Compound Action Potential Latency Research Articles

Is neuronal inhibition or excitability controlled by Na+K+2CL– transporters?

Is neuronal inhibition or excitability controlled by Na⁺K⁺2CL^– transporters?

Effects of tanshinone ⅡA sulfonate on nerve regeneration after cryopreserved sciatic nerve allograft in rats

In order to investigate the protective effect of tanshinone ⅡA sulfonate on the sciatic nerve activty in rats after cryopreservation as well as the nerve regeneration and functional recovery after allograft and its possible mechanism, Sprague-Dawley (SD) rats were divded into four groups at different doses of tanshinone ⅡA sulfonate (A 0 mg·L⁻¹, B 80 mg·L⁻¹, C 160 mg·L⁻¹, D 480 mg·L⁻¹) cryopreserved at -80 °C for 24 weeks. Fresh control group nerve segments were harvested without cryopreservation. The ultrastructure and the viable cells of the nerve segments after cryopreservation were observed by electron microscopy, calcein-AM/propidium iodide staining, respectively. The expression of Bax and Bcl-2 was detected by Western blot. After cryopreservation, the nerve segments were cultured in vitro for one week, the mRNA and protein level of NGF and GDNF were detected by PCR and Western blot respectively. In addition, the above four cryopreserved groups transplanted to the Wistar rats by allografting (A', B', C', D'). At 16-week postoperation, muscle compound action potential latency and nerve conduction velocity were examined by electrophysiological. The number and the thickness of myelinated nerve fibers were analyzed by toluidine blue staining. The ultrastructure of the sciatic nerve by electron microscopy was observed. According to the results, after the cryopreserved for 24 weeks, compared with groups A and B, the nerve demyelination and vacuolation were weak, and the more viable cells, the decreased Bax and increased Bcl-2, the increased NGF and GDNF in group C and D. At 16-week poseoperation, the results demonstrated that the more larger and thickly regenerated myelinated axons, the shorter latency of muscle compound action potentials and higher nerve conduction velocity in groups C' and D' compared with groups A' and B'. According to these results, tanshinone ⅡA sulfonate exerted a significant protective effect on the viability of the nerves during cryopreservation at -80 °C and promoted nerve regeneration and functional recovery after transplantation especially in middle- and high-dose of tanshinone ⅡA sulfonate.

SEVERE CEREBRAL HYPOPERFUSION INDUCES A DEFICIT IN WHITE MATTER FUNCTION THAT IS ATTENUATED BY MODULATING MICROGLIA WITH DIMETHYL FUMARATE

Chronic cerebral hypoperfusion contributes to age-related cognitive decline and dementia such as Alzheimer's disease. The causative link is not yet defined but is proposed to involve damage to the brain's white matter mediated by oxidative stress and inflammation. Dimethyl fumarate (DMF) is an anti-inflammatory drug which acts in part by activating Nrf2-signalling, a master regulator of anti-oxidant and anti-inflammatory pathways. In the present study, the effects of severe cerebral hypoperfusion and DMF administration were investigated on white matter function and inflammation. Male C57Bl/6J mice underwent bilateral common carotid artery stenosis and white matter function was assessed at 7 days with electrophysiology in response to evoked compound action potentials (CAPs) in the corpus callosum. DMF (100mg/kg) or vehicle was administered twice daily by oral gavage. White matter structural and inflammatory alterations were assessed using immunohistochemistry, and cytokine/chemokine levels were assessed with inflammatory-related protein multiplex. The peak latency of CAPs and axonal refractoriness was increased in the corpus callosum in response to hypoperfusion, indicating a marked functional impairment in white matter, which was paralleled by axonal and myelin pathology. The functional impairment in peak latency was significantly correlated with increased microglial numbers in response to hypoperfusion. Dimethyl fumarate was found to ameliorate the functional deficits in peak latency but not axonal refractoriness. DMF had no effect on hypoperfusion-induced axonal and myelin pathology. Hypoperfusion decreased levels of mature oligodendrocytes and increased levels of immature oligodendrocytes, however these were unaffected by DMF treatment. The density of microglia was significantly increased after hypoperfusion whereas DMF treated mice had similar levels to that of vehicle treated mice, indicating that DMF reduced microgliosis. Chemokine and growth factor molecules were increased in response to severe hypoperfusion and their levels were further modulated by DMF. The study suggests that microglial activation following severe cerebral hypoperfusion contributes to the functional impairment in white matter and that DMF, by reducing microglia, may be of potential therapeutic benefit in cerebral vascular disease. We are currently investigating the therapeutic potential of boosting Nrf2-signalling pathway to prevent neurodegeneration caused by cerebral hypoperfusion using transgenic and novel pharmacological approaches.

