Cell-cell coupling in cultures of striatal and cortical astrocytes of the monkey Cebus apella

Astrocytes were cultured from striatum and neocortex of fetal (embryonic day 90) monkeys (Cebus apella). The cultures grew well, and the cells retained viability after freeze-storage and thawing. The cells displayed depolarized membrane potentials (-19 and -33 mV, for striatal and cortical cells, respectively) and the vast majority of cells were dye-coupled to a mean of 7 (1-18) neighbouring cells. Cell coupling was blocked by octanol (0.25 and 0.5 mM) but was independent of high K+ (10 and 50 mM) and glutamate (500 microm). Thus, cultures of fetal primate astrocytic cells are established as a model system for studies on astroglial cell-cell coupling.

Regionally distinct regulation of astroglial neurotransmitter receptors by fibroblast growth factor-2

Fibroblast growth factor (FGF)-2 is an abundant astroglial cytokine. We have previously shown that FGF-2 downregulates gap junctions in primary astroglial cultures (B. Reuss et al., 1998, Glia 22, 19-30). We demonstrate now that FGF-2 induces astroglial dopamine (DA) sensitivity and D1 dopamine-receptor (D1DR) antigen and message in cortical and striatal astroglial cultures. On the functional level 10 micromol/L DA triggered transient increases in astroglial [Ca(2+)](i). In gap-junction-coupled cells, no FGF-2-dependent changes in proportions of DA-responsive cells were observable. However, uncoupling with octanol or 18alpha-glycirrhetinic acid isolated the smaller population of astrocytes intrinsically sensitive to DA which was significantly increased by FGF-2 in cortical and striatal cultures. Administration of DR-specific substances revealed that FGF-2 upregulated D1DR. These results indicate that downregulation of astroglial gap junctions by FGF-2 is accompanied by an upregulation of D1DR and DA sensitivity, adding a new aspect to the role of FGF-2 in the regulation of brain functions.

ATP release by mechanically loaded porcine chondrons in pellet culture

OBJECTIVE

To determine whether ATP is released from chondrocytes during mechanical stimulation and whether degradation of ATP generates inorganic pyrophosphate in chondron pellet cultures.

METHODS

Chondron pellets were formed from 1.6 x 10(6) cells that had been enzymatically isolated from porcine articular cartilage. ATP was measured in media from cultures at rest and during fluid movement and cyclic compression. ATP hydrolysis was examined by high-performance liquid chromatography following the addition of gamma32P-ATP to resting cultures.

RESULTS

Pellet cultures at rest maintained a steady-state concentration of 2-4 nM ATP in 2 ml of medium. The ATP concentration increased 5-12-fold with cyclic compression (7.5 and 15 kPa at 0.5 Hz), then decreased to preloading levels within 60 minutes despite continued loading. A subsequent increase in pressure stimulated a further increase in ATP release, suggesting that chondrocytes desensitize to load. Cell viability was similar for pellets at rest and up to 24 hours after compression. ATP released in response to mechanical stimulation was inhibited 50% by 0.5 mM octanol, suggesting a regulated mechanism for ATP release. Exogenous ATP was rapidly hydrolyzed to pyrophosphate in resting cultures.

CONCLUSION

The occurrence of basal levels of extracellular ATP in the presence of pyrophosphohydrolase activity indicates that ATP was continuously released by chondrocytes at rest. Considering that chondrocytes express purinoceptors that respond to ATP, we suggest a role for ATP in extracellular signaling by chondrocytes in response to mechanical load. ATP released by chondrocytes in response to mechanical load is a likely source of pyrophosphate in calcium pyrophosphate dihydrate crystal deposition diseases.

