Potential Preventive Strategies for Amyotrophic Lateral Sclerosis.

It may seem useless to propose preventive measures for a disease without established pathogenesis and successful therapy, such as amyotrophic lateral sclerosis (ALS). However, we will show that ALS shares essential molecular mechanisms with aging and that established anti-aging strategies, such as healthy diet or individually adjusted exercise, may be successfully applied to ameliorate the condition of ALS patients. These strategies might be applied for prevention if persons at ALS risk could be identified early enough. Recent research advances indicate that this may happen soon.

[Provision of assistive devices in amyotrophic lateral sclerosis. Analysis of 3 years case management in an internet-based supply network]. / Hilfsmittelversorgung bei der amyotrophen Lateralsklerose. Analyse aus 3 Jahren Fallmanagement in einem internetunterstützten Versorgungsnetzwerk.

BACKGROUND: The provision of assistive devices (PAD) is a key element of care in amyotrophic lateral sclerosis (ALS). Since 2011, assistive devices (AD) have been coordinated in an internet-supported care network at university-based ALS centers in Berlin, Bochum, Hannover and Jena. The digitization of PAD processes has facilitated the evaluation of real-life ALS care. OBJECTIVES: Orthotics (OT), augmentative and alternative communication (AAC), supported treadmill (ST) and powered wheelchair (PW) were the PAD groups analyzed for delivery rates (proportion of delivered AD vs. medically indicated AD), rejection by patients and payers and latency of provision of care. RESULTS: Between June 2011 and October 2014 a total of 1479 patients and 12,478 AD were coordinated, among which 3313 PAD were related to OT, AAC, ST or EM. The median delivery rate was 64.3 %. The mean rejection rate by patients was 9.8 % (OT 5.4 %, AAC 9.8 %, ST 10.2 % and PW 15.6 %). Marked differences were noted in the rejection rate by payers and in care provision latency: OT (16.2 %, 68 days, n = 734), AAC (30.4 %, 96 days, n = 392), ST (34.8 %, 113 days, n = 164) and PW (35.6 %, 129 days, n = 259). Analysis of rejection rates showed significant differences among insurers. CONCLUSION: Only two thirds of the medically indicated AD reached the patients. Rejection rates by patients and payers and latency of provision of care were high. The PAD can substantially vary among health insurance companies. The establishment of consented criteria for PAD and their integration into treatment regimens and guidelines are crucial tasks for the future.

Muscle ultrasonography as an additional diagnostic tool for the diagnosis of amyotrophic lateral sclerosis.

OBJECTIVE: We aimed to determine the utility of muscle ultrasonography (MUS) in addition to electromyography (EMG) in the diagnosis of amyotrophic lateral sclerosis (ALS). METHODS: In all, 60 patients with ALS and 20 with other neuromuscular disorders underwent MUS and EMG. In addition, 30 healthy controls underwent only MUS. Occurrence of fasciculations and fibrillations was evaluated. Ultrasonic echogenicity was graded semiquantitatively. RESULTS: The incidence of fasciculations was significantly higher in patients undergoing MUS than in those undergoing EMG (p<0.05), even in muscles of full strength (p<0.001). However, EMG was more sensitive in detecting fibrillations (p<0.05). MUS had an overall higher sensitivity in detecting spontaneous activity in the tongue (p<0.05). Patients with ALS showed significantly increased muscle echo intensity (EI) compared to patients who were initially suspected as having ALS and normal controls (p<0.05), irrespective of the clinical or electrophysiological status. CONCLUSION: Our results showed that the sensitivity and specificity of MUS in diagnosing ALS was almost equivalent to those of EMG, using the Awaji criteria. Combination of MUS and EMG enhances the diagnostic accuracy compared to EMG alone (p<0.05). SIGNIFICANCE: The combination of EMG and MUS can be used to evaluate the lower motor neuron affection by reducing the use of the often painful and uncomfortable EMG examinations but without decreasing the diagnostic sensitivity and specificity.

