Amphetamine-haloperidol discrimination: effects of chronic drug treatment

A Randomized, Controlled Laboratory Classroom Study of Serdexmethylphenidate and d-Methylphenidate Capsules in Children with Attention-Deficit/Hyperactivity Disorder

Objectives: To evaluate the efficacy and safety of once-daily serdexmethylphenidate/dexmethylphenidate (SDX/d-MPH) capsules (Azstarys^™) compared with placebo in children with attention-deficit/hyperactivity disorder (ADHD) in a randomized, double-blind, dose-optimized laboratory classroom study. Methods: Children ages 6-12 with ADHD were enrolled. During a 3-week, open-label, Dose Optimization Phase, subjects initiated treatment with 39.2 mg/7.8 mg/day of SDX/d-MPH and were titrated weekly to an optimal dose (maximum dose of 52.3/10.4 mg). During the double-blind Treatment Phase, subjects were randomized to receive their optimal dose of SDX/d-MPH or placebo for 7 days. On day 7, efficacy was assessed in the laboratory classroom using the Swanson, Kotkin, Agler, M-Flynn, and Pelham (SKAMP) Rating Scale and Permanent Product Measure of Performance (PERMP). To evaluate safety, adverse events (AEs), vital signs, and electrocardiograms were assessed, and suicide risk was assessed. Results: A total of 149 subjects completed the study. In the primary efficacy analysis, the mean postdose change from baseline in SKAMP-Combined scores averaged over the laboratory classroom day was significantly improved with SDX/d-MPH versus placebo (least-squares mean treatment difference [95% confidence interval]: -5.41 [-7.10 to -3.71]; p < 0.001). A significant treatment effect for SDX/d-MPH compared with placebo was observed from 1 to 10 hours postdose. A post hoc analysis more comparable with that conducted in similar studies indicated a 0.5- to 13-hour onset and duration of efficacy. Both average postdose PERMP-Attempted and PERMP-Correct score changes from baseline were significantly improved among those treated with SDX/d-MPH versus placebo (p < 0.001 for both). No serious AEs were reported. During the Dose Optimization Phase, two-thirds of subjects reported AEs; the most common being insomnia and decreased appetite. Conclusions: SDX/d-MPH showed significant improvement in ADHD symptoms compared with placebo in children 6-12 years of age, with a rapid onset and extended duration of treatment effect. SDX/d-MPH was safe, with AEs comparable with those observed with other stimulant treatments.

Translational Value of Drug Discrimination with Typical and Atypical Antipsychotic Drugs

This chapter focuses on the translational value of drug discrimination as a preclinical assay for drug development. In particular, the importance of two factors, i.e., training dose and species, for drug discrimination studies with the atypical antipsychotic clozapine is examined. Serotonin receptors appear to be an important pharmacological mechanism mediating clozapine's discriminative cue in both rats and mice, although differences are clearly evident as antagonism of cholinergic muscarinic receptors is important in rats at a higher training dose (5.0 mg/kg) of clozapine, but not at a lower training dose (1.25 mg/kg). Antagonism of α1 adrenoceptors is a sufficient mechanism in C57BL/6 and 129S2 mice to mimic clozapine's cue, but not in DBA/2 and B6129S mice, and only produces partial substitution in low-dose clozapine discrimination in rats. Dopamine antagonism produces partial substitution for clozapine in DBA/2, 129S2, and B6129S mice, but not in C57BL/6 mice, and partial substitution is seen with D4 antagonism in low-dose clozapine drug discrimination in rats. Thus, it is evident that clozapine has a complex mixture of receptor contributions towards its discriminative cue based on the data from the four mouse strains that have been tested that is similar to the results from rat studies. A further examination of antipsychotic stimulus properties in humans, particularly in patients with schizophrenia, would go far in evaluating the translational value of the drug discrimination paradigm for antipsychotic drugs.

Stimulus generalization by fenfluramine in a quipazine-ketanserin drug discrimination is not dependent on indirect serotonin release

