Neurotoxicity Endpoints

The use of SPMDs to sequester hydrophobic contaminants for incorporation into bioindicator test-based screening is increasing in both frequency of application and in the array of modes of action. Eor example, as a focused part of a broader 

Following dialysis and treatment by SEC, the sample extracts were solvent exchanged into sterile DMSO. Subsequently, four rainbow trout Oncorhynchus mykiss [RBT]) were placed in each of seven tanks (each tank is considered as a treatment and a replicate is an individual fish within a tank) in 18 °C well water (280 mg hardness as CaCOs) using flow-through conditions. RBT were fed once daily throughout the study. Following a 48 hour acclimation, RBT were injected interperitoneally with 100 pL of a 1 1 mixture of an SPMD extract or appropriate controls in DMSO or corn oil. Controls included non-deployed SPMD extracts, SEC blanks, and DMSO blanks. The same injection procedure was repeated 6 days later. RBT were sacrificed 11 days after initial exposure to the extracts, and the plasma, liver, gills, and brain were immediately removed from each fish and maintained at -80 °C until assayed. 

An examination of the data reveals that the average values of plasma cholinesterase were depressed in fish exposed to the SPMD sample extracts from the IWWTP site compared to all controls, although only significantly lower compared to the SEC blanks (p 0.05). A similar trend was observed with the Nogales Wash sample extracts. A number of chemicals determined to be present in the 

SPMD sample extracts, e.g., certain organochlorine pesticides (OCPs), are known to inhibit cholinesterase activity. Therefore, these results were not unexpected. However, it was surprising that a similar response was not observed with brain cholinesterase activity. It is possible that brain cells can more readily metabolize the chemicals, that the chemicals did not pass the brain blood barrier or that the effects occurred earlier in the exposure period, effectively allowing the activity to recover. Considering the numerous neurotoxic chemicals potentially entering aquatic ecosystems or present as airborne vapor phase chemicals, the neurotoxic mode of action related to exposure to contaminants is of increasing interest. Evidence presented in this work demonstrate that SPMDs concentrate members of this class of toxicants. 

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Based upon the review of the toxicological data, reliable lifetime TDI values for the organotin species in question cannot be derived, since long-term studies at the appropriate doses and in the appropriate species are not available. Medium-term exposure results have therefore been used to derive TDIs for preliminary risk characterization. For dimethyltin, there is a reliable NOAEL as a basis for setting a TDI against a neurotoxicity endpoint. For the remaining compounds, best estimates of amedium-term exposure TDI for preliminary risk characterization have been derived from the available studies (Table 25). 

A two-generation reproduction study is needed with at least 6 hr/d exposure continuing to pups after weaning and into the F2 generation to determine the effects of long-term exposures. Developmental neurotoxicity endpoints should be included in this study based on the types of effects seen in the peri- and postnatal study with the snout-only 1 hr/d exposure. 

Child Study Group (N) Pb Exposure History Neurotoxic Endpoints References... 

The temporality criterion among the Hill causality proofs can be readily answered in the affirmative. For example, one aspect of temporality that was once of concern was the potential existence of reverse causality, i.e., is elevated blood lead concurrent with a developmental neurotoxic endpoint because the neurotoxic impairment enhances lead exposure rather than the reverse The outcomes of multiple prospective epidemiological studies of lead neurotoxicity in children effectively ruled out the likelihood of reverse causality. 

CBs, like OPs, can cause a variety of sublethal neurotoxic and behavioral effects. In one study with goldfish Carrasius auratus), Bretaud et al. (2002) showed effects of carbofuran on behavioral end points after prolonged exposure to 5 pg/L of the insecticide. At higher levels of exposure (50 or 500 pg/L), biochemical effects were also recorded, including increases in the levels of norepinephrine and dopamine in the brain. The behavioral endpoints related to both swimming pattern and social interactions. Effects of CBs on the behavior of fish will be discussed further in Chapter 16, Section 16.6.1. 

Most insecticides, especially the organophosphate group, cause neurotoxicity as their major mode of action. Assessment of the neurotoxicity includes neurochemical endpoints such as cholinesterase (including acetylcholinesterase, which is the major neurotransmitter in vertebrates such as fish, and other enzymes such as butyrylcholinesterase) inhibition and behavioral endpoints such as swimming speed [79]. Studies done in rats show the neurotoxic action of insecticides such as dimethoate, methyl parathion, dichlorvos, ethyl parathion or propoxur after a prolonged exposure [80,81]. 

Screens almost always focus on detecting a single endpoint of effect (such as mutagenicity, lethality, neurotoxicity, or development toxicity), and have a particular set of operating characteristics in common. 

Additionally, there are specialized studies designed to address endpoints of concern for almost all drugs (carcinogenicity, reproductive or developmental toxicity) or concerns specific to a compound or family of compounds (local irritation, neurotoxicity, or immunotoxicity, for example). When these are done, timing also requires careful consideration. It must always be kept in mind that the intention is to ensure the safety of people in whom the drug is to be evaluated (clinical trials) or used therapeutically. An understanding of special concerns for both populations should be considered essential. 

Many of the studies done in safety assessment are multiple endpoint screens. Such study types as a 90-day toxicity study or immunotox or neurotox screens are designed to measure multiple endpoints with the desire of increasing both sensitivity and reliability (by correspondence-correlation checks between multiple data sets). 