Endoplasmic reticulum stress is accompanied by activation of NF-κB in amyotrophic lateral sclerosis.

BACKGROUND: Recent studies have indicated that endoplasmic reticulum (ER) stress is involved in the pathogenesis of amyotrophic lateral sclerosis (ALS). ER stress occurs when the ER-mitochondria calcium cycle is disturbed and misfolded proteins accumulate in the ER. To cope with ER stress, cells activate the unfolded protein response (UPR). Accumulating evidence from non-neuronal cell models suggests that there is extensive cross-talk between the UPR and the NF-κB pathway. METHODS: Here we investigated the expression of NF-κB and the main UPR markers X-box binding protein 1 (XBP1), basic leucine-zipper transcription factor 6 (ATF6) and phosphorylated eukaryotic initiation factor-2α (p-eIF2) in mutated SOD1(G93A) cell models of ALS, as well as their modulation by lipopolysaccharide and ER-stressing (tunicamycin) stimuli. RESULTS: Expression of NF-κB was enhanced in the presence of SOD1(G93A). Lipopolysaccharide did not induce the UPR in NSC34 cells and motor neurons in a mixed motor neuron-glia coculture system. The induction of the UPR by tunicamycin was accompanied by activation of NF-κB in NSC34 cells and motor neurons. CONCLUSION: Our data linked two important pathogenic mechanisms of ALS, ER stress and NF-κB signalling, in motor neurons.

The unfolded protein response in models of human mutant G93A amyotrophic lateral sclerosis.

Recent studies indicate that endoplasmic reticulum (ER) stress is involved in the pathogenesis of familial and sporadic amyotrophic lateral sclerosis (ALS). ER stress occurs when the ER-mitochondria calcium cycle (ERMCC) is disturbed and misfolded proteins accumulate in the ER. To cope with ER stress, the cell engages the unfolded protein response (UPR). While activation of the UPR has been shown in some ALS models and tissues, ER stress elements have not been studied directly in motor neurons. Here we investigated the expression of XBP1 and ATF6α and phosphorylation of eIF2α, and their modulation, in mutated SOD1(G93A) NSC34 and animal model of ALS. Expression of XBP1 and ATF6α mRNA and protein was enhanced in SOD1(G93A) NSC34 cells. Activation of ATF6α and XBP1 and phosphorylation of eIF2α were detectable in mutated SOD1(G93A) motor but not in wild-type motor neurons. Treatment with the ER stressor thapsigargin enhanced phosphorylation of eIF2α and activated proteolysis of ATF6α and splicing of XBP1 in NSC34 and motor neurons in a time-dependent manner. The present study thus provides direct evidence of activated UPR in motor neurons which overexpress human pathogenic mutant SOD1(G93A) , providing evidence that ER stress plays a major role in ALS.

EFNS guidelines on the use of neuroimaging in the management of motor neuron diseases.

BACKGROUND AND PURPOSE: These European Federation of Neurological Societies guidelines on neuroimaging of motor neuron diseases (MNDs) are designed to provide practical help for the neurologists to make appropriate use of neuroimaging techniques in patients with MNDs, which ranges from diagnostic and monitoring aspects to the in vivo study of the pathobiology of such conditions. METHODS: Literature searches were performed before expert members of the Task Force wrote proposal. Then, consensus was reached by circulating drafts of the manuscript to the Task Force members and by discussion of the classification of evidence and recommendations. RESULTS AND CONCLUSIONS: The use of conventional MRI in patients suspected of having a MND is yet restricted to exclude other causes of signs and symptoms of MN pathology [class IV, level good clinical practice point (GCPP)]. Although the detection of corticospinal tract hyperintensities on conventional MRI and a T2-hypointense rim in the pre-central gyrus can support a pre-existing suspicion of MND, the specific search of these abnormalities for the purpose of making a firm diagnosis of MND is not recommended (class IV, level GCPP). At present, advanced neuroimaging techniques, including diffusion tensor imaging and proton magnetic resonance spectroscopic imaging, do not have a role in the diagnosis or routine monitoring of MNDs yet (class IV, level GCPP). However, it is strongly advisable to incorporate measures derived from these techniques into new clinical trials as exploratory outcomes to gain additional insights into disease pathophysiology and into the value of these techniques in the (longitudinal) assessment of MNDs (class IV, level GCPP).

