Low concentrations of amitriptyline inhibit nicotinic receptors in unmyelinated axons of human peripheral nerve.

BACKGROUND AND PURPOSE: Amitriptyline is often prescribed as a first-line treatment for neuropathic pain but its precise mode of analgesic action remains uncertain. Amitriptyline is known to inhibit voltage-dependent ion channels and also to act as an antagonist at ligand-gated ion channels, such as nicotinic acetylcholine receptors (nAChRs). In the present study, we tested the effect of amitriptyline on nicotinic responses of unmyelinated axons in isolated segments of human peripheral nerve. In particular, a comparison was made between the concentrations of amitriptyline necessary for inhibition of nAChRs and those required for inhibition of the compound C-fibre action potential. EXPERIMENTAL APPROACH: Isolated axon fascicles were prepared from short segments of human sural nerve, and multiple measures of axonal excitability were recorded using computer-controlled threshold tracking software. KEY RESULTS: Amitriptyline (EC(50) 2.6 microM) reduced the nicotine-induced increase in C-fibre excitability but only slightly altered the amplitude and latency to onset of the compound action potential. In contrast, tetrodotoxin produced a clear reduction in the amplitude and a prolongation of action potential onset latency but was without effect on the nicotine-induced increase in axonal excitability. CONCLUSIONS AND IMPLICATIONS: These data demonstrate that low concentrations of amitriptyline suppress the response of human peripheral C-type axons to nicotine by directly inhibiting nAChRs. Blockade of tetrodotoxin-sensitive, voltage-dependent sodium channels does not contribute to this effect. An inhibitory action of amitriptyline on nAChRs in unmyelinated nociceptive axons may be an important component of amitriptyline's therapeutic effect in the treatment of neuropathic pain.

Retigabine reduces the excitability of unmyelinated peripheral human axons.

Enhancement of membrane K(+) conductance may reduce the abnormal excitability of primary afferent nociceptive neurons in neuropathic pain. It has been shown that retigabine, a novel anticonvulsant, activates Kv7 (KCNQ/M) channels in the axonal/nodal membrane of peripheral myelinated axons. In this study, we have tested the effects of retigabine on excitability parameters of C-type nerve fibers in isolated fascicles of human sural nerve. Application of retigabine (3-10 microM) produced an increase in membrane threshold. This effect was pronounced in depolarized axons and small in hyperpolarized axons. This finding indicates that retigabine produces a membrane hyperpolarization which is limited by the K(+) equilibrium potential. The retigabine-induced reduction in excitability was accompanied by modifications of the post-spike recovery cycle. Most notable is the development of a late subexcitability at 250-400 ms following a short burst of action potentials. All effects of retigabine were blocked in the presence of XE991 (10 microM). The data show that Kv7 channels are present on axons of unmyelinated, including nociceptive, peripheral human nerve fibers. It is likely that activation of these channels by retigabine may reduce the ectopic generation of action potentials in neuropathic pain.

Kinetics of ATP release following compression injury of a peripheral nerve trunk.

Compression and/or contusion of a peripheral nerve trunk can result in painful sensations. It is possible that release of ATP into the extracellular space may contribute to this symptom. In the present study, we used real-time measurements of ATP-induced bioluminescence together with electrophysiological recordings of compound action potentials to follow changes in the extracellular ATP concentration of isolated rat spinal roots exposed to mechanical stimuli. Nerve compression for about 8 s resulted in an immediate release of ATP into the extracellular space and in a decrease in the amplitude of compound action potentials. On average, a rise in ATP to 60 nM was observed when nerve compression blocked 50% of the myelinated axons. After the compression, the extracellular concentration of ATP returned to the resting level within a few minutes. The importance of ecto-nucleotidases for the recovery period was determined by exposure of isolated spinal roots to high concentrations of ATP and by use of inhibitors of ecto-nucleotidases. It was observed that spinal roots have a high capacity for ATP hydrolysis which is only partially blocked by betagamma-methylene ATP and ARL 67156. In conclusion, acute nerve compression produces an increase in the extracellular concentration of ATP and of its metabolites which may be sufficient for activation of purinergic P2 and/or P1 receptors on axons of nociceptive afferent neurons.

