Oral administration of D-alanine in monkeys robustly increases plasma and cerebrospinal fluid levels but experimental D-amino acid oxidase inhibitors had minimal effect.

Hypofunction of the N-methyl-d-aspartate (NMDA) receptor is thought to exacerbate psychosis in patients diagnosed with schizophrenia. Consistent with this hypothesis, D-alanine, a co-agonist at the glycine site of the NMDA receptor, was shown to improve positive and cognitive symptoms when used as add-on therapy for schizophrenia treatment. However, D-alanine had to be administered at high doses (~7 g) to observe clinical effects. One possible reason for the high dose is that D-alanine could be undergoing oxidation by D-amino acid oxidase (DAAO) before it reaches the brain. If this is the case, the dose could be reduced by co-administration of D-alanine with a DAAO inhibitor (DAAOi). Early studies with rodents showed that co-administration of D-alanine with 5-chloro-benzo[d]isoxazol-3-ol (CBIO), a prototype DAAOi, significantly enhanced the levels of extracellular D-alanine in the frontal cortex compared with D-alanine alone. Further, the use of CBIO reduced the dose of D-alanine needed to attenuate prepulse inhibition deficits induced by dizocilpine. The objective of the work reported herein was to confirm the hypothesis that DAAO inhibition can enhance D-alanine exposure in a species closer to humans: non-human primates. We report that while oral D-alanine administration to baboons (10 mg/kg) enhanced D-alanine plasma and CSF levels over 20-fold versus endogenous levels, addition of experimental DAAOi to the regimen exhibited a 2.2-fold enhancement in plasma and no measurable effect on CSF levels. The results provide caution regarding the utility of DAAO inhibition to increase D-amino acid levels as treatment for patients with schizophrenia.

D-Amino-Acid Oxidase Inhibition Increases D-Serine Plasma Levels in Mouse But not in Monkey or Dog.

D-serine has been shown to improve positive, negative, and cognitive symptoms when used as add-on therapy for the treatment of schizophrenia. However, D-serine has to be administered at high doses to observe clinical effects. This is thought to be due to D-serine undergoing oxidation by D-amino-acid oxidase (DAAO) before it reaches the brain. Consequently, co-administration of D-serine with a DAAO inhibitor could be a way to lower the D-serine dose required to treat schizophrenia. Early studies in rodents to evaluate this hypothesis showed that concomitant administration of structurally distinct DAAO inhibitors with D-serine enhanced plasma and brain D-serine levels in rodents compared with administration of D-serine alone. In the present work we used three potent DAAO inhibitors and confirmed previous results in mice. In a follow-up effort, we evaluated plasma D-serine levels in monkeys after oral administration of D-serine in the presence or absence of these DAAO inhibitors. Even though the compounds reached steady state plasma concentrations exceeding their Ki values by >60-fold, plasma D-serine levels remained the same as those in the absence of DAAO inhibitors. Similar results were obtained with dogs. In summary, in contrast to rodents, DAAO inhibition in monkeys and dogs did not influence the exposure to exogenously administered D-serine. Results could be due to differences in D-serine metabolism and/or clearance mechanisms and suggest that the role of DAAO in the metabolism of D-serine is different across species. These data provide caution regarding the utility of DAAO inhibition for patients with schizophrenia.

Allosteric modulation of GABA(A) receptor subtypes:effects on visual recognition and visuospatial working memory in rhesus monkeys [corrected].

