Differential sensitivity to midazolam discriminative-stimulus effects following self-administered versus response-independent midazolam

Interactions between the discriminative and reinforcing effects of midazolam were studied in two baboons trained to discriminate midazolam (0.32 mg/kg, IV) from saline. The midazolam generalization gradient determined after the baboons were permitted to self-administer midazolam (IV) was shifted to the left of that determined before self-administration. In contrast, the midazolam generalization gradient determined after the same doses of midazolam were delivered response-independently, but in the same order and with the same temporal pattern as during self-administration, was shifted to the right of that determined just before the response-independent phase. These data suggest that sensitivity to the discriminative-stimulus effects of a drug can be modulated by behavioral experience with that drug.

Withdrawal syndrome after the double-blind cessation of caffeine consumption

BACKGROUND

People who stop consuming caffeine may have symptoms, but the incidence and severity of caffeine withdrawal are not known. This study was performed to determine the effects in the general population of ending one's dietary intake of caffeine.

METHODS

We studied 62 normal adults whose intake of caffeine was low to moderate (mean amount, 235 mg--the equivalent of 2.5 cups of coffee--per day). They completed questionnaires about symptoms and tests of their mood and performance when consuming their normal diets (base-line period) and at the end of each of two two-day periods during which they consumed caffeine-free diets and under double-blind conditions received capsules containing placebo (placebo period) or caffeine (caffeine period) in amounts equal to their daily caffeine consumption.

RESULTS

More subjects had abnormally high Beck Depression Inventory scores (11 percent), high scores on the trait scale of the State-Trait Anxiety Inventory (8 percent), low vigor scores (11 percent) and high fatigue scores (8 percent) on the Profile of Mood States, and moderate or severe headache (52 percent) during the placebo period than during either the base-line period (2, 0, 0, 0, and 2 percent, respectively; P less than 0.05) or the caffeine period (3, 2, 2, 0, and 6 percent; P less than 0.05). More subjects reported unauthorized use of medications during the placebo period (13 percent) than during the caffeine period (2 percent, P = 0.017). Performance of a tapping task was slower during the placebo period than during the base-line and caffeine periods (P less than 0.01).

CONCLUSIONS

Persons who consume low or moderate amounts of caffeine may have a withdrawal syndrome after their daily consumption of caffeine ceases.

Subjective and behavioral effects of diphenhydramine, lorazepam and methocarbamol: evaluation of abuse liability

The effects of orally administered placebo, diphenhydramine, lorazepam, methocarbamol and placebo were studied in volunteers with histories of recreational substance abuse including sedative/hypnotics. Placebo, diphenhydramine (100, 200 and 400 mg), lorazepam (1 and 4 mg) and methocarbamol (2.25 and 9 g) were tested in a randomized, double-blind crossover study using 14 subjects. Psychomotor and cognitive performance and subject- and observer-rated responses were measured daily before and for 5.5 hr after drug administration. The results showed that each of the drugs exhibited a different profile of effects on the test battery. Lorazepam produced significant increases in subjects' ratings of drug effect and liking, increases in measures of sedation and impairment of psychomotor performance. Methocarbamol also produced significant increases in subjects' ratings of drug effect and liking and measures of sedation, but it produced only minor impairment of psychomotor and cognitive performance. Diphenhydramine increased subjects' and observers' ratings of drug effect and measures of sedation, but it produced less psychomotor performance impairment and liking than lorazepam. Diphenhydramine produced the most side effects. The present study clearly differentiated the behavioral and subjective profiles of diphenhydramine, lorazepam and methocarbamol. Consistent with its recognized low abuse liability, diphenhydramine produced fewer increases in measures of positive mood and more adverse effects. The considerable overlap in subjective effect measures of positive mood make further differentiation with respect to abuse liability difficult.

