Prolonged fear responses in mice lacking dopamine D1 receptor.
Dopamine is an important neurotransmitter involved in learning and memory including emotional memory. The involvement of dopamine in conditioned fear has been widely documented. However, little is known about the molecular mechanisms that underlie contextual fear conditioning and memory consolidation. To address this issue, we used dopamine D1-deficient mice (D1-/-) and their wild-type (D1+/+) and heterozygote (D1+/-) siblings to assess aversive learning and memory. We quantified two different aspects of fear responses to an environment where the mice have previously received unsignaled footshocks. Using one-trial step-through passive avoidance and conditioned freezing paradigms, mice were conditioned to receive mild inescapable footshocks then tested for acquisition, retention and extinction of conditioned fear responses 5 min after and up to 45-90 days post-training. No differences were observed among any of the genotypes in the acquisition of passive avoidance response or fear-induced freezing behavior. However, with extended testing, D1-/- mice exhibited prolonged retention and delayed extinction of conditioned fear responses in both tasks, suggesting that D1-/- mice are capable of acquiring aversive learning normally. These findings demonstrate that the dopamine D1 receptor is not important for acquisition or consolidation of aversive learning and memory but has an important role in modulating the extinction of fear memory.