Inhalant use among incarcerated adolescents in the United States: prevalence, characteristics, and correlates of use

OBJECTIVE

To characterize patterns and correlates of inhalant use among incarcerated youth.

METHOD

Residents (N=723) of 27 Missouri Division of Youth Services facilities completed interviews assessing substance use, psychiatric symptoms, antisocial traits, trauma, suicidality, and criminality.

RESULTS

Participants averaged 15.5 (S.D.=1.2) years of age, were ethnically diverse, and predominantly male. More than one-third (36.9%) reported lifetime inhalant use; 47.9% of users had tried four or more inhalant products. Comparatively high rates of use were observed for Hispanic and small town/rural youth. Commonly abused agents included gasoline (22%), permanent markers (15%), computer "air duster," (15%) and spray paint (12%). Inhalant users evidenced significantly higher levels of criminal behavior, antisocial attitudes, current psychiatric symptoms, earlier onset of offending and substance use, and more extensive histories of head injury, kidney disease, hormonal problems, mental illness, suicidality, trauma, and substance-related problems than nonusers. In multiple logistic regression models, race/ethnicity, geographic area of residence, fearlessness, suicidality, and polydrug use distinguished inhalant users and nonusers. Measures of cognitive impairment, impulsivity, fearlessness, blame externalization, polydrug use, and substance-related problems were positively associated with lifetime frequency of inhalant use.

CONCLUSIONS

Inhalant use was widespread in this sample and associated with serious physical and mental health impairments.

Characterizing early cigarette use episodes in novice smokers

Retrospective self-report data indicate that early cigarette use episodes may be important predictors of smoking. Unfortunately, recall of early experiences are confounded with current smoking. The current study is the first to examine early cigarette use episodes (EUEs) prospectively in novice smokers (less than 15 lifetime cigarettes). Smoking amount, context and subjective experiences for up to five of the first cigarette episodes during their first year of college were collected using weekly internet-based questionnaires and structured interviews. Data were obtained on 538 EUEs from 163 students. EUEs generally occurred within a social/party context; over 90% of EUEs occurred when participants were with other people who were smoking and over 65% occurred while participants were drinking alcohol. Subjective effects across episodes were reported as generally mild and factor analysis yielded Positive, Negative and Sensory/Peripheral effects scales. Subjective effects were related to the amount smoked and inhalation, whereas EUE context, including alcohol use and social context, was not. This study demonstrates that it is possible to study EUEs in college students within days or weeks of their occurrence and that most of these occur in social settings with the concurrent use of alcohol.

Addiction epidemiology in adolescents receiving inpatient psychiatric treatment

This study sought to characterize adolescent psychiatric inpatient populations from two sites and to determine correlates of substance use disorders (SUD). Screening procedures for SUD differ substantially between these sites. A retrospective review of adolescent inpatients (n=636) revealed that the populations were similar in gender, race and age. Rates of SUD at the site with a formalized SUD screening regimen were higher (39%) than those at the other site (16.5%). Similar correlates of SUD were observed across sites, including older age, legal involvement, sexual activity, childhood disruptive disorder, and tobacco use. These results suggest that SUD is a major issue in adolescent psychiatric patients. More rigorous screening for SUD and its correlates may facilitate earlier detection of substance use in this vulnerable population.

An exploratory study of drug abuse and dependence information in package inserts

Information about drug abuse and dependence from package inserts of centrally acting drugs was evaluated for content. Of the 77 labels reviewed, 40 were opiate agonists, 18 were stimulants, and the remainder fell into other selected categories. The amount of information ranged from 0-66 sentences, with greatest variability found in the opiate agonists (range 9-66). Information amount was significantly correlated with the year of drug approval (p < 0.001) but not with the latest label revision (p = 0.749). Information amount did not differ significantly with warning strength or schedule. While most package inserts explain physical dependence, tolerance, and withdrawal, there is a lack of information about psychological dependence. Variability in information about abuse and dependence potential is high and can affect prescribing by physicians and counseling by pharmacists, underscoring the need for further studies.

