The D2 receptor is critical in mediating opiate motivation only in opiate-dependent and withdrawn mice

Connexin-36-positive gap junctions in ventral tegmental area GABA neurons sustain opiate dependence

Drug dependence is characterized by a switch in motivation wherein a positively reinforcing substance can become negatively reinforcing. Put differently, drug use can transform from a form of pleasure-seeking to a form of relief-seeking. Ventral tegmental area (VTA) GABA neurons form an anatomical point of divergence between two double dissociable pathways that have been shown to be functionally implicated and necessary for these respective motivations to seek drugs. The tegmental pedunculopontine nucleus (TPP) is necessary for opiate conditioned place preferences (CPP) in previously drug-naïve rats and mice, whereas dopaminergic (DA) transmission in the nucleus accumbens (NAc) is necessary for opiate CPP in opiate-dependent and withdrawn (ODW) rats and mice. Here, we show that this switch in functional anatomy is contingent upon the gap junction-forming protein, connexin-36 (Cx36), in VTA GABA neurons. Intra-VTA infusions of the Cx36 blocker, mefloquine, in ODW rats resulted in a reversion to a drug-naïve-like state wherein the TPP was necessary for opiate CPP and where opiate withdrawal aversions were lost. Consistent with these data, conditional knockout mice lacking Cx36 in GABA neurons (GAD65-Cre;Cx36 fl(CFP)/fl(CFP)) exhibited a perpetual drug-naïve-like state wherein opiate CPP was always DA independent, and opiate withdrawal aversions were absent even in mice subjected to an opiate dependence and withdrawal induction protocol. Further, viral-mediated rescue of Cx36 in VTA GABA neurons was sufficient to restore their susceptibility to an ODW state wherein opiate CPP was DA dependent. Our findings reveal a functional role for VTA gap junctions that has eluded prevailing circuit models of addiction.

In Vitro and In Vivo Pharmacological Profiles of LENART01, a Dermorphin-Ranatensin Hybrid Peptide

Diverse chemical and pharmacological strategies are currently being explored to minimize the unwanted side effects of currently used opioid analgesics while achieving effective pain relief. The use of multitarget ligands with activity at more than one receptor represents a promising therapeutic approach. We recently reported a bifunctional peptide-based hybrid LENART01 combining dermorphin and ranatensin pharmacophores, which displays activity to the mu-opioid receptor (MOR) and dopamine D2 receptor (D2R) in rat brains and spinal cords. In this study, we investigated the in vitro binding and functional activities to the human MOR and the in vivo pharmacology of LENART01 in mice after subcutaneous administration. In vitro binding assays showed LENART01 to bind and be selective to the human MOR over the other opioid receptor subtypes and delta, kappa and nociceptin receptors. In the [35S]GTPγS binding assay, LENART01 acted as a potent and full agonist to the human MOR. In mice, LENART01 produced dose-dependent antinociceptive effects in formalin-induced inflammatory pain, with increased potency than morphine. Antinociceptive effects were reversed by naloxone, indicating MOR activation in vivo. Behavioral studies also demonstrated LENART01's properties to induce less adverse effects without locomotor dysfunction and withdrawal syndrome compared to conventional opioid analgesics, such as morphine. LENART01 is the first peptide-based MOR-D2R ligand known to date and the first dual MOR-dopamine D2R ligand for which in vivo pharmacology is reported with antinociceptive efficacy and reduced opioid-related side effects. Our current findings may pave the way to new pain therapeutics with limited side effects in acute and chronic use.

Dopaminergic dysfunction: Role for genetic & epigenetic testing in the new psychiatry

Reward Deficiency Syndrome (RDS), particularly linked to addictive disorders, costs billions of dollars globally and has resulted in over one million deaths in the United States (US). Illicit substance use has been steadily rising and in 2021 approximately 21.9% (61.2 million) of individuals living in the US aged 12 or older had used illicit drugs in the past year. However, only 1.5% (4.1 million) of these individuals had received any substance use treatment. This increase in use and failure to adequately treat or provide treatment to these individuals resulted in 106,699 overdose deaths in 2021 and increased in 2022. This article presents an alternative non-pharmaceutical treatment approach tied to gene-guided therapy, the subject of many decades of research. The cornerstone of this paradigm shift is the brain reward circuitry, brain stem physiology, and neurotransmitter deficits due to the effects of genetic and epigenetic insults on the interrelated cascade of neurotransmission and the net release of dopamine at the Ventral Tegmental Area -Nucleus Accumbens (VTA-NAc) reward site. The Genetic Addiction Risk Severity (GARS) test and pro-dopamine regulator nutraceutical KB220 were combined to induce "dopamine homeostasis" across the brain reward circuitry. This article aims to encourage four future actionable items: 1) the neurophysiologically accurate designation of, for example, "Hyperdopameism /Hyperdopameism" to replace the blaming nomenclature like alcoholism; 2) encouraging continued research into the nature of dysfunctional brainstem neurotransmitters across the brain reward circuitry; 3) early identification of people at risk for all RDS behaviors as a brain check (cognitive testing); 4) induction of dopamine homeostasis using "precision behavioral management" along with the coupling of GARS and precision Kb220 variants; 5) utilization of promising potential treatments include neuromodulating modalities such as Transmagnetic stimulation (TMS) and Deep Brain Stimulation(DBS), which target different areas of the neural circuitry involved in addiction and even neuroimmune agents like N-acetyl-cysteine.

