Average Membrane Potential Research Articles

Regulation of action-potential firing in spiny neurons of the rat neostriatum in vivo.

Both silent and spontaneously firing spiny projection neurons have been described in the neostriatum, but the reason for their differences in firing activity are unknown. We compared properties of spontaneously firing and silent spiny neurons in urethan-anesthetized rats. Neurons were identified as spiny projection neurons after labeling by intracellular injection of biocytin. The threshold for action-potential firing was measured under three different conditions: 1) electrical stimulation of the contralateral cerebral cortex, 2) brief directly applied current pulses, and 3) spontaneous action-potentials occurring during spontaneous episodes of depolarization ( state). The average membrane potential and the amplitude of noiselike fluctuations of membrane potential in the state were determined by fitting a Gaussian curve to the membrane-potential distribution. All neurons in the sample exhibited spontaneous membrane potential shifts between a hyperpolarized state and a depolarized state, but not all fired action potentials while in the state. The difference between the spontaneously firing and the silent spiny neurons was in the average membrane potential in the state, which was significantly more depolarized in the spontaneously firing than in the silent spiny neurons. There were no significant differences in the threshold, the amplitude of the noiselike fluctuations of membrane potential in the state, or in the proportion of time that the membrane potential was in the state. In both spontaneously firing and silent neurons, the threshold for action potentials evoked by current pulses was significantly higher than for those evoked by cortical stimulation. Application of more intense current pulses that reproduced the excitatory postsynaptic potential rate of rise produced firing at correspondingly lower thresholds. Because the membrane potential in the state is mainly determined by the balance between the synaptic drive and the outward potassium conductances activated in the subthreshold range of membrane potentials, either or both of these factors may determine whether firing occurs in response to spontaneous afferent activity.

Passive properties of swimmeret motor neurons.

Four different functional types of motor neurons innervate each swimmeret: return-stroke excitors (RSEs), power-stroke excitors (PSEs), return-stroke inhibitors (RSIs), and power-stroke inhibitors (PSIs). We studied the structures and passive electrical properties of these neurons, and tested the hypothesis that different types of motor neurons would have different passive properties that influenced generation of the swimmeret motor pattern. Cell bodies of neurons innervating one swimmeret were clustered in two anatomic groups in the same ganglion. The shapes of motor neurons in both groups were similar, despite the differences in locations of their cell bodies and in their functions. Diameters of their axons in the swimmeret nerve ranged from <2 to approximately 35 microm. Resting membrane potentials, input resistances, and membrane time constants were recorded with microelectrodes in the processes of swimmeret motor neurons in isolated abdominal nerve cord preparations. Membrane potentials had a median of -59 mV, with 25th and 75th percentiles of -66.0 and -53 mV. The median input resistance was 6.4 M omega, with 25th and 75th percentiles of 3.4 and 13.7 M omega. Membrane time constants had a median of 9.3 ms, with 25th and 75th percentiles of 5.7 and 15.0 ms. Excitatory and inhibitory motor neurons had similar passive properties. RSE motor neurons were typically more depolarized than the other types, but the passive properties of RSE, PSE, RSI, and PSI neurons were not significantly different. Membrane time constants measured from cell bodies were briefer than those measured from neuropil processes, but membrane potentials and input resistances were not significantly different. The relative sizes of different motor neurons were measured from the sizes of their impulses recorded extracellularly from the swimmeret nerve. Smaller motor neurons had lower membrane potentials and were more likely to be active in the motor pattern than were large motor neurons. Motor neurons of different sizes had similar input resistances and membrane time constants. Motor neurons that were either oscillating or oscillating and firing in phase with the swimmeret motor pattern had lower average membrane potentials and longer time constants than those that were not oscillating. When the state of the swimmeret system changed from quiescence to continuous production of the motor pattern, the resting potentials, input resistances, and membrane time constants of individual swimmeret motor neurons changed only slightly. On average, both input resistance and membrane time constant increased. These similarities are considered in light of the functional task each motor neuron performs, and a hypothesis is developed that links the brief time constants of these neurons and graded synaptic transmission by premotor interneurons to control of the swimmeret muscles and the performance of the swimmeret system.

Polarization anatomy of a kainic acid seizure.