Blocking gap junctional intercellular communication in myoblasts inhibits myogenin and MRF4 expression

Cells rely heavily on cues from their extracellular environment and other cells to coordinate normal physiological processes, and the exchange of molecules via gap junctions has been suggested as on important avenue for cell-cell communication. Gap junctions are found in virtually all mammalian tissues with the notable exception of adult skeletal muscle. However, since functional gap junctions have been detected during the early stages of muscle development, gap junctional intercellular communication (GJIC) may play on important role in myogenesis. In this study, GJIC in normal 16 myoblasts was inhibited using the known blockers l-octanol and beta-glycyrrhetinic acid (beta-GA). Under differentiation promoting conditions, 16 cells fused to form multinucleated myotubes, but when treated with either octanol or beta-GA, no fusion was observed. The expression of two muscle regulatory factors (MRFs), myogenin and MRF4, was examined in both the blocked and control cells. As expected, the activation of both the myogenin and MRF4 genes coincided with the onset of differentiation in the control 16 cells. Neither of these genes were turned on in the blocked cells, even when grown under low serum conditions. This inhibition of differentiation by octanol and beta-GA was reversible, since the activation of both MRF genes as well as myoblast fusion were observed when the blocking medium was replaced with normal differentiating medium. These results suggest that intercellular communication via gap junctions plays an important role in skeletal muscle development and perhaps in the cell signaling events that trigger the activation of muscle-specific MRF genes.

Multiple agents potentiate alpha1-adrenoceptor-induced conduction depression in canine cardiac purkinje fibers

BACKGROUND

Halothane more so than isoflurane potentiates an alpha1-adrenoceptor (alpha1-AR)-mediated action of epinephrine that abnormally slows conduction in Purkinje fibers and may facilitate reentrant arrhythmias. This adverse drug interaction was further evaluated by examining conduction responses to epinephrine in combination with thiopental and propofol, which "sensitize" or reduce the dose of epinephrine required to induce arrhythmias in the heart, and with etomidate, which does not, and responses to epinephrine with verapamil, lidocaine, and l-palmitoyl carnitine, a potential ischemic metabolite.

METHODS

Action potentials and conduction times were measured in vitro using two microelectrodes in groups of canine Purkinje fibers stimulated at 150 pulses/min. Conduction was evaluated each minute after exposure to 5 microm epinephrine (or phenylephrine) alone or with the test drugs. Changes in the rate of phase 0 depolarization (Vmax) and the electrotonic spread of intracellular current were measured during exposure to epinephrine with octanol to evaluate the role of inhibition of active and passive (intercellular coupling) membrane properties in the transient depression of conduction velocity.

RESULTS

Lidocaine (20 microm) and octanol (0.2 mm) potentiated alpha1-AR-induced conduction depression like halothane (0.4 mm), with maximum depression at 3-5 min of agonist exposure, no decrease of Vmax, and little accentuation at a rapid (250 vs. 150 pulses/min) stimulation rate. Thiopental (95 microm), propofol (50 microm), and verapamil (2 microm) similarly potentiated epinephrine responses, whereas etomidate (10 microm) did not. Between groups, the decrease of velocity induced by epinephrine in the presence of (10 microm) l-palmitoyl carnitine (-18%) was significantly greater than that resulting from epinephrine alone (-6%; 0.05 </= P </= 0.10). Current injection experiments were consistent with marked transient inhibition of cell-to-cell coupling correlating with alpha1-AR conduction depression in fibers exposed to octanol.

CONCLUSIONS

Anesthetic "sensitization" to the arrhythmogenic effects of catecholamines may be a special case of a more general phenomenon by which not only some anesthetics and antiarrhythmic drugs but also possible ischemic fatty acid metabolites potentiate conduction depression due to acute alpha1-AR-mediated cell-to-cell uncoupling.

GABAergic modulation of gap junction communication in slice cultures of the rat suprachiasmatic nucleus

We employed morphological and electrophysiological methods in order to elucidate mechanisms which are responsible for communication between cellular oscillators in the cultured rat suprachiasmatic nucleus, the site of the endogenous biological clock that regulates circadian rhythms in mammals. When a gap junction-permeable dye, Lucifer Yellow, was injected into single neurons in the suprachiasmatic nucleus culture, the dye was transferred to neighboring cells in a gap junction blocker-sensitive manner. Optical imaging of neural activity evoked by electrical stimulation in the culture revealed that the spread of depolarization was inhibited by gap junction blockers but not by a blocker of voltage-dependent Na(+) channels. Depolarization propagation was inhibited by muscimol, a GABA(A) receptor agonist, in a dose-dependent manner and the inhibition was reversed by bicuculline, a GABA(A) receptor antagonist. Furthermore, muscimol inhibited dye-transfer between neurons in the suprachiasmatic nucleus culture in a dose-dependent fashion.These independent lines of evidence suggest that the gap junction communication is involved in interneuronal communication in the suprachiasmatic nucleus slice culture and that the coupling state between neurons is not static but dynamically regulated via GABA(A) receptor systems.

