Axonal release of glutamate analog, d-2,3-3H-Aspartic acid and l-14C-proline from segments of sciatic nerve following electrical and magnetic stimulation

Glutamate receptors and glutamatergic signalling in the peripheral nerves

In the peripheral nervous system, the vast majority of axons are accommodated within the fibre bundles that constitute the peripheral nerves. Axons within the nerves are in close contact with myelinating glia, the Schwann cells that are ideally placed to respond to, and possibly shape, axonal activity. The mechanisms of intercellular communication in the peripheral nerves may involve direct contact between the cells, as well as signalling via diffusible substances. Neurotransmitter glutamate has been proposed as a candidate extracellular molecule mediating the cross-talk between cells in the peripheral nerves. Two types of experimental findings support this idea: first, glutamate has been detected in the nerves and can be released upon electrical or chemical stimulation of the nerves; second, axons and Schwann cells in the peripheral nerves express glutamate receptors. Yet, the studies providing direct experimental evidence that intercellular glutamatergic signalling takes place in the peripheral nerves during physiological or pathological conditions are largely missing. Remarkably, in the central nervous system, axons and myelinating glia are involved in glutamatergic signalling. This signalling occurs via different mechanisms, the most intriguing of which is fast synaptic communication between axons and oligodendrocyte precursor cells. Glutamate receptors and/or synaptic axon-glia signalling are involved in regulation of proliferation, migration, and differentiation of oligodendrocyte precursor cells, survival of oligodendrocytes, and re-myelination of axons after damage. Does synaptic signalling exist between axons and Schwann cells in the peripheral nerves? What is the functional role of glutamate receptors in the peripheral nerves? Is activation of glutamate receptors in the nerves beneficial or harmful during diseases? In this review, we summarise the limited information regarding glutamate release and glutamate receptors in the peripheral nerves and speculate about possible mechanisms of glutamatergic signalling in the nerves. We highlight the necessity of further research on this topic because it should help to understand the mechanisms of peripheral nervous system development and nerve regeneration during diseases.

Glutamate Activates AMPA Receptor Conductance in the Developing Schwann Cells of the Mammalian Peripheral Nerves

Schwann cells (SCs) are myelinating cells of the PNS. Although SCs are known to express different channels and receptors on their surface, little is known about the activation and function of these proteins. Ionotropic glutamate receptors are thought to play an essential role during development of SC lineage and during peripheral nerve injury, so we sought to study their functional properties. We established a novel preparation of living peripheral nerve slices with preserved cellular architecture and used a patch-clamp technique to study AMPA-receptor (AMPAR)-mediated currents in SCs for the first time. We found that the majority of SCs in the nerves dissected from embryonic and neonatal mice of both sexes respond to the application of glutamate with inward current mediated by Ca²⁺-permeable AMPARs. Using stationary fluctuation analysis (SFA), we demonstrate that single-channel conductance of AMPARs in SCs is 8-11 pS, which is comparable to that in neurons. We further show that, when SCs become myelinating, they downregulate functional AMPARs. This study is the first to demonstrate AMPAR-mediated conductance in SCs of vertebrates, to investigate elementary properties of AMPARs in these cells, and to provide detailed electrophysiological and morphological characterization of SCs at different stages of development.SIGNIFICANCE STATEMENT We provide several important conceptual and technical advances in research on the PNS. We pioneer the first description of AMPA receptor (AMPAR)-mediated currents in the PNS glia of vertebrates and provide new insights into the properties of AMPAR channels in peripheral glia; for example, their Ca²⁺ permeability and single-channel conductance. We describe for the first time the electrophysiological and morphological properties of Schwann cells (SCs) at different stages of development and show that functional AMPARs are expressed only in developing, not mature, SCs. Finally, we introduce a preparation of peripheral nerve slices for patch-clamp recordings. This preparation opens new possibilities for studying the physiology of SCs in animal models and in surgical human samples.

