Anatomical localization of Ca_v3.1 calcium channels and electrophysiological effects of T-type calcium channel blockade in the motor thalamus of MPTP-treated monkeys

Abstract

-Conventional anti-Parkinsonian dopamine replacement therapy is often complicated by side effects that limit the use of these medications. There is a continuing need to develop nondopaminergic approaches to treat Parkinsonism. One such approach is to use medications that normalize dopamine depletion-related firing abnormalities in the basal ganglia-thalamocortical circuitry. In this study, we assessed the potential of a specific T-type calcium channel blocker (ML218) to eliminate pathologic burst patterns of firing in the basal gangliareceiving territory of the motor thalamus in Parkinsonian monkeys. We also carried out an anatomical study, demonstrating that the immunoreactivity for T-type calcium channels is strongly expressed in the motor thalamus in normal and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys. At the electron microscopic level, dendrites accounted for Ͼ90% of all tissue elements that were immunoreactive for voltage-gated calcium channel, type 3.2-containing T-type calcium channels in normal and Parkinsonian monkeys. Subsequent in vivo electrophysiologic studies in awake MPTP-treated Parkinsonian monkeys demonstrated that intrathalamic microinjections of ML218 (0.5 l of a 2.5-mM solution, injected at 0.1-0.2 l/min) partially normalized the thalamic activity by reducing the proportion of rebound bursts and increasing the proportion of spikes in non-rebound bursts. The drug also attenuated oscillatory activity in the 3-13-Hz frequency range and increased gamma frequency oscillations. However, ML218 did not normalize Parkinsonism-related changes in firing rates and oscillatory activity in the beta frequency range. Whereas the described changes are promising, a more complete assessment of the cellular and behavioral effects of ML218 (or similar drugs) is needed for a full appraisal of their anti-Parkinsonian potential.

Sub-synaptic localization of Cav3.1 T-type calcium channels in the thalamus of normal and parkinsonian monkeys

“…However, we cannot rule out that functional properties of T-type channels might be affected in parkinsonism. On the other hand, the fact that intracerebral injections of a specific T-type calcium channel blocker in the basal ganglia-receiving region of the thalamus did not significantly affect the rate and pattern of neuronal activity of BGMT neurons in our previous study (Devergnas et al, 2016) suggests that altered T-type channel activity may not be the sole source of alterations in thalamic firing in the parkinsonian state. A thorough assessment of the functional properties of T-type calcium channels in thalamic slices from normal and dopamine-depleted animals is warranted to further address this issue.…”

“…Thus, despite clear evidence for increased thalamic rebound bursting in the parkinsonian state (Zirh et al, 1998; Magnin et al, 2000; Pessiglione et al, 2005; Devergnas et al, 2016), these functional changes are unlikely to result from abnormal cellular, subcellular and subsynaptic expression of Ca v 3.1 channels. However, we cannot rule out that functional properties of T-type channels might be affected in parkinsonism.…”

“…According to the supplier, this antibody reacts with the >250kDa molecular weight protein associated with Ca v 3.1, does not cross-react with other subtypes of Ca v 3 channels, and does not result in any significant immunostaining or Western blot band labeling in tissue from Ca v 3.1 knockout mice (NeuroMab; Accession # Q9WUT2). The quality and specificity of this antibody for its use in the present study was demonstrated via Western blot analysis performed on fresh monkey brain tissue (Devergnas et al, 2016). Overall, the pattern of immunostaining throughout the monkey brain with this antibody was consistent with findings from previous anatomical and electrophysiological studies suggesting the functional expression or lack of T-type calcium channels (Cavelier and Bossu, 2003; McKay et al, 2006; Hildebrand et al, 2009; Isope et al, 2012).…”

“…The intensity of Ca v 3.1 immunoreactivity was determined by measuring the optical density of Ca v 3.1 immunoperoxidase labeling in the basal ganglia-receiving territory of the VA and CM/Pf complex of control and MPTP-treated monkeys. Adjacent vGluT2-immunostained sections were used to help separate the cerebellar- from the basal ganglia-receiving regions of the ventral motor nuclei (Kuramoto et al 2011; Devergnas et al, 2016). In each selected nuclear region of the three control and three MPTP-treated monkeys, measurements of optical density were taken from three immediately adjacent 2.08 mm 2 areas of Ca v 3.1-immunostained tissue and averaged, yielding three values for each nucleus per treatment condition.…”

Lack of Antiparkinsonian Effects of Systemic Injections of the Specific T-Type Calcium Channel Blocker ML218 in MPTP-Treated Monkeys

“…STN neurons, at least in rodents, express high levels of mRNA for Cav3.3 and Cav3.2, while thalamic relay neurons have high expressions of Cav3.1. 43, 49 …”