Characterizing withdrawal in rats following repeated drug administration using an amphetamine-vehicle-haloperidol drug discrimination

Distinct cognitive and discriminative stimulus effects of ketamine enantiomers in rats

Although (S)-ketamine was approved for use in treatment-resistant depression in 2019, new preclinical findings suggest that (R)-ketamine might produce better efficacy and tolerability relative to (S)-ketamine. Here we evaluated the effects of (R)-, (S)-, and (R,S)-ketamine on executive functions as measured in the attentional set shifting task (ASST) and on their discriminative stimulus effects in rats. Earlier data demonstrated that cognitive flexibility is compromised by (R,S)-ketamine, but the effects of enantiomers in rats are unknown. Separate cohorts of rats were tested in ASST and trained to discriminate either (R,S)-ketamine, (S)-ketamine, or (R)-ketamine (all at 10 mg/kg) from saline; in order to maintain the discrimination, a higher (R)-ketamine dose (17.5 mg/kg) was subsequently instituted. In ASST, all three forms increased the trials to criterion measure at reversal learning and extra-dimensional set-shifting phases. However, in contrast to (R)- and (S)-ketamine, (R,S)-ketamine prolonged the mean time to complete a single trial during early stages, suggesting increased reaction time, and/or unspecific side-effects related to motor or motivational impairments. In the drug discriminations, all rats acquired their respective discriminations between drug and saline. In (R,S)-ketamine-trained rats, (R)-ketamine and (S)-ketamine only partially substituted for the training dose of (R,S)-ketamine. Further, (R)-ketamine did not fully substitute in rats trained to (S)-ketamine. The data suggest more serious cognitive deficits produced by (R,S)-ketamine than its enantiomers. Furthermore, (R,S)-ketamine and its isomers share overlapping but not isomorphic discriminative stimulus effects predicting distinct subjective responses to (R)- vs. (S)-ketamine in humans.

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.

Time-dependent effects of amphetamine on feeding in rats

Following administration of a moderate dose of amphetamine, rats appear to pass through a sequence of physiological/psychological states, including stimulant and depressant states. The present research evaluated whether these states could be inferred from time-dependent changes in feeding-related measures. Male rats were housed in individual stations (light-dark 12-12 h, free access to water) where, at 3-h intervals, they could respond for food for 1 h. The work requirement was fixed ratio 1, and each lever press produced six 94-mg food pellets. When the pattern of responding for food stabilized across the light-dark cycle, a series of 6 or 7 tests was run. During each test, rats received a saline treatment (1.0 ml/kg, subcutaneously) followed by a 48-h monitoring period, and then they received an amphetamine treatment (2.0 mg/kg, subcutaneously) followed by a 72-h monitoring period. Different groups were treated at either light onset or light offset. Lever presses and head-in-feeding-bin responses were monitored throughout these tests. Administration of amphetamine at light onset and at light offset produced cumulative food intake functions having four regions: post-treatment hours 1-6 (hypophagia), 7-12 (normal intake), 13-27 (hypophagia), and 28 and beyond (normal intake). The sequence, duration, and quality of the amphetamine-induced changes in food intake resembled those formerly seen in cue state and activity, and provided further evidence of a transient withdrawal state 20-24 h post-amphetamine treatment.

An activity indicator of acute withdrawal depends on amphetamine dose in rats

A moderate dose of amphetamine (AMPH) produces hypoactivity around 20 h post-administration. This hypoactivity may be an indicator of an acute withdrawal state. The purpose was to see how AMPH doses affected the expression of this hypoactivity and, by inference, AMPH-induced acute withdrawal. Rats were housed in individual open fields, with free access to food and water. Light-dark cycles were scheduled such that drug-elicited patterns could be readily detected. Animals first received a series of eight control treatments, and then a series of 10 experimental treatments spaced at 33-h intervals. Different experimental treatment groups received saline, 1.0 mg/kg, 2.0 mg/kg, or 4.0 mg/kg AMPH. The effects of these treatments on 33-h patterns of locomotor activity were observed. Control treatments produced no systematic time-dependent changes in activity beyond the first hour post-treatment. All doses of AMPH produced typical short-term effects: They markedly increased locomotion and/or stereotypy during the first 3 to 6 h post-treatment. Acute and chronic administrations of the 2.0 and 4.0 mg/kg doses also produced similar changes in longer term activity patterns: They produced hypoactivity 20 h later, followed by a recovery of activity around hour 25 post-treatment. The timing of amphetamine-induced hypoactivity and acute withdrawal may be independent of dose over a wide range of doses. Time-dependent changes in AMPH-induced state may influence motivation and drug-related assessments. The methodology described here may provide an easy and rapid way to investigate the determinants of AMPH-induced hypoactivity and acute withdrawal.