Exercise can restore behavioural and molecular changes of intergenerational morphine effects

In our previous studies, the offspring of morphine-exposed parents (MEO) showed pharmacological tolerance to the morphine's reinforcing effect. According to the role of exercise in treatment of morphine addiction, the current study was designed to utilize exercise to improve the effect of parental morphine exposure on the morphine's reinforcing effect. Male and female rats received morphine for 10 days and were drug-free for another 10 days. Each morphine-exposed animal was allowed to mate either with a drug-naïve or a morphine-exposed rat. The offspring were divided into two groups: (1) offspring that were subjected to treadmill exercise and (2) offspring that were not subjected to exercise. The reinforcing effect of morphine was evaluated using conditioned place preference (CPP) and two-bottle choice (TBC) tests. Levels of dopamine receptors (D1DR and D2DR), μ-opioid receptor (MOR), and ΔFosB were evaluated in the nucleus accumbens. The MEO obtained lower preference scores in CPP and consumed morphine more than the control group in TBC. After 3 weeks of exercise, the reinforcing effect of morphine in the MEO was similar to the control. D1DR, D2DR, and MOR were increased in MEO compared with the controls before exercise. Levels of D1DR and MOR were decreased after exercise in the MEO; however, D1DR was increased in control. D2DR level did not change after exercise in MEO, but it increased in control group. Moreover, the level of ΔFosB was decreased among MEO while it was increased after exercise. In conclusion, exercise might modulate the reinforcing effect of morphine via alteration in levels of D1DR, MOR, and ΔFosB.

Reward Deficiency Syndrome (RDS) Surprisingly Is Evolutionary and Found Everywhere: Is It "Blowin' in the Wind"?

Reward Deficiency Syndrome (RDS) encompasses many mental health disorders, including a wide range of addictions and compulsive and impulsive behaviors. Described as an octopus of behavioral dysfunction, RDS refers to abnormal behavior caused by a breakdown of the cascade of reward in neurotransmission due to genetic and epigenetic influences. The resultant reward neurotransmission deficiencies interfere with the pleasure derived from satisfying powerful human physiological drives. Epigenetic repair may be possible with precision gene-guided therapy using formulations of KB220, a nutraceutical that has demonstrated pro-dopamine regulatory function in animal and human neuroimaging and clinical trials. Recently, large GWAS studies have revealed a significant dopaminergic gene risk polymorphic allele overlap between depressed and schizophrenic cohorts. A large volume of literature has also identified ADHD, PTSD, and spectrum disorders as having the known neurogenetic and psychological underpinnings of RDS. The hypothesis is that the true phenotype is RDS, and behavioral disorders are endophenotypes. Is it logical to wonder if RDS exists everywhere? Although complex, “the answer is blowin’ in the wind,” and rather than intangible, RDS may be foundational in species evolution and survival, with an array of many neurotransmitters and polymorphic loci influencing behavioral functionality.

Dopamine Supersensitivity: A Novel Hypothesis of Opioid-Induced Neurobiological Mechanisms Underlying Opioid-Stimulant Co-use and Opioid Relapse

Emergent harms presented by the co-use of opioids and methamphetamine highlight the broader public health challenge of preventing and treating opioid and stimulant co-use. Development of effective therapeutics requires an understanding of the physiological mechanisms that may be driving co-use patterns, specifically the underlying neurobiology of co-use and how they may facilitate (or be leveraged to prevent) continued use patterns. This narrative review summarizes largely preclinical data that demonstrate clinically-meaningful relationships between the dopamine and opioid systems with direct implications for opioid and stimulant co-use. Synthesized conclusions of this body of research include evidence that changes in the dopamine system occur only once physical dependence to opioids develops, that the chronicity of opioid exposure is associated with the severity of changes, and that withdrawal leaves the organism in a state of substantive dopamine deficit that persists long after the somatic or observed signs of opioid withdrawal appear to have resolved. Evidence also suggests that dopamine supersensitivity develops soon after opioid abstinence and results in increased response to dopamine agonists that increases in magnitude as the abstinence period continues and is evident several weeks into protracted withdrawal. Mechanistically, this supersensitivity appears to be mediated by changes in the sensitivity, not quantity, of dopamine D2 receptors. Here we propose a neural circuit mechanism unique to withdrawal from opioid use with implications for increased stimulant sensitivity in previously stimulant-naïve or inexperienced populations. These hypothesized effects collectively delineate a mechanism by which stimulants would be uniquely reinforcing to persons with opioid physical dependence, would contribute to the acute opioid withdrawal syndrome, and could manifest subjectively as craving and/or motivation to use that could prompt opioid relapse during acute and protracted withdrawal. Preclinical research is needed to directly test these hypothesized mechanisms. Human laboratory and clinical trial research is needed to explore these clinical predictions and to advance the goal of developing treatments for opioid-stimulant co-use and/or opioid relapse prevention and withdrawal remediation.

Differential Effects of Oxycodone, Hydrocodone, and Morphine on Activation Levels of Signaling Molecules

BACKGROUND

Opioids alter the responses of D2-like dopamine receptors (D2DRs), known to be involved in the pathology of addiction and other mental illnesses. Importantly, our recent results demonstrated that various opioids differentially modulate the behavioral responses of D2DRs.

OBJECTIVE

To examine the effect of various opioids on striatal activation levels of Akt and ERK1/2, as well as the signaling responses of D2DRs following opioid exposure.

METHODS

Mice were pre-treated with 20 mg/kg morphine, hydrocodone, oxycodone, or saline for 6 days. Twenty-four hours later, mice were injected with vehicle or a D2/D3 receptor agonist, quinpirole. Thirty minutes later, dorsal striatum was collected and analyzed using Western blot.

RESULTS

In morphine-pretreated animals, baseline Akt activation level was unchanged, but was reduced in response to quinpirole. In contrast, baseline Akt activation levels were reduced in mice pretreated with hydrocodone and oxycodone, but were unchanged in response to quinpirole. In mice pretreated with all opioids, baseline ERK2 activation levels were unchanged and increased in response to quinpirole. However, quinpirole-induced ERK2 activation was significantly higher than drug naïve animals only in the morphine-pretreated mice.

