Effects of kappa-opioid receptor agonists on long-term cocaine use and dopamine neurotransmission

DARK Classics in Chemical Neuroscience: Salvinorin A

Salvinorin A is the main bioactive compound in Salvia divinorum, an endemic plant with ancestral use by the inhabitants of the Mazateca mountain range (Sierra Mazateca) in Oaxaca, México. The main use of la pastora, as locally known, is in spiritual rites due to its extraordinary hallucinogenic effects. Being the first known nonalkaloidal opioid-mediated psychotropic molecule, salvinorin A set new research areas in neuroscience. The absence of a protonated amine group, common to all previously known opioids, results in a fast metabolism with the concomitant fast elimination and swift loss of activity. The worldwide spread and psychotropic effects of salvinorin A account for its misuse and classification as a drug of abuse. Consequently, salvinorin A and Salvia divinorum are now banned in many countries. Several synthetic efforts have been focused on the improvement of physicochemical and biological properties of salvinorin A: from total synthesis to hundreds of analogues. In this Review, we discuss the impact of salvinorin A in chemistry and neuroscience covering the historical relevance, isolation from natural sources, synthetic efforts, and pharmacological and safety profiles. Altogether, the chemistry behind and the taboo that encloses salvinorin A makes it one of the most exquisite naturally occurring drugs.

Kappa opioid agonists reduce oxycodone self-administration in male rhesus monkeys

RATIONALE

Combinations of mu and kappa opioid receptor (KOR) agonists have been proposed as potential analgesic formulations with reduced abuse liability. The current studies extend previous work by investigating the typical KOR agonist, salvinorin A, and the atypical KOR agonist, nalfurafine, as deterrents of oxycodone self-administration using a progressive ratio (PR) schedule of reinforcement.

METHODS

In separate experiments, adult male rhesus monkeys (N = 4/experiment) were trained under a PR schedule of reinforcement to self-administer cocaine (0.1 mg/kg/injection) and saline on alternating days. Oxycodone (0.01-0.1 mg/kg/injection) alone and combined with salvinorin A (experiment 1; 0.006, 0.012 mg/kg/injection) or nalfurafine (experiment 2; 0.0001-0.00032 mg/kg/injection) were tested within the alternating cocaine and saline baseline. The mechanism of nalfurafine's effects on oxycodone self-administration was investigated via pretreatment with the KOR antagonist, nor-binaltorphimine (nor-BNI; 10 mg/kg; i.m.).

RESULTS

All subjects self-administered oxycodone alone above saline levels at sufficiently large doses, and combining salvinorin A or nalfurafine with oxycodone reduced the mean number of injections per session to saline levels (experiment 1) or to levels that were significantly lower than oxycodone alone (experiment 2). The ability of nalfurafine to reduce oxycodone self-administration was reversed by pretreatment with nor-BNI.

CONCLUSIONS

These results demonstrate that KOR agonists, including the clinically used KOR agonist, nalfurafine, can punish self-administration of a prescription opioid analgesic, oxycodone, in rhesus monkeys and that nalfurafine's punishing effect is KOR-dependent. Combinations of KOR agonists with prescription opioids may have reduced abuse liability.

The Role of Dynorphin and the Kappa Opioid Receptor in the Symptomatology of Schizophrenia: A Review of the Evidence

Schizophrenia is a debilitating mental illness that affects approximately 1% of the world's population. Despite much research in its neurobiology to aid in developing new treatments, little progress has been made. One system that has not received adequate attention is the kappa opioid system and its potential role in the emergence of symptoms, as well as its therapeutic potential. Here we present an overview of the kappa system and review various lines of evidence derived from clinical studies for dynorphin and kappa opioid receptor involvement in the pathology of both the positive and negative symptoms of schizophrenia. This overview includes evidence for the psychotomimetic effects of kappa opioid receptor agonists in healthy volunteers and their reversal by the pan-opioid antagonists naloxone and naltrexone and evidence for a therapeutic benefit in schizophrenia for 4 pan-opioid antagonists. We describe the interactions between kappa opioid receptors and the dopaminergic pathways that are disrupted in schizophrenia and the histologic evidence suggesting abnormal kappa opioid receptor signaling in schizophrenia. We conclude by discussing future directions.

