Interlaboratory comparison of motor activity experiments: implications for neurotoxicological assessments

Motor activity is an important functional measure used in neurotoxicology. The effects of chemicals on motor activity, however, may depend on variables such as type of measurement apparatus, physical and environmental testing conditions, and many other experimental protocol and organismic variables. Due to the increasing use of motor activity in neurotoxicology, a major question concerns the potential for differences in experimental findings due to variations in sensitivity and reliability between different laboratories and devices used to measure motor activity. This study examined historical data from a number of laboratories that employed different devices and experimental protocols to measure motor activity. Four aspects of the motor activity data were compared: 1) within-laboratory control variability across time; 2) within-laboratory replicability of control data; 3) between-laboratory variability in the effects of chemicals; and 4) between-laboratory comparison of the control rates of habituation. The analyses indicated that there was a relatively restricted range of within-laboratory variability and reliability in control values, and that these ranges were comparable across laboratories. Similar profiles of habituation were also seen across the different laboratories. Moreover, in virtually every case, all laboratories were capable of detecting qualitatively similar changes in motor activity following acute exposure to a variety of chemicals. These data indicate a high degree of comparability in the data generated by the different devices and experimental protocols.

Investigations of amitraz neurotoxicity in rats. IV. Assessment of toxicity syndrome using a functional observational battery

A Functional observational battery (FOB) was utilized to provide a semiquantitative description of the hyperreactivity, excitability, and debilitation produced by amitraz. Adult male Long-Evans rats were administered either vehicle or 10, 25, 50, 100, or 200 mg/kg amitraz ip. They were tested with the FOB immediately before dosing, at 1 and 4 hr, and at 1, 2, 4, and 8 days after dosing. Higher doses (100-200 mg/kg) produced increased reactivity to manipulation, tenseness, and aggression. Most or all doses produced depressed arousal and rearing activity, hypothermia, body weight loss, and autonomic changes including ptosis, chromodacryorrhea resulting in facial crustiness, loss of the pupil reflex, and decreased defecation. Altered gait and decreased landing foot splay were also produced by amitraz. For the most part, effects of lower doses (10-50 mg/kg) were reversible by 2 to 4 days after treatment. In the higher dose groups, however, signs of toxicity were evident, and in some cases even more prominent (e.g., handling hyperreactivity), 8 days after a single dose of amitraz. The FOB thus provided a semiquantitative description of the magnitude and time course of many features of the amitraz toxicity syndrome.

Rat strain and stock comparisons using a functional observational battery: baseline values and effects of amitraz

A functional observational battery (FOB) was utilized to assess the effects of 3-day exposure to the formamidine pesticide amitraz in outbred Sprague-Dawley-derived and inbred Fischer-344-derived (F344) rats (both from Charles River Laboratories) and in outbred Long-Evans rats obtained from two commercial suppliers (Charles River Breeding Laboratories and Blue Spruce Farms). Significant strain and stock differences were obtained in baseline values for one-third of the FOB measures. In most cases, F344 rats were different from the others. Characteristic signs of amitraz exposure consisting of increased excitability, hyper-reactivity, and physiological and autonomic changes were evident in all treated rats. These effects increased with repeated dosing, and many were still present 6 days after dosing. On individual measures, there were differences between the strains and stocks in terms of sensitivity and time course of amitraz effects. In general, Blue Spruce Long-Evans rats displayed more effects of amitraz and F344 rats recovered more quickly than others. Although Sprague-Dawley rats showed the least effect overall, they displayed the largest increases in the sensorimotor responses to stimuli. These data indicate that although some behavioral and physiological parameters showed strain and supplier differences, in both baseline values and the effects of amitraz, conclusions concerning its neurotoxic potential in a screening context would be similar.