CONCLUSIONS

Various opioids differentially modulate the baseline activation levels of signaling molecules, which in turn results in ligand-selective effects on the responses to a D2/D3 dopamine receptor agonist. This demonstrates a complex interplay between opioid receptors and D2DRs, and supports the notion that various opioids carry differential risks to the dopamine reward system. This information should be considered when prescribing opioid pain medication, to balance effectiveness with minimal risk.

Differential effects of oxycodone, hydrocodone, and morphine on the responses of D2/D3 dopamine receptors

Oxycodone and hydrocodone are opioids which are widely used for pain management and are also commonly misused and abused. The exposure to opioid analgesics has been associated with altered responses of D2-like dopamine receptors (D2DRs). Our recent results suggest that various opioids will differentially modulate the responses of D2DRs. The D2DRs are known to be involved in the pathology of addiction and other mental illnesses, indicating the need to improve our understanding of the effects of opioid analgesics on the responses of the D2DRs. Thus, in this study, we first established equianalgesic oral doses of oxycodone, hydrocodone, and morphine using the tail withdrawal assay. Then, mice were orally administered (gavage) with the various opioids or saline once daily for 6 days. Twenty-four hours later, the mice were tested for their locomotor response to quinpirole, a D2/D3 dopamine receptor agonist. Mice pretreated with oxycodone showed significantly greater locomotor supersensitivity to quinpirole than did morphine-pretreated mice, while hydrocodone-pretreated mice showed sensitivity in between that of mice treated with morphine and oxycodone. This finding suggests that various opioids differentially modulate the responses of D2DRs. It provides further evidence supporting of the notion that various opioids carry differential risks to the dopamine reward system.

Effects of pramipexole on the acquisition of responding with opioid-conditioned reinforcement in the rat

RATIONALE

Dopamine D3 receptor-preferring ligands may be able to modify the conditioned reinforcing effects of drug-associated stimuli. In evaluating the effects of these compounds, it is important to clarify the extent to which responding depends on (1) conditioned reinforcement vs. other behavioral mechanisms and (2) dopamine D3 vs. D2 receptor activity.

OBJECTIVES

To use behaviorally stringent new-response acquisition procedures to characterize the effects of the D3-preferring agonist, pramipexole, on the conditioned reinforcing effects of a stimulus paired with the opioid agonist, remifentanil.

METHODS

First, in Pavlovian conditioning (PAV) sessions, rats received response-independent IV injections of remifentanil and presentations of a light-noise stimulus. In separate groups, injections and stimuli either always co-occurred ("paired PAV") or occurred with no consistent relationship ("random PAV" control). Next, in instrumental acquisition (ACQ) sessions, all animals could respond in two nose-poke manipulanda: an active nose-poke, which produced the stimulus alone, or an inactive nose-poke. Pramipexole was injected SC prior to ACQ sessions with or without pretreatments of the D3-preferring antagonist, SB-277011A, or the D2-preferring antagonist, L-741,626.

RESULTS

After paired PAV, but not random PAV, rats acquired nose-poke responding during ACQ (i.e., active > inactive). Pramipexole dose-dependently increased active responding without changing inactive responding. Pramipexole-induced increases in responding were blocked by pretreatment with L-741,626, but not SB-277011A.

CONCLUSIONS

Pramipexole specifically enhanced remifentanil-conditioned reinforcement: active responding was selectively increased only after the stimulus was paired with remifentanil. Although pramipexole is D3-preferring, the antagonist effects obtained presently suggest an important role for the D2 receptor in opioid-conditioned reinforcement.

BDNF signaling in the VTA links the drug-dependent state to drug withdrawal aversions

Drug administration to avoid unpleasant drug withdrawal symptoms has been hypothesized to be a crucial factor that leads to compulsive drug-taking behavior. However, the neural relationship between the aversive motivational state produced by drug withdrawal and the development of the drug-dependent state still remains elusive. It has been observed that chronic exposure to drugs of abuse increases brain-derived neurotrophic factor (BDNF) levels in ventral tegmental area (VTA) neurons. In particular, BDNF expression is dramatically increased during drug withdrawal, which would suggest a direct connection between the aversive state of withdrawal and BDNF-induced neuronal plasticity. Using lentivirus-mediated gene transfer to locally knock down the expression of the BDNF receptor tropomyosin-receptor-kinase type B in rats and mice, we observed that chronic opiate administration activates BDNF-related neuronal plasticity in the VTA that is necessary for both the establishment of an opiate-dependent state and aversive withdrawal motivation. Our findings highlight the importance of a bivalent, plastic mechanism that drives the negative reinforcement underlying addiction.

Differential effects of methadone and buprenorphine on the response of D2/D3 dopamine receptors in adolescent mice

BACKGROUND

There is a lack of studies that examine the effects of opioid maintenance drugs on the developing adolescent brain, limiting the ability of physicians to conduct a science-based risk assessment on the appropriateness of these treatments for that age group. Our recent observations indicate higher potential risks in repeated exposure to morphine during adolescence, specifically to the D2/D3 dopamine receptors' signaling. Disturbances in dopaminergic signaling could have broader implications for long-term mental health. Thus, this study examined whether buprenorphine and methadone differentially alter the responses of the D2/D3 dopamine receptors in adolescents.

METHODS

Adolescent mice were orally administered buprenorphine (0.1-0.4 mg/kg), methadone (25-100 mg/kg), or saline once daily for 6 days. Two hours or three days later, the mice were tested for their locomotor response to 10 mg/kg quinpirole, a D2/D3 dopamine receptor agonist.

RESULTS

Buprenorphine-treated adolescent mice did not significantly differ from control drug-naïve animals in their response to quinpirole. However, an enhanced response was observed in methadone-treated adolescent animals. This enhanced locomotion was significantly higher two hours following the final dose of methadone, as compared to three days afterwards.

