Developmental and behavioral changes in the rat during chronic exposure to lead.

Young male and female Sprague-Dawley rats were given drinking water containing 5 or 50 ppm Pb for 40 days prior to mating. Pregnant females were continued on these regimens throughout gestation and lactation. After weaning the offspring were similarly exposed through adulthood. Reflex development, body weights, and locomotor activity were measured in the offspring. Significant delays were noted in the development of the righting reflex at 5 and 50 ppm and in eye opening at 50 ppm. No difference was observed in development of the startle reflex at either dose. Mean body weights of treatment groups during this developmental period were not significantly different from controls. Locomotor activity was measured in adult males utilizing a residential maze. Both levels of lead produced a significant reduction in locomotor activity. When groups were treated with d-amphetamine (4.0 mg/kg subcutaneous), lead treatment caused a dose-related diminution in the amphetamine-induced hyperactivity. These results indicate that rats exposed to low levels of lead from conception until adulthood show a delay in nervous system development. As adults, these animals exhibit hypoactivity and decreased responsiveness to amphetamine.

The U.S. federal framework for research on endocrine disruptors and an analysis of research programs supported during fiscal year 1996.

The potential health and ecological effects of endocrine disrupting chemicals has become a high visibility environmental issue. The 1990s have witnessed a growing concern, both on the part of the scientific community and the public, that environmental chemicals may be causing widespread effects in humans and in a variety of fish and wildlife species. This growing concern led the Committee on the Environment and Natural Resources (CENR) of the National Science and Technology Council to identify the endocrine disruptor issue as a major research initiative in early 1995 and subsequently establish an ad hoc Working Group on Endocrine Disruptors. The objectives of the working group are to 1) develop a planning framework for federal research related to human and ecological health effects of endocrine disrupting chemicals; 2) conduct an inventory of ongoing federal research programs; and 3) identify research gaps and develop a coordinated interagency plan to address priority research needs. This communication summarizes the activities of the federal government in defining a common framework for planning an endocrine disruptor research program and in assessing the status of the current effort. After developing the research framework and compiling an inventory of active research projects supported by the federal government in fiscal year 1996, the CENR working group evaluated the current federal effort by comparing the ongoing activities with the research needs identified in the framework. The analysis showed that the federal government supports considerable research on human health effects, ecological effects, and exposure assessment, with a predominance of activity occurring under human health effects. The analysis also indicates that studies on reproductive development and carcinogenesis are more prevalent than studies on neurotoxicity and immunotoxicity, that mammals (mostly laboratory animals) are the main species under study, and that chlorinated dibenzodioxins and polychlorinated biphenyls are the most commonly studied chemical classes. Comparison of the inventory with the research needs should allow identification of underrepresented research areas in need of attention.

Effects of noinionizing radiation on the central nervous system, behavior, and blood: a progress report.

This paper presents a progress report on the U. S. research which has been designated as collaborative research with the Soviet Union to study the biological effects of nonionizing radiation on the central nervous system, behavior, and blood. Results of investigations to study the effects of microwaves on isolated nerves, synaptic function, transmission of neural impulses, electroencephalographic recordings, behavior, and on chemical, cytochemical and immunological properties of the blood are presented. Specifically, the effects of microwave exposure on chick brain and cat spinal cords, on EEG patterns of rats, on behavioral of neonatal rats exposed during development, on behavior of adult rats, on behavior of rhesus monkeys and on the pathology, hematology, and immunology of rabbits will be reported in a summary format. Much of the information is new and has not been published previously.

Ontogeny of the acoustic startle response and sensitization to background noise in the rat.

