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Drug-discrimination procedures empirically evaluate the control that internal drug states have over behavior. They provide a highly selective method to investigate the neuropharmacological underpinnings of the interoceptive effects of drugs in vivo. As a result, drug discrimination has been one of the most widely used assays in the field of behavioral pharmacology. Drug-discrimination procedures have been adapted for use with humans and are conceptually similar to preclinical drug-discrimination techniques in that a behavior is differentially reinforced contingent on the presence or absence of a specific interoceptive drug stimulus. This chapter provides a basic overview of human drug-discrimination procedures and reviews the extant literature concerning the use of these procedures to elucidate the underlying neuropharmacological mechanisms of commonly abused illicit drugs (i.e., stimulants, opioids, and cannabis) in humans. This chapter is not intended to review every available study that used drug-discrimination procedures in humans. Instead, when possible, exemplary studies that used a stimulant, opioid, or Δ9-tetrahydrocannabinol (the primary psychoactive constituent of cannabis) to assess the discriminative-stimulus effects of drugs in humans are reviewed for illustrative purposes. We conclude by commenting on the current state and future of human drug-discrimination research.
Drug-discrimination procedures empirically evaluate the control internal drug states have over behavior. They provide a highly selective method to investigate the neuropharmacological underpinnings of the interoceptive effects of drugs in vivo. As a result, drug discrimination has been one of the most widely used assays in the field of behavioral pharmacology. Since the publication of one of the earliest studies to suggest the control of behavior by the presence or absence of the interoceptive-stimulus effects of alcohol in rats (Conger, 1951), there has been substantial work investigating the discriminative-stimulus effects of drugs spanning more than four decades (e.g., Porter and Prus, 2009). Drug-discrimination procedures have also been adapted for use with humans and remain conceptually similar to preclinical drug-discrimination procedures in that a behavior is differentially reinforced contingent on the presence or absence of a specific interoceptive drug stimulus (see Chapter 1; also see Preston, 1991). A PubMed search using the quoted search phrase “drug discrimination” yields 1,284 peer-reviewed publications dating back to the mid 1940s (i.e., Jellinek, 1946). Of the total number of published drug-discrimination studies, those concerning human drug discrimination comprise approximately 16% (i.e., 205 reports). Figure 1 shows the total number of drug-discrimination publications per year since 1973 and the relative proportion of those concerning human drug discrimination.
As noted above and described in previous chapters, the interoceptive-stimulus effects of drugs and the ensuing stimulus control of behavior have been widely studied in non-human laboratory animals using drug-discrimination procedures. Below, the extant literature that assessed the discriminative-stimulus effects of stimulants, opioids, and Δ9-tetrahydrocannabinol (Δ9-THC; the primary pharmacological constituent in cannabis) in humans is reviewed. Since the adaptation of drug-discrimination procedures for use with humans, a number of reviews have been published. These reviews focused on: (a) the relationship between the discriminative-stimulus and subjective effects of drugs (e.g., Preston and Bigelow, 1991; Schuster and Johanson, 1988; Schuster et al., 1981); (b) the concordance between preclinical and human drug-discrimination experiments (Kamien et al. 1993); and (c) the neuropharmacological selectivity of drug-discrimination procedures relative to subjective drug-effect questionnaires (Kelly et al., 2003). Although the present chapter provides some general discussion of these previously reviewed topics, it differs from earlier reviews in that it primarily focuses on the utility of human drug-discrimination procedures to elucidate the underlying neuropharmacological mechanisms of commonly abused illicit drugs (i.e., stimulants, opioids, and cannabis). This chapter is not intended to review every available study that used human drug-discrimination procedures. Instead, when possible, studies that used a stimulant, opioid, or Δ9-THC to assess the discriminative-stimulus effects of drugs in humans are reviewed for illustrative purposes. Lastly, we conclude by commenting on the current state and future of human drug-discrimination research.
Potential subjects are typically recruited through formal advertisements in local newspapers, online classified ads (e.g., Craigslist), flyers posted in public areas, and by word-of-mouth referral. Volunteers who may qualify upon initial screening complete a rigorous in-person screening that includes a complete medical history, physical health screen, and psychiatric assessment. Volunteers also provide basic demographic information (e.g., age, sex, socioeconomic status) and complete a battery of questionnaires that assess drug-use history and severity as well as symptomology for other clinically relevant conditions such as depression and Attention-Deficit Hyperactivity Disorder. Responses on these instruments are used to determine whether volunteers satisfy the study inclusion criteria or meet criteria that would exclude them from participation (e.g., active disease process, psychiatric disorder, prescribed medication(s) contraindicated with the study medication). Given the substantial time commitment required by human drug-discrimination studies, another important consideration is whether a potential subject is able to dedicate the time necessary to complete the study. A physician reviews all screening materials to determine whether the volunteer is physically and psychologically eligible for participation. Thorough physical and mental health screening is absolutely imperative to ensure subject safety in any study involving the administration of pharmacological agents to human subjects.
The discriminative-stimulus effects of various drugs have been assessed in normal healthy volunteers (e.g., Rush et al., 1995; Silverman & Griffiths, 1992), drug-dependent individuals (e.g., Lile et al., 2011a; Oliveto et al., 2013), and individuals with a history of drug dependence who are currently abstinent/detoxified (e.g., Preston et al., 1989). However, there are no published studies in which the discriminative-stimulus effects of particular drugs have been prospectively compared between these populations. Several factors should be considered when selecting the most appropriate population of subjects given the specific research question(s) and the primary aim(s) of the study. For example, participants with an extensive history of substance abuse may be most appropriate in the context of testing whether a novel compound has potential for abuse itself or may effectively attenuate the discriminative-stimulus effects of a drug with known abuse potential. An important caveat, however, is that their extensive drug-use history may complicate interpretation of the results because of differences in expectancies, conditioning history, and tolerance (Brauer, Goudie, and de Wit, 1997). Although there are advantages and disadvantages to using various populations, research in individuals with and without histories of substance abuse is necessary to gain a more complete understanding of the neuropharmacological mechanisms that underlie the discriminative-stimulus effects of drugs (Brauer, Goudie, and de Wit, 1997).
