Effects of the noradrenergic system in rat white matter exposed to oxygen-glucose deprivation in vitro

Glutamate receptors and white matter stroke

White matter (WM) damage during ischemia occurs at multiple sites including myelin, oligodendrocytes, astrocytes and axons. A major driver of WM demise is excitoxicity as a consequence of excessive glutamate release by vesicular and non-vesicular mechanisms from axons and glial cells. This results in over-activation of ionotropic glutamate receptors (GluRs) profusely expressed by all cell compartments in WM. Thus, blocking excitotoxicity in WM with selective antagonists of those receptors has a potential therapeutic value. The significance of WM GluR expression for WM stroke injury is the focus of this review, and we will examine the role of GluRs in injury to myelin, oligodendrocytes, astrocytes and the axon cylinder.

The optimal distance between two electrode tips during recording of compound nerve action potentials in the rat median nerve

The distance between the two electrode tips can greatly influence the parameters used for recording compound nerve action potentials. To investigate the optimal parameters for these recordings in the rat median nerve, we dissociated the nerve using different methods and compound nerve action potentials were orthodromically or antidromically recorded with different electrode spacings. Compound nerve action potentials could be consistently recorded using a method in which the middle part of the median nerve was intact, with both ends dissociated from the surrounding fascia and a ground wire inserted into the muscle close to the intact part. When the distance between two stimulating electrode tips was increased, the threshold and supramaximal stimulating intensity of compound nerve action potentials were gradually decreased, but the amplitude was not changed significantly. When the distance between two recording electrode tips was increased, the amplitude was gradually increased, but the threshold and supramaximal stimulating intensity exhibited no significant change. Different distances between recording and stimulating sites did not produce significant effects on the aforementioned parameters. A distance of 5 mm between recording and stimulating electrodes and a distance of 10 mm between recording and stimulating sites were found to be optimal for compound nerve action potential recording in the rat median nerve. In addition, the orthodromic compound action potential, with a biphasic waveform that was more stable and displayed less interference (however also required a higher threshold and higher supramaximal stimulus), was found to be superior to the antidromic compound action potential.

Neurotransmitter signaling in white matter

White matter (WM) tracts are bundles of myelinated axons that provide for rapid communication throughout the CNS and integration in grey matter (GM). The main cells in myelinated tracts are oligodendrocytes and astrocytes, with small populations of microglia and oligodendrocyte precursor cells. The prominence of neurotransmitter signaling in WM, which largely exclude neuronal cell bodies, indicates it must have physiological functions other than neuron-to-neuron communication. A surprising aspect is the diversity of neurotransmitter signaling in WM, with evidence for glutamatergic, purinergic (ATP and adenosine), GABAergic, glycinergic, adrenergic, cholinergic, dopaminergic and serotonergic signaling, acting via a wide range of ionotropic and metabotropic receptors. Both axons and glia are potential sources of neurotransmitters and may express the respective receptors. The physiological functions of neurotransmitter signaling in WM are subject to debate, but glutamate and ATP-mediated signaling have been shown to evoke Ca(2+) signals in glia and modulate axonal conduction. Experimental findings support a model of neurotransmitters being released from axons during action potential propagation acting on glial receptors to regulate the homeostatic functions of astrocytes and myelination by oligodendrocytes. Astrocytes also release neurotransmitters, which act on axonal receptors to strengthen action potential propagation, maintaining signaling along potentially long axon tracts. The co-existence of multiple neurotransmitters in WM tracts suggests they may have diverse functions that are important for information processing. Furthermore, the neurotransmitter signaling phenomena described in WM most likely apply to myelinated axons of the cerebral cortex and GM areas, where they are doubtless important for higher cognitive function.

Β-adrenergic-stimulated L-type channel Ca²+ entry mediates hypoxic Ca²+ overload in intact heart

