THE DEVELOPMENT OF SENSITIZATION TO THE PSYCHOMOTOR STIMULANT EFFECTSOF AMPHETAMINE IS ENHANCED IN A NOVEL ENVIRONMENT

Two experiments were designed to assess the effect of a ''novel'' envi ronment on the development of sensitization to the psychomotor activat ing effects of d-amphetamine. In the first experiment, rats with a uni lateral 6-hydroxydopamine lesion of the mesostriatal dopamine system r eceived ten daily injections of amphetamine (2 mg/kg), either in their home cages or in novel test cages. The home and novel cages were phys ically identical (cylindrical transparent buckets), but one group live d and were tested in these cages, whereas the other group was transpor ted from the stainless steel hanging cages where they lived to these n ovel test cages, for each test session. The first injection of ampheta mine produced significantly more rotational behavior in animals tested in a novel environment than in animals tested at home. In addition, a nimals tested in a novel environment showed greater sensitization than animals tested at home, so the difference between the two groups was even more pronounced following the Last injection. In a second experim ent, locomotor activity was quantified in rats that received ten injec tions of either saline or 1.5 mg/kg amphetamine, in their home cages o r in a physically identical novel environment. Again, there was a sign ificantly greater locomotor response to the first injection of ampheta mine, and greater sensitization, in animals tested in a novel environm ent than in animals tested at home. These data indicate that environme ntal factors can exert a large effect on the susceptibility to sensiti zation, and mechanisms by which this may occur are discussed.