Zolpidem behavioral pharmacology in baboons: self-injection, discrimination, tolerance and withdrawal

This study examined in baboons various behavioral effects of zolpidem, a short-acting imidazopyridine hypnotic which has selectivity for subtypes of the benzodiazepine receptor. Intravenous drug self-injection was studied under a fixed-ratio 80- or 160-response schedule with a 3-hr timeout after each injection. Maximal rates of self-injection maintained by zolpidem (0.01-1 mg/kg) were consistently higher than those maintained by vehicle and the benzodiazepine hypnotic triazolam. Substitution of vehicle after about 2 weeks of zolpidem self-injection (7-8 mg/kg/day) resulted in a time-limited suppression of food pellet intake, indicating a drug withdrawal effect. In a drug discrimination study, baboons were trained to discriminate either lorazepam (1.8 mg/kg p.o.) or pentobarbital (10 mg/kg p.o.) from the no-drug condition. Zolpidem (0.1-18 mg/kg p.o.) occasioned both lorazepam- and pentobarbital-appropriate responding (greater than 80%) in a dose-dependent manner. In a final experiment, zolpidem (3.2 or 5.6 mg/kg i.m.) produced ataxia and sedation that progressively decreased over 7 consecutive days of administration. The withdrawal, discriminative stimulus effects and tolerance shown with zolpidem were similar to those shown previously with benzodiazepines under similar conditions. The rates of self-injection of zolpidem were similar to those maintained by intermediate duration barbiturates (e.g., pentobarbital) and higher than those maintained by 11 benzodiazepines studied previously under similar conditions. Further research on the reinforcing effects of zolpidem may provide useful insights into mechanisms underlying the maintenance of behavior by compounds acting through the benzodiazepine receptor.

Low-dose caffeine discrimination and self-reported mood effects in normal volunteers

A caffeine versus placebo discrimination procedure was used to determine the lowest caffeine dose that could produce discrimination and self-reported mood effects in normal volunteers. During daily sessions under double-blind conditions, caffeine-abstinent subjects orally ingested a capsule containing 178 mg caffeine or placebo. Before beginning discrimination training, the compounds were identified to subjects by letter codes. Fifteen, 30, and 45 min after capsule ingestion, subjects guessed the capsule's letter code. Correct guesses at 45 min earned money. After each session, subjects received a supplementary capsule containing caffeine or placebo to ensure that, within each phase of the study, subjects received the same daily dose of caffeine equal to the training dose. Five of the 15 subjects acquired the caffeine versus placebo discrimination within the first 20 sessions (greater than or equal to 75% correct); 6 other subjects acquired the discrimination with additional training. Nine subjects who acquired the discrimination were subsequently trained at progressively lower caffeine doses. In general, the lowest dose to produce discrimination (greater than or equal to 75% correct) was also the lowest dose to produce self-reported mood effects: 4 subjects showed discrimination and self-reported mood effects at 100 mg caffeine, 2 at 56 mg, 1 at 32 mg, and 1 at 18 mg. One of these subjects also showed self-reported mood effects at 10 mg. The present study documents discriminative stimulus and self-reported mood effects of caffeine at doses below those previously shown to affect any behavior in normal volunteers.

Caffeine tolerance and choice in humans

Thirty-two healthy subjects with histories of moderate caffeine consumption abstained from dietary caffeine throughout the study. Subjects were stratified into two groups based on several factors including caffeine preference, which was assessed using a caffeine versus placebo choice procedure. Subsequently, subjects received either caffeine (300 mg t.i.d.) or placebo (placebo t.i.d.) for 18 consecutive days, and thereafter were exposed again to a caffeine versus placebo choice procedure. The study documented tolerance development to the subjective effects of caffeine: after chronic dosing, administration of caffeine produced significant subjective effects in the chronic placebo group but not in the chronic caffeine group. The study also provided indirect evidence for tolerance development: during chronic dosing, the chronic caffeine and placebo groups did not differ meaningfully on ratings of mood and subjective effect. When subjects were categorized into caffeine choosers or nonchoosers, caffeine choosers tended to report positive subjective effects of caffeine and negative subjective effects of placebo. Nonchoosers, in contrast, tended to report negative subjective effects of caffeine. Chronic caffeine did not alter the reinforcing effects of caffeine as assessed by caffeine versus placebo choice, possibly because the relatively short duration of caffeine abstinence in the placebo condition was not sufficient to result in maximal withdrawal effects after termination of the relatively high caffeine dose. This study provides the clearest evidence to date of complete tolerance development to a CNS effect of caffeine in humans.