Chemical mediators enhance the excitability of unmyelinated sensory axons in normal and injured peripheral nerve of the rat.

Ectopic excitation of nociceptive axons by chemical mediators may contribute to symptoms in neuropathic pain. In this study, we have measured the excitability of unmyelinated rat C-fiber axons in isolated segments of sural nerves under different experimental conditions. (1) We demonstrate in normal rats that several mediators including ATP, serotonin (5-HT), 1-(3-chlorophenyl)biguanide (5-HT3 receptor agonist), norepinephrine, acetylcholine and capsaicin alter electrophysiological parameters of C-fibers which indicate an increase of axonal excitability. Other mediators such as histamine, glutamate, prostaglandin E(2) and the cytokines tumor necrosis factor alpha, interleukin-1beta and interleukin-6 did not produce such effects. (2) The effects of several mediators were tested after peripheral nerve injury (partial ligation or spared nerve injury). Sural nerves from such animals did not show significant changes when compared with controls. (3) We tested whether the effects of chemical mediators on axonal excitability are due to actions on the sensory C-fiber afferents or the postganglionic sympathetic efferents. In order to distinguish these effects, we performed surgical sympathectomy of the lumbar sympathetic chain, including the L3, L4 and L5 ganglia. Sympathectomy did not markedly influence the effects of mediators on axonal excitability (except that the norepinephrine effect was significantly diminished). In conclusion, our data suggest a constitutive rather than inducible expression of axonal receptors for some chemical mediators on the axonal membrane of unmyelinated fibers. Most of the changes in axonal excitability take place in sensory C-fiber afferents rather than in postganglionic sympathetic efferents. Thus, it is possible that certain immune and glial cell mediators released in or around the nerve following injury or inflammation influence the excitability of intact nociceptive fibers. This mechanism could contribute to ectopic excitation of axons in neuropathic pain.

Characterization of neuronal nicotinic acetylcholine receptors in the membrane of unmyelinated human C-fiber axons by in vitro studies.

Application of acetylcholine to peripheral nerve terminals in the skin is a widely used test in studies of human small-fiber functions. However, a detailed pharmacological profile and the subunit composition of nicotinic acetylcholine receptors in human C-fiber axons are not known. In the present study, we recorded acetylcholine-induced changes of the excitability and of the intracellular Ca2+ concentration in C-fiber axons of isolated human nerve segments. In addition, using immunohistochemistry, an antibody of a subtype of nicotinic acetylcholine receptor was tested. Acetylcholine and agonists reduced the current necessary for the generation of action potentials in C fibers by > 5-Iodo-A-85380 > 1,1-dimethyl-4-phenylpiperazinium iodide > nicotine > cytisine > acetylcholine; choline had no effect. The epibatidine-induced increase in axonal excitability was blocked by mecamylamine and, less efficiently, by methyllycacontine and dihydro-beta-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the alpha5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of alpha3, alpha5, and beta4 but not of alpha4/beta2 or of alpha7 subunits in the axonal membrane of unmyelinated human C fibers. In addition, the observations suggest that the axonal membrane of C fibers in isolated segments of human sural nerve can be used as a model for presumed cholinergic chemosensitivity of axonal terminals.

ATP stimulates peripheral axons in human, rat and mouse--differential involvement of A(2B) adenosine and P2X purinergic receptors.