The purpose of this study was to determine if animals trained to discriminate a serotonin2A (5-HT2A) receptor agonist from a 5-HT2A receptor antagonist would also be sensitive to alterations in serotonin neurotransmission brought about by 5-HT reuptake inhibitors and releasers. Previous work from our laboratory has shown that the quipazine-ketanserin discrimination is mediated solely by the 5-HT2A receptor, thus providing a behavioral continuum of 5-HT2A receptor function. Rats were trained to discriminate quipazine (0.35 mg/kg) from ketanserin (1.0 mg/kg) on a variable interval-30 schedule of reinforcement. Following acquisition, substitution tests were conducted with the training drug, quipazine, and agents that have been shown to alter the synaptic levels of 5-HT, including fenfluramine, norfenfluramine, 5-methoxy-6-methyl-2-aminoindan (MMAI) and fluoxetine. All compounds substituted, except fluoxetine. Antagonist tests with mianserin and MDL 100,907 indicated that fenfluramine's and MMAI's substitution for quipazine was mediated by the 5-HT2A receptor. Animals were pretreated with PCPA to determine whether 5-HT release or direct agonism mediated the discriminative stimulus effects of fenfluramine and MMAI. PCPA blocked the substitution of MMAI but not of fenfluramine for quipazine. Analysis of 3H-IP formation in cells showed that norfenfluramine dose-dependently stimulated phosphoinositide hydrolysis to levels similar to that of serotonin and quipazine. These results indicate that fenfluramine's substitution for quipazine in rats trained on a quipazine-ketanserin discrimination are due to direct agonism at the 5-HT2A receptor likely mediated by norfenfluramine, an active metabolite.

Tolerance to the anticonflict effects of diazepam: importance of methodological considerations

The present study examines the effects of chronic diazepam treatment on conflict behavior in rats using the Geller-Seifter paradigm. A dose-response function for the effects of diazepam (DZ) on punished and unpunished responding was determined (0.0, 0.63, 1.25, 2.5, and 5.0 mg/kg DZ intraperitoneally) using five independent groups. The test doses of DZ produced an inverted U-shaped function where punished responding increased as a function of dose up to 2.5 mg/kg and then decreased at 5.0 mg/kg. All groups were then treated with 2 x 5 mg/kg DZ per day for 5 days. When the dose-response function was redetermined at 36 h post-chronic treatment, it was found that the function had shifted to the right, indicating tolerance. Because of the inverted U-shaped nature of the original function, tolerance was manifested as a decrease in responding on the ascending portion of the function and as an increase in responding on the dose (5 mg/kg) representing the descending side of the inverted U.

A three-choice haloperidol-saline-cocaine drug discrimination task in rats

This study was conducted to test whether rats could be trained and successfully maintain a three-choice drug discrimination task using 0.1 mg/kg haloperidol (SC, 2-h pretreatment), saline (IP or SC, 2 h and 15 min pretreatment), and 10 mg/kg cocaine (IP, 15-min pretreatment) as training stimuli. Six male Sprague-Dawley rats achieved criterion performance for stimulus control by these training stimuli under a fixed-ratio-5 schedule of food reinforced lever-press responding in an average of 164 training sessions. Dose-response functions for cocaine and haloperidol demonstrated both quantitative and qualitative specificity of the training stimuli. The data also are presented along a single pharmacological continuum (agonist-antagonist) that we hypothesize to represent a parallel subjective or interoceptive stimulus continuum associated with the drug injections. Based on the previous multidimensional model of drug stimuli dimensionality (3), this specific stimulus dimension is characterized as an unidimensional bipolar continuum represented by the hypothetical states of hedonia or euphoria on one end (cocaine) and anhedonia or depression on the opponent end (haloperidol), with a neutral (saline) centroid region. We propose that this specific three-choice drug discrimination task in rats may function as an animal analog of the subjective states associated with cocaine abuse and the subsequent withdrawal or, crash, in humans (7,8,21).

Drug discrimination is a continuous rather than a quantal process following training on a VI-TO schedule of reinforcement

Debate continues as to whether drug discrimination in animals is an inherently quantal or continuous process. This issue is important in determining the appropriate interpretation of results from drug discrimination studies designed to assess the nature of drug-induced interoceptive cues. The quantal approach holds that subjects perceive a drug cue in an all-or-none manner, while the continuous view proposes that when appropriate training and testing procedures are used, subjects can discriminate along a continuum of interoceptive cues. Data consistent with the quantal view have consistently been generated by animals trained to respond on schedules of reinforcement having an FR component. Since quantal responding is a characteristic of these schedules, results from drug discrimination studies using training schedules with FR components are of little value in empirically determining whether drug discrimination reflects a quantal or continuous process. Use of variable schedules of reinforcement might be more appropriate because the pattern of responding generated does not preclude results consistent with either of the competing views. Data from the following studies that trained subjects using VI schedules with a concurrent TO for incorrect lever responding were analyzed: Barrett et al. (1982): L-5-hydroxytryptophan versus saline; Smith (1990): diazepam versus pentylenetetrazol; Barrett et al. (1992): amphetamine versus haloperidol; Barrett and Steranka (1983): amphetamine versus haloperidol. In every case, when experimental conditions produced a group mean intermediate to that for the training drugs, the distribution of scores for individual animals was normally rather than bimodally distributed.