Oral self-administration of triazolam, diazepam and ethanol in the baboon: drug reinforcement and benzodiazepine physical dependence

Reinforcing and physical dependence-producing effects of oral diazepam and triazolam (0.01-1.28 mg/ml) were studied in four non-water-deprived baboons in daily 2-3-h sessions. Drinking initially was food-induced, but subsequently it was maintained for greater than year without the inducing procedures; drug intake greater than 10 mg/kg per session was attained. Triazolam and diazepam reinforcement (compared to vehicle) was concluded for only one baboon for each drug under a single-spout procedure and for two baboons for each drug under a two-spout procedure. However, all baboons showed ethanol reinforcement under a two-spout procedure. When a lever-pressing requirement was imposed for each drink (one-spout procedure), ethanol maintained requirements of 128 or 256 responses/drink, and volume of ethanol consumed was greater than vehicle. Neither benzodiazepine maintained lever pressing better than vehicle at any response requirement and drinking was suppressed by requirements of 1-32. Physical dependence to triazolam and diazepam developed after approximately 1 month of daily ingestion, evidenced by a precipitated withdrawal syndrome after injection of the benzodiazepine antagonist flumazenil. A mild spontaneous withdrawal syndrome occurred after substitution of vehicle for triazolam or diazepam. These data indicate a clear dissociation between the reinforcing and physical dependence-producing effects of triazolam and diazepam.

Physical dependence on benzodiazepines: differences within the class

Concern about physical dependence seems to be the major limiting factor in the long-term treatment with benzodiazepines (BZD). The severity of the withdrawal syndrome is determined by multiple factors, e.g. dose, duration of use, frequency of dose interval, mode of discontinuation, the pharmacologic characteristics of the BZD, personality and previous or concurrent use of cross-dependent drugs and/or alcohol. There is evidence that BZD with a short elimination half-life cause a more severe withdrawal syndrome than those with a long elimination half-life. Besides pharmacokinetic properties, pharmacodynamic factors such as potency may also covary with the liability of a BZD to induce physical dependence. There is an increasing body of literature indicating that quickly eliminated, high potency BZD such as alprazolam and lorazepam may be more likely to cause severe withdrawal reactions than slowly eliminated compounds such as diazepam or less potent derivatives such as oxazepam. Alprazolam seems to play an exceptional role, insofar as relatively soon after its introduction to the market a number of case reports of withdrawal psychoses, seizures and intense rebound anxiety were published. Data reviewed from the literature correspond well with the results of interviews conducted with 31 clinicians across the United States with experience in detoxifying patients dependent on BZD, 84% of these physicians mentioning alprazolam as especially problematic with respect to the intensity and/or duration of the withdrawal syndrome.

Abuse liability assessment of anxiolytics/hypnotics: rationale and laboratory lore

This paper describes the rationale, practical details, and laboratory lore relevant to conducting an acute dose-effect crossover evaluation for abuse liability of a novel anxiolytic/hypnotic compound relative to a standard compound. Basic principles underlying meaningful abuse liability assessments are presented and illustrated with examples from a comparative evaluation of the hypnotics zolpidem and triazolam. These principles include using a double-blind placebo-controlled design, selecting an appropriate comparison compound, evaluating a wide range of doses including supratherapeutic doses, using subjects with histories of sedative drug abuse, and using a range of dependent measures. The dose-effect crossover evaluation is proposed as a crucial first step in any rigorous drug abuse liability evaluation of a novel anxiolytic or hypnotic drug. Such a study can be expected to generate a broad data base which not only provides a prediction about the likelihood of abuse of the novel compound, but provides an assessment of some of the adverse consequences of recreational abuse. The crossover study also establishes parameters for subsequent studies such as direct assessment of reinforcing effects with drug self-administration testing.

Behavioral effects of benzodiazepine ligands in non-dependent, diazepam-dependent and diazepam-withdrawn baboons

Acute i.m. injections of benzodiazepine receptor ligands were administered to baboons before 1-h observational sessions. The agonist midazolam produced sedative effects, the antagonist flumazenil produced no behavioral effects, the inverse agonist FG7142 produced tremor and the inverse agonist 3-carboethoxy-beta-carboline hydrochloride (beta CCE) produced tremor, vomiting, jerks and seizures. Co-administration of these drugs (midazolam + beta CCE, midazolam + flumazenil or flumazenil + beta CCE) produced a mutual antagonism of these effects. Compared to the non-dependent condition, in the diazepam-dependent condition (baboons maintained on 20 mg/kg per day diazepam) and withdrawn condition (dependent baboons tested during withdrawal), midazolam produced decreased sedative effects, flumazenil produced increased effects (i.e., tremor, vomiting and jerks), and beta CCE produced increased frequency of seizures. Taken together, these data suggest that (1) benzodiazepine receptor ligands lie on a continuum of behavioral activity, and (2) chronic diazepam administration alters the behavioral effects of these benzodiazepine ligands, producing a shift in the direction of the inverse agonist.