The purpose of this study was to characterize the ontogeny of the acoustic startle response (ASR) and response sensitization to background noise in preweanling rats. Animals were tested daily from 11 to 21 days of age using one of four sets of background white noise levels [45-80 dB(A)]. With constant low-level (45 dB, SPL) background noise, response latency decreased steadily with age, whereas both response incidence and amplitude increased nonmonotonically with age. Two approaches were used to examine the ontogeny of sensitization to background noise: The first compared the ASR of animals tested at 75 dB background noise with ones tested at 45 dB; the second compared the ASR of animals tested at three background levels (30 dB range) within the test session. Sensitization was not evident before 15-16 days of age. By comparing these results with the results from naive animals, it was found that daily test experience does not alter ASR amplitude, latency, incidence, or the development of sensitization.

Research to strengthen the scientific basis for health risk assessment: a survey of the context and rationale for mechanistically based methods and models.

Assessment of health risks is an integral part of regulatory decision-making that occurs at the interface between science (e.g. facts) and policy (e.g. values). Because existing scientific knowledge and understanding are often inadequate to answer the most critical risk-related questions, regulatory agencies have developed sets of formalized 'science policies' to extrapolate from existing data to real-life events and situations. These science policies, as, for example, the use of default assumptions or exposure scenarios, can introduce significant uncertainties into the final risk estimate. We survey the rationale for research to reduce extrapolation-related uncertainties, focusing specifically on the need to develop mechanistically based methods and models, including test methods to identify and characterize health effects, integrated human exposure models, physiologically based pharmacokinetic (PBPK) models and biologically based dose-response (BBDR) models.

Behavioral toxicity of trialkyltin compounds: a review.

Triethyltin (TET) and trimethyltin (TMT) are neurotoxic organotin compounds which produce different patterns of toxicity in adult animals. Exposure to TET produces behavioral toxicity (decreased motor activity, grip strength, operant response rate and startle response amplitude) which reflects impaired neuromotor function. These deficits are consistent with the reported myelin vacuolation and cerebral edema produced by TET, and with its direct effects on muscle. Exposure to TMT produces both hyperactivity and impaired learning and performance. These impairments are consistent with reported neuronal cell death produced by TMT, particularly in limbic system structures. While the behavioral deficits produced by repeated exposure to TET are reversible when dosing is terminated, the behavioral impairments produced by a single exposure to TMT appears to be irreversible.

Neurobehavioral toxicity of triethyltin in rats as a function of age at postnatal exposure.

Triethyltin (TET) has been shown to be neurotoxic when injected on postnatal day (PND) 5. In the present experiment we examined the toxicity of a single exposure to TET at several postnatal ages. Rat pups were injected ip with 0 (saline), 1.5, 3.0, or 6.0 mg/kg TET bromide on PND 1, 5, 10 or 15. In agreement with our previous data, PND-5 exposure to 6 mg/kg TET produced behavioral toxicity and decreased adult brain weight. High dose pups were less successful in descending on a rope at 20 and 21 days of age, and were hyperactive in figure-eight mazes at 29-30 and 57-58 days of age. The spatial distribution of activity was also altered: photocell counts were increased primarily in the figure-eight area of the maze. The size of the milk bands was reduced in 6 mg/kg pups injected on either PND 1 or PND 5. Preweaning growth was decreased following all injection ages; this reduction was most pronounced for pups exposed to TET on PND 1 and PND 5. Mating behavior was disrupted in 6 mg/kg males irrespective of age at exposure. These data demonstrate a differential sensitivity to the toxicity of TET during postnatal life, with maximal susceptibility on PND 5.

Neonatal triethyltin exposure alters adult electrophysiology in rats.

In adults, triethyltin (TET) produces degeneration of white matter, edema, vacuolization on myelin and histotoxic hypoxia. To determine the functional consequences of perinatal exposure to TET, albino rats were administered either 0, 3, 6, or 9 mg/kg TET on postnatal day 5. Upon reaching adulthood, the rats were implanted with electrodes for recording visual evoked potentials (VEPs) and hippocampal afterdischarges (ADs). In addition to these tests, 17 days of kindling trials were administered to the rats followed by testing with pentylenetetrazol and picrotoxin for seizure susceptibility. TET increased latencies of P2, P3, and N3 of the VEP in a dose dependent fashion. TET also decreases N1P2 amplitudes and produced gender-specific alterations in both P1, N1, and N2 latencies and N2P3 amplitudes. TET produced alterations in duration of the AD recorded from cortex during kindling, but did not produce significant alterations in any of the other variables tested. The results support previous studies, since they show that the adult VEP is sensitive to perinatal toxicant exposure.

