Repeated treatments with 7-OH-DPAT: Context-independent behavioral sensitization and conditioned hyperactivity

The primary objective of this study was to determine whether the expression of behavioral sensitization to the putative dopamine D-3 receptor agonist 7-OH-DPAT is context dependent. Three groups (n = 8 each) of male Wistar ra ts (250-350 g) were given nine injections (at 48-h intervals) of 7-OH-DPAT (1.0 mg/kg, SC) or vehicle 15 min before and after activity testing. The pa ired group received 7-OH-DPAT before activity testing and vehicle after tes ting. The unpaired group received vehicle before and 7-OH-DPAT after testin g, and the vehicle control group received two vehicle injections. Locomotor activity was measured in photocell arenas for 2 h. After the first seven s essions, all rats were tested for activity following a vehicle injection to test for possible conditioning effects. Prior to the 11th session, all rat s were given a challenge injection of 7-OH-DPAT (1.0 mg/kg, SC) to test for sensitization. Major findings were as follows: (a) the 7-OH-DPAT/paired gr oup displayed a progressively greater increase in locomotor activity with r epeated treatments; (b) the 7-OH-DPAT/paired group was significantly more a ctive than either the vehicle control group or the 7-OH-DPAT/unpaired group during the vehicle test session; and (c) after the 7-OH-DPAT challenge inj ection, the paired and unpaired 7-OH-DPAT groups were significantly, and eq ually, more active than the vehicle control group. In contrast to previous findings with the D-2-type dopamine agonists bromocriptine and quinpirole, these results suggest that the expression of behavioral sensitization to 7- OH-DPAT is not context dependent. Moreover, these results suggest that the apparent conditioned hyperactivity and context dependency often observed af ter repeated dopamine agonist treatments may not be related to the same ass ociative and/or nonassociative mechanisms. (C) 2000 Elsevier Science Inc.