Receptors for ATP have been reported on peripheral nerve terminals. It is a widespread assumption that the axonal membrane does not possess this kind of chemosensitivity, although P2X purinoceptors have been found in isolated rat vagus nerve. Therefore, in the present study, effects of ATP and analogues were tested on the excitability of unmyelinated axons in isolated rat sural nerve, mouse dorsal roots, and human sural nerve. Bath application of ATP to all three types of axonal preparations increased axonal excitability, but the underlying receptors appear to differ in the various preparations. In rat sural nerve, alpha,beta-adenosine-5'-methylene triphosphate produced the strongest excitation. This effect was blocked by pyridoxal-phosphate-6-azophenyl-2',5'-disulphonic acid and indicates the presence of P2X receptors. In mouse dorsal roots, differences were found between fast and slow C-fibres. The latter responded to both P2X receptor and adenosine receptor agonists. In contrast, effects of ATP on faster-conducting C-fibres seem to be caused exclusively by effects of ATP on adenosine receptors. Application of ATP also excited C-fibres in fascicles of isolated human nerve. The pharmacological profile indicates activation of A(2B) adenosine receptors. However, we could not detect P2X receptors in this preparation with our techniques. These data show that the ATP sensitivity of sensory neurones is not restricted to their terminals. Activation of axonal purinergic receptors may contribute to the transduction of sensory, including nociceptive, stimuli.

Diadenosine pentaphosphate is more potent than ATP at P2X receptors in isolated rat vagus nerve.

The effects of diadenosine pentaphosphate (Ap5A), diadenosine tetraphosphate (Ap4A), alpha,beta-methyleneATP (alpha,beta-meATP), and ATP were studied on the excitability of unmyelinated axons in isolated rat vagus nerve by means of a computerized threshold tracking technique. All purinergic compounds produced an increase in excitability, however, only the effects of alpha,beta-meATP and of Ap5A were strongly reduced by 2'- (or 3') -O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP), a selective blocker for P2X1, P2X3, and heteromeric P2X2/3 receptors. The rank order of potency for TNP-ATP-sensitive excitation was determined as follows (30 microM each): alpha,beta-meATP >Ap5A >> Ap4A = ATP. These data suggest that Ap5A might be an important naturally occurring agonist for P2X receptors at the axonal membrane of unmyelinated, including nociceptive, nerve fibres.

Enhanced proliferation and potassium conductance of Schwann cells isolated from NF2 schwannomas can be reduced by quinidine.

Neurofibromatosis type 2 (NF2) is an autosomal dominant disease that is characterized mainly by schwannomas, as well as menigiomas and gliomas. The NF2 gene product merlin/schwannomin acts as a tumor suppressor. Schwann cells derived from NF2 schwannomas showed an enhanced proliferation rate, and electrophysological studies revealed larger K(+) outward currents as compared with controls. Schwann cells isolated from schwannomas of NF2 patients or multiorgan donors were treated with different concentrations of the K(+) current blockers quinidine, tetraethylammonium chloride, and 4-aminopyridine and K(+) outward currents and proliferation rates of these cells were compared. K(+) outward currents of both cell types can be blocked by quinidine. Importantly, treatment with quinidine reduces proliferation of NF2 Schwann cells in a concentration dependent manner but did not reduce proliferation of normal Schwann cells. Therefore, the use of quinidine or quinidine-like components would possibly provide a novel adjuvant therapeutic option for NF2 patients to slow down or freeze growth of schwannomas.

Neuroligand-mediated calcium signaling in cultured human schwannoma cells.

Intracellular Ca2+ release regulates proliferation of nonexcitable cells, however, it is not known whether and which neuroligands modulate the free intracellular Ca2+ concentration in human schwannoma cells. Confocal laser scanning microscopy was used for the study of neuroligand-induced Ca2+ signaling in cultured human schwannoma cells loaded with the Ca2+-sensitive fluorescent dyes Calcium Green-1 and Fura Red. Intracellular Ca2+ transients were observed during bath application of ATP (90% of cells tested), endothelin (60%), and norepinephrine (20%); histamine, serotonin, glutamate, and bradykinin did not have this effect. Two types of P2Y nucleotide receptor subtypes involved in ATP-induced Ca2+ transients could be separated by application of different P2Y receptor agonists and cross desensitization experiments: P2Y1 (rank order of potency: 2-MeSADP > 2-MeSATP > ADP > ATP) and P2Y2 (sensitive to UTP and ATP). Endothelin-induced Ca2+ signaling was also seen during application of [Ala(1,3,11,15)]ET-1; this suggests the presence of an ET(B) receptor subtype. The results indicate that the presence of receptors linked to neuroligand-triggered Ca2+ signaling does not seem to be abnormal in a human Schwann cell tumor, i.e. schwannoma cells retain this characteristic of peripheral glia.

