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

Inhibition of the endocannabinoid-regulating enzyme monoacylglycerol lipase elicits a CB1 receptor-mediated discriminative stimulus in mice

Substantial challenges exist for investigating the cannabinoid receptor type 1 (CB₁)-mediated discriminative stimulus effects of the endocannabinoids, 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (anandamide; AEA), compared with exogenous CB₁ receptor agonists, such as Δ⁹-tetrahydrocannabinol (THC) and the synthetic cannabinoid CP55,940. Specifically, each endocannabinoid is rapidly degraded by the respective hydrolytic enzymes, monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH). Whereas MAGL inhibitors partially substitute for THC and fully substitute for CP55,940, FAAH inhibitors do not substitute for either drug. Interestingly, combined FAAH-MAGL inhibition results in full THC substitution, and the dual FAAH-MAGL inhibitor SA-57 serves as its own discriminative training stimulus. Because MAGL inhibitors fully substitute for SA-57, we tested whether the selective MAGL inhibitor MJN110 would serve as a training stimulus. Twelve of 13 C57BL/6J mice learned to discriminate MJN110 from vehicle, and the CB₁ receptor antagonist rimonabant dose-dependently blocked its discriminative stimulus. CP55,940, SA-57, and another MAGL inhibitor JZL184, fully substituted for MJN110. In contrast, the FAAH inhibitor PF-3845 failed to substitute for the MJN110 discriminative stimulus, but produced a 1.6 (1.1-2.2; 95% confidence interval) leftward shift in the MJN110 dose-response curve. Inhibitors of other relevant enzymes (i.e., ABHD6, COX-2) and nicotine did not engender substitution. Diazepam partially substituted for MJN110, but rimonabant failed to block this partial effect. These findings suggest that MAGL normally throttles 2-AG stimulation of CB₁ receptors to a magnitude insufficient to produce cannabimimetic subjective effects. Accordingly, inhibitors of this enzyme may release this endogenous brake producing effects akin to those produced by exogenously administered cannabinoids.

Current FDA regulatory guidance on the conduct of drug discrimination studies for NDA review: Does the scientific literature support recent recommendations?

BACKGROUND

The Controlled Substances Staff of the Center for Drug Evaluation and Research at the US Food and Drug Administration and the Pharmaceutical Research Manufacturers Association (PhRMA) conducted a series of open forum dialog sessions between 2006 and 2016. A Cross Company Abuse Liability Council (CCALC) was formed during the process of this unique collaborative effort between Industry and Federal Regulators whose goals were to establish the development of standards for the preclinical screening of new molecular entities for schedule control actions required as part of every New Drug Application process. The draft guidance document was published and disseminated in 2010, which allowed for alternative approaches to each study protocol requirement needed for NDA review, if the approach satisfied the requirements of the applicable statutes and regulations (i.e., the controlled substance act). In a series of recent pre-study protocol reviews requested by confidential Pharmaceutical Sponsors of MPI Research, the CSS staff appeared to change its policy and set forth to require all drug discrimination study data to be generated under "extinction" test sessions. MPI Research is a Contract Research Organization acting on behalf of pharmaceutical companies and bound under separate confidentiality agreements.

PURPOSE

The purpose of this review is to highlight the data appearing in peer-reviewed scientific journals that do not support the regulatory administrative constraints on one specific testing methodology (extinction) to the exclusion of another (reinforced test sessions).

CONCLUSION

This mind shift represents a restrictive administrative policy by the FDA that is not supported by the published data.

A translational pharmacology approach to understanding the predictive value of abuse potential assessments

Within the drug development industry the assessment of abuse potential for novel molecules involves the generation and review of data from multiple sources, ranging from in-vitro binding and functional assays through to in-vivo nonclinical models in mammals, as well as collection of information from studies in humans. This breadth of data aligns with current expectations from regulatory agencies in both the USA and Europe. To date, there have been a limited number of reviews on the predictive value of individual models within this sequence, but there has been no systematic review on how each of these models contributes to our overall understanding of abuse potential risk. To address this, we analyzed data from 100 small molecules to compare the predictive validity for drug scheduling status of a number of models that typically contribute to the abuse potential assessment package. These models range from the assessment of in-vitro binding and functional profiles at receptors or transporters typically associated with abuse through in-vivo models including locomotor activity, drug discrimination, and self-administration in rodents. Data from subjective report assessments in humans following acute dosing of compounds were also included. The predictive value of each model was then evaluated relative to the scheduling status of each drug in the USA. In recognition of the fact that drug scheduling can be influenced by factors other than the pharmacology of the drug, we also evaluated the predictive value of each assay for the outcome of the human subjective effects assessment. This approach provides an objective and statistical assessment of the predictive value of many of the models typically applied within the pharmaceutical industry to evaluate abuse potential risk. In addition, the impact of combining information from multiple models was examined. This analysis adds to our understanding of the predictive value of each model, allows us to critically evaluate the benefits and limitations of each model, and provides a method for identifying opportunities for improving our assessment and prediction of abuse liability risk in the future.

Role of training dose in drug discrimination: a review

Drug discrimination has been an important technique in behavioural pharmacology for at least 40 years. The characteristics of drug-produced discriminative stimuli are influenced by behavioural and pharmacological variables, including the doses used to establish discriminations. This review covers studies on the effects of varying the training dose of a drug in a search for general principles that are applicable across different drug classes and methodological approaches. With respect to quantitative changes, relationships between training dose and the rate of acquisition or magnitude of stimulus control were found for most drug classes. Acquisition accelerated with dose up to a point beyond which drug-induced impairments of performance had a deleterious impact. Sensitivity to the training drug as measured by ED(50) values typically increased when the training dose was reduced. Qualitative changes were more complex and appeared to fall into three categories: (a) changes in profiles of generalization between partial and full agonists; (b) reduced specificity of some discriminations at small training doses; and (c) changes in the relative salience of actions mediated through different neurotransmitter systems or from central and peripheral sites. Three-lever discrimination procedures incorporating 'drug versus drug' or 'dose versus dose' contingencies enabled detection of more subtle differences than the simple 'drug versus no drug' approach when applied to the opioid, hallucinogen and barbiturate classes of drugs. These conclusions have implications for the interpretation of data from studies that use either within-subject or between-subject designs for studying the discriminative stimulus effects of drugs.

