The L-type channel antagonist isradipine is neuroprotective in a mouse model of Parkinson's disease

Abstract

The motor symptoms of Parkinson's disease (PD) are due to the progressive loss of dopamine (DA) neurons in substantia nigra pars compacta (SNc). Nothing is known to slow the progression of the disease, making the identification of potential neuroprotective agents of great clinical importance. Previous studies using the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of PD have shown that antagonism of L-type Ca²⁺ channels protects SNc DA neurons. However, this was not true in a 6-hydroxydopamine (6-OHDA) model. One potential explanation for this discrepancy is that protection in the 6-OHDA model requires greater antagonism of Cav1.3 L-type Ca²⁺ channels thought to underlie vulnerability and this was not achievable with the low affinity dihydropyridine (DHP) antagonist used. To test this hypothesis, the DHP with the highest affinity for Cav1.3 L-type channels—isradipine—was systemically administered and then the DA toxin 6-OHDA injected intrastriatally. Twenty-five days later, neuroprotection and plasma concentration of isradipine were determined. This analysis revealed that isradipine produced a dose-dependent sparing of DA fibers and cell bodies at concentrations achievable in humans, suggesting that isradipine is a potentially viable neuroprotective agent for PD.

Introduction

PD is a slowly progressing neurodegenerative condition whose presenting motor symptoms—bradykinesia, rigidity and tremor—are attributable to the loss of DA neurons (Riederer and Wuketich, 1976). Postmortem analysis of PD patient brains has revealed a striking loss of tyrosine hydroxylase (TH) immunoreactive, DA neurons in the SNc and relative sparing of DA neurons in the neighboring ventral tegmental area (VTA) (Hirsch et al., 1988). As nothing is known to slow the progression of the disease, the identification of neuroprotective agents in PD is of considerable importance.

One potential target for neuroprotective therapies in PD is the L-type Ca²⁺ channel with a Cav1.3 pore-forming subunit. The rationale for targeting these channels comes from an appreciation of the potentially deleterious consequences of elevations in intracellular Ca²⁺ (Gleichmann and Mattson, 2010) and the recent discovery that vulnerable SNc DA neurons have an usually strong engagement of Cav1.3 L-type Ca²⁺ channels during autonomous pacemaking (Guzman et al., 2010, Khaliq and Bean, 2010). This influx has been shown to increase mitochondrial oxidant stress in SNc DA neurons and this stress is exacerbated in a genetic model of PD (Guzman et al., 2010). In principle, this stress should increase the sensitivity to mitochondrial toxins used to create animal models of PD. In agreement with this inference, previous work has shown that antagonizing L-type Ca²⁺ channels by systemic administration of the DHP nimodipine increased the resistance of SNc DA neurons to both acute and chronic challenge with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration (Chan et al., 2007, Kupsch et al., 1995, Kupsch et al., 1996). However, systemic administration of nimodipine did not protect SNc DA neurons against a challenge with 6-hydroxydopamine (6-OHDA) (Sautter et al., 1997).

The reason for this apparent discrepancy is unclear. Although both 6-OHDA and MPTP disrupt mitochondrial function, they do so through different mechanisms (Bove et al., 2005). As a consequence, it is possible that protection against 6-OHDA toxicity requires a greater reduction in Ca²⁺ influx through L-type channels. The efficacy of systemically administered DHPs in antagonizing Cav1.3 Ca²⁺ channels in SNc DA neurons depends upon their bioavailability and potency. In this regard, DHPs are heterogeneous (Eisenberg et al., 2004). Although nimodipine has good brain bioavailability (Kupsch et al., 1996), it has a relatively low affinity for Cav1.3 Ca²⁺ channels (Sinnegger-Brauns et al., 2009). In contrast, isradipine has much higher (> 40 fold) affinity for Cav1.3 channels as well as good brain bioavailability (Bonci et al., 1998, Fitton and Benfield, 1990, Sinnegger-Brauns et al., 2009). Another relevant factor is DHP potency at Cav1.2 Ca²⁺ channels found in the cardiovascular system (Simuni et al., 2010). The antagonism of these channels limits the dose of DHPs that can be used for neuroprotective purposes. Theoretically, the ideal DHP for protecting SNc DA neurons would be one that was selective for Cav1.3 channels. Although none of the known DHPs have a higher affinity for Cav1.3 channels than Cav1.2 channels, the DHP that comes the closest is isradipine, which has nearly equal potency at Cav1.2 and Cav1.3 channels (Sinnegger-Brauns et al., 2009).

The studies reported were designed to test the hypothesis that isradipine would protect SNc DA neurons and fibers in a progressive model of PD created by intrastriatal injection of 6-OHDA. We found that indeed isradipine produced a dose-dependent protection in this model. Moreover, it was found that the plasma concentrations of isradipine that led to protection were within those that can be safely achieved in humans.