General anesthetics attenuate gap junction coupling in P19 cell line

Effects of general anesthetics on synaptic transmission and plasticity

General anesthetics depress excitatory and/or enhance inhibitory synaptic transmission principally by modulating the function of glutamatergic or GABAergic synapses, respectively, with relative anesthetic agent-specific mechanisms. Synaptic signaling proteins, including ligand- and voltage-gated ion channels, are targeted by general anesthetics to modulate various synaptic mechanisms, including presynaptic neurotransmitter release, postsynaptic receptor signaling, and dendritic spine dynamics to produce their characteristic acute neurophysiological effects. As synaptic structure and plasticity mediate higher-order functions such as learning and memory, long-term synaptic dysfunction following anesthesia may lead to undesirable neurocognitive consequences depending on the specific anesthetic agent and the vulnerability of the population. Here we review the cellular and molecular mechanisms of transient and persistent general anesthetic alterations of synaptic transmission and plasticity.

Neocortical in vivo focal and spreading potassium responses and the influence of astrocytic gap junctional coupling

Raised extracellular potassium ion (K⁺) concentration is associated with several disorders including migraine, stroke, neurotrauma and epilepsy. K⁺ spatial buffering is a well-known mechanism for extracellular K⁺ regulation/distribution. Astrocytic gap junction-mediated buffering is a controversial candidate for K⁺ spatial buffering. To further investigate the existence of a K⁺ spatial buffering and to assess the involvement of astrocytic gap junctional coupling in K⁺ redistribution, we hypothesized that neocortical K⁺ and concomitant spreading depolarization (SD)-like responses are controlled by powerful local K⁺ buffering mechanisms and that K⁺ buffering/redistribution occurs partially through gap junctional coupling. Herein, we show, in vivo, that a threshold amount of focally applied KCl is required to trigger local and/or distal K⁺ responses, accompanied by a SD-like response. This observation indicates the presence of powerful local K⁺ buffering which mediates a rapid return of extracellular K⁺ to the baseline. Application of gap junctional blockers, carbenoxolone and Gap27, partially modulated the amplitude and shape of the K⁺ response and noticeably decreased the velocity of the spreading K⁺ and SD-like responses. Opening of gap junctions by trimethylamine, slightly decreased the amplitude of the K⁺ response and markedly increased the velocity of redistribution of K⁺ and SD-like events. We conclude that spreading K⁺ responses reflect powerful local K⁺ buffering mechanisms which are partially modulated by gap junctional communication. Gap junctional coupling mainly affected the velocity of the K⁺ and SD-like responses.

Antiarrhythmic effect of sevoflurane as an additive to HTK solution on reperfusion arrhythmias induced by hypothermia and ischaemia is associated with the phosphorylation of connexin 43 at serine 368

Background

Reperfusion ventricular arrhythmia (RA) associated with hypothermic ischaemic storage is increasingly recognized as a substantial contributor to adverse consequences after heart transplantation. Ischemia- or hypothermia-induced gap junction (GJ) remodelling is closely linked to RA. Reducing GJ remodelling contributes to RA attenuation and is important in heart transplantation. However, sevoflurane has an antiarrhythmic effect associated with the connexin 43 (Cx43) protein that has not yet been fully established.

Methods

Hearts were divided into two groups according to a random number table: all hearts were arrested by an infusion of histidine-tryptophan-ketoglutarate (HTK) solution (4 °C) followed by (1) storage in HTK solution (4 °C) alone for 6 h (n = 8, Control group) or (2) storage in HTK solution supplemented with sevoflurane (2.5%) (4 °C) for 6 h (n = 8, Sevo-HTK group). First, the total Cx43 level and the phosphorylation of Cx43 at Ser368 (Cx43-pS368) were assessed by Western blotting, and the distribution of Cx43 was assessed by immunohistochemistry. Second, programmed electrical stimulation (PES) and monophasic action potential (MAP) recording were used to analyse the MAP duration (MAPD), conduction velocity (CV) and transmural repolarization dispersion (TDR). In addition, haematoxylin and eosin (HE) and terminal deoxynucleotidyl transferase-dUTP nick end labelling (TUNEL) staining were individually used to investigate the degree of myocardial pathological damage and cell apoptosis. Finally, bipolar electrograms were used to record the graft re-beating time and monitor RA during reperfusion for 15 to 30 min.

