Characterizing tolerance to trichloroethylene (TCE): effects of repeated inhalation of TCE on performance of a signal detection task in rats

Long-term perchloroethylene exposure: a meta-analysis of neurobehavioral deficits in occupationally and residentially exposed groups

The literature concerning the neurobehavioral and neurophysiological effects of long-term exposure to perchloroethylene (PERC) in humans was meta-analyzed to provide a quantitative review and synthesis in the form of dose-effect curves. The useable database from this literature comprised studies reporting effects of long-term exposure to PERC, effects that included slowed reaction times, cognitive deficits, impaired color vision, and reduced visual contrast sensitivity. For the meta-analyses, dose was defined as the product of the concentration inhaled PERC and the duration of exposure, expressed in unites of ppm-h/1000 (for numerical convenience). Dose-related results were highly variable across studies. Reports involving low exposure concentrations characteristic of nonoccupational exposures consistently produced effects of a magnitude that were comparable to those reported for higher concentration occupational studies. If this finding is reliable and general, studies of occupationally exposed persons may underestimate the magnitude of effects of PERC and other chemicals in the total population. Given the limited scope of the available data for PERC and its methodological and reporting problems (small sample sizes, testers were not blind to the subjects' exposure conditions, and the timing and location of testing were insufficiently documented), it seems important to test this conclusion with a well-documented study of two groups (occupational and nonoccupational exposure) in which subjects are evaluated in randomized order, using the same procedures and with the testers kept blind to the status of the subjects.

Abuse pattern of toluene exposure alters mouse behavior in a waiting-for-reward operant task

Inhaling solvents for recreational purposes continues to be a world-wide public health concern. Toluene, a volatile solvent in many abused products, adversely affects the central nervous system. However, the long-term neurobehavioral effects of exposure to high-concentration, binge patterns typical of toluene abuse remain understudied. We studied the behavioral effects of repeated toluene exposure on cognitive function following binge toluene exposure on behavioral impulse control in Swiss Webster mice using a "wait-for-reward" operant task. Mice were trained on a fixed-ratio (FR) schedule using sweetened milk as a reward. Upon achieving FR15, a wait component was added which delivered free rewards in the absence of responses at increasing time intervals (2s, 4s, 6s, etc...). Mice continued to receive free rewards until they pressed a lever that reinstated the FR component (FR Reset). Once proficient in the FR-Wait task, mice were exposed to either 1000 ppm, 3600 ppm or 6000 ppm toluene, or 0ppm (air controls) for 30 min per day for 40 days. To avoid acute effects of toluene exposure, behavior was assessed approximately 22-23 h later. Repeated toluene exposure decreased response rates, the number of FR resets, and increased mean wait time, resulting in a higher response-to-reinforcer ratio than exhibited by controls. Mice receiving the higher exposure level (6000 ppm) showed a dramatic decrease in the number of rewards received, which was reversed when toluene exposure ceased. Mice receiving the lower exposure level (1000 ppm) showed little change in the number of rewards. These results indicate that repeated binge exposures to high concentrations of toluene can significantly interfere with performance as measured by a waiting-for-reward task, suggesting a significant impact on cognitive and/or psychomotor function.

Characterization of the effects of inhaled perchloroethylene on sustained attention in rats performing a visual signal detection task

