Cocaine-induced Increases In Extracellular Dopamine Research Articles

Cannabidiol attenuates the rewarding effects of cocaine in rats by CB2, 5-HT1A and TRPV1 receptor mechanisms

Cocaine abuse continues to be a serious health problem worldwide. Despite intense research there is currently no FDA-approved medication to treat cocaine use disorder. The recent search has been focused on agents targeting primarily the dopamine system, while limited success has been achieved at the clinical level. Cannabidiol (CBD) is a U.S. FDA-approved cannabinoid for the treatment of epilepsy and recently was reported to have therapeutic potential for other disorders. Here we systemically evaluated its potential utility for the treatment of cocaine use disorder and explored the underlying receptor mechanisms in experimental animals. Systemic administration (10–40 mg/kg) of CBD dose-dependently inhibited cocaine self-administration, shifted a cocaine dose-response curve downward, and lowered break-points for cocaine self-administration under a progressive-ratio schedule of reinforcement. CBD inhibited cocaine self-administration maintained by low, but not high, doses of cocaine. In addition, CBD (3–20 mg/kg) dose-dependently attenuated cocaine-enhanced brain-stimulation reward (BSR) in rats. Strikingly, this reduction in both cocaine self-administration and BSR was blocked by AM630 (a cannabinoid CB2 receptor antagonist), WAY100135 (a 5-HT1A receptor antagonist), or capsazepine (a TRPV1 channel blocker), but not by AM251 (a CB1 receptor antagonist), CID16020046 (a GPR55 antagonist), or naloxone (an opioid receptor antagonist), suggesting the involvement of CB2, 5-HT1A, and TRPV1 receptors in CBD action. In vivo microdialysis indicated that pretreatment with CBD (10–20 mg/kg) attenuated cocaine-induced increases in extracellular dopamine (DA) in the nucleus accumbens, while CBD alone failed to alter extracellular DA. These findings suggest that CBD may have certain therapeutic utility by blunting the acute rewarding effects of cocaine via a DA-dependent mechanism.

The Dopamine Transporter Expression Level Differentially Affects Responses to Cocaine and Amphetamine

Although both cocaine and amphetamine mainly target the dopamine transporter (DAT) and cause psychomotor effects, they have very different mechanisms of actions. The authors examined whether responses to cocaine and amphetamine were affected differentially by changes in DAT expression levels using transgenic mice with different DAT expression levels. In the constitutive DAT knockdown mice, reduced DAT expression enhanced cocaine's locomotor stimulatory effects and at the same time diminished amphetamine's locomotor stimulatory effects. Similar effects were observed in the inducible DAT knockdown mice, ruling out the contribution of developmental compensations in DAT knockdown mice. Extracellular dopamine levels in response to psychostimulants were assessed by in vivo microdialysis. Whereas amphetamine-induced increase in extracellular dopamine was drastically diminished in constitutive DAT knockdown mice, cocaine-induced increase in extracellular dopamine had a faster onset in knockdown mice compared with wild-type controls. Postsynaptically, D1 agonist–stimulated c-fos expression was significantly attenuated in constitutive DAT knockdown mice compared with wild-type controls. The authors propose that responses to cocaine and amphetamine depend on psychostimulant drug type, drug dose, as well as DAT expression level. DAT expression level affects presynaptic responses to psychostimulants directly and postsynaptic responses to psychostimulants indirectly via changes in receptor signaling. These data imply that individual differences in DAT expression (either genetically or pharmacologically induced) may affect susceptibility to addiction of different types of psychostimulants.

