Synaptotagmin-1-dependent phasic axonal dopamine release is dispensable for basic motor behaviors in mice.

In Parkinson's disease (PD), motor dysfunctions only become apparent after extensive loss of DA innervation. This resilience has been hypothesized to be due to the ability of many motor behaviors to be sustained through a diffuse basal tone of DA; but experimental evidence for this is limited. Here we show that conditional deletion of the calcium sensor synaptotagmin-1 (Syt1) in DA neurons (Syt1 cKODA mice) abrogates most activity-dependent axonal DA release in the striatum and mesencephalon, leaving somatodendritic (STD) DA release intact. Strikingly, Syt1 cKODA mice showed intact performance in multiple unconditioned DA-dependent motor tasks and even in a task evaluating conditioned motivation for food. Considering that basal extracellular DA levels in the striatum were unchanged, our findings suggest that activity-dependent DA release is dispensable for such tasks and that they can be sustained by a basal tone of extracellular DA. Taken together, our findings reveal the striking resilience of DA-dependent motor functions in the context of a near-abolition of phasic DA release, shedding new light on why extensive loss of DA innervation is required to reveal motor dysfunctions in PD.

Amphetamine disrupts dopamine axon growth in adolescence by a sex-specific mechanism in mice.

Initiating drug use during adolescence increases the risk of developing addiction or other psychopathologies later in life, with long-term outcomes varying according to sex and exact timing of use. The cellular and molecular underpinnings explaining this differential sensitivity to detrimental drug effects remain unexplained. The Netrin-1/DCC guidance cue system segregates cortical and limbic dopamine pathways in adolescence. Here we show that amphetamine, by dysregulating Netrin-1/DCC signaling, triggers ectopic growth of mesolimbic dopamine axons to the prefrontal cortex, only in early-adolescent male mice, underlying a male-specific vulnerability to enduring cognitive deficits. In adolescent females, compensatory changes in Netrin-1 protect against the deleterious consequences of amphetamine on dopamine connectivity and cognitive outcomes. Netrin-1/DCC signaling functions as a molecular switch which can be differentially regulated by the same drug experience as function of an individual's sex and adolescent age, and lead to divergent long-term outcomes associated with vulnerable or resilient phenotypes.

The reverse transcriptase inhibitor 3TC protects against age-related cognitive dysfunction.

Aging is the primary risk factor for most neurodegenerative diseases, including Alzheimer's disease. Major hallmarks of brain aging include neuroinflammation/immune activation and reduced neuronal health/function. These processes contribute to cognitive dysfunction (a key risk factor for Alzheimer's disease), but their upstream causes are incompletely understood. Age-related increases in transposable element (TE) transcripts might contribute to reduced cognitive function with brain aging, as the reverse transcriptase inhibitor 3TC reduces inflammation in peripheral tissues and TE transcripts have been linked with tau pathology in Alzheimer's disease. However, the effects of 3TC on cognitive function with aging have not been investigated. Here, in support of a role for TE transcripts in brain aging/cognitive decline, we show that 3TC: (a) improves cognitive function and reduces neuroinflammation in old wild-type mice; (b) preserves neuronal health with aging in mice and Caenorhabditis elegans; and (c) enhances cognitive function in a mouse model of tauopathy. We also provide insight on potential underlying mechanisms, as well as evidence of translational relevance for these observations by showing that TE transcripts accumulate with brain aging in humans, and that these age-related increases intersect with those observed in Alzheimer's disease. Collectively, our results suggest that TE transcript accumulation during aging may contribute to cognitive decline and neurodegeneration, and that targeting these events with reverse transcriptase inhibitors like 3TC could be a viable therapeutic strategy.

Axonal Domain Structure as a Putative Identifier of Neuron-Specific Vulnerability to Oxidative Stress in Cultured Neurons.

Several populations of neurons are purported to degenerate in Parkinson's disease (PD). One current hypothesis suggests that vulnerable neurons in PD share common characteristics including projecting to voluminous territories and having extremely long and branched axonal domains with large numbers of neurotransmitter release sites. In this study, we used a mouse in vitro culture system to compare the axonal domain of neuronal populations suspected to be vulnerable in PD to that of neuronal populations considered at a lesser risk. In the first category, we included dopamine (DA) neurons of the substantia nigra, noradrenergic neurons of the locus coeruleus (LC), serotonin neurons of the raphe nuclei (R), and cholinergic neurons of the dorsal motor nucleus of the vagus (DMV). In the second category, we included DA neurons of the ventral tegmental area, cholinergic neurons of the hypoglossal nucleus, and cholinergic interneurons of the dorsal striatum. Validating their differential vulnerability, we find that, when compared with neurons presumed to be resilient in PD, a larger proportion of neurons presumed to be vulnerable in PD degenerate in response to cell stress induced by hydrogen peroxide. We also find that they are endowed with larger axonal domains, that are more complex, have more axonal varicosities with a higher proportion of varicosities that are positive for synaptotagmin 1 (Syt-1). Notwithstanding the obvious limitations related to the dissection of small brain nuclei and to the growth of these neurons in vitro, these findings support the hypothesis that axonal domain structure is a key characteristic of neuronal vulnerability to oxidative stress.

Macular Retinal Sublayer Thicknesses in G11778A Leber Hereditary Optic Neuropathy.

BACKGROUND AND OBJECTIVE: To determine macular retinal sublayer thickness changes in G11778A Leber hereditary optic neuropathy (LHON). PATIENTS AND METHODS: The authors performed a cross-sectional study by segmenting spectral-domain Cirrus OCT (Carl Zeiss Meditec, Dublin, CA) 512 × 128 macular cube scans from a prospective, observational study of G11778A LHON. The thickness of the retinal sublayers of LHON affected subjects and asymptomatic carriers were compared to those of a normal group. RESULTS: The study included 20 LHON-affected subjects (13 males; age: 31 years ± 14 years, range: 10 years to 61 years; time since onset of visual loss: 5.9 years ± 8.7 years; 0.4-29.8), 31 asymptomatic LHON carriers (five males; age: 38 years ± 18 years, range: 9 years to 65 years), and 14 normal subjects (five males; age: 39 years ± 13 years, range: 23 years to 61 years). The retinal sublayer thickness parameters were not significantly correlated with age in any of the groups. There were no differences between carriers and normal subjects for thickness of total retina or any sublayer. Affected LHON retinal nerve fiber layer (RNFL) and ganglion cell layer plus inner plexiform layer (GCL+IPL) were thinner, whereas the photoreceptor outer segment (OS) layer was thicker than carriers and normal subjects (P values ranged from .042 to < .001), except for the OS layer in the inferior inner ring and temporal outer ring. Differences between groups were not significant in the inner nuclear layer plus outer plexiform layer (INL+OPL). The affected LHON outer nuclear layer plus inner segment layer was thicker in some quadrants, and the affected LHON choroid layer was generally thinner than carriers and normal subjects; however, these differences were not significant after accounting for age. CONCLUSION: LHON-affected patients have thickened photoreceptor OS layers in spite of having thinner RNFL and GCL+IPL layers. The findings indicate LHON also has an effect on the morphology of the photoreceptors. [Ophthalmic Surg Lasers Imaging Retina. 2016;47:802-810.].