Roger D. Spealman

Cocaine Administered into the Medial Prefrontal Cortex Reinstates Cocaine-Seeking Behavior by Increasing AMPA Receptor-Mediated Glutamate Transmission in the Nucleus Accumbens

One of the major determinants of reinstatement to cocaine use among human addicts is acute reexposure to the drug, which often precipitates cocaine craving and relapse. We used an animal model of cocaine relapse to determine the role of the glutamatergic pathway from the medial prefrontal cortex (mPFC) to the nucleus accumbens in the reinstatement of cocaine-seeking behavior after a cocaine priming injection. Rats were trained to self-administer cocaine intravenously on a second order schedule. Responding was extinguished subsequently by substituting saline for cocaine. During subsequent reinstatement sessions, drug-seeking behavior was assessed after noncontingent priming injections. Results indicated that reinstatement induced by a systemic cocaine injection was blocked by intra-mPFC administration of the dopamine antagonist flupenthixol. Consistent with this finding, administration of cocaine directly into the mPFC reinstated cocaine-seeking behavior. Administration of cocaine into the nucleus accumbens also reinstated drug seeking, whereas microinjection of cocaine into the neostriatum or lateral septum did not. Reinstatement of cocaine seeking induced by intra-mPFC cocaine was blocked by administration of the AMPA receptor antagonist CNQX into the nucleus accumbens. Administration of the NMDA receptor antagonist AP-5 into the nucleus accumbens had variable effects on reinstatement induced by intra-mPFC cocaine in that AP-5 had no effect in some animals but augmented reinstatement in others. Subsequent experiments showed that intra-accumbal microinjection of AP-5 alone dose-dependently reinstated cocaine seeking. These data indicate that the glutamatergic pathway from the mPFC to the nucleus accumbens plays an important role in cocaine priming-induced reinstatement of drug seeking. Moreover, the present results demonstrate that AMPA and NMDA receptors in the nucleus accumbens have opposing roles in the reinstatement of cocaine-seeking behavior.

Antagonism of cocaine self-administration by selective dopamine D(1) and D(2) antagonists.

Effects of the dopamine D1 antagonist SCH 39166 were compared with those of the D2 antagonist eticlopride in squirrel monkeys responding under a second-order fixed-interval schedule of i.v. self-administration of cocaine. Dose-response curves were determined for a range of doses of self-administered cocaine (0.01–1.7 mg/kg/injection) alone and after pretreatment with SCH 39166 (0.01-O.lmg/kg) or eticlopride (0.001–0.006 mg/kg). Cocaine maintained self-administration behavior in a dose-related manner; as the dose of cocaine was increased, rates of responding first increased and then either decreased or leveled off. Optimum doses (0.03–0.3 mg/kg) maintained high rates of responding (0.7–1.7 responses per second) among the different monkeys, and patterns of responding that were characteristic for second-order schedules. Pretreatment with either SCH 39166 or eticlopride altered self-administration behavior in all monkeys. In most cases, dose-response curves for cocaine were shifted to the right, indicative of surmountable antagonism, and a 3 to 6-fold increase in dose of cocaine was necessary to restore optimal performances. In some instances, dose-response curves were shifted either downward or downward and to the right, indicating that the antagonistic effects of SCH 39166 and eticlopride were not always fully surmountable. These results show that self-administration of cocaine can be comparably modified by drugs that selectively block dopamine D1 or D2 receptors.

Controversies in translational research: drug self-administration.

