Effect of lactulose on METH-induced monoamine depletions. Rats were treated with METH (10 mg/kg, i.p., every 2 h × 4) or saline (1 ml/kg, i.p., every 2 h × 4) and lactulose (5.3 g/kg, p.o., every 12 h) or vehicle (8 ml/kg, p.o., every 12 h). Tissue content of dopamine or 5-HT in the striatum was measured 7 d later. A, Treatment with lactulose significantly attenuated the striatal dopamine depletions caused by METH as METH depleted dopamine by only 34.5 ± 7.9% (mean ± SEM) in lactulose-treated rats. B, METH depleted 5-HT by 15.2 ± 8.2% (mean ± SEM) in lactulose-treated rats, which was not statistically significant compared with saline-treated rats (*p < 0.001 compared with Lac Sal; &p < 0.001 compared with Veh METH) (n = 6–9 rats per group). Lac, lactulose; Sal, saline; Veh, vehicle.