Argon: Neuroprotection in in vitro models of cerebral ischemia and traumatic brain injury

The first biological effects of argon were demonstrated as early as 1939 [1]. Behnke et al. described the narcotic effects of argon as experienced by deep sea divers at high pressures. Half a century later Soldatov and co-workers [2] were the first to show argon's protective effects under hypoxic conditions. Thereafter, it was reported that argon shields hair cells from ototoxic process [3] and protects cell cultures from ischemia [4]. In contrast to argon, xenon's organ protective effects have been investigated in various settings and models, ranging from cell cultures to clinical trials. Xenon has proven to be a safe anaesthetic agent and xenon's organoprotective properties have been demonstrated in many fields [5–14].

Stroke and traumatic brain injury (TBI) are two very common causes of death and disability worldwide and create a significant economic and social burden [15–17]. While the acute treatment of stroke today is highly standardized and secondary prevention is effective, an efficient protection of the cells at risk in the penumbra is lacking. This is particularly evident in regard to TBI. Although an estimated 1.5 million people in the United States suffer from TBI annually [15, 17] due to the diverse mechanisms of the initial trauma itself and the following molecular pathways, a specific treatment is still absent.

When compared to xenon argon has some conspicuous advantages: low cost; no narcotic effects at normobaric pressures. Yet, data on argon's effects on cells are sparse. Therefore, we tested the effect of argon in in vitro models that involved either a focal mechanical trauma or oxygen-glucose deprivation of cultured hippocampal slices.

All experiments were performed in compliance with the local Institutional Ethical Review Committee and have been approved by the animal protection representative at the Institute of Animal Research at the RWTH Aachen University Hospital, according to the German animal protection law §4, Section 3. Unless otherwise stated, all chemicals were obtained from PAA Laboratories GmbH (Pasching, Austria).

Assessment of cell injury

The fluorescence images were digitalized at eight bit, allowing us to differentiate between a spectrum of 256 (from 0 to 255) grey scale levels. Damaged regions with a high PI uptake emitted at a high grey scale level, while vital regions showed only minor emissions. The red channel of each image was analyzed with ImageJ software (National Institutes of Health, Bethesda, MD, USA) [24]. ImageJ generated a histogram for each image which showed the absolute number of pixels with the same grey scale value. Histograms from non-traumatized slices (see Figure 1) showed that the vast majority of all pixels had grey scale values between 10 and 100, representing mostly background fluorescence. In contrast, traumatized slices showed, in addition to their background fluorescence, a well-defined peak between 160 and 185. As in previous publications [8, 19, 25, 26] we established a threshold (in this instance at a grey scale value of 100) which proved to be valid to distinguish between traumatized and non-traumatized cells. The integration of all pixels exceeding the threshold therefore was a sound quantification of cell injury.

Tissue damage after 14 days of incubation was negligible (as has been shown before, see Rossaint et al. [19]) otherwise slices were excluded. A distinct pattern of distribution of pixel values was evident for traumatized and non-traumatized groups 72 hours after inducing the experimental injury. Both groups shared a certain background fluorescence level after OGD or TBI (Figure 1). Above a threshold of a grey scale value of 100, the traumatized groups showed a characteristic peak in fluorescence between pixel values of 160 to 185. Non-traumatized slices in both OGD- and TBI-models displayed a minor, but still detectable, rise in emission at a similar luminance. All pixels above the threshold were summarized for each group. The integral for OGD and TBI trauma control groups was set as one and the sums for all further groups were normalized to the OGD and TBI trauma control group respectively as a quantitative measure for trauma intensity (see inserts in Figure 1).

TBI produced less absolute tissue damage than OGD due to the focal nature of this injury. Nevertheless the difference between traumatized and non-traumatized groups was still greater in TBI due to the very low damage found in non-traumatized TBI-slices. More than likely this can be attributed to a longer and more strenuous procedure during OGD. While there were only two medium changes necessary during TBI, OGD slices were subjected to three exchanges. Furthermore the pressure chamber had to be flushed twice for OGD, exposing to a certain extent the surface of the slices to dehydration. Therefore, the TBI control group reached only 6% of the total trauma while the OGD control group showed almost 27.3% of maximum trauma intensity. After establishing a valid measurement for tissue injury, we tested the effects of argon on traumatized slices. We treated groups of slices with 25%, 50% or 74% argon after TBI or OGD was induced. Figure 2 demonstrates that argon provided a significant protective effect in OGD as well as in TBI. After OGD, argon decreased tissue injury by at least 40%. Figure 2, panel A shows the relationship between argon concentration and damage reduction at 37°C. A concentration of 74% argon was most effective (0.52 ± 0.05), yet at concentrations of 25% (0.60 ± 0.05) or 50% (0.56 ± 0.03) a significant reduction of trauma was observed.