Extended Access to Nicotine Self-Administration Leads to Dependence: Circadian Measures, Withdrawal Measures, and Extinction Behavior in Rats

The present study characterized nicotine intake, circadian patterns of food and water intake, precipitated somatic signs of withdrawal, and extinction of nicotine-seeking behavior in rats with 23-h access to intravenous self-administration (IVSA). Separate groups of animals were allowed access to nicotine IVSA (0.015, n = 9; 0.03, n = 14; 0.06, n = 16; mg/kg/0.1 ml infusion/s; fixed ratio 1) and trained to nosepoke for food and water 23 h/day for 40 consecutive days. Somatic signs of nicotine withdrawal were examined following saline or mecamylamine administration (1.5 mg/kg i.p.), and extinction of nicotine-seeking behavior was assessed. A dose-dependent decrease in lever responding and an increase in nicotine intake were observed, with the highest nicotine dose producing the lowest amount of lever responding and the highest amount of nicotine intake. Nicotine acutely reduced diurnal and nocturnal food intake, producing smaller and fewer meals, and an increased rate of eating. Differences in rate of nicotine intake between the light and dark phase decreased significantly, especially in rats receiving higher unit nicotine doses (0.03 and 0.06 mg/kg), along with long-term decreases in the circadian profile and amplitude of feeding. Mecamylamine precipitated robust withdrawal signs, the magnitude of which was positively correlated with the total amount of self-administered nicotine. Extinction of nicotine-seeking behavior was observed and was facilitated by removal of nicotine-associated cues. The results demonstrate that rats will self-administer nicotine to the point of producing dependence, as measured by somatic signs, resistance to extinction, and measures of food intake.

Impact of formulation on the abuse liability, safety and regulation of medications: the expert panel report

A scientific meeting was held in April 2005 to consider how the formulation of medications might impact on their potential for abuse. The background papers prepared for this meeting, as well as abstracts of volunteered presentations, are published in this supplemental issue of Drug and Alcohol Dependence. This paper is the Expert Panel Report summarizing the discussions held following the formal presentations and including the suggested recommendations for additional research that emerged from these discussions. There was overwhelming consensus that formulation does play a role in prescription drug abuse, i.e., a formulation of an abused substance can be developed that will decrease its abuse potential, and several examples were cited. Nevertheless, it is imperative that new formulations have similar efficacy and in no way compromise medication access to doctors and patients. However, there was also consensus that a great deal of research and discussion was needed to fully implement a program of risk management through reformulation of existing products or tailoring the formulation of new products to retain clinical efficacy and safety while minimizing potential for abuse. Those who need to take part in this discussion include scientific groups, pharmaceutical companies, as well as governmental and regulatory agencies. The areas where more research is needed include development of standards for assessing tamper-resistance, improved animal models that can address formulation-related variables (e.g., onset, duration), the redesign of human laboratory studies providing appropriate models for comparing formulations, and improved post-marketing surveillance. Finally, knowledge and experience are needed to translate scientific work into a predictable, transparent and reliable regulatory process.

Tolerance and sensitization to inhaled 1,1,1-trichloroethane in mice: results from open-field behavior and a functional observational battery

RATIONALE

1,1,1-Trichloroethane (TCE), a representative abused solvent, has well described acute behavioral effects in animals. Much less is known about repeated high-concentration exposures as would be encountered in inhalant abusers. Tolerance has been demonstrated in some, but not all, studies with TCE while sensitization has also been seen with other abused solvents.

OBJECTIVE

The present study was designed to further characterize changes in the effects of repeated exposure to TCE on a variety of mouse behaviors.

METHODS

Mice were tested using locomotor activity as well as a functional observational battery (FOB) both before and after a regimen of daily exposures to various concentrations of TCE.

