Effects of electric stimulation and previous nerve injury on motor function recovery in rats

Can Chromatin Accessibility be Exploited for Axon Regeneration?

Several studies have demonstrated that the intrinsic ability of neurons to regenerate their axons can be stimulated by maneuvers that favor the open state of chromatin, such as inhibiting histone deacetylase activity or increasing histone acetyltransferase activity. Taken together, these experiments suggest that axon regenerative ability can be increased by promoting chromatin accessibility. In this article, we assess the direct evidence in the literature for this hypothesis and re-examine other axon regeneration-promoting manipulations to see if they provide additional support. We find that several interventions known to enhance intrinsic axonal growth capability also increase chromatin accessibility. Although the support for this correlation is strong in the literature, we conclude with a word of caution about therapeutics attempting to exploit this relationship.

The Effects of Different Electrical Stimulation Protocols on Nerve Regeneration Through Silicone Conduits

BACKGROUND

The effects of electrical stimulation on the regeneration of transected nerves through silicone rubber conduits may depend on the stimulation protocol.

METHODS

Rat sciatic nerve was transected and reconnected using a silicone rubber conduit with a 7-mm gap. The subjects were divided into nine groups. Each group received one protocol of electrical stimulation 30 minutes each day for 3 weeks. The effects of electrical stimulation were evaluated through morphologic methods.

RESULTS

Direct current stimulation increased nerve fiber bundle area without significantly affecting the number of myelinated fibers, resulting in decreased nerve fiber density. All the pulse current stimulation protocols as a group decreased the nerve fiber bundle area without affecting the number of myelinated fibers, resulting in increased nerve fiber density. Within the pulse current stimulation protocols, stimulation frequency affected nerve fiber density, whereas current strength did not.

CONCLUSIONS

Stimulation protocols had significant influences on the benefit of electrical stimulation in nerve regeneration.

Microscopic changes at the neuromuscular junction in free muscle transfer

Free muscle transfers do not generate the same force after transfer as that at the original sites. Light and electron microscopy were used to study serially during 30 weeks the changes at the neuromuscular junction after free muscle transfer of the gracilis muscle in the adult Wistar rat. Under light microscopy, after staining with acetylthiocholine the neuromuscular junction showed changes of degeneration with withdrawal of the innervating axon terminal followed by regeneration and reconstitution of the neuromuscular junction. The newly formed neuromuscular junction still lacked the structural detail seen in the control neuromuscular junction, even after 30 weeks. With the electron microscope, mitochondrial swelling and clumping of the synaptic vesicles were followed by withdrawal of the axon terminal from the muscle membrane on denervation. The infolding of the muscle membrane at the neuromuscular junction became less prominent. With reinnervation the ultrastructure of the junction was only partially reestablished with poorly reconstituted primary and secondary folds of the muscle membrane 30 weeks after the transfer. Failure of complete reformation of the ultrastructure of the neuromuscular junction may provide another explanation for failure of full recovery of skeletal muscle function after free muscle transfer.

Influence of electrical stimulation on regeneration of the radial nerve in dogs

The effects of biphasic electric fields on nerve regeneration that follows injury to the left radial nerve were studied in dogs by electromyography (EMG). Left and right radial nerves were crushed with a serrated haemostat. Stimulating electrodes were positioned proximally and distally to the site of the injury. The left nerves received rectangular, biphasic and current pulses (30 microA, 0.5 Hz) through the injury for two months. The right radial nerves were treated as controls and regenerated without electrical stimulation. EMG activities were recorded intramuscularly from the left and right musculus extensor digitalis communis (MEDC). Results obtained at the end of the two-month stimulation period showed a significant difference in EMG activity between the left (stimulated) and the right (non-stimulated) MEDC, suggesting that electrical treatment enhanced nerve regeneration.

Recovery of original nerve supply after hypoglossal-facial anastomosis causes permanent motor hyperinnervation of the whisker-pad muscles in the rat