Non-selective positive allosteric modulators (PAMs) of GABAA receptors (GABAARs) are known to impair anterograde memory. The role of the various GABAAR subtypes in the memory-impairing effects of non-selective GABAAR PAMs has not been fully elucidated. The current study assessed, in rhesus monkeys, effects of modulation of α1, α2/3, and α5GABAARs on visual recognition and spatial working memory using delayed matching-to-sample (DMTS) and self-ordered spatial search (SOSS) procedures, respectively. The DMTS procedure (n=8) involved selecting a previously presented 'sample' image from a set of multiple images presented after a delay. The SOSS procedure (n=6) involved touching a number of boxes without repeats. The non-selective GABAAR PAM triazolam and the α1GABAA preferential PAMS zolpidem and zaleplon reduced accuracy in both procedures, whereas the α5GABAA preferential PAMs SH-053-2'F-R-CH3 and SH-053-2'F-S-CH3, and the α2/3GABAA preferential PAM TPA023B were without effects on accuracy or trial completion. The low-efficacy α5GABAAR negative allosteric modulator (NAM) PWZ-029 slightly increased only DMTS accuracy, whereas the high-efficacy α5GABAAR NAMs RY-23 and RY-24 did not affect accuracy under either procedure. Finally, the slopes of the accuracy dose-effect curves for triazolam, zolpidem, and zaleplon increased with box number in the SOSS procedure, but were equivalent across DMTS delays. The present results suggest that (1) α1GABAARs, compared with α2/3 and α5GABAARs, are primarily involved in the impairment, by non-selective GABAAR PAMs, of visual recognition and visuospatial working memory in nonhuman primates; and (2) relative cognitive impairment produced by positive modulation of GABAARs increases with number of locations to be remembered, but not with the delay for remembering.

Behavioral effects and pharmacokinetics of (±)-3,4-methylenedioxymethamphetamine (MDMA, Ecstasy) after intragastric administration to baboons.

(±)-3,4-Methylenedioxymethamphetamine (MDMA, "Ecstasy") is a popular drug of abuse. We aimed to characterize the behavioral effects of intragastric MDMA in a species closely related to humans and to relate behavioral effects to plasma MDMA and metabolite concentrations. Single doses of MDMA (0.32-7.8 mg/kg) were administered via an intragastric catheter to adult male baboons (N = 4). Effects of MDMA on food-maintained responding were assessed over a 20-hour period, whereas untrained behaviors and fine-motor coordination were characterized every 30 minutes until 3 hours postadministration. Levels of MDMA and metabolites in plasma were measured in the same animals (n = 3) after dosing on a separate occasion. MDMA decreased food-maintained responding over the 20-hour period, and systematic behavioral observations revealed increased frequency of bruxism as the dose of MDMA was increased. Drug blood level determinations showed no MDMA after the lower doses of MDMA tested (0.32-1.0 mg/kg) and modest levels after higher MDMA doses (3.2-7.8 mg/kg). High levels of 3,4-dihydroxymethamphetamine (HHMA) were detected after all doses of MDMA, suggesting extensive first-pass metabolism of MDMA in the baboon. The present results demonstrate that MDMA administered via an intragastric catheter produced behavioral effects that have also been reported in humans. Similar to humans, blood levels of MDMA after oral administration may not be predictive of the behavioral effects of MDMA. Metabolites, particularly HHMA, may play a significant role in the behavioral effects of MDMA.

A critical examination of best dose analysis for determining cognitive-enhancing potential of drugs: studies with rhesus monkeys and computer simulations.

RATIONALE: Best dose analysis involves identifying the dose associated with the greatest improvement in performance for each subject and comparing performances associated with these individually determined best doses to control performances. OBJECTIVES: The current experiments were conducted to examine whether significant best dose effects might result from the selective analysis of data rather than an actual drug effect. METHODS: Experiment 1 examined the effects of nicotine and methylphenidate on delayed matching-to-sample (DMTS) and self-ordered spatial search (SOSS) performances in rhesus monkeys (DMTS: n = 7; SOSS: n = 6) to determine the validity and reliability of best dose effects. Experiment 2 used Monte Carlo computer simulations to estimate the likelihood of obtaining a significant outcome when the best dose method was applied to randomly generated data sets for which no difference existed. RESULTS: Significant effects were obtained when the best dose analysis was applied to performances from nondrug sessions, and best dose performances were not significantly different from the best nondrug performances. The doses identified as best doses from two nicotine dose-response curve determinations were unrelated, and the improvement associated with the best dose observed during the first dose-response curve determination was not reliable when the dose was administered repeatedly. Finally, there was a high likelihood of obtaining a statistically significant difference when no real difference existed. CONCLUSIONS: Best dose analysis for the identification of potential therapeutic agents should be replaced by single-subject designs.

Long-term exposure to oral methylphenidate or dl-amphetamine mixture in peri-adolescent rhesus monkeys: effects on physiology, behavior, and dopamine system development.

