The speed of cocaine delivery determines the subsequent motivation to self-administer the drug

Attentional ERPs in consumers of smoked and insufflated cocaine associated with neuropsychological performance

BACKGROUND

Cocaine consumption is associated with reduced attentional event-related potentials (ERPs), namely P3a and P3b, indicating bottom-up and top-down deficits respectively. At cognitive level, these impairments are larger for faster routes of administration (e.g., smoked cocaine [SC]) than slower routes (e.g., insufflated cocaine [IC]). Here we assess these ERPs considering the route of cocaine administration. We hypothesized that SC dependent (SCD) would exhibit reduced amplitude of the P3a, while both SCD and IC dependent (ICD) would show reduced amplitude of the P3b.

METHODS

We examined 25 SCD, 22 ICD matched by poly-consumption profiles, and 25 controls matched by demographic variables. We combined EEG data from the Global-Local task with behavioral data from attentional cognitive tasks.

RESULTS

At the behavioral level, SCD exhibited attentional deficits in both bottom-up and top-down processes, while ICD only showed a tendency for top-down deficits. The amplitude of P3a and P3b was lower in Users groups. We observed subtle route-based differences, with larger differences in the P3a for SCD and in the P3b for ICD. Neurophysiological and behavioral data converged, with the P3a associated to bottom-up performance and P3b to top-down.

CONCLUSIONS

Different routes of administration lead to distinct attentional neurocognitive profiles. Specifically, SCD showed greater attentional impairment, mainly at bottom-up/P3a, while ICD showed a trend of top-down/P3b deficits. These findings emphasize the crucial role of considering the route of administration in both clinical and research settings and support the use of attentional ERPs as valid measures for assessing attentional deficits in substance Dependence.

Brain connectivity changes to fast versus slow dopamine increases

The rewarding effects of stimulant drugs such as methylphenidate (MP) depend crucially on how fast they raise dopamine in the brain. Yet how the rate of drug-induced dopamine increases impacts brain network communication remains unresolved. We manipulated route of MP administration to generate fast versus slow dopamine increases. We hypothesized that fast versus slow dopamine increases would result in a differential pattern of global brain connectivity (GBC) in association with regional levels of dopamine D1 receptors, which are critical for drug reward. Twenty healthy adults received MP intravenously (0.5 mg/kg; fast dopamine increases) and orally (60 mg; slow dopamine increases) during simultaneous [11C]raclopride PET-fMRI scans (double-blind, placebo-controlled). We tested how GBC was temporally associated with slow and fast dopamine increases on a minute-to-minute basis. Connectivity patterns were strikingly different for slow versus fast dopamine increases, and whole-brain spatial patterns were negatively correlated with one another (rho = -0.54, pspin < 0.001). GBC showed "fast>slow" associations in dorsal prefrontal cortex, insula, posterior thalamus and brainstem, caudate and precuneus; and "slow>fast" associations in ventral striatum, orbitofrontal cortex, and frontopolar cortex (pFDR < 0.05). "Fast>slow" GBC patterns showed significant spatial correspondence with D1 receptor availability (estimated via normative maps of [11C]SCH23390 binding; rho = 0.22, pspin < 0.05). Further, hippocampal GBC to fast dopamine increases was significantly negatively correlated with self-reported 'high' ratings to intravenous MP across individuals (r(19) = -0.68, pbonferroni = 0.015). Different routes of MP administration produce divergent patterns of brain connectivity. Fast dopamine increases are uniquely associated with connectivity patterns that have relevance for the subjective experience of drug reward.

Neural circuit selective for fast but not slow dopamine increases in drug reward

The faster a drug enters the brain, the greater its addictive potential, yet the brain circuits underlying the rate dependency to drug reward remain unresolved. With simultaneous PET-fMRI we linked dynamics of dopamine signaling, brain activity/connectivity, and self-reported 'high' in 20 adults receiving methylphenidate orally (results in slow delivery) and intravenously (results in fast delivery) (trial NCT03326245). We estimated speed of striatal dopamine increases to oral and IV methylphenidate and then tested where brain activity was associated with slow and fast dopamine dynamics (primary endpoint). We then tested whether these brain circuits were temporally associated with individual 'high' ratings to methylphenidate (secondary endpoint). A corticostriatal circuit comprising the dorsal anterior cingulate cortex and insula and their connections with dorsal caudate was activated by fast (but not slow) dopamine increases and paralleled 'high' ratings. These data provide evidence in humans for a link between dACC/insula activation and fast but not slow dopamine increases and document a critical role of the salience network in drug reward.