Approaches for assessing the validity of a functional observational battery

As neurobehavioral assessments during the preliminary stages of chemical testing are more widely undertaken, it is critical that the screening procedures utilized be valid indicators of neurobehavioral function and that they be sensitive, specific, and reliable. Efforts in this laboratory have been directed towards assessing these features in the use of a functional observational battery (FOB). For the purpose of assessing validity, we have examined FOB data which addresses the issues of criterion, predictive, concurrent, and construct validities. The FOB appears to be valid for detecting chemical-induced neurological dysfunction in rats, i.e., shows a good degree of criterion validity. Furthermore, in many instances the effects observed with the FOB may be predictive of symptomatology in humans. When comparisons can be made between effects detected with the FOB and other methods of measuring neurotoxicity (e.g., neuropathology), concurrent validity can also be established. To assess construct validity, effects of neurotoxicants can be classified into functional domains which are described by various measures in the FOB. Approaches for assessing the validity of the test method thus include answering specific research questions directed at assessing criterion, predictive, concurrent, and construct validity. Available data indicate that, in these aspects, the FOB is a valid screening method for the detection of neurotoxicity.

Neurobehavioral effect of triadimefon, a triazole fungicide, in male and female rats

Triadimefon is a widely used systemic fungicide, yet there is little published information on its effects in mammals. This study describes the effects of triadimefon in male and female rats using a functional observational battery (FOB), motor activity (measured in a figure-eight maze), and operant performance (responding under a fixed-interval 3-min schedule). For the FOB, Long-Evans hooded rats were tested immediately before dosing and 0.5, 4, 24, and 48 hr after IP dosing with either vehicle, 30, 100, or 300 mg/kg triadimefon. Prominent effects of triadimefon (100 and 300 mg/kg) included increased arousal, stereotypies involving repetitive sniffing, head bobbing, and pacing, and self-mutilation. Dose-related handling-induced convulsions, changes in reflexes and sensory reactivity, hypothermia, and body weight loss were also significant findings. Doses of 30, 75 and 150 mg/kg triadimefon increased figure-eight maze activity whereas 300 mg/kg decreased activity. Habituation of activity during the session as well as the spatial distribution within the maze were also affected by triadimefon. Overall rates of responding maintained by fixed-interval milk reinforcement were increased at 30 and 56 mg/kg, and decreased at 100 and 200 mg/kg. Responding within the 3-min fixed-interval was also affected, with low rates normally occurring early in the interval markedly increased. These effects on operant performance were similar to those seen following d-amphetamine, and were attenuated by pretreatment with chlorpromazine (0.5 mg/kg). On many measures, female rats appeared to be somewhat more sensitive than males. Recovery was evident in some measures the day after dosing, but the effects of high doses (greater than or equal to 100 mg/kg) were typically prolonged (several days). Thus triadimefon produced a unique neurotoxic syndrome which is similar in many aspects to that produced by CNS stimulants.

Investigations of amitraz neurotoxicity in rats. III. Effects on motor activity and inhibition of monoamine oxidase

The formamidine pesticide amitraz (AMZ) produces many behavioral and physiological changes in rats. We examined the dose effect and time course of AMZ on motor activity, monoamine oxidase (MAO) activity, and acetylcholinesterase (AChE) activity to evaluate possible neurochemical mechanisms for the behavioral effects of AMZ. For motor activity studies, male Long-Evans hooded rats were tested in photocell activity measurement devices. AMZ produced dose-related decreases in motor activity of rats allowed free access to food and rats maintained at a stable body weight through food restriction. Lowest effective doses of AMZ tested were 1-3 mg/kg, administered 20 min before testing. AMZ appeared to be about three times more potent in food-restricted rats, indicating that amount of body fat may play a significant role in the pharmacokinetics of AMZ. Motor activity returned to control levels over 4-5 days after dosing with 100-200 mg/kg AMZ, whereas recovery was evident the day after administration of low doses (1-30 mg/kg). Inhibition of MAO was measured in whole brain of rats sacrificed at various times after dosing with AMZ. Only greater than or equal to 100 mg/kg AMZ inhibited MAO, which was measurable within 2 hr of dosing and lasted up to 7 days. AMZ appeared to be more selective for type B MAO when given in vivo, although MAO-A was also inhibited at doses greater than or equal to 300 mg/kg. However, no selectivity was indicated by the IC50 values determined in vitro (IC50 = 31 and 28 microM for MAO-A and MAO-B, respectively). AMZ produced only negligible inhibition of AChE at the highest doses administered in vivo or at 10 mM in vitro. These data indicate that while AMZ does inhibit MAO, the dose range over which it produces this action is much higher than that which suppressed motor activity. Thus MAO inhibition is probably not responsible for AMZ-induced alterations in motor activity.