Dynorphin/Kappa Opioid Receptor Signaling in Preclinical Models of Alcohol, Drug, and Food Addiction

The dynorphin/kappa opioid receptor (KOR) system is implicated in the "dark side" of addiction, in which stress exacerbates maladaptive responses to drug and alcohol exposure. For example, acute stress and acute ethanol exposure result in an elevation in dynorphin, the KOR endogenous ligand. Activation of KORs results in modulation of several neurotransmitters; however, this chapter will focus on its regulatory effects on dopamine in mesolimbic areas. Specifically, KOR activation has an inhibitory effect on dopamine release, thereby influencing reward processing. Repeated stimulation of KORs, for example, via chronic drug and/or stress exposure, results in increased function of the dynorphin/KOR system. This augmentation in KOR function shifts the homeostatic balance in favor of an overall reduction in dopamine signaling via either by reducing dopamine release or by increasing dopamine transporter function. This chapter examines the effects of chronic ethanol exposure on KOR function and the downstream effects on dopamine transmission. Additionally, the impact of chronic cocaine exposure and its effects on KOR function will be explored. Further, KORs may also be involved in driving excessive consumption of food, contributing to the risk of developing obesity. While some studies have shown that KOR agonists reduce drug intake, other studies have shown that antagonists reduce addiction-like behaviors, demonstrating therapeutic potential. For example, KOR inhibition reduces ethanol intake in dependent animals, motivation to self-administer cocaine in chronic stress-exposed animals, and food consumption in obese animals. This chapter will delve into the mechanisms by which modulation of the dynorphin/KOR system may be therapeutic.

The opioid receptors as targets for drug abuse medication

The endogenous opioid system is largely expressed in the brain, and both endogenous opioid peptides and receptors are present in areas associated with reward and motivation. It is well known that this endogenous system plays a key role in many aspects of addictive behaviours. The present review summarizes the modifications of the opioid system induced by chronic treatment with drugs of abuse reported in preclinical and clinical studies, as well as the action of opioid antagonists and agonists on the reinforcing effects of drugs of abuse, with therapeutic perspectives. We have focused on the effects of chronic psychostimulants, alcohol and nicotine exposure. Taken together, the changes in both opioid peptides and opioid receptors in different brain structures following acute or chronic exposure to these drugs of abuse clearly identify the opioid system as a potential target for the development of effective pharmacotherapy for the treatment of addiction and the prevention of relapse.

Salvinorin A analogs and other κ-opioid receptor compounds as treatments for cocaine abuse

Acute activation of kappa-opioid receptors produces anti-addictive effects by regulating dopamine levels in the brain. Unfortunately, classic kappa-opioid agonists have undesired side effects such as sedation, aversion, and depression, which restrict their clinical use. Salvinorin A (Sal A), a novel kappa-opioid receptor agonist extracted from the plant Salvia divinorum, has been identified as a potential therapy for drug abuse and addiction. Here, we review the preclinical effects of Sal A in comparison with traditional kappa-opioid agonists and several new analogs. Sal A retains the anti-addictive properties of traditional kappa-opioid receptor agonists with several improvements including reduced side effects. However, the rapid metabolism of Sal A makes it undesirable for clinical development. In an effort to improve the pharmacokinetics and tolerability of this compound, kappa-opioid receptor agonists based on the structure of Sal A have been synthesized. While work in this field is still in progress, several analogs with improved pharmacokinetic profiles have been shown to have anti-addictive effects. While in its infancy, it is clear that these compounds hold promise for the future development of anti-addictive therapeutics.

Effects of ATPM-ET, a novel κ agonist with partial μ activity, on physical dependence and behavior sensitization in mice

AIM

to investigate the effects of ATPM-ET [(-)-3-N-Ethylaminothiazolo [5,4-b]-N-cyclopropylmethylmorphinan hydrochloride] on physical dependence and behavioral sensitization to morphine in mice.

METHODS

the pharmacological profile of ATPM-ET was characterized using competitive binding and GTPγS binding assays. We then examined the antinociceptive effects of ATPM-ET in the hot plate test. Morphine dependence assay and behavioral sensitization assay were used to determine the effect of ATPM-ET on physical dependence and behavior sensitization to morphine in mice.

RESULTS

the binding assay indicated that ATPM-ET ATPM-ET exhibited a high affinity to both κ- and μ-opioid receptors with K(i) values of 0.15 nmol/L and 4.7 nmol/L, respectively, indicating it was a full κ-opioid receptor agonist and a partial μ-opioid receptor agonist. In the hot plate test, ATPM-ET produced a dose-dependent antinociceptive effect, with an ED(50) value of 2.68 (2.34-3.07) mg/kg. Administration of ATPM-ET (1 and 2 mg/kg, sc) prior to naloxone (3.0 mg/kg, sc) injection significantly inhibited withdrawal jumping of mice. In addition, ATPM-ET (1 and 2 mg/kg, sc) also showed a trend toward decreasing morphine withdrawal-induced weight loss. ATPM-ET (1.5 and 3 mg/kg, sc) 15 min before the morphine challenge significantly inhibited the morphine-induced behavior sensitization (P<0.05).

CONCLUSION

ATPM-ET may have potential as a therapeutic agent for the treatment of drug abuse.

