Dopaminergic mechanisms mediating the long-term expression of locomotor sensitization following pre-exposure to morphine or amphetamine

The role of dopaminergic mechanisms in opiate- and psychostimulant- induced long-term locomotor sensitization was investigated. To that aim, rats were behaviourally sensitized with morphine or amphetamine and 3 weeks after cessation of treatment challenged with various direct and indirect dopamine agonists. Both morphine- and amphetamine-pretreated rats displayed sensitization of the locomotor effects of amphetamine, cocaine, and the selective dopamine reuptake inhibitor GBR-12909. Sensitization of the locomotor stimulant effects of the dopamine D2/D3 receptor agonist quinpirole was observed in amphetamine- but not morphine-pretreated rats. In contrast, morphine-, but not amphetamine-pretreated rats appeared hyposensitive to the locomotor inhibitory effects of a low, presumably D2- autoreceptor selective, dose of quinpirole. Neither pretreatment induced sensitization to the dopamine D1/D2 agonist apomorphine or the dopamine D1 agonist SKF-82958. In fact, the locomotor stimulant effects of SKF-82958 appeared to be decreased in animals pre-exposed to amphetamine. These results suggest that functional changes in presynaptic dopamine release mechanisms represent common neuroadaptations involved in the long-term expression of morphine- and amphetamine-induced locomotor sensitization. Presynaptic dopamine D2 and postsynaptic D2 and/or D3 receptors are differentially involved in the expression of morphine- and amphetamine-induced locomotor sensitization. In a parallel study, we report that all of the drugs that elicited sensitized locomotor responses in morphine- or amphetamine- pretreated rats caused reinstatement of previously extinguished heroin- or cocaine-seeking behaviour, respectively. Taken together, these data suggest a marked relationship between drug-seeking behaviour and drug sensitization

Trait Impulsivity Predicts Escalation of Sucrose Seeking and Hypersensitivity to Sucrose-Associated Stimuli

Poor impulse control has been associated with compulsive drug seeking and an enhanced risk of relapse, suggesting that impulsivity is causally related to addiction proneness and relapse vulnerability. However, whether this association is specific to drugs of abuse or whether heightened impulsivity relates to a general increase in sensitivity to rewards and reward-associated stimuli is unknown. To address this issue, the authors selected rats on the basis of individual differences in impulsive action in the 5-choice serial reaction time task, after which they were subjected to an operant sucrose self-administration paradigm. High-impulsive rats displayed a progressive increase in responding on the active hole (including responses emitted during the time-out period) in comparison with low-impulsive rats, which reflects escalation of sucrose-seeking behavior. Once sucrose and sucrose-associated stimuli were omitted (extinction training), nose-poke responding ceased rapidly, an effect that was independent of impulsivity level. In contrast, on reintroduction of sucrose-associated stimuli, sucrose seeking was successfully reinstated in high-impulsive but not in low-impulsive rats. Collectively, the results suggest that impaired response inhibition is associated with enhanced responsiveness to reward-associated stimuli. As such, elevated impulsivity might constitute a risk factor for the initiation and maintenance of addictive behaviors

A single exposure to morphine induces long-lasting behavioural and neurochemical sensitization in rats

Repeated exposure to drugs of abuse causes persistent behavioural sensitization and associated adaptations in striatal neurotransmission, which is thought to play an important role in certain aspects of drug addiction. Remarkably, even a single exposure to psychostimulant drugs such as amphetamine or cocaine can be sufficient to elicit long-lasting sensitization. The present study was designed to evaluate whether long-lasting behavioural and neurochemical sensitization can also be evoked by a single exposure to morphine, an opiate drug of abuse. Rats were pretreated once with morphine (2, 10 or 30 mg/kg). Three weeks later, the locomotor effects of morphine and amphetamine, as well as the electrically evoked release of [3H]dopamine and [14C]acetylcholine from slices of nucleus accumbens and caudate-putamen, was assessed. In morphine-pretreated rats, the psychomotor effects of morphine and amphetamine were sensitized. In addition, the electrically evoked release of [3H]dopamine and [14C]acetylcholine was augmented in slices of nucleus accumbens and caudate-putamen from morphine-pretreated animals. Although the sensitization of the locomotor effect of morphine was less profound than previously observed after repeated intermittent morphine treatment, the enduring behavioural and neurochemical consequences of a single and repeated intermittent morphine treatment appear to be highly comparable. We therefore conclude that a single exposure to morphine induces long-lasting behavioural sensitization and associated neuroadaptations

Involvement of dopamine D1 and D2 receptors in the nucleus accumbens core and shell in inhibitory response control

Rationale: Impaired inhibitory control over behavior is a key feature in various psychiatric disorders, and recent studies indicated an important role for dopamine D1 and D2 receptors and the nucleus accumbens (Acb) in this respect. Objective: The present experiments were designed to study the role of dopamine D1 and D2 receptors in the Acb in inhibitory response control. Methods: Rats were trained in a five-choice serial reaction time task and received bilateral infusions into the Acb core or shell of either SCH 23390 or eticlopride (representing selective dopamine D 1 and D2 receptor antagonists, respectively). Subsequently, the effects of systemic amphetamine on inhibitory response control were examined. Results: Eticlopride into either the Acb core or shell did not affect premature responding, a measure for inhibitory response control, but increased reaction time and errors of omission. In contrast, SCH 23390 into both regions reduced premature responding, slightly improved attentional performance in the core and increased errors of omission in the shell. Amphetamine robustly increased premature responding which was dose-dependently blocked by eticlopride in the Acb core and attenuated by eticlopride in the shell. In addition, amphetamine slightly decreased accuracy and reaction time, and these effects were inhibited by eticlopride in both regions. SCH 23390 infusion into the Acb core or shell did not alter amphetamine's effects. Conclusion: Our data provide evidence for the involvement of dopamine D1 and D2 receptors in the Acb core and shell in inhibitory response control and attentional performance

