Remyelination of nerve fibers in the transected frog sciatic nerve

Determination of Nerve Fiber Diameter Distribution From Compound Action Potential: A Continuous Approach

When a signal is initiated in the nerve, it is transmitted along each nerve fiber via an action potential (called single fiber action potential (SFAP)) which travels with a velocity that is related with the diameter of the fiber. The additive superposition of SFAPs constitutes the compound action potential (CAP) of the nerve. The fiber diameter distribution (FDD) in the nerve can be computed from the CAP data by solving an inverse problem. This is usually achieved by dividing the fibers into a finite number of diameter groups and solve a corresponding linear system to optimize FDD. However, number of fibers in a nerve can be measured sometimes in thousands and it is possible to assume a continuous distribution for the fiber diameters which leads to a gradient optimization problem. In this paper, we have evaluated this continuous approach to the solution of the inverse problem. We have utilized an analytical function for SFAP and an assumed a polynomial form for FDD. The inverse problem involves the optimization of polynomial coefficients to obtain the best estimate for the FDD. We have observed that an eighth order polynomial for FDD can capture both unimodal and bimodal fiber distributions present in vivo, even in case of noisy CAP data. The assumed FDD distribution regularizes the ill-conditioned inverse problem and produces good results.

Effect of Sciatic Nerve Transection on acetylcholinesterase activity in spinal cord and skeletal muscles of the bullfrog Lithobates catesbeianus

Sciatic nerve transection (SNT), a model for studying neuropathic pain, mimics the clinical symptoms of "phantom limb", a pain condition that arises in humans after amputation or transverse spinal lesions. In some vertebrate tissues, this condition decreases acetylcholinesterase (AChE) activity, the enzyme responsible for fast hydrolysis of released acetylcholine in cholinergic synapses. In spinal cord of frog Rana pipiens, this enzyme's activity was not significantly changed in the first days following ventral root transection, another model for studying neuropathic pain. An answerable question is whether SNT decreases AChE activity in spinal cord of frog Lithobates catesbeianus, a species that has been used as a model for studying SNT-induced neuropathic pain. Since each animal model has been created with a specific methodology, and the findings tend to vary widely with slight changes in the method used to induce pain, our study assessed AChE activity 3 and 10 days after complete SNT in lumbosacral spinal cord of adult male bullfrog Lithobates catesbeianus. Because there are time scale differences of motor endplate maturation in rat skeletal muscles, our study also measured the AChE activity in bullfrog tibial posticus (a postural muscle) and gastrocnemius (a typical skeletal muscle that is frequently used to study the motor system) muscles. AChE activity did not show significant changes 3 and 10 days following SNT in spinal cord. Also, no significant change occurred in AChE activity in tibial posticus and gastrocnemius muscles at day 3. However, a significant decrease was found at day 10, with reductions of 18% and 20% in tibial posticus and gastrocnemius, respectively. At present we cannot explain this change in AChE activity. While temporally different, the direction of the change was similar to that described for rats. This similarity indicates that bullfrog is a valid model for investigating AChE activity following SNT.

Delayed nerve stimulation promotes axon-protective neurofilament phosphorylation, accelerates immune cell clearance and enhances remyelination in vivo in focally demyelinated nerves

Rapid and efficient axon remyelination aids in restoring strong electrochemical communication with end organs and in preventing axonal degeneration often observed in demyelinating neuropathies. The signals from axons that can trigger more effective remyelination in vivo are still being elucidated. Here we report the remarkable effect of delayed brief electrical nerve stimulation (ES; 1 hour @ 20 Hz 5 days post-demyelination) on ensuing reparative events in a focally demyelinated adult rat peripheral nerve. ES impacted many parameters underlying successful remyelination. It effected increased neurofilament expression and phosphorylation, both implicated in axon protection. ES increased expression of myelin basic protein (MBP) and promoted node of Ranvier re-organization, both of which coincided with the early reappearance of remyelinated axons, effects not observed at the same time points in non-stimulated demyelinated nerves. The improved ES-associated remyelination was accompanied by enhanced clearance of ED-1 positive macrophages and attenuation of glial fibrillary acidic protein expression in accompanying Schwann cells, suggesting a more rapid clearance of myelin debris and return of Schwann cells to a nonreactive myelinating state. These benefits of ES correlated with increased levels of brain derived neurotrophic factor (BDNF) in the acute demyelination zone, a key molecule in the initiation of the myelination program. In conclusion, the tremendous impact of delayed brief nerve stimulation on enhancement of the innate capacity of a focally demyelinated nerve to successfully remyelinate identifies manipulation of this axis as a novel therapeutic target for demyelinating pathologies.