Trimethyltin disrupts acoustic startle responding in adult rats.

Trimethyltin (TMT) is a limbic-system toxicant which also produces sensory dysfunction in adult animals. In the present experiment, we examined the effects of TMT on the acoustic startle response. Adult male, Long-Evans rats (N = 12/dose) received a single i.p. injection of either 0, 4.0, 5.0 or 6.0 mg/kg TMT hydroxide as the base. The number of responses, latency and peak amplitude of the startle response to a 13 kHz, 120 dB tone were measured 2 h, 2 weeks, and 4 weeks after dosing. For each test session, 10 stimuli were presented at each of three background noise levels (50, 65 and 80 dB). By 2 h after dosing, the number of responses and response amplitude were decreased following 4.0-6.0 mg/kg TMT; these treatment effects persisted through 4 weeks after dosing. Increases in latency were also seen following all dosages of TMT. These data suggest that TMT produces disruption of function within the acoustic-startle pathway.

Pyrethroid insecticides and radioligand displacement from the GABA receptor chloride ionophore complex.

Radioligand binding displacement studies were conducted to determine the effects of Type I and II pyrethroids on [3H]flunitrazepam (FLU), [3H]muscimol (MUS), and [35S]t-butylbicyclophosphorothionate (TBPS) binding. Competition experiments with [3H]FLU and [3H]MUS indicate a lack of competition for binding by the pyrethroids. Type I pyrethroids failed to compete for the binding of [35S]TBPS at concentrations as high as 50 microM. Type II pyrethroids inhibited [35S]TBPS binding to rat brain synaptosomes with Ki values ranging from 5-10 microM. The data presented here suggest that the interaction of Type II pyrethroids with the gamma-aminobutyric acid (GABA) receptor-ionophore complex is restricted to a site near the TBPS/picrotoxinin binding site.

Serotonergic modulation of the acoustic startle response in rats during preweaning development.

The involvement of serotonin (5-HT) in modulating the acoustic startle response (ASR) is well established in adult rats, but 5-HT involvement during the preweaning period, when 5-HT neurons undergo extensive development, has not previously been described. Three 5-HT receptor subtypes are reported to modulate the ASR in adult rats: 5-HT1A and 5-HT2 receptor agonists facilitate the ASR, whereas 5-HT1B agonists decrease the response. In the present study, the effects of 5-HT agonists and generalized 5-HT depletion on the ASR were studied in preweanling animals, using independent groups of Long-Evans rats tested on postnatal day (PND) 13, 17 and 21. 8-Hydroxy-2-(di-n-propylamino) tetralin (8OHDPAT, 62-1000 micrograms/kg), a 5-HT1A receptor agonist, and 5-methoxy-N,N-dimethyl tryptamine (MeODMT, 2-4 mg/kg), a nonselective 5-HT agonist, had no effect on PND 13 and then increased the ASR on PND 17 and 21. The 5-HT2 receptor antagonists cyproheptadine (5 mg/kg) and ketanserin (5 mg/kg) blocked the effect of MeODMT at both ages, providing some evidence that MeODMT increased the ASR through 5-HT2 receptors. 1-(m-Chlorophenyl) piperazine (mCPP, 1-5 mg/kg), a 5-HT1B agonist, had no effect on ASR amplitude on PND 13 or 17 and then produced a dose-related decrease in the response on PND 21. Generalized depletion of 5-HT by 80-90% in whole-brain and spinal cord, using p-chlorophenylalanine (PCPA, 300 mg/kg 24 hr prior to testing), did not alter ASR amplitude at any age.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute behavioral toxicity of carbaryl and propoxur in adult rats.

