Reorganization of actin in neurons after propofol exposure

Propofol-induced age-different hypocampal long-term potentiation is associated with F-actin polymerization in rats

Elderly patients may experience a decline in cognition after a surgery performed under anesthesia. Propofol (2,6-diisopropylphenol), a common intravenous anesthetic agent, has been reported to mediate the long-term potentiation (LTP), a major form of synaptic plasticity. The present study was conducted to investigate the underlying mechanisms in young (3-month-old) and elderly (20-month-old) male rats. A decline of theta-burst stimulation (TBS)-induced LTP in the hippocampal CA1 area was found in the young rats at 72 h post-anesthesia, and this alteration almost disappeared after 2-week-recovery as compared with their age-matched control rats. On the other hand, the propofol-induced CA1 LTP reduction was persistent in the aged rats during the whole experimental process. Moreover, TBS-induced increases in CA 1 filamentous-actin (F-actin) polymerization and phospho-cofilin expression were enhanced at 72 h post-anesthesia in young rats, and this change was significantly attenuated after 2 weeks. However, in anesthetic elderly rats, the alterations in F-actin and phospho-cofilin of the CA1 region were still presented at the end of the experiments. Taken together, our results indicate that the discrepant responses between young and aged rats to propofol anesthesia may be associated with the differential polymerization of F-actin.

Possible augmentation of neuromuscular blockade by propofol during recovery from rocuronium

Propofol is a widely used drug in anesthesia practice, and its pharmacological characteristics are well known. However, propofol is not known for neuromuscular effects. As part of clinical neuromuscular monitoring, the neuromuscular responses to train-of-four (TOF) stimulation were monitored and recorded. We observed, in two cases of balanced anesthesia maintained by desflurane and fentanyl, that administration of a small dose of propofol during almost complete recovery from rocuronium in two patients resulted in marked decreases of both T1 (first twitch response of the TOF) and the TOF ratio. This neuromuscular block dissipated in both patients without any subsequent neuromuscular effects. These two observations provide visual confirmation of the possible impact of propofol on recovery from a rocuronium neuromuscular blockade.

Sedative drug modulates T-cell and lymphocyte function-associated antigen-1 function

BACKGROUND

Sedative drugs modify immune cell functions via several mechanisms. However, the effects of sedatives on immune function have been primarily investigated in neutrophils and macrophages, and to the lesser extent lymphocytes. Lymphocyte function-associated antigen-1 (LFA-1) is an adhesion molecule that has a central role in regulating immune function of lymphocytes including interleukin-2 (IL-2) production and lymphocyte proliferation. Previous clinical studies reported that propofol and isoflurane reduced IL-2 level in patients, but midazolam did not. We previously demonstrated that isoflurane inhibited LFA-1 binding to its counter ligand, intercellular adhesion molecule-1 (ICAM-1), which might contribute to the reduction of IL-2 levels. In the current study, we examined the effect of propofol, midazolam, and dexmedetomidine on LFA-1/ICAM-1 binding, and the subsequent biological effects.

METHODS

The effect of sedative drugs on T-cell proliferation and IL-2 production was measured by calorimetric assays on human peripheral blood mononuclear cells. Because LFA-1/ICAM-1 binding has an important role in T-cell proliferation and IL-2 production, we measured the effect of sedative drugs on ICAM-1 binding to LFA-1 protein (cell-free assay). This analysis was followed by flow cytometric analysis of LFA-1 expressing T-cell binding to ICAM-1 (cell-based assay). To determine whether the drug/LFA-1 interaction is caused by competitive or allosteric inhibition, we analyzed the sedative drug effect on wild-type and high-affinity LFA-1 and a panel of monoclonal antibodies that bind to different regions of LFA-1.

RESULTS

Propofol at 10 to 100 μM inhibited ICAM-1 binding to LFA-1 in cell-free assays and cell-based assays (P < 0.05). However, dexmedetomidine and midazolam did not affect LFA-1/ICAM-1 binding. Propofol directly inhibits LFA-1 binding to ICAM-1 by binding near the ICAM-1 contact area in a competitive manner. At clinically relevant concentrations, propofol, but not dexmedetomidine or midazolam, inhibited IL-2 production (P < 0.05). Additionally, propofol inhibited lymphocyte proliferation (P < 0.05).

CONCLUSIONS

Our study suggests that propofol competitively inhibits LFA-1 binding to ICAM-1 on T-cells and suppresses T-cell proliferation and IL-2 production, whereas dexmedetomidine and midazolam do not significantly influence these immunological assays.