Carisoprodol-mediated modulation of GABAA receptors: in vitro and in vivo studies

Carisoprodol is a frequently prescribed muscle relaxant. In recent years, this drug has been increasingly abused. The effects of carisoprodol have been attributed to its metabolite, meprobamate, a controlled substance that produces sedation via GABA(A) receptors (GABA(A)Rs). Given the structural similarities between carisoprodol and meprobamate, we used electrophysiological and behavioral approaches to investigate whether carisoprodol directly affects GABA(A)R function. In whole-cell patch-clamp studies, carisoprodol allosterically modulated and directly activated human alpha1beta2gamma2 GABA(A)R function in a barbiturate-like manner. At millimolar concentrations, inhibitory effects were apparent. Similar allosteric effects were not observed for homomeric rho1 GABA or glycine alpha1 receptors. In the absence of GABA, carisoprodol produced picrotoxin-sensitive, inward currents that were significantly larger than those produced by meprobamate, suggesting carisoprodol may directly produce GABAergic effects in vivo. When administered to mice via intraperitoneal or oral routes, carisoprodol elicited locomotor depression within 8 to 12 min after injection. Intraperitoneal administration of meprobamate depressed locomotor activity in the same time frame. In drug discrimination studies with carisoprodol-trained rats, the GABAergic ligands pentobarbital, chlordiazepoxide, and meprobamate each substituted for carisoprodol in a dose-dependent manner. In accordance with findings in vitro, the discriminative stimulus effects of carisoprodol were antagonized by a barbiturate antagonist, bemegride, but not by the benzodiazepine site antagonist, flumazenil. The results of our studies in vivo and in vitro collectively suggest the barbiturate-like effects of carisoprodol may not be due solely to its metabolite, meprobamate. Furthermore, the functional traits we have identified probably contribute to the abuse potential of carisoprodol.

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.

Discriminative stimulus and self-reported effects of methylphenidate, d-amphetamine, and triazolam in methylphenidate-trained humans

Asymmetrical generalization between drugs on drug-discrimination procedures has been demonstrated for sedative and stimulant drugs in animals and to some extent with sedative drugs in humans. The aim of this experiment was to examine the discriminative-stimulus effects of d-amphetamine in methylphenidate-trained humans. A previous study demonstrated that methylphenidate substitutes for d-amphetamine in d-amphetamine-trained humans. Six healthy human participants first learned to discriminate 30 mg oral methylphenidate. Doses of oral methylphenidate, d-amphetamine, triazolam, and placebo were then tested to determine whether they share discriminative-stimulus and self-reported effects with 30 mg methylphenidate. Methylphenidate and d-amphetamine dose-dependently increased methylphenidate-appropriate responding and produced prototypical stimulant-like effects. Triazolam produced low levels of methylphenidate-appropriate responding and prototypical sedative-like effects. The results of this experiment are concordant with previous studies and suggest that the behavioral effects of oral methylphenidate and d-amphetamine overlap extensively and that the discriminative-stimulus effects of methylphenidate and d-amphetamine are symmetrical.

Effects of modulators of N-methyl-D-aspartate receptor-mediated neurotransmission on diazepam discrimination in rats

N-methyl-D-aspartate (NMDA) antagonists share a number of pharmacological effects with GABA(A) agonists, including anxiolytic and anticonvulsant effects. This study evaluated the effects of site-selective NMDA antagonists in rats trained to discriminate the benzodiazepine diazepam from vehicle. As expected, diazepam produced robust discriminative stimulus effects and dose-dependently substituted for the training dose. Mixed results were obtained with competitive NMDA antagonists: whereas NPC 17742 partially substituted for diazepam, SDZ EAA 494 did not elicit responding on the diazepam-associated lever. Other site-selective NMDA antagonists, including the open channel blocker phencyclidine, the glycine-site antagonists ACEA 1021 and MDL 102,288, the polyamine-site antagonist arcaine, and the glutamate release inhibitor riluzole, failed to substitute for diazepam. Agonists at nonbenzodiazepine sites of the GABA(A) receptor complex were also tested for comparison purposes. The barbiturate pentobarbital and the neurosteroid Co 2-1068 partially substituted for diazepam. In contrast, the anticonvulsant carbamazepine failed to substitute even at a dose that substantially reduced response rates. These results suggest that substitution of NMDA antagonists for GABA(A) agonists is dependent upon the site at which the NMDA antagonist binds. Further, they suggest that similarities between the stimulus properties of GABA(A) agonists and NMDA antagonists are at least as strong as similarities among agonists acting at different sites on GABA(A) receptors.