Dimethyl fumarate improves white matter function following severe hypoperfusion: Involvement of microglia/macrophages and inflammatory mediators

The brain’s white matter is highly vulnerable to reductions in cerebral blood flow via mechanisms that may involve elevated microgliosis and pro-inflammatory pathways. In the present study, the effects of severe cerebral hypoperfusion were investigated on white matter function and inflammation. Male C57Bl/6J mice underwent bilateral common carotid artery stenosis and white matter function was assessed at seven days with electrophysiology in response to evoked compound action potentials (CAPs) in the corpus callosum. The peak latency of CAPs and axonal refractoriness was increased following hypoperfusion, indicating a marked functional impairment in white matter, which was paralleled by axonal and myelin pathology and increased density and numbers of microglia/macrophages. The functional impairment in peak latency was significantly correlated with increased microglia/macrophages. Dimethyl fumarate (DMF; 100 mg/kg), a drug with anti-inflammatory properties, was found to reduce peak latency but not axonal refractoriness. DMF had no effect on hypoperfusion-induced axonal and myelin pathology. The density of microglia/macrophages was significantly increased in vehicle-treated hypoperfused mice, whereas DMF-treated hypoperfused mice had similar levels to that of sham-treated mice. The study suggests that increased microglia/macrophages following cerebral hypoperfusion contributes to the functional impairment in white matter that may be amenable to modulation by DMF.

MTOR regulates peripheral nerve response to tensile strain.

While excessive tensile strain can be detrimental to nerve function, strain can be a positive regulator of neuronal outgrowth. We used an in vivo rat model of sciatic nerve strain to investigate signaling mechanisms underlying peripheral nerve response to deformation. Nerves were deformed by 11% and did not demonstrate deficits in compound action potential latency or amplitude during or after 6 h of strain. As revealed by Western blotting, application of strain resulted in significant upregulation of mammalian target of rapamycin (mTOR) and S6 signaling in nerves, increased myelin basic protein (MBP) and β-actin levels, and increased phosphorylation of neurofilament subunit H (NF-H) compared with unstrained (sham) contralateral nerves (P < 0.05 for all comparisons, paired two-tailed t-test). Strain did not alter neuron-specific β3-tubulin or overall nerve tubulin levels compared with unstrained controls. Systemic rapamycin treatment, thought to selectively target mTOR complex 1 (mTORC1), suppressed mTOR/S6 signaling, reduced levels of MBP and overall tubulin, and decreased NF-H phosphorylation in nerves strained for 6 h, revealing a role for mTOR in increasing MBP expression and NF-H phosphorylation, and maintaining tubulin levels. Consistent with stretch-induced increases in MBP, immunolabeling revealed increased S6 signaling in Schwann cells of stretched nerves compared with unstretched nerves. In addition, application of strain to cultured adult dorsal root ganglion neurons showed an increase in axonal protein synthesis based on a puromycin incorporation assay, suggesting that neuronal translational pathways also respond to strain. This work has important implications for understanding mechanisms underlying nerve response to strain during development and regeneration.NEW & NOTEWORTHY Peripheral nerves experience tensile strain (stretch) during development and movement. Excessive strain impairs neuronal function, but moderate strains are accommodated by nerves and can promote neuronal growth; mechanisms underlying these phenomena are not well understood. We demonstrated that levels of several structural proteins increase following physiological levels of nerve strain and that expression of a subset of these proteins is regulated by mTOR. Our work has important implications for understanding nerve development and strain-based regenerative strategies.