Precision of the pacemaker nucleus in a weakly electric fish: network versus cellular influences

We investigated the relative influence of cellular and network properties on the extreme spike timing precision observed in the medullary pacemaker nucleus (Pn) of the weakly electric fish Apteronotus leptorhynchus. Of all known biological rhythms, the electric organ discharge of this and related species is the most temporally precise, with a coefficient of variation (CV = standard deviation/mean period) of 2 x 10(-4) and standard deviation (SD) of 0.12-1.0 micros. The timing of the electric organ discharge is commanded by neurons of the Pn, individual cells of which we show in an in vitro preparation to have only a slightly lesser degree of precision. Among the 100-150 Pn neurons, dye injection into a pacemaker cell resulted in dye coupling in one to five other pacemaker cells and one to three relay cells, consistent with previous results. Relay cell fills, however, showed profuse dendrites and contacts never seen before: relay cell dendrites dye-coupled to one to seven pacemaker and one to seven relay cells. Moderate (0.1-10 nA) intracellular current injection had no effect on a neuron's spiking period, and only slightly modulated its spike amplitude, but could reset the spike phase. In contrast, massive hyperpolarizing current injections (15-25 nA) could force the cell to skip spikes. The relative timing of subthreshold and full spikes suggested that at least some pacemaker cells are likely to be intrinsic oscillators. The relative amplitudes of the subthreshold and full spikes gave a lower bound to the gap junctional coupling coefficient of 0.01-0.08. Three drugs, called gap junction blockers for their mode of action in other preparations, caused immediate and substantial reduction in frequency, altered the phase lag between pairs of neurons, and later caused the spike amplitude to drop, without altering the spike timing precision. Thus we conclude that the high precision of the normal Pn rhythm does not require maximal gap junction conductances between neurons that have ordinary cellular precision. Rather, the spiking precision can be explained as an intrinsic cellular property while the gap junctions act to frequency- and phase-lock the network oscillations.

Inverse expression of peroxisomes and connexin-43 in the granulosa cells of the quail follicle

Studying the regulation of peroxisome (Px) expression could improve our understanding of human peroxisomal disorders. The granulosa of the largest preovulatory quail follicles proved to be a relevant model because (a) Px expression changes according to the follicular maturation stage and (b) Px expression varies regionally according to the distance of the granulosa relative to the germinal disc region containing the female gamete (oocyte). The question was asked whether Px expression is related to the extent of metabolic cell coupling and whether zonal Px variation is causally related to oocytal factors. This was evaluated by the presence of catalase and Cx-43 (marker proteins for peroxisomes and gap junctions, respectively) and by in vitro experiments with granulosa explants. The data obtained show that the expression of Cx-43 and Px is inversely correlated both temporally and spatially. Uncoupling of gap junctions results in an upregulation of alpha-catalase immunofluorescence. This is in agreement with reports that gap junctions are often negatively affected by Px proliferators. The zonal gradient in Px expression appears to be imposed by the oocyte, as is the case for steroidogenesis and proliferative capacity in the granulosa epithelium. (J Histochem Cytochem 48:167-177, 2000)

Lipid-dependent activation of protein kinase C-alpha by normal alcohols

Significant stimulation of protein kinase C-alpha (PKCalpha) by n-alcohols was observed in characterized lipid systems composed of phosphatidylcholine/phosphatidylserine/dioleoylglycerol (PC/PS/DO). The logarithm of the alcohol concentrations to achieve half-maximal PKC stimulation (ED(50)) and of the maximal PKC stimulation by alcohols were both linear functions of alcohol chain length, consistent with the Meyer-Overton effect. Binding of phorbol esters to PKC was not significantly affected by octanol. Octanol increased, up to 4-fold, the affinity of PKC binding to the lipid bilayers in both the absence and presence of DO. However, octanol increased PKC activity much more significantly than it enhanced binding of the enzyme to the lipid bilayers, suggesting that the stimulation of PKC is not merely a reflection of the increase in PKC bilayer binding affinity. (31)P NMR experiments did not reveal formation of non-lamellar phases with octanol. Differential scanning calorimetry suggested that alcohols, like diacylglycerol, induce formation of compositionally distinct domains and the maximal enzyme activity with alcohol resided roughly in the putative domain-coexistence region. These results suggest that alcohols are mimicking diacylglycerol in activating PKC, not by binding to the high affinity phorbol ester binding site, but by altering lipid structure and by enhancing PKC-bilayer binding.