RESULTS

The initial locomotor effects of acute 30-min exposures to TCE were biphasic with concentration-dependent increases in activity at lower concentrations and decreases observed at higher concentrations. The profile of acute effects as measured by the FOB included changes in posture, decreased arousal, disturbances in gait, delayed righting reflexes, and decreased sensorimotor reactivity. Animals were then divided into five groups and exposed 30 min/day to either air or one of four concentrations of TCE (2,000, 6,000, 10,000, or 13,300 ppm) for 15 consecutive days. The TCE concentration used primarily affected the magnitude of change, not whether tolerance or sensitization occurred. Tolerance developed on the measures of forelimb grip strength, inverted screen, and number of rears. Conversely, sensitization developed to measures of locomotor activity.

CONCLUSION

Depending on the behavioral measure, both tolerance and sensitization can occur in mice with repeated exposure to TCE. Both of these phenomena are characteristic of drugs of abuse.

Dose, duration, and pattern of nicotine administration as determinants of behavioral dependence in rats

RATIONALE

Relatively little is known about the role of dose, duration, and pattern of nicotine exposure in the development of dependence. Disruption of learned behavior during antagonist-precipitated withdrawal can be a sensitive, quantitative measure of behavioral dependence.

OBJECTIVES

The present study sought to determine whether behavioral dependence upon nicotine could be induced in rats and, if so, what exposure conditions were essential for inducing it. Our primary focus was on whether continuous exposure over several days was necessary to produce dependence.

METHODS

Male Sprague-Dawley rats were trained to lever press under fixed-ratio 10 schedules of food reinforcement during daily, 15-min experimental sessions. Nicotine was then administered s.c. via osmotic minipumps that delivered various nicotine dosage regimens, some including 24-h nicotine-free periods, to manipulate pattern of exposure. The presence of dependence was tested with challenges with the nicotinic acetylcholine receptor antagonist, mecamylamine, or during spontaneous withdrawal.

RESULTS

After 7 days of 3, 6, and 12 mg kg(-1) day(-1) nicotine administration, response rates were significantly reduced in nicotinized, but not in saline-treated rats following mecamylamine challenges. Subsequent studies demonstrated that 4 days, but not 3 days, of cumulative 3 mg kg(-1) day(-1) nicotine administration was sufficient to induce dependence. The induction of dependence could be prevented by imposing a nicotine-free period between the first and second days during these 4-day regimens but not at other times.

CONCLUSION

Behavioral dependence upon nicotine can be induced in the rat, and its induction is dependent upon its cumulative duration and pattern of exposure suggesting that tobacco dependencies could be controlled by similar determinants.

Intravenous self-administration of abused solvents and anesthetics in mice

Volatile organic solvents, fuels and anesthetics are subject to abuse. The aim of the present study was to evaluate i.v. self-administration of several of these chemicals in drug- and experiment-naive mice using a commercially available vehicle, intralipid. Two strains of mice (DBA/2 and Swiss) were allowed to self-administer toluene (0.0017-0.17 micromol/infusion), 1,1,1-trichloroethane (0.006-0.19 micromol/infusion), ethanol (0.32-1.6 micromol/infusion), cyclohexane (0.0017-0.052 micromol/infusion), propofol (0.01-0.53 micromol/infusion) and flurothyl (0.00042-0.072 micromol/infusion) or their vehicles during 30-min tests. During the test, each nose-poke of the master mouse resulted in a 1.88-microl i.v. infusion to the master mouse and a yoked control mouse. When the delivery line was loaded with a reinforcing drug solution, the number of nose-pokes of the master mice significantly exceeded that for yoked control mice. In the present experiments, significant differences in rates of nose-poking were observed between mice receiving response-contingent and response-noncontingent deliveries of ethanol and toluene in both strains of mice and of 1,1,1-trichloroethane in Swiss mice. These data suggest that the reinforcing effects of abused inhalants can be studied using i.v. self-administration procedures.

