A mechanosensitive K+ channel with fast-gating kinetics on human axons blocked by gadolinium ions

The peripheral origin of tap-induced muscle contraction revealed by multi-electrode surface electromyography in human vastus medialis

It is well established that muscle percussion may lead to the excitation of muscle fibres. It is still debated, however, whether the excitation arises directly at the percussion site or reflexively, at the end plates. Here we sampled surface electromyograms (EMGs) from multiple locations along human vastus medialis fibres to address this issue. In five healthy subjects, contractions were elicited by percussing the distal fibre endings at different intensities (5-50 N), and the patellar tendon. EMGs were detected with two 32-electrode arrays, positioned longitudinally and transversally to the percussed fibres, to detect the origin and the propagation of action potentials and their spatial distribution across vastus medialis. During muscle percussion, compound action potentials were first observed at the electrode closest to the tapping site with latency smaller than 5 ms, and spatial extension confined to the percussed strip. Conversely, during tendon tap (and voluntary contractions), action potentials were first detected by electrodes closest to end plates and at a greater latency (mean ± s.d., 28.2 ± 1.7 ms, p < 0.001). No evidence of reflex responses to muscle tap was observed. Multi-electrode surface EMGs allowed for the first time to unequivocally and quantitatively describe the non-reflex nature of the response evoked by a muscle tap.

Optogenetics, the intersection between physics and neuroscience: light stimulation of neurons in physiological conditions

Neuronal stimulation by light is a novel approach in the emerging field of optogenetics, where genetic engineering is used to introduce light-activated channels. However, light is also capable of stimulating neurons even in the absence of genetic modifications through a range of physical and biological mechanisms. As a result, rigorous design of optogenetic experiments needs to take note of alternative and parallel effects of light illumination of neuronal tissues. Thus all matters relating to light penetration are critical to the development of studies using light-activated proteins. This paper discusses ways to quantify light, light penetration in tissue, as well as light stimulation of neurons in physiological conditions. We also describe the direct effect of light on neurons investigated at different sites.

Acute nerve stretch and the compound motor action potential

In this paper, the acute changes in the compound motor action potential (CMAP) during mechanical stretch were studied in hamster sciatic nerve and compared to the changes that occur during compression.

In response to stretch, the nerve physically broke when a mean force of 331 gm (3.3 N) was applied while the CMAP disappeared at an average stretch force of 73 gm (0.73 N). There were 5 primary measures of the CMAP used to describe the changes during the experiment: the normalized peak to peak amplitude, the normalized area under the curve (AUC), the normalized duration, the normalized velocity and the normalized velocity corrected for the additional path length the impulses travel when the nerve is stretched. Each of these measures was shown to contain information not available in the others.

During stretch, the earliest change is a reduction in conduction velocity followed at higher stretch forces by declines in the amplitude of the CMAP. This is associated with the appearance of spontaneous EMG activity. With stretch forces < 40 gm (0.40 N), there is evidence of increased excitability since the corrected velocities increase above baseline values. In addition, there is a remarkable increase in the peak to peak amplitude of the CMAP after recovery from stretch < 40 gm, often to 20% above baseline.

Multiple means of predicting when a change in the CMAP suggests a significant stretch are discussed and it is clear that a multifactorial approach using both velocity and amplitude parameters is important. In the case of pure compression, it is only the amplitude of the CMAP that is critical in predicting which changes in the CMAP are associated with significant compression.

Inhibition of chloride outward transport by gadolinium in cultured rat spinal cord neurons

Gadolinium is a rare-earth lanthanide metal ion and is used as organic gadolinium complexes in magnetic resonance imaging (MRI). Although gadolinium-based MRI agents are thought to be safe in clinical use, the in vivo release of the toxic free inorganic gadolinium (Gd3+) has been reported in some patients with kidney disease. In central nervous system neurons, the inhibitory action of GABA is a consequence of relatively hyperpolarized Cl- equilibrium potential (ECl), which results from the activity of K+-Cl- co-transporter (KCC). The lanthanide ions are reported to affect GABAA receptors. However, little is known about the effect of Gd3+ on GABAA receptor function with intact intracellular Cl- concentration. In the present study, we investigated the effect of Gd3+ on GABAA receptor-mediated currents using gramicidin perforated patch recording method in cultured rat spinal cord neurons. The application of muscimol, a GABAA receptor agonist, caused outward current at a holding potential of -50 mV. Gd3+ inhibited the muscimol-induced outward current in a concentration-dependent and reversible manner. Gd3+ inhibited the maximum muscimol response but had no effect on the half-maximum concentration. The Gd3+ inhibition was accompanied by a depolarizing shift of the reversal potential. The Gd3+ action was blocked by furosemide, a blocker of both KCC and Na+-K+-Cl- co-transporter (NKCC), but not bumetanide, a specific blocker of NKCC. Gd3+ failed to inhibit the muscimol-induced outward currents recorded by conventional whole-cell patch-clamp method which cannot retain intact intracellular Cl- concentration. These results suggest that Gd3+ inhibits a KCC function and gives rise to increase in intracellular Cl- concentration. The reduction of outward chloride transport could be related to the neurotoxic effects of Gd3+.