Intermittent access training produces greater motivation for a non-drug reinforcer than long access training

It has recently been proposed that the intermittent access (IntA) drug self-administration procedure better produces behavioral changes relevant to addiction than the long access (LgA) procedure. In this version of the IntA procedure, the drug is made available for a 5-min period during each half hour of a 6-h session. In contrast, on the LgA procedure, the drug is available continuously for 6 h. Previous studies have found that IntA drug self-administration produces greater drug motivation, measured by increased progressive ratio breakpoints, than LgA self-administration. It has been hypothesized that this effect is due to the rapid, "spiking" brain levels of the drug, and consequent neuroadaptations, experienced by rats during IntA sessions. However, no study has compared the effects of IntA versus LgA training on reinforcer motivation when using a non-drug reinforcer. The present study compared motivation for a saccharin reinforcer after IntA or LgA training. In Experiment 1, separate groups of rats lever-pressed for saccharin on the IntA or LgA procedures. In Experiment 2, a within-subjects design was used where rats pressed one lever on the IntA procedure and another lever on the LgA procedure for saccharin. In both experiments, IntA training produced greater breakpoints than LgA training. As no drug was used here, spiking drug levels could not have been responsible for the increased saccharin motivation observed after IntA training. Instead, it is proposed that differences in stimulus-reinforcer associations learned during IntA versus LgA training may be responsible for the effect. Future research is needed to determine the extent to which such learning factors may contribute to the increased motivation observed after IntA training with drug reinforcers.

Impact of delivery rate on the acute response to intravenous nicotine: A human laboratory study with implications for regulatory science

Faster delivery rate enhances the abuse potential of drugs of abuse, yet systematic studies on the impact of delivery rate on the acute effects of nicotine in humans are lacking. Using an intravenous (IV) nicotine infusion procedure that allows precise control of rate of delivery, we examined the impact of nicotine delivery rate on the positive subjective drug effects, smoking urges, withdrawal, heart rate, blood pressure and attention function in smokers. Twenty-four male and female (ages 21-35) dependent smokers attended five experimental sessions, following overnight abstinence from smoking. Using a crossover design, participants attended five sessions, where they were assigned to a random sequence of saline infusion or 1 mg nicotine delivered over 1, 2.5, 5 or 10 min at rates of 1, 0.4, 0.2 or 0.1 mg/min, respectively. The positive subjective effects of nicotine were most robust under the two faster delivery rate conditions, 1- and 0.4-mg nicotine/min. In contrast, all nicotine delivery rates were equally more effective than saline in alleviating urges to smoke. Likewise, nicotine-induced heart rate increases did not vary with the rate of nicotine delivery. Lastly, the cognitive enhancing effects of nicotine were observed only under the two slowest delivery rate conditions-0.1- and 0.2-mg nicotine/min. Collectively, these findings support the critical role of delivery rate in optimizing nicotine's abuse potential versus potential therapeutic effects and have timely implications for developing novel therapeutics for nicotine dependence, as well as for tobacco regulatory science.

The Critical Role of Peripheral Targets in Triggering Rapid Neural Effects of Intravenous Cocaine