RationaleLaboratory animal and human models of drug self-administration are used to evaluate potential pharmacotherapies for drug abuse, yet the utility of these models in predicting clinically useful medications is variable.ObjectiveThe objective of this study was to track how antagonist, agonist, and partial agonist medication approaches influence heroin and cocaine self-administration by rodents, non-human primates, and humans and to compare these results to clinical outcomes.ResultsAcross species, heroin self-administration was decreased by all three medication approaches, paralleling their demonstrated clinical utility. The heroin data emphasize the importance of assessing a medication’s abuse liability preclinically to predict medication abuse and compliance and of considering subject characteristics (e.g., opioid dependence) when interpreting medication effects. For cocaine, the effects of ecopipam, modafinil, and aripiprazole were consistent in the laboratory and clinic, provided that the medications were administered repeatedly before self-administration sessions. Modafinil attenuated cocaine’s reinforcing effects in the human laboratory and improved treatment outcome, while ecopipam and aripiprazole increased the reinforcing effects of cocaine and do not appear promising in the clinic.ConclusionsThe self-administration model has reliably identified medications to treat opioid dependence, and the recent data with modafinil suggest that the human laboratory model also identifies medications to treat cocaine dependence. There have been numerous false positives when subjective effects are the primary outcome measure, but not when self-administration is the outcome. Factors relevant to the predictive validity of self-administration procedures include medication maintenance and the concurrent assessment of a range of behaviors to determine abuse liability and the specificity of effect.

Pharmacological and Environmental Determinants of Relapse to Cocaine-Seeking Behavior

Animal models have been developed that simulate relevant features of relapse to cocaine-seeking behavior in humans. These models have provided valuable information about pharmacological and environmental factors that precipitate reinstatement of extinguished cocaine-seeking in rats and monkeys, as well as new insights about potential pharmacotherapies for relapse prevention. Reinstatement of cocaine-seeking behavior in animals can be induced by cocaine priming or by cocaine-paired environmental stimuli: however, maximum reinstatement of drug-seeking appears to be induced when cocaine priming and cocaine-paired stimuli are combined. Drugs that share cocaines indirect dopamine agonist properties or that act as direct agonists at D2-like dopamine receptors also induce reinstatement of cocaine-seeking behavior, whereas with some exceptions (e.g., caffeine, morphine) drugs from other pharmacological classes do not. D1-like receptor agonists block rather than mimic the priming effects of cocaine, suggesting different roles for D1- and D2-like receptor mechanisms in cocaine relapse. Although considerable overlap exists, drugs that exhibit cocaine-like discriminative stimulus and/ or reinforcing effects in other situations do not invariably induce cocaine-like reinstatement of drug-seeking and vice versa, implying that these effects are not simply different behavioral expressions of a unitary neurobiological process. Finally, recent findings with D1-like receptor agonists, partial agonists, and antagonists suggest that some of these drugs may be viable candidates for development as antirelapse pharmacotherapies.

Pharmacological blockade of α2-Adrenoceptors induces reinstatement of cocaine-seeking behavior in squirrel monkeys

Converging evidence suggests a role for noradrenergic mechanisms in stress-induced reinstatement of cocaine seeking in animals. Yohimbine, an α2-adrenoceptor antagonist, is known to be anxiogenic and induce stress-related responses in humans and animals. Here, we tested the ability of yohimbine to reinstate cocaine-seeking behavior and induce behavioral and physiological signs characteristic of stress in squirrel monkeys. Monkeys were trained to self-administer cocaine under a second-order schedule of i.v. drug injection. Drug seeking subsequently was extinguished by substituting saline for cocaine injections and omitting the cocaine-paired stimulus. The ability of yohimbine and the structurally distinct α2-adrenoceptor antagonist RS-79948 to reinstate cocaine-seeking behavior was assessed by administering priming injections immediately before test sessions in which the cocaine-paired stimulus was either present or absent. Priming injections of yohimbine (0.1–0.56 mg/kg, i.m.) or RS-79948 (0.01–0.1 mg/kg, i.m.) induced dose-related reinstatement of cocaine-seeking behavior. The magnitude of yohimbine-induced reinstatement was similar regardless of the presence or absence of the cocaine-paired stimulus. Yohimbine also significantly increased salivary cortisol levels, a physiological marker of stress, as well as scratching and self-grooming, behavioral markers of stress in nonhuman primates. In drug interaction experiments, pretreatment with the α2-adrenoceptor agonist clonidine (0.1–0.3 mg/kg, i.m.) dose-dependently inhibited yohimbine-induced reinstatement of cocaine seeking. In contrast, pretreatment with the dopamine receptor antagonist flupenthixol failed to inhibit yohimbine-induced reinstatement of cocaine seeking. The results show that pharmacological blockade of α2-adrenoceptors can induce reinstatement of cocaine-seeking behavior and characteristic stress responses in squirrel monkeys, providing a potentially useful model of stress-induced relapse to drug seeking.

