Publications

UNLABELLED: Pain management in opioid abusers engenders ethical and practical difficulties for clinicians, often resulting in pain mismanagement. Although chronic opioid administration may alter pain states, the presence of pain itself may alter the propensity to self-administer opioids, and previous history of drug abuse comorbid with chronic pain promotes higher rates of opioid misuse. Here, we tested the hypothesis that inflammatory pain leads to increased heroin self-administration resulting from altered mu opioid receptor (MOR) regulation of mesolimbic dopamine (DA) transmission. To this end, the complete Freund's adjuvant (CFA) model of inflammation was used to assess the neurochemical and functional changes induced by inflammatory pain on MOR-mediated mesolimbic DA transmission and on rat intravenous heroin self-administration under fixed ratio (FR) and progressive ratio (PR) schedules of reinforcement. In the presence of inflammatory pain, heroin intake under an FR schedule was increased for high, but attenuated for low, heroin doses with concomitant alterations in mesolimbic MOR function suggested by DA microdialysis. Consistent with the reduction in low dose FR heroin self-administration, inflammatory pain reduced motivation for a low dose of heroin, as measured by responding under a PR schedule of reinforcement, an effect dissociable from high heroin dose PR responding. Together, these results identify a connection between inflammatory pain and loss of MOR function in the mesolimbic dopaminergic pathway that increases intake of high doses of heroin. These findings suggest that pain-induced loss of MOR function in the mesolimbic pathway may promote opioid dose escalation and contribute to opioid abuse-associated phenotypes.

SIGNIFICANCE STATEMENT: This study provides critical new insights that show that inflammatory pain alters heroin intake through a desensitization of MORs located within the VTA. These findings expand our knowledge of the interactions between inflammatory pain and opioid abuse liability, and should help to facilitate the development of novel and safer opioid-based strategies for treating chronic pain.

BACKGROUND: The dynorphin (DYN)/kappa-opioid receptor (KOR) system undergoes neuroadaptations following chronic alcohol exposure that promote excessive operant self-administration and negative affective-like states; however, the exact mechanisms are unknown. The present studies tested the hypothesis that an upregulated DYN/KOR system mediates excessive alcohol self-administration that occurs during withdrawal in alcohol-dependent rats by assessing DYN A peptide expression and KOR function, in combination with site-specific pharmacologic manipulations.

METHODS: Male Wistar rats were trained to self-administer alcohol using operant behavioral strategies and subjected to intermittent alcohol vapor or air exposure. Changes in self-administration were assessed by pharmacologic challenges during acute withdrawal. In addition, 22-kHz ultrasonic vocalizations were utilized to measure negative affective-like states. Immunohistochemical techniques assessed DYN A peptide expression and [(35)S]GTPγS coupling assays were performed to assess KOR function.

RESULTS: Alcohol-dependent rats displayed increased alcohol self-administration, negative affective-like behavior, DYN A-like immunoreactivity, and KOR signaling in the amygdala compared with nondependent control rats. Site-specific infusions of a KOR antagonist selectively attenuated self-administration in dependent rats, whereas a mu-opioid receptor/delta-opioid receptor antagonist cocktail selectively reduced self-administration in nondependent rats. A mu-opioid receptor antagonist/partial KOR agonist attenuated self-administration in both cohorts.

CONCLUSIONS: Increased DYN A and increased KOR signaling could set the stage for a one-two punch during withdrawal that drives excessive alcohol consumption in alcohol dependence. Importantly, intracentral nucleus of the amygdala pharmacologic challenges functionally confirmed a DYN/KOR system involvement in the escalated alcohol self-administration. Together, the DYN/KOR system is heavily dysregulated in alcohol dependence and contributes to the excessive alcohol consumption during withdrawal.