CONCLUSIONS

This study suggests that exposure to various opioids carries differential probabilities of altering the highly sensitive neurochemistry of adolescent brains. Methadone exposure disturbs the D2-like receptor's response, indicating a potential risk in administering methadone to adolescents (either for the treatment of opioid dependency/abuse or for pain management). In contrast, buprenorphine appears to have a significantly lower effect on the behavioral sensitivity of D2/D3 dopamine receptors in adolescents.

Morphine alters the locomotor responses to a D2/D3 dopamine receptor agonist differentially in adolescent and adult mice

The D2-like dopamine receptors mediate the emotional/aversive state during morphine withdrawal. Given age-dependent differences in the affective responses to withdrawal, this study examined whether the response to dopamine receptor agonists is altered differentially across ages following morphine administration. Adolescent and adult mice were injected with morphine (twice daily, 10-40 mg/kg, s.c.) or saline for 6 days. Subsequently, they were examined for their locomotor response to quinpirole, a D2/D3 receptor agonist, and SKF 38393, a D1 receptor agonist. Quinpirole dose-dependently reduced locomotion in drug-naïve animals. Initial suppression was also observed in morphine-treated animals, but was followed by enhanced locomotion. Notably, this enhanced locomotion was markedly greater in adolescents than adults. Quinpirole-induced hypo-locomotion is thought to be mediated by the presynaptic D2Short receptors, whereas its activating effect is mediated by postsynaptic D2Long/D3 receptors. This suggests that following morphine administration, the postsynaptic, but not the presynaptic, dopaminergic signaling is differentially modulated across ages. This locomotor supersensitivity was not observed for SKF 38393, a D1 dopamine receptor agonist. The D2/D3 receptors are involved in the pathophysiology of many mental illnesses. Thus, this study offers a potential explanation for the increased psychiatric disorder co-morbidities when drug use begins during adolescence.

The neurobiology of opiate motivation

Opiates are a highly addictive class of drugs that have been reported to possess both dopamine-dependent and dopamine-independent rewarding properties. The search for how, if at all, these distinct mechanisms of motivation are related is of great interest in drug addiction research. Recent electrophysiological, molecular, and behavioral work has greatly improved our understanding of this process. In particular, the signaling properties of GABA(A) receptors located on GABA neurons in the ventral tegmental area (VTA) appear to be crucial to understanding the interplay between dopamine-dependent and dopamine-independent mechanisms of opiate motivation.

Dopaminergic signaling mediates the motivational response underlying the opponent process to chronic but not acute nicotine

The mesolimbic dopamine (DA) system is implicated in the processing of the positive reinforcing effect of all drugs of abuse, including nicotine. It has been suggested that the dopaminergic system is also involved in the aversive motivational response to drug withdrawal, particularly for opiates, however, the role for dopaminergic signaling in the processing of the negative motivational properties of nicotine withdrawal is largely unknown. We hypothesized that signaling at dopaminergic receptors mediates chronic nicotine withdrawal aversions and that dopaminergic signaling would differentially mediate acute vs dependent nicotine motivation. We report that nicotine-dependent rats and mice showed conditioned place aversions to an environment paired with abstinence from chronic nicotine that were blocked by the DA receptor antagonist alpha-flupenthixol (alpha-flu) and in DA D(2) receptor knockout mice. Conversely, alpha-flu pretreatment had no effect on preferences for an environment paired with abstinence from acute nicotine. Taken together, these results suggest that dopaminergic signaling is necessary for the opponent motivational response to nicotine in dependent, but not non-dependent, rodents. Further, signaling at the DA D(2) receptor is critical in mediating withdrawal aversions in nicotine-dependent animals. We suggest that the alleviation of nicotine withdrawal primarily may be driving nicotine motivation in dependent animals.

Environmental enrichment decreases the rewarding but not the activating effects of heroin

RATIONALE

Environmental conditions during adolescence, a critical period of brain maturation, can have important consequences on subsequent vulnerability to drugs of abuse. We have recently found that the behavioral effects of cocaine as well as its ability to increase expression of zif-268 are reduced in mice reared in enriched environments (EE).

OBJECTIVES

The present experiments examined whether environmental enrichment has protective influences on the effects of heroin, a drug of addiction whose mechanism of action differs from that of cocaine.

MATERIALS AND METHODS

Mice were housed either in standard environments (SE) or in EE from weaning to adulthood before any drug exposure. EE were constituted by big housing cages and contained constantly a running wheel and a small house and four to five toys that were changed once a week with new toys of different shapes and colors. We assessed the influence of EE on the ability of heroin to (1) induce conditioned place preferences, (2) induce behavioral sensitization, (3) increase dopamine levels in the nucleus accumbens (NAc), and (4) increase expression of the immediate early gene zif-268 in the striatum.

RESULTS

Conditioned place preference but not behavioral sensitization was reduced in EE mice compared to SE mice. Heroin induced similar increases in dopamine levels and in the expression of zif-268 in the NAc of EE and SE mice.

CONCLUSIONS

The rewarding effects of heroin are blunted by EE and appear to be, at least in part, independent from activation of the mesolimbic system.

Genetics of dopamine receptors and drug addiction: a comprehensive review

Drug dependence is a chronic, relapsing disorder in which compulsive drug-seeking and drug-taking behaviours persist despite serious negative consequences. Addictive substances, such as opioids, ethanol, psychostimulants and nicotine, induce pleasant states or relieve distress, effects that contribute to their recreational use. Dopamine is critically involved in drug addiction processes. However, the role of the various dopaminergic receptor subtypes has been difficult to delineate. Here, we will review the information collected implicating the receptors of the D1 family (DRD1 and DRD5) and of the D2 family (DRD2, DRD3 and DRD4) in drug addiction. We will summarize the distribution of these receptors in the brain, the preclinical experiments carried out with pharmacological and transgenic approaches and the genetic studies carried out linking genetic variants of these receptors to drug addiction phenotypes. A meta-analysis of the studies carried out evaluating DRD2 and alcohol dependence is also provided, which indicates a significant association. Overall, this review indicates that different aspects of the addiction phenotype are critically influenced by dopaminergic receptors and that variants of those genes seem to influence some addiction phenotypes in humans.

