Effect of presynaptic membrane potential on electrical vs. chemical synaptic transmission

The growing realization that electrical coupling is present in the mammalian brain has sparked renewed interest in determining its functional significance and contrasting it with chemical transmission. One question of interest is whether the two types of transmission can be selectively regulated, e.g., if a cell makes both types of connections can electrical transmission occur in the absence of chemical transmission? We explore this issue in an experimentally advantageous preparation. B21, the neuron we study, is an Aplysia sensory neuron involved in feeding that makes electrical and chemical connections with other identified cells. Previously we demonstrated that chemical synaptic transmission is membrane potential dependent. It occurs when B21 is centrally depolarized prior to and during peripheral activation, but does not occur if B21 is peripherally activated at its resting membrane potential. In this article we study effects of membrane potential on electrical transmission. We demonstrate that maximal potentiation occurs in different voltage ranges for the two types of transmission, with potentiation of electrical transmission occurring at more hyperpolarized potentials (i.e., requiring less central depolarization). Furthermore, we describe a physiologically relevant type of stimulus that induces both spiking and an envelope of depolarization in the somatic region of B21. This depolarization does not induce functional chemical synaptic transmission but is comparable to the depolarization needed to maximally potentiate electrical transmission. In this study we therefore characterize a situation in which electrical and chemical transmission can be selectively controlled by membrane potential.     

Effect of oxytocin on electrical potential and ionic permeability of the apical membrane of frog gall bladder epithelial cells

The effect of oxytocin on the intracellular Na⁺ and K⁺ concentrations, the level of the transmembrane potential difference, and the relative ionic permeability (P_Na/P_K) of the apical parts of the surface membrane of the epithelial cells was studied in experiments on the isolated frog gall bladder. In a dose of 20 milliunits/ml, oxytocin, added to the external incubation solution, reduced the transmembrane potential difference, increased the P_Na/P_K ratio, and led to a very small shift in the Na⁺ and K⁺ concentrations in the intracellular fluid. After exposure of the organ to the hormone for 30 min the membrane potential (MP) of the cells fell from 52.7 to 38.7 mV at P<0.001 (negative charge inside the cell), whereas the value of P_Na/P was increased from 0.083 (control) to 0.175 (experiment) at P<0.001. Meanwhile the intracellular Na⁺ concentration was increased by 18.3 meq/kg and the K⁺ concentration reduced by 5.2 meq/kg intracellular water. This shift in the intracellular concentrations of Na⁺ and K⁺ could result in an evident decrease of only 0.7 mV in MP whereas in fact MP fell by 14.0 mV. Consequently, the decrease in the transmembrane potential difference takes place as a result of an increase in P_Na/P_K under the influence of oxytocin. Electrogenic ion transport through the apical membranes of the frog gall bladder epithelial cells could not be detected.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. (Presented by Academician of the Academy of Medical Sciences of the USSR N. N. Gorev.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 85, No. 2, pp. 136–139, February, 1978.     

Effects of membrane potential changes on electrical cell-to-cell coupling in proximal tubule

Single proximal convoluted tubules (PCT) of Necturus kidney were impaled with three microelectrodes in the sequence M1, M2, M3. M1 was used for injecting short DC current pulses, M2 for recording peritubular membrane potential, V, and M3 for injecting longer DC current steps and thereby shifting V to a new baseline potential, V. We define the p.d. changes at M2 due to M1-induced pulses as V and V (for baselines V and V, respectively). Our objective was to test whether V was equal to V. The main finding is that when V depolarized by 10 to 80 mV V/V remained close to 1.00. Care was taken to ensure that this apparent stability of the pulse ratio was not due (i) to opposite changes of apical and basolateral membrane conductances (g(A) and g(B) respectively), (ii) to changes of the sum g(A)+g(B) compensated for by changes of the cell-to-cell junctional conductance, g(j), or (iii) to a distortion of the V/V ratio as a function of interelectrode distance, masking voltage-dependent changes of cell membrane conductances. Hyperpolarization of V produced gradual electrical uncoupling between cells as V became increasingly negative, by a mechanism yet to be determined.Supported by GRECO 24 (CNRS).     

Effective synaptic current can be estimated from measurements of neuronal discharge.

1. The basic question of how motoneurons transform synaptic inputs into spike train outputs remains unresolved, despite detailed knowledge of their morphology, electrophysiology, and synaptic connectivity. We have approached this problem by making measurements of a synaptic input under steady-state conditions and combining them with quantitative assessments of their effects on the discharge rates of cat spinal motoneurons. 2. We used a modified voltage-clamp technique to measure the steady-state effective synaptic currents (IN) produced by rubrospinal input to cat triceps surae motoneurons. In the same motoneurons we measured the slope of the firing rate-injected current (f-i) relation in the primary range. We then reactivated the rubrospinal input during steady, repetitive firing to assess its effect on motoneuron discharge rate. 3. We found that changes in the steady-state discharge rate of a motoneuron produced by this synaptic input could be described simply as the product of the net effective synaptic current measured at the soma and the slope of the motoneuron's f-i relation. This expression essentially redefines synaptic efficacy in terms of a cell's basic input-output function. Further, measurements of effective synaptic current simplify the task of estimating synaptic efficacy, because detailed knowledge of neither the electrotonic architecture of the postsynaptic cell nor of the locations of the presynaptic boutons is required.     