Hypoglossal-facial anastomosis (HFA), used in humans for the treatment of facial palsy, was experimentally performed in adult female Wistar rats. The time course of facial reinnervation and the extent of the new motor nerve supply of the vibrissal muscles that develops after HFA were estimated by counting all motoneurons in the brainstem labeled by injection of horseradish peroxidase (HRP) into the whisker pad; muscle innervation by motor endplates was not studied. In untreated animals, HRP injection labels 1,254 +/- 54 (mean +/- S.D.; n = 6) motoneurons, localized exclusively in the lateral subdivision of the facial nucleus. Immediately following HFA, this number drops to zero. The first HRP-labeled motoneurons appear in the hypoglossal nucleus at 28 days postoperation (dpo) and at 56 dpo their number reaches 1,096 +/- 48. Unexpectedly, the facial nerve, whose proximal stump has been left as blind end during surgery, additionally sends axons to the facial periphery. This resprouting is first detected at 42 dpo with HRP-marked neurons throughout the facial nucleus lacking somatotopic organization. The number of these labeled neurons also rises with time, and at 56 dpo, a total of 1,797 +/- 142 facial and hypoglossal motoneurons, that is, 43% more motoneurons than in normal animals, supplies the whisker pad. This hyperinnervation, that is, the projection of more motoneurons into the target muscle than under normal conditions--further increases to 1,978 +/- 92 motoneurons at 224 dpo and may provide a new animal model for studying the competitive relationships between motoneurons in their search for peripheral targets.

Electrical stimulation of nerve regeneration in the rat: the early effects evaluated by a vibrating probe and electron microscopy

This study examines the effect of applied d.c. electric fields on nerve regeneration following injury to the rat sciatic nerve using the circularly vibrating probe and electron microscopy. The transected and treated nerve which received a d.c. electrical stimulator (0.6 mu A) was compared with untreated transected and crushed nerves. At one week postoperative, the probe was used to measure in vivo the current density along the nerve length. All nerves studied had a proximal peak at the lesion site and a second peak at varying distal locations: crushed/untreated (13.3 mm), transected/untreated (9.7 mm) and transected/treated (16.3 mm). A significant difference (69%) between the distal peak distances in the two transection groups suggests that the electrical treatment enhanced the progress of nerve regeneration. There were no significant differences between the mean peak amplitudes (1.6-2.2 mu A/cm2). Applied verapamil reduced the peaks, suggesting they are associated in part with a calcium-dependent current. Electron microscopy at selected nerve regions indicated that the peaks correspond to regenerating axonal growth cones. The results suggest the potential clinical application of d.c. electric fields in the treatment of nerve injuries.

The effect of a conditioning lesion on sudomotor axon regeneration

The effects of a conditioning lesion on the rate of sudomotor axon regeneration were judged by the recovery of sweat gland (SG) secretion after cholinergic stimulation. Three groups of mice were given a conditioning lesion by crushing the sciatic nerve at mid-thigh 4, 7, and 14 days before a test lesion. A 4th group received a conditioning crush of the tibial nerve at the ankle 7 days before the test lesion. Control mice had a single test lesion. SG reinnervation in control mice began 19 days after the test lesion, and was functionally complete by 41 days. In groups with the conditioning lesion 4, 7, and 14 days before the test operation, the first reactive SGs reappeared at 16, 15, and 16 days respectively after the test lesion, and maximal recovery occurred by 33, 32, and 39 days. In mice with the distal conditioning lesion, reinnervation began at 19 days and was maximal by 36 days. In summary, a nerve conditioning lesion placed from 4 to 14 days prior to and at the same site as a test lesion significantly accelerated the growth rate of the fastest regenerating unmyelinated sudomotor axons and reduced the time until most SGs were reinnervated. A more distally placed test lesion reduced the interval for recovery.

Problems with the use of the toe-spreading reflex in rats as an assay in nerve regeneration studies

Stages in the return of the toe-spreading reflex after sciatic nerve injury were examined using the rat. It was found that the earliest stages in the return of the reflex do not indicate nerve regeneration, but rather reflect the development of adrenalin sensitivity in denervated muscles. Probably systemic adrenalin released during the reflex-testing procedure causes muscle contractions which imitate a nerve-induced toe-spread reflex.

A twitch tension method to assess motor nerve function in rat

A simple and quantitative method is described which assesses the motor function of the rat tibial nerve by recording the twitch tension developed by the digital flexors. Stimulating electrodes are placed at the proximal end of the sciatic nerve of anesthetized rats, and the associated foot is immobilized on a sliding stage. The middle digit is connected via a thread to a force-displacement transducer and the sciatic nerve is stimulated supramaximally. The resulting twitch tension curve is recorded on an oscilloscope and/or signal averager. Measurement of the area and amplitude of the twitch tension curve provides an estimate of the recovery of motor function. The method is illustrated by examining the return of motor function following either sciatic nerve crush, or sciatic transection and repair. The results obtained are compared with other assessments of sciatic nerve function, such as the toe-spreading reflex and the analysis of walking tracks. The results from the twitch tension method are more sensitive, unaffected by changes in sensory function, and less prone to variation than the other assessments.