The stimulants methylphenidate and amphetamine are used to treat children with attention deficit/hyperactivity disorder over important developmental periods, prompting concerns regarding possible long-term health impact. This study assessed the effects of such a regimen in male, peri-adolescent rhesus monkeys on a variety of cognitive/behavioral, physiological, and in vivo neurochemical imaging parameters. Twice daily (0900 and 1200 hours), for a total of 18 months, juvenile male monkeys (8 per group) consumed either an unadulterated orange-flavored solution, a methylphenidate solution, or a dl-amphetamine mixture. Doses were titrated to reach blood/plasma levels comparable to therapeutic levels in children. [¹¹C]MPH and [¹¹C]raclopride dynamic PET scans were performed to image dopamine transporter and D2-like receptors, respectively. Binding potential (BP(ND)), an index of tracer-specific binding, and amphetamine-induced changes in BP(ND) of [¹¹C]raclopride were estimated by kinetic modeling. There were no consistent differences among groups on the vast majority of measures, including cognitive (psychomotor speed, timing, inhibitory control, cognitive flexibility), general activity, physiological (body weight, head circumference, crown-to-rump length), and neurochemical (ie, developmental changes in dopamine transporter, dopamine D2 receptor density, and amphetamine-stimulated dopamine release were as expected). Cytogenetic studies indicated that neither drug was a clastogen in rhesus monkeys. Thus, methylphenidate and amphetamine at therapeutic blood/plasma levels during peri-adolescence in non-human primates have little effect on physiological or behavioral/cognitive development.

GABA site agonist gaboxadol induces addiction-predicting persistent changes in ventral tegmental area dopamine neurons but is not rewarding in mice or baboons.

Dopamine neurons of the ventral tegmental area (VTA) are involved at early phases of drug addiction. Even the first in vivo dose of various abused drugs induces glutamate receptor plasticity at the excitatory synapses of these neurons. Benzodiazepines that suppress the inhibitory GABAergic interneurons in the VTA via facilitation of synaptic GABA(A) receptors have induced neuroplasticity in dopamine neurons due to this disinhibitory mechanism. Here, we have tested a non-benzodiazepine direct GABA site agonist 4,5,6,7-tetrahydroisoxazolol[4,5-c]pyridine-3-ol (THIP) (also known as gaboxadol) that acts preferentially via high-affinity extrasynaptic GABA(A) receptors. A single sedative dose of THIP (6 mg/kg) to mice induced glutamate receptor plasticity for at least 6 d after administration. Increased AMPA/NMDA receptor current ratio and increased frequency, amplitude, and rectification of AMPA receptor responses suggested persistent targeting of GluA2-lacking AMPA receptors in excitatory synapses of VTA dopamine neurons ex vivo after THIP administration. This effect was abolished in GABA(A) receptor δ(-/-) mice, which have a loss of extrasynaptic GABA(A) receptors. In behavioral experiments, we found neither acute reinforcement in intravenous self-administration sessions with THIP at relevant doses using a yoked control paradigm in mice nor in baboons using a standard paradigm for assessing drug abuse liability; nor was any place preference found after conditioning sessions with various doses of THIP but rather a persistent aversion in 6 mg/kg THIP-conditioned mice. In summary, we found that activation of extrasynaptic δ-subunit-containing GABA(A) receptors leads to glutamate receptor plasticity of VTA dopamine neurons, but is not rewarding, and, instead, induces aversion.

Joseph Vincent Brady (1922-2011).

Joseph Vincent Brady was born in New York City on March 28, 1922. Joe died on July 29, 2011. Joe was a pioneer in bringing the methods and philosophy of behavior analysis to the emerging field of behavioral pharmacology. In 1960, with David Rioch, Joe founded the nonprofit Institute for Behavioral Research (later the Institutes for Behavioral Resources, or IBR), which continues to the present day. True to his belief in the importance of the environmental determinants of behavior, when interviewed about his accomplishments in so many varied fields, Joe replied that in each area he had been "the beneficiary of a fortuitous environment."

Metabolism and disposition of 3,4-methylenedioxymethamphetamine ("ecstasy") in baboons after oral administration: comparison with humans reveals marked differences.

