Cocaine, reward, movement and monoamine transporters

Endocannabinoid-dependent decrease of GABAergic transmission on dopaminergic neurons is associated with susceptibility to cocaine stimulant effects in pre-adolescent male MAOA hypomorphic mice exposed to early life stress

Vulnerability to cocaine use disorder depends upon a combination of genetic and environmental risk factors. While early life adversity is a critical environmental vulnerability factor for drug misuse, allelic variants of the monoamine oxidase A (MAOA) gene have been shown to moderate its influence on the risk of drug-related problems. However, data on the interactions between MAOA variants and early life stress (ES) with respect to predisposition to cocaine abuse are limited. Here, we show that a mouse model capturing the interaction of genetic (low-activity alleles of the Maoa gene; MAOA^Neo) and environmental (i.e., ES) vulnerability factors displays an increased sensitivity to repeated in vivo cocaine psychomotor stimulant actions associated with a reduction of GABAA receptor-mediated inhibition of dopamine neurons of the ventral tegmental area (VTA). Depolarization-induced suppression of inhibition (DSI), a 2-arachidonoylglycerol (2AG)-dependent form of short-term plasticity, also becomes readily expressed by dopamine neurons from male MAOA^Neo ES mice repeatedly treated with cocaine. The activation of either dopamine D2 or CB1 receptors contributes to cocaine-induced DSI expression, decreased GABA synaptic efficacy, and hyperlocomotion. Next, in vivo pharmacological enhancement of 2AG signaling during repeated cocaine exposure occludes its actions both in vivo and ex vivo. This data extends our knowledge of the multifaceted sequelae imposed by this gene-environment interaction in VTA dopamine neurons of male pre-adolescent mice and contributes to our understanding of neural mechanisms of vulnerability for early onset cocaine use.

Betel quid: New insights into an ancient addiction

The use of areca nuts (areca) in the form of betel quids constitutes the fourth most common addiction in the world, associated with high risk for oral disease and cancer. Areca is a complex natural product, making it difficult to identify specific components associated with the addictive and carcinogenic properties. It is commonly believed that the muscarinic agonist arecoline is at the core of the addiction. However, muscarinic receptor activation is not generally believed to support drug-taking behaviour. Subjective accounts of areca use include descriptions of both sedative and stimulatory effects, consistent with the presence of multiple psychoactive agents. We have previously reported partial agonism of α4-containing nicotinic acetylcholine receptors by arecoline and subsequent inhibition of those receptors by whole areca broth. In the present study, we report the inhibition of nicotinic acetylcholine receptors and other types of neurotransmitter receptors with compounds of high molecular weight in areca and the ability of low molecular weight areca extract to activate GABA and glutamate receptors. We confirm the presence of a high concentration of GABA and glutamate in areca. Additionally, data also indicate the presence of a dopamine and serotonin transporter blocking activity in areca that could account for the reported stimulant and antidepressant activity. Our data suggest that toxic elements of high molecular weight may contribute to the oral health liability of betel quid use, while two distinct low molecular weight components may provide elements of reward, and the nicotinic activity of arecoline contributes to the physical dependence of addiction.

Cannabidiol prevents several of the behavioral alterations related to cocaine addiction in mice

Cocaine dependence is a highly prevalent disease in modern society and lacks an effective treatment. Cannabidiol (CBD), a major non-psychoactive constituent of Cannabis sativa, has been shown to be a promising tool in the management of some neuropsychiatric disorders, including cocaine abuse. However, its therapeutic effects on the behavioral outcomes related to cocaine addiction remain unclear. The present research evaluates the effects of CBD (30, 60 and 120 mg/kg; injected intraperitoneally) on the acquisition, expression, extinction and reinstatement of cocaine (10 mg/kg)-induced conditioned place preference (CPP; Study 1); cocaine (25 mg/kg)-induced locomotor stimulation (Study 2); and cocaine withdrawal symptoms (Study 3) in male C57BL/6 J mice. The results show that CBD does not possess motivational properties in itself and does not modify the acquisition, expression or extinction of cocaine-induced CPP. Interestingly, when administered during the extinction phase of the cocaine-induced CPP, CBD (30 and 60 mg/kg) prevented priming-induced reinstatement of CPP. Moreover, CBD abolished cocaine-induced hyperactivity without altering the spontaneous locomotion of the animals. Furthermore, CBD (120 mg/kg) reduced the memory deficits induced by cocaine withdrawal in the object recognition test, though it did not reverse depressive-like symptoms measured in the tail suspension test. Overall, our data suggest that CBD can prevent the development of cocaine addiction, and, when administered during cocaine abstinence, may be of help in avoiding relapse to drug-seeking and in ameliorating the memory disturbances provoked by chronic consumption of cocaine.

Computational Design and Crystal Structure of a Highly Efficient Benzoylecgonine Hydrolase

Benzoylecgonine (BZE) is the major toxic metabolite of cocaine and is responsible for the long-term cocaine-induced toxicity owing to its long residence time in humans. BZE is also the main contaminant following cocaine consumption. Here, we identified the bacterial cocaine esterase (CocE) as a BZE-metabolizing enzyme (BZEase), which can degrade BZE into biological inactive metabolites (ecgonine and benzoic acid). CocE was redesigned by a reactant-state-based enzyme design theory. An encouraging mutant denoted as BZEase2, presented a >400-fold improved catalytic efficiency against BZE compared with wild-type (WT) CocE. In vivo, a single dose of BZEase2 (1 mg kg^-1 , IV) could eliminate nearly all BZE within only two minutes, suggesting the enzyme has the potential for cocaine overdose treatment and BZE elimination in the environment by accelerating BZE clearance. The crystal structure of a designed BZEase was also determined.

A "Drug-Dependent" Immune System Can Compromise Protection against Infection: The Relationships between Psychostimulants and HIV

Psychostimulant use is a major comorbidity in people living with HIV, which was initially explained by them adopting risky behaviors that facilitate HIV transmission. However, the effects of drug use on the immune system might also influence this phenomenon. Psychostimulants act on peripheral immune cells even before they reach the central nervous system (CNS) and their effects on immunity are likely to influence HIV infection. Beyond their canonical activities, classic neurotransmitters and neuromodulators are expressed by peripheral immune cells (e.g., dopamine and enkephalins), which display immunomodulatory properties and could be influenced by psychostimulants. Immune receptors, like Toll-like receptors (TLRs) on microglia, are modulated by cocaine and amphetamine exposure. Since peripheral immunocytes also express TLRs, they may be similarly affected by psychostimulants. In this review, we will summarize how psychostimulants are currently thought to influence peripheral immunity, mainly focusing on catecholamines, enkephalins and TLR4, and shed light on how these drugs might affect HIV infection. We will try to shift from the classic CNS perspective and adopt a more holistic view, addressing the potential impact of psychostimulants on the peripheral immune system and how their systemic effects could influence HIV infection.

Cocaine Reduces the Neuronal Population While Upregulating Dopamine D2-Receptor-Expressing Neurons in Brain Reward Regions: Sex-Effects

Addiction to cocaine is associated with dysfunction of the dopamine mesocortical system including impaired dopamine-2 receptor (D2r) signaling. However, the effects of chronic cocaine on neuronal adaptations in this system have not been systematically examined and data available is mostly from males. Here, we investigated changes in the total neuronal density and relative concentration of D2r-expressing neurons in the medial prefrontal cortex (mPFC), dorsal striatum (Dstr), nucleus accumbens (NAc), and ventral tegmental area (VTA) in both male and female mice passively exposed to cocaine for two weeks. In parallel experiments, we measured mRNA levels for Drd2 and for opioid peptides (mPenk and mPdyn). Through a combination of large field of view fluorescent imaging with BAC transgenic D2r-eGFP mice and immunostaining, we observed that cocaine exposed mice had a higher density of D2r-positive cells that was most prominent in mPFC and VTA and larger for females than for males. This occurred amidst an overall significant decrease in neuronal density (measured with NeuN) in both sexes. However, increases in Drd2 mRNA levels with cocaine were only observed in mPFC and Dstr in females, which might reflect the limited sensitivity of the method. Our findings, which contrast with previous findings of cocaine-induced downregulation of D2r binding availability, could reflect a phenotypic shift in neurons that did not previously express Drd2 and merits further investigation. Additionally, the neuronal loss particularly in mPFC with chronic cocaine might contribute to the cognitive impairments observed with cocaine use disorder.

