Increased locomotor activity of CN mutant mice in the open field test. Distance traveled (cm) (A), vertical activity (B), and stereotypic behavior counts (C) were significantly greater in CN mutants than in controls. Time spent in center (D) was shorter during the first 30 min in CN mutants relative to controls.

Impaired prepulse inhibition (A and B) and latent inhibition (C–F) of CN mutant mice. Startle amplitude was not different between genotypes (A), but the percentage prepulse inhibition was significantly smaller in CN mutants compared with controls (B). Percent freezing during the conditioning phase was significantly less in CN mutants (D) compared with controls (C), most likely due to their hyperactivity. CN mutants traveled longer distances in response to shocks than controls (Insets in D and C, respectively). Freezing during contextual testing was not significantly different between genotypes (C and D). In cued testing, the percent freezing for the P group was significantly lower than that of the NP group in control mice, indicating significant latent inhibition in control mice (E). In contrast, the CN mutants failed to show a significant latent inhibition (F).

Locomotor activating effects of amphetamine and MK-801. Locomotor stimulation after amphetamine injection was observed in both genotypes (A and B), although CN mutants were less sensitive to amphetamine in terms of the ratio between activity before injection and activity after injection (E). On the other hand, the locomotor stimulatory effect of MK-801 was significantly greater in CN mutants compared with controls (C, D, and F; 7–10 animals were used for each group).

Decreased social interaction in home cage. Number of particles in home cage (A), an index of social activity, and activity level (B) are shown. (C and D) All frames taken during an hour were averaged. Control and CN mutants were active during the dark cycle (C; 11:00 p.m. to 12:00 p.m.). During the light cycle (D; 11:00 a.m. to 12:00 a.m.), control and CN mutants were inactive. During inactive periods, control mice stayed in contact with their cage mates, whereas mutant mice tended to stay separated from each other.

Impaired nesting behavior. Representative pictures of the cages of control mice (A and B) and CN mutant mice (C and D).

Normal dopamine release. Despite their pronounced locomotor hyperactivity, CN mutants had no increase in either extracellular levels of dopamine (A) or its metabolites (B) in striatum compared with controls (n = 5 and n = 4, respectively).