Differential reinforcing effects of cocaine and GBR-12909: biochemical evidence for divergent neuroadaptive changes in the mesolimbic dopaminergic system

Are We Getting High Cause the Thrill is Gone?

In the USA alone, opioid use disorder (OUD) affects approximately 27 million people. While the number of prescriptions may be declining due to increased CDC guidance and prescriber education, fatalities due to fentanyl-laced street heroin are still rising. Our laboratory has extended the overall concept of both substance and non-substance addictive behaviors, calling it "Reward Deficiency Syndrome (RDS)." Who are its victims, and how do we get this unwanted disorder? Is RDS caused by genes (Nature), environment (Neuro-epigenetics, Nurture), or both? Recent research identifies resting-state functional connectivity in the brain reward circuitry as a crucial factor. Analogously, it is of importance to acknowledge that the cumulative discharge of dopamine, governed by the nucleus accumbens (NAc) and modulated by an array of additional neurotransmitters, constitutes a cornerstone of an individual's overall well-being. Neuroimaging reveals that high-risk individuals exhibit a blunted response to stimuli, potentially due to DNA polymorphisms or epigenetic alterations. This discovery has given rise to the idea of a diminished 'thrill,' though we must consider whether this 'thrill' may have been absent from birth due to high-risk genetic predispositions for addiction. This article reviews this issue and suggests the general concept of the importance of "induction of dopamine homeostasis." We suggest coupling a validated genetic assessment (e.g., GARS) with pro-dopamine regulation (KB220) as one possible frontline modality in place of prescribing potent addictive opioids for OUD except for short time harm reduction. Could gene editing offer a 'cure' for this undesirable genetic modification at birth, influenced by the environment and carried over generations, leading to impaired dopamine and other neurotransmitter imbalances, as seen in RDS? Through dedicated global scientific exploration, we hope for a future where individuals are liberated from pain and disease, achieving an optimal state of well-being akin to the proverbial 'Garden of Eden'.

Behavioral profile of adult zebrafish acutely exposed to a selective dopamine uptake inhibitor, GBR 12909

BACKGROUND

The dopamine transporter (DAT) is the main regulator of dopamine concentration in the extrasynaptic space. The pharmacological inhibition of the DAT results in a wide spectrum of behavioral manifestations, which have been identified so far in a limited number of species, mostly in rodents.

AIM

Here, we used another well-recognized model organism, the zebrafish (Danio rerio), to explore the behavioral effects of GBR 12909, a highly-affine selective DAT blocker.

METHODS

We evaluated zebrafish locomotion, novelty-related exploration, spatial cognition, and social phenotypes in the novel tank, habituation and shoaling tests, following acute 20-min water immersion in GBR 12909.

RESULTS

Our findings show hypolocomotion, anxiety-like state, and impaired spatial cognition in fish acutely treated with GBR 12909. This behavioral profile generally parallels that of the DAT knockout rodents and zebrafish, and it overlaps with behavioral effects of other DAT-inhibiting drugs of abuse, such as cocaine and D-amphetamine.

CONCLUSION

Collectively, our data support the utility of zebrafish in translational studies on DAT targeting neuropharmacology and strongly implicate DAT aberration as an important mechanisms involved in neurological and psychiatric diseases.

A New Method for the Visualization of Living Dopaminergic Neurons and Prospects for Using It to Develop Targeted Drug Delivery to These Cells

This is the first study aiming to develop a method for the long-term visualization of living nigrostriatal dopaminergic neurons using 1-(2-(bis(4-fluorophenyl)methoxy)ethyl)-4-(3-phenylpropyl)piperazine-BODIPY (GBR-BP), the original fluorescent substance, which is a derivative of GBR-12909, a dopamine uptake inhibitor. This method is based on the authors’ hypothesis about the possibility of specifically internalizing into dopaminergic neurons substances with a high affinity for the dopamine transporter (DAT). Using a culture of mouse embryonic mesencephalic and LUHMES cells (human embryonic mesencephalic cells), as well as slices of the substantia nigra of adult mice, we have obtained evidence that GBR-BP is internalized specifically into dopaminergic neurons in association with DAT via a clathrin-dependent mechanism. Moreover, GBR-BP has been proven to be nontoxic. As we have shown in a primary culture of mouse metencephalon, GBR-BP is also specifically internalized into some noradrenergic and serotonergic neurons, but is not delivered to nonmonoaminergic neurons. Our data hold great promise for visualization of dopaminergic neurons in a mixed cell population to study their functioning, and can also be considered a new approach for the development of targeted drug delivery to dopaminergic neurons in pathology, including Parkinson’s disease.

Analysis of membrane transport mechanisms of endogenous substrates using chromatographic techniques

Membrane transporters are expressed in various bodily tissues and play essential roles in the homeostasis of endogenous substances and the absortion, distribution and/or excretion of xenobiotics. For transporter assays, radioisotope-labeled compounds have been mainly used. However, commercially available radioisotope-labeled compounds are limited in number and relatively expensive. Chromatographic analyses such as high-performance liquid chromatography with ultraviolet absorptiometry and liquid chromatography with tandem mass spectrometry have also been applied for transport assays. To elucidate the transport properties of endogenous substrates, although there is no difficulty in performing assays using radioisotope-labeled probes, the endogenous background and the metabolism of the compound after its translocation across cell membranes must be considered when the intact compound is assayed. In this review, the current state of knowledge about the transport of endogenous substrates via membrane transporters as determined by chromatographic techniques is summarized. Chromatographic techniques have contributed to our understanding of the transport of endogenous substances including amino acids, catecholamines, bile acids, prostanoids and uremic toxins via membrane transporters.

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

The dopamine transporter is responsible for recycling dopamine after release. Inhibitors of the dopamine transporter, such as cocaine, will stop the reuptake of dopamine and allow it to stay extracellularly, causing prominent changes at the molecular, cellular, and behavioral levels. There is much left to be known about the mechanism and site(s) of binding, as well as the effect that cocaine administration does to dopamine transporter-cocaine binding sites and gene expression which also plays a strong role in cocaine abusers and their behavioral characteristics. Thus, if more light is shed on the dopamine transporter-cocaine interaction, treatments for addiction and even other diseases of the dopaminergic system may not be too far ahead. As today's ongoing research expands on the shoulders of classic research done in the 1990s and 2000s, the foundation of core research done in that time period will be reviewed, which forms the basis of today's work and tomorrow's therapies.

Changes in dopamine transporter binding in nucleus accumbens following chronic self-administration cocaine: heroin combinations

Concurrent use of cocaine and heroin (speedball) has been shown to exert synergistic effects on dopamine neurotransmission in the nucleus accumbens (NAc), as observed by significant increases in extracellular dopamine levels and compensatory elevations in the maximal reuptake rate of dopamine. The present studies were undertaken to determine whether chronic self-administration of cocaine, heroin or a combination of cocaine:heroin led to compensatory changes in the abundance and/or affinity of high- and low-affinity DAT binding sites. Saturation binding of the cocaine analog [(125) I] 3β-(4-iodophenyl)tropan-2β-carboxylic acid methyl ester ([(125) I]RTI-55) in rat NAc membranes resulted in binding curves that were best fit to two-site binding models, allowing calculation of dissociation constant (Kd ) and binding density (Bmax ) values corresponding to high- and low-affinity DAT binding sites. Scatchard analysis of the saturation binding curves clearly demonstrate the presence of high- and low- affinity binding sites in the NAc, with low-affinity sites comprising 85 to 94% of the binding sites. DAT binding analyses revealed that self-administration of cocaine and a cocaine:heroin combination increased the affinity of the low-affinity site for the cocaine congener RTI-55 compared to saline. These results indicate that the alterations observed following chronic speedball self-administration are likely due to the cocaine component alone; thus further studies are necessary to elaborate upon the synergistic effect of cocaine:heroin combinations on the dopamine system in the NAc.

