Long-range intercellular signalling in glial cells of the peripheral nerve.

Schwann cells are considered to be electrically silent satellite cells surrounding axons, although they exhibit ionic channels, some of which are similar to those employed by axons for the generation and transmission of nerve impulses. Here, we show that Schwann cells generate, in response to a short and gentle electrical stimulus, a long-lasting depolarizing potential, slowly propagating along the Schwann cell synsitium. This electrical signal, which in situ might be generated by the Schwann cells in response to the axonal electrical activity, constitutes in the peripheral glia a novel form of long-range intercellular signalling, which may be involved in the regulation and modulation of the axonal excitability.

Hyperpolarizing afterpotentials in C fibers and local anesthetic effects of clonidine and lidocaine.

Effects of clonidine and lidocaine on the hyperpolarizing after-potential (HAP) and frequency-dependent block in C fibers were examined on desheathed rabbit vagus nerves, using the sucrose gap technique. A single action potential (AP) was followed by a fast and a slow HAP. Clonidine, at concentrations from 0.05 to 50 mumol/l, decreased the fast HAP, while the AP amplitude was unchanged. At a 500 mumol/l concentration of clonidine, the fast HAP amplitude was similar to control, the slow HAP was increased, and the AP amplitude decreased. Lidocaine at 500 mumol/l delayed and broadened the HAP, making a distinction between fast and slow HAP impossible, and decreased and delayed the AP amplitude. In the presence of lidocaine (500 mumol/l), clonidine at concentrations from 0.05 to 500 mumol/l decreased the HAP amplitude, without modifying the lidocaine-induced shape of the HAP. The modifications of the HAP, however, do not contribute to the local anesthetic effects of clonidine, as the addition of clonidine (0.5 and 500 mumol/l) to Locke or lidocaine (500 mumol/l) solution does not enhance the frequency-dependent block (3 and 10 Hz) observed with either Locke or lidocaine solution alone.

Clonidine enhances the effects of lidocaine on C-fiber action potential.

We examined local anesthetic effects of clonidine and its interaction with lidocaine with regard to tonic inhibition of the C-fiber action potential (AP) on the isolated, desheathed rabbit vagus nerve by the sucrose gap method. Clonidine and lidocaine at 500 microM concentrations caused a comparable degree of C-fiber inhibition, corresponding to an AP area under the curve of 75.8% +/- 9.4% (mean +/- SE) and 82.2% +/- 5.9% of control, respectively. Concentrations of clonidine less than 500 microM did not inhibit C-fiber AP. Clonidine, added in concentrations of 500 nM, 500 microM, and 5 mM to a 500 microM lidocaine perfusion, caused a significant decrease in fiber blockade of 18%, 20%, and 54%, respectively, as compared with clonidine added to Locke perfusion (P less than 0.05). The sodium channel blocker tetrodotoxin (3 microM) decreased the AP area to 9.3% +/- 1.3% of control. The remaining tetrodotoxin-resistant AP was almost completely blocked by clonidine (500 microM) and lidocaine (500 microM), indicating a higher susceptibility of tetrodotoxin-resistant fibers to the two drugs than the C-fiber population as a whole. The enhancing effect of a low dose of clonidine (500 nM) on lidocaine-induced (500 microM) inhibition of C-fiber AP might explain the clinical observation that clonidine, at approximately 1000-fold lower concentrations than lidocaine, prolongs the action of lidocaine in peripheral nerve block.

After potentials in nonmyelinated nerve fibers.

1. The sucrose-gap technique was employed to examine the different types of after potentials that follow, in desheathed rabbit vagus nerves, a single action potential (AP) elicited by a short (0.4 ms) supramaximal depolarizing pulse. 2. A fast and a slow hyperpolarizing after potential (fHAP and sHAP) as well as a depolarizing after potential (DAP) followed a single spike. Both the fHAP and the sHAP showed a dependence on the K+ electrochemical gradient, indicating that they are due to an outwardly oriented current of K+ ions. 3. The fHAP was sensitive to low concentrations of tetraethylammonium (TEA; 1 mM) and 4-aminopyridine (4-AP; 10 microM) and to millimolar concentrations of Ba2+. We conclude that the fHAP reflects the tail of the delayed rectifier K+ current. 4. The sHAP contained a Ca(2+)-sensitive component that showed a requirement for voltage-dependent Ca2+ entry during the AP. This component was completely blocked by low concentration of TEA (1 mM) and by Cd2+ (1 mM), but unaffected by 4-AP. These observations suggest that it reflects a current flowing through Ca(2+)-activated K+ channels. The remaining, apparently Ca(2+)-insensitive, component was insensitive to 4-AP and could be blocked by TEA only at concentrations greater than 50 mM. 5. The DAP usually appeared when the external concentration of K+ was increased to above approximately 8 mM, but sometimes it was clearly visible even at lower [K+]o. The DAP was TEA insensitive and entirely Ca2+ dependent. This latter property is inconsistent with the widely accepted hypothesis according to which the DAP reflect the accumulation of K+ in the extracellular space during the AP. 6. The origins of both the Ca(2+)-insensitive component of the sHAP and the DAP are not clear. However, in view of the fact that the sucrose-gap technique records not only the membrane potential of the nerve fibers but also of the surrounding glia, there is the possibility that these after potentials reflect changes in the electrical properties of the satellite Schwann cells.

Effects of substance P, calcitonin gene-related peptide and capsaicin on tension and membrane potential of pig coronary artery in vitro.