Temporospatial coupling of networked synaptic activation of AMPA-type glutamate receptor channels and calcium transients in cultured motoneurons.

AMPA-type glutamate receptor (GluR) channels provide fast excitatory synaptic transmission in the CNS, but mediate also cytotoxic insults. It could be shown that AMPA-type GluR channel-mediated chronic excitotoxicity leads to an increased intracellular calcium concentration and plays an important role in neurodegenerative diseases like for example amyotrophic lateral sclerosis (ALS). As calcium is an important mediator of various processes in the cell and calcium signals have to be very precise in the temporospatial resolution, excessive intracellular calcium increases can seriously impair cell function. It is still unclear if AMPA-type receptors can directly interact with the intracellular calcium homeostasis or if other mechanisms are involved in this process. The objective of this study was therefore to investigate the calcium homeostasis in rat motoneurons under physiological stimulation of AMPA-type GluR channels using calcium imaging techniques and patch-clamp recordings simultaneously. It was found that spontaneous excitatory postsynaptic currents of cultured motoneurons did not elicit significant intracellular calcium transients. Large intracellular calcium transients occurred only when preceding fast sodium currents were observed. Pharmacological experiments showed that activation of AMPA-type GluR channels during synaptic transmission has a great functional impact on the calcium homeostasis in motoneurons as all kinds of activity was completely blocked by application of the selective kainate- and AMPA-type GluR channel blocker 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Furthermore we suggest from our experiments that calcium transients of several hundred milliseconds' duration result from release of calcium from the endoplasmic reticulum via activation of ryanodine receptors (calcium-induced calcium release, CICR). Our results help to understand the regulatory function of AMPA-type GluR channels in the intracellular calcium homeostasis which is known to be disturbed in neurodegenerative diseases.

The cellular mRNA expression of GABA and glutamate receptors in spinal motor neurons of SOD1 mice.

ALS is a fatal neurodegenerative disorder characterized by a selective loss of upper motor neurons in the motor cortex and lower motor neurons in the brain stem and spinal cord. About 10% of ALS cases are familial, in 10-20% of these, mutations in the gene coding for superoxide dismutase 1 (SOD1) can be detected. Overexpression of mutated SOD1 in mice created animal models which clinically resemble ALS. Abnormalities in glutamatergic and GABAergic neurotransmission presumably contribute to the selective motor neuron damage in ALS. By in situ hybridization histochemistry (ISH), we investigated the spinal mRNA expression of the GABAA and AMPA type glutamate receptor subunits at different disease stages on spinal cord sections of mutant SOD1 mice and control animals overexpressing wild-type SOD1 aged 40, 80, 120 days and at disease end-stage, i.e. around 140 days) (n=5, respectively). We detected a slight but statistically significant decrease of the AMPA receptor subunits GluR3 and GluR4 only in end stage disease animals.

Ligand-gated channels in early mesencephalic neuronal precursors: immunocytochemical and electrophysiological analysis.

Neuronal precursors play an important role in potential regenerative therapeutic strategies in different neurodegenerative diseases, e.g. Parkinson's disease. To understand proliferation and differentiation of these cells in vitro and in vivo, it is important to characterize functional properties of neuronal precursors in detail. The aim of the present study was to analyse the electrophysiological characteristics of ligand-gated channels of neuronal precursors prepared from the rat ventral mesencephalon (VM) of embryonic stage 12.5 during their in vitro differentiation. For the experiments we used the patch-clamp technique in combination with a system for ultrafast solution exchange and immunocytochemistry. It could be shown that functional active AMPA-type glutamate as well as GABA(A) receptor channels are expressed at an early stage of neuronal development. In culture we observed excitatory as well as inhibitory postsynaptic currents (defined by their different kinetics) which correspond to the activation of AMPAergic and GABAergic receptor channels. Two populations of glutamate-activated currents could be differentiated by their different time course of desensitization whereas the time course of resensitization and deactivation was normally distributed in all cells. GABAergic currents could be blocked by bicuculline and their kinetics correspond to that of GABA(A) receptor channel currents. Summarizing the results, in the present study it was shown for the first time that neuronal embryonic precursors of the rat VM express both functional AMPA-type glutamate and functional GABA(A) receptor channels in vitro.

