Effect of repeated intraperitoneal injections of soman on schedule-controlled behavior in the rat

Long-term behavioral consequences of soman poisoning in mice

We investigated the long-term (up to 90 days) consequences of soman intoxication in mice on weight, motor performances (grip strength, rotarod) and mnemonic cognitive processes (T-maze, Morris water maze test). First, a relative weight loss of 20%, measured 3 days after intoxication, was evidenced as a threshold beyond which neuropathological damage was observed in the hippocampus. Animals were then distributed into either low weight loss (LWL) or high weight loss (HWL) groups according to the relative 20% weight loss threshold. Compared to controls, both groups of poisoned mice quickly exhibited a decrease in their motor performance subsequent to an acute soman toxicity phase. Then, total motor recovery occurred for the LWL group. Comparatively, HWL mice showed only transient recovery prior to a second decrease phase due to soman-induced delayed toxicity. One month after intoxication, mnemonic cognitive performances of the LWL group were similar to controls while the HWL group did not exhibit any learning skill. Three months after poisoning, compared to controls, the LWL group showed similar mnemonic performances in the maze test but a mild deficit in the Morris water maze task. At the same time, learning skills slightly recovered in the HWL group. Mnemonic cognitive data are discussed in relation to the neuropathology, neurogenesis and sprouting occurring in the hippocampus of soman-intoxicated animals.

Effects of sarin on the operant behavior of guinea pigs

The present study evaluated the dose-response effects of subacute exposure to sublethal doses of the organophosphorus (OP) chemical warfare nerve agent (CWNA) sarin (GB) on the operant behavior of guinea pigs. Dietary restricted guinea pigs, trained to respond for food under a progressive ratio (PR) schedule of reinforcement, were injected five times per week (Monday-Friday) for 2 weeks with fractions (0.1, 0.2, and 0.4) of the established LD(50) of GB (42 microg/kg). Changes in body weight, whole blood (WB) acetylcholinesterase (AChE) levels, and operant performances were monitored over the 2 weeks of GB exposure and for an additional 2 weeks following the termination of exposures. There were dose-related changes in body weight and WB AChE levels throughout the exposure and post-exposure periods. Several parameters of PR performance were disrupted during exposure to 0.4 LD50 GB, however, concurrent weight loss indicated the presence of overt toxicity. PR performance recovered following the termination of exposures. Lower doses (0.1 and 0.2 LD50) of GB failed to produce reliable effects on operant performance during the exposure period. Overall responding decreased during exposure to 0.4 LD50 GB, resulting in reduced response rates and break points. The decrease in overall response rates was attributed to an increase in pausing since there was no decrease in running rate. Motor effects of 0.4 LD50 GB were evident as an increase in the proportion of lever press durations > or = 1.0 s. In the present study, doses of GB lower than 0.4 LD50 produced no marked alteration of operant performance in guinea pigs, although WB AChE levels were maximally inhibited to 20% of control.

The dose-response effects of repeated subacute sarin exposure on guinea pigs

The present study assessed the effects of repeated subacute exposure to the organophosphorous nerve agent, sarin. Guinea pigs were injected five times per week (Monday-Friday) for 2 weeks with fractions of the established LD(50) dose of sarin (42 microg/kg sc). The animals were assessed for the development of cortical EEG seizures. Changes in body weight, red blood cell (RBC) acetylcholinesterase (AChE) levels and neurobehavioral reactions to a functional observational battery were monitored over the 2 weeks of sarin exposure and for an extended postinjection period. There were dose-related changes in body weight and RBC AChE levels. No guinea pigs receiving 0.3, 0.4 or 0.5 x LD(50) of sarin showed signs of cortical EEG seizures despite decreases in RBC AChE levels to as low as 10% of baseline. Seizures were evident in animals receiving 0.6 x LD(50) of sarin as early as the second day, and subsequent injections led to incapacitation and death. Animals receiving 0.5 x LD(50) sarin showed obvious signs of cholinergic toxicity, which included a significant increase in their angle of gait. Overall, 2/13 animals receiving 0.5 x LD(50) sarin died before all 10 injections were given. By the 10th day of injections, the animals receiving saline were significantly easier to remove from their cages and handle as compared to the first day of injections. They were also significantly less responsive to an approaching pencil and touch on the rump in comparison to the first day of testing. In contrast, the animals receiving 0.4 x LD(50) sarin failed to show any significant reductions in their responses to an approaching pencil and a touch on the rump as compared to the first day. The 0.5 x LD(50) sarin animals failed to show any significant changes to the approach response and touch response and did not adjust to handling or cage removal from the first day of injections to the last day of handling. In summary, the guinea pigs receiving the 0.4 x LD(50) and 0.5 x LD(50) doses of sarin failed to habituate to some aspects of the functional observational battery testing.