Comparison of chlordimeform and carbaryl using a functional observational battery

The effects of the formamidine pesticide chlordimeform (CDM), and the carbamate carbaryl (CAR) were compared using a functional observational battery (FOB). The FOB, a series of observations and measurements that can be rapidly administered to toxicant-treated rats, includes home-cage and open-field observations, neuromuscular and sensorimotor tests, and physiological measures. Evaluations were made according to U.S. Environmental Protection Agency testing guidelines so as to determine dose-, time-, and sex-related toxicant effects. Long-Evans hooded rats of both sexes were tested initially and then dosed ip with either vehicle, CDM (1, 25, 56 mg/kg) or CAR (3, 10, 30 mg/kg), and tested at various times after dosing (for CDM 1, 5, 24 hr; for CAR 0.5, 3, 24, 48 hr). Both compounds affected general activity (home-cage and open-field), equilibrium, CNS excitability, and sensory responsiveness. Whereas similar decreases were obtained on rearing, gait, and arousal, there were important qualitative differences in the effects of CAR and CDM on reactions to handling and the reflex tests in that CDM increased excitability and enhanced responses to several stimuli but CAR either had no effect or decreased these measures. Only CDM produced an increase in muscle tone as measured by grip strength, and only CAR produced cholinergic autonomic signs of intoxication. Body weight and temperature were decreased by both compounds. Thus, the profiles of effect produced by these two pesticides could be clearly differentiated using the FOB.

An alpha 2-adrenergic mode of action of chlordimeform on rat visual function

The hypothesis that chlordimeform increased the amplitude of components N1P1 and P1N3 in rat pattern-reversal visual evoked potentials through actions on alpha 2-adrenergic receptors was tested with two sets of experiments. First, rats received single injections of either vehicle, an alpha 2-adrenergic antagonist yohimbine (0.1, 0.5, or 2.0 mg/kg), or an alpha 2-adrenergic agonist clonidine (0.05, 0.1, or 0.5 mg/kg). Yohimbine alone had no effect on pattern-reversal evoked potential amplitude. Clonidine treatment produced a dosage related increase in amplitude of both components similar to that produced by chlordimeform (W.K. Boyes and R.S. Dyer, 1984, Brain Res. Bull., 10, 817-823). Second, rats were given double injections of either vehicle or yohimbine (0.05, 0.5, 2.0, or 5.0 mg/kg) followed by either vehicle, clonidine (0.1 or 0.2 mg/kg) or chlordimeform (10, 20, or 40 mg/kg). Yohimbine pretreatment attenuated the effects of subsequent treatment with either clonidine or chlordimeform. These results support the hypothesis that chlordimeform alters rat pattern-reversal evoked potentials through actions as a central nervous system alpha 2-adrenergic agonist.

Investigations of amitraz neurotoxicity in rats. II. Effects on visual evoked potentials

As a part of a series of studies investigating the possible neurotoxicity of amitraz (AMZ), a formamidine pesticide, visual evoked potentials were recorded from Long-Evans rats following acute and short-term repeated exposures to AMZ. The first of three experiments examined the relationship between a single ip injection of AMZ (0, 50, and 100 mg/kg) and the latency and peak-to-peak amplitude of pattern-reversal (PREP) and flash-evoked potentials (FEP). The effects of another formamidine, chlordimeform (CDM; 40 mg/kg), were also studied for comparison purposes. Two hours after treatment, AMZ exposure produced large, dose-related increases in PREP amplitudes. Exposure to CDM produced similar changes. Neither compound changed FEP amplitudes. Body temperatures were reduced and evoked potential peak latencies were increased by both compounds. The latency increases were probably a secondary consequence of hypothermia. In the second experiment, PREPs were recorded before and 2, 24, 48, and 72 hr after treatment with AMZ (100 mg/kg). The time course of changes was biphasic in nature, with increases in amplitudes (N1P1, P1N2, and N2P3) 2 hr after treatment followed by subsequent depression in amplitude (P2N3) at 48 hr. Recovery occurred by 72 hr after treatment. The third experiment examined the effects of three daily treatments with either vehicle or 50 or 100 mg/kg AMZ. Body weights and body temperatures showed dose-related reductions which progressed with each additional treatment and recovered partially by 6 days after cessation of treatment. The PREPs of AMZ-treated rats agains showed biphasic changes, with N1P1 and P1N2 amplitudes significantly increased on each day of treatment and 1-2 days following the third treatment. Amplitude P2N3 showed an initial increase on the first 2 days of treatment, followed by subsequent, progressive amplitude reductions. In summary, AMZ produced two phases of change in visual evoked potentials. The first phase was characterized by large increases in PREP amplitudes without increasing FEP amplitudes in the same rats. The second phase was characterized by suppression of PREP P2N3 amplitude. Short-term repeated exposure produced signs of accumulating intoxication including progressive loss of body weight, lowered body temperature, and prolonged duration of evoked potential changes.