The effect of electrical stimulation on reinnervation of rat muscle: contractile properties and endplate morphometry

Denervated extensor digitorum longus muscles of Wistar rats were electrically stimulated in vivo for 4 days (2h per day) after peroneal nerve crush 1 cm from the muscle. Isometric contractile properties and endplate ultrastructure were measured on days 11 and 18. On day 11, the time to peak (116% of control) and 1/2-relaxation time (136% of control) for the twitch tensions of stimulated muscles measured in vivo were significantly less than those (127% and 157% of controls, respectively) of non-stimulated muscles. Peak twitch and tetanic tensions were not significantly different. The postsynaptic area of endplates for stimulated muscles were closer in size to controls than those for the non-stimulated ones. On day 18, no difference was found in the contractile responses between stimulated and non-stimulated groups. Similarly, the postsynaptic areas were the same for both groups. These results demonstrate that denervated muscle stimulated electrically for 4 days prior to reinnervation can preserve the structure of the endplate as well as accelerate recovery of normal function in reinnervated muscle fibers after 11 days of denervation.

Cytofluorometric quantification of somatopetal axonal transport: effects of a conditioning lesion and 2,5-hexanedione

Cytofluorometric quantification of axonally transported fluorescein isothiocyanate (FITC)-labelled wheat germ agglutinin (WGA) from the injection site in the snout area was performed in the facial nucleus at various times during regeneration of one of the facial nerves. Measurements were made on single neurons on both operated and non-operated sides in three different groups of mice 8, 12 and 16 days after a nerve crush, Group 1: (control group) animals with a nerve crush, Group 2: animals with a conditioning lesion (nerve injury) made 3 days before the nerve crush, and Group 3: animals exposed daily to 2,5-hexanedione from 2 weeks before nerve crush until killing. A conditioning lesion caused a more rapid return of transport in regenerating nerves but there was no evidence for an increase in the total amount of transported FITC-WGA. For mice exposed to 2,5-hexanedione a transient increase of tracer transport in regenerating nerves could be demonstrated on day 12 after nerve crush. On day 16, however, a reduction of transport was seen in both operated and non-operated nerves. This study shows that it is possible experimentally to manipulate the influx of macromolecules to the nerve cell body from the periphery during nerve regeneration, and the present method offers the opportunity to study quantitatively the effects of various treatments on reinnervation of a muscle.

Enhancement of the conditioning lesion effect in rat sciatic motor axons after superimposition of conditioning and test lesions

In previous studies on sensory axons we reported that the effect of a conditioning lesion on increasing regeneration rate was enhanced if the two lesions were superimposed, rather than made at separate sites on the nerve, and proposed that this was due to the growth of axons through nerve predegenerated by the conditioning lesion. We now find that the regeneration of motor axons, determined by labeling with fast axonally transported protein, is also enhanced by superimposed conditioning and test lesions, to a greater extent than by separated lesions. However, the regeneration rate of the conditioned motor axons (5.40 +/- 0.44 mm/day) was less than that of conditioned sensory axons in the same nerves (6.65 +/- 0.56 mm/day). Recovery of motor function after the test lesion was assessed by computing a "sciatic functional index" from measurements of hind footprints made by the rats while walking. Recovery began earlier in the conditioned animals, with the time to half-maximum recovery being 13 days, compared with 18 days in animals that had received a test lesion only. In both groups of animals recovery was complete. Although these results are consistent with the proposal that regenerating motor axons elongate more rapidly through nerve predegenerated following the conditioning lesion, we cannot eliminate the possibility that the enhanced regeneration rate in motoneurons was a result of a more vigorous metabolic response to the conditioning lesion when placed more proximally on their axons.

Enzyme changes in the rat facial nucleus following a conditioning lesion

The enzymatic changes in the facial nucleus of the rat occurring after single nerve transection were compared with those after double lesion. In a first operation the left facial nerve was transected and 2 weeks later, both the left and the right facial nerves were axotomized. The double or "conditioning" lesion produced a complex pattern of changes that differed from those after a single lesion. Three enzymes were investigated both biochemically and histochemically. Acetylcholinesterase is representative of the group of transmitter-related enzymes which in general showed a decrease after a single lesion. The hexose monophosphate shunt enzymes, represented here by glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase, are known to increase in the perikaryon. 5'-Nucleotidase is a marker enzyme for the perineuronal satellite glia which also increase in number during chromatolysis. The following results were obtained: (i) In comparison with the single-lesion side the conditioning-lesion side exhibited less activity of the pentose phosphate shunt enzymes on days 7 and 12 after the second operation. On the conditioning-lesion side the amount of enzyme per perikaryon was higher on days 1 and 3, approximately the same on day 7, and less on day 12 compared with the single-lesion side. (ii) The conditioning-lesion side displayed a more pronounced decrease of acetylcholinesterase. (iii) 5'-Nucleotidase increased again after a second axotomy and reached the same level of activity as after a single lesion. These data suggest that a conditioning lesion does not simply amplify the ongoing axonal reaction of the cells in a linear fashion, but that it leads to a complex response. The data are in favor of a shorter initial delay prior to the axonal outgrowth which occurs after a conditioning lesion. However, our data could not explain an enhancement of axonal outgrowth velocity after the second operation.

