Fig. 1.

Effects of S33138 and cocaine on electrical BSR. Panel A shows representative rate-frequency function curves for BSR, indicating the BSR threshold (θ₀) and Y_max (the maximal work output achieved by the animal). Cocaine (2 mg/kg, i.p.) shifted the rate-frequency function curve to the left, lowering the BSR threshold θ₀ value and, thus, enhancing brain reward. Pretreatment with S33138 (2.5 mg/kg, i.p.) significantly blocked the cocaine-induced decrease in threshold (θ₀), and (at this dose) elevated the BSR threshold by itself. Panel B shows dose-dependent enhancement by cocaine of BSR. Panel C shows that 2 mg/kg cocaine-enhanced BSR was dose-dependently attenuated by S33138 (0.156–2.5 mg/kg, i.p.). Panel D shows that S33138 pretreatment did not significantly alter Y_max levels. Panel E shows that S33138 alone, at 2.5 mg/kg, but not lower doses, produced an aversive-like inhibition of BSR. Panel F shows that S33138 alone did not alter Y_max levels. *p < 0.05, ***p < 0.001, when compared with vehicle (Panels B and E) or cocaine alone treatment group (Panel C).