The orexin (hypocretin) neuropeptide system is a target for novel therapeutics to treat cocaine use disorder with alcohol coabuse.

An estimated 50-90% of individuals with cocaine use disorder (CUD) also report using alcohol. Cocaine users report coabusing alcohol to 'self-medicate' against the negative emotional side effects of the cocaine 'crash', including the onset of anxiety. Thus, pharmaceutical strategies to treat CUD would ideally reduce the motivational properties of cocaine, alcohol, and their combination, as well as reduce the onset of anxiety during drug withdrawal. The hypothalamic orexin (hypocretin) neuropeptide system offers a promising target, as orexin neurons are critically involved in activating behavioral and physiological states to respond to both positive and negative motivators. Here, we seek to describe studies demonstrating efficacy of orexin receptor antagonists in reducing cocaine, alcohol- and stress-related behaviors, but note that these studies have largely focused on each of these phenomena in isolation. For orexin-based compounds to be viable in the clinical setting, we argue that it is imperative that their efficacy be tested in animal models that account for polysubstance use patterns. To begin to examine this, we present new data showing that rats' preferred level of cocaine intake is significantly increased following chronic homecage access to alcohol. We also report that cocaine intake and motivation are reduced by a selective orexin-1 receptor antagonist when rats have a history of cocaine + alcohol, but not a limited history of cocaine alone. In light of these proof-of-principle data, we outline what we believe to be the key priorities going forward with respect to further examining the orexin system in models of polysubstance use. This article is part of the special issue on Neurocircuitry Modulating Drug and Alcohol Abuse.

Hold-down as an alternative to unit dose in cocaine self-administration experiments: Characterization using a progressive ratio schedule.

RATIONALE: Virtually all cocaine self-administration studies have used a "unit dose" as a reinforcing stimulus; the subject is a passive recipient of an experimenter-selected dose. OBJECTIVES: The present experiments examined the consequence of requiring the subject to actively determine the dose and speed of each injection. METHODS: A two-lever procedure was used in which responding on a progressive ratio (PR) schedule provided access to cocaine on a hold down (HD) schedule. With HD, the pump is turned on for the duration that the lever is held down, thus the dose and speed of injection is determined by the behavior of the subject. The procedure allows for the evaluation of both drug taking and drug seeking responses. RESULTS: The results were qualitatively different from PR self-administration studies using unit dose. The self-administered HD dose varied across the session; the self-administered dose was found to inversely correlate with drug levels at the time of access. Importantly, the 2 L-PR-HD procedure identified a subpopulation of subjects that showed extremes in both drug seeking and drug taking. Subjects at the top end of the distribution displayed unprecedented final ratios (> 900) and rapidly self-administered very large doses (> 1.4 mg; ~ 4.2 mg/kg). Manipulation of drug-taking variables (HD access duration and concentration of drug in the pump) showed that the immediacy of a cocaine bolus, not the duration of access, is the major determinant of drug seeking. CONCLUSIONS: Incorporating a consummatory response into a PR procedure provides a unique perspective on the interactions of drug-seeking and drug-taking.

Increased Number and Activity of a Lateral Subpopulation of Hypothalamic Orexin/Hypocretin Neurons Underlies the Expression of an Addicted State in Rats.

BACKGROUND: The orexin (hypocretin) system is important for reward-driven motivation but has not been implicated in the expression of a multiphenotype addicted state. METHODS: Rats were assessed for economic demand for cocaine before and after 14 days of short access, long access, or intermittent access (IntA) to cocaine. Rats were also assessed for a number of other DSM-5-relevant addiction criteria following differential access conditions. Orexin system function was assessed by quantification of numbers and activity of orexin cells, pharmacological blockade of the orexin-1 receptor, and subregion-specific knockdown of orexin cell populations. RESULTS: IntA produced a cluster of addiction-like behaviors that closely recapitulate key diagnostic criteria for addiction to a greater extent than long access or short access. IntA was accompanied by an increase in number and activity of orexin-expressing neurons within the lateral hypothalamic subregion. This increase in orexin cell number and activity persisted during protracted withdrawal from cocaine for at least 150 days and was accompanied by enhanced incubation of craving in the same rats. Selective knockdown of lateral hypothalamic orexin neurons reduced motivation for cocaine, and orexin-1 receptor signaling played a larger role in drug seeking after IntA. CONCLUSIONS: We provide the first evidence that lateral hypothalamic orexin system function extends beyond general reward seeking to play a critical role in expression of a multiphenotype addiction-like state. Thus, the orexin system is a potential novel target for pharmacotherapies designed to treat cocaine addiction. In addition, these data point to the IntA model as a preferred approach to modeling addiction-like behavior in rats.