Activity and stability of chemically modified Candida antarctica lipase B adsorbed on solid supports

The effect of various covalent chemical modifications on the transesterification activity and stability of adsorbed lipase B from Candida antarctica (CALB) was studied in 2-butanone and o-xylene. CALB species modified with either polyethylene glycol 2000 monomethyl ether (MPEG), polyethylene glycol 300 mono-octyl ether (OPEG) or n-octanol (OCT) were used in combination with a hydrophobic (Accurel) and a hydrophilic (Duolite) support. The thermostabilities of adsorbed CALB in both solvents, and that of free CALB in o-xylene were not influenced by the modifications. In contrast, the thermostability of free CALB in 2-butanone decreased 2.5-fold after MPEG modification and increased 1.5-fold after modification with OPEG and n-octanol, compared to that of native CALB. The activities of the native and modified CALB species were up to 9-fold higher after adsorption onto Accurel than those of the corresponding free enzymes. Adsorption of these enzyme species onto Duolite only resulted in a 2- to 3-fold increase in the activity of OPEG- and OCT-modified CALB. The modified CALB species adsorbed onto Accurel show similar or up to 2-fold lower activities than do native adsorbed CALB species, while 1.5- to 6-fold higher activities were found for modified CALB species adsorbed onto Duolite. We propose that hydrophobic modifiers induce conformational changes of CALB during adsorption on a hydrophobic support whereas all three modifiers protect CALB from structural alterations during adsorption onto a hydrophilic support.

Modulation of glycinergic synaptic current kinetics by octanol in mouse hypoglossal motoneurons

Octanol-induced changes in the kinetics of glycinergic inhibitory postsynaptic currents (IPSCs) were investigated by whole-cell recording from hypoglossal motoneurons in mouse brainstem slices. Octanol (1 mM) prolonged the decay time constants (tau(decay)) of stimulus-evoked IPSCs (e-IPSCs) by 202+/-67% (SE). The depression of e-IPSC amplitudes was dose-dependent with an EC50 of 475 microM. Octanol also reduced the amplitude and prolonged the decay time constant of glycinergic currents evoked by local pressure ejection of glycine (I(gly)). Replacement of extracellular Na+ by choline and application of the specific glycine transporter GLYT1 inhibitor, sarcosine, lengthened tau(decay) of I(gly), but did not change the decay time constants of e-IPSCs. Intracellular acidification by the weak organic acid salt sodium propionate (30 mM) reduced the e-IPSC amplitude by 22+/-9% and prolonged tau by 18+/-6%. Sodium propionate also prolonged the decay time constants of I(gly) by 28+/-11%. The observed effects on decay kinetics were much smaller than those caused by octanol. The data show that octanol prolongs the decay time course of glycinergic synaptic currents by mechanisms independent of glycine uptake or intracellular acidification. We conclude that the effects were most probably due to direct action on postsynaptic glycine receptors.

Synthesis and properties of 3-(2-hydroxyethyl)-3-n-pentyldiazirine, a photoactivable general anesthetic

To overcome the difficulties of locating the molecular sites of general anesthetic action, we synthesized a novel photoactivable general anesthetic, 3-(2-hydroxyethyl)-3-n-pentyldiazirine (3-diazirinyloctanol), which anesthetized tadpoles with an ED(50) of 160 microM. Subanesthetic concentrations of 3-diazirinyloctanol enhanced GABA-induced currents in GABA(A) receptors, an effect that has been implicated in general anesthetic action. It also enhanced [(3)H]muscimol binding to this receptor. In muscle nicotinic acetylcholine receptors (nAcChoR), it inhibited the response to acetylcholine with an IC(50) of 33 microM. 3-Diazirinyloctanol's pharmacological actions were comparable to those of octanol. 3-(2-Hydroxyethyl)-3-[4,5-(3)H(2)]-n-pentyldiazirine photoincorporated into Torpedo nAcChoR-rich membranes mainly in the alpha subunit with 70% being in a proteolytic fragment containing the M4 transmembrane segment. Agonist enhanced the photolabeling 10-fold in a fragment containing the M1, M2, and M3 transmembrane segments. Thus, 3-diazirinyloctanol is a novel general anesthetic that acts on, and can be photoincorporated into, postsynaptic receptors.