The test environment and experimental materials required to conduct a human drug-discrimination experiment generally consists of a test room containing a desk, chair, a computer with a mouse, numeric keypad and programming to present the drug-discrimination task and record the data, and equipment that is used to monitor participants’ vital signs. Although the use of a computer is more typical, pen and paper could also be used for task presentation and data collection. The room may also be equipped with a television and other recreational materials (e.g., magazines, books, games, craft supplies) that volunteers may use when not engaged in experimental activities.
In one example of a two-choice drug-discrimination task, the volunteer is presented with two response options (e.g., Drug A and Not Drug A) on the computer screen and instructed to indicate which drug condition that they think they received by distributing 100 points between the two options using the numeric keypad. For example, if a volunteer is relatively confident that they received Drug A, they might allocate 80 points to the Drug A option and 20 points to the Not Drug A option. Volunteers complete the drug-discrimination task multiple times at regular intervals throughout the session: usually every 30 minutes to an hour depending on the pharmacokinetics of the drug(s) under study. The total number of points allocated to the correct response option out of all possible points is exchanged for money at a constant rate. For example, points have been exchanged for money at rate of $0.04–$0.08 per point in previous drug-discrimination studies conducted in our laboratory (Rush et al., 2002; Sevak et al., 2009). Participants can earn $20-$40 per session but the specific rate with which points are exchanged for money (i.e., $0.04 vs. $0.08) does not appear to significantly alter performance on the task (Rush et al., 2002; Sevak et al., 2009).
The use of money as the reinforcer in human drug-discrimination studies is a primary difference from preclinical drug-discrimination studies. In preclinical studies, subjects are often food restricted so that food reinforcers effectively maintain behavior. Another notable difference between preclinical and human drug-discrimination studies is that some human studies do not utilize a formal schedule of reinforcement, at least as typically conceptualized, and reinforcement is withheld until the end of the session when subjects are paid. In contrast, responding by animals is typically maintained by a fixed-ratio schedule of reinforcement and reinforcers are delivered or withheld upon completion of each response requirement.
This section of the chapter provides a general experimental overview and highlights the basic methodological elements of human drug-discrimination procedures. Notable procedural variations between drug-discrimination studies, more complex drug-discrimination procedures, and the advantages and limitations of these approaches are then discussed. As noted above, the methods used in human drug-discrimination studies are very similar to those used in preclinical drug-discrimination research. Although a standardized human drug-discrimination procedure has not been established, these experiments often consist of three phases that are completed in a fixed order: (1) Sampling Phase; (2) Acquisition Phase; and (3) Test Phase.
During the sampling phase, participants complete several experimental sessions to acquaint them with the interoceptive-stimulus effects of the training dose. The training dose is usually identified to participants by a specific code (e.g., Drug A or Red Drug). Participants may also complete sampling sessions during which they receive placebo. In this case, placebo is identified with a unique code (e.g., Not Drug A; Drug B; or Blue Drug). During the sampling sessions, participants are verbally instructed to attend to the effects of the drug because correctly identifying the drug they received will determine the amount of monetary compensation that they earn in future sessions.
Following the sampling phase, an acquisition phase (sometimes referred to as the Test-of-Acquisition or Control Phase) is conducted in which the training dose and placebo are administered once per day across several sessions (e.g., 4–12 total sessions) in random order. During each session in this phase, volunteers ingest drug or placebo under blinded conditions and then complete the drug-discrimination task along with subjective drug-effect questionnaires periodically for several hours after drug administration. Although participants are asked to identify which treatment they received on the drug-discrimination task periodically throughout the session, the correct treatment code (i.e., Drug A vs. Not Drug A; Drug A vs. Drug B; Red Drug vs. Blue Drug) is not revealed to the participant until the conclusion of the session. The percentage of correct responses (i.e., correct identification of the treatment) is then converted to money and the participant is told immediately how much bonus money they earned during the experimental session. The performance criterion for having acquired the discrimination is predetermined (e.g., 80% correct responding on four consecutive days), and only those participants that meet the criterion in a specified number of sessions (e.g., 12) advance beyond the acquisition phase. The extensive training associated with human drug-discrimination procedures provides participants with similar recent behavioral and pharmacological histories, which is thought to reduce variability both within and across participants.
The final phase is the test phase, during which the discriminative-stimulus effects of different doses of the training drug, novel drugs, or drug combinations are determined. Sessions involving the administration of doses or drugs other than the training condition are deemed to be “test sessions”. Participants are not told the purpose of test sessions, nor do they know when these sessions are scheduled until completing the session. As is the case in preclinical studies, there is no correct response per se during these test sessions, so participants usually receive all of the available money that is contingent on correctly identifying the drug condition that was administered. Test-of-acquisition sessions that are identical to those in the acquisition phase are interspersed among test sessions to ensure that participants continue to accurately discriminate the training dose versus placebo. Additional sessions are inserted to re-establish accurate discrimination if the participant fails to correctly identify the training condition they received during a test-of-acquisition session conducted during the test phase. The number of test-of-acquisition sessions included in the test phase varies but is usually fewer than the total number of test sessions (e.g., 25–50%).
In general, there are two strategies in the choice of drug conditions administered in the test phase with the goal of elucidating the neuropharmacological mechanisms that mediate the discriminative-stimulus effects of the training drug. The first is the use of substitution procedures, in which a range of doses of other drugs is tested to determine if they share discriminative-stimulus effects with the training drug. Based on the drugs that produce significant drug-appropriate responding, inferences can be made regarding the neuropharmacological mechanisms that mediate the effects of the training drug. The second approach is to determine a dose-response curve for the training drug alone and in combination with pharmacologically selective compounds. These compounds can be administered concurrently with the training drug or one given as a pretreatment to the other, depending on the pharmacokinetic profiles of the training and test drugs. Inferences are made regarding the neuropharmacological mechanisms that mediate the discriminative-stimulus effects of the training drug based on the mechanism of action of the test drugs that shift the training-drug dose-response curve.
Human drug-discrimination procedures offer a number of advantages relative to other assays commonly used in behavioral pharmacology. As mentioned previously, three strengths of human drug discrimination are that it produces data that are orderly and dose-dependent, is pharmacologically selective, and that subjects have virtually identical training and recent drug-exposure histories prior to testing novel drugs and/or drug doses. In addition to these strengths, the relationship between the subjective- and discriminative-effects of drugs may be directly evaluated in human drug-discrimination studies.