Direct interaction of cocaine with centrally located monoamine transporters is the primary mechanism underlying its reinforcing properties. It is also often assumed that this drug action is responsible for all the physiological and behavioral effects of this drug. The goal of this review is to challenge this basic mechanism and demonstrate the importance of peripheral actions of cocaine in inducing its initial, rapid neural effects. The use of high-resolution electrophysiological, neurochemical and physiological techniques revealed that the effects of intravenous cocaine at behaviorally relevant doses are exceptionally rapid and transient correlating with strong, quick, and transient increases in blood cocaine levels. Some of these effects are mimicked by cocaine-methiodide, a cocaine analog that cannot cross the blood-brain barrier and they are resistant to dopamine (DA) receptor blockade. Therefore, it appears that rapid neural effects of cocaine result from its direct interaction with receptive sites on afferents of sensory nerves densely innervating blood vessels. This interaction creates a rapid neural signal to the CNS that results in generalized neural activation and subsequent changes in different physiological parameters. This drug's action appears to be independent from cocaine's action on central neurons, which requires a definite time to occur and induce neural and physiological effects with longer latencies and durations. The co-existence in the same drug on two timely distinct actions with their subsequent interaction in the CNS could explain consistent changes in physiological and behavioral effects of cocaine following their repeated use, playing a role in the development of drug-seeking and drug-taking behavior.

Sex differences in cocaine self-administration behaviour under long access versus intermittent access conditions

Studies in humans suggest that women progress more rapidly from initial cocaine use to addiction. Similarly, female rats can show more incentive motivation for cocaine than male rats do. Most preclinical studies on this issue have used self-administration procedures that provide continuous cocaine access during each session ("long-access" or LgA and "short-access"). However, intermittent access (IntA) cocaine self-administration better models the intermittency of human cocaine use. Here, we compared cocaine use in female and male rats that received ten, daily 6-hour LgA or IntA sessions. Cocaine intake was greatest under LgA, and female LgA rats escalated their intake. Only IntA rats (both sexes) developed locomotor sensitization to self-administered cocaine, and sensitization was greatest in females. Five and 25 days after the last self-administration session, we quantified responding for cocaine (0.083-0.75 mg/kg/infusion) under a progressive ratio (PR) schedule, a measure of motivation for drug. Across conditions, females earned more cocaine infusions than males under the PR schedule. Across sexes, IntA rats earned more infusions than LgA rats, even though IntA rats had previously taken much less cocaine. Cumulative cocaine intake significantly predicted responding for cocaine under the PR schedule in male LgA rats only. In IntA rats, the extent of locomotor sensitization significantly predicted responding under the PR schedule. Thus, LgA might be appropriate to study sex differences in cocaine intake, whereas IntA might be best suited to study sex differences in sensitization-related neuroadaptations involved in cocaine addiction. This has implications for modelling distinct features of cocaine addiction in preclinical studies.

Taking Rapid and Intermittent Cocaine Infusions Enhances Both Incentive Motivation for the Drug and Cocaine-induced Gene Regulation in Corticostriatal Regions

A goal in addiction research is to distinguish forms of neuroplasticity that are involved in the transition to addiction from those involved in mere drug taking. Animal models of drug self-administration are essential in this context. Here, we compared in male rats two cocaine self-administration procedures that differ in the extent to which they evoke addiction-like behaviours. We measured both incentive motivation for cocaine using progressive ratio procedures, and cocaine-induced c-fos mRNA expression, a marker of neuronal activity. Rats self-administered intravenous cocaine (0.25 mg/kg/infusion) for seven daily 6-hour sessions. One group had intermittent access (IntA; 6 minutes ON, 26 min OFF × 12) to rapid infusions (delivered over 5 s). This models the temporal kinetics of human cocaine use and produces robust addiction-like behaviour. The other group had Long access (LgA) to slower infusions (90 s). This produces high levels of intake without promoting robust addiction-like behaviour. LgA-90 s rats took twice as much cocaine as IntA-5 s rats did, but IntA-5 s rats showed greater incentive motivation for the drug. Following a final self-administration session, we quantified c-fos mRNA expression in corticostriatal regions. Compared to LgA-90 s rats, IntA-5 s rats had more cocaine-induced c-fos mRNA in the orbitofrontal and prelimbic cortices and the caudate-putamen. Thus, a cocaine self-administration procedure (intermittent intake of rapid infusions) that promotes increased incentive motivation for the drug also enhances cocaine-induced gene regulation in corticostriatal regions. This suggests that increased drug-induced recruitment of these regions could contribute to the neural and behavioural plasticity underlying the transition to addiction.