The effects of substance P (SP), calcitonin gene-related peptide (CGRP) and capsaicin were studied on isometric tension and membrane potential of pig coronary arterial strips in vitro. CGRP induced an endothelium-independent relaxation without change in the smooth muscle membrane potential whereas SP relaxed and hyperpolarized the strip via the endothelium. Applied together, the mechanical effects of SP plus CGRP were additive. CGRP did not affect the hyperpolarization due to SP. In order to examine a possible role of endogenous SP and CGRP, capsaicin was used. It provoked a contraction which was adventitia-dependent, and was inhibited by indomethacin. In presence of indomethacin, capsaicin caused a relaxation. It was accompanied by a hyperpolarization of smooth muscle membrane potential only when the strip had an intact endothelium. When the strip was de-endothelialized capsaicin relaxation subsisted. This indicates that capsaicin produced a relaxation of indomethacin-treated strip by releasing a hyperpolarizing endothelial factor and probably endogenous CGRP.

Hemoglobin causes both endothelium-dependent and endothelium-independent contraction of the pig coronary arteries, independently of an inhibition of EDRF effects.

Hemoglobin is widely used as an inhibitor of EDRF effects. Hemoglobin contracts pig coronary arteries in vitro. However, during this contraction, effects of substance P and bradykinin which act via the EDRF are not inhibited. This means that the hemoglobin contraction is not caused by inhibition of the EDRF. This contraction is caused by a substance released from the endothelium, and by eicosanoïds released from the smooth muscles.

Substance P and bradykinin hyperpolarize pig coronary artery endothelial cells in primary culture.

Substance P (SP) and bradykinin (BK) relax and hyperpolarize smooth muscles of pig coronary arteries in an endothelium-dependent manner. We investigated the effect of both peptides on the membrane potential of endothelial cells in culture. The membrane potential of pig coronary artery endothelial cells in primary culture was -44 mV. SP and BK hyperpolarized it transiently by 23 mV. These hyperpolarizations were dependent on the potassium gradient across the cellular membrane. They had similar time courses as the SP and BK endothelium-dependent hyperpolarizations already observed in smooth muscles.

Electrophysiological and mechanical effects of substance P and acetylcholine on rabbit aorta.

1. The mechanical and electrical properties of smooth muscle cells of the rabbit aorta were recorded simultaneously using respectively a force transducer and a 3 M-KCl-filled glass microelectrode. 2. Acetylcholine had two effects depending on concentration. At low concentration, it caused a persistent endothelium-dependent relaxation and hyperpolarization. At higher concentrations the acetylcholine endothelium-dependent relaxation summed with an endothelium-independent contraction. 3. Substance P caused a transient endothelium-dependent relaxation and hyperpolarization. 4. Acetylcholine and substance P depolarized and contracted de-endothelialized smooth muscle. When the de-endothelialized strip was pre-contracted by noradrenaline, acetylcholine depolarized the muscle but substance P did not. 5. In a 'cascade' experiment, the perfusate from an upstream intact aorta passed over a downstream de-endothelialized strip. Acetylcholine and substance P relaxed the downstream strip showing that they released an endothelial humoral factor which relaxes smooth muscle. 6. The results suggest a constant release of a factor from the endothelial cells which hyperpolarizes the smooth muscle cells in the media. Activation of acetylcholine and substance P receptors on the endothelium accelerates the release of this factor and causes vasodilatation.

Effect of mechanical stimulation, substance P and vasoactive intestinal polypeptide on the electrical and mechanical activities of circular smooth muscles from pig coronary arteries contracted with acetylcholine: role of endothelium.

Mechanical stimulation, substance P and vasoactive intestinal polypeptide (VIP) were found to relax the transversal strip of anterior descending branches of pig coronary arteries precontracted by acetylcholine. The effects of mechanical stimulation and substance P required the presence of intact endothelium, while VIP did not. The effect of VIP did not appear to be mediated by catecholamines. Simultaneous measurements of intracellular membrane potential and tension developed by coronary smooth muscle precontracted with Ach showed that the smooth muscle relaxation by substance P is accompanied by membrane hyperpolarization. In contrast VIP relaxed the same tissue without affecting the membrane potential. In a cascade experiment, the fluid perfused intraluminally in intact segments of coronary arteries was dropped over a de-endothelialized strip which relaxed in response to substance P and mechanical stimulation. This indicates that substance P and mechanical stimulation act by releasing from the endothelium a humoral factor that produces arterial smooth muscle relaxation.

3-O-hydrosulphato-4-hydroxyphenethylamine (dopamine 3-O-sulphate), a metabolite involved in the sclerotization of insect cuticle.

Dopamine 3-O-sulphate (3-O-hydrosulphato-4-hydroxyphenethylamine) was isolated from newly ecdysed cockroaches, Periplaneta americana (L.), and its structure established by chemical and physical techniques and by synthesis. Relatively high concentrations (about 1mumol/g wet. wt.) of dopamine 3-O-sulphate exist in the newly ecdysed insect, and these concentrations decrease sharply as sclerotization of the cuticle proceeds. At least 40% of the radioactivity of (14)C-labelled dopamine 3-O-sulphate injected into newly ecdysed nymphs was recovered in the sclerotized cuticle 7-12 days after the injection. However, less than 1% of the radioactivity of injected dopamine 3-O-[(35)S]sulphate was recovered, and this value was not appreciably different from that for the incorporation of Na(2) (35)SO(4). Apparently, little or none of the sulphate moiety of dopamine 3-O-sulphate is incorporated directly into the cuticle as the intact sulphate ester. These observations are discussed in relation to current concepts of cuticular sclerotization in insects.