Corticosterone Acts in the Nucleus Accumbens to Enhance Dopamine Signaling and Potentiate Reinstatement of Cocaine Seeking

Stressful life events are important contributors to relapse in recovering cocaine addicts, but the mechanisms by which they influence motivational systems are poorly understood. Studies suggest that stress may "set the stage" for relapse by increasing the sensitivity of brain reward circuits to drug-associated stimuli. We examined the effects of stress and corticosterone on behavioral and neurochemical responses of rats to a cocaine prime after cocaine self-administration and extinction. Exposure of rats to acute electric footshock stress did not by itself reinstate drug-seeking behavior but potentiated reinstatement in response to a subthreshold dose of cocaine. This effect of stress was not observed in adrenalectomized animals, and was reproduced in nonstressed animals by administration of corticosterone at a dose that reproduced stress-induced plasma levels. Pretreatment with the glucocorticoid receptor antagonist RU38486 did not block the corticosterone effect. Corticosterone potentiated cocaine-induced increases in extracellular dopamine in the nucleus accumbens (NAc), and pharmacological blockade of NAc dopamine receptors blocked corticosterone-induced potentiation of reinstatement. Intra-accumbens administration of corticosterone reproduced the behavioral effects of stress and systemic corticosterone. Corticosterone treatment acutely decreased NAc dopamine clearance measured by fast-scan cyclic voltammetry, suggesting that inhibition of uptake₂-mediated dopamine clearance may underlie corticosterone effects. Consistent with this hypothesis, intra-accumbens administration of the uptake₂ inhibitor normetanephrine potentiated cocaine-induced reinstatement. Expression of organic cation transporter 3, a corticosterone-sensitive uptake₂ transporter, was detected on NAc neurons. These findings reveal a novel mechanism by which stress hormones can rapidly regulate dopamine signaling and contribute to the impact of stress on drug intake.

Retrodialysis of N/OFQ into the nucleus accumbens shell blocks cocaine-induced increases in extracellular dopamine and locomotor activity

Nociceptin (N/OFQ) has been implicated in a variety of neurological disorders, most notably in reward processes and drug abuse. N/OFQ suppresses extracellular dopamine in the nucleus accumbens (NAc) after intracerebroventricular injection. This study sought to examine the effects of retrodialyzed N/OFQ on the cocaine-induced increase in extracellular dopamine levels in the NAc, as well as locomotor activity, in freely moving rats. 1.0μM, 10μM, and 1mM N/OFQ, in the NAc shell, significantly suppressed the cocaine-induced dopamine increase in the NAc, while N/OFQ alone had no significant effect on dopamine levels. Co-delivery of the selective NOP receptor antagonist SB612111 ([(–)-cis-1-Methyl-7-[[4-(2,6-dichlorophenyl)piperidin-1-yl]methyl]-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ol] reversed the N/OFQ suppression of cocaine-induced dopamine in the NAc, suggesting that this is an NOP receptor-mediated effect. Using a novel system to assess locomotion, we measured various motor activities of the animals with simultaneous microdialysis from the home cage. Cocaine produced an expected increase in total activity, including horizontal movement and rearing behavior. Retrodialysis of N/OFQ with cocaine administration affected all motor activities, initially showing no effect on behavior, but over time inhibiting cocaine-induced motor behaviors. These results suggest that N/OFQ can act directly in the NAc shell to block cocaine-induced increases in extracellular dopamine levels. Extracellular dopamine and locomotor activity can be dissociated within the NAc and may reflect motor output differences in shell versus core regions of the NAc. These studies confirm the widespread involvement of NOP receptors in drug addiction and further validate the utility of an NOP receptor agonist as a medication for treatment of drug addiction.

