Time course of the effect of 20 Hz dorsal raphe nucleus (DRN) electrical stimulation on DA extracellular levels in the nucleus accumbens (NAC) and the striatum in saline- and ondansetron-treated rats. Each column represents the mean percentage ± SEM of the results from four to six animals per group of the three dialysate fractions collected during and after DRN stimulation, performed during the first dialysate fraction (15 min). Ondansetron, subcutaneously injected at 0.1 or 1 mg/kg 30 min before DRN stimulation, significantly reduced the overall effect of DRN stimulation on DA release only in the NAC (p< 0.05 and p < 0.01, respectively, two-way ANOVA; see Results). ***p < 0.001 versus the sham stim + saline group; ^#p < 0.05, ^##p < 0.01, and^###p < 0.001 versus the 20 Hz stim + saline group; °p < 0.05 versus the sham stim + ondansetron 0.1 group;⁺p < 0.05 and⁺⁺p < 0.01 versus the sham stim + ondansetron 1 group (Tukey’s test for each time point).