Brief intermittent cocaine self-administration and abstinence sensitizes cocaine effects on the dopamine transporter and increases drug seeking.

Although traditional sensitization paradigms, which result in an augmentation of cocaine-induced locomotor behavior and dopamine (DA) overflow following repeated experimenter-delivered cocaine injections, are often used as a model to study drug addiction, similar effects have been difficult to demonstrate following cocaine self-administration. We have recently shown that intermittent access (IntA) to cocaine can result in increased cocaine potency at the DA transporter (DAT); however, traditional sensitization paradigms often show enhanced effects following withdrawal/abstinence periods. Therefore, we determined a time course of IntA-induced sensitization by examining the effects of 1 or 3 days of IntA, as well as a 7-day abstinence period on DA function, cocaine potency, and reinforcement. Here we show that cocaine potency is increased following as little as 3 days of IntA and further augmented following an abstinence period. In addition, IntA plus abstinence produced greater evoked DA release in the presence of cocaine as compared with all other groups, demonstrating that following abstinence, both cocaine's ability to increase DA release and inhibit uptake at the DAT, two separate mechanisms for increasing DA levels, are enhanced. Finally, we found that IntA-induced sensitization of the DA system resulted in an increased reinforcing efficacy of cocaine, an effect that was augmented after the 7-day abstinence period. These results suggest that sensitization of the DA system may have an important role in the early stages of drug abuse and may drive the increased drug seeking and taking that characterize the transition to uncontrolled drug use. Human data suggest that intermittency, sensitization, and periods of abstinence have an integral role in the process of addiction, highlighting the importance of utilizing pre-clinical models that integrate these phenomena, and suggesting that IntA paradigms may serve as novel models of human addiction.

Intermittent cocaine self-administration produces sensitization of stimulant effects at the dopamine transporter.

Previous literature investigating neurobiological adaptations following cocaine self-administration has shown that high, continuous levels of cocaine intake (long access; LgA) results in reduced potency of cocaine at the dopamine transporter (DAT), whereas an intermittent pattern of cocaine administration (intermittent access; IntA) results in sensitization of cocaine potency at the DAT. Here, we aimed to determine whether these changes are specific to cocaine or translate to other psychostimulants. Psychostimulant potency was assessed by fast-scan cyclic voltammetry in brain slices containing the nucleus accumbens following IntA, short access, and LgA cocaine self-administration, as well as in brain slices from naive animals. We assessed the potency of amphetamine (a releaser), and methylphenidate (a DAT blocker, MPH). MPH was selected because it is functionally similar to cocaine and structurally related to amphetamine. We found that MPH and amphetamine potencies were increased following IntA, whereas neither was changed following LgA or short access cocaine self-administration. Therefore, whereas LgA-induced tolerance at the DAT is specific to cocaine as shown in previous work, the sensitizing effects of IntA apply to cocaine, MPH, and amphetamine. This demonstrates that the pattern with which cocaine is administered is important in determining the neurochemical consequences of not only cocaine effects but potential cross-sensitization/cross-tolerance effects of other psychostimulants as well.

Reduction of the reinforcing effectiveness of cocaine by continuous D-amphetamine treatment in rats: importance of active self-administration during treatment period.

RATIONALE: Continuous administration of D-amphetamine has shown promise as a treatment for psychostimulant addiction. In rodent studies, constant infusion of D-amphetamine (5 mg/kg/day) has been shown to reduce cocaine-reinforced responding in the dose range of 0.19-0.75 mg/kg/inf. OBJECTIVES: The present study tested whether these effects were a reflection of pharmacological interactions between D-amphetamine and cocaine or if they resulted from associative learning mechanisms METHODS: After stable progressive ratio (PR) baselines were established, rats were implanted with subcutaneous osmotic minipumps filled with either D-amphetamine (5 mg/kg/day-groups 1 and 2) or saline (group 3). During the treatment period, groups 1 and 3 self-administered cocaine at a dose that was previously shown to produce the most robust effects in combination with D-amphetamine treatment (0.19 mg/kg/inf), while group 2 received passive cocaine infusions. RESULTS: In replication of previous studies, D-amphetamine treatment resulted in a significant (35 %) decrease in breakpoints relative to saline controls. By contrast, no reductions in breakpoints were observed in animals that received passive cocaine infusions during the treatment period (group 2). CONCLUSIONS: Active self-administration of cocaine during the treatment period appears to be an important factor in reducing cocaine-reinforced breakpoints. These findings suggest learning mechanisms are involved in the therapeutic effects of continuous D-amphetamine, and pharmacological interaction mechanisms such as cross-tolerance cannot completely account for the observed decreases in cocaine seeking.

