Why does the rapid delivery of drugs to the brain promote addiction?

Caffeine Induces a Stimulant Effect and Increases Dopamine Release in the Nucleus Accumbens Shell Through the Pulmonary Inhalation Route of Administration in Rats

Oral, intraperitoneal, or intravenous have been the common routes of administration used to study the behavioral and neurochemical pharmacology of caffeine, one of the most widely used psychoactive substances worldwide. We have reported that caffeine is an active adulterant frequently found in coca-paste (CP)-seized samples, a highly addictive form of smokable cocaine. The role of caffeine in the psychostimulant and neurochemical effects induced by CP remains under study. No preclinical animal studies have been performed so far to characterize the effects of caffeine when it is administered through the pulmonary inhalation route. Caffeine (10, 25, and 50 mg) was volatilized and rats were exposed to one inhalation session of its vapor. The stimulant effect was automatically recorded and plasmatic levels of caffeine were measured. Caffeine capability (50 mg) to increase extracellular dopamine (DA) levels in nucleus accumbens shell was also studied by in vivo microdialysis in non-anesthetized animals. A dose-dependent stimulant effect induced by volatilized caffeine was observed and this effect was directly related with caffeine plasmatic levels. A significant increase in the extracellular DA was achieved after 50 mg of volatilized caffeine exposure. This is the first report showing pharmacological acute effects of caffeine through the pulmonary inhalation route of administration and suggests that this could be a condition under which caffeine can elevate its weak reinforcing effect and even enhance the psychostimulant effect and abuse liability of smokable adulterated psychostimulant drugs.

Metabotropic Glutamate Receptor and Fragile X Signaling in a Female Model of Escalated Aggression

BACKGROUND

Escalated aggression is a behavioral sign of numerous psychiatric disorders characterized by a loss of control. The neurobiology underlying escalated aggression is unknown and is particularly understudied in females. Research in our laboratory demonstrated that repeated aggressive experience in female hamsters resulted in an escalated response to future aggressive encounters and an increase in dendritic spine density on nucleus accumbens (NAc) neurons. We hypothesized that the activation of group I metabotropic glutamate receptor signaling though the fragile X mental retardation protein (FMRP) pathway may underlie synaptic plasticity associated with aggression escalation.

METHODS

Female hamsters were given five daily aggression tests with or without prior treatment with the metabotropic glutamate receptor 5 (mGluR5) antagonist 2-methyl-6-(phenylethynyl)-pyridine. Following aggression testing, messenger RNA expression and protein levels were measured in the nucleus accumbens for postsynaptic density protein 95 (PSD-95) and SAP90/PSD-95-associated protein 3, as well as the levels of phosphorylated FMRP.

RESULTS

Experience-dependent escalation of aggression in female hamsters depends on activation of mGluR5 receptors. Furthermore, aggressive experience decreases phosphorylation of FMRP in the NAc, which is coupled to a long-term increase in the expression of the synaptic scaffolding proteins PSD-95 and SAP90/PSD-95-associated protein 3. Finally, the experience-dependent increase in PSD-95 is prevented by antagonism of the mGluR5 receptor.

CONCLUSIONS

Activation of the FMRP pathway by group I metabotropic glutamate receptors is involved in regulating synaptic plasticity following aggressive experience. The NAc is a novel target for preclinical studies of the treatment of escalated aggression, with the added benefit that emerging therapeutic approaches are likely to be effective in treating pathologic aggression in both female and male subjects.

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.

[What brings neurobiology to addictions?]

Addictions are multifactorial, and there are no experimental models replicating all aspects of this pathology. The development of animal models reproducing the clinical symptoms of addictions allows significant advances in the knowledge of the neurobiological processes involved in addiction. Preclinical data highlight different neuroadaptations according to the routes of administration, speeds of injection and frequencies of exposure to drugs of abuse. The neuroadaptations induced by an exposure to drugs of abuse follow dynamic processes in time. Despite significant progresses in the knowledge of neurobiology of addictions allowing to propose new therapeutic targets, the passage of new drugs in clinical is often disappointing. The lack of treatment efficacy reported in clinical trials is probably due to a very important heterogeneity of patients with distinct biological and genetic factors, but also with different patterns of consumption that can lead to different neuroadaptations, as clearly observed in preclinical studies.

Predictors of transition to heroin use among initially non-opioid dependent illicit pharmaceutical opioid users: A natural history study

BACKGROUND

Increases in illicit pharmaceutical opioid (PO) use have been associated with risk for transition to heroin use. We identify predictors of transition to heroin use among young, illicit PO users with no history of opioid dependence or heroin use at baseline.

METHODS

Respondent-driven sampling recruited 383 participants; 362 returned for at least one biannual structured interview over 36 months. Cox regression was used to test for associations between lagged predictors and hazard of transition to heroin use. Potential predictors were based on those suggested in the literature. We also computed population attributable risk (PAR) and the rate of heroin transition.

RESULTS

Over 36 months, 27 (7.5%) participants initiated heroin use; all were white, and the rate of heroin initiation was 2.8% per year (95% CI=1.9%-4.1%). Mean length of PO at first reported heroin use was 6.2 years (SD=1.9). Lifetime PO dependence (AHR=2.39, 95% CI=1.07-5.48; PAR=32%, 95% CI=-2% to 64%), early age of PO initiation (AHR=3.08, 95%; CI=1.26-7.47; PAR=30%, 95% CI=2%-59%), using illicit POs to get high but not to self-medicate a health problem (AHR=4.83, 95% CI=2.11-11.0; PAR=38%, 95% CI=12%-65%), and ever using PO non-orally most often (AHR=6.57, 95% CI=2.81-17.2; PAR=63%, 95% CI=31%-86%) were significant predictors.

CONCLUSION

This is one of the first prospective studies to test observations from previous cross-sectional and retrospective research on the relationship between illicit PO use and heroin initiation among young, initially non-opioid dependent PO users. The results provide insights into targets for the design of urgently needed prevention interventions.

Pharmacophore-based discovery of inhibitors of a novel drug/proton antiporter in human brain endothelial hCMEC/D3 cell line

BACKGROUND AND PURPOSE

An influx drug/proton antiporter of unknown structure has been functionally demonstrated at the blood-brain barrier. This transporter, which handles some psychoactive drugs like diphenhydramine, clonidine, oxycodone, nicotine and cocaine, could represent a new pharmacological target in drug addiction therapy. However, at present there are no known drugs/inhibitors that effectively inhibit/modulate this transporter in vivo.

EXPERIMENTAL APPROACH

The FLAPpharm approach was used to establish a pharmacophore model for inhibitors of this transporter. The inhibitory potency of 44 selected compounds was determined against the specific substrate, [(3)H]-clonidine, in the human cerebral endothelial cell line hCMEC/D3 and ranked as good, medium, weak or non-inhibitor.

KEY RESULTS

The pharmacophore model obtained was used as a template to screen xenobiotic and endogenous compounds from databases [Specs, Recon2, Human Metabolome Database (HMDB), human intestinal transporter database], and hypothetical candidates were tested in vitro to determine their inhibitory capacity with [(3)H]-clonidine. According to the transporter database, 80% of the proton antiporter inhibitor candidates could inhibit P-glycoprotein/MDR1/ABCB1 and specificity is improved by reducing inhibitor size/shape and increasing water solubility. Virtual screening results using HMDB and Recon2 for endogenous compounds appropriately scored tryptamine as an inhibitor.

CONCLUSIONS AND IMPLICATIONS

The pharmacophore model for the proton-antiporter inhibitors was a good predictor of known inhibitors and allowed us to identify new good inhibitors. This model marks a new step towards the discovery of this drug/proton antiporter and will be of great use for the discovery and design of potent inhibitors that could potentially help to assess and validate its pharmacological role in drug addiction in vivo.

