Evaluation of the abuse potential of methocarbamol

The subjective and behavioral effects of p.o. administered methocarbamol, lorazepam and placebo were studied in a nonresidential group of adult male volunteers with histories of recreational substance abuse including sedative/hypnotics. In the first phase of the investigation, a dose run-up of methocarbamol (up to 12 g) was conducted in six subjects to determine appropriate doses. In the second phase, a randomized block cross-over study using 14 subjects was conducted. The following drug conditions were tested in the cross-over phase: placebo, lorazepam 1, 2 and 4 mg, and methocarbamol 2.25, 4.5 and 9 g. Drug conditions were tested under double-blind conditions. Psychomotor and cognitive performance measures and subject- and observer-rated behavioral responses were measured daily before dosing and for 5.5 hr after drug administration. The results showed that both lorazepam and methocarbamol produced statistically significant dose-related increases in subjects' ratings of drug effect and liking, although only lorazepam increased morphine-benzedrine group (MGB) scale scores. Methocarbamol also increased ratings on measures indicating the emergence of dysphoric and other side effects at high doses. Both drugs impaired psychomotor and cognitive performance, with lorazepam generally producing greater effects than methocarbamol. The results indicate that methocarbamol, at doses well above those used therapeutically, has some potential to be abused by persons with histories of sedative/hypnotic abuse; however, this potential for abuse is probably decreased by the accompanying side effects at high doses and is probably less than that of lorazepam.

Diazepam and triazolam self-administration in sedative abusers: concordance of subject ratings, performance and drug self-administration

The reinforcing effects of diazepam (DZ), triazolam (TZ) and placebo were examined in eight male subjects with histories of sedative abuse. DZ (40 or 80 mg), TZ (1.0 or 2.0 mg) and placebo were each individually available for oral self-administration using a double-blind within-subject design. After an initial sampling exposure, subjects could self-administer a single dose of drug on each of 6 days by completing a progressively increasing bicycle riding requirement. All subjects initially self-administered DZ and TZ but a decreasing number of subjects continued to self-administer drugs on the remaining days; there was no difference between DZ and TZ in the total number of self-administrations. Placebo was self-administered only by one subject on two occasions. Performance measures showed that TZ produced greater memory impairment than DZ and that DZ produced residual psychomotor impairment on the next day. With repeated dosing, evidence of tolerance was seen for both drugs on psychomotor and memory performance and subject ratings of drug liking. A few modest correlations of drug self-administration and subject-ratings were obtained, suggesting some correspondence of subject verbal and drug self-administration behaviors, but these measures did not covary in a completely consistent manner.

Differential generalization to pentobarbital in rats trained to discriminate lorazepam, chlordiazepoxide, diazepam, or triazolam

In drug discrimination studies benzodiazepine-trained animals have typically responded on the drug lever when tested with barbiturates. In a recent study, greater specificity appeared to be shown when lorazepam was used as a training drug. The generality and limits of this finding were explored in the present set of experiments. The asymmetrical cross-generalization found in lorazepam- and pentobarbital-trained baboons was replicated in rats and was shown not to be a function of either lorazepam (0.1., 0.32, or 1.0 mg/kg) or pentobarbital (10 or 25 mg/kg) training dose (i.e., pentobarbital-trained rats responded on the drug lever in tests with lorazepam, but lorazepam-trained rats did not show comparable pentobarbital generalization). In the next experiment, groups of rats were trained to discriminate chlordiazepoxide (10 mg/kg), triazolam (0.1 mg/kg), or diazepam (1.0 mg/kg). Generalization to both lorazepam and pentobarbital was shown by these rats. Finally after daily pentobarbital administration, lorazepam-trained rats made a sufficient number of responses after high pentobarbital doses to permit extension of the range of pentobarbital doses tested. Pentobarbital generalization increased, but still did not occur in all rats and was unreliable in successive tests in the same rats. These results suggest less homogeneity in the discriminative stimulus effects of "depressant drugs" than generally has been recognized.

