Magnesium deficiency differentially affects the retina and visual cortex of intact rats

Burst-Suppression EEG Reactivity to Photic Stimulation-A Translational Biomarker in Hypoxic-Ischemic Brain Injury

The reactivity of an electroencephalogram (EEG) to external stimuli is impaired in comatose patients showing burst-suppression (BS) patterns following hypoxic-ischemic brain injury (HIBI). We explored the reactivity of BS induced by isoflurane in rat models of HIBI and controls using intermittent photic stimulation (IPS) delivered to one eye. The relative time spent in suppression referred to as the suppression ratio (SR) was measured on the contralateral fronto-occipital cortical EEG channel. The BS reactivity (BSR) was defined as the decrease in the SR during IPS from the baseline before stimulation (SRPRE). We found that BSR increased with SRPRE. To standardize by anesthetic depth, we derived the BSR index (BSRi) as BSR divided by SRPRE. We found that the BSRi was decreased at 3 days after transient global cerebral ischemia in rats, which is a model of brain injury after cardiac arrest. The BSRi was also reduced 2 months after experimental perinatal asphyxia in rats, a model of birth asphyxia, which is a frequent neonatal complication in humans. Furthermore, Oxytocin attenuated BSRi impairment, consistent with a neuroprotective effect in this model. Our data suggest that the BSRi is a promising translational marker in HIBI which should be considered in future neuroprotection studies.

Hypomagnesemia and Cardiovascular Risk in Type 2 Diabetes

Hypomagnesemia is tenfold more common in individuals with type 2 diabetes (T2D), compared to the healthy population. Factors that are involved in this high prevalence are low Mg2+ intake, gut microbiome composition, medication use and presumably genetics. Hypomagnesemia is associated with insulin resistance, which subsequently increases the risk to develop T2D or deteriorates glycaemic control in existing diabetes. Mg2+ supplementation decreases T2D associated features like dyslipidaemia and inflammation; which are important risk factors for cardiovascular disease (CVD). Epidemiological studies have shown an inverse association between serum Mg2+ and the risk to develop heart failure (HF), atrial fibrillation (AF) and microvascular disease in T2D. The potential protective effect of Mg2+ on HF and AF may be explained by reduced oxidative stress, fibrosis and electrical remodeling in the heart. In microvascular disease, Mg2+ reduces the detrimental effects of hyperglycemia and improves endothelial dysfunction. Though, clinical studies assessing the effect of long-term Mg2+ supplementation on CVD incidents are lacking and gaps remain on how Mg2+ may reduce CVD risk in T2D. Despite the high prevalence of hypomagnesemia in people with T2D, routine screening of Mg2+ deficiency to provide Mg2+ supplementation when needed is not implemented in clinical care as sufficient clinical evidence is lacking. In conclusion, hypomagnesemia is common in people with T2D and is both involved as cause, probably through molecular mechanisms leading to insulin resistance, and consequence and is prospectively associated with development of HF, AF and microvascular complications. Whether long-term supplementation of Mg2+ is beneficial, however, remains to be determined.

Myocilin is involved in NgR1/Lingo-1-mediated oligodendrocyte differentiation and myelination of the optic nerve

Myocilin is a secreted glycoprotein that belongs to a family of olfactomedin domain-containing proteins. Although myocilin is detected in several ocular and nonocular tissues, the only reported human pathology related to mutations in the MYOCILIN gene is primary open-angle glaucoma. Functions of myocilin are poorly understood. Here we demonstrate that myocilin is a mediator of oligodendrocyte differentiation and is involved in the myelination of the optic nerve in mice. Myocilin is expressed and secreted by optic nerve astrocytes. Differentiation of optic nerve oligodendrocytes is delayed in Myocilin-null mice. Optic nerves of Myocilin-null mice contain reduced levels of several myelin-associated proteins including myelin basic protein, myelin proteolipid protein, and 2'3'-cyclic nucleotide 3'-phosphodiesterase compared with those of wild-type littermates. This leads to reduced myelin sheath thickness of optic nerve axons in Myocilin-null mice compared with wild-type littermates, and this difference is more pronounced at early postnatal stages compared with adult mice. Myocilin also affects differentiation of oligodendrocyte precursors in vitro. Its addition to primary cultures of differentiating oligodendrocyte precursors increases levels of tested markers of oligodendrocyte differentiation and stimulates elongation of oligodendrocyte processes. Myocilin stimulation of oligodendrocyte differentiation occurs through the NgR1/Lingo-1 receptor complex. Myocilin physically interacts with Lingo-1 and may be considered as a Lingo-1 ligand. Myocilin-induced elongation of oligodendrocyte processes may be mediated by activation of FYN and suppression of RhoA GTPase.