Biophysical mechanisms of transient optical stimulation of peripheral nerve

A new method for in vivo neural activation using low-intensity, pulsed infrared light exhibits advantages over standard electrical means by providing contact-free, spatially selective, artifact-free stimulation. Here we investigate the biophysical mechanism underlying this phenomenon by careful examination of possible photobiological effects after absorption-driven light-tissue interaction. The rat sciatic nerve preparation was stimulated in vivo with a Holmium:yttrium aluminum garnet laser (2.12 microm), free electron laser (2.1 microm), alexandrite laser (750 nm), and prototype solid-state laser nerve stimulator (1.87 microm). We systematically determined relative contributions from a list of plausible interaction types resulting in optical stimulation, including thermal, pressure, electric field, and photochemical effects. Collectively, the results support our hypothesis that direct neural activation with pulsed laser light is induced by a thermal transient. We then present data that characterize and quantify the spatial and temporal nature of this required temperature rise, including a measured surface temperature change required for stimulation of the peripheral nerve (6 degrees C-10 degrees C). This interaction is a photothermal effect from moderate, transient tissue heating, a temporally and spatially mediated temperature gradient at the axon level (3.8 degrees C-6.4 degrees C), resulting in direct or indirect activation of transmembrane ion channels causing action potential generation.

Magnetic micro- and nanoparticle mediated activation of mechanosensitive ion channels

Most cells are known to respond to mechanical cues, which initiate biochemical signalling pathways and play a role in cell membrane electrodynamics. These cues can be transduced either via direct activation of mechanosensitive (MS) ion channels or through deformation of the cell membrane and cytoskeleton. Investigation of the function and role of these ion channels is a fertile area of research and studies aimed at characterizing and understanding the mechanoactive regions of these channels and how they interact with the cytoskeleton are fundamental to discovering the specific role that mechanical cues play in cells. In this review, we will focus on novel techniques, which use magnetic micro- and nanoparticles coupled to external applied magnetic fields for activating and investigating MS ion channels and cytoskeletal mechanics.

Effects of gadolinium chloride on slowly adapting and rapidly adapting receptors of the rabbit lung

Effects of gadolinium chloride, an inhibitor of stretch-activated channels, on the responses of slowly adapting receptors (SARs) and rapidly adapting receptors (RARs) to hyperinflation were investigated. The increase in activity of RARs resulting from sustained elevations of left atrial pressure (LAP) was also assessed with gadolinium chloride application. Action potentials (AP) of SARs and RARs during hyperinflation were recorded from the vagus nerve of anesthetized New Zealand White rabbits before and after application of gadolinium chloride (20mM) directly on the receptor area of the nerve endings. There was a significant reduction of activity in SARs (n = 9) and RARs (n = 7) after application of gadolinium chloride. Activity of RARs (n = 6) increased when the LAP was elevated by 5 and 10 mmHg. This effect was abolished after gadolinium chloride was applied to receptor endings and the activity was restored when gadolinium chloride was removed. This suggests that stretch-activated channels play a role in SARs and RARs activity.

A mechanogated nonselective cation channel in proximal tubule that is ATP sensitive

Ion channels that are gated in response to membrane deformation or "stretch" are empirically designated stretch-activated channels. Here we describe a stretch-activated nonselective cation channel in the basolateral membrane (BLM) of the proximal tubule (PT) that is nucleotide sensitive. Single channels were studied in cell-intact and cell-free patches from the BLM of PT cells that maintain their epithelial polarity. The limiting inward Cs+ conductance is ~28 pS, and channel activity persists after excision into a Ca2+- and ATP-free bath. The stretch-dose response is sigmoidal, with half-maximal activation of about -19 mmHg at -40 mV, and the channel is activated by depolarization. The inward conductance sequence is: NH ~ Cs+ ~ Rb+ > K+ ~ Na+ ~ Li+ > Ca2+ ~ Ba2+ > N-methyl-D-glucamine ~ tetraethylammonium. The venom of the common Chilean tarantula, Grammostola spatulata, completely blocks channel activity in cell-attached patches. Hypotonic swelling reversibly activates the channel. Intracellular ATP concentration ([ATP]i) reversibly blocks the channel (inhibitory constant approximately 0.48 mM), suggesting that channel function is coupled to the metabolic state of the cell. We conclude that this channel may function as a Ca2+ entry pathway and/or be involved in regulation of cell volume. We speculate this channel may be important when [ATP]i is depleted, as occurs during periods of increased transepithelial transport or with ischemic injury.

