Fig. 4.

Locomotor responses to repeated amphetamine treatment in congenic D₁^+/+ and D₁^−/− mice (n=8 per genotype) recorded for 1-h daily sessions. a Baseline locomotor activity in D₁^−/− mice was significantly higher (**P<0.0001) than in D₁^+/+ mice. Repeated exposure of mice to amphetamine for three consecutive days and again after 3 weeks of abstinence (day 25) induced locomotor stimulation and sensitization in D₁^+/+ mice b, but not in D₁^−/− mice c as compared with saline-pretreated mice and their own responses to saline pretreatment (left panels). Amphetamine-induced locomotor activation was significantly enhanced in amphetamine-pretreated mice from both genotypes on day 33 when all mice were given only 1-h session following saline or amphetamine treatment without saline pre-habituation (as indicated by dashed rectangular boxes). D₁^+/+ mice maintained sensitized responses to amphetamine on days 35 and 47 compared to their initial response on day 1 and to saline-pretreated controls as well as to their own response to saline (left panel). D₁^−/− mice failed to show sensitization when compared to their saline-pretreated controls, although their response to amphetamine was significantly different from their own response to saline (left panel). *P<0.01–0.03; **P<0.002–0.0001 compared to saline-pretreated mice. ^#P<0.01–0.03; ^##P<0.002–0.0001 compared to their responses to saline 1 h prior to amphetamine. Values are means + SEM