Ultrasonic vocalizations induced by sex and amphetamine in M2, M4, M5 muscarinic and D2 dopamine receptor knockout mice

Adult mice communicate by emitting ultrasonic vocalizations (USVs) during the appetitive phases of sexual behavior. However, little is known about the genes important in controlling call production. Here, we study the induction and regulation of USVs in muscarinic and dopaminergic receptor knockout (KO) mice as well as wild-type controls during sexual behavior. Female mouse urine, but not female rat or human urine, induced USVs in male mice, whereas male urine did not induce USVs in females. Direct contact of males with females is required for eliciting high level of USVs in males. USVs (25 to120 kHz) were emitted only by males, suggesting positive state; however human-audible squeaks were produced only by females, implying negative state during male-female pairing. USVs were divided into flat and frequency-modulated calls. Male USVs often changed from continuous to broken frequency-modulated calls after initiation of mounting. In M2 KO mice, USVs were lost in about 70–80% of the mice, correlating with a loss of sexual interaction. In M5 KO mice, mean USVs were reduced by almost 80% even though sexual interaction was vigorous. In D2 KOs, the duration of USVs was extended by 20%. In M4 KOs, no significant differences were observed. Amphetamine dose-dependently induced USVs in wild-type males (most at 0.5 mg/kg i.p.), but did not elicit USVs in M5 KO or female mice. These studies suggest that M2 and M5 muscarinic receptors are needed for male USV production during male-female interactions, likely via their roles in dopamine activation. These findings are important for the understanding of the neural substrates for positive affect.

The effects of two highly selective dopamine D3 receptor antagonists (SB-277011A and NGB-2904) on food self-administration in a rodent model of obesity

In the current study, we examined the effect of the selective D(3) receptor antagonists SB-277011A and NGB 2904 on operant food self-administration (FSA) in Zucker obese and lean rats. Obese (Ob) and lean (Le) Zucker rats were maintained under a restricted feeding regimen (70% of ad-libitum rat chow) and were trained to lever press for food during daily, 2 hour fixed-ratio 4 (FR4) schedules. Once rats reached a stable baseline for FSA, they were injected with vehicle until a stable FSA criterion was achieved. Animals then received daily injections of different random doses of SB-277011A (3, 10, and 30 mg/kg i.p.), and NGB-2904 (0.3, 1 and 3 mg/kg i.p.). SB-277011A produced a significant decrease in both food intake and active lever responses in both Ob and Le rats. In contrast, NGB-2904 did not decrease food intake levels or lever presses for food in Ob and Le rats. These results suggest that along with its involvement in seeking behavior for drugs of abuse, the D(3) dopamine receptor may also be involved in seeking behavior for natural reinforcers such as food.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.

A D2 antagonist enhances the rewarding and priming effects of a gambling episode in pathological gamblers

Previous research indicated shared neurochemical substrates for gambling and psychostimulant reward. This suggests that dopamine substrates may directly govern the reinforcement process in pathological gambling. To investigate this issue, the present study assessed the effects of the relatively selective dopamine D2 antagonist, haloperidol (3 mg, oral) on responses to actual gambling (15 min on a slot machine) in 20 non-comorbid pathological gamblers and 18 non-gambler controls in a placebo-controlled, double-blind, counterbalanced design. In gamblers, haloperidol significantly increased self-reported rewarding effects of gambling, post-game priming of desire to gamble, facilitation of reading speed to Gambling words, and gambling-induced elevation in blood pressure. In controls, haloperidol augmented gambling-induced elevation in blood pressure, but had no effect on other indices. The findings provide direct experimental evidence that the D2 substrate modulates gambling reinforcement in pathological gamblers.

Neurobiology of addiction. An integrative review

Evidence that psychoactive substance use disorders, bulimia nervosa, pathological gambling, and sexual addiction share an underlying biopsychological process is summarized. Definitions are offered for addiction and addictive process, the latter being the proposed designation for the underlying biopsychological process that addictive disorders are hypothesized to share. The addictive process is introduced as an interaction of impairments in three functional systems: motivation-reward, affect regulation, and behavioral inhibition. An integrative review of the literature that addresses the neurobiology of addiction is then presented, organized according to the three functional systems that constitute the addictive process. The review is directed toward identifying candidate neurochemical substrates for the impairments in motivation-reward, affect regulation, and behavioral inhibition that could contribute to an addictive process.

A test of the opponent-process theory of motivation using lesions that selectively block morphine reward

The opponent-process theory of motivation postulates that motivational stimuli activate a rewarding process that is followed by an opposed aversive process in a homeostatic control mechanism. Thus, an acute injection of morphine in nondependent animals should evoke an acute rewarding response, followed by a later aversive response. Indeed, the tegmental pedunculopontine nucleus (TPP) mediates the rewarding effects of opiates in previously morphine-naive animals, but not other unconditioned effects of opiates, or learning ability. The aversive opponent process for acute morphine reward was revealed using a place-conditioning paradigm. The conditioned place aversion induced by 16-h spontaneous morphine withdrawal from an acute morphine injection in nondependent rats was abolished by TPP lesions performed prior to drug experience. However, TPP-lesioned rats did show conditioned aversions for an environment paired with the acute administration of the opioid antagonist naloxone, which blocks endogenous opioids. The results show that blocking the rewarding effects of morphine with TPP lesions also blocked the opponent aversive effects of acute morphine withdrawal in nondependent animals. Thus, this spontaneous withdrawal aversion (the opponent process) is induced by the acute rewarding effects of morphine and not by other unconditioned effects of morphine, the pharmacological effects of morphine or endogenous opioids being displaced from opiate receptors.