Cocaine-induced locomotor stimulation involves autophagic degradation of the dopamine transporter

Cocaine exerts its stimulant effect by inhibiting dopamine reuptake leading to increased dopamine signaling. This action is thought to reflect binding of cocaine to the dopamine transporter (DAT) to inhibit its function. However, cocaine is a relatively weak inhibitor of DAT, and many DAT inhibitors do not share the behavioral actions of cocaine. We previously showed that toxic levels of cocaine induce autophagic neuronal cell death. Here, we show that subnanomolar concentrations of cocaine elicit neural autophagy in vitro and in vivo. Autophagy inhibitors reduce the locomotor stimulant effect of cocaine in mice. Cocaine-induced autophagy degrades transporters for dopamine but not serotonin in the nucleus accumbens. Autophagy inhibition impairs cocaine conditioned place preference in mice. Our findings indicate that autophagic degradation of DAT modulates behavioral actions of cocaine.

Amphetamine-like Neurochemical and Cardiovascular Effects of α-Ethylphenethylamine Analogs Found in Dietary Supplements

Dietary supplements often contain additives not listed on the label, including α-ethyl homologs of amphetamine such as N,α-diethylphenethylamine (DEPEA). Here, we examined the neurochemical and cardiovascular effects of α-ethylphenethylamine (AEPEA), N-methyl-α-ethylphenethylamine (MEPEA), and DEPEA as compared with the effects of amphetamine. All drugs were tested in vitro using uptake inhibition and release assays for monoamine transporters. As expected, amphetamine acted as a potent and efficacious releasing agent at dopamine transporters (DAT) and norepinephrine transporters (NET) in vitro. AEPEA and MEPEA were also releasers at catecholamine transporters, with greater potency at NET than DAT. DEPEA displayed fully efficacious release at NET but weak partial release at DAT (i.e., 40% of maximal effect). In freely moving, conscious male rats fitted with biotelemetry transmitters for physiologic monitoring, amphetamine (0.1–3.0 mg/kg, s.c.) produced robust dose-related increases in blood pressure (BP), heart rate (HR), and motor activity. AEPEA (1–10 mg/kg, s.c.) produced significant increases in BP but not HR or activity, whereas DEPEA and MEPEA (1–10 mg/kg, s.c.) increased BP, HR, and activity. In general, the phenethylamine analogs were approximately 10-fold less potent than amphetamine. Our results show that α-ethylphenethylamine analogs are biologically active. Although less potent than amphetamine, they produce cardiovascular effects that could pose risks to humans. Given that MEPEA and DEPEA increased locomotor activity, these substances may also have significant abuse potential.

Organic cation transporter 3 and the dopamine transporter differentially regulate catecholamine uptake in the basolateral amygdala and nucleus accumbens

Regional alterations in kinetics of catecholamine uptake are due in part to variations in clearance mechanisms. The rate of clearance is a critical determinant of the strength of catecholamine signaling. Catecholamine transmission in the nucleus accumbens core (NAcc) and basolateral amygdala (BLA) is of particular interest due to involvement of these regions in cognition and motivation. Previous work has shown that catecholamine clearance in the NAcc is largely mediated by the dopamine transporter (DAT), but clearance in the BLA is less DAT-dependent. A growing body of literature suggests that organic cation transporter 3 (OCT3) also contributes to catecholamine clearance in both regions. Consistent with different clearance mechanisms between regions, catecholamine clearance is more rapid in the NAcc than in the BLA, though mechanisms underlying this have not been resolved. We compared the expression of DAT and OCT3 and their contributions to catecholamine clearance in the NAcc and BLA. We found DAT protein levels were ~ 4-fold higher in the NAcc than in the BLA, while OCT3 protein expression was similar between the two regions. Immunofluorescent labeling of the two transporters in brain sections confirmed these findings. Ex vivo voltammetry demonstrated that the magnitude of catecholamine release was greater, and the clearance rate was faster in the NAcc than in the BLA. Additionally, catecholamine clearance in the BLA was more sensitive to the OCT3 inhibitor corticosterone, while clearance in the NAcc was more cocaine sensitive. These distinctions in catecholamine clearance may underlie differential effects of catecholamines on behavioral outputs mediated by these regions.

Cholinergic Receptors and Addiction

Human behavior can be controlled by physical or psychological dependencies associated with addiction. One of the most insidious addictions in our society is the use of tobacco products which contain nicotine. This addiction can be associated with specific receptors in the brain that respond to the natural neurotransmitter acetylcholine. These nicotinic acetylcholine receptors (nAChR) are ligand-gated ion channels formed by the assembly of one or multiple types of nAChR receptor subunits. In this paper, we review the structure and diversity of nAChR subunits and our understanding for how different nAChR subtypes play specific roles in the phenomenon of nicotine addiction. We focus on receptors containing β2 and/or α6 subunits and the special significance of α5-containing receptors. These subtypes all have roles in regulating dopamine-mediated neurotransmission in the mesolimbic reward pathways of the brain. We also discuss the unique roles of homomeric α7 nAChR in behavioral responses to nicotine and how our knowledge of nAChR functional diversity may help guide pharmacotherapeutic approaches for treating nicotine addiction. While nicotine addiction is a truly global problem, the use of areca nut (betel) products is also a serious addiction associated with public health issues across most of South Asia, impacting as many as 600 million people. We discuss how cholinergic receptors of the brain are also involved with areca addiction and the unique challenges for dealing with addiction to this substance.

Roles Played by the Na+/Ca2+ Exchanger and Hypothermia in the Prevention of Ischemia-Induced Carrier-Mediated Efflux of Catecholamines into the Extracellular Space: Implications for Stroke Therapy

The release of [³H]dopamine ([³H]DA) and [³H]noradrenaline ([³H]NA) in acutely perfused rat striatal and cortical slice preparations was measured at 37 °C and 17 °C under ischemic conditions. The ischemia was simulated by the removal of oxygen and glucose from the Krebs solution. At 37 °C, resting release rates in response to ischemia were increased; in contrast, at 17 °C, resting release rates were significantly reduced, or resting release was completely prevented. The removal of extracellular Ca²⁺ further increased the release rates of [³H]DA and [³H]NA induced by ischemic conditions. This finding indicated that the Na⁺/Ca²⁺ exchanger (NCX), working in reverse in the absence of extracellular Ca²⁺, fails to trigger the influx of Ca²⁺ in exchange for Na⁺ and fails to counteract ischemia by further increasing the intracellular Na⁺ concentration ([Na⁺]_i). KB-R7943, an inhibitor of NCX, significantly reduced the cytoplasmic resting release rate of catecholamines under ischemic conditions and under conditions where Ca²⁺ was removed. Hypothermia inhibited the excessive release of [³H]DA in response to ischemia, even in the absence of Ca²⁺. These findings further indicate that the NCX plays an important role in maintaining a high [Na⁺]_i, a condition that may lead to the reversal of monoamine transporter functions; this effect consequently leads to the excessive cytoplasmic tonic release of monoamines and the reversal of the NCX. Using HPLC combined with scintillation spectrometry, hypothermia, which enhances the stimulation-evoked release of DA, was found to inhibit the efflux of toxic DA metabolites, such as 3,4-dihydroxyphenylacetaldehyde (DOPAL). In slices prepared from human cortical brain tissue removed during elective neurosurgery, the uptake and release values for [³H]NA did not differ from those measured at 37 °C in slices that were previously maintained under hypoxic conditions at 8 °C for 20 h. This result indicates that hypothermia preserves the functions of the transport and release mechanisms, even under hypoxic conditions. Oxidative stress (H₂O₂), a mediator of ischemic brain injury enhanced the striatal resting release of [³H]DA and its toxic metabolites (DOPAL, quinone). The study supports our earlier findings that during ischemia transmitters are released from the cytoplasm. In addition, the major findings of this study that hypothermia of brain slice preparations prevents the extracellular calcium concentration ([Ca²⁺]_o)-independent non-vesicular transmitter release induced by ischemic insults, inhibiting Na⁺/Cl⁻-dependent membrane transport of monoamines and their toxic metabolites into the extracellular space, where they can exert toxic effects.

Lack of correlation between the activity of the mesolimbic dopaminergic system and the rewarding properties of pregabalin in mouse

RATIONALE

Pregabalin is a psychoactive drug indicated in the treatment of epilepsy, neuropathic pain, and generalized anxiety disorders. Pregabalin acts on different neurotransmission systems by inactivating the alpha2-delta subunit of voltage-gated calcium channels. In light of this pharmacological property, the hypothesis has been raised that pregabalin may regulate the mesolimbic dopamine pathway and thereby display a potential for misuse or abuse as recently observed in humans. Although some preclinical data support this possibility, the rewarding properties of gabapentinoid are still a matter for debate.

OBJECTIVE

The aim of this work was to evaluate the rewarding properties of pregabalin and to determine its putative mechanism of action in healthy mice.