Motor activity and neuromotor function were examined in adult CD rats exposed to either carbaryl or propoxur, and behavioral effects were compared with the time course of cholinesterase inhibition. Rats received an IP injection of either 0, 2, 4, 6 or 8 mg/kg propoxur or 0, 4, 8, 16 or 28 mg/kg carbaryl in corn oil 20 min before testing. All doses of propoxur reduced 2 hr activity in a figure-eight maze, and crossovers and rears in an open field. For carbaryl, dosages of 8, 16 and 28 mg/kg decreased maze activity whereas 16 and 28 mg/kg reduced open field activity. In order to determine the time course of effects, rats received a single IP injection of either corn oil, 2 mg/kg propoxur or 16 mg/kg carbaryl, and were tested for 5 min in a figure-eight maze either 15, 30, 60, 120 or 240 min post-injection. Immediately after testing, animals were sacrificed and total cholinesterase was measured. Maximum effects of propoxur and carbaryl on blood and brain cholinesterase and motor activity were seen within 15 min. Maze activity had returned to control levels within 30 and 60 min whereas cholinesterase levels remained depressed for 120 and 240 min for propoxur and carbaryl, respectively. These results indicate that both carbamates decrease motor activity, but behavioral recovery occurs prior to that of cholinesterase following acute exposure.

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.

Chemical exposures and animal activity: utility of the figure-eight maze.

In 1975, Norton and coworkers (11) described a figure-eight maze system for measuring motor activity in rodents. Since their original report, a number of studies have appeared in the literature which have utilized the figure-eight maze to evaluate the behavioral toxicity of a variety of chemical and physical agents (Table 1). The present paper describes a number of salient features of the maze which have emerged from these studies which support its continued use in toxicological studies. These features are described in the context of EPA's Test Guidelines for motor activity (4).

Neurotoxicology in regulation and risk assessment.

It is well recognized that a wide variety of chemicals exert toxic effects on the structure and function of the nervous system. Regulatory agencies, charged with the protection of human health from toxicant exposures, should therefore consider neurotoxicological evaluations in the risk assessment process. Over the last decade, several expert panels have recommended testing schemes for evaluating neurobehavioral toxicity. In general, these panels have recommended behavioral measures performed in tandem with neuropathological evaluations. In keeping with these recommendations, the Environmental Protection Agency has developed seven neurotoxicity test guidelines for use in evaluating new and existing chemicals. Further refinement and extension of these test guidelines will flow from a better understanding of the cellular/molecular events which underly neurotoxicant-induced functional alterations.

Effects of two pyrethroid insecticides on motor activity and the acoustic startle response in the rat.

To better characterize the behavioral toxicity of pyrethroid insecticides, comparisons were made of the effects of cismethrin and deltamethrin exposure on motor activity and the acoustic startle response in male Long-Evans rats. Acute dose-effect, acute time course, and 30-day repeated-exposure determinations of 1-hr motor activity were made using figure-eight mazes. The acoustic startle response was measured to a 13-kHz, 120-dB(A), 40-msec tone at each of three background white noise levels (50, 65, and 80 dB). Deltamethrin (0, 2, 6, or 8 mg/kg) or cismethrin (0, 6, 12, 18, or 24 mg/kg) were administered po in 0.2 ml/kg corn oil. Cismethrin and deltamethrin produced similar dosage-dependent decreases in motor activity. The time course of onset and recovery for this decreased activity was rapid (1 to 4 hr) No cumulative effects on motor activity of a 30-day exposure to 2 mg/kg/day deltamethrin or 6 mg/kg/day cismethrin were found. The effects of cismethrin and deltamethrin on the acoustic startle response were dissimilar: deltamethrin produced a dosage-dependent decrease in amplitude and an increase in latency, and cismethrin produced an increase in amplitude and no change in latency. The differential effects of cismethrin and deltamethrin on the acoustic startle response may be related to the contrasting effects previously shown with neurophysiological and/or neurochemical techniques.

Motor activity: a survey of methods with potential use in toxicity testing.