The aliphatic hydrocarbon perchloroethylene (PCE) has been associated with neurobehavioral dysfunction including reduced attention in humans. The current study sought to assess the effects of inhaled PCE on sustained attention in rats performing a visual signal detection task (SDT). Due to its similarities in physiological effect to toluene and trichloroethylene (TCE), two other commonly used volatile organic compounds (VOCs) known to reduce attention in rats, we hypothesized (1) that acute inhalation of PCE (0, 500, 1000, 1500 ppm) would disrupt performance of the SDT in rats; (2) that impaired accuracy would result from changes in attention to the visual signal; and (3) that these acute effects would diminish upon repetition of exposure. PCE impaired performance of the sustained attention task as evidenced by reduced accuracy [P(correct): 500 to 1500 ppm], elevated response time [RT: 1000 and 1500 ppm] and reduced number of trials completed [1500 ppm]. These effects were concentration-related and either increased (RT and trial completions) or remained constant [P(correct)] across the 60-min test session. The PCE-induced reduction in accuracy was primarily due to an increase in false alarms, a pattern consistent with reduced attention to the signal. A repeat of the exposures resulted in smaller effects on these performance measures. Thus, like toluene and TCE, inhaled PCE acutely impaired sustained attention in rats, and its potency weakened upon repetition of the exposure.

Acute Toluene Exposure and Rat Visual Function in Proportion to Momentary Brain Concentration

Acute exposure to toluene was assessed in two experiments to determine the relationship between brain toluene concentration and changes in neurophysiological function. The concentration of toluene in brain tissue at the time of assessment was estimated using a physiologically based pharmacokinetic model. Brain neurophysiological function was measured using pattern-elicited visual evoked potentials (VEP) recorded from electrodes located over visual cortex of adult male Long-Evans rats. In the first experiment, VEPs were recorded before and during exposure to control air or toluene at 1000 ppm for 4 h, 2000 ppm for 2 h, 3000 ppm for 1.3 h, or 4000 ppm for 1 h. In the second experiment, VEPs were recorded during and after exposure to clean air or 3000 or 4000 ppm toluene. In both experiments, the response amplitude of the major spectral component of the VEP (F2 at twice the stimulus rate in steady-state responses) was reduced by toluene. A logistic function was fit to baseline-adjusted F2 amplitudes from the first experiment that described a significant relationship between brain toluene concentration and VEP amplitude deficits. In the second experiment, 3000 ppm caused equivalent VEP deficits during or after exposure as a function of estimated brain concentration, but 4000 ppm showed a rapid partial adaptation to the acute effects of toluene after exposure. In general, however, the neurophysiological deficits caused by acute toluene exposure could be described by estimates of the momentary concentration of toluene in the brain at the time of VEP evaluation.

Repeated inhalation of toluene by rats performing a signal detection task leads to behavioral tolerance on some performance measures

Previous work showed that trichloroethylene (TCE) impairs sustained attention as evidenced by a reduction in accuracy and elevation of response latencies in rats trained to perform a visual signal detection task (SDT). This work also showed that these effects abate during repeated exposures if rats inhale TCE while performing the SDT. The present experiment sought to determine whether toluene, another commonly-used solvent, would induce tolerance similarly if inhaled repeatedly during SDT testing. Sixteen male, Long-Evans rats were trained to perform the SDT. Upon completion of training, rats were divided into 2 groups. In Phase I, concentration-effect functions were determined for toluene (0, 1200, 1600, 2000, 2400 ppm) in both groups. Toluene reduced the proportion of correct responses [P(correct)], and increased response time (RT) and response failures. In Phase II, Group-Tol inhaled 1600 ppm toluene while Group-Air inhaled clean air during 11 daily SDT sessions. In Group-Tol the effect of toluene on P(correct) abated after 3 days, while RT remained elevated for the duration of the repeated exposures. In Phase III, toluene concentration-effect functions were re-determined for both groups. Group-Air remained impaired on all test measures, whereas for Group-Tol, toluene did not reduce P(correct), but continued to increase RT. These data confirm our previous hypothesis that animals can develop tolerance to chemical exposures that impair appetitively-motivated behaviors if that impairment leads to loss of reinforcement.