Predicting abuse potential of stimulants and other dopaminergic drugs: overview and recommendations

Examination of a drug's abuse potential at multiple levels of analysis (molecular/cellular action, whole-organism behavior, epidemiological data) is an essential component to regulating controlled substances under the Controlled Substances Act (CSA). We reviewed studies that examined several central nervous system (CNS) stimulants, focusing on those with primarily dopaminergic actions, in drug self-administration, drug discrimination, and physical dependence. For drug self-administration and drug discrimination, we distinguished between experiments conducted with rats and nonhuman primates (NHP) to highlight the common and unique attributes of each model in the assessment of abuse potential. Our review of drug self-administration studies suggests that this procedure is important in predicting abuse potential of dopaminergic compounds, but there were many false positives. We recommended that tests to determine how reinforcing a drug is relative to a known drug of abuse may be more predictive of abuse potential than tests that yield a binary, yes-or-no classification. Several false positives also occurred with drug discrimination. With this procedure, we recommended that future research follow a standard decision-tree approach that may require examining the drug being tested for abuse potential as the training stimulus. This approach would also allow several known drugs of abuse to be tested for substitution, and this may reduce false positives. Finally, we reviewed evidence of physical dependence with stimulants and discussed the feasibility of modeling these phenomena in nonhuman animals in a rational and practical fashion. This article is part of the Special Issue entitled 'CNS Stimulants'.

Monoamine transporter inhibitors and substrates as treatments for stimulant abuse

The acute and chronic effects of abused psychostimulants on monoamine transporters and associated neurobiology have encouraged development of candidate medications that target these transporters. Monoamine transporters, in general, and dopamine transporters, in particular, are critical molecular targets that mediate abuse-related effects of psychostimulants such as cocaine and amphetamine. Moreover, chronic administration of psychostimulants can cause enduring changes in neurobiology reflected in dysregulation of monoamine neurochemistry and behavior. The current review will evaluate evidence for the efficacy of monoamine transporter inhibitors and substrates to reduce abuse-related effects of stimulants in preclinical assays of stimulant self-administration, drug discrimination, and reinstatement. In considering deployment of monoamine transport inhibitors and substrates as agonist-type medications to treat stimulant abuse, the safety and abuse liability of the medications are an obvious concern, and this will also be addressed. Future directions in drug discovery should identify novel medications that retain efficacy to decrease stimulant use but possess lower abuse liability and evaluate the degree to which efficacious medications can attenuate or reverse neurobiological effects of chronic stimulant use.

Differential regulation of accumbal dopamine transmission in rats following cocaine, heroin and speedball self-administration

Cocaine/heroin combinations (speedball) exert synergistic neurochemical and behavioral effects that are thought to contribute to the increased abuse potential and subjective effects reported by polydrug users. In vivo fast-scan cyclic voltammetry was used to examine the effects of chronic intravenous self-administration (25 consecutive sessions) of cocaine (250 μg/inf), heroin (4.95 μg/inf) and speedball (250/4.95 μg/inf cocaine/heroin) on changes in electrically evoked dopamine (DA) efflux, maximal rate of DA uptake (V(max)) and the apparent affinity (K(m)) of the DA transporter in the nucleus accumbens. The increase in electrically evoked DA was comparable following cocaine and speedball injection; however, heroin did not increase evoked DA. DA transporter K(m) values were similarly elevated following cocaine and speedball, but unaffected by heroin. However, speedball self-administration significantly increased baseline V(max), while heroin and cocaine did not change baseline V(max), compared with the baseline V(max) values of drug-naïve animals. Overall, elevated DA clearance is a likely consequence of synergistic elevations of nucleus accumbens extracellular DA concentrations by chronic speedball self-administration, as reported previously in microdialysis studies. The present results indicate neuroadaptive processes that are unique to cocaine/heroin combinations and cannot be readily explained by simple additivity of changes observed with cocaine and heroin alone.

Simultaneous measurement of extracellular dopamine and dopamine transporter occupancy by cocaine analogs in squirrel monkeys

Several classes of drugs bind to the dopamine transporter (DAT) with high affinity, but some are weaker positive reinforcers than cocaine, suggesting that affinity for and occupancy of the DAT is not the only determinant of a drug's reinforcing effectiveness. Other factors such as the rate of onset have been positively and strongly correlated with the reinforcing effects of DAT inhibitors in nonhuman primates. In the current studies, we examined the effects of acute systemic administration of cocaine and three cocaine analogs (RTI-150, RTI-177, and RTI-366) on binding to DAT in squirrel monkey brain using positron emission tomography (PET) neuroimaging. During the PET scan, we also measured drug effects on dopamine (DA) levels in the caudate using in vivo microdialysis. In general, our results suggest a lack of concordance between drug occupancy at DAT and changes in DA levels. These studies also indicate that acute cocaine administration decreases the availability of plasma membrane DAT for binding, even after cocaine is no longer blocking DA uptake as evidence by a return to basal DA levels.

The high-affinity nAChR partial agonists varenicline and sazetidine-A exhibit reinforcing properties in rats

Varenicline (Chantix®, Champix®) is a nicotinic acetylcholine receptor (nAChR) partial agonist clinically approved for smoking cessation, yet its potential abuse liability properties have not been fully characterized. The nAChR ligand sazetidine-A has been reported as a selective full or partial agonist at α4β2* nAChR subtypes in in vitro studies. In the present studies, varenicline, sazetidine-A and nicotine exhibited inverted U-shaped dose-response functions under fixed-ratio (peak responding at 30, 60 and 10-30 μg/kg/inf, respectively) or progressive-ratio (peak responding at 30-60, 30-100 and 30 μg/kg/inf, respectively) schedules in rats trained to self-administer nicotine. Varenicline (ED(50) 0.2 mg/kg) and sazetidine-A (ED(50) 0.44 mg/kg) fully substituted for nicotine (ED(50) 0.09 mg/kg) in rats trained to discriminate nicotine (0.4 mg/kg, i.p.) from saline. The reinforcing and discriminative stimulus (DS) properties of sazetidine-A, varenicline and nicotine were attenuated by acute pretreatment with the non-selective neuronal non-competitive nAChR antagonist mecamylamine or the α4* nAChR-selective antagonist dihydro-β-erythroidine, but not by the α7 nAChR subtype antagonist methyllycaconitine. Drug-naïve rats acquired stable self-administration of varenicline (30 μg/kg/inf), and sazetidine-A (60 μg/kg/inf), at doses that supported peak responding under a fixed-ratio 3 schedule in nicotine-trained rats. Nonetheless, self-administration and re-acquisition of varenicline and sazetidine-A were less robust than nicotine. Thus, partial activation of α4β2* nAChRs by varenicline or sazetidine-A is sufficient to mimic the DS and reinforcing properties of nicotine in nicotine-experienced rats, although the reinforcing properties of partial agonists are diminished in nicotine-naïve rats. Future studies should assess nicotine withdrawal measures in animals chronically exposed to varenicline or sazetidine-A.