Role of the orbitofrontal cortex and the dorsal striatum in incentive motivation for cocaine

Drug addiction involves increased incentive motivation for drug. Intermittent access to cocaine (IntA; 5-6 minutes ON, 25-26 minutes OFF, for 5-6 hours/session) enhances motivation to take the drug. The orbitofrontal cortex (OFC) and the dorsal striatum (DS) are part of a corticolimbic circuit that encodes incentive value and regulates reward-directed behaviour. We predicted that inactivation of the OFC, DS or both suppresses incentive motivation for cocaine after IntA experience. Male Wistar rats had IntA to cocaine (0.25 mg/kg/infusion) for 10 sessions. The rats developed a 'loading' pattern of intake, taking most of their cocaine in the first minute of each drug-available period. They also developed psychomotor sensitization to self-administered cocaine. We then measured incentive motivation for cocaine using a progressive ratio schedule of reinforcement (PR). Before some PR sessions, rats received microinfusions of a baclofen/muscimol cocktail (0.3 and 0.03 nmol/hemisphere, respectively, or saline) to temporarily inactivate the OFC or DS, or to disconnect the two regions. None of these treatments changed spontaneous locomotion in cocaine-naïve rats. However, both baclofen/muscimol and saline infusions influenced cocaine self-administration behaviour. Infusing baclofen/muscimol or saline into the OFC or into the OFC and contralateral DS decreased responding for cocaine under PR, with baclofen/muscimol and saline having similar effects, except that only OFC-DS disconnection with baclofen/muscimol slowed the pace of cocaine intake. Baclofen/muscimol or saline into the DS also reduced responding for cocaine under PR, but baclofen/muscimol was more effective. We conclude that neuronal activity in the OFC and DS might regulate incentive motivation for cocaine.

Intermittent intake of rapid cocaine injections promotes the risk of relapse and increases mesocorticolimbic BDNF levels during abstinence

Cocaine is thought to be more addictive when it reaches the brain rapidly. We predicted that variation in the speed of cocaine delivery influences the likelihood of addiction in part by determining the risk of relapse after abstinence. Under an intermittent-access schedule, rats pressed a lever for rapid (injected over 5 s) or slower (90 s) intravenous cocaine injections (0.5 mg/kg/injection). Control rats self-administered food pellets. A tone-light cue accompanied each self-administered reward. The 5s- and 90s-rats took a similar average amount of cocaine. One or 45 days after withdrawal from cocaine/forced abstinence, lever-pressing behaviour was extinguished during a 6-h session. Immediately thereafter, cue- or cocaine (10 mg/kg, i.p.)-induced reinstatement was assessed for 1 h. One or 45 days after withdrawal, only 5s-rats showed significant cocaine-induced reinstatement of reward-seeking behaviour. In both cocaine groups, cue-induced reinstatement behaviour was more pronounced after 45 days than after 1 day of withdrawal from cocaine, indicating incubation of conditioned drug craving. However, cue-induced reinstatement after extended abstinence was greatest in the 5s-rats. Brain-derived neurotrophic factor (BDNF) activity in the brain regulates reinstatement behaviour. Thus, 24 h after reinstatement tests, we measured BDNF protein concentrations in mesocorticolimbic regions. Only 5s-rats showed time-dependent increases in BDNF concentrations in the prelimbic cortex, nucleus accumbens core and ventral tegmental area after withdrawal from cocaine (day 45 > day 1). Thus, rapidly rising brain cocaine levels might facilitate addiction by evoking changes in the brain that intensify drug craving after abstinence, and these changes persist long after the last bout of cocaine use.

Characterization of locomotor response to psychostimulants in the parthenogenetic marbled crayfish (Procambarus fallax forma virginalis): A promising model for studying the neural and molecular mechanisms of drug addiction

