Effects of selective dopamine D1- and D2-type receptor antagonists on the development of behavioral sensitization to 7-OH-DPAT.

The primary objective of this study was to determine whether the development of behavioral sensitization to the putative dopamine D3 receptor agonist 7-OH-DPAT could be prevented by either selective D1-type or D2-type dopamine receptor antagonists. In three experiments, male Wistar rats (250-350 g) were given seven to nine injections (at 48-h intervals) of 7-OH-DPAT (1.0 mg/kg, SC) or vehicle in combination with the D2-type dopamine antagonist eticlopride (0.3 mg/kg, SC), the D1-type dopamine antagonist SCH 23390 (0.1 or 0.2 mg/kg, SC), or vehicle. After the injections, the rats were tested for locomotor activity in photocell arenas for 2 h. In the first two experiments, after seven injections, all rats were tested for activity following vehicle injections to test for possible conditioning effects. In each experiment, after the last pre-exposure session, all rats were given a challenge injection of 7-OH-DPAT (1.0 mg/kg, SC) and tested for activity. Major findings were as follows: a) 7-OH-DPAT treatments produced a progressively greater increase in locomotor activity with repeated treatment; b) concurrent treatment with eticlopride or SCH 23390 (0.1 and 0.2 mg/kg) blocked the acute locomotor-activating effects of 7-OH-DPAT across days; c) eticlopride, but not SCH 23390, completely blocked the development of behavioral sensitization to 7-OH-DPAT. Although the low dose of SCH 23390 (0.1 mg/kg) produced a partial attenuation of sensitization, the higher dose (0.2 mg/kg) of SCH 23390 appeared to augment, rather than block, sensitization to 7-OH-DPAT; d) rats previously treated with SCH 23390 (0.2 mg/kg, but not 0.1 mg/kg) without 7-OH-DPAT displayed a hyperactive response to the 7-OH-DPAT challenge injection; and e) after vehicle injections, rats previously given 7-OH-DPAT, SCH 23390, or eticlopride either alone or in combination were more active than vehicle control rats. These findings suggest that the neurochemical mechanisms mediating the development of behavioral sensitization to 7-OH-DPAT may differ from those of other dopamine D2-type agonists such as quinpirole or bromocriptine. Moreover, these results demonstrate that hyperactivity responses following vehicle injections in drug-pretreated animals do not necessarily reflect conditioning mechanisms.