Relative abuse liability of lorazepam and diazepam: an evaluation in 'recreational' drug users

The subjective, psychomotor and cognitive effects of oral doses of lorazepam (0, 1.5, 3 and 6 mg) and diazepam (0, 10, 20 and 40 mg) were studied under double-blind conditions in 14 volunteers with histories of 'recreational' benzodiazepine use/abuse. For each subject, drug was administered over 4 test days in a 2 (drug) by 4 (dose level) mixed design. Drug was the between-groups factor while dose was the within-subjects factor. Test days were separated by at least 1 week. The results showed that subjective ratings of drug 'liking' and the psychomotor and cognitive effects of lorazepam were generally similar to those of diazepam over the range of doses studied. Lorazepam, however, tended to produce effects of longer duration than diazepam. Since previous studies have shown that diazepam has a relatively high abuse liability among the benzodiazepines, the present findings suggest that lorazepam shares this property with diazepam is subjects with a history of 'recreational' drug use/abuse.

Amantadine: evaluation of reinforcing properties and effect on cocaine self-injection in baboons

The ability of amantadine to maintain self-injection behavior and to alter self-administration of cocaine was examined in baboons using a standard intravenous cocaine self-injection procedure. Responding was maintained under a FR 80- or 160-response schedule of intravenous cocaine delivery (0.32 mg/kg per injection). Each drug injection was followed by a 3-h time-out allowing a maximum of 8 injections/day. Vehicle or amantadine doses were substituted for cocaine for a period of 15 or more days. Evaluation of a wide range of amantadine doses (0.32-32 mg/kg per injection) showed that this compound did not maintain self-administration behavior above vehicle control levels. In another experiment using the cocaine self-injection baseline, amantadine (10 or 32 mg/kg per day) was administered via a chronic intravenous infusion. Cocaine self-injection behavior was maintained and re-initiated during chronic amantadine exposure, suggesting that the reinforcing efficacy of cocaine was not modified by chronic amantadine administration.

Reinforcing effects of caffeine in humans

The reinforcing and subjective effects of caffeine were studied under double-blind conditions in 12 normal humans. After 2 forced exposure days on which subjects received color-coded capsules containing either caffeine (100, 200, 400 or 600 mg) or placebo, subjects had a choice day on which they chose which one of the two types of color-coded capsules would be ingested. Subjects were exposed to 10 experimentally independent choices (i.e., involving exposure and choice between novel color-coded capsule conditions) at each of several dose levels. All forced exposure and choice opportunities occurred when subjects were overnight abstinent from their normal dietary caffeine intake (mean, 116 mg/day). Significant caffeine positive reinforcement was demonstrated in 5 of 12 subjects at one or more doses. Percentage of selection of caffeine was inversely related to dose, with four subjects showing significant caffeine avoidance at 400 and/or 600 mg. Choice behavior was correlated positively with feelings of contentedness and was correlated negatively with prestudy trait anxiety scores and with ratings of capsule disliking. Compared to placebo, caffeine produced increases in subjective ratings indicating arousal while producing decreases in headache and "craving" for caffeine-containing foods, even at the lowest dose of 100 mg. At higher doses caffeine produced dysphoric anxiety-like subjective effects. Overall, this study provides the first demonstration in humans of the positive reinforcing effects of caffeine alone (i.e., in capsules) and documents individual differences among normal subjects in both caffeine positive reinforcement and caffeine avoidance.

Reinforcing properties of caffeine: studies in humans and laboratory animals

Three types of experimental studies are reviewed: (1) intravenous and oral caffeine self-administration by laboratory animals, (2) oral caffeine self-administration by humans, and (3) human subjective effects of caffeine relevant to reinforcing effects. These studies show that, under appropriate conditions, caffeine can serve as a reinforcer and can produce elevations in subjective drug liking and/or euphoria. In this regard, caffeine can be distinguished from a wide range of behaviorally active compounds, such as the amphetamine analog fenfluramine and the major tranquilizer chlorpromazine, which do not produce such effects. Caffeine can also be distinguished from classic drugs of abuse such as cocaine, d-amphetamine or pentobarbital which generally maintain high levels of self-administration (or liking) in contrast to caffeine which tends to maintain lower levels of self-administration (or liking) or maintain self-administration under a more narrow range of parametric conditions. Several human studies and one animal experiment suggest that physical dependence substantially potentiates the reinforcing effects of caffeine. Other human and animal studies indicate that there may be substantial differences between individual subjects in the reinforcing effects of caffeine. An important challenge for future human and animal drug self-administration research will be to delineate more precisely the conditions under which caffeine does and does not serve reliably as a reinforcer.

