ΔFosB immunoreactivity in the nucleus accumbens 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 nucleus accumbens are shown after chronic amphetamine (A) or cocaine (B) administration. Representative immunoblots from saline- (S), amphetamine- (A), and cocaine- (C) injected post-weanling (P24), periadolescent (P33), and adult mice are shown in the top panels. Bottom panels show mean ± SEM percentage of basal ΔFosB expression. n values for each group are shown in the bars. Significant increases in ΔFosB were found in the nucleus accumbens of only the periadolescent mice. *p < 0.05; **p < 0.01 (Student's t test; saline vs drug).