Confocal calcium imaging reveals an ionotropic P2 nucleotide receptor in the paranodal membrane of rat Schwann cells.

1. The paranodal Schwann cell region is of major importance for the function of a myelinated axon. In the present study we searched for a possible ionotropic effect of extracellular ATP in this Schwann cell compartment. 2. Whole-cell patch-clamp recordings from cultured rat Schwann cells revealed that ATP and 2'-3'-O-(4-benzoylbenzoyl)-adenosine 5'-triphosphate (BzATP) induced a non-specific cation current. The effect of ATP was much enhanced in a Ca2+- and Mg2+-free solution. ADP, UTP and alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP) had no effect. 3. Confocal Ca2+ imaging of myelinating Schwann cells in isolated rat spinal roots showed a BzATP-induced rise in the free intracellular Ca2+ concentration in the paranodal Schwann cell cytoplasm whereas alpha,beta-meATP and 2-(methylthio)-adenosine 5'-triphosphate were without effect. In contrast to the known metabotropic effect of UTP on these Schwann cell regions, the BzATP-induced Ca2+ signal was not transient, was unaffected by depletion of intracellular Ca2+ stores and dependent on the presence of extracellular Ca2+. 4. These results suggest that an ionotropic ATP receptor with electrophysiological and pharmacological characteristics of the P2X7 subtype of nucleotide receptors is functionally active in myelinating Schwann cells of peripheral nerves. Such a receptor might contribute to Schwann cell reactions in nerve injury or neuropathy.

Voltage-dependent membrane currents of cultured human neurofibromatosis type 2 Schwann cells.

Previous experimental observations indicate that inhibition of voltage-dependent K+ currents suppresses proliferation of normal Schwann cells. In the present study we tested the opposite relationship, i.e., whether Schwann cells from tumors with abnormally high rates of proliferation would have an increase in membrane K+ currents. Whole-cell membrane currents were studied in cultured cells from schwannomas of two neurofibromatosis type 2 (NF2) patients (n = 53), one patient with a sporadic schwannoma (n = 22), and two control subjects (n = 41). Five different types of voltage-dependent membrane currents were found in all of the Schwann cells tested. Membrane depolarization activated outward K+ and Cl- currents; quinidine was found to block the K+ current (IC50 approximately 1 microM), and NPPB reduced the Cl- current. Ba2+-sensitive inward rectifier K+ currents, fast Na+ currents, and a transient, inactivating K+ current were less frequently observed. On average, NF2 cells were found to have statistically significant higher membrane potential and larger non-inactivating K+ outward current as compared to controls. Electrophysiological parameters of Schwann cells from a sporadic schwannoma showed a tendency for larger outward currents; however, the difference did not reach statistical significance. Together the data support the suggestion of a possible link between K+ outward current and proliferation of Schwann cells.

Differences in the sensitivity to purinergic stimulation of myelinating and non-myelinating Schwann cells in peripheral human and rat nerve.