Despite these notable strengths, human drug-discrimination procedures also have several potential limitations that warrant consideration. First, drug-discrimination procedures require extensive training before testing can begin and require a considerable investment of time and resources on the part of both volunteers and investigators. An offsetting strength is that fewer subjects are required to achieve adequate statistical power in drug-discrimination studies relative to other procedures that rely more heavily on subjective-effects measures. Second, drug-discrimination tasks specifically provide a relatively limited amount of information (i.e., typically a single outcome measure such as discrimination accuracy) as compared to other behavioral measures that provide information across an array of dimensions (e.g., subjective-effects measures; Kelly et al., 2003). However, the interpretation of drug-discrimination data is somewhat less complicated because conclusions may be drawn directly from performance on the discrimination task. The likelihood of Type I errors is also decreased because drug-discrimination procedures rely on a single primary-outcome measure. Third, drug-discrimination performance is relatively insensitive to changes in circulating levels of drug across the time-course of drug effects in that the allocation of responses to the drug-appropriate option does not typically decrease as blood levels decrease (e.g., Kelly et al., 1997). Fourth, the investigation of the specific role of various molecular sites of action (e.g., transporters, receptor systems, and specific receptor subtypes) to the discriminative-stimulus effects of drugs in humans are relatively limited because medications that are approved for use with humans by the U.S. Food and Drug Administration are typically used in human drug-discrimination studies. Fifth, as noted above, a significant challenge relative to animal models is that humans vary in their behavioral and pharmacological histories, which can affect study results and complicate the interpretation of the findings. Finally, in the context of the study of substance-use disorders, drug-discrimination procedures lack the face validity of other experimental approaches such as drug self-administration (e.g., McMahon, 2015). Although the drug-discrimination paradigm may lack a certain degree of face validity relative to other experimental approaches, it has predictive validity with respect to the underlying neurobiological and neuropharmacological mechanisms of drugs and determination of the abuse potential of novel compounds (e.g., Colpaert, 1999; Brauer, Goudie, and de Wit, 1997; Holtzman and Locke, 1988; Huskinson et al., 2014; Kelly et al., 2003).
As indicated in previous chapters, drug-discrimination procedures are pharmacologically selective and, as a result, have been used to assess the underlying neuropharmacology of centrally acting drugs. In addition, findings from human drug-discrimination studies are, in many cases, consistent with the hypothesized neuropharmacological mechanisms of actions of those drugs. According to the most recent epidemiological findings, the three most-used substances in 2013 among persons age 12 years or older were cannabis (19.8 million), psychotherapeutics (including prescription stimulants and opioid pain relievers; 6.5 million), and cocaine (1.5 million; Substance Abuse and Mental Health Services Administration [SAMHSA], 2014). Therefore, in this section of the chapter, we have chosen to review a portion of the human drug-discrimination literature that demonstrates the utility of this behavioral assay to elucidate the underlying neuropharmacology of stimulants, opioids, and cannabis.
Abused stimulants exert their pharmacodynamic effects via interactions with monoamine transporters (e.g., dopamine [DA], serotonin [5-HT], and norepinephrine [NE]; reviewed in Fleckenstein et al. 2000; Johanson and Fischman, 1989; Rothman and Glowa, 1995; Seiden et al., 1993). Prior ex vivo studies suggest that stimulants can be classified into two groups based on their differential regulation of these transporters. Amphetamines (e.g., d-amphetamine and methamphetamine) act as substrates for monoamine transporters and are transported into the nerve terminal where they prevent accumulation of neurotransmitter in storage vesicles, inhibit metabolic degradation by monoamine oxidase, and promote neurotransmitter release via carrier-mediated exchange (Seiden et al. 1993). Although amphetamines can also function as reuptake inhibitors, these effects are more moderate compared to their actions as transporter substrates (Rothman et al. 2001). By contrast, cocaine is a reuptake inhibitor and may cause firing-dependent reversal of the transporter thereby promoting the accumulation of neurotransmitter in the synapse (for a review, see Heal et al., 2014; Fleckenstein et al. 2000). Central monoamine systems (e.g., DA, 5-HT and NE) are implicated in the discriminative-stimulus effects of abused stimulants (Barrett and Appel, 1989; Callahan et al., 1991; Callahan et al., 1995; Callahan and Cunningham, 1995; Colpaert et al., 1979; Johanson and Barrett, 1993; Spealman et al., 1991; Spealman, 1995; Terry et al., 1994). The evidence for the involvement of central monoamine systems, namely DA, in the interoceptive effects of abused stimulants is reviewed below.
Substitution tests in prior human drug-discrimination studies suggest a prominent role for central monoamine systems in the interoceptive effects of stimulants. For example, in participants discriminating d-amphetamine (i.e., 10 mg) from placebo (Chait et al., 1986b), the D2 receptor partial agonist phenylpropanolamine (i.e., 25 and 75 mg) and monoamine reuptake inhibitor mazindol (i.e., 0.5 and 2.0 mg) substituted for d-amphetamine, suggesting that central monoamine systems are critically involved in the discriminative-stimulus effects of d-amphetamine. Other studies have shown that drugs that directly modulate monoaminergic tone (e.g., caffeine and methylphenidate; Garrett and Griffiths, 1997; Cauli et al., 2003) engender d-amphetamine-appropriate responding; whereas, drugs that do not (e.g., diazepam, hydromorphone, and diazepam) produce partial to minimal drug-appropriate responding (Chait and Johanson, 1988; Chait et al., 1984; Chait et al., 1985; Chait et al., 1986a, 1986b; Heishman and Henningfield, 1991; Kollins and Rush, 1999; Lamb and Henningfield, 1994; Rush et al., 1998; Rush et al., 2003). These studies demonstrate that d-amphetamine functions as a discriminative-stimulus via complex interactions at central monoamine systems.