Tolerance, withdrawal, and supersensitivity to dopamine mediated cues in a drug-drug discrimination

Rats were trained to discriminate between 0.25 mg/kg amphetamine (AMPH) and 0.03 mg/kg haloperidol (HAL) in a two-lever drug discrimination task. In order to test for a drug-induced withdrawal state, animals were assigned to one of three chronic treatment groups and given injections of AMPH, HAL, or distilled water (DW) for 10 consecutive days. Subjects from each treatment condition were then tested at 24, 48, or 72 h after the final injection. At the 24 h retest interval, subjects injected with AMPH responded as though administered an acute dose of HAL (0.028 mg/kg) and subjects injected with chronic HAL responded as though administered an acute dose of AMPH (0.15 mg/kg). By 72 h choice behavior had returned to pretreatment values. To determine whether the rebound observed after 10 days of drug treatment was present after a single injection, independent groups of subjects were injected with single doses of either 10 mg/kg AMPH or 1.0 mg/kg HAL and then retested from 4 h to 48 h later. Single doses of both AMPH and HAL produced significant rebounds that peaked between 20 h (AMPH) and 24 h (HAL) following administration. In a third experiment, animals were tested with or without acute doses of drug following pretreatment with either HAL or AMPH. Receptor supersensitivity accounts for the tolerance observed to HAL 24 h after treatment with 1.0 mg/kg HAL, whereas receptor subsensitivity accounts for the tolerance observed 20 h after treatment with 10 mg/kg AMPH.

Rebound cue state following a single dose of haloperidol

It has been reported that chronic administration of haloperidol produces an amphetamine-like rebound cue state. The experiments reported here were designed to assess whether a similar rebound phenomenon would result from a single dose of haloperidol. Rats were trained to discriminate .5 mg/kg amphetamine from distilled water. Five groups were formed to allow testing of haloperidol's effect at 12, 18, 24, 30, and 36 hr postinjection. Each animal was given 0, .5, 1.0, and 1.5 mg/kg haloperidol at its appropriate injection time in a counterbalanced fashion with one week between each test. A shift in the dose-response function of amphetamine that occurred during these weeks, however, precluded appropriate analysis of haloperidol's effects. Given this result, a second experiment was conducted using a between-subjects design. Half of the animals were injected with 1 mg/kg haloperidol 23 hr prior to testing, whereas the others were injected with distilled water. When tested, the haloperidol group responded 33% of the time on the amphetamine-correct lever, whereas the control group responded at 20%. The observation of posthaloperidol rebound in the between-subjects study and the failure to find significant temporal patterns of rebound phenomena using a within-subjects design have both theoretical and methodological importance.

Conditioned opponent processes in the development of tolerance to psychoactive drugs

1. Tolerance may involve classical conditioning processes. A conditioned drug-opponent response is thought to increase with drug exposures and summate with the unconditioned response to the drug - resulting in tolerance. 2. Support for this theory comes from reports that tolerance exhibits many basic features of classical conditioning phenomena. 3. Problems for this account of tolerance arise from the fact that some, but not all, empirical attempts to demonstrate conditioned opponent responses have failed. 4. It has consequently been suggested that tolerance might best be conceptualised in terms of theories of habituation, which do not involve opponent processes. 5. The controversy over the role of opponent processes is theoretically important, because conditioned opponent processes are believed to be involved in dependence. Resolution of the controversy requires that parametric studies are conducted, in which the optimal conditions for obtaining conditioned tolerance are defined, and an efficient "model system" developed for analysis of the possible role of opponent processes in tolerance. 6. Research in this area highlights the importance of interdisciplinary studies of tolerance and dependence.

Failure of SCH 23390 to function as a discriminative stimulus in rats

Four rats were studied in a two-lever, food-reinforced drug discrimination paradigm using the Dl dopamine antagonist SCH 23390 (0.03 mg/kg, IP, 30 minutes prior to the session) and saline as the training stimuli. After at least 100 training sessions there was no evidence of stimulus control over responding by SCH 23390 in 3 of the 4 rats, and only briefly in the fourth. On the other hand, food delivery exerted control over behavior indicating that SCH 23390 did not disrupt control of behavior by the reinforcing stimulus. An increase in training dose to 0.06 mg/kg for an additional 12 sessions did not improve discriminative accuracy although this dose reduced rate of responding to an extent that made further training using 0.06 mg/kg untenable. The results provide no evidence of stimulus control of behavior by SCH 23390 and suggest that SCH 23390 does not function as a discriminative stimulus in rats.