Performance-enhancing and thermoregulatory effects of intracerebroventricular dopamine in running rats

To assess the role of central dopamine on metabolic rate, heat balance and running performance, 2.0 microL of 5 x 10(-3)M dopamine solution (DA) or 0.15M NaCl (SAL) was intracerebroventricularly injected in Wistar rats 1 min before running on a motor-driven treadmill, according to a graded exercise protocol, until fatigue. Oxygen consumption (VO(2)) and body temperature (T(b)) were recorded at rest, during exercise, and after 30 min of recovery. DA induced a marked increase in workload (approximately 45%, p<0.05). At fatigue point, DA-injected rats attained approximately 29% higher maximum oxygen consumption (VO(2max)) and approximately 0.75 degrees C higher T(b) than SAL-injected rats. Despite the higher VO(2max) and T(b) attained during exercise, DA-treated rats reached VO(2) basal values within the same recovery period and dissipated heat approximately 33% faster than SAL-treated rats (p<0.05). The mechanical efficiency loss rate was approximately 40% lower in DA than in SAL-treated rats (p<0.05), however, the heat storage was approximately 35% higher in the DA group (p<0.05). Our results demonstrate that increased DA availability in the brain has a performance-enhancing effect, which is mediated by improvements in the tolerance to heat storage and increases in the metabolic rate induced by graded exercise. These data provide further evidence that central activation of dopaminergic pathways plays an important role in exercise performance.

Dopamine uptake changes associated with cocaine self-administration

The present study was designed to reveal the relationship between cocaine-induced dopamine uptake changes and patterns of cocaine self-administration observed under a fixed-ratio schedule. Cocaine was intravenously infused into anesthetized rats, according to inter-infusion intervals obtained from self-administering animals, and dopamine uptake changes (apparent K(m)) were assessed in the nucleus accumbens using voltammetry. The data demonstrate that cocaine-induced dopamine transporter (DAT) inhibition accounts for the accumbal dopamine fluctuations, which are associated with the cyclic regularity of cocaine intake observed during self-administration. Specifically, the inter-infusion intervals that are maintained during cocaine self-administration correlate with the maintenance of a rapidly changing level of dopamine uptake inhibition, which appears to be tightly regulated. Furthermore, this maintained level of dopamine uptake inhibition was found to shift upward using intervals from animals that had shown an escalation in the rate of cocaine self-administration. Although no significant change in the apparent K(m) was revealed in animals that exhibited an escalation in the rate of cocaine intake, an increased dopamine uptake rate was found suggesting an upregulation of DAT number in response to a history of high cocaine intake. This is the first demonstration of the tight correlation that exists between the level of dopamine uptake inhibition and rates of cocaine self-administration. Moreover, a new mathematical model was created that quantitatively describes the changes in cocaine-induced dopamine uptake and correctly predicts the level of dopamine uptake inhibition. This model permits a computational interpretation of cocaine-induced dopamine uptake changes during cocaine self-administration.

Acute norepinephrine reuptake inhibition decreases performance in normal and high ambient temperature

Combined inhibition of dopamine (DA)/norepinephrine (NE) reuptake improves exercise performance and increases core temperature in the heat. A recent study demonstrated that this effect may primarily be related to increased DA activity. NE reuptake inhibition (NERI), however, has received little attention in humans, certainly in the heat, where central fatigue appears to be a main factor influencing performance. Therefore the present study examines the effect of NERI (reboxetine) on exercise capacity, thermoregulation, and hormonal response in normal and high temperature. Nine healthy well-trained male cyclists participated in this study. Subjects ingested either placebo (Pla; 2 x 8 mg) or reboxetine (Rebox; 2 x 8 mg). Subjects exercised in temperate (18 degrees C) or warm (30 degrees C) conditions and cycled for 60 min at 55% W(max) immediately followed by a time trial (TT; Pla18/Rebox18; Pla30/Rebox30) to measure exercise performance. Acute NERI decreased power output and consequently exercise performance in temperate (P = 0.018) and warm (P = 0.007) conditions. Resting heart rate was significantly elevated by NERI (18 degrees C: P = 0.02; 30 degrees C: P = 0.018). In Rebox18, heart rate was significantly higher than in the Pla18, while in the heat no effect of the drug treatment was reported during exercise. In Rebox30, all hormone concentrations increased during exercise, except for growth hormone (GH), which was significantly lower during exercise. In Rebox18, prolactin (PRL) concentrations were significantly elevated; GH was significantly higher at rest, but significantly lower during exercise. In conclusion, manipulation of the NE system decreases performance and modifies hormone concentrations, thereby indicating a central NE effect of the drug. These findings confirm results from previous studies that predominantly increased DA activity is important in improving performance.

Influence of brain catecholamines on the development of fatigue in exercising rats in the heat

The purpose of the present study was to identify the effects of an acute injection of a dual dopamine (DA)/noradrenaline (NA) reuptake inhibitor (bupropion) on exercise performance, thermoregulation and neurotransmitters in the preoptic area and anterior hypothalamus (PO/AH) of the rat during exercise in the heat. Body core temperature (T(core)), brain temperature (T(brain)) and tail skin temperature (T(tail)) were measured. A microdialysis probe was inserted in the PO/AH, and samples for measurement of extracellular DA, NA and serotonin (5-HT) levels were collected. Rats received either bupropion (17 mg kg(-1); hot-BUP) or saline (1 ml kg(-1); hot) 20 min before the start of exercise and ran at a speed of 26 m min(-1) until exhaustion in a warm environment (30 degrees C). Rats also ran until exhaustion in a cool environment (18 degrees C; cool). Running time to exhaustion was significantly influenced by the ambient temperature, and it was increased by bupropion in the heat (cool, 143.6 +/- 21 min; hot, 65.8 +/- 13 min; hot-BUP, 86.3 +/- 7.2 min). T(core) and T(brain) at exhaustion were significantly higher in the bupropion group compared to the cool and hot groups, respectively. T(tail) measured at exhaustion was not significantly different between the two hot conditions. Extracellular concentrations of DA and NA in the PO/AH increased during exercise, and was significantly higher in the bupropion than in cool and hot groups (P < 0.05). No differences were observed between groups for 5-HT levels. These results suggest that DA and NA in the PO/AH might be responsible for the increase in exercise performance and T(core) and T(brain) in the bupropion group in hyperthermia. Moreover, these results support previous findings in humans that acute bupropion ingestion increases T(core) during exercise in the heat, indicating the possibility of an important role for DA and NA in thermoregulation.

Chronic treatment with a dopamine uptake blocker changes dopamine and acetylcholine but not glutamate and GABA concentrations in prefrontal cortex, striatum and nucleus accumbens of the awake rat

The present study was aimed to investigate the effects of a chronic treatment with the dopamine uptake blocker nomifensine on the in vivo extracellular concentrations of dopamine, acetylcholine, glutamate and GABA in the prefrontal cortex, striatum and nucleus accumbens. Male Wistar rats received intraperitoneal (i.p.) daily injections of nomifensine (10 mg/kg) or saline for 22 days. Microdialysis experiments were performed on days 1, 8, 15 and 22 of treatment to evaluate the effects of the injection of nomifensine or saline. Motor activity of the animals was monitored during microdialysis experiments. Injections of nomifensine increased extracellular concentration of dopamine in striatum and nucleus accumbens, but not in prefrontal cortex. Acetylcholine concentrations in striatum but not in nucleus accumbens were increased by nomifensine on days 15 and 22 of treatment. In prefrontal cortex, nomifensine increased acetylcholine levels without differences among days. No changes were found on glutamate and GABA concentrations in the three areas studied. Injections of nomifensine also increased spontaneous motor activity and stereotyped behaviour without differences among days. These results show that systemic chronic treatment with a dopamine uptake blocker produces differential effects on extracellular concentrations of dopamine and acetylcholine, but not glutamate and GABA, in different areas of the brain.