Conflation of cocaine seeking and cocaine taking responses in IV self-administration experiments in rats: methodological and interpretational considerations.

IV drug self-administration is a special case of an operant task. In most operant experiments, the instrumental response that completes the schedule requirement is separate and distinct from the consumptive response (e.g. eating or drinking) that follows the delivery of the reinforcing stimulus. In most IV self-administration studies drug seeking and drug taking responses are conflated. The instrumental lever press or nose poke is also a consumptive response. The conflation of these two response classes has important implications for interpretation of the data as they are differentially regulated by dose and price. The types of pharmacological pretreatments that affect appetitive responses are not necessarily the same as those that affect consumptive responses suggesting that the neurobiology of the two response classes are to some extent controlled by different mechanisms. This review discusses how schedules of reinforcement and behavioral economic analyses can be used to assess the regulation of drug seeking and drug taking separately. New methods are described that allow the examination of appetitive or consumptive responding in isolation and provide subjects with greater control over the self-administered dose. These procedures provide novel insights into the regulation of drug intake. Cocaine intake patterns that result in large, intermittent spikes in cocaine levels are shown to produce increases in appetitive responding (i.e. drug seeking). The mechanisms that control drug intake should be considered distinct from appetitive and motivational processes and should be taken into consideration in future IV self-administration studies.

Temporal pattern of cocaine intake determines tolerance vs sensitization of cocaine effects at the dopamine transporter.

The dopamine transporter (DAT) is responsible for terminating dopamine (DA) signaling and is the primary site of cocaine's reinforcing actions. Cocaine self-administration has been shown previously to result in changes in cocaine potency at the DAT. To determine whether the DAT changes associated with self-administration are due to differences in intake levels or temporal patterns of cocaine-induced DAT inhibition, we manipulated cocaine access to produce either continuous or intermittent elevations in cocaine brain levels. Long-access (LgA, 6 h) and short-access (ShA, 2 h) continuous self-administration produced similar temporal profiles of cocaine intake that were sustained throughout the session; however, LgA had greater intake. ShA and intermittent-access (IntA, 6 h) produced the same intake, but different temporal profiles, with 'spiking' brain levels in IntA compared with constant levels in ShA. IntA consisted of 5-min access periods alternating with 25-min timeouts, which resulted in bursts of high responding followed by periods of no responding. DA release and uptake, as well as the potency of cocaine for DAT inhibition, were assessed by voltammetry in the nucleus accumbens slices following control, IntA, ShA, and LgA self-administration. Continuous-access protocols (LgA and ShA) did not change DA parameters, but the 'spiking' protocol (IntA) increased both release and uptake of DA. In addition, high continuous intake (LgA) produced tolerance to cocaine, while 'spiking' (IntA) produced sensitization, relative to ShA and naive controls. Thus, intake and pattern can both influence cocaine potency, and tolerance seems to be produced by high intake, while sensitization is produced by intermittent temporal patterns of intake.

Ritualistic chewing behavior induced by mCPP in the rat is an animal model of obsessive compulsive disorder.

Obsessive Compulsive Disorder (OCD) is characterized by recurrent, anxiety-producing thoughts accompanied by unwanted, overwhelming urges to perform ritualistic behaviors. Pharmacological treatments for this disorder (serotonin uptake inhibitors) are problematic because there is a 6-8 week delayed onset and half of the patients do not adequately respond. The present study evaluated whether Ritualistic Chewing Behaviors (RCBs) induced by the serotonin agonist mCPP in the rat is a behavioral model for OCD. The effects upon the RCBs induced by mCPP (1 mg/kg) were evaluated following treatments with either the serotonin antagonist mianserin (3 mg/kg), the dopamine antagonist haloperidol (1 mg/kg), the GABA modulator diazepam (10 mg/kg), or the serotonin uptake inhibitors clomipramine and fluvoxamine (15 mg/kg). The response to mCPP was blocked by acute treatment with mianserin, but not with acute haloperidol or diazepam. Further experiments revealed that the effects of mCPP were blocked by chronic, but not acute, treatment with clomipramine and fluvoxamine. A time-course demonstrated that 14 days of chronic treatment were required for blockade of the mCPP-evoked response. The current study demonstrates that mCPP-evoked RCBs may be a rodent model for OCD that can be used to predict the clinical efficacy and time course of novel OCD treatment. Future investigations may be able to use the current model as a tool for bench-marking corresponding changes in other measures of neurological activity that may provide insight into the mechanisms underlying OCD.

