Erik B. Oleson

The hypocretin–orexin system regulates cocaine self-administration via actions on the mesolimbic dopamine system

Recent evidence suggests that the hypocretin–orexin system participates in the regulation of reinforcement processes. The current studies examined the extent to which hypocretin neurotransmission regulates behavioral and neurochemical responses to cocaine, and behavioral responses to food reinforcement. These studies used a combination of fixed ratio, discrete trials, progressive ratio and threshold self‐administration procedures to assess whether the hypocretin 1 receptor antagonist, SB‐334867, reduces cocaine self‐administration in rats. Progressive ratio sucrose self‐administration procedures were also used to assess the extent to which SB‐334867 reduces responding to a natural reinforcer in food‐restricted and food‐sated rats. Additionally, these studies used microdialysis and in vivo voltammetry in rats to examine whether SB‐334867 attenuates the effects of cocaine on dopamine signaling within the nucleus accumbens core. Furthermore, in vitro voltammetry was used to examine whether hypocretin knockout mice display attenuated dopamine responses to cocaine. Results indicate that when SB‐334867 was administered peripherally or within the ventral tegmental area, it reduced the motivation to self‐administer cocaine and attenuated cocaine‐induced enhancement of dopamine signaling. SB‐334867 also reduced the motivation to self‐administer sucrose in food‐sated but not food‐restricted rats. Finally, hypocretin knockout mice displayed altered baseline dopamine signaling and reduced dopamine responses to cocaine. Combined, these studies suggest that hypocretin neurotransmission participates in reinforcement processes, likely through modulation of the mesolimbic dopamine system. Additionally, the current observations suggest that the hypocretin system may provide a target for pharmacotherapies to treat cocaine addiction.

Homer2 Is Necessary for EtOH-Induced Neuroplasticity

Homer proteins are integral to the assembly of proteins regulating glutamate signaling and synaptic plasticity. Constitutive Homer2 gene deletion [knock-out (KO)] and rescue with adeno-associated viral (AAV) transfection of Homer2b was used to demonstrate the importance of Homer proteins in neuroplasticity produced by repeated ethanol (EtOH) administration. Homer2 KO mice avoided drinking high concentrations of EtOH and did not develop place preference or locomotor sensitization after repeated EtOH administration. The deficient behavioral plasticity to EtOH after Homer2 deletion was paralleled by a lack of augmentation in the rise in extracellular dopamine and glutamate elicited by repeated EtOH injections. The genotypic differences in EtOH-induced change in behavior and neurochemistry were essentially reversed by AAV-mediated transfection of Homer2b into accumbens cells including, differences in EtOH preference, locomotor sensitization, and EtOH-induced elevations in extracellular glutamate and dopamine. These data demonstrate a necessary and active role for accumbens Homer2 expression in regulating EtOH-induced behavioral and cellular neuroplasticity.

Behavioral and neurochemical phenotyping of Homer1 mutant mice: possible relevance to schizophrenia

Homer proteins are involved in the functional assembly of postsynaptic density proteins at glutamatergic synapses and are implicated in learning, memory and drug addiction. Here, we report that Homer1‐knockout (Homer1‐KO) mice exhibit behavioral and neurochemical abnormalities that are consistent with the animal models of schizophrenia. Relative to wild‐type mice, Homer1‐KO mice exhibited deficits in radial arm maze performance, impaired prepulse inhibition, enhanced ‘behavioral despair’, increased anxiety in a novel objects test, enhanced reactivity to novel environments, decreased instrumental responding for sucrose and enhanced MK‐801‐ and methamphetamine‐stimulated motor behavior. No‐net‐flux in vivo microdialysis revealed a decrease in extracellular glutamate content in the nucleus accumbens and an increase in the prefrontal cortex. Moreover, in Homer1‐KO mice, cocaine did not stimulate a rise in frontal cortex extracellular glutamate levels, suggesting hypofrontality. These behavioral and neurochemical data derived from Homer1 mutant mice are consistent with the recent association of schizophrenia with a single‐nucleotide polymorphism in the Homer1 gene and suggest that the regulation of extracellular levels of glutamate within limbo‐corticostriatal structures by Homer1 gene products may be involved in the pathogenesis of this neuropsychiatric disorder.