RESULTS

Pregabalin alone (60 mg/kg; s.c.) produced a rewarding effect in the conditioned place preference (CPP) test albeit to a lower extent than cocaine (30 mg/kg; s.c.). Interestingly, when assessing locomotor activity in the CPP, the PGB60 group, similarly to the cocaine group, showed an increased locomotor activity. In vivo single unit extracellular recording showed that pregabalin had mixed effects on dopamine (DA) neuronal activity in the ventral tegmental area since it decreased the activity of 50% of neurons and increased 28.5% of them. In contrast, cocaine decreased 75% of VTA DA neuronal activity whereas none of the neurons were activated. Intracerebal microdialysis was then conducted in awake freely mice to determine to what extent such electrophysiological parameters influence the extracellular DA concentrations ([DA]ext) in the nucleus accumbens. Although pregabalin failed to modify this parameter, cocaine produced a robust increase (800%) in [DA]ext.

CONCLUSIONS

Collectively, these electrophysiological and neurochemical experiments suggest that the rewarding properties of pregabalin result from a different mode of action than that observed with cocaine. Further experiments are warranted to determine whether such undesirable effects can be potentiated under pathological conditions such as neuropathic pain, mood disorders, or addiction and to identify the key neurotransmitter system involved.

Dopaminergic Effects of Major Bath Salt Constituents 3,4-Methylenedioxypyrovalerone (MDPV), Mephedrone, and Methylone Are Enhanced Following Co-exposure

Designer drug mixtures popularized as "bath salts" often contain the synthetic cathinones 3,4 methylenedioxypyrovalerone (MDPV), mephedrone, and methylone in various combinations. However, most preclinical investigations have only assessed the effects of individual bath salt constituents, and little is known about whether co-exposure to MDPV, mephedrone, and methylone produces significant neuropharmacological interactions. This study evaluated and compared how MDPV, mephedrone, and methylone influence discrete brain tissue dopamine (DA) levels and motor stimulant responses in mice when administered alone and as a ternary mixture. Male adolescent Swiss-Webster mice received intraperitoneal injections of saline or 1 or 10 mg/kg doses of MDPV, mephedrone, or methylone, or a cocktail of all three cathinones at doses of 1, 3.3, or 10 mg/kg each. The effect of each treatment on DA and DA metabolite levels in mesolimbic and nigrostriatal brain tissue was quantified 15 min after a single exposure using HPLC-ECD. Additionally, locomotor activity was recorded in mice after acute (day 1) and chronic intermittent (day 7) dosing. MDPV, mephedrone, and methylone produced dose-related increases in mesolimbic and nigrostriatal DA levels that were significantly enhanced following their co-administration. In addition, mice treated with the cathinone cocktail displayed decreased locomotor activity on day 1 that was exacerbated by day 7 and not observed with any of the drugs alone. Our findings demonstrate a significant enhanced effect of MDPV, mephedrone, and methylone on both DA, and these effects on DA result in significant alterations in locomotor activity.

Regulators of G-protein signaling 2 and 4 differentially regulate cocaine-induced rewarding effects

There is a need to identify new therapeutic targets for the treatment of cocaine addiction due to the rise in cocaine abuse and deaths due to cocaine overdose. Regulator of G protein signaling (RGS) proteins such as RGS2 and RGS4 are widely distributed in brain regions that play a role in drug reward. Importantly, RGS2 and RGS4 negatively regulate G-protein coupled receptor signaling pathways of monoaminergic neurotransmitters that play a role in the rewarding effects of cocaine by enhancing the rate of hydrolysis of Gα-bound guanine nucleotide triphosphate. Thus, the objective of this study was to investigate the effects of cocaine on conditioned place preference (CPP) and locomotor activity in mice that lacked either RGS2 or RGS4 (i.e. knockout (KO) mice) and their wildtype (WT) littermates. Moreover recent studies have reported influence of sex on RGS functioning and hence studies were conducted in both male and female mice. Cocaine-induced CPP was attenuated in male, but not female RGS4 KO mice compared to respective RGS4 WT mice. Cocaine-induced CPP was not influenced by deletion of RGS2 in either male or female mice. Similarly, cocaine-induced locomotor activity was not influenced by deletion of either RGS2 or RGS4 irrespective of sex. Together, the data indicate that the rewarding effects of cocaine were attenuated in the absence of RGS4 expression, but not in the absence of RGS2 expression in a sex-dependent manner. Importantly, these data suggest that RGS4 can serve as a potential target for medications that can be used to treat cocaine addiction.

Median and Dorsal Raphe Serotonergic Neurons Control Moderate Versus Compulsive Cocaine Intake

BACKGROUND

Reduced expression of the serotonin transporter (SERT) promotes anxiety and cocaine intake in both humans and rats. We tested the hypothesis that median raphe nucleus (MRN) and dorsal raphe nucleus (DRN) serotonergic projections differentially mediate these phenotypes.

METHODS

We used virally mediated RNA interference to locally downregulate SERT expression and compared the results with those of constitutive SERT knockout. Rats were allowed either short access (ShA) (1 hour) or long access (LgA) (6 hours) to cocaine self-administration to model moderate versus compulsive-like cocaine taking.

RESULTS

SERT knockdown in the MRN increased cocaine intake selectively under ShA conditions and, like ShA cocaine self-administration, reduced corticotropin-releasing factor (CRF) immunodensity in the paraventricular nucleus of the hypothalamus. In contrast, SERT knockdown in the DRN increased cocaine intake selectively under LgA conditions and, like LgA cocaine self-administration, reduced CRF immunodensity in the central nucleus of the amygdala. SERT knockdown in the MRN or DRN produced anxiety-like behavior, as did withdrawal from ShA or LgA cocaine self-administration. The phenotype of SERT knockout rats was a summation of the phenotypes generated by MRN- and DRN-specific SERT knockdown.

CONCLUSIONS

Our results highlight a differential role of serotonergic projections arising from the MRN and DRN in the regulation of cocaine intake. We propose that a cocaine-induced shift from MRN-driven serotonergic control of CRF levels in the hypothalamus to DRN-driven serotonergic control of CRF levels in the amygdala may contribute to the transition from moderate to compulsive intake of cocaine.

Role of monoaminergic systems and ambient temperature in bath salts constituent 3,4-methylenedioxypyrovalerone (MDPV)-elicited hyperthermia and locomotor stimulation in mice

3,4-Methylenedioxypyrovalerone (MDPV) is a common constituent of illicit bath salts products, and in vitro studies implicate monoamine transporters as mediators of its pharmacological effects. Locomotor and thermoregulatory effects of MDPV depend on ambient temperature, so the current studies aimed to gauge the involvement of dopamine (DA), norepinephrine (NE), and serotonin (5-HT) in MDPV-induced locomotor stimulation and hyperthermia in the mouse at different ambient temperatures. Mice were pretreated with the selective 5-HT-reuptake inhibitor fluoxetine (3 mg/kg), the NE-reuptake inhibitor desipramine (3 mg/kg), the DA-reuptake inhibitor bupropion (10 mg/kg), or saline, followed by 10 mg/kg MDPV while thermoregulation and locomotor activity were monitored via radiotelemetry. In other studies, mice were pretreated for three days with saline, 100 mg/kg of the tryptophan hydroxylase inhibitor para-chlorophenylalanine (p-CPA), or 100 mg/kg of the tyrosine hydroxylase inhibitor α-methyl-para-tyrosine (α-MPT) before receiving 10 mg/kg MDPV on the fourth day. All manipulations were conducted at both 20 °C and 28 °C ambient temperatures. MDPV increased locomotor activity under both ambient conditions and modestly increased core body temperature at 20 °C; however, neither pretreatment with monoamine reuptake inhibitors nor monoamine synthesis inhibitors significantly altered these effects. At 28 °C, MDPV induced a more pronounced hyperthermic effect which was attenuated by bupropion, desipramine, or fluoxetine pretreatment, but not by the monoamine synthesis inhibitors. These results suggest that MDPV may have a more complex pharmacological profile than suggested by in vitro studies, perhaps extending beyond interactions with monoamine transporters. A more thorough binding profile of MDPV at various brain recognition sites should be developed. This article is part of the Special Issue entitled 'Designer Drugs and Legal Highs.'