Slow voltage-sensitive dyes work by accumulating in brain tissue and report the average membrane potential of neurons and glia. The voltage-sensitive dye diO-C2-5 was used to monitor the polarization state of 27 brain structures in the rat during a systemically induced, behaviorally mild, kainic acid seizure using a 20 s recording period. The effects of the anesthetic agent used in the experiment were minimized by delaying the dye injection and seizure mapping for one day. Eleven areas were depolarized during the seizure, but 16 other areas did not change their polarization state compared to controls. The effects of pentobarbital appear to have no measurable effect on seizure propagation once the animal has behaviorally recovered from the anesthesia. The technique allows for mapping areas of seizure involvement with a unique combination of spatial and temporal resolution.

Dissociation of postnatal hippocampal neurons for short term culture

A method is described to isolate hippocampal neurons from postnatal day 7 to postnatal day 21 rats for use in short term cultures using a combination of enzymatic and mechanical dissociation. A significant fraction of the cells (37.5±2.3%, n=10 cultures) were labelled with anti-GABA antibodies and the neurons survived a minimum of 8 days in culture. Patch-clamp recording in the current clamp mode revealed an average membrane potential and input resistance of −47.6±2.5 mV and 737±147 MΩ, respectively, after 24 h in culture. The cells exhibited normal excitability and fired multiple action potentials in response to a depolarizing pulse. The technique described here provides a good alternative to either the use of embryonic brain tissue for cultures used in electrophysiological studies which may not exhibit the mature phenotype or use of acutely isolated cells which may be unstable or have their channels and receptors modified by enzymatic treatment.

The relationship between light-evoked synaptic excitation and spiking behaviour of salamander retinal ganglion cells.

1. Light-evoked input-output characteristics of ganglion cells in dark-adapted tiger salamander retina were studied in the slice preparation using patch-clamp techniques. Excitatory postsynaptic currents (EPSCs), isolated by blocking inhibitory inputs and evoked by a range of light stimulus intensities, were recorded under whole-cell voltage clamp. Spike responses, evoked by the same light intensities, were recorded extracellularly from the same cells using the cell-attached patch-clamp technique. 2. When N-methyl-D-aspartate (NMDA) receptor-mediated input was blocked by the competitive NMDA antagonist DL-2-amino-5-phosphonoheptanoate (AP7), light-evoked EPSC amplitude and peak firing rate were reduced at all light intensities. In both cases, the data obtained in the presence of AP7 scaled linearly to control data, indicating that NMDA and non-NMDA receptors are activated in the same proportions across the entire 2 log unit stimulus response range of these ganglion cells. 3. The relationship between light-evoked spike frequency and light-evoked EPSC amplitude was linear. The slope of the light-evoked synaptic current-spike frequency relationship was close to the slope of the injected current-spike frequency relationship, indicating that synaptic current and injected current drive spiking in a similar manner. The linearity of the synaptic current-spike frequency relationship was not compromised when NMDA input was blocked by AP7. 4. Light-evoked voltage responses, recorded under whole-cell current clamp, revealed that the average membrane potential during a spike response was depolarized only slightly with increased firing rate. Once the membrane potential surpassed spike threshold, it was maintained by the voltage-gated, spike-generating conductances at a depolarized plateau upon which action potentials were fired. The potential of this plateau varied only slightly with spike frequency. We conclude that the voltage control exerted by the spike-generating currents in ganglion cells prevents a substantial response-dependent decrease in the electrical driving force of the excitatory currents, obviating the need for the voltage-independent synaptic efficacy provided by the combination of NMDA and non-NMDA inputs.

Properties of unitary IPSPs evoked by anatomically identified basket cells in the rat hippocampus.

