Prior experience with bromocriptine in the home cage attenuates locomotor sensitization in rats

The dopamine D(2) agonist bromocriptine has a long lasting, biphasic effect on locomotion, causing decreased activity during the first 60-90 min after the injection, followed by increased activity that peaks 90-180 min after the injection. In Experiment 1, repeated injections of 5.0 mg/kg bromocriptine produced rapid development of locomotor sensitization when rats were placed in activity chambers for 90 min immediately following each injection. Once sensitization had occurred, expression of sensitization was enhanced when the rats were tested for 90 min beginning 90 min after the injection. In contrast, when rats were placed in the activity chambers for 90 min after spending the first 90 min following each injection of bromocriptine in their home cage, the onset of locomotor sensitization was delayed and expression of sensitization was reduced. In Experiment 2, this inhibition of sensitization did not occur when rats spent the first 90 min following each injection of bromocriptine in a cage that was identical to their home cage, but was not their home cage. Thus, spending the first 90 min after the injection of bromocriptine in the home cage somehow inhibits the development of locomotor sensitization during the subsequent 90 min of each session. These data are consistent with other data indicating that repeated injections of drugs in the home cage often fail to produce the locomotor sensitization that is observed when these injections occur in novel cages.