Block of receptor response in the stretch receptor neuron of the crayfish by gadolinium

Antidromic potential spread modulates the receptor responses in the stretch receptor neurons of the crayfish

The effects of antidromic potential spread were investigated in the stretch receptor neurons of the crayfish. Current and potential responses to conductance changes were recorded in the dynamic clamp condition and compared to those obtained by using some conventional clamp methods and a compartmental neuron model. An analogue circuit was used for dynamic calculation of the injected receptor current as a function of the membrane potential and the given conductance change. Alternatively, receptor current responses to a mechanical stimulus were recorded and compared when the cell was voltage clamped to a previously recorded impulse wave form and the resting potential, respectively. Under dynamic clamp, the receptor current had an oscillating waveform which contrasts with the conventional recordings. Frequency, amplitude and sign of the oscillations were dependent on the applied conductance level, reversal potential and electrotonic attenuation. Mean current amplitude and frequency of the evoked impulse responses were smaller under dynamic clamp, especially for large conductance increases. However, firing frequency was larger if plotted against the mean current response. Recorded responses were similar to those calculated in the model. It was not possible to evoke any adaptation in the slowly adapting neuron by using the dynamic clamp. Evoked potential change served as a self limiting response, preventing the depolarization block. However, impulse duration was significantly shorter in the rapidly adapting neuron when the dynamic clamp was used. It was concluded that, in the stretch receptor neurons during a conductance increase, antidromic potential spread modulates the receptor responses and contributes to adaptation.

Mechanotransduction and the crayfish stretch receptor

Mechanotransduction or mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates. Mechanosensitivity covers a wide spectrum of functions from osmosensing, cell attachment, classical sensory mechanisms like tactile senses in the skin, detection of sound in hair cells of the hearing apparatus, proprioceptive functions like recording of muscle length and tension in the muscle spindle and tendon organ, respectively, and pressure detection in the circulation etc. Since most development regarding the molecular aspects of the mechanosensitive channel has been made in nonsensory systems it is important to focus on mechanosensitivity of sensory organs where the functional importance is undisputed. The stretch receptor organ of the crustaceans is a suitable preparation for such studies. The receptor organ is experimentally accessible to mechanical manipulation and electrophysiological recordings from the sensory neuron using intracellular microelectrode or patch clamp techniques. It is also relatively easy to inject substances into the neuron, which also makes the neuron accessible to measurements with fluorescent techniques. The aim of the present paper is to give an up to date summary of observations made on the transducer properties of the crayfish stretch receptor (Astacus astacus and Pacifastacus leniusculus) including some recent unpublished findings. Finally some aspects on future line of research will be presented.

Ion channels for mechanotransduction in the crayfish stretch receptor

Mechanosensitivity is found in almost every cell in all organisms from bacteria to vertebrates and covers a wide spectrum of function from osmosensing to mechanical sensing in the specialized receptors, such as the hair cells of the cochlea. The molecular substrate for such mechanosensitivity is thought to be mechanosensitive ion channels (MSCs). Because most development regarding the molecular aspects of the MSC has been made in nonsensory or sensory systems, which have not been accessible to recordings from ion channels, it is important to focus on the mechanosensitivity of sensory organs where their functional importance is undisputed. The stretch receptor organ (SRO) of the crustaceans is a suitable preparation for such studies. Each organ contains two receptors: one slowly and one rapidly adapting receptor neurons. The primary mechanosensitivity is generated by two types of MSC of hitherto unknown molecular type located in the neuronal dendrites, which are inserted into a receptor muscle fiber. In addition to the MSCs, the neurons contain voltage-gated Na(+) channels, which seem to be differently located in the slowly and rapidly adapting neurons. At least three types of voltage-gated K(+) channels are present in the sensory neurons, the location of which is not known. The spatial distribution of ion channels and the kinetics of the channels, together with the viscoelastic properties of the receptor muscles, determine the overall transducer properties and impulse firing of the two receptor neurons, including their typical adaptive characteristics.

Inhibition of mechanotransducer currents in crayfish sensory neuron by CGS 9343B, a calmodulin antagonist

The effects of CGS 9343B (zaldaride maleate), a calmodulin antagonist, on mechanosensitive channels were examined in crayfish slowly adapting sensory neurons using the two-electrode voltage clamp technique. In addition to its inhibition of voltage-gated Na(+) and K(+) currents, CGS 9343B (<30 microM) blocked reversibly the receptor current in a dose-dependent and voltage-dependent manner with a dissociation constant (K(d)) of 26.8 microM. The time course of the block was 265 s. Within the extension range of 3-30%, the reduction in receptor current was stimulus-independent and the gating mechanisms were not affected. Extracellular Ca(2+) was not necessary for its blocking effects. No changes in passive muscle tension were observed in the presence of 20 microM CGS 9343B. These results suggest that CGS 9343B, as a calmodulin antagonist, can also block mechanosensitive channels, possibly by being incorporated into the lipid membrane and/or interacting with the channel protein.

