Pioneer in behavioral pharmacology: a tribute to Joseph V. Brady

The contributions of Joseph V. Brady to behavioral pharmacology span more than 50 years and range from early studies using the Estes-Skinner (conditioned emotional response) procedure to examine drug effects and various physiological processes in experimental animals to the implementation of mobile methadone treatment services and to small group behavioral analyses in simulated space environments. This expansive range of activities is based on Brady's insight and innovative use of behavioral procedures, his spirited and unabashed enthusiasm for the discipline and its philosophical underpinnings, together with a collegiality and commitment to the experimental analysis of behavior that is both legendary and inspirational. These contributions are summarized and highlighted in this tribute that focuses primarily on Brady's contributions to behavioral pharmacology but which also acknowledges his conceptual and technical contributions spanning multiple disciplines.

Incorporating the assessment of abuse liability into the drug discovery and development process

Evaluation of abuse liability is one of many obligations incurred by industrial sponsors in the development of medications acting on substrates in the central nervous system. In addition to providing the information necessary for a scheduling recommendation in the marketing application, the abuse liability assessment allows sponsors to estimate safety and commercial risks associated with scheduling, as well as to tailor their pre- and post-approval programs to collect information relevant to product misuse, illicit diversion and physical dependence. There are several important factors to consider before embarking on an abuse liability assessment, including the compound's primary and secondary biochemical activities, its absorption and metabolism, its final formulation, and its intended clinical population. Each of these factors will temper the timing and extent of the abuse liability program in animals and humans. Although every drug development program is unique in some way, a decision-making process may be applied to abuse liability assessment that will serve to better utilize limited resources and inform decisions regarding subsequent steps in the process. The emerging properties of the product will define the unique procedures best applied to assess it.

Guidelines and methodological reviews concerning drug abuse liability assessment

Regulatory control of drugs with abuse liability is an important component of drug control policy and is believed to help prevent nonmedical use. To be maximally effective, this requires a scientific assessment of abuse liability of drugs considered for regulatory control. These assessments have relied extensively on laboratory-based animal and human testing, but also utilize information from clinical trials, actual abuse and other sources. Here, we discuss recommendations and guidelines that have been proposed for abuse liability assessment and describe important review papers and conference proceedings that have addressed this matter, focusing primarily on drugs with medical usefulness. Historically, there is substantial consensus about how to approach abuse liability evaluation of drugs with actions similar to those of abused opiates, stimulants, depressants, and to a somewhat lesser extent, cannabinoids and hallucinogens, and much of what has been recommended for abuse potential assessment in the past remains valid and useful. On the other hand, novel CNS-active medications which cannot be readily classified with these traditional drugs of abuse are increasingly under development. In addition, advances in the science of abuse liability assessment need to be incorporated into future guidelines and recommendations on this subject. Developers of new medications need guidance on how to utilize scientific research to maximize therapeutic benefit while minimizing risk for abuse. Thus, another goal of this review has been to identify areas where critical thinking and new guideline development are needed.

Assessing the abuse potential of methylphenidate in nonhuman and human subjects: a review

Methylphenidate (MPH) is widely used for the treatment of Attention Deficit Hyperactivity Disorder (ADHD) in children, adolescents, and adults. Methylphenidate is clearly effective for the treatment of ADHD, but there is controversy as to whether it has significant abuse potential like other psychostimulants (e.g., D-amphetamine and cocaine). In general, the drug is believed to be abused at rates much lower than those for other stimulants. The present review examines studies that investigated the behavioral pharmacological profile of methylphenidate and discusses how results from these studies address its abuse liability. Using MEDLINE search terms methylphenidate, drug discrimination, reinforcement, self-administration, subjective effects, subject-rated effects, abuse potential, and abuse liability, along with a review of the references from identified articles, 60 studies were located in which the reinforcing, discriminative-stimulus, or subjective effects of methylphenidate were directly assessed in nonhumans or humans. Forty-eight (80.0%) of the studies reviewed indicate that methylphenidate either functions in a manner similar to D-amphetamine or cocaine (e.g., functions as a reinforcer, substitutes fully in drug discrimination experiments), or produces a pattern of subjective effects suggestive of abuse potential. The results are discussed as they pertain to factors that may account for the apparent discrepancy in abuse rates between methylphenidate and other stimulants, including characterization of actual abuse rates, defining abuse and misuse, pharmacokinetic factors, and validity of abuse liability assays.

