Context determines the type of sensitized behaviour: a brief review and a hypothesis on the role of environment in behavioural sensitization.

Behavioural sensitization to psychostimulants may develop context-dependency in certain circumstances. Animals given a stimulant repeatedly in a test cage but not in other environments may show enhanced drug-induced behaviour in the test cage. Conditioning mechanisms have been claimed to be responsible for these phenomena. However, several recent findings are not properly accounted for by conditioning. In addition, growing evidence supports the hypothesis that behavioural sensitization reflects neural changes induced by repeated exposure to psychostimulants (the pharmacological hypothesis). However, the pharmacological hypothesis itself fails to account for environmental influences. In this paper, we propose a hypothesis on the role of environment that is complementary to the pharmacological hypothesis. According to our hypothesis, environment does not have a causal role in the development of sensitization, but it modifies the mode of expression of the sensitized behaviour. Sensitization primarily reflects a neuroadaptive change induced by repeated exposure of the neural system to psychostimulants. However, psychostimulants are known to induce different behaviours in different environments. Therefore, repeated administration of a psychostimulant in different environments would result in augmentation of different behaviours. Our hypothesis potentially accommodates various previous observations. We briefly review the literature and present our hypothesis.