Kinetic analysis of recombinant mammalian alpha(1) and alpha(1)beta glycine receptor channels.

To analyze the influence of the beta-subunit on the kinetic properties of GlyR channel currents, alpha(1)-subunits and alpha(1)beta-subunits were transiently expressed in HEK 293 cells. A piezo dimorph was used for fast application of glycine to outside-out patches. The rise time of activation was dose dependent for both receptors and decreased with increasing glycine concentrations. Subunit composition had no effect on the time course of activation. Coexpression of alpha(1)- and beta-subunits resulted in a significantly lower EC(50) and a reduced slope of the dose-response curve of glycine compared with expression of alpha(1)-subunits alone. For both receptor subtypes, the time course of desensitization was concentration dependent. Desensitization was best fitted with a single time constant at 10-30 micro M, with two at 0.1 mM, and at saturating concentrations (0.3-3 mM) with three time constants. Desensitization of homomeric alpha(1)-receptor channels was significantly slower than that of alpha(1)beta-receptor channels. The time course of current decay after the end of glycine pulses was tested at different pulse durations of 1 mM glycine. It was best fitted with two time constants for both alpha(1) and alpha(1)beta GlyR channels, and increased significantly with increasing pulse duration.

Voltage-dependent block of neuronal and skeletal muscle sodium channels by thymol and menthol.

BACKGROUND AND OBJECTIVE: Thymol is a naturally occurring phenol derivative used in anaesthetic practice as a stabilizer and preservative of halothane, usually at a concentration of 0.01%. Although analgesic effects have long been described for thymol and its structural homologue menthol, a molecular basis for these effects is still lacking. We studied the blocking effects of thymol and menthol on voltage-activated sodium currents in vitro as possible molecular target sites. METHODS: Whole cell sodium inward currents via heterologously (HEK293 cells) expressed rat neuronal (rat type IIA) and human skeletal muscle (hSkM1) sodium channels were recorded in the absence and presence of definite concentrations of either thymol or menthol. RESULTS: When depolarizing pulses to 0 mV were started from a holding potential of -70 mV, half-maximum blocking concentrations (IC50) for the skeletal muscle and the neuronal sodium channel were 104 and 149 mumol for thymol and 376 and 571 mumol for menthol. The blocking potency of both compounds increased at depolarized holding potentials with the fraction of inactivated channels. The estimated dissociation constant Kd for thymol and menthol from the inactivated state was 22 and 106 mumol for the neuronal and 23 and 97 mumol for the skeletal muscle sodium channel, respectively. CONCLUSIONS: The results suggest that antinociceptive and local anaesthetic effects of thymol and menthol might be mediated via blockade of voltage-operated sodium channels with the phenol derivative thymol being as potent as the local anaesthetic lidocaine.

The chemotherapeutic oxaliplatin alters voltage-gated Na(+) channel kinetics on rat sensory neurons.

The chemotherapeutic oxaliplatin causes a sensory-motor neuropathy with predominantly hyperpathic symptoms. The mechanism underlying this hyperexcitability was investigated using rat sensory nerve preparations, dorsal root ganglia and hippocampal neurons. Oxaliplatin resulted in an increase of the amplitude and duration of compound action potentials. It lengthened the refractory period of peripheral nerves suggesting an interaction with voltage-gated Na(+) channels. Application of oxaliplatin to dorsal root ganglion neurons resulted in an increase of the Na(+) current, a block of the maximal amplitude and a shift of the voltage-response relationship towards more negative membrane potentials. The effect was detectable on 13 of 18 tested cells. This observation, together with the absence of any effect on Na(+) currents of hippocampal neurons, suggests that the interaction of oxaliplatin is restricted to one or more channel subtypes. The effect of oxaliplatin could be antagonised by the Na(+) channel blocker carbamazepine which could be used to reduce side effects of oxaliplatin therapy in patients.