Depletion of serotonin decreases the effects of the kappa-opioid receptor agonist U-69593 on cocaine-stimulated activity

Treatment with a kappa-opioid receptor agonist for 5 days decreases locomotor activity and reduces activity in response to a cocaine challenge 3 days later. In addition, chronic cocaine increases kappa-opioid receptor density, striatal dynorphin, and dynorphin gene expression in the striatum. The upregulation of kappa-opioid receptors after cocaine treatment occurs predominantly in brain regions that are highly innervated by serotonin. To determine if serotonin plays a role in the effects of kappa-opioid receptor agonists on cocaine-stimulated activity, parachloroamphetamine (PCA), which depleted serotonin by 53%-66%, or saline, was given prior to a five-day treatment with U-69593 or vehicle. Three days later each rat received a single injection of cocaine and locomotor activity was measured. Treatment with PCA had no effect on the ability of U-69593 alone to decrease locomotor activity. Thus, the behavioral effects of U-69593 alone were not dependent upon serotonin. In rats pretreated with saline, U-69593 treatment significantly blocked the locomotor-activating effects of cocaine. Following PCA pretreatment, however, there were no significant differences in locomotor activity in rats challenged with an injection of cocaine after treatment with U-69593 or vehicle. Thus, serotonin depletion prevented the long-lasting blockade of the locomotor-activating effects of cocaine subsequent to repeated administration of U-69593 but did not alter the effects of cocaine in rats that were treated with vehicle. Thus, the effects of PCA on U-69593 are not due to non-specific alterations in cocaine-induced locomotor activity. These findings suggest that serotonin plays an important role in mediating the effects of kappa-opioid receptor agonists on the behavioral response to cocaine.

Dynorphin and the pathophysiology of drug addiction

Drug addiction is a chronic relapsing disease in which drug administration becomes the primary stimulus that drives behavior regardless of the adverse consequence that may ensue. As drug use becomes more compulsive, motivation for natural rewards that normally drive behavior decreases. The discontinuation of drug use is associated with somatic signs of withdrawal, dysphoria, anxiety, and anhedonia. These consequences of drug use are thought to contribute to the maintenance of drug use and to the reinstatement of compulsive drug use that occurs during the early phase of abstinence. Even, however, after prolonged periods of abstinence, 80-90% of human addicts relapse to addiction, suggesting that repeated drug use produces enduring changes in brain circuits that subserve incentive motivation and stimulus-response (habit) learning. A major goal of addiction research is the identification of the neural mechanisms by which drugs of abuse produce these effects. This article will review data showing that the dynorphin/kappa-opioid receptor (KOPr) system serves an essential function in opposing alterations in behavior and brain neurochemistry that occur as a consequence of repeated drug use and that aberrant activity of this system may not only contribute to the dysregulation of behavior that characterizes addiction but to individual differences in vulnerability to the pharmacological actions of cocaine and alcohol. We will provide evidence that the repeated administration of cocaine and alcohol up-regulates the dynorphin/KOPr system and that pharmacological treatments that target this system may prove effective in the treatment of drug addiction.

Cotreatment with the kappa opioid agonist U69593 enhances locomotor sensitization to the D2/D3 dopamine agonist quinpirole and alters dopamine D2 receptor and prodynorphin mRNA expression in rats

RATIONALE

The repeated coadministration of the kappa opioid receptor agonist U69593 with the D2/D3 dopamine (DA) agonist quinpirole (QNP) potentiates locomotor sensitization induced by QNP. Behavioral evidence has implicated both pre- and postsynaptic changes as being involved in this augmentation.

OBJECTIVES

The objectives of this study were to obtain supporting molecular evidence of pre- and/or postsynaptic alterations in the DA system with U69593/QNP cotreatment and to examine the relationship of such changes to locomotor sensitization.

MATERIALS AND METHODS

Gene expression of D1 and D2 receptors (D1R and D2R), the DA transporter, as well as the endogenous opioid prodynorphin (DYN), in the basal ganglia was examined by in situ hybridization in rats after one or ten drug injections.

RESULTS

After one injection, changes that were specific to U69593/QNP cotreatment were decreased D1R and D2R messenger RNA (mRNA) in the nucleus accumbens (Acb) shell and increased DYN mRNA in the dorsal striatum (STR). After ten injections, U69593/QNP-specific changes were decreased D2R mRNA in substantia nigra (SN) and increased DYN mRNA in STR and Acb core. Only in U69593/QNP rats was the sensitized locomotor performance on injection ten positively correlated with DYN mRNA levels in Acb and STR.

CONCLUSIONS

Distinct alterations of D2R and DYN mRNA levels in SN and Acb/STR, respectively, strengthen the evidence implicating pre- and postsynaptic changes in augmented locomotor sensitization to U69593/QNP cotreatment. It is suggested that repeated U69593/QNP cotreatment may augment locomotor sensitization to QNP by activating D1R-expressing DYN neurons and attenuating presynaptic D2R function.