Insights into the Role of Dopamine Receptor Systems in Learning and Memory

It is well established that learning and memory are complex processes involving and recruiting different brain modulatory neurotransmitter systems. Considerable evidence points to the involvement of dopamine in various aspects of cognition, and interest has been focused on investigating the clinical relevance of dopamine systems to age-related cognitive decline and manifestations of cognitive impairment in schizophrenia, Alzheimer's disease, Parkinson's disease and other neurodegenerative diseases. In the past decade or so, in spite of the molecular cloning of the five dopamine receptor subtypes, their specific roles in brain function remained inconclusive due to the lack of completely selective ligands that could distinguish between the members of the D1-like and D2-like dopamine receptor families. One of the most important advances in the field of dopamine research has been the generation of mutant mouse models permitting evaluation of the dopaminergic system using gene targeting technologies. These mouse models represent an important approach to explore the functional roles of closely related receptor subtypes. In this review, we present and discuss evidence on the role of dopamine receptors in different aspects of learning and memory at the cellular, molecular and behavioral levels. We compare evidence using conventional pharmacological, lesion or electrophysiological studies with results from mice with targeted deletions of different subtypes of dopamine receptor genes. We particularly focus on dopamine D1 and D2 receptors in an effort to delineate their specific roles in various aspects of cognitive function. We provide strong evidence, from our own recent work as well as others, that dopamine is part of the network that plays a very important role in cognitive function, and that although multiple dopamine receptor subtypes contribute to different aspects of learning and memory, the D1 receptor seems to play a more prominent role in mediating plasticity and specific aspects of cognitive function, including spatial learning and memory processes, reversal learning, extinction learning, and incentive learning.

Morphine reward in dopamine-deficient mice

Dopamine has been widely implicated as a mediator of many of the behavioural responses to drugs of abuse. To test the hypothesis that dopamine is an essential mediator of various opiate-induced responses, we administered morphine to mice unable to synthesize dopamine. We found that dopamine-deficient mice are unable to mount a normal locomotor response to morphine, but a small dopamine-independent increase in locomotion remains. Dopamine-deficient mice have a rightward shift in the dose-response curve to morphine on the tail-flick test (a pain sensitivity assay), suggesting either a decreased sensitivity to the analgesic effects of morphine and/or basal hyperalgesia. In contrast, dopamine-deficient mice display a robust conditioned place preference for morphine when given either caffeine or l-dihydroxyphenylalanine (a dopamine precursor that restores dopamine throughout the brain) during the testing phases. Together, these data demonstrate that dopamine is a crucial component of morphine-induced locomotion, dopamine may contribute to morphine analgesia, but that dopamine is not required for morphine-induced reward as measured by conditioned place preference.

Brain stimulation and morphine reward deficits in dopamine D2 receptor-deficient mice

RATIONALE

The rewarding effects of lateral hypothalamic brain stimulation, various natural rewards, and several drugs of abuse are attenuated by D1 or D2 dopamine receptor (D1R or D2R) antagonists. Much of the evidence for dopaminergic involvement in rewards is based on pharmacological agents with limited or "relative" selectivity for dopamine receptor subtypes. Genetically engineered animal models provide a complementary approach to pharmacological investigations.

OBJECTIVES

In the present study, we explored the contribution of dopamine D2Rs to (1) brain stimulation reward (BSR) and (2) the potentiation of this behavior by morphine and amphetamine using D2R-deficient mice.

METHODS

Wild-type (D2Rwt), heterozygous (D2Rhet), and D2R knockout (D2Rko) mice were trained to turn a wheel for rewarding brain stimulation. Once equivalent rate-frequency curves were established, morphine-induced (0, 1.0, 3.0, and 5.6 mg/kg s.c.) and amphetamine-induced (0, 1.0, 2.0, and 4.0 mg/kg i.p.) potentiations of BSR were determined.

RESULTS

The D2Rko mice required approximately 50% more stimulation than the D2Rwt mice did. With the equi-rewarding levels of stimulation current, amphetamine potentiated BSR equally across the three genotypes. In contrast, morphine potentiated rewarding stimulation in the D2Rwt, had no effect in the D2Rhet, and antagonized rewarding stimulation in the D2Rko mice.

CONCLUSIONS

D2R elimination decreases, but does not eliminate, the rewarding effects of lateral hypothalamic stimulation. After compensation for this deficit, amphetamine continues to potentiate BSR, while morphine does not.

Naloxone's suppression of spontaneous and food-conditioned locomotor activity is diminished in mice lacking either the dopamine D(2) receptor or enkephalin

Mice lacking the D2 dopamine receptor (D2(-/-)) and congenic to the C57BL/6J background were tested for opioid-mediated locomotor activity to examine the involvement of the D2 dopamine receptor in opioid pharmacology. Morphine-stimulated locomotor activity did not significantly differ between the two genotypes. The opioid antagonist naloxone dose-dependently decreased spontaneous motor activity in wild-type mice but was without significant effect in D2(-/-) mice. The magnitude of food-conditioned increases in locomotor activity in wild-type mice and D2(-/-) mice was similar but naloxone did not decrease conditioned motor activity in D2(-/-) mice. Spontaneous locomotor activity of mice lacking the endogenous opioids beta-endorphin and/or enkephalin was also tested and we found that naloxone did not reduce activity in mice specifically lacking enkephalin. We suggest that the D2 dopamine receptor is necessary for modulation of spontaneous locomotor activity stimulated by the endogenous opioid enkephalin.

Phenotypic analysis of dopamine receptor knockout mice; recent insights into the functional specificity of dopamine receptor subtypes

The functional specificity of dopamine receptor subtypes remains incompletely understood, in part due to the absence of highly selective agonists and antagonists. Phenotypic analysis of dopamine receptor knockout mice has been instrumental in identifying the role of dopamine receptor subtypes in mediating dopamine's effects on motor function, cognition, reward, and emotional behaviors. In this article, we provide an update of recent studies in dopamine receptor knockout mice and discuss the limitations and future promise of this approach.