Noise-induced damage to ribbon synapses without permanent threshold shifts in neonatal mice

Recently, ribbon synapses to the hair cells (HCs) in the cochlea have become a novel site of interest in the investigation of noise-induced cochlear lesions in adult rodents (Kujawa and Liberman, 2009; Lin et al., 2011; Liu et al., 2012; Shi et al., 2013). Permanent noise-induced damage to this type of synapse can result in subsequent degeneration of spiral ganglion neurons (SGNs) in the absence of permanent changes to hearing sensitivity. To verify whether noise exposure during an early developmental period produces a similar impact on ribbon synapses, the present study examined the damaging effects of noise exposure in neonatal Kunming mice. The animals received exposure to broadband noise at 105-decibel (dB) sound pressure level (SPL) for 2h on either postnatal day 10 (P10d) or postnatal day 14 (P14d), and then hearing function (based on the auditory brainstem response (ABR)) and cochlear morphology were evaluated during either postnatal weeks 3–4 (P4w) or postnatal weeks 7–8 (P8w). There were no significant differences in the hearing threshold between noise-exposed and control animals, which suggests that noise did not cause permanent loss of hearing sensitivity. However, noise exposure did produce a significant loss of ribbon synapses, particularly in P14d mice, which continued to increase from P4w to P8w. Additionally, a corresponding reduction in the amplitude of compound action potential (CAP) was observed in the noise-exposed groups at P4w and P8w, and the CAP latency was elongated, indicating a change in synaptic function.

Effects of early and delayed laser application on nerve regeneration

The aim of this study is to analyze the differences between early and delayed use of low-level laser therapy (LLLT) in functional and morphological recovery of the peripheral nerve. Thirty male Wistar rats were divided into three groups after the sciatic nerve was crushed: (1) control group without laser treatment, (2) early group with laser treatment started immediately after surgery and lasted 14 days, and (3) delayed group with laser treatment starting on the postoperative day 7 and lasted until day 21. A 650-nm diode laser (model: DH650-24-3(5), Huanic, China) with an output power of 25 mW exposed transcutaneously at three equidistant points on the surgical mark corresponding to the crushed nerve. The length of the laser application was calculated as 57 s to satisfy approximately 10 J/cm(2). A Sciatic Functional Index (SFI) was used to evaluate functional improvement in groups at pre- and post-surgery (on days 7, 14, and 21). Compound action potential (CAP) was measured after the sacrifice and histological examination was performed for all groups. SFI results showed that there was no significant difference between groups at different days (p > 0.05). On the other hand, the latency of CAP decreased significantly (p < 0.05) in the delayed group. Histological examination confirmed that the number of mononuclear cells was lower (p < 0.05) in both early and delayed groups. In conclusion, results supported the hypothesis that LLLT could accelerate the rate of recovery of injured peripheral nerves in this animal model. Though both laser groups had positive outcomes, delayed group showed better recovery.

Induced endolymphatic flow from the endolymphatic sac to the cochlea in Ménière's disease

ObjectiveThe aim of the present study was to verify whether drugs injected into the endolymphatic sac (ES) can reach the cochlea and possibly treat inner ear disorders. Study DesignProspective cohort study. SettingTertiary referral center, Otolaryngology Department, University of Verona. Subjects and MethodsPatients with Ménière's disease (MD) who were candidates for ES decompression were selected. Nineteen subjects received dexamethasone (DEX) via injection into the ES. To objectively define whether substances administered into the ES could reach the cochlea, we added gadolinium (GD) in three patients. All subjects had intraoperative electrocorticogram recordings and an audiologic follow-up. The three subjects who underwent injection of the DEX-GD solution were followed-up with magnetic resonance imaging. The audiological data are presented during a follow-up period of 12 months. ResultsIntraoperative electrocochleography recordings revealed no changes in two patients and summating potentials and compound action potential latency and wave-form modifications in all the other subjects. GD distribution was observed from 48 hours to one week after ES injection into the cochlea of the three subjects injected with DEX-GD. GD-related enhancement of inner ear structures lasted more than two weeks in all subjects. Pure tone average results showed hearing improvement of at least 20 dB HL in 42 percent of patients (8 of 19) at the 12-month follow-up. Statistically significant differences emerged between the mean pure tone average of the ES procedure subjects at one and 12 months after surgery (P = 0.0096). ConclusionThis novel approach might reveal new prospects for treating viral, metabolic, autoimmune, and genetic disorders of the cochlea.