Testing the abuse liability of anxiolytic and hypnotic drugs in humans

This paper represents a comprehensive review of laboratory studies investigating the abuse liability of anxiolytic and hypnotic drugs in humans. The subjective effects of these drugs, most of which are either barbiturates or benzodiazepines, have been measured using various self-report questionnaires and their reinforcing effects have been studied using self-administration and choice procedures. Studies using both subjective and reinforcing effects reveal orderly relations between the two main chemical classes of anxiolytic/hypnotics (e.g. barbiturates are associated with higher abuse liability than benzodiazepines), between different doses of the drugs (e.g. higher doses are usually associated with higher abuse liability) and among different compounds within a class. The subjective and reinforcing effects of barbiturates and benzodiazepines depend critically upon the subject populations that are tested and it is argued that individuals with histories of drug abuse provide a more sensitive indicator of abuse liability than healthy volunteers. Several principles of abuse liability testing are discussed, including the selection of an appropriate subject population, the use of blind drug administration procedures, the comparison of a test compound to an appropriate standard, the inclusion of a placebo and a wide range of doses of the test drug and the use of multiple measures of likelihood of abuse.

Self-injection of barbiturates, benzodiazepines and other sedative-anxiolytics in baboons

Self-injection of 12 sedative-anxiolytics was examined in baboons. Intravenous injections and initiation of a 3-h time-out were dependent upon completion of a fixed-ratio schedule requirement, permitting eight injections per day. Before testing each dose of drug, self-injection performance was established with cocaine. Subsequently, a test dose was substituted for cocaine. At some doses, all five of the benzodiazepines examined (alprazolam, bromazepam, chlordiazepoxide, lorazepam, triazolam) maintained rates (number of injections per day) of drug self-injection above vehicle control in each of the baboons tested. Maximum rates of benzodiazepine self-injection were generally submaximal. Of the benzodiazepines examined, triazolam maintained the highest rates of self-injection. Among the three barbiturates tested, methohexital generally maintained high rates of self-injection in contrast to hexobarbital and phenobarbital, which only maintained low rates. Of the four non-benzodiazepine non-barbiturate sedatives examined, both chloral hydrate and methyprylon occasionally maintained high rates of self-injection. Although there were differences within and across animals, baclofen maintained intermediate rates of self-injection. The novel anxiolytic buspirone maintained only low rates of self-injection that were not different from vehicle. This study further validates the self-injection methodology for assessing sedative-anxiolytic abuse liability and provides new information about drug elimination rate as a determinant of drug self-administration.

Comparison of the discriminative stimulus effects of midazolam after intracranial and peripheral administration in the rat

Rats were trained in a two-lever drug discrimination paradigm to discriminate midazolam (0.32 mg/kg, i.p. or 1.0 mg/kg, i.p.) from the no-drug condition. After completion of i.p. and s.c. midazolam generalization gradients (0.032-1.0 mg/kg), rats were surgically implanted with unilateral cannulae into the lateral ventricles. Intracerebroventricular (i.c.v.) doses of 1.1-44.2 micrograms midazolam were delivered to unrestrained rats. Midazolam produced dose-dependent increases in drug-appropriate responding by all three routes of administration, but was 2.4- to 4.3-fold more potent when given i.c.v. than when given s.c. or i.p. Midazolam, over the dose range tested, did not produce substantial decreases in response rate by any route of administration. The discriminative-stimulus effect of i.c.v. midazolam was blocked by peripherally administered flumazenil, and such antagonism was surmounted by a 2- to 5-fold increase in the i.c.v. midazolam dose. Taken together, these data suggest that the discriminative-stimulus effects of midazolam are mediated via central benzodiazepine (BZ) receptors.

