Evidence for cross-hemispheric preconditioning in experimental Parkinson's disease

The cross-hemispheric nigrostriatal pathway prevents the expression of levodopa-induced dyskinesias

Parkinson's disease (PD) is a neurodegenerative movement disorder that is routinely treated with levodopa. Unfortunately, long-term dopamine replacement therapy using levodopa leads to levodopa-induced dyskinesias (LID), a significant and disabling side-effect. Clinical findings indicate that LID typically only occurs following the progression of PD motor symptoms from the unilateral (Hoehn and Yahr (HY) Stage I) to the bilateral stage (HY Stage II). This suggests the presence of some compensatory interhemispheric mechanisms that delay the occurrence of LID. We therefore investigated the role of interhemispheric connections of the nigrostriatal pathway on LID expression in a rat model of PD. The striatum of one hemisphere of rats was first injected with a retrograde tracer to label the ipsi- and cross-hemispheric nigrostriatal pathways. Rats were then split into groups and unilaterally lesioned in the striatum or medial forebrain bundle of the tracer-injected hemisphere to induce varying levels of hemiparkinsonism. Finally, rats were treated with levodopa and tested for the expression of LID. Distinct subsets emerged from rats that underwent the same lesioning paradigm based on LID. Strikingly, non-dyskinetic rats had significant sparing of their cross-hemispheric nigrostriatal pathway projecting from the unlesioned hemisphere. In contrast, dyskinetic rats only had a small proportion of this cross-hemispheric nigrostriatal pathway survive lesioning. Crucially, both non-dyskinetic and dyskinetic rats had nearly identical levels of ipsi-hemispheric nigrostriatal pathway survival and parkinsonian motor deficits. Our data suggest that the survival of the cross-hemispheric nigrostriatal pathway plays a crucial role in preventing the expression of LID and represents a potentially novel target to halt the progression of this devastating side-effect of a common anti-PD therapeutic.

Pronounced α-Synuclein Pathology in a Seeding-Based Mouse Model Is Not Sufficient to Induce Mitochondrial Respiration Deficits in the Striatum and Amygdala

Increasing evidence suggests that cross talk between α-synuclein pathology formation and mitochondrial dysfunction plays a central role in the pathogenesis of Parkinson’s disease (PD) and related synucleinopathies. While mitochondrial dysfunction is a well-studied phenomenon in the substantia nigra, which is selectively vulnerable in PD and some models thereof, less information is available in other brain regions that are also affected by synuclein pathology. Therefore, we sought to test the hypothesis that early α-synuclein pathology causes mitochondrial dysfunction and that this effect might be exacerbated in conditions of increased vulnerability in affected brain regions, such as the amygdala. We combined a model of intracerebral α-synuclein pathology seeding with chronic glucocorticoid treatment, which models non-motor symptoms of PD and affects amygdala physiology. We measured mitochondrial respiration, reactive oxygen species (ROS) generation and protein abundance as well as α-synuclein pathology in male mice. Chronic corticosterone administration induced mitochondrial hyperactivity in the amygdala. Although injection of α-synuclein preformed fibrils (PFFs) into the striatum resulted in pronounced α-synuclein pathology in both striatum and amygdala, mitochondrial respiration in these brain regions was not compromised, regardless of corticosterone treatment. Our results suggest that early stage α-synuclein pathology does not influence mitochondrial respiration in the striatum and amygdala, even in corticosterone-induced respirational hyperactivity. We discuss our findings in light of relevant literature, warn of a potential publication bias and encourage scientists to report their negative results within the framework of this model.

Proenkephalin Derived Peptides Are Involved in the Modulation of Mitochondrial Respiratory Control During Epileptogenesis

Epilepsies are a group of common neurological diseases exerting a strong burden on patients and society, often lacking clear etiology and effective therapeutical strategies. Early intervention during the development of epilepsy (epileptogenesis) is of great medical interest, though hampered by poorly characterized epileptogenetic processes. Using the intrahippocampal kainic acid mouse model of temporal lobe epilepsy, we investigated the functional role of the endogenous opioid enkephalin during epileptogenesis. We addressed three sequential questions: (1) How does enkephalin affect seizure threshold and how is it regulated during epileptogenesis? (2) Does enkephalin influence detrimental effects during epileptogenesis? (3) How is enkephalin linked to mitochondrial function during epileptogenesis?. In contrast to other neuropeptides, the expression of enkephalin is not regulated in a seizure dependent manner. The pattern of regulation, and enkephalin's proconvulsive effects suggested it as a potential driving force in epileptogenesis. Surprisingly, enkephalin deficiency aggravated progressive granule cell dispersion in kainic acid induced epileptogenesis. Based on reported beneficial effects of enkephalin on mitochondrial function in hypoxic/ischemic states, we hypothesized that enkephalin may be involved in the adaptation of mitochondrial respiration during epileptogenesis. Using high-resolution respirometry, we observed dynamic improvement of hippocampal mitochondrial respiration after kainic acid-injections in wild-type, but not in enkephalin-deficient mice. Thus, wild-type mice displayed higher efficiency in the use of mitochondrial capacity as compared to enkephalin-deficient mice. Our data demonstrate a Janus-headed role of enkephalin in epileptogenesis. In naive mice, enkephalin facilitates seizures, but in subsequent stages it contributes to neuronal survival through improved mitochondrial respiration.

Enhancing and Extending Biological Performance and Resilience

Human performance, endurance, and resilience have biological limits that are genetically and epigenetically predetermined but perhaps not yet optimized. There are few systematic, rigorous studies on how to raise these limits and reach the true maxima. Achieving this goal might accelerate translation of the theoretical concepts of conditioning, hormesis, and stress adaptation into technological advancements. In 2017, an Air Force-sponsored conference was held at the University of Massachusetts for discipline experts to display data showing that the amplitude and duration of biological performance might be magnified and to discuss whether there might be harmful consequences of exceeding typical maxima. The charge of the workshop was "to examine and discuss and, if possible, recommend approaches to control and exploit endogenous defense mechanisms to enhance the structure and function of biological tissues." The goal of this white paper is to fulfill and extend this workshop charge. First, a few of the established methods to exploit endogenous defense mechanisms are described, based on workshop presentations. Next, the white paper accomplishes the following goals to provide: (1) synthesis and critical analysis of concepts across some of the published work on endogenous defenses, (2) generation of new ideas on augmenting biological performance and resilience, and (3) specific recommendations for researchers to not only examine a wider range of stimulus doses but to also systematically modify the temporal dimension in stimulus inputs (timing, number, frequency, and duration of exposures) and in measurement outputs (interval until assay end point, and lifespan). Thus, a path forward is proposed for researchers hoping to optimize protocols that support human health and longevity, whether in civilians, soldiers, athletes, or the elderly patients. The long-term goal of these specific recommendations is to accelerate the discovery of practical methods to conquer what were once considered intractable constraints on performance maxima.