Cumulative biochemical effects of repeated subclinical hydrogen sulfide intoxication in mouse brain

Application of adverse outcome pathway networks to integrate mechanistic data informing the choice of a point of departure for hydrogen sulfide exposure limits

Acute exposure to hydrogen sulfide initiates a series of hallmark biological effects that occur progressively at increasing exposure levels: odor perception, conjunctivitis, olfactory paralysis, "knockdown," pulmonary edema, and apnea. Although effects of exposure to high concentrations of hydrogen sulfide are clear, effects associated with chronic, low-level exposure in humans is under debate, leading to uncertainty in the critical effect used in regulatory risk assessments addressing low dose exposures. This study integrates experimental animal, observational epidemiology, and occupational exposure evidence by applying a pathway-based approach. A hypothesized adverse outcome pathway (AOP) network was developed from 34 studies, composed of 4 AOPs sharing 1 molecular initiating events (MIE) and culminating in 4 adverse outcomes. A comparative assessment of effect levels and weight of evidence identified an AOP leading to a biologically-plausible, low-dose outcome relative to the other outcomes (nasal lesions, 30 ppm versus olfactory paralysis, >100 ppm; neurological effects, >80 ppm; pulmonary edema, >80 ppm). This AOP (i.e. AOP1) consists of the following key events: cytochrome oxidase inhibition (>10 ppm), neuronal cell loss (>30 ppm), and olfactory nasal lesions (defined as both neuronal cell loss and basal cell hyperplasia; >30 ppm) in rodents. The key event relationships in this pathway were supported by moderate empirical evidence and have high biological plausibility due to known mechanistic understanding and consistency in observations for diverse chemicals.

Characterizing a mouse model for evaluation of countermeasures against hydrogen sulfide-induced neurotoxicity and neurological sequelae

Hydrogen sulfide (H₂ S) is a highly neurotoxic gas. It is the second most common cause of gas-induced deaths. Beyond mortality, surviving victims of acute exposure may suffer long-term neurological sequelae. There is a need to develop countermeasures against H₂ S poisoning. However, no translational animal model of H₂ S-induced neurological sequelae exists. Here, we describe a novel mouse model of H₂ S-induced neurotoxicity for translational research. In paradigm I, C57/BL6 mice were exposed to 765 ppm H₂ S for 40 min on day 1, followed by 15-min daily exposures for periods ranging from 1 to 6 days. In paradigm II, mice were exposed once to 1000 ppm H₂ S for 60 minutes. Mice were assessed for behavioral, neurochemical, biochemical, and histopathological changes. H₂ S intoxication caused seizures, dyspnea, respiratory depression, knockdowns, and death. H₂ S-exposed mice showed significant impairment in locomotor and coordinated motor movement activity compared with controls. Histopathology revealed neurodegenerative lesions in the collicular, thalamic, and cortical brain regions. H₂ S significantly increased dopamine and serotonin concentration in several brain regions and caused time-dependent decreases in GABA and glutamate concentrations. Furthermore, H₂ S significantly suppressed cytochrome c oxidase activity and caused significant loss in body weight. Overall, male mice were more sensitive than females. This novel translational mouse model of H₂ S-induced neurotoxicity is reliable, reproducible, and recapitulates acute H₂ S poisoning in humans.

Acute hydrogen sulfide-induced neuropathology and neurological sequelae: challenges for translational neuroprotective research

Hydrogen sulfide (H2 S), the gas with the odor of rotten eggs, was formally discovered in 1777, over 239 years ago. For many years, it was considered an environmental pollutant and a health concern only in occupational settings. Recently, however, it was discovered that H2 S is produced endogenously and plays critical physiological roles as a gasotransmitter. Although at low physiological concentrations it is physiologically beneficial, exposure to high concentrations of H2 S is known to cause brain damage, leading to neurodegeneration and long-term neurological sequelae or death. Neurological sequelae include motor, behavioral, and cognitive deficits, which are incapacitating. Currently, there are concerns about accidental or malicious acute mass civilian exposure to H2 S. There is a major unmet need for an ideal neuroprotective treatment, for use in the field, in the event of mass civilian exposure to high H2 S concentrations. This review focuses on the neuropathology of high acute H2 S exposure, knowledge gaps, and the challenges associated with development of effective neuroprotective therapy to counteract H2 S-induced neurodegeneration.