Narcotic antagonists in drug dependence: pilot study showing enhancement of compliance with SYN-10, amino-acid precursors and enkephalinase inhibition therapy

We decided to test the hypothesis that possibly by combining a narcotic antagonist and amino-acid therapy consisting of an enkephalinase inhibitor (D-phenylalanine) and neurotransmitter precursors (L-amino- acids) to promote neuronal dopamine release might enhance compliance in methadone patients rapidly detoxified with the narcotic antagonist Trexan (Dupont, Delaware). In this regard, Thanos et al. [J. Neurochem. 78 (2001) 1094] and associates found increases in the dopamine D2 receptors (DRD2) via adenoviral vector delivery of the DRD2 gene into the nucleus accumbens, significantly reduced both ethanol preference (43%) and alcohol intake (64%) of ethanol preferring rats, which recovered as the DRD2, returned to baseline levels. This DRD2 overexpression similarly produced significant reductions in ethanol non-preferring rats, in both alcohol preference (16%) and alcohol intake (75%). This work further suggests that high levels of DRD2 may be protective against alcohol abuse [JAMA 263 (1990) 2055; Arch, Gen. Psychiatr. 48 (1991) 648]. The DRD2 A1 allele has also been shown to associate with heroin addicts in a number of studies. In addition, other dopaminergic receptor gene polymorphisms have also associated with opioid dependence. For example, Kotler et al. [Mol. Phychiatr. 3 (1997) 251] showed that the 7 repeat allele of the DRD4 receptor is significantly overpresented in the opioid-dependent cohort and confers a relative risk of 2.46. This has been confirmed by Li et al. [Mol. Psychiatry 2 (1997) 413] for both the 5 and 7 repeat alleles in Han Chinese case control sample of heroin addicts. Similarly Duaux et al. [Mol. Psychiatry 3 (1998) 333] in French Heroin addicts, found a significant association with homozygotes alleles of the DRD3-Bal 1. A study from NIAAA, provided evidence which strongly suggests that DRD2 is a susceptibility gene for substance abusers across multiple populations (2003). Moreover, there are a number of studies utilizing amino-acid and enkephalinase inhibition therapy showing reduction of alcohol, opiate, cocaine and sugar craving behavior in human trials (see Table 1). Over the last decade, a new rapid method to detoxify either methadone or heroin addicts utilizing Trexan sparked interest in many treatment centers throughout the United States, Canada, as well as many countries on a worldwide basis. In using the combination of Trexan and amino-acids, results were dramatic in terms of significantly enhancing compliance to continue taking Trexan. The average number of days of compliance calculated on 1000 patients, without amino-acid therapy, using this rapid detoxification method is only 37 days. In contrast, the 12 subjects tested, receiving both the Trexan and amino-acid therapy was relapse-free or reported taking the combination for an average of 262 days (p < 0.0001F). Thus coupling amino-acid therapy and enkephalinase inhibition while blocking the delta-receptors with a pure narcotic antagonist may be quite promising as a novel method to induce rapid detox in chronic methadone patients. This may also have important ramifications in the treatment of both opiate and alcohol-dependent individuals, especially as a relapse prevention tool. It may also be interesting too further test this hypothesis with the sublingual combination of the partial opiate mu receptor agonist buprenorphrine.

Mu opioid receptor: a gateway to drug addiction

Mu opioid receptors mediate positive reinforcement following direct (morphine) or indirect (alcohol, cannabinoids, nicotine) activation, and our understanding of mu receptor function is central to the development of addiction therapies. Recent data obtained in native neurons confirm that mu receptor signaling and regulation are strongly agonist-dependent. Current functional mapping reveals morphine-activated neurons in the extended amygdala and early genomic approaches have identified novel mu receptor-associated proteins. A classification of about 30 genes either promoting or counteracting the addictive properties of morphine is proposed from the analysis of knockout mice data. The targeting of effectors or regulatory proteins, beyond the mu receptor itself, might provide valuable strategies to treat addictive disorders.

Wheel running use in dopamine D2L receptor knockout mice

The present study investigated the role of dopamine and the opioid system in the acquisition and maintenance of wheel running use (WR), employing D2L receptor-deficient (D2L-/-) mice as a model system. The daily administration of 1mg/kg of naloxone virtually abolished the acquisition of WR in D2L-/-, but did not modify the number of wheel turns after the consolidation of this behavior. These findings imply that both dopamine and opioid reward systems are necessary for WR consolidation, and they also suggest that reward systems in WR could vary according to the motivational stage of the animal.

DREAM ablation selectively alters THC place aversion and analgesia but leaves intact the motivational and analgesic effects of morphine

DREAM (downstream regulatory element antagonistic modulator) is a novel transcriptional repressor for the prodynorphin gene, and genetic deletion of DREAM in mice results in a phenotype of ongoing analgesia by virtue of its effect on opioid gene expression. In the present study, we evaluated the motivational effects of opioids (morphine), cannabinoids [Delta(9)-tetrahydrocannabinol (THC)] and cocaine in mice lacking the dream gene (dream(-/-)). The aversive effects of THC were potentiated in dream(-/-) mice in a kappa-opioid receptor-dependent fashion, whereas morphine reward and the aversive effects of morphine withdrawal remained intact. The rewarding and aversive effects of cocaine were likewise unperturbed in dream(-/-) mice. Moreover, the aversive properties of lithium chloride and naloxone were unaffected by the absence of DREAM, indicating that the effect of DREAM on THC-induced dysphoria is not due to a general involvement in the behavioral response to aversive stimuli. Additionally, physical dependence to morphine and the locomotor-sensitizing effects of cocaine were unaltered in these animals. Finally, whereas the absence of DREAM reduced the analgesic efficacy of THC, morphine analgesia was unaffected in dream(-/-) mice.