Ca(2+) mishandling plays a key role in ischemia- and hypoxia-related cardiac dysfunction and injury. However, the cellular and molecular mechanisms underlying hypoxic intracellular Ca(2+) ([Ca(2+)]i) overload remain incompletely understood. This study aimed to investigate possible mechanisms of [Ca(2+)]i overload during hypoxia in the intact heart. In Langendorff-perfused heart expressing the Ca(2+) indicator GCaMP2, confocal microscopy was used to simultaneously visualize [Ca(2+)]i, mitochondrial membrane potential (ΔΨm, by tetramethylrhodamine methyl ester) and sarcolemmal integrity (by Evans blue). Upon hypoxia (pO2 ~20 mmHg in glucose-free perfusate), [Ca(2+)]i transients were initially enhanced and then became depressed, arrhythmic, and completely abolished within 12 min. At ~20 min, basal [Ca(2+)]i rose to its first peak at a supraphysiological level, coincident with loss of ΔΨm and onset of rigor. A greater [Ca(2+)]i rise occurred at ~2h and was linked to the loss of sarcolemmal integrity. Removal of extracellular Ca(2+) or blockade of the l-type Ca(2+) channel (LTCC) (10 μM diltiazem or nifedipine) prevented [Ca(2+)]i overload and markedly delayed the loss of ΔΨm; by contrast, depletion of the sarcoplasmic reticulum Ca(2+) store by thapsigargin did not have any significant effect. Importantly, β-adrenergic blockade or depletion of the sympathetic catecholamine store by reserpine slowed the Ca(2+) and mitochondrial responses to hypoxia in intact heart. This LTCC-mediated hypoxic [Ca(2+)]i overload was reproduced in isolated cardiomyocytes when β-adrenergic agonist was present. Taken together, we conclude that Ca(2+) entry through β-adrenergic-stimulated LTCC underlies hypoxia-induced [Ca(2+)]i overload and the ensuing loss of mitochondrial function in intact heart.

Roles of white matter in central nervous system pathophysiologies

The phylogenetic enlargement of cerebral cortex culminating in the human brain imposed greater communication needs that have been met by the massive expansion of WM (white matter). Damage to WM alters brain function, and numerous neurological diseases feature WM involvement. In the current review, we discuss the major features of WM, the contributions of WM compromise to brain pathophysiology, and some of the mechanisms mediating WM injury. We will emphasize the newly appreciated importance of neurotransmitter signalling in WM, particularly glutamate and ATP signalling, to understanding both normal and abnormal brain functions. A deeper understanding of the mechanisms leading to WM damage will generate much-needed insights for developing therapies for acute and chronic diseases with WM involvement.

Diffusion tensor imaging detected optic nerve injury correlates with decreased compound action potentials after murine retinal ischemia

PURPOSE

This study evaluated the function of mouse optic nerves after transient retinal ischemia using in vitro electrophysiologic recordings of compound action potentials (CAPs) correlated with diffusion tensor imaging (DTI) injury markers with confirmation by immunohistochemistry-determined pathology.

METHODS

Retinal ischemia was induced in 7- to 8-week-old female C57BL/6 mice by elevating intraocular pressure to 110 mm Hg for 60 minutes. At 3 and 7 days after retinal ischemia, optic nerves were removed for CAP measurements. The CAP amplitude was recorded using suction electrodes in isolated control and injured optic nerves followed by ex vivo DTI evaluation. After DTI, optic nerves were embedded in paraffin and cut for immunohistochemical analyses.

RESULTS

Consistent with previous in vivo DTI measurements, a 25% decrease in axial diffusivity with normal radial diffusivity was seen at 3 days after retinal ischemia, suggesting axonal injury without myelin damage. At 7 days, there was no additional change in axial diffusivity compared with that at 3 days, but radial diffusivity significantly increased by 50%, suggestive of significant myelin damage due to sustained axonal injury. The relative anisotropy (RA) progressively decreased after retinal ischemia when compared with that of the controls. The CAP amplitude in injured nerves also progressively decreased after retinal ischemia, which correlated with the reduced RA (r = 0.80).

CONCLUSIONS

This study suggests that CAP amplitude reflects both axonal and myelin integrity and RA is an optimal parameter for functional assessment compared with axial or radial diffusivity alone in murine optic nerves after retinal ischemia.

The axo-myelinic synapse

Axons have evolved to acquire myelination, enabling denser packing and speedier transmission. Although myelin is considered a passive insulator, recent reports suggest a more dynamic role. Axons, in turn, are endowed with neurotransmitter release and uptake systems along their trunks. Based on these observations, I argue that there may exist a new type of chemical synapse between axon and myelin, one that supports activity-dependent communication between the two. This raises intriguing possibilities of dynamic fine-tuning of the myelin sheath even in adulthood, efficient recruitment of resources for myelin maintenance and bi-directional signaling, whereby the axon informs its myelinating cell of its metabolic needs proportionally to the electrical traffic it is transmitting. This would also have implications for de- and dysmyelinating diseases should this axo-myelinic synapse become dysfunctional.

