Enduring Changes in Brain and Behavior Produced by Chronic Amphetamine Administration: A Review and Evaluation of Animal Models of Amphetamine Psychosis

Some people who repeatedly use stimulant drugs, such as amphetamine (AMPH), develop an AMPH-induced psychosis that is similar to paranoid schizophrenia. There has been, therefore, considerable interest in characterizing the effects of chronic stimulant drug treatment on brain and behavior in non-human animals, and in developing an animal model of AMPH psychosis. A review of this literature shows that in non-human animals chronic AMPH treatment can produce at least two different syndromes, and both of these have been proposed as animal models of AMPH psychosis. The first syndrome is called ‘AMPH neurotoxicity’, and is produced by maintaining elevated brain concentrations of AMPH for prolonged periods of time. AMPH neurotoxicity is characterized by what has been termed ‘hallucinatory-like’ behavior, which occurs in association with brain damage resulting in the depletion of striatal DA and other brain monoamines. The second syndrome is called ‘behavioral sensitization’, and is produced by the repeated intermittent administration of lower doses of AMPH. Behavioral sensitization is characterized by a progressive and enduring enhancement in many AMPH-induced behaviors, and is not accompanied by brain damage or monoamine depletion. It is argued that the changes in the brain and behavior associated with the phenomenon of behavioral sensitization provide a better ‘model’ of AMPH psychosis than those associated with AMPH neurotoxicity. Much of the review involves a critical analysis of hypotheses regarding the biological basis of behavioral sensitization. Research on this question has focused on mesotelencephalic DA systems, and suggestions that behavioral sensitization is accompanied by: (1) an increase in postsynaptic DA receptors (2) an increase in DA synthesis (3) an increase in DA utilization and/or release (4) a decrease in DA autoreceptors, are evaluated. It is concluded that there is not convincing evidence for an increase in postsynaptic DA receptors or in DA synthesis in animals sensitized to AMPH. In contrast, there is strong evidence to support the notion that behavioral sensitization is due to enhanced mesotelencephalic DA release, especially upon re-exposure to the drug. The evidence that this enhancement in DA release is due to autoreceptor subsensitivity was found to be equivocal, and therefore other hypotheses should be entertained. Lastly, evidence is discussed in support of the idea that behavioral sensitization is not unique to the psychopharmacology of stimulant drugs, but may be produced by many environmental stimuli that directly or indirectly activate brain catecholamine systems. For example, there are many studies showing that AMPH and stress are to some extent interchangeable in producing both behavioral sensitization and long-term changes in brain DA systems. It is concluded that sensitized animals may be hyperresponsive to any stimulus that activates brain catecholamine systems, and that the effects of sensitization are not obvious in the absence of such stimuli. This may be related to the fact that psychosis only tends to recur in former AMPH addicts following re-exposure to AMPH or stress, and that stress is considered a precipitating factor in psychiatric disorders thought to involve brain catecholamine dysfunction.