Central monoamine systems also play a prominent role in the discriminative-stimulus effects of methamphetamine and cocaine. In one study, participants learned to discriminate oral methamphetamine (i.e., 10 mg) from placebo (Sevak et al., 2009). A range of oral doses of methamphetamine (i.e., 2.5–15 mg), d-amphetamine (i.e., 2.5–15 mg), methylphenidate (i.e., 5–30 mg), and γ-aminobutyric acid-A (GABAA) modulator triazolam (i.e., 0.0625–0.375 mg) was then tested. Figure 2 shows that d-amphetamine and methylphenidate dose-dependently increased methamphetamine-appropriate responding; whereas, triazolam failed to engender methamphetamine-appropriate responding. Similarly, Figure 3 shows that cocaine and methylphenidate produced similar discriminative-stimulus effects in participants who had learned to discriminate oral cocaine (i.e., 150 mg) from placebo (Rush et al., 2002). In contrast, neither modafinil, a NE releaser with weak affinity for the DA transporter (Akaoka et al., 1991; Ferraro et al., 1997), or the sedative hypnotic drug triazolam fully substituted for cocaine in this study. These findings collectively suggest that drugs that preferentially increase synaptic DA substitute for commonly abused stimulants across a range of doses; whereas, drugs that exert their primary effects through other neurotransmitter systems (e.g., triazolam and modafinil) do not produce discriminative-stimulus effects similar to commonly abused stimulants in humans.
The results of substitution tests in preclinical drug-discrimination studies are consistent with the notion that central monoamine systems mediate the discriminative effects of abused stimulants. For example, a range of doses of methamphetamine, cocaine, methylphenidate, d-amphetamine, and GBR 12909 were tested to determine if they shared discriminative-stimulus effects with methamphetamine in rats trained to discriminate 0.3 mg/kg methamphetamine from saline (Desai et al., 2010). GBR 12909 is a high-affinity DA transport blocker that is considered to be selective for DA transporters (Baumann et al., 2002; Howell and Kimmel, 2008). Each test drug substituted for methamphetamine in a dose-dependent manner suggesting that that DA neurotransmission contributes to the discriminative-stimulus effects of methamphetamine. Other studies have shown that DA reuptake inhibitors (e.g., bupropion, GBR 12909, and mazindol) fully substitute for cocaine whereas 5-HT and NE reuptake inhibitors do not (Baker et al., 1993; Broadbent et al., 1991; Cunningham and Callahan, 1991; Spealman, 1995; Terrry et al., 1994). In addition, D1- and D2-receptor agonists (e.g., SKF 38393 and quinpirole, respectively) engender cocaine-appropriate responding (Callahan et al., 1991; Callahan and Cunningham, 1993), suggesting a prominent role for DA signaling in the discriminative-stimulus effects of abused stimulants that are concordant with the results of substitution tests in human drug-discrimination studies.
Although the lack of selective compounds available for use with humans limits the conclusions that may be made about the specific roles of particular monoamine systems, the results of pretreatment tests in human drug-discrimination studies also suggest that central monoamine systems mediate the discriminative-stimulus effects of commonly abused stimulants. The effects of a range of doses of d-amphetamine (i.e., 0, 2.5, 5, 10, and 15 mg), alone and following pretreatment with the D2 receptor antagonist fluphenazine (i.e., 0, 3, and 6 mg) were assessed in participants who learned to discriminate 15 mg oral d-amphetamine from placebo (Stoops et al. unpublished data). Lower doses of fluphenazine (i.e., 3 mg) did not significantly alter the discriminative-stimulus effects of d-amphetamine in this study, but a higher dose (i.e., 6 mg) produced a marked rightward shift in the d-amphetamine dose-response curve in the one participant that completed the study (Figure 4). These findings suggest that central DA systems mediate the discriminative-stimulus effects of d-amphetamine in humans. However, these results should be interpreted cautiously because only a single subject completed the study due to the negative side-effect profile of fluphenazine.
Aripiprazole is an atypical antipsychotic that functions as a partial D2 receptor agonist (Burris et al., 2002) and is also known to exert effects at 5-HT1A, 5-HT2A, 5-HT2B, and 5-HT7 receptors (Shapiro et al., 2003). Partial agonists can either activate receptors with decreased efficacy relative to full agonists, or conversely function as an antagonist, depending on synaptic neurotransmitter levels. To determine the effects of aripiprazole on the discriminative-stimulus effects of d-amphetamine, a range of doses of d-amphetamine (i.e., 0, 2.5, 5, 10, and 15 mg) were assessed, alone and in combination with aripiprazole (0 and 20 mg), in participants who learned to discriminate oral d-amphetamine (i.e., 15 mg) from placebo (Lile et al., 2005a). d-Amphetamine functioned as a discriminative stimulus, but aripiprazole did not engender d-amphetamine-appropriate responding when tested alone. Aripiprazole pretreatment significantly attenuated the discriminative-stimulus effects of d-amphetamine, suggesting a role for DA and 5-HT in the interoceptive effects of d-amphetamine. These results are consistent with the ability of a D2-receptor partial agonist to function as an antagonist in the presence of a drug that elevates synaptic monoamine levels (Exner and Clark, 1992). Other studies have shown similar effects with other antipsychotics and GABAA modulators such as risperidone and alprazolam, respectively (Rush et al., 2003, 2004).
Pretreatment tests with agonists and antagonists in humans discriminating methamphetamine and cocaine further suggest that central monoamine systems are involved in the discriminative-stimulus effects of commonly abused stimulants (e.g., Lile et al., 2011a; Sevak et al., 2011; Vansickel et al., [2009a, 2009b] unpublished data). For example, Sevak and colleagues (2011) determined the influence of aripiprazole (0 and 20 mg) on the discriminative-stimulus effects of a range of doses of methamphetamine (0, 2.5, 5, and 10 mg) in participants who had learned to discriminate 10 mg methamphetamine. Methamphetamine functioned as a discriminative stimulus and dose-dependently increased drug-appropriate responding. Aripiprazole pretreatment significantly attenuated methamphetamine-appropriate responding (Figure 5), suggesting that monoamine systems play a role in the discriminative-stimulus effects of methamphetamine. To assess the role of monoamine systems in the discriminative-stimulus effects of cocaine, Lile and colleagues (2011a) tested a range of doses of oral cocaine (0, 25, 50, 100, and 200 mg) alone and in combination with aripiprazole (15 mg) in participants who had learned to discriminate 150 mg oral cocaine from placebo (Lile et al., 2011a). Although few effects of aripiprazole were observed, it appeared to attenuate the discriminative-stimulus effects of cocaine. These data collectively suggest that the discriminative-stimulus effects of commonly abused stimulants in humans are mediated by monoamine systems, namely DA and 5-HT.