Preprodynorphin mediates locomotion and D2 dopamine and mu-opioid receptor changes induced by chronic 'binge' cocaine administration

Evidence suggests that the kappa-opioid receptor (KOP-r) system plays an important role in cocaine addiction. Indeed, cocaine induces endogenous KOP activity, which is a mechanism that opposes alterations in behaviour and brain function resulting from repeated cocaine use. In this study, we have examined the influence of deletion of preprodynorphin (ppDYN) on cocaine-induced behavioural effects and on hypothalamic-pituitary-adrenal axis activity. Furthermore, we have measured mu-opioid receptor (MOP-r) agonist-stimulated [(35)S]GTPgammaS, dopamine D(1), D(2) receptor and dopamine transporter (DAT) binding. Male wild-type (WT) and ppDYN knockout (KO) mice were injected with saline or cocaine (45 mg/kg/day) in a 'binge' administration paradigm for 14 days. Chronic cocaine produced an enhancement of locomotor sensitisation in KO. No genotype effect was found on stereotypy behaviour. Cocaine-enhanced MOP-r activation in WT but not in KO. There was an overall decrease in D(2) receptor binding in cocaine-treated KO but not in WT mice. No changes were observed in D(1) and DAT binding. Cocaine increased plasma corticosterone levels in WT but not in KO. The data confirms that the endogenous KOP system inhibits dopamine neurotransmission and that ppDYN may mediate the enhancement of MOP-r activity and the activation of the hypothalamic-pituitary-adrenal axis after chronic cocaine treatment.

The kappa-opioid receptor agonist U-69593 prevents cocaine-induced phosphorylation of DARPP-32 at Thr(34) in the rat brain

DARPP-32 (dopamine- and cAMP-regulated phosphoprotein) is a potent endogenous inhibitor of protein phosphatase-1, which plays an important role in dopaminergic transmission. A large body of evidence supports the key role of DARPP-32-dependent signalling in mediating the actions of multiple drugs of abuse, including cocaine, which, when acutely administered, increases the Thr(34) phosphorylation of DARPP-32 in the striatal and cortical areas. In this study, we have examined the contribution of the kappa opioid system to the regulation of DARPP-32 phosphorylation at Thr(34), following acute cocaine administration, in selected rat brain areas. Results showed that a single injection of cocaine induces a significant increase in DARPP-32 phosphorylation at Thr(34) in the hippocampus, caudate putamen and prefrontal cortex. In addition, pretreatment with the kappa opioid receptor agonist U-69593 prevented cocaine effects in all the investigated areas. These data could be considered consistent with the ability of kappa opioid agonists to attenuate many behavioural and neurochemical effects of cocaine.

Chronic cocaine produces decreases in N/OFQ peptide levels in select rat brain regions

The interaction of opioids and stimulants is well established; however, the mechanisms that underlie the role that opioid receptors play in psychostimulant action are not. Nociceptin/orphaninFQ (N/OFQ), the endogenous agonist at NOP receptors, attenuates the behavioral effects of cocaine. The effects of cocaine on N/OFQ were examined in rats using immunoautoradiographic and RIA techniques. Chronic administration of cocaine decreased N/OFQ in medial regions of the caudate putamen, the nucleus accumbens shell, and the substantia nigra. These studies show that N/OFQ levels are altered by treatment with cocaine. Furthermore, the changes in N/OFQ parallel those seen for kappa-opioid receptors, suggesting that the interactions between cocaine and these systems might be similar.

Role of serotonin in the regulation of the dynorphinergic system by a kappa-opioid agonist and cocaine treatment in rat CNS

It has been shown that chronic cocaine increases prodynorphin mRNA in the caudate putamen and decreases it in the hypothalamus. In addition, treatment with a kappa-opioid receptor agonist produced the opposite effect on prodynorphin gene expression in these brain regions and also evoked a decrease in the hippocampus. It is already known that kappa-opioid receptor agonists decrease the development of sensitization to some of the behavioral effects of cocaine. The serotonin system has also been shown to regulate dynorphin gene expression and a continuous infusion of fluoxetine induced prodynorphin gene expression in the same pattern as the kappa-opioid agonist (+)(5a,7a,8b)-N-methyl-N-[7-(1-pyrrolidinyl)-1 oxaspiro[4.5]dec-8-yl]-benzeneacetamide (U-69593) in the brain regions investigated. It is interesting to note that treatment with a continuous infusion of cocaine produced different effects on this parameter. To determine whether serotonin plays a role in the regulation of prodynorphin mRNA by kappa-opioid agonists or cocaine, rats were treated with the serotonin depleter parachloroamphetamine (PCA). Beginning 24 h later, rats were treated with the selective kappa-opioid agonist U-69593 for 5 days or continuously with cocaine for 7 days and prodynorphin mRNA was measured. Prodynorphin mRNA was decreased significantly in the hypothalamus, caudate putamen, and hippocampus of rats treated with a single injection of PCA. Subsequent to PCA administration the effects of U-69593 or cocaine on prodynorphin mRNA were differentially affected across brain regions. Prodynorphin gene expression was still increased by U-69593 treatment in the hypothalamus and decreased in the caudate putamen. Cocaine treatment still produced a decrease in this parameter in the hypothalamus and an increase in the caudate putamen. In contrast, in the hippocampus, the decrease in prodynorphin mRNA produced by U-69593 was no longer evident after PCA and cocaine, which previously had no effect, now increased it in the serotonin-depleted group. These findings suggest that serotonin is necessary to maintain normal levels of dynorphin mRNA in all of the investigated brain areas and that the regulation of prodynorphin mRNA expression by chronic treatment with a kappa-opioid receptor agonist or cocaine requires serotonin in the hippocampus, but not in the hypothalamus or caudate putamen.