Millisecond analyses of Ca2+ initiation sites evoked by muscarinic receptor stimulation in exocrine acinar cells

High speed laser confocal microscopy (8 ms/image) was applied to the dissociated parotid acini as a model to study Ca2+ signaling mechanisms in non-excitable exocrine secretory cells. Immunofluorescence microscopy showed the localization of IP3 receptor type 2 along the apical membrane region. Muscarinic stimulation with carbachol evoked a rise in [Ca2+]i that was initiated from apical region and propagated into basal region as Ca2+ waves. This was most clearly observed when extracellular Ca2+ was omitted. Carbachol also triggered the abrupt increase of [Ca2+]i simultaneously at both basal and apical regions in many acini. Within an acinus, each cell responded synchronously. The present results suggest that one Ca2+ initiation site in the rat parotid acinar cell is apical region, corresponding to the localization of IP3 receptors. Another Ca2+ initiation site is basal region, which seems to be related to Ca2+ entry from extracellular medium and/or Ca2+ release from basally located organelles such as nuclei and endoplasmic reticulum.

Properties of T-type calcium current in enkephalinergic neurones in guinea-pig hypothalamic slices

The guinea-pig hypothalamic magnocellular dorsal nucleus (mdn) exclusively contains enkephalinergic neurones providing inputs to the septum. This nucleus is believed to play a role in hippocampo-septo-hypothalamic relationships. mdn neurones display prominent low-threshold Ca2+ spikes, which differ in their propensity to trigger either a burst of Na+ spikes or a single spike. In the present study, whole-cell voltage-clamp experiments were carried out on thick slices at 34 degrees C to characterize the pharmacological and physical properties of the transient Ca2+ current (IT) underlying the low-threshold spikes. Recorded cells were dye-labelled and identified as belonging to the mdn. In bursting and non-bursting neurones, IT was reduced by amiloride (1 microM) and octanol (1 mM), and during replacement of Ca2+ by Ba2+. The Ca2+ channel blocker mibefradil (10 microM) had only a slight blocking action. Nifedipine (100 microM) and flunarizine (1 microM) had no effect. IT activated between -80 mV and -50 mV and the mean peak current was 1050 pA. Steady-state activation and inactivation curves were fitted by a Boltzmann equation. The half-activation voltage was -70 mV, slope factor=3.6, and half-inactivation voltage was about -80 mV, slope factor=4.5. Time-to-peak and time constant of inactivation were voltage dependent. Recovery from activation occurred within 500 ms. When compared with results on other IT, the present data show that the current possesses distinct pharmacological and physical properties. Nevertheless, all investigated cells displayed a homogenous profile of IT, suggesting that the differences in spike pattern between mdn neurones are not due to different populations of Ca2+ channels.

Octanol blocks fluid secretion by inhibition of capacitative calcium entry in rat mandibular salivary acinar cells

The aliphatic alcohol octanol is thought to modulate enzyme secretion from the exocrine pancreas by the inhibition of gap junction permeability. We have now investigated the effects of octanol on salivary secretion and intracellular calcium concentration ([Ca2+]i), measured in isolated perfused rat mandibular glands and in isolated mandibular acinar cells respectively. Stimulation of perfused glands with 10 microM carbachol (CCh) evoked a rapid increase in fluid secretion followed by a decrease to a sustained elevated level. Application of 1 mM octanol during CCh stimulation inhibited fluid secretion reversibly. In isolated acini, the CCh-induced [Ca2+]i increase was reversibly inhibited by the same concentration of octanol. However, octanol also inhibited the increase in [Ca2+]i in single acinar cells where gap junctions were no longer functional, indicating that octanol directly affected the intracellular Ca2+ signalling pathway. The initial increase in [Ca2+]i induced by 0.5-10 microM CCh, which is due to Ca2+ release from IP3-sensitive Ca2+ stores, was not affected by pretreatment with octanol. In contrast, CCh-, phenylephrine- or thapsigargin-induced Ca2+ entry was almost completely and reversibly inhibited by octanol. Octanol also blocked agonist-evoked Ca2+ entry in pancreatic acinar cells, and thapsigargin-evoked Ca2+ entry in fibroblasts. These data strongly suggest that octanol blocks salivary secretion from mandibular gland by the inhibition of capacitative Ca2+ entry, and raise the possibility that octanol may be a useful tool for inhibiting agonist-evoked Ca2+ entry pathways.