The baboon is potentially an attractive animal for modeling 3,4-methylenedioxymethamphetamine (MDMA) effects in humans. Baboons self-administer MDMA, are susceptible to MDMA neurotoxicity, and are suitable for positron emission tomography, the method most often used to probe for MDMA neurotoxicity in humans. Because pharmacokinetic equivalence is a key feature of a good predictive animal model, we compared the pharmacokinetics of MDMA in baboons and humans. Baboons were trained to orally consume MDMA. Then, pharmacokinetic profiles of MDMA and its major metabolites were determined after various oral MDMA doses using the same analytical method recently used to perform similar studies in humans. Results indicate that MDMA pharmacokinetics after oral ingestion differ markedly between baboons and humans. Baboons had little or no MDMA in their plasma but had high plasma concentrations of 3,4-dihydroxymethamphetamine (HHMA), pointing to much more extensive first-pass metabolism of MDMA in baboons than in humans. Other less prominent differences included less O-methylation of HHMA to 4-hydroxy-3-methoxymethamphetamine, greater N-demethylation of MDMA to 3,4-methylenedioxyamphetamine, and a shorter half-life of HHMA in the baboon. To our knowledge, this is the first study to characterize MDMA metabolism and disposition in the baboon. Differences in MDMA pharmacokinetics between baboons and humans suggest that the baboon may not be ideal for modeling human MDMA exposure. However, the unusually rapid conversion of MDMA to HHMA in the baboon may render this animal uniquely useful for clarifying the relative role of the parent compound (MDMA) versus metabolites (particularly HHMA) in the biological actions of MDMA.

Intravenous self-administration of γ-hydroxybutyrate (GHB) in baboons.

BACKGROUND: Abuse of gamma-hydroxybutyrate (GHB) poses a public health concern. In previous studies, intravenous (IV) self-administration of GHB doses up to 10 mg/kg was not maintained in non-human primates under limited-access conditions, which was inconsistent with the usual good correspondence between drugs abused by humans and those self-injected by laboratory animals. METHODS: Self-administration of GHB was studied in 10 baboons using procedures standard for our laboratory to assess drug abuse liability. Each self-injection depended on completion of 120 or 160 lever responses. Sessions ran continuously; a 3-h timeout limited the number of injections per 24h to 8. Self-injection was established at 6-8 injections/day with cocaine (0.32 mg/kg/injection) prior to substitution of each GHB dose (3.2-178 mg/kg/injection) or vehicle for 15 days. Food pellets were available 24h/day. RESULTS: GHB maintained significantly greater numbers of injections when compared to vehicle in 6 of the 9 baboons that completed GHB evaluations that included 32 mg/kg/injection or higher. The baboons that self-administered GHB at high rates were ones for which GHB was the first drug each had tested under the 24-h/day cocaine baseline procedure. Self-injection of the highest doses of GHB decreased food-maintained responding. CONCLUSIONS: High-dose GHB can function as a reinforcer in non-human primates under 24-h access, but self-administration history may be important. The findings are consistent with the demonstrated abuse liability of GHB in humans, and remove GHB as an exception to the typical good correspondence between those drugs abused by humans and those self-administered by nonhuman primates.

Reducing abuse liability of GABAA/benzodiazepine ligands via selective partial agonist efficacy at alpha1 and alpha2/3 subtypes.