Electroretinogram and visual-evoked potential assessment of retinal and central visual function in a rat ocular hypertension model of glaucoma

PURPOSE/AIM

The aim of the study was to investigate the long-term functional changes that may occur in the retina and visual cortex in a rat ocular hypertension (OHT) model of glaucoma, used in our lab for treatment studies, using electroretinogram (ERG) and visual-evoked potential (VEP) cortical recordings in order to test the hypothesis that experimental glaucoma has differential retinal and central effects.

MATERIALS AND METHODS

Experimental glaucoma was induced unilaterally in Dark Agouti rats using hypertonic saline injection into the episcleral veins. After 3, 8, 16 and 26 weeks, ERGs and VEPs were recorded under scotopic conditions using brief full-field white flashes (10 μcd s m(-2) to 10.4 cd s m(-2)) and under photopic conditions using a rod-adapting background and white light flashes (0.13-10.4 cd s m(-2)).

RESULTS

At 16 and 26 weeks after OHT induction, there was a significant reduction in the amplitudes of the a- (50% and 30% of unoperated eye values, respectively) and b-waves (55% and 40%, respectively) of the scotopic ERG and the b-waves of the photopic ERG (55% and 45%, respectively) in the glaucomatous eyes. However, no significant changes in the VEPs simultaneously recorded over the visual cortex were seen at any of the time points.

CONCLUSIONS

The reductions in ERG amplitudes suggest that this model of glaucoma not only causes retinal ganglion cell (RGC) degeneration but also degeneration of the outer retinal cells, and this was confirmed by histology showing a reduction in the outer retinal layers in the glaucomatous eyes. Cortical VEPs did not show detrimental effects suggesting that the retinal damage in this model was not extensive enough to be detected with the VEP methods used or that there could be central compensation in this model of glaucoma.

Bral1: its role in diffusion barrier formation and conduction velocity in the CNS

At the nodes of Ranvier, excitable axon membranes are exposed directly to the extracellular fluid. Cations are accumulated and depleted in the local extracellular nodal region during action potential propagation, but the impact of the extranodal micromilieu on signal propagation still remains unclear. Brain-specific hyaluronan-binding link protein, Bral1, colocalizes and forms complexes with negatively charged extracellular matrix (ECM) proteins, such as versican V2 and brevican, at the nodes of Ranvier in the myelinated white matter. The link protein family, including Bral1, appears to be the linchpin of these hyaluronan-bound ECM complexes. Here we report that the hyaluronan-associated ECM no longer shows a nodal pattern and that CNS nerve conduction is markedly decreased in Bral1-deficient mice even though there were no differences between wild-type and mutant mice in the clustering or transition of ion channels at the nodes or in the tissue morphology around the nodes of Ranvier. However, changes in the extracellular space diffusion parameters, measured by the real-time iontophoretic method and diffusion-weighted magnetic resonance imaging (MRI), suggest a reduction in the diffusion hindrances in the white matter of mutant mice. These findings provide a better understanding of the mechanisms underlying the accumulation of cations due to diffusion barriers around the nodes during saltatory conduction, which further implies the importance of the Bral1-based extramilieu for neuronal conductivity.