Mechanosensitive whole-cell currents in cultured rat somatosensory neurons

Mechanosensitive currents in cultured rat dorsal root ganglion (DRG) neurons were analyzed by whole-cell patch clamp experiments. A positive pressure applied through a patch electrode induced an inward current in neurons 20 microm or larger in diameter. Ca(2+)- and Na(+)-depletion experiments revealed two kinds of channels involved in the mechanotransduction. One type of cells (type A) displayed blockade of the inward current in the absence of external Ca(2+); namely, the positive pressure of 10 cmH(2)O induced an inward current of 0.45+/-0.14 nA (mean+/-S.D., n=6) in the normal medium, and 0.08+/-0.07 nA (n=6) in the Ca(2+)-free solution. The current was influenced only a little by the depletion of external Na(+) in type A; the positive pressure induced an inward current of 0.48+/-0.04 nA (n=6) in the normal medium, and 0.39+/-0.06 nA (n=6) in the Na(+)-free solution. In the other type of cells (type B), the current persisted in the absence of Ca(2+); 0.52+/-0.09 nA (n=6) in the normal medium, and 0.35+/-0.09 nA (n=6) in the Ca(2+)-free solution. This type displayed a significant decrease in the inward current in the absence of Na(+); 0.42+/-0.09 nA (n=6) in the normal medium, and 0.23+/-0.08 nA (n=6) in the Na(+)-free solution. We concluded that there are two types of mechanosensitive channels in cultured DRG neurons, a Ca(2+)-selective channel and a non-selective cation channel.

Involvement of stretch-sensitive calcium flux in mechanical transduction in visceral afferents

The spinal and vagal visceral innervation to the gastrointestinal tract contains mechanosensitive afferents that are activated by contraction, distension of smooth muscle or movement in the receptive field. The mechanism by which free nerve endings detect changes in smooth muscle tension is not clear. The present study investigated the effects of mechanical stimulation on dorsal root ganglion neurons in vitro. Neurons were cultured using standard techniques and used in experiments after 24-72 h. Intracellular calcium [Ca2+]i was visualized using a video microscopic technique (Attoflour) in Fura-2 loaded neurons. DRG neurons innervating the stomach or colon were identified by the presence of a retrograde tracer, dextran-conjugated Texas Red, injected into the visceral wall 14-28 days previously. Increases in [Ca2+]i were measured in response to transient (0.5 s) mechanical stimulation of the cell soma using a flame polished probe. Approximately 25% of the whole population of DRG neurons (n = 199) were mechanosensitive, showing a transient rise in [Ca2+]i. In labeled afferents (n = 12), approximately 40% of neurons were mechanosensitive. The increase in [Ca2+]i in response to mechanical stimulation was reduced (100 microM) or abolished (250 microM) by superfusion with gadolinium or by removal of extracellular calcium. Cell somata of visceral spinal afferents show a stretch-sensitive calcium flux that may be involved in sensory transduction of mechanical stimuli that lead to autonomic and sensory reflexes.

Using gadolinium to identify stretch-activated channels: technical considerations

Gadolinium (Gd3+) blocks cation-selective stretch-activated ion channels (SACs) and thereby inhibits a variety of physiological and pathophysiological processes. Gd3+ sensitivity has become a simple and widely used method for detecting the involvement of SACs, and, conversely, Gd3+ insensitivity has been used to infer that processes are not dependent on SACs. The limitations of this approach are not adequately appreciated, however. Avid binding of Gd3+ to anions commonly present in physiological salt solutions and culture media, including phosphate- and bicarbonate-buffered solutions and EGTA in intracellular solutions, often is not taken into account. Failure to detect an effect of Gd3+ in such solutions may reflect the vanishingly low concentrations of free Gd3+ rather than the lack of a role for SACs. Moreover, certain SACs are insensitive to Gd3+, and Gd3+ also blocks other ion channels. Gd3+ remains a useful tool for studying SACs, but appropriate care must be taken in experimental design and interpretation to avoid both false negative and false positive conclusions.