Hydrogen Sulfide

Hydrogen sulfide is a relatively common, frequently lethal, and unique occupational hazard for which research since 1990 has uncovered many anomalies and subtleties and a previously unsuspected physiological role for the endogenous agent. The result has been uncertainty and misunderstanding, particularly for persons new to the literature. This review addresses evidence that settles past controversies, guides practical issues in evaluating human toxicity, addresses unresolved issues involving chronic exposure, and points the way to a deeper understanding of the agent and its effects.

Low-level hydrogen sulfide and central nervous system dysfunction

Forty-nine adults living in Lovington, Tatum, and Artesia, the sour gas/oil sector of Southeastern New Mexico, were tested for neurobehavioral impairment. Contributing hydrogen sulfide were (1) an anaerobic sewage plant; (2) two oil refineries; (3) natural gas/oil wells and (4) a cheese-manufacturing plant and its waste lagoons. Comparisons were to unexposed Wickenburg, Arizona, adults. Neurobehavioral functions were measured in 26 Lovington adults including 23 people from Tatum and Artesia, New Mexico, and 42 unexposed Arizona people. Participants completed questionnaires including chemical exposures, symptom frequencies and the Profile of Mood States. Measurements included balance, reaction time, color discrimination, blink reflex, visual fields, grip strength, hearing, vibration, problem solving, verbal recall, long-term memory, peg placement, trail making and fingertip number writing errors (FTNWE). Average numbers of abnormalities and test scores were adjusted for age, gender, educational level, height and weight, expressed as percent predicted (% pred) and compared by analysis of variance (ANOVA). Ages and educational attainment of the three groups were not statistically significantly different (ssd). Mean values of Lovington residents were ssd from the unexposed Arizona people for simple and choice reaction times, balance with eyes open and closed, visual field score, hearing and grip strength. Culture Fair, digit symbol substitution, vocabulary, verbal recall, peg placement, trail making A and B, FTNWE, information, picture completion and similarities were also ssd. The Lovington adults who averaged 11.8 abnormalities were ssd from, Tatum—Artesia adults who had 3.6 and from unexposed subjects with 2.0. Multiple source community hydrogen sulfide exposures impaired neurobehavioral functions.

Olfactory mucosal necrosis in male CD rats following acute inhalation exposure to hydrogen sulfide: reversibility and the possible role of regional metabolism

Hydrogen sulfide (H2S) is a potent inhibitor of cytochrome oxidase (CO) and is associated with dysosmia and anosmia in humans and nasal lesions in exposed rodents. An improved understanding of the pathogenesis of these lesions is needed to determine their toxicological relevance. We exposed 10-week-old male CD rats to 0, 30, 80, 200, or 400 ppm H2S for 3 hours/day for 1 or 5 days consecutively. The nose was histologically examined 24 hours after H2S exposure, and lesion recovery was assessed at 2 and 6 weeks following the 5-day exposure. A single 3-hour exposure to > or = 80 ppm H2S resulted in regeneration of the respiratory mucosa and full thickness necrosis of the olfactory mucosa localized to the ventral and dorsal meatus, respectively. Repeated exposure to the same concentrations caused necrosis of the olfactory mucosa with early mucosal regeneration that extended from the dorsal medial meatus to the caudal regions of the ethmoid recess. Acute exposure to 400 ppm H2S induced severe mitochondrial swelling in sustentacular cells and olfactory neurons, which progressed to olfactory epithelial necrosis and sloughing. CO immunoreactive cells were more frequently observed in regions of the olfactory mucosa commonly affected by H2S than in regions that were not. These findings demonstrate that acute exposure to >80 ppm H2S resulted in reversible lesions in the respiratory and olfactory mucosae of the CD rat and that CO immunoreactivity may be a susceptibility factor for H2S-induced olfactory toxicity in the rat.

Neuropsychological effects of occupational exposures to carbon disulfide and hydrogen sulfide

In the framework of an extensive health survey of viscose rayon workers in Belgium, 187 workers underwent a neuropsychological examination. Of these, 120 had been exposed for at least a year to carbon disulfide (CS&inf2;) and hydrogen sulfide (H2S), and 67 served as a non-exposed control group. Measurements showed that many of the 17 jobs in the factory involved exposures to CS&inf2; ranging from 3 mg/m(3) (centrifuge operator) to 147 mg/m(3) (spinning cake regulator), far in excess of the threshold limit value (TLV) of 31 mg/m(3); H2S exposures remained below the recommended TLV of 14 mg/m(3). The neuropsychological investigation included subtests of the Wechsler Adult Intelligence Scale, the entire Wechsler Memory Scale, the Bourdon-Wiersma Test, the Santa Ana Dexterity Test, the Gibson Spiral Maze, and the Bimanual Sinusoidal Movement Test. Specific questions were included to account for the effects of age, educational level, eye complaints, alcohol consumption, medication intake, and test motivation. Only the group exposed to values exceeding three times the recommended TLV for CS2; had significant impairments in both the speed and the quality of psychomotor performance. Exposure to CS2; and H2S had no significant effect on memory and attention. Covariance analysis revealed the confounding influences of educational level and eye complaints for explaining observed "differences" in memory and attention tasks found by univariate analysis.