Ischaemic changes in refractoriness of human cutaneous afferents under threshold-clamp conditions.

1. A technique was developed to counteract the changes in threshold to electrical stimuli of large myelinated cutaneous afferents in the human median nerve induced by ischaemia for 13 min. Intermittent application of polarizing currents was used in five subjects, in whom refractoriness, supernormality and the strength-duration time constant (tauSD) were tracked to determine whether compensating for the ischaemia-induced changes in threshold also controlled the ischaemic changes in these excitability parameters. 2. The threshold compensation prevented the ischaemic changes in tauSD, an excitability parameter dependent on nodal Na+ channels. Threshold compensation did not prevent the changes in refractoriness and supernormality, whether the compensation began 10, 100 or 200 ms prior to the test stimuli. 3. In three subjects, continuous polarizing current was injected for 13 min to compensate for the ischaemic change in threshold, thus clamping threshold at the pre-ischaemic level. Again, tauSD was effectively controlled, but there were still ischaemic changes in refractoriness and supernormality. 4. The effective control of tauSD suggests that both the intermittent threshold compensation and the continuous threshold clamp effectively controlled membrane potential at the node of Ranvier. 5. The ischaemic increase in refractoriness when threshold was kept constant could be due to interference with the processes responsible for refractoriness by a metabolic product of ischaemia. The ischaemic change in supernormality during effective compensation probably results from the intrusion of refractoriness into the conditioning-test intervals normally associated with maximal supernormality. 6. The present results indicate that ischaemia has effects on axonal excitability that cannot be readily explained by changes in membrane potential. Specifically, it is suggested that ischaemic metabolites interfere with the recovery of Na+ channels from inactivation.

Reproducibility of indices of axonal excitability in human subjects.

OBJECTIVES: Different indices of axonal excitability are now being measured in human subjects, both normal volunteers undergoing some test manoeuvre and patients with a variety of peripheral nerve disorders. The reproducibility of these indices has not previously been established, and was determined for cutaneous afferents in the median nerve of 12 healthy subjects, using threshold tracking techniques. METHODS: Refractoriness and supernormality were determined as the change in stimulus current required to produce a predetermined target potential when conditioned by a supramaximal stimulus at appropriate conditioning-test intervals. Strength-duration time constant was calculated from the threshold currents using unconditioned test stimuli of 0.1 ms and 1.0 ms. The effects of changes in membrane potential on these indices was assessed by applying subthreshold DC currents (from 50% depolarizing to 50% hyperpolarizing), using the reciprocal of threshold (i.e., 'excitability') as an indicator of membrane potential. The intraindividual reproducibility was determined by repeating the study on each subject up to 10 times. RESULTS: Refractoriness and supernormality were variable between subjects (mean +/- SD of 31.5 +/- 9.5% and 13.2 +/- 3.8%, respectively) and within subjects (coefficient of variation 0.2104 and 0.21849, respectively). TauSD showed even greater interindividual variability (499.2 +/- 115 micros) and intraindividual variability (coefficient of variation 0.2339). The slopes of relationships between each of the indices and axonal 'excitability' suggest that refractoriness is extremely sensitive to changes in excitability (0.9767 +/- 0.1907), tauSD less so (0.3766 +/- 0.1322), supernormality least (0.2223 +/- 0.1268). CONCLUSIONS: Under controlled conditions, refractoriness is the most sensitive and least variable of the indices of axonal excitability. However, small decreases in temperature greatly increase refractoriness but have little effect on tauSD. Given that 3 indices reflect different biophysical mechanisms, nodal and internodal, greater insight into the functional state of peripheral nerve axons will come when there are coherent changes in all 3 indices.