N- and L-Type Voltage-Gated Calcium Channels Mediate Fast Calcium Transients in Axonal Shafts of Mouse Peripheral Nerve

In the peripheral nervous system (PNS) a vast number of axons are accommodated within fiber bundles that constitute peripheral nerves. A major function of peripheral axons is to propagate action potentials along their length, and hence they are equipped with Na(+) and K(+) channels, which ensure successful generation, conduction and termination of each action potential. However little is known about Ca(2+) ion channels expressed along peripheral axons and their possible functional significance. The goal of the present study was to test whether voltage-gated Ca(2+) channels (VGCCs) are present along peripheral nerve axons in situ and mediate rapid activity-dependent Ca(2+) elevations under physiological circumstances. To address this question we used mouse sciatic nerve slices, Ca(2+) indicator Oregon Green BAPTA-1, and 2-photon Ca(2+) imaging in fast line scan mode (500 Hz). We report that transient increases in intra-axonal Ca(2+) concentration take place along peripheral nerve axons in situ when axons are stimulated electrically with single pulses. Furthermore, we show for the first time that Ca(2+) transients in peripheral nerves are fast, i.e., occur in a millisecond time-domain. Combining Ca(2+) imaging and pharmacology with specific blockers of different VGCCs subtypes we demonstrate that Ca(2+) transients in peripheral nerves are mediated mainly by N-type and L-type VGCCs. Discovery of fast Ca(2+) entry into the axonal shafts through VGCCs in peripheral nerves suggests that Ca(2+) may be involved in regulation of action potential propagation and/or properties in this system, or mediate neurotransmitter release along peripheral axons as it occurs in the optic nerve and white matter of the central nervous system (CNS).

Trans-spinal direct current enhances corticospinal output and stimulation-evoked release of glutamate analog, D-2,3-³H-aspartic acid

Trans-spinal direct current (tsDC) stimulation is a modulator of spinal excitability and can influence cortically elicited muscle contraction in a polarity-dependent fashion. When combined with low-frequency repetitive cortical stimulation, cathodal tsDC [tsDC(-)] produces a long-term facilitation of cortically elicited muscle actions. We investigated the ability of this combined stimulation paradigm to facilitate cortically elicited muscle actions in spinal cord-injured and noninjured animals. The effect of tsDC-applied alone or in combination with repetitive spinal stimulation (rSS) on the release of the glutamate analog, D-2,3-(3)H-aspartate (D-Asp), from spinal cord preparations in vitro-was also tested. In noninjured animals, tsDC (-2 mA) reproducibly potentiated cortically elicited contractions of contralateral and ipsilateral muscles tested at various levels of baseline muscle contraction forces. Cortically elicited muscle responses in animals with contusive and hemisectioned spinal cord injuries (SCIs) were similarly potentiated. The combined paradigm of stimulation caused long-lasting potentiation of cortically elicited bilateral muscle contraction in injured and noninjured animals. Additional analysis suggests that at higher baseline forces, tsDC(-) application does not increase the rising slope of the muscle contraction but causes repeated firing of the same motor units. Both cathodal and anodal stimulations induced a significant increase of D-Asp release in vitro. The effect of the combined paradigm of stimulation (tsDC and rSS) on the concentration of extracellular D-Asp was polarity dependent. These results indicate that tsDC can powerfully modulate the responsiveness of spinal cord neurons. The results obtained from the in vitro preparation suggest that the changes in neuronal excitability were correlated with an increased concentration of extracellular glutamate. The combined paradigm of stimulation, used in our experiments, could be noninvasively applied to restore motor control in humans with SCI.

Distribution of radiolabeled l-glutamate and d-aspartate from blood into peripheral tissues in naive rats: significance for brain neuroprotection