The effects of octanol on the late photointermediates of rhodopsin

Membrane suspensions of unperturbed rhodopsin and rhodopsin perturbed with 2.5 mM octanol were photolyzed with 477 nm laser pulses at 20 degrees C and 35 degrees C. Changes in absorbance were monitored at times ranging from 1 microsecond to 80 ms after excitation. The data were analyzed using singular value decomposition, global exponential fitting and kinetic modeling. A recently proposed model involving the photointermediate Meta-I380 (T. E. Thorgeirsson, J. W. Lewis, S. E. Wallace-Williams, and D. S. Kliger, Biochemistry 32, 13861-13872, 1993) fits data for samples with and without octanol. Comparison of the microscopic rates shows this alcohol accelerates the formation of Meta-II via Meta-I380. Activation and equilibrium thermodynamic parameters obtained from Arrhenius plots suggest that octanol reduces the entropy increase in forming both Meta-I380 and Meta-II. It also lowers the enthalpy of Meta-I380 relative to Lumi and of Meta-II relative to Meta-I480. To help determine whether octanol affects the protein directly or indirectly through the lipid bilayer, similar experiments were conducted using rhodopsin solubilized in 0.13% dodecyl maltoside with and without octanol. Spectral shifts in the presence of octanol suggest that a direct protein interaction exists in addition to previously reported effects dependent on membrane free volume.

Modulation of agonist and antagonist interactions in serotonin 1A receptors by alcohols

The serotonin type IA (5-HT1A) receptors are members of a superfamily of seven transmembrane domain receptors that couple to GTP binding regulatory proteins (G-proteins). Serotonergic signalling has been shown to play an important role in alcohol tolerance and dependence. We have studied the effects of alcohols on ligand (agonist and antagonist) binding to bovine hippocampal 5-HT1A receptor in native as well as solubilized membranes. Our results show that alcohols inhibit the specific binding of the agonist OH-DPAT and the antagonist p-MPPF to 5-HT1A receptors in a concentration-dependent manner.

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.

Similar propagation of SD and hypoxic SD-like depolarization in rat hippocampus recorded optically and electrically

Neuron membrane changes and ion redistribution during normoxic spreading depression (SD) induced, for example, by potassium injection, closely resemble those that occur during hypoxic SD-like depolarization (HSD) induced by oxygen withdrawal, but the degree to which the two phenomena are related is controversial. We used extracellular electrical recording and imaging of intrinsic optical signals in hippocampal tissue slices to compare 1) initiation and spread of these two phenomena and 2) the effects of putative gap junction blocking agents, heptanol and octanol. Both events arose focally, after which a clear advancing wave front of increased reflectance and DC shift spread along the CA1 stratum radiatum and s. oriens. The rate of spread was similar: conduction velocity of normoxic SD was 8.73 +/- 0.92 mm/min (mean +/- SE) measured electrically and 5.84 +/- 0.63 mm/min measured optically, whereas HSD showed values of 7.22 +/- 1.60 mm/min (electrical) and 6.79 +/- 0.42 mm/min (optical). When initiated in CA1, normoxic SD consistently failed to enter the CA3 region (7/7 slices) and could not be initiated by direct KC1 injection in the CA3 region (n = 3). Likewise, the hypoxic SD-like optical signal showed onset in the CA1 region and halted at the CA1/CA3 boundary (9/9 slices), but in some (4/9) slices the dentate gyrus region showed a separate onset of signal changes. Microinjection into CA1 stratum radiatum of octanol (1 mM), which when bath applied arrests the spread of normoxic SD, created a small focus that appeared to be protected from hypoxic depolarization. However, bath application of heptanol (3 mM) or octanol (2 mM) did not prevent the spread of HSD, although the onset was delayed. This suggests that, although gap junctions may be essential for the spread of normoxic SD, they may play a less important role in the spread of HSD.