Determination of dopamine concentrations in brain extracellular fluid using microdialysis with short sampling intervals, analyzed by ultra high performance liquid chromatography tandem mass spectrometry

INTRODUCTION

An increase in striatal dopamine is considered essential for the rewarding and reinforcing effects of drugs of abuse. We have developed and validated an ultra high performance liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) method for the analysis of dopamine in rat brain extracellular fluid (ECF) sampled with microdialysis. The method was applied to monitor changes in dopamine concentrations over time after an intravenous bolus injection of heroin.

METHODS

Dopamine and dopamine-d3 were analyzed using a 2.1×100mm Aquity T3 column, 1.7μm particle size, with a formic acid and methanol gradient. The run time of the method was 2.5min including equilibration time.

RESULTS

The method had an LOQ of 0.15ng/mL, which equals 0.55pg on column. The calibration curves were linear in the tested area of 0.15 to 16ng/mL. Inter-assay coefficients of variation varied between 5-17%, with an accuracy expressed as bias of -10 to 5%. The intra-assay coefficients of variation varied between 9-15%, with an accuracy of -3-7%.

DISCUSSION

Heroin metabolism is very rapid. Sampling intervals of only 2min were thus required to obtain an adequate number of samples of dopamine analysis accompanying the concentration-time profile of opioids in the brain. Applying a flow of 2μL/min, 4μL of dialysate were sampled at 2min intervals, in 7μL internal standard. The injection volume onto the UPLC column was 10μL. Analyses of microdialysate samples from a rat given heroin i.v. showed that it was possible to measure baseline levels and rapid changes in dopamine concentrations with very short sampling periods.

Caffeine enhances and accelerates the expression of sensitization induced by coca paste indicating its relevance as a main adulterant

BACKGROUND AND OBJECTIVES

Caffeine is an active adulterant found in several drugs of abuse including coca paste (CP). We had previously demonstrated that caffeine potentiated the acute stimulant effect induced by CP seized samples. The role of caffeine in the expression of sensitization elicited by a CP seized sample (CP1) was here evaluated.

METHODS

CP1 (equivalent dose of 10 mg/kg of cocaine), cocaine (pure, 10 mg/kg), a combination of cocaine 10 mg/kg plus caffeine 2.5 mg/kg (CP1-surrogate) and saline (control) were intraperitoneally injected in male rats under two different sensitization schedules. Ambulatory locomotion was recorded in 58 animals.

RESULTS

After five daily CP1 injections and 5 days of withdrawal, CP1-challenged animals displayed a more robust sensitization than cocaine-treated animals. When a 3 injections-regime of CP1-surrogate or cocaine was assayed, only CP1-surrogate was able to elicit sensitization.

DISCUSSION AND CONCLUSIONS

Caffeine enhances and accelerates the CP1-induced sensitization.

SCIENTIFIC SIGNIFICANCE

Results may shed light on the fast and high dependence observed in CP users.

Behavioral mechanisms underlying nicotine reinforcement

Cigarette smoking is the leading cause of preventable deaths worldwide, and nicotine, the primary psychoactive constituent in tobacco, drives sustained use. The behavioral actions of nicotine are complex and extend well beyond the actions of the drug as a primary reinforcer. Stimuli that are consistently paired with nicotine can, through associative learning, take on reinforcing properties as conditioned stimuli. These conditioned stimuli can then impact the rate and probability of behavior and even function as conditioning reinforcers that maintain behavior in the absence of nicotine. Nicotine can also act as a conditioned stimulus (CS), predicting the delivery of other reinforcers, which may allow nicotine to acquire value as a conditioned reinforcer. These associative effects, establishing non-nicotine stimuli as conditioned stimuli with discriminative stimulus and conditioned reinforcing properties as well as establishing nicotine as a CS, are predicted by basic conditioning principles. However, nicotine can also act non-associatively. Nicotine directly enhances the reinforcing efficacy of other reinforcing stimuli in the environment, an effect that does not require a temporal or predictive relationship between nicotine and either the stimulus or the behavior. Hence, the reinforcing actions of nicotine stem both from the primary reinforcing actions of the drug (and the subsequent associative learning effects) as well as the reinforcement enhancement action of nicotine which is non-associative in nature. Gaining a better understanding of how nicotine impacts behavior will allow for maximally effective tobacco control efforts aimed at reducing the harm associated with tobacco use by reducing and/or treating its addictiveness.

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.

Potentiated gene regulation by methylphenidate plus fluoxetine treatment: Long-term gene blunting (Zif268, Homer1a) and behavioral correlates

Use of psychostimulants such as methylphenidate (Ritalin) in medical treatments and as cognitive enhancers in the healthy is increasing. Methylphenidate produces some addiction-related gene regulation in animal models. Recent findings show that combining selective serotonin reuptake inhibitor (SSRI) antidepressants such as fluoxetine with methylphenidate potentiates methylphenidate-induced gene regulation. We investigated the endurance of such abnormal gene regulation by assessing an established marker for altered gene regulation after drug treatments - blunting (repression) of immediate-early gene (IEG) inducibility - 14 days after repeated methylphenidate+fluoxetine treatment in adolescent rats. Thus, we measured the effects of a 6-day repeated treatment with methylphenidate (5 mg/kg), fluoxetine (5 mg/kg) or their combination on the inducibility (by cocaine) of neuroplasticity-related IEGs (Zif268, Homer1a) in the striatum, by in situ hybridization histochemistry. Repeated methylphenidate treatment alone produced modest gene blunting, while fluoxetine alone had no effect. In contrast, fluoxetine given in conjunction with methylphenidate produced pronounced potentiation of methylphenidate-induced blunting for both genes. This potentiation was seen in many functional domains of the striatum, but was most robust in the lateral, sensorimotor striatum. These enduring molecular changes were associated with potentiated induction of behavioral stereotypies in an open-field test. For illicit psychostimulants, blunting of gene expression is considered part of the molecular basis of addiction. Our results thus suggest that SSRIs such as fluoxetine may increase the addiction liability of methylphenidate. Key words: cognitive enhancer, dopamine, serotonin, gene expression, psychostimulant, SSRI antidepressant, striatum.

Cocaine affects foraging behaviour and biogenic amine modulated behavioural reflexes in honey bees

In humans and other mammals, drugs of abuse alter the function of biogenic amine pathways in the brain leading to the subjective experience of reward and euphoria. Biogenic amine pathways are involved in reward processing across diverse animal phyla, however whether cocaine acts on these neurochemical pathways to cause similar rewarding behavioural effects in animal phyla other than mammals is unclear. Previously, it has been shown that bees are more likely to dance (a signal of perceived reward) when returning from a sucrose feeder after cocaine treatment. Here we examined more broadly whether cocaine altered reward-related behaviour, and biogenic amine modulated behavioural responses in bees. Bees developed a preference for locations at which they received cocaine, and when foraging at low quality sucrose feeders increase their foraging rate in response to cocaine treatment. Cocaine also increased reflexive proboscis extension to sucrose, and sting extension to electric shock. Both of these simple reflexes are modulated by biogenic amines. This shows that systemic cocaine treatment alters behavioural responses that are modulated by biogenic amines in insects. Since insect reward responses involve both octopamine and dopamine signalling, we conclude that cocaine treatment altered diverse reward-related aspects of behaviour in bees. We discuss the implications of these results for understanding the ecology of cocaine as a plant defence compound. Our findings further validate the honey bee as a model system for understanding the behavioural impacts of cocaine, and potentially other drugs of abuse.