Results

Sevo-HTK solution relatively increased the total Cx43 (P < 0.01) and Cx43-pS368 (P < 0.01) levels and prevented Cx43 redistribution (P < 0.05) and CV slowing (P < 0.001) but did not change TDR (P > 0.05). Additionally, the Cx43-pS368/total Cx43 ratio (P>0.05) was similar in the two groups. However, with Sevo-HTK solution, the graft re-beating times were shortened, myocardial pathological damage was ameliorated, and the number of apoptotic cells was markedly decreased.

Conclusion

The reduction in hypothermia and ischaemia-induced reperfusion arrhythmias by the addition of sevoflurane to HTK solution may be related to the phosphorylation of Cx43 at serine 368.

Propofol depresses cisplatin cytotoxicity via the inhibition of gap junctions

The general anesthetic, propofol, affects chemotherapeutic activity, however, the mechanism underlying its effects remains to be fully elucidated. Our previous study showed that tramadol and flurbiprofen depressed the cytotoxicity of cisplatin via the inhibition of gap junction (GJ) intercellular communication (GJIC) in connexin (Cx)32 HeLa cells. The present study investigated whether the effects of propofol on the cytotoxicity of cisplatin were mediated by GJ in U87 glioma cells and Cx26‑transfected HeLa cells. Standard colony formation assay was used to determine the cytotoxicity of cisplatin. Parachute dye coupling assay was used to measure GJ function, and western blot analysis was used to determine the expression levels of Cx32. The results revealed that exposure of the U87 glioma cells and the Cx26-transfected HeLa cells to cisplatin for 1 h reduced clonogenic survival in low density cultures (without GJs) and high density cultures (with GJs). However, the toxic effect was higher in the high density culture. In addition, pretreatment of the cells with propofol significantly reduced cisplatin‑induced cytotoxicity, but only in the presence of functional GJs. Furthermore, propofol significantly inhibited dye coupling through junctional channels, and a long duration of exposure of the cells to propofol downregulated the expression levels of Cx43 and Cx26. These results demonstrated that the inhibition of GJIC by propofol affected the therapeutic efficacy of chemotherapeutic drugs. The present study provides evidence of a novel mechanism underlying the effects of analgesics in counteracting chemotherapeutic efficiency.

The gap junction blocker carbenoxolone enhances propofol and sevoflurane-induced loss of consciousness

General anesthetics induce loss of consciousness by inhibiting ascending arousal pathways, and they interfere with gap junction electrical coupling. The present study aimed to determine whether inhibition of gap junction-mediated signaling could influence general anesthetic-induced loss of consciousness. The general anesthetics sevoflurane and propofol were used. Intracerebroventricular administration of carbenoxolone, a gap junction blocker, significantly decreased the time to loss of the righting reflex (P < 0.05), but prolonged the time to recovery of the reflex (P < 0.05). Moreover, intracerebroventricular administration of carbenoxolone increased the sensitivity to sevoflurane, with a leftward shift of the loss of righting reflex dose-response curve, and decreased the 50% effective concentration of sevoflurane. These results suggest that the gap junction blocker carbenoxolone enhances propofol and sevoflurane-mediated general anesthesia.

Fast-spiking Cell to Pyramidal Cell Connections Are the Most Sensitive to Propofol-induced Facilitation of GABAergic Currents in Rat Insular Cortex

Background:

Propofol facilitates γ-aminobutyric acid–mediated inhibitory synaptic transmission. In the cerebral cortex, γ-aminobutyric acidergic interneurons target both excitatory pyramidal cells (Pyr) and fast-spiking (FS) and non-FS interneurons. Therefore, the propofol-induced facilitation of inhibitory transmission results in a change in the balance of excitatory and inhibitory inputs to Pyr. However, it is still unknown how propofol modulates γ-aminobutyric acidergic synaptic transmission in each combination of Pyr and interneurons.

Methods:

The authors examined whether propofol differentially regulates inhibitory postsynaptic currents (IPSCs) depending on the presynaptic and postsynaptic cell subtypes using multiple whole cell patch clamp recording from γ-aminobutyric acidergic interneurons and Pyr in rat insular cortex.