Effects of abused inhalants and GABA-positive modulators in dizocilpine discriminating inbred mice

There is in vitro evidence that some of the effects of abused volatile solvents may be produced by actions at the NMDA receptor. In addition, some solvents produce phencyclidine-like discriminative stimulus effects. The major goal of the present study was to further compare abused solvents to NMDA antagonists by testing them in two strains of mice trained to discriminate 0.17 mg/kg of the very selective uncompetitive NMDA antagonist, dizocilpine, from saline and contrast those results with several GABA(A)-positive modulators, PCP and ethanol. The results indicated that the discriminative stimulus produced by 0.17 mg/kg dizocilpine was highly specific in both mouse strains. PCP produced 91% dizocilpine-lever responding in C57BL/6J mice, but only 56% dizocilpine-lever responding in DBA/2J mice. Pentobarbital, midazolam and ethanol produced at least some overlap in discriminative stimulus effects with dizocilpine in one or both mouse strains. In contrast, toluene, 1,1,1-trichloroethane (TCE), xylene and methoxyflurane produced saline-appropriate responding almost exclusively. These data indicate that, at least under the specific conditions tested, abused volatile solvents do not have substantial dizocilpine-like discriminative stimulus effects in either C57BL/6J or DBA/2J mice, providing little support that NMDA antagonism plays a central role in the production of this abuse-related effect.

Using growth models to relate acquisition of nicotine self-administration to break point and nicotinic receptor binding

Growth modeling can be used to characterize individual and mean acquisition trajectories for drug self-administration. Individual characteristics can also be incorporated into the growth model, providing a powerful tool for investigating the relationship between acquisition and other behavioral and biological measures. We illustrate the utility of this method by examining the relationship between acquisition of nicotine self-administration and (1) break point on a progressive ratio schedule of reinforcement, and (2) the density of brain nicotinic receptors (B(max)). Daily infusion rates from male and female Sprague-Dawley rats were modeled with break point or B(max) as time-invariant covariates. Use of this model led to two novel findings regarding individual differences in acquisition. First, greater rates of change in infusions early in acquisition were related to higher break points; this relationship was mediated by a similar effect of increasing the number of responses required to obtain nicotine. Second, animals displaying more resistance to increases in the response requirement during acquisition, as indicated by a smaller drop in the rate of nicotine self-administration, generally had fewer nicotinic receptors at the end of the experiment. The relationships revealed demonstrate the usefulness of growth models in the quantitative analysis of individual differences in drug self-administration behavior.

Analyzing the acquisition of drug self-administration using growth curve models

Current approaches to studying acquisition of drug self-administration have modest power to detect individual differences in the pattern of acquisition or to efficiently and accurately describe trajectories of behavior change. Methodological advances in human research have elucidated approaches to describing repeated measure data that focus on modeling the behavior of individual subjects. In this article, we re-analyzed data published in using growth curve modeling to characterize the acquisition of nicotine-taking in rats. Change over time in the infusion rate was examined, revealing that the acquisition process could be described with a quadratic equation represented by intercept, slope, and acceleration parameters. Unit dose of nicotine, sex and fixed ratio (FR) schedule of reinforcement had significant effects on the acquisition curves. Dose altered the absolute rate of infusions, but not the slope or acceleration, indicating that, when an effective dose was available, the shape of acquisition trajectories was not affected by dose. In addition, dose impacted acquisition by moderating the disruption in infusion rates after an increase in the response requirement. Thus, the role of a higher dose may not be to accelerate the acquisition process but to lead to behavior that is more resistant to change. Trajectories differed between males and females at the smallest dose, but these differences dissipated by the end of acquisition. Growth curve modeling captures the process of acquisition of drug self-administration and facilitates a greater understanding of the individual differences in change in drug-taking behavior over time.

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.

Evaluation of the phencyclidine-like discriminative stimulus effects of novel NMDA channel blockers in rats

RATIONALE

Because of their potential therapeutic effects, N-methyl-D-aspartate (NMDA) receptor antagonists have been investigated for clinical use. Unfortunately, many channel-blocking antagonists have been associated with the production of side effects, including motor impairment and phencyclidine (PCP)-like subjective effects.

OBJECTIVE

This study investigated the relationship between NMDA receptor channel blockade and production of PCP-like side effects by evaluating a variety of NMDA channel blockers with different binding characteristics for the production of PCP-like discriminative stimulus effects.