(1S, 3S)-3-Amino-4-difluoromethylenyl-1-cyclopentanoic Acid (CPP-115), a Potent γ-Aminobutyric Acid Aminotransferase Inactivator for the Treatment of Cocaine Addiction

Vigabatrin, a GABA aminotransferase (GABA-AT) inactivator, is used to treat infantile spasms and refractory complex partial seizures and is in clinical trials to treat addiction. We evaluated a novel GABA-AT inactivator (1S, 3S)-3-amino-4-difluoromethylenyl-1-cyclopentanoic acid (CPP-115, compound 1) and observed that it does not exhibit other GABAergic or off-target activities and is rapidly and completely orally absorbed and eliminated. By use of in vivo microdialysis techniques in freely moving rats and microPET imaging techniques, 1 produced similar inhibition of cocaine-induced increases in extracellular dopamine and in synaptic dopamine in the nucleus accumbens at (1)/(300) to (1)/(600) the dose of vigabatrin. It also blocks expression of cocaine-induced conditioned place preference at a dose (1)/(300) that of vigabatrin. Electroretinographic (ERG) responses in rats treated with 1, at doses 20-40 times higher than those needed to treat addiction in rats, exhibited reductions in ERG responses, which were less than the reductions observed in rats treated with vigabatrin at the same dose needed to treat addiction in rats. In conclusion, 1 can be administered at significantly lower doses than vigabatrin, which suggests a potential new treatment for addiction with a significantly reduced risk of visual field defects.

Tyrosine kinase B and C receptors in the neostriatum and nucleus accumbens are co-localized in enkephalin-positive and enkephalin-negative neuronal profiles and their expression is influenced by cocaine

Single- and double-label immunohistochemistry were used to determine the extent to which the tyrosine kinase B and C receptors, are expressed in enkephalin-immunopositive or enkephalin-immunonegative neuronal profiles in the rat neostriatum and nucleus accumbens. Results indicate that tyrosine kinase B and C receptors are co-localized in both enkephalin-positive and enkephalin-negative neurons in both of these nuclei, which suggests that these receptors influence both the striatal–pallidal (enkephalin) and striatal–ventral mesencephalic (substance P/dynorphin) pathways. We also examined the influence of acute or repeated injections of cocaine on the number of tyrosine kinase B and C receptors immunoreactive neuronal profiles in the rat neostriatum and nucleus accumbens. Following an acute injection of cocaine (15 mg/kg, i.p.), there were significant decreases in the number of tyrosine kinase B and C receptors immunoreactive profiles in specific regions of the neostriatum and nucleus accumbens relative to saline-pretreated rats. One or 14 days following the last of seven daily injections of 15 mg/kg cocaine or saline there were no differences in the numbers of tyrosine kinase B or C receptors immunoreactive neuronal profiles between these treatment groups. Collectively, the present results indicate that tyrosine kinase B and C receptors in the neostriatum and nucleus accumbens are co-localized in enkephalin-positive and enkephalin-negative neuronal profiles, which suggests that the striatal medium spiny neurons expressing tyrosine kinase B and C receptors include those that project to the pallidum or the ventral mesencephalon. The current results also show that an acute injection of cocaine results in a decrease in the number of tyrosine kinase B and C receptors immunoreactive neuronal profiles in specific regions of the nucleus accumbens and neostriatum, indicating that cocaine-induced increases in extracellular dopamine in the striatal complex result in compensatory decreases in the expression of tyrosine kinase B and C receptors.

Lack of Cocaine Effect on Dopamine Clearance in the Core and Shell of the Nucleus Accumbens of Dopamine Transporter Knock-Out Mice

Cocaine produces its reinforcing effects primarily by inhibiting the dopamine transporter (DAT) at the level of presynaptic terminals and increasing extracellular levels of dopamine (DA). Surprisingly, in mice genetically lacking the DAT, cocaine was still able to elevate DA in the nucleus accumbens (NAc). This finding is critically important for explaining the persistence of cocaine reinforcement in DAT knock-out (DAT-KO) mice. However, the mechanism by which cocaine elevates DA is unclear. Here, we tested the recently proposed hypothesis that in the absence of the DAT, the norepinephrine transporter (NET) could provide an alternative uptake site for DA clearance. If true, cocaine could elevate DA levels through its inhibition of the NET. In vitro voltammetry, a technique well suited for evaluating the effects of drugs on DA uptake, was used in the present study. We report that both cocaine and desipramine, a potent NET inhibitor, failed to change DA clearance or evoked release in the NAc of mutant mice. Additionally, fluoxetine, a serotonin transporter (SERT) inhibitor, also had no effect on these parameters. These data rule out the involvement of accumbal NET or SERT in the cocaine-induced increase in extracellular DA in DAT-KO mice. Moreover, the present findings suggest that in the DAT-KO mice, cocaine acts primarily outside the NAc to produce its effects.