Low-dose caffeine physical dependence in humans

This study investigated the effects of terminating low dose levels of caffeine (100 mg/day) in 7 normal humans. Substitution of placebo capsules for caffeine capsules occurred under double-blind conditions while subjects rated various dimensions of their mood and behavior. In the first phase of the study, substitution of placebo for 12 consecutive days resulted in an orderly withdrawal syndrome in 4 subjects which peaked on days 1 or 2 and progressively decreased toward prewithdrawal levels over about 1 week. Data from the remaining three subjects provided no evidence of withdrawal. In the second phase of the study, the generality of the withdrawal effect was examined by repeatedly substituting placebo for 100 mg/day of caffeine for 1-day periods separated by an average of 9 days. Despite differences within and across subjects with respect to the presence, nature and magnitude of symptoms, each of the seven subjects demonstrated a statistically significant withdrawal effect. Although the phenomenon of caffeine withdrawal has been described previously, the present report documents that the incidence of caffeine withdrawal is higher (100% of subjects), the daily dose level at which withdrawal occurs is lower (roughly equivalent to the amount of caffeine in a single cup of strong brewed coffee or 3 cans of caffeinated soft drink) and the range of symptoms experienced is broader (including headache, fatigue and other dysphoric mood changes, muscle pain/stiffness, flu-like feelings, nausea/vomiting and craving for caffeine) than heretofore recognized.

Zolpidem and triazolam in humans: behavioral and subjective effects and abuse liability

Zolpidem, which is currently marketed in Europe as a hypnotic, is a short-duration imidazopyridine whose actions are mediated at the gamma-aminobutyric acid benzodiazepine receptor complex. However, zolpidem produces a variety of biochemical differences from classic benzodiazepine agonists including showing selectivity for the central BZ1 (omega 1) receptor subtype as well as showing a different pattern of distribution of binding sites. This study compared zolpidem to the benzodiazepine hypnotic triazolam in 15 healthy male volunteers with histories of sedative drug abuse. Placebo, zolpidem (15, 30 and 45 mg) and triazolam (0.25, 0.5 and 0.75 mg) were administered p.o. in a mixed sequence in a double-blind, cross-over design. The onset time with zolpidem was faster than with triazolam, with peak effects of both drugs occurring at 1 to 2 hr after administration. Both zolpidem and triazolam produced dose-related decrements in performance on various performance tasks including circular lights, reaction time, balance, number recall and the digit symbol substitution test. Both drugs also produced similar dose-related changes on various observer ratings including overall strength of drug effect. Triazolam, but not zolpidem, increased subject- and observer-rated sleepiness and produced greater impairment on a picture memory task. Zolpidem, but not triazolam, produced increases in subject ratings of various somatic symptoms (e.g., dizzy, anxious and queasy) and there were 9 days on which subjects vomited after zolpidem, but none after triazolam. Although the highest dose of both drugs was identified by subjects as being active, the highest dose of triazolam was identified as being barbiturate, benzodiazepine or alcohol, almost twice as often as the highest dose of zolpidem. Overall, this study shows that although zolpidem produces many effects in common with triazolam, it also has a unique profile of effects distinguishable from classic benzodiazepine agonists. The mechanism(s) underlying these differences is unclear, but may be related to the atypical biochemical profile of zolpidem.

Intravenous self-administration of 4-methylaminorex in primates

The reinforcing effects of (+/-)-cis-2-Amino-4-methyl-5-phenyl-2-oxazoline (4-methylaminorex) were determined in two models of intravenous drug self-administration in primates. In baboons, lever pressing was maintained under a fixed-ratio (FR) 80- or 160-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 per day. Vehicle or 4-methylaminorex doses were substituted for cocaine for a period of 15 or more days. One of the two 4-methylaminorex doses evaluated (0.32 mg/kg per injection) maintained self-administration behavior above vehicle control levels in all four animals. This dose of 4-methylaminorex maintained cyclic patterns of self-injection behavior across days and produced signs of psychomotor stimulant toxicity. In rhesus monkeys, 4-methylaminorex (0.0003-0.1 mg/kg per injection) was made available to animals trained to self-administer cocaine (0.01 or 0.033 mg/kg per injection) under an FR 10 schedule of reinforcement during daily 1-h sessions. Each of the three monkeys self-administered at least two doses of 4-methylaminorex at rates exceeding those maintained by vehicle injections. Taken together with reports of recreational abuse of 4-methylaminorex, the present results indicate that this drug has a potential for abuse similar to that of other psychomotor stimulants.

Relative abuse liability of different benzodiazepines in drug abusers

There is a convergence of data from various sources suggesting that there are meaningful differences among the benzodiazepines with respect to their attractiveness as drugs of abuse for drug abusers. Laboratory studies of subjective and reinforcing effects in drug abusers, interviews with drug abusers, clinical judgment of medical professionals, and epidemiological studies all indicate that diazepam, in particular, has a greater abuse liability than many of the other benzodiazepines. Some of the available data also suggest that lorazepam and alprazolam are more diazepam-like in having relatively high abuse liability, while oxazepam, halazepam, and possibly chlordiazepoxide, are relatively low in this regard. These differences in abuse liability among benzodiazepines are analogous to the widely recognized differences in abuse liability within the barbiturate class that have proved to be important in helping guide clinicians' drug prescribing practices.

