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Neurotoxicity

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Description of key information

Key value for chemical safety assessment

Effect on neurotoxicity: via inhalation route

Endpoint conclusion
Endpoint conclusion:
adverse effect observed
Dose descriptor:
NOAEC
19 000 mg/m³
Study duration:
subchronic
Species:
rat

Additional information

Specific information - Neurotoxicity.

In a developmental neurotoxicity study, rats were exposed both pre- and/or postnatally to ethanol at a dose of 1 g/kg of body weight of dams/day via maternal peroral intubation. This dose resulted in a maternal blood ethanol level of 29-41mg/100ml. This dose significantly increased the mortality rate in offspring exposed to ethanol during pregnancy with a continued postnatal exposure having no additional effect. Growth of the offspring was initially delayed but recovered within 9 weeks after exposure ceased. A two-way active avoidance test carried out on offspring from dams exposed to ethanol during gestation and lactation showed an impairment, compared with the controls, of learning and memory in both male and female adolescent rats, as well as in male, but not in female, 5-month-old rats indicated by an almost doubling of the number of "poor learners," compared to the control group. Results from this study suggest that ethanol at a dose of 1 g/kg/day administered to dams during gestation and lactation produced growth and behavioral changes in the offspring.

Male Sprague-Dawley rats were exposed 7 hours per day for six weeks to 10,000 or 16,000ppm ethanol by inhalation and then mated with untreated rats. Pregnant females received the same experimental treatment from day 1 -19 of gestation and were allowed to deliver their offspring. Treatment with ethanol did not affect the weight gain of parental animals. Offspring from paternally or maternally exposed animals performed as well as controls in tests of neuromotor coordination, activity levels, and learning ability. Levels of acetylcholine, dopamine, substance P, and beta-endorphin were unchanged in the brain in the offspring of ethanol exposed rats. Complex, but significant changes in the levels of norepinephrine occurred only in paternally exposed offspring. 5 -Hydroxytryptamin levels were reduced in the cerebrum, and met-enkephalin levels were increased in all brain regions of offspring from both maternally and paternally exposed rats. The same authors also exposed male rats to ethanol in an inhalation chamber at concentrations of 11,000ppm or 16,000ppm for six weeks. After a two day non-exposure period they were mated to untreated females for 5 days. Pregnant females received the same experimental treatment from day 1 -20 of gestation and were allowed to deliver their litters. Fertility or litter sizes were not affected by the ethanol exposure and showed no difference from controls in any of the behavioural tests conducted.

In a study designed to assess whether acute ethanol administration selectively impairs the memory of certain spatial tasks, groups of male mice were assessed in spatial and non-spatial tasks using a Morris water maze before and following intraperitoneal injections of 1.25, 1.75 or 2.25 g ethanol/kg bw. Spatial and non-spatial memory were assessed using latency to locate a hidden or visible escape platform, or path length. A single acute injection of ethanol at 1.75 g/kg bw and above was shown to selectively impair spatial memory in male mice; non-spatial memory was impaired at the higher dose of 2.25 g/kg bw. No significant impairment of spatial or non-spatial memory (measured as latency, path lengths or swim speeds to reach the escape platform) was seen in mice following administration of a single injection of 1.25 g ethanol/kg bw (the lowest tested dose).

Behavioural and neurochemical alterations were investigated in 21 -day-old pups born from female Wistar rats administered (by gavage) 0.5 or 4.0 g ethanol/kg bw/day, for 30 days before mating, throughout their gestational period, and to weaning (at 21 days). Effects of ethanol on locomotion, anxiety, and CNS depression were evaluated using open field, elevated plus maze, and forced swimming tests, respectively. Binding assays were used to identify dopaminergic and muscarinic receptors. Significant and dose-dependant increases in the number of entrances in the open arms and in the time of permanence in the open arms were observed in the plus maze test in the offspring of rats exposed to ethanol as compared to controls, indicating an anxiolytic effect. Spontaneous locomotor activity and occurrences of rearing and grooming were decreased in the offspring of ethanol exposed rats in the open field test. Exposed offspring also showed dose-dependant increases in their immobility time in the forced swimming test, indicative of CNS depression (despair behaviour). In the offspring of rats exposed at the higher ethanol level, decreases in the hippocampal and striatal dopaminergic binding were detected, whilst there were significant increases in muscarinic binding in the hippocampus as well as in the striatum in these rats, compared to the offspring of unexposed controls. It is unclear from this study, whether the effects seen may be due to a developmental effect on the offspring, or merely result from exposure to a high level of ethanol via the dams milk up to weaning.

A range of tests was carried out to compare the neurobehavioural (CNS) effects of a single dose of ethanol between rats and humans, including standardised observational measures, spontaneous motor activity assessments, and learned visual discrimination performance. Groups of rats were given ethanol at levels of approximately 0.5, 1.0 or 2.0 g/kg bw. In a human volunteer study, 12 healthy male subjects were given 0.65 g ethanol/kg bw and neurobehavioural effects were measured prior to and 1 and 3 hours after ethanol administration, with a computerised neurobehavioural test battery. Results of the behavioural tests in rats provided evidence of ethanol-induced changes in neuromuscular, sensori-motor, and activity domains. There were also significant changes in visual discrimination, particularly in the areas of general measures of responding and psychomotor speed. In humans, there were small but statistically significant effects on learning and memory, psychomotor skills and attention. These effects were subtle, however, and not all parameters within given domains were affected. The studies demonstrated a qualitative similarity in response to ethanol between rats and humans.