Novel approaches for the treatment of psychostimulant and opioid abuse - focus on opioid receptor-based therapies

INTRODUCTION

Psychostimulant and opioid addiction are poorly treated. The majority of abstinent users relapse back to drug-taking within a year of abstinence, making 'anti-relapse' therapies the focus of much current research. There are two fundamental challenges to developing novel treatments for drug addiction. First, there are three key stimuli that precipitate relapse back to drug-taking: stress, presentation of drug-conditioned cue, taking a small dose of drug. The most successful novel treatment would be effective against all three stimuli. Second, a large number of drug users are poly-drug users: taking more than one drug of abuse at a time. The ideal anti-addiction treatment would, therefore, be effective against all classes of drugs of abuse.

AREAS COVERED

In this review, the authors discuss the clinical need and animal models used to uncover potential novel treatments. There is a very broad range of potential treatment approaches and targets currently being examined as potential anti-relapse therapies. These broadly fit into two categories: 'memory-based' and 'receptor-based' and the authors discuss the key targets here within.

EXPERT OPINION

Opioid receptors and ligands have been widely studied, and research into how different opioid subtypes affect behaviours related to addiction (reward, dysphoria, motivation) suggests that they are tractable targets as anti-relapse treatments. Regarding opioid ligands as novel 'anti-relapse' medication targets, research suggests that a 'non-selective' approach to targeting opioid receptors will be the most effective.

αCaMKII controls the establishment of cocaine's reinforcing effects in mice and humans

Although addiction develops in a considerable number of regular cocaine users, molecular risk factors for cocaine dependence are still unknown. It was proposed that establishing drug use and memory formation might share molecular and anatomical pathways. Alpha-Ca(2+)/calmodulin-dependent protein kinase-II (αCaMKII) is a key mediator of learning and memory also involved in drug-related plasticity. The autophosphorylation of αCaMKII was shown to accelerate learning. Thus, we investigated the role of αCaMKII autophosphorylation in the time course of establishing cocaine use-related behavior in mice. We found that αCaMKII autophosphorylation-deficient αCaMKII(T286A) mice show delayed establishment of conditioned place preference, but no changes in acute behavioral activation, sensitization or conditioned hyperlocomotion to cocaine (20 mg kg(-1), intraperitoneal). In vivo microdialysis revealed that αCaMKII(T286A) mice have blunted dopamine (DA) and blocked serotonin (5-HT) responses in the nucleus accumbens (NAcc) and prefrontal cortex after acute cocaine administration (20 mg kg(-1), intraperitoneal), whereas noradrenaline responses were preserved. Under cocaine, the attenuated DA and 5-HT activation in αCaMKII(T286A) mice was followed by impaired c-Fos activation in the NAcc. To translate the rodent findings to human conditions, several CAMK2A gene polymorphisms were tested regarding their risk for a fast establishment of cocaine dependence in two independent samples of regular cocaine users from Brazil (n=688) and Switzerland (n=141). A meta-analysis across both samples confirmed that CAMK2A rs3776823 TT-allele carriers display a faster transition to severe cocaine use than C-allele carriers. Together, these data suggest that αCaMKII controls the speed for the establishment of cocaine's reinforcing effects.

Frequency of cocaine self-administration influences drug seeking in the rat: optogenetic evidence for a role of the prelimbic cortex

High-frequency intake and high drug-induced seeking are associated with cocaine addiction in both human and animals. However, their relationships and neurobiological underpinnings remain hypothetical. The medial prefrontal cortex (mPFC), basolateral amygdala (BLA), and nucleus accumbens (NAc) have been shown to have a role in cocaine seeking. However, their involvement in regulating high-frequency intake and high cocaine-induced seeking is unclear. We manipulated frequency of cocaine self-administration and investigated whether it influenced cocaine seeking. The contribution of the aforementioned structures was evaluated using changes in expression of the immediate early gene c-Fos and targeted optogenetic manipulations. Rats that self-administered at High frequency (short inter-infusion intervals allowed by short time-out) showed higher cocaine-induced seeking than low frequency rats (long inter-infusions intervals imposed by long time-out), as measured with cocaine-induced reinstatement. c-Fos was enhanced in High frequency rats in the prelimbic (PL) and infralimbic (IL) areas of the mPFC, the BLA, and the NAc core and shell. Correlational analysis of c-Fos revealed that the PL was a critical node strongly correlated with both the IL and NAc core in High frequency rats. Targeted optogenetic inactivation of the PL decreased cocaine-induced reinstatement, but increased cocaine self-administration, in High frequency rats. In contrast, optogenetic activation of the PL had no effect on Low frequency rats. Thus, high-frequency intake promotes a PL-dependent control of cocaine seeking, with the PL exerting a facilitatory or inhibitory effect, depending on operant contingencies. Individual differences in cocaine-induced PL activation might be a source of vulnerability for poorly controlled cocaine-induced seeking and/or cocaine intake.

Role of 6-monoacetylmorphine in the acute release of striatal dopamine induced by intravenous heroin

After injection, heroin is rapidly metabolized to 6-monoacetylmorphine (6-MAM) and further to morphine. As morphine has been shown to increase striatal dopamine, whereas 6-MAM has not been studied in this respect, we gave i.v. injections of 3 μmol 6-MAM, morphine or heroin to rats. Opioids were measured in blood, and dopamine and opioids in microdialysate from brain striatal extracellular fluid (ECF), by UPLC-MS/MS. After 6-MAM injection, 6-MAM ECF concentrations increased rapidly, and reached Cmax of 4.4 μM after 8 min. After heroin injection, 6-MAM increased rapidly in blood and reached Cmax of 6.4 μM in ECF after 8 min, while ECF Cmax for heroin was 1.2 μM after 2 min. T max for morphine in ECF was 29 and 24 min following 6-MAM and heroin administration, respectively, with corresponding Cmax levels of 1 and 2 μM. Dopamine levels peaked after 8 and 14 min following 6-MAM and heroin administration, respectively. The dopamine responses were equal, indicating no dopamine release by heroin per se. Furthermore, 6-MAM, and not morphine, appeared to mediate the early dopamine response, whereas morphine administration, giving rise to morphine ECF concentrations similar to those observed shortly after 6-MAM injection, did not increase ECF dopamine. 6-MAM appeared accordingly to be the substance responsible for the early increase in dopamine observed after heroin injection. As 6-MAM was formed rapidly from heroin in blood, and was the major substance reaching the brain after heroin administration, this also indicates that factors influencing blood 6-MAM concentrations might change the behavioural effects of heroin.

Expectancies for cigarettes, e-cigarettes, and nicotine replacement therapies among e-cigarette users (aka vapers)

INTRODUCTION

Use of e-cigarettes has been increasing exponentially, with the primary motivation reported as smoking cessation. To understand why smokers choose e-cigarettes as an alternative to cigarettes, as well as to US Food and Drug Administration (FDA)--approved nicotine replacement therapies (NRT), we compared outcome expectancies (beliefs about the results of drug use) for the three nicotine delivery systems among vapers, i.e., e-cigarette users, who were former smokers.

METHODS

Vapers (N = 1,434) completed an online survey assessing 14 expectancy domains as well as perceived cost and convenience. We focused on comparisons between e-cigarettes and cigarettes to determine the attraction of e-cigarettes as a smoking alternative and between e-cigarettes and NRT to determine perceived advantages of e-cigarettes over FDA-approved pharmacotherapy.

RESULTS

Participants believed that e-cigarettes, in comparison to conventional cigarettes, had fewer health risks; caused less craving, withdrawal, addiction, and negative physical feelings; tasted better; and were more satisfying. In contrast, conventional cigarettes were perceived as better than e-cigarettes for reducing negative affect, controlling weight, providing stimulation, and reducing stress. E-cigarettes, compared to NRT, were perceived to be less risky, cost less, cause fewer negative physical feelings, taste better, provide more satisfaction, and be better at reducing craving, negative affect, and stress. Moderator analyses indicated history with ad libitum forms of NRT was associated with less positive NRT expectancies.

