Dopamine D2 receptor mechanisms in the expression of conditioned fear

The increase in the startle reflex in the presence of a stimulus that has been previously paired to foot shock is taken as an index of anxiety and named fear potentiated startle (FPS). Freezing behavior, defined as a cessation of all observable movements except those associated with respiration, has also been used as an index of fear in rats. Recently, a growing body of evidence has suggested that dopaminergic mechanisms are significant for different aspects of affective memory, namely its formation, expression or retrieval. However, the results of studies that have directly examined the ability of the dopaminergic system to influence fear have been inconsistent. This work is aimed at examining the involvement of D1 and D2 receptors in the acquisition and expression of conditioned fear using the fear potentiated startle test and the freezing behavior. We evaluated the effects of systemic administration of the D1 antagonist SCH 23390, the D1 agonist SKF 38393, the D2 antagonist sulpiride and the D2 agonist quinpirole before and after conditioning on FPS and freezing as indices of fear memory. The motor activity of the animals was also evaluated in an open field test. Injections of SCH 23390, SKF 38393, sulpiride and quinpirole before conditioning sessions did not produce any significant effect on FPS, but SCH 23390 decreased freezing. Injections of SCH 23390, SKF 38393 and sulpiride before testing session did not produce any significant effect on FPS or freezing. Quinpirole caused significant reduction in FPS and freezing when injected before testing. Drugs' actions were not due to nonspecific impairments in the ability to respond to the CS because the identical drug treatments had no effect in an open field test. Our findings indicate that DA mechanisms mediated by D2 receptors are mainly involved in the expression rather than in the acquisition of fear.