Although scientific research using mammalian models has made great strides in uncovering the enigmatic neural and molecular mechanisms orchestrating the state of drug addiction, a complete understanding has thus far eluded researchers. The complexity of the task has led to the use of invertebrate model systems to complement the research of drug-induced reward in mammalian systems. Invertebrates, such as crayfish, offer excellent model systems to help reveal the underlying mechanisms of drug addiction as they retain the ancestral neural reward circuit that is evolutionarily conserved across taxa, and they possess relatively few, large neurons, laid out in an accessible, modularly organized nervous system. Crayfish offer the benefits of delineated developmental life stages, a large body size suitable for a variety of experimental methods, and stereotyped behaviors. Unique among crayfish is the parthenogenetic marbled crayfish (Procambarus fallax forma virginalis), a species of asexually reproducing, genetically identical clones. With the benefits of reduced individual variation, high fecundity, and easy lab husbandry, the marbled crayfish would make a particularly powerful addition to the animal model repertoire. Here we characterize the locomotor response of juvenile P. f. f. virginalis exposed to the psychostimulant, d-amphetamine sulfate. Custom video-tracking software was used to record the movement patterns of juveniles exposed to water infused with varying concentrations of d-amphetamine sulfate. ANOVA demonstrated that crayfish locomotion was significantly impacted by drug concentration. These psychostimulant effects provide the foundation of P. f. f. virginalis as a model for parsing the neural and molecular mechanisms of drug addiction.

Varying the rate of intravenous cocaine infusion influences the temporal dynamics of both drug and dopamine concentrations in the striatum

The faster drugs of abuse reach the brain, the greater is the risk of addiction. Even small differences in the rate of drug delivery can influence outcome. Infusing cocaine intravenously over 5 vs. 90-100 s promotes sensitization to the psychomotor and incentive motivational effects of the drug and preferentially recruits mesocorticolimbic regions. It remains unclear whether these effects are due to differences in how fast and/or how much drug reaches the brain. Here, we predicted that varying the rate of intravenous cocaine infusion between 5 and 90 s produces different rates of rise of brain drug concentrations, while producing similar peak concentrations. Freely moving male Wistar rats received acute intravenous cocaine infusions (2.0 mg/kg/infusion) over 5, 45 and 90 s. We measured cocaine concentrations in the dorsal striatum using rapid-sampling microdialysis (1 sample/min) and high-performance liquid chromatography-tandem mass spectrometry. We also measured extracellular concentrations of dopamine and other neurochemicals. Regardless of infusion rate, acute cocaine did not change concentrations of non-dopaminergic neurochemicals. Infusion rate did not significantly influence peak concentrations of cocaine or dopamine, but concentrations increased faster following 5-s infusions. We also assessed psychomotor activity as a function of cocaine infusion rate. Infusion rate did not significantly influence total locomotion, but locomotion increased earlier following 5-s infusions. Thus, small differences in the rate of cocaine delivery influence both the rate of rise of drug and dopamine concentrations, and psychomotor activity. A faster rate of rise of drug and dopamine concentrations might be an important issue in making rapidly delivered cocaine more addictive.

High and escalating levels of cocaine intake are dissociable from subsequent incentive motivation for the drug in rats

RATIONALE

Taking high and increasing amounts of cocaine is thought to be necessary for the development of addiction. Consequently, a widely used animal model of drug self-administration involves giving animals continuous drug access during long sessions (LgA), as this produces high and escalating levels of intake. However, human cocaine addicts likely use the drug with an intermittent rather than continuous pattern, producing spiking brain cocaine levels.

OBJECTIVES

Using an intermittent-access (IntA) cocaine self-administration procedure in rats, we studied the relationship between escalation of cocaine intake and later incentive motivation for the drug, as measured by responding under a progressive ratio schedule of cocaine reinforcement.

RESULTS

First, under IntA, rats escalated their cocaine use both within and between sessions. However, escalation did not predict later incentive motivation for the drug. Second, incentive motivation for cocaine was similar in IntA-rats limited to low- and non-escalating levels of drug intake (IntA-Lim) and in IntA-rats that took high and escalating levels of drug. Finally, IntA-Lim rats took much less cocaine than rats given continuous drug access during each self-administration session (LgA-rats). However, IntA-Lim rats later responded more for cocaine under a progressive ratio schedule of reinforcement.

CONCLUSIONS

Taking large and escalating quantities of cocaine does not appear necessary to increase incentive motivation for the drug. Taking cocaine in an intermittent pattern-even in small amounts-is more effective in producing this addiction-relevant change. Thus, beyond the amount of drug taken, the temporal kinetics of drug use predict change in drug use over time.