The kappa-opioid receptor (KOR) is the primary target for the endogenous opioid peptide dynorphin (DYN), and KORs reside within brain circuitry underlying the complex integration of information related to different behavioral domains such as motivation, negative affect, and decision-making. Alterations in extended amygdala DYNs and KOR function following chronic alcohol exposure have been shown to mediate escalated alcohol self-administration during acute withdrawal. In addition to excessive alcohol consumption and increased negative affect, other symptoms of alcohol dependence include compromised impulse control. Given that DYN and KOR expressions are dysregulated within prefrontal brain circuitry associated with decision-making and impulse control in alcohol-dependent humans and rodents, and have been shown to modify multiple neurotransmitter systems associated with impulse-control disorders, we hypothesized that KOR activation could contribute to impulsive phenotypes. To test this hypothesis, separate cohorts of male Wistar rats were trained in one of the two animal models of impulsivity: delay-discounting (DD) or stop-signal reaction time (SSRT) tasks, and once stable responding was observed, received intracerebroventricular (ICV) infusions of the KOR agonist U50,488 (0-50 μg) according to a within-subject dosing regimen. The results demonstrated a dissociable effect of U50,488 on impulsive phenotypes related to intolerance to delay or response inhibition, with selective effects in the SSRT. Furthermore, the pro-impulsive effects of KOR activation were rescued by pretreatment with the KOR antagonist nor-binaltorphimine (nor-BNI). Therefore, KOR activation was shown to induce an impulsive phenotype that was nor-BNI-sensitive. Dysregulation of impulsive behavior by increased DYN/KOR activity could serve to increase vulnerability for the initiation, or perpetuate existing patterns of excessive alcohol abuse and can enhance the probability of relapse in dependent individuals. Furthermore, KOR-mediated impulsivity has implications for numerous neuropsychiatric disorders.

Negative affect promotes dysregulated alcohol consumption in non-dependent and alcohol-dependent animals, and cues associated with negative affective states induce withdrawal-like symptoms in rats. This study was designed to test the hypotheses that: (1) the kappa-opioid receptor (KOR) system mediates phenotypes related to alcohol withdrawal and withdrawal-like negative affective states and (2) cues associated with negative affective states would result in dysregulated alcohol consumption when subsequently presented alone. To accomplish these goals, intracerebroventricular infusion of the KOR antagonist nor-binaltorphimine (nor-BNI) was assessed for the ability to attenuate the increase in 22-kHz ultrasonic vocalizations (USVs) associated with alcohol withdrawal and KOR activation in adult male wistar rats. Furthermore, cues associated with a KOR agonist-induced negative affective state were assessed for the ability to dysregulate alcohol consumption and the efficacy of intracerebroventricular KOR antagonism to reduce such dysregulation was evaluated. KOR antagonism blocked the increased number of 22-kHz USVs observed during acute alcohol withdrawal and a KOR agonist (U50,488) resulted in a nor-BNI reversible increase in 22-kHz USVs (mimicking an alcohol-dependent state). Additionally, cues associated with negative affective states resulted in escalated alcohol self-administration, an effect that was nor-BNI sensitive. Taken together, this study implicates negative affective states induced by both alcohol withdrawal and conditioned stimuli as being produced, in part, by activity of the DYN/KOR system.

This review represents the focus of a symposium that was presented at the "Alcoholism and Stress: A Framework for Future Treatment Strategies" conference in Volterra, Italy on May 3-6, 2011 and organized/chaired by Dr. Brendan M. Walker. The primary goal of the symposium was to evaluate and disseminate contemporary findings regarding the emerging role of kappa-opioid receptors (KORs) and their endogenous ligands dynorphins (DYNs) in the regulation of escalated alcohol consumption, negative affect and cognitive dysfunction associated with alcohol dependence, as well as DYN/KOR mediation of the effects of chronic stress on alcohol reward and seeking behaviors. Dr. Glenn Valdez described a role for KORs in the anxiogenic effects of alcohol withdrawal. Dr. Jay McLaughlin focused on the role of KORs in repeated stress-induced potentiation of alcohol reward and increased alcohol consumption. Dr. Brendan Walker presented data characterizing the effects of KOR antagonism within the extended amygdala on withdrawal-induced escalation of alcohol self-administration in dependent animals. Dr. Georgy Bakalkin concluded with data indicative of altered DYNs and KORs in the prefrontal cortex of alcohol dependent humans that could underlie diminished cognitive performance. Collectively, the data presented within this symposium identified the multifaceted contribution of KORs to the characteristics of acute and chronic alcohol-induced behavioral dysregulation and provided a foundation for the development of pharmacotherapeutic strategies to treat certain aspects of alcohol use disorders.

