Behavioural sensitization in addiction, schizophrenia, Parkinson's disease and dyskinesia.

Incentive learning takes place when dopaminergic neurons are activated, usually by rewards. As a result, previously neutral stimuli associated with reward acquire incentive salience and thus the ability to elicit approach or other responses in the future. Incentive learning is assumed to underlie psychostimulant-induced context-dependent sensitization that may play a prominent role in the development of addiction, in dyskinesia, and in amphetamine-induced psychosis. Assuming that these pathological states are due to the gradual process of sensitization, the effects of therapeutics might be manifested as a gradual desensitization. This assumption could explain the delay between onset of cellular effects of drugs (e.g., dopamine receptor blockade) and the improvement in symptoms (e.g., decreases in psychotic symptoms). Reduced dopamine activity results in behavioural changes that are opposite to psychostimulant-induced sensitization, i.e., rewarded behaviours decline in an extinction-like fashion despite the presence and consumption of rewards. We show here that also non reward-related behaviour, i.e., motor activity and catalepsy, follows the same rules: motility is not switched off by dopamine receptor blockade or by 6-hydroxydopamine lesions, but shows a test-to-test extinction-like decline. Thus, psychostimulant-induced sensitization and dopamine-deficiency induced decline of behaviour follows similar rules but in opposite directions.