Ammonium sulfate induced uncouplings of crayfish septate axons with and without increased junctional resistance

Calmodulin-Mediated Regulation of Gap Junction Channels

Evidence that neighboring cells uncouple from each other as one dies surfaced in the late 19th century, but it took almost a century for scientists to start understanding the uncoupling mechanism (chemical gating). The role of cytosolic free calcium (Ca2+i) in cell-cell channel gating was first reported in the mid-sixties. In these studies, only micromolar [Ca2+]i were believed to affect gating-concentrations reachable only in cell death, which would discard Ca2+i as a fine modulator of cell coupling. More recently, however, numerous researchers, including us, have reported the effectiveness of nanomolar [Ca2+]i. Since connexins do not have high-affinity calcium sites, the effectiveness of nanomolar [Ca2+]i suggests the role of Ca-modulated proteins, with calmodulin (CaM) being most obvious. Indeed, in 1981 we first reported that a CaM-inhibitor prevents chemical gating. Since then, the CaM role in gating has been confirmed by studies that tested it with a variety of approaches such as treatments with CaM-inhibitors, inhibition of CaM expression, expression of CaM mutants, immunofluorescent co-localization of CaM and gap junctions, and binding of CaM to peptides mimicking connexin domains identified as CaM targets. Our gating model envisions Ca2+-CaM to directly gate the channels by acting as a plug ("Cork" gating model), and probably also by affecting connexin conformation.

Modulation of Connexin-36 Gap Junction Channels by Intracellular pH and Magnesium Ions

Connexin-36 (Cx36) protein forms gap junction (GJ) channels in pancreatic beta cells and is also the main Cx isoform forming electrical synapses in the adult mammalian brain. Cx36 GJs can be regulated by intracellular pH (pHi) and cytosolic magnesium ion concentration ([Mg2+]i), which can vary significantly under various physiological and pathological conditions. However, the combined effect and relationship of these two factors over Cx36-dependent coupling have not been previously studied in detail. Our experimental results in HeLa cells expressing Cx36 show that changes in both pHi and [Mg2+]i affect junctional conductance (gj) in an interdependent manner; in other words, intracellular acidification cause increase or decay in gj depending on whether [Mg2+]i is high or low, respectively, and intracellular alkalization cause reduction in gj independently of [Mg2+]i. Our experimental and modelling data support the hypothesis that Cx36 GJ channels contain two separate gating mechanisms, and both are differentially sensitive to changes in pHi and [Mg2+]i. Using recombinant Cx36 we found that two glutamate residues in the N-terminus could be partly responsible for the observed interrelated effect of pHi and [Mg2+]i. Mutation of glutamate at position 8 attenuated the stimulatory effect of intracellular acidification at high [Mg2+]i, while mutation at position 12 and double mutation at both positions reversed stimulatory effect to inhibition. Moreover, Cx36*E8Q lost the initial increase of gj at low [Mg2+]i and double mutation lost the sensitivity to high [Mg2+]i. These results suggest that E8 and E12 are involved in regulation of Cx36 GJ channels by Mg2+ and H+ ions.

Chemical gating of gap junction channels; roles of calcium, pH and calmodulin

Both Ca(2+) and H(+) play a role in chemical gating of gap junction channels, but, with the possible exception of Cx46 hemichannels, neither of them is likely to induce gating by a direct interaction with connexins. Some evidence suggests that low pH(i) affects gating via an increase in [Ca(2+)](i); in turn, Ca(2+) is likely to induce gating by activation of CaM, which may act directly as a gating particle. The effective concentrations of both Ca(2+) and H(+) vary depending on cell type, type of connexin expressed and procedure employed to increase their cytosolic concentrations; however, pH(i) as high as 7.2 and [Ca(2+)](i) as low as 150 nM or lower have been reported to be effective in some cells. Some data suggest that Ca(2+) and H(+) affect gating by acting synergistically, but other data do not support synergism. Chemical gating follows the activation of a slow gate distinct from the fast V(j)-sensitive gate, and there is evidence that the chemical/slow gate is V(j)-sensitive. At the single channel level, the chemical/slow gate closes the channels slowly and completely, whereas the fast V(j) gate closes the channels rapidly and incompletely. At least three molecular models of channel gating have been proposed, but all of them are mostly based on circumstantial evidence.

