Passive electrical properties of spontaneously active neurons in the nucleus reticularis gigantocellularis of the cat

Mitochondrial Dysfunction and Parkinson's Disease-Near-Infrared Photobiomodulation as a Potential Therapeutic Strategy

As the main driver of energy production in eukaryotes, mitochondria are invariably implicated in disorders of cellular bioenergetics. Given that dopaminergic neurons affected in Parkinson's disease (PD) are particularly susceptible to energy fluctuations by their high basal energy demand, it is not surprising to note that mitochondrial dysfunction has emerged as a compelling candidate underlying PD. A recent approach towards forestalling dopaminergic neurodegeneration in PD involves near-infrared (NIR) photobiomodulation (PBM), which is thought to enhance mitochondrial function of stimulated cells through augmenting the activity of cytochrome C oxidase. Notwithstanding this, our understanding of the neuroprotective mechanism of PBM remains far from complete. For example, studies focusing on the effects of PBM on gene transcription are limited, and the mechanism through which PBM exerts its effects on distant sites (i.e., its "abscopal effect") remains unclear. Also, the clinical application of NIR in PD proves to be challenging. Efficacious delivery of NIR light to the substantia nigra pars compacta (SNpc), the primary site of disease pathology in PD, is fraught with technical challenges. Concerted efforts focused on understanding the biological effects of PBM and improving the efficiency of intracranial NIR delivery are therefore essential for its successful clinical translation. Nonetheless, PBM represents a potential novel therapy for PD. In this review, we provide an update on the role of mitochondrial dysfunction in PD and how PBM may help mitigate the neurodegenerative process. We also discussed clinical translation aspects of this treatment modality using intracranially implanted NIR delivery devices.

Physiological phenotype and vulnerability in Parkinson's disease

This review will focus on the principles underlying the hypothesis that neuronal physiological phenotype-how a neuron generates and regulates action potentials-makes a significant contribution to its vulnerability in Parkinson's disease (PD) and aging. A cornerstone of this hypothesis is that the maintenance of ionic gradients underlying excitability can pose a significant energetic burden for neurons, particularly those that have sustained residence times at depolarized membrane potentials, broad action potentials, prominent Ca(2+) entry, and modest intrinsic Ca(2+) buffering capacity. This energetic burden is shouldered in neurons primarily by mitochondria, the sites of cellular respiration. Mitochondrial respiration increases the production of damaging superoxide and other reactive oxygen species (ROS) that have widely been postulated to contribute to cellular aging and PD. Many of the genetic mutations and toxins associated with PD compromise mitochondrial function, providing a mechanistic linkage between known risk factors and cellular physiology that could explain the pattern of pathology in PD. Because much of the mitochondrial burden created by this at-risk phenotype is created by Ca(2+) entry through L-type voltage-dependent channels for which there are antagonists approved for human use, a neuroprotective strategy to reduce this burden is feasible.

Neuronal vulnerability, pathogenesis, and Parkinson's disease

Although there have been significant advances, pathogenesis in Parkinson's disease (PD) is still poorly understood. Potential clues about pathogenesis that have not been systematically pursued are suggested by the restricted pattern of neuronal pathology in the disease. In addition to dopaminergic neurons in the substantia nigra pars compacta (SNc), a significant number of other central and peripheral neuronal populations exhibit Lewy pathology (LP), phenotypic dysregulation, or frank degeneration in PD patients. Drawing on this literature, there appears to be a small number of risk factors contributing to vulnerability. These include autonomous activity, broad action potentials, low intrinsic calcium buffering capacity, poorly myelinated long highly branched axons and terminal fields, and use of a catecholamine neurotransmitter, often with the catecholamine-derived neuromelanin pigment. Of these phenotypic traits, only the physiological ones appear to provide a reachable therapeutic target at present.

The pathology roadmap in Parkinson disease

An under-appreciated clue about pathogenesis in Parkinson disease (PD) is the distribution of pathology in the early and middle stages of the disease. This pathological 'roadmap' shows that in addition to dopaminergic neurons in the substantia nigra pars compacta (SNc), a significant number of other central and peripheral neuronal populations exhibit Lewy pathology, phenotypic dysregulation or frank degeneration in PD patients. This spatially distributed, at-risk population of neurons shares a number of features, including autonomously generated activity, broad action potentials, low intrinsic calcium buffering capacity and long, poorly myelinated, highly branched axons. Many, and perhaps all, of these traits add to the metabolic burden in these neurons, suggesting that mitochondrial deficits could drive pathogenesis in PD-in agreement with a large segment of the literature. What is less clear is how this neuronal phenotype might shape the susceptibility to proteostatic dysfunction or to the spread of α-synuclein fibrils deposited in the extracellular space. The review explores the literature on these issues and their translational implications.

Calcium, bioenergetics, and neuronal vulnerability in Parkinson's disease

The most distinguishing feature of neurons is their capacity for regenerative electrical activity. This activity imposes a significant mitochondrial burden, especially in neurons that are autonomously active, have broad action potentials, and exhibit prominent Ca(2+) entry. Many of the genetic mutations and toxins associated with Parkinson's disease compromise mitochondrial function, providing a mechanistic explanation for the pattern of neuronal pathology in this disease. Because much of the neuronal mitochondrial burden can be traced to L-type voltage-dependent channels (channels for which there are brain-penetrant antagonists approved for human use), a neuroprotective strategy to reduce this burden is available.

Neuronal vulnerability, pathogenesis, and Parkinson's disease

Although there have been significant advances, pathogenesis in Parkinson's disease (PD) is still poorly understood. Potential clues about pathogenesis that have not been systematically pursued are suggested by the restricted pattern of neuronal pathology in the disease. In addition to dopaminergic neurons in the substantia nigra pars compacta (SNc), a significant number of other central and peripheral neuronal populations exhibit Lewy pathology (LP), phenotypic dysregulation, or frank degeneration in PD patients. Drawing on this literature, there appear to be a small number of risk factors contributing to vulnerability. These include autonomous activity, broad action potentials, low intrinsic calcium-buffering capacity, poorly myelinated long highly branched axons and terminal fields, and use of a monoamine neurotransmitter, often with the catecholamine-derived neuromelanin pigment. Of these phenotypic traits, only the physiological ones appear to provide a reachable therapeutic target at present.

Passive biophysical membrane properties of nucleus reticularis gigantocellularis neurons in brain slices from the rat

This study employed intracellular recording coupled with the current clamp technique to characterize the passive biophysical membrane properties of neurons in the nucleus reticularis gigantocellularis (NRGC) of brain slices from adult, male Sprague-Dawley rats. We found that NRGC neurons possessed highly depolarized transmembrane potentials (-22.9 +/- 0.6 mV, n = 189). The 29 NRGC neurons that received further evaluation showed that they could be separated into two clusters, with significantly different membrane input resistances (3.51 +/- 0.89 vs 77.50 +/- 8.82 M omega) and membrane time constants (0.56 +/- 0.05 vs 1.27 +/- 0.12 ms). These membrane properties, which resembled qualitatively those we observed previously in cats in vivo, will form the basis for further evaluation of the ionic mechanisms that may underlie the actions of pharmacologic agents on the NRGC neurons.