An Epidemiologic Study of the Effects of Carbon Disulfide on the Peripheral Nerves

Although the labor inspection had disclosed considerable exposure to carbon disulfide (CS&inf2;) in a Belgian viscose rayon factory, the company medical officer had not diagnosed any case of polyneuropathy in association with CS&inf2; exposure, although this finding had been extensively reported in the literature. Personal monitoring of CS&inf2; exposure was performed in 17 jobs. Because the working conditions in the factory had not changed since 1932, a CS&inf2; cumulative exposure index (CS&inf2; index) could be calculated for each individual. Examination of the exposed subjects (n = 111) included a self-administered questionnaire, a clinical neurologic examination, and electroneuromyography. Seventy-four workers from other plants, not exposed to CS&inf2; or to any other neurotoxic agent, served as referents. The average CS&inf2; exposures of the study group ranged from 4 to 112 mg/m(3). The data were analyzed with multiple regression methods, adjusting the effect of exposure for a number of possible confounders. Significant associations were found between the cumulative CS&inf2; index and symptoms consistent with polyneuropathy in the legs and with abnormal recruitment pattern and decrease of motor conduction velocities of the peroneal nerves. Exposures to CS&inf2; at levels below the present threshold limit value (31 mg/m(3)) were associated with significant decreases of motor conduction velocity.

Polyneuropathy induced by carbon disulphide in viscose rayon workers

OBJECTIVES

To understand the prevalence of polyneuropathy and correlations among the clinical manifestations, electrophysiological findings, and degree of exposure to carbon disulphide (CS2) in workers who were exposed to variable concentrations of CS2 in a viscose rayon factory.

METHODS

All the 163 workers received a detailed physical and neurological evaluation. Fixed point air samples were analysed for CS2. Nerve conduction velocity was studied in 26 workers with symptoms similar to neuropathy.

RESULTS

Nine workers (53%) with overt polyneuropathy from the fibre cutting department and 19 workers (13%) with oligosymptoms similar to polyneuropathy from various jobs were noted. The fixed point air concentrations of CS2 were 150-300 ppm in the cutting areas and 15 to 100 ppm in the spinning areas. The estimated eight hour time weighted averages in the fibre cutting areas were 40-67 ppm. The occurrence of polyneuropathy was generally correlated with the degree of exposure to CS2. Nerve conduction velocities (NCVs) were significantly different in the overt polyneuropathy and subclinical polyneuropathy groups from the normal controls. The sensitive indicators for CS2 polyneuropathy were distal latency, motor NCV, and amplitude of sensory nerve action potentials in sensory NCVs.

CONCLUSION

The outbreak of polyneuropathy was attributed to higher concentrations of CS2 in fibre cutting areas. Even in other jobs with relatively lower concentrations of CS2, the hazard of subclinical polyneuropathy cannot be overlooked.

Occupational exposure to hydrogen sulfide in the sour gas industry: some unresolved issues

Occupational exposure to hydrogen sulfide (H2S) and the medical management of H2S-associated toxicity remains a problem in the sour gas industry and some other industrial settings. The acute effects of exposure to H2S are well recognized, but accurate exposure-response data are limited to acutely lethal effects, even in animal studies. Odor followed by olfactory paralysis and keratoconjunctivitis are the characteristics effects of H2S at lower concentrations. H2S-induced acute central toxicity leading to reversible unconsciousness is a "knockdown"; it is controversial whether repeated or prolonged knockdowns are associated with chronic neurologic sequelae but the evidence is suggestive. Knockdowns can be acutely fatal as a consequence of respiratory paralysis and cellular anoxia. Pulmonary edema is also a well-recognized acute effect of H2S toxicity. Human studies of sublethal exposure with satisfactory exposure assessment are almost nonexistent. There are indications, poorly documented at present, of other chronic health problems associated with H2S exposure, including neurotoxicity, cardiac arrhythmia, and chronic eye irritation but apparently not cancer. Rigorous and comprehensive studies in the sour gas industry are difficult, in part because of confounding exposures and uncertain end points.