Individual differences in timing of peak positive subjective responses to d-amphetamine: Relationship to pharmacokinetics and physiology

Rate of delivery of psychostimulants has been associated with their positive euphoric effects and potential addiction liability. However, information on individual differences in onset of d-amphetamine's effects remains scarce. We examined individual differences in the time to peak subjective and physiological effects and the pharmacokinetics/pharmacodynamics of oral d-amphetamine. We considered two independent studies that used different dosing regimens where subjects completed the drug effects questionnaire at multiple time points post d-amphetamine. Based on the observation of distinct individual differences in time course of drug effects questionnaire "feel", "high", and "like" ratings (DEQH+L+F) in Study 1, subjects in both studies were categorized as early peak responders (peak within 60 minutes), late peak responders (peak > 60 minutes) or nonresponders; 20-25% of participants were categorized as early peak responders, 50-55% as late peak responders and 20-30% as nonresponders. Physiological (both studies) and plasma d-amphetamine (Study 1) were compared among these groups. Early peak responders exhibited an earlier rise in plasma d-amphetamine levels and more sustained elevation in heart rate compared to late peak responders. The present data illustrate the presence of significant individual differences in the temporal pattern of responses to oral d-amphetamine, which may contribute to heightened abuse potential.

Blood-brain and retinal barriers show dissimilar ABC transporter impacts and concealed effect of P-glycoprotein on a novel verapamil influx carrier

BACKGROUND AND PURPOSE

The respective impact and interplay between ABC (P-glycoprotein/P-gp/Abcb1a, BCRP/ABCG2, MRP/ABCC) and SLC transporter functions at the blood-brain barrier (BBB) and blood-retinal barriers (BRB) are incompletely understood.

EXPERIMENTAL APPROACH

We measured the initial cerebral and retinal distribution of selected ABC substrates by in situ carotid perfusion using P-gp/Bcrp knockout mice and chemical ABC/SLC modulation strategies. P-gp, Bcrp, Mrp1 and Mrp4 were studied by confocal retina imaging.

KEY RESULTS

Chemical or physical disruption of P-gp increased [(3) H]-verapamil transport by ~10-fold at the BBB and ~1.5-fold at the BRB. [(3) H]-Verapamil transport involved influx-mediated by an organic cation clonidine-sensitive/diphenhydramine-sensitive proton antiporter at both barriers; this effect was unmasked when P-gp was partially or fully inhibited/disrupted at the BBB. Studies of [(3) H]-mitoxantrone and [(3) H]-zidovudine transport suggested, respectively, that Bcrp efflux was less involved at the BRB than BBB, whereas Mrps were significantly and similarly involved at both barriers. Confocal imaging showed that P-gp and Bcrp were expressed in intra-retinal vessels (inner BRB/iBRB) but absent from the blood/basal membrane of cells of the retinal pigment epithelium (outer BRB/oBRB/RPE) where, in contrast, Mrp1 and Mrp4 were localized.

CONCLUSIONS AND IMPLICATIONS

P-gp, Bcrp, Mrp1 and Mrp4 are differentially expressed at the outer and inner BRB, resulting in an altered ability to limit substrate distribution at the retina as compared with the BBB. [(3) H]-Verapamil distribution is not P-gp-specific and involves a proton antiporter at both the BBB and BRB. However, this transport is concealed by P-gp at the BBB, but not at the BRB, where P-gp activity is reduced.

Cocaine activates Rac1 to control structural and behavioral plasticity in caudate putamen

Repeated exposure to cocaine was previously found to cause sensitized behavioral responses and structural remodeling on medium spiny neurons of the nucleus accumbens (NAc) and caudate putamen (CPu). Rac1 has emerged as a key integrator of environmental cues that regulates dendritic cytoskeletons. In this study, we investigated the role of Rac1 in cocaine-induced dendritic and behavioral plasticity in the CPu. We found that Rac1 activation was reduced in the NAc but increased in the CPu following repeated cocaine treatment. Inhibition of Rac1 activity by a Rac1-specific inhibitor NSC23766, overexpression of a dominant negative mutant of Rac1 (T17N-Rac1) or local knockout of Rac1 attenuated the cocaine-induced increase in dendrites and spine density in the CPu, whereas overexpression of a constitutively active Rac1 exert the opposite effect. Moreover, NSC23766 reversed the increased number of asymmetric spine synapses in the CPu following chronic cocaine exposure. Downregulation of Rac1 activity likewise attenuates behavioral reward responses to cocaine exposure, with activation of Rac1 producing the opposite effect. Thus, Rac1 signaling is differentially regulated in the NAc and CPu after repeated cocaine treatment, and induction of Rac1 activation in the CPu is important for cocaine exposure-induced dendritic remodeling and behavioral plasticity.