Dose-response evaluation of the amnestic effects of triazolam and pentobarbital in normal subjects

The effects of placebo, triazolam (0.25, 0.5, and 0.75 mg), and pentobarbital (100, 200, and 300 mg) were examined in seven normal male volunteers using a double-blind, Latin Square, balanced crossover design. Before and at hourly intervals after oral drug administration at approximately 10 a.m., subjects completed subject ratings of drug effects, psychomotor performance tasks, and two versions of a number recall task in which eight-digit number stimuli were recalled by reproduction on a numeric keypad. In one version, number stimuli were displayed on a video screen for varying lengths of time (3, 6, or 9 seconds) before immediate recall. In another version, subjects initially reproduced a continuously displayed number and then recalled the number following an immediate or a 16-second delay interval. Both triazolam and pentobarbital produced dose-related effects on all measures. Relative potency comparisons showed that triazolam was 270-384 times more potent than pentobarbital on subject ratings and psychomotor measures but was 406-647 times more potent than pentobarbital on measures of recall impairment. Recall performance deficits produced by both triazolam and pentobarbital at short (3-second) stimulus presentation times were attenuated at longer presentation times. With the variable delay task, triazolam but not pentobarbital interacted with the delay interval and produced impairments only at the 16-second delay condition. These data indicate that both triazolam and pentobarbital quantitatively impair acquisitional processes involved in short-term recall performance. Furthermore, triazolam may have a greater potential than pentobarbital to produce memory impairment, as reflected by its greater relative potency on these measures and its tendency to interfere with retention over short delay intervals.

Low-dose caffeine discrimination in humans

A novel drug discrimination procedure was used to study the discriminability and subjective effects of caffeine in seven human volunteers who abstained from dietary sources of caffeine. Daily sessions involved p.o. ingestion of two capsules sequentially, one of which contained caffeine and the other placebo, under double-blind conditions. Each day subjects attempted to identify and were later informed which capsule contained caffeine and which contained placebo. All subjects acquired rapidly the initial discrimination (100 or 178 mg vs. placebo). Examination of progressively lower caffeine doses showed that accuracy and ratings of confidence in accuracy were increasing functions of dose. There were individual differences in the lowest discriminable dose: three subjects discriminated 56 mg, three discriminated 18 mg and one discriminated 10 mg. Discrimination accuracy was usually higher after the second capsule than after the first capsule. All subjects indicated that they believed that they made the discrimination predominantly on the basis of central effects of caffeine vs. placebo, such as changes in mood and socializing. Compared to placebo, 100 mg of caffeine increased ratings of alertness, well-being, social disposition, motivation for work, concentration, energy and self-confidence and decreased ratings of headache and sleepiness. This dose of caffeine also produced a large increase in a measure of "euphoria." The present study documents biological activity of caffeine at lower doses than heretofore recognized. The general approach to investigating the effects of low drug doses may have broad application in human psychopharmacology research for characterizing other subtle psychotropic effects.

Self-administration in baboons and the discriminative stimulus effects in rats of bupropion, nomifensine, diclofensine and imipramine

The behavioral effects of the antidepressants nomifensine, diclofensine, bupropion, and imipramine were examined using a cocaine substitution drug self-administration procedure in baboons and a cocaine drug discrimination procedure in rats. Intravenous self-administration of the antidepressants 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 fixed-ratio 80-response or 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. The antidepressants or their vehicles were substituted for cocaine for a period of 15 days, followed by a return to the cocaine baseline. Nomifensine, diclofensine and bupropion all maintained self-administration behavior at levels above those maintained by their respective vehicles. Some doses of nomifensine, diclofensine, and bupropion maintained levels of behavior similar to those maintained under baseline cocaine conditions. High doses of imipramine maintained levels of behavior above those maintained by its vehicle, but the amount of behavior maintained under these conditions was extremely small. In a second experiment rats were trained to discriminate 32 mumol/kg cocaine (IP 10 min presession) from no drug in a two-lever food reinforced drug discrimination procedure in which responding on one lever was reinforced following ten consecutive responses when the session was preceded by cocaine administration, while responding on the other lever was similarly reinforced in the absence of cocaine pretreatment. Cocaine, nomifensine, diclofensine, and bupropion all dose-dependently occasioned cocaine-appropriate responding. Imipramine did not occasion cocaine-appropriate responding over a range of behaviorally active doses.