Gadolinium chloride inhibition of pulmonary nitric oxide production and effects on pulmonary circulation in the rabbit

Nitric oxide is an important regulator of pulmonary vascular resistance. Pulmonary nitric oxide formation is detectable in exhaled air and the synthesis is partly stretch-dependent. Gadolinium chloride (GdCl3) reduces pulmonary nitric oxide formation, possibly by interference with stretch-activated cellular calcium influx, but the effect on pulmonary circulation is not known. We therefore measured exhaled nitric oxide and pulmonary vascular resistance in anaesthetised rabbits, and compared the effects of GdCl3 with those of an nitric oxide-synthase inhibitor (L-N omega-nitro-arginine methyl ester, L-NAME). Both GdCl3 and L-NAME reduced nitric oxide in exhaled air and increased pulmonary vascular resistance. However, the increase in pulmonary vascular resistance was more pronounced with GdCl3 than with L-NAME. A 50% reduction of exhaled nitric oxide caused by either GdCl3 or L-NAME was accompanied by a 90% or 17% increase in pulmonary vascular resistance respectively. Inhaled nitric oxide (40 ppm) reduced pulmonary vascular resistance after L-NAME, but not after GdCl3 infusion. Infusion of glyceryltrinitrate reduced pulmonary vascular resistance after GdCl3 infusion. GdCl3 caused hypoxaemia, probably due to vasoconstriction since lung weight was unaltered. Thus GdCl3 can induce a marked increase in pulmonary vascular resistance, which partly may be caused by inhibition of pulmonary nitric oxide formation. Intact stretch-activated calcium channels may be important for maintenance of normal pulmonary vascular function.

Effects of hyposmolar solutions on membrane currents of hippocampal interneurons and mossy cells in vitro

Whole cell voltage-clamp recordings in rat hippocampal slices were used to investigate the effect of changes in extracellular osmolarity on voltage-activated potassium currents. Currents were evoked from oriens/alveus (O/A) interneurons, hilar interneurons, and mossy cells. Hyposmolar external solutions produced a significant potentiation of K+ current recorded from O/A and hilar interneurons, but not from mossy cells. Hyposmolar solutions also dramatically potentiated the spontaneous excitatory postsynaptic currents recorded from mossy cells. These results suggest that hippocampal excitability can be modulated by the complex actions exerted by changes in extracellular osmolarity.

Stretch-induced stimulation of lower airway nitric oxide formation in the guinea-pig: inhibition by gadolinium chloride

The effect of stretch on lower airway nitric oxide formation was studied in normoxic tracheostomized anaesthetized guinea-pigs. Increase of level of positive end-expiratory pressure caused increased lower airway nitric oxide formation, as measured by its presence in exhaled tracheal air. The L-type calcium channel blocker, verapamil, did not decrease lower airway nitric oxide formation. Neither the local anaesthetic xylocaine nor the ganglion blocker trimetaphan affected exhaled nitric oxide, excluding local and centrally-mediated neuronal reflexes. Intravenous administration of gadolinium chloride (GdCl3, 50 mg/kg) induced a rapid and pronounced decrease (75%) in the basal level of exhaled nitric oxide. GdCl3 completely abolished lower airway nitric oxide formation induced by ventilation with positive end-expiratory pressure (7 cm H2O). GdCl3 induced hypoxaemia, but there was no indication for the development of lung oedema. The results indicate that positive end-expiratory pressure stimulates lower airway nitric oxide formation in the guinea-pig. GdCl3 inhibits lower airway nitric oxide formation in the guinea-pig in vivo, perhaps by interference with stretch-induced cellular calcium-influx.