Reinforcing Doses of Intravenous Cocaine Produce Only Modest Dopamine Uptake Inhibition

The reinforcing efficacy of cocaine is thought to stem from inhibition of the dopamine transporter (DAT) and subsequent increases in extracellular dopamine concentrations in the brain. In humans, this hypothesis has generally been supported by positron emission tomography imaging studies where the percent of DATs occupied by cocaine is used as a measure of cocaine activity in the brain. Interpretation of these studies, however, often relies on the assumption that measures of DAT occupancy directly correspond with functional DAT blockade. In the current studies, we used in vivo and in vitro fast scan cyclic voltammetry in mice to measure dopamine uptake inhibition following varying doses of cocaine as well as two high affinity DAT inhibitors. We then compared dopamine clearance rates following these drug treatments to dopamine clearance obtained from DAT knockout mice as a proxy for complete DAT blockade. We found that administration of abused doses of cocaine resulted in approximately 2% of maximal DAT blockade. Overall, our data indicate that abused doses of cocaine produce a relatively modest degree of DA uptake inhibition, and suggest that the relationship between DAT occupancy and functional blockade of the DAT is more complex than originally posited.

NMDAR dependent intracellular responses associated with cocaine conditioned place preference behavior

The aim of this study was to investigate the intracellular responses associated with the acquisition and expression of cocaine-context associations. ERK (extracellular regulated kinase), CREB (cAMP responsive element binding protein), FosB and ΔFosB proteins were of particular interest due to their involvement in cocaine reward and in synaptic plasticity underlying learning and memory. We used the conditioned place preference (CPP) paradigm, which employs a Pavlovian conditioning procedure to establish an association between a drug-paired environment and the drug's rewarding effects, to study the role of these signaling pathways in cocaine-context associations. N-methyl-D-aspartate receptor (NMDAR) antagonism prior to cocaine administration during conditioning blocked the acquisition of cocaine CPP and reduced Nucleus Accumbens (NAc) phosphorylated-ERK (pERK) and phosphorylated CREB (pCREB) levels following the CPP test (drug-free). We also show that cocaine-induced increases in Caudate Putamen (CPu) FosB and ΔFosB levels are decreased after MK-801 pre-treatment during conditioning. In addition, our results provide evidence for the involvement of striatal SIRT (Silent Information Regulator of Transcription) proteins in cocaine-CPP. These results will aid in the advancement of general knowledge about the molecular formation and retrieval of cocaine-associated memories that can be used in the future when designing treatments for cocaine addiction that target both prevention and relapse.

N-phenylpropyl-N'-substituted piperazines occupy sigma receptors and alter methamphetamine-induced hyperactivity in mice

This study examined the effect of the N-phenylpropyl-N'-substituted piperazine ligands SA4503 (3.4-dimethoxyphenethyl), YZ-067 (4-methoxyphenethyl), YZ-185 (3-methoxyphenethyl) and Nahas-3h (4-methoxybenzyl) on methamphetamine-induced hyperactivity in mice. In a previous study in rats, SA4503 increased methamphetamine-induced hyperactivity at a lower ligand dose and enhanced it at a higher dose. The other ligands have not been investigated in this assay. Presently, mice were administered sigma ligands, and specific [¹²⁵I]E-IA-DM-PE-PIPZE and [¹²⁵I]RTI-121 binding was measured to determine σ1 sigma receptor and dopamine transporter occupancy, respectively. Mice were also administered sigma ligands followed by methamphetamine, and locomotor activity was measured. Each of the ligands occupied σ1 sigma receptors (ED₅₀=0.2-0.6μmol/kg) with similar potency, but none occupied the transporter (ED₅₀>10μmol/kg). At the highest dose tested (31.6μmol/kg) all four sigma ligands significantly attenuated methamphetamine-induced hyperactivity. Interestingly, SA4503, YZ-067 and Nahas-3h, but not YZ-185, enhanced methamphetamine-induced hyperactivity at lower ligand doses (1-3.16μmol/kg). These results suggest that these ligands function as stimulant agonists at lower doses and as antagonists at higher does, with subtle changes in the substitution pattern at the 3- and 4-positions of the phenethyl group contributing to the nature of the interactions. Overall, these data indicate a complex role for σ1 sigma receptor ligands in methamphetamine's behavioral effects.

Fluorine-18 Radiolabeled PET Tracers for Imaging Monoamine Transporters: Dopamine, Serotonin, and Norepinephrine

This review focuses on the development of fluorine-18 radiolabeled PET tracers for imaging the dopamine transporter (DAT), serotonin transporter (SERT), and norepinephrine transporter (NET). All successful DAT PET tracers reported to date are members of the 3β-phenyl tropane class and are synthesized from cocaine. Currently available carbon-11 SERT PET tracers come from both the diphenylsulfide and 3β-phenyl nortropane class, but so far only the nortropanes have found success with fluorine-18 derivatives. NET imaging has so far employed carbon-11 and fluorine-18 derivatives of reboxetine but due to defluorination of the fluorine-18 derivatives further research is still necessary.

Cyclin-Dependent Kinase Inhibitor 1a (p21) Modulates Response to Cocaine and Motivated Behaviors

This study investigated the functional role of cyclin-dependent kinase inhibitor 1a (Cdkn1a or p21) in cocaine-induced responses using a knockout mouse model. Acute locomotor activity after cocaine administration (15 mg/kg, i.p.) was decreased in p21(-/-) mice, whereas cocaine-induced place preference was enhanced. Interestingly, κ-opioid-induced place aversion was also significantly enhanced. Concentration-dependent analysis of locomotor activity in response to cocaine demonstrated a rightward shift in the p21(-/-) mice. Pretreatment with a 5-hydroxytryptamine receptor antagonist did not alter the enhancement of cocaine-induced conditioned place preference in p21(-/-) mice, indicating a lack of involvement of serotonergic signaling in this response. Cocaine exposure increased p21 expression exclusively in the ventral sector of the hippocampus of rodents after either contingent or noncontingent drug administration. Increased p21 expression was accompanied by increased histone acetylation of the p21 promoter region in rats. Finally, increased neurogenesis in the dorsal hippocampus of p21(-/-) mice was also observed. These results show that functional loss of p21 altered the acute locomotor response to cocaine and the conditioned responses to either rewarding or aversive stimuli. Collectively, these findings demonstrate a previously unreported involvement of p21 in modulating responses to cocaine and in motivated behaviors.

Classic Studies on the Interaction of Cocaine and the Dopamine Transporter

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

Negative affective states and cognitive impairments in nicotine dependence

Smokers have substantial individual differences in quit success in response to current treatments for nicotine dependence. This observation may suggest that different underlying motivations for continued tobacco use across individuals and nicotine cessation may require different treatments in different individuals. Although most animal models of nicotine dependence emphasize the positive reinforcing effects of nicotine as the major motivational force behind nicotine use, smokers generally report that other consequences of nicotine use, including the ability of nicotine to alleviate negative affective states or cognitive impairments, as reasons for continued smoking. These states could result from nicotine withdrawal, but also may be associated with premorbid differences in affective and/or cognitive function. Effects of nicotine on cognition and affect may alleviate these impairments regardless of their premorbid or postmorbid origin (e.g., before or after the development of nicotine dependence). The ability of nicotine to alleviate these symptoms would thus negatively reinforce behavior, and thus maintain subsequent nicotine use, contributing to the initiation of smoking, the progression to dependence and relapse during quit attempts. The human and animal studies reviewed here support the idea that self-medication for pre-morbid and withdrawal-induced impairments may be more important factors in nicotine addiction and relapse than has been previously appreciated in preclinical research into nicotine dependence. Given the diverse beneficial effects of nicotine under these conditions, individuals might smoke for quite different reasons. This review suggests that inter-individual differences in the diverse effects of nicotine associated with self-medication and negative reinforcement are an important consideration in studies attempting to understand the causes of nicotine addiction, as well as in the development of effective, individualized nicotine cessation treatments.

The effect of quetiapine (Seroquel™) on conditioned place preference and elevated plus maze tests in rats when administered alone and in combination with (+)-amphetamine

RATIONALE

Recent case reports describe recreational use of quetiapine and drug-seeking behaviour to obtain quetiapine, an atypical antipsychotic.

OBJECTIVE

We examined the hypothesis that quetiapine (10, 20 or 40 mg/kg) alone or co-administered with (+)-amphetamine (0.25, 0.5, 0.75 or 2.0 mg/kg) will affect reward and/or decrease anxiety in rats, as measured by conditioned place preference (CPP) and elevated plus maze (EPM) test, respectively.

RESULTS

Quetiapine (20 mg/kg) produced greater open arm time and entries in the EPM test compared to 10 and 40 mg/kg, and quetiapine (10 mg/kg) significantly increased open arm entries and time when co-administered with (+)-amphetamine (0.5 mg/kg) compared to (+)-amphetamine (0.5 mg/kg) alone, suggesting decreased anxiety. Quetiapine (10, 20 or 40 mg/kg) produced no CPP when administered alone; the lowest dose of quetiapine (10 mg/kg) reduced CPP produced by a low dose of (+)-amphetamine (0.25 mg/kg), but had no significant effect on CPP produced by a higher dose (0.5 mg/kg).