CONCLUSIONS

The degree to which expectancies for e-cigarettes differed from expectancies for either tobacco cigarettes or NRT offers insight into the motivation of e-cigarette users and provides guidance for public health and clinical interventions to encourage smoking-related behavior change.

Levels of heroin and its metabolites in blood and brain extracellular fluid after i.v. heroin administration to freely moving rats

BACKGROUND AND PURPOSE

Heroin, with low affinity for μ-opioid receptors, has been considered to act as a prodrug. In order to study the pharmacokinetics of heroin and its active metabolites after i.v. administration, we gave a bolus injection of heroin to rats and measured the concentration of heroin and its metabolites in blood and brain extracellular fluid (ECF).

EXPERIMENTAL APPROACH

After an i.v. bolus injection of heroin to freely moving Sprague-Dawley rats, the concentrations of heroin and metabolites in blood samples from the vena jugularis and in microdialysis samples from striatal brain ECF were measured by ultraperformance LC-MS/MS.

KEY RESULTS

Heroin levels decreased very fast, both in blood and brain ECF, and could not be detected after 18 and 10 min respectively. 6-Monoacetylmorphine (6-MAM) increased very rapidly, reaching its maximal concentrations after 2.0 and 4.3 min, respectively, and falling thereafter. Morphine increased very slowly, reaching its maximal levels, which were six times lower than the highest 6-MAM concentrations, after 12.6 and 21.3 min, with a very slow decline during the rest of the experiment and only surpassing 6-MAM levels at least 30 min after injection.

CONCLUSIONS AND IMPLICATIONS

After an i.v. heroin injection, 6-MAM was the predominant opioid present shortly after injection and during the first 30 min, not only in the blood but also in rat brain ECF. 6-MAM might therefore mediate most of the effects observed shortly after heroin intake, and this finding questions the general assumption that morphine is the main and most important metabolite of heroin.

Single trial nicotine conditioned place preference in pre-adolescent male and female rats

The mean age of first voluntary tobacco inhalation is 12.3 years (DiFranza et al., 2004). 60% of smokers start smoking before the age of 14 and 90% are dependent before reaching the age of 19. Females are typically more sensitive to nicotine than males yet few studies examine the effects of nicotine on the reward systems in pre-adolescent female subjects. This study utilized the single trial conditioned place preference (CPP) test in very young (postnatal day 25-27) rats of both sexes. Latent effects on anxiety and amphetamine response were determined 5 and 7 days following a second nicotine exposure. Results show that 0.05 mg/kg nicotine induced CPP in females following a single trial while both sexes showed CPP following the 0.5 mg/kg dose. Five days later, rats dosed with 0.05 mg/kg show increased time on the open arm of the elevated plus maze, an anxiolytic response. While baseline activity was increased in nicotine-exposed males 7 days following dosing, amphetamine response was not affected by the treatments in either sex. Therefore, our data suggest that young females are more sensitive to nicotine reward than males supporting a heightened sensitivity of the mesolimbic dopamine system in very young females. However, alterations in baseline activity were only seen in males suggesting that different components of the system are affected by nicotine in each sex. An anxiolytic response to nicotine 5 days after dosing may suggest that this very young age group is uniquely affected by this very low nicotine dose. Clearly, nicotine has substantial acute and lasting effects during pre-adolescence at doses substantially lower than seen at older ages as reported by others. These effects, which could potentially result from cigarette or e-cigarette smoking by 11-12 year old children , focus attention on the vulnerability of this age group to nicotine.

Human abuse liability evaluation of CNS stimulant drugs

Psychoactive drugs that increase alertness, attention and concentration and energy, while also elevating mood, heart rate and blood pressure are referred to as stimulants. Despite some overlapping similarities, stimulants cannot be easily categorized by their chemical structure, mechanism of action, receptor binding profile, effects on monoamine uptake, behavioral pharmacology (e.g., effects on locomotion, temperature, and blood pressure), therapeutic indication or efficacy. Because of their abuse liability, a pre-market assessment of abuse potential is required for drugs that show stimulant properties; this review article focuses on the clinical aspects of this evaluation. This includes clinical trial adverse events, evidence of diversion or tampering, overdoses and the results of a human abuse potential study. While there are different types of human experimental studies that can be employed to evaluate stimulant abuse potential (e.g., drug discrimination, self-administration), only the human abuse potential study and clinical trial adverse event data are required for drug approval. The principal advances that have improved human abuse potential studies include using study enrichment strategies (pharmacologic qualification), larger sample sizes, better selection of endpoints and measurement strategies and more carefully considered interpretation of data. Because of the methodological advances, comparisons of newer studies with historical data is problematic and may contribute to a biased regulatory framework for the evaluation of newer stimulant-like drugs, such as A2 antagonists. This article is part of the Special Issue entitled 'CNS Stimulants'.

[Chronic non-cancer-related pain. Long-term treatment with rapid-release and short-acting opioids in the context of misuse and dependency]

Annually published data show a continual increase in the volume of opioid prescriptions in Germany, thus indicating an intensification of opioid therapy. The majority of opioids are prescribed to treat chronic non-cancer-related pain. On the basis of current guidelines, as well as in terms of the lack of data regarding long-term use of opioids and their effectiveness beyond a period of 3 months, this development must be viewed critically. With reference to four case reports, we discuss and evaluate opioid therapy in relation to medication misuse and the development of drug dependency. Particular emphasis is placed on the administration of rapid-release and short-acting opioid preparations, which we consider to be particularly problematic.

A novel procedure for evaluating the reinforcing properties of tastants in laboratory rats: operant intraoral self-administration

This paper describes a novel method for studying the bio-behavioral basis of addiction to food. This method combines the surgical component of taste reactivity with the behavioral aspects of operant self-administration of drugs. Under very brief general anaesthesia, rats are implanted with an intraoral (IO) cannula that allows delivery of test solutions directly in the oral cavity. Animals are then tested in operant self-administration chambers whereby they can press a lever to receive IO infusions of test solutions. IO self-administration has several advantages over experimental procedures that involve drinking a solution from a spout or operant responding for solid pellets or solutions delivered in a receptacle. Here, we show that IO self-administration can be employed to study self-administration of high fructose corn syrup (HFCS). Rats were first tested for self-administration on a progressive ratio (PR) schedule, which assesses the maximum amount of operant behavior that will be emitted for different concentrations of HFCS (i.e. 8%, 25%, and 50%). Following this test, rats self-administered these concentrations on a continuous schedule of reinforcement (i.e. one infusion for each lever press) for 10 consecutive days (1 session/day; each lasting 3 hr), and then they were retested on the PR schedule. On the continuous reinforcement schedule, rats took fewer infusions of higher concentrations, although the lowest concentration of HFCS (8%) maintained more variable self-administration. Furthermore, the PR tests revealed that 8% had lower reinforcing value than 25% and 50%. These results indicate that IO self-administration can be employed to study acquisition and maintenance of responding for sweet solutions. The sensitivity of the operant response to differences in concentration and schedule of reinforcement makes IO self-administration an ideal procedure to investigate the neurobiology of voluntary intake of sweets.

A novel procedure for evaluating the reinforcing properties of tastants in laboratory rats: operant intraoral self-administration

This paper describes a novel method for studying the bio-behavioral basis of addiction to food. This method combines the surgical component of taste reactivity with the behavioral aspects of operant self-administration of drugs. Under very brief general anaesthesia, rats are implanted with an intraoral (IO) cannula that allows delivery of test solutions directly in the oral cavity. Animals are then tested in operant self-administration chambers whereby they can press a lever to receive IO infusions of test solutions. IO self-administration has several advantages over experimental procedures that involve drinking a solution from a spout or operant responding for solid pellets or solutions delivered in a receptacle. Here, we show that IO self-administration can be employed to study self-administration of high fructose corn syrup (HFCS). Rats were first tested for self-administration on a progressive ratio (PR) schedule, which assesses the maximum amount of operant behavior that will be emitted for different concentrations of HFCS (i.e. 8%, 25%, and 50%). Following this test, rats self-administered these concentrations on a continuous schedule of reinforcement (i.e. one infusion for each lever press) for 10 consecutive days (1 session/day; each lasting 3 hr), and then they were retested on the PR schedule. On the continuous reinforcement schedule, rats took fewer infusions of higher concentrations, although the lowest concentration of HFCS (8%) maintained more variable self-administration. Furthermore, the PR tests revealed that 8% had lower reinforcing value than 25% and 50%. These results indicate that IO self-administration can be employed to study acquisition and maintenance of responding for sweet solutions. The sensitivity of the operant response to differences in concentration and schedule of reinforcement makes IO self-administration an ideal procedure to investigate the neurobiology of voluntary intake of sweets.