Evidence of Gadolinium Distribution from the Endolymphatic Sac to the Endolymphatic Compartments of the Human Inner Ear

To verify whether injection of substances into the endolymphatic sac (ES) diffuses into the endolymphatic compartments of the human inner ear and in particular to the endolymphatic space of the scala media (ESp-SM), as demonstrated in animals, an exploratory investigation with magnetic resonance imaging (MRI) and intraoperative electrocochleographic recordings (ECoG) was conducted in patients with Ménière’s disease (MD) treated with ES decompression. A mixture of dexamethasone and gadolinium (GD) in solution was injected into the ES of 4 patients. The results of the ES injection procedure were compared with administration of the same solution intratympanically (IT, 1 patient) and via a platinotomy in 2 patients. The study was conducted retrospectively at a tertiary referral center. Main outcomes measures were pre- and postintervention complete audiological and neuro-otological evaluation; intraoperative ECoG investigation with evaluation of the morphology of acoustically elicited compound action potentials (CAPs) and 1.5 T MRI evaluations at different follow-up times. Distribution of GD from the ES injection procedure was observed first in the ES, after 24 h in the vestibule and semicircular canals, and after 24–48 h in the ESp-SM in all patients. High signal was detected within the inner ear for 1 week or more (mean: 10 days; range: 7–16 days). Changes in morphology and latency of CAPs were observed within 30 min of the dilatory injection into the ES in all patients. Administration of GD into the vestibule and the IT approach did not distribute the contrast in the ES and GD was observed in the perilymphatic space of the vestibule, cochlea and semicircular canals. No side effects relating to administration of GD into the ES, IT or into the vestibule were observed. To the best of our knowledge this is the first demonstration in humans that drugs injected into the endolymphatic structure of the ES diffuse to the cochlea, presumably into the ESp-SM. The possibility of injecting substances into the endolymphatic space might open up new prospects in the treatment of inner ear disorders. Further studies will be needed to define the limitations of this approach.

Preventing Hearing Damage Using Topical Dexamethasone During Reversible Cochlear Ischemia

Local application of dexamethasone to the round window (RW) niche prevents cochlear damage caused by local reversible ischemia. Cochlear ischemia induced by internal auditory artery (IAA) compression/stretching is thought to cause postoperative sensory hearing loss after attempted hearing preservation removal of acoustic neuroma tumors. Dexamethasone administered to the RW niche traveling through the membrane to the cochlear fluids may prevent ischemic damage. Ten young albino rabbits were used for this study. Ischemic episodes were induced by compressing the IAA. Laser Doppler cochlear blood flow was measured using a probe positioned at the RW niche. Transtympanic electrocochleography was measured at 4, 8, and 12 kHz. In 5 test ears, dexamethasone was administered topically at the RW for approximately 50 minutes before the IAA compressions, whereas in 5 control ears, saline was applied in the same way. Each ear underwent one 10-minute IAA compression with a 60-minute postischemic period of transtympanic electrocochleography monitoring. In both control- and dexamethasone-treated ears, ischemic episodes measured by Laser Doppler cochlear blood flow were comparable. Fifty minutes after IAA decompression, in dexamethasone-pretreated ears, cochlear microphonic and compound action potential amplitudes at all test frequencies were 10 to 15% less reduced than those in control ears. Compound action potential latencies in dexamethasone-pretreated ears resulted in shorter latency delay than in control ears. The RW seems to be an efficacious route for the administration of dexamethasone into the inner ear. Dexamethasone showed a protective effect on cochlear function after local ischemia. Transtympanic electrocochleography was found to be a sufficient and effective tool in monitoring hearing.