Schwann cells of the peripheral nervous system are distinguished by morphological and functional criteria in myelinating and non-myelinating subtypes. We and others have previously reported that Schwann cells in isolated peripheral human and rat nerve respond to extracellular application of ATP with a rise in the intracellular free calcium concentration [Ca2+]i. In the present study, the receptors mediating these Ca2+ transients have been investigated in myelinating and non-myelinating Schwann cells of intact fascicles of isolated human sural nerves, rat ventral roots, and rat vagus nerves. Microfluorometry and confocal laser scanning was used on preparations stained with the Ca2+-sensitive dyes Calcium Green-1 and Fura Red. In myelinating Schwann cells of human and rat nerves, the ATP-induced rise of [Ca2+]i resulted from the activation of a P2Y2 purinoceptor subtype (rank order of potency: UTP > or = ATP >> 2-MeSATP = ADP). In contrast, in non-myelinating Schwann cells, Ca2+ transients were produced by activation of a P2Y1 purinoceptor subtype (rank order of potency: 2-MeSATP > ATP > ADP >> UTP). The P1 agonist adenosine and alpha,alpha-meATP did not evoke Ca2+ signals. Ca2+ transients in both types of Schwann cells were found to be due to Ca2+ release from cyclopiazonic acid-sensitive intracellular stores. However, inhibition by suramin was only found in non-myelinating Schwann cells. These findings indicate that mammalian Schwann cells express phenotype-specific P2Y receptor subtypes.

Activity-dependent intracellular Ca2+ transients in unmyelinated nerve fibres of the isolated adult rat vagus nerve.

Confocal laser scanning microscopy was used to follow changes in the free intracellular calcium concentration ([Ca2+]i) in nerve fibres and adjacent Schwann cells in isolated rat vagus nerves. [Ca2+]i was monitored by the Ca2+-sensitive fluorescent dyes Calcium Green-1 and Fura Red. Intracellular Ca2+ transients were observed during repetitive (1-50 Hz) supramaximal electrical stimulation or by bath application of ATP. Trains of action potentials were more effective at elongated, fibre-like structures of the vagus nerves, whereas ATP-induced Ca2+ transients were found predominantly in regions of Schwann cell bodies. Activity-induced Ca2+ signals were unaffected by pharmacological manipulation of intracellular Ca2+ stores, during long-lasting application of purinergic receptor agonists, or by substitution of extracellular Na+ with Li+. However, they were abolished in the presence of Ca2+-free bathing solution or after the blocking of Ca2+ channels with Cd2+. Ca2+ transients were also observed during Ca2+ action potentials. Such "Ca2+ spikes" were elicited by electrical stimulation in the presence of a combination of tetrodotoxin and K+ channel blockers. These data suggest that voltage-dependent Ca2+ channels, activated during short trains of Na+ action potentials, produce an increase in intra-axonal [Ca2+] of rat vagus nerves. We did not find evidence for activity-dependent Ca2+ transients in the Schwann cells surrounding the unmyelinated axons.

A special blocker reveals the presence and function of the hyperpolarization-activated cation current IH in peripheral mammalian nerve fibres.

Electrotonic responses recorded extra- or intracellularly from peripheral nerve preparations show a "sag" to hyperpolarizing current pulses. The biophysical nature of this "inward rectification" is still under discussion since the phenomenon has not been noted at voltage-clamped single nerve fibres, and since Cs+, which reduces inward rectification, is not a specific ion channel blocker. In this study, we found that low micromolar concentrations of ZD 7288, a specific blocker of the hyperpolarization-activated cationic current (Ih) in the soma of central mammalian neurons, result in a complete block of inward rectification in the electrotonic responses of isolated rat spinal dorsal roots. In addition, ZD 7288 enhanced the activity-dependent slowing of conduction seen in compound C fibre action potentials of isolated rat vagus nerves and augmented the post-tetanic hyperpolarization following trains of action potentials in unmyelinated and myelinated axons. The data suggest that ZD 7288 is a potent blocker and a useful research tool for the study of hyperpolarization-activated inward rectification (Ih) of peripheral nerve preparations.

Intracellular calcium transients mediated by P2 receptors in the paranodal Schwann cell region of myelinated rat spinal root axons.