Results:

Propofol (10 μM) consistently prolonged decay kinetics of unitary IPSCs (uIPSCs) in all types of inhibitory connections without changing paired-pulse ratio of the second to first uIPSC amplitude or failure rate. The FS→Pyr connections exhibited greater enhancement of uIPSC charge transfer (2.2 ± 0.5 pC, n = 36) compared with that of FS→FS/non-FS connections (0.9 ± 0.2 pC, n = 37), whereas the enhancement of charge transfer in non-FS→Pyr (0.3 ± 0.1 pC, n = 15) and non-FS→FS/non-FS connections (0.2 ± 0.1 pC, n = 36) was smaller to those in FS→Pyr/FS/non-FS. Electrical synapses between FS pairs were not affected by propofol.

Conclusions:

The principal inhibitory connections (FS→Pyr) are the most sensitive to propofol-induced facilitation of uIPSCs, which is likely mediated by postsynaptic mechanisms. This preferential uIPSC enhancement in FS→Pyr connections may result in suppressed neural activities of projection neurons, which in turn reduces excitatory outputs from cortical local circuits.

Propofol inhibits gap junctions by attenuating sevoflurane-induced cytotoxicity against rat liver cells in vitro

BACKGROUND

Liver abnormalities are seen in a small proportion of patients following anaesthesia with sevoflurane.

OBJECTIVES

To investigate whether the cytotoxicity of sevoflurane against rat liver cells was mediated by gap junction intercellular communications, and the effect of propofol on sevoflurane-induced cytotoxicity.

DESIGN

Experimental study.

SETTING

The study was carried out in the central laboratory of The Third Affiliated Hospital, Sun Yat-sen University.

CELL LINE

BRL-3A rat liver cells.

METHODS

Immortal rat liver cells BRL-3A were grown at low and high density. Colony-forming assays were performed to determine clonogenic growth of these cells. To investigate the effect of oleamide and propofol on gap junction function, we measured fluorescence transmission between cells using parachute dye-coupling assays. Immunoblotting assays were performed to determine connexin32 and connexin43 expression.

RESULTS

Our colony formation assays revealed that, in low-density culture, sevoflurane caused no apparent inhibition of clonogenic growth of BRL-3A cells. In high-density culture, 2.2 to 4.4% sevoflurane markedly inhibited clonogenic growth of BRL-3A cells with 67.6 (0.34)% and 61.2 (0.17)% of the cells being viable, respectively (P = 0.003 vs. low-density culture), suggesting cell density dependency of sevoflurane-induced cytotoxicity. Our colony formation assays revealed that propofol markedly attenuated the suppression by sevoflurane of the clonogenic growth of BRL-3A cells (viability: propofol and sevoflurane, 91.5 (0.014)% vs. sevoflurane, 56.6 (0.019)%; P <0.01). Blocking gap junctions with 10 μmol l oleamide significantly attenuated 4.4% sevoflurane-induced suppression with a viability of 83.6 ± 0.138% (oleamide and sevoflurane vs. sevoflurane, P < 0.01). Immunoblotting assays further showed that propofol (3.2 μg ml) markedly reduced CX32 levels and significantly inhibited gap junctional intercellular communications as revealed by parachute dye-coupling assays. Values are mean (SD).

CONCLUSION

This study provides the first direct evidence that sevoflurane-induced cytotoxicity, which is mediated through gap junctions, is attenuated by propofol, possibly by its action on Cx32 homomeric or heteromeric complexes.

Gap junctions facilitate propagation of synchronous firing in the cortical neural population: a numerical simulation study

This study investigates the effect of gap junctions on firing propagation in a feedforward neural network by a numerical simulation with biologically plausible parameters. Gap junctions are electrical couplings between two cells connected by a binding protein, connexin. Recent electrophysiological studies have reported that a large number of inhibitory neurons in the mammalian cortex are mutually connected by gap junctions, and synchronization of gap junctions, spread over several hundred microns, suggests that these have a strong effect on the dynamics of the cortical network. However, the effect of gap junctions on firing propagation in cortical circuits has not been examined systematically. In this study, we perform numerical simulations using biologically plausible parameters to clarify this effect on population firing in a feedforward neural network. The results suggest that gap junctions switch the temporally uniform firing in a layer to temporally clustered firing in subsequent layers, resulting in an enhancement in the propagation of population firing in the feedforward network. Because gap junctions are often modulated in physiological conditions, we speculate that gap junctions could be related to a gating function of population firing in the brain.