The Discriminative Stimulus Effects of γ-Hydroxybutyrate and Related Compounds in Rats Discriminating Baclofen or Diazepam: The Role of GABAB and GABAA Receptors

The discriminative stimulus effects of gamma-hydroxybutyrate (GHB) can be mimicked by GABA(A) receptor-positive modulators (e.g., diazepam) and GABA(B) receptor agonists (e.g., baclofen). The purposes of this study were to see whether stimulus control could be established with baclofen and to further characterize the role of GABAergic mechanisms in the behavioral actions of GHB by evaluating GHB and related compounds in rats discriminating either diazepam or baclofen. Training criteria were satisfied with baclofen and diazepam after 69 and 44 sessions, respectively. GHB and its precursors gamma-butyrolactone and 1,4-butanediol occasioned >80% responding on the drug-associated lever in rats discriminating baclofen and <11% in rats discriminating diazepam. Diazepam and other GABA(A) receptor-positive modulators occasioned intermediate levels of responding on the baclofen lever, whereas baclofen occasioned less than 4% responding on the diazepam lever. The GABA(B) receptor antagonist CGP 35348 [(3-aminopropyl)(diethoxymethyl) phosphinic acid] partially antagonized the effects of baclofen as well as the baclofen-like effects of GHB, and flumazenil partially antagonized the effects of diazepam. This study established stimulus control with baclofen, and substitution data provided direct evidence for a role of GABAergic, especially GABA(B), mechanisms in the discriminative stimulus effects of GHB. The lack of substitution by GHB or its metabolic precursors for diazepam indicates a comparatively smaller role of GABA(A) mechanisms in these effects of GHB. The inability of CGP 35348 to completely attenuate the effects of baclofen and GHB suggests that multiple receptors could be involved in the discriminative stimulus effects of GHB.

Principles of drug abuse liability assessment in laboratory animals

This paper describes the rationale for use of preclinical assessments of abuse liability in laboratory animals, and then discusses "cross-cutting" methodological issues that apply to behavioral evaluations intended to contribute to an abuse liability evaluation package. Issues include use of: (1) positive and negative control conditions; (2) full dose-effect evaluations, (3) multiple dependent measures, (4) pharmacokinetic evaluations to guide choice of dose ranges, (5) a species for which good methodological and comparative data are available to aid interpretation of results, and (6) appropriate methods for the group or single-subject experimental design selected. The remainder of the paper describes basic methodology by which three core pieces of behavioral data required by the Food and Drug Administration for its use in the overall abuse liability analysis can be obtained preclinically. Reinforcing effects are assessed in study of drug self-administration; drug discrimination assesses degree of overlap of interoceptive stimulus effects with relevant comparison drugs; physical dependence potential is determined by assessing whether a withdrawal syndrome occurs after chronic drug administration. Background and methodological issues specific to each procedure are discussed. A key consideration for cross-cutting and specific methodological issues is that choices made enable confident interpretation of both positive and negative results.

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.

Prenatal protein malnutrition enhances stimulus control by CDP, but not a CDP/THIP combination in rats

In the present study, the effects of prenatal protein malnutrition on stimulus control exerted by the benzodiazepine (BZ), chlordiazepoxide (CDP) and the GABA-A receptor agonist 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) were characterized. The adult, male offspring of female Sprague-Dawley rats fed either low (6% casein) or adequate (25% casein) protein diets 5 weeks prior to mating and throughout pregnancy served as subjects. Subjects were first trained to discriminate CDP (8.0 mg/kg ip) from saline using drug discrimination procedures. Once a criterion level of performance was achieved, generalization tests were performed to lower doses of CDP (4.0, 2.0, 1.0, 0.5 and 0.25 mg/kg) and then to several doses of THIP (10.0, 7.5, 5.6 and 3.2 mg/kg). Lastly, the ability of a single dose of THIP (3.0 mg/kg) to enhance discriminative control by several low doses of CDP (4.0, 2.0, 1.0 and 0.5 mg/kg) was assessed. Although both diet groups acquired the original CDP/saline discrimination at the same rate, malnourished rats exhibited significantly more generalization to low doses of CDP than their well-nourished counterparts. Neither diet group exhibited significant generalization to THIP nor a difference in THIP's ability to enhance the CDP cue. These results suggest that a subject's sensitivity to the stimulus properties of drugs can be selectively modified by prenatal malnutrition.

Effects of alprazolam, caffeine, and zolpidem in humans trained to discriminate triazolam from placebo

This study compared the discriminative stimulus effects of zolpidem, a nonbenzodiazepine hypnotic, to benzodiazepines. Eight participants learned to discriminate triazolam (0.35 mg/70 kg) from placebo. The discriminative stimulus effects, self-reported subjective effects, and performance effects of triazolam (0.05-0.35 mg/70 kg), alprazolam (0.25-1.75 mg/70 kg), zolpidem (2.5-35 mg/70 kg) and caffeine (75-525 mg/70 kg) were assessed under two-response and novel-response drug discrimination procedures. Under the two-response procedure, triazolam, alprazolam and zolpidem fully substituted for triazolam and caffeine did not. Under the novel-response procedure, triazolam and alprazolam substituted for triazolam and zolpidem partially substituted for triazolam. Zolpidem, but not triazolam or alprazolam, also produced some novel responding. Caffeine produced both placebo-appropriate and novel responding. The self-reported effects of triazolam, alprazolam and zolpidem were similar. Overall, zolpidem produced similar, but not identical, effects as the benzodiazepines.

Zaleplon and triazolam: drug discrimination, plasma levels, and self-administration in baboons

Zaleplon is a chemically novel hypnotic that preferentially binds alpha(1)-subunit containing subtypes of the alphabetagamma configuration of the gamma-aminobutyric acid (GABA)(A) receptor. Zaleplon and the non-subtype-selective hypnotic triazolam occasioned 100% drug-appropriate responding in baboons trained to discriminate lorazepam or pentobarbital from vehicle. Flumazenil shifted the zaleplon generalization gradient at least five-fold to the right. A plasma elimination half-life of 6-8 h for oral 10 mg/kg zaleplon and 0.32 mg/kg triazolam was paralleled by discriminative control for 7 h. Zaleplon maintained self-injection greater than vehicle, as did comparison doses of the similarly selective hypnotic zolpidem and triazolam. Concurrent food-maintained responding increased during self-injection of all three drugs. Preferential binding at this alpha(1)-containing GABA(A) subtype did not diminish the benzodiazepine (Bzs)-like behavioral effects of zaleplon.

