Assessing pentobarbital preference in normal volunteers using a cumulative dosing procedure

Characterizing the subjective and psychomotor effects of carisoprodol in healthy volunteers

Carisoprodol is a centrally acting drug used to relieve skeletal muscle spasms and associated pain in acute musculoskeletal conditions. There is evidence from different sources that this oral muscle relaxant is abused and that it is associated with impairment leading to arrests for "driving under the influence" as well as increased risk of automobile accidents. Its subjective and psychomotor effects in healthy volunteers at therapeutic and supratherapeutic doses have not been well-characterized, and form the basis of this report. Fifteen healthy volunteers (8 males, 7 females) were administered 0, 350, and 700 mg of carisoprodol in separate sessions and for 6h afterwards they completed a battery of tests at fixed time intervals so as to assess the subjective and psychomotor effects of the drug. The supratherapeutic dose, 700 mg, increased visual analog scale ratings of terms that were more reflective of sedation (e.g., "sleepy," "heavy, sluggish feeling") than those of abuse liability, and produced impaired performance on several psychomotor tests. The therapeutic dose, 350 mg, while producing few and mild subjective effects, still produced psychomotor impairment. The fact that the therapeutic dose of carisoprodol produced minimal subjective effects while adversely affecting performance is of concern in that patients prescribed this drug may feel relatively normal and engage in tasks (driving) that could put themselves and others at risk.

A therapeutic dose of zolpidem has limited abuse-like effects in drug-naïve females: a pilot study

Zolpidem has abuse potential, particularly among individuals with histories of drug abuse. This double-blind, placebo-controlled, cross over pilot study investigated the subjective effects of zolpidem (10 mg) in drug-naïve females. Over the course of a 5-h period vital signs were monitored and a series of computerized questionnaires was administered. Results indicate that zolpidem engendered subjective effects characteristic of hypnotic drugs, but reduced ratings of drug liking, willing to take again, and willing to pay for, relative to placebo. Thus, a therapeutic dose of zolpidem may have limited potential for misuse among females who have no experience with drugs of abuse.

Individual differences in aggressive responding to intravenous flumazenil administration in adult male parolees

Nonhuman and human studies have shown that benzodiazepine (BZD) receptor agonists can modify aggressive behaviour. However, it is unknown whether flumazenil, a BZD receptor antagonist, enhances or inhibits aggressive behaviour. The present study was designed to investigate the effects of acute administrations of flumazenil on aggressive responding in adult humans. Six adult males with histories of childhood conduct disorder (DSM IV R) participated in experimental sessions. Aggression was measured using the Point Subtraction Aggression Paradigm (PSAP; Cherek 1992), which provided subjects with aggressive and monetary-reinforced response options. Acute doses of flumazenil (2 and 3mg) did not produce statistically significant changes in either monetary-reinforced responding or aggressive responding. The analysis of individual subjects data revealed that aggressive responses varied across subjects. The results are discussed in terms of individual differences based on the previous history of BZD abuse. Additional laboratory research is needed to better clarify the behavioural mechanisms by which BZD receptor antagonists modify human aggressive responding.

Reinforcing and subjective effects of the volatile anesthetic, sevoflurane

Sevoflurane is a volatile anesthetic that is chemically similar to volatile substances of abuse and can be safely administered to humans in laboratory research. In this study, the reinforcing and subjective effects of five concentrations of sevoflurane (0, 0.2, 0.4, 0.6, 0.8% sevoflurane in O2) were studied in 20 non-drug-abusers. During each of five sessions, subjects sampled a concentration of sevoflurane and 100% O2 (placebo) for 10 min each. Later, within the session, they chose nine times, once every 5 min, among sevoflurane (e.g. "Agent A"), placebo (e.g. "Agent B"), or neither (and were administered 100% O2, identified as "drug-free air"). Although "neither" was selected most frequently, mean preference ratios (sevoflurane choices/[sevoflurane choices+placebo choices]) and total sevoflurane choice peaked at the 0.4% concentration. Choice patterns varied across subjects, with some subjects never choosing sevoflurane and other subjects showing monotonic increasing or bitonic concentration-choice functions. Concentration-related increases in subjective effects were observed, including effects that are putatively associated with abuse liability. Ratings of drug liking and of wanting to inhale the drug again were positively correlated with sevoflurane choice. This study shows that sevoflurane can function as a reinforcer and produce abuse liability-related subjective effects in some healthy volunteers.