The role of connexin36 gap junctions in modulating the hypnotic effects of isoflurane and propofol in mice

Gap junction blockade is a possible mechanism by which general anaesthetic drugs cause unconsciousness. We measured the sensitivity of connexin36 knockout mice to the hypnotic effects of isoflurane and propofol. The experimental endpoint was recovery of the righting reflex of the anaesthetised animals during 0.2% step-reductions in isoflurane concentration, or following intraperitoneal injection of propofol (100 mg.kg(-1) ). Connexin36 knockout animals were more sensitive to the hypnotic effects of isoflurane than 'normal' wild-type animals. The half maximal effective concentration (EC50) for recovery of righting reflex was 0.37% for connexin36 knockout vs 0.49% for wild-type animals (p < 0.001). For propofol, connnexin36 knockout animals showed more rapid loss of righting reflex than wild-type animals (mean (SD) 2.8 (0.13) vs 3.8 (0.27) min); and young (< 60 days) connexin36 knockout animals remained anaesthetised for longer than young wild-type mice (47.2 (2.9) vs 30.5 (1.7) min; p < 0.00001). These findings suggest that the hypnotic effects of anaesthetic drugs may be moderately enhanced by gap junction blockade.

Propofol depresses the cytotoxicity of X-ray irradiation through inhibition of gap junctions

BACKGROUND

General anesthetics (e.g., propofol) influence the therapeutic activity of intraoperative radiotherapy but the mechanism of the effects is largely unknown. It has been reported that propofol inhibits gap junction (GJ) function briefly, and a functional GJ enhances the efficacy of radiotherapy in some cancer cells. Yet the mechanisms underlying the inhibition of GJ function by propofol and the influence of propofol on therapeutic activity of intraoperative radiotherapy are unknown.

METHODS

The role of propofol at clinically relevant concentrations in the modulation of radiograph-induced cytotoxicity in HeLa cells transfected with connexin 32 (Cx32) plasmid was explored by manipulation of connexin expression, GJ presence, and function. GJ function, Cx32 protein level, and Cx32 mRNA expression were determined by "Parachute" dye-coupling assay, Western blotting, and reverse transcriptase-polymerase chain reaction, respectively.

RESULTS

Propofol significantly reduced radiograph-induced cytotoxicity only in the presence of functional GJ. Four-hour propofol exposure inhibited GJ function mainly by diminution of Cx32 protein levels but without influence on Cx32 mRNA expression.

CONCLUSIONS

These results suggest that propofol inhibits the function of the GJ through the reduction of Cx32 protein levels by a transcription-independent mechanism. They further indicate that propofol depresses the cytotoxicity of radiograph irradiation through inhibition of GJ activity.

Rhythmic episodes of subthreshold membrane potential oscillations in the rat inferior olive nuclei in vivo

In vitro studies of inferior olive neurons demonstrate that they are intrinsically active, generating periodic spatiotemporal patterns. These self-generated patterns of activity extend the role of olivary neurons beyond that of a deliverer of teaching or error signals. However, autorhythmicity or patterned activity of complex spikes in the cerebellar cortex was observed in only a few studies. This discrepancy between the self-generated rhythmicity in the inferior olive observed in vitro and the sporadic reports on rhythmicity of complex spikes can be reconciled by recording intracellularly from inferior olive neurons in situ. To this end, we recorded intracellularly from olivary neurons of anesthetized rats. We demonstrate that, in vivo, olivary neurons show both slow and fast rhythmic processes. The slow process (0.2-2 Hz) is expressed as rhythmic transitions from quiescent periods to periods of fast rhythm, manifested as subthreshold oscillations of 6-12 Hz. Spikes, if they occur, are locked to the depolarized phase of these subthreshold oscillations and, therefore, hold and transfer rhythmic information. The transient nature of these oscillatory epochs accounts for the difficulties to uncover them by prolonged recordings of complex spikes activity in the cerebellar cortex.