Endogenous opioids mediate basal hedonic tone independent of dopamine D-1 or D-2 receptor activation

Exogenously administered opiates are recognized as rewarding and the involvement of dopamine systems in mediating their apparent pleasurable effects is contentious. The aversive response to naloxone administration observed in animal studies suggests the presence of an endogenous opioid tone regulating hedonic state. We sought evidence for the requirement for dopamine systems in mediating this action of endogenous opioids by determining whether mice deficient in dopamine D-1 or D-2 receptors were able to display conditioned place aversion to naloxone. Mice received saline in the morning in one chamber and either saline or naloxone (10 mg/kg, s.c.) in the afternoon in another chamber, each day for 3 days. On the test day they were given free access to the testing chambers in the afternoon. Similar to their wild-type littermates, D-1 and D-2 receptor knockout mice receiving naloxone in the afternoon spent significantly less time on the test day in the compartment in which they previously received naloxone, compared with animals receiving saline in the afternoon. The persistence of naloxone-conditioned place aversion in D-1 and D-2 knockout mice suggests that endogenous opioid peptides maintain a basal level of positive affect that is not dependent on downstream activation of dopamine systems involving D-1 or D-2 receptors.

Opiate state controls bi-directional reward signaling via GABAA receptors in the ventral tegmental area

The neural mechanisms that mediate the transition from a drug-naive state to a state of drug dependence and addiction are not yet known. Here we show that a discrete population of GABA(A) receptors in the mammalian ventral tegmental area (VTA) serves as a potential addiction switching mechanism by gating reward transmission through one of two neural motivational systems: either a dopamine-independent (opiate-naive) or a dopaminergic (opiate-dependent or opiate-withdrawn) system. Bi-directional transmission of reward signals through this GABA(A) receptor substrate is dynamically controlled by the opiate state of the organism and involves a molecular alteration of the GABA(A) receptor. After opiate exposure and subsequent withdrawal, the functional conductance properties of the rat VTA GABA(A) receptor switch from an inhibitory to an excitatory signaling mode.

Addicting drugs utilize a synergistic molecular mechanism in common requiring adenosine and Gi-beta gamma dimers

The mesolimbic dopamine system and cAMP-dependent/protein kinase A (PKA) pathways are strongly implicated in addictive behaviors. Here we determine the role of dopamine D2 receptors (D2) in PKA signaling responses to delta-opioid (DOR) and cannabinoid (CB1) receptors. We find in NG108-15/D2 cells and in cultured primary neurons that a brief exposure to saturating concentrations of DOR and CB1 agonists increases cAMP, promotes PKA C alpha translocation and increases cAMP-dependent gene expression. Activation of PKA signaling is mediated by Gi-beta gamma dimers. Importantly, subthreshold concentrations of DOR or CB1 agonists with D2 agonists, which are without effect when added separately, together activate cAMP/PKA signaling synergistically. There is also synergy between DOR or CB1 with ethanol, another addicting agent. In all instances, synergy requires adenosine activation of adenosine A2 receptors and is mediated by beta gamma dimers. Synergy by this molecular mechanism appears to confer hypersensitivity to opioids and cannabinoids while simultaneously increasing the sensitivity of D2 signaling when receptors are expressed on the same cells. This mechanism may account, in part, for drug-induced activation of medium spiny neurons in the nucleus accumbens.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

DREAM is a critical transcriptional repressor for pain modulation

Control and treatment of chronic pain remain major clinical challenges. Progress may be facilitated by a greater understanding of the mechanisms underlying pain processing. Here we show that the calcium-sensing protein DREAM is a transcriptional repressor involved in modulating pain. dream(-/-) mice displayed markedly reduced responses in models of acute thermal, mechanical, and visceral pain. dream(-/-) mice also exhibited reduced pain behaviors in models of chronic neuropathic and inflammatory pain. However, dream(-/-) mice showed no major defects in motor function or learning and memory. Mice lacking DREAM had elevated levels of prodynorphin mRNA and dynorphin A peptides in the spinal cord, and the reduction of pain behaviors in dream(-/-) mice was mediated through dynorphin-selective kappa (kappa)-opiate receptors. Thus, DREAM appears to be a critical transcriptional repressor in pain processing.

Mouse strain differences in opiate reward learning are explained by differences in anxiety, not reward or learning

Gene-targeting techniques to produce null mutations provide a powerful method for evaluating the contribution of particular candidate genes involved in motivation. The embryonic stem cell lines in which homologous recombination is undertaken are derived from 129 mice, but because of the impoverished performance of 129 mice on a number of behavioral tasks, mice chimeric for the mutation are often bred with a C57BL/6 mouse strain. Thus, an examination of both parental strains is important in the study of the knock-out mice. Although the C57BL/6 behavioral phenotype is well documented, details of the 129 phenotype have not been the focus of study until recently. We investigated opiate motivation in both 129/SvJ and C57BL/6J mouse strains to determine whether, and under what circumstances, the 129/SvJ mouse exhibited motivated behavior toward opiates. 129/SvJ mice required both drug and contextual cues to demonstrate morphine conditioned place preferences on test day, whereas C57BL/6J mice required only contextual cues to express opiate place conditioning. Pentobarbital and diazepam but not saline, cocaine, or naloxone could substitute for morphine on test day in 129/SvJ mice, demonstrating that morphine indeed has rewarding motivational valence in the 129/SvJ mouse strain. This critical, interoceptive cue in 129/SvJ mice on test day may be the anxiolytic properties of the effective drugs. Therefore, some deficits observed in 129 mice and mice harboring this genetic background may be attributed to high levels of anxiety during the retrieval period rather than to sensory, learning, or motivational deficits.