Caffeine physical dependence: a review of human and laboratory animal studies

Although caffeine is the most widely used behaviorally active drug in the world, caffeine physical dependence has been poorly characterized in laboratory animals and only moderately well characterized in humans. In humans, a review of 37 clinical reports and experimental studies dating back to 1833 shows that headache and fatigue are the most frequent withdrawal symptoms, with a wide variety of other signs and symptoms occurring at lower frequency (e.g. anxiety, impaired psychomotor performance, nausea/vomiting and craving). When caffeine withdrawal occurs, severity can vary from mild to extreme (i.e. incapacitating). The withdrawal syndrome has an onset at 12-24 h, peak at 20-48 h, and duration of about 1 week. The pharmacological specificity of caffeine withdrawal has been established. The proportion of heavy caffeine users who will experience withdrawal symptoms has been estimated from experimental studies to range from 25% to 100%. Withdrawal symptoms have been documented after relatively short-term exposure to high doses of caffeine (i.e. 6-15 days of greater than or equal to 600 mg/day). Although animal and human studies suggest that physical dependence may potentiate the reinforcing effects of caffeine, human studies also demonstrate that a history of substantial caffeine intake is not a necessary condition for caffeine to function as a reinforcer. The similarities and differences between caffeine and classic drugs of abuse are discussed.

Lorazepam and meprobamate dose effects in humans: behavioral effects and abuse liability

On a residential research ward, the acute effects of placebo, lorazepam (LZ) (1.5-9.0 mg) and meprobamate (MEP) (600-3600 mg) were examined using a within-subject double-blind Latin Square design in nine male subjects with histories of drug abuse. Drug effects were assessed with objective performance tasks, subject estimates of performance, staff ratings of drug effects and subject ratings of drug effects, sleep, mood, drug liking and monetary street value. Generally, both LZ and MEP produced comparable dose-related effects; LZ had a more rapid onset of action and on several measures showed a more shallow dose-response curve than MEP. With LZ, but not MEP, subjects under-estimated the degree to which their performance was impaired and under-rated drug effects as compared to analogous staff ratings. Both drugs produced sedation-like subject ratings of mood and sleep but generally did not produce tranquilization-like ratings. MEP produced subject ratings of drug liking and monetary street value which were equal to or in some cases greater than those of LZ. Relative potency estimations showed that LZ was 510 to 783 times more potent than MEP in producing performance impairment but was only 370 times more potent than MEP in producing subject ratings of drug liking. Overall, these data indicate that the likelihood of abuse of MEP is at least equal to if not greater than that of LZ although LZ may have a greater likelihood of producing adverse behavioral effects such as a performance impairment and under-estimates of the degree of impairment. These data in conjunction with previous results from this laboratory show that the behavioral effects of benzodiazepines can be differentiated from those of other types of sedative/anxiolytics and that MEP displays characteristics of a barbiturate-like profile of action.

Effects of Ro 15-1788 and CGS 8216 in diazepam-dependent baboons

Baboons received continuous intragastric infusions of diazepam (20 mg/kg per day) for one or more months. While diazepam treatment continued, baboons received intragastric doses of Ro 15-1788 (0.032-32.0 mg/kg) or CGS 8216 (1.0-100.0 mg/kg) at intervals of two or more weeks. Baboons were observed following administration of these antagonists for the presence of precipitated withdrawal signs. The following results were obtained: (1) both Ro 15-1788 and CGS 8216 produced signs of precipitated withdrawal in the baboon; (2) a more severe overall withdrawal syndrome was precipitated with Ro 15-1788 than with CGS 8216 at testable doses; (3) Ro 15-1788 produced dose-related increases in the overall severity of withdrawal, while CGS 8216 did not produce a clear dose-related increase in the overall severity of withdrawal; (4) dose-effect curves for Ro 15-1788 for certain signs (e.g. limb-tremor) were monotonicly increasing, while for other signs dose-effect curves plateaued at lower doses of Ro 15-1788 (e.g. retching and vomiting) or were an inverted U-shape (e.g. scratching). CGS 8216 precipitated withdrawal signs were less clearly dose-dependent; (5) onset of Ro 15-1788 precipitated withdrawal signs were rapid (5-15 min) and reliable, while the onset of CGS 8216 precipitated withdrawal signs were generally slower (approximately 30 min) and more variable; (6) at doses of Ro 15-1788 and CGS 8216 that produced equal levels of vomiting and retching, Ro 15-1788 produced more limb-tremor than CGS 8216. These studies indicate that Ro 15-1788 and CGS 8216 may produce quantitatively and qualitatively different precipitated withdrawal syndromes.