Dopamine transport inhibitors based on GBR12909 and benztropine as potential medications to treat cocaine addiction

The discovery and development of medications to treat addiction and notably, cocaine addiction, have been frustrated by both the complexity of the disorder and the lack of target validation in human subjects. The dopamine transporter has historically been a primary target for cocaine abuse medication development, but addictive liability and other confounds of such inhibitors of dopamine uptake have limited clinical evaluation and validation. Herein we describe efforts to develop analogues of the dopamine uptake inhibitors GBR 12909 and benztropine that show promising profiles in animal models of cocaine abuse that contrast to that of cocaine. Their unique pharmacological profiles have provided important insights into the reinforcing actions of cocaine and we propose that clinical investigation of novel dopamine uptake inhibitors will facilitate the discovery of cocaine-abuse medications.

Monoamine transporters and psychostimulant addiction

Psychostimulants are a broadly defined class of drugs that stimulate the central and peripheral nervous systems as their primary pharmacological effect. The abuse liability of psychostimulants is well established and represents a significant public health concern. An extensive literature documents the critical importance of monoamines (dopamine, serotonin and norepinephrine) in the behavioral pharmacology and addictive properties of psychostimulants. In particular, the dopamine transporter plays a primary role in the reinforcing and behavioral-stimulant effects of psychostimulants in animals and humans. Moreover, both serotonin and norepinephrine systems can reliably modulate the neurochemical and behavioral effects of psychostimulants. However, there is a growing body of evidence that highlights complex interactions among additional neurotransmitter systems. Cortical glutamatergic systems provide important regulation of dopamine function, and inhibitory amino acid gamma-aminobutyric acid (GABA) systems can modulate basal dopamine and glutamate release. Repeated exposure to psychostimulants can lead to robust and enduring changes in neurobiological substrates, including monoamines, and corresponding changes in sensitivity to acute drug effects on neurochemistry and behavior. Significant advances in the understanding of neurobiological mechanisms underlying psychostimulant abuse and dependence have guided pharmacological treatment strategies to improve clinical outcome. In particular, functional agonist treatments may be used effectively to stabilize monoamine neurochemistry, influence behavior and lead to long-term abstinence. However, additional clinical studies are required in order to identify safe and efficacious pharmacotherapies.

Cocaine-induced oxidative stress precedes cell death in human neuronal progenitor cells

By 2003, an estimated 34 million Americans had used cocaine according to the National Survey on Drug Use & Health. About 5.9 million of those had used in the past 12 months. Chronic cocaine users often develop addiction, dependency and tolerance to the drug. The psychological and physical effects of cocaine are due to the disruption of the limbic system in the central nervous system (CNS). Increased oxidative stress reported in the frontal cortex and the striatum of rats exposed to cocaine suggests that oxidative damage plays a significant role in cocaine-induced disruption of the CNS. Although it is evident that cocaine induces oxidative stress in the CNS, little has been learned about whether such increased oxidative stress is also relevant to apoptosis in cocaine-exposed models. To gain insight into the role of cocaine-induced oxidative stress in apoptosis, we hypothesized that oxidative stress precedes cell death when cocaine is administrated. To test this hypothesis, we have monitored the oxidative stress and apoptotic effects of acute cocaine exposure in human neuronal progenitor cells (HNPC). We found that oxidative stress was significantly increased at 48h after a 30min cocaine exposure compared to control cells, and that this was followed by cell death at 72h. Using the same experimental paradigm we have previously shown that pro-inflammatory genes are up-regulated in cocaine-exposed HNPC at 24h. Therefore, we suggest that the increased oxidative stress (possibly mediated by inflammatory responses) precedes cell death in cocaine-exposed HNPC. This may have implications for the consequences of cocaine abuse in situations where antioxidant capacity is compromised, as in the aging brain.

Effects of cocaine on monoamine uptake as measured ex vivo

The increase in extracellular dopamine (DA) following cocaine administration plays a major role in cocaine abuse. In vitro, cocaine binds to DA transporters (DAT) and blocks DA uptake. Moreover, cocaine can increase extracellular DA concentration as measured by in vivo neurochemical methods. The present study examined the effects of cocaine and other drugs on DA, NE and 5-HT uptake using an ex vivo assay. Rats were injected i.v. with saline or drug and sacrificed at various time points after injections. Brains were dissected for regional monoamine uptake studies ex vivo. In most brain regions, cocaine given in vivo blocked monoamine uptake as expected. [3H]DA uptake in nucleus accumbens was inhibited with an ED50=22.3 micromol/kg. Cocaine fully inhibited [3H]NE uptake (ED50=4.58 micromol/kg) in the occipital cortex and partially inhibited [3H]5-HT uptake (33% at 30 micromol/kg) in the midbrain. However, under the same conditions [3H]DA uptake in the striatum was not inhibited after injections of cocaine up to 56 micromol/kg. Although the mechanism for this discrepancy is unclear, DA binding and uptake sites may be distinct and/or there may be regional differences in DA transporters.

Effects of prenatal cocaine, morphine, or both on postnatal opioid (μ) receptor development

We studied the effects of prenatal cocaine and morphine given separately and in combination on the (1) postnatal brain mu-opioid receptor development and (2) interaction of dopamine with mu receptors. Pregnant rats received single daily intraperitoneal (I.P.) injections of saline, cocaine (20 mg/kg), morphine (2 mg/kg), or the combination of both drugs from day 13 to day 20 of gestation. Postnatal days (P) 1, 7, 14, and 28, whole brains were analyzed for opioid receptor binding and mu mRNA. Prenatal cocaine administered by itself had no significant effect on the ontogeny of brain mu receptors on all the days studied when compared to controls. The morphine-treated group showed a significant increase in mu receptor binding on P1 and P7. Exposure to both cocaine and morphine showed a significant increase in mu receptor density on P1 and P7. In addition, there was also a significant increase in MOR mRNA in both the morphine alone and combination groups. Pretreatment with dopamine D2 receptor antagonist (sulpiride, 20 mg/kg) prior to drug administration showed decreased mu receptor binding on P1 and P7. These results suggest that prenatal exposure to morphine or a combination of cocaine and morphine significantly increases mu receptor density. By P14, mu-opioid receptor binding was no longer different than the control. This may suggest that the effect on receptor may be short-lived and that other key intracellular events may be activated to mediate the long-term effects. Also, the data show that dopaminergic mechanisms are (or opioid-dopamine interaction is) involved in the effects of morphine alone or morphine in combination with cocaine on mu receptor regulation.

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.