Hippocampal pyramidal cells receive GABA-mediated synaptic input from several distinct interneurons. In order to define the effect of perisomatic synapses, intracellular recordings were made with biocytin-containing microelectrodes from synaptically connected inhibitory and pyramidal cell pairs in subfields CA1 and CA3 of the rat hippocampus. Subsequent physiological analysis were restricted to the category of cells, here referred to as basket cells (n = 14), which had an efferent synaptic target profile (n = 282 synaptic contacts) of predominantly somatic (48.2%) and proximal dendritic synapses (45.0%). Electron microscopic analysis revealed that in two instances identified postsynaptic pyramidal cells received a total of 10 and 12 labelled basket cell synapses respectively. At an average membrane potential of -57.8 +/- 4.6 mV, unitary inhibitory postsynaptic potentials (IPSPs; n = 24) had a mean amplitude of 450 +/- 238 microV, a 10-90% rise time of 4.6 +/- 3.2 ms and, measured at half-amplitude, a mean duration of 31.6 +/- 18.2 ms. In most instances (n = 19) the IPSP decay could be fitted with a single exponential with a mean time constant of 32.4 +/- 18.0 ms. Unitary basket cell-evoked IPSPs (n = 5) was extrapolated to be at -74.9 +/- 6.0 mV. Averages of unitary IPSPs had a mean calculated conductance of 0.95 +/- 0.29 nS, ranging from 0.52 to 1.16 nS. Unitary basket cell IPSPs (n = 3) increased in amplitude by 26.6 +/- 19.9% following bath application of the GABAB receptor antagonist CGP 55845A [correction of CGP 35845A] (1-4 microM), whereas subsequent addition of the GABAA receptor antagonist bicuculline (10-13 microM) reduced the IPSP amplitude to 13.5 +/- 3.1% of the control response. Rapid presynaptic trains of basket cell action potentials resulted in the summation of up to four postsynaptic responses (n = 5). However, any increase in the rate of tonic firing (2- to 10-fold) led to a > 50% reduction of the postsynaptic response amplitude. At depolarized membrane potentials, averaged IPSPs could be followed by a distinct depolarizing overshoot or postinhibitory facilitation (n = 4). At firing threshold, pyramidal cells fired postinhibitory rebound-like action potentials, the latter in close temporal overlap with the depolarizing overshoot. In conclusion, hippocampal basket cells have been identified as one source of fast, GABAA receptor-evoked perisomatic inhibition. Unitary events are mediated by multiple synaptic release sites, thus providing an effective mechanism to avoid total transmission failures.

A spectral analysis of the integration of artificial synaptic potentials in mammalian central neurons

In order to stimulate the interaction between synaptic input and intrinsic membrane properties in mammalian central neurons a well-defined current was injected into the neurons through a recording electrode. The stimulus was white noise bandpass filtered at 0.5 and 75 Hz and the power spectra of the responses were calculated. Recordings were obtained from neurons of the neocortex, the hippocampus, the thalamus and the cerebellar cortex. The neurons were either located in newly cut slices from adult guinea pig brains or in 3–10-weeks-old slice cultures from brains of newborn rats. In hippocampal and cortical cells the passive membrane properties dominated the shape of the power spectra. In general, when the average membrane potential was made more positive the power of the response increased. When neurons had active subthreshold responses like delayed rectification, sag-and-hump responses or delayed depolarization there was a depression of the response power at frequencies below 10–20 Hz. The depression was voltage dependent in the same way as the current that produced the active subthreshold response. In thalamic cells with a low-threshold Ca 2+ spike (lts) the power of the responses grew in the 3–20-Hz range with hyperpolarization. The spectra of the responses of thalamic neurons had multiple peaks indicating multiple frequencies of resonance. Purkinje cells of the cerebellar cortex have prominent plateau potentials. When these cells were stimulated with the white noise at levels where the plateau potentials could be activated the spectra were dominated by a large peak at the lowest frequencies, i.e, below 5 Hz. Few cells in our data base generated spontaneous membrane potential oscillations. When the current stimulus was injected into such neurons the intrinsic rhythm was unaffected by the input and the power spectrum showed a marked peak at the frequency of the intrinsic oscillations. We conclude that bandpass filtered white noise as simulation of synaptic input is valuable for quantification of how passive and active membrane properties affect synaptic intergration. The technique can also provide information on the role of transmitters and modulators in the CNS.