Functional gap junctions in the schwann cell myelin sheath

The Schwann cell myelin sheath is a multilamellar structure with distinct structural domains in which different proteins are localized. Intracellular dye injection and video microscopy were used to show that functional gap junctions are present within the myelin sheath that allow small molecules to diffuse between the adaxonal and perinuclear Schwann cell cytoplasm. Gap junctions are localized to periodic interruptions in the compact myelin called Schmidt-Lanterman incisures and to paranodes; these regions contain at least one gap junction protein, connexin32 (Cx32). The radial diffusion of low molecular weight dyes across the myelin sheath was not interrupted in myelinating Schwann cells from cx32-null mice, indicating that other connexins participate in forming gap junctions in these cells. Owing to the unique geometry of myelinating Schwann cells, a gap junction-mediated radial pathway may be essential for rapid diffusion between the adaxonal and perinuclear cytoplasm, since this radial pathway is approximately one million times faster than the circumferential pathway.

Intercellular bridges between epithelial cells in the Drosophila ovarian follicle: a possible aid to localized signaling

In the epithelium of Drosophila ovarian follicles, cytoplasm-filled intercellular bridges connect epithelial cells. This study presents further descriptive information about the morphology of these intercellular bridges and the extent of their distribution. We also offer speculations concerning the possible developmental importance of the epithelial bridges. These bridges, whose luminal diameters averaged 0.25 microm, are smaller than those forming the ring canals joining germline cells: nor do they increase their size over time. The membranes limiting the bridges are lined on the cytoplasmic side with an electron-dense material to which is attached a monolayer of filaments which encircle the bridge. By decoration with the S1 fragment of myosin, these filaments are confirmed as actin filaments. Following disruption of gap junctional dye coupling by treatment with 1 mM octanol, microinjection of Lucifer yellow CH revealed the extent and distribution of follicle cell intercellular bridges to be confined to arrays of no more than eight cells/cluster, with many such independent clusters comprising the epithelium. Thus cell-to-cell movement throughout the epithelium of cytosolic regulatory molecules cannot occur via these intercellular bridges. However, weak signals affecting only one or a few cells in each cluster would be amplified throughout the group by spread through the intercellular bridges.

Astrocytic gap junction blockage and neuronal Ca2+ oscillation in neuron-astrocyte cocultures in vitro

We have investigated the effects of gap junction inhibitors, octanol, halothane, sodium propionate and lindane, on neuronal periodic Ca2+ transients in neuron-astrocyte coculture systems. Octanol reduced the amplitude and frequency of Ca2+ oscillations in dose-dependent manner. One mM octanol caused a complete disappearance of Ca2+ oscillations. Similar suppressions were obtained by halothane (1 mM) and sodium propionate (25 mM). In contrast, lindane (300 nM) uniquely raised the basal level of [Ca2+], in oscillating neurons as well as the height of apparent amplitude without changes in the frequency. The current results imply that octanol, halothane and sodium propionate might lower the frequency of spontaneous Ca2+ oscillations by blocking the gap junctional communication of neighboring astrocytes and that lindane, though also blocking the gap junctions, might not affect the frequency but reversely increase both the basal [Ca2+]i and the amplitude, probably due to an increase of neuronal [Ins (1.4.5)P3]i. These findings strongly suggest that astrocytes contribute to the generation of periodic neuronal Ca2+ oscillations through astrocytic gap junctional communications and/or other signaling components between astrocytes and neurons.