Differential effects of dopamine and psychoactive drugs on dopamine transporter phosphorylation and regulation

The dopamine transporter (DAT) is a phosphoprotein whose activity and phosphorylation state are acutely regulated by both protein kinase C (PKC) and substrate transport. DAT is a major site of action for psychostimulant and therapeutic drugs that either block transport or are transported substrates, but the effects of such drugs on DAT phosphorylation and regulation are not well understood. To examine these issues we subjected rDAT LLC-PK(1) cells to acute in vitro pretreatments with the endogenous, psychostimulant, and therapeutic compounds dopamine (DA), (-)-cocaine, 2 beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (beta-CFT), GBR 12909, mazindol, and methylphenidate (MPH), in the presence or absence of the PKC activator phorbol 12 myristate 13 acetate (PMA), followed by analysis of DAT metabolic phosphorylation and transport activity. Basal phosphorylation of DAT was not affected by any of the uptake blockers tested, and PMA-stimulated phosphorylation was not affected by cocaine, beta-CFT, mazindol or MPH, but was strongly suppressed by GBR 12909. Pretreatment of cells with cocaine or MPH had no effect on subsequent DA transport activity or the extent of PMA-induced transport down-regulation, whereas GBR 12909 inhibited PMA-induced DAT internalization. These findings indicate that these DAT phosphorylation and down-regulation properties are unaffected by some classes of uptake blocking drugs, but that differential regulatory effects may be exerted by GBR compounds. Pretreatment of cells with DA had no obvious effect on basal or PMA-stimulated DAT phosphorylation but led to cocaine-blockable transport down-regulation. DA-induced down-regulation was blocked by the PKC inhibitor bisindoylmaleimide I and was not additive with down-regulation induced by PMA, consistent with PKC serving as a common step and point of integration for these DA and PMA induced processes. The results of this study provide information on the potential for endogenous and psychoactive compounds to modulate or be modulated by DAT phosphorylation-mediated regulatory mechanisms that may contribute to drug behavioral or therapeutic properties.

Varenicline: an alpha4beta2 nicotinic receptor partial agonist for smoking cessation

Herein we describe a novel series of compounds from which varenicline (1, 6,7,8,9-tetrahydro-6,10-methano-6H-pyrazino[2,3-h][3]benzazepine) has been identified for smoking cessation. Neuronal nicotinic acetylcholine receptors (nAChRs) mediate the dependence-producing effects of nicotine. We have pursued alpha4beta2 nicotinic receptor partial agonists to inhibit dopaminergic activation produced by smoking while simultaneously providing relief from the craving and withdrawal syndrome that accompanies cessation attempts. Varenicline displays high alpha4beta2 nAChR affinity and the desired in vivo dopaminergic profile.

Cocaine-induced alterations in dopamine receptor signaling: Implications for reinforcement and reinstatement

The transition from casual drug use to addiction, and the intense drug craving that accompanies it, has been postulated to result from neuroadaptations within the limbic system caused by repeated drug exposure. This review will examine the implications of cocaine-induced alterations in mesolimbic dopamine receptor signaling within the context of several widely used animal models of addiction. Extensive evidence indicates that dopaminergic mechanisms critically mediate behavioral sensitization to cocaine, cocaine-induced conditioned place preference, cocaine self-administration, and the drug prime-induced reinstatement of cocaine-seeking behavior. The propagation of the long-term neuronal changes associated with recurring cocaine use appears to occur at the level of postreceptor signal transduction. Repeated cocaine treatment causes an up-regulation of the 3',5'-cyclic adenosine monophosphate (cAMP)-signaling pathway within the nucleus accumbens, resulting in a dys-regulation of balanced D1/D2 dopamine-like receptor signaling. The intracellular events arising from enhanced D1-like postsynaptic signaling mediate both facilitatory and compensatory responses to the further reinforcing effects of cocaine.

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.

Molecular mechanisms underlying the rewarding effects of cocaine

The initially surprising observation that cocaine retains its rewarding effects in dopamine transporter (DAT) knockout (KO) mice led our laboratory to examine the effects of deletion of other monoaminergic genes on cocaine reward. Our initial approach to this problem was to combine DAT KO mice with serotonin transporter (SERT) KO mice to make combined DAT/SERT KO mice. The combination of these knockouts eliminates cocaine reward as assessed in the conditioned place preference (CPP) paradigm. We have also identified evidence that, in the absence of DAT, there is greater participation in cocaine reward by serotonin (SERT) and norepinephrine (NET) transporters. Both NET and SERT blockers (nisoxetine and fluoxetine) produced significant CPPs in DAT KO mice, but not in wild-type (WT) mice. The striking elimination of cocaine CPP in combined DAT/SERT KO mice contrasts with effects that we have identified in combined NET/SERT knockout mice, which display increases in cocaine reward, and with recent reports that suggest that DAT/NET combined KOs retain substantial cocaine CPP. Overall, these studies indicate important requirements for several monoaminergic system genes to fully explain cocaine reward, in particular those expressed by dopamine and serotonin systems.

Sucrose intake enhances behavioral sensitization produced by cocaine

Many studies have reported relationships between the intake or preference for sweets and the effects or self-administration of drugs of abuse. This study was conducted to determine whether intermittent access to sucrose would alter the activity response to an injection of cocaine (15 mg/kg, i.p.) or the degree to which repeated cocaine injections produce behavioral sensitization. Nondeprived rats were given 1 h access to granulated sucrose, ground chow, or alternating sucrose and chow for 38 days. Activity levels were measured after injections of saline and cocaine. Rats were also tested after a total of seven cocaine injections, and again 14 days later with no intervening treatments. There was an on overall facilitation of the response to cocaine in rats exposed to sucrose, compared to rats exposed only to ground chow. Subsequent analyses indicated that after the seventh cocaine injection, there was a significant increase in activity of the sucrose group early in the session (compared to the chow group). When tested 14 days later, there was a prolongation of the effect of cocaine in the sucrose group. These results are in partial agreement with the results of others on amphetamine-elicited activity and suggest that some degree of potentiation or cross-sensitization between sucrose and psychostimulants is possible.

Effects of dopamine indirect agonists and selective D1-like and D2-like agonists and antagonists on cocaine self-administration and food maintained responding in rats

A procedure is described for comprehensive evaluation of the effects of acute drug pretreatments on the reinforcing effects of cocaine using the rat self-administration assay in combination with a novel control assay of liquid-food maintained responding. In sessions comprised of five 20-min components, either complete dose-effect functions for cocaine self-administration or complete concentration-effect functions for liquid-food maintained responding were evaluated. The schedule of reinforcement (FR 5 TO 20-s), drug pretreatment doses and time intervals (0-30 min), and duration of sessions (108 min) were identical for cocaine- and food-reinforced test sessions. Whereas acute pretreatment with indirect dopamine agonists (D-amphetamine, GBR 12909) and D2-like agonists (7-OH-DPAT, quinelorane) produced dose-dependent leftward shifts in dose-effect functions for cocaine self-administration, D1-like agonists (SKF 82958, R-6-Br-APB) and dopamine antagonists (D1-like, SCH 39166; D2-like, eticlopride) shifted dose-effect functions for cocaine downward and rightward, respectively. Interestingly, with the indirect dopamine agonists but not the D2-like agonists, increased responding maintained by low cocaine doses was paralleled by increased responding maintained by low food concentrations. Moreover, three of the four direct agonists were moderately selective (< or =5-fold more potent) in decreasing cocaine self-administration relative to food maintained responding. When data were analyzed according to alterations in total cocaine intake, all of the agonists uniformly decreased total cocaine intake, whereas both antagonists increased total cocaine intake. Overall, this procedure was sensitive to leftward, downward and rightward shifts in cocaine dose-effect functions and should be useful for evaluating the nature of pharmacological interactions between novel compounds and self-administered cocaine, as well as the potential for altering cocaine self-administration selectively with candidate treatments for cocaine abuse and dependence.

