Effects of repeated administration of low doses of apomorphine in three behavioural models in the rat.

A low dose of the dopamine (DA) receptor agonist apomorphine (APO 0.05 mg/kg) was given repetitively and the effects were tested in three different behavioural models: reduction of spontaneous locomotion, induction of yawning and decrease in water intake in water-deprived animals. The APO-induced suppression of exploration and decrease in water intake were not affected by a previous injection of APO given 1 or 3 hours before the test dose of APO. There was a small, but significant, decrease in the induction of yawning by a previous dose of APO given 1 hour or 30 min before the test dose. However, pretreatment with APO 3 hours before the test dose did not diminish the yawning response. It is suggested that the dopaminergic mechanisms mediating APO induced yawning are different from those mediating decrease in water intake and suppression of exploration. The results are also discussed in relation to the proposed efficiency of low doses of DA agonists in the treatment of various neurological and psychiatric disorders.