Effects of benzodiazepine receptor ligands and ethanol in rats trained to discriminate pregnanolone

Although GABA(A) receptor positive modulators share many behavioral effects, subtle differences have been detected among their discriminative stimulus effects. The purpose of the present study was to determine the extent of shared discriminative stimulus effects of pregnanolone with various benzodiazepine receptor ligands and with ethanol. Naive male Sprague-Dawley rats were trained to discriminate the endogenous neuroactive steroid pregnanolone (5.6 or 8.0 mg/kg) from vehicle. The benzodiazepine receptor agonists, triazolam and lorazepam, the benzodiazepine receptor partial agonist, bretazenil, the benzodiazepine1 (BZ1) receptor subtype selective agonists, zolpidem and zaleplon and ethanol were tested. Triazolam, lorazepam and bretazenil substituted for pregnanolone. Lorazepam, but not triazolam or bretazenil, decreased response rates at the highest dose tested. Zaleplon completely substituted for pregnanolone with no effect on response rates. Zolpidem substituted for pregnanolone only at a dose that severely disrupted response rates. Ethanol partially substituted for pregnanolone and decreased response rates. The results are consistent with GABA(A) receptor mediation of the discriminative stimulus effects of pregnanolone. The effects on response rates suggest subtle differentiation among the GABA(A) receptor-mediated cues.

Drug discrimination: stimulus control during repeated testing in extinction

Rats were trained, under a two-lever drug-discrimination procedure, to respond differentially depending upon whether lorazepam (1.0 mg/kg) or no injection had been administered before the session. Responses on the appropriate lever produced a food pellet under a modified fixed-ratio (FR) 10 schedule, in which the 10 responses had to be emitted consecutively. In reinforcement tests, completing an FR 10 on either lever produced a pellet. In extinction tests, stimulus changes paired with reinforcement occurred but no pellet was delivered. Training sessions were conducted between test sessions. Each of four extinction phases consisted of six tests preceded by one stimulus (e.g., lorazepam). Repeated exposures to extinction reduced response rates for all rats, but stimulus control, as inferred from either percentage of total responses or percentage of total FR 10s on the drug-appropriate lever, remained high. The percentage of total FR 10s measure was less subject to skewing under low-rate conditions than was the percentage of total responses measure and provided an evaluation of stimulus control in terms of meeting the consecutive response contingency. These results demonstrate a level of independence between response rate and stimulus control in drug discrimination, which has positive implications for the validity of interpreting discriminative effects of novel test conditions in well-trained animals, even when overall response rates are low.

Differentiating benzodiazepine- and barbiturate-like discriminative stimulus effects of lorazepam, diazepam, pentobarbital, imidazenil and zaleplon in two- versus three-lever procedures

Previous studies found that animals trained to discriminate pentobarbital show a relatively inclusive generalization profile. They generalize to sedative-hypnotics and anxiolytics, regardless of differences among such drugs in molecular mechanism of action. In contrast, animals trained to discriminate lorazepam have shown a generalization profile that appears selective for compounds with in-vitro profiles as full agonists at the benzodiazepine modulatory site. The present study investigated whether benzodiazepine receptor ligands, to which pentobarbital-trained rats had generalized under a two-lever procedure, would occasion pentobarbital- or lorazepam-appropriate responding when the rats were retrained to discriminate among pentobarbital, lorazepam and the no-drug condition under a three-lever procedure. A second group of rats was trained first to discriminate lorazepam and then retrained under the same three-lever procedure. Under the two-lever procedure, all pentobarbital-trained rats showed dose-dependent generalization to lorazepam, but not all lorazepam-trained rats showed full generalization to pentobarbital. Both groups showed full generalization to diazepam and zaleplon, a novel hypnotic that is selective for alpha, 1-subunit-containing subtypes of the gamma-aminobutyric acid (GABA)A receptor. Pentobarbital-trained rats, but not all lorazepam-trained rats, generalized to imidazenil. Under the three-lever procedure, dose-dependent generalization to lorazepam and pentobarbital was demonstrated on the appropriate levers. Diazepam shared discriminative effects with pentobarbital, zaleplon shared discriminative effects with lorazepam, and imidazenil shared discriminative effects with lorazepam and pentobarbital. These results show that when the opportunity for finer differentiation of discriminative effects of GABAergic drugs is provided, a generalization profile more in line with differential in-vitro profiles can be revealed.

High-dose discrimination training with midazolam: context determines generalization profile

In previous work, greater differentiation among ligands for the benzodiazepine site was found in rats trained to discriminate among vehicle, 0.32, and 3.2 mg/kg midazolam than in animals trained to discriminate a single midazolam dose from vehicle (i.e., virtually all test drugs occasioned low-dose midazolam-appropriate responding, but most did not occasion high-dose midazolam-appropriate responding even at high test doses). A possibility was that merely training with 3.2 mg/kg-midazolam (not previously studied) would result in greater selectivity than training with lower midazolam doses. In the present study, rats were trained to discriminate 3.2 mg/kg i.p. midazolam from no drug under a two-lever, food-maintained, procedure; and drugs from the previous three-lever studies were tested. Triazolam, bretazenil, clonazepam, lorazepam, midazolam, zolpidem, chlordiazepoxide, pentobarbital, and flurazepam all dose-dependently occasioned >80% responding on the midazolam-appropriate lever in roughly that order of potency. Only triazolam had occasioned midazolam 3.2 mg/kg-appropriate responding in the previous work. The greater differentiation among these drugs in the dose-vs.-dose procedure likely was due to a training dose context rather than to the high training dose per se.