DISCUSSION

The quetiapine-induced anxiolytic effect in the EPM might explain why humans are misusing quetiapine and combining it with (+)-amphetamine. It is possible that humans experience an anxiolytic effect of the combined drugs and relatively unaltered rewarding effects of (+)-amphetamine. The results shed some light on the question of why humans are abusing and misusing quetiapine, despite its dopamine (DA) D2 receptor antagonism; it will be the task of future studies to identify the pharmacological mechanism mediating this behaviour.

Deficient Wnt signalling triggers striatal synaptic degeneration and impaired motor behaviour in adult mice

Synapse degeneration is an early and invariant feature of neurodegenerative diseases. Indeed, synapse loss occurs prior to neuronal degeneration and correlates with the symptom severity of these diseases. However, the molecular mechanisms that trigger synaptic loss remain poorly understood. Here we demonstrate that deficient Wnt signalling elicits synaptic degeneration in the adult striatum. Inducible expression of the secreted Wnt antagonist Dickkopf1 (Dkk1) in adult mice (iDkk1) decreases the number of cortico-striatal glutamatergic synapses and of D1 and D2 dopamine receptor clusters. Synapse loss occurs in the absence of axon retraction or cell death. The remaining excitatory terminals contain fewer synaptic vesicles and have a reduced probability of evoked transmitter release. IDkk1 mice show impaired motor coordination and are irresponsive to amphetamine. These studies identify Wnts as key endogenous regulators of synaptic maintenance and suggest that dysfunction in Wnt signalling contributes to synaptic degeneration at early stages in neurodegenerative diseases.

Disruption of dopamine D1 receptor phosphorylation at serine 421 attenuates cocaine-induced behaviors in mice

Dopamine D1 receptors (D1Rs) play a key role in cocaine addiction, and multiple protein kinases such as GRKs, PKA, and PKC are involved in their phosphorylation. Recently, we reported that protein kinase D1 phosphorylates the D1R at S421 and promotes its membrane localization. Moreover, this phosphorylation of S421 is required for cocaineinduced behaviors in rats. In the present study, we generated transgenic mice over-expressing S421A-D1R in the forebrain. These transgenic mice showed reduced phospho-D1R (S421) and its membrane localization, and reduced downstream ERK1/2 activation in the striatum. Importantly, acute and chronic cocaine-induced locomotor hyperactivity and conditioned place preference were significantly attenuated in these mice. These findings provide in vivo evidence for the critical role of S421 phosphorylation of the D1R in its membrane localization and in cocaine-induced behaviors. Thus, S421 on the D1R represents a potential pharmacotherapeutic target for cocaine addiction and other drug-abuse disorders.

The κ-opioid receptor gene as a predictor of response in a cocaine vaccine clinical trial

OBJECTIVES

We examined the pharmacogenetic association between a variant in the κ-opioid receptor (OPRK1) gene and the response to treatment with a cocaine vaccine tested in a recent clinical trial (October 2003 to April 2005). This gene has a protective allele for opioid addiction that may act by the inhibiting dopamine activation associated with reinforcement.

METHODS

Sixty-nine DNA samples were obtained from 114 cocaine-dependent and opioid-dependent patients who were enrolled in a 16-week phase IIb randomized double-blind placebo-controlled trial and received five vaccinations over the first 12 weeks. We genotyped 66 of these patients for the rs6473797 variant of the OPRK1 gene and compared vaccine patients with placebo patients in terms of cocaine-free urines over time.

RESULTS

Using repeated measures analysis of variance corrected for population structure, it was seen that vaccine pharmacotherapy reduced cocaine-positive urines significantly on the basis of the OPRK1 genotype. Among patients treated with the cocaine vaccine, those who were homozygous for the protective A allele of rs6473797 had the proportion of positive urines drop from 78 to 51% on using the vaccine (point-wise P<0.0001, experiment-wise P<0.005), whereas the positive urines of individuals carrying the nonprotective, risk G allele dropped from 82 to 77%. Strong treatment by single nucleotide polymorphism interactions reflected a lower baseline and significant reduction for placebo patients with the risk G allele (P<0.00001).

CONCLUSION

This study indicates that a patient's OPRK1 genotype could be used to identify a subset of individuals for whom vaccine treatment may be an effective pharmacotherapy for cocaine dependence.

Restoration of cocaine stimulation and reward by reintroducing wild type dopamine transporter in adult knock-in mice with a cocaine-insensitive dopamine transporter

In previous studies, we generated knock-in mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) and found cocaine does not stimulate locomotion or produce reward in these mice, indicating DAT inhibition is necessary for cocaine stimulation and reward. However, DAT uptake is reduced in DAT-CI mice and thus the lack of cocaine responses could be due to adaptive changes. To test this, we used adeno-associated virus (AAV) to reintroduce the cocaine-sensitive wild type DAT (AAV-DATwt) back into adult DAT-CI mice, which restores cocaine inhibition of DAT in affected brain regions but does not reverse the adaptive changes. In an earlier study we showed that AAV-DATwt injections in regions covering the lateral nucleus accumbens (NAc) and lateral caudate-putamen (CPu) restored cocaine stimulation but not cocaine reward. In the current study, we expanded the AAV-DATwt infected areas to cover the olfactory tubercle (Tu) and the ventral midbrain (vMB) containing the ventral tegmental area (VTA) and substantia nigra (SN) in addition to CPu and NAc with multiple injections. These mice displayed the restoration of both locomotor stimulation and cocaine reward. We further found that AAV-DATwt injection in the vMB alone was sufficient to restore both cocaine stimulation and reward in DAT-CI mice. AAV injected in the VTA and SN resulted in DATwt expression and distribution to the DA terminal regions. In summary, cocaine induced locomotion and reward can be restored in fully developed DAT-CI mice, and cocaine inhibition of DAT expressed in dopaminergic neurons originated from the ventral midbrain mediates cocaine reward and stimulation.

Methylphenidate and cocaine self-administration produce distinct dopamine terminal alterations

Methylphenidate (MPH) is a commonly abused psychostimulant prescribed for the treatment of attention deficit hyperactivity disorder. MPH has a mechanism of action similar to cocaine (COC) and is commonly characterized as a dopamine transporter (DAT) blocker. While there has been extensive work aimed at understanding dopamine (DA) nerve terminal changes following COC self-administration, very little is known about the effects of MPH self-administration on the DA system. We used fast scan cyclic voltammetry in nucleus accumbens core slices from animals with a 5-day self-administration history of 40 injections/day of either MPH (0.56 mg/kg) or COC (1.5 mg/kg) to explore alterations in baseline DA release and uptake kinetics as well as alterations in the interaction of each compound with the DAT. Although MPH and COC have similar behavioral effects, the consequences of self-administration on DA system parameters were found to be divergent. We show that COC self-administration reduced DAT levels and maximal rates of DA uptake, as well as reducing electrically stimulated release, suggesting decreased DA terminal function. In contrast, MPH self-administration increased DAT levels, DA uptake rates and DA release, suggesting enhanced terminal function, which was supported by findings of increased metabolite/DA tissue content ratios. Tyrosine hydroxylase messenger RNA, protein and phosphorylation levels were also assessed in both groups. Additionally, COC self-administration reduced COC-induced DAT inhibition, while MPH self-administration increased MPH-induced DAT inhibition, suggesting opposite pharmacodynamic effects of these two drugs. These findings suggest that the factors governing DA system adaptations are more complicated than simple DA uptake blockade.

A selective sigma-2 receptor ligand antagonizes cocaine-induced hyperlocomotion in mice

Cocaine functions, in part, through agonist actions at sigma-1 (σ1 ) receptors, while roles played by sigma-2 (σ2 ) receptors are less established. Attempts to discriminate σ2 receptor-mediated effects of cocaine in locomotor hyperactivity assays have been hampered by the lack of potent and selective antagonists. Certain tetrahydroisoquinolinyl benzamides display high σ2 receptor affinity, and excellent selectivity for binding to σ2 over σ1 receptors. The behavioral properties of this structural class of σ ligands have not yet been investigated. The present study evaluated 5-bromo-N-[4-(6,7-dimethoxy-3,4-dihydro-1H-isoquinolin-2-yl)-butyl)]-2,3-dimethoxy-benzamide, 1, a ligand shown by others to bind preferentially to σ2 over σ1 receptors, as well as dopamine D2 and D3 sites. First, we determined binding to monoamine transporters and opioid receptors, and noted 57-fold selectivity for σ2 receptors over the serotonin transporter, and >800-fold selectivity for σ2 receptors over the other sites tested. We then examined 1 in locomotor activity studies using male CD-1® mice, and saw no alteration of basal activity at doses up to 31.6 µmol/kg. Cocaine produced a fivefold increase in locomotor activity, which was attenuated by 66% upon pretreatment of mice with 1 at 31.6 µmol/kg. In vivo radioligand binding studies also were performed, and showed no occupancy of σ1 receptors or the dopamine transporter by 1, or its possible metabolites, at the 31.6 µmol/kg dose. Thus, ligand 1 profiles behaviorally as a σ2 receptor-selective antagonist that is able to counteract cocaine's motor stimulatory effects.