Non-voltage-gated Ca2+ influx through mechanosensitive ion channels in aortic baroreceptor neurons

The mechanisms underlying mechanotransduction in baroreceptor neurons (BRNs) are undefined. In this study, we specifically identified aortic baroreceptor neurons in primary neuronal cell cultures from nodose ganglia of rats. Aortic baroreceptor neurons were identified by labeling their soma with the fluorescent dye 1,1'-dioleyl-3,3,3',3'-tetramethylin-docarbocyanine (DiI) applied to the aortic arch. Using Ca2+ imaging with fura 2, we examined these BRNs for evidence of Ca2+ influx and determined its mechanosensitivity and voltage dependence. Mechanical stimuli were produced by ejecting buffer from a micropipette onto the cell surface with a pneumatic picopump, producing a shift in the center of mass of the cell that was related to intensity of stimulation. Ninety-three percent of DiI-labeled neurons responded to mechanical stimulation with an increase in [Ca2+]i. The magnitude of the increases in [Ca2+]i was directly related to the intensity of the stimulus and required the presence of external Ca2+. The trivalent cations Gd3+ or La3+ in equimolar concentrations (20 mumol/L) eliminated the K(+)-induced rises in [Ca2+]i, demonstrating that both trivalent cations are equally effective at blocking voltage-gated Ca2+ channels in these baroreceptor neurons. In contrast, the mechanically induced increases in [Ca2+]i were blocked by Gd3+ (20 mumol/L) only and not by La3+ (20 mumol/L). Stretch-activated channels (SACs) have been shown in other preparations to be blocked by Gd3+ specifically. Our data demonstrate that (1) BRNs, specifically identified as projecting to the aortic arch, have ion channels that are sensitive to mechanical stimuli; (2) mechanically induced Ca2+ influx in these cells is mediated by a Gd(3+)-sensitive ion channel and not by voltage-gated Ca2+ channels; (3) the magnitude of the Ca2+ influx is dependent on the intensity of the stimulus and the degree and duration of deformation; and (4) repeated stimuli of the same intensity result in comparable increases in [Ca2+]i. We conclude that mechanical stimulation increases Ca2+ influx into aortic BRNs independent of voltage-gated Ca2+ channels. The results suggest that Gd(3+)-sensitive SACs are the mechanoelectrical transducers in baroreceptors.

Mechanical stimulation of neurites generates an inward current in putative aortic baroreceptor neurons in vitro

We investigated the responses of putative aortic baroreceptor neurons to mechanical stimulation of their processes. Putative aortic baroreceptor neurons were identified by applying the carbocyanine dye DiI to the adventitia of the aortic arch of anesthetized rats. After at least 1 week, the nodose ganglia were removed and the neurons were cultured. Within 2-3 days, neurite outgrowth was evident on many neurons. The soma was voltage-clamped using whole cell patch clamp techniques while the neurites were deformed with pneumatic ejection of bath solution at 5-15 psi using a glass pipette (7-15 microm) positioned at least 50 microm from the neurite. Mechanical stimulation induced an inward current in 15 out of 17 putative aortic baroreceptor neurons. The magnitude of the current was related to the intensity of stimulation. The current was blocked by 20 microM gadolinium (n = 11), a reported blocker of mechanically sensitive ion channels, or by incubating the cells overnight in 10 microM phalloidin, which binds to actin filaments (n = 5). We conclude that mechanical deformation of neurites of putative baroreceptor neurons activates a mechanosensitive inward current in the soma and that the cytoskeletal actin filaments are involved in the generation of this current.

Integrins and modulation of transmitter release from motor nerve terminals by stretch

The stretch of a frog muscle within the physiological range can more than double the spontaneous and evoked release of neurotransmitter from its motor nerve terminals. Here, stretch enhancement of release was suppressed by peptides containing the sequence arginine-glycine-aspartic acid (RGD), which blocks integrin binding. Integrin antibodies also inhibited the enhancement obtained by stretching. Stretch enhancement depended on intraterminal calcium derived both from external calcium and from internal stores. Muscle stretch thus might enhance the release of neurotransmitters either by elevating internal calcium concentrations or by increasing the sensitivity of transmitter release to calcium in the nerve terminal.

Mechanical stimulation increases intracellular calcium concentration in nodose sensory neurons

The cellular mechanisms involved in activation of mechanosensitive visceral sensory nerves are poorly understood. The major goal of this study was to determine the effect of mechanical stimulation on intracellular calcium concentration ([Ca2+]i) using nodose sensory neurons grown in culture. Primary cultures of nodose sensory neurons were prepared by enzymatic dispersion from nodose ganglia of 4-8 week old Sprague-Dawley rats. Whole cell [Ca2+]i was measured by a microscopic digital image analysis system in fura-2 loaded single neurons. Brief mechanical stimulation of individual nodose sensory neurons was achieved by deformation of the cell surface with a glass micropipette. In 31 of 50 neurons (62%), mechanical stimulation increased [Ca2+]i from 125 +/- 8 to 763 +/- 89 nM measured approximately 10 s after stimulation. [Ca2+]i then declined gradually, returning to near basal levels over a period of minutes. [Ca2+]i failed to increase after mechanical stimulation in the remaining 19 neurons. The mechanically-induced rise in [Ca2+]i was essentially abolished after the neurons were incubated for 5-10 min in zero Ca2+ buffer (n = 7) or after addition of gadolinium (10 microM), a blocker of stretch-activated ion channels (n = 5). The effect of gadolinium was reversed after removal of gadolinium. The results indicate that: (1) mechanical stretch increases [Ca2+]i in a subpopulation of nodose sensory neurons in culture, and (2) the stretch-induced increase in [Ca2+]i is dependent on influx of Ca2+ from extracellular fluid and is reversibly blocked by gadolinium. The findings suggest that opening of stretch-activated ion channels in response to mechanical deformation leads to an increase in Ca2+ concentration in visceral sensory neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Smooth muscle stretch-activated phospholipase C activity