Argon showed neuroprotective potential in TBI (Figure 2, panel B). The protection was most effective at a concentration of 50% (0.14 ± 0.03); however, it was still effective at 25% (0.37 ± 0.04) and 74% (0.66 ± 0.07) argon (see exemplary fluorescence images for traumatized and non-traumatized control slices and slices treated with 50% argon in Figure 3).

To adapt our laboratory setting more closely to a typical clinical situation, we decided to apply argon two and three hours after trauma. We incubated the slices in an atmosphere containing 50% argon because this concentration showed the best neuroprotective effect after TBI and there was no significant difference detectable between these three concentrations in OGD. As before, we could show that Argon strongly reduced cell damage in OGD and likewise in TBI (see Figure 4, panels A and B, respectively). Although there was an increase in tissue damage when argon application was delayed, argon was still significantly neuroprotective even two and three hours after injury.

We investigated the potential neuroprotective effects of the noble gas argon in two in vitro models of OGD and TBI. Our methods involved depriving cultured hippocampal slices of oxygen and glucose or producing a focal mechanical trauma on the CA1 region. Our data demonstrate argon's neuroprotective effect when it was applied directly as well as two and three hours after trauma.

Cultured organotypic hippocampal slices are a well established [27, 28] in vitro model to gain easy access to nerve tissue. This model presents a reasonable compromise between dissociated cell cultures and models using intact living animals as most neuronal and glial cells survive [27] and their cytoarchitecture and connective organisation are well preserved [18, 23, 28–32]. Reducing the complex functions of the in vivo state to in vitro settings bears both benefits and disadvantages which have to be taken into account when interpreting our findings. However, this model mirrors to a certain extent the in vivo characteristics, when complicating systemic factors like blood pressure are excluded.

Utilization and outcome of OGD as a model of ischemia have proven to be very reproducible and are widely used [4, 33–36]. While the complete pathogenic pathways of stroke are still incompletely understood, several mechanisms (including increased glutamate, calcium overload, mitochondrial dysfunction and oxidative stress) have been proposed to contribute to neuronal damage [28]. OGD, in contrast to other in vitro ischemic models such as glutamate excitotoxicity, might be more suitable to mimic this in vivo situation, as it allows for more than one pathomechanism elicited by energy depletion to occur. Although a wide range of possible neuroprotective compounds such as glutamate receptor antagonists [37], caspase inhibitors [38], anticonvulsants [39] and volatile anaesthetics [40] have been tested in an OGD setting, inert gases other than xenon have heretofore been scarcely investigated. Consequently, limited data are available on argon's organ-protective potential. Yarin [3] showed that argon protects rat's hair cells against ototoxic processes. In another rat model, Soldatov and co-workers [2] found that a gas mixture containing 25% argon improved the animals' survival under hypoxic conditions compared to a similar respiratory gas mixture without argon.

Jawad et al. [4] were the first investigators to show that 75% argon, administered during OGD and 24 hours thereafter, had neuroprotective effects. However, these results were limited to cultures of dissociated neurons. Therefore we used slice cultures in our study as a more complex and lifelike model. We could confirm argon's neuroprotective potential, even when administered after trauma. Furthermore, we could establish a concentration-dependent effect using three different argon concentrations. There was no significant difference in neuroprotective efficacy between the different argon concentrations in the OGD setting. However, there was a peak effect at 50% argon in the TBI-model. Interestingly, a similar observation about a peak effect of 50% xenon in the same in vitro model has been made by Coburn and colleagues. Yet, this was a theoretical assumption based on extrapolated data [8]. More importantly, with regard to typical clinical situations, we could demonstrate that argon significantly reduced neuronal damage even when applied two or three hours after OGD.