Brain damage caused by hydrogen sulfide: a follow-up study of six patients

Hydrogen sulfide (H2S) poisoning involves a risk of hypoxic brain damage. Six patients who lost consciousness due to H2S poisoning are described. The symptoms varied from anosmia in the patient with the shortest but highest exposure to delayed neurological deterioration in the patient with the longest exposure. The two patients with the most serious symptoms developed pulmonary edema, which may have prolonged the hypoxia. The patients were reexaminated 5 years or more after the poisoning. The five patients who had been unconscious in H2S atmosphere for from 5 to 15-20 min showed persisting impairment at neurological and neuropsychological re-examination. Memory and motor function were most affected. One patient was seriously demented. Recent reports of large groups of H2S-poisoned workers probably underestimate the risk of sequelae, due to the inclusion of cases with exposure of short duration and lack of follow-up.

Growth and development in the rat during sub-chronic exposure to low levels of hydrogen sulfide

The effects of low levels of hydrogen sulfide (H2S) on mammalian growth and development are unknown although it has long been postulated that H2S can inhibit critical developmental functions through the cleavage of disulfide bonds and chelation of essential metal ions. Gravid rat dams exposed to H2S (less than or equal to 75 PPM) from day 6 of gestation until day 21 postpartum (PP) demonstrated normal reproductive parameters until parturition. At parturition, however, delivery time was extended in a dose dependent manner with a maximum increase of 42% at 75 PPM. Maternal liver cholesterol content was elevated significantly on day 21 postpartum following exposure to 75 PPM H2S each day for 6 weeks. Pups which were exposed in utero and neonatally to day 21 postpartum developed with a subtle decrease in time of ear detachment and hair development and with no other observed change in growth and development through day 21 postpartum.

Metabolic changes in rat brain synaptosomes after exposure to sulfide in vivo

The effects of exposure of rats to sulfide and the action of exogenous heme were studied in rat-brain synaptosomes. Exposure to sulfide impaired the respiration of synaptosomes which was reversed by heme (4 mg/kg body weight). Sodium sulfide caused partial inhibition of gamma-aminobutyric acid (GABA) and dopamine uptake, strongly inhibited veratridine-dependent release of these neurotransmitters and reduced veratridine-dependent changes in transmembrane electrical potential. Heme treatment did not reverse these changes.

The influence of simultaneous exposure to carbon disulfide and hydrogen sulfide on the peripheral nerve toxicity and metabolism of carbon disulfide in rats

Three groups of 10 male Sprague-Dawley rats were exposed daily, 5 days a week for 25 weeks, either to 500 ppm carbon disulfide (CS2), 50 ppm hydrogen sulfide (H2S), or to both of them as a mixture and were periodically examined for sensory and motor tail nerve conduction velocity (SNCV, MNCV). A concomitant control group of 10 rats was used. In addition, rats exposed to 500 ppm CS2, and those simultaneously exposed to 500 ppm CS2 and 50 ppm H2S, were twice examined for 24-h urine excretion of 2-thio-thiazolidine-4-carboxylic acid (TTCA) in the course of the experimental period. Simultaneous exposure to CS2 and H2S had no significant interactive effect on nerve conduction velocities. A significant time-dependent slowing down of MNCV and SNCV occurred as the result of chronic exposure to CS2, including exposure to 500 ppm CS2 and to the mixture of 500 ppm CS2 and 50 ppm H2S, but did not occur after chronic exposure to 50 ppm H2S. With combined exposure to 500 ppm CS2 and 50 ppm H2S, the quantity of TTCA excreted in 24-h urine was not significantly different from that occurring in response to CS2 exposure alone. On the basis of these results it is suggested that chronic exposure to H2S would neither influence CS2-induced peripheral nerve toxicity nor obscure the interpretation of the measurement of urinary TTCA as a biological indicator of CS2 exposure.

A critical review of the literature on hydrogen sulfide toxicity

The information available on the biological activity of hydrogen sulfide has been examined for present status of critical results pertaining to the toxicity of hydrogen sulfide. This review of the literature is intended as an evaluative report rather than an annotated bibliography of all the source material examined on hydrogen sulfide. The information was selected as it might relate to potential toxic effects of hydrogen sulfide to man and summarized, noting information gaps that may require further investigation. Several recommendations are listed for possible consideration for either toxicological research or additional short- and long-term tests. Two bibliographies have been provided to assist in locating references considered in this report: (1) literature examined but not cited and (2) reference citations. The majority of the references in the first bibliography were considered peripheral information and less appropriate for inclusion in this report.

