Carbidopa

All stereotaxic surgeries (prevention and reversibility studies) were performed as previously detailed.22 To test the hypothesis that shRNA‐mediated silencing of striatal CaV1.3 channels prior to levodopa would prevent the development and provide sustained amelioration of LIDs, rats received unilateral injections of either rAAV‐CaV1.3‐shRNA or the control rAAV‐Scrambled (Scr)‐shRNA (1.0 × 10¹³ vg/mL) into 2 dorsolateral sites within the left striatum (AP0.0, ML+3.0, DV‐5.2; and AP+1.6, ML+2.7, DV‐4.9). Vector surgery preceded by 1 week 6‐hydroxydopamine (6‐OHDA) neurotoxin surgery used to induce unilateral parkinsonism (Fig. 1A). Parkinsonism was induced using stereotaxic injection of 6‐OHDA into substantia nigra (SN) and medial forebrain bundle per our usual protocol.10, 23 Vector delivery prior to 6‐OHDA was used to minimize spine loss associated with CaV1.3 dysregulation, which occurs secondary to striatal DA depletion. However, because AAV is retrogradely transported to nigral DA neurons in which CaV1.3 silencing could interfere with 6‐OHDA‐induced cell death,24 timing for these proof‐of‐principle experiments was systematically worked out in pilot feasibility studies. Three weeks following 6‐OHDA (4 weeks postvector surgery), rats began receiving daily levodopa injections given at escalating doses ranging from low (6 mg/kg) to moderate (9 mg/kg) to high (12 mg/kg) to what is referred to here as “extreme” (18 mg/kg); see Figure 1A. Each dose was given daily (Monday through Friday) for 2 weeks with a constant dose of carbidopa (12 mg/kg), a peripheral decarboxylase inhibitor. LID behaviors were rated on days 1, 6, and 10 of each dose.