Regional and temporal profiles of calpain and caspase-3 activities in postnatal rat brain following repeated propofol administration

Exposure of newborn rats to a variety of anesthetics has been shown to induce apoptotic neurodegeneration in the developing brain. We investigated the effect of the general anesthetic propofol on the brain of 7-day-old (P7) Wistar rats during the peak of synaptic growth. Caspase and calpain protease families most likely participate in neuronal cell death. Our objective was to examine regional and temporal patterns of caspase-3 and calpain activity following repeated propofol administration (20 mg/kg). P7 rats were exposed for 2, 4 or 6 h to propofol and killed 0, 4, 16 and 24 h after exposure. Relative caspase-3 and calpain activities were estimated by Western blot analysis of the proteolytic cleavage products of α-II-spectrin, protein kinase C and poly(ADP-ribose) polymerase 1. Caspase-3 activity and expression displayed a biphasic pattern of activation. Calpain activity changed in a region- and time-specific manner that was distinct from that observed for caspase-3. The time profile of calpain activity exhibited substrate specificity. Fluoro-Jade B staining revealed an immediate neurodegenerative response that was in direct relationship to the duration of anesthesia in the cortex and inversely related to the duration of anesthesia in the thalamus. At later post-treatment intervals, dead neurons were detected only in the thalamus 24 h following the 6-hour propofol exposure. Strong caspase-3 expression that was detected at 24 h was not followed by cell death after 2- and 4-hour exposures to propofol. These results revealed complex patterns of caspase-3 and calpain activities following prolonged propofol anesthesia and suggest that both are a manifestation of propofol neurotoxicity at a critical developmental stage.

Cholinergic neural signals to the spleen down-regulate leukocyte trafficking via CD11b

The cholinergic anti-inflammatory pathway is a physiological mechanism that inhibits cytokine production and diminishes tissue injury during inflammation. Recent studies demonstrate that cholinergic signaling reduces adhesion molecule expression and chemokine production by endothelial cells and suppresses leukocyte migration during inflammation. It is unclear how vagus nerve stimulation regulates leukocyte trafficking because the vagus nerve does not innervate endothelial cells. Using mouse models of leukocyte trafficking, we show that the spleen, which is a major point of control for cholinergic modulation of cytokine production, is essential for vagus nerve-mediated regulation of neutrophil activation and migration. Administration of nicotine, a pharmacologic agonist of the cholinergic anti-inflammatory pathway, significantly reduces levels of CD11b, a beta(2)-integrin involved in cell adhesion and leukocyte chemotaxis, on the surface of neutrophils in a dose-dependent manner and this function requires the spleen. Similarly, vagus nerve stimulation significantly attenuates neutrophil surface CD11b levels only in the presence of an intact and innervated spleen. Further mechanistic studies reveal that nicotine suppresses F-actin polymerization, the rate-limiting step for CD11b surface expression. These studies demonstrate that modulation of leukocyte trafficking via cholinergic signaling to the spleen is a specific, centralized neural pathway positioned to suppress the excessive accumulation of neutrophils at inflammatory sites. Activating this mechanism may have important therapeutic potential for preventing tissue injury during inflammation.

GABAergic mechanism of propofol toxicity in immature neurons

Certain anesthetics exhibit neurotoxicity in the brains of immature but not mature animals. Gamma-aminobutyric acid (GABA), the primary inhibitory neurotransmitter in the adult brain, is excitatory on immature neurons via its action at the GABAA receptor, due to a reversed transmembrane chloride gradient. GABAA receptor activation in immature neurons is sufficient to open L-type voltage-gated calcium channels. As propofol is a GABAA agonist, we hypothesized that it and more specific GABAA modulators would increase intracellular free calcium ([Ca2+]i), resulting in the death of neonatal rat hippocampal neurons. Neuronal [Ca2+]i was monitored using Fura2-AM fluorescence imaging. Cell death was assessed by double staining with propidium iodide and Hoechst 33258 at 1 hour (acute) and 48 hours (delayed) after 5 hours exposure of neurons to propofol or the GABAA receptor agonist, muscimol, in the presence and absence of the GABA receptor antagonist, bicuculline, or the L-type Ca2+ channel blocker, nifedipine. Fluorescent measurements of caspase-3,-7 activities were performed at 1 hour after exposure. Both muscimol and propofol induced a rapid increase in [Ca2+]i in days in vitro (DIV) 4, but not in DIV 8 neurons, that was inhibited by nifedipine and bicuculline. Caspase-3,-7 activities and cell death increased significantly in DIV 4 but not DIV 8 hippocampal neuronal cultures 1 hour after 5 hours exposure to propofol, but not muscimol, and were inhibited by the presence of bicuculline or nifedipine. We conclude that an increase in [Ca2+]i, due to activation of GABAA receptors and opening of L-type calcium channels, is necessary for propofol-induced death of immature rat hippocampal neurons but that additional mechanisms not elicited by GABAA activation alone also contribute to cell death.