Alcohol choice and amphetamine effects in light and moderate drinkers

BACKGROUND

The results of previously published reports suggest that light and moderate drinkers respond differently to the effects of commonly abused sedatives (e.g., diazepam or ethanol). The purpose of this experiment was to determine whether light and moderate drinkers respond differentially to the effects of ethanol and d-amphetamine.

METHODS

In the first phase of this experiment, volunteers (eight light drinkers and eight moderate drinkers) randomly sampled 0.5 g/kg of ethanol and placebo across two separate sessions. In the second phase, volunteers completed three sessions in which they chose either ethanol or placebo. In the third phase, volunteers received 0, 5, 10, and 15 mg of d-amphetamine. Each dose was tested twice. After drug administration in each phase, volunteers completed a battery of subject-rated, performance, and physiologic measures periodically for 5 hr.

RESULTS

Ethanol produced prototypical subject-rated effects (e.g., increased ratings on the Alcohol Sensation Scale), but it was chosen over placebo infrequently. Light and moderate drinkers did not differ in terms of the self-reported or reinforcing effects of ethanol. d-Amphetamine produced prototypical subject-rated stimulant-like effects (e.g., dose-dependent increases in ratings of High and Rush). Moderate drinkers reported significantly greater drug effects than light drinkers. Responses to ethanol reliably predicted subsequent responses to d-amphetamine on several measures.

CONCLUSIONS

The results of this experiment suggest that even moderate ethanol use may increase an individual's vulnerability to abuse drugs. Future studies should determine whether light and moderate drinkers respond differentially to other commonly abused drugs (e.g., opioids) and whether behavioral responses to ethanol also predict responses to these compounds.

Bitonic dose-response functions for reinforcing and self-reported effects of nitrous oxide in humans

The reinforcing and self-reported effects of nitrous oxide (10%, 30%, and 50% N(2)O in O(2)) were examined in 13 humans. During each of three sessions, subjects sampled one dose of N(2)O and 100% O(2) (placebo) for 10 min each, separated by 30-min recovery periods. The agents were identified by letter code, and later in the session, subjects chose nine times, once every 5 min, among N(2)O (e.g., "Agent A"), placebo (e.g., "Agent B"), or "neither" (also 100% O(2), identified as "drug-free air"). Self-reported and psychomotor effects were measured at various times. Dose-response functions varied across subjects and included bitonic, monotonic increasing, monotonic decreasing, U-shaped, and flat dose-response functions for reinforcing and/or self-reported effects. For subjects who showed bitonic reinforcing effects, the descending limb of the dose-response function could not be attributed to behavioral impairment. This study replicates previous studies showing dose-dependent effects of N(2)O, as well as between-subject variability in N(2)O effects. Bitonic dose-response functions for some subjects extend the generality of the phenomenon of bitonicity of drug effects to N(2)O effects in humans.

Analysis of the reinforcing and subjective effects of different doses of nitrous oxide using a free-choice procedure