Effects of Acrylamide on the Nervous Tissue Antioxidant System and Sciatic Nerve Electrophysiology in the Rat

To investigate the time-dependent effects of acrylamide (ACR) on the antioxidative status in rat nerve tissues, adult male Wistar rats were given ACR (40 mg/kg, i.p., 3 times/week) for 2, 4, 6 and 10 weeks, respectively. The time-dependent changes of the lipid peroxidation (malondialdehyde, MDA) and antioxidative status (glutathione, GSH; glutathione peroxidase, GSH-Px; glutathione reductase, GR; superoxide dismutase, SOD and anti-reactive oxygen species, anti-ROS) in nerve tissues were investigated. The electrophysiology indices (nerve conduction velocity, NCV; compound action potential duration, CAPD; compound action potential amplitude, CAPA; compound action potential latency, CAPL) in the sciatic nerve were determined using BL-420E Biologic Function Determining System. The results showed that MDA levels increased significantly (P < 0.05) in nerve tissues, while GSH levels markedly decreased (P < 0.05) in a time-dependent manner. SOD activity (in the spinal cord and sciatic nerve) and GR activity (in the sciatic nerve) increased significantly after 4 weeks ACR treatment (P < 0.01), but then decreased (P < 0.05). The anti-ROS activity in the sciatic nerve was markedly decreased at the end of week 6 and 10 (P < 0.01). The above indices changed most in the sciatic nerve. The levels of GSH, MDA and anti-ROS in rat sciatic nerve were in high correlation (P < 0.05, |r| > 0.80) with the electrophysiology indices according to the exposure time. Thus, ACR-induced neurotoxicity may be associated with the enhancement of lipid peroxidation and reduction of the antioxidative capacity. Depletion of neural GSH level might be one of the primary events in ACR-induced neuropathy.

Evoked cochlear potentials in the barn owl

Gross electrical responses to tone bursts were measured in adult barn owls, using a single-ended wire electrode placed onto the round window. Cochlear microphonic (CM) and compound action potential (CAP) responses were evaluated separately. Both potentials were physiologically vulnerable. Selective abolishment of neural responses at high frequencies confirmed that the CAP was of neural origin, while the CM remained unaffected. CAP latencies decreased with increasing stimulus frequency and CAP amplitudes were correlated with known variations in afferent fibre numbers from the different papillar regions. This suggests a local origin of the CAP along the tonotopic gradient within the basilar papilla. The audiograms derived from CAP and CM threshold responses both showed a broad frequency region of optimal sensitivity, very similar to behavioural and single-unit data, but shifted upward in absolute sensitivity. CAP thresholds rose above 8 kHz, while CM responses showed unchanged sensitivity up to 10 kHz.

Sepsis induces early phrenic nerve neuropathy in rats

The aim of the present study was to investigate the electrophysiology of the phrenic nerve and the diaphragm muscle during sepsis. In total, 26 rats underwent either sham laparotomy or caecal ligation and puncture (CLP). Electrophysiology was evaluated via a phrenic nerve conduction study and needle electromyography of the diaphragm, prior to CLP, 6 and 24 h post-CLP and on day 7. The histopathology of the diaphragm muscle and phrenic nerve was also examined on day 7. In the sepsis group, the phrenic nerve conduction study showed decreased amplitude of compound action potential (CMAP), and prolongation in the duration and the latency of CMAP. The diaphragmatic needle electromyography showed decreased amplitude and frequency of the motor unit action potential (MUP), and prolongation in the duration of MUP, at all time points, compared with the pre-CLP values. The electrophysiological abnormalities were consistent with axonal and demyelinating phrenic nerve neuropathy. Electrophysiological abnormalities were present at 6 h with worsening at 24 h and on day 7. Histopathological examination showed normal muscular fibres and focally slight myelin degenerations of the phrenic nerve fibres. In conclusion, sepsis induced phrenic nerve neuropathy as early as the 6th h in rats.

Chronically implanted epineural electrodes for repeated assessment of nerve conduction velocity and compound action potential amplitude in rodents

Implanted epineural electrodes were used for the longitudinal assessment of peripheral nerve conduction velocity (NCV) and compound action potential (CAP) amplitude in rats. Custom-fabricated stimulating and recording electrodes were sutured over the tibial and sciatic nerves, respectively, and were used for weekly recordings of CAP latency and amplitude. Intra-day variability of nerve conduction velocity measurements had coefficients of variation of less than 2% for same day recordings from individual subjects. A clear trend in recovery of the NCV values following implant was observed over the 7-week trial period. These results demonstrate that implanted epineural electrodes provide a reliable method for chronic, in vivo monitoring of nerve conduction parameters in rodents.

Conduction latency along CA3 hippocampal axons from rat.