Dopamine transporter as target for drug development of cocaine dependence medications

Because much evidence implicates the dopamine transporter in the reinforcing effects of cocaine, development of potential medications for cocaine dependence has included the dopamine transporter as a target. The present overview covers progress in the drug development area regarding several classes of dopamine uptake inhibitors, with an emphasis on structure-activity relationships that enhance potency and selectivity at transporters for dopamine compared with those for serotonin or norepinephrine. The following categories of compounds are covered: tropane, benztropine, 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine (GBR), methylphenidate, mazindol, and phencyclidine analogs. Activity at transporters as well as on behavior is highlighted.

Restricted feeding with scheduled sucrose access results in an upregulation of the rat dopamine transporter

Recent studies suggest that the mesoaccumbens dopamine system undergoes neurochemical alterations as a result of restricted feeding conditions with access to sugars. This effect appears to be similar to the neuroadaptation resulting from drugs of abuse and may underlay some pathological feeding behaviors. To further investigate the cellular mechanisms of these alterations, the present study used quantitative autoradiography and in situ hybridization to assess dopamine membrane transporter (DAT) protein density and mRNA expression in restricted-fed and free-fed adult male rats. The restricted feeding regimen consisted of daily limited access to either a normally preferred sucrose solution (0.3 M) or a less preferred chow in a scheduled (i.e., contingent) fashion for 7 days. Restricted-fed rats with the contingent sucrose access lost less body weight, ate more total food, and drank more fluid than free-fed, contingent food, or noncontingent controls. In addition, these animals had selectively higher DAT binding in the nucleus accumbens and ventral tegmental area. This increase in protein binding also was accompanied by an increase in DAT mRNA levels in the ventral tegmental area. In contrast to the restricted-fed groups, no differential effect in DAT regulation was observed across free-fed groups. The observed alteration in behavior and DAT regulation suggest that neuroadaptation in the mesoaccumbens dopamine system develops in response to repeated feeding on palatable foods under dietary constraints. This supports the notion that similar cellular changes may be involved in restrictive eating disorders and bingeing.

Mixed cocaine agonist/antagonist properties of (+)-methyl 4beta-(4-chlorophenyl)-1-methylpiperidine-3alpha-carboxylate, a piperidine-based analog of cocaine

The present study investigated the pharmacological properties of a piperidine-based novel cocaine analog, namely, (+)-methyl 4beta-(4-chlorophenyl)-1-methylpiperidine-3alpha-carboxylic acid [(+)-CPCA]. Like cocaine, (+)-CPCA inhibited rat synaptosomal dopamine and norepinephrine uptake with high affinity, but was 33-fold less potent than cocaine in inhibiting serotonin uptake. Like cocaine, (+)-CPCA is a locomotor stimulant, although it was less potent and efficacious than cocaine. Importantly, pretreatment with (+)-CPCA dose dependently blocked the locomotor stimulant effects of cocaine in rats. (+)-CPCA completely substituted for cocaine in drug discrimination tests, although it was about 3 times less potent than cocaine. It was also self-administered by rats. Unexpectedly, (+)-CPCA did not enhance cocaine-induced convulsions in mice. As expected from rodent studies, rhesus monkeys readily self-administered (+)-CPCA. However, compared with cocaine, (+)-CPCA showed limited reinforcing properties in rats as assessed by both fixed and progressive ratio intravenous drug self-administration tests. These results collectively suggest that (+)-CPCA has an atypical pharmacological profile having both cocaine-like "agonist" and some cocaine "antagonist" properties. These properties of (+)-CPCA suggest that it may have utility in the treatment of cocaine craving and dependence.

Novel (bisarylmethoxy)butylpiperidine analogues as neurotransmitter transporter inhibitors with activity at dopamine receptor sites

A series of (bisarylmethoxy)butylpiperidine derivatives was prepared and evaluated in vitro and in vivo to determine the structural requirements necessary for dual activity at the DAT and DA/5-HT receptor sites. These hybrid ligands, constructed by combining pharmacophores specific for the DAT and DA/5-HT receptors, could be useful drugs for treating cocaine addiction by assisting cocaine addicts in maintaining abstinence. The series was evaluated in vitro for DAT and DA/5-HT receptor activity and then selected compounds were tested in vivo for their effects on cocaine-induced hyperlocomotor activity (LMA). The majority of the new compounds demonstrated high to moderate affinity (4-191 nM) for the DAT with 4-hydroxy-4-phenylpiperidine analogues 14 and 15 possessing the greatest affinity. Compounds 15 and 22 exhibited the highest ratio of reuptake inhibition-to-binding (discrimination ratio, DR), 111 and 323, respectively. These derivatives had modest affinity and antagonistic activity for dopamine D(2)/D(3) receptors. Compounds 9 and 15 (DR=0.9 and 111, respectively) stimulated locomotor activity, whereas the other compounds suppressed this response. All compounds tested except for 17 and 21 attenuated cocaine-induced hyperlocomotion.

Reinforcing strength of a novel dopamine transporter ligand: pharmacodynamic and pharmacokinetic mechanisms

Drugs that block dopamine uptake often function as positive reinforcers but can differ along the dimension of strength or effectiveness as a positive reinforcer. The present study was designed to examine pharmacological mechanisms that might contribute to differences in reinforcing strength between the piperidine-based cocaine analog (+)-methyl 4beta-(4-chlorophenyl)-1-methylpiperidine-3-alpha-carboxylate [(+)-CPCA] and cocaine. Drugs were made available to rhesus monkeys (n = 5) for i.v. self-administration under a progressive ratio schedule. Both compounds maintained responding with sigmoidal or biphasic dose-response functions (0.1-1.0 mg/kg/injection). (+)-CPCA was one-fourth as potent as cocaine and maintained fewer injections per session, at maximum. For in vitro binding in monkey brain tissue, (+)-CPCA was about one-half as potent as cocaine at the dopamine transporter (DAT), and the two compounds had similar affinities at the norepinephrine transporter. (+)-CPCA was less than 1/10 as potent as cocaine at the serotonin transporter. In ex vivo binding in rat striatum, occupancy of the DAT increased directly with dose to a maximum of approximately 80% for both compounds, and (+)-CPCA was about one-fourth as potent as cocaine. Ex vivo DAT occupancy was significantly higher for cocaine than (+)-CPCA at 2 min after injection but similar at other times. Thus, the primary differences between these compounds were in serotonin transporter affinity and the kinetics of DAT binding. These results suggest that (+)-CPCA is a weaker positive reinforcer than cocaine because it has a slower onset of action over the first few minutes after i.v. injection.

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.

