Increased sensitivity to (+)amphetamine self-administered by rats following meso-cortico-limbic dopamine neurone destruction

Abstract

DRUG self-administration (SA) in laboratory animals shows some of the characteristics of intracranial self-stimulation and of other natural reinforcers (for review see ref. 1). Animals will self-administer many of the same drugs abused by human subjects. Recent evidence has shown that (+)amphetamine reinforces behaviour in the same manner as conventional reinforcers1–3 that it acts mainly on dopamine (DA) transmission, and that locomotor activation and stereotypy are mediated by the mesolimbic–mesocortical (A10 group) and nigro-striatal (A9 group) DA systems, respectively^4,5. Using various specific noradrenergic and dopaminergic blocking agents in a (+)amphetamine SA retention paradigm, DA transmission blockade was shown to produce effects similar to reward reduction and reward termination^3,6,7. DA transmission in the frontal-mesolimbic system has been implicated in brain stimulation reward⁸, and the link between self-administration of (+)amphetamine and reward mechanisms, both of which depend on DA transmission, may be related to the craving and early dependence that certain subjects display for addictive drugs. However, the mechanisms which lead to addiction are still unclear, and it is possible that in drug addicts the particularly strong and rapid addicting effect of the drug could be due to a pre-existing imbalance in some homeostatic mechanism. Thus, the use of healthy animals in a steady-state paradigm is not necessarily a good model for the study of the psychological or neurobiological vulnerability to drug addiction acquisition. Because the meso-cortico-limbic DA system may be an essential target for psychostimulant drugs, the lesion of this system could induce disruption of SA behaviour.