An examination of the intravenous self-administration of phenylpropanolamine using a cocaine substitution procedure in the baboon

Intravenous self-administration of phenylpropanolamine HCl (0.10 to 10.0 mg/kg/injection) was examined in baboons under conditions in which baseline responding was maintained by intravenous injections of cocaine HCl (0.32 mg/kg/injection). Drug was available under a FR 160-response schedule of intravenous injection. Each drug injection was followed by a 3-hr time-out allowing a maximum of eight injections per day. Phenylpropanolamine or phenylpropanolamine vehicle (saline) was substituted for cocaine for a period of 15 days followed by a return to the cocaine baseline. Response rates after phenylpropanolamine substitution were similar to those maintained by saline substitution, and lower than those maintained under cocaine baseline conditions. At the two highest doses of phenylpropanolamine tested (3.2 and 10.0 mg/kg/injection) concurrent food maintained behavior was suppressed.

Phenylpropanolamine: effects on subjective and cardiovascular variables at recommended over-the-counter dose levels

Two controlled clinical studies evaluated the effects of phenylpropanolamine HCL (PPA) on measures of blood pressure, pulse, and subjective state (mood). One hundred fifty subjects participated in a parallel groups design that compared a 75-mg sustained release (SR) preparation with a 25-mg tid. dosing regimen and placebo. Fifty-nine of these subjects participated in an additional cross-over component that compared SR PPA 75 mg with placebo. Measures of blood pressure, pulse, and subjective drug effect were obtained nine times throughout the course of a 12-hour session. Data analysis revealed no clinically and few statistically significant effects due to drug treatment. As expected, most measures showed circadian changes on both the cardiovascular and mood variables, which were not related to drug treatment. No euphorogenic or "amphetamine-like" effects were noted. Although further work is warranted regarding the effects of chronic or higher-than-normal doses of PPA, the current studies suggest that PPA, at currently recommended dose levels, is not associated with adverse effects on either cardiovascular or subjective functioning.

Self-administration of barbiturates and benzodiazepines: a review

Studies of barbiturate and benzodiazepine self-administration are categorized by species and route of administration. Reinforcement, defined as self-administration of drug greater than of a non-drug control, has been demonstrated most often in studies employing the IV route, and there has been greater reliability in this result for a given drug among barbiturates rather than among benzodiazepines. Most studies of PO self-administration in rodents have not demonstrated reinforcement, despite a number of behavioral manipulations to induce drug intake. Studies of PO barbiturate self-administration in monkeys have demonstrated reinforcement but recent studies of PO benzodiazepine self-administration in baboons have not, although physical dependence was demonstrated. Reinforcement via the IG route has not been reliably demonstrated. Behavioral variables, including interreinforcement interval and drug self-administration history, appear to be important determinants of whether or not reinforcement will be demonstrated, particularly among the benzodiazepines; but the range of conditions under which behavioral and pharmacological variables interact to promote or lessen the likelihood of self-administration of these drugs remains to be determined experimentally.

Interactions of diazepam and caffeine: behavioral and subjective dose effects in humans

The effects of diazepam (DZ) (0, 10, and 20 mg) and caffeine (CAF) (0, 200, 400, and 600 mg) alone and in combination were examined in nine healthy male subjects using a within-subject experimental design in which all subjects received all twelve possible dose combinations. Drug effects were assessed using various psychomotor and cognitive performance tasks, staff (observer) ratings of subject behavior, and subject ratings of mood and drug effect. DZ treatment alone impaired performance on all tasks and produced staff and subject ratings indicative of sedative drug effects. CAF treatment alone facilitated performance on two psychomotor tasks requiring rapid reaction speed and increased staff ratings of subject restlessness and subject ratings of tension, alertness, arousal, and CAF symptoms. CAF generally antagonized the DZ-induced ratings of sedation and impairment of psychomotor performance; however, CAF did not consistently antagonize the DZ impairment of immediate recall or delayed recognition memory performance. DZ antagonized the CAF-induced staff-rated restlessness, and subject-ratings of tension, alertness, arousal and CAF symptoms. The results generally support the hypothesis that DZ and CAF produce antagonistic effects through functionally opposing mechanisms, however, the observed effects of drug combinations are dependent on the specific doses being tested and on the measures of drug effect being examined.

