Nitrous oxide, but not xenon, affects the signaling in the neuronal growth cone

A health-based recommended occupational exposure limit for nitrous oxide using experimental animal data based on a systematic review and dose-response analysis

BACKGROUND

Nitrous oxide (N₂O) is a common inhalation anaesthetic used in medical, paramedical, and veterinary practice. Since the mid 1950's, concerns have been raised regarding occupational exposure to N₂O, leading to many epidemiological and experimental animal studies. Previous evaluations resulted in the classification of N₂O as a possible risk factor for adverse reproductive health outcomes based on animal data. Human data were deemed inadequate primarily because of simultaneous co-exposures to other risk factors for adverse reproductive and developmental outcomes, including other anaesthetic gases. Since previous evaluations, controversies regarding N₂O use remained and new approaches for dose response modelling have been adopted, calling for an update and re-evaluation of the body of evidence. This review aims to assess available animal evidence on N₂O reproductive and developmental outcomes to inform a health-based recommended occupational exposure limit (OEL) for N₂O with a benchmark dose-response modelling (BMD) approach.

METHODS

Comprehensive searches in PubMed, EMBASE, and Web of Science were performed to retrieve all relevant studies addressing reproductive and developmental outcomes related to inhalation of N₂O in animals. The articles retrieved were screened based on title-abstract and full text by two independent reviewers. After data extraction, an overview of all studies was created for the different endpoints, namely foetal outcomes (e.g., resorption), female outcomes (e.g. implantations), and male outcomes (e.g. sperm count). A subset of studies reporting on exposure relevant to workplace settings and with a sufficient number of tested doses were included in dose-response modelling using the BMD approach.

RESULTS

In total, 15.816 articles were retrieved, of which 47 articles were finally included while 4 of those were used for the quantitative data synthesis. The overall risk of bias was judged to be probably high (using OHAT risk of bias tool) and unclear (using SYRCLE's risk of bias tool). From eligible rat studies, three studies provided an acceptable result by fitting a Hill model to the dose-response data. The resulting benchmark dose lower bounds (BMDLs) from three studies converged to an average (±sd) exposure level of 925 ± 2 mg/m³ at an additional risk of one standard deviation of implantation losses above those observed in the control group (i.e. reduced number of live foetuses/mother). For extrapolation from rats to humans, an uncertainty factor of 10 was used and an additional factor of 5 was applied to account for interindividual variability within the population of workers.

CONCLUSION

With this systematic review, all available evidence for reproductive toxicity and adverse developmental outcomes in animals resulting from inhalation exposure to N₂O was used to derive a health-based OEL recommendation of 20 mg/m³ as 8-h time-weighted average.

LOTUS Inhibits Neuronal Apoptosis and Promotes Tract Regeneration in Contusive Spinal Cord Injury Model Mice

Nogo receptor-1 (NgR1) signaling is involved in the limitation of axonal regeneration following spinal cord injury (SCI) through collapsing the growth cone and inhibiting neurite outgrowth. Lateral olfactory tract usher substance (LOTUS), a NgR antagonist, suppresses these pathological conditions. A previous report demonstrated the positive effects of LOTUS expression on motor function through raphespinal tract regeneration using pan-neuronally LOTUS-overexpressing transgenic mice. However, this report used a hemi-section model, which does not represent the majority of clinical SCI cases, and lacked a detailed histological analysis of other descending tracts. To determine the true therapeutic effects of LOTUS, we used a more clinically relevant contusive SCI model in female transgenic mice. Definitive tracing analyses revealed that LOTUS promoted the extensive regeneration of the reticulospinal tract across the lesion site and suppressed axonal dieback of corticospinal tract (CST). A significant increase in raphespinal tract fibers was seen from the subacute to the chronic phase after the injury, strongly suggesting that LOTUS promoted translesional elongation of this tract. Furthermore, histological analyses revealed that LOTUS had a neuroprotective effect on the injured spinal cord through suppressing cellular apoptosis during the acute phase. These neuroprotective and regenerative effects contributed to significant motor functional recovery and restoration of the motor evoked potential (MEP). Therefore, LOTUS application could prove beneficial in the treatment of SCI by promoting axonal regeneration of some descending fibers, reducing axonal dieback of CST fibers and encouraging motor function recovery.