The results of pretreatment tests in preclinical drug-discrimination studies with commonly abused stimulants correspond with those from human drug-discrimination studies and support the hypothesis that central monoamine systems underlie the interoceptive effects of abused stimulants. For example, Mechanic and colleagues (2002) determined whether the D2 and 5-HT2 antagonist olanzapine would attenuate the interoceptive cues elicited by d-amphetamine in rats that were trained to discriminate d-amphetamine (1.0 mg/kg) from saline. Olanzapine (1.5 mg/kg) significantly blunted the discriminative-stimulus effects of d-amphetamine. Similar findings have been obtained with selective D1 (e.g., SCH39166) and D2 antagonists (e.g., remoxipride and nemonapride; Tidey and Bergman, 1998), as well as the high-affinity dopamine transport blocker GBR 12909 (Czoty et al., 2004) to suggest a role for DA signaling in the discriminative-stimulus effects of stimulants in laboratory animals. In addition, these DA systems are under the inhibitory control of GABA systems (e.g., Kita and Kitai, 1988; Kalivas et al., 1990; Zetterstrom and Fillenz, 1990; Dewey et al., 1997). For example, Druhan and colleagues (1991) showed that pretreatment with the GABAA receptor modulator midazolam (i.e., 0–0.2 mg/kg) significantly attenuated drug-appropriate responding in rats trained to discriminate d-amphetamine (i.e., 1.0 mg/kg). In sum, the results of drug-discrimination studies with humans and non-human animals suggest that the neuropharmacological mechanisms of the discriminative-stimulus effects of abused stimulants are generally consistent (Rush et al., 2011).
In general, data from preclinical and human drug-discrimination studies demonstrate that abused stimulants produce their interoceptive effects via activation of DA and other monoamine systems. Abused stimulants function as discriminative stimuli and readily substitute for one another under a wide range of laboratory conditions and across species. Drugs that share discriminative-stimulus effects with abused drugs might function as effective agonist-replacement therapies to treat stimulant-use disorders (Klee et al., 2001; Shearer et al., 2001; 2002; Tiihonen et al., 2007). Alternatively, drugs that attenuate the discriminative-stimulus effects of abused drugs might function as effective pharmacotherapies for stimulant-use disorder by blunting the interoceptive effects of the drug (de Wit and Stewart, 1981; for a review, see Stoops and Rush, 2014).
Collectively, these studies suggest that human drug-discrimination procedures are rigorous behavioral assays that may be used to elucidate the underlying neuropharmacology of the discriminative-stimulus effects of stimulants. Future studies are needed to more fully elucidate the neuropharmacological mechanisms underlying the interoceptive-stimulus effects of abused stimulants in humans. These studies might test blockers of other catecholamines or drug-combinations that may have promise as pharmacotherapies (see Stoops and Rush, 2014 for a review). A more comprehensive understanding of the neuropharmacological mechanisms that mediate the interoceptive effects of stimulants in humans will inform the development of putative pharmacotherapies to manage stimulant-use disorders.
The basic neuropharmacology of opioid receptors is well known (for reviews see Janecka et al., 2004; Waldhoer et al., 2004). Briefly, the mu, kappa, and delta opioid receptors belong to the class A (rhodopsin) family of Gi/o protein-coupled receptors and are found throughout the central and peripheral nervous systems. These three receptor families mediate the analgesic effects of endogenous opioid peptides and opioid drugs (Borg and Kreek, 1998; Kelly et al., 2003; Waldhoer et al., 2004). Opioid drugs are naturally occurring, semi-synthetic, or synthetic formulations (e.g., morphine, hydromorphone and fentanyl, respectively). They are further classified as full agonists, partial or mixed agonists/antagonists, and full antagonists based on their pharmacological actions, selectivity, affinity and efficacy at the three primary receptor families (Kelly et al., 2003). The majority of prescribed opioid analgesics are agonists at the mu receptor with relatively limited activity at the other receptor types. The abuse-related behavioral effects of prototypical opioids like morphine, heroin, or hydromorphone have largely been attributed to their interaction with the mu receptor family (Mello et al., 1981; Sullivan et al., 2006; Walsh et al., 1996). The mu receptor family, in particular, is known to modulate the neuropharmacological activity of monoamine and GABAergic neurotransmitter systems resulting in increased synaptic dopamine levels (Vaughan et al., 1997; Baldauf et al., 2005; Chefer et al., 2009). The kappa and delta opioid receptor families are structurally and functionally similar to mu opioid receptors (Waldhoer et al., 2004). However, the behavioral effects of drugs that activate kappa and delta opioid receptors differ from those that preferentially activate mu receptors. For example, kappa agonists can produce dysphoria and hallucinations and there is evidence that the kappa receptor family is involved in stress responses (Land et al., 2008). Delta receptor agonists are less susceptible to analgesic tolerance compared to mu receptor agonists suggesting that these receptors may produce analgesic effects via different pharmacological mechanisms (Varga et al., 2004). This section of the chapter focuses on the mu receptor because most opioids that have been tested affect mu activity and the mu receptor is most clinically relevant with regard to opioid dependence in humans.