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

The rapid delivery of drugs of abuse to the brain is associated with an increased likelihood and severity of addiction. Here we evaluated the hypothesis that rapidly delivered cocaine facilitates the addiction process by promoting the development of enhanced motivation for the drug. Rats lever-pressed for cocaine delivered intravenously over 5 or 90 s under fixed ratio (FR) during 6-h sessions. The motivation for cocaine was subsequently assessed using a progressive ratio (PR) schedule, where each successive drug injection cost an exponentially greater number of lever presses, until the cessation of responding. Throughout all self-administration sessions, all rats could only take one injection every 90 s. The 5-s groups self-administered more drug than the 90-s groups across the FR sessions. Under PR, animals that had chronically self-administered rapidly delivered cocaine took more cocaine across a range of doses and regardless of whether the drug was delivered over 5 or 90 s during PR testing. The speed of delivery also determined the long-term neurobiological impact of cocaine. Fourteen days following cocaine withdrawal, caudate-putamen D2 levels were decreased only in the 90-s rats, and quinpirole-mediated Gα(i/o)-protein activation was increased to a greater extent in the 90- vs 5-s rats. Thus, rapid delivery promotes the pursuit of cocaine in the face of rising costs and alters cocaine-induced changes in striatal D2 receptor number and function. As such, rapidly delivered cocaine might facilitate addiction because it more readily alters brain motivation circuits in ways that contribute to the compulsive pursuit of the drug.

Smokers versus snorters: do treatment outcomes differ according to route of cocaine administration?

Smoking cocaine achieves maximal concentration and effect far more rapidly than through the intranasal ("snorting") route, and it is associated with greater propensity for dependence and more severe consequences. However, very little is known about differences in treatment outcome according to route of administration. This study compared treatment outcomes, such as frequency of cocaine use and Addiction Severity Index (ASI) composite scores, by primary route of cocaine administration (smoking vs. intranasal) among a pooled sample of 412 cocaine-dependent individuals participating in 1 of 5 randomized clinical trials. The majority (80%) reported smoking as their primary route of cocaine administration. Overall, results indicated better cocaine use outcomes both during the treatment phase and through a 12-month follow-up period for intranasal users compared to smokers, although not all differences reached statistical significance. Intranasal users remained in treatment longer, F(1, 408) = 3.55, p < .05, and showed a trend toward achieving longer periods of sustained abstinence within treatment, F(1, 378) = 2.68, p = .08, as well as less use over time during the follow-up period than smokers (Time × Route: t = 1.87, p = .06). Also, intranasal users' ASI cocaine composite score decreased more than smokers, but there were overall decreases in the other ASI domains for all participants over the course of the study period. These results suggest that intranasal users may achieve better cocaine use outcomes than smokers, yet this doesn't appear to translate to differential changes in the severity of problems experienced in other life areas.

Chronic cocaine dampens dopamine signaling during cocaine intoxication and unbalances D1 over D2 receptor signaling

Dopamine increases triggered by cocaine and consequent stimulation of dopamine receptors (including D1 and D2) are associated with its rewarding effects. However, while facilitation of D1 receptor (D1R) signaling enhances the rewarding effects of cocaine, facilitation of D2R signaling decreases it, which indicates that for cocaine to be rewarding it must result in a predominance of D1R over D2R signaling. Moreover, the transition to compulsive cocaine intake might result from an imbalance between D1R and D2R signaling. To test the hypothesis that chronic cocaine use unbalances D1R over D2R signaling during cocaine intoxication, we used microprobe optical imaging to compare dynamic changes in intracellular calcium ([Ca(2+)]i, marker of neuronal activation) to acute cocaine in striatal D1R-EGFP and D2R-EGFP-expressing neurons between control and chronically treated mice. Chronic cocaine attenuated responses to acute cocaine in D1R (blunting Ca(2+) increases by 67 ± 16%) and D2R (blunting Ca(2+) decrease by 72 ± 17%) neurons in most D1R and D2R neurons (~75%). However, the dynamics of this attenuation during cocaine intoxication was longer lasting for D2R than for D1R. Thus, whereas control mice showed a fast but short-lasting predominance of D1R over D2R signaling (peaking at ~8 min) during acute cocaine intoxication, in chronically treated mice D1R predominance was sustained for >30 min (throughout the measurement period). Thus, chronic cocaine use dramatically reduced cocaine-induced DA signaling, shifting the balance between D1R and D2R signaling during intoxication to a predominance of D1R (stimulatory) over D2R (inhibitory) signaling, which might facilitate compulsive intake in addiction.

Invertebrate models in addiction research

While drug addiction is a uniquely human problem, most research examining the biological mechanisms of the transition from substance use to addiction is conducted with vertebrate animal models. Many other fields of neuroscience have greatly benefitted from contributions from simple and manipulable invertebrate model systems. However, the potential of invertebrate research has yet to be fully capitalised on in the field of addiction neuroscience. This may be because of the complexity of addiction and the clinical imperative of addiction research. We argue that the homocentric diagnostic criteria of addiction are no more a hindrance to the use of invertebrate models than they are to vertebrate models. We highlight the strengths of the diversity of different invertebrate model systems in terms of neuroanatomy and molecular machinery, and stress that working with a range of different models will aid in understanding addiction and not be a disadvantage. Finally, we discuss the specific advantages of utilising invertebrate animals for addiction research and highlight key areas in which invertebrates are suited for making unique and meaningful contributions to this field.

Neural mechanisms of attention-deficit/hyperactivity disorder symptoms are stratified by MAOA genotype

BACKGROUND

Attention-deficit/hyperactivity disorder (ADHD) is characterized by deficits in reward sensitivity and response inhibition. The relative contribution of these frontostriatal mechanisms to ADHD symptoms and their genetic determinants is largely unexplored.

METHODS

Using functional magnetic resonance imaging and genetic analysis of the monoamine oxidase A (MAOA) gene, we investigated how striatal and inferior frontal activation patterns contribute to ADHD symptoms depending on MAOA genotype in a sample of adolescent boys (n = 190).

RESULTS

We demonstrate an association of ADHD symptoms with distinct blood oxygen level-dependent (BOLD) responses depending on MAOA genotype. In A hemizygotes of the expression single nucleotide polymorphism rs12843268, which express lower levels of MAOA, ADHD symptoms are associated with lower ventral striatal BOLD response during the monetary incentive delay task and lower inferior frontal gyrus BOLD response during the stop signal task. In G hemizygotes, ADHD symptoms are associated with increased inferior frontal gyrus BOLD response during the stop signal task in the presence of increased ventral striatal BOLD response during the monetary incentive delay task.

CONCLUSIONS

Depending on MAOA genotype, ADHD symptoms in adolescent boys are associated with either reward deficiency or insufficient response inhibition. Apart from its mechanistic interest, our finding may aid in developing pharmacogenetic markers for ADHD.