Amphetamine and Cocaine Suppresses Social Play Behavior in Rats Through Distinct Mechanisms

Rationale: Social play behavior is a characteristic form of social behavior displayed by juvenile and adolescent mammals. This social play behavior is highly rewarding and of major importance for social and cognitive development. Social play is known to be modulated by neurotransmitter systems involved in reward and motivation. Interestingly, psychostimulant drugs, such as amphetamine and cocaine, profoundly suppress social play, but the neural mechanisms underlying these effects remain to be elucidated. Objective: In this study, we investigated the pharmacological underpinnings of amphetamine- and cocaine-induced suppression of social play behavior in rats. Results: The play-suppressant effects of amphetamine were antagonized by the alpha-2 adrenoreceptor antagonist RX821002 but not by the dopamine receptor antagonist alpha-flupenthixol. Remarkably, the effects of cocaine on social play were not antagonized by alpha-2 noradrenergic, dopaminergic, or serotonergic receptor antagonists, administered either alone or in combination. The effects of a subeffective dose of cocaine were enhanced by a combination of subeffective doses of the serotonin reuptake inhibitor fluoxetine, the dopamine reuptake inhibitor GBR12909, and the noradrenaline reuptake inhibitor atomoxetine. Conclusions: Amphetamine, like methylphenidate, exerts its play-suppressant effect through alpha-2 noradrenergic receptors. On the other hand, cocaine reduces social play by simultaneous increases in dopamine, noradrenaline, and serotonin neurotransmission. In conclusion, psychostimulant drugs with different pharmacological profiles suppress social play behavior through distinct mechanisms. These data contribute to our understanding of the neural mechanisms of social behavior during an important developmental period, and of the deleterious effects of psychostimulant exposure thereon

Morphine-induced long-term sensitization to the locomotor effects of morphine and amphetamine depends on the temporal pattern of the pretreatment regimen

The development of behavioural sensitization is thought to depend on the dose and temporal pattern of drug treatment. Previous studies have shown that two distinct morphine pretreatment regimens cause different long-term neuroadaptations in rat striatum. Therefore, in the present study the ability of these pretreatment regimens to induce long-term behavioural sensitization was investigated. One pretreatment regimen, termed 'chronic', consisted of three daily injections, for 5 days, with escalating doses (10 50 mg/kg) of morphine, and the other, termed 'intermittent', of 14 daily injections with morphine (10 mg/kg). Both intermittent and chronic morphine pretreatment caused sensitization to the locomotor effects of morphine, 3 weeks post- treatment, although the former induced a far greater level of sensitization. Moreover, 3 weeks post-treatment, intermittent, but not chronic, morphine pretreatment induced cross-sensitization to the locomotor stimulant effects of amphetamine. Behavioural sensitization following intermittent morphine pretreatment was clear-cut both 1 day and 3 weeks post-treatment, while after 9 weeks, the locomotor effects of morphine were still slightly augmented. It is concluded that intermittent morphine pre-treatment is far more effective in inducing long-term behavioural sensitization than chronic morphine pre- treatment

A single exposure to amphetamine is sufficient to induce long-term behavioral, neuroendocrine, and neurochemical sensitization in rats

Repeated treatment with psychostimulant drugs causes long-lasting behavioral sensitization and associated neuroadaptations. Although sensitization induced by a single psychostimulant exposure has also been reported, information on the behavioral and neurochemical consequences of a single psychostimulant exposure is sparse. Therefore, to evaluate whether behavioral sensitization evoked by single and repeated psychostimulant pretreatment regimens represent the same neurobiological phenomenon, the time-dependent expression of behavioral, neurochemical, and neuroendocrine sensitization after a single exposure to amphetamine was investigated in rats. A single exposure to amphetamine (5 mg/kg, i.p.) caused context- independent sensitization of the Iocomotor effects of amphetamine, which intensified over time. Thus, sensitization to amphetamine was marginal at 3 d after treatment and more evident after 1 week, whereas 3 weeks after treatment, profound sensitization, as well as cross-sensitization, to cocaine was observed. Amphetamine pretreatment caused an increase in the electrically evoked release of [3H]dopamine from nucleus accumbens, caudate putamen, and medial prefrontal cortex slices and of [14C]acetylcholine from accumbens and caudate slices. The hyperreactivity of dopaminergic nerve terminals appeared to parallel the development of locomotor sensitization, i.e., whereas hyperreactivity of accumbens dopaminergic terminals increased between 3 d and 3 weeks after treatment, the hyperreactivity of medial prefrontal dopaminergic terminals decreased. Pre-exposure to amphetamine also sensitized the hypothalamus-pituitary-adrenal axis response to amphetamine at 1 and 3 weeks, but not at 3 d after treatment. Because these data closely resemble those reported previously for repeated amphetamine pretreatment, it is concluded that a single exposure to amphetamine is sufficient to induce long- term behavioral, neurochemical, and neuroendocrine sensitization in rats