Relatively few physiological studies have been carried out on intrahippocampal axons. We have recorded compound potentials from fiber groups and the activity of individual axons at 22-25 degrees C to characterize the conduction in subsets of the broad fan-shaped CA3 pyramidal axonal tree, including the Schaffer collaterals and longitudinal branches. The same wide axonal branching was indicated by antidromic activation of individual CA3 pyramidal cells. The average compound action potential latency from the CA3 to the CA1 area (Schaffer collaterals) increased by 4.16 +/- 0.06 ms/mm separation between the stimulation and registration electrodes. The impulses spread 31% faster in the 45-degree oblique temporal than in the transverse direction across CA1. The latency of the longitudinal axons in the CA3 area increased by 6.19 +/- 0.19 ms/mm. More impressive than these direction-dependent differences in latency were the large differences between individual axons running in the same direction. For both the longitudinal axons and the Schaffer collaterals, there was a broad distribution of antidromic latencies for a given distance between the stimulation and recording points. Typically, the fastest impulses arrived in half the time of the slowest. The distribution of compound action potential latencies between two points in the tissue could be made narrower by surgical restriction of the thickness and width of the preparation. By comparison, the cerebellar parallel fibers showed a narrower distribution of their latencies than the Schaffer collaterals. Because the cerebellar fibers run more straight than Schaffer collaterals, this suggested that some of the latency differences of the latter were due to differences in the path length of the axons. One consequence of our findings is that synchronous firing of neighboring CA3 pyramidal cells does not necessarily give synchronous inputs to common target CA1 neurons.

Sensitivity of peripheral nerve fibres to sodium pentobarbital anaesthesia in rat.

The sensitivity of different classes of peripheral nerve fibres to anaesthesia with sodium pentobarbital in rat was tested. The basic approach was to examine changes in compound action potential (CAP) in a rat sural nerve induced by sodium pentobarbital. Rats were either sacrificed by cervical dislocation (control) or anaesthetized with sodium pentobarbital (100 mg/kg, i.p.), and a 30 mm long sural nerve segment excised and placed on electrodes in a thermostatically controlled recording chamber. CAPs were evoked by electrically stimulating the sural nerve segment with supra-threshold stimuli. CAP in control sural nerves consisted of the early component (A axons: 32.73 ± 2.91 m/s) and the late component (C axons: 0.92 ± 0.05 m/s) with peak voltage amplitudes 4.9 ± 1.0 mV and 0.1 ± 0.03 mV, respectively. Anaesthesia with sodium pentobarbital had no effect on CAP latency. However, the amplitude of CAP of both A and C axons was reduced by approximately 40 % and 50 %, respectively. The depressant effect of pentobarbital on CAPs was statistically significant for both groups of axons (p<0.01). Non-selective sensitivity of A and C axons to pentobarbital suggests even distribution of receptors for GABA in these two populations of axons in the rat peripheral nerve.

Twenty-four-hour oscillations of facial electroneurography parameters and their relationship to body temperature.

The purpose of the present study was to assess the temporal variation of the facial electroneurography (ENoG) parameters recorded from the nasolabial crease and to compare it to the temporal oscillations of oral and facial cutaneous temperatures. Twelve healthy individuals were examined at 4-hour intervals for 2 consecutive days. Twenty-four-hour rhythms of oral and facial cutaneous temperatures were found in all subjects (troughs on average at 4:20 and 4:00 AM, respectively). Significant 24-hour oscillations of ENoG amplitude were found in all subjects, which were positively correlated to the oscillations of oral and facial cutaneous temperatures (trough on average at 2:40 AM). The compound action potential (CAP) latency fluctuated during the day and was negatively correlated with local temperatures. However, these fluctuations and correlations did not reach statistical significance. We also found a significant reduction of ENoG amplitude in 5 patients with artificial cooling of the face near the facial nerve. Also, there was a small increase in CAP latency with facial cooling. Interpreting ENoG recordings in accordance with oral temperature and recording time of day reduce the intertest variation.

In vitro frog sciatic nerve as a peripheral nerve model for studies of the mechanism of action of low energy lasers: Part one.