Excess l-glutamate (glutamate) levels in brain interstitial and cerebrospinal fluids (ISF and CSF, respectively) are the hallmark of several neurodegenerative conditions such as stroke, traumatic brain injury or amyotrophic lateral sclerosis. Its removal could prevent the glutamate excitotoxicity that causes long-lasting neurological deficits. As in previous studies, we have established the role of blood glutamate levels in brain neuroprotection, we have now investigated the contribution of the peripheral organs to the homeostasis of glutamate in blood. We have administered naive rats with intravenous injections of either l-[1-(14)C] Glutamic acid (l-[1-(14)C] Glu), l-[G-(3)H] Glutamic acid (l-[G-(3)H] Glu) or d-[2,3-(3)H] Aspartic acid (d-[2,3-(3)H] Asp), a non-metabolized analog of glutamate, and have followed their distribution into peripheral organs. We have observed that the decay of the radioactivity associated with l-[1-(14)C] Glu and l-[G-(3)H] Glu was faster than that associated with glutamate non-metabolized analog, d-[2,3-(3)H] Asp. l-[1-(14)C] Glu was subjected in blood to a rapid decarboxylation with the loss of (14)CO(2). The three major sequestrating organs, serving as depots for the eliminated glutamate and/or its metabolites were skeletal muscle, liver and gut, contributing together 92% or 87% of total l-[U-(14)C] Glu or d-[2,3-(3)H] Asp radioactivity capture. l-[U-(14)C] Glu and d-[2,3-(3)H] Asp showed a different organ sequestration pattern. We conclude that glutamate is rapidly eliminated from the blood into peripheral tissues, mainly in non-metabolized form. The liver plays a central role in glutamate metabolism and serves as an origin for glutamate metabolites that redistribute into skeletal muscle and gut. The findings of this study suggest now that pharmacological manipulations that reduce the liver glutamate release rate or cause a boosting of the skeletal muscle glutamate pumping rate are likely to cause brain neuroprotection.

Excitability changes in the sciatic nerve and triceps surae muscle after spinal cord injury in mice

BACKGROUND

From the onset to the chronic phase of spinal cord injury (SCI), peripheral axons and muscles are subjected to abnormal states of activity. This starts with very intense spasms during the first instant of SCI, through a no activity flaccidity phase, to a chronic hyperactivity phase. It remains unclear how the nature of this sequence may affect the peripheral axons and muscles.

METHODS

We set out to investigate the changes in excitability of the sciatic nerve and to characterize the properties of muscle contractility after contusive injury of the mouse thoracic spinal cord.

RESULTS

The following changes were observed in animals after SCI: 1) The sciatic nerve compound action potential was of higher amplitudes and lower threshold, with the longer strength-duration time constant and faster conduction velocity; 2) The latency of the onset of muscle contraction of the triceps surae muscle was significantly shorter in animals with SCI; 3) The muscle twitches expressed slower rising and falling slopes, which were accompanied by prolonged contraction duration in SCI animals compared to controls.

CONCLUSION

These findings suggest that in peripheral nerves SCI promotes hyperexcitability, which might contribute to mechanisms of spastic syndrome.

The influence of pulsed magnetic fields (PMFs) on nonsynaptic potentials recorded from the central and peripheral nervous systems in vitro

The influence of pulsed magnetic fields (PMFs) on nonsynaptic potentials recorded from the central and peripheral nervous system in vitro has been investigated. The population spikes (PSs) recorded from hippocampal slices during antidromic stimulation and compound action potentials (CAPs) recorded from the segments of the sciatic nerve were used as indicators of neuronal activity. The potentials recorded from both preparations were significantly and permanently enhanced following PMF (0.16 Hz, 15 mT) exposure. The increase in the antidromic PS occurred even in the presence of potassium channel blocker tetraethylammonium (TEA) and was accompanied by multiple spiking. Among all frequencies of PMF tested (0.5, 0.16, 0.07, 0.03, 0.0 Hz), the frequency of 0.5 Hz was the most effective in enhancement of potential amplitude. The influence of PMF on the amplitude of two CAPs evoked by the pair of electrical stimuli applied in rapid succession has also been evaluated. In control conditions the potential triggered by the second stimuli was slightly smaller expressing the phenomenon of short-term depression (STD). Although PMF exposure amplified the amplitude of both potentials, the increase in the size of the first potential was significantly greater increasing further the magnitude of STD. The blocking of potassium channels reversed STD into facilitation. One of the possible mechanisms involved in PMF action could be the modification of the axonal threshold, which was significantly reduced following exposure to PMF.