Effects of extravascular acidification and extravascular alkalinization on constriction and depolarization in rat cerebral arterioles in vitro

The relationship between cell membrane potential, vessel diameter, and pH in small cerebral arterioles is not completely understood. This study involved direct, simultaneous measurement of cell membrane potential and vessel diameter at various extracellular pH levels. Arterioles ranging from 44 to 91 microns in diameter were isolated, transferred to a temperature-controlled microscope chamber, which was used as an organ bath, and observed through an inverted videomicroscope. Two vessel cannulation procedures were used: a single-sided cannulation with the other side occluded, and a double-sided and perfused cannulation. After cannulation, the vessels were pressurized to 60 mm Hg intraluminally and the bath temperature was raised to 37 degrees C. Cell membrane potentials of vessel wall cells were obtained after the bath temperature reached 37 degrees C with the vessels partly constricted and again after spontaneous tone (constriction) of the healthy vessels had developed. The effect of extraluminal pH on cell membrane potentials was studied by changing the bath pH from 7.3 to either 7.65 or 6.8 in the single-sided cannulation. The average cell membrane potential for vessels at 37 degrees C, with 60 mm Hg of intraluminal pressure and pH 7.3, was -37.5 mV. The cell membrane potential depolarized to -30.9 mV at pH 7.65 and hyperpolarized to -58.4 mV at pH 6.8, with a slope of 25.8 mV/pH unit. The effect of depolarizing extracellular potassium ions on the cell membrane potential was examined by perfusing two vessels with modified Ringer's solution containing 70 mM KCl. This perfusion method decreased the vessel diameter by 48% and depolarized the observed cell membrane potential from -41.9 to -19.8 mV, with a slope of -0.42 mV per percentage diameter change. These data provide the first measurements of membrane potentials of isolated penetrating arteriole wall cells in vitro. The results indicate that the cell membrane potential relates linearly to the vessel diameter. This new technique opens the possibility for studying vessel response to stimuli under controlled conditions and regulatory mechanisms such as the propagation of vasomotor responses.

Voltage-Activated Ionic Currents in Goldfish Pituitary Cells

The release of gonadotropin and growth hormone from goldfish pituitary cells has been shown to be dependent on the entry of extracellular Ca2+ through voltage-sensitive Ca2+ channels by pharmacological studies. As a first step to further investigate the involvement of voltage-dependent ion channels in the regulation of anterior pituitary hormone release in the goldfish, cell excitability and voltage-dependent ion currents were characterized using tight-seal whole-cell recordings in dispersed goldfish pituitary cells. Cultured goldfish pituitary cells had an average membrane potential of -36 ± 3 mV. When held at membrane potentials more negative than -60 mV, these cells were excitable, responding to depolarizing current pulses or anode break with the firing of single action potentials. Results from total current voltage-clamp recordings suggested that all goldfish pituitary cells possess voltage-dependent Na+ , Ca2+, and K+ currents. These currents were further characterized independently under isolated current recording conditions. The rapid, transient Na+ current activated at voltages more positive than -40 mV and was sensitive to tetrodotoxin. The steady state inactivation of this Na+ current was also voltage-dependent; at the measured resting potential, ⩾50% of the Na+ current was not available for activation. The voltage-dependence and activation kinetics of the tetraethylammonium-sensitive K+ current resembled those of the delayed rectifier K+ current. The K+ current activated slowly at potentials more positive than -40 mV, and showed little inactivation over the duration of a 1-sec depolarizing pulse. Steady-state inactivation characteristics indicated that ≤50% of the K+ current was inactivated at resting potentials. Experiments with 4-aminopyridine indicated the presence of an early transient K+ current that activated in a similar voltage range as the delayed rectifier current. Using barium as the charge carrier to measure Ca2+ currents, a high-voltage activated, long-lasting Ca2+ current was revealed. This "L-type" Ca2+ current activated at potentials more positive than -30 mV and was inhibited by verapamil and nifedipine. This study indicates that goldfish pituitary cells possess the electrophysiological properties required for the participation of voltage-sensitive ion channels in the regulation of hormone release.

Central respiratory modulation of facial motoneurons in rats

Facial motoneurons (FMN) were recorded intracellularly in Sprague-Dawley rats anesthetized with halothane. The animals were vagotomized, paralyzed, and artificially ventilated. The average membrane potential of the cells was 62.6 ± 1.9 mV and their input impedance ranged from 5 to 30 Mω (9.8 ± 1.1 M ω, n = 38). The membrane potential of most FMNs varied throughout the central respiratory cycle and four distinct patterns were detected. Type I (post-inspiratory) cells ( 21 44 ) showed a two-phase C1 −-mediated hyperpolarization during the respiratory cycle, one during central inspiration and the second during late expiration. Type II cells (early inspiratory, n = 10) showed early inspiratory depolarization. Type III ( n = 2, stage-2 expiratory) cells displayed late expiratory depolarization and one cell (type IV or throughout inspiratory) exhibited expiratory C1 −-mediated hyperpolarization. The remaining 10 cells showed no detectable respiratory modulation. The results reflect the heterogeneity of the central respiratory modulation of FMNs and suggest that these cells receive both excitatory and inhibitory inputs from elements of the central respiratory pattern generating network.

