GBR 12909 fails to antagonize cocaine-induced elevation of dopamine in striatal slices

Antidepressant treatment facilitates dopamine release and drug seeking behavior in a genetic animal model of depression

Anhedonia and lack of motivation are core symptoms of depression. In contrast, hyper-motivation and euphoria characterize intoxicated states. In order to explore the relationship between these two behavioral states we examined cocaine self-administration tasks in an animal model of depression [Flinders Sensitive Line (FSL) rats]. We found that FSL rats exhibit sub-sensitivity in their cocaine-seeking behavior, which was normalized following a chronic treatment with the antidepressant desipramine. However, when the cocaine dosage was increased, FSL rats demonstrated a similar cocaine-seeking behavior to that of controls. In light of dopamine's central role in modulating cocaine reinforcement, we examined dopaminergic neurotransmission in the nucleus accumbens, a brain region implicated in the rewarding and hedonic effects of substances of misuse. FSL rats exhibited low but dose-dependent increases in extracellular levels of dopamine in the nucleus accumbens after acute intravenous cocaine injection. Furthermore, by using the dopamine transporter blocker GBR-12909 we were able to demonstrate that the low extracellular dopamine levels, observed in FSL rats, were a consequence of low dopamine release in the nucleus accumbens, as opposed to the possibility of increased uptake. Treatment of FSL rats with the antidepressant desipramine raised cocaine- and GBR-12909-induced dopamine release to the level of controls. This treatment also resulted in increased cocaine-seeking behavior.

Combinations of cocaine with other dopamine uptake inhibitors: assessment of additivity

Drugs that inhibit dopamine (DA) reuptake through actions at the dopamine transporter (DAT) have been proposed as candidates for development as pharmacotherapies for cocaine abuse. Accordingly, it is important to understand the potential pharmacological interactions of cocaine with other drugs acting at the DAT. Effects of combinations of cocaine with a cocaine analog, 2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane (WIN 35,428), were compared quantitatively with the combinations of cocaine with the N-butyl,4',4''-diF benztropine analog, 3-(bis(4-fluorophenyl)methoxy)-8-butyl-8-azabicyclo[3.2.1]octane (JHW 007), to determine whether their effects on DA levels in the shell of the nucleus accumbens (NAC) in mice differed. Each of the drugs alone produced dose-related elevations in NAC DA levels. In contrast to the other drugs, JHW 007 was less effective, producing maximal effects that approached 400% of control versus approximately 700% with the other drugs. In addition, the JHW 007 dose-effect curve was not as steep as those for cocaine and WIN 35,428. Combinations of cocaine with its analog, WIN 35,428, were most often greater than those predicted based on dose additivity. In contrast, combinations of cocaine with JHW 007 were most often subadditive. This outcome is consistent with recent studies suggesting that structurally divergent DA uptake inhibitors bind to different domains of the DAT, which can result in different DAT conformations. The conformational changes occurring with JHW 007 binding may result in functional outcomes that alter its abuse liability and its effects in combination with cocaine.

Novel in vivo electrophysiological assay for the effects of cocaine and putative "cocaine antagonists" on dopamine transporter activity of substantia nigra and ventral tegmental area dopamine neurons

The aim of these studies was to establish a rapid in vivo assay for evaluating potential "cocaine antagonists," i.e., drugs postulated to block cocaine binding to the dopamine transporter (DAT) without corresponding blockade of dopamine reuptake. The assay is based on the ability of dopamine, and drugs that elevate synaptic dopamine levels, to inhibit the extracellular single unit activities of midbrain dopamine neurons in chloral hydrate-anesthetized rats. As expected, cocaine itself (0.06-16 mg/kg, i.v.) caused a dose-dependent inhibition of firing of both substantia nigra and ventral tegmental area (VTA) dopamine neurons, but had a significantly higher potency on VTA than nigral dopamine cells (ED(50)'s 1.2 and 8.8 mg/kg, respectively). VTA cells were also inhibited to a greater extent (to 4.7 +/- 4.5% vs. 41.3 +/- 6.3% of baseline rates at 16 mg/kg, respectively). We next evaluated GBR12909, a piperazine analog promoted as a "cocaine antagonist" because of its ability to bind with high affinity to the DAT, while only modestly elevating extracellular dopamine levels. The agonist- and antagonist-like properties of GBR12909 were evaluated on only VTA dopamine cells since these neurons were more fully inhibited by cocaine and have been implicated in its rewarding effects. Given alone, GBR12909 exhibited modest "cocaine-like" activity insofar as it partially inhibited VTA dopamine neurons (to 59.0 +/- 4.6% of baseline at 8 mg/kg). However, consistent with an antagonist profile, pretreatment with a low (0.5 mg/kg) dose of GBR12909, which depressed firing only slightly, resulted in a >2-fold rightward shift in the dose-response curve to cocaine (ED(50) 2.6 mg/kg). We conclude that electrophysiological testing of putative "anti-cocaine" drugs for their abilities to inhibit the firing of VTA dopamine neurons, and to block their inhibitory responses to cocaine, may provide a rapid in vivo screen for compounds expected to behave as functional cocaine antagonists in the dopamine reward system.