The role of phosphatidylethanolamine methyltransferase in a mouse model of intrahepatic cholestasis

Intrahepatic cholestasis eventually leads to liver failure. We report here a condition that decreases liver damage in intrahepatic cholestasis based on a mouse model that lacks multiple drug resistant protein 2 (ABCB4). We found that lack of phosphatidylethanolamine N-methyltransferase (PEMT) decreased liver damage in Abcb4(-/-) mice caused by exposure of the liver to excess bile acids. The protective effect was not related to hepatic ratio of phosphatidylcholine to phosphatidylethanolamine or the level of cholesterol. The decreased concentration of bile acids in liver was related to impaired re-absorption of bile acids in intestine and increased disposal of bile acids in feces in Abcb4(-/-)/Pemt(-/-) mice as compared to Abcb4(-/-) mice. PEMT deficiency affected intestinal Na(+) absorption resulting in an impaired Na(+) concentration gradient along the length of the small intestine and abnormal absorption of bile acids mediated by apical sodium-dependent bile acid transporter (ASBT). The findings of this study suggest that inhibition of PEMT and/or reduction of intestinal sodium concentration may be helpful in attenuating liver damage and prolonging hepatic function in intrahepatic cholestasis.

Identification of four functional NR3B isoforms in developing white matter reveals unexpected diversity among glutamate receptors

Functional neurotransmitter receptors are expressed in central white matter, where they mediate ischemic damage to glia and may be involved in cell-cell signalling. In this study, we analysed NMDA receptor NR1, NR2B-C and NR3A-B subunit expression in the brain and optic nerve by molecular cloning. In addition to the canonical forms of NR1 and NR2, four previously unknown NR3B variants, generated by alternative splicing, were identified. The variants encoded for isoforms with deletions of 8/15 amino acids in the N-terminal domain or 200/375 amino acids removing one or three transmembrane domains and part of the C-terminal domain, as compared with the previously characterized NR3B isoform. Co-expression of NR3B isoforms with NR1/NR2A-C modulated the amplitude and Mg(2+)-sensitivity of glutamate responses in a NR2 subunit-dependent fashion, with significant variations in the effects produced by different isoforms. These effects were not the result of reduced surface expression of the receptor complex since all NR3B isoforms reduced surface expression by a similar degree. These data reveal previously uncharacterized regulation of NMDA receptor function by alternative splicing of the NR3B subunit.

Glia as transmitter sources and sensors in health and disease

Glial cells express a bewildering array of neurotransmitter receptors. To illustrate the complexity of expression, we have assayed non-glutamatergic neurotransmitter receptor mRNA in isolated rat optic nerve, a preparation devoid of neurons and neuronal synapses and from which relatively pure "glial" RNA can be isolated. Of the 44 receptor subunits examined which span the GABA-A, nicotinic, adreno- and glycine receptor families, over three quarters were robustly expressed in this mixed population of white matter glial cells, with several expressed at higher levels than found in control whole brain RNA. In addition to the complexity of glial receptor expression, numerous neurotransmitter release mechanisms have been identified. We have focused on glutamate release from astrocytes, which can occur via at least seven distinct pathways and which is implicated in excitotoxic injury and are neurons and glia. Recent findings suggest that non-glutamatergic receptors can also mediate acute glial injury are also discussed.

Progressive multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory, demyelinating disease of the central nervous system, which starts in the majority of patients with a relapsing/remitting MS (RRMS) course , which after several years of disease duration converts into a progressive disease. Since anti-inflammatory therapies and immune modulation exert a beneficial effect at the relapsing/remitting stage of the disease, but not in the progressive stage, the question was raised whether inflammation drives tissue damage in progressive MS at all. We show here that also in progressive MS, inflammation is the driving force for brain injury and that the discrepancy between inflammation-driven tissue injury and response to immunomodulatory therapies can be explained by different pathomechanisms acting in RRMS and progressive MS.

Mechanisms of axonal injury: internodal nanocomplexes and calcium deregulation

Axonal degeneration causes morbidity in many neurological conditions including stroke, neurotrauma and multiple sclerosis. The limited ability of central nervous system (CNS) neurons to regenerate, combined with the observation that axonal damage causes clinical disability, has spurred efforts to investigate the mechanisms of axonal degeneration. Ca influx from outside the axon is a key mediator of injury. More recently, substantial pools of intra-axonal Ca sequestered in the 'axoplasmic reticulum' have been reported. These Ca stores are under the control of multimolecular 'nanocomplexes' located along the internodes under the myelin. The overactivation of these complexes during disease can lead to a lethal release of Ca from intra-axonal stores. Rich receptor pharmacology offers tantalizing therapeutic options targeting these nanocomplexes in the many diseases where axonal degeneration is prominent.