Abuse-liability-related effects of subtype-selective GABA(A) modulators were explored relative to the prototypic benzodiazepine lorazepam. 7-Cyclobutyl-6-(2-methyl-2H-1,2,4-triazol-3-ylmethoxy)-3-phenyl-1,2,4-triazolo[4,3-b]pyridazine (TPA123) has weak partial agonist efficacy at alpha(1)-, alpha(2)-, alpha(3)-, and alpha(5)-containing GABA(A) receptors, whereas 7-(1,1-dimethylethyl)-6-(2-ethyl-2H-1,2,4-triazol-3-ylmethoxy)-3-(2-fluorophenyl)-1,2,4-triazolo[4,3-b]pyridazine (TPA023) has weaker partial agonist efficacy at alpha(2) and alpha(3) and none at alpha(1) and alpha(5) subtypes. For both compounds, preclinical data suggested efficacy as nonsedating anxiolytics. Self-injection of TPA123 (0.0032-0.1 mg/kg) and TPA023 (0.0032-0.32 mg/kg) was compared with lorazepam (0.01-0.32 mg/kg) in baboons. TPA123 and lorazepam maintained self-injection higher than vehicle at two or more doses in each baboon; peak rate of self-injection of lorazepam was higher than TPA123. Self-injected lorazepam and TPA123 also increased rates of concurrently occurring food-maintained behavior. After the availability of self-administered TPA123 doses ended, an effect consistent with a mild benzodiazepine-like withdrawal syndrome occurred. In contrast with lorazepam and TPA123, TPA023 did not maintain self-administration. Positron emission tomography studies showed that TPA023 produced a dose-dependent inhibition in the binding of [(11)C]flumazenil to the benzodiazepine binding site in the baboon, which was essentially complete (i.e., 100% occupancy) at the highest TPA023 dose (0.32 mg/kg). In a physical dependence study, TPA023 (32 mg/kg/24 h) was delivered as a continuous intragastric drip. Neither flumazenil at 14 days nor stopping TPA023 after 30 to 31 days resulted in the marked withdrawal syndrome characteristic of benzodiazepines in baboons. In the context of other data, elimination of efficacy at the alpha(1) subtype of the GABA/benzodiazepine receptor is not sufficient to eliminate abuse liability but may do so when coupled with reduced alpha(2/3) subtype efficacy.

Consistent, high-level ethanol consumption in pig-tailed macaques via a multiple-session, limited-intake, oral self-dosing procedure.

BACKGROUND: Alcohol abuse is a major public health burden that can lead to many adverse health effects such as impaired hepatic, gastrointestinal, central nervous system and immune system function. Preclinical animal models of alcohol abuse allow for experimental control over variables often difficult to control in human clinical studies (e.g., ethanol exposure before or during the study, history of other drug use, access to medical care, nutritional status, etc). Nonhuman primate models in particular provide increased genetic, anatomic and physiologic similarity to humans, relative to rodent models. A small percentage of macaques will spontaneously consume large quantities of ethanol; however, most nonhuman primate models of "voluntary" ethanol intake produce relatively low daily ethanol intake in the majority of monkeys. METHODS: To facilitate study of chronic exposure to high levels of ethanol intake, a macaque model has been developed that induces consistent, daily high-level ethanol consumption. This multiple-session procedure employed 4 drinking sessions per day, with sessions occurring once every 6 hours. RESULTS: The group average alcohol consumption was 4.6 g/kg/d (SEM 0.4), roughly twice the group average consumption of previous reports. Ethanol drinking sessions produced group mean blood ethanol levels of 95 mg/dl after 60 minutes, and fine motor control was impaired up to 90 minutes after a drinking session. CONCLUSION: This model of multiple-session, limited access, oral ethanol self-dosing produced consistent, high-level ethanol consumption with each session qualifying as a "binge" drinking session using the definition of "binge" provided by the NIAAA (>80 mg/dl/session). This model of ethanol drinking in macaques will be of great utility in the study of immunological, physiological and behavioral effects of ethanol in nonhuman primates.

Novel discriminative stimulus effects of TPA023B, subtype-selective gamma-aminobutyric-acid(A)/benzodiazepine modulator: comparisons with zolpidem, lorazepam, and TPA023.

Anxiolytics with fewer unwanted effects may be created by varying GABAergic efficacy at the BZ binding site across GABA(A) receptor subtypes. TPA023 and TPA023B have in vitro antagonist efficacy at alpha(1) subtypes and partial-agonist efficacy at alpha(2/3) subtypes. TPA023B has partial-agonist efficacy at alpha(5); TPA023 has none. Drug discrimination procedures were used to determine whether the novel GABA(A) receptor efficacy profiles would be reflected in a model of subjective effects of BZ-site ligands. Rats were trained to discriminate TPA023, TPA023B, the nonselective BZ anxiolytic lorazepam, or the alpha(1)-selective hypnotic zolpidem. The lorazepam, zolpidem, and TPA023 discriminations were learned in < 50 sessions. The TPA023B training group showed no evidence of acquiring the TPA023B discrimination after 160 sessions despite various procedural manipulations. Neither zolpidem- nor lorazepam-trained rats generalized to TPA023B. Within the same dose range, however, TPA023-trained rats generalized fully and dose-dependently to TPA023B. Number of training sessions to regain criterion discrimination performance following TPA023B tests in the lorazepam, zolpidem, and TPA023 groups increased as a function of dose, likely due to effects of residual TPA023B. Together with previous data, the present results suggest that elimination of alpha(1) efficacy plus reductions in alpha(2/3) efficacy permits anxiolysis but decreases BZ-like interoceptive stimulus effects.