Neurotoxicity of industrial chemicals and contaminants: aspects of biochemical mechanisms and effects

Nervous system toxicity by industrial chemicals and contaminants cannot be explained by a single mechanism because of the complex functional capacity of the brain and spinal cord. The nervous system is dependent on the adequate supply of oxygen, and therefore, its lack in the ambient air causes rapid dysfunction. Similar effects to hypoxia are caused by agents which reduce oxygen availability at the cellular level by histotoxic mechanisms, e.g., through the inhibition of mitochondrial respiration. Hypoxic effects are rapidly produced, and in cases where a significant number of cells are fatally damaged permanent disability may remain. Acutely harmful effects are also brought about by organic solvents. The typical mode of exposure is that of vapour inhalation. The lipid-soluble molecules pass the blood-brain barrier rapidly and depress the nerve cell membrane functions. This hinders the formation of action potentials. Neuronal cells acquire tolerance towards the membrane-depressant effects in repeated or prolonged exposure so that similar doses do not produce equivalent effects. The development of the tolerance can be regarded as one of the long-term effects of lipophilic chemicals. They may also produce metabolic adaptation so that their biotransformation is enhanced. This may increase the risk of producing more toxic intermediates. The toxic effects thus created can be cumulative since neurons do not multiply by cell division in the postnatal life. The neurotoxicity of metals is more clearly associated with the accumulation of the dose. Adult brain possesses a blood-brain barrier to many water-soluble compounds such that a threshold concentration must be overcome before appreciable toxic effects are seen. Children are in this respect more vulnerable because of their immature barrier function.

Dose-dependent effects of peroral dimethylformamide administration on rat brain

Three-month-old Wistar rats were given dimethylformamide in their drinking water at three concentrations. Succinate dehydrogenase activity decreased at the two higher doses in brain after 2 or 7 weeks. Decreased glutathione concentration occurred at the highest dose. Cerebral azoreductase activity was below the control range after 7 weeks at all doses. Glial cell succinate dehydrogenase activity was below the control range in all animals. No qualitative changes in the spinal cord axon protein composition were detected. It is postulated that formic acid generated in the dimethylformamide metabolism might have led to a significant derangement of cerebral energy metabolism.

Effects of single and repeated exposures to thermo-oxidative degradation products of poly(acrylonitrile-butadiene-styrene) (ABS) on rat lung, liver, kidney, and brain

Male Wistar rats were exposed to thermo-oxidative degradation products of heated poly(acrylonitrile-butadiene-styrene) (ABS). The exposures were conducted once, three times or ten times (5 nights/week, 6 h/night) in the nighttime. The degradation products included styrene, various nitriles, aldehydes, acids, and a significant aerosol fraction. The oxygen concentration in the exposure chamber was constantly above 20%. The shortest exposures caused a significant reduction of the 0-deethylation activity in lung and kidney but not in liver, as well as a decrease in tissue reduced glutathione concentration in liver and kidney but not in lung. These effects well-nigh disappeared during the two-week exposure. In these rats the cerebral glutathione was below the control range. Superoxide dismutase activity increased in liver and brain during the three-day exposure. In liver the activity reached the control value after the two-week exposure but the cerebral activity was significantly lower than in controls. The complex mixture of noxious compounds in the ABS fumes does not readily allow identification of causative agents. Nitrile-dependent histotoxic, peroxidative and reactive metabolite mediated mechanisms may be involved.

Biochemical effects of carbon monoxide poisoning in rat brain with special reference to blood carboxyhemoglobin and cerebral cytochrome oxidase activity

Male Wistar rats exposed to 1000 ppm carbon monoxide for 3 h showed a rapid removal of carbon monoxide from the blood, and a cerebral cytochrome oxidase activity within the control range immediately after the end of the exposure. The cytochrome oxidase activity decreased while carboxyhemoglobin concentration diminished during the reoxygenation period. The effect might have been caused through a loss of mitochondria by increased lipid peroxidation as cerebral glutathione concentration decreased and lysosomal acid proteinase activity increased in glial cell fractions. The present results seem to indicate that the cerebral cytochrome oxidase may not be specifically inhibited in non-lethal carbon monoxide poisoning despite its proven interactions in vitro.