The possible effects of argon on TBI were completely unknown. Therefore we tested argon's impact in an in vitro model by inducing a focal mechanical trauma. This model has been widely used before by us and others when testing possible treatments [8, 19, 25] for traumatic brain injury. Nevertheless this is a simplified imitation of brain trauma, which lacks pathomechanisms involving systemic variables (for example, blood pressure) or local swelling, inflammation, ischemia and/or hypoxia. Yet, despite these obvious limitations, it approximates the in vivo situation thereby validating its clinical feasibility [41]. It is generally accepted that TBI damage is caused by two main factors. The initial lesion is mediated through direct mechanical damage at the impact site. Subsequently, several cellular and molecular processes expand the local injury. The so-called secondary injury is amongst others caused by excitotoxicity [42], up-regulation of cell-death genes [43], the formation of free radicals and the activation of pro-apoptotic mediator pathways [44–46].

Since medical intervention cannot rescue directly traumatized, dying cells, cells near the impact site surviving the initial assault are the main target for the neuroprotective potential of drugs [43]. Indeed, our experiments showed that argon was able to reduce cell death significantly, whether it was applied directly after the trauma or two and three hours afterwards. Of particular significance is argon's potential in protecting neuronal cells when argon administration was delayed. One of the many reasons why positive in vitro results do not transfer favourably to clinical trials [47] is that in many laboratory models treatment is only applied during the course of injury or directly thereafter. We decided to explore the outcome of delayed argon application solely with a gas mixture containing 50% argon for two reasons. First, in the TBI setting 50% argon was most effective. Secondly, and clinically more relevant, 50% of argon allows a higher inspiratory oxygen concentration for patients who require it.

Of consequence, especially in times of cost reduction, argon is the most abundant inert gas which is already widely used in other industries and therefore available at a relatively low price (nine cents/l) compared to xenon (20 €/l). Furthermore, argon has no anaesthetic properties at normobaric conditions [48]. It may therefore be used when sedation would be inappropriate.

While to date little is known about argon's mechanism of action, it has been proposed that argon triggers gamma-aminobutyric acid (GABA) neurotransmission by acting at the benzodiazepine binding site and possibly at multiple other discrete sites of the GABA_A receptor [49]. The activation of GABA receptors has been shown to be neuroprotective in in vitro and in vivo ischemia models and several potential mechanisms have been proposed [40, 50]. First, glutamatergic and GABAergic activity counterbalance the function of each other. On an electrochemical level, stimulation of the ionotropic GABA_A receptor and the following chloride-based membrane hyperpolarization inhibit N-methyl-D-aspartate (NMDA) receptors [51]. In an in vivo rat model, Zhang et al. could show that GABA receptor activation diminished the phosphorylation of the NR2A subunit of the NMDA receptor, thus directly attenuating the receptors functionality [52]. NMDA receptor activation is generally seen as a key element in the development of neuronal death following ischemic events by, among others, increasing calcium influx [53, 54]. Second, GABA receptor activation directly influences downstream pathogenic pathways. Galeffi et al. showed that diazepam counteracted ATP depletion and disabled cytochrome c release in rat hippocampal slices after OGD, two main events in the ischemic cascade [55]. Xu and co-workers demonstrated that GABA agonists inhibited pro-apoptotic pathways through activation of the phosphoinositide 3-kinase/protein kinase B cascade [56]. These findings can be a starting point for further studies to describe argons definite mode of action.

Nevertheless it is surprising that noble gases are able to generate an effect as it requires forming a chemical bond with another molecule. The gas molecules' outer valence shell is completely filled with electrons, making the gas inert to basic chemical reactions. However, these freely vacillating electrons can be polarized. Trudell et al. suggest that a charged element of the binding site itself can induce a bipole in the gas molecules, thus generating enough binding energy to form a bond with the binding site. Another component of the binding energy might be the London dispersion force which is generated by changes in electron density. When the distribution of electrons in one molecule fluctuates to produce an instantaneous dipole, this dipole can produce a dipole in a second molecule [57]. Thus, the previously uncharged gas molecules are temporarily polarized and therefore enabled to interact with the binding site.

This study shows that argon bears surprisingly effective neuroprotective potential in both in vitro models of ischemia and traumatic brain injury. Protection was observed with three different concentrations of argon (25, 50 and 74%), either directly applied after the trauma or when administered at a concentration of 50% two and three hours after the injuries. Considering these promising results, despite the inherent simplifications of any in vitro model, further animal research, preferably using a whole animal model, seems appropriate.