Lack of cocaine effect on dopamine clearance in the core and shell of the nucleus accumbens of dopamine transporter knock-out mice.

Cocaine produces its reinforcing effects primarily by inhibiting the dopamine transporter (DAT) at the level of presynaptic terminals and increasing extracellular levels of dopamine (DA). Surprisingly, in mice genetically lacking the DAT, cocaine was still able to elevate DA in the nucleus accumbens (NAc). This finding is critically important for explaining the persistence of cocaine reinforcement in DAT knock-out (DAT-KO) mice. However, the mechanism by which cocaine elevates DA is unclear. Here, we tested the recently proposed hypothesis that in the absence of the DAT, the norepinephrine transporter (NET) could provide an alternative uptake site for DA clearance. If true, cocaine could elevate DA levels through its inhibition of the NET. In vitro voltammetry, a technique well suited for evaluating the effects of drugs on DA uptake, was used in the present study. We report that both cocaine and desipramine, a potent NET inhibitor, failed to change DA clearance or evoked release in the NAc of mutant mice. Additionally, fluoxetine, a serotonin transporter (SERT) inhibitor, also had no effect on these parameters. These data rule out the involvement of accumbal NET or SERT in the cocaine-induced increase in extracellular DA in DAT-KO mice. Moreover, the present findings suggest that in the DAT-KO mice, cocaine acts primarily outside the NAc to produce its effects.

Serotonergic attenuation of the reinforcing and neurochemical effects of cocaine in squirrel monkeys.

Preclinical studies have documented that serotonin (5-HT) can modulate the behavioral effects of cocaine. The present study examined the ability of 5-HT to attenuate the reinforcing and neurochemical effects of cocaine in nonhuman primates. In squirrel monkeys trained to self-administer cocaine (0.1 and 0.3 mg/injection) under a second-order schedule of i.v. drug delivery, the 5-HT uptake inhibitor alaproclate (3.0 and 10.0 mg/kg) and the 5-HT direct agonist quipazine (0.3-1.0 mg/kg) decreased response rates at doses that had no significant effect on behavior maintained by an identical schedule of stimulus termination. The neurochemical bases of the observed drug interactions on behavior were investigated further using in vivo microdialysis techniques in a separate group of awake monkeys to monitor drug-induced changes in extracellular dopamine (DA). Cocaine (1.0 mg/kg) elevated the concentration of DA in the caudate nucleus to approximately 300% of basal levels. Pretreatment with alaproclate or quipazine attenuated cocaine-induced increases in extracellular DA at the same pretreatment doses that decreased cocaine self-administration. The results obtained suggest that increasing brain 5-HT activity can attenuate the reinforcing effects of cocaine, ostensibly by decreasing the ability of cocaine to elevate extracellular DA in brain areas that mediate the behavioral effects. These findings extend those reported previously for the behavioral-stimulant effects of cocaine and identify a potential neurochemical mechanism underlying drug interactions on behavior.

