Dopamine D1/D2 antagonist combinations as antagonists of the discriminative stimulus effects of cocaine

Drug discrimination learning: Interoceptive stimulus control of behavior and its implications for regulated and dysregulated drug intake

Drug discrimination research has generated rich evidence for the capacity of interoceptive drug stimuli to control behavior by serving as discriminative cues. Owing to its neuropharmacological specificity, drug discrimination learning has been widely used to characterize the stimulus effects and neuropharmacological underpinning of drugs. Apart from such utility, discriminative drug stimuli may help regulate drug use by disambiguating conditioned associations and post-intake outcomes. First, this review summarizes the evidence supporting interoceptive regulation of drug intake from the literature of exteroceptive discriminative control of drug-related behavior, effects of drug priming, and self-titration of drug intake. Second, an overview of interoceptive control of reward-seeking and the animal model of discriminated goal-tracking is provided to illustrate interoceptive stimulus control of the initiation and patterning of drug intake. Third, we highlight the importance of interoceptive control of aversion-avoidance in the termination of drug-use episodes and describe the animal model of discriminated taste avoidance that supports such a position. In bridging these discriminative functions of drug stimuli, we propose that interoceptive drug stimuli help regulate intake by disambiguating whether intake will be rewarding, nonrewarding, or aversive. The reflection and discussion on current theoretical formulations of interoceptive control of drug intake may further scientific advances to improve animal models to study the mechanisms by which interoceptive stimuli regulate drug intake, as well as how alterations of interoceptive processes may contribute to the transition to dysregulated drug use.

Interactions of cocaine with dopamine D₂-like antagonists in squirrel monkeys

RATIONALE

Studies investigating dopamine D₂ receptor antagonism of cocaine's discriminative stimulus effects have resulted in varied effects possibly due to the use of different antagonists, species, and procedures.

OBJECTIVES

The present study sought to further investigate D₂ antagonism of cocaine's discriminative stimulus effects using a variety of D₂ antagonists and multiple doses of the antagonists in combination with cocaine.

METHODS

The benzamide D₂ antagonists, eticlopride, raclopride, and sulpiride, and the butyrophenone D₂ antagonists haloperidol and spiperone were administered alone and in combination with cocaine in squirrel monkeys trained to discriminate cocaine from saline under a fixed-ratio food reinforcement procedure.

RESULTS

All the D₂ antagonists, except haloperidol, antagonized the discriminative stimulus effects of the cocaine training dose. However, only the benzamide D₂ antagonists produced significant rightward shifts in the cocaine discriminative stimulus dose-effect curve and they only did so within a narrow dose range and time after administration. In contrast, the D₂ antagonists failed to antagonize the rate-suppressant effects of cocaine, and in some cases, cocaine appeared to antagonize the rate-suppressant effects of the antagonists.

CONCLUSIONS

The present results suggest (1) that D₂ antagonism of cocaine's discriminative stimulus effects depends critically on the selected antagonist, antagonist dose, and time of administration, as well as how antagonism is assessed (i.e., in terms of effects on training dose or on the cocaine dose-effect curve), (2) that the maximal shift in cocaine's discriminative stimulus dose-effect curve possible with D₂ antagonists under these procedures is ~two- to threefold, and (3) that different effects of cocaine are differentially sensitive to dopamine receptor antagonism.

Brain-derived neurotrophic factor signaling modulates cocaine induction of reward-associated ultrasonic vocalization in rats

Cocaine exhibits high liability for inducing addictive behaviors, but the mechanisms of neuroplasticity underlying the behavioral effects remain unclear. As a crucial mediator of neuroplasticity in diverse functional models, brain-derived neurotrophic factor (BDNF) could contribute to the mechanisms of addiction-related neuroplasticity. Here, we addressed the hypothesis that cocaine increases synaptic dopamine, which induces BDNF protein expression to initiate addiction-related behavior in the rat. An enzyme-linked immunosorbent assay was used to measure BDNF protein expression in rat striatal tissues. For behavioral readout, we used a noninvasive measurement system to measure the emission of 50-kHz ultrasonic vocalization (USV), a response that correlates with electrical brain stimulation and conditioned place preference behavior in rodents. A single injection of cocaine significantly increased BDNF protein expression, but this effect was not further augmented by repeated cocaine administration. A single administration of cocaine elicited significant and dose-related USV responses, and the magnitude of the behavior increased with repeated drug administration. R-(+)-7-Chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH23390), but not raclopride, significantly attenuated cocaine-induced BDNF protein expression, whereas either the D(1)-like or D(2)-like receptor antagonist blocked cocaine-induced USV behavior. Furthermore, significant USV behavior was elicited by the nonselective dopamine agonist, apomorphine, but not by agonists that are selective for D(1)-like or D(2)-like receptors. Intracerebroventricular injection of the neurotrophin TrkB receptor inhibitor, K252a, blocked cocaine-induced USV behavior but not locomotor activity. These results suggest that neurotrophin signaling downstream of dopamine receptor function probably constitutes a crucial link in cocaine induction of USV behavior and may contribute to the mechanisms underlying the development of addiction-related behaviors.