Investigations of amitraz neurotoxicity in rats. I. Effects on operant performance

Amitraz (AMZ) is a formamidine pesticide which is often compared to chlordimeform (CDM). The effects of AMZ (6.25-75 mg/kg) and CDM (5-20 mg/kg) on the schedule-controlled performance of rats were examined using a multiple fixed-ratio (FR) 10 fixed-interval (FI) 300-sec schedule of milk reinforcement. Following dose-effect determinations, rats received three daily doses of AMZ (50 mg/kg). Low to intermediate doses of AMZ (6.25-25 mg/kg), administered 20 min presession, significantly decreased FI responding but not FR responding. In contrast, CDM appeared to decrease responding similarly under both components. Both compounds disrupted the temporal pattern of responding within the interval; AMZ had only a moderate influence at all effective doses but CDM produced a marked dose-dependent disruption of temporal response patterning. The effects of the high doses of AMZ (50-75 mg/kg) were more pronounced 24 hr after dosing, whereas the rats had recovered from lower doses at this time. Performance was progressively disrupted and the rats' health rapidly deteriorated during the course of three daily injections of AMZ (50 mg/kg). Operant performance recovered more quickly than did the general health of the rats. Thus AMZ produced effects of multiple-schedule performance that were distinct from the effects of CDM. Moreover, the signs of intoxication and the effects on schedule-controlled behavior following a high dose (50 mg/kg) were augmented and persistent with short-term repeated administration.

Cholinergic involvement in the action of formetanate on operant behavior in rats

Formetanate (FMT) is a formamidine acaricide/insecticide with a carbamate moiety in its molecular structure. FMT-induced lethality is reportedly due to inhibition of acetylcholinesterase. Here we report evidence of the neurochemical basis for the sublethal, behavioral effects of FMT in rats. In this experiment, 0.5 mg/kg of FMT (5 min before the 55-min test session) produced a pronounced suppression of response rates in rats trained to lever-press under a multiple fixed-interval 1-min fixed-interval 5-min schedule of milk reinforcement. Injections of scopolamine (0.1 mg/kg) and methylscopolamine (0.1 mg/kg) 15 min before FMT blocked the response rate suppression, whereas pretreatment with either mecamylamine (2 mg/kg) or hexamethonium (2 mg/kg) did not. These data suggest that FMT acts as an indirect agonist on central and peripheral muscarinic receptors, by inhibiting acetylcholinesterase, to produce changes in schedule-controlled responding.

The effects of inhaled toluene, halothane, 1,1,1-trichloroethane, and ethanol on fixed-interval responding in mice

Adult male mice were trained to lever press under a fixed-interval (FI) 60-sec schedule for milk presentation during 15-min sessions. Concentration-effect curves were then determined at the termination of 30-min inhalation exposures to various volatile agents--halothane, toluene, 1,1,1-trichloroethane (1,1,1-TCE), and ethanol. The mice were also tested after ethanol administered orally. Each compound produced both response rate increases and decreases, initially or during the recovery phase. Recovery from the response rate decreasing effects generally occurred during the 15-min sessions. The highest concentration of toluene and the high dose of oral ethanol produced the longest duration of effects. Response rates in the first 3 min of the session were used to evaluate peak effects following inhalation. Linear regression analyses of these data indicated the potency order to be toluene greater than halothane greater than 1,1,1-TCE greater than ethanol. Patterning of responding characteristics of FI performance was affected after high concentrations of all but ethanol vapor. This study provided evidence that the effects of two industrial solvents, toluene and 1,1,1-TCE, were qualitatively similar to those of two volatile CNS depressants, halothane and ethanol, as well as to those reported for other centrally-acting drugs.