The novel-response procedure in humans

The novel-response drug discrimination procedure is one of several three-choice procedures developed to address interpretational difficulties that can occur under standard two-response procedures. The novel-response procedure is unique among three-choice discrimination procedures by using instructions, rather than explicit training procedures. With the novel-response procedure, participants are trained under a standard two-response (drug vs. placebo) discrimination, and then instructed that in the presence of a drug stimulus unlike either of the training drugs, responses should be made on the novel-response alternative. Several studies have assessed the utility of the novel-response procedure by comparing effects under a standard two-response and the novel-response procedure in participants trained to discriminate triazolam from placebo. Results indicate that the novel-response procedure can increase the selectivity of both placebo- and drug-appropriate responding, and in this way, allows for finer distinctions to be made among sedatives than a standard two-response procedure.

Defeat engenders pentylenetetrazole-appropriate responding in rats: antagonism by midazolam

Defeat and the threat of defeat by an aggressive conspecific is stressful and may engender an anxiety- or fear-like state in animals; the present experiment investigated whether defeat generalized to the discriminative stimulus properties of PTZ and how benzodiazepine receptors were involved in this generalization. Separate groups of male Long-Evans rats (Rattus norvegicus) were trained to discriminate 20 mg/kg pentylenetetrazole (PTZ) or 0.4 mg/kg midazolam (MDZ) from saline in a two-choice drug-discrimination task. After establishing stimulus control, PTZ- and MDZ-trained rats were exposed to an aggressive conspecific which resulted in defeat, as defined by the display of defensive and submissive postures as well as audible and ultrasonic vocalizations. Administration of saline after defeat resulted in greater than 80% PTZ lever selection in 15 out of 25 PTZ-trained rats; this effect was attenuated through pretreatment with MDZ (1 mg/kg). Furthermore, short-term defeat substitution for the PTZ discriminative stimulus was not accompanied by long-term changes in the post-defeat generalization curves for PTZ and MDZ when compared to pre-defeat generalization curves. Nor did defeat alter the antagonism of PTZ by diazepam (2.5 mg/kg) or MDZ by flumazenil (10 mg/kg). In order further to characterize the necessary features for defeat substitution for the PTZ discriminative stimulus, exposure to a threatening conspecific was also attempted by PTZ-trained rats protected from physical contact with a wire mesh cage. In these tests, saline continued to engender greater than 50% PTZ lever responding in 15 of 25 rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Recent developments in the behavioral pharmacology of benzodiazepine (omega) receptors: evidence for the functional significance of receptor subtypes

Recent research in molecular biology has demonstrated the complexity of GABAA receptors and shown that benzodiazepine (BZ-omega) receptor subtypes have a structural reality. It is therefore appropriate to ask whether the different pharmacological effects produced by benzodiazepines (anticonvulsant activity, anxiety reduction, motor incoordination, learning deficits, characteristic discriminative stimulus effects, tolerance and dependence) are associated with activity at different receptor subtypes. The present paper reviews the literature dealing with the behavioral effects of novel BZ (omega) receptor ligands relevant to the question of the functional significance of the BZ1 (omega 1) and BZ2 (omega 2) receptor subtypes. The only drugs currently available with a considerable degree of selectivity are alpidem and zolpidem. These compounds have relatively high affinity for GABAA receptors containing the alpha 1 subunit (corresponding to the BZ1 (omega 1) subtype) and very low affinity for receptors with the alpha 5 subunit (corresponding to one type of BZ2 (omega 2) receptor). Pharmacological effects observed with these, and other, less selective compounds allow several tentative conclusions to be drawn: (a) Little is known of the role of subtype selectivity in anxiolytic or amnestic effects but compounds with low intrinsic activity may reduce anxiety without giving rise to sedation or motor incoordination and BZ1 (omega 1) selective drugs appear to disrupt memory only at sedative doses; (b) Selectivity for BZ1 (omega 1) receptors may be associated with sleep-inducing activity but not with motor incoordination, suggesting that BZ2 (omega 2) receptors may be of particular importance in mechanisms of muscle relaxation; (c) The discriminative stimulus effects of different BZ (omega) receptor ligands are not identical and differences may be related to receptor selectivity; (d) Compounds with BZ1 (omega 1) selectivity and compounds with low intrinsic activity produce little or no tolerance and dependence. A wider range of selective compounds will be necessary to investigate these factors in detail and many different pharmacological profiles can be expected from drugs with selectivity and different levels of intrinsic activity.

Evaluation of the discriminative stimulus effects of the novel sedative-hypnotic CL 284,846

CL 284,846, N-[3-(3-cyanopyrazolo[1, 5-a]pyrimidin-7-yl)phenyl)]-N- ethylacetamide, is a novel non-benzodiazepine sedative-hypnotic with benzodiazepine-like sedative effects, but with less apparent liability for accompanying undesired side effects. In an effort to further characterize its pharmacological activity, CL 284,846 (3.0 mg/kg, IP, 30 min pretreatment) was established as a discriminative stimulus (DS) in rats (n = 7). CL 284,846 (0.3-10.0 mg/kg) showed a dose-related increase in drug-appropriate responding up to the training dose and a dose-related decrease in response rate. The benzodiazepine agonist triazolam (0.1-1.0 mg/kg), the benzodiazepine partial agonist Ro 17-1812 (0.3-3.0 mg/kg) and the triazolopyridazine CL 218,872 (1.0-3.0 mg/kg) substituted for CL 284,846 in all rats, whereas the imidazopyridines zolpidem (3.0-10.0 mg/kg) and alpidem (10.0-30.0 mg/kg), the benzodiazepine partial agonist bretazenil (0.03-10.0 mg/kg) and the novel putative anxiolytic CL 273,547 (10.0-56.0 mg/kg) substituted in most, but not all, rats. Ro 17-1812, bretazenil, and CL 218,872 had no effect on response rate while the other drugs showed a concomitant decrease in rate. The 5-HT1A agonist buspirone (1.0-10.0 mg/kg) and the barbiturate pentobarbital (3.0-17.0 mg/kg) failed to substitute for CL 284,846 up to rate-decreasing doses. The benzodiazepine antagonist flumazenil (3.0-10.0 mg/kg) blocked the DS effects of CL 284,846 in most rats with no effect on response rate. Taken together, these results suggest that the DS effects of CL 284,846 are mediated via benzodiazepine receptors; however, the DS profile of CL 284,846 remains distinct from both benzodiazepine and non-benzodiazepine sedative-hypnotic drugs.