Effects of chronic nicotine treatment on the accumulation of [3H]tetraphenylphosphonium by cerebral cortical synaptosomes.

Chronic exposure of rats to nicotine increases the number of [3H]nicotine binding sites in the brain; however, it is not clear whether nicotinic cholinergic receptor function is altered as well. In this study, we have used [3H]tetraphenylphosphonium as a probe of synaptosomal membrane potential to investigate whether exposure to nicotine in vivo alters the ability of cerebral cortical synaptosomes to maintain a potential difference and to depolarize in response to in vitro nicotine. Treatment of rats for 14 days with 0.475 mg of nicotine base/day via subcutaneously implanted minipumps resulted in a decrease in the synaptosomal accumulation of [3H]tetraphenylphosphonium in physiological buffer, corresponding to a decrease in estimated membrane potential from -55 mV to -50 mV. The onset of the decrease in membrane potential occurred after 7 days of in vivo nicotine treatment and was significantly correlated with an increase in [3H]nicotine binding to cerebral cortical synaptosomal (P2) membranes. Nicotine, at in vitro concentrations of 3-1,000 microM, decreased [3H]tetraphenylphosphonium accumulation in cerebral cortical synaptosomes from control animals. When compared to accumulation in buffer alone, in vitro nicotine and other nicotinic agonists did not significantly decrease [3H]tetraphenylphosphonium accumulation in cerebral cortical synaptosomes prepared from rats treated with nicotine in vivo. These studies provide evidence that chronic treatment with nicotine results in an average lower membrane potential in cerebral cortical synaptosomes and in functional down-regulation of the depolarization response to nicotinic cholinergic receptor stimulation.

Inositol tetrakisphosphate isomers and elevation of cytosolic Ca2+ in vasopressin-stimulated insulin-secreting RINm5F cells.

Signal generation during the stimulation of insulin secretion by arginine vasopressin (AVP) was investigated in RINm5F cells. AVP (0.1 microM) caused a biphasic cytosolic Ca2+ ([Ca2+]i) rise, namely a rapid transient marked elevation after stimulation followed by a series of oscillations. In the absence of extracellular Ca2+, the sustained oscillations were abolished, while the initial [Ca2+]i transient was only partly decreased, indicating that the former are due to Ca2+ influx and the latter due mainly to mobilization from internal Ca2+ stores. AVP also evoked a transient depolarization of the average membrane potential. AVP-induced Ca2+ influx during the sustained phase, which was strictly dependent on receptor occupancy, was attenuated by membrane hyperpolarization with diazoxide. However, blockade of Ca2+ channels of the L- or T-type was ineffective. AVP stimulated production of diacylglycerol and inositol phosphates; for the latter both [3H] inositol labeling and mass determinations were performed. A transient increase in Ins(1,4,5)P3 was followed by a marked enhancement of Ins(1,3,4,5)P4 (8-fold) peaking at 15 s and gradually returning to basal values. Ins(1,3,4,6)P4 and Ins(3,4,5,6)P4 exhibited the most long-lasting augmentation (4- and 1.7-fold, respectively), and therefore correlated best with the period of sustained [Ca2+]i oscillations. InsP5 and InsP6 were not elevated. The effects of AVP, including the stimulation of insulin secretion from perifused cells, were obliterated by a V1 receptor antagonist. In conclusion, AVP induces protracted [Ca2+]i elevation in RINm5F cells which is associated with long-lasting increases in InsP4 isomers. The accumulation of InsP4 isomers reflects receptor occupancy and accelerated metabolism of the inositol phosphates. Activation of second messenger-operated Ca2+ channels is not necessarily implicated because of the attenuating effect of membrane hyperpolarization.