Subadditive withdrawal from cocaine/κ-opioid agonist combinations in Planaria

We have previously developed and extensively characterized a convenient and sensitive metric for the quantification of withdrawal responses using Planaria. Planaria are particularly valuable for these studies because of their permeable exteriors and their relevant neurotransmitter systems (e.g., dopaminergic, opioid, and serotonergic). In the present study, we used this metric and mathematically rigorous joint-action analysis to investigate poly-drug withdrawal from fixed-ratio cocaine/kappa-opioid agonist combinations. The D50 (concentration producing half-maximal effect) for cocaine and U-50,488H was 10.3 and 1.02 microg, respectively. The D50 for 19:1 or 1:19 combinations did not differ significantly (p>0.05) from expected additive values (11.6+/-3.0 vs. 9.9+/-1.4 and 1.1+/-0.2 vs. 1.5+/-0.1, respectively), but the 3:1, 1:1, and 1:3 ratios did (34.5+/-6.9 vs. 7.7+/-1.1; 55.1+/-10.0 vs. 5.7+/-0.7; and 40.8+/-8.9 vs. 3.3+/-0.4, respectively), indicating subadditive interaction at these ratios. The finding of subadditivity in this model suggests that abstinence-induced withdrawal from the combination is less intense than that predicted from the individual drug potencies. The concept that certain combinations of drugs leads to attenuated withdrawal might generalize to humans.

Repeated administration of the selective kappa-opioid receptor agonist U-69593 increases stimulated dopamine extracellular levels in the rat nucleus accumbens

Reinforcing properties of drugs of abuse are reduced by the coadministration of kappa opioid receptor (KOR) agonists. This effect is related to the inhibition of dopamine (DA) release in the nucleus accumbens (NAc) produced by the acute administration of KOR agonists. The present study was undertaken to investigate the in vivo effect of the repeated administration of KOR agonist on extracellular DA levels in the NAc. Rats were injected once daily with the selective KOR agonist U-69593 (0.16-0.32 mg/kg) or vehicle for 4 days. Microdialysis studies assessing extracellular concentration of DA in the NAc under basal and K(+)-stimulatory conditions were conducted 1 day later. The microdialysis studies revealed that preexposure to U-69593 had no effect on basal extracellular DA levels but significantly augmented the amount of extracellular DA induced by high K(+) compared with vehicle pretreated rats. The D2 receptor agonist quinpirole perfused through the dialysis probe in the NAc, although it produced a significant decrease on basal and K(+)-stimulated DA levels in control rats, it did not decrease significantly either basal or K(+)-stimulated DA levels in U-69593 preexposed rats. Preexposure to U-69593 did not alter the expression of tyrosine hydroxylase or dopamine transporter in the ventral tegmental area. These results show that repeated administration of U-696593 increases the amount of extracellular DA induced by high K in the NAc, an effect that may be related to decreased D2 autoreceptor function. It is suggested that repeated activation of KOR changes the response status of dopaminergic neurons in the NAc.

Kappa-opioid agonist U69593 potentiates locomotor sensitization to the D2/D3 agonist quinpirole: pre- and postsynaptic mechanisms

To assess whether the development and expression of behavioral sensitization to the dopamine D2/D3 agonist quinpirole (QNP) is influenced by coadministration of the kappa opioid receptor agonist U69593, rats received every 3-4 days for a total of 10 treatments an injection of U69593 (0.3 mg/kg) together with an injection of either a postsynaptic (0.5 mg/kg) or a presynaptic dose of QNP (0.05 mg/kg); locomotor activity was measured after each treatment. Control rats were injected as appropriate with QNP, U69593, and vehicle/saline. Following chronic treatment, dose-response profiles to QNP were obtained to assess the expression of sensitization; the effect of U69593 on locomotor activity in animals already sensitized to QNP was also assessed. Results showed that cotreatment of U69593 with a postsynaptic dose of QNP doubled the speed and magnitude of sensitization to QNP, while U69593 cotreatment with a presynaptic dose of QNP switched the effects of QNP from locomotor depression to locomotor sensitization. However, U69593 cotreatment with a presynaptic dose of QNP changed a different set of measures of sensitization than did cotreatment with a postsynaptic dose of the dopamine agonist. Together, findings suggest that sensitization to QNP is not a unitary phenomenon but has components that are relatively independent, mediated by distinct pre- and postsynaptic mechanisms and modulated by kappa receptor activity.