Delayed ischemic electrocortical suppression during rapid repeated cerebral ischemia and kainate-induced seizures in rat

Global cerebral ischemia induces, within seconds, suppression of spontaneous electrocortical activity, partly due to alterations in synaptic transmission. In vitro studies have found that repeated brief hypoxic episodes prolong the persistence of synaptic transmission due to weakened adenosine release. The aim of this study was to investigate in vivo whether the time to ischemic electrocortical suppression (T(ES)) could be altered during energy stress conditions such as rapid repeated global cerebral ischemia and kainate-induced seizures. Experiments were carried out in adult rats under chloral hydrate anaesthesia. Repeated episodes of 1 min of ischemia were induced by transiently clamping the carotid arteries in a 'four-vessel occlusion' model. We devised an automatic method of T(ES) estimation based on the decay of the root mean square of two-channel electrocorticographic recordings. To distinguish the alterations in spontaneous electrocortical activity we compared T(ES) with the ischemic suppression of visual evoked potentials (VEP). During the first ischemic episode, T(ES) was approximately 15 s and remained unchanged when five ischemic episodes were separated by 10-min reperfusion intervals. When ischemia was repeated after 2 min of reperfusion T(ES) progressively increased, reaching a plateau value of approximately 24 s. A similar plateau was reached during kainate-induced seizures. The T(ES) plateau occurred prior to ischemic suppression of VEP. Our data suggest that, under conditions of acute metabolic stress in vivo, the ischemic suppression of spontaneous electrocortical activity may be delayed up to a plateau value. These findings are consistent with the hypothesis of a depletable adenosine pool; however, the restoration of synaptic transmission may be faster in vivo than in vitro.

Simian lentiviral vector-mediated retinal gene transfer of pigment epithelium-derived factor protects retinal degeneration and electrical defect in Royal College of Surgeons rats

Retinitis pigmentosa (RP) is a heterogenous group of inherited retinal diseases resulting in adult blindness caused by mutations of various genes. Although it is difficult to cure the blindness that results from these diseases, delaying the disease progression may be of great benefit, since the majority of RP diseases are seen in middle age or later. To test a gene therapy strategy for RP using a neurotrophic factor gene, we assessed the effect of simian lentivirus (SIV)-mediated subretinal gene transfer of pigment epithelium-derived factor (PEDF), a potent neurotrophic factor, during the disease progression in Royal College of Surgeons (RCS) rats, a well-accepted animal model of RP. Regional gene transfer via SIV into the peripheral subretinal space at the nasal hemisphere was performed in all animals to monitor site-specific transgene expression as well as the therapeutic effect in each retina. Gene transfer of lacZ and PEDF was observed in the regional pigment epithelium corresponding to the regional gene transfer. Histologically, PEDF gene transfer significantly protected the loss of photoreceptor cells (PCs) corresponding to the regions of the gene transfer, compared to those of control groups during the course of the experiment. The antiapoptotic effect of PEDF on PCs is likely to be a related mechanism, because a significant reduction of terminal dUTP-nicked end labeling-positive PC numbers was found in PEDF-treated eyes compared to those of the control group (P<0.05). PEDF-treated eyes also retained a significant sensitivity to light flash during the experimental course. These findings clearly show that neuroprotective gene therapy using PEDF can protect retinal degeneration and functional defects in individuals with RP.

The long-term effects of antiepileptic drugs on the visual system in rats: electrophysiological and histopathological studies

OBJECTIVE

We quantified the long-term effects of antiepileptic drugs (AEDs) on the visual system of rats using electroretinograms (ERGs) and visual evoked potentials (VEPs).

METHODS

Twenty adult Sprague-Dawley rats were divided into 4 groups (n=5). Each animal was treated by monotherapy of phenytoin (PHT), valproic acid (VPA), zonisamide (ZNS) or physiological saline as control. The AEDs were injected intraperitoneally daily for 180 days. ERGs and VEPs were recorded before the medication and on Days 30 and 180.

RESULTS

There were no significant changes in the 4 groups on Day 30. On Day 180, the amplitudes of a- and b-waves of dark-adapted (DA) ERGs were reduced in the PHT group compared with those of the control group. In the VPA group, the amplitudes of the DA ERG a- and b-waves, light-adapted ERG b-wave and the DA VEP were reduced. No significant changes were observed in the ZNS group. There were no histopathological changes of the retina and visual cortex in all groups.

CONCLUSIONS

Our results indicate that neurons along the visual pathways have different sensitivity to each AED. This may result from the differential pharmacological actions of each AED on visual neurons.

SIGNIFICANCE

Our findings suggest that epileptic patients on long-term use of AEDs might have subclinical influences to the visual systems.