Five male volunteers were exposed for six hours to air containing ethanol at 0, 250, 500 and 1000 ppm. Sensitive markers of neuromotor disturbance (including reaction times, body sway, hand tremour and rapid alternating movements) were measured before and after each exposure. A statistically significant decrease in diadochokinesia velocity (maximum speed) and harmonic index (left hand), and increase in sway velocity (closed eyes), was seen in volunteers exposed to air containing ethanol at 500 ppm (about 942 mg/m3), when compared to pre-exposure measures. However, no statistically significant changes were seen at any other exposure concentration (including 1000 ppm). Therefore, the NOAEC for neuromotor effects of ethanol in this acute human volunteer study can be considered as 1000 ppm.

In a study to assess the potential for developmental neurotoxicity in the offspring of rats exposed to ethanol, pregnant Long-Evans rats were exposed to ethanol vapour concentrations of 5000, 10000 and 21000ppm from GD9 -20. The offspring were subjected to motor locomotor and functional observation battery (FOB) assessment up to post natal day 180 and cognitive development assessments from post natal day 63 onwards. In addition, biochemical and blood pressure measurements were also taken. No overt toxicity in the dams was observed. Ethanol did not affect litter size or weight, or postnatal weight gain in the pups. Motor activity was normal in offspring through postnatal day (PND) 29. On PND 62, the 5000 and 21,000 ppm groups were more active than controls. On PND 29 and 62, FOB assessment of the offspring revealed small changes in the neuromuscular and sensorimotor domains that were not systematically related to dose. In the cognitive development assessments, the only response of the many assessed that was seen to increase with dose was the anticipatory response to choice reaction time seen in the high dose seen in males (females not tested). All other responses showed no effect with ethanol exposure or effects were only seen in one of the lower dose groups, or similarly in all dose groups (possibly due to confounding) and can therefore be concluded as not biologically significant. Systolic blood pressure was increased by 10,000 ppm ethanol in males at PND 90 but not at PND 180. No differences in lipoprotein profile, liver function, or kidney function were observed. In summary, prenatal exposure to inhaled ethanol caused some mild changes in physiological and behavioral development in offspring that were not clearly related to inhaled concentration or BEC and not deemed significant plus some slight reductions in anticipatory response times of mature offspringThe apparent low toxicity by inhalation may result from the slow rise in BEC for exposure by this route.

General information

Ethanol can evoke a reduction in anxiety and a feeling of pleasure as well as sedation, motor incoordination, aggression, changes in other forms of social interaction and aberrations in cognition. Certain neurotransmitter systems are more or less sensitive to ethanol with both glutamate and GABA systems targets for ethanol's activity. Particularly sensitive components are certain receptor-gated ion channels including GABA(A) and the N-methyl-D-aspartate subtype of the glutamate receptor, the nicotinic cholinergic receptor and the serotonin-3 receptor. Ethanol also elicits an increase of dopamine in the nucleus accumbents. Some of these systems alter their function (adapt) during periods of chronic drinking and such maladaptation may generate manifestations of tolerance, physical dependence and craving on cessation of intake. Maladaptations in systems that gate calcium ion entry into neurons can contribute to brain damage in some alcoholics. (Reference “Adenylyl cylases and alcohol”. Tabakoff, B., Grant, K.A., Hoffman, P.L., Little, H.J. In Cooper DMF (ed) Advances insystems are more or less sensitive to ethanol with both glutamate and GABA systems targets for ethanol's activity. Particularly sensitive components are certain receptor-gated ion channels including GABA(A) and the N-methyl-D-aspartate subtype of the glutamate receptor, the nicotinic cholinergic receptor and the serotonin-3 receptor. Ethanol also elicits an increase of dopamine in the nucleus accumbents. Some of these systems alter their function (adapt) during periods of chronic drinking and such maladaptation may generate manifestations of tolerance, physical dependence and craving on cessation of intake. Maladaptations in systems that gate calcium ion entry into neurons can contribute to brain damage in some alcoholics. (Reference “Adenylyl cylases and alcohol”. Tabakoff, B., Grant, K.A., Hoffman, P.L., Little, H.J. In Cooper DMF (ed) Advances in  secondary messenger and phoshoprotein research, vol 32, Lippincott-Raven,  Philadelphia, PA p173-93. (1988))

Drinking to a level that results in a blood ethanol concentrations of 50 -60 mg/l to 900 mg/l (20 mM) defined as 'moderate'. Such consumption selectively affects the function of the GABA, glutamatergic, serotonergic, dopaminergic, cholinergic, and opioid neuronal systems. Behavioural consequences are dose and time related and can even change on the rising and falling phases of the blood ethanol curve. A number of studies have noted a measurable diminution in neuropsychologic parameters in habitual consumers of moderate amounts of ethanol, but others have not found such changes. Some have even noted positive effects on cognitive effects in aging humans. Consumption by pregnant women can have significant consequences on the developing fetal nervous system.  

It can be concluded however that all such neurotoxic effects only manifest at blood alcohol leves that are associated with significant deliberate consumption of alcoholic beverages and that there is a large margin of safety between such blood concentrations and those that are seen following exposure via inhalation or accidental consumption.

(References from the following review; Eckardt, MJ; File, SE; Gessa GL; Grant KA; Guerri, C; Hoffman, P; Kalant,  H; Koob, GF; Li TK; Tabakoff, B  (1998) Effects of moderate alcohol  consumption on the central nervous system. Alcoholism Clin Exper Res,  22, 5, 998-1041.)

Justification for classification or non-classification

There is no specific classification for the neurotoxicity end point. Developmental neurotoxicity effects are seen only at very high doses, insufficient to trigger classification. Similarly, repeat dose effects are only seen at concentrations well above the levels required for STOT(RE) classification. Acute effects in humans are only seen at high concentrations and are subtle. They are not considered to be sufficient to trigger a STOT(SE) classification.