Inhibition of MAO-A fails to alter cocaine-induced increases in extracellular dopamine and norepinephrine in rat nucleus accumbens

Monoamine oxidase (MAO) inhibitors are being investigated as possible medications for cocaine dependence, but there are potential problems with this approach. In the present study, we tested the hypothesis that inhibition of catecholamine metabolism with the MAO-A inhibitor, clorgyline, might enhance cocaine-induced increases in extracellular dopamine and norepinephrine in rat nucleus accumbens. Male rats were pretreated with clorgyline (1 mg/kg, s.c.) or its saline vehicle (1 ml/kg, s.c.), and microdialysis probes were inserted into previously implanted guide cannulae. After overnight perfusion of the probes in situ, rats received an acute challenge injection of either cocaine (1 mg/kg, i.v.) or its saline vehicle (1 ml/kg, i.v.). Clorgyline pretreatment alone caused significant elevations in basal levels of dialysate norepinephrine but not dopamine. Cocaine administration elicited significant increases in extracellular dopamine and norepinephrine in all groups of rats, and this effect was not altered by clorgyline pretreatment. The 1 mg/kg dose of clorgyline decreased dopamine metabolites in postmortem brain tissue by more than 80%. Our data are consistent with clinical studies that demonstrate pretreatment with the MAO-B selective inhibitor, selegeline, fails to alter cocaine-induced subjective effects in human drug users. Moreover, these findings suggest that adverse consequences related to altered catecholamine transmission would not occur if patients taking phenelzine, a non-selective MAO inhibitor, relapsed and used cocaine.

Orphanin FQ/nociceptin attenuates motor stimulation and changes in nucleus accumbens extracellular dopamine induced by cocaine in rats.

Orphanin FQ (OFQ; also known as nociceptin), the endogenous ligand of the opioid receptor-like receptor, injected intracerebroventricularly (i.c.v.) decreases basal motor activity and basal extracellular levels of dopamine (DA) in the nucleus accumbens (Nuc Acc) in rats. The present study was designed to determine if OFQ similarly attenuates cocaine-induced motor stimulation and to determine if this effect is dependent on attenuation of the increase in extracellular DA. After a 1-h adaptation period, rats were injected with either artificial cerebrospinal fluid or OFQ (3-30 nmol, i.c.v.) 5 min prior to cocaine (10 mg/kg, i.p.) or apomorphine (3 mg/kg, i.p.) administration and the total distance traveled was measured for a further 1 h. In a separate experiment, changes in extracellular DA were monitored by microdialysis following cocaine and OFQ treatment in anesthetized rats. OFQ dose-dependently attenuated both basal and cocaine-induced motor stimulation. OFQ (30 nmol, i.c.v.) also attenuated both the basal and the cocaine-induced increase in extracellular DA in the Nuc Acc. OFQ, at the highest dose, also decreased the motor stimulation induced by apomorphine. Our results suggest that the modulatory effect of OFQ on locomotor activity is not solely due to its inhibitory action on extracellular DA in the Nuc Acc.

Cocaine Intake by Rats Correlates with Cocaine-Induced Dopamine Changes in the Nucleus Accumbens Shell

Extracellular dopamine levels were determined by microdialysis in the core and shell of the nucleus accumbens and the frontal cortex of rats before and after an injection of cocaine (20 mg/kg, IP). After removal of the probes, these same animals were then tested for their voluntary intake of cocaine using the two-bottle, free-choice paradigm. Baseline dopamine levels and their responses to an injection of cocaine differed among the three brain areas. No significant correlations were found between baseline dopamine levels in any of the three brain regions and the voluntary cocaine consumption. A significant negative correlation was found between cocaine-induced increases in extracellular dopamine in the shell of the nucleus accumbens and the voluntary intake of cocaine ( r = −0.73, p < 0.01). No such correlations were observed in the accumbens core region or the frontal cortex. These results provide further evidence of the role of the accumbal shell region in cocaine preference, and indicate that cocaine-induced increases in dopamine levels play a role in oral cocaine self-administration or preference. In addition, this relatively novel approach in using the same animals for both cocaine induced neurotransmitter responses and cocaine preference studies can also be applied for the study of other neurotransmitters and drugs of abuse.