Effects of diazepam and hydromorphone in triazolam-trained humans under a novel-response drug discrimination procedure

Seven healthy normal male and female volunteers (21-31 years) were trained to discriminate between the benzodiazepine triazolam (0.32 mg/70 kg, PO; e.g., drug A) and placebo (e.g., drug B) under a three-choice, instructed novel response drug discrimination procedure. Once the criterion for discrimination was met (i.e., > 85% correct responding on four consecutive sessions), dose-effect curves were determined for triazolam (0.1-0.56 mg/70 kg), the benzodiazepine diazepam (10-32 mg/70 kg) and the opioid agonist hydromorphone (1-6 mg/70 kg). Subjects met the criterion for discrimination within four to six sessions. Triazolam and diazepam produced dose-related increases in triazolam-appropriate responding and no novel-appropriate responding at any dose tested. In contrast, hydromorphone generally increased novel-appropriate responding in a dose-related manner with placebo-appropriate responding and some triazolam-appropriate responding at intermediate doses occurring also. Triazolam and diazepam produced qualitatively similar increases on several measures of sedative drug effects; hydromorphone increased ratings of "like novel" and sedative-like effects in subjects who discriminated hydromorphone as novel relative to those who did not. These results indicate that the novel response drug discrimination procedure enhances the specificity of the triazolam-placebo discrimination.

Discriminative stimulus effects of alpidem, a new imidazopyridine anxiolytic

Alpidem in an imidazopyridine derivative which binds selectively to the omega 1 (BZ1) receptor subtype. It is active in some, but not all, behavioural tests sensitive to benzodiazepine anxiolytics and has clinical anti-anxiety effects. However, in a previous study, it was shown that alpidem did not substitute for chlordiazepoxide in rats trained to discriminate this benzodiazepine. The present experiments were carried out to investigate the discriminative stimulus properties of alpidem in greater detail. In the first experiment rats learned to discriminate a dose of 10 mg/kg alpidem from saline. Acquisition of the discrimination was long and performance unstable. Chlordiazepoxide, clorazepate and zolpidem substituted only partially for alpidem but the effects of the training dose of alpidem were blocked by 10 mg/kg flumazenil. The second experiment established stimulus control more rapidly to a dose of 30 mg/kg alpidem. Alpidem induced dose-related stimulus control, and dose-related and complete substitution for alpidem was produced by zolpidem, abecarnil, CL 218,872, triazolam and suriclone. Partial substitution occurred with chlordiazepoxide, clorazepate and pentobarbital. In most cases, high levels of substitution were produced only by doses which greatly reduced response rates even though the training dose of alpidem produced only modest decreases in rates. Ethanol, buspirone and bretazenil produced very little substitution for alpidem and both flumazenil and bretazenil antagonised the effects of alpidem. In two further experiments alpidem was found to substitute for the stimulus produced by zolpidem (2 mg/kg) but not for that produced by ethanol (1.5 g/kg).(ABSTRACT TRUNCATED AT 250 WORDS)

Discriminative stimulus effects of presynaptic GABA agonists in pentobarbital-trained rats

The discriminative stimulus effects of indirect-acting GABAergic drugs were compared to those of pentobarbital (PB) and midazolam in rats trained to discriminate 5 mg/kg PB from saline under a two-lever fixed-ratio 32 schedule of food reinforcement. PB and midazolam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Valproic acid, an antiepileptic drug and GABA transaminase inhibitor, substituted for PB but only at a dose that produced response rate suppression. Vigabatrin, an irreversible GABA transaminase inhibitor, failed to substitute for PB, but did produce a dose-dependent decrease in response rates. The GABA uptake inhibitors, 1-[2-[bis[4-(trifluoromethyl)phenyl]-methoxy]ethyl]-1,2,5,6- tetrahydro-3-pyridinecarboxylic acid (CI-966) and (R(-)-N-[4,4-bis(3-methylthien-2-yl)but-3-enyl] nipecotic acid HCl (tiagabine), produced no greater than 40% PB-lever responding. Aminooxyacetic acid (AOAA), which is described as a nonselective presynaptic GABA agonist, yielded a maximum of 43% PB-lever responding. These results indicate that the discriminative stimulus effects of the indirect GABAA agonists, PB and midazolam, although similar to one another, differ from those of presynaptic GABAergic drugs. Differences in the discriminative stimulus properties of GABA transaminase inhibitors and uptake inhibitors also exist, indicating that not all presynaptic GABA agonists have similar behavioral profiles. These results contribute to a further understanding of the similarities and differences in the behavioral effects of drugs that enhance GABAergic neurotransmission.