Neurotoxicology. Table 7.6 presents the FDA s current draft criteria (FDA, 1993, 2000) for endpoints to be incorporated in studies as a screen for neurotoxicity. IN practice, a functional observation battery is employed at several endpoints (usually one and three months in to the study) to fill these requirements. 

Information regarding health effects of fuel oils in humans and animals is available for the inhalation, oral, and dermal routes of exposure. Most of the information in humans is from cases of accidental ingestion of kerosene that resulted in respiratory, neurotoxic, and to a lesser extent, gastrointestinal effects. In addition, a few case studies have identified these effects as well as cardiovascular, hematological, and renal effects in humans after inhalation and/or dermal exposures to fuel oils. Fuel oils appear to be eye and skin irritants in both animals and humans following direct contact. Animal data exist for most systemic effects however, the data are inconclusive for many of the endpoints. Further, a number of the animal studies... 

The 1998 OECD test guidelines for the oral 28-/90-day studies (see Table 4.12) examine a number of simple nervous system endpoints, e.g., clinical observations of motor and autonomous nervous system activity, and histopathology of nerve tissue. It should be recognized that the standard 28-/90-day tests measure only some aspects of nervous system stmcture and function, while other aspects, e.g., learning and memory and sensory function is not or only superficially tested. Primarily the standard 28-/90-day tests are intended as a screening for neurotoxicity and depending on the results, further testing may be needed. 

A number of studies have been undertaken to investigate whether different endpoints of concern, which might give rise to effects at low doses like immunotoxic, endocrinologic, neurotoxic, and developmentally toxic effects could be included in the TTC values. The Nordic group considered that not aU these endpoints have been adequately covered by the analyses performed today. 

Repeat-dose neurotoxicity studies may identify behavioral effects or impaired nerve functions that can interfere with mating or maternal care. Developmental neurotoxicity studies have been conducted for specific pesticide classes, following requirements of US-EPA. If such a study is available it can be examined not only for the study-specific endpoints on the developing brain but also compared to the prenatal toxicity study and the two-generation smdy with respect to general endpoints of pre- and postnatal development, respectively. 

Winneke G (1995) Endpoints of developmental neurotoxicity in environmentally exposed children. Toxicol Lett, 77(1-3) 127-136. 

QSAR models exist for a wide range of biological and toxicological endpoints, e.g. published QSAR models for toxicity cover over 30 different endpoints, from carcinogenicity, mutagenicity, skin sensitization, eye irritation, neurotoxicity to gastric irritancy etc. Dearden (2003) and Barratt and Rodford (2001) review in their publications some of the recent QSAR models for various endpoints. 

European Commission 2003a). However, the risk assessors seem to be more reluctant to use this default assumption to assess the relevance of the observed indications of developmental neurotoxicity in rodents. For this endpoint additional data are requested. 

The UEL is only for effects that are observed at birth and only for a short term exposure. No long-term follow-up studies (e.g., on neurotoxicity) have been conducted. UELs for other reproductive endpoints cannot be calculated. A UEL for chronic exposure to JP-8 was not calculated because there are no chronic toxicity studies on JP-8 reported in the literature. Conversion from mg/ kg/day by the oral route to the equivalent concentration in inhaled air to achieve the same daily... 

Data on the toxicity and disposition of JP-8 in animals are sparse, and no data are available for humans. No reproductive toxicity studies have been done in experimental animals. One adequate study demonstrated developmental toxicity in rats treated orally at 1,500-2,000 mg/kg/d (Cooper and Mattie 1996). A study in a second species should be supplemented with a multiple-generation reproductive toxicity study in rats or mice, including an evaluation of postnatal endpoints, such as developmental neurotoxicity, immunotoxicity, and hematological, hepatic, and renal effects, that could result from prenatal exposures. 

Repeated inhalation or oral exposures to moderate to high doses of -butyl acetate and -butanol are well tolerated. These aforementioned molecules are readily and rapidly metabolized to -butyric acid. The no-observed-effect level (NOEL) for repeated dose oral exposure to -butanol was 125 mg kg day. In a 90 day inhalation study in rats with -butyl acetate a NOEL of 500 ppm was reported for systemic effects, and a NOEL of 3000 ppm (highest dose tested) was reported for postexposure neurotoxicity based on functional observational battery endpoints, quantitative motor activity, neuropathy, and sched-uled-controlled operant behavior endpoints. Results of inhalation studies conducted on -butanol and -butyl acetate were negative for inducing reproductive and developmental toxicity. The NOEL for female reproductive toxicity was 6000 ppm with -butanol and 1500 ppm for -butyl acetate. In a 90 day repeated-dose inhalation toxicity study with butyl acetate the NOEL for male reproductive toxicity was 3000 ppm. For developmental toxicity, a NOEL of 3500 ppm was observed with -butanol and a NOEL of 1500 ppm (the highest exposure tested) was seen in both rats and rabbits following exposure to -butyl acetate. 

The oral LD50 in rats for tetrahydrofuran is 2.3 ml kg A single 4h inhalation study of tetrahydrofuran in rabbits at 100-12 000 ppm produced a transient dose-related decrease of tracheal ciliary activity. Concentrations of tetrahydrofuran 25 000 ppm produced anesthesia. Tetrahydrofuran was irritating to rabbit skin when applied topically in solutions exceeding a 20% concentration. No lasting acute adverse effects on neurological endpoints were seen with THF exposures of 0, 500, 2500, or 5000 ppm for 6 h in rats except for sedation. The no-observed-effect level (NOEL) for the acute neurotoxicity in rats was 500 ppm. 

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