Temporal response of multiple nicotine administration on SPLI content in VTA-related brain regions such as the prefrontal cortex (a), nucleus accumbens shell (b), and nucleus accumbens core (c). Animals were given injections of (±) nicotine (as described for Fig. 1) or saline (control) and killed 12, 18 or 48 h following treatment. Results are expressed as percentages of control and represent mean values ± S.E.M. (n= 8 for control and 9 for drug-treated animals per group). The average control value of SPLI concentration for prefrontal cortex, nucleus accumbens shell and nucleus accumbens core were 319 ± 46, 2,189 ± 159, and 1,418 ± 153 pg/mg protein, respectively. *P < 0.05 vs. control.