Human cigarette smoking: effects of puff and inhalation parameters on smoke exposure

This study determined effects of three smoking behavior components: puff volume, inhalation volume and lung exposure duration on biological measures of smoke exposure. A microcomputer-based auditory feedback system allowed subjects (N = 9 or 10 per experiment) to control puff and inhalation parameters as they smoked usual brand cigarettes. In each of four experiments, one smoking parameter was manipulated across sessions while two other parameters were held constant. Biological samples were obtained before and after each 8-puff smoking session conducted under a given set of behavioral parameters for analysis of plasma nicotine and expired air carbon monoxide (CO) levels. In Experiment I, both nicotine and CO levels were influenced systematically as puff volume was varied from 15 to 60 ml (inhalation volume = 50% of vital capacity, lung exposure time = about 9 sec). Nicotine boost (post- minus presession levels) increased 4-fold and CO boost increased 9-fold over this range of puff volume values. In Experiment II, nicotine levels were unaffected when average lung exposure times varied from 5 to 21 sec (puff volume = 50 ml, inhalation volume = 50% of vital capacity), suggesting that all the nicotine available may be absorbed during a normal smoking inhalation cycle with no breathholding. CO levels increased systematically with longer breathholds. In Experiments III and IV, inhalation volumes from 10% and 20% to 60% of vital capacity had no effect on either nicotine or CO levels, and this was true whether lung exposure time was about 8 sec (Experiment III) or about 4 sec (Experiment IV). This series of studies has shown that puff volume is an important determinant of tobacco smoke exposure, but that inhalation components of smoking behavior, at least within the range of parameters tested, have no effect on nicotine exposure levels.

Self-injection of d,1-3,4-methylenedioxymethamphetamine (MDMA) in the baboon

MDMA (d,1-3,4-Methylenedioxymethamphetamine HCl; "ecstasy") self-injection (0.1-3.2 mg/kg/injection) was examined in baboons under conditions in which baseline responding was maintained by intravenous injections of cocaine HCl (0.32 mg/kg/injection). Drug was available under a FR 160-response schedule of intravenous injection. Each drug injection was followed by a 3-h time out allowing a maximum of eight injections per day. MDMA or MDMA vehicle (saline) was substituted for cocaine for a period of 14 or more days followed by a return to the cocaine baseline. MDMA (0.32-3.2 mg/kg/inj) maintained more injections and higher responses rates than were maintained by saline. The maximal number of injections maintained by MDMA and the maximal response rate maintained by MDMA were less than those maintained under baseline conditions with cocaine. The highest dose of MDMA tested maintained a cyclic pattern of self-injection, i.e., days of high numbers of injections intermixed with days of low numbers of injections. At the highest dose of MDMA tested, concurrent food maintained behavior was suppressed to an extent that food intake was also decreased.

Nicotine gum: dose-related effects on cigarette smoking and subjective ratings

The effects of nicotine-containing chewing gum on cigarette smoking and subjective and physiological response were evaluated in eight normal volunteers. Isolated subjects smoked their regular brand of cigarettes freely in a naturalistic laboratory environment while watching television or reading. Before 90-min smoking sessions subjects chewed two pieces of placebo or nicotine-containing gum (0, 2, 4, or 8 mg) under double-blind conditions. Each subject received each treatment four times in a mixed order across days. Analysis of the chewed gum for remaining nicotine revealed that the mean delivered nicotine doses were 0, 1.02, 2.39, and 5.20 mg nicotine. Nicotine preloading produced dose-related increases in plasma nicotine, while producing dose-related decreases in various measures of cigarette smoking including number of cigarettes smoked, number of puffs taken, expired air carbon monoxide level, and ratings of smoking satisfaction. Nicotine preloading produced dose-related increases in ratings of gum dose-strength, while producing decreases in ratings of gum dose acceptability and liking. Heart rate and blood pressure were not significantly affected by nicotine gum. Taken together, the present results confirm that responses to nicotine in the gum preparation are orderly and related to dose, and the results suggest that the efficacy of treating tobacco dependence with nicotine gum may be enhanced by increasing the administered dose.