Effect of tramadol on the rat sciatic nerve conduction: a numerical analysis and conduction velocity distribution study

The aim of this study was to document the effect of tramadol as an opioid on individual fibers of rat sciatic nerve. To accomplish this objective, compound action potentials (CAPs) were recorded from isolated nerves treated with tramadol from five different concentration levels. Then recorded CAPs and the control group were analyzed by numerical methods namely Conduction Velocity Distribution (CVD) and Fast Fourier Transform (FFT). The results show that the area under CAP and the time derivative of CAP curves decreases, and the excitability of the nerve trunk falls as well (rheobase and chronaxie increases) with increasing tramadol concentration. CVD deduced by model study was divided into subgroups as SLOW (8-26 m/s), MODERATE (26-44 m/s), MEDIUM (44-60 m/s) and FAST (60-78 m/s). The decrement in percentage relative contribution of these conduction velocity groups starts with a concentration of 0.25 mM tramadol, especially in the subgroup named FAST. The power spectrum shifts from higher frequency region to lower frequency region as the tramadol concentration increases. These findings show that fast conducting fibers are more susceptible to tramadol than medium and moderate groups and tramadol possibly acts on channel activity rather than passive properties (such as space and time constant) of nerve fibers.

Does Collagenase Affect the Electrophysiological Parameters of Nerve Trunk?

Collegenase is widely used in the process of teasing a nerve in order to perform single fiber action potential (SFAP) recordings. In this study, the effects of collagenase on nerve conduction parameters were investigated. To accomplish this, normal compound action potentials (nCAPs) were recorded from isolated frog sciatic nerve at various distances using the suction technique. Then, the same nerve was treated with collagenased Ringer's solution (3.5 mg/ml, Sigma Type XI) for 90 minutes and action potentials (cCAPs) were recorded again. Numerical analysis of these records was performed and the results were compared. Using the nCAP and cCAP recordings, the conduction velocity distributions (CVD) of the individual nerve trunks were determined by a method that we have previously described. Statistical results indicated significant differences (p < 0.05) between the nCAP and cCAP CVD data. From these findings it is concluded that, when used for teasing the nerve fibers, collagenase may affect the nerve trunk conduction parameters. Specifically, a significant amount of decrease has been observed in conduction velocities of myelinated fibers having diameters smaller than 8 microns.

Polarized domains of myelinated axons

The entire length of myelinated axons is organized into a series of polarized domains that center around nodes of Ranvier. These domains, which are crucial for normal saltatory conduction, consist of distinct multiprotein complexes of cell adhesion molecules, ion channels, and scaffolding molecules; they also differ in their diameter, organelle content, and rates of axonal transport. Juxtacrine signals from myelinating glia direct their sequential assembly. The composition, mechanisms of assembly, and function of these molecular domains will be reviewed. I also discuss similarities of this domain organization to that of polarized epithelia and present emerging evidence that disorders of domain organization and function contribute to the axonopathies of myelin and other neurologic disorders.

A correction procedure for the volume conductor effect in the compound action potential recorded from isolated nerve trunk

The shape and magnitude of the compound action potential (CAP), which is the linear summation of the single fiber action potentials, depend strongly on the recording conditions. Volume conductor effect should be eliminated or corrected in order to get reliable information about the functional state of the nerve trunk. In the case of monophasic extracellular recordings, the integral of CAP recorded extracellularly tends to decrease with the distance, because the extracellular resistance between the stimulating and recording electrodes changes. To compensate for this effect, we took into account the spatial deviation of the integral of CAP versus distance and defined a spatial correcting factor, g(x). By applying g(x) to all CAPs, we get corrected CAP (cCAP) data for further evaluations. It is well known that the slope of the maximum derivative of CAP versus distance curve would be a measure of conduction velocity distribution for the fast conducting nerves in a nerve trunk. The slopes of these curves for extracellular and suction techniques on the same nerves are compared; we concluded that the difference between the two techniques was not important for the correction procedure on extracellular records.