The acute effects of monoamine reuptake inhibitors on the stimulus effects of hallucinogens

In a previous study it was observed that fluoxetine potentiates the stimulus effects of lysergic acid diethylamide (LSD). In the present investigation, stimulus control was established in groups of rats using as training drugs the hallucinogens lysergic acid diethylamide (LSD); 0.1 mg/kg), (-)-2,5-dimethoxy-4-methylamphetamine [(-)-DOM; 0.56 mg/kg], ibogaine (10 mg/kg), and 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT; 3 mg/kg). A two-lever, fixed-ratio 10, positively reinforced task with saline controls was employed. The hypotheses tested were that (a) monoamine uptake inhibitors other than fluoxetine potentiate the discriminative effects of LSD, and (b) hallucinogens other than LSD are potentiated by acute pretreatment with monoamine uptake inhibitors. The effects of a range of doses of each of the training drugs were determined both alone and following pretreatment with the monoamine reuptake inhibitors fluoxetine, fluvoxamine, and venlafaxine. In LSD-trained subjects, all three reuptake inhibitors caused a significant increase in LSD-appropriate responding. Similar results were observed in rats trained with (-)-DOM and with ibogaine. In 5-MeO-DMT-trained subjects, only fluoxetine resulted in an enhancement of drug-appropriate responding. The reuptake inhibitors given alone elicited varying degrees of responses appropriate for the respective training drugs. For fluoxetine in rats trained with LSD and ibogaine, for venlafaxine in LSD trained, and for fluvoxamine in (-)-DOM trained, the degree of responding met our criterion for intermediate responding, i.e., significantly different from both training conditions. Subsequent experiments in (-)-DOM-trained subjects examined a range of doses of each of the reuptake inhibitors in combination with a fixed dose of (-)-DOM (0.1 mg/kg), which alone yielded about 50% (-)-DOM-appropriate responding. With the exception of the point obtained with the highest dose of venlafaxine, all data were compatible with additivity of effects rather than true potentiation. In summary, the present data extend our previous observation of the augmentation of the stimulus effects of LSD by fluoxetine to include other hallucinogens. The mechanisms by which these interactions arise and possible differential effects of acute and chronic treatment remain to be established.

The interaction of clozapine with the meta-chlorophenylpiperazine (mCPP) discriminative stimulus

The psychotropic effects of the 5-HT2C agonist mCPP in human subjects are blocked by the atypical antipsychotic clozapine, but not by typical antipsychotics. An understanding of the mechanistic basis for the interaction of clozapine and mCPP would provide further insight into the basis for its unique therapeutic effects in humans. Drug-induced stimulus control provides an animal model for the subjective effects of psychotropic agents in humans. In the present study, the interaction of the atypical antipsychotic clozapine and the typical antipsychotic fluphenazine with the mCPP-stimulus were defined. Neither drug antagonized the stimulus effects of mCPP in vivo. In contrast, clozapine fully antagonized the mCPP-stimulated phosphoinositide turnover at the 5-HT2C receptor in vitro. The present data indicate that the paradigm of mCPP-induced stimulus control does not facilitate the differentiation of atypical and typical antipsychotic activities.

5-HT2C receptor-mediated phosphoinositide turnover and the stimulus effects of m-chlorophenylpiperazine

The present study was designed to investigate the hypothesis that agonist interactions at 5-HT2C receptors mediate the discriminative stimulus properties of m-chlorophenylpiperazine (mCPP). Three structural classes of compounds have been described to stimulate increases in phosphoinositide (PI) hydrolysis at the 5-HT2C receptor site: phenylpiperazines, phenylalkylamines, and indolamines. Four representative phenylpiperazines, mCPP, TFMPP, MK-212 and quipazine, one phenylalkylamine, (-)DOM, and one indolamine, LSD, were employed in the present study. The efficacies of these compounds were defined (1) in vitro, with respect to their abilities to stimulate increases in PI hydrolysis in the choroid plexus, and (2) in vivo with respect to their abilities to substitute for the mCPP discriminative stimulus. In vitro intrinsic activity at the 5-HT2C site was expressed as a fraction of the maximal PI hydrolysis response elicited by serotonin (5-HT). MK-212 (fractional efficacy = 1.1) and (-)DOM (0.77) were full agonists, while mCPP (0.72), LSD (0.27), quipazine (0.24), and TFMPP (0.22) were partial agonists with respect to the stimulation of PI hydrolysis at the 5-HT2C receptor. In vivo, each of the phenylpiperazines fully substituted for the mCPP stimulus, while (-)DOM (75%), and LSD (67%) elicited only partial substitution. While compounds with agonist activity at the 5-HT2C receptor in vitro substitute for the mCPP stimulus in vivo, no clear relationship exists between in vitro intrinsic activity at the 5-HT2C receptor with respect to the stimulation of PI turnover and maximal substitution for the mCPP stimulus in vivo. The present data suggest that mCPP elicits a compound stimulus which is mediated by agonist interactions at the 5-HT2C receptor and possibly additional interactions with 5-HT2A, 5-HT3, and/or 5-HT1B receptors.