Carbenoxolone depresses spontaneous epileptiform activity in the CA1 region of rat hippocampal slices

An important contributor to the generation of epileptiform activity is the synchronization of burst firing in a group of neurons. The aim of this study was to investigate whether gap junctions are involved in this synchrony using an in vitro model of epileptiform activity. Hippocampal slices (400 microm) were prepared from female Sprague-Dawley rats (120-170 g). The perfusion of slices with a medium containing no added magnesium and 4-aminopyridine (50 microM) resulted in the generation of spontaneous bursts of population spikes of a fast frequency along with less frequent negative-going bursts. The frequency of the bursts produced was consistent over a 3h period. Carbenoxolone (100 microM), a gap junction blocker and mineralocorticoid agonist, perfused for 75 min, reduced the frequency of both types of spontaneous burst activity. Perfusion of spironolactone (1 microM), a mineralocorticosteroid antagonist, for 15 min prior to and during carbenoxolone perfusion did not alter the ability of carbenoxolone to depress the frequency of spontaneous activity. The incubation of hippocampal slices in carbenoxolone prior to recording increased the time taken for the spontaneous activity to start on change to the zero magnesium/4-aminopyridine medium and decreased the total number of spontaneous bursts over the first 60 min period. The ability of carbenoxolone to delay induction of epileptiform activity and reduce established epileptiform activity suggests that gap junctions contribute to the synchronization of neuronal firing in this model.

Activity-dependent pH shifts and periodic recurrence of spontaneous interictal spikes in a model of focal epileptogenesis

The mechanisms that control the periodicity of spontaneous epileptiform cortical potentials were investigated in the in vitro isolated guinea pig brain preparation. A brief intracortical application of bicuculline in the piriform cortex induced spontaneous interictal spikes (sISs) that recurred with high periodicity (8.5 +/- 3.1 sec, mean +/- SD). Intracellular recordings from principal neurons showed that the early phase of the inter-sIS period is caused by a GABAb receptor-mediated inhibitory potential. The late component of the interspike period correlated to a slowly decaying depolarization abolished at membrane potentials positive to -32.1 +/- 5.3 mV and was not associated with membrane conductance changes. Specific pharmacological tests excluded the contribution of synaptic and intrinsic conductances to the late inter-sIS interval. Recordings with ion-sensitive electrodes demonstrated that sISs determined both a rapid increase in extracellular K+ concentration (0.5-1 mM) and an extracellular alkalinization (0.05-0.08 pH units) that slowly decayed during the inter-sIS period and returned to control values just before a subsequent sIS was generated. These observations were not congruous with the presence of a silent period, because both extracellular increase in K+ and alkalinization are commonly associated with an increase in neuronal excitability. Extracellular alkalinization could be correlated to an sIS-induced intracellular acidification, a phenomenon that reduces cell coupling by impairing gap junction function. When intracellular acidification was transiently prevented by arterial perfusion with NH4Cl (10-20 mM), spontaneous ictal-like epileptiform discharges were induced. In addition, the gap junction blockers octanol (0.2-2 mM) and 18-alpha-glycyrrethinic acid (20 microM) applied either via the arterial system or locally in the cortex completely and reversibly abolished the sIS. The results reported here suggest that the massive cell discharge associated with an sIS induce a strong inhibition, possibly secondary to a pH-dependent uncoupling of gap junctions, that regulates sIS periodicity.