Human coffee drinking: reinforcing and physical dependence producing effects of caffeine

In a residential research ward coffee drinking was studied in nine volunteer human subjects with histories of heavy coffee drinking. The presence or absence of caffeine in the coffee was manipulated under double-blind conditions by using caffeinated (C) or decaffeinated (D) coffee. When subjects were switched alternately for 10 or more consecutive days between C and D, the daily number of cups consumed tended to be relatively stable. In a different experiment, preference for C vs. D was assessed. After experimenter-scheduled exposures, subjects were given choices between C and D. When subjects were presumably caffeine tolerant/dependent, C was rated as being better liked than D and was reliably preferred to D in choice tests. When subjects were not caffeine tolerant/dependent, C was not reliably preferred to D, nor were there pronounced differences in ratings of liking. Under these conditions, some subjects preferred D to C, citing adverse symptoms (suggesting caffeine toxicity) as reasons for avoiding C. The effects of caffeine withdrawal were studied by abruptly substituting D for C for 10 or more days. This resulted in an orderly withdrawal syndrome, having an onset latency of 19 hr, peaking on days 1 and 2, and decreasing progressively over the next 5 or 6 days. The withdrawal syndrome, which was detected on subject-rated, staff-rated and objective behavioral measures, was characterized by increased headache, sleepiness and laziness and decreased alertness and activeness. The present study demonstrates the reinforcing effects of caffeine in humans and also documents the severity of the caffeine withdrawal syndrome. It is concluded that caffeine has the cardinal features of a prototypic drug of abuse.

Diazepam and methadone blood levels following concurrent administration of diazepam and methadone

Results of a previous study indicated that the opioid effects of methadone were enhanced by the concurrent administration of diazepam in methadone-maintained subjects. To determine whether a pharmacokinetic interaction might account for this methadone-diazepam interaction, the plasma levels of methadone, diazepam and diazepam metabolites were determined in blood samples collected during that study. Five adult male patients on methadone maintenance (50-60 mg/day) were administrated single doses of placebo, diazepam (20 and 40 mg), methadone (100%, 150% and 200% of the maintenance dose), and four diazepam-methadone dose combinations (20 and 40 mg diazepam in combination with 100% and 150% of the maintenance dose). The results showed that the concurrent administration of methadone and diazepam did not significantly change the time-course or areas under the plasma concentration-time curve of methadone, diazepam or N-desmethyl-diazepam compared to the levels following the administration of either drug alone. Thus, plasma drug level analysis does not indicate a pharmacokinetic interaction between diazepam and methadone.

Comparison of opioid self-injection and disruption of schedule-controlled performance in the baboon

Eight opioid agonist, mixed agonist/antagonist or antagonist compounds were compared in baboons. In the first experiment, i.v. drug self-injection procedures involved a fixed-ratio schedule with a 3 hr time-out after each injection. Doses of a test drug were substituted for cocaine for 12 or 15 days. Codeine, morphine, butorphanol, nalbuphine and pentazocine all maintained self-injection above vehicle control levels. These compounds differed in their relative potencies, and morphine did not maintain maximal drug self-injection rates. Buprenorphine maintained self-injection in only one of four baboons, and SKF-10,047 and naloxone did not maintain self-injection. In a second experiment, another group of baboons responded on a fixed-ratio 50 schedule of food pellet delivery. Intravenous injections were given 30 min before test sessions that lasted 30 min. All eight drugs produced dose-related decreases in response rates, and the buprenorphine dose-response curve was more shallow and not parallel to the others. Comparison across experiments suggested that the failure of morphine to maintain maximal self-injection rates is due to its relatively high potency in suppressing schedule-controlled performance. The results of this study confirm those of previous studies demonstrating that opioids with morphine-like subjective effects in man are self-administered by laboratory animals.

Effects of marijuana smoking on subjective ratings and tobacco smoking

Multiple measures of tobacco cigarette smoking and subjective and physiological effect were collected during 90 minute test sessions in volunteer cigarette smokers who also had histories of recreational marijuana use. Before sessions, subjects smoked one marijuana cigarette (placebo or 1.29%, 2.84%, 4.00%) using a standardized puffing procedure. Each dose and placebo was given four times to each subject in a randomized block sequence. Marijuana smoking produced dose-related increases in heart rate, ratings of dose strength and drug liking. However, marijuana produced no significant alterations in tobacco cigarette smoking. Puff duration within each marijuana cigarette varied in a fashion similar to that observed in previous studies of tobacco cigarette smoking: puff duration progressively decreased as the cigarette was smoked. This effect is probably due to progressive decreases in resistance to draw as the cigarette is smoked. Expired air carbon monoxide (CO) levels following marijuana smoking were inversely related to marijuana dose, suggesting the occurrence of some compensatory changes in marijuana smoking in response to dose manipulations. It is concluded that, although marijuana produces dose-related effects on physiological and subjective effects and on marijuana smoking behavior, marijuana differs from a variety of other psychoactive drugs previously studied in this paradigm in that no reliable changes in tobacco smoking were produced.