Eleven published clinical studies have examined the discriminative-stimulus effects of opioid drugs (Bickel et al., 1989; Jones et al., 1999; Duke et al., 2011; Preston and Bigelow, 1994; 1998; 2000; Preston et al., 1987; 1989; 1990; 1992; Strickland et al., 2015). A seminal study by Preston and Bigelow (1994) illustrates that opioid agonists with similar efficacy and affinity for the mu receptor generalize other mu receptor agonists but do not generalize opioid agonists that differ in these respects. Volunteers with a history of regular opioid use learned to discriminate intramuscular saline, hydromorphone, and butorphanol using the three-choice discrimination procedure (i.e., Drug A, Drug B, or Drug C) to investigate the discriminative-stimulus effects of hydromorphone and other opioid drugs with varying degrees of affinity for mu and kappa opioid receptors. The opioid drugs that were tested included hydromorphone (0.375–3.0 mg), the partial mu and kappa receptor agonist pentazocine (7.5–60 mg), the mu and kappa receptor mixed agonist-antagonist butorphanol (0.75–6 mg), the non-selective opioid agonist nalbuphine (3.0–24 mg), and the partial mu receptor agonist buprenorphine (0.075–0.6 mg). Opioids with greater affinity for the mu receptor fully substituted for hydromorphone regardless of whether the drug was a partial or full agonist. Opioids with lower intrinsic activity at mu receptors did not substitute for the mu agonist hydromorphone. Figure 6 shows that hydromorphone occasioned dose-related increases in hydromorphone-appropriate responding but did not substitute for butorphanol, consistent with their hypothesized neuropharmacological actions at the mu opioid receptor.
Preclinical research with pigeons (Morgan & Picker, 1998; Picker et al., 1993), rats (Beardsley et al., 2004; Shannon and Holtzman, 1977a; 1977b; 1979; Morgan et al., 1999), and non-human primates (Platt et al., 2001, 2004) have consistently shown that the discriminative-stimulus effects of opioids are concordant across species and that these effects follow with their in vitro neuropharmacology. For example, Platt and colleagues (2001) investigated the discriminative-stimulus effects of heroin in non-human primates and showed that the interoceptive effects of heroin were largely attributable to mu opioid receptor activation. Substitution tests with the major metabolites of heroin (i.e., 6-monoacetylmorphine, morphine, morphine-6-glucuronide, and morphine-3-glucuronide) and the mu opioid receptor agonists fentanyl and methadone were conducted with rhesus monkeys trained to discriminate heroin from saline. Each of these drugs occasioned dose-dependent increases in heroin-appropriate responding and, on average, engendered full substitution for heroin.
We know of two published clinical studies that have used pretreatment strategies to investigate the discriminative-stimulus effects of opioid drugs (Oliveto et al., 1998b; Strickland et al., 2015). For example, Strickland and colleagues (2015) utilized antagonist pretreatment in conjunction with substitution strategies to demonstrate that some of the discriminative-stimulus effects of the atypical opioid tramadol are mediated by mu receptor activation. Figure 7 shows representative drug-discrimination data for two subjects following administration of hydromorphone or a range of doses of tramadol alone (circles) or in combination with 50 mg naltrexone (squares). Tramadol occasioned dose-related increases in drug-appropriate responding for tramadol and a test dose of hydromorphone occasioned partial or full substitution for tramadol. Pretreatment with naltrexone (50 mg, p.o.) significantly attenuated the discriminative-stimulus effects of tramadol and hydromorphone. The use of opioid antagonists in human drug-discrimination procedures is an important strategy that provides additional information about the underlying neuropharmacological mechanisms of opioid drugs. Further, the use of this strategy bridges preclinical and clinical research; thereby, strengthening the translational validity of findings from drug-discrimination studies. Unfortunately, there are few clinical studies that have used antagonist pretreatment procedures to elucidate the neuropharmacological underpinnings of the discriminative-stimulus effects of opioids.
Preclinical work using pretreatment strategies has been crucial for examining the neuropharmacology of the discriminative-stimulus effects of opioid drugs. For example, France and colleagues (1984) trained pigeons to discriminate morphine from placebo and then performed substitution tests with morphine and oxymorphazone (a mu opioid receptor agonist). Morphine and oxymorphazone occasioned morphine-appropriate responding in a dose-dependent manner. Pretreatment with naltrexone shifted the dose-response curves to the right, indicating that naltrexone attenuated the discriminative-stimulus effects of these drugs. Antagonism of the discriminative-stimulus effects of opioid drugs by naltrexone pretreatment has also been observed in rhesus monkeys that were trained to discriminate heroin or morphine from vehicle (Bowen et al., 2002; Platt et al., 2001; 2004).
Opioid drug-discrimination studies in both human and non-human animals using substitution and pretreatment procedures are remarkably consistent with their neuropharmacological binding profiles for the mu receptor. These studies have revealed that although the discriminative-stimulus effects of opioid drugs are not limited to activity at opioid receptors, they are primarily mediated by mu receptor activity. These results are consistent with a primary role for the mu receptor in the ability of repeated opioid administration and dosing cessation to induce dependence and withdrawal, respectively (reviewed in Bailey and Connor, 2005). This neuropharmacological overlap in clinically relevant effects suggests that opioid drug-discrimination procedures could be used for medications development (McMahon, 2015). Opioid drugs with decreased abuse potential that share discriminative-stimulus effects with abused opioids might be effective pharmacotherapies for opioid dependence.
Of the more than 60 cannabinoid compounds found in cannabis, Δ9-tetrahydrocannabinol (Δ9-THC) is widely considered to be primarily responsible for its psychoactive effects (Ashton, 2001). The behavioral effects of Δ9-THC are mediated through the endogenous cannabinoid neurotransmitter system, which is composed of two known receptor subtypes: CB1 and CB2 (Matsuda et al., 1990; Munro et al., 1993). Both cannabinoid receptor subtypes are G-protein coupled receptors that inhibit adenylate cyclase activity and activate mitogen-activated protein kinase, but they differ to some degree in their interactions with certain ion channels and other G-proteins (e.g., Onaivi, 2006; Pertwee, 1997, 2006). CB1 and CB2 receptors also differ in their distribution such that CB1 receptors are primarily expressed on presynaptic nerve terminals throughout the central and peripheral nervous systems; whereas, CB2 receptors are expressed on immune cells (Pertwee, 2006). Although Δ9-THC is a non-selective partial agonist at CB1 and CB2 receptors, at least four lines of evidence suggest that the central effects of Δ9-THC are primarily mediated through CB1 receptors. First, the in vivo potency of Δ9-THC correlates with its binding affinity at the CB1 receptor (Compton et al., 1993). Second, the CB1 receptor subtype is localized in areas of the central nervous system that correspond with Δ9-THC effects (Breivogel and Childers, 1998). Third, agonists that are selective for CB1 receptors produce behavioral effects more similar to Δ9-THC than selective CB2 agonists (Järbe et al., 2006a; McMahon, 2006; Valenzano et al., 2005). Lastly, the centrally mediated effects of Δ9-THC are blocked by the administration of CB1-selective antagonists, but not those selective for CB2 receptors (Compton et al., 1996; Huestis et al., 2001; Järbe et al., 2006b; Zuurman et al., 2010). Given that another principal function of cannabinoid receptors is the modulation of non-cannabinoid neurotransmitter release via retrograde signaling (Szabo and Schlicker, 2005), other neurotransmitter systems also likely play a role in the behavioral effects of cannabinoids.