Ultrastructure of electrical synapses: review

This article reviews studies providing information on the ultrastructure of electrical synapses. Although the review focuses on electron-microscopic investigations, its aim is to examine how the structure of an electrical synapse relates to its function. It begins by presenting a historical overview of the early studies which were responsible for the recognition of electrical synapses. The structure of gap junctions which are the morphological correlates of electrical synapses is illustrated and the ultrastructure and function of the two types of electrical synapse, rectifying and non-rectifying, described. Recent papers investigating the ultrastructure of electrical and mixed electrical-chemical synapses in invertebrates and vertebrates are reviewed. For earlier references, the reader is directed to previous reviews on the subject. Much new information, however, on the structure and formation of electrical synapses has been obtained from work on cultured neurons and from electron-microscopic, immunocytochemical, conformational and molecular studies. This article reviews those studies and in light of their findings, re-examines the relationships of the structure of electrical synapses with their function.

Increase in gap junction resistance with acidification in crayfish septate axons is closely related to changes in intracellular calcium but not hydrogen ion concentration

Neutral-carrier pH- and Ca-sensitive microelectrodes were used to investigate the relationship between junctional electrical resistance and either pHi or [Ca2+]i in crayfish septate axons uncoupled by acidification. For measuring [Ca2+]i a new neutral carrier sensor sensitive to picomolar [Ca2+] and virtually insensitive to other ions was used. Uncoupling was induced by superfusing the axons with Na-acetate solutions (pH 6.3). With acetate, the time course of changes in junctional resistance differed markedly from that of pHi or [H+]i, and [H+]i peaked 40-90 sec before junctional resistance. The difference in shape and peak time between pHi and junctional resistance curves caused significant hysteresis in the pHi versus junctional resistance relationship. In addition, junctional resistance maxima reached with slow acidification rates were 3-4 times greater than those with fast acidification of similar magnitude. With acetate, [Ca2+]i increased by approximately one order of magnitude from basal values of 0.1-0.3 microM. The curves describing the time course of changes in [Ca2+]i and junctional resistance matched well with each other in shape, peak time and magnitude. Both junctional resistance and [Ca2+]i recovered following a single exponential decay with a time constant of approximately 2 min. Different rates of acidification caused increases in [Ca2+]i and junctional resistance comparable in magnitude. The data indicate that the increase in junctional resistance induced by acidification is more closely related to [Ca2+]i than to [H+]i.

Voltage-clamp analysis of a crayfish rectifying synapse

1. The rectifying crayfish giant motor synapse has been studied in the second abdominal ganglion, using the double-voltage-clamp technique which allowed direct measurements of junctional current at various fixed transjunctional potentials. 2. The transjunctional potential (Vj), defined as the difference between the voltages recorded in the lateral giant axon and the giant motor fibre, was varied from -70 to +50 mV, the minimum and maximum junctional chord conductances (gmin and gmax, respectively) were found to be 1.2 +/- 1.3 microS (n = 10) and 22.9 +/- 6.3 microS (n = 10), respectively. 3. For a given Vj, changes in the lateral giant axon or giant motor fibre membrane potential over a range of +/- 30 mV around their resting levels did not influence the junctional permeability (gj), indicating that the inside-outside potential of the junctional channel does not control gj. 4. Therefore, the steady-state junctional chord conductances were dependent only upon Vj. 5. The voltage dependence of the chord conductance was well fitted by a modified Boltzmann relation given by the equation (Formula: see text) with the constants: A = 0.15 +/- 0.03 mV-1 (n = 10) and V0 = 28 +/- 4 mV (n = 10); the latter two parameters were also found to be independent of both transmembrane potentials. 6. The junctional currents were already constant 1 ms after step changes in the junctional voltage; this was three orders of magnitude faster than the other known examples of voltage-controlled gap junctions between embryonic cells. 7. Our results may be interpreted by a highly voltage-dependent probability of opening of the junctional channels. They also suggest that the gap-junction channels forming the giant motor synapse respond very rapidly to potential and that the hemi-channels which constitute them may not be symmetric.