Strength-duration properties and their voltage dependence at different sites along the median nerve.

OBJECTIVES: There is some evidence that the ease with which ectopic activity can be induced varies systematically along the course of a nerve and is greater at more proximal sites. Recent studies have implicated a non-inactivating threshold conductance, possibly due to persistent Na+ channels, in ectopic activity associated with ischaemia and hyperventilation. This conductance is largely responsible for the voltage dependence of strength-duration time constant (tauSD), and changes in it can explain the time constant changes that occur during hyperventilation and ischaemia. METHODS: To determine whether the strength-duration properties of motor axons of the median nerve vary along the course of the nerve, tauSD and rheobase were calculated at wrist, elbow and axilla in 15 healthy subjects, and the relationship of these properties to threshold was assessed using DC polarizing current to change axonal excitability. RESULTS: tauSD was similar at the 3 stimulating sites but increased less at the axilla with depolarizing current. CONCLUSIONS: These data indicate that the greater tendency for ectopic activity to arise from proximal segments of motor axons cannot be explained by differences in the conductances that contribute to tauSD and underlie its dependence on axonal excitability. The findings provide further support for the view that the precise relationship of the stimulating electrodes to the nerve has little effect on tauSD, at least when it is measured in the forearm.

Changes in excitability indices of cutaneous afferents produced by ischaemia in human subjects.

1. The present study was undertaken to determine whether mechanisms other than membrane depolarization contribute to the changes in excitability of cutaneous afferents of the median nerve under ischaemic conditions. 2. In six healthy subjects, axonal excitability was measured as the reciprocal of the threshold for a compound sensory action potential (CSAP) of 50% maximal amplitude. Refractoriness and supernormality were measured as threshold changes 2 and 7 ms, respectively, after supramaximal conditioning stimuli. The strength-duration time constant (tauSD) was calculated from the thresholds for unconditioned CSAPs using test stimuli of 0.1 and 1.0 ms duration. Changes in these indices were measured when subthreshold polarizing currents lasting 10 or 100 ms were applied, before, during and after ischaemia for 13 min. 3. At rest, the change in supernormality produced by polarizing currents was greater with the longer polarizing current, indicating that it took up to 100 ms to charge the internodal capacitance. 4. Refractoriness and its dependence on excitability increased more than expected during ischaemia. Supernormality was abolished during ischaemia, and reached a maximum after ischaemia but was then barely altered by polarizing current. tauSD had a similar relationship to excitability before, during and after ischaemia. 5. By contrast, during continuous depolarizing current for 8 min to mimic the depolarization produced by ischaemia, the relationship between excitability and refractoriness was the same during the depolarization as before it. 6. It is suggested that the large increase in refractoriness during ischaemia might be due to interference with the recovery from inactivation of transient sodium channels by an intra-axonal substrate of ischaemia. The post-ischaemic increase in supernormality and the lack of change with changes in axonal excitability can be explained by blockage of voltage-dependent potassium channels.

Function of the hyperpolarization-activated inward rectification in nonmyelinated peripheral rat and human axons.

The function of time-dependent, hyperpolarization-activated inward rectification was analyzed on compound potentials of nonmyelinated axons in the mammalian peripheral nervous system. Isolated rat vagus nerves and fascicles of biopsied human sural nerve were tested in a three-chambered, Vaseline-gap organ bath at 37 degrees C. Inward rectification was assessed by recording the effects of long-lasting hyperpolarizing currents on electrical excitability with the use of the method of threshold electrotonus (program QTRAC, copyright Institute of Neurology, London, UK) and by measuring activity-dependent changes in conduction velocity and membrane potential. Prominent time-dependent, cesium-sensitive inward rectification was revealed in rat vagus and human sural nerve by recording threshold electrotonus to 200-ms hyperpolarizing current pulses. A slowing of compound action potential conduction was observed during a gradual increase in the stimulation frequency from 0.1 to 3 Hz. Above a stimulation frequency of 0.3 Hz, this slowing of conduction was enhanced during bath application of 1 mM cesium. Cesium did not alter action potential waveforms during stimulation at frequencies < 1 Hz. Cesium-induced slowing in action potential conduction was correlated with membrane hyperpolarization. The hyperpolarization by cesium was stronger during higher stimulation frequencies and small in unstimulated nerves. These data show that a cesium-sensitive, time-dependent inward rectification in peripheral rat and human nonmyelinated nerve fibers limits the slowing in conduction seen in such axons at action potential frequencies higher than approximately 0.3 Hz.