Regulation of spontaneous activity and oscillatory spike firing in rat midbrain dopamine neurons recorded in vitro

Intracellular recordings were obtained from identified dopamine (DA) neurons in rat midbrain slices maintained in vitro. DA neuron membranes exhibited pronounced instantaneous and time-dependent anomalous rectification that showed evidence of maximal activation at average membrane potentials of -63 and -78 mV, respectively. Action potentials were followed by prominent afterhyperpolarizations (AHP) that consisted of two components. The fast component showed evidence of inactivation at -63 mV independent of the initial membrane potential, whereas the longer-duration, later component increased in amplitude at hyperpolarized potentials. Unlike DA neurons recorded in vivo, there was no evidence of spike frequency adaptation or summation of AHPs with prolonged depolarization-induced spike trains. Spontaneous spike discharge occurred via an endogenous pacemaker potential that was dependent on both TTX-sensitive and cobalt-sensitive processes. Hyperpolarizing prepulses could activate rebound pacemaker discharge, but this rebound activity was progressively blocked with larger-amplitude hyperpolarizing prepulses. DA neurons recorded in the anesthetized animal, freely moving animal, and in vitro preparations have been shown to exist in two states of activity: 1) spontaneously discharging action potentials or 2) hyperpolarized, quiescent, and nonfiring. Furthermore, although it is rare to find DA neurons in the untreated animal in transitional states of activity, quiescent neurons can be activated by stimuli that place a demand on the DA system. The evidence presented here is consistent with the hypothesis that the special combination of membrane properties of DA neurons contribute to the segregation of their activity into active or inactive states.

Electrical and mechanical responses to inhibition of cell respiration in vascular smooth muscle of the rat portal vein.

Metabolic regulation of contractility in vascular smooth muscle was studied in the spontaneously active rat portal vein using respiratory depression by cyanide (0.2-2.0 mM) as a model for tissue hypoxia. Intracellular recordings of electrical activity were done with concomitant registration of force development. Average membrane potential in the absence of cyanide was -61 +/- 1 mV (n = 27). Addition of cyanide to normal Krebs solution resulted in a reduction of force amplitude and the number of action potentials per burst, with a relatively more pronounced effect on the mechanical activity. At moderate levels of inhibition of force amplitude the frequency of spontaneous bursts of action potentials transiently increased concomitant with a slight depolarization, but after prolonged (15-20 min) exposure to cyanide the membrane repolarized to the level prior to cyanide addition and the burst frequency decreased to be equal to or lower than that in the absence of cyanide. Higher concentrations of cyanide totally inhibited spontaneous mechanical and electrical activity. In contrast to the results with glucose, it was found that when beta-hydroxybutyrate was used as substrate the addition of 2 mM cyanide led to a marked hyperpolarization (13 +/- 1 mV) after total inhibition of spontaneous activity. The hyperpolarization was not prevented by administration of 4-aminopyridine (2.5 mM) or tetraethylammonium (4-6 mM) prior to the addition of cyanide. To investigate the effects of increased metabolic demand on the relation between force and membrane potential in cyanide-treated muscle, high-K+ (40 mM) contractures were studied. Contractures were associated with depolarization of 34 +/- 3 mV (n = 5). 1 mM cyanide reduced the amplitude of the contractures to about 9% of control with a moderate reduction in the amount of depolarization (28 +/- 1 mV, n = 5). It is concluded that the decrease of mechanical activity during respiratory inhibition may partly reflect a reduction in the number of spikes per burst but that other mechanisms, independent of membrane activity, also contribute to the inhibition. The increase of glycolysis during respiratory inhibition seems to prevent more pronounced changes in membrane potential.

