The atypical antipsychotic risperidone is often prescribed to pediatric patients with neuropsychiatric disorders, though its effects on the developing brain remain unclear. Accordingly, we studied the effects of repeated treatment of risperidone on dopamine receptors in brain regions of juvenile rat. Levels of dopamine receptors (D 1 , D 2 , D 3 , D 4 ) in forebrain regions of juvenile rats were quantified after 3 weeks of treatment with three different doses of risperidone (0.3, 1.0 and 3.0 mg/kg) and compared findings to those in adult rats treated with risperidone (3.0 mg/kg/day) previously. Risperidone (at 1.0 and 3.0 mg/kg/day) increased levels of D 1 receptors in nucleus accumbens and caudate-putamen of juvenile, but not adult rats. Conversely, all three doses of risperidone dosedependently increased D 2 labeling in medial prefrontal cortex and hippocampus, and D 4 receptor in nucleus accumbens, caudate-putamen and hippocampus of juvenile animals as well as in adults. Only the high dose of risperidone (3.0 mg/kg) increased D 2 receptors in caudate-putamen in both juvenile and adult brain. D 3 receptors were not altered by risperidone in any brain region at any dose or age. The findings indicate dose-dependent effects of risperidone on dopamine receptors in developing animals, and that juvenile animals are more sensitive than adults to the cerebral effects of risperidone.

Working memory is regulated by neurotransmitters in prefrontal cortex (PFC), including dopamine and norepinephrine. Previous studies of dopamine function in working memory have focused on the D 1 and D 2 receptors, with most evidence suggesting a dominant role for the D 1 receptor. Since the dopamine D 4 receptor is highly expressed in PFC, we hypothesize that it may also contribute to working memory. To test this hypothesis, we examined behavioral effects of L-745,870, a highly selective, centrally active, D 4 antagonist, using a delayed alternation task in rats. Task performance was dose-dependently affected by the D 4 antagonist, depending on individual baseline functional status of working memory. In rats with good baseline performance, the D 4 antagonist had no effects at low doses, whereas high doses disrupted working memory. In rats with poor baseline working memory, the D 4 antagonist significantly improved working memory at low doses, and higher doses were not distinguishable from vehicle controls. Effects of the D 4 antagonist among poor performers were most robust when task demand for working memory was high, with lesser effects at lower demand level, suggesting that such effects were selective for working memory. The present findings indicate a significant role of the D 4 receptor in working memory, and suggest innovative, D 4 -based, treatment of cognitive deficits associated with neuropsychiatric disorders.