METHODS

The NMDA channel blockers were tested in rats trained to discriminate 2 mg/kg PCP, i.p., from saline using a standard two-lever drug discrimination procedure with responding under a fixed ratio (FR) 32 schedule of food reinforcement.

RESULTS

The high-affinity channel blockers PD 138289, PD 137889 and FR 115427, produced full, dose-dependent substitution for PCP. Of the moderate-affinity channel blockers, MRZ 2/579 fully substituted for PCP while 1-(4-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline, 8-(2-methoxyphenyl)-1,2,3,4-tetrahydroisoquinoline and alaproclate produced partial substitution. Drugs with the lowest affinity for the channel site and/or higher affinity for non-NMDA CNS sites, antazoline, idazoxan, 1-phenyl-1,2,3,4-tetrahydroisoquinoline, alpha-benzyl- N-methylphenethylamine and orphenadrine, failed to substitute for PCP.

CONCLUSIONS

The results demonstrate that the cellular actions of the individual channel-blocking NMDA antagonists, in particular affinity for the channel site and NMDA receptor specificity, are important determinants of their discriminative stimulus effects. While higher affinity channel blockers show a correlation between affinity and PCP-like discriminative stimulus effects, behavioral disruption through action at non-NMDA receptors probably prevents achieving sufficient concentrations of the lower affinity compounds at NMDA receptors to produce PCP-like discriminative stimulus effects.

Toluene inhalation produces a conditioned place preference in rats

Toluene is a widely abused solvent with demonstrated addictive potential in humans. Here we explore if conditioned place preference can be used to study the abuse-related effects of inhaled toluene in rats. Animals were confined to a distinctive compartment of a three-compartment chamber while exposed to toluene vapor and later tested for preference for that compartment compared to appropriate control subjects. In this study, a flame ionization detector was used for on-line monitoring of toluene vapor concentrations inside the conditioning apparatus coupled with computerized recording of the time spent by the animals on the test day in each of the chambers. Sprague-Dawley rats were exposed to 810, 1895 or 4950 ppm of toluene vapors in either the black or white compartment during 30-min pairing sessions given every other day alternating with air exposure for the total of six pairings for each treatment. Rats that received air in both sides (control group) did not show any preference for either side with approximately equal time spent in each compartment on the test day (241 +/- 33 and 234 +/- 34 s, for white and black box, respectively). However, the 1895- and 4950-ppm test groups, but not the 810-ppm group, demonstrated a significant preference for the side paired with toluene exposure. When a subsequent test session was performed during toluene exposures, no conditioned place preference was observed. Thus, toluene produced a clear conditioned place preference that appears to be most evident when animals are not intoxicated. This procedure should be useful for further studies of the abuse-related effects of abused inhalants.

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.

Evaluation of toluene dependence and cross-sensitization to diazepam

Acute effects of the abused inhalant toluene resemble those of CNS depressant drugs. Since abuse of toluene involves repeated use, the purpose of the present study was to evaluate the effects of repeated or continuous exposure to toluene and to compare these effects to those of other inhalants and depressants. In experiment 1, ICR mice exposed continuously to 250 ppm toluene via inhalation for four days developed mild dependence upon termination that was characterized by an increase in severity of handling-induced convulsions. However, administration of the convulsants, N-methyl-D-aspartate (NMDA) or pentylenetetrazole (PTZ), did not differentially affect toluene- vs. air-exposed mice. In experiment 2, CFW mice (but not ICR mice) developed cross-sensitization to the initial locomotor stimulatory effects of toluene following four days of injections with 10 mg/kg/day diazepam. Previous findings have shown that 1,1,1-trichloroethane (TCE) produced robust dependence and cross-sensitization to diazepam's locomotor effects when tested under similar conditions. The present results suggest that the dependence and cross-sensitization with diazepam produced by toluene are milder than those induced by TCE. Further, these studies add to increasing evidence that abused inhalants do not have identical pharmacological effects.