Rabbit aortic muscles were stretched from a holding length of 0.6 maximum length (Lmax) to lengths as great as 1.0 Lmax and the new length maintained. When muscles were stretched to 1.0 Lmax, inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] and inositol 1,4-bisphosphate [Ins(1,4)P2] contents were increased at 375 ms (uncorrected for freezing time) poststretch to 209 +/- 27 and 139.8 +/- 12% (SE), respectively, of control values. Increases in Ins(1,4,5)P3 and Ins(1,4)P2 contents reached an apparent maximum at approximately 500 ms, i.e., to 243.7 +/- 15.8 and 180.9 +/- 16.2% of control, and were decreased to near control levels at 1,700 ms poststretch. The stretch threshold for phospholipase C (PLC) activation was 0.85 Lmax. The latency to onset of PLC activation, correcting for the time for freezing, was 275 to 375 ms. Maximal PLC activity was 91 pmol.s-1.100 nmol total lipid P(i)-1, which corresponded to 10% of total phosphatidylinositol bisphosphate being hydrolyzed per second. The mechanism of stretch-activated PLC activity involved influx of Ca2+ via gadolinium-sensitive ion channels, but not via nifedipine-sensitive ion channels.

Mechanisms of baroreceptor activation

The determinants of the nerve activity generated at the baroreceptor endings have been examined. 1) In the isolated carotid sinus, the placement of activated bovine aortic endothelial cells decreased baroreceptor activity (BRA) in a reversible manner. Both endothelin and nitric oxide (NO) suppress BRA, whereas prostacyclin (PGI2) increases activity. 2) The BRA in single units declines and often ceases during non-pulsatile increases in carotid sinus pressure sustained over several minutes. This "adaptation" is attenuated by the transient potassium channel (IA) blocker 4-aminopyridine (4-AP) and not by inhibition of the Na+/K+ pump. 3) In preliminary studies, mechano-electrical transduction was examined in isolated and cultured nodose ganglion neurons. Opening of stretch-activated (SA) channels by suction on the cell-attached patch was seen infrequently; however, probing the neurons consistently increased their intracellular calcium [Ca++]i measured with fura-2. This increase in [Ca++]i is blocked by gadolinium (Gd3+), a trivalent lanthanide reported to block SA channels. Gd3+ also blocks the BRA in the carotid sinus. We conclude that paracrine factors significantly modulate BR sensitivity, that selective ionic mechanisms (the 4-AP sensitive K+ channels) determine the degree of "adaptation" of BR to elevated pressure, and that SA channels sensitive to Gd3+ may be the mechano-electrical transducers in BR neurons.

Effects of gadolinium on ion channels in the myelinated axon of Xenopus laevis: four sites of action

The action of gadolinium (Gd3+) on ion currents in myelinated axons of Xenopus laevis was investigated with the voltage clamp technique. The analysis revealed the following effects. (i) The potential-dependent parameters of both Na and K channels were shifted. The shift was equally large for activation, inactivation, and activation time constant curves (+9 mV for 100 microM Gd3+). The effects could be explained by screening of fixed surface charges at a density of -1.2 e nm-2. (ii) The rate of gating for both Na and K channels was reduced more than predicted from the shift. This effect could be quantified as a scaling (by a factor 3 and 5 respectively at 100 microM Gd3+) of the activation time constant curves. (iii) An activation- and inactivation-independent block of both Na and K channels, obeying 1:1 stoichiometry with a Kd value of about 70 microM potential-independent block of leakage current, obeying 1:2 stoichiometry with a Kd value of 600 microM. (iv) The analysis suggests separate binding sites for the effects, comprising high affinity modulatory and blocking sites on the channel proteins and low affinity receptors on phospholipids, associated with the effect on the leakage current.