A possible mechanism of halocarbon-induced cardiac sensitization arrhythmias

Cardiac sensitization is the term used for malignant ventricular arrhythmias associated with exposure to inhaled halocarbons in the presence of catecholamines. We investigated the electrophysiological changes associated with cardiomyocyte exposure to epinephrine and a halocarbon known to be associated with cardiac sensitization (halon 1301, CF3Br). Cardiomyocytes (CMs) were isolated from neonatal rats and grown on multielectrode arrays (MEAs). Upon exposure to epinephrine, the CM inter-spike interval (ISI) was decreased 14% at 10 microg/L (P<0.05) and 27% at 100 microg/L (P<0.05) as compared to baseline. Halon alone (50 mg/L) mildly prolonged the field potential (FP) duration (7%). CMs exposed to combinations of epinephrine (100 microg/L) and halon (50 mg/L) for 15 min showed a blunted increase in the ISI (35+/-12%) and a 38% decrease in conduction velocity (P<0.05) when compared to epinephrine alone. There was no change in field potential properties, but dephosphorylated connexin 43 (Cx43) was increased 60+/-16% with the combination as compared to epinephrine alone (P<0.05). Treatment with okadaic acid, a phosphatase inhibitor, prevented the Cx43 dephosphorylation and the reduction in conduction velocity upon exposure to halon and epinephrine. Moreover, the electrophysiological changes induced by epinephrine and halon were indistinguishable from those seen with the gap junction inhibitor heptanol. In conclusion, the combination of a halocarbon and epinephrine results in a unique electrophysiological signature including slow conduction that may explain, in part, the basis for cardiac sensitization. The slowing of conduction is most likely related to changes in the phosphorylation state of Cx43.

Flow cytometry analysis of gap junction-mediated cell-cell communication: advantages and pitfalls

BACKGROUND

Since the first morphological description of the gap junctions use electron microscopy, a considerable number of techniques has been introduced to evaluate gap junction channel functionality, many of which use dye transfer techniques, such as dye injection and fluorescent dye transfer, analyzed by flow cytometry.

METHODS

To analyze dye transfer, generally one population of cells is incubated with calcein-AM (0.5 microM) for 30 min at 37 degrees C, and the other population was incubated with the lipophilic dye DiIC(18) (3) (10 microM) for 1 h at 37 degrees C; after incubation, these cells were washed five times with PBS and cocultured for different times, and then the dye transfer was analyzed by flow cytometry.

RESULTS

In this short overview, we focus on some advantages and disadvantages of flow cytometry as a technique to investigate gap junction-mediated intercellular communication (GJIC). In addition, we point out some technical pitfalls that we have encountered when applying this technique to study gap junctions in immune system cells.

CONCLUSIONS

Analysis of fluorescent dye transfer by flow cytometry is a useful tool to investigate GJIC. However, some points must be taken into consideration before using this methodology, which are discussed herein.

Memory in astrocytes: a hypothesis

Background

Recent work has indicated an increasingly complex role for astrocytes in the central nervous system. Astrocytes are now known to exchange information with neurons at synaptic junctions and to alter the information processing capabilities of the neurons. As an extension of this trend a hypothesis was proposed that astrocytes function to store information. To explore this idea the ion channels in biological membranes were compared to models known as cellular automata. These comparisons were made to test the hypothesis that ion channels in the membranes of astrocytes form a dynamic information storage device.

Results

Two dimensional cellular automata were found to behave similarly to ion channels in a membrane when they function at the boundary between order and chaos. The length of time information is stored in this class of cellular automata is exponentially related to the number of units. Therefore the length of time biological ion channels store information was plotted versus the estimated number of ion channels in the tissue. This analysis indicates that there is an exponential relationship between memory and the number of ion channels. Extrapolation of this relationship to the estimated number of ion channels in the astrocytes of a human brain indicates that memory can be stored in this system for an entire life span. Interestingly, this information is not affixed to any physical structure, but is stored as an organization of the activity of the ion channels. Further analysis of two dimensional cellular automata also demonstrates that these systems have both associative and temporal memory capabilities.

Conclusion

It is concluded that astrocytes may serve as a dynamic information sink for neurons. The memory in the astrocytes is stored by organizing the activity of ion channels and is not associated with a physical location such as a synapse. In order for this form of memory to be of significant duration it is necessary that the ion channels in the astrocyte syncytium be electrically in contact with each other. This function may be served by astrocyte gap junctions and suggests that agents that selectively block these gap junctions should disrupt memory.