. Better understanding when and how stress contributes to drug seeking should guide the development of more effective interventions, particularly for those whose drug use is stress related.

Medial prefrontal cortical control of reward- and aversion-based behavioral output: Bottom-up modulation

How does the brain guide our actions? This is a complex issue, where the medial prefrontal cortex (mPFC) plays a crucial role. The mPFC is essential for cognitive flexibility and decision making. These functions are related to reward- and aversion-based learning, which ultimately drive behavior. Though, cortical projections and modulatory systems that may regulate those processes in the mPFC are less understood. How does the mPFC regulate approach-avoidance behavior in the case of conflicting aversive and appetitive stimuli? This is likely dependent on the bottom-up neuromodulation of the mPFC projection neurons. In this review, we integrate behavioral-, pharmacological-, and viral-based circuit manipulation data showing the involvement of mPFC dopaminergic, noradrenergic, cholinergic, and serotoninergic inputs in reward and aversion processing. Given that an incorrect balance of reward and aversion value could be a key problem in mental diseases such as substance use disorders, we discuss outstanding questions for future research on the role of mPFC modulation in reward and aversion.

Hyperfunction of the stress response system and novelty-induced hyperactivity correlate with enhanced cocaine-induced conditioned place preference in NCAM-deficient mice

Several studies in humans and rodents suggest an association between impulsivity and activity of the stress response on the one hand and addiction vulnerability on the other. The neural cell adhesion molecule (NCAM) has been related to several neuropsychiatric disorders in humans. Constitutively NCAM-deficient (-/-) mice display enhanced novelty-induced behavior and hyperfunction of the hypothalamic-pituitary-adrenal axis. Here we hypothesize that NCAM deficiency causes an altered response to cocaine. Cocaine-induced behaviors of NCAM-/- mice and wild-type (+/+) littermates were analyzed in the conditioned place preference (CPP) test. c-fos mRNA levels were investigated by quantitative polymerase chain reaction (qPCR) to measure neural activation after exposure to the cocaine-associated context. NCAM-/- mice showed an elevated cocaine-induced sensitization, enhanced CPP, impaired extinction, and potentiated cocaine-induced hyperlocomotion and CPP after extinction. NCAM-/- showed no potentiated CPP as compared with NCAM+/+ littermates when a natural rewarding stimulus (ie, an unfamiliar female) was used, suggesting that the behavioral alterations of NCAM-/- mice observed in the CPP test are specific to the effects of cocaine. Activation of the prefrontal cortex and nucleus accumbens induced by the cocaine-associated context was enhanced in NCAM-/- compared with NCAM+/+ mice. Finally, cocaine-induced behavior correlated positively with novelty-induced behavior and plasma corticosterone levels in NCAM-/- mice and negatively with NCAM mRNA levels in the hippocampus and nucleus accumbens in wild-type mice. Our findings indicate that NCAM deficiency affects cocaine-induced CPP in mice and support the view that hyperfunction of the stress response system and reactivity to novelty predict the behavioral responses to cocaine.

Prefrontal cortex nicotinic receptor inhibition by methyllycaconitine impaired cocaine-associated memory acquisition and retrieval

Cocaine administration has been shown to induce plastic changes in the medial prefrontal cortex (mPFC), which could represent a mechanism by which cocaine facilitates the association between cocaine rewarding effects with contextual cues. Nicotinic acetylcholine receptors (nAChRs) in the mPFC have critical roles in cognitive function including attention and memory and are key players in plasticity processes. However, whether nAChRs in the mPFC are required for the acquisition and maintenance of cocaine-associated memories is still unknown. To assess this question, we used the conditioning place preference (CPP) model to study the effect of intra-mPFC infusion of methyllycaconitine, a selective antagonist of α7 nAChRs, on the acquisition, consolidation and expression of cocaine-associated memory in adult rats. Our findings reveal that mPFC α7 nAChRs activation is necessary for the acquisition and retrieval, but not consolidation, of cocaine induced CPP. Moreover, cocaine-induced sensitization during CPP conditioning sessions was abolished by methyllycaconitine infusion in the mPFC. Together, these results identify mPFC α7 nAChRs as critical players involved in both acquiring and retrieving cocaine-associated memories. Considering that drug seeking often depends on the association between drug-paired cues and the rewarding effects of the drug, α7 nAChRs in the mPFC could be considered as potential targets for the prevention or treatment of cocaine use disorder.

Endocannabinoid-Like Lipid Neuromodulators in the Regulation of Dopamine Signaling: Relevance for Drug Addiction

The family of lipid neuromodulators has been rapidly growing, as the use of different -omics techniques led to the discovery of a large number of naturally occurring N-acylethanolamines (NAEs) and N-acyl amino acids belonging to the complex lipid signaling system termed endocannabinoidome. These molecules exert a variety of biological activities in the central nervous system, as they modulate physiological processes in neurons and glial cells and are involved in the pathophysiology of neurological and psychiatric disorders. Their effects on dopamine cells have attracted attention, as dysfunctions of dopamine systems characterize a range of psychiatric disorders, i.e., schizophrenia and substance use disorders (SUD). While canonical endocannabinoids are known to regulate excitatory and inhibitory synaptic inputs impinging on dopamine cells and modulate several dopamine-mediated behaviors, such as reward and addiction, the effects of other lipid neuromodulators are far less clear. Here, we review the emerging role of endocannabinoid-like neuromodulators in dopamine signaling, with a focus on non-cannabinoid N-acylethanolamines and their receptors. Mounting evidence suggests that these neuromodulators contribute to modulate synaptic transmission in dopamine regions and might represent a target for novel medications in alcohol and nicotine use disorder.