Dopamine D1 receptors have subcellular distributions conducive to interactions with prodynorphin in the rat nucleus accumbens shell

Activation of dopamine (DA) D1 receptors (D1Rs) in the nucleus accumbens (Acb) markedly affects the levels of prodynorphin, the precursor of aversion-associated dynorphin peptides. The location of prodynorphin, specifically as related to the dopaminergic inputs and D1Rs in the Acb, is fundamental for establishing the physiologically relevant sites. To determine these sites, we examined the electron microscopic dual-immunolabeling of prodynorphin and D1R or tyrosine hydroxylase (TH), a marker of catecholamine terminals in the rat Acb shell. This subregion is targeted by mesolimbic dopaminergic inputs affecting reward-aversion responses and locomotor activity. Prodynorphin was prominently localized to large (100-200 nm) granular aggregates in somatodendritic and axonal profiles, some of which expressed dynorphin A/B. In somata and dendrites, prodynorphin was often found in punctate clusters in the cytoplasm. Of the total prodynorphin-labeled dendrites, approximately 63% expressed D1Rs, which were largely located on the plasma membranes. In comparison with dendrites, many more axon terminals contained prodynorphin, although only 15% of these terminals contained D1R-labeling. Prodynorphin terminals formed symmetric synapses with D1R-labeled or unlabeled dendrites, and also apposed TH-containing axon terminals. Our results provide ultrastructural evidence that in the Acb shell, the prodynorphin is available for cleavage to physiologically active peptides in both dendrites and terminals of neurons that express D1Rs. They also indicate that dynorphin peptides have distributions that would enable their participation in modulation of DA release or D1R-mediated postsynaptic responses in Acb shell neurons.

Kappa opioids as potential treatments for stimulant dependence

Stimulant abuse is a major problem in the United States and the development of pharmacological treatments for stimulant abuse remains an important therapeutic goal. Classically, the "dopamine hypothesis" has been used to explain the development of addiction and dependence of stimulants. This hypothesis involves the direct increase of dopamine as the major factor in mediating the abuse effects. Therefore, most treatments have focused on directly influencing the dopamine system. Another approach, which has been explored for potential treatments of stimulant abuse, is the use of kappa opioid agonists. The kappa receptor is known to be involved, via indirect effects, in synaptic dopamine levels. This review covers several classes of kappa opioid ligands that have been explored for this purpose.

Cocaine-induced sex differences in D1 receptor activation and binding levels after acute cocaine administration

Although it is established that female rats have a more robust behavioral response to acute cocaine administration than male rats, the neurobiological mechanisms underlying these differences remain unclear. The purpose of the present study was to determine whether dopamine (DA) receptor activation influences sex differences in cocaine-induced behaviors. A second study was performed to determine sex differences in D1/D2 receptor levels prior to and post-cocaine administration. Male and female Fischer rats were pre-treated with the D1 antagonist SCH-23390 (0.05, 0.1, and 0.25 mg/kg, i.p.), the D2 antagonist eticlopride (0.03, 0.1 mg/kg, i.p.), or vehicle (saline) 15 min before acute cocaine (20 mg/kg, i.p.) or saline administration. Cocaine-induced ambulatory and rearing activity was greater in female than male rats. Pre-treatment with SCH-23390 affected cocaine-induced ambulatory, rearing, and stereotypic activity in a sex-dependent manner; cocaine-induced ambulatory and stereotypic behavior in female rats was reduced by the lowest dose of SCH-23390. Eticlopride did not alter behavioral responses to cocaine in male or female rats. These results suggest that in both male and female rats, activation of the D1, but not the D2, receptor modulates cocaine's motor effects. There were no sex differences in baseline levels of D1, D2, and DA transporter binding in the caudate putamen (CPu) and the nucleus accumbens (NAc). Cocaine administration reduced D1 binding levels in the CPu only in male rats. Our findings suggest that the regulation of striatal D1 binding levels after acute cocaine administration is a sexually dimorphic process. We also hypothesize that the greater sensitivity to D1 receptor blockade in female rats, as compared to male rats, may contribute to their overall increased hyperactivity in response to acute cocaine. Taken together, the D1 receptor may be an important substrate in the regulation of sex differences to cocaine-induced locomotor activity.

Neurochemical alterations produced by daily nicotine exposure in periadolescent vs. adult male rats

Chronic treatment with nicotine differentially alters behavior in adolescent rats compared to adult rats. It is not known, however, whether the effects of nicotine on the neurochemical pathways with which it interacts differ in adolescents vs. adults. In the current study, the effects of a 7-day treatment with nicotine on nicotinic, dopaminergic, and serotonergic neurochemistry were examined in the caudate putamen and nucleus accumbens in periadolescent vs. adult male rats. Nicotine treatment increased dopamine transporter densities and decreased serotonin transporter densities in periadolescent rats. There was no change in nicotinic acetylcholine receptor densities or dopamine D1 or D2 receptor densities in nicotine-pretreated periadolescent rats. In adult rats pretreated with nicotine, there was an increase in nicotinic acetylcholine densities, but no change in dopamine transporter, dopamine D1 or D2 receptor, or serotonin transporter densities. Overall, these findings show that periadolescent rats have neurochemical adaptations to nicotine different from adult rats. These alterations may explain, at least in part, the differential behavioral effects of chronic nicotine in adult and adolescent male rats.