The effects of xenon and nitrous oxide gases on alcohol relapse

BACKGROUND

In recent years, the glutamate theory of alcoholism has emerged as a major theory in the addiction research field and N-methyl-d-aspartate (NMDA) receptors have been shown to play a major role in alcohol craving and relapse. The NMDA receptors are considered as the primary side of action of the anesthetic gases xenon (Xe) and nitrous oxide (N2 O). Despite the rapid on/off kinetics of these gases on the NMDA receptor, a brief gas exposure can induce an analgesic or antireward effect lasting several days. The aim of this study was to examine the effect of both Xe and N2 O on alcohol-seeking and relapse-like drinking behavior (measured as the alcohol deprivation effect) in Wistar rats.

METHODS

We used 2 standard procedures-the alcohol deprivation model with repeated deprivation phases and the cue-induced reinstatement model of alcohol seeking-to study the effect of 2 brief gas exposures of either Xe, N2 O, or control gas on relapse-like drinking and alcohol-seeking behavior.

RESULTS

Here, we show that exposure to Xe during the last 24 hours of abstinence produced a trend toward reduced ethanol intake during the first alcohol re-exposure days. In addition, Xe gas exposure significantly decreased the cue-induced reinstatement of alcohol-seeking behavior. N2 O had no effect on either behavior.

CONCLUSIONS

Xe reduces alcohol-seeking behavior in rats and may therefore also interfere with craving in human alcoholics.

Involvement of brain protein kinase C in nitrous oxide-induced antinociception in mice

Exposure of mice to the anesthetic gas nitrous oxide (N(2)O) produces a marked antinociceptive effect. Protein kinase C is a key regulatory enzyme that may be targeted by general anesthetics. However, a relationship between N(2)O-induced antinociception and protein kinase C has yet to be established. The present study was conducted to identify whether protein kinase C might influence N(2)O-induced antinociception in mice. Regular exposure (11 min) to N(2)O produced concentration-dependent antinociception in mice, as determined using the abdominal constriction test. N(2)O-induced antinociception was attenuated by i.c.v. pretreatment with phorbol 12,13-dibutyrate, a protein kinase C activator. This phorbol 12,13-dibutyrate antagonism of N(2)O-induced antinociception was reversed by i.c.v. pretreatment with calphostin C, a protein kinase C inhibitor. Long-term exposure (41 min in total, including 30 min prior to, and 11 min of analgesic testing) to 70% N(2)O produced reduced analgesic effects, compared with regular exposure to 70% N(2)O, thus indicating acute tolerance to N(2)O-induced antinociception. However, mice pretreated with calphostin C, chelerythrine, which is another protein kinase C inhibitor, and phorbol 12,13-dibutyrate, did not develop acute tolerance. Regarding activation of protein kinase C, regular exposure to 70% N(2)O did not increase protein kinase C within the membrane fraction of brain tissue, as determined by immunoblot analysis, but long-term exposure to 70% N(2)O did. The i.c.v. pretreatment with calphostin C and phorbol 12,13-dibutyrate prevented the increase in protein kinase C observed with long-term exposure to 70% N(2)O. These results suggest that brain protein kinase C negatively regulates the antinociceptive effect of N(2)O, and that activation of brain protein kinase C is related to the development of acute tolerance to N(2)O-induced antinociception in mice.

Xenon preconditioning reduces brain damage from neonatal asphyxia in rats

Xenon attenuates on-going neuronal injury in both in vitro and in vivo models of hypoxic-ischaemic injury when administered during and after the insult. In the present study, we sought to investigate whether the neuroprotective efficacy of xenon can be observed when administered before an insult, referred to as 'preconditioning'. In a neuronal-glial cell coculture, preexposure to xenon for 2 h caused a concentration-dependent reduction of lactate dehydrogenase release from cells deprived of oxygen and glucose 24 h later; xenon's preconditioning effect was abolished by cycloheximide, a protein synthesis inhibitor. Preconditioning with xenon decreased propidium iodide staining in a hippocampal slice culture model subjected to oxygen and glucose deprivation. In an in vivo model of neonatal asphyxia involving hypoxic-ischaemic injury to 7-day-old rats, preconditioning with xenon reduced infarction size when assessed 7 days after injury. Furthermore, a sustained improvement in neurologic function was also evident 30 days after injury. Phosphorylated cAMP (cyclic adenosine 3',5'-monophosphate)-response element binding protein (pCREB) was increased by xenon exposure. Also, the prosurvival proteins Bcl-2 and brain-derived neurotrophic factor were upregulated by xenon treatment. These studies provide evidence for xenon's preconditioning effect, which might be caused by a pCREB-regulated synthesis of proteins that promote survival against neuronal injury.