Effect of chronic MK-801 on development of L-DOPA-induced AIMs. Control and MK-801-treated animals developed AIMs that worsened over repeated L-DOPA injections, as indicated by increasing scores on limb + axial + oral AIMs (A), limb AIMs (B), axial AIMs (C), oral AIMs (D), and contraversive rotation (E). There was no effect of MK-801 treatment on the development of sensitized responses to L-DOPA (F). Animals were treated with 0.1 mg/kg MK-801, i.p., 30 min prior to L-DOPA, on Days 1–14 and 16–21. Days 0, 15, and 22 (see arrows) were L-DOPA challenge days when all animals received vehicle + L-DOPA. LAO, limb + axial + oral AIMs.