This article represents one of five contributions focusing on the topic "Plasticity and neuroadaptive responses within the extended amygdala in response to chronic or excessive alcohol exposure" that were developed by awardees participating in the Young Investigator Award Symposium at the "Alcoholism and Stress: A Framework for Future Treatment Strategies" conference in Volterra, Italy on May 3-6, 2011 that was organized/chaired by Drs. Antonio Noronha and Fulton Crews and sponsored by the National Institute on Alcohol Abuse and Alcoholism. This review discusses the dependence-induced neuroadaptations in affective systems that provide a basis for negative reinforcement learning and presents evidence demonstrating that escalated alcohol consumption during withdrawal is a learned, plasticity-dependent process. The review concludes by identifying changes within extended amygdala dynorphin/kappa-opioid receptor systems that could serve as the foundation for the occurrence of negative reinforcement processes. While some evidence contained herein may be specific to alcohol dependence-related learning and plasticity, much of the information will be of relevance to any addictive disorder involving negative reinforcement mechanisms. Collectively, the information presented within this review provides a framework to assess the negative reinforcing effects of alcohol in a manner that distinguishes neuroadaptations produced by chronic alcohol exposure from the actual plasticity that is associated with negative reinforcement learning in dependent organisms.

Alcoholism is a chronic relapsing disorder characterized by continued alcohol use despite numerous adverse consequences. Alcohol has been shown to interact with numerous neurotransmitter systems to exert its pharmacological effects. The endogenous opioid system (EOS) has been strongly implicated in the positive and negative reinforcing effects of alcohol. Traditionally recognized as dysphoric/anhedonic in nature, the dynorphin/kappa-opioid receptor (DYN/KOR) system has recently received considerable attention due to evidence suggesting that an upregulated DYN/KOR system may be a critical contributor to the complex factors that result in escalated alcohol consumption once dependent. The present review will discuss alcohol-induced plasticity in the DYN/KOR system and how these neuroadaptations could contribute to excessive alcohol seeking and consumption.

RATIONALE: Once dependent on alcohol or opioids, negative affect may accompany withdrawal. Dependent individuals are hypothesized to "self-medicate" in order to cope with withdrawal, which promotes escalated alcohol and drug use.

OBJECTIVES: The current study aimed to develop a reliable animal model to assess symptoms that occur during spontaneous alcohol and opioid withdrawal.

METHODS: Dependence was induced using intermittent alcohol exposure or pulsatile heroin delivery and assessed for the presence of withdrawal symptoms during acute withdrawal by measuring somatic signs, behavior in the forced swim test (FST), and air-puff-induced 22-kHz ultrasonic vocalizations (USVs). Additional animals subjected to 8 weeks of alcohol vapor exposure were evaluated for altered somatic signs, operant alcohol self-administration, and 22-kHz USV production, as well as performance in the elevated plus maze (EPM).

RESULTS: During spontaneous withdrawal from pulsatile heroin or intermittent alcohol vapor, animals displayed increased somatic withdrawal signs, FST immobility, and 22-kHz USV production but did not show any behavioral change in the EPM unless the duration of alcohol exposure was extended to 4 weeks. Following 8 weeks of alcohol vapor exposure, animals displayed somatic withdrawal signs, escalated alcohol self-administration, and increased 22-kHz USVs.

CONCLUSIONS: These paradigms provide consistent methods to evaluate the behavioral ramifications, and neurobiological substrates, of alcohol and opioid dependence during spontaneous withdrawal. As immobility in the FST and percent open-arm time in the EPM were dissociable, with 22-kHz USVs paralleling immobility in the FST, assessment of air-puff-induced 22-kHz USVs could provide an ethologically valid alternative to the FST.

Investigations into the physiological mechanisms of sleep control require an animal psychomotor vigilance task (PVT) with fast response times (<300 ms). Rats provide a good PVT model since whisker stimulation produces a rapid and robust cortical evoked response, and animals can be trained to lick following stimulation. Our prior experiments used deprivation-based approaches to maximize motivation for operant conditioned responses. However, deprivation can influence physiological and neurobehavioral effects. In order to maintain motivation without water deprivation, we conditioned rats for immobilization and head restraint, then trained them to lick for a 10% sucrose solution in response to whisker stimulation. After approximately 8 training sessions, animals produced greater than 80% correct hits to the stimulus. Over the course of training, reaction times became faster and correct hits increased. Performance in the PVT was examined after 3, 6 and 12 h of sleep deprivation achieved by gentle handling. A significant decrease in percent correct hits occurred following 6 and 12 h of sleep deprivation and reaction times increased significantly following 12 h of sleep deprivation. While behaviorally the animals appeared to be awake, we observed significant increases in EEG delta power prior to misses. The rat PVT with fast response times allows investigation of sleep deprivation effects, time-on-task and pharmacological agents. Fast response times also allow closer parallel studies to ongoing human protocols.