Glutamate-induced c-Jun expression in neuronal PC12 cells: the effects of ketamine and propofol

Transcription factor c-Jun affects neuronal cell death and survival in mammalian brain. As general anesthetics, such as ketamine and propofol, are thought to provide some degree of neuroprotection, this study was intended to test whether the protection of injured neuronal PC12 cells by ketamine and propofol is related to the inhibition of phospho-c-Jun. Using neuronal PC12 cells from rat pheochromocytoma cells differentiated with nerve growth factor, we found that 24 hours of exposure to glutamate (1 to 100 mM) induced concentration-dependent cell death as determined by an ability to reduce the tetrazolium derivative, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) into a blue formazan salt. Neuronal PC12 cells were exposed to ketamine (0.1, 1.0 mM) or propofol (0.5, 5.0 microM) and glutamate (0, 20 mM) for 24 hours. Cell injury was assessed using MTT, in situ terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling, and c-Jun activity assay. Glutamate, 20 mM, induced about 70% of cell death as determined by MTT and terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-biotin nick end labeling staining. Glutamate-induced cell death was related to an increase in expression of phospho-c-Jun. Glutamate-induced cell death was reduced by ketamine (0.1, 1.0 mM) in a dose-dependent manner and also by propofol (0.5, 5.0 microM). In addition, the expression of phospho-c-Jun was substantially reduced by ketamine (0.1, 1.0 mM) and propofol (0.5, 5.0 microM), respectively, as determined by Western blot assay. These results suggest that inhibition of c-Jun activity is involved in the neuroprotective effects of ketamine and propofol on glutamate-induced injury in neuronal PC12 cells.

The effect of propofol on actin, ERK-1/2 and GABAA receptor content in neurones

AIM

Interaction with the gamma-aminobutyric acid receptor (GABA(A)R) complex is recognized as an important component of the mechanism of many anaesthetic agents, including propofol. The aims of this study were to investigate the effect of propofol on GABA(A)R, to determine whether exposure of neurones to propofol influences the localization of GABA(A)R within the cell and to look for cytoskeletal changes that may be connected with activation, such as the mitogen-activated protein kinase (MAPK) pathway.

METHODS

Primary cortical cell cultures from rat, with and without pre-incubation with the GABA(A)R antagonist bicuculline, were exposed to propofol. The cells were lysed and separated into membrane and cytosolic fractions. Immunoblot analyses of filamentous actin (F-actin), the GABA(A)beta(2)-subunit receptor and extracellular signal-regulated kinase-1/2 (ERK-1/2) were performed.

RESULTS

Propofol triggers an increase in GABA(A)R, actin content and ERK-1/2 phosphorylation in the cytosolic fraction. In the membrane fraction, there is a decrease in GABA(A)beta(2)-subunit content and an increase in both actin content and ERK-1/2 phosphorylation. The GABA(A)R antagonist bicuculline blocks the propofol-induced changes in F-actin, ERK and GABA(A)beta(2)-subunit content, and ERK-1/2 phosphorylation.

CONCLUSION

We believe that propofol triggers a dose-dependent internalization of the GABA(A)beta(2)-subunit. The increase in internal GABA(A)beta(2)-subunit content exhibits a close relationship to actin polymerization and to an increase in ERK-1/2 activation. Actin contributes to the internalization sequestering of the GABA(A)beta(2)-subunit.

Potentially toxic effects of anaesthetics on the developing central nervous system

A growing body of experimental evidence suggests that anaesthetics, by influencing GABAergic and glutaminergic neural signalling, can have adverse effects on the developing central nervous system. The biological foundation for this is that gamma-aminobutyric acid and glutamate could act non-synaptically, in addition to their role in neurotransmission in the adult brain, in the regulation of neuronal development in the central nervous system. These neurotransmitters and their receptors are expressed from very early stages of central nervous system development and appear to influence neural progenitor proliferation, cell migration and neuronal differentiation. During the synaptogenetic period, pharmacological blockade of N-methyl-d-aspartate (NMDA)-type glutamate receptors as well as stimulation of GABAA receptors has been reported to be associated with increased apoptosis in the developing brain. Importantly, recent data suggest that even low, non-apoptogenic concentrations of anaesthetics can perturb neuronal dendritic development and thus could potentially lead to impairment of developing neuronal networks. The extrapolation of these experimental observations to clinical practice is of course very difficult and requires extreme caution as differences in drug concentrations and exposure times as well as interspecies variations are all important confounding variables. While clinicians should clearly not withhold anaesthesia based on current animal studies, these observations should urge more laboratory and clinical research to further elucidate this issue.

Interactions of anesthetics with their targets: non-specific, specific or both?