Chemical warfare agents: estimating oral reference doses

Health risk assessments for sites contaminated with chemical warfare agents require a comparison of the potential levels of exposure with a characterization of the toxic potency of each chemical. For noncancer health effects, toxic potency is expressed in terms of Reference Doses (RfD). A RfD is a daily exposure level or dose (usually expressed in units of milligrams of chemical per kilogram body weight per day) for the human population, including sensitive subpopulations, that is likely to be without an appreciable risk of deleterious effects. A daily exposure at or below the RfD is not likely to be associated with health risks, but as the amount of chemical that an individual is exposed to increases above the RfD, the probability that an adverse effect will occur also increases. A RfD is derived by first examining the available human or animal toxicity data to identify a dose or exposure that corresponds to a no-observed-adverse-effect level (NOAEL) or a lowest-observed-adverse-effect level (LOAEL). The NOAEL is the exposure level at which there are no statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control. Effects may be produced at this level, but they are not considered to be adverse if they do not result in functional impairment or pathological lesions that affect the performance of the whole organism or which reduce an organism's ability to cope with additional challenge. The LOAEL is the lowest exposure level at which there are statistically or biologically significant increases in frequency or severity of adverse effects between the exposed population and its appropriate control. If only a LOAEL is identified by the toxicity data, a NOAEL is estimated by dividing the LOAEL by a factor no greater than 10. This extrapolation factor of 10 or less is termed the LOAEL-to-NOAEL Uncertainty Factor (UFL). The NOAEL is also adjusted by the application of other Uncertainty Factors, including (1) a UFH < or = 10 to ensure that the resulting RfD protects segments of the human population that may be more sensitive to the chemical than the average person; (2) a UFA < or = 10 to extrapolate from the experimental animal species to humans; (3) a UFS < or = 10 to extrapolate from an experimental subchronic exposure study to a potential chronic exposure; and (4) a UFD < or = 10 to ensure that the resulting RfD is protective for all possible adverse effects, particularly those that may not have been adequately evaluated in the available studies. A Modifying Factor (MF), based on a qualitative professional assessment of the data, may also be used to account for other factors (e.g., deficiencies in the critical study) that are not adequately covered by the standard Uncertainty Factors. 1. Agent HD (Sulfur Mustard). RfDe = 7 x 10(-6) mg kg-1 d-1. A LOAEL was identified in a two-generation reproductive toxicity study conducted in rats. A total uncertainty factor of 3000 was applied to account for protection of sensitive subpopulations (10), animal-to-human extrapolation (10), LOAEL-to-NOAEL extrapolation (3), and extrapolation from a subchronic to chronic exposure (10). A LOAEL-to-NOAEL UF of 3, instead of the default value of 10, was used because the critical effect (stomach lesions) was considered to be "mild" in severity and may have been enhanced by the vehicle used (sesame oil in which sulfur mustard is fully soluble) and the route of administration (gavage), which is more likely to result in localized irritant effects. The key study did identify a toxic effect that is consistent with the vesicant properties of sulfur mustard. In none of the other available studies was there any indication of a different effect occurring at a lower exposure level.

Behavioral toxicity of anticholinesterases in primates: effects of daily repeated soman exposure

The effects of repeated daily exposure to soman, an organophosphate nerve agent, on the performance of a well-learned, compensatory tracking task were tested in rhesus monkeys. The ED50 daily exposure required to produce a performance decrement on or before the fifth daily exposure (0.97 microgram/kg) was about 40% of the acute dose required to produce a similar performance decrement. After repeated, low-dose exposures, performance decrements appeared when serum cholinesterase (ChE) activity was inhibited 85-90%. Acute exposures that produced similar performance effects were associated with lower levels of ChE inhibition (65-70%), suggesting that repeated daily exposure may lead to the development of a tolerance (physiological or behavioral) to low levels of ChE activity.

Behavioural toxicity of anticholinesterases in humans and animals--a review

1. Available information describing the behavioural changes induced by anticholinesterases in humans and animals is reviewed. 2. Very little confidence can be placed in existing descriptions of the behavioural effects of anticholinesterases in humans. 3. Although data from animal experimental studies is reliable, the information obtained is relatively superficial. 4. It is concluded therefore, although much information is available, surprisingly little is known of the behavioural changes induced by anticholinesterases.

Differential sensitivity of CNS regions to acetylcholinesterase inhibition following chronic low-dose soman treatment in the rat

Several discrete brain regions were analyzed for regional brain sensitivity to acetylcholinesterase (AChE) inhibition following administration of repeated low-dose soman (39 micrograms/kg, SC; once daily for 5 days). The same brain regions were also assayed on days 3 through 7 after the last injection of soman, to examine possible recovery of AChE. The observed regional brain sensitivity to AChE inhibition can be described in the following order: frontal cortex = piriform cortex greater than hippocampus = cerebellum = thalamus greater than striatum = hypothalamus. Most brain areas showed only a small recovery from this inhibition through the seventh day following the last soman injection. The progressive nature of the AChE inhibition during the week of soman administration and the failure to find a significant interaction between injection and day indicates that tolerance to soman exposure did not develop in this paradigm. The protocol described here appears to provide a useful method to study prophylactic measures for low-dose repeated soman exposure and evaluation of differential CNS inhibition and recovery from AChE inhibition.