ΔFosB immunoreactivity in the caudate putamen after chronic psychostimulant administration. CD-1 mice were injected once daily with saline, amphetamine, or cocaine for 7 d beginning on day 24 (P24; post-weanling), day 33 (P33; periadolescent), or day 60 (Adult). Levels of ΔFosB (35 kDa) immunoreactivity in the caudate putamen are shown after chronic amphetamine (A) or cocaine (B) administration. Representative immunoblots from saline- (S), amphetamine- (A), and cocaine- (C) injected periadolescent mice (P33) are shown in thetop panels. Bottom panels show mean ± SEM percentage of basal ΔFosB expression. n values for each group are shown in the bars. Significant amphetamine-induced increases in ΔFosB immunoreactivity were found in the caudate putamen of only periadolescent mice (A). Chronic cocaine administration produced increases in ΔFosB in all three age groups (B). *p < 0.05; **p < 0.01 (Student's t test; saline vs drug).