There have been numerous reports of modulation of peripheral nerve action potential characteristics through application of low energy laser irradiation (LELI), although no mechanism has yet been advanced to explain these observations. In order to investigate the mechanism of LELI effects in peripheral nerve tissue, a well-characterized, reliable, and robust peripheral nerve preparation is required. The objective of this study was to evaluate the in vitro frog sciatic nerve as a candidate model for future LELI mechanism studies. Following 60-minute baseline recordings of compound action potential (CAP) amplitude, latency, depolarization rate, and repolarization rate, helium-neon (HeNe) laser irradiation (632 nm, 15 min, 1-7 J, 44-320 J/cm2) was delivered to one of two sites on the nerve. Laser-induced changes in CAP parameters were analyzed during irradiation and for 60 minutes post-irradiation using a repeated measures linear regression model. In the treatment group that received 7 J of HeNe energy over the recording electrode, CAP latency increased relative to nonirradiated controls during the postirradiation period. No other treatment group demonstrated laser-induced changes in CAP characteristics at any time during the experiment. HeNe irradiation demonstrated limited ability to alter the CAP under these conditions. As such, the in vitro frog sciatic nerve is an inappropriate model for mechanism of action studies.

Search for frequency-specific effects of millimeter-wave radiation on isolated nerve function.

Effects of a short-term exposure to millimeter waves (CW, 40-52 GHz, 0.24-3.0 mW/cm2) on the compound action potential (CAP) conduction were studied in an isolated frog sciatic nerve preparation. CAPs were evoked by either a low-rate or a high-rate electrical stimulation of the nerve (4 and 20 paired pulses/s, respectively). The low-rate stimulation did not alter the functional state of the nerve, and the amplitude, latency, and peak latency of CAPs could stay virtually stable for hours. Microwave irradiation for 10-60 min at 0.24-1.5 mW/cm2, either at various constant frequencies or with a stepwise frequency change (0.1 or 0.01 GHz/min), did not cause any detectable changes in CAP conduction or nerve refractoriness. The effect observed under irradiation at a higher field intensity of 2-3 mW/cm2 was a subtle and transient reduction of CAP latency and peak latency along with a rise of the test CAP amplitude. These changes could be evoked by any tested frequency of the radiation; they reversed shortly after cessation of exposure and were both qualitatively and quantitatively similar to the effect of conventional heating of 0.3-0.4 degree C. The high-rate electrical stimulation caused gradual and reversible decrease of the amplitude of conditioning and test CAPs and increased their latencies and peak latencies. These changes were essentially the same with and without irradiation (2.0-2.7 or 0.24-0.28 mW/cm2), except for attenuation of the decrease of the test CAP amplitude. This effect was observed at both field intensities, but was statistically significant only for certain frequencies of the radiation. Within the studied limits, this effect appeared to be dependent on the frequency rather than on the intensity of the radiation, but this observation requires additional experimental confirmation.

Correlation between electrophysiological effects of mexiletine and ischemic protection in central nervous system white matter.

Protection of CNS white matter tracts in brain and spinal cord is essential for maximizing clinical recovery from disorders such as stroke or spinal cord injury. Central myelinated axons are damaged by anoxia/ischemia in a Ca(2+)-dependent manner. Leakage of Na+ into the axoplasm through Na+ channels causes Ca2+ overload mainly by reverse Na(+)-Ca2+ exchange. Na+ channel blockers have thus been shown to be protective in an in vitro anoxic rat optic nerve model. Mexiletine (10 microM-1 mM), an antiarrhythmic and use-dependent Na+ channel blocker, was also significantly protective, as measured by recovery of the compound action potential after a 60 min anoxic exposure in vitro. More importantly, mexiletine (80 mg/kg, i.p.) also significantly protected optic nerves from injury in a model of in situ ischemia. This in situ model is more clinically relevant as it addresses drug pharmacokinetics, toxicity and CNS penetration. Optic nerve recovery cycles (defined as shifts in latency of compound action potentials with paired stimulation) were used to measure the concentration of mexiletine in optic nerves after systemic administration, estimated at approximately 42 microM 1 h after a single dose of 80 mg/kg, i.p. These results indicate that mexiletine is able to penetrate into the CNS at concentrations sufficient to confer significant protection. Na+ channel blockers such as mexiletine may prove to be effective clinical therapeutic agents for protecting CNS white matter tracts against anoxic/ischemic injury.