Immunohistochemical and electrophysiological evidence for omega-conotoxin-sensitive calcium channels in unmyelinated C-fibres of biopsied human sural nerve.

In vitro electrophysiological measurements of Ca2+ potentials in human sural nerve fascicles revealed that Ca2+ conductances might be present on unmyelinated C-fibres. Furthermore, these Ca2+ potentials were partially blocked by omega-conotoxin, a calcium antagonist for the N-type Ca2+ channels. Therefore, immunohistochemical staining with indirect immunofluorescent omega-conotoxin GVIA was used to localize N-type Ca2+ channels in intact and in enzymatically dissociated human sural nerve fascicles. Densities of toxin binding sites were highly heterogeneous throughout the different nerve fascicles investigated and putative N-type Ca2+ channels were localized in about 20% of the unmyelinated C-fibres. Myelinating Schwann cells as well as enzymatically demyelinated axons displayed no specific binding indicating the absence of N-type Ca2+ channels.

Capsaicin blocks tetrodotoxin-resistant sodium potentials and calcium potentials in unmyelinated C fibres of biopsied human sural nerve in vitro.

Topical application of capsaicin has been tested recently for treatment of painful peripheral neuropathy. In the present study, effects of capsaicin were explored on compound action potentials of isolated fascicles from human sural nerve biopsies. Capsaicin reduced the C fibre component by 30-60%; the remaining C fibres were not sensitive to the drug. A good correlation was found between the sensitivity of C fibres to capsaicin and their resistance to tetrodotoxin (TTX), i.e. C fibre action potentials recorded in the presence of TTX were completely blocked by capsaicin. Calcium action potentials seen after inhibition of axonal potassium conductances were also completely suppressed. The data indicate that application of capsaicin nearby human peripheral nerves might prevent action potential conduction in specific subtypes of C fibres.

Calcium potentials and tetrodotoxin-resistant sodium potentials in unmyelinated C fibres of biopsied human sural nerve.

Compound action potentials and electrotonic responses to 150 ms current pulses were recorded from isolated nerve fascicles of human sural nerve biopsies. Compound action potentials in normal bathing solution were characterized by previously described A beta, A delta and C fibre components. In addition, tetrodotoxin-resistant sodium- or calcium-dependent potential components were found when a mixture of tetrodotoxin and the potassium channel blockers 4-aminopyridine and tetraethylammonium was added to the bathing solution. In contrast to tetrodotoxin-sensitive action potentials, tetrodotoxin-resistant sodium- or calcium-dependent potentials could be recorded in the presence of high extracellular potassium concentrations (10-20 mM). Calcium action potentials were found to be sensitive to specific pharmacological antagonists or agonists of L-, N- and P-type calcium channels. Lidocaine, cadmium, verapamil and capsaicin showed unspecific blocking effects on calcium and tetrodotoxin-resistant potentials. Tetrodotoxin-resistant action potentials seem to originate from unmyelinated C fibres since a clear correlation was found between the number of C fibres and the amplitude of tetrodotoxin-resistant calcium and sodium spikes in preparations with different axon type composition. The evidence for tetrodotoxin-resistant Na+ and Ca2+ spikes in peripheral human axons offers new possibilities for a better understanding and/or treatment of abnormalities in the excitability of damaged or diseased peripheral nerves.