Receptors for neuroligands in the paranodal Schwann cell region of a myelinated nerve fiber could have important functions. We have used confocal laser scanning microscopy in combination with Ca(2+)-sensitive fluorescent dyes to study the possible effects of purinergic agonists on the free intracellular calcium concentration ([Ca2+]i) in paranodes of isolated rat spinal roots. Application of ATP in concentrations of 100 and 300 microM resulted in a transient rise in [Ca2+]i in about 57% of the paranodal Schwann cell regions studied. UTP was equipotent to ATP whereas adenosine, beta,gamma-methylene ATP, and elevation of the extracellular K+ concentration by 10 mM had no effect on [Ca2+]i. These data indicate the presence of the P2Y2 (previously termed P2U) subtype of P2 receptors in the paranodal Schwann cell membrane of rat spinal root myelinated nerve fibers.

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.

Abnormal axonal inward rectification in diabetic neuropathy.

An abnormal axonal membrane conductance might contribute to human diabetic neuropathy. To test this idea, we have compared the threshold changes produced by long-lasting (100-200 ms) de- and hyperpolarizing currents applied to median motor and sensory axons at the wrist in 63 diabetic patients with those from 50 normal controls and 27 amyotrophic lateral sclerosis (ALS) patients. Averages of the threshold electrotonus plots for motor and sensory axons of diabetic patients showed more subexcitability during, and slower recovery following, the application of hyperpolarizing currents. Such alterations have been previously found in isolated rat nerves after inhibition of axonal inward rectification by means of cesium ions. The abnormalities in diabetics were positively correlated with the age of patients and the presence of neuropathy. Threshold electrotonus seen in diabetes differed strongly from the effects of acute ischemia and were unlike changes recorded in ALS. The data indicate that an abnormal inward rectification of peripheral axons is associated with diabetic neuropathy. A better understanding of the neurobiology of this conductance might provide information about the pathophysiology of this disease.

Glucose availability alters ischaemia-induced changes in intracellular pH and calcium of isolated rat spinal roots.

Peripheral nerves in diabetic patients show an enhanced liability to ischaemic lesions. Using an in vitro model, we have now analysed the possible role of intracellular proton (pHi) and calcium concentrations ([Ca2+]i) for the pathophysiology of this phenomenon. Isolated rat spinal roots were preincubated for 3 to 6 h in either 5 or 25 mM of D-glucose before transient exposure to gaseous hypoxia or cyanide. Intracellular pH and Ca2+ concentrations were measured photometrically by means of the fluorescent dyes carboxy-SNARF-1 and a combination of Calcium Green-1 and Fura Red, respectively. The following observations were made. (a) The presence of 25 mM D-glucose resulted in stronger intracellular acidification and much slower post-hypoxic recovery of pHi as compared to 5 mM D-glucose. (b) Intracellular calcium increased during hypoxia and recovered quickly on reoxygenation. There were no statistically significant differences between the Ca2+ signals in either high or normal concentrations of D-glucose, although on average less rise was seen in high glucose. (c) Inhibition of glycolysis with iodoacetate reduced the acidification but amplified the rise in [Ca2+]i seen during transient hypoxia. These data suggest that hypoxia-induced nerve acidification rather than a rise in [Ca2+]i might contribute to ischaemic lesions found in diabetic neuropathy.

P2 purinoceptor-mediated intracellular Ca2+ transients in human sural nerve.

Segments of biopsied human sural nerve were stained with the Ca(2+)-sensitive fluorescent dyes Calcium Green-1 and Fura Red. The emission ratio was used to follow changes in the intracellular free Ca2+ concentration ([Ca2+]i). Application of ATP and analogues in concentrations between 0.3 and 300 microM via the bathing solution resulted in a transient rise in [Ca2+]i. The rank order of agonist potency, 2-methylthioATP > ATP > UTP, and the failure of adenosine, alpha,beta-MeATP and beta,gamma-MeATP to evoke rises in [Ca2+]i indicate the presence of P2Y/U subtypes of purinoceptors in this preparation. ATP-induced Ca2+ transients in biopsied human nerve preparations might serve as a diagnostic tool in neuropathies.

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.