Apomorphine sensitization effects: evidence for environmentally contingent behavioral reorganization processes

Apomorphine-induced behavioral sensitization was investigated with a Pavlovian conditioning paradigm. Rats were administered apomorphine (2.0 mg/kg SC) daily for 7 days either paired or unpaired with a 10-min test environment placement. Initially, apomorphine induced hypolocomotion, but by treatment day 5, hyperlocomotion developed. Utilizing a videoimage analysis program which quantitated angular movement, it was determined that the increase in locomotion induced by repeated apomorphine treatment was due to an increase in rotational locomotion. Critically, rotation per se did not increase, but rather wide angle rotation toward the periphery of the test environment increased. Furthermore, a directional bias of rotation developed and stabilized which was unrelated to the animal's initial asymmetry bias. This emergence of a new locomotion pattern in conjunction with hyperlocomotion pointed to the need to reconceptualize behavioral sensitization phenomena into a new framework consistent with a progressive change in behavioral structure. Behavioral reorganization is presented as an alternative formulation to that of behavioral sensitization, as a drug-environment interactive process which is more compatible with the behavioral dynamics that emerge with repeated intermittent dopaminergic psychostimulant drug treatment.