γ-Aminobutyric acid mimetic drugs differentially inhibit the dopaminergic response to cocaine

Dopaminergic activity in the mesocorticolimbic system is associated with reinforcing properties of psychostimulant drugs. We previously demonstrated that increased γ-aminobutyric acid (GABA)-ergic activity produced by γ-vinyl GABA [ d, l-4-amino-hex-5-enoic acid (Vigabatrin®)], an irreversible inhibitor of GABA-transaminase, attenuated cocaine, nicotine, heroin, alcohol, and methamphetamine-induced increases in extracellular nucleus accumbens dopamine as well as behaviors associated with these biochemical changes. In the present study, using in vivo microdialysis techniques, we compared three different strategies to increase GABAergic activity in order to modulate cocaine-induced increase in extracellular dopamine. Our data demonstrate that the anticonvulsant 1-(2-(((diphenylmethylene)amino)oxy)ethyl)-1,2,5,6-tetrahydro-3-pyridinecarboxylic acid hydrochloride (NNC-711), a GABA uptake inhibitor, dose and time dependently diminished increases in extracellular dopamine following acute cocaine challenge. Furthermore, we demonstrated that cyclized analogue of vigabatrin, a competitive reversible GABA-transaminase inhibitor, is a more potent inhibitor of cocaine-induced dopamine increase than vigabatrin. Our data suggest that in addition to irreversible inhibition of GABA transaminase, inhibition of GABA uptake represent another potentially effective, indirect strategy for the treatment of cocaine abuse.

Doses of GBR12909 that suppress cocaine self-administration in non-human primates substantially occupy dopamine transporters as measured by [11C] WIN35,428 PET scans.

GBR12909 (GBR) is a high-affinity, selective, and long-acting inhibitor of dopamine (DA) uptake that produces a persistent and noncompetitive blockade of DA transporters and substantially reduces cocaine-induced increases in extracellular DA in the nucleus accumbens of rats. Prior studies showed that intravenous infusion of GBR to Rhesus monkeys selectively reduced (1 mg/kg) and eliminated (3 mg/kg) cocaine self-administration. This study tested the hypothesis that doses of GBR that reduce cocaine self-administration in nonhuman primates produce significant occupation of DA transporters. DA transporters were quantitated in two baboons using [11C]WIN35,428 and positron emission tomography (PET). Each baboon underwent paired control/blocked PET scans (performed on three separate study days, 3-4 weeks apart). On the first scan the baboon received saline (3 ml/kg) 90 minutes before the injection of the radiotracer. GBR (1 mg/kg i.v.) was infused 90 minutes before the second [11C]WIN 35,428 study. The same experimental design was repeated with GBR doses of 3 and 10 mg/kg, respectively. Doses of 1 (n = 2), 3 mg/kg (n = 2), and 10 mg/kg (n = 2) reduced binding potential by 26, 53, and 72%, respectively. GBR was well tolerated in all baboons. These results demonstrate that doses of GBR that suppress cocaine self-administration in nonhuman primates also produce high occupancy of the DA transporter. These data strongly suggest that occupancy for the DA transporter by GBR explains its ability to attenuate cocaine-induced increases in extracellular DA and to suppress cocaine self-administration. Moreover, these data suggest that experimental human studies of orally administered GBR to test the DA hypothesis of cocaine addiction should use doses that produce at least 70% occupancy of the DA transporter.

Serotonin 5-HT 2C agonists selectively inhibit morphine-induced dopamine efflux in the nucleus accumbens