Efflux pumps involved in toluene tolerance in Pseudomonas putida DOT-T1E

The basic mechanisms underlying solvent tolerance in Pseudomonas putida DOT-T1E are efflux pumps that remove the solvent from bacterial cell membranes. The solvent-tolerant P. putida DOT-T1E grows in the presence of high concentrations (e.g., 1% [vol/vol]) of toluene and octanol. Growth of P. putida DOT-T1E cells in LB in the presence of toluene supplied via the gas phase has a clear effect on cell survival: the sudden addition of 0.3% (vol/vol) toluene to P. putida DOT-T1E pregrown with toluene in the gas phase resulted in survival of almost 100% of the initial cell number, whereas only 0.01% of cells pregrown in the absence of toluene tolerated exposure to this aromatic hydrocarbon. One class of toluene-sensitive octanol-tolerant mutant was isolated after Tn5-'phoA mutagenesis of wild-type P. putida DOT-T1E cells. The mutant, called P. putida DOT-T1E-18, was extremely sensitive to 0.3% (vol/vol) toluene added when cells were pregrown in the absence of toluene, whereas pregrowth on toluene supplied via the gas phase resulted in survival of about 0.0001% of the initial number. Solvent exclusion was tested with 1,2,4-[14C]trichlorobenzene. The levels of radiochemical accumulated in wild-type cells grown in the absence and in the presence of toluene were not significantly different. In contrast, the mutant was unable to remove 1,2,4-[14C]trichlorobenzene from the cell membranes when grown on Luria-Bertani (LB) medium but was able to remove the aromatic compound when pregrown on LB medium with toluene supplied via the gas phase. The amount of 14C-labeled substrate in whole cells increased in competition assays in which toluene-and xylenes were the unlabeled competitors, whereas this was not the case when benzene was the competitor. This finding suggests that the exclusion system works specifically with certain aromatic substrates. The mutation in P. putida DOT-T1E-18 was cloned, and the knockedout gene was sequenced and found to be homologous to the drug exclusion gene mexB, which belongs to the efflux pump family of the resistant nodulator division type.

Electrophysiological properties of frog olfactory supporting cells

Cells, identified as supporting cells by Lucifer Yellow injection, were recorded from slices of frog olfactory epithelium using patch-clamp recordings. Cell-attached single-channel recordings indicated that the intracellular potential (IP) was -68 +/- 7 mV (n = 22) with 4 mM K+ in the bath ([K+]o). IP was -67 +/- 4 mV (n = 32) in whole-cell conditions with 100 mM KCl inside the cell, suggesting a low membrane permeability for Cl-. IP depended on [K+]o in a manner described by the Goldman-Hodgkin-Katz equation with a permeability ratio pk+:PNa+ of 40. The input resistance was 32 +/- 14 M omega (n = 15), indicating a high membrane conductance at rest. Odorant stimulations evoked passive membrane depolarizations, probably reflecting an increase in [K+]o due to the neuronal activation. Whole-cell recordings with 100 mM CsCl instead of KCl in the pipette, together with the block of gap-junctions with octanol, indicated the existence of an electrical coupling between supporting cells. The electrical coupling between these glial-like cells could facilitate the clearance of K+ ions released by olfactory receptor neurons during odorant stimulation.

Gap junctional communication coordinates vasopressin-induced glycogenolysis in rat hepatocytes

Because hepatocytes communicate via gap junctions, it has been proposed that Ca2+ waves propagate through this pathway and in the process activate Ca2+-dependent cellular responses. We testedthis hypothesis by measuring vasopressin-induced glycogenolysis in short-term cultures of rat hepatocytes. A 15-min vasopressin (10(-8) M) stimulation induced a reduction of glycogen content that reached a maximum 1-3 h later. Gap junction blockers, octanol or 18alpha-glycyrrhetinic acid, reduced the effect by 70%. The glycogenolytic response induced by Ca2+ ionophore 8-bromo-A-21387, which acts on each hepatocyte, was not affected by gap junction blockers. Moreover, the vasopressin-induced glycogenolysis was lower (70%) in dispersed than in reaggregated hepatocytes and in dispersed hepatocytes was not affected by gap junction blockers. In hepatocytes reaggregated in the presence of a synthetic peptide homologous to a domain of the extracellular loop 1 of the main hepatocyte gap junctional protein, vasopressin-induced glycogenolysis and incidence of dye coupling were drastically reduced. Moreover, gap junctional communication was detected between reaggregated cells, suggesting that hepatocytes with different vasopressin receptor densities become coupled to each other. The vasopressin-induced effect was not affected by suramin, ruling out ATP as a paracrine mediator. We propose that gap junctions allow for a coordinated vasopressin-induced glycogenolytic response despite the heterogeneity among hepatocytes.