Morphine withdrawal dramatically reduces lymphocytes in morphine-dependent macaques.

The immune effects of chronic opiate exposure and/or opiate withdrawal are not well understood. The results of human studies with opiate abusers are variable and may not be able to control for important factors such as subjects' drug histories, health and nutritional status. Nonhuman primate models are necessary to control these important factors. A model of opiate dependence in macaques was developed to study the effects of opiate dependence and withdrawal on measures of immune function. Four pigtailed macaques drank a mixture of morphine (20 mg/kg/session) and orange-flavored drink every 6 h for several months. During stable morphine dependence, absolute numbers of neutrophils, monocytes and lymphocytes did not change relative to pre-morphine levels. However, there was a significant decrease in the absolute number and percentage of natural killer (NK) cells in morphine dependence. Either precipitated withdrawal or abstinence for 24 h resulted in behavioral withdrawal signs in all animals. Absolute lymphocyte counts decreased and absolute netrophil counts increased significantly in withdrawal, relative to levels during morphine dependence. Lymphocyte subset (CD4+, CD8+, CD20+) cells were also decreased in absolute numbers with little change in their percentage distributions. There was, however, a significant increase in the percentage of NK cells in withdrawal relative to levels during morphine dependence. This study demonstrates the usefulness of voluntary oral self-dosing procedures for maintaining morphine dependence in nonhuman primates and demonstrates that the morphine withdrawal syndrome includes large alterations in blood parameters of immune system function, including nearly 50% reduction in numbers of CD4+, CD8+ and CD20+ cells.

Comparison of the behavioral effects of bretazenil and flumazenil in triazolam-dependent and non-dependent baboons.

Behavioral effects of the benzodiazepine receptor partial agonist bretazenil were compared with those of the benzodiazepine receptor antagonist flumazenil under conditions in which three baboons received continuous intragastric (i.g.) infusion of vehicle and then continuous i.g. infusion of triazolam (1.0 mg/kg/day). In each condition, acute doses of flumazenil (0.01-3.2 mg/kg) and bretazenil (0.01-10.0 mg/kg) were administered every 2 weeks (beginning after 30 days of treatment in the triazolam-dependent condition). Food pellets were available during daily 20-h sessions. Following test injections, 60-min behavioral observations were conducted followed by a fine motor assessment. During chronic vehicle administration, neither drug produced changes in observed behaviors. Bretazenil increased pellets earned and time to complete the fine-motor task (10.0 mg/kg dose). During chronic triazolam dosing, both bretazenil and flumazenil precipitated benzodiazepine withdrawal syndromes, characterized by vomiting, tremors/jerks, and a decrease in pellets earned. Thus, bretazenil can function as an antagonist under conditions of benzodiazepine physical dependence.

Self-injection of flunitrazepam alone and in the context of methadone maintenance in baboons.

Patients in methadone maintenance programs use benzodiazepines to "boost" methadone's subjective effects, and flunitrazepam has been prominent in this context. Self-administration of flunitrazepam (0.001-0.32 mg/kg i.v.) alone and during daily oral methadone administration was evaluated in three baboons. Flunitrazepam maintained self-injection as an inverted U-shaped function of dose at rates higher than those maintained by most other benzodiazepines under the same procedure. In the context of demonstrated physical dependence on 3.2 mg/kg/day p.o. methadone, flunitrazepam doses on the ascending limb of the dose-effect curve maintained greater rates of self-injection than before methadone in two baboons. When the methadone dose decreased to 1.8 mg/kg/day, self-injection remained higher for those baboons and became higher than before methadone for the third baboon. Self-injection remained higher when methadone decreased to 1.0 mg/kg/day, except self-injection of the lowest flunitrazepam dose returned to or below the pre-methadone rate for two baboons. After methadone was discontinued, the dose-effect curve shifted to the right in one baboon but remained to the left in two. Flunitrazepam thus served as a reinforcer alone and in the context of methadone maintenance. Lower doses maintained higher self-injection during and shortly after methadone maintenance. Further research should explore the duration of higher self-injection rates following methadone maintenance.