Cocaine-induced changes of synaptic transmission in the striatum are modulated by adenosine A2A receptors and involve the tyrosine phosphatase STEP

The striatum is a brain area implicated in the pharmacological action of drugs of abuse. Adenosine A2A receptors (A2ARs) are highly expressed in the striatum and mediate, at least in part, cocaine-induced psychomotor effects in vivo. Here we studied the synaptic mechanisms implicated in the pharmacological action of cocaine in the striatum and investigated the influence of A2ARs. We found that synaptic transmission was depressed in corticostriatal slices after perfusion with cocaine (10 μM). This effect was reduced by the A2AR antagonist ZM241385 and almost abolished in striatal A2AR-knockout mice (mice lacking A2ARs in striatal neurons, stA2ARKO). The effect of cocaine on synaptic transmission was also prevented by the protein tyrosine phosphatases (PTPs) inhibitor sodium orthovanadate (Na3VO4). In synaptosomes prepared from striatal slices, we found that the activity of striatal-enriched protein tyrosine phosphatase (STEP) was upregulated by cocaine, prevented by ZM241385, and absent in synaptosomes from stA2ARKO. The role played by STEP in cocaine modulation of synaptic transmission was investigated in whole-cell voltage clamp recordings from medium spiny neurons of the striatum. We found that TAT-STEP, a peptide that renders STEP enzymatically inactive, prevented cocaine-induced reduction in AMPA- and NMDA-mediated excitatory post-synaptic currents, whereas the control peptide, TAT-myc, had no effect. These results demonstrate that striatal A2ARs modulate cocaine-induced synaptic depression in the striatum and highlight the potential role of PTPs and specifically STEP in the effects of cocaine.

Behavior of knock-in mice with a cocaine-insensitive dopamine transporter after virogenetic restoration of cocaine sensitivity in the striatum

Cocaine's main pharmacological actions are the inhibition of the dopamine, serotonin, and norepinephrine transporters. Its main behavioral effects are reward and locomotor stimulation, potentially leading to addiction. Using knock-in mice with a cocaine-insensitive dopamine transporter (DAT-CI mice) we have shown previously that inhibition of the dopamine transporter (DAT) is necessary for both of these behaviors. In this study, we sought to determine brain regions in which DAT inhibition by cocaine stimulates locomotor activity and/or produces reward. We used adeno-associated viral vectors to re-introduce the cocaine-sensitive wild-type DAT in specific brain regions of DAT-CI mice, which otherwise only express a cocaine-insensitive DAT globally. Viral-mediated expression of wild-type DAT in the rostrolateral striatum restored cocaine-induced locomotor stimulation and sensitization in DAT-CI mice. In contrast, the expression of wild-type DAT in the dorsal striatum, or in the medial nucleus accumbens, did not restore cocaine-induced locomotor stimulation. These data help to determine cocaine's molecular actions and anatomical loci that cause hyperlocomotion. Interestingly, cocaine did not produce significant reward - as measured by conditioned place-preference - in any of the three cohorts of DAT-CI mice with the virus injections. Therefore, the locus or loci underlying cocaine-induced reward remain underdetermined. It is possible that multiple dopamine-related brain regions are involved in producing the robust rewarding effect of cocaine.

Rhodiola rosea Impairs Acquisition and Expression of Conditioned Place Preference Induced by Cocaine

A novel approach to the treatment of adverse effects of drugs of abuse is one which makes use of natural products. The present study investigated the effect of Rhodiola rosea L. hydroalcoholic extract (RHO) on cocaine-induced hyperactivity and conditioned place preference (CPP) in mice. In a first experiment, mice received RHO (15, 20 or 25 mg/kg, IG), cocaine (25 mg/kg, i.p.) (COC), or a combination of both drugs (COC + RHO15, COC + RHO20, and COC + RHO25), and their locomotor activity was evaluated. In a second experiment, the effects of RHO on the acquisition, expression, and reinstatement of cocaine CPP (induced by drug priming or social defeat stress) were evaluated. RHO alone did not increase activity but potentiated the hyperactivity induced by cocaine. Rhodiola did not induce motivational effects by itself but attenuated the acquisition and expression of cocaine-induced CPP. Moreover, it was found that RHO did not block reinstatement. The results indicate that RHO is effective in reducing the rewarding properties of cocaine but is ineffective in preventing priming or stress-induced cocaine reinstatement. In light of these findings, the benefits of Rhodiola rosea L. as a treatment of cocaine addiction would seem to be limited.

The dopamine antagonist cis-flupenthixol blocks the expression of the conditioned positive but not the negative effects of cocaine in rats

Human cocaine users report that the initial "high" produced by cocaine administration is followed by an anxiogenic "crash". Given that cocaine has such robust and opposing properties, it is likely that both positive and negative effects of cocaine contribute to an individual's motivation to administer the drug. Despite this likelihood, the neurobiology underlying cocaine's dual processes remains unclear. While much literature supports a role for dopamine (DA) in cocaine reward, it is uncertain if DA also contributes to the drug's negative effects. Our laboratory has extensively utilized a modified conditioned place test to explore cocaine's opponent processes. In this paradigm rats develop conditioned place preferences (CPPs) for an environment paired with the immediate/positive effects of cocaine, and conditioned place aversions (CPAs) for an environment paired with the delayed/negative effects present 15-min after i.v. injection. In the current study rats were conditioned to associate an environment with either the immediate or delayed effects of i.v. cocaine (1mg/kg/0.1ml) 3h after i.p. pre-treatment with either the DA D1/D2 receptor antagonist cis-flupenthixol (0.5mg/kg/ml) or saline vehicle. As expected, vehicle-treated control animals developed the normal pattern of CPPs for cocaine's immediate effects or CPAs for the delayed effects of cocaine. However, while DA receptor antagonism prevented the expression of cocaine CPPs it did not alter the expression of cocaine-induced CPAs. These data confirm a role for DA transmission in cocaine reward but suggest that different neural pathways mediate the drug's negative/anxiogenic properties.

Effects of N-phenylpropyl-N'-substituted piperazine sigma receptor ligands on cocaine-induced hyperactivity in mice

The present study examined N-phenylpropyl-N'-substituted piperazine sigma receptor ligands on cocaine-induced changes in locomotor activity in mice. Previous reports indicate that N-phenylpropyl-N'-(4-methoxybenzyl)piperazine (Nahas-3h), N-phenylpropyl-N'-(4-methoxyphenethyl)piperazine (YZ-067), and N-phenylpropyl-N'-(3-methoxyphenethyl)piperazine (YZ-185) bind with high affinity (Ki values≈1 nM) to σ1 sigma receptors. YZ-067 and YZ-185 are known to attenuate cocaine-induced convulsions, while Nahas-3h has not been tested in behavioral studies. Nahas-3h significantly attenuated cocaine-induced hyperactivity. YZ-067 decreased the effect of cocaine in a dose-dependent manner. Interestingly, YZ-185 inhibited cocaine's effect at higher doses, but enhanced cocaine's effect at a low dose. The YZ-185 inhibition of cocaine-induced hyperactivity was not surmounted by increasing the cocaine dose. Overall, this study is consistent with previous work showing the ability of certain sigma receptor ligands to affect cocaine-induced hyperactivity. Further, subtle alterations of ligand structure and the specific dosage levels employed influence the behavioral effects observed, with a 3-methoxy substituent apparently conferring the ability of a ligand to enhance cocaine's locomotor stimulatory effects.

Implications of genome wide association studies for addiction: are our a priori assumptions all wrong?