Effect of cocaine, nomifensine, GBR 12909 and WIN 35428 on carbon fiber microelectrode sensitivity for voltammetric recording of dopamine

Electrochemical measurements using voltammetry or amperometry at carbon-fiber microelectrodes have been used in vitro and in vivo to examine regulatory mechanisms for the central dopamine system. In many of these experiments, dopamine efflux concentrations under control conditions are determined followed by their alterations in response to a drug treatment. The present study demonstrates that some drugs can affect dopamine measurements, not only by their expected pharmacological action but also by directly altering the microelectrode responsivity. The commonly used reuptake inhibitors GBR 12909 (10 microM) and nomifensine (5 microM) drastically reduce electrode sensitivity and, in the case of nomifensine, increase the time to reach a plateau in response to dopamine boluses (i.e. reduced 'frequency response'). Cocaine (10 microM) and WIN 35428 (2 microM) have negligible effect on these indices. This decrease in sensitivity was found in both nafion and non-nafion coated electrodes. Further, the reduction in sensitivity seen in non-nafion coated electrodes was not prevented by increasing the reversal potential (from +1.0 to +1.3 V) and voltage scan rate (from 350 to 450 V/s). These data suggest that care must be taken when interpreting data from voltammetric or amporometric experiments using carbon electrodes where GBR 12909 or nomifensine are used, especially at high concentrations. Furthermore, wherever possible, direct effects of a drug on electrode sensitivity and frequency response should be determined.

Dopaminergic agents for the treatment of cocaine abuse

Cocaine is a major drug of abuse whose devastating effects have captured the attention of health officials and policy makers. Based upon the alarming health and crime-related costs associated with the use of this powerful reinforcing drug, immediate therapies are needed for the treatment of cocaine addiction. In this review, some of the small-molecule-based approaches that have been pursued in the search for such medications are highlighted. Because the pharmacological actions of cocaine stem laargely from its ability to block the dopamine transporter, many intervention strategies have focused on the dopaminergic pathway.

Opioid receptor agonists and antagonists alter GBR12909-induced turning in the rat

We extended observations on cocaine-induced turning and its interactions with mu-opioid receptor agonists in nigrally-lesioned rats to GBR12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-[3-phenyl-propyl]-piperazine) , a selective dopamine reuptake inhibitor. GBR12909 produced turning that was potentiated by the mu-opioid receptor agonists morphine and methadone. The effects of these opioids were blocked by the general opioid receptor antagonist naloxone, which did not affect the action of GBR12909. The reuptake inhibitors nisoxetine (norepinephrine) and fluoxetine (serotonin) did not produce turning alone or in combination with morphine. Antagonists selective for each opioid receptor subtype did not alter GBR12909-induced turning. However, naltrexone, another general opioid receptor antagonist, potentiated turning induced by GBR12909. This was blocked by naloxone, suggesting that naltrexone has opioid receptor agonist actions, in contrast to naloxone. These results indicate that cocaine-induced turning and its potentiation by mu-opioid receptor agonists are dependent upon the inhibition of dopamine reuptake.

Relations between heterogeneity of dopamine transporter binding and function and the behavioral pharmacology of cocaine

Both in vitro binding studies and studies of dopamine uptake have indicated that there is a heterogeneity of action of cocaine and cocaine analogs. Both high- and low-affinity binding sites have been identified. Some drugs that bind to the dopamine transporter show both high- and low-affinity components whereas others do not. Behavioral studies have indicated that the high-affinity component appears to be the one most directly involved in the actions of cocaine related to abuse. These conclusions are based on correlations of affinities and psychomotor stimulant effects. In addition, tolerance to the psychomotor stimulant effects of cocaine occurs with a concomitant change in only the high-affinity component for dopamine uptake. Certain dopamine uptake inhibitors may have only actions mediated by the low-affinity component. These drugs bind to the dopamine transporter and inhibit dopamine uptake; however, they do not have behavioral effects like those of cocaine. This finding is a critical point of inquiry for the dopamine hypothesis because, based on the neurochemical data, these drugs should have behavioral actions like those of cocaine. In contrast, some of these drugs antagonize the behavioral effects of cocaine, suggesting that the low-affinity site somehow modulates the actions mediated by the high-affinity site. Recently, some benztropine analogs have been discovered that bind to the dopamine transporter and inhibit dopamine uptake monophasically but have behavioral effects that are dissimilar to those of cocaine. These compounds may prove useful in determining the behavioral significance of heterogeneity of actions at the dopamine transporter. Further, these studies may provide leads to novel therapeutics for the treatment of cocaine abuse.

Evidence that pure uptake inhibitors including cocaine interact slowly with the dopamine neuronal carrier

We have studied the ability of various uptake blockers to protect the dopamine neuronal carrier labeled with [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-(propenyl)-piperazine) against N-ethylmaleimide-induced alkylation, using membrane preparations obtained from rat striatum. Pure uptake inhibitors such as mazindol, pyrovalerone, nomifensine and methylphenidate, and substrates (dopamine, d-amphetamine, m-tyramine) protected the [3H]GBR 12783 binding site in a concentration-dependent manner. Preincubation of the membranes with these agents prior to N-ethylmaleimide treatment did not modify the protecting ability of substrates, whereas it significantly improved that of pure uptake inhibitors including cocaine. When the preincubation was omitted, the concentration dependence of the protection observed with pure uptake inhibitors decreased and a maximal 40% protection was observed for 10 microM to 1 mM cocaine concentrations. Effective protecting concentrations of blockers are correlated with their Ki determined in standard binding studies. These results reveal that all pure uptake inhibitors bind slowly to the dopamine neuronal carrier whereas substrates interact with it rapidly.