Subsecond Dopamine Release in the Nucleus Accumbens Predicts Conditioned Punishment and Its Successful Avoidance

The mesolimbic dopamine system is believed to be a pathway that processes rewarding information. While previous studies have also implicated a general role for dopamine in punishment and its avoidance, the precise nature of subsecond dopamine release during these phenomena remains unknown. Here, we used fast-scan cyclic voltammetry to investigate whether subsecond dopamine release events in the nucleus accumbens encode cues predicting the avoidance of punishment during behavior maintained in a signaled footshock avoidance procedure. In this task, rats could initiate an avoidance response by pressing a lever within a warning period, preventing footshock. Alternatively, once footshocks commenced, animals could initiate an escape response by pressing the lever, terminating footshock. This design allowed us to assess subsecond dopamine release events during the presentation of a warning signal, safety periods, and two distinct behavioral responses. We found that release consistently increased upon presentation of the warning signal in a manner that reliably predicted successful punishment avoidance. We also observed subsecond dopamine release during the safety period, as occurs following the receipt of reward. Conversely, we observed a decrease in release at the warning signal during escape responses. Because of this finding, we next assessed dopamine release in a conditioned fear model. As seen during escape responses, we observed a time-locked decrease in dopamine release upon presentation of a cue conditioned to inescapable footshock. Together, these data show that subsecond fluctuations in mesolimbic dopamine release predict when rats will successfully avoid punishment and differentially encode cues related to aversive outcomes.

Endocannabinoids Shape Accumbal Encoding of Cue-Motivated Behavior via CB1 Receptor Activation in the Ventral Tegmentum

Transient increases in nucleus accumbens (NAc) dopamine concentration are observed when animals are presented with motivationally salient stimuli and are theorized to energize reward seeking. They arise from high-frequency firing of dopamine neurons in the ventral tegmental area (VTA), which also results in the release of endocannabinoids from dopamine cell bodies. In this context, endocannabinoids are thought to regulate reward seeking by modulating dopamine signaling, although a direct link has never been demonstrated. To test this, we pharmacologically manipulated endocannabinoid neurotransmission in the VTA while measuring transient changes in dopamine concentration in the NAc during reward seeking. Disrupting endocannabinoid signaling dramatically reduced, whereas augmenting levels of the endocannabinoid 2-arachidonoylglycerol (2AG) increased, cue-evoked dopamine concentrations and reward seeking. These data suggest that 2AG in the VTA regulates reward seeking by sculpting ethologically relevant patterns of dopamine release during reward-directed behavior.

Distinct Roles for Different Homer1 Isoforms in Behaviors and Associated Prefrontal Cortex Function

Homer1 mutant mice exhibit behavioral and neurochemical abnormalities that are consistent with an animal model of schizophrenia. Because the Homer1 gene encodes both immediate early gene (IEG) and constitutively expressed (CC) gene products, we used the local infusion of adeno-associated viral vectors carrying different Homer1 transcriptional variants into the prefrontal cortex (PFC) to distinguish between the roles for IEG and CC Homer1 isoforms in the “schizophrenia-like” phenotype of Homer1 mutant mice. PFC overexpression of the IEG Homer1 isoform Homer1a reversed the genotypic differences in behavioral adaptation to repeated stress, whereas overexpression of the constitutively expressed Homer1 isoform Homer1c reversed the genotypic differences in sensorimotor and cognitive processing, as well as cocaine behavioral sensitivity. Homer1a overexpression did not influence PFC basal glutamate content but blunted the glutamate response to cocaine in wild-type mice. In contrast, Homer1c overexpression reversed the genotypic difference in PFC basal glutamate content and enhanced cocaine-induced elevations in glutamate. These data demonstrate active and distinct roles for Homer1a and Homer1c isoforms in the PFC in the mediation of behavior, in the maintenance of basal extracellular glutamate, and in the regulation of PFC glutamate release relevant to schizophrenia and stimulant abuse comorbidity.

Homer isoforms differentially regulate cocaine-induced neuroplasticity.