Successful Function of Autologous iPSC-Derived Dopamine Neurons following Transplantation in a Non-Human Primate Model of Parkinson’s Disease

Autologous transplantation of patient-specific induced pluripotent stem cell (iPSC)-derived neurons is a potential clinical approach for treatment of neurological disease. Preclinical demonstration of long-term efficacy, feasibility, and safety of iPSC-derived dopamine neurons in non-human primate models will be an important step in clinical development of cell therapy. Here, we analyzed cynomolgus monkey (CM) iPSC-derived midbrain dopamine neurons for up to 2 years following autologous transplantation in a Parkinsons disease (PD) model. In one animal, with the most successful protocol, we found that unilateral engraftment of CM-iPSCs could provide a gradual onset of functional motor improvement contralateral to the side of dopamine neuron transplantation, and increased motor activity, without a need for immunosuppression. Postmortem analyses demonstrated robust survival of midbrain-like dopaminergic neurons and extensive outgrowth into the transplanted putamen. Our proof of concept findings support further development of autologous iPSC-derived cell transplantation for treatment of PD.

Improved cell therapy protocols for Parkinson's disease based on differentiation efficiency and safety of hESC-, hiPSC-, and non-human primate iPSC-derived dopaminergic neurons

The main motor symptoms of Parkinsons disease are due to the loss of dopaminergic (DA) neurons in the ventral midbrain (VM). For the future treatment of Parkinsons disease with cell transplantation it is important to develop efficient differentiation methods for production of human iPSCs and hESCs‐derived midbrain‐type DA neurons. Here we describe an efficient differentiation and sorting strategy for DA neurons from both human ES/iPS cells and non‐human primate iPSCs. The use of non‐human primate iPSCs for neuronal differentiation and autologous transplantation is important for preclinical evaluation of safety and efficacy of stem cell‐derived DA neurons. The aim of this study was to improve the safety of human‐ and non‐human primate iPSC (PiPSC)‐derived DA neurons. According to our results, NCAM+/CD29low sorting enriched VM DA neurons from pluripotent stem cell‐derived neural cell populations. NCAM+/CD29low DA neurons were positive for FOXA2/TH and EN1/TH and this cell population had increased expression levels of FOXA2, LMX1A, TH, GIRK2, PITX3, EN1, NURR1 mRNA compared to unsorted neural cell populations. PiPSC‐derived NCAM+/CD29low DA neurons were able to restore motor function of 6‐hydroxydopamine (6‐OHDA) lesioned rats 16 weeks after transplantation. The transplanted sorted cells also integrated in the rodent brain tissue, with robust TH+/hNCAM+ neuritic innervation of the host striatum. One year after autologous transplantation, the primate iPSC‐derived neural cells survived in the striatum of one primate without any immunosuppression. These neural cell grafts contained FOXA2/TH‐positive neurons in the graft site. This is an important proof of concept for the feasibility and safety of iPSC‐derived cell transplantation therapies in the future. STEM Cells 2013;31:1548–1562

Behavioral effects of cocaine and dopaminergic strategies for preclinical medication development