Activity measurements are expected to have widespread use in toxicity testing. The multifaceted nature of motor activity will directly influence the selection of a measurement technique since the relative contribution of various motor acts to any particular measurement will depend upon the detection method. Because of the apparatus-dependent nature of motor activity measurements, it is recommended that consideration be given to how accurately the various devices measure locomotor activity. In the present paper, two types of body movement will be considered as locomotor activity: ambulation (horizontally directed movement) and rearing (vertically directed movement). Discussion focuses on the various methods currently used to record motor activity, the various components of motor activity which are likely to be recorded, and the advantages and disadvantages of these techniques for the measurement of locomotor activity. Finally, consideration is given to studies which have compared treatment effects on motor activity derived from two or more measurement techniques.

The effects of type I and II pyrethroids on motor activity and the acoustic startle response in the rat.

Recent data have demonstrated that the in vivo effects of low dosages of two pyrethroids, cismethrin and deltamethrin, can be differentiated. Two behavioral tests, locomotor activity and the acoustic startle response (ASR), were utilized to separate the behavioral actions of Type I and II pyrethroids using permethrin, RU11679, cypermethrin, RU26607, fenvalerate, cyfluthrin, flucythrinate, fluvalinate and p,p'-DDT. Dosage-effect functions for all compounds were determined for both figure-eight-maze activity and the ASR in the rat. All compounds were administered po in 1 ml/kg corn oil 1.5-3 hr prior to testing. All compounds produced dosage-dependent decreases in locomotor activity. The Type I compounds, permethrin and RU11679, along with p,p'-DDT, increased amplitude and had no effect on latency to onset of the ASR. In contrast, the Type II pyrethroids, cypermethrin, cyfluthrin, and flucythrinate, decreased amplitude and increased the latency to onset of the ASR. Fenvalerate increased the amplitude, had no effect on latency, but unlike the other compounds tested, increased ASR sensitization. Fluvalinate had no effect on any measure of the ASR. These data provide further evidence of the differences between the in vivo effects of low dosages of Type I and II pyrethroids, and extend the findings of our previous work to other representatives of the two classes of pyrethroids.

Pyrethroid insecticides and the gamma-aminobutyric acidA receptor complex: motor activity and the acoustic startle response in the rat.

Two behavioral tests, motor activity and the acoustic startle response (ASR), were used to test for dose-addition of cismethrin, a Type I, or deltamethrin, a Type II pyrethroid, with compounds active at the gamma-aminobutyric acid (GABAA) receptor complex (picrotoxin, muscimol and chlordiazepoxide). Additivity was assessed using a simplified version of isobolographic analysis using chlorpromazine and haloperidol as positive controls for dose-additivity. Dosage-effect functions for all compounds were determined for both motor activity and the ASR. The effects of various combinations of chlorpromazine (0.5-4.0 mg/kg) and haloperidol (0.05-0.2 mg/kg) on motor activity indicate dose-addition. To test for dose-addition of pyrethroids and GABAergic compounds, cismethrin (3-18 mg/kg) or deltamethrin (2-6 mg/kg) were administered 90 min before testing, either alone, or before treatment with picrotoxin (0.25-2.0 mg/kg), muscimol (0.6-2.5 mg/kg) or chlordiazepoxide (2.5-10 mg/kg) administered 20 to 30 min before testing. All compounds produced dosage-dependent decreases in motor activity. Muscimol and picrotoxin decreased ASR amplitude, increased ASR latency and reduced ASR sensitization to increasing background noise levels. Chlordiazepoxide had no effect on any measure of the ASR. Results from the interaction studies indicate dose-addition of the effects of picrotoxin and deltamethrin on motor activity and the ASR. Additivity of dose was not seen with any other combination. These data suggest that the in vivo effects of the Type II pyrethroid deltamethrin may be due in part to interaction with the picrotoxinin binding site of the GABAA receptor-ionophore complex. In addition, these results are consistent with reported differential effects of the two classes of pyrethroids on the GABAA receptor complex.