Lower Affinity of Isradipine for L-Type Ca2+ Channels during Substantia Nigra Dopamine Neuron-Like Activity: Implications for Neuroprotection in Parkinson's Disease

Ca2+ -influx through L-type Ca2+ -channels (LTCCs) is associated with activity-related stressful oscillations of Ca2+ levels within dopaminergic (DA) neurons in the substantia nigra (SN), which may contribute to their selective degeneration in Parkinson's disease (PD). LTCC blockers were neuroprotective in mouse neurotoxin models of PD, and isradipine is currently undergoing testing in a phase III clinical trial in early PD. We report no evidence for neuroprotection by in vivo pretreatment with therapeutically relevant isradipine plasma levels, or Ca(v)1.3 LTCC deficiency in 6-OHDA-treated male mice. To explain this finding, we investigated the pharmacological properties of human LTCCs during SN DA-like and arterial smooth muscle (aSM)-like activity patterns using whole-cell patch-clamp recordings in HEK293 cells (Ca(v)1.2 alpha 1-subunit, long and short Cav1.3 1-subunit splice variants; beta 3/alpha 2 delta 1). During SN DA-like pacemaking, only Ca(v)1.3 variants conducted Ca2+ current (I-Ca) at subthreshold potentials between action potentials. SN DA-like burst activity increased integrated I-Ca during (Ca(v)1.2 plus Ca(v)1.3) and after (Ca(v)1.3) the burst. Isradipine inhibition was splice variant and isoform dependent, with a 5to11-fold lower sensitivity to Ca(v)1.3 variants during SN DA-like pacemaking compared with Ca(v)1.2 during aSM-like activity. Supratherapeutic isradipineconcentrationsreducedthepacemakerprecisionofadultmouseSNDAneuronsbutdidnotaffecttheirsomaticCa(2+) oscillations. Our data predict that Ca(v)1.2 and Ca(v)1.3 splice variants contribute differentially to Ca2+ load inSNDAneurons, with prominent Ca(v)1.3-mediated ICa between action potentials and after bursts. The failure of therapeutically relevant isradipine levels to protectSNDAneurons can be explained by weaker state-dependent inhibition of SN DA LTCCs compared with aSM Ca(v)1.2