Carrier-mediated cocaine transport at the blood-brain barrier as a putative mechanism in addiction liability

BACKGROUND

The rate of entry of cocaine into the brain is a critical factor that influences neuronal plasticity and the development of cocaine addiction. Until now, passive diffusion has been considered the unique mechanism known by which cocaine crosses the blood-brain barrier.

METHODS

We reassessed mechanisms of transport of cocaine at the blood-brain barrier using a human cerebral capillary endothelial cell line (hCMEC/D3) and in situ mouse carotid perfusion.

RESULTS

Both in vivo and in vitro cocaine transport studies demonstrated the coexistence of a carrier-mediated process with passive diffusion. At pharmacological exposure level, passive diffusion of cocaine accounted for only 22.5% of the total cocaine influx in mice and 5.9% in hCMEC/D3 cells, whereas the carrier-mediated influx rate was 3.4 times greater than its passive diffusion rate in vivo. The functional identification of this carrier-mediated transport demonstrated the involvement of a proton antiporter that shared the properties of the previously characterized clonidine and nicotine transporter. The functionnal characterization suggests that the solute carrier (SLC) transporters Oct (Slc22a1-3), Mate (Slc47a1) and Octn (Slc22a4-5) are not involved in the cocaine transport in vivo and in vitro. Diphenhydramine, heroin, tramadol, cocaethylene, and norcocaine all strongly inhibited cocaine transport, unlike benzoylecgonine. Trans-stimulation studies indicated that diphenhydramine, nicotine, 3,4-methylenedioxyamphetamine (ecstasy) and the cathinone compound 3,4-methylenedioxypyrovalerone (MDPV) were also substrates of the cocaine transporter.

CONCLUSIONS

Cocaine transport at the BBB involves a proton-antiporter flux that is quantitatively much more important than its passive diffusion. The molecular identification and characterization of this transporter will provide new tools to understand its role in addictive mechanisms.

The self-administration of rapidly delivered cocaine promotes increased motivation to take the drug: contributions of prior levels of operant responding and cocaine intake

RATIONALE

Rapid drug delivery to the brain might increase the risk for developing addiction. In rats, increasing the speed of intravenous cocaine delivery (5 vs. 90 s) increases drug intake and the subsequent motivation to self-administer cocaine. Increased motivation for cocaine could result not only from more extensive prior drug intake and operant responding for drug, but also from neuroplasticity evoked by rapid drug uptake.

OBJECTIVE

We determined the contributions of prior drug intake and operant responding to the increased motivation for cocaine evoked by rapid delivery. We also investigated the effects of cocaine delivery speed on corticostriatal expression of brain-derived neurotrophic factor (BDNF) and tropomyosin receptor kinase B (TrkB) mRNA.

METHODS

Rats self-administered cocaine (0.25 mg/kg/infusion) delivered over 5 or 90 s during short-access (1 h/session; ShA) or long-access (6 h; LgA) sessions. Motivation for cocaine was then assessed by measuring responding under a progressive ratio schedule of reinforcement. Next, BDNF and TrkB mRNA levels were measured in 5- and 90-s rats.

RESULTS

Five-second ShA and 5-s-LgA rats were more motivated for cocaine than their 90-s counterparts. This effect was dissociable from previous levels of drug intake or of operant responding for cocaine. In parallel, only rats self-administering rapid cocaine injections had altered BDNF and TrkB mRNA levels in corticostriatal regions.

CONCLUSIONS

Rapid drug delivery augments the motivation for cocaine independently of effects on the levels of drug intake or operant responding for drug. We suggest that rapid delivery might increase the motivation for drug by promoting neuroplasticity within reward pathways. This neuroplasticity could involve increased regulation of BDNF/TrkB.