The time-dependent stimulus effects of R(-)-2,5-dimethoxy-4-methamphetamine (DOM): implications for drug-induced stimulus control as a method for the study of hallucinogenic agents

The pharmacodynamic characteristics of the stimulus effects of the hallucinogens d-LSD and (-)DOM were investigated in the rat. The stimulus control induced by (-)DOM (0.56 mg/kg) was significantly less stable at the 15-min pretreatment time than at the 75-min pretreatment time. In addition, (-)DOM (0.8 mg/kg) produced a time-dependent substitution for the LSD stimulus in LSD trained subjects (0.1 mg/kg, 15-min pretreatment time). As pretreatment times were increased, the substitution of (-)DOM (0.8 mg/kg) for the LSD stimulus increased, culminating in a maximal level of 99.5% LSD-appropriate responding at the 75-min pre-treatment time. A dose-response relationship for the substitution of (-)DOM (75-min pretreatment time) for the LSD stimulus, indicated that 0.2 mg/kg (-)DOM was the minimum dose which elicited greater than 90% LSD-appropriate responding. LSD (0.32 mg/kg, 15-min pretreatment time) fully substituted for (-)DOM in the (-)DOM trained subjects (0.56 mg/kg, 75-min pretreatment time). These findings suggest that the pharmacodynamic parameters of d-LSD and (-)DOM-induced stimulus control differ. The time of onset for the stimulus effects of (-)DOM is markedly longer than that of LSD in the rat.

The role of the 5-HT2A and 5-HT2C receptors in the stimulus effects of m-chlorophenylpiperazine

m-Chlorophenylpiperazine (mCPP), a major metabolite of the atypical antidepressant trazadone, has been observed to produce marked physiological and behavioral effects in both humans and animals. These effects have been attributed to the interaction of mCPP with serotonergic receptors. The present study was designed to characterize those interactions of mCPP with central serotonergic receptors which mediate mCPP-induced stimulus control. A series of serotonergic antagonists (mesulergine, pizotyline, ketanserin, spiperone, risperidone, ritanserin, metergoline, pirenpirone, and LY53857) was tested for the ability to block the mCPP stimulus. The affinity of these antagonists for 5-HT2A and 5-HT2C receptors was then correlated with maximal percent inhibition of the mCPP stimulus. Kd at the 5-HT2C receptor was inversely proportional (r = -0.75, P < 0.05), and Kd at the 5-HT2A receptor directly proportional (r = +0.67, P < 0.05) to the maximal percent inhibition of the mCPP stimulus. The 5-HT2C selectivity ratio [Kd(5-HT2A)/Kd(5-HT2C)] of the antagonists was directly proportional (r = +0.86, P < 0.01) to maximal percent inhibition of the mCPP stimulus. A multiple regressions analysis indicated that 81% of the variance in the ability of a given antagonist to block the mCPP stimulus could be predicted on the basis of its affinity for 5-HT2A and 5-HT2C receptors. It is concluded that the stimulus effects of mCPP are mediated predominantly by a combination of agonist activity at 5-HT2C receptors and antagonist activity at 5-HT2A receptors.

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.