Drug discrimination analysis of endogenous neuroactive steroids in rats

Rats were trained in a two-lever procedure to discriminate either pentobarbital (10 mg/kg), ethanol (1.5 g/kg), diazepam (1 mg/kg), or lorazepam (1 mg/kg) from the no-drug condition. Consistent with previous reports, rats in the pentobarbital, ethanol, and diazepam training conditions all showed complete dose-dependent generalization to pentobarbital under test conditions, but rats trained to discriminate lorazepam did not. Administration of the neuroactive steroids 3 alpha,21-dihydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-THDOC) and 3 alpha-hydroxy-5 alpha-pregnan-20-one (3 alpha,5 alpha-P) also produced complete generalization in rats trained to discriminate pentobarbital, ethanol, and diazepam, but not in rats trained to discriminate lorazepam. These results further indicate the specificity of the lorazepam training condition and are consistent with neurochemical data indicating that these neuroactive steroids are similar to barbiturates in modulating gamma-aminobutyric acid (GABA)A receptors. In the context of previous data, the results from the four training groups suggest that the discriminative-stimulus effects of the neuroactive steroids are sedative/anxiolytic in nature and probably mediated through a non-benzodiazepine GABAA site.

Tolerance/cross-tolerance to the discriminative stimulus effects of chlordiazepoxide and bretazenil

Rats were trained to discriminate 10 mg/kg chlordiazepoxide (CDAP) from saline in a two-lever drug discrimination procedure. Both CDAP and the nonsedative benzodiazepine partial agonist, bretazenil, dose-dependently substituted for the training dose of CDAP. Training was then suspended and half of the rats were placed on chronic CDAP, while the other half received water. Tolerance to the discriminative stimulus effects of CDAP developed after 1 mo and a final dose of approximately 110 mg/kg/d, as evidenced by the fact that the training dose of CDAP no longer produced drug-appropriate responding. An insurmountable tolerance also developed to bretazenil, as no rat responded on the drug-appropriate lever at doses as high as 56 mg/kg, whereas in the prechronic dose-effect curve, 1 mg/kg of bretazenil produced 100% drug-appropriate responding. One week after chronic CDAP was discontinued, the dose-effect curve for CDAP in the chronic CDAP group was comparable to that obtained in the prechronic phase, indicating that the rats were no longer tolerant to CDAP. In contrast to CDAP, the dose-effect curve for bretazenil did not return to its prechronic level, with higher doses being required for substitution. In the chronic water group, the dose-effect curves for CDAP and bretazenil were essentially the same before, during, and after the chronic regimen. Thus, suspension of training for 6 wk does not result in loss of the discriminative stimulus control. Chronic exposure to CDAP, however, does result in tolerance to the discriminative stimulus effects of CDAP, with an accompanying insurmountable cross-tolerance to the partial benzodiazepine agonist, bretazenil. These results support findings by other investigators who find tolerance to the discriminative stimulus properties of a wide range of psychoactive drugs (Young and Sannerud, 1989).

Drug discrimination by humans compared to nonhumans: current status and future directions

In drug discrimination (DD) procedures, behavior is differentially reinforced depending on the presence or absence of specific drug stimuli. The DD paradigm has been widely adopted by behavioral pharmacologists because of its specificity of stimulus control, concordance with drug action at cellular levels and its use as a preclinical model of subject-rated effects in humans. With the successful extension of DD to humans, a comparison of human and nonhuman DD will help place each in the context of the other. Twenty-eight studies of DD in humans are reviewed, including studies of amphetamine, opioid, benzodiazepine, caffeine, nicotine, marijuana and ethanol discriminative stimuli. Comparison of procedures between studies in humans and nonhumans reveals a common tradition, except the use of instructions appears to facilitate greatly DD acquisition in humans. Findings were qualitatively similar between humans and nonhumans. Potency relationships were quantitatively similar between humans and most, but not all, other species. Areas of human DD needing additional empirical evaluation include the influence of instructions, the effects of training dose and the effects of antagonists. Additionally, antihistamines, barbiturates, nicotine and marijuana are under-represented in human DD.

Comparative amnesic and sedative effects of lorazepam and oxazepam in healthy volunteers

Oxazepam and its chlorinated derivative, lorazepam, have similar half-lives but differing potencies. This study compared the effects of these two benzodiazepines with a placebo on memory, mood and psychomotor function. Thirty six volunteers took part in a double-blind, independent groups design. Subjects completed a battery of tests before and 2.5 h after drug administration. Lorazepam 2 mg produced more profound subjective and motor sedation than oxazepam 30 mg, and this in turn produced a similar, global pattern of impairments across a wide range of tasks. However, lorazepam produced greater decrements than oxazepam on a task involving episodic memory even when sedative effects were partialled out. We suggest that this finding may reflect either differential task sensitivities or a contribution of priming to performance on the explicit memory task.

Discriminative stimulus effects of omega (BZ) receptor ligands: correlation with in vivo inhibition of [3H]-flumazenil binding in different regions of the rat central nervous system

Rats can be trained to discriminate benzodiazepines (BZ) from vehicle and there is considerable evidence that the stimulus effects of these drugs are mediated by activity at omega (BZ) modulatory sites of the GABAA receptor complex. A number of recent studies, however, have indicated that differences may exist between the discriminative stimulus effects of benzodiazepines and those of certain non-benzodiazepine ligands for the omega (BZ) receptors (e.g. zolpidem, abecarnil). As it is known that several subtypes of omega (BZ) sites are found in the central nervous system, and that drugs such as zolpidem have selectivity for certain subtypes, it is possible that differential stimulus effects may be associated with receptor selectivity. In the present study, correlations were calculated between the potencies of nine compounds with affinity for omega receptors (diazepam, lorazepam, triazolam, clonazepam, alprazolam, zopiclone, suriclone, CL 218, 872 and zolpidem) to substitute for chlordiazepoxide in rats trained to discriminate a dose (5 mg/kg) of this benzodiazepine and the ability of the same compounds to inhibit the binding of [3H]-flumazenil from different structures in the rat central nervous system in vivo. The correlations obtained were: cerebellum 0.46, cortex 0.39, striatum 0.78 (P < 0.05), hippocampus 0.79 (P < 0.05) and spinal cord 0.95 (P < 0.001). These different structures are known to contain different relative concentrations of omega 1 (BZ1) and omega 2 (BZ2) sites with the spinal cord containing the greatest (80%) and cerebellum the lowest (5%) concentration of omega 2 (BZ2) sites.(ABSTRACT TRUNCATED AT 250 WORDS)