Kappa-opioid receptors are differentially labeled by arylacetamides and benzomorphans

Using Chinese Hamster Ovary cell membranes that stably expressed the human kappa-opioid receptor, we investigated the hypothesis that kappa(1)- and kappa(2)-opioid receptors, historically defined by their pharmacological selectivity for either arylacetamides or benzomorphans are, in fact, different affinity states or binding sites on the same kappa-opioid receptors. Receptor binding studies showed that GTP gamma S potently inhibited [3H](5 alpha,7 alpha,8 beta)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro [4.5]dec-8-yl)-benzeneacetamide (U69,593) binding, compared to virtually no inhibition of [3H]bremazocine binding. Saturation binding experiments showed a three-fold decrease in [3H]U69,593 affinity in the presence of GTP gamma S, but GTP gamma S had no effect on [3H]bremazocine affinity. The kappa-opioid receptor antagonist nor-binaltorphimine had a four-fold higher affinity for [3H]U69,593-labeled receptors than for [3H]bremazocine-labeled receptors. Functional selectivity studies, measuring the stimulation of [35S]GTP gamma S agonist-induced binding, showed a significantly higher U69,593-induced G protein-receptor activation in comparison to the stimulation observed with bremazocine. These results suggest that pharmacologically defined 1 kappa-opioid receptor subtypes may be different affinity states of the same receptor.

Cyclazocine: comparison to hydromorphone and interaction with cocaine

Kappa-opioid agonists produce neurobiological and behavioral effects opposite to those of cocaine and may be useful for the treatment of cocaine dependence. To evaluate the kappa- and mu-agonist effects of cyclazocine and to test whether cyclazocine pretreatment would attenuate the effects of cocaine, healthy, male and female, experienced opiate and cocaine users (n = 13) were enrolled in a two-phase study. In Phase 1, placebo, cyclazocine (0.2, 0.4 and 0.8 mg) and the mu-agonist hydromorphone (5 and 15 mg) were administered orally in six 4.5-hour sessions separated by at least 72 h. In Phase 2, cocaine (100 mg intranasal) was given 2 h after oral pretreatment with cyclazocine (0, 0.1, 0.2, 0.4, 0.8 and 0 mg, in that order) in each of six sessions conducted daily Monday to Friday and the following Monday. Physiological, subjective and behavioral measures were collected in each session. Nine participants completed Phase 1; eight completed Phase 2. Hydromorphone (15 mg) produced prototypic mu-agonist effects. Cyclazocine exhibited only modest kappa-like effects. Cyclazocine also had only modest, non-dose-related effects on response to cocaine. However, cocaine effects were consistently lower on the last administration (cyclazocine 0 mg pretreatment) following 4 days of cyclazocine pretreatment, compared to the first administration (0 mg pretreatment). This finding is unlikely to be fully attributable to cocaine tolerance and is not accounted for by pharmacokinetic changes; plasma concentrations of cocaine were not altered by cyclazocine. This study is suggestive but not strongly supportive for the use of kappa-opiate drugs to diminish acute effects of cocaine administration or for the use of these kappa agonists in drug abuse treatment applications.

κ-Opioid Receptor Ligands Inhibit Cocaine-Induced HIV-1 Expression in Microglial Cells

Cocaine abuse has been implicated as a cofactor in human immunodeficiency virus (HIV)-1-associated dementia (HAD). In this study, we tested the hypothesis that exposure of microglial cells, the resident macrophages of the brain, to cocaine would potentiate HIV-1 expression. Because kappa-opioid receptor (KOR) agonists have been shown to suppress neurochemical and neurobehavioral responses to cocaine and to inhibit HIV-1 expression in microglial cell cultures, we also postulated that KOR ligands would inhibit cocaine-induced potentiation of HIV-1 expression. Human microglial cells were infected with HIV-1(SF162), an R5 isolate, and viral expression was quantified by measurement of p24 antigen in culture supernatants. Treatment of microglia with the KOR agonists trans-(+/-)-3,4-dichlor-N-methyl-N-(2[1-pyrrolidnyl])benzeneacetamide methanesulfonate and 8-carboxamidocyclazocine inhibited viral expression (maximal suppression of 42 and 48%, respectively). Consistent with the hypotheses, treatment of microglia with cocaine promoted HIV-1 expression (maximal enhancement of 54%), and pretreatment of microglia with these KOR agonists as well as with the KOR-selective antagonist nor-binaltorphimine abrogated cocaine-induced potentiation of viral expression. Results of flow cytometry studies suggested that the mechanism whereby KOR ligands inhibit cocaine's stimulatory effect on viral expression involves the suppression of cocaine-induced activation of extracellular signal-regulated kinase1/2, thereby blunting cocaine-enhanced up-regulation of the HIV-1 entry chemokine coreceptor CCR5. The findings of this study suggest that in addition to its neurotoxic effects, cocaine could foster development of HAD by potentiating viral expression in the brain and that this phenomenon is inhibited by KOR ligands.