Discriminative stimulus effects of atypical anxiolytics in baboons and rats

Baboons and rats were trained to discriminate lorazepam and pentobarbital in a food-maintained two-lever drug vs. no-drug discrimination procedure. Previous research showed that benzodiazepines, but not barbiturates, occasioned drug lever responding in the lorazepam-trained animals. Lorazepam and six nonbenzodiazepines that have been proposed as anxiolytics (CGS 9896, CL 218,872, PK 9084, zopiclone, buspirone and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol) were studied in test sessions in which responding on either level produced food. Of the nonbenzodiazepine compounds studied that displace 3H-benzodiazepines in vitro, CL 218,872 and zopiclone occasioned drug lever responding in all animals; PK 9084 did not occasion drug lever responding in any animal; and CGS 9896 did not occasion drug lever responding in lorazepam- or pentobarbital-trained baboons or in lorazepam-trained rats, but did so in the pentobarbital-trained rats. Buspirone, a nonbenzodiazepine anxiolytic with prominent dopaminergic activity, and 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol, a gamma-aminobutyric acid agonist, also did not occasion drug lever responding in either baboons or rats, regardless of training drug. Time course studies in baboons with CGS 9896, PK 9084, 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol and buspirone did not reveal delayed onset of drug stimulus generalization. Some differences in potency as a function of route of administration were found with CL 218,872, zopiclone and buspirone. The discriminative stimulus effects of lorazepam, CL 218,872 and zopiclone were antagonized by the benzodiazepine receptor antagonist Ro 15-1788. It is concluded that the discriminative stimulus properties of these nonbenzodiazepine compounds thus do not co-vary with their antipunishment effects, with their clinical efficacy as anxiolytics or with benzodiazepine receptor binding.

Repeated administration of diazepam and triazolam to subjects with histories of drug abuse

The present study examined the effects of repeated administration of diazepam (DZ) and triazolam (TZ) on psychomotor performance and subject-rated drug liking. Subjects were 11 males (30-41 years) who had documented histories of drug abuse and who resided on a behavioral pharmacology research ward. Six subjects received 80 mg DZ every third day (3 subjects) or every sixth day (3 subjects) for a total of 3-6 dosing occasions and six subjects received TZ (2.0 or 3.0 mg) every second day (4 subjects) or every third day (2 subjects) for a total of 3-5 dosing occasions. The results showed that on the first dose occasion, the two drugs produced generally similar degrees of psychomotor impairment and subject-rated drug liking. Following the first DZ dose, subsequent doses produced less of an effect (i.e. single-dose tolerance). Across at least the first three dose occasions, progressive tolerance development was observed with DZ but no tolerance was observed with TZ. It is hypothesized that pharmacokinetic differences between DZ and TZ may account for the difference in the development of tolerance.

Human coffee drinking: manipulation of concentration and caffeine dose

In a residential research ward coffee drinking was studied in 9 volunteer human subjects with histories of heavy coffee drinking. A series of five experiments was undertaken to characterize adlibitum coffee consumption and to investigate the effects of manipulating coffee concentration, caffeine dose per cup, and caffeine preloads prior to coffee drinking. Manipulations were double-blind and scheduled in randomized sequences across days. When cups of coffee were freely available, coffee drinking tended to be rather regularly spaced during the day with intercup intervals becoming progressively longer throughout the day; experimental manipulations showed that this lengthening of intercup intervals was not due to accumulating caffeine levels. Number of cups of coffee consumed was an inverted U-shaped function of both coffee concentration and caffeine dose per cup; however, coffee-concentration and dose-per-cup manipulations did not produce similar effects on other measures of coffee drinking (intercup interval, time to drink a cup, within-day distribution of cups). Caffeine preload produced dose-related decreases in number of cups consumed. As a whole, these experiments provide some limited evidence for both the suppressive and the reinforcing effects of caffeine on coffee consumption. Examination of total daily coffee and caffeine intake across experiments, however, provides no evidence for precise regulation (i.e., titration) of coffee or caffeine intake.