The published literature concerning the discriminative-stimulus effects of Δ9-THC in humans is much smaller in comparison to the other drug classes discussed in this chapter. To the best of our knowledge, only 8 studies have been published that evaluated the discriminative-stimulus effects of Δ9-THC in humans (Chait et al., 1988; Lile, Kelly, Pinksy, and Hays, 2009; Lile, Kelly, and Hays, 2010, 2011b, 2012a, 2012b, 2014; Lile, Wesley, Kelly, and Hays, 2015). In more recent studies, participants learned to discriminate orally administered Δ9-THC versus placebo. The use of orally administered Δ9-THC in lieu of smoked cannabis improves pharmacological selectivity (as cannabis contains other cannabinoids), allows better control of dosing parameters, and eliminates peripheral cues associated with smoked cannabis (e.g., Chait et al., 1988). The available literature on the discriminative-stimulus effects of orally administered Δ9-THC and its underlying neuropharmacology as determined with human drug-discrimination procedures is reviewed below.
The substitution of other drugs for the discriminative-stimulus effects of Δ9-THC in humans has been determined in several studies (Lile et al., 2009; Lile et al., 2010, 2011b, 2012a, 2012b, 2014). However, most of these studies determined the effects of a test drug alone (i.e., substitution) and in combination (i.e., pretreatment) with Δ9-THC (i.e., Lile et al., 2011b, 2012a, 2012b, 2014). The results of pretreatment tests are discussed below in a separate section for ease of comparison. In the first study by Lile and colleagues (2009), eight cannabis users learned to discriminate 25 mg oral Δ9-THC versus placebo. After learning the discrimination, a range of oral doses of Δ9-THC (5–25 mg), triazolam (0.0675–0.375 mg), hydromorphone (0.75–4.5 mg), and methylphenidate (5–30 mg) was substituted for the training dose. Figure 8 shows that oral Δ9-THC engendered dose-related increases in drug-appropriate responding, whereas none of the other drugs occasioned significant Δ9-THC-like responding. Worth mentioning is that each of the drugs tested produced measurable effects on other study outcomes, confirming that biologically relevant doses were tested. Lile and colleagues (2010) determined the substitution profile of the mixed CB receptor agonist nabilone in 6 human cannabis users who learned to discriminate 25 mg Δ9-THC from placebo. As shown in Figure 9, nabilone dose-dependently substituted for the interoceptive stimulus effects of Δ9-THC with the highest doses of nabilone (3 and 5 mg) fully substituting for the training dose. In contrast, methylphenidate did not significantly increase drug-appropriate responding, similar to a previous study (Lile et al., 2009). These findings demonstrate the pharmacological selectivity of the discriminative-stimulus effects of Δ9-THC and suggest that cannabinoid receptors are central to the Δ9-THC discriminative stimulus but other receptor systems (e.g., GABA) are not.
The results of substitution tests with human subjects discriminating Δ9-THC are relatively consistent with the results of non-human animal studies. Specifically, cannabinoid agonists occasion drug-appropriate responding in animals discriminating Δ9-THC (e.g., Järbe et al., 2006a, 2006b, 2010, 2012; De Vry and Jentzsch, 2003), but mu-opioid agonists (e.g., heroin and morphine) generally do not share discriminative-stimulus effects with Δ9-THC in animals (Browne and Weissman, 1981; Järbe and Hiltunen, 1988; Järbe et al., 2006b; Solinas et al., 2004; Solinas and Goldberg, 2005; McMahon, 2006; Wiley et al., 1995). Preclinical studies have also shown that dopaminergic drugs generally do not substitute for the discriminative-stimulus effects of Δ9-THC (Bueno et al., 1976; Järbe et al., 2006b; McMahon, 2006). However, the results with triazolam and diazepam in humans (Lile et al., 2009; Lile et al., 2014) do not agree with the preclinical findings that positive modulators of the GABAA receptor partially substitute for the discriminative-stimulus effects of Δ9-THC (Barrett et al., 1995; Browne and Weissman, 1981; Järbe and Hiltunen, 1988; Mokler et al., 1986; Wiley et al., 1995; Wiley and Martin, 1999).
Five studies have used drug-discrimination procedures to investigate the underlying neuropharmacology of the Δ9-THC discriminative stimulus in humans (Lile et al., 2011b, 2012a, 2012b, 2014, 2015). These studies used similar procedures to determine the role of the cannabinoid and GABA neurotransmitter systems in the discriminative-stimulus effects of Δ9-THC. Briefly, participants in these studies learned to discriminate 30 mg of oral Δ9-THC versus placebo in a two-choice (i.e., Drug vs. Not Drug) procedure. During testing, participants received three doses of nabilone (0, 1, and 3 mg p.o.), tiagabine (0, 6, and 12 mg p.o.), diazepam (0, 5, and 10 mg p.o.), and baclofen (0, 25, and 50 mg p.o.) alone and in combination with oral Δ9-THC (5, 15, and 30 mg). Figure 10 shows that nabilone occasioned Δ9-THC-appropriate responding when administered alone and shifted the Δ9-THC dose-effect function upward and leftward when co-administered with Δ9-THC (Lile et al., 2011b). Similarly, the GABA reuptake inhibitor tiagabine fully substituted for the Δ9-THC discriminative stimulus at the highest dose tested (12 mg) when administered alone and shifted the Δ9-THC dose-response curve upward and leftward in a dose-related manner (Lile et al., 2012a). In subsequent studies, the GABAA positive modulator diazepam did not occasion Δ9-THC-like responding when administered alone, in agreement with earlier triazolam results (Lile et al., 2009), and did not systematically affect the discriminative-stimulus effects of Δ9-THC when administered in combination (Lile et al., 2014). In contrast, a high dose of the GABAB agonist baclofen (50 mg) partially substituted for the Δ9-THC discriminative stimulus and both doses of baclofen significantly enhanced Δ9-THC-appropriate responding when co-administered (Lile et al., 2012b). These findings collectively demonstrate the involvement of GABAB receptor subtype, in the discriminative-stimulus effects of Δ9-THC in humans.