Psychobiology of cocaine addiction: Contribution of a multi-symptomatic animal model of loss of control

Transition to addiction is the shift from controlled to uncontrolled drug use that occurs after prolonged drug intake in a limited number of drug users. A major challenge of addiction research in recent years has been to develop models for studying this pathological transition. Toward this goal, a DSM-IV/5-based multi-symptomatic model of cocaine addiction has been developed in the rat. It is based on an operational translation of the main features of the disease. 1. Addiction is not just taking drug; it is a non-adaptive drug use: The procedure models addiction in relation to its clinical definition. 2. All drug users do not face the same individual risk of developing addiction: The model includes an individual-based approach. 3. Addiction develops after protracted periods of controlled drug use: This procedure allows for the study of the long-term shift from controlled drug use to addiction. We describe this model in detail and show how it can contribute to our understanding of the pathophysiology of cocaine addiction. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Imaging opioid analgesia in the human brain and its potential relevance for understanding opioid use in chronic pain

Opioids play an important role for the management of acute pain and in palliative care. The role of long-term opioid therapy in chronic non-malignant pain remains unclear and is the focus of much clinical research. There are concerns regarding analgesic tolerance, paradoxical pain and issues with dependence that can occur with chronic opioid use in the susceptible patient. In this review, we discuss how far human neuroimaging research has come in providing a mechanistic understanding of pain relief provided by opioids, and suggest avenues for further studies that are relevant to the management of chronic pain with opioids.

This article is part of the Special Issue Section entitled ‘Neuroimaging in Neuropharmacology’.

•

Brain mechanisms are crucial to opioid analgesia in humans.

•

Opioids can have a direct effect on brain mechanisms for pain perception.

•

Opioids can also engage descending inhibition of spinal nociception.

•

Drug-induced tolerance, dependence and paradoxical pain may limit chronic opioid analgesic therapy.

Cocaine tolerance in honey bees

Increasingly invertebrates are being used to investigate the molecular and cellular effects of drugs of abuse to explore basic mechanisms of addiction. However, in mammals the principle factors contributing to addiction are long-term adaptive responses to repeated drug use. Here we examined whether adaptive responses to cocaine are also seen in invertebrates using the honey bee model system. Repeated topical treatment with a low dose of cocaine rendered bees resistant to the deleterious motor effects of a higher cocaine dose, indicating the development of physiological tolerance to cocaine in bees. Cocaine inhibits biogenic amine reuptake transporters, but neither acute nor repeated cocaine treatments caused measurable changes in levels of biogenic amines measured in whole bee brains. Our data show clear short and long-term behavioural responses of bees to cocaine administration, but caution that, despite the small size of the bee brain, measures of biogenic amines conducted at the whole-brain level may not reveal neurochemical effects of the drug.

Increase in opiate prescription in Germany between 2000 and 2010: a study based on insurance data

BACKGROUND

Insufficient data have been available to date on the prevalence of opioid treatment in Germany, physicians' prescribing habits, and the percentages of cancer patients and non-cancer patients among those receiving opioids for an evaluation of the quality of care and an assessment of possible underuse or misuse.

METHODS

The data analyzed in this study were derived from the statutory health insurance sample of the AOK health insurance company in the German state of Hesse / ASHIP Hesse for the years 2000-2010. For the purpose of this study, prevalence was defined as the percentage of insurees who received at least one outpatient prescription of an opioid (ATC N02A, excluding codeine, levomethadone and methadone). In order to control for population aging, the prevalence was standardized to the German population on December 31(st) of the preceding year and to the age-structure of the population as it was in 1999. Opioid prescribing for cancer was assumed when a cancer diagnosis was documented in the same year in which the opioid prescription was issued.

RESULTS

The percentage of insurees receiving at least one opioid prescription rose over the period of the study from 3.31% in 2000 to 4.53% in 2010, a relative gain of 37%. Opioids were mostly prescribed to patients with non-cancer pain (2010: about 77% of opioid recipients). The percentage of non-cancer patients receiving long-term opioid treatment has also increased over the period of the study.

CONCLUSION

As opioids are frequently prescribed for non-cancer pain, it cannot be inferred from the observed increase in opioid prescribing that cancer patients are now receiving better opioid treatment than they were before. Further issues of concern are the observed increases in the prescribing of potent immediate release opioids and in the long-term opioid treatment for non-cancer patients, the benefit of which is currently debated.

Coexistence of passive and proton antiporter-mediated processes in nicotine transport at the mouse blood-brain barrier

Nicotine, the main tobacco alkaloid leading to smoking dependence, rapidly crosses the blood-brain barrier (BBB) to become concentrated in the brain. Recently, it has been shown that nicotine interacts with some organic cation transporters (OCT), but their influence at the BBB has not yet been assessed in vivo. In this study, we characterized the transport of nicotine at the mouse luminal BBB by in situ brain perfusion. Its influx was saturable and followed the Michaelis-Menten kinetics (K(m)=2.60 mM, V(max)=37.60 nmol/s/g at pH 7.40). At its usual micromolar concentrations in the plasma, most (79%) of the net transport of nicotine at the BBB was carrier-mediated, while passive diffusion accounted for 21%. Studies on knockout mice showed that the OCT Oct1-3, P-gp, and Bcrp did not alter [(3)H]-nicotine transport at the BBB. Neither did inhibiting the transporters Mate1, Octn, or Pmat. The in vivo manipulation of intracellular and/or extracellular pH, the chemical inhibition profile, and the trans-stimulation experiments demonstrated that the nicotine transporter at the BBB shared the properties of the clonidine/proton antiporter. The molecular features of this proton-coupled antiporter have not yet been identified, but it also transports diphenhydramine and tramadol and helps nicotine cross the BBB at a faster rate and to a greater extent. The pharmacological inhibition of this nicotine/proton antiporter could represent a new strategy to reduce nicotine uptake by the brain and thus help curb addiction to smoking.

Addiction-related gene regulation: risks of exposure to cognitive enhancers vs. other psychostimulants

The psychostimulants methylphenidate (Ritalin, Concerta), amphetamine (Adderall), and modafinil (Provigil) are widely used in the treatment of medical conditions such as attention-deficit hyperactivity disorder and narcolepsy and, increasingly, as "cognitive enhancers" by healthy people. The long-term neuronal effects of these drugs, however, are poorly understood. A substantial amount of research over the past two decades has investigated the effects of psychostimulants such as cocaine and amphetamines on gene regulation in the brain because these molecular changes are considered critical for psychostimulant addiction. This work has determined in some detail the neurochemical and cellular mechanisms that mediate psychostimulant-induced gene regulation and has also identified the neuronal systems altered by these drugs. Among the most affected brain systems are corticostriatal circuits, which are part of cortico-basal ganglia-cortical loops that mediate motivated behavior. The neurotransmitters critical for such gene regulation are dopamine in interaction with glutamate, while other neurotransmitters (e.g., serotonin) play modulatory roles. This review presents (1) an overview of the main findings on cocaine- and amphetamine-induced gene regulation in corticostriatal circuits in an effort to provide a cellular framework for (2) an assessment of the molecular changes produced by methylphenidate, medical amphetamine (Adderall), and modafinil. The findings lead to the conclusion that protracted exposure to these cognitive enhancers can induce gene regulation effects in corticostriatal circuits that are qualitatively similar to those of cocaine and other amphetamines. These neuronal changes may contribute to the addiction liability of the psychostimulant cognitive enhancers.