Reinforcing effects of caffeine in coffee and capsules

In a residential research ward the reinforcing and subjective effects of caffeine were studied under double-blind conditions in volunteer subjects with histories of heavy coffee drinking. In Experiment 1, 6 subjects had 13 opportunities each day to self-administer either a caffeine (100 mg) or a placebo capsule for periods of 14 to 61 days. All subjects developed a clear preference for caffeine, with intake of caffeine becoming relatively stable after preference had been attained. Preference for caffeine was demonstrated whether or not preference testing was preceded by a period of 10 to 37 days of caffeine abstinence, suggesting that a recent history of heavy caffeine intake (tolerance/dependence) was not a necessary condition for caffeine to function as a reinforcer. In Experiment 2, 6 subjects had 10 opportunities each day to self-administer a cup of coffee or (on different days) a capsule, dependent upon completing a work requirement that progressively increased and then decreased over days. Each day, one of four conditions was studied: caffeinated coffee (100 mg/cup), decaffeinated coffee, caffeine capsules (100 mg/capsule), or placebo capsules. Caffeinated coffee maintained the most self-administration, significantly higher than decaffeinated coffee and placebo capsules but not different from caffeine capsules. Both decaffeinated coffee and caffeine capsules were significantly higher than placebo capsules but not different from each other. In both experiments, subject ratings of "linking" of coffee or capsules covaried with the self-administration measures. These experiments provide the clearest demonstrations to date of the reinforcing effects of caffeine in capsules and in coffee.

Behavioral differentiation of benzodiazepine ligands after repeated administration in baboons

Baboons received repeated daily administration of saline, 5.6 mg/kg midazolam, 5.0 mg/kg flumazenil (Ro15-1788), 3.2 mg/kg 3-carboethoxy-beta-carboline hydrochloride (beta CCE) or 10 mg/kg beta CCE for 5 days. Behavioral signs of sedation and excitation were scored for 1 h after i.m. injections. Daily administration of these benzodiazepine-receptor ligands differentiated their behavioral effects; repeated midazolam resulted in tolerance to the sedative and ataxic effects; repeated beta CCE resulted in sensitization to its convulsant properties; and repeated flumazenil or saline produced no changes in behavior. In a second study, baboons received repeated injections of midazolam (5.6, 11.2 or 20 mg/kg per day) for 6 days. All three groups became tolerant to the sedative and ataxic effects of midazolam. Acute injections of flumazenil (5.0 mg/kg) on day 5 produced a dose-dependent withdrawal syndrome. This flumazenil treatment produced a slight attenuation in the degree of tolerance to midazolam on day 6, suggesting that receptor sensitivity to the benzodiazepine agonist may have partially reset.

Repeated diazepam administration: effects on the acquisition and performance of response chains in humans

The effects of repeated diazepam administration (80 mg) were assessed across a 12-hr time course with humans responding under a two-component multiple schedule of repeated acquisition and performance of response sequences. Subjects resided in an inpatient clinical research ward for the duration of the study. In each component of the multiple schedule, subjects completed sequences of 10 responses in a predetermined order using three keys of a numeric keypad. In the acquisition component, a new response sequence was to be acquired each session. In the performance component, the response sequence always remained the same. After stable responding was obtained and the effects of the placebo assessed, diazepam was administered for 3 consecutive days. The effects of repeated diazepam administration on overall percentage of errors across the two components of the multiple schedule were selective. In the acquisition component, the first dose of diazepam increased percentage errors with the magnitude of effects decreasing across the second and third days of diazepam administration. In the performance component, the percentage of errors was either minimally affected across all 3 days of diazepam administration or substantively increased on Day 1 with subsequent diazepam administrations having minimal effects. Effects on response rate were not selective. Diazepam decreased rates of responding in both schedule components, with the magnitude of effects decreasing across successive administrations. These results replicate previous findings in humans and nonhumans on the selective effects of diazepam on acquisition versus performance baselines. Also, the results suggest that the selective effects do not result from differences in reinforcement rate. Finally, the present results demonstrate that the selective recovery from repeated drug administration previously demonstrated in nonhumans using a repeated acquisition arrangement has generality to human behavior.