The impact of early environmental rearing condition on the discriminative stimulus effects and Fos expression induced by cocaine in adult male and female rats

RATIONALE

A number of environmental manipulations, including maternal separation (MS), have been shown to alter behavioral responses to drugs of abuse.

OBJECTIVES

This study assessed if MS affected the stimulus and Fos-inducing effects of cocaine.

MATERIALS AND METHODS

In experiment 1, male and female Sprague-Dawley rats were exposed to brief maternal separations (BMS), long maternal separations (LMS), or animal facility rearing (AFR) and then trained as adults to discriminate cocaine (10 mg/kg, intraperitoneally) from saline. Following training, generalization tests to novel doses of cocaine and other dopaminergic compounds were performed. Assessments of variations in training dose pretreatment times were also made. In experiment 2, male and female rats exposed to MS conditions were administered cocaine or saline for 14 days, and Fos expression in the mesolimbic system was measured.

RESULTS

In males, BMS retarded the acquisition of the cocaine discrimination. Generalization to novel doses of cocaine did not differ among rearing conditions, but the training dose cue lasted longer in LMS. Distinct generalization and ED(50) profiles were found between male rearing conditions for all dopamine compounds. While BMS females had higher cocaine ED(50) estimates, no other differences were found in females. LMS males and females, as well as AFR females, had significant increases in Fos expression after cocaine in a region-specific manner. No differences were found with other rearing groups.

CONCLUSION

Early environmental variables altered the stimulus effects (in a sex-dependent manner) as well as the neuronal responsiveness to cocaine, which may be mediated by the dopamine system.

Cocaine-conditioned behavioral effects: a role for habituation processes

Cocaine has potent locomotor stimulant effects in rodents, which seemingly can become conditioned to test environment cues. In two experimental protocols, we measured the effects of cocaine on locomotor activity and grooming behavior, and subsequently tested whether these cocaine effects became conditioned to contextual cues. In the first experiment, three groups of rats received 14 injections of either saline or cocaine (10 mg/kg) paired or unpaired to the test environment. Cocaine increased locomotion and decreased grooming during treatment and on the conditioning test. Over the course of the treatment phase, however, the saline- and cocaine-unpaired groups but not the cocaine paired group developed progressively lower locomotion and higher grooming scores indicative of substantial habituation effects. To examine whether the cocaine may have impaired the acquisition of habituation effects rather than induce a Pavlovian cocaine conditioned response, an additional experiment was conducted in which two additional non-habituation saline and cocaine control groups were added to the experimental design. On a conditioning test, the two non-habituation control groups were equivalent in activity and grooming behavior to the cocaine-paired group. The findings were consistent with a failure by cocaine-paired animals to acquire habituation effects, which could transfer to the non-cocaine state. The connection between cocaine and novelty/habituation may have substantial importance for understanding cocaine effects.

Substance abuse among patients with schizophrenia

Substance use in the general population is a significant public health problem. Problems associated with substance use are aggravated by concomitant psychiatric illness, particularly schizophrenia and schizoaffective disorder. Although there is a general agreement on the need to address this problem, a wide range of opinions exists on exactly what is the best modality. In this article, we provide a brief overview of the etiology and consequences of substance use in individuals with schizophrenia, followed by a more detailed review of pharmacological and psychotherapeutic trends in the treatment of this population. Research studies indicate that, while some evidence supports the self-medication hypothesis, individuals with schizophrenia or schizoaffective disorder frequently use substances for the same reasons and in the same manner as the general population. In the pharmacotherapy section, we briefly discuss the rationale for current medication strategies, their efficacy, and directions for future research. This is followed by an assessment of current psychotherapeutic interventions, their limitations, and potential modifications to improve treatment outcome. The research literature suggests that integrated treatment and well-tailored interventions that take into account psychosocial factors and motivation offer the most promise for the future. More controlled trials are necessary to validate this hypothesis.

Olanzapine attenuates the reinforcing effects of cocaine

The possibility that the atypical neuroleptic olanzapine can antagonize the ability of cocaine to produce both conditioned place preference and self-administration in rats was investigated. Pre-treatment with olanzapine (3.0, 4.5 mg/kg, but not 1.5 mg/kg) significantly attenuated conditioned place preference produced by cocaine (10 mg/kg). However, the higher dose of olanzapine administered alone resulted in conditioned place aversion. Pre-treatment with olanzapine also produced a dose-dependent decrease in cocaine self-administration (0.33 mg/infusion) under a fixed-ratio 2 schedule of reinforcement. Olanzapine produced a similar dose-responsive attenuation in operant responding for food (fixed-ratio 10) suggesting that olanzapine produces a nonspecific decrease in operant behavior. Pre-treatment with 4.5 mg/kg olanzapine significantly attenuated cocaine-induced hyperactivity, whereas lower olanzapine doses had little effect upon cocaine-induced hyperactivity. These results suggest that pre-treatment with olanzapine is capable of blocking the reinforcing effects of cocaine and illustrates the value of using multiple tests of reinforcement when evaluating the pharmacological effects of newer psychotherapeutic agents.