Abstract Rationale. The illicit use of cocaine is a persistent health problem worldwide. Currently, there are no broadly effective pharmacotherapies to treat cocaine addiction. A prerequisite for development of useful anti-cocaine medications is an understanding of the pharmacological basis of cocaines effects. The functional analysis of behavior in laboratory animals has allowed for the development of strategies identifying candidate medications to treat cocaine addiction. Objectives. This review summarizes the current status of dopaminergic compounds as cocaine pharmacotherapies in animal models of cocaine addiction. Results. Maintenance medications should share key subjective effects with cocaine, yet have limited abuse liability and side effects. However, maintenance medications often have reinforcing effects that could contribute to abuse potential and side effects that could deter patient compliance. Combined with cocaine, these drugs enhance cocaines effects. Cocaine antagonists should block the effects of cocaine and have no cocaine-like effects or side effects on their own. However, the cocaine-modulating effects of candidate cocaine antagonists are often surmountable, and, on their own, these drugs produce severe motoric side effects. In contrast, dopamine (DA) partial agonists should exhibit reduced abuse potential relative to agonists, as well as less severe motoric effects relative to antagonists. Combined with cocaine, these drugs should antagonize cocaines effects. Conclusions. DA partial agonists, in particular the D3-selective and the D1-like partial agonists, offer a more encouraging profile for novel anti-cocaine medications. Neither class of drug is self-administered, and side effects are often less severe and only observed at doses above those that antagonize the effects of cocaine.

Dopaminergic Signaling in Dendritic Spines

Dopamine regulates movement, motivation, reward, and learning and is implicated in numerous neuropsychiatric and neurological disorders. The action of dopamine is mediated by a family of seven-transmembrane G protein-coupled receptors encoded by at least five dopamine receptor genes (D1, D2, D3, D4, and D5), some of which are major molecular targets for diverse neuropsychiatric medications. Dopamine receptors are present throughout the soma and dendrites of the neuron, but accumulating ultrastructural and biochemical evidence indicates that they are concentrated in dendritic spines, where most of the glutamatergic synapses are established. By modulating local channels, receptors, and signaling modules in spines, this unique population of postsynaptic receptors is strategically positioned to control the excitability and synaptic properties of spines and mediate both the tonic and phasic aspects of dopaminergic signaling with remarkable precision and versatility. The molecular mechanisms that underlie the trafficking, targeting, anchorage, and signaling of dopamine receptors in spines are, however, largely unknown. The present commentary focuses on this important subpopulation of postsynaptic dopamine receptors with emphases on recent molecular, biochemical, pharmacological, ultrastructural, and physiological studies that provide new insights about their regulatory mechanisms and unique roles in dopamine signaling.

Modification of behavioral effects of cocaine by selective serotonin and dopamine uptake inhibitors in squirrel monkeys

Modification of the behavioral effects of cocaine by the selective serotonin (5-HT) uptake inhibitors citalopram and fluoxetine and the selective dopamine (DA) uptake inhibitor GBR 12909 was investigated in squirrel monkeys trained under a fixed-interval schedule of reinforcement or a two-lever cocaine-discrimination procedure. Under the fixed-interval schedule cocaine (0.03–1.78 mg/kg) produced dose-related increases in response rate, reaching an average maximum of 215% of control after a dose of 0.3 mg/kg. Similar rate-increasing effects were seen with GBR 12909 (3.0 or 10.0 mg/kg), but not citalopram (10.0 or 17.8 mg/kg) or fluoxetine (10.0 mg/kg). Pretreatment with citalopram or fluoxetine attenuated the rate-increasing effects of cocaine and produced an overall downward shift in the cocaine dose-response function. Pretreatment with GBR 12909, on the other hand, produced an overall leftward shift in the cocaine dose-response function. Under the drug-discrimination procedure cocaine (0.03–1.78 mg/kg) engendered dose-related increases in the percentage of cocaine-appropriate responses, as did GBR 12909 (1.0–17.8 mg/kg) but not citalopram (1.0–17.8 mg/kg). Pretreatment with citalopram attenuated the discriminative stimulus effects of cocaine and produced an overall rightward shift in the cocaine dose-response function, whereas pretreatment with GBR 12909 produced an overall leftward shift in the cocaine dose-response function. The results show that selective 5-HT and DA uptake inhibitors can modify the rate-altering and discriminative stimulus effects of cocaine in qualitatively different ways and suggest a modulatory role for 5-HT uptake inhibition in the behavioral effects of cocaine.