Does electrical stimulation of denervated muscle, continued after reinnervation, influence recovery of contractile function?

The study was conducted to determine if daily electrical stimulation of denervated muscle, initiated the day following crush denervation and continued for 8 weeks (i.e., 5 weeks after presumptive reinnervation), would influence denervation-associated alterations in muscle size and in situ contractile properties of rat gastrocnemius. A stimulation protocol of brief, strong, isometric contractions was designed to maximize the beneficial effects as described by previous authors. By 8 weeks after crush, unstimulated muscles were still significantly lighter in wet weight, were tetanically weaker, and showed slower isometric contractile responses in situ than controls. Denervated muscles which had been stimulated daily were heavier and tetanically stronger (the latter not different from controls) than those in the nonstimulated group. Muscle weights from groups of animals killed at 2 or 4 weeks after nerve crush indicated the major benefit of stimulation occurred during this initial 4-week period. In situ fatigue properties were unaffected by denervation or stimulation. A protocol of electrical stimulation-evoked strong contractions, initiated soon after denervation and continued after reinnervation, was effective in attenuating the strength-related, but not speed-related, changes in neuromuscular function resulting from denervation. These latter changes are presumably the result of loss of "neurotrophic influence" and/or continuous low-tension muscle activity lost as a result of denervation.

Electrical stimulation of regenerating nerve and its effect on motor recovery

The rate of recovery of motor function, after axonotmesis of the motor nerve innervating the soleus muscle in the rabbit, was evaluated. In a chronic study over a period of 4 weeks, contraction parameters and muscle action potentials were recorded. A group of rabbits, whose soleus nerves were stimulated with 4 pps for 24 h daily, was compared with a control group. The electrically stimulated animals showed a faster improvement in motor function and reached their initial values a week earlier than the controls. Electrical stimulation proved to have a positive effect on the regeneration and motor recovery of nerves.

Protein synthesis and axonal transport in goldfish retinal ganglion cells during regeneration accelerated by a conditioning lesion

Axonal outgrowth in goldfish retinal ganglion cells following a testing lesion of the optic axons is accelerated by a prior conditioning lesion. Changes in protein synthesis and axonal transport were examined during the accelerated regeneration. The conditioning lesion was an optic tract cut made 2 weeks prior to the testing lesion, which consisted of a tract cut at the chiasma, so that nerves subjected to either a conditioning lesion ('conditioned nerves') or a sham operation ('sham-conditioned nerves') could be examined in the same animal. In the retinal ganglion cells of conditioned nerves, the incorporation of [3H]proline into protein began to increase between 1 and 8 days after the testing lesion. The amount of fast-transported labeled protein was elevated to about 8 X normal by 1 day after the testing lesion but had decreased to about 3-5X normal at 8 and 22 days. The 8 and 22 day values were not significantly different from those in sham-conditioned nerves or nerves that had received a testing lesion alone. For slow protein transport, the instantaneous amount transported was 15-16 X normal in the conditioned nerves at 1 and 8 days after the testing lesion, and the velocity of slow transport, which was already elevated above normal by 1 day after the testing lesion, was elevated still further by 8 days--to a value in excess of 1.5 mm/day (compared to 0.2-0.4 mm/day in normal animals). We believe that the enhanced outgrowth resulting from the conditioning lesion is due to a transient increase in the amount of fast transport (possibly responsible for a decreased delay in the initiation of sprouting), and a sustained increase in the amount and velocity of slow transport (which may account for an increased rate of elongation).

The effect of low-frequency electrical stimulation on the denervated extensor digitorum longus muscle of the rabbit

Both extensores digitorum longi (EDL) muscles of rabbits were denervated by crushing the common peroneal nerves. The EDL muscle on one side was directly stimulated at 10-12 Hz via implanted electrodes. This treatment reduced the changes of twitch/tetanus ratios produced by denervation and prevented the slowing of contraction and relaxation that follows denervation. It is concluded that the stimulation reduced the duration of the active state of denervated muscles. These effects of stimulation were reduced after 5 weeks, probably because by that time the slowing effect of low-frequency activity on the fast muscles became apparent.