Substantial genetic contributions to addiction vulnerability are supported by data from twin studies, linkage studies, candidate gene association studies and, more recently, Genome Wide Association Studies (GWAS). Parallel to this work, animal studies have attempted to identify the genes that may contribute to responses to addictive drugs and addiction liability, initially focusing upon genes for the targets of the major drugs of abuse. These studies identified genes/proteins that affect responses to drugs of abuse; however, this does not necessarily mean that variation in these genes contributes to the genetic component of addiction liability. One of the major problems with initial linkage and candidate gene studies was an a priori focus on the genes thought to be involved in addiction based upon the known contributions of those proteins to drug actions, making the identification of novel genes unlikely. The GWAS approach is systematic and agnostic to such a priori assumptions. From the numerous GWAS now completed several conclusions may be drawn: (1) addiction is highly polygenic; each allelic variant contributing in a small, additive fashion to addiction vulnerability; (2) unexpected, compared to our a priori assumptions, classes of genes are most important in explaining addiction vulnerability; (3) although substantial genetic heterogeneity exists, there is substantial convergence of GWAS signals on particular genes. This review traces the history of this research; from initial transgenic mouse models based upon candidate gene and linkage studies, through the progression of GWAS for addiction and nicotine cessation, to the current human and transgenic mouse studies post-GWAS.

Mice expressing markedly reduced striatal dopamine transporters exhibit increased locomotor activity, dopamine uptake turnover rate, and cocaine responsiveness

Variations in the expression levels of the dopamine transporter (DAT) can influence responsiveness to psychostimulant drugs like cocaine. To better understand this relationship, we studied a new DAT-low expresser (DAT-LE) mouse model and performed behavioral and biochemical studies with it. Immunoblotting and [(3) H]WIN 35,428 binding analyses revealed that these mice express ∼35% of wildtype (WT) mouse striatal DAT levels. Compared to WT mice, DAT-LE mice were hyperactive in a novel open-field environment. Despite their higher basal locomotor activity, cocaine (10 or 20 mg/kg, i.p.) induced greater locomotor activation in DAT-LE mice than in WT mice. The maximal velocity (Vmax ) of DAT-mediated [(3) H]DA uptake into striatal synaptosomes was reduced by 46% in DAT-LE mice, as compared to WT. Overall, considering the reduced number of DAT binding sites (Bmax ) along with the reduced Vmax in DAT-LE mice, a 2-fold increase in DA uptake turnover rate (Vmax /Bmax ) was found, relative to WT mice. This suggests that neuroadaptive changes have occurred in the DAT-LE mice that would help to compensate for their low DAT numbers. Interestingly, these changes do not include a reduction in tyrosine hydroxylase levels, as was previously reported in DAT knockout homozygous and heterozygous animals. Further, these changes are not sufficient to prevent elevated novelty- and cocaine-induced locomotor activity. Hence, these mice represent a unique model for studying changes of in vivo DAT function and regulation that result from markedly reduced levels of DAT expression.

A brief history of oxytocin and its role in modulating psychostimulant effects

Over the past century, the polypeptide oxytocin has played an important role in medicine with major highlights including the identification of its involvement in parturition and the milk let-down reflex. Oxytocin is now implicated in an extensive range of psychological phenomena including reward and memory processes and has been investigated as a treatment for several psychiatric disorders including addiction, anxiety, autism, and schizophrenia. In this review, we first provide an historical overview of oxytocin and describe key aspects of its physiological activity. We then outline some pharmacological limitations in this field of research before highlighting the role of oxytocin in a wide range of behavioral and neuronal processes. Finally, we review evidence for a modulatory role of oxytocin with regard to psychostimulant effects. Key findings suggest that oxytocin attenuates a broad number of cocaine and methamphetamine induced behaviors and associated neuronal activity in rodents. Evidence also outlines a role for oxytocin in the prosocial effects of 3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) in both rodents and humans. Clinical trials should now investigate the effectiveness of oxytocin as a novel intervention for psychostimulant addiction and should aim to determine its specific role in the therapeutic properties of MDMA that are currently being investigated.

The effects of the 5-HT3 receptor antagonist tropisetron on cocaine-induced conditioned taste aversions

Although cocaine readily induces taste aversions, little is known about the mechanisms underlying this effect. Recent work has shown that cocaine's actions on serotonin (5-HT) may be involved. To address this possibility, the present experiments examined a role of the specific 5-HT receptor, 5-HT3, in this effect given that it is implicated in a variety of behavioral effects of cocaine. This series of investigations first assessed the aversive effects of the 5-HT3 receptor antagonist tropisetron alone (Experiment 1). Specifically, in Experiment 1 male Sprague-Dawley rats were given repeated pairings of a novel saccharin solution and tropisetron (0, 0.056, 0.18 and 0.56mg/kg). Following this, a non-aversion-inducing dose of tropisetron (0.18mg/kg) was assessed for its ability to block aversions induced by a range of doses of cocaine (Experiment 2). Specifically, in Experiment 2 animals were given access to a novel saccharin solution and then injected with tropisetron (0 or 0.18mg/kg) followed by an injection of various doses of cocaine (0, 10, 18 and 32mg/kg). Cocaine induced dose-dependent taste aversions that were not blocked by tropisetron, suggesting that cocaine's aversive effects are not mediated by 5-HT, at least at this specific receptor subtype. At the intermediate dose of cocaine, aversions appeared to be potentiated, suggesting 5-HT3 may play a limiting role in cocaine's aversive effects. These data are discussed in the context of previous examinations of the roles of serotonin, dopamine, and norepinephrine in cocaine-induced aversions.

Bacterial cocaine esterase: a protein-based therapy for cocaine overdose and addiction

Cocaine is highly addictive and there are no pharmacotherapeutic drugs available to treat acute cocaine toxicity or chronic abuse. Antagonizing an inhibitor such as cocaine using a small molecule has proven difficult. The alternative approach is to modify cocaine's pharmacokinetic properties by sequestering or hydrolyzing it in serum and limiting access to its sites of action. We took advantage of a bacterial esterase (CocE) that has evolved to hydrolyze cocaine and have developed it as a therapeutic that rapidly and specifically clears cocaine from the subject. Native enzyme was unstable at 37°C, thus limiting CocE's potential. Innovative computational methods based on the protein's structure helped elucidate its mechanism of destabilization. Novel protein engineering methodologies were applied to substantially improve its stability in vitro and in vivo. These improvements rendered CocE as a powerful and efficacious therapeutic to treat cocaine intoxication and lead the way towards developing a therapy for addiction.

Chronic fluoxetine selectively upregulates dopamine D₁-like receptors in the hippocampus

The dentate gyrus of the hippocampus has been implicated in mechanisms of action of selective serotonin reuptake inhibitors (SSRIs). We have recently demonstrated that the SSRI fluoxetine can reverse the state of maturation of the adult dentate granule cells and enhances serotonin 5-HT₄ receptor-mediated synaptic potentiation at the synapses formed by their mossy fiber axons. Here, we show that fluoxetine can induce long-lasting enhancement of dopaminergic modulation at the mossy fiber synapse. Synaptic responses arising from the mossy fiber-CA3 pyramidal cell synapse were recorded using acute mouse hippocampal slices. Dopamine potentiates mossy fiber synaptic transmission by activating D₁-like receptors. Chronic fluoxetine treatment induced a prominent increase in the magnitude of dopamine-induced synaptic potentiation, and this effect was maintained at least up to 1 month after withdrawal of fluoxetine. Quantitative autoradiography revealed that binding of the D₁-like receptor ligand [³H]SCH23390 was selectively increased in the dentate gyrus and along the mossy fiber in fluoxetine-treated mice. However, binding of the 5-HT₄ receptor ligand [³H]GR113808 was not significantly changed. These results suggest that chronic fluoxetine enhanced the dopaminergic modulation at least in part by upregulating expression of D₁-like receptors, while the enhanced serotonergic modulation may be mediated by modifications of downstream signaling pathways. These enhanced monoaminergic modulations would greatly increase excitatory drive to the hippocampal circuit through the dentate gyrus. The highly localized upregulation of D₁-like receptors further supports the importance of the dentate gyrus in the mechanism of action of SSRIs.

Candidate pathway association study in cocaine dependence: the control of neurotransmitter release

OBJECTIVES

Cocaine is the second most used illegal drug in Europe. The transition from use to dependence involves both genetic and environmental factors. Genetic variation in neurotransmitter systems is involved in the susceptibility to cocaine dependence. We examined the possible contribution to cocaine dependence of 16 genes involved in the cellular machinery that controls neurotransmitter release: genes encoding proteins of the SNARE complex (STX1A, SNAP25, VAMP1 and VAMP2), fusion control elements (SYT1, SYT2, CPLX1, CPLX2, CPLX3 and CPLX4) and regulatory elements (STXBP1, SYP, SNPH, NSF, NAPA and RAB3A).

METHODS

We genotyped 121 SNPs, selected according to genetic coverage criteria, in 360 cocaine-dependent patients and 360 controls from Spain.