In vivo microdialysis was used to compare the effects of serotonergic drugs on morphine- and cocaine-induced increases in extracellular dopamine (DA) concentrations in the rat nucleus accumbens (NAc). Systemic administration of the 5-HT 2A/2C receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (2.5 mg/kg, s.c.) prevented the increase in extracellular DA in the NAc produced by morphine (5 mg/kg, i.p.). In contrast, this dose of DOI had no effect on the ability of cocaine (10 mg/kg, i.p.) to increase extracellular DA concentrations in the NAc. A preferential 5-HT 2C receptor agonist, 6-chloro-2-[1-piperazinyl]-pyrazine (MK-212, 5 mg/kg, s.c.) also inhibited morphine-induced increases in extracellular DA concentrations in the NAc. Pretreatment of rats with the selective 5-HT 2A antagonist, amperozide, had no effect on morphine-induced elevation of NAc DA concentrations. In order to determine if inhibition of the firing of 5-HT neurons contributes to the serotonin agonist-mediated inhibition of morphine-induced accumbens DA release, rats were pretreated with the 5-HT 1A agonist, 8-OHDPAT. At a dose of 100 μg/kg (s.c.), 8-OHDPAT did not interfere with morphine's ability to increase DA concentrations in the NAc. These results suggest that the activation of 5-HT 2C receptors selectively inhibits morphine-induced DA release in the NAc in a manner which is independent of the inhibition of 5-HT neurons.

Effects of pharmacologic increases in brain GABA levels on cocaine-induced changes in extracellular dopamine.

Cocaine-induced increases in extracellular dopamine (DA) concentrations were measured using in vivo microdialysis techniques in the nucleus accumbens (NACC) of freely moving rats. In control animals, cocaine increased extracellular DA concentrations approximately 482% 60 min following administration, returning to baseline values 200 min later. When administered 2 h following an acute dose of gamma-vinyl-GABA (GVG, Vigabatrin), cocaine-induced increases in extracellular DA were reduced to approximately 365% of baseline values. Chronic GVG administration further dose-dependently attenuated the effects of cocaine but did not alter the rate of increase or the rate of return to baseline values. These results indicate that GVG, a drug that increases brain GABA concentrations, is effective in attenuating the effect of cocaine on NACC DA. Taken with our earlier findings, these results support the targeting of brain GABAergic systems as a potentially effective pharmacologic treatment strategy for cocaine addiction.

Gamma-vinyl GABA attenuates cocaine-induced lowering of brain stimulation reward thresholds.

Gamma-vinyl GABA (GVG, also referred to as vigabatrin), an irreversible inhibitor of GABA transaminase (GABA-T), raises levels of GABA in nerve terminals, inhibits striatal dopamine release, and attenuates cocaine-induced increases in extracellular dopamine in the striatum and nucleus accumbens. In order to determine the action of GVG on dopamine-mediated reward, we examined its effects on the threshold for rewarding brain stimulation in male F-344 rats. GVG dose-dependently raised brain stimulation reward (BSR) thresholds at doses of 200, 300, and 400 mg/kg without significant effects on motor performance as measured by response latencies. In order to determine if GVG had similar modulatory effects on cocaine-induced lowering of BSR thresholds, the effective doses of GVG were co-administered with 2.5 and 5.0 mg/kg cocaine, doses that significantly lower BSR thresholds. The 400 mg/kg dose of GVG significantly blocked the lowering of thresholds seen at each dose of cocaine. Cocaine in combination with 200 or 300 mg/kg GVG, doses of GVG that significantly raise BSR thresholds, resulted in thresholds not significantly different from those obtained with cocaine alone. These data demonstrate that, at the doses tested, GVG is more effective at modulating basal reward thresholds that at modulating thresholds lowered by cocaine, implying that as dopaminergic activity increases, GABAergic activity must also increase in order to exert its inhibitory influence on dopaminergic activity.