The reinforcing and subjective effects of five doses of nitrous oxide (0, 10, 20, 30, 40% N(2)O in O(2)) were studied in 20 non-drug-abusers using a free-choice procedure. During each of five sessions, subjects sampled a dose of N(2)O and 100% O(2) (placebo) for 10 min each. Later they chose nine times, once every 5 min, among N(2)O (e.g. 'Agent A'), placebo (e.g. 'Agent B'), or a no-drug option. Mean preference ratios (N(2)O choices/[N(2)O choices+placebo choices]) and total N(2)O choice increased with increasing N(2)O dose. Individual preference ratios suggested that at least one active dose of N(2)O functioned as a reinforcer in 80% of subjects, and the doses that functioned as reinforcers varied across subjects. N(2)O choice was positively correlated with end-of-session and post-session ratings of N(2)O liking and of wanting to inhale N(2)O again, but not with ratings of those effects during sampling. Placebo was chosen significantly less than the no-drug option, even though both were 100% O(2). More robust reinforcing effects of N(2)O were observed in this subject population than in previous studies. Choice data emphasize the importance of examining a range of doses, and of examining those effects within-subject, when assessing reinforcing effects of drugs. Inclusion of the no-drug option eliminated the 'forced' choice of placebo, making preference ratios easier to interpret than in previous, forced-choice procedures. Reinforcing effects were more correlated with subjective effects assessed after the session than with subjective effects obtained while subjects were under the influence of the drug.

Within- and between-subject variability in the reinforcing and subjective effects of nitrous oxide in healthy volunteers

Within- and between-subject variability in the reinforcing and subjective effects of nitrous oxide (N(2)O) was studied across five sessions. Twelve volunteers with no history of drug dependence sampled 30% N(2)O and 100% oxygen for 10 min each, then chose nine times, once every 5 min, among N(2)O (e.g. "Agent A"), oxygen (e.g. "Agent B"), or "drug-free air." Choice varied across subjects but was stable within subjects. Quantitative differences in subjective effects occurred within and across subjects. Some subjective effects were correlated with choice and/or differed between subjects who were consistent choosers of N(2)O versus those who were not. However, drug liking and euphoria, two face-valid measures of abuse liability, were unrelated to choice. Thus, the present study found individual differences (i.e. between-subject variability) in subjective and reinforcing effects of N(2)O and, in terms of within-subject variability, suggested that subjective effects fluctuate across sessions to a relatively greater extent than do reinforcing effects. The varying degrees of correlation between N(2)O choice and its subjective effects emphasize the need for obtaining multiple measures when characterizing abuse liability of this drug.

The effects of a cold-water immersion stressor on the reinforcing and subjective effects of fentanyl in healthy volunteers

The reinforcing and subjective effects of fentanyl, an opioid analgesic, were tested in ten healthy volunteers without histories of drug abuse, as a function of the temperature of a water bath in which the volunteers' forearms were immersed. The temperatures were body-temperature (37 degrees C), moderately cold (10 degrees C), and very cold (2 degrees C). A discrete-trial choice procedure was used in which, in each session, volunteers sampled 50 micrograms of fentanyl (delivered as a bolus via an infusion pump) and saline, and then on three successive trials, chose between the two. Volunteers then had to immerse their non-dominant forearm in the water bath 5 min after a drug delivery. Fentanyl was chosen on 77% of choice occasions in the 10 degrees C and 2 degrees C water conditions, which was significantly different from chance levels, and on 60% of choice occasions in the 37 degrees C water condition, which did not differ from chance levels. Several subjective effects of fentanyl were also modulated by the temperature of the water bath. We conclude that in the context of a painful stimulus, 50 micrograms of fentanyl functions as a reinforcer in non-drug abusers.

Effects of marijuana history on the subjective, psychomotor, and reinforcing effects of nitrous oxide in humans

An experiment using marijuana users and non-users was conducted to assess whether the reinforcing, subjective, or psychomotor effects of nitrous oxide were influenced by a subject's drug history. Subjects in the first four sessions sampled 40% nitrous oxide in oxygen and 100% oxygen (placebo), and then over the next three sessions, chose which agent they wished to inhale. Choice distributions between the two groups did not differ significantly, and nitrous oxide choice rates were less than 50% in both groups. However, a history of marijuana use appeared to intensify some of the subjective effects induced by nitrous oxide inhalation.