Examination of behavioral strategies regulating cocaine intake in rats.

RATIONALE: It has long been observed that rats self-administer psychostimulants in a highly regular pattern. The inverse relationship between dose and rate of drug intake has been interpreted as a titration phenomenon wherein brain-cocaine levels are maintained within a range. Most studies examining this phenomenon have used fixed, unit doses in which case the only titration strategy available to the animal is to adjust inter-infusion intervals. OBJECTIVES: In this study, we examined whether selection of dose size could also be a factor in regulation of intake. We used a schedule of reinforcement, under which the dose can vary through a wide range and is determined by the behavior of the animal. METHODS: Rats self-administered cocaine using a behaviorally dependent dosing schedule of reinforcement, under which the size of each dose was determined by the length of time the lever was held down. The concentration of cocaine was changed across sessions. RESULTS: Total pump-time self-administered decreased by 56 % following each doubling of the concentration, which led to an average 11 % increase in total intake. Similarly, estimated brain levels of cocaine increased by 12 % for each doubling of concentration. These adjustments were the result of manipulation of both the size and spacing of infusions. CONCLUSIONS: In agreement with previous studies, the regular pattern of intake appears to be the result of a titration mechanism in which animals maintain brain levels of cocaine above some threshold. Compensatory regulation appeared to involve both the selection of dose size and inter-infusion intervals.

The motivation to self-administer is increased after a history of spiking brain levels of cocaine.

Recent attempts to model the addiction process in rodents have focused on cocaine self-administration procedures that provide extended daily access. Such procedures produce a characteristic loading phase during which blood levels rapidly rise and then are maintained within an elevated range for the duration of the session. The present experiments tested the hypothesis that multiple fast-rising spikes in cocaine levels contribute to the addiction process more robustly than constant, maintained drug levels. Here, we compared the effects of various cocaine self-administration procedures that produced very different patterns of drug intake and drug dynamics on Pmax, a behavioral economic measure of the motivation to self-administer drug. Two groups received intermittent access (IntA) to cocaine during daily 6-h sessions. Access was limited to twelve 5-min trials that alternated with 25-min timeout periods, using either a hold-down procedure or a fixed ratio 1 (FR1). Cocaine levels could not be maintained with this procedure; instead the animals experienced 12 fast-rising spikes in cocaine levels each day. The IntA groups were compared with groups given 6-h FR1 long access and 2-h short access sessions and two other control groups. Here, we report that cocaine self-administration procedures resulting in repeatedly spiking drug levels produce more robust increases in Pmax than procedures resulting in maintained high levels of cocaine. These results suggest that rapid spiking of brain-cocaine levels is sufficient to increase the motivation to self-administer cocaine.

Cocaine self-administration in rats: hold-down procedures.

For decades, researchers have used animal self-administration models to examine the effects drugs of abuse have on physiology and behavior. Sophisticated self-administration procedures have been developed to model many different aspects of drug addiction. The hold-down procedure provides animals with control over the amount of each injection. Holding the lever down turns the syringe pump on and subsequently releasing the lever turns the pump off. In this way, animals can hold the lever down for any duration of time thereby self-administering any dose on a continuous spectrum. This procedure eliminates some of the ambiguity in translating results from effects only observed at one unit dose and allows examination of which dose the animal "prefers" at different times.

Brain-cocaine concentrations determine the dose self-administered by rats on a novel behaviorally dependent dosing schedule.

A novel behaviorally dependent dosing (BDD) schedule was used to examine the relationship between doses of cocaine self-administered by rats and brain drug levels within a session. The BDD schedule used a hold-down response to activate a syringe pump. The length of time the lever was held down determined the duration that the syringe pump was activated. In the first experiment, rats self-administered cocaine for daily 3 h sessions and brain levels of cocaine were modeled using well-established parameters. Although analysis revealed that rats self-administered doses within a predicted range, one extremely large dose was consistently observed at the beginning of each session when brain levels of cocaine were low. In the second experiment, we introduced a range of timeout periods (10-25 min) in order to produce variability in brain-cocaine concentrations. Animals self-administered larger doses immediately following each timeout period and the dose size was inversely correlated with the length of the timeout. These results show that the dose of cocaine that rats self-administer within a session is inversely related to the amount of drug on board.