What makes a general anesthetic a general anesthetic? We shall review first what general anesthesia is all about and which drugs are being used as anesthetics. There is neither a unique definition of general anesthesia nor any consensus on how to measure it. Diverse drugs and combinations of drugs generate general anesthetic states of sometimes very different clinical quality. Yet the principal drugs are still considered to belong to the same class of 'general anesthetics'. Effective concentrations of inhalation anesthetics are in the high micromolar range and above, and even for intravenous anesthetics they do not go below the micromolar range. At these concentrations, many molecular and higher level targets are affected by inhalation anesthetics, fewer probably by intravenous anesthetics. The only physicochemical characteristic shared by anesthetics is the correlation of their anesthetic potencies with hydrophobicity. These correlations depend on the group of general anesthetics considered. In this review, anesthetic potencies for many different targets are plotted against octanol/water partition coefficients as measure of hydrophobicity. Qualitatively, similar correlations result, suggesting several but weak interactions with proteins as being characteristic of anesthetic actions. The polar interactions involved are weak, being roughly equal in magnitude to hydrophobic interactions. Generally, intravenous anesthetics are noticeably more potent than inhalation anesthetics. They differ considerably more between each other in their interactions with various targets than inhalation anesthetics do, making it difficult to come to a decision which of these should be used in future studies as representative 'prototypical general anesthetics'.

Propofol-induced calcium signalling and actin reorganization within breast carcinoma cells

BACKGROUND AND OBJECTIVE

MDA-MB-468 breast carcinoma cells respond to non-volatile anaesthetics such as propofol with an increased migration. Here we investigated the relationship between GABA-A receptor modulators, the mode of calcium oscillation and actin reorganization with regard to breast carcinoma cell migration.

METHODS

Expression of the GABA-A receptor was determined by Western blot analysis. Calcium-imaging experiments of individual MDA-MB-468 cells as well as visualization of the F-actin distribution were performed by confocal laser scanning microscopy. Cell migration was investigated in a three-dimensional collagen matrix by time-lapse video microscopy. The GABA agonist propofol was used in a final concentration of 6 microg mL(-1). GABA-A receptor antagonist bicuculline (50 micromol) and selective L-type calcium channel blocker verapamil (5 micromol) were used to modulate the propofol effects.

RESULTS

A functional GABA-A receptor is expressed by MDA-MB-468 cells. Activation with propofol resulted in sustained increased intracellular calcium concentrations concomitant with actin reorganization and induction of migration in MDA-MB-468 cells. These propofol effects were completely blocked by verapamil. Spontaneous migration of MDA-MB-468 cells (64.4 +/- 7.0%) was significantly increased by propofol to 85.0 +/- 5.0%. MDA-MB-468 cells co-treated with propofol and verapamil showed a migratory activity of 63.0 +/- 2.0% indicating that verapamil blocked the propofol effect. Similar results were achieved with the GABA-A receptor inhibitor bicuculline (control: 56.3 +/- 8.5%; propofol: 80.5 +/- 7.1%; propofol + bicuculline: 52.5 +/- 8.6%).

CONCLUSION

Activation of GABA-A receptor by propofol correlated with an increased migration of MDA-MB-468 breast carcinoma cells, mediated by calcium influx via L-type calcium channels and reorganization of the actin cytoskeleton.

The difference between sleep and anaesthesia is in the intracellular signal: propofol and GABA use different subtypes of the GABA(A) receptor beta subunit and vary in their interaction with actin

BACKGROUND

Propofol is known to interact with the gamma-aminobutyric acidA (GABA(A)) receptor, however, activating the receptor alone is not sufficient for producing anaesthesia.

METHODS

To compare propofol and GABA, their interaction with the GABAA receptor beta subunit and actin were studied in three cellular fractions of cultured rat neurons using Western blot technique.

RESULTS

Propofol tyrosine phosphorylated the GABA(A) receptor beta2 (MW 54 and 56 kDa) and beta3 (MW 57 kDa) subtypes. The increase was shown in both the cytoskeleton (beta2(54) and beta2(56) subtypes) and the cell membrane (beta2(54) and beta3 subtypes). Concurrently the 56 kDa beta2 subtype was reduced in the cytosol. Propofol, but not GABA, also tyrosine phosphorylated actin in the cell membrane and cytoskeletal fraction. Without extracellular calcium available, the amount of actin decreased in the cytoskeleton, but tyrosine phosphorylation was unchanged. GABA caused increased tyrosine phosphorylation of beta2(56) and beta3 subtypes in the membrane and both beta2 subtypes in the cytoskeleton but no cytosolic tyrosine phosphorylation.

CONCLUSION

The difference between propofol and GABA at the GABA(A) receptor was shown to take place in the membrane, where the beta2(54) was increased by propofol and instead the beta2(56) subtype was increased by GABA. Only propofol also tyrosine phosphorylated actin in the cell membrane and cytoskeletal fraction. This interaction between the GABAA receptor and actin might explain the difference between anaesthesia and physiological neuronal inhibition.