Differential effects of formamidine pesticides on fixed-interval behavior in rats

Chlordimeform (CDM), amitraz (AMZ), and formetanate (FMT) are members of the formamidine class of pesticides. To date, effects on operant behavior have been determined only for CDM. This experiment compared the effects of CDM, AMZ, and FMT on schedule-controlled responding. Nine male Long-Evans rats were trained during 1-hr sessions to lever-press under a multiple fixed-interval (FI)-1-min FI-5-min schedule of milk reinforcement. Dose-effect determinations for each compound administered ip, 20 min presession, were carried out in each subject. The dose ranges were: CDM HCl, 0.3-20 mg/kg; FMT HCl, 0.03-0.75 mg/kg; and AMZ, 5-75 mg/kg. Under baseline conditions response rates were higher under the FI-1-min than under FI-5-min, and index of curvature (IOC) values (a measure of within-interval response patterning) were generally higher under FI-5-min. All compounds produced dose-dependent decreases in response rate. CDM significantly decreased only FI-1-min response rates; a similar effect of AMZ was seen only at an intermediate dose. FMT decreased responding to the same extent in both components. CDM produced pronounced changes in the pattern of responding in both components, with IOC decreased more under FI-5-min than under FI-1-min. AMZ produced significant decreases in IOC only under FI-5-min. FMT did not appreciably decrease IOC in either component. High doses of AMZ produced general signs of poor health that persisted for several days. In addition, a greater effect on response rates and IOC in both components was obtained when AMZ (75 mg/kg) was given more than 10 days following another dose compared to when it was given 7 days or less after another dose. Formamidine pesticides produce differential effects on FI schedule-controlled behavior that are in turn modulated by the parameter value of the FI schedule.

Yohimbine attenuates the delayed lethality induced in mice by amitraz, a formamidine pesticide

We have found that a single dose of amitraz, a formamidine pesticide, produces death in mice 2-5 days after dosing. To further examine this phenomenon, adult albino mice of both sexes were treated with either yohimbine (10 mg/kg, i.p.) or deionized water (6 ml/kg, i.p.), immediately before an injection of amitraz (600 mg/kg, i.p.) and twice daily thereafter for 8 days. Male mice treated with water were more susceptible than water-treated females to the lethal effects of amitraz. In addition, yohimbine treatment significantly decreased the number of deaths in both sexes. These data suggest that an alpha 2-adrenergic mechanism is involved in the delayed lethality produced by amitraz.

Effects of xylene isomers on operant responding and motor performance in mice

Xylene, a widely used industrial solvent, is a mixture of the ortho-, meta-, and paraisomers. In this study we examined the effects of each individual isomer, as well as a commercial-grade mixture of xylenes, on two behavioral measures: (1) operant performance of 15 mice trained to lever-press under a DRL (differential reinforcement of low rates) 10-sec schedule, and (2) motor performance of mice on an inverted screen test. The 15-min operant sessions immediately followed 30-min exposures to solvent vapors (500 to 7000 ppm), or air, in static inhalation chambers. All mice were exposed to each isomer in a counterbalanced order, and the mixture was given last in all cases. Ortho-, meta-, para-, and mixed xylenes produced similar biphasic effects on response rates, and concentration-dependent decreases in reinforcement rates. The lowest significantly effective concentration for each isomer on any variable was 1400 ppm, where increases in response rates occurred. Half-maximal response rate decreases were produced by 5179 ppm (ortho-xylene) to 6176 ppm (meta-xylene). The temporal distribution of responses was only moderately disrupted, even at high concentrations. In other groups of mice, motor coordination was also disrupted by the xylenes in a concentration-dependent manner. Half-maximal effective concentrations were 2676 ppm (para-xylene) to 3790 ppm (meta-xylene). Minimally effective concentrations were 2000 to 3000 ppm, higher than those seen in the operant studies. Xylene produced pronounced behavioral actions following acute exposure and no substantial differences in overall effects, and only slight potency differences, were obtained between the individual isomers or the commercial mixture.