Clusters of axonal Na+ channels adjacent to remyelinating Schwann cells

Rat sciatic nerve fibres were demyelinated by injection of lysolecithin and examined at several stages as Schwann cells proliferated, adhered, and initiated remyelination. Immunoperoxidase EM has been used to follow the clustering of Na+ channels that represents an early step in the formation of new nodes of Ranvier. At the peak of demyelination, 1 week post-injection, only isolated sites, suggestive of the original nodes, were labelled. As Schwann cells adhered and extended processes along the axons, regions of axonal Na+ channel immunoreactivity were often found just beyond their leading edges. These channel aggregates were associated only with the axolemma and Na+ channels were not detected on glial membranes. Sites with more than one cluster in close proximity and broadly labelled aggregates between Schwann cells suggested that new nodes of Ranvier formed as neighbouring Na+ channel groups merged. Schwann cells thus seem to play a major role in ion channel distributions in the axolemma. In all of these stages Na+ channel label was found primarily just outside the region of close contact between axon and Schwann cell. This suggests that Schwann cell adherence acts in part to exclude Na+ channels, or that diffusible substances are involved and can act some distance from regions of direct contact.

Na+ channel aggregation in remyelinating mouse sciatic axons following transection

Mouse sciatic nerves from the degeneration-resistant strain C57BL/6/Wld (Ola) were surgically injected with lysolecithin to induce focal demyelination. Three days later they were transected adjacent to the spinal cord to eliminate contact of the axons with their cell bodies. The Na+ channel distribution was assessed by immunocytochemistry and followed at several stages of remyelination. Control experiments were performed on nerves that were injected but not cut. At (3 + 4) days, namely, nerves cut 3 days post-injection and examined 4 days after cutting, axons contained fully demyelinated regions. Na+ channel clusters appeared only at heminodes and at isolated sites that are likely to represent original nodes of Ranvier. During the next few days proliferating Schwann cells adhered to the axons and extended their processes. Clusters of Na+ channels appeared at their edges, and as the Schwann cells elongated the distance between these aggregates increased. A few clusters associated with neighboring Schwann cells approached each other and appeared to coalesce at sites where presumably new nodes of Ranvier would be formed. Beyond (3 + 6) days excessive degeneration of the transected axons precluded further observations. In the uncut controls, the spatio-temporal sequence of Schwann cell proliferation and channel patch formation and movement was similar to that described above, although myelin formation was somewhat faster than in the cut axons. It is concluded that Na+ channel aggregation associated with the early stages of remyelination is not dependent upon continuous communication of the axon with its cell body and is under local control.

In vitro studies of axonally-regulated Schwann cell genes during Wallerian degeneration

Wallerian degeneration in vivo is associated with marked downregulation of myelin protein genes such as P(o) and upregulation of other genes such as nerve growth factor receptor (NGF-R), glial fibrillary acidic protein (GFAP) and neural cell adhesion molecule (N-CAM). This study examines the expression of these genes during Wallerian degeneration in vitro and how manipulating Ca2+ affects this response. Small explants of sciatic nerve from normal young adult rats cultured for five days show similar reversal of the myelinating phenotype as found in vivo. If Ca++ is removed from the culture medium through the addition of EGTA, expression of the nerve growth factor receptor and glial fibrillary acidic protein genes is inhibited but downregulation of the P(o) gene still occurs. Explants cultured in medium containing EGTA are still capable of expressing nerve growth factor receptor if the medium is replaced by one containing Ca2+. Supplementation of normal medium with drugs modulating Ca2+, such as Bepridil which blocks the Na+Ca2+ exchanger or compound 48/80 which inhibits calmodulin, also prevent the expression of the nerve growth factor receptor gene during Wallerian degeneration in vitro. Treatment of the cervical sympathetic trunk with Bepridil leads to loss of the nerve growth factor receptor immunoreactivity which is normally present. The results indicate that Ca2+ may play a role in the expression of the nerve growth factor receptor gene during Wallerian degeneration and provide some indication that this effect may be directly on the Schwann cell rather than operating indirectly via the axon.

Increase of sodium channels in demyelinated lesions of multiple sclerosis

Redistribution of sodium channels along demyelinated pathways in multiple sclerosis (MS) could be an important event in restoring conduction prior to other reparative mechanisms such as remyelination. Sodium channels in human multiple sclerosis lesions were identified by quantitative light microscopic autoradiography using tritiated saxitoxin (STX), a highly specific sodium channel ligand. Demyelinated areas in various central nervous system regions containing denuded but vital axons exhibited a high increase of STX-binding sites by up to a factor of 4 as compared to normal human white matter. This important finding could explain aspects of fast clinical remissions and 'silent' MS lesions on functional and morphological properties. Demyelinated axons may functionally reorganize their membranes and adapt properties similar to those of slow conducting unmyelinated nerve fibres which have a higher amount and a more diffuse distribution of STX binding sites. This report is the first description of an altered distribution of voltage-sensitive sodium channels in human multiple sclerosis lesions.