The last decade of solvent research in animal models of abuse: mechanistic and behavioral studies

The abuse of volatile organic solvents (inhalants) leads to diverse sequelae at levels ranging from the cell to the whole organism. This paper reviews findings from the last 10 years of animal models investigating the behavioral and mechanistic effects of solvent abuse. In research with animal models of inhalant abuse, NMDA, GABA(A), glycine, nicotine, and 5HT(3) receptors appear to be important targets of action for several abused solvents with emerging evidence suggesting that other receptor subtypes and nerve membrane ion channels may be involved as well. The behavioral effects vary in magnitude and duration among the solvents investigated. The behavioral effects of acute and chronic inhalant abuse include motor impairment, alterations in spontaneous motor activity, anticonvulsant effects, anxiolytic effects, sensory effects, and effects on learning, memory and operant behavior (e.g., response rates and discriminative stimulus effects). In addition, repeated exposure to these solvents may produce tolerance, dependence and/or sensitization to these effects.

Tolerance and sensitization to inhaled 1,1,1-trichloroethane in mice: results from open-field behavior and a functional observational battery

RATIONALE

1,1,1-Trichloroethane (TCE), a representative abused solvent, has well described acute behavioral effects in animals. Much less is known about repeated high-concentration exposures as would be encountered in inhalant abusers. Tolerance has been demonstrated in some, but not all, studies with TCE while sensitization has also been seen with other abused solvents.

OBJECTIVE

The present study was designed to further characterize changes in the effects of repeated exposure to TCE on a variety of mouse behaviors.

METHODS

Mice were tested using locomotor activity as well as a functional observational battery (FOB) both before and after a regimen of daily exposures to various concentrations of TCE.

RESULTS

The initial locomotor effects of acute 30-min exposures to TCE were biphasic with concentration-dependent increases in activity at lower concentrations and decreases observed at higher concentrations. The profile of acute effects as measured by the FOB included changes in posture, decreased arousal, disturbances in gait, delayed righting reflexes, and decreased sensorimotor reactivity. Animals were then divided into five groups and exposed 30 min/day to either air or one of four concentrations of TCE (2,000, 6,000, 10,000, or 13,300 ppm) for 15 consecutive days. The TCE concentration used primarily affected the magnitude of change, not whether tolerance or sensitization occurred. Tolerance developed on the measures of forelimb grip strength, inverted screen, and number of rears. Conversely, sensitization developed to measures of locomotor activity.

CONCLUSION

Depending on the behavioral measure, both tolerance and sensitization can occur in mice with repeated exposure to TCE. Both of these phenomena are characteristic of drugs of abuse.

Sustained attention in the mouse: A study of the relationship with the cerebellum

To explore the role of the cerebellum in sustained attention, the authors tested lurcher, wildtype, and lurcher chimeric mice with zero, normal, and variable numbers of Purkinje cells, respectively, in a previously validated task of sustained attention. Results indicate that lurcher mice had a deficit in performance likely related to their motor disability, whereas lurcher chimeras performed similarly to wildtype controls. Presentation of auditory or visual distracters caused deficits in the performance of all mice that were specific to either signal (auditory) or non-signal (visual) events. The authors' results do not support a role of the cerebellum in sustained attention, instead indicating that behavioral changes are an indirect result of motor deficits.

Momentary Brain Concentration of Trichloroethylene Predicts the Effects on Rat Visual Function