Procedural differences preclude the direct comparison of preclinical and human laboratory studies because most preclinical studies have determined the effects of pretreatment with cannabinoid antagonists on the discriminative-stimulus effects of Δ9-THC instead of cannabinoid agonists or GABA ligands. For example, pretreatment with the cannabinoid receptor antagonist rimonabant attenuates the discriminative-stimulus effects of Δ9-THC in laboratory animals (e.g., Järbe, Gifford, & Makriyannis, 2010; Järbe et al., 2010; 2014; Wiley et al., 2011). Despite these differences, some consistent findings emerge. First, drugs that activate the cannabinoid receptor system engender Δ9-THC-appropriate responding in humans and animals supporting the assertion that the cannabinoid receptor system is critically involved in the discriminative-stimulus effects of Δ9-THC (e.g., Lile et al., 2010, 2011b; Järbe et al., 2006a, 2006b, 2010, 2012; De Vry and Jentzsch, 2003). Second, stimulation of GABA neurotransmission appears to play a role in the discriminative-stimulus effects of Δ9-THC in both humans and preclinical animal models but the mechanisms that mediate these effects may differ between species (Lile et al., 2012a, 2012b, 2014; Barrett et al., 1995; Browne and Weissman, 1981; Järbe and Hiltunen, 1988; Mokler et al., 1986; Wiley et al., 1995; Wiley and Martin, 1999).
Although the body of research that has examined the underlying neuropharmacology of Δ9-THC in human subjects is relatively small, the extant literature demonstrates that cannabinoid and GABA neurotransmitter systems are important contributors to the discriminative-stimulus effects of Δ9-THC in humans. However, there appear to be species differences in the GABA-specific receptor mechanisms between humans and non-human animals. Lastly, the activation of monoamine (e.g., DA) and mu-opioid receptors does not appear to be involved in the interoceptive effects of Δ9-THC in humans. These studies also provide insight into potential therapeutic targets for the treatment of cannabis-use disorders. More specifically, these findings suggest that GABA could be targeted in the development of medications for cannabis dependence. In fact, gabapentin, a GABA analog that is approved for treating neuropathic pain and seizures, has recently emerged as a promising candidate pharmacotherapy for cannabis-use disorder (Mason et al., 2012) and, to date, is the only medication that has demonstrated initial pharmacotherapeutic efficacy in clinical trials in adults. Future research is needed to disentangle the mechanism by which gabapentin reduces cannabis use and also to determine whether a GABA reuptake inhibitor or GABAB agonist would be useful for managing cannabis dependence. In sum, drug-discrimination studies have greatly enhanced our understanding of the underlying neuropharmacology of Δ9-THC in humans and have helped to identify potential neuropharmacological targets for the treatment of cannabis dependence.
This section reviewed a number of studies that used human drug-discrimination techniques to investigate the underlying neuropharmacology of stimulants, opioids, and the primary psychoactive constituent in cannabis, Δ9-THC. At least four overarching conclusions can be drawn from the drug-discrimination literature reviewed above: (1) drugs in each of these classes function as discriminative stimuli in humans, (2) the discriminative-stimulus effects of these drugs are generally consistent with their underlying neuropharmacology, (3) the discriminative-stimulus effects of drugs in these classes are conserved across species, and (4) drug-discrimination techniques allow the determination of the underlying neuropharmacology of commonly abused illicit drugs to identify potential therapeutic targets that may guide the development and evaluation of putative pharmacotherapies for substance-use disorders.
The primary objective of this chapter was to provide a basic procedural overview of human drug-discrimination procedures and summarize the extant literature regarding the underlying neuropharmacology of commonly abused drugs (i.e., stimulants, opioids, and cannabis) as determined via human drug-discrimination studies. Although the extant literature firmly establishes human drug discrimination as a highly versatile and useful behavioral assay of in vivo neuropharmacology, interest in human drug-discrimination research and drug-discrimination research in general, has waned somewhat since its peak in the late 1990s. One factor that has potentially led to the decrease in enthusiasm for drug-discrimination studies in substance-abuse research is that the role of discriminative-stimulus effects in substance abuse may be less apparent relative to behavioral processes that are the focus of other experimental approaches. McMahon (2015) articulates a particularly poignant example when addressing the downward trend in the publication of drug-discrimination compared with the continued increase in the publication of drug self-administration research. Specifically, he cites that drug discrimination lacks the strong face validity of drug self-administration with regard to substance abuse because operant behavior maintained by a drug reinforcer more closely resembles the behavioral phenomenon of substance abuse (McMahon, 2015). Although behavioral models that have high face validity are intuitively appealing, whether or not they effectively predict the outcome of a manipulation on the phenomenon that they are intended to model is more important. The validity of the drug-discrimination paradigm for identifying the underlying neuropharmacology of centrally acting drugs in whole organisms is virtually unparalleled. However, less research has centered on the role that the discriminative-stimulus effects of drugs play in substance abuse but they may play a particularly important role in relapse and the resumption of problematic drug use.
Although the use of human drug-discrimination procedures in the future is uncertain, the emergence and growing popularity of designer drugs (i.e., bath salts), synthetic marijuana (i.e., spice), and devices that are used to vaporize nicotine (e.g., e-cigarettes) and cannabis will create new opportunities for additional drug-discrimination research. Furthermore, creative thinking about the application of human and laboratory animal drug-discrimination procedures to the investigation of interoceptive events that may contribute to substance abuse (e.g., drug withdrawal, anxiety, stress, etc.) may also provide opportunities for the use of these procedures to investigate the abuse-related behavioral effects of drugs in addition to underlying neuropharmacology.