Probes for the dopamine transporter: new leads toward a cocaine-abuse therapeutic--A focus on analogues of benztropine and rimcazole

In an attempt to discover a cocaine-abuse pharmacotherapeutic, extensive investigation has been directed toward elucidating the molecular mechanisms underlying the reinforcing effects of this psychostimulant drug. The results of these studies have been consistent with the inhibition of dopamine uptake, at the dopamine transporter (DAT), which results in a rapid and excessive accumulation of extracellular dopamine in the synapse as being the mechanism primarily responsible for the locomotor stimulant actions of cocaine. Nevertheless, investigation of the serotonin (SERT) and norepinephrine (NET) transporters, as well as other receptor systems, with which cocaine either directly or indirectly interacts, has suggested that the DAT is not solely responsible for the reinforcing effects of cocaine. In an attempt to further elucidate the roles of these systems in the reinforcing effects of cocaine, selective molecular probes, in the form of drug molecules, have been designed, synthesized, and characterized. Many of these compounds bind potently and selectively to the DAT, block dopamine reuptake, and are behaviorally cocaine-like in animal models of psychostimulant abuse. However, there have been exceptions noted in several classes of dopamine uptake inhibitors that demonstrate behavioral profiles that are distinctive from cocaine. Structure-activity relationships between chemically diverse dopamine uptake inhibitors have suggested that different binding interactions, at the molecular level on the DAT, as well as divergent actions at the other monoamine transporters may be related to the differing pharmacological actions of these compounds, in vivo. These studies suggest that novel dopamine uptake inhibitors, which are structurally and pharmacologically distinct from cocaine, may be developed as potential cocaine-abuse therapeutics.

Persistent antagonism of methamphetamine-induced dopamine release in rats pretreated with GBR12909 decanoate

Methamphetamine abuse is a serious global health problem, and no effective treatments for methamphetamine dependence have been developed. In animals, the addictive properties of methamphetamine are mediated via release of dopamine (DA) from nerve terminals in mesolimbic reward circuits. At the molecular level, methamphetamine promotes DA release by a nonexocytotic diffusion-exchange process involving DA transporter (DAT) proteins. We have shown that blocking DAT activity with high-affinity DA uptake inhibitors, such as 1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl) piperazine (GBR12909), can substantially reduce amphetamine-induced DA release in vivo. In the present study, we examined the ability of a long-acting depot formulation of GBR12909 decanoate (GBR-decanoate) to influence neurochemical actions of methamphetamine in the nucleus accumbens of rats. Rats received single injections of GBR-decanoate (480 mg/kg i.m.) and were subjected to in vivo microdialysis testing 1 and 2 weeks later. Pretreatment with GBR-decanoate produced modest elevations in basal extracellular levels of DA, but not 5-hydroxytryptamine (5-HT), at both time points. GBR-decanoate nearly eliminated the DA-releasing ability of methamphetamine (0.3 and 1.0 mg/kg i.v.) for 2 weeks, whereas methamphetamine-induced 5-HT release was unaffected. Autoradiographic analysis revealed that GBR-decanoate caused long-term decreases in DAT binding in the brain. Our data suggest that GBR-decanoate, or similar agents, may be useful adjuncts in treating methamphetamine dependence. This therapeutic strategy would be especially useful for noncompliant patient populations.

Dopamine re-uptake inhibitor GBR-12909 induction of aberrant behaviors in animal models of dopamine dysfunction

Many individuals with mental retardation exhibit chronic aberrant behaviors (CABs) that includes hyperactive, stereotyped, aggressive, and self-injurious behaviors. Brain imaging studies have found that several of these individuals have abnormalities in their dopaminergic neurotransmitter systems that are thought to be responsible in part, for the development of these behaviors. The present study evaluated the effects of a selective dopamine re-uptake blocker, GBR-12909 in three animal models of varying striatal dopamine concentrations. The three animal models included the neonatal 6-hydroxydopamine (6-OHDA)-lesioned rat, a model of dopamine neuronal depletion, the prenatal methylazoxymethanol (MAM)-exposed rat, a model of hyper-dopaminergic innervation and control rats, a model of normal dopaminergic function. The animals were given five daily injections of GBR-12909 and videotaped observations were conducted immediately following the injections and 6h later. The results of the study indicate that the MAM-treated rats exhibited more hyperactive behaviors than either the 6-OHDA or the control animals in response to the GBR-12909 injections. However, the 6-OHDA and control rats exhibited more self-injurious behaviors than the MAM rats. Interestingly, the topography of the self-injurious behavior exhibited differed from that we have previously observed in 6-OHDA lesioned rats following dopamine agonists and resembles the mouthing behaviors seen in some individuals with mental retardation, in particular those with Rett syndrome. These findings indicate the models of varying dopaminergic function interact differently with a dopamine re-uptake blocker than dopamine agonists and that the partially dopamine depleted model may model the behaviors seen in individuals with Rett syndrome.

Cocaine and amphetamine attenuate the discriminative stimulus effects of naltrexone in opioid-dependent rhesus monkeys

This study tested the hypothesis that stimulants (indirect dopamine agonists) attenuate the discriminative stimulus of naltrexone in monkeys chronically treated with L-alpha-acetylmethadol (LAAM). Four rhesus monkeys (Macaca mulatta) received LAAM (1.0 mg/kg s.c.) twice daily and discriminated a withdrawal-precipitating dose of naltrexone (0.0178 mg/kg s.c.) from saline. Cocaine (0.1-1.78 mg/kg), amphetamine (0.32-1.78 mg/kg), haloperidol (0.01-0.1 mg/kg), sulpiride (1.0-10.0 mg/kg), propranolol (0.32-3.2 mg/kg), clonidine (0.001-0.1 mg/kg), desipramine (0.32-3.2 mg/kg), and imipramine (1.0-10.0 mg/kg) were given s.c. before cumulative doses of naltrexone. Cocaine and amphetamine antagonized the discriminative stimulus effects of naltrexone, each shifting the naltrexone dose-effect curve significantly (e.g., 100-fold) rightward or downward. In contrast, the dopamine antagonist haloperidol shifted the naltrexone dose-effect curve 5-fold leftward. Sulpiride, desipramine, clonidine, and propranolol had comparatively less effect on the naltrexone discriminative stimulus, whereas some doses of imipramine attenuated the naltrexone stimulus in a manner similar to that of cocaine and amphetamine. These results support the notion that multiple neurotransmitter systems are involved in the discriminative stimulus effects of opioid withdrawal. Furthermore, these data are consistent with reports that dopamine levels decrease during opioid withdrawal and provide evidence that enhancing dopamine or other monoamine levels may attenuate subjective effects of opioid withdrawal.

Preclinical evaluation of GBR12909 decanoate as a long-acting medication for methamphetamine dependence

Methamphetamine (METH) abuse is a growing health problem, and no treatments for METH dependence have been identified. The powerful addictive properties of METH are mediated by release of dopamine (DA) from nerve terminals in mesolimbic reward pathways. METH stimulates DA release by acting as a substrate for DA transporter (DAT) proteins, thereby triggering efflux of DA from cells into the synapse. We have shown that blocking DAT activity with high-affinity DA uptake inhibitors, like GBR12909, can substantially reduce METH-evoked DA release in vitro, suggesting GBR12909 may have potential as a pharmacotherapy for METH dependence. The purpose of the present study was to examine the neurobiological effects of a long-acting oil-soluble preparation of GBR12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-hydroxy-3-phenylpropyl) piperazinyl decanoate, or GBR-decanoate). Male rats received GBR-decanoate (480 mg/kg, i.m.) or its oil vehicle, and were tested using a variety of methods one and two weeks later. Ex vivo autoradiography showed that GBR-decanoate decreases DAT binding in DA-rich brain regions. In vivo microdialysis in the nucleus accumbens revealed that GBR-decanoate elevates baseline levels of extracellular DA and antagonizes the ability of METH to evoke DA release. The dopaminergic effects of GBR-decanoate were sustained, lasting for at least two weeks. Rats pretreated with GBR-decanoate displayed enhanced locomotor responses to novelty at one week, but not two weeks, postinjection. Administration of the D(2)/D(3) receptor agonist quinpirole (10 and 100 microg/kg, s.c.) decreased locomotor activity and suppressed plasma prolactin levels; quinpirole-induced responses were not altered by GBR-decanoate. Thus, GBR-decanoate is able to elevate basal synaptic DA levels and block METH-evoked DA release in a persistent manner, without significant perturbation of DA receptor function. The findings suggest that GBR-decanoate, or similar long-acting agents, should be evaluated further as potential treatment adjuncts in the management of METH addiction in humans.