Jendrassik maneuver facilitates soleus H-reflex without change in average soleus motoneuron pool membrane potential

Facilitation of spinal reflex amplitude by remote muscle contraction, otherwise known as the Jendrassik maneuver (JM), was first shown over 100 years ago, yet the mechanism by which this facilitation operates remains undetermined. Earlier work has eliminated participation of the muscle spindle in JM-induced spinal reflex facilitation, leaving changes in postsynaptic (e.g., change in average soleus motoneuron membrane potential) and presynaptic (e.g., inhibition of presynaptic inhibition) mechanisms as viable candidates. We recorded background EMG in the soleus muscle during JM-induced soleus H-reflex facilitation in humans. The JM in this experiment consisted of wrist muscle contraction. Soleus background EMG was maintained by the subject at either a zero level (e.g., relaxed) or a specified moderate level prior to and during the JM. The JM increased H-reflex amplitude by comparable amounts in both situations, but had no effect on soleus background EMG. Given the well-known relationship between the average motoneuron pool membrane potential and background EMG, we conclude that JM facilitation of the soleus H-reflex is not caused by an increase in background excitatory input to the soleus motoneuron pool. Remaining candidates for mediation of JM induced H-reflex facilitation include change in stimulus-evoked afferent input at some point proximal to the muscle spindle, such as reduction in presynaptic inhibition, or a change in motoneuron input resistance.

Effet du vieillissement sur la défense rapide contre la polyspermie chez l'ovule d'oursin

Aging of unfertilized ova of Echinoids is marked by a lowered fertilizability and an increased susceptibility to polyspermy. Since the fast block to polyspermy is electrically mediated in these species, we have studied the electrophysiological events triggered by fertilization in aging ova of Strongylocentrotus purpuratus. Aging of unfertilized ova is accompanied by a decrease in the resting potential. This could be explained by a decreased activity of membrane ionic pumps and could account for the gradual unfertilizability of aged eggs. Similarly, the depolarization plateau, which follows fertilization in freshly shed eggs, is abolished in eggs older than 80 min. Such aged eggs become vulnerable to polyspermy until the definitive, complete block is established by the cortical reaction. Finally, irrespective of age, fertilized ova repolarize at the same average membrane potential of −72 mV. This could reflect the general metabolic activation that follows fertilization.

The mesangial cell culture: a tool for the study of the electrophysiological and pharmacological properties of the glomerular mesangial cell

Abstract Cultured rat glomerular mesangial cells (MC) were evaluated as a tool for reliable electrophysiological measurements as well as for fluorimetric determinations of intracellular Ca ++. They had a resting potential similar to that observed in cultured vascular smooth muscle cells (VSMCs), in VSMCs of mouse kidney arterioles, or in glomerular - presumably mesangial - cells of kidney slices. The comparison with the other cell types was carried out in order to look for features distinguishing them from these cells, e.g., active and passive electrical membrane properties or electrical membrane responses to vasoactive pharmacological agents. In MCs, as well as in the other cell types, the average membrane potential was approx. − 50 mV. The vasoconstrictor peptides angiotensin II (ANG II) and arginine-vasopressin (AVP) caused depolarizations that could be blocked by the respective specific inhibitors of these compounds. The agonist-induced depolarizations have to be attributed, at least in part, to a Ca + + inward current. Norepi-nephrine, if any, had only a weak action upon MCs, whereas isoproterenol either did not influence the membrane potential or hyperpolarized the cells. Other substances tested, which had no influences upon the membrane potential, were neuropeptide Y and atriopeptin 3. As to their resting electrical properties and their responses to pharmacological agents, cultured mesangial cells did not differ from glomerular, i.e., most probably mesangial, cells in the kidney slice. The difference between mesangial cells and VSMCs consists in their reaction to noradrenaline. Whereas VSMCs respond with a marked depolarization, the noradrenaline effect upon MCs in culture and in the kidney slice is either absent or very weak. Repeated passage of the cells (more than six passages) led to a gradual loss of their responsiveness to the agonists, indicating reduced receptor expression which may be interpreted as dedifferentiation. This held for both cultured MCs and VSMCs. Fluorimetric measurements using the Ca + +-specific indicators quin-2 and fura-2 were performed with a purpose-developed, ultrasensitive photon-counting microspectro-fluorimeter. Individual MCs as well as isolated glomeruli responded to the vasoconstrictors ANG II and AVP with an increase in Ca + +-dependent fluorescence indicating that these agents indeed depolarize the cells partly via a Ca + + influx and increase cytosolic free Ca + + Owing to the absence of visual criteria for the discrimination of MCs from VSMCs, the establishment of electrophysiological differences, e.g., in the response to noradrenaline, may prove to be very helpful for cell identification, especially when questions are addressed that require recording from the cells in their microenvironment, e.g., in the kidney slice.