Mixed cocaine agonist/antagonist properties of (+)-methyl 4beta-(4-chlorophenyl)-1-methylpiperidine-3alpha-carboxylate, a piperidine-based analog of cocaine

The present study investigated the pharmacological properties of a piperidine-based novel cocaine analog, namely, (+)-methyl 4beta-(4-chlorophenyl)-1-methylpiperidine-3alpha-carboxylic acid [(+)-CPCA]. Like cocaine, (+)-CPCA inhibited rat synaptosomal dopamine and norepinephrine uptake with high affinity, but was 33-fold less potent than cocaine in inhibiting serotonin uptake. Like cocaine, (+)-CPCA is a locomotor stimulant, although it was less potent and efficacious than cocaine. Importantly, pretreatment with (+)-CPCA dose dependently blocked the locomotor stimulant effects of cocaine in rats. (+)-CPCA completely substituted for cocaine in drug discrimination tests, although it was about 3 times less potent than cocaine. It was also self-administered by rats. Unexpectedly, (+)-CPCA did not enhance cocaine-induced convulsions in mice. As expected from rodent studies, rhesus monkeys readily self-administered (+)-CPCA. However, compared with cocaine, (+)-CPCA showed limited reinforcing properties in rats as assessed by both fixed and progressive ratio intravenous drug self-administration tests. These results collectively suggest that (+)-CPCA has an atypical pharmacological profile having both cocaine-like "agonist" and some cocaine "antagonist" properties. These properties of (+)-CPCA suggest that it may have utility in the treatment of cocaine craving and dependence.

Effects of inhaled toluene and 1,1,1-trichloroethane on seizures and death produced by N-methyl-D-aspartic acid in mice

Evidence exists that some abused solvents have N-methyl-D-aspartic acid (NMDA) antagonist activity, although which of their effects may be related to this mechanism is not well understood. The effects of toluene and 1,1,1-trichloroethane (TCE) on NMDA-induced seizures in mice were studied using three experimental protocols: (a) animals injected i.p. with 120 or 170 mg/kg NMDA and immediately afterwards exposed to solvent vapors or air for 30 min (co-exposure protocol); (b) mice exposed for 30 min to solvent or air, then injected with NMDA and placed in the chamber for a second 30-min exposure (pre-exposure+co-exposure protocol); and (c) mice that inhaled 4000 ppm toluene or air for 30 min twice a day, 6 h apart, for 7 days, and were injected with 120 mg/kg NMDA immediately before a 30-min toluene exposure (repeated exposure protocol). When given acutely, toluene, but not TCE, produced concentration-dependent protection against NMDA-induced seizures. Higher concentrations of toluene were also effective against the lethal effects produced by 170 mg/kg NMDA. Clearer effects were seen when the pre-exposure+co-exposure protocol was followed. Under these conditions the IC(50) for toluene was 739 ppm (653-825) against seizure occurrence and 2127 ppm (1966-2288) against lethality. Repeated exposure to toluene did not result in tolerance to its anticonvulsant effects. These results are consistent with the in vitro effects described for toluene as a noncompetitive NMDA antagonist and as a compound that enhances GABAergic transmission. The lack of protective effects of TCE is not consistent with its in vitro actions.

Affinity and specificity of N-methyl- D-aspartate channel blockers affect their ability to disrupt prepulse inhibition of acoustic startle in rats

RATIONALE

Phencyclidine (PCP) binds with high affinity to a site located within the ionophore of N-methyl- D-aspartate (NMDA) receptors. Previous studies have demonstrated that PCP and other high-affinity NMDA channel blockers reliably disrupt prepulse inhibition (PPI) of acoustic startle, an animal model of sensorimotor gating used to study attentional deficits associated with schizophrenia. Recently, a number of low-affinity NMDA channel blockers that exhibit minimal PCP-like effects in humans at therapeutic doses have been developed.

OBJECTIVES

The purpose of this study was to evaluate the effects on PPI of NMDA channel blockers with varying affinities for the channel site as well as different specificities for NMDA receptors.