Pentobarbital-like discriminative stimulus effects of direct GABA agonists in rats

The discriminative stimulus effects of direct and indirect-acting GABAergic drugs were investigated in rats trained to discriminate 5 mg/kg pentobarbital (PB) from saline under a two-lever fixed ratio (FR) 32 schedule of food reinforcement. PB and diazepam produced dose-dependent substitution for the training dose of PB with response rate reduction only at doses above those producing full substitution. Muscimol, thiomuscimol and 4,5,6,7-tetrahydroisoxazolo [5,4-c]-pyridin-3-ol (THIP) produced intermediate levels of pentobarbital-lever responding (40-60%), accompanied by dose-dependent decreases in rates of responding following THIP and muscimol administration. The GABAA agonist progabide and its metabolite 4-([(4-chlorophenyl) (5-fluoro-2-hydroxyphenyl)methylene]amino)] butyric acid (SL 75102) also partially substituted for PB, producing means of 39-73% PB-lever responding. The GABAB agonist, baclofen, completely failed to substitute for PB even at doses that decreased rates of responding. These results show that the discriminative stimulus effects of indirect GABAA agonists, PB and diazepam, although similar to one another, differ from those of direct GABAA receptor agonists, which produced only partial substitution for PB. The GABAB agonist, baclofen, can be distinguished by lacking any ability to substitute for PB. These results contribute to a further understanding of the similarities and differences in the behavioral effects of different types of GABA agonists.

Drug discrimination using a conditioned taste-aversion paradigm in rhesus monkeys

The development of drug discrimination was assessed in rhesus monkeys using the conditioned taste-aversion paradigm. Monkeys were initially trained to respond under a fixed-ratio 30-response schedule of food-pellet delivery to assess the rate-decreasing effects of alprazolam (0.03 to 3 mg/kg, i.m., 60 min presession). Alprazolam decreased responding at doses greater than 0.1 mg/kg. Discriminative stimulus effects of alprazolam were then assessed by giving 0.03 mg/kg before sessions in which 1.8 mEq/kg lithium chloride was given immediately after the session (alprazolam/lithium session). On intervening days, saline was given before and after the session (saline/saline session). Rates of responding decreased over successive alprazolam/lithium sessions and also during the saline/saline session that immediately followed an alprazolam/lithium session. During subsequent saline/saline sessions, rates of responding returned to levels near baseline rates within two to four sessions. The discriminative stimulus effects of alprazolam were then assessed by giving 0.1 mg/kg before sessions in which 1 mg/kg d-amphetamine was given immediately after the session (alprazolam/d-amphetamine session). Rates of responding decreased during subsequent alprazolam/d-amphetamine sessions in drug-experienced monkeys, but did not decrease during intervening saline/saline sessions. These findings demonstrate that drug stimuli associated with postsession drug injections can rapidly develop control over behavior and suggest that similar methods be explored in the assessment of drug discrimination.

Animal models for assessing drugs of abuse

A major toxic effect that has limited the clinical usefulness of medicinal drugs has been their susceptibility to nonmedical use and abuse by significant segments of the population. Major research efforts have been directed toward the development of safer and more effective therapeutic agents that would not be subject to such misuse, and laboratory animal assessment models have contributed importantly to the evaluation of such compounds. There are now several converging lines of evidence that testify to the reliability and broad generality of observations concerning drug abuse liability in humans based upon such animal laboratory models. The most important point of contact that characterizes the interaction between such animal assessment models and the human drug abuse arena is the demonstrated relationship between the biochemical/pharmacological/toxic properties of drugs on the one hand, and their environmental/behavioral stimulus functions on the other. As a result of these developments in animal model research technology and the consequent advances in knowledge of drug action, an operational basis has been provided for redefining the bewildering range of phenomena and experiential pseudo-phenomena loosely identify with such terms as "addition," "dependence" and "abuse."

Drug discrimination models in anxiety and depression

Drug discrimination is a technique for investigating the stimulus properties of centrally active drugs. Although many studies have employed animals to investigate the stimulus properties of substances used clinically for the treatment of anxiety and depression, it would be a mistake to consider the internal discriminative stimuli as being related specifically to the anxiolytic or antidepressant properties of these drugs. Rather drug cues are better considered as relating to the pharmacological action of classes of compounds. Thus, benzodiazepine cues generalize to other compounds acting at benzodiazepine receptors, but not to substances (anxiolytic or otherwise) acting at 5-HT1A receptors. Similarly, antidepressants with different pharmacological properties, for example the tricyclic imipramine, or the phenylaminoketone buproprion produce distinct, unrelated discriminative stimuli. For this reason, the limits of drug discrimination techniques for investigating novel anxiolytic or antidepressant drugs should be clearly recognized. Attempts to identify an anxiogenic discriminative stimulus using pentylenetetrazole have also been misguided. In this technique it has proven difficult to separate unequivocally the pharmacological proconvulsant effects of the drug from the psychological construct anxiety. Nevertheless, drug discrimination remains a valuable technique for investigating pharmacological interactions in animals and man.