The Bulla ocular circadian pacemaker. II. Chronic changes in membrane potential lengthen free running period.

We have used intracellular recording to directly measure the effects of three experimental agents, light, elevated potassium seawater, and lowered sodium seawater on the membrane potential of the putative circadian pacemaker neurons of the Bulla eye. These agents were subsequently tested for effects on the free running period of the circadian pacemaker. We report that: 1. When applied to the eye, light and elevated potassium seawater depolarized the putative pacemaker neurons, while lowered sodium seawater hyperpolarized them. The membrane potential changes induced by these agents are sustained for at least one hour, suggesting that they produce persistent changes in the average membrane potential of the putative pacemaker neurons. 2. The amplitude of the membrane potential response to the depolarizing agents varies with the phase of the circadian cycle. Depolarizations induced by light and elevated potassium seawater are twice as large during the subjective night than they are during the subjective day. No significant difference was found in the response to lowered sodium seawater at different phases. 3. Continuous application of each of these agents caused a lengthening of the free running period of the Bulla eye. Constant light increased the period by 0.9 h, while the other depolarizing treatment (elevated potassium seawater) increased the free running period by 0.6 h. Both treatments increased the mean peak impulse frequency of treated eyes. The hyperpolarizing treatment also increased the period of the ocular pacemaker (+0.8 h), but had little effect on peak impulse frequency.(ABSTRACT TRUNCATED AT 250 WORDS)

An analysis of the action of pentobarbitone on the excitatory postsynaptic potentials and membrane properties of neurones in the guinea-pig olfactory cortex.

Intracellular recordings were made from neurones in slices of guinea-pig olfactory cortex maintained in vitro at 37 degrees C. The average membrane potential was 63 +/- 12 mV and the input resistance of these cells was 42 +/- 20 M omega (mean +/- s.d.). Stimulation of the lateral olfactory tract (l.o.t.) generated a transient depolarization in these cells which had the characteristics of an excitatory postsynaptic potential (e.p.s.p.). If the e.p.s.p. was of sufficient amplitude it culminated in an action potential. The e.p.s.p. was potentiated by repetitive stimulation at 10-50 Hz and showed post-tetanic potentiation after a prolonged period of high frequency stimulation (50-100 Hz for 30-60 s). Pentobarbitone (0.1-0.5 mM) depressed the e.p.s.p. reversibly but was without effect on the resting membrane potential, input resistance or time constant of the neurones. Pentobarbitone did not inhibit potentiation of the e.p.s.p. by a preceding conditioning shock. It is concluded that pentobarbitone does not affect the passive membrane properties of neurones in the olfactory cortex. The depressant action of pentobarbitone on synaptic transmission results from a decrease in the amount of transmitter released in response to a nerve impulse, or a decrease in the sensitivity of the postsynaptic membrane to the transmitter or a combination of both effects.

Ion fluxes changes during early stages of Schistosoma mansoni. Evaluation of complement effect.

The relationship between the average membrane potential (delta psi av) and sensitivity to complement action of the Schistosoma mansoni parasite was explored. The average membrane potential was estimated by measuring the uptake of [3H]tetraphenyl phosphonium ([3H]Ph4P+). The parasites take up Ph4P+ indicating the existence of a negative internal plasma potential which is in part dependent on the transmembrane K+ gradient, maintained by an active Na+/K+-ATPase. Values for Ph4P+ uptake could be corrected for mitochondrial accumulation by employing the protonophore carbonylcyanide m-chlorophenylhydrazone (CCCP), which collapses the mitochondrial potential. The plasma membrane potential derived by this technique was in the range of -60 mV. Transformation of this parasite, from its early cercaria stage to the adult worm, was associated with changes in the average membrane potential. The apparent hyperpolarization, which accompanies transformation, may be related to changes in ionic permeability and morphology which occur concomitantly. Complement acting through both the classical and alternative pathways was found to affect the potential of the parasite in its early development stages. The correlation between effects on delta psi av and sensitivity to complement action, indicates that the complement-induced changes in delta psi av are indeed tightly associated with its mode of action. Treatment of the parasite with complement resulted in net hyperpolarization of the membrane indicating that hyperpolarization rather than depolarization of the membrane is linked to the primary non-lethal action of complement.