Aggressive experience increases dendritic spine density within the nucleus accumbens core in female Syrian hamsters

Activity within the mesolimbic dopamine system is associated with the performance of naturally motivated behaviors, one of which is aggression. In male rats, aggressive behavior induces neurochemical changes within the nucleus accumbens, a key structure within the mesolimbic dopamine system. Corresponding studies have not been done in females. Female Syrian hamsters live as isolates and when not sexually responsive are aggressive toward either male or female intruders, making them an excellent model for studying aggression in females. We took advantage of this naturally expressed behavior to examine the effects of repeated aggressive experience on the morphology of medium spiny neurons in the nucleus accumbens and caudate nucleus, utilizing a DiOlistic labeling approach. We found that repeated aggressive experience significantly increased spine density within the nucleus accumbens core, with no significant changes in any other brain region examined. At the same time, significant changes in spine morphology were observed in all brain regions following repeated aggressive experience. These data are significant in that they demonstrate that repeated exposure to behaviors that form part of an animal's life history will alter neuronal structure in a way that may shift neurobiological responses to impact future social interactions.

Selection for high alcohol preference drinking in mice results in heightened sensitivity and rapid development of acute functional tolerance to alcohol's ataxic effects

Propensity to develop acute functional (or within session) tolerance to alcohol (ethanol) may influence the amount of alcohol consumed, with higher drinking associated with greater acute functional tolerance (AFT). The goal of this study was to assess this potential correlated response between alcohol preference and AFT in second and third replicate lines of mice selectively bred for high (HAP2 and HAP3) and low (LAP2 and LAP3) alcohol preference drinking. Male and female mice were tested for development of AFT on a static dowel task, which requires that animals maintain balance on a wooden dowel in order to prevent falling. On test day, each mouse received one (1.75 g/kg; Experiment 1) or two (1.75 and 2.0 g/kg; Experiment 2) injections of ethanol; an initial administration before being placed on the dowel and in Experiment 2, an additional administration after the first regain of balance on the dowel. Blood samples were taken immediately after loss of balance [when blood ethanol concentrations (BECs) were rising] and at recovery (during falling BECs) in Experiment 1, and after first and second recovery in Experiment 2. It was found that HAP mice fell from the dowel significantly earlier and at lower BECs than LAP mice following the initial injection of ethanol and were therefore more sensitive to its early effects. Furthermore, Experiment 1 detected significantly greater AFT development (BECfalling--BECrising) in HAP mice when compared with LAP mice, which occurred within ~30 min, supporting our hypothesis. However, AFT was not different between lines in Experiment 2, indicating that ~30-60 min following alcohol administration, AFT development was similar in both lines. These data show that high alcohol drinking genetically associates with both high initial sensitivity and very early tolerance to the ataxic effects of ethanol.

Cues paired with either rapid or slower self-administered cocaine injections acquire similar conditioned rewarding properties

The faster drugs of abuse reach the brain, the more addictive they can be. It is not known why this is. Environmental stimuli associated with drugs can promote the development and persistence of addiction by invigorating and precipitating drug-seeking behaviour. We determined, therefore, whether cues associated with the self-administration of rapidly delivered cocaine (injected intravenously over 5 versus 90 seconds) would acquire greater conditioned rewarding properties, as assessed by the performance of an operant response reinforced solely by the cues. Rats nose-poked for intravenous cocaine infusions delivered either over 5 or 90 seconds. Discrete visual cues accompanied each infusion. The rats could then press a lever to obtain the cues—now a conditioned reward—or an inactive lever. Rats in both the 5- and 90-second groups pressed more on the active versus inactive lever following extensive (24 sessions) but not following limited (3 sessions) self-administration training. There were no group differences in this behaviour. Following withdrawal from cocaine self-administration, lever discrimination progressively abated in both groups and was lost by withdrawal day 30. However, the rewarding properties of the cues were not “forgotten” because on withdrawal days 32–33, amphetamine selectively enhanced active-lever pressing, and did so to a similar extent in both groups. Thus, cues paired with rapid or slower cocaine delivery acquire similar conditioned rewarding properties. We conclude, therefore, that the rapid delivery of cocaine to the brain promotes addiction by mechanisms that might not involve a greater ability of drug cues to control behaviour.

Probability and predictors of transition from first use to dependence on nicotine, alcohol, cannabis, and cocaine: results of the National Epidemiologic Survey on Alcohol and Related Conditions (NESARC)

BACKGROUND

This study aims to estimate general and racial-ethnic specific cumulative probability of developing dependence among nicotine, alcohol, cannabis or cocaine users, and to identify predictors of transition to substance dependence.

METHODS

Analyses were done for the subsample of lifetime nicotine (n=15,918), alcohol (n=28,907), cannabis (n=7389) or cocaine (n=2259) users who participated in the first and second wave of the National Epidemiological Survey on Alcohol and Related Conditions (NESARC). Discrete-time survival analyses were implemented to estimate the cumulative probability of transitioning from use to dependence and to identify predictors of transition to dependence.

RESULTS

The cumulative probability estimate of transition to dependence was 67.5% for nicotine users, 22.7% for alcohol users, 20.9% for cocaine users, and 8.9% for cannabis users. Half of the cases of dependence on nicotine, alcohol, cannabis and cocaine were observed approximately 27, 13, 5 and 4 years after use onset, respectively. Significant racial-ethnic differences were observed in the probability of transition to dependence across the four substances. Several predictors of dependence were common across the four substances assessed.

CONCLUSIONS

Transition from use to dependence was highest for nicotine users, followed by cocaine, alcohol and cannabis users. Transition to cannabis or cocaine dependence occurred faster than transition to nicotine or alcohol dependence. The existence of common predictors of transition dependence across substances suggests that shared mechanisms are involved. The increased risk of transition to dependence among individuals from minorities or those with psychiatric or dependence comorbidity highlights the importance of promoting outreach and treatment of these populations.

Low and high affinity dopamine transporter inhibitors block dopamine uptake within 5 sec of intravenous injection

Extensive evidence suggests that the reinforcing effects of cocaine involve inhibition of dopamine transporters (DAT) and subsequent increases in dopamine (DA) levels in the striatum. We have previously reported that cocaine inhibits the DAT within 4-5 s of i.v. injection, matching the temporal profile of the behavioral and subjective effects of cocaine. Intravenous injection of GBR-12909, a high affinity, long-acting DAT inhibitor, also inhibits DA uptake within 5 s. Given that high affinity, long-acting drugs are considered to have relatively low abuse potential, we found it intriguing that GBR-12909 had an onset profile similar to that of cocaine. To further explore the onset kinetics of both low and high affinity DAT inhibitors, we examined the effects of i.v. cocaine (1.5 mg/kg), methylphenidate (1.5 mg/kg), nomifensine (1.5 mg/kg), GBR-12909 (1.5 mg/kg), PTT (0.5 mg/kg), and WF23 (0.5 mg/kg) on electrically-evoked DA release and uptake in the nucleus accumbens core. Results indicate that all of the DAT inhibitors significantly inhibited DA uptake within 5 s of injection. However, the timing of peak uptake inhibition varied greatly between the low and high affinity uptake inhibitors. Uptake inhibition following cocaine, methylphenidate, and nomifensine peaked 30 s following injection. In contrast, peak effects for GBR-12909, PTT, and WF23 occurred between 20 and 60 min following injection. These observations suggest that the initial onset for i.v. DAT inhibitors is extremely rapid and does not appear to be dictated by a drug's affinity.

Coca-paste seized samples characterization: chemical analysis, stimulating effect in rats and relevance of caffeine as a major adulterant

Coca-paste (CP) is a drug of abuse that so far has not been extensively characterized. CP is an intermediate product of the cocaine alkaloid extraction process from coca leaves, hence it has a high content of cocaine base mixed with other chemical substances (impurities) and it is probably adulterated when it reaches the consumers. Despite its high prevalence and distribution through South America, little is known about its effects on the central nervous system. In the present study, a chemical analysis of CP samples from different police seizures was performed to determine the cocaine base content and the presence and content of impurities and adulterants. Some CP representative samples were selected to study the effects on the locomotor activity induced after acute systemic administration in rats as a measure of its stimulant action. The behavioral response was compared to equivalent doses of cocaine. As expected, cocaine was the main component in most of the CP samples assayed. Caffeine was the only active adulterant detected. Interestingly, several CP samples elicited a higher stimulant effect compared to that observed after cocaine when administered at equivalent doses of cocaine base. Combined treatment of cocaine and caffeine, as surrogate of different CP samples mimicked their stimulant effect. We demonstrated that cocaine and caffeine are the main components responsible for the CP-induced stimulant action while the contribution of the impurities was imperceptible.