Cellular electrophysiology of canine pulmonary vein cardiomyocytes: action potential and ionic current properties

Pulmonary vein (PV) cardiomyocytes play an important role in atrial fibrillation; however, little is known about their specific cellular electrophysiological properties. We applied standard microelectrode recording and whole-cell patch-clamp to evaluate action potentials and ionic currents in canine PVs and left atrium (LA) free wall. Resting membrane potential (RMP) averaged -66 ± 1 mV in PVs and -74 ± 1 mV in LA ( P < 0.0001) and action potential amplitude averaged 76 ± 2 mV in PVs vs. 95 ± 2 mV in LA ( P < 0.0001). PVs had smaller maximum phase 0 upstroke velocity ( V _max: 98 ± 9 vs. 259 ± 16 V s⁻¹, P < 0.0001) and action potential duration (APD): e.g. at 2 Hz, APD to 90 % repolarization in PVs was 84 % of LA ( P < 0.05). Na⁺ current density under voltage-clamp conditions was similar in PV and LA, suggesting that smaller V _max in PVs was due to reduced RMP. Inward rectifier current density in the PV cardiomyocytes was ˜58 % that in the LA, potentially accounting for the less negative RMP in PVs. Slow and rapid delayed rectifier currents were greater in the PV (by ˜60 and ˜50 %, respectively), whereas transient outward K⁺ current and L-type Ca²⁺ current were significantly smaller (by ˜25 and ˜30 %, respectively). Na⁺-Ca²⁺-exchange (NCX) current and T-type Ca²⁺ current were not significantly different. In conclusion, PV cardiomyocytes have a discrete distribution of transmembrane ion currents associated with specific action potential properties, with potential implications for understanding PV electrical activity in cardiac arrhythmias.     

Astrocyte control of synaptic transmission and neurovascular coupling

From a structural perspective, the predominant glial cell of the central nervous system, the astrocyte, is positioned to regulate synaptic transmission and neurovascular coupling: the processes of one astrocyte contact tens of thousands of synapses, while other processes of the same cell form endfeet on capillaries and arterioles. The application of subcellular imaging of Ca2+ signaling to astrocytes now provides functional data to support this structural notion. Astrocytes express receptors for many neurotransmitters, and their activation leads to oscillations in internal Ca2+. These oscillations induce the accumulation of arachidonic acid and the release of the chemical transmitters glutamate, d-serine, and ATP. Ca2+ oscillations in astrocytic endfeet can control cerebral microcirculation through the arachidonic acid metabolites prostaglandin E2 and epoxyeicosatrienoic acids that induce arteriole dilation, and 20-HETE that induces arteriole constriction. In addition to actions on the vasculature, the release of chemical transmitters from astrocytes regulates neuronal function. Astrocyte-derived glutamate, which preferentially acts on extrasynaptic receptors, can promote neuronal synchrony, enhance neuronal excitability, and modulate synaptic transmission. Astrocyte-derived d-serine, by acting on the glycine-binding site of the N-methyl-d-aspartate receptor, can modulate synaptic plasticity. Astrocyte-derived ATP, which is hydrolyzed to adenosine in the extracellular space, has inhibitory actions and mediates synaptic cross-talk underlying heterosynaptic depression. Now that we appreciate this range of actions of astrocytic signaling, some of the immediate challenges are to determine how the astrocyte regulates neuronal integration and how both excitatory (glutamate) and inhibitory signals (adenosine) provided by the same glial cell act in concert to regulate neuronal function.     

Effect of ethanol on action potential and ionic membrane currents in rat ventricular myocytes

Aim: Even though alcohol intoxication is often linked to arrhythmias, data describing ethanol effect on cardiac ionic channels are rare. In addition, ethanol is used as a solvent of hydrophobic compounds in experimental studies. We investigated changes of the action potential (AP) configuration and main ionic membrane currents in rat cardiomyocytes under 20–1500 mm ethanol. Methods: Experiments were performed on enzymatically isolated rat right ventricular myocytes using the whole cell patch-clamp technique at room temperature. Results: Ethanol reversibly decelerated the upstroke velocity and decreased AP amplitude and duration at 0.2 and 3 Hz. The fast sodium current I_Na, l -type calcium current I_Ca and transient outward potassium current I_to were reversibly inhibited in a concentration-dependent manner (50% inhibition at 446 ± 12, 553 ± 49 and 1954 ± 234 mm , respectively, with corresponding Hill coefficients 3.1 ± 0.3, 1.1 ± 0.2 and 0.9 ± 0.1). Suppression of I_Na and I_Ca magnitude was slightly voltage dependent. The effect on I_Ca and I_to was manifested mainly as an acceleration of their apparent inactivations with a decreased slow and fast time constant respectively. As a consequence of marked differences in n_H, sensitivity of the currents to ethanol at 10% inhibition decreases in the following order: I_Ca (75 mm , 3.5‰), I_to (170 mm , 7.8‰) and I_Na (220 mm , 10.1‰). Conclusion: Our results suggest a slight inhibition of all the currents at ethanol concentrations relevant to deep alcohol intoxication. The concentration dependence measured over a wide range may serve as a guideline when using ethanol as a solvent.     