Abuse liability of alprazolam relative to other commonly used benzodiazepines: a review

Nonhuman and human studies comparing the abuse liability of alprazolam to other commonly used benzodiazepines are reviewed. These studies are reviewed to determine to what extent prospective, controlled, experimental studies support opinions that alprazolam's abuse liability is greater than that of other commonly used benzodiazepines. Studies comparing the self-administration of alprazolam, the discriminative stimulus effects of alprazolam, self-reported effects of alprazolam, physiological dependence on alprazolam, and adverse effects of alprazolam relative to other benzodiazepines are reviewed. Overall, the experimental literature does not support the widely held belief that alprazolam's abuse liability is greater than that of other benzodiazepines, but much more research is needed. Such research should focus explicitly on alprazolam's reinforcing effects, and the nature and severity of the discontinuation syndrome associated with its long-term use. Important issues such as selection of an appropriate comparison drug, selection of an appropriate population, dosing regimen and test doses need to be considered in future studies.

Neurobiology of alcohol abuse

Excessive consumption of beverage alcohol (ethanol) is a major health concern worldwide. Understanding the mechanisms by which ethanol affects neural functioning, after both acute and chronic exposure, has become a major goal in the study of alcoholism. With such an understanding, we should be able to institute more effective treatments and preventative measures for alcohol abuse problems. Recent studies have found, contrary to earlier assumptions, that ethanol has selective, dose-dependent effects on various neurotransmitter systems within the CNS. These effects are observed at all levels of analysis, from molecular to behavioral. This review by Herman Samson and Adron Harris covers these recent findings, with the intent of generating questions that will focus further research efforts.

Molecular Mechanisms Associated with Cocaine Effects

Cocaine has been shown to be a highly addictive and toxic drug. It produces these effects and a variety of other physiological and behavioral effects through its interactions with several distinct central nervous system receptor sites. We present the results of a series of studies that utilized multiple site analyses to elucidate which cocaine binding sites influence the reinforcing and toxic effects of cocaine and with what proportion of influence. The nature of cocaine interactions with monoamine transporters is also discussed, especially with the dopamine transporter, which has been shown to be the cocaine binding site that is primarily associated with the reinforcing effects of cocaine. We also provide evidence that vulnerability to both the toxic and addictive effects of cocaine may be significantly influenced by genetic differences in both humans and animals. In view of the fact that cocaine is commonly abused in a polydrug situation, we present the results of both behavioral and biochemical experiments which suggest that common biochemical pathways may mediate the reinforcing or addictive properties of drugs of abuse. Finally, we discuss research on the biochemical mechanisms associated with effects of ethanol, particularly those which may also influence cocaine self-administration and speculate on pharmacotherapeutic strategies for concurrent abuse of cocaine and ethanol.

Stimulus functions of caffeine in humans: relation to dependence potential

The interoceptive stimulus functions common to drugs of dependence include positive subjective effects, discriminative functions, and reinforcing functions. Data from studies measuring these stimulus functions constitute the objective assessment of a drug's dependence potential. This paper reviews the subjective effects, discriminative stimulus, and reinforcing stimulus functions of caffeine in humans to assess the dependence potential of caffeine. The stimulus effects of caffeine are compared with those of d-amphetamine, a prototypic CNS stimulant that has been studied under similar conditions, to evaluate the relative dependence potential of caffeine. Finally, caffeine's effects are evaluated in terms of generally accepted criteria for defining drug dependence. It is concluded that caffeine partially meets the primary criteria of drug dependence: 1) the majority of caffeine use is highly controlled, but not compulsive; 2) caffeine is psychoactive; and 3) caffeine functions as a reinforcer under certain conditions in humans, but not in animals. Caffeine thus has limited dependence potential. Additionally, although caffeine shares stimulus functions with d-amphetamine, it does so under limited conditions and should be considered to have a relatively lower dependence potential.

Drug abuse potential evaluation in animals

Animal laboratory studies can provide useful information concerning the potential of drugs for abuse. Over the past 50 years, methods have been developed for use with animal subjects which allow the evaluation of pharmacological properties of drugs which are particularly relevant to their abuse. The methods for preclinical drug abuse liability testing are reviewed under six heading: (1) establishment of the degree of pharmacological equivalence to known drugs of abuse, (2) drug discrimination studies, (3) tests of tolerance and cross-tolerance, (4) tests of physical dependence capacity, (5) drug self-administration tests of reinforcing effects, and (6) evaluation of toxicity and performance impairment at self-administered doses. Preclinical studies can be helpful early in drug development to select lead compounds with low abuse potential for further study. In the case of new or already marketed medications, animal testing can often compliment and extend abuse liability evaluation in human subjects. The results of abuse potential evaluation studies can be useful in making decisions about the possible need for regulation under national and international drug laws, and thus play an important role in drug abuse prevention.

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."