Monoamine oxidase inhibition cannot account for changes in visual evoked potentials produced by chlordimeform

Chlordimeform (CDM), a formamidine insecticide and monoamine oxidase (MAO) inhibitor, has recently been shown to produce large changes in visual evoked potentials of hooded rats (Boyes and Dyer, 1984a). Two experiments were performed to determine if the changes in evoked potentials were a result of the inhibition of MAO. In the first, the degree of inhibition of MAO in the brains of rats treated with chlordimeform (1.0-100 mg/kg, i.p.) was compared with that produced by pargyline (0.3-30 mg/kg, i.p.). Both compounds preferentially inhibited MAO-B, although MAO-A was substantially inhibited at larger doses. Pargyline was a relatively more potent inhibitor of MAO than chlordimeform, but not more efficacious. In the second experiment, pattern reversal evoked potentials (PREPs) and flash-evoked potentials (FEPs) were recorded from groups of rats after treatment with either saline, 0.4 mg/kg pargyline, 20 mg/kg pargyline or 40 mg/kg chlordimeform. The latter two groups were selected so as to have similar levels of inhibition of MAO, about 90% inhibition of MAO-B and 60% inhibition of MAO-A. The results showed a doubling of the amplitude of pattern reversal evoked potentials and increased latencies of the pattern reversal evoked potential and the flash-evoked-potentials in the chlordimeform-treated group, but no significant changes from saline control values in the pargyline-treated groups. These results confirm that chlordimeform is a MAO inhibitor at doses which produce behavioral and electrophysiological changes, but demonstrate further that the changes in visual evoked potentials produced by chlordimeform are not a direct result of the inhibition of MAO.

Effects of toluene, halothane and ethanol vapor on fixed-ratio performance in mice

The behavioral effects of inhalation of the vapors of volatile compounds representative of different chemical groups were studied in mice under conditions where behavior and exposure concentrations could be concurrently monitored. The magnitude and time course of the effects of toluene, halothane and ethanol inhalation on fixed-ratio (FR) responding were compared. The subjects were trained to lever-press under a FR-100 schedule of water reinforcement. Daily operant sessions took place in the exposure chambers, and solvent exposures were conducted once a week. The test exposures lasted for 20 min, and the sessions continued until the subjects resumed baseline rates of responding to give a measure of recovery. All solvents produced concentration-dependent response rate decreases, and only halothane showed any evidence of response rate increases at low concentrations. Halothane quickly produced maximal response rate-decreasing effects and recovery was rapid, while the effects of toluene became progressively greater during the exposure and recovery was prolonged. Ethanol displayed the most rapid onset and recovery of effects. Thus, these solvents produced somewhat similar effects on FR responding but displayed potency and time course differences.

Acute motor and lethal effects of inhaled toluene, 1,1,1-trichloroethane, halothane, and ethanol in mice: effects of exposure duration

Some acute effects of inhalation exposure to toluene, 1,1,1-trichloroethane (1,1,1-TCE), halothane, and ethanol were examined in mice. Lethality and performance on an inverted screen test of motor performance were measured following 10-, 30-, and 60-min exposures. Concentration-dependent effects were obtained on both measures for all solvents except that lethal concentrations of ethanol could not be produced under these exposure conditions. Lethality increased with longer exposures for toluene, 1,1,1-TCE, and halothane. Sensitivity to the motor effects of 1,1,1-TCE, halothane, and ethanol increased when exposure duration was increased from 10 to 30 min, with no further change with 60-min exposures. In contrast, behavioral sensitivity to toluene increased over the entire range of exposure durations. The relative lipid solubilities of the solvents correlate with potency for behavioral activity but not as well with potency for lethality. The ratio of potency for motor and lethal effects depended on the chemical studied and in some cases exposure duration. These results demonstrate that both concentration and exposure duration determine the effects of inhaled compounds, but a simple linear relationship does not exist and it depends upon the effect measured.

Concurrent measurement of solvent vapor concentrations and effects on operant behavior using a dynamic exposure system

A method is described for exposing mice, lever-pressing on an operant schedule for water reinforcement, to continuously-generated solvent vapors. Concurrent monitoring of chamber concentration and schedule-controlled behavior allows a correlation of the magnitude and time course for behavioral effects with changes in the levels of solvent exposure. Data are presented showing the acute effects of 1,1,1-trichloroethane over the range of 1000-8000 ppm on behavior maintained by a fixed-ratio 100 schedule of water presentation. Concentration-dependent decreases in response rates with a rapid onset and recovery were obtained.