RESULTS

Single and multiple-marker analyses revealed a strong association between cocaine dependence and the NSF gene, encoding the N-ethylmaleimide-sensitive factor (P = 5.1e-04, OR = 2.44 (1.45-4.00) and P = 0.001, OR = 1.82 (1.28-2.59), respectively). The presence and absence of psychotic symptoms were also studied. Interestingly, when we considered the time between initial consumption and the onset of cocaine dependence, we observed that the association was mainly restricted to the group of patients that rapidly developed drug dependence (≤ 2 years; P = 2.98e-06, OR = 1.33 (1.20-1.47)).

CONCLUSIONS

Our data show preliminary evidence that NSF may predispose not only to cocaine dependence, but also to an early onset of the dependence.

Behavioural genetics of the serotonin transporter

The serotonin transporter is a key regulator of the bioavailability of serotonin and therefore any modulation in the expression or action of the transporter would be expected to have consequences on behaviour. The transporter has therefore become a target for pharmaceutical intervention in behavioural and mood disorders. The search for polymorphic variants in the transporter that would associate with neurological disorders has been extensive but has become focused on two domains which are both termed variable number tandem repeat (VNTR)polymorphisms. Both of these VNTRs are in non-coding DNA and therefore proposed to be mechanistically involved in a disorder through their ability to modulate transcriptional or post-transcriptional regulation of the transporter. The most extensively studied is in the promoter and is a bi-allelic insertion/deletion found in the 50 promoter region of the gene 1.2 kb upstream of the transcriptional start site. This VNTR, termed, 5-HTTLPR was initially identified as two variants containing either, 14 (short/deletion) or 16 (long/insertion) copies of a 22 bp repeat. A second widely studied VNTR found in the non-coding region of the transporter is located within intron 2 and comprises 9, 10 or 12 copies of a16–17 bp repeat termed, STin2.9, STin2.10 and STin2.12, respectively. These VNTR polymorphisms have been associated with a range of behavioural and psychiatric disorders including depression, OCD, anxiety and schizophrenia, however often the lack of reproducibility in different cohorts has led to debate on the actual association of the polymorphisms with this extensive range of neurological conditions. Here we review these two polymorphic VNTRs in depth and relate that to pharmaceutical response, their ability to regulate differential transporter expression, their core involvement in gene-environment interaction and their genetic association with specific disorders.

The SLC6A4 VNTR genotype determines transcription factor binding and epigenetic variation of this gene in response to cocaine in vitro

We demonstrated that the genotype of the variable number tandem repeats (VNTRs) in the linked polymorphic region (LPR) of the 5' promoter and in the intron 2 (Stin2) transcriptional regulatory domains of the serotonin transporter SLC6A4 gene determined its promoter interactions with transcription factors and co-activators in response to cocaine in the JAr cell line. The LPR variants contain 14 (short, s) or 16 (long, l) copies of a 22-23 bp repeat element, whereas the Stin2 VNTR exists as three variants containing 9, 10 or 12 copies of a 16-17 bp repeat. We observed a differential effect of cocaine on the association of the promoter with the transcription factor CTCF, which bound to both LPR alleles prior to cocaine exposure but only to the l-allele following exposure. Significantly, this differential effect of cocaine was correlated with the binding of the transcriptional regulator MeCP2 specifically to the s-allele and recruiting the histone deacetylase complex (HDAC). Concurrently, cocaine increased the association of positive histone marks over the SLC6A4 gene locus. At the Stin2 domain, we lost binding of the transcription factor YB-1, while CTCF remained bound. Our biochemical data are consistent with differential reporter gene activity directed by the individual or dual domains in response to cocaine in an Epstein-Barr virus-based episome model of stable transfections. These observations suggest that exposure of JAr cells to cocaine may result in differential binding of transcription factors and activators based on a specific genotype that might alter epigenetic parameters affecting gene expression after the initial challenge.

Insights to drug addiction derived from ultrastructural views of the mesocorticolimbic system

Drugs of abuse increase the release of dopamine from mesocorticolimbic neurons in the ventral tegmental area. Thus, insights into the cytoarchitecture and the synaptic circuitry affecting the activity of dopaminergic neurons in this area are fundamental for understanding the commonalities produced by mechanistically distinct drugs of abuse. Electron microscopic immunolabeling has provided these insights and also shown the critical relationships between the dopaminergic axon terminals and their targeted neurons in the prefrontal cortex and in the both the dorsal and ventral striatum. These brain regions are among those where dopamine and associated neurotransmitters are most implicated in the transition from recreational to compulsive consumption of reinforcing drugs. Thus, the synaptic circuitry and drug-induced plasticity occurring in the ventral tegmental area and in dopamine-targeted regions are reviewed, as both are essential for understanding the long-lasting changes produced by addictive substances.

Smoking, dopamine transporter, and hand tremor

AIM

The purpose of the study was to evaluate the relationship between the number of packs smoked per day and specific uptake ratio (SUR) in the striatum on Tc-99m TRODAT, and frequency of hand tremor.

METHODS

It was a prospective, cross-sectional study. In all, 23 healthy nonsmokers and 37 current smokers were recruited in the study. All subjects underwent Tc-99m TRODAT SPECT, brain CT scan, thyroid function test, tremor measurement system, and neurologic examinations.

RESULTS

There were significant differences in the SUR in the striatum on Tc-99m TRODAT and in the frequency of hand tremor in rest state and in arm extended state among nonsmokers (grade I), current smokers with less than 1 pack smoked per day (grade II), and current smokers with equal or more than 1 pack smoked per day (grade III) by ANOVA (all P < 0.001). After adjusting for age and gender, there was a significantly negative correlation between smoke grade and SUR in the striatum on Tc-99m TRODAT by multiple linear regression (β = -0.45, P < 0.001). Smoke grade was the significant predictor for the frequency of hand tremor in rest state and in arm extended state, after adjusting for age and gender by multiple linear regression (β = 14.70, P < 0.001; β = 15.37, P < 0.001).

CONCLUSIONS

There is a dose-response relationship between the number of packs smoked per day and SUR in the striatum, and the frequency of hand tremor. Decreased dopamine transporter binding in the striatum and increased frequency of hand tremor in smokers may have important implications for evaluating the impact of smoking on the central and peripheral nerve systems.

Enhanced dopamine D1 and BDNF signaling in the adult dorsal striatum but not nucleus accumbens of prenatal cocaine treated mice

Previous work from our group and others utilizing animal models have demonstrated long-lasting structural and functional alterations in the meso-cortico-striatal dopamine pathway following prenatal cocaine (PCOC) treatment. We have shown that PCOC treatment results in augmented D1-induced cyclic AMP (cAMP) and cocaine-induced immediate-early gene expression in the striatum of adult mice. In this study we further examined basal as well as cocaine or D1-induced activation of a set of molecules known to be mediators of neuronal plasticity following psychostimulant treatment, with emphasis in the dorsal striatum (Str) and nucleus accumbens (NAc) of adult mice exposed to cocaine in utero. Basally, in the Str of PCOC treated mice there were significantly higher levels of (1) CREB and Ser133 P-CREB (2) Thr34 P-DARPP-32 and (3) GluA1 and Ser 845 P-GluA1 when compared to prenatal saline (PSAL) treated mice. In the NAc there were significantly higher basal levels of (1) CREB and Ser133 P-CREB, (2) Thr202/Tyr204 P-ERK2, and (3) Ser845 P-GluA1. Following acute administration of cocaine (15 mg/kg, i.p.) or D1 agonist (SKF 82958; 1 mg/kg, i.p.) there were significantly higher levels of Ser133 P-CREB, Thr34 P-DARPP-32, and Thr202/Tyr204 P-ERK2 in the Str that were evident in all animals tested. However, these cocaine-induced increases in phosphorylation were significantly augmented in PCOC mice compared to PSAL mice. In sharp contrast to the observations in the Str, in the NAc, acute administration of cocaine or D1 agonist significantly increased P-CREB and P-ERK2 in PSAL mice, a response that was not evident in PCOC mice. Examination of Ser 845 P-GluA1 revealed that cocaine or D1 agonist significantly increased levels in PSAL mice, but significantly decreased levels in the PCOC mice in both the Str and NAc. We also examined changes in brain-derived neurotrophic factor (BDNF). Our studies revealed significantly higher levels of the BDNF precursor, pro-BDNF, and one of its receptors, TrkB in the Str of PCOC mice compared to PSAL mice. These results suggest a persistent up-regulation of molecules critical to D1 and BDNF signaling in the Str of adult mice exposed to cocaine in utero. These molecular adaptations may underlie components of the behavioral deficits evident in exposed animals and a subset of exposed humans, and may represent a therapeutic target for ameliorating aspects of the PCOC-induced phenotype.