Role of 5-HT1A receptor-mediated serotonergic transmission in the medial prefrontal cortex in acute restraint stress-induced augmentation of rewarding memory of cocaine in mice

Stress enhances cocaine craving. We recently reported that acute restraint stress increases cocaine conditioned place preference (CPP) in mice; however, the underlying mechanisms remain unclear. This study aimed to examine the role of serotonergic transmission in the medial prefrontal cortex (mPFC) in cocaine CPP enhancement by acute restraint stress, which increases extracellular serotonin (5-HT) levels in the mPFC. Intra-mPFC infusion of the selective serotonin reuptake inhibitor (S)-citalopram prior to the test session significantly increased the cocaine CPP score under non-stressed conditions. This is indicative of the substantial role of increased mPFC 5-HT levels in cocaine CPP enhancement. Moreover, intra-mPFC and systemic administration of the 5-HT_1A receptor antagonist WAY100635 immediately before restraint stress exposure significantly attenuated stress-induced cocaine CPP enhancement. Our findings suggest that enhanced serotonergic transmission via 5-HT_1A receptors in the mPFC is involved in acute stress-induced augmentation of rewarding memory of cocaine; moreover, the 5-HT_1A receptor could be a therapeutic target for stress-induced cocaine craving.

Molecular and circuit mechanisms regulating cocaine memory

Risk of relapse is a major challenge in the treatment of substance use disorders. Several types of learning and memory mechanisms are involved in substance use and have implications for relapse. Associative memories form between the effects of drugs and the surrounding environmental stimuli, and exposure to these stimuli during abstinence causes stress and triggers drug craving, which can lead to relapse. Understanding the neural underpinnings of how these associations are formed and maintained will inform future advances in treatment practices. A large body of research has expanded our knowledge of how associative memories are acquired and consolidated, how they are updated through reactivation and reconsolidation, and how competing extinction memories are formed. This review will focus on the vast literature examining the mechanisms of cocaine Pavlovian associative memories with an emphasis on the molecular memory mechanisms and circuits involved in the consolidation, reconsolidation, and extinction of these memories. Additional research elucidating the specific signaling pathways, mechanisms of synaptic plasticity, and epigenetic regulation of gene expression in the circuits involved in associative learning will reveal more distinctions between consolidation, reconsolidation, and extinction learning that can be applied to the treatment of substance use disorders.

Noradrenaline enhances the excitatory effects of dopamine on medial prefrontal cortex pyramidal neurons in rats

Aim

Our previous studies showed that exposure to acute restraint stress enhanced cocaine‐induced conditioned place preference (cocaine‐CPP) and suggested the possibility that co‐activation of adrenergic transmission boosts the increase in medial prefrontal cortex (mPFC) neuronal activity by the activation of dopaminergic transmission. To examine this possibility, the effects of the co‐treatment with dopamine (DA) and noradrenaline (NA) on mPFC neurons were compared with those of treatment with DA alone using whole‐cell patch‐clamp recordings.

Methods

The effects of DA alone and a mixture of DA and NA on the membrane potentials and spontaneous excitatory postsynaptic currents (sEPSCs) were examined by electrophysiological recordings of mPFC pyramidal neurons in brain slices of male Sprague Dawley rats. Extracellular DA and NA levels in the mPFC during and after restraint stress exposure were also examined by in vivo microdialysis.

Results

Dopamine significantly produced depolarizing effects on mPFC neurons and tended to increase sEPSC frequency. Co‐administration of NA with DA produced stronger depolarizing effects and significantly increased sEPSC frequency. The findings suggest that the additional depolarizing effect of NA on DA‐responsive neurons, rather than the excitation of DA‐nonresponsive neurons by NA, contributes to the stronger effect of co‐treatment of NA with DA.

Conclusion

The present study suggests that NA released by restraint stress exposure cooperates with DA to stimulate DA‐responsive neurons in the mPFC, thereby causing the stress‐induced enhancement of cocaine‐CPP.

Our previous studies showed that exposure to acute restraint stress enhanced cocaine‐induced conditioned place preference (cocaine‐CPP). The present study revealed that noradrenaline enhanced the excitatory effects of dopamine on medial prefrontal cortex (mPFC) pyramidal neurons in rats. Noradrenaline released by restraint stress exposure may cooperate with dopamine to stimulate dopamine‐responsive neurons in the mPFC, thereby causing the stress‐induced enhancement of cocaine‐CPP.