Dopamine manipulations limited to preexposure are sufficient to modulate latent inhibition

Four experiments are reported that demonstrated that dopamine (DA) transmission is involved in the acquisition of latent inhibition (LI) of a conditioned taste aversion. LI refers to weaker conditioning as a consequence of nonreinforced preexposure (PE) of the future conditioned stimulus. Although it is known to depend on DA transmission during the conditioning phase, it is usually thought that the cognitive processes involved in the establishment of LI (during the PE phase) are DA independent. Either amphetamine (AMPH; 0.5 or 1.0 mg/kg) or haloperidol (HAL; 0.1 mg/kg) were injected before 1 or all of the 3 PE sessions. AMPH blocked the acquisition of LI if it was injected before each or before only the last PE session and HAL potentiated LI.

Psychostimulant withdrawal as an inducing condition in animal models of depression

A large body of evidence indicates that the withdrawal from high doses of psychostimulant drugs in humans induces a transient syndrome, with symptoms that appear isomorphic to those of major depressive disorder. Pharmacological treatment strategies for psychostimulant withdrawal in humans have focused mainly on compounds with antidepressant properties. Animal models of psychostimulant withdrawal have been shown to demonstrate a wide range of deficits, including changes in homeostatic, affective and cognitive behaviors, as well as numerous physiological changes. Many of these behavioral and physiological sequelae parallel specific symptoms of major depressive disorder, and have been reversed by treatment with antidepressant drugs. These combined findings provide strong support for the use of psychostimulant withdrawal as an inducing condition in animal models of depression. In the current review we propound that the psychostimulant withdrawal model displays high levels of predictive and construct validity. Recent progress and limitations in the development of this model, as well as future directions for research, are evaluated and discussed.

A 'crash' course on psychostimulant withdrawal as a model of depression

Most drugs of abuse generate diverse behavioral and neurochemical effects in mammals. However, one feature common to many such drugs is the phenomenon of the withdrawal syndrome that results from termination of drug administration. Early drug withdrawal, often referred to as the 'crash' phase in humans, is characterized by adverse psychological and/or somatic symptoms. Withdrawal from psychostimulant drugs precipitates a transient and primarily psychological condition that bears remarkable similarity to the symptoms of major depressive disorder in humans. Rodent paradigms of psychostimulant withdrawal faithfully model the human condition. Associated behavioral deficits in these animals can be reversed by treatments with antidepressant properties, suggesting that psychostimulant withdrawal might provide the basis for an animal model of depression. Current advances and limitations in the development of this model, together with recent evidence that psychostimulant withdrawal in rodents can be used to screen for novel, rapidly acting antidepressant treatments, are discussed.

Effects of training dose on amphetamine drug discrimination: dose-response functions and generalization to cocaine

Rats were trained to discriminate one of three doses of amphetamine (AM), 0.5, 1, or 2 mg/kg, from vehicle (VEH) in a two-lever, food-reinforced, drug-discrimination task. The purpose of the study was to investigate the nature of the shift of the dose-response curve and generalization to cocaine (COC) as a function of training dose. In order to preclude potential differences among the groups in stimulus control, the three training-dose groups were required to perform the discrimination at high and equivalent levels of accuracy. The shift of the dose-response functions to the right as a function of increasing training dose was not parallel. The slope decreased as training dose increased. There was a dose-dependent increase in AM lever responding to test doses of COC that tended to be affected by training dose. The results suggest that proper evaluation of training-dose effects requires that groups be trained to equivalent levels of stimulus control.