The relationship between the concentration of trichloroethylene (TCE) in the brain and changes in brain function, indicated by the amplitude of steady-state pattern-elicited visual evoked potentials (VEP), was evaluated in Long-Evans rats. VEPs were recorded from visual cortex following stimulation of the eyes and, thus, reflect the function of the afferent visual pathway and, in broad terms, may be indicative of overall brain function. The concentration of TCE in the brain at the time of VEP testing (i.e., momentary brain concentration) was hypothesized to predict the amplitude of the VEP across a range of inhalation concentrations, both during and after exposure. Awake restrained rats were exposed to clean air or TCE in the following combinations of concentration and duration: 500 ppm (4 h), 1000 ppm (4 h), 2000 (2 h), 3000 ppm (1.3 h), 4000 ppm (1 h), and 5000 ppm (0.8 h). VEPs were recorded several times during the exposure session, and afterward for experimental sessions of less than 4 h total duration (i.e., concentrations from 2000 to 5000 ppm). The sample collection time for each VEP was about 1 min. Brain concentrations of TCE were predicted using a physiologically based pharmacokinetic (PBPK) model. VEP waveforms were submitted to spectral analysis, and the amplitude of the largest response component, occurring at twice the temporal stimulation rate (F2), was measured. Exposure to all air concentrations of TCE in the study reduced F2 amplitude. The reduction of F2 amplitude was proportional to momentary brain TCE concentration during and after exposure. A logistical function fit to the combined data from all exposure conditions described a statistically significant relationship with 95% confidence limits between brain TCE concentration and F2 amplitude. The results support the hypothesis that momentary brain concentration of TCE is an appropriate dose metric to describe the effects of acute TCE inhalation exposure on rat VEPs. The combination of the PBPK model predicting brain TCE concentration from the exposure conditions with the logistical function predicting F2 amplitude from the brain TCE concentration constitute a quantitative exposure-dose-response model describing an acute change in neurological function following exposure to an important hazardous air pollutant.

A search for residual behavioral effects of trichloroethylene (TCE) in rats exposed as young adults

Trichloroethylene (TCE) is an organic solvent with robust acute effects on the nervous system, but poorly documented long-term effects. This study employed a signal detection task (SDT) to assess the persistence of effects of repeated daily inhalation of TCE on sustained attention in rats. Adult male Long-Evans rats inhaled TCE at 0, 1600, or 2400 ppm, 6 h/day for 20 days (n=8/group) and began learning the SDT 3 weeks later. Rats earned food by pressing one retractable response lever in a signal trial and a second lever in a blank (no signal) trial. TCE did not affect acquisition of the response rule or performance of the SDT after the intertrial interval (ITI) was changed from a constant value to a variable one. Increasing the trial presentation rate reduced accuracy equivalently in all groups. Injections of ethanol (0, 0.5, 1.0, 1.5 g/kg ip) and d-amphetamine (0, 0.1, 0.3, 1.0 mg/kg sc) systematically impaired performance as functions of drug dose. d-Amphetamine (1.0 mg/kg) reduced P(hit) more in the 2400-ppm TCE group than in the other groups. All rats required remedial training to learn a reversal of the response contingencies, which TCE did not interfere with. Thus, a history of exposure to TCE did not significantly alter learning or sustained attention in the absence of drugs. Although ethanol did not differentially affect the TCE groups, the effect of d-amphetamine is consistent with solvent-induced changes in dopaminergic functions in the CNS. Calculations indicated power values of 0.5 to 0.8 to detect main effects of TCE for the three primary endpoints.

Signal detection behavior in humans and rats: a comparison with matched tasks

Animal models of human cognitive processes are essential for studying the neurobiological mechanisms of these processes and for developing therapies for intoxication and neurodegenerative diseases. A discrete-trial signal detection task was developed for assessing sustained attention in rats; a previous study showed that rats perform as predicted from the human sustained attention literature. In this study, we measured the behavior of humans in a task formally homologous to the task for rats, varying two of the three parameters previously shown to affect performance in rats. Signal quality was manipulated by varying the increment in the intensity of a lamp. Trial rate was varied among values of 4, 7, and 10 trials/min. Accuracy of signal detection was quantified by the proportion of correct detections of the signal (P(hit)) and the proportion of false alarms (P(fa), i.e. incorrect responses on non-signal trials). As with rats, P(hit) in humans increased with increasing signal intensity whereas P(fa) did not. Like rats, humans were sensitive to the trial rate, though the change in behavior depended on the sex of the subject. These data show that visual signal detection behavior in rats and humans is controlled similarly by two important parameters, and suggest that this task assesses similar processes of sustained attention in the two species.