Opposite effects of ammonia and carbon dioxide on dye coupling between horizontal cells in the carp retina

Effects of ammonia (NH3) and carbon dioxide (CO2) on the membrane potential of horizontal cells and on dye coupling between the cells in isolated retinas of the carp (Cyprinus carpio) were investigated. Ammonia (less than 300 ppm NH3 in air) initially depolarized and subsequently hyperpolarized, while CO2 (10% in air) hyperpolarized the membrane potential of horizontal cells, accompanied by a diminution of both center and surround responses to spot and annular light stimuli. During the course of amplitude diminution, the center response consistently became smaller with NH3 and larger with CO2 than the surround response. In the presence of intravitreally applied DA (50 microM) or amphetamine (100 microM), a fluorescent dye Lucifer Yellow CH (LY) was found to be restricted to single injected horizontal cells. The presence of intravitreal haloperidol (100 microM) for 20-25 min or an exposure of the retina to NH3 for 5-10 min diffused the restricted LY from single injected cells to numerous neighboring cells. On the other hand, CO2 was found to restrict the injected dye to single cells, an effect similar to that of DA and opposite to that of NH3 and haloperidol. The results suggest that NH3 appears to act as a coupler while CO2 acts as an uncoupler on gap junctions between horizontal cells in the carp retina, presumably by changing the intracellular pH. In addition, a brief exposure of cells, marked with LY in the presence of DA, to the exciting light 426 nm was found to prevent the NH3-induced dye diffusion from single cells to their neighbors; the reason is unknown.

Effects of ammonium salts on synaptic transmission to hippocampal CA1 and CA3 pyramidal cells in vivo

The effects of ammonium acetate or chloride, perfused through the lateral ventricle, were studied on the hippocampal formation of the rat. During perfusion with ammonia, the population spikes, evoked by stimuli delivered to the fimbria, were first increased and then reduced. On the other hand, the late positive wave gradually decreased throughout the application of ammonia. The inhibition, studied by the paired-pulse test, was found to be reduced when the population spike was transiently enhanced, indicating that disinhibition could be responsible for the enhancement of synaptically evoked responses. Neither antidromically evoked population spikes nor the typical effects of iontophoretically applied glutamate, aspartate or gamma-aminobutyrate were changed by ammonia. These findings can be accounted for by a single action of ammonia, a depression of excitatory synaptic transmission, the excitatory synapses on inhibitory interneurons being more readily depressed than those on the pyramidal cells. Both effects, early hyperexcitability and late depression, are probably due to a reduction in the release of the excitatory neurotransmitter, glutamate and/or aspartate. We tentatively suggest that these mechanisms are responsible for some of the symptoms observed during the development of hyperammonemic encephalopathies.

pH dependence of transmission at electrotonic synapses of the crayfish septate axon

Gap junctions between segments of the crayfish septate axon mediate electrotonic transmission of impulses propagating along the length of the nerve cord. We simultaneously measured intracellular pH (pHi) and gap junctional conductance (gj) while axons were exposed to saline equilibrated with CO2, weak acids, and the weak base ammonium chloride. Normal pHi is about 7.1. When pHi is elevated, gj is unaffected. When pHi is reduced, gj declines with an apparent pK of about 6.7 and a Hill coefficient of about 2.7. We also measured effects of pHi on non-junctional conductance (gnj) and on the coupling coefficient, k. Over the pHi range 6.2-8, gnj increases approximately linearly with pHi. Since k is a function of gj and gnj, it reached a maximum at about pHi 7.1, decreasing at higher and lower pHi. The pHi dependence of gj in crayfish septate axon is less steep and has a lower apparent pK than the gj-pHi relation in two vertebrate embryos previously examined. This finding illustrates a difference in gating among analogous and possibly homologous membrane channels.

Application of antimony microelectrodes to intracellular pH monitoring

Some novel studies of the properties of the antimony microelectrode used for intracellular pH measurements are described. First, it is shown that currents in the picoampere range, such as those encountered as leakage in some electrometers, induce important changes in pH sensitivity. The response time of the electrode has also been measured and indicates that the electrode exhibits a rapid time course which would be very useful for dynamic cytoplasmic pH investigations. An example of internal pH recording during cellular acidification in Xenopus laevis oocyte is also presented.