Reappraisal of methods in clinical length measurements: The potential of current technology

Some basic considerations for improved length measurements for clinical paediatric use using the supine position are presented. These lead to a design philosophy for possible future measurement systems. A system which has been built in accordance with this philosophy is described.     

Effect of temperature on membrane potential and ionic fluxes in intact and dialysed barnacle muscle fibres

1. The temperature-dependent component of the resting potential in intact, cannulated and dialysed fibres from the muscle of the barnacle Balanus nubilus was studied under a variety of different experimental conditions. A decrease in temperature from 22 to 12 degrees C produced a mean depolarization of 10 mV.2. Neither addition of strophanthidin, nor replacement of external sodium by lithium affect the voltage shift induced by temperature. However, the magnitude of the voltage shift depends on the external chloride and potassium concentration.3. The dialysis technique was applied to measure the potassium, chloride and sodium fluxes as a function of temperature. The Q(10) for the passive fluxes of these ions was 1.9, 1.7, and 1.4 respectively.4. The temperature-dependent changes in the passive ionic fluxes combined with the inability of inhibitors of the sodium pump to alter the temperature dependence of the resting potential suggest that the change induced by temperature on the resting potential is primarily caused by a change in the passive permeability ratios, and is not related to active ion transport.     

Effect of inhibitors and diuretics on electrical potential differences in rat kidney proximal tubule

Active transport potentials were studied across early loops of rat proximal tubule during luminal perfusion and peritubular superfusion with HCO3- Ringer's solution of identical ionic composition. From the effects of the carbonic anhydrase inhibitor acetazolamide and of ouabain it is concluded 1. that the lumen-positive active transport potential indicates an excess of active H+ secretion/HCO3- absorption over active Na+ absorption and 2. that the lumen-negative active transport potential, which develops in the presence of glucose (and/or aminoacids) in the tubular lumen, indicates stimulation of active Na+ absorption. Ouabain did not abolish the lumen-positive potential difference suggesting that active H+/HCO3- transport and active Na+ transport may be to some extent independent. Among the diuretics tested the mercurial diuretic mersalyl acted primarily on Na+ transport, and furosemide acted on HCO3- transport, whereas the effect of ethacrynic acid appeared to be unspecific.     

Theory and cardiac applications of electrical impedance measurements

The methodology of the two-electrode, four-electrode, and guard-ring techniques is presented following a brief history of impedance plethysmography. The theoretical basis for predicting the sampling fields for conductivity and volume changes is presented. Theoretical and experimental studies of the sampling field associated with various electrode arrays are reviewed. With this background, the use of impedance plethysmography for cardiac monitoring and diagnosis is reviewed. The basic methodology is presented and models used to interpret the signal are reviewed. Theoretical and experimental studies of what is sampled are summarized. The accuracy of impedance stroke volume estimates is evaluated by surveying the results of human studies and examining critical animal studies. The usefulness of impedance cardiography for ventricular performance evaluation is also reviewed. Additional uses for cardiopulmonary diagnosis are briefly presented.     

A study on synaptic coupling between single orofacial mechanoreceptors and human masseter muscle

The connection between individual orofacial mechanoreceptive afferents and the motoneurones that innervate jaw muscles is not well established. For example, although electrical and mechanical stimulation of orofacial afferents in bulk evokes responses in the jaw closers, whether similar responses can be evoked in the jaw muscles from the discharge of type identified single orofacial mechanoreceptive afferents is not known. Using tungsten microelectrodes, we have recorded from 28 afferents in the inferior alveolar nerve and 21 afferents in the lingual nerve of human volunteers. We have used discharges of single orofacial afferents as the triggers and the electromyogram (EMG) of the masseter as the source to generate spike-triggered averaged records to illustrate time-based EMG modulation by the nerve discharge. We have then used cross correlation analysis to quantify the coupling. Furthermore, we have also used coherence analysis to study frequency-based relationship between the nerve spike trains and the EMG. The discharge patterns of the skin and mucosa receptors around the lip and the gingiva generated significant modulation in EMGs with a success rate of 40% for both cross correlation and coherence analyses. The discharge patterns of the periodontal mechanoreceptors (PMRs) generated more coupling with a success rate of 70% for cross correlation and about 35% for coherence analyses. Finally, the discharges of the tongue receptors displayed significant coupling with the jaw muscle motoneurones with a success rate of about 40% for both analyses. Significant modulation of the jaw muscles by single orofacial receptors suggests that they play important roles in controlling the jaw muscle activity so that mastication and speech functions are executed successfully.