Intra-Ventral Tegmental Area Administration of H7 Delays, But Does Not Prevent the Development of Cocaine-Induced Sensitization

Previous studies have suggested that increased protein kinase C activity in the ventral tegmental area (VTA) may play a role in the acute and development of the sensitized behavioral responses to cocaine. The present study was conducted to further characterize the role of protein kinases in the development of sensitization. Animals received injections of saline or the nonspecific protein kinase inhibitor H7 into the VTA before each of their four daily systemic injections of saline or cocaine. Animals were tested for sensitization with a challenge injection of systemic cocaine after a withdrawal period of 24 h or 1 week. Tests for sensitization included monitoring cocaine-induced motor activity and/or dopamine concentrations in the nucleus accumbens, as measured by in vivo microdialysis. Pretreatment with H7 in the VTA attenuated the acute motor stimulant response to cocaine as well as the cocaine-induced increase in extracellular dopamine in the nucleus accumbens. In addition, the augmented increase in dopamine in the nucleus accumbens of cocaine-sensitized animals was prevented in animals pretreated with H7 before each of their daily cocaine injections, when tested after a 24 h withdrawal. However, when tested after a 1 week withdrawal, animals demonstrated sensitization to both the cocaine-induced increase in motor activity and the cocaine-induced increase in dopamine in the nucleus accumbens regardless of whether they received intra-VTA saline or H7 before each of their daily cocaine injections. These data suggest that injection of a protein kinase inhibitor into the VTA delays, but does not prevent the development of cocaine-induced behavioral sensitization.

Naloxone reduces the neurochemical and behavioral effects of amphetamine but not those of cocaine

The specific opioid receptor antagonist naloxone modifies the effects of amphetamine in a wide variety of behavioral paradigms. Naloxone also attenuates the amphetamine-induced increase in extracellular dopamine in the brain of rats. Therefore, these experiments were designed to replicate the neurochemical and behavioral interactions between naloxone and amphetamine, and to extend these observations to interactions between naloxone and cocaine. Microdialysis was performed on adult male rats of Sprague-Dawley descent. Rats were pretreated with a subcutaneous injection of 5.0 mg/kg naloxone or vehicle, which was followed 30 min later by cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, 6.4 mg/kg) or intraperitoneal cocaine (0, 3, 10, 30, 56 mg/kg) at 30 min intervals. The microdialysis probes were perfused at a flow rate of 0.6 μ1/min with artificial cerebrospinal fluid. Dialysate samples were collected every 10 min from either the nucleus accumbens or striatum and analyzed for dopamine content by high-performance liquid chromatography (HPLC). Locomotor activity (photobeam breaks) was monitored simultaneously with the collection of dialysate samples. Amphetamine and cocaine dose-dependently increased extracellular dopamine in both the nucleus accumbens and striatum. Naloxone pretreatment significantly reduced the amphetamine-induced increase in extracellular dopamine in both brain regions and also attenuated the increase in locomotor activity elicited by amphetamine. Naloxone pretreatment had no effect, however, on the cocaine-induced increase in extracellular dopamine or locomotor activity. These findings suggest that endogenous opioid systems play a role in mediating the neurochemical and behavioral effects of amphetamine, but not those of cocaine.

U50,488, a κ opioid receptor agonist, attenuates cocaine-induced increases in extracellular dopamine in the nucleus accumbens of rats

Because an increase in extracellular levels of dopamine in the nucleus accumbens has been associated with the reinforcing effects of addictive drugs, we investigated whether U50,488, a selective κ opioid receptor agonist, would alter cocaine-induced increases in extracellular dopamine in the nucleus accumbens using in vivo microdialysis in awake and freely moving rats. Cocaine (20 mg/kg i.p.) produced a 10-fold increase in extracellular dopamine levels. Pretreatment (20 min beforehand) with U50,488 (10 mg/kg i.p.), which alone caused a modest decrease in dopamine levels, produced a 50% decrease in the effect of cocaine on dopamine levels. This attenuation was completely reversed by administration of a κ opioid receptor antagonist, nor-binaltorphimine (10 mg/kg s.c.), 20 min before the agonist challenge. Treatment with nor-binaltorphimine alone induced a brief increase in dopamine levels. These findings indicate that activation of κ receptors attenuates cocaine's effects and that κ opioid receptor agonists may, therefore, be useful as functional cocaine antagonists.