Contributions of GABAA receptor subtype selectivity to abuse liability and dependence potential of pharmacological treatments for anxiety and sleep disorders.

When benzodiazepines (BZs) supplanted barbiturates as a favored, safer treatment for anxiety and sleep disorders in the 1960s, the abuse liability and dependence potential of these drugs were little understood. Widespread recognition of the difficulty of stopping use of chronically taken BZs emerged through the popular press in the late 1970s, which resulted in reluctance to prescribe these otherwise clinically useful compounds. Evolution of the understanding of the biochemical basis for BZ effects in the 1980s and 1990s, coupled with regulatory emphasis on collection of data used in legal scheduling decisions, made possible a targeted search for drugs that would provide effective treatment for anxiety disorders in the absence of abuse liability or dependence potential. Compounds that have selective efficacy at subtypes of the gamma-aminobutyric acid type A receptor, are active in preclinical anxiolytic screens, but negative in preclinical studies of behavior relevant to evaluation of abuse liability appear to be one promising means for achieving this end.

Selectivity in generalization to GABAergic drugs in midazolam-trained baboons.

When barbiturates have been tested in animals trained to discriminate the intravenous benzodiazepine (Bz) anesthetic midazolam, squirrel monkeys and pigeons did not reliably generalize to barbiturates but rats did. To explore this unexpected phenomenon in another species and to extend the midazolam generalization profile to GABAergic compounds not previously tested, five baboons were trained to discriminate midazolam maleate (0.32 mg/kg i.v.) from saline under a two-lever procedure. In tests 10 min after dose delivery, the partial agonist imidazenil, the full agonist chlordiazepoxide, and the receptor-subtype-selective hypnotic zolpidem fully shared discriminative effects with midazolam. The barbiturate pentobarbital did so in only one of five baboons, and the intravenous anesthetic propofol failed to do so in the three baboons tested. Testing 1 min after dose delivery shifted midazolam and zolpidem curves to the left and increased generalization to propofol but not pentobarbital. Taken together with previous published data, partial or full agonism at the Bz binding site appears sufficient for midazolam-like discriminative effects in nonhuman primates, pigeons, and rodents, and modulation through the anesthetic site is sufficient in baboons. However, to date, positive modulation of GABA through the barbiturate site is not generally sufficient for this effect in nonhuman primates and pigeons although it is in rodents.

Principles of initial experimental drug abuse liability assessment in humans.

This paper describes the rationale and procedures for conducting what is considered by many to be the current "gold standard" for initial abuse liability testing of a novel compound: the classic acute dose-effect comparison study in volunteers with histories of drug abuse. Such a trial is most appropriate for predicting the likelihood of abuse by drug abusers and, in turn, the extent of drug diversion and illicit street sales if the novel compound became available in the community. The dose-effect abuse liability trial typically involves a double-blind complete crossover design in 10-14 subjects with histories of polydrug abuse in a controlled clinical pharmacology laboratory setting. Drug conditions usually involve placebo, three doses of the novel compound and three doses of an appropriate reference compound of known abuse liability. In each session, the time-course of effects of a single drug dose are evaluated. Intervals between experimental sessions are typically 1 to several days. The importance of testing high supra-therapeutic doses of the novel drug for the validity of the trial is emphasized, and the use of a dose run-up pilot study for selecting maximal doses and matching doses between the novel and comparison compound is explained. The rationale and description of outcome measures is discussed, including measures that reflect likelihood of abuse (e.g. drug vs. money choice and subject ratings of liking, good effects, estimated monetary street value), secondary measures that should be considered in interpreting likelihood of abuse (e.g. drug identification, subject-rated side effects and mood changes), and additional concurrent measures to establish equivalence of the novel and comparison compound (e.g. behavioral performance, observer-rated assessments, physiological measures).