Gadolinium blocks mechano-electric transducer current in chick cochlear hair cells

We investigated the effects of gadolinium ion (Gd3+) on the mechano-electrical transduction (MET) current using a whole-cell patch electrode voltage clamp technique in dissociated cochlear hair cells of chicks. Gd3+ blocked the MET channel in a concentration- and voltage-dependent manner. At -50 mV, Gd3+ blocked the MET channel, with a Hill coefficient of 1.14 and a dissociation constant (KD) of 1.01 x 10(-5) M. Adaptation of the MET current disappeared after the introduction of Gd3+, a change that may be due to a decrease in inward going MET currents, specifically the Ca2+ component.

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.

Osmo- and mechanosensitivity of the transient outward K+ current in a mammalian neuronal cell line

1. The transient outward current in NG108-15 cells was investigated with the whole-cell patch-clamp technique. The current was inhibited by external 4-aminopyridine or tetra-ethylammonium. The reversal potential shifted rightward with increased external K+ concentrations. 2. Current inactivation was markedly accelerated in hyperosmotic media (+30 mosmol l-1) and after nearby ejection of isosmotic solution with maximal acceleration occurring after 15-20 s and full recovery within 2-4 min, thus demonstrating an osmo- and mechanosensitivity of this current. Voltage-dependent Na+ and Ca2+ currents were unaffected. 3. Hyperosmotic solution shifted the voltage dependence of inactivation leftward. Inactivation was sensitive to reducing and oxidizing intracellular conditions. Reduction blocked the acceleration of current inactivation induced by hyperosmotic media, while oxidation did not hamper the response. 4. Action potentials had a decreased amplitude and a slower repolarization after hyperosmotic ejections. 5. It is concluded that the transient K+ current is osmo- and mechanosensitive, thus providing a mechanism for extracellular osmolarity to modulate neuronal excitability. The response appeared to be mediated through a changed sensitivity of the inactivating principle to the membrane electric field and was dependent on the redox state of the cell.

Mechanical sensitivity of regenerating myelinated skin and muscle afferents in the cat

These experiments describe the responses of myelinated skin and muscle afferent nerve fibres at a neuroma to stretch, local pressure and vibration in the anaesthetised cat. The sural nerve and the nerve supplying the medial gastrocnemius were studied. Neuroma formation was encouraged by placing the cut end of the nerve in a cuff made of synthetic material (Gore-tex). By 6 days after nerve section, the two nerves contained mechanically sensitive afferents. No motor fibres appeared to be mechanically sensitive. Mechanically sensitive sural afferents responded to ramp stretch of the nerve, applied at the cuff, with a single impulse or brief burst of impulses. The majority of gastrocnemius afferents responded to stretch with slowly adapting trains of impulses. Many muscle group II afferents exhibited a steady resting discharge, while group I afferents had an intermittent or bursting resting discharge or were silent. Those group I axons which showed resting activity had a low stretch threshold and were probably Ia fibres. Many of the silent units were also stretch sensitive. It is proposed that the spontaneously active units and silent units with low stretch thresholds were Ia fibres, while silent units with high stretch thresholds were Ib fibres. Both sural and gastrocnemius afferents responded to locally applied vibration. The mean peak response frequency for sural units was 170 Hz (+/- 70 Hz SD). For gastrocnemius units it was 325 Hz (+/- 86 Hz SD). Group I muscle afferents responded to higher frequencies of vibration than group II afferents. In four experiments the nerve was treated at a site a few millimetres proximal to the point of section with the axonal transport blocker colchicine. Twenty-five millimolar colchicine blocked impulse conduction at its point of application. Nevertheless, mechanically sensitive areas developed in the nerve just proximal to the treated region. Ten millimolar colchicine did not block impulse conduction, but led to dispersion of mechanosensitive areas to more proximal regions of the mechanosensitive areas to more proximal regions of the nerve. This result suggests that the disruption of orthograde axonal transport by colchicine leads to development of mechanically sensitive areas in axons further back from their cut ends. Local application of the drugs succinyl choline, tetra-ethyl ammonium and gadolinium had no effect on levels of resting activity or on mechanical sensitivity of afferents in the cuff. The potassium channel blocker 4-aminopyridine, on the other hand, produced an increase in the levels of resting activity and in the stretch responses of afferents.(ABSTRACT TRUNCATED AT 400 WORDS)