Reinforcing effects of extended inhalation of a low nitrous oxide concentration in humans

The reinforcing, subjective, and psychomotor effects of 30 min of inhalation of 20% nitrous oxide were determined in 12 healthy volunteers using a choice paradigm with 100% oxygen as placebo. Nitrous oxide was chosen on only 22% of choice occasions, indicating that, in general, this concentration did not function as a reinforcer. Nitrous oxide produced changes in mood, but had no effect on psychomotor performance. Three out of the 12 subjects chose nitrous oxide on at least two out of the three choice sessions, and during a poststudy debriefing interview, reported pleasant effects of the drug. The other nine subjects reported unpleasant acute effects of the drug (e.g., drowsiness) or residual (postsession) effects of the drug which, they said, influenced their drug choice. The present results are compared to those results obtained in a previous study in which higher concentrations of nitrous oxide (30 and 40%) also produced relatively low choice rates. The apparent lack of reinforcing effects of extended inhalation of nitrous oxide is discussed.

Subjective, behavioral and physiological responses to intravenous meperidine in healthy volunteers

Meperidine is a mu opiate agonist that is frequently used to treat pain. We examined in healthy volunteers (N = 10) the effects of intravenous meperidine (0, 0.25, 0.5, and 1.0 mg/kg) on mood and psychomotor performance. A randomized, placebo-controlled, crossover design was used in which subjects were injected with meperidine or saline in a double-blind fashion. Subjects completed several subjective effects questionnaires commonly used in abuse liability testing studies before drug injection and at periodic intervals for up to 5 h after drug injection. Subjects also completed several psychomotor tests. Meperidine produced a constellation of subjective effects in a dose-related fashion, including increases in ratings of "sedated," "coasting or spaced out" and "feel drug effect" ratings. Many of the drug's subjective effects persisted up to 4 or 5 h after administration of the 1.0 mg/kg dose. Drug liking ratings assessed on a visual analog scale were increased after meperidine injection in about half of the subjects (P = 0.09). Eye-hand coordination was affected slightly by meperidine but other indices of psychomotor functioning were unaffected. Miosis increased in a dose-related fashion. Other physiological parameters, such as vital signs, were not affected by meperidine. We conclude that meperidine in healthy volunteers has robust and long-lasting effects on mood, but may have weaker effects on psychomotor performance.

Self-administration of pentobarbital in light and moderate alcohol drinkers

Preference for a moderate dose of pentobarbital was assessed in light and moderate alcohol drinkers using a double-blind, placebo-controlled laboratory choice procedure. Sixteen light drinkers (less than six alcoholic drinks per week) and 13 moderate drinkers (six or more drinks per week) participated in a seven-session study in which they first sampled capsules containing pentobarbital (150 mg) or placebo and then chose and ingested the capsule they preferred. Subjective and behavioral measures were obtained at regular intervals during each session to characterize the drug's effects. Both groups chose pentobarbital less often than placebo: Mean pentobarbital choice in light drinkers was 20.8% and in moderate drinkers was 38.5%. Pentobarbital choice and drug liking ratings were highest among male moderate drinkers but still did not exceed placebo levels. The drug did not increase scores on standardized measures of drug euphoria, even among the most frequent choosers or the heaviest alcohol consumers. The results extend previous reports showing that individuals without histories of drug abuse, even those who are moderate consumers of alcohol, do not self-administer sedative/anxiolytic drugs or experience their effects as euphorigenic.

Preference procedures for testing the abuse liability of drugs in humans

Drug preference procedures may be used to assess the abuse liability of drugs in human volunteers. These procedures usually measure subjects' choice of a drug versus a placebo, under controlled, double-blind conditions. Depending upon the conditions of the studies (e.g. subject population, environmental conditions of testing), drugs that are known to be abused are also preferred in these laboratory procedures. To assess the abuse liability of an unknown test drug, the frequency of choice of the test drug is compared to choice of a standard drug with known abuse potential: preference for the test drug, relative to the standard, is then taken as an indicator of its relative potential for abuse. A number of methodological variables must be considered when designing and interpreting choice studies, including the characteristics of the subjects, the range of doses tested, whether choices were spaced widely apart (e.g. one a day) or closely together (e.g. one immediately after another), and whether the study was conducted under controlled laboratory conditions or in the subjects' normal daily environments. The conditions for testing the abuse liability of an unknown compound must be selected based on the existing literature with similar, known drugs of abuse.