Effects of single and dual administration of cocaine and ethanol on opioid and ORL1 receptor expression in rat CNS: an autoradiographic study

The co-abuse of cocaine and ethanol is common among human addicts and has been reported to produce a stronger increase of euphoria as compared to either drug given alone. Both cocaine and ethanol increase the extracellular dopamine concentration in the nucleus accumbens, a terminal region in the mesolimbic dopamine pathway. In addition, both cocaine and ethanol affect the endogenous opioid system, which in turn alters the activity of the mesolimbic dopamine pathway. We have carried out quantitative autoradiography mapping of the opioid receptors as well as the opioid receptor-like 1 receptor in the brains of rats treated with both single and dual cocaine and ethanol. Rats received acute cocaine, ethanol or both drugs in combination. Ethanol alone or in combination with cocaine modulated the receptor densities in rat central nervous system. The kappa receptor densities were generally decreased, while both the mu and the opioid receptor-like 1 receptors were up-regulated. The mu opioid receptor levels were mainly increased in non-cortical regions, whereas the opioid receptor-like 1 receptors were increased in cortical structures. No changes in delta opioid receptors were observed. Cocaine alone did not influence the receptor levels in any of the treatment groups.

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).

Cocaine differentially alters behavior and neurochemistry in periadolescent versus adult rats

This study examined whether there are differences in the behavioral and neurochemical effects of cocaine in periadolescent rats compared to adult rats. Periadolescent (postnatal days 28-35) and adult rats were injected with cocaine or vehicle for 7 days. Ten days later (day 17), rats either were challenged with cocaine, or dopamine transporter and receptor and serotonin transporter binding were examined. Adult rats became sensitized to the locomotor-activating effects of cocaine and there were increases in dopamine transporter density in the caudate putamen compared to vehicle-treated adult rats. In addition, serotonin transporter densities were increased in the ventromedial caudate putamen, nucleus accumbens shell, and the olfactory tubercle in cocaine-treated adult rats compared to vehicle-treated adult rats. In contrast, periadolescent rats did not show sensitization to cocaine and there was no effect of cocaine on either dopamine or serotonin transporter densities. These findings suggest that there are different neurochemical and behavioral adaptations to repeated cocaine administration in periadolescent versus adult rats.

Chronic cocaine increases kappa-opioid receptor density: lack of effect by selective dopamine uptake inhibitors

Continuous infusion of cocaine or the selective dopamine uptake inhibitors GBR 12909 or RTI-117 increases locomotor stimulation, to which partial tolerance occurs. In addition, all three drugs produce significant decreases in tyrosine hydroxylase immunoreactivity in caudate putamen and nucleus accumbens core, suggesting a decreased dopaminergic tone. An interaction between cocaine and opioids has long been documented. Chronic cocaine significantly increases mu and kappa-opioid receptors and treatment with a kappa-opioid agonist markedly reduces the behavioral effects of cocaine. In addition, chronic cocaine, but not GBR 12909, increases prodynorphin gene expression in caudate putamen. To further understand the interaction between cocaine and the kappa-opioid system, the effects of a chronic continuous infusion for 14 days of cocaine or one of the selective dopamine uptake inhibitors GBR 12909 or RTI-117 via osmotic minipump were examined on kappa-opioid receptors using the selective kappa-opioid ligand [3H] U-69593. [3H] U-69593 binding density was significantly increased in caudate putamen, nucleus accumbens shell, claustrum, and endopiriform nucleus after cocaine, while neither GBR 12909 nor RTI-117 had any effect. The increased kappa-opioid receptor densities observed following cocaine are likely not related to dopamine uptake inhibition, since they were not produced by selective dopamine uptake inhibitors. These findings suggest that regulation of kappa-opioid receptors by cocaine may be via inhibition of serotonin or norepinephrine uptake, by a combination of effects on two or three monoamine transporters, or by a mechanism unrelated to transporter inhibition.

Regulation of dynorphin gene expression by kappa-opioid agonist treatment

The effects of K-opioid agonist treatment on prodynorphin mRNA expression in the rat brain were studied. Rats were treated with the selective kappa-opioid agonist U-69593 or vehicle for 5 days and prodynorphin mRNA was measured on day 8 (3 days after the last injection) or 22 (17 days after the last injection). On day 8 prodynorphin mRNA was increased in the hypothalamus and decreased in the striatum, frontal cortex, and hippocampus of rats treated with U-69593. On day 22, prodynorphin mRNA was increased in the hypothalamus, frontal cortex and striatum of U-69593 treated rats. These findings suggests that kappa-opioid receptor agonist treatment has long-term, continually changing effects on prodynorphin mRNA expression.