Episodic withdrawal promotes psychomotor sensitization to morphine

The relative intermittency or continuity of drug delivery is a major determinant of addictive liability, and also influences the impact of drug exposure on brain function and behavior. Events that occur during the offset of drug action (ie, acute withdrawal) may have an important role in the consequences of intermittent drug exposure. We assessed whether recurrent episodes of acute withdrawal contribute to the development of psychomotor sensitization in rodents during daily morphine exposure. The acoustic startle reflex--a measure of anxiety induced by opiate withdrawal-was used to resolve and quantify discrete withdrawal episodes, and pharmacological interventions were used to manipulate withdrawal severity. Startle potentiation was observed during spontaneous withdrawal from a single morphine exposure, and individual differences in initial withdrawal severity positively predicted the subsequent development of sensitization. Manipulations that reduce or exacerbate withdrawal severity also produced parallel changes in the degree of sensitization. These results demonstrate that the episodic experience of withdrawal during daily drug exposure has a novel role in promoting the development of psychomotor sensitization--a prominent model of drug-induced neurobehavioral plasticity. Episodic withdrawal may have a pervasive role in many effects of intermittent drug exposure and contribute to the development of addiction.

The novel triple reuptake inhibitor JZAD-IV-22 exhibits an antidepressant pharmacological profile without locomotor stimulant or sensitization properties

Triple reuptake inhibitors (TRIs) that block the dopamine transporter (DAT), norepinephrine transporter, and serotonin transporter are being developed as a new class of antidepressant that may have better efficacy and fewer side effects compared with traditional antidepressants. We describe a novel TRI, 2-[4-(4-chlorophenyl)-1-methylpiperidin-3-ylmethylsulfanyl]-1-(3-methylpiperidin-1-yl)-ethanone (JZAD-IV-22), that inhibits all three monoamine transporters with approximately equal potency in vitro. (+/-)-1-(3,4-dichlorophenyl)-3-azabicyclo-[3.1.0]hexane hydrochloride (DOV 216,303), a TRI shown to be an effective antidepressant in a clinical trial, shows reuptake inhibition similar to that of JZAD-IV-22 in vitro. Furthermore, both JZAD-IV-22 and DOV 216,303 increase levels of dopamine, norepinephrine, and serotonin in the mouse prefrontal cortex when administered by peripheral injection. JZAD-IV-22 and DOV 216,303 exhibited antidepressant-like efficacy in the mouse forced-swim and tail-suspension tests at doses that increased neurotransmitter levels. Because development of DAT inhibitors could be hindered by abuse liability, both JZAD-IV-22 and DOV 216,303 were compared in two assays that are markers of abuse potential. Both JZAD-IV-22 and DOV 216,303 partially substituted for cocaine in a drug discrimination assay in rats, and high doses of DOV 216,303 produced locomotor sensitization in mice. JZAD-IV-22 showed no evidence of sensitization at any dose tested. These results demonstrate that JZAD-IV-22 is a TRI with antidepressant-like activity similar to that of DOV 216,303. The striking feature that distinguishes the two TRIs is that locomotor sensitization, a common underlying feature of drugs of abuse, is seen with DOV 216,303 but is completely lacking in JZAD-IV-22. These findings may have implications for the potential for abuse liability in humans.

Rats markedly escalate their intake and show a persistent susceptibility to reinstatement only when cocaine is injected rapidly

When drugs enter the brain rapidly, liability for addiction is increased, but why this is the case is not well understood. Here we examined the influence of varying the speed of intravenous cocaine delivery on self-administration behavior in rats given limited or extended opportunity to take drug. The speed of cocaine delivery had no effect on self-administration behavior when rats were given only 1 h each day to take cocaine. When given sixfold more time to take cocaine, rats that received cocaine rapidly (5-45 s) increased their total intake eightfold. However, rats that received cocaine more slowly (>90 s) did not avail themselves of the opportunity to take much more drug: they increased their intake only twofold. Furthermore, when tested 45 d after the last self-administration session, a drug-priming injection reinstated drug-seeking behavior only in rats that in the past had cocaine injected rapidly (5 s), and this was associated with a persistent suppression in the ability of cocaine to induce immediate early gene expression. Cocaine may be potentially more addictive when it reaches the brain rapidly because (1) this promotes a marked escalation in intake and (2) it renders individuals more susceptible to relapse long after the discontinuation of drug use. This is presumably because the rapid uptake of drug to the brain preferentially promotes persistent changes in brain systems that regulate motivation for drug, and continuing exposure to large amounts of drug produces a vicious cycle of additional maladaptive changes in brain and behavior.

N-methyl d-aspartate receptors are involved in the induction, but not expression stage of amphetamine sensitization in schedule-induced polydipsia in rats

1. The aim of the present study was to examine the role of dopaminergic and glutamatergic receptors on different stages of the amphetamine (AMPH) sensitized effect in schedule-induced polydipsia (SIP) in rats. 2. Three experiments were designed to evaluate the roles of DAD2 receptor antagonist haloperidol (HAL) and glutamatergic N-methyl d-aspartate receptor antagonist MK-801 on both the induction and the expression stage of AMPH sensitization in SIP rats. First, the induction of AMPH sensitization in the SIP model was tested again to confirm previous findings. Second, HAL or MK-801 was co-administered with AMPH on five consecutive days and their effect on induction was examined 14 days after withdrawal. Finally, HAL or MK-801 was co-administered with AMPH on the final day of testing in SIP rats in which AMPH sensitization had been established previously. 3. The present results showed that HAL and MK-801 affected the effect of AMPH differently during the process of sensitization. Whereas HAL influenced the sensitization during both the induction and the expression phases, MK-801 affected only the induction phase; thus, once the sensitization had been established, MK-801 had no further influence. 4. These results suggest that the SIP model could be considered useful for the study of sensitization. In addition, the induction and expression of AMPH sensitization is influenced differently by the dopaminergic and glutamatergic systems.

Particle size distribution of nicotine in mainstream smoke from 2R4F, Marlboro Medium, and Quest1 cigarettes under different puffing regimens

Nicotine's dose and rate of delivery to the brain play an important role in its addiction and cardiovascular effects. Nicotine is mainly present in the particulate phase of cigarette smoke, and since particle size distribution controls the deposition behavior of particles in the respiratory tract, changes in the particle size distribution can produce variations in its regional and total dose to the lung. These variations can change its absorption rate and delivery to the brain. The particle size distribution of mainstream smoke (MS) varies with changes in puffing regimen and cigarette design and composition. This study examined nicotine in different particle size fractions of MS generated from 2R4F, Marlboro Medium, and Quest1 cigarettes using 3 puffing regimens: (1) FTC-like puff, 35 ml over 2 s; (2) short puff, 50 ml over 2 s; and (3) long puff, 100 ml over 10 s. MS was generated in a chamber at 37 degrees C and >95% relative humidity (RH), and size-segregated particles were collected using RJR cascade impactors. Particle size distribution was determined by spectrophotometry. Nicotine was analyzed using gas chromatography and mass spectrometry. Results showed that nicotine speciates in larger particles (1.1-1.9 microm diameter) under the long puffing regimen and in smaller particles (0.4-1.1 microm diameter) under the short puffing regimen, while mass median aerodynamic diameter of mainstream smoke particles was found to be approximately constant (0.9-1.0 microm) for the three puffing regimens. Overall, changes in puffing regimen have a significant effect on particle size distribution of nicotine and its deposited dose.