Homer proteins modulate neuroplasticity in excitatory synapses and are dynamically regulated by cocaine. Whereas acute cocaine elevates immediate-early gene (short) isoforms of Homer1 in the nucleus accumbens, withdrawal from repeated cocaine administration downregulates the expression of constitutive Homer1 isoforms. The present study determined whether or not this downregulation in constitutive Homer expression in the accumbens is necessary for enduring alterations in cocaine-induced changes in the brain and behavior. The long vs short Homer isoforms were overexpressed in the rat nucleus accumbens during drug abstinence, and the adaptations elicited by repeated cocaine on glutamate transmission and motor behavior were measured. It was found that both chronic and acute overexpression of constitutive, but not short, Homer isoforms abolished cocaine-induced sensitization of locomotor hyperactivity and prevented the development of glutamate abnormalities in the accumbens, including the reduction in basal extracellular glutamate content and the sensitized glutamate response to a subsequent cocaine challenge injection. Together, these data indicate that the enduring reduction of long Homer isoforms in the nucleus accumbens of cocaine-withdrawn rats is necessary for the expression of cocaine-induced neuroplasticity.

A Brain on Cannabinoids: The Role of Dopamine Release in Reward Seeking

Increases in mesolimbic dopamine transmission are observed when animals are treated with all known drugs of abuse, including cannabis, and to conditioned stimuli predicting their availability. In contrast, decreases in mesolimbic dopamine function are observed during drug withdrawal, including cannabis-withdrawal syndrome. Thus, despite general misconceptions that cannabis is unique from other drugs of abuse, cannabis exerts identical effects on the mesolimbic dopamine system. The recent discovery that endogenous cannabinoids modulate the mesolimbic dopamine system, however, might be exploited for the development of potential pharmacotherapies designed to treat disorders of motivation. Indeed, disrupting endocannabinoid signaling decreases drug-induced increases in dopamine release in addition to dopamine concentrations evoked by conditioned stimuli during reward seeking.

Behavioral economic assessment of price and cocaine consumption following self-administration histories that produce escalation of either final ratios or intake.

Various self-administration procedures are being developed to model specific aspects of the addiction process. For example, ‘increased cocaine intake over time’ has been modeled by providing long access (LgA) to cocaine during daily self-administration sessions under a fixed-ratio (FR1) reinforcement schedule. In addition, ‘increased time and energy devoted to acquire cocaine’ has been modeled by providing access to cocaine during daily self-administration sessions under a progressive-ratio (PR) schedule. To investigate the distinctiveness of these models, the behavioral economics variables of consumption and price were applied to cocaine self-administration data. To assess changes in consumption and price, cocaine self-administration was tested across a descending series of doses (0.237–0.001 mg per injection) under an FR1 reinforcement schedule to measure drug intake in the high dose range and thresholds in the low range. Cocaine consumption remained relatively stable across doses until a threshold was reached, at which maximal responding was observed. It was found that a history of LgA training produced an increase in cocaine consumption; whereas a history of PR training produced an increase in the maximal price (Pmax) expended for cocaine. Importantly, the concepts of consumption and price were found to be dissociable. That is, LgA training produced an increase in consumption but a decrease in Pmax, whereas PR training produced an increase in Pmax without increasing consumption. These results suggest that distinct aspects of the addiction process can be parsed using self-administration models, thereby facilitating the investigation of specific neurobiological adaptations that occur through the addiction process.

Effects of chronic alcohol exposure on dopamine uptake in rat nucleus accumbens and caudate putamen

RationaleExisting data strongly suggest that alcohol affects dopamine (DA) neurotransmission in the brain. However, many questions remain about the effects of alcohol on the delicate equilibrium between such neurochemical processes as DA release and uptake. Dysregulation of these processes in the mesolimbic and nigrostriatal systems after chronic alcohol ingestion could be a neuroadaptation contributing to dependence.ObjectivesIn the present study, we have employed an alcohol vapor inhalation model to characterize the effects of chronic alcohol exposure on DA dynamics in rat nucleus accumbens (NAc) and caudate putamen (CP) using fast-scan cyclic voltammetry (FSCV) in brain slices. This method provides a unique view of real-time, spatially resolved changes in DA concentration.ResultsWe found that chronic alcohol exposure enhanced DA uptake rates in rat NAc and CP. These changes would have the effect of down-regulating extracellular DA levels, presumably a compensatory effect related to increased DA release by repeated alcohol exposure. The sensitivity of terminal release-regulating DA autoreceptors was not different in alcohol-exposed rats compared with alcohol-naïve animals.ConclusionsThe DA uptake changes after chronic alcohol exposure documented here using FSCV may be associated with a compensatory response of the DA system aimed at decreasing DA signaling. Alterations in autoreceptor function may require relatively long lasting alcohol exposure.