METHODS

Sprague-Dawley rats were presented with multiple stimulus presentation trials, including pulse-alone and PPI trials.

RESULTS

As expected, the high-affinity ligands dizocilpine and dextrorphan disrupted PPI at doses that did not affect the response during pulse-alone trials. Low-affinity drugs produced a mixed pattern of results. Whereas dextromethorphan and memantine disrupted PPI, orphenadrine, amantadine, desipramine, and alaproclate did not affect this response. Ibogaine also disrupted PPI, but only within a dose range that severely decreased the startle response during pulse-alone trials.

CONCLUSIONS

These results suggest that not all NMDA channel blockers share PCP's effect of PPI disruption. In addition, they suggest caution in the use of supratherapeutic doses of these compounds and in their use in vulnerable populations (e.g., schizophrenic patients).

Evaluation of the discriminative stimulus effects of the low-affinity N-methyl-D-aspartate channel blockers AR-R 13950AA and AR-R 16283AA in rats and rhesus monkeys

Low-affinity channel-blocking -methyl-D-aspartate (NMDA) antagonists have been of interest for clinical development because they are purported to produce few phencyclidine (PCP)-like side-effects, particularly at therapeutic doses. In the current study, two low-affinity NMDA channel blockers, AR-R 13950AA and AR-R 16283AA, were evaluated for NMDA antagonist-associated behavioral effects. The drugs were tested in rats and rhesus monkeys trained to discriminate PCP from saline, using a standard two-lever drug discrimination paradigm, under a fixed-ratio (FR) schedule of food reinforcement. Both drugs were also tested in rats trained to discriminate NPC 17742, a competitive NMDA antagonist, from saline in a similar experimental procedure. In rats, both AR-R 13950AA and AR-R 16283AA resulted in intermediate levels of PCP-lever selection (up to 60%). Testing in NPC 17742-trained rats produced at most 30% NPC 17742-lever responding. In rhesus monkeys, AR-R 13950AA produced virtually no PCP-lever responding at any dose, while AR-R 16283AA produced a dose-dependent substitution for PCP in all four subjects. The results with AR-R 16283AA in monkeys suggest that, at doses above therapeutic levels, it may produce PCP-like intoxication in humans. Overall, the results suggest that, while there is some overlap of the discriminative stimulus effects produced by the AR-R compounds with those of PCP, there are also important differences.

The selective glycine antagonist gavestinel lacks phencyclidine-like behavioral effects

Gavestinel [GV150526A; ( E)-3[(phenylcarbamoil)ethenyl]-4,6-dichloroindole-2-carboxylic acid sodium salt] is a selective antagonist at the strychnine-insensitive glycine site of the -methyl-D-aspartate (NMDA) receptor. It was tested for its ability to substitute for phencyclidine (PCP) in rats and rhesus monkeys trained to discriminate PCP from saline, under a two-lever fixed-ratio (FR) food reinforcement schedule, and for its ability to maintain responding in rhesus monkeys trained to self-administer PCP under a FR reinforcement schedule. No PCP-lever responding was observed after gavestinel (1-56 mg/kg i.p.) administration to rats discriminating PCP (2.0 mg/kg i.p.) from saline. The highest dose of gavestinel (100 mg/kg i.p.) tested eliminated responding. Likewise, no PCP-lever responding was observed after gavestinel (1-30 mg/kg s.c.) administration to rhesus monkeys discriminating PCP (0.08 or 0.1 mg/kg i.m.) from saline; the highest dose of gavestinel (30 mg/kg s.c.) tested reduced response rates to approximately 50% of those observed after its vehicle ( -cyclodextrin in 0.9% saline). Gavestinel (0.1-1 mg/kg per i.v. infusion) was not self-administered by rhesus monkeys that reliably self-administered PCP (0.0056 or 0.01 mg/kg per i.v. infusion). Infusion rates at the highest dose were typically lower than those for vehicle or saline, suggesting behavioral activity. Together, these results suggest that at behaviorally active doses gavestinel is not PCP-like and is likely to have low abuse liability.