Generation of an activating Zn(2+) switch in the dopamine transporter: mutation of an intracellular tyrosine constitutively alters the conformational equilibrium of the transport cycle

Binding of Zn(2+) to the endogenous Zn(2+) binding site in the human dopamine transporter leads to potent inhibition of [(3)H]dopamine uptake. Here we show that mutation of an intracellular tyrosine to alanine (Y335A) converts this inhibitory Zn(2+) switch into an activating Zn(2+) switch, allowing Zn(2+)-dependent activation of the transporter. The tyrosine is part of a conserved YXX Phi trafficking motif (X is any residue and Phi is a residue with a bulky hydrophobic group), but Y335A did not show alterations in surface targeting or protein kinase C-mediated internalization. Despite wild-type levels of surface expression, Y335A displayed a dramatic decrease in [(3)H]dopamine uptake velocity (V(max)) to less than 1% of the wild type. In addition, Y335A showed up to 150-fold decreases in the apparent affinity for cocaine, mazindol, and related inhibitors whereas the apparent affinity for several substrates was increased. However, the presence of Zn(2+) in micromolar concentrations increased the V(max) up to 24-fold and partially restored the apparent affinities. The capability of Zn(2+) to restore transport is consistent with a reversible, constitutive shift in the distribution of conformational states in the transport cycle upon mutation of Tyr-335. We propose that this shift is caused by disruption of intramolecular interactions important for stabilizing the transporter in a conformation in which extracellular substrate can bind and initiate transport, and accordingly that Tyr-335 is critical for regulating isomerization between discrete states in the transport cycle.

Expansion of structure-activity studies of piperidine analogues of 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)piperazine (GBR 12935) compounds by altering substitutions in the N-benzyl moiety and behavioral pharmacology of selected molecules

A series of substituted N-benzyl analogues of the dopamine transporter (DAT) specific compound, 4-[2-(diphenylmethoxy)ethyl]-1-benzylpiperidine were synthesized and biologically characterized. Different 4'-alkyl, 4'-alkenyl, and 4'-alkynyl substituents were introduced in the phenyl ring of the benzyl moiety along with the replacement of the same phenyl ring by the isomeric alpha- and beta-naphthyl groups. Different polar substitutions at the 3'- and 4'-position were also introduced. Novel compounds were tested for their binding affinity at the dopamine, serotonin, and norepinephrine transporter systems in the brain by competing for [(3)H]WIN 35 428, [(3)H]citalopram, and [(3)H]nisoxetine, respectively. Selected compounds were also evaluated for their activity in inhibiting the uptake of [(3)H]dopamine. Binding results demonstrated that alkenyl and alkynyl substitutions at the 4'-position produced potent compounds in which compound 6 with a vinyl substitution was the most potent. In vivo evaluation of three selected compounds indicated that despite their high potency at the DAT, these compounds stimulated locomotor activity (LMA) less than cocaine when tested across similar dose ranges. In a drug discrimination study procedure, none of these three compounds generalized from cocaine in mice trained to discriminate 10 mg/kg cocaine from vehicle. In a 4 h time course LMA experiment, one of our previous lead piperidine derivatives (1a) showed considerable prolonged action. Thus, in this report, we describe a structure-activity relationship study of novel piperidine analogues assessed by both in vitro transporter assays and in vivo behavioral activity measurements.

Excessive sugar intake alters binding to dopamine and mu-opioid receptors in the brain

Palatable food stimulates neural systems implicated in drug dependence; thus sugar might have effects like a drug of abuse. Rats were given 25% glucose solution with chow for 12 h followed by 12 h of food deprivation each day. They doubled their glucose intake in 10 days and developed a pattern of excessive intake in the first hour of daily access. After 30 days, receptor binding was compared to chow-fed controls. Dopamine D-1 receptor binding increased significantly in the accumbens core and shell. In contrast, D-2 binding decreased in the dorsal striatum. Binding to dopamine transporter increased in the midbrain. Opioid mu-1 receptor binding increased significantly in the cingulate cortex, hippocampus, locus coeruleus and accumbens shell. Thus, intermittent, excessive sugar intake sensitized D-1 and mu-1 receptors much like some drugs of abuse.

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

kappa-Opioid receptor agonists have been suggested as treatments for cocaine addiction based on studies showing that they block cocaine-related behaviors. To determine the effects of kappa-opioid receptor agonists on long-term behavioral effects associated with cocaine and the neurochemical bases underlying these effects, rats were treated with the selective kappa-opioid receptor agonist U-69593 ((+)(5alpha,7alpha,8beta)-N-methyl-N-[7-(1-pyrrolidinyl)-1 oxaspiro[4.5]dec-8-yl]-benzeneacetamide) alone or in combination with cocaine and locomotor activity was measured daily. In addition, dopamine transporter and dopamine receptor densities were measured using autoradiographic techniques, and tyrosine hydroxylase was measured using immunoautoradiographic techniques. Treatment with U-69593 with or without cocaine decreased locomotor activity. When challenged with cocaine after a 5-day treatment period, the effects of cocaine were markedly reduced in rats initially treated with U-69593 compared to vehicle. When U-69593 was administered five times with 3-day intervals, it alone had no effect on locomotor activity but still reduced activity associated with a cocaine injection. After five daily injections, U-69593 decreased dopamine transporter and dopamine D(2) receptor densities and increased tyrosine hydroxylase levels. These changes were not seen after the 3-day interval regimen, even though cocaine-induced activity was greatly reduced. These findings show that the effects associated with daily U-69593 treatment are attenuated if the drug is administered with a greater interval, while maintaining a blockade of cocaine-induced activity. In addition, U-69593 can block cocaine-induced locomotor effects without major perturbation of the dopamine system.

The acute effects of norharman on cocaine self-administration and sensorimotor function in male Wistar rats

The purpose of the present study was to determine the acute effects of the beta-carboline norharman on cocaine dependence. Male Wistar rats were allowed to self-administer cocaine for 3 h for seven sessions. A single injection of norharman (0.2-20 mg/kg, i.p.) was given 30 min before the onset of the seventh session. It was shown that norharman decreased the cocaine intake in a U-shaped manner with the dose of 10 mg/kg having the most potent effect. These results indicate that several receptor mechanisms mediate the effects of norharman. In addition, 15 min following the administration of norharman on session 7, motor and sensory skill tests were performed. Six naïve animals were tested with the dose, which has the most pronounced effect on cocaine self-administration intake, in order to examine whether the observed effects were due to norharman or due to the combination of norharman and cocaine. It was observed that norharman in both the naïve